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Christian Schorr, Mika Muinonen & Fiia Nurminen

TORREFACTION OF BIOMASS

Julkaisu 1 / 2012

Publication 1 / 2012

6.3.2012

LAAJENNETTU TIIVISTELMÄ

Taustaa

Tämä julkaisu on selvitys puun torrefioinnista eli paahtamisesta, jolla puubiomassasta tuotetaan biohiiltä. Biohiili on uusiutuva polttoaine ja tulevaisuudessa sillä tulee olemaan merkittävä osuus fossiilisen hiilen korvaamisessa energiantuotannossa. Selvitys keskittyy torrefiointiprosessin perustoimintoihin, eli mekanismeihin jotka ovat läsnä kaikissa torre-fioinnin teknisissä sovellutuksissa, sekä pelletointiin.

Torrefiointiprosessi

Torrefiointi tarkoittaa biomassan paahtamista korkeassa lämpötilassa (250 °C) hapetto-missa olosuhteissa niin, että veden lisäksi osa haihtuvista yhdisteistä poistuu. Prosessin aikana biomassan painosta katoaa 30 %, mutta sen sisältämästä energiasta vain 10 %. Torrefioidulla biomassalla on samankaltaiset käsittelyominaisuudet kuin kivihiilellä. Torre-fioinnin tarkoitus on parantaa biomassan palamisominaisuuksia ja lämpöarvoa sekä kehit-tää sen käsittelyominaisuuksia niin, että sitä voidaan polttaa olemassa olevissa kivihiili-voimaloissa. Torrefiointiprosessi voidaan jakaa viiteen vaiheeseen, joista ensimmäinen on alkulämmi-tys (initial heating). Lämmitysvaiheessa biomassa lämmitetään pisteeseen, jossa vesi ei vielä haihdu. Lämpöä käytetään ainoastaan nostamaan biomassan lämpötilaa, ja vaihe loppuu kun vesi alkaa haihtua. Seuraava vaihe on esikuivaus (pre-drying), jossa lämpötila säilyy muuttumattomana, mutta vapaa vesi haihtuu biomassasta. Esikuivatusvaiheen en-simmäisessä vaiheessa biomassan kosteuspitoisuus laskee lineaarisesti, jolloin haihtumis-nopeus riippuu ympäristön olosuhteista. Kun massan kosteuspitoisuus laskee kriittiseen pisteeseen, alkaa toinen vaihe, jossa haihtuvan veden täytyy tunkeutua materiaalin läpi, mikä tarvitsee enemmän energiaa ja aikaa, koska vahvemmat kapillaarivoimat on voitet-tava. Kun kaikki massaan sitoutunut vesi on haihtunut, alkaa jälkikuivatus- ja keskitason-lämmitysvaihe (post-drying and intermediate heating). Lämpötila alkaa jälleen nousta, ja fyysisesti sitoutunut vesi vapautuu hitaasti, kunnes biomassassa ei käytännössä ole enää kosteutta. Tässä vaiheessa ensimmäiset haihtuvat yhdisteet, kuten terpeenit, voivat haih-tua, mikä tarkoittaa että ensimmäiset kiinteät yhdisteet kaasuuntuvat ja massa pienenee. Tämä vaihe lakkaa, kun lämpötila nousee 200 °C:een, jossa määritelmän mukaan alkaa torrefiointivaihe. Torrefiointivaiheen ollessa koko prosessin ydin, vaiheeseen kuuluu sekä lämmitys että jäähdytys (cooling). Välissä on myös vaihe, jolloin lämpötila säilyy muuttu-mattomana, ja yleensä tämä on koko prosessin korkein lämpötila. Lämpötilan noustessa jopa 300 °C:een, tapahtuu pyrolyyttistä hajoamista, ja biomassa paahtuu. Torrefiointivai-heessa massasta häviää merkittävä osa. Prosessi on periaatteessa endoterminen, mutta käytännössä jotkut hiukkaset vastaanottavat enemmän lämpöä ja saavuttavat lämpötila-tason, jossa reaktiot ovat eksotermisiä. Määritelmän mukaan torrefiointivaihe loppuu, kun lämpötila laskee jälleen 200 °C:een. Reaktioaika käsittää ajan, jolloin materiaali on

lämmitetty 200 °C:sta haluttuun lämpötilaan, joka pidetään vakiona halutun ajan. Aika, joka kuluu materiaalin viilentyessä jälleen 200 °C:een, on jätetty pois reaktioajasta, vaikka se kuuluu torrefiointivaiheeseen. Tämä siksi, että suurin osa termisesti epävakaista yhdis-teistä biomassassa on jo hajonnut, ja lämpötilan alkaessa laskea niitä on enää hyvin vähän tai ei ollenkaan jäljellä. Reaktion voidaan siis katsoa loppuneen, kun lämpötila alkaa las-kea. Viimeinen vaihe on kiinteän aineen jäähdyttäminen, jolloin massa jäähdytetään ha-luttuun loppulämpötilaan. Tämä tehdään anaerobisissa olosuhteissa johtuen syttymis- tai jopa räjähdysvaarasta. Torrefiointiprosessin lopputuotteet riippuvat käytetystä biomassasta. Lopputuotteet voi-daan jakaa kiinteisiin ja kaasumaisiin yhdisteisiin. Kaasumaiset yhdisteet voidaan jakaa edelleen tiivistyviin tai nestemäisiin ja ei-tiivistyviin tai pysyvästi kaasumaisiin aineisiin. Hemiselluloosan hajoamisesta syntyy pääasiassa hiilimonoksidia ja hiilidioksidia. Karbok-syyliryhmien lämmittämisestä syntyy karbonyylejä, kuten metanolia, propioaldehydiä ja muita hiilivetyjä. Nestemäiseksi tiivistyvät lopputuotteet jakautuvat vedeksi, orgaanisiksi aineiksi ja rasvoiksi. Nestemäisiä aineita, joita tutkimuksissa on todettu, ovat mm. maito-happo, muurahaishappo, furfuraali, hydroksyyliasetoni ja metanoli, mutta myös fenoleja on löydetty. Nestemäiset lopputuotteet ovat kuitenkin pääasiassa vettä ja etikkahappoa. Kiinteät aineet ovat mm. sokereita ja uudelleen muodostuneita tai uusia polymeerejä. Aromaattisen renkaan muodostuminen on mahdollista, kuten myös kivihiilen tapaisten hiilirakenteiden ja tuhkan muodostuminen. Pysyvät kaasut ovat pääasiassa hiilimonoksi-dia ja hiilidioksidia, joiden happi on vapautunut alkuperäisistä biomassan yhdisteistä, sekä pienissä määrin molekyylivetyä ja metaania. Vaikka prosessissa syntyvä kaasuseos yleensä poltetaan prosessin lämmittämiseksi ennen kun nesteeksi tiivistyvät aineet ehtivät pois-tua seoksesta, on kuitenkin tärkeä tuntea kaasun koostumus, jotta sen palamisominai-suuksia voidaan tutkia. Yleisesti biomassamolekyylit voivat lämmön vaikutuksesta reagoi-da ja muodostaa uusia yhdisteitä, tai pysyä muuttumattomina. Torrefioinnissa tapahtuvat reaktiot ovat kuitenkin monimutkaisia eikä niitä vielä täysin ymmärretä, joten selvitys keskittyy vain täysin ymmärrettyihin ja todennettuihin reaktioihin. Ideaalitilanteessa torrefiointi on autoterminen prosessi, mikä tarkoittaa tasapainoa pro-sessin aikana vapautuvien haihtuvien yhdisteiden sisältämän kemiallisen energian ja sen polttoprosessin myötä systeemin palauttaman lämpöenergian välillä. Näin ollen energia kiertää systeemissä, eikä ulkoisia energialähteitä tarvita. Tämän saavuttamiseksi prosessin parametrit on optimoitava viipymäajan ja torrefiointilämpötilan suhteen, käytetty bio-massa sekä prosessin malli huomioon ottaen. Jos torrefiointikaasun energiasisältö ei ole riittävä, on sekaan lisättävä toista polttoainetta, esimerkiksi maakaasua. Tämä kuitenkin nostaa operointikustannuksia. On myös mahdollista polttaa käsittelemätöntä biomassaa lämmön tuomiseksi systeemiin. Torrefioinnin taloudellista potentiaalia voidaan kasvattaa tiivistämällä torrefioitua bio-massaa, esimerkiksi puristamalla siitä brikettejä tai pellettejä. Koska energiatiiviyden kas-vaessa kuljetuskustannukset pienenevät, on biohiilipellettejä mahdollista hyödyntää kau-empana tuotantopaikasta.

Kivihiilen osittainen korvaaminen biohiilipelleteillä

Soveltuvimmat käyttökohteet biohiilipelleteille liittyvät sen rinnakkaispolttoon olemassa olevissa kivihiilivoimalaitoksissa. Maailman kivihiilivoimaloista 90 % on hiilipölykattiloita, ja suuri osa lopusta 10 %:sta on leijupetikattiloita, ja nämä ovatkin kaksi tärkeintä tekno-logiaa rinnakkaispolttoon liittyen. Yleisesti ottaen rinnakkaispoltto voidaan jakaa suoraan ja epäsuoraan rinnakkaispolttoon. Suorassa yhteispoltossa biomassa yhdistetään kivihiilen syöttöön, jolloin kattilaan syötetään biomassan ja kivihiilen sekoitusta. Biomassaa voidaan siis polttaa olemassa olevissa hiilikattiloissa, mutta monet sen osat saattavat vaatia suuria muokkauksia. Epäsuorassa yhteispoltossa biomassa ja biohiili poltetaan erillisissä katti-loissa, ja vasta prosessien höyryt tuodaan yhteen turbiinien pyörittämiseksi. Jälkimmäinen vaihtoehto vaatii suuremmat investointikustannukset, mutta myös käyttökustannukset voivat olla korkeammat. Useimmissa tapauksissa suora yhteispoltto on edullisempi vaih-toehto, joten se on yleisesti suositeltavampi. Biomassan rinnakkaispolton järjestäminen hiilivoimaloissa on helpointa kun toissijainen polttoaine vastaa ominaisuuksiltaan kivihiiltä. Koska torrefioidulla biomassalla on korke-ampi lämpöarvo kuin käsittelemättömällä biomassalla, se voi ratkaista ongelman jossain määrin. Kun tarkastellaan polttoaineiden ominaisuuksia, bitumisen kivihiilen kanssa tulisi käyttää vain torrefioitua, kuivaa puuta, kun taas kostea biomassa sopii paremmin poltet-tavaksi ruskohiilen kanssa. Biohiilipellettien rinnakkaispoltto kivihiilen kanssa vähentää kivihiilen poltossa syntyviä päästöjä. Biomassan rikkipitoisuus on luontaisesti matalampi, mutta se myös sitoo rikin kemiallisesti tuhkaansa, joten rikkidioksidia muodostuu vähemmän. Savukaasukoostu-muksen muuttuminen voi vaikuttaa esimerkiksi kattilakorroosioon, varsinkin jos savukaa-suissa on liikaa klooria. Korkea klooripitoisuus ei kuitenkaan ole todennäköinen kun käy-tetään puuta raaka-aineena, koska puun ja kivihiilen klooripitoisuudet ovat samankaltai-set. Tutkimukset ovat osoittaneet, että typenoksidipäästöjä voidaan laskea biomassan rinnakkaispoltolla. Hiilimonoksidipitoisuuksien ei odoteta nousevan. Päästöt huomioon ottaen hiilivoimalan päästötasapaino ei heikkene biomassan rinnakkaispolton myötä, jois-sain tapauksissa biomassan lisääminen voi jopa olla edullista. Biomassan laatu on kuiten-kin oleellista, ja puu onkin paras raaka-aine verrattuna olkibiomassaan tai lietteeseen, joka on kaikista ongelmallisinta. Biohiilipellettien (TOP-pelletti) rinnakkaispolton vaikutuksia ei ole vielä päästy suuressa mittakaavassa tutkimaan, mutta tämän hetken tietämyksen mukaan biohiilipellettien osuus kivihiilen rinnakkaispoltossa voi olla useita kymmeniä prosentteja, jopa puolet, kun tavallisilla puupelleteillä osuus on noin 15 – 20 %. Myös biohiilipellettien biomassateho on moninkertainen puupelletteihin nähden. Kun pyritään nostamaan uusiutuvien energialäh-teiden osuus käytetystä polttoaineesta kivihiilikattiloissa merkittäväksi (30 – 50 %), torre-fioitu biomassa on yksi parhaista vaihtoehdoista. Suomalaisillakin energiayhtiöillä on bio-hiilipelletteihin liittyen meneillään useita projekteja, joissa näitä vaikutuksia pyritään sel-vittämään.

Teknologiset vaihtoehdot

Torrefiointilaitos voidaan sijoittaa kivihiiltä käyttävän voimalaitoksen yhteyteen, jolloin lopputuote hyödynnetään sähkön ja lämmön tuotannossa. Voimalaitoksen hukkalämpöä voidaan hyödyntää torrefiontireaktorissa ja torrefiontilaitoksen mahdolliset kaasut voi-daan puhdistaa voimalaitoksen savukaasunpuhdistusprosessissa. Yhteistyön toteuttami-seksi olemassa olevat voimalaitokset vaatisivat kuitenkin muutoksia, mikä voi heikentää sähkötuotannon tehokkuutta. Lisäksi prosessit voivat tulla liian riippuvaisiksi toisistaan, jolloin ongelmat toisessa prosessissa voivat heijastua toiseen. Suurin torrefioinnin hyöty jäisi näin kuitenkin saavuttamatta, eli edullisemmat kuljetuskustannukset. Voimalaitoksesta erillisen torrefiointilaitoksen sijainti vaatii hyvää infrastruktuuria ja suu-ria biomassalähteitä laitoksen läheisyydessä. Prosessin täytyy tällöin tuottaa itse vaati-mansa lämpöenergia, mikä voi tarkoittaa raakabiomassan polttoa ja torrefiointikaasun hyödyntämistä. Hyötynä voidaan kuitenkin pitää pienempiä kuljetuskustannuksia niin raakabiomassan kuljetuksessa torrefiointilaitokselle kuin valmiin polttoaineen kuljetuk-sessa voimalaitoksille johtuen suuremmasta energiatiheydestä. Kolmas vaihtoehto on siirrettävä torrefiontilaitos, joka on sijoitettu esimerkiksi kuorma-autoon. Etuna on, että prosessi voidaan toteuttaa missä tahansa, missä on paljon (jäte)biomassaa. Mitä toden-näköisimmin tällainen systeemi ei voi saavuttaa kiinteän laitoksen tehokkuutta ja kapasi-teetin voidaan olettaa olevan hyvin rajoittunut. Tällaiselle systeemille on kuitenkin kysyn-tää erityisesti alueilla, joilla on paljon biomassaa mutta ongelmallinen infrastruktuuri. Toinen torrefiointitekniikkaan liittyvä tekijä on tarvittavan lämmön tuominen prosessiin ja partikkeleihin pyrolyyttisen hajoamisen mahdollistamiseksi. Reaktorin rakenne määritte-lee, minkälainen lämmityssysteemi tulee kysymykseen, mutta pääkategoriat ovat suora ja epäsuora lämmitys. Suorassa lämmityksessä lämmin torrefiointikaasun polton savukaasut tai torrefiointikaasu itsessään johdetaan suoraan biomassan sekaan sen kuivatusvaihees-sa. Epäsuorassa lämmityksessä sen sijaan lämpö johdetaan kiinteän seinän läpi reaktoriin. Torrefiointikaasu johdetaan polttokammioon ja savukaasut antavat lämpönsä välittäjäai-neelle, joka kiertää torrefiointireaktorin ja lämmönvaihtimen välillä. Epäsuora lämmitys vaatii enemmän panostusta, mutta se estää lämmönvälityksen kielteiset vaikutukset. Keskeiset teknologiset erot eri ratkaisuissa ovat torrefiointireaktoreissa, ja mahdollisia reaktoriteknologioita on useita. Mahdollisia ovat mm. pyörivä rumpureaktori, ruuvikulje-tin reaktori, mikroaaltoreaktori, liikkuva peti ja värähtelevä hihnakuljetin. On kuitenkin myös erityisesti pyrolyyttisiin sovelluksiin, kuten torrefiointiin, kehitettyjä reaktoreja, esimerkiksi Torbed –reaktori, monikerrosuuni (Multiple Hearth Furnance) ja Wyssmont-Turbo-Dryer® –reaktori. Kaupallisen kokoluokan biohiilipelletin tuotantolaitoksia ei juuri ole vielä käynnissä, mutta demonstrointilaitoksia on jo käynnissä. Hollannissa on meneillään käyttöönottovaihe lai-toksesta, jonka tuotantokapasiteetti on 60 000 tonnia vuodessa. Johtavaa tutkimusta tehdään erityisesti Hollannissa ja Belgiassa sekä Pohjois-Amerikassa, ja laitoksia on suun-nitteilla ympäri maailmaa. Tällä hetkellä on ainakin 60 yritystä, joilla on torrefiointiin liit-tyviä hankkeita kehitysasteella. Suurin osa näistä on pieniä yrityksiä, jotka eivät pysty vas-taamaan suuren kokoluokan torrefiointilaitoksen pystyttämisestä, mutta myös osa suuris-

ta insinööritoimistoista on aktiivisia tällä sektorilla, kuten Stamproy Green ja KEMA. Lisäk-si monet eurooppalaiset sähköyhtiöt kehittävät torrefiointisysteemejä yhdessä tytär- ja yhteistyöyritysten kanssa. Merkittävimpiä yrityksiä torrenfiointisektorilla ovat mm. ECN, FoxCoal, Topell Energy, Stramproy Green ja 4Energy Invest Hollannista ja Belgiasta, rans-kalainen Thermya, yhdysvaltalaiset Integro Earth Fuels, Zilkha Biomass, Airex Energy ja Terra Green Energy sekä kanadalainen Biomass Secure Power. Torrefiontilaitoksen perustamisessa keskeinen haaste on sopivan menetelmän valinta. Eri teknologioista ei vielä ole riittävästi tietoa hyötyjen ja puutteiden arvioimiseksi, ja toisaal-ta lopputuotteen laatu riippuu sekä alkuperäisen biomassan laadusta että sen käsittelystä reaktorissa. Tästä syystä on tärkeää tietää käytettävän biomassan raaka-ainekoostumus jo hyvissä ajoin tarkoituksenmukaisen teknologian valitsemiseksi. Varsinkin kuoreen varas-toituneiden haitallisten aineiden vapautuminen päästöihin voidaan ehkäistä sopivalla teknologialla.

Yhteenveto

Varsinkin Euroopassa fossiilisten polttoaineiden käytön lainsäädäntö muuttuu tiukem-maksi ja monimutkaisemmaksi, joten uusiutuvien energialähteiden käyttö voimaloissa voi johtaa taloudellisiin hyötyihin päästökaupan ja kansallisten tukien myötä. Vihreiden ener-gialähteiden käyttö voi myös hyödyttää markkinointia, kun voidaan osoittaa että ympäris-tön suojelua on toteutettu. Kivihiilen korvaamiseksi biomassoilla on eri vaihtoehtoja, tärkeimpänä kaasutus, torrefi-ointi ja bioöljy. Torrefioidun biomassan keskeisiä etuja on sen sopivuus hiilipölykattiloissa käytettäväksi sellaisenaan, edut kuljetuksessa ja varastoinnissa sekä käsittelyssä hiilimyl-lyssä ja tulipesässä. Torrefioidun biomassan arvioidaan korvaavan noin 50 % käytetystä hiilestä. Biohiili soveltuu käytettäväksi olemassa olevissa CHP-kattiloissa, eikä sen käyt-töönotto vaadi suuria muutosinvestointeja. (VTT) Torrefioidusta biomassasta voidaan myös puristaa biohiilipellettejä (TOP-pelletti), mikä helpottaa tuotteen varastointia ja kul-jetusta. TOP-pellettien etuna on suurempi energiatiiviys, mikä johtaa pienempiin kulje-tuskustannuksiin ja soveltumiseen kivihiilen rinnakkaispolttoon. Biohiili jauhautuu helpos-ti, mikä pienentää energian kulutusta pelletointivaiheessa ja toisaalta edesauttaa hiilipö-lykattiloissa polttamista. Hiileen verrattuna biohiilen käyttö alentaa hiilidioksidipäästöjä merkittävästi, koska materiaali on hiilidioksidineutraalia. Torrefioitu biomassa on hydro-fobista, jolloin se ei ime kosteutta säilytyksessäkään, eikä se hajoa tai syty itsestään ja on näin helpompi kuljettaa ja varastoida. Ominaisuuksiltaan biohiili vastaa kivihiiltä, mikä helpottaa sen käyttöä olemassa olevissa pölypolttolaitoksissa. Prosessoinnista johtuen raaka-aineen laatuvaatimukset ovat matalat.

CONTENTS

LAAJENNETTU TIIVISTELMÄ ..............................................................................................

ALKUSANAT .......................................................................................................................

PREFACE ............................................................................................................................

1 INTRODUCTION ....................................................................................................... 1

1.1 Historical development of pelletizing and torrefaction ................................ 1

1.2 Problems with the status quo and state of affairs ........................................ 2

2 TORREFACTION ....................................................................................................... 5

2.1 Introduction ................................................................................................... 5

2.2 Basic torrefaction pattern ............................................................................. 7

2.3 Lignocellulose biomass .................................................................................. 8

2.4 Thermo-chemical conversion of lignocellulose biomass ............................ 13

2.5 Fuel Refinement .......................................................................................... 23

3 ALTERNATIVE PROCESS CONCEPTS AND SPECIFICATIONS ................................... 29

3.1 Autothermal operation ............................................................................... 29

3.2 On-site and off-site ...................................................................................... 29

3.3 Comparison of direct- and indirect heating ................................................ 31

3.4 Future trends ............................................................................................... 33

4 CO-FIRING OF UNTREATED BIOMASS AND TORREFIED WOOD IN COAL POWER

PLANTS ......................................................................................................................... 34

4.1 Biomass co-firing in general ........................................................................ 34

4.2 Pretreatment ............................................................................................... 35

4.3 Milling .......................................................................................................... 36

4.4 Expected performance of torrefied wood in coal power plants ................. 37

5 CONCLUSION ........................................................................................................ 42

PICTURE LIST .....................................................................................................................

REFERENCES ......................................................................................................................

ALKUSANAT

Helmikuussa 2012 Mikkelin seudulla käynnisti toimintansa Biosaimaa -klusteri, joka on alueen yritysten, tutkimuslaitosten, rahoittajien, viranomaisten ja muiden sidosryhmien yhteinen toimija. Toiminnan painopisteenä on metsäenergia ja keskeisenä tavoitteena on bioenergialiiketoiminnan kasvattaminen. Klusterin toimintaa koordinoi Miktech Oy. Klus-terin yhtenä kärkihankkeena on Ristiinan biologistiikkakeskus, jonka yhteyteen on suun-nitteilla suuren kokoluokan biohiilipellettitehdas. Metsähaketta raaka-aineena käyttävän laitoksen tuotantokapasiteetti tulee olemaan 200 000 t/a. Tämä teknologiaselvitys on osa investointihankkeen esiselvitystä. Selvityksen ohjaajana on toiminut kehityspäällikkö Mika Muinonen Miktech Oy:stä. Selvi-tyksen on laatinut Christian Schorr Bingenin ammattikorkeakoulusta Saksasta. Suomen-kielisen tiivistelmän laati insinööriopiskelija Fiia Nurminen Mikkelin ammattikorkeakou-lusta. Selvitystyö on toteutettu osana Osaamiskeskusohjelmaa ja Uusiutuva metsäteollisuus –osaamisklusteria. Mikkelissä 6.3.2012 Tekijät

PREFACE

In February 2012, the Biosaimaa-cluster was formed in Mikkeli region in Finland. Bi-osaimaa is a joint actor of the region’s businesses, research institutions, funding agencies, authorities and other stakeholders. The focus is on forest energy and the main goal is to increase the bio-energy business. The cluster is coordinated by Miktech Ltd. One of the main projects of the cluster is launching the large scale torrefied pellet (TOP-pellet) facto-ry acting in association with the biologistic centre in Ristiina, Southern Savonia. The facto-ry would use the forest chips as a raw material, and its production capacity will be 200 000 t/a. This technology study is part of the Ristiina’s biologistic centre project. The supervisor of the survey has been the Development Manager Mika Muinonen from Miktech Ltd. The research is drawn up by Christian Schorr from Bingen University of Ap-plied Sciences in Germany. The Finnish abstract is written by the engineering student Fiia Nurminen from Mikkeli University of Applied Sciences in Finland.. This survey was conducted as a part of Centre of Expertise-programme and the Forest Industry Future cluster programme. Mikkeli, 6.3.2012 Authors

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1 INTRODUCTION

1.1 Historical development of pelletizing and torrefaction

The increasing demand for renewable fuels – due to the scarcity of resources, the partly abandonment of nuclear power and environmental consciousness being mainly a conse-quence of climate change – forces the energy industry to find suitable alternatives for fossil energy carriers. Bioenergy has been used by men since the very verifiability of human existence and still is – in the natural form of raw biomass – the primary energy source in poor countries today. With the coming of fossil fuels the industrial countries no longer had great needs for bio-mass as an energy carrier. Not until the occurrence of the oil crisis in 1970s bioenergy attracted attention again, first in the USA and Canada, where the pelleting of wood – a rather simple mechanical process – was developed to commercial scale for purposes of domestic and industrial heating. In contrast to America the European countries entered the pellet market not until the early 1980s, with Sweden and Denmark leading the way. Today wood pellets are a common fuel for domestic heating with a steadily growing mar-ket especially in the European Union. However, against the background of the energy turnaround, the industry is looking for new ways to use woody biomass and pellets more effectively in power generation and heat supply.1 In consequence of the rising complexity of energy supply on the one hand and the in-crease of power consumption on the other, a variety of processes has been developed for effective ways of biomass-utilization besides standard combustion, including amongst others liquefaction, gasification, carbonization and pyrolysis, differing in both tempera-ture and excess air ratio. All of them are thermal-chemical conversion processes, resulting in the production of enhanced secondary fuels that can be transformed into useable en-ergy under both time-wise and spatial decoupling. One relatively new process – in terms of bioenergy – is torrefaction, a technology that was developed from the coffee industry, where the coffee beans get roasted to make them brittle and to gain their special flavors for the final product. More detailed infor-mation of the process itself shall not be given at this point. A pilot plant for biomass torrefaction was engineered and built in France by the company Pechiney in the mid-1980s, though the torrefied biomass served the purpose of a reduc-ing agent in an aluminium production process and not for energy reasons. Nevertheless, the plant with a production capacity of roughly 12,000 t/a worked well in terms of the technology, but still was demounted due to economic aspects in the early 1990s, for the energy efficiency of approximately 70 % caused high total productions costs in the range

1DOLZAN, P. et al.: Global Wood Pellets Markets and Industry – Policy Drivers, Market Status and Raw Mate-

rial Potential; 2007

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of 150 to 180 €/t. The investment amounted to nearly 3 Mio. €, resulting in specific costs of 240 €/t of installed annual capacity, respectively 25 €/t.2

1.2 Problems with the status quo and state of affairs

This survey in hand deals with a relative new process within the frame of energy engi-neering – named torrefaction – that allows converting raw biomass into a high-efficient fuel with similar handling qualities to fossil coal. Torrefaction is not yet commercially op-erated, mainly due to processing challenges, but also caused by the technology’s unclear sector acceptance and its unassigned position in the supply chain of biomass. The follow-ing explication shall reveal the current state of torrefaction activities and the general problems of the technology’s development on a global scale. Worldwide there are several projects on torrefaction aiming at an extension of the ther-mo-chemical process`s capacity, that would allow it to become a serious alternative in the large-scale industry environment. International Standardization is currently being pre-pared under the number ISO 17255-1, which will include special regulations for thermally treated materials and thereby differ from the standard for biofuels ISO TC 238.3 In a global view there is one location where R&D referring to torrefaction is focused. That is the Netherlands, together with Belgium, where several demonstration plants are either being planned or already being operated in test procedures. But there are also a few mi-nor projects from small companies in the rest of Europe – mostly in France, Spain and Scandinavia. In the United States there are also several companies working on the innova-tion. So far there are about sixty companies recognized to deal with the torrefaction pro-cess with all of them having projects still in a pre-commercial state. Unfortunately, for most cases there is no information available on the size of those groups. The impression gained by enquiry is that quite a big amount of those known companies has less than twenty employees and consequently won´t be capable to deliver the full service of engi-neering work for an industrial torrefaction plant`s construction. But at the same time there are some big engineering companies with strong reputation and well-known con-sultant firms active in this sector (Stramproy Green, KEMA and more). Towards bigger companies from the coal and boiler industries can only be guessed whether they are bus-ying themselves with torrefaction. Besides, major European electric power companies commissioned some of their subsidiaries – for example RWE Innogy working with the Dutch company Topell Energy4 and Nuon of Vattenfall cooperating with ECN5to advance their torrefaction systems. Several companies applied patents for their own developed torrefaction technologies, now waiting for clients to contract. For many firms this is the obstacle before they are

2BERGMAN, P.C.A. et al.: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”;

2005 3ALAKANGAS, E. et al.: Preliminary results of EUBIONET III industrial pellet questionnaire; 2011

4http://www.rwe.com/web/cms/de/37110/rwe/presse-news/pressemitteilung/?pmid=4002215;

08.01.2012 5http://www.nuon.com/press/newsfacts/20100607/; 08.01.2012

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able to manufacture big-scale torrefaction plants: By doing so, they perhaps would bring the development of the process into the next big and final phase, possibly the last step to have reliable and well-engineered torrefaction processes. The Netherlands seems to be ahead in this case: Most of the torrefaction production units being in planning phase now are developed in this West European region and so the operators and developers of these demonstration plants may have significant advantages towards other firms, due to the fact that they will gain experiences with the operating of torrefaction factory components in industrial scale much earlier. That lead in know-how could make them hard to overtake for the rest of the world.

Picture 1 Torrefaction development activities in Europe [red marker: developing com-pany headquarters; blue marker: planned or realized pilot plant]. Created with Google maps; 2011

But also in the US-state of Georgia the construction of one torrefaction plant will take place in the near future for sure. Furthermore there are an increasing number of torrefac-tion developers in the United States –although, in contrast with Europe, widely scattered over the continent.

4

Picture 2 Torrefaction development activities in the USA and Canada [red marker: de-veloping company headquarters; blue marker: planned or realized pilot plant]. Created with Google maps; 2011

R&D is also made in Canada where the pellet and energy industry recognized the im-mense potential of torrefaction and initiate the possible change in the market`s struc-ture.6The research is strongly focused in the region around Vancouver, where most de-veloping companies’ headquarters are. Torrefaction referring company or institute names you may hear most often these days are among others ECN, FoxCoal, Topell Energy, Stramproy Green, 4Energy Invest from the Netherlands and Belgium, Thermya from France and Integro Earth Fuels, Zilkha Biomass, Airex Energy and Terra Green Energy from the United States and Biomass Secure Power from Canada. That might be caused by announcements of those companies to build torre-faction demo-plants.7 So it seems that what the torrefaction branch is lacking at the mo-ment is investment, so that developers can overcome the existing difficulties and can get aware of those that will not occur until large-scale torrefaction has been tested. For companies planning to work with torrefaction-technology from the developers this results in another challenge: To choose the right technology for their most likely unique demands is not an easy task, because firstly there is not much known of the real weak-nesses and benefits of the different technologies nowadays and secondly the qualities of the torrefaction products depend crucially on the input material as well as the specially therefore applied treatment in the reactor. There are first statements and recommenda-tions about which technologies are most promising, though there still is quite an absence

6 LAPOINTE, D. et al.: Overview of Torrefaction Activities in Canada;2011

7 MEIJER, R.: Overview of European torrefaction landscape; 2011

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of information to find a well-considered decision8. The companies dealing with torrefac-tion, especially the ones not having a contracted client and therefore making no progress, claim their secret own-developed technology to be superior over competitive or promote torrefaction with arguments that obviously are whitewashing the truth. Due to that, the current state of the torrefaction technology can hardly be estimated and badly needs an independent and reliable comparative analysis9. Against this background the objectives of this report were to explain the torrefaction technology’s current state of the art, the reactions, phenomenona and mechanisms of its processing, according to present knowledge and to show the latest trends in its develop-ment.

2 TORREFACTION

2.1 Introduction

One synonym for torrefaction is mild pyrolysis, which can be derived from the fact that torrefaction actually is simply the first step of this thermo-chemical conversion under modified process parameters and it takes place in a reactor that can be designed in dif-ferent ways. Nevertheless, as torrefaction still is under development various references present different values for the process parameters and the product’s properties, that are dependent on the input biomass and the concrete realization of the respective testing plant. Thus the data for the process’s temperature range differ slightly – yet in general set within the scope of 200 to 320 °C - the pressure level is in the neighborhood of atmos-pheric conditions while the residence time in the reactor is usually significantly higher than in the original pyrolysis treatment, resulting in low heating rates of < 50 °C/min. The values for optimal residence times lie in the range of thirty to ninety minutes. Still, a few numbers of experiments were made with short residence times of only a few minutes but higher temperatures which won’t be majorly considered in this survey, due to the rarity of such processing. However the torrefaction takes place, the process always has to be realized in the absence of oxygen in an inert atmosphere, due to the hazards of ignition and explosion of the modified material.10 The primary goal is to refine raw biomass to an upgraded solid fuel, including better han-dling qualities and enhanced combustible properties simile to fossil coal’s, leading to de-creased costs, but financial gains. The essential principle in this respect is to increase the energy density of the biomass, requiring a growth of the ratio between energy and mass. Consequently the calorific value of the torrefied biomass increments as well, since it is a specific value reflecting the released energy per mass unit for solid fuels. At this point the difference between the LHV and the HHV shall be shortly defined so there are no misunderstandings possible during further reading of this report. As woody

8 CIOLKOSZ, D. et al.: A review of torrefaction for bioenergy feedstock production; 2011

9 DHUNGANA, A.: Torrefaction of Biomass; 2011

10 BERGMAN, P.C.A.: Combined torrefaction and pelletisation - The TOP Process; 2005

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biomass contains hydrogen, water will be formed as one of the products of combustion. In contrast to condensation where energy is liberated during the change of phase, the vaporization of water requires a specific amount of energy defined as the enthalpy of evaporation. As appropriate it is absolutely crucial for the energy balance of an incinera-tion process – especially for fuels with high hydrogen contents – whether the water re-leased during combustion has the form of steam or condensate.

m mass of water formed by combustion hfg enthalpy of vaporization of water in kJ/kg ug specific internal energy of vapor in kJ/kg uf specific internal energy of liquid in kJ/kg

However, since for all practical purposes the water released by combustion escapes in the form of vapor, in this survey the LHV is applied and meant by the term of calorific value whenever it occurs. Still, it must be kept in mind that the rate of combustion is not taken account of by neither LHV nor HHV. A fuel’s calorific value can be measured by chemical analysis in accord with the Dulong’s formula or with the aid of bomb calorimeters in la-boratory.11 During the torrefaction process the input biomass loses about 30 % of its mass, but only 10 % of its energy, due to the degassing of low-energy volatile compounds and the escape of moisture, eventuating in a higher energy density of the biomass of roughly 30 % more energy per mass unit. However, there are even more advantages of the torrefied bio-mass, when compared to the untreated feedstock biomass or conventional wood pellets. Biochemical torrefaction mechanisms cause the biomass’s changed structure, leading to new properties that make the handling of the final product much easier and also offers the possibility to utilize it in existing coal-fired boilers as will be shown in the related chapter. To anticipate the benefits shortly: The grindability of the input biomass can be increased significantly by torrefaction due to the modification of its molecular structure, so that existing problems arising with untreated biomass in the milling component of a coal pow-er plant are overcome. Also the biomass exchanges its hydrophilic properties to hydro-phobicity that allows an effortless storage that goes hand in hand with a greater re-sistance against biological degradation, self-ignition and physical decomposition in gen-eral. However, the risk of biological degradation is not overcome completely, but fungal growth and microbial activity is reduced, since the torrefied material stays very dry.12Since the torrefied product already loses a great amount of volatiles during the thermo-chemical conversion, there are less remaining for the following combustion step.

11

RAJPUT, R.K.: Engineering Thermodynamics, 2009 12

BERGMAN, P.C.A.: Combined torrefaction and pelletisation – The TOP Process; 2005

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That might lead, maybe even more than for conventional biomass, to lower emissions in terms of “sulfur dioxide (SO2), nitrogen oxides (NOx) and net greenhouse gas emissions of carbon dioxide (CO2)”13, but also a diminished level of ash formation, respectively a new composition of the inorganic residues. As one can expect, each of these issues has a tre-mendous potential in both ecological and economical views. However, there still are many unproven torrefaction technologies, whose benefits and disadvantages can hardly be guessed for practical reality. For example the ideas of direct or indirect co-firing. Nevertheless, the technical differences are quite obvious. The unit operations and process arrangements are predominantly the same, although the perfor-mance conditions vary strongly. Another feature that differs crucially is the component design, especially the reactor’s, since the process’s development has not yet progressed to a point of such knowledge that allows statements based on facts, on which design is really superior to other ones at industrial scale. The resulting perception is simply that more research and an independent comparison of the technologies still is needed.14 Real-izing the fact that it is not possible to deal with all the different technology types for every component or to consider all deviances from the common process arrangement in this text, the report will show the fundamental torrefaction process, expanded with optional pelleting, meaning the mechanisms that take place in any of the process’s modification on one hand and the basic process arrangement that seems to be the most effective one from a present-day perspective on the other. However, there will be one chapter that shortly shows the technology trends, the alternative ways on how torrefaction may be conducted and realized in detail, to fulfill the responsibility of considering the current state of the art in a fast moving industry.

2.2 Basic torrefaction pattern

13

DIETENBERGER et al.: The encyclopedia of materials – science and technology; 2001 14

MELIN, S.: Torrefied wood – a new emerging energy carrier; 2011

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Picture 3 Basic exemplary operation arrangement for biomass torrefaction

The figure above shows a possible basic arrangement of operations in a wood refinement plant via torrefaction. From the storage of feedstock biomass the material firstly gets sift-ed (and usually screened) for impurities and is subsequently ground to smaller particles of a required size. Following this, the biomass is actively dried, which is one crucial operation in the process, since the drying requires remarkable amounts of heat. The torrefaction afterwards roasts the material as explained above in the absence of oxygen. Usually the torrefied material must be cooled down, since there may occur high reactive fine matter that may explode when it gets in contact with oxygen. Optionally, yet probably recom-mendable, is a downstream densification, to further improve the handling qualities of the product and to increase the volumetric energy density of the solid product. Afterwards there is a screening to check the product quality, preparing the biomass for the further handling and delivery to clients.

2.3 Lignocellulose biomass

One important method in applied natural science is to reduce complex issues to a certain scale by defining a suitable system that serves the understanding of the specific problem. For a better insight of the torrefaction process later, this chapter will show up the compo-sition and properties of wood - as far as it is relevant for its transformation to a solid bio-fuel via torrefaction - by presenting its most important compounds and structures in this context.

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The chemical composition of wood can be described as a mixture of organic polymers, significantly smaller amounts of minerals and a minority of other inorganic and organic compounds. These organic polymers can be assigned to three main groups, as they are hemicellulose, cellulose and lignin, combined together known as the lignocellulose frac-tion. The distribution of these three components differs from wood type to wood type. Nevertheless, all wood species being in line for torrefaction in Finland share the same property of a lignocellulose mass fraction in excess of 90 %. It is important to state that each of these three main groups represents a vast number of different polymers, but all having common features which are essential for certain qualities of the wood. From a biological perspective the plant cell wall, where the lignocellulose substances oc-cur exclusively, is one crucial component of the plant’s structure and co-determines the plants characteristics as it causes the size and form of the cell and tissue, but is also re-sponsible for the plants statics and stability. Most important for torrefaction is the fact, that each of these main groups has a specific temperature range where they show signs of thermal degradation due to their different molecular structures. Another meaningful aspect is the nature of cellulose to bundle to fibrils that cause the biomass`s fibrous structure, making it resistant to mechanical forces. In the following subchapters these three main components will be illustrated in more de-tail15.

2.3.1 Hemicellulose

As will be explained later the hemicellulose fraction is the most important group for the torrefaction process. Hemicellulose is one major component in the eukaryotic cell walls - more precisely in the primary and in the case of wood also in the secondary and tertiary cell wall – forming the matrix for cellulose fibrils pervading it. Thus the hemicellulose sub-stances mould the framework of the woody plant cell, providing the biomass with stabili-zation structures and mechanical strength. From a chemical point of view hemicellulose belongs to the polysaccharide class, meaning long chains of sugar molecules like galac-tose, mannose, glucose, and xylose. To be more accurate hemicellulose is an amorphous mixture polymer in forms of chains built by 500 to 1000 sugar units consisting mainly of C5-monomers and partly of C6-monomers. A view on one hemicellulose molecule shows that it is of a ramose structure, with irregular branches.

2.3.2 Cellulose

With about 40 – 50 % on mass basis it is the main component of wood and the most common organic compound on earth: Approximately one half of the biosphere’s organic carbon is bound in cellulose. Like hemicellulose it belongs to the group of polysaccha-rides, although it consists of longer, rigid and linear polymeric chains with basic units of C6-monomers – alternating α- and β-glucose – joining more than 10,000 monomers to-gether. Due to the alternation of α- and β-glucose the three dimensional structure is strongly influenced so that – unlike to hemicellulose - no molecular branches can be built. Cellulose congregates to fibrils inside a matrix of hemicellulose and pectins, whereby the

15

EICHHORN, S.E: Biology of Plants; 2005

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cellulose’s polar OH- and H+ groups lead to hydrogen bonds to the matrix substances and by doing so causing the biomass’ fibrous nature. Furthermore cellulose molecules at-tached to each other - micro- and macro fibrils – form highly ordered areas and providing the cell wall with crystalline properties. 2.3.3 Lignin

The word lignin can be derived from the Latin word for wood, “lignum”, which can be explained by the fact that every woody biomass contains this substance in great quanti-ties and it is the origin of woody tissue. Lignin can exist in diverse molecular forms, thus it is hard to phrase a general description. It also is a polymer, though there is no clear order of chain link compounds and further it is of a very heterogeneous outer structure with irregular branches and interchangeable chemical groups. Beside the features of providing stiffness to the cell wall, acting as glue between the cells, lignin also causes hydrophobi-city of the cell wall and protects the wood against biological degradation. In fact, lignin is the least significant group of the lignocellulose fraction in respect to the active reactions in the wood during torrefaction. Nevertheless it is absolutely crucial for the principle of torrefaction due to its nature of a higher resistance to thermal degradation and thus the quality to keep almost its entire energy content during the process. That goes hand in hand with the – in contrast to hemicellulose and cellulose - large amount of carbon - on mass basis around 60 % - bound in the molecule and remaining for oxidation after the process. Also the product’s beneficial qualities are massively dependent on this sub-stance.16 According to SCHWARZOTT, 1993 the lignocellulose fraction for the two wood types, hard-wood and sorftood, differs slightly, as shown by the benchmark values in the table be-low.17 Table 1 Distribution of lignocellulose fractions for deciduous and coniferous wood, based on SCHWARZOTT, J.: Stickoxidemissionen bei der Holzverbrennung; 1993

Wood category Hardwood Softwood

Cellulose [% of dry matter] 40-50 40-45

Hemicellulose [% of dry matter] 22-40 24-37

Lignin [% of dry matter] 30-35 26-38

Additionally must be stated that these amounts differ again from wood type to wood type in these ranges and naturally also between individual trees. Furthermore the de-composition intensity of the respective groups may differ between the distinctive mole-cules of the respective group. In the case of hemicelluloses, for instance, the kinetics of

16

www.waechtershaeuser.de; 23.11.2011 17

SCHWARZOTT, J.: Stickoxidemissionen bei der Holzverbrennung; 1993

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thermal degradation reactions of xylan and arabinogalactanare expected to be distin-guishable.18

2.3.4 Elemental analysis of wood

As indicated above the lignocellulose fraction distribution is one important property of wood. However, there are many more decisive features that determine the performance in industrial processing. It is absolutely crucial to have detail knowledge of the biomass feedstock even before the phase of basic for a wood processing plant hence that know-how is required for an efficient plant design respectively for the choice of a suitable tech-nology. Roughly90 % of the wood’s dry matter is made up of C and O, 6 % of H and the residual 4 % are composed of a variety of different elements, which most times are bound in nutrients. Important elements of those are N, K, Ca, P, Mg, S and Fe. Inconveniently wood also contains irregular amounts of unwanted heavy metals, often concentrated in the bark, that are released during combustion.19Above organism-specific concentrations these group of substances have toxic effects and must be avoided as far as technically possible, also expressed by strict emission limit values. Furthermore, almost in any bio-mass Cl, Si and Na is detectable, whereby particularly Cl has negative effects in the incin-eration process, causing acid emissions and can lead, in cases of incomplete combustion, to formation of toxic PCDD and PCDF.

2.3.5 Characteristics and associated effects of biomass

According to KALTSCHMITT et al., 2009 there are some other characteristics beside the chemical composition determining important effects during pre-treatment and combus-tion of biomass. Some of them influence the fuel quality of biomass and other, being physical-mechanical properties; have impacts on the processing, handling and economics. The table below shows the most important properties of biomass related to its energetic utilization.

Table 2 Quality characteristics and important effects of biomass, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

Quality characteristic Important effects

Chemical composition

carbon calorific value, air demand, particle emissions

hydrogen calorific value, air demand

oxygen calorific value, air demand

nitrogen NOx- and N2O-emissions

Potassium Ash-bonding-behavior, high-temperature corrosion, particle emissions

18

PRINS, M.J. et al.: Torrefaction of wood part 1 – Weight loss kinetics; 2006 19

GUDERIAN, R.: Terrestrische Ökosysteme – Wirkungen auf Pflanzen, Diagnose und Überwachung, Wir-

kungen auf Tiere; 2001

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magnesium Ash-bonding-behavior, bonding of pollutants, ash utili-zation, particle emissions

calcium Ash-bonding-behavior, bonding of pollutants, ash utili-zation, particle emissions

sulphur SOX-emissions, high-temperature corrosion, particle emissions

chlorine emission of HCl and halogenorganic compounds, high-temperature corrosion, particle emissions

heavy metals ash utilization, heavy metal emissions, catalytic ef-fects, particle emissions

Fuel characteristics

moisture content calorific value, storage life, fuel weight, combustion temperature

calorific value energy content, plant design

ash content particle emission, residue generation

Ash-bonding-behavior bottom ash generation, residues, operational safety, maintenance requirements

Physical-mechanical properties

rubble size need for preparation, ignition properties, drying

particle size distribution disturbances in conveyors, pourability, bridging, dust, explosion hazard

bridging propensity flowability, conveying conditions

bulk density storage- and transport costs, power of conveyor ele-ments

gross density bulk density, power of conveyors

abrasion resistance fines

2.3.6 Definition of water- and moisture content

For the following explanations it is necessary at this point to state some definitions con-cerning the terms of wood, respectively fuel moisture content and water content in the context of bioenergy. The quantity of water that can be removed from the fuel under de-fined conditions is called water content w. It is the ratio of the mass of water mw to the investigated material’s mass on wet basis, which consequently implies that the denomi-nator is made up of the sum of the feed’s mass on dry matter and the water in it

However, there is one other term existing that is often used falsely as a synonym to the water content w. The fuel moisture content u, from time to time also referred as wood moisture, is the ratio between the mass of water and the fuel’s mass on dry basis, which makes it possible to calculate the moisture from the water content.

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Hence, moisture content can exceed 100 % and a water content of 50 % corresponds with a moisture value of 100 %. In the framework of biogenic energy technology the water content has established, so if values refer to the moisture content, it has to be declared clearly. The water content’s influence on the calorific value is larger than the biomass type’s. As a consequence of that, a meaningful comparison of different wood types as biofuels requires calorific values on dry basis. From time to time even the water-and ash-free value is used. However, there is a linear correlation between the calorific value and the water content as shown in Picture 4 below.

Picture 4 Linear correlation between water content and calorific value

2.4 Thermo-chemical conversion of lignocellulose biomass

According to KALTSCHMITT et al., 2009 the objective of this kind of treatment may be the production of easily transportable intermediates, but in most cases it is the supply of bio-energy carriers with well qualified properties for one very specific application. Thermo-chemical refinement processes transform solid bioenergy carriers under heat influence in solid, gaseous or liquid secondary fuels, in other words a biogenic solid is chemically changed by heat input for a later release of thermal energy. In addition to this, plain com-bustion is also one form of thermochemical conversion. This means that all products of such processes are eventually brought to an oxidation step that may take place decou-pled in both spatial and time-wise respect, meaning that the actual and final use of the modified fuel can be at another location and also at another time. The products of the oxidation are exhaust gases and incombustible, inorganic residues in forms of ashes. Though the products and processes technics differ between the main thermochemical conversion processes, the basic mechanisms of each agree in principle while the essential disparity is the air-ratio. This chapter will shortly show and explain the principles of the

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most important alternatives beside torrefaction, with the goal to enable a better under-standing for that process in focus by revealing the technical environment to the reader.

2.4.1 Air-ratio

As mentioned above the air-ratio is one crucial characteristic for each thermochemical conversion process respectively for any technical incineration. To secure a complete combustion of a fuel by totally oxidizing all oxidative, organic compounds, it is necessary to have a stoichiometric excess of oxygen, respectively more air than actually required. A deficiency of oxygen would lead to a suboptimal energy yield and furthermore to higher pollutant emission. The level of air access is described by the air-number λ. It is defined as the ratio between the real quantity of air L added to an oxidation process and the stoichi-ometrically required quantity of air for complete combustion Lmin, under the widespread assume that the percentage of oxygen in the gas mixture of air is 21 %.

Vmin can be interpreted in this case as the amount of exhaust gas in addition to the fuel’s steam. Consequently λ has to be greater or equal 1 for complete combustion and equals 0 if the reactions take place in the absence of oxygen; such reactions are also known as pyrolytic decomposition. Thus, an air ratio between 1 and 0 leads to incomplete combus-tion.

2.4.2 Alternative processes

Combustion Considering the biofuels’ composition of hydrocarbons exclusively, the products of com-plete combustion are carbon dioxide and water and the release of thermal energy. The basic stoichiometric equation for this reaction of complete combustion can be seen be-low.

OHm

COnOm

nHC mn 2222

)4

(

In cases of complete combustion, λ has to be greater or equal 1. If not, the fuel residues will still contain oxidative compounds like hydrocarbons or carbon monoxide. In fact, as raw biomass contains also oxygen in its chemical compounds, the added air can be ex-pected to be a little lower than in cases of pure hydrocarbons. More detailed values can be achieved by combustion calculations.

Pyrolysis The distinctive feature of pyrolysis is that the characterizing reactions occur only under the influence of heat and the absence of oxygen.

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15

Hence biomass can contain large percentages of oxygen it is consequently possible that the reactions leading to the decomposition of the biogenic tissue can include oxidation. For wood, with roughly 44 % of oxygen on mass basis, this is not negligible. The long chains of lignocellulose polymers are broken due to the strong molecular vibration caused by the heat transfer, resulting in shorter polymers, the degassing of volatile compounds and also the escape of liquid substances. Like in KALTSCHMITT et al., 2009 the term of pyrolytic decomposition is in this report not used as a synonym for pyrolysis, but for the heat-caused thermo-chemical transformation mechanisms of organic substances without additional oxygen feed. Pyrolysis on the other hand labels all technical implementations that apply those pyrolytic decomposition reac-tions and aim at the production of liquid or solid energy carriers. Pyrolysis forms with primary solid products are carbonization and torrefaction. Since the pyrolytic decomposi-tion is crucial for torrefaction, it will be explained in more detail in one following subchap-ter.

Gasification or Partial Combustion In the gasification process the solid feedstock biomass is transformed into a gaseous sec-ondary fuel, when the combustion or oxidation is incomplete. This is achieved by keeping the air ratio between 0 and 1. As introduced above, such combustible gases can be converted into useable energy in special applications, independent from the time when and the place where the gasifica-tion takes place. There are also some processes that were designed to generate electricity from such gases and can reach a higher stage of efficiency than, for example, standard feedstock combustion.

Liquefaction The main goal of this kind of process is the production of liquid energy carriers, like meth-anol or pyrolysis oil. In most cases this is achieved by a combination of other thermo-chemical treatments like gasification, pyrolysis or combustion. Hence the technics differ greatly it is impossible to define a universally valid air ratio. One promising technique for the future could be the Fischer-Tropsch-Synthesis, which is strongly discussed these days.

Carbonization In the carbonization process, being another representative of pyrolysis, the feedstock biomass is transformed to charcoal. The temperature of the input material is brought up to higher levels than those of torrefaction, as a general rule above 500 °C, though the heat source then is no longer external, since the reactions taking place change from endo-thermic to exothermic. As a result the energy yield is decreased, since the chemical ener-gy is released in the form of heat. In contrast to torrefaction, the pyrolytic decomposition reactions in the carbonization process occur extensively exhaustive.

2.4.3 Definitions mass yield and energy yield

To have meaningful and characterizing performance data that describe a torrefaction process’s efficiency, it is necessary to define certain parameters. BERGMAN et al.; 2005

16

introduced the mass and energy yield that several younger publications applied, respec-tively referred to.

(

)

yM = Mass yield

(

)

yE = Energy yield

These parameters describe the shift of mass and chemical energy from the biomass’s re-active part into the solid torrefied material. The term of reactive part means in this case that the amounts of inorganic substances remaining as ash and also of the free water are excluded in the definition. Moreover, the LHV is preferred to the HHV since as a rule a plant’s design for energetic utilization of solid biogenic fuels is not applied for an efficient use of the condensation heat, which leads to the fact that the LHV is the much more ap-plication-related and informative value.

2.4.4 Phases of torrefaction

BERGMAN et al., 2005 defined furthermore five stages, through which the biomass is pro-cessed during torrefaction. The locations for these stages are the upstream drying kiln, the torrefaction reactor and the downstream cooling unit. The advantage of defining these phases is caused by the fact that it offers the possibility to understand better what happens to biomass particles, when going through the process. Furthermore it is neces-sary to avoid obscurities concerning similar sounding terms and the sectioning the pro-cess in these different phases allows exact definitions of the reaction or torrefaction time, respectively the reactor residence time.

Initial heating Is defined as the phase in which the biomass is initially heated, lasting as long as no water is evaporated. In consequence the heat is solely used to increase the temperature, so the stage’s end is reached when moisture starts to escape.

Pre-drying The term of pre-drying refers to the drying process that can be divided into two sections as will be explained later. During this stage the temperature remains unchanged and the free water evaporates at a constant rate from the biomass. Hence the evaporation of this water is an endothermic reaction, the input thermal energy entirely performs the func-tion of the vaporization enthalpy. That is the amount of energy that is thermodynamically required to transform the water isothermally and isobarically from the liquid to the gase-ous state. Consequently this is also the reason for the stagnation of the temperature lev-el. The end of this stage is marked by the critical moisture content that can be well ex-plained by illustrating the drying process in Picture 6. As one can see, the moisture con-tent decreases linearly in the pre-drying phase, but at the stage’s end the function passes through a bending point which marks the critical moisture content. The further trajectory shows the diminishing of the moisture loss until all water is evaporated. That curve shape can basically be registered during any drying process. However, the critical moisture con-tent divides two phases of the drying progress: the first intercept presents the time in

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which the adhesive water evaporates, whereby its evaporation speed depends on the ambient conditions. During the second phase the water must diffuse through the materi-al’s pores, which needs more energy and time, since stronger capillary forces must be overcome. Picture 5 illustrates these two.

Picture 5 Illustration of adhesive and capillary water, based on SCHEFFOLD, K.H.: Prak-tikumsskript Umwelttechnik – Trocknung; University of Applied Sciences Bingen; 2010

Post-drying and intermediate heating When all adhesive water is evaporated, the temperature begins to increase again. Now all the physically bound water is released slowly until the biomass is practically free of mois-ture. At this stage also first volatile compounds like terpenes may escape, meaning the first solid compounds undergo a phase transition to the gaseous state, affecting the mass yield. The end of this stage is reached, when the temperature level attains 200 °C, charac-terizing per definition the beginning of the torrefaction phase.

Torrefaction Being the core of the entire process, this stage consists of both a heating and a cooling phase. In between, there is also a period in which the temperature remains constant, usually this temperature represents also the top temperature level of the whole process. With increasing temperatures up to 300 °C the pyrolytic decomposition occurs and the biomass gets in the truest sense of the word torrefied, though the intensity of this step is strongly dependent on the process parameters. That is accompanied by the significant reduction of the mass yield, as described in the section dealing with pyrolytic decomposi-tion. Since the pyrolytic decomposition in the applied temperature level is endothermic in theory, the stop of the devolatilization should be well controllable, for only the heat supply has to be interrupted or the reactor must be cooled actively. In practice, however, it happens that some particles receive more heat and reach a temperature level so the reactions are transited to exothermic ones. Furthermore, exothermic reactions may occur at lower temperatures for certain types of biomass and the respective decomposition regimes. In this case, the control of the process may be highly problematic. According to definition, the torrefaction phase ends when the temperature falls down to 200 °C once more. The reaction time trea is comprised of the time when the material is heated from 200 °C to the required temperature level Ttor that is kept constant after that for the time period ttor. Consequently the period that follows afterwards from the sinking from Ttor to 200 °C is excluded from the reaction time, though it belongs to the torrefaction phase.

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The reason behind is that most of the thermally unstable compounds of the feedstock biomass has already been decomposed, thus in the time period ttor,c there are none or only few reactive substances left, so decomposition is expected to stop as soon as the temperature is decreased. As shown in picture 6 the mass loss is also predominantly neg-ligible.

Cooling of the solids During this period the further cooling to the desired final temperature is completed. In any cause this must be executed in the absence of oxygen, due to the hazards of ignition or even explosion of the high reactive dust that may occur during the process.20

Picture 6 Phases of torrefaction, based on BERGMAN: Combined Torrefaction for bio-mass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005

20

BERGMAN, P.C.A. et al.: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”;

2005

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2.4.5 Phases of thermo-chemical conversion

Depending on the conversion process, there are four phases of thermo-chemical conver-sion, that can proceed in combination with each other or individually and separated from each other, in other words independent stages. The differences between these phases are the altering chemical and physical reactions, but also the distinct temperature levels that initiate the particular heat-induced process. Another important characteristic is the air ratio. The basic phases are the drying and heat-up phase and the pyrolytic decomposi-tion phase, representing the periods in the absence of oxygen and at the same time the relevant steps for torrefaction. Still, the following phases may emerge after torrefaction. Executing gasification can make sense to further refine the torrefied material for certain fields of use whereas the oxidation phase always represents the final step in energetic utilization of a biofuel, that may occur time-wise and spatial decoupled for an effective transformation of the chemically bound energy to useful energy. The area that is primari-ly relevant for the torrefaction decomposition mechanisms is marked by the red border-lines in the comprehensive picture 7 below.

Picture 7 Stages of oxidation, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

Heat-up and drying The first stage occurs until temperatures up to 200 °C and consists mainly of the evapora-tion of free water. As a consequence of the enthalpy of evaporation the increase of tem-perature is strongly inhibited and may stay constant until greater amounts of water have vaporized. Another result of this is the fact that there is nearly no energy used to modify the organic matter. The decomposition mechanisms occur not until the temperature level reaches a sufficient scale, or, since coupled to it, the free water has been lost. The heating rate of the biomass must not exceed certain values, for the water’s volume increases dramatically when undergoing phase transition to steam. That may lead to a

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blasting of the plant cell wall, especially in the case of coniferous wood, since resins may plug the radial vascular tissue, if heated too intensely. Also extension values of wood can be different for each direction in space, leading only to tension forces in the wood struc-ture at first, but eventually to cracks.

Picture 8 Exemplary thermal gravimetric analysis of wood, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

Picture 8 shows the thermal behavior of wet wood, when heated in the absence of oxy-gen. Taking the explanations of the water content above into account it gets clear that the wet fuel mass can exceed 100 %. Thermal gravimetric analysis, which is the principle for such diagrams, detects the mass loss of a sample over time and temperature. The re-sidual product, charcoal, sometimes also referred to as coke, is what remains after pyrol-ysis and has huge carbon content. Pyrolytic decomposition The next step to complete oxidation of the fuel is the pyrolytic decomposition, the break-ing of the treated material’s molecules under heat influence in the absence of oxygen. Even if oxygen occurs in a fuel particle’s atmosphere during this thermo-chemical conver-sion, it does normally not affect the process if it is executed until its final stage, since the decomposition products flow from the particle’s core to the outside. The principle of torrefaction is based on the property of lignocellulose material, to lose its three different main components at distinct temperature ranges at varying intensities. As torrefaction takes place at temperatures between 200 and 300 °C, the hemicellulose is the major constituent that is decomposed. Due to its ramose molecular structure it is very fragile when it comes to greater heat transfers. In fact, the decomposition of hemicellu-lose is well described as a two-step mechanism, as stated by DI BLASI et al., 1997. Accord-ing to their published knowledge, in the first step the sugar structure is changed and rear-ranged as a result of depolymerization, whereas the second step consists mainly of de-composition reactions leading to the loss of these decomposition products respectively the formation of chars, steam, CO and CO2 as well as the purging of light volatiles like car-bonyl compounds out of the carbon skeleton. The first step’s reactions occur at tempera-tures up to 250 °C which furthermore is accompanied by a very low mass loss which is significantly increased with higher temperatures that consequently initiate the second step’s decomposition reactions.

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Picture 9 Thermal decomposition of lignocellulose fractions and wood, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

However, there is already a certain mass loss before the second step takes place. Accord-ing to KALTSCHMITT et al., 2009, solid biofuel materials start slowly to decompose even at 150 °C, when first macro molecules are destroyed irreversibly under heat influence. Still, the appreciable pyrolytic decomposition begins at a temperature of 200 °C, when the organic material starts to disband and the formation of water, CO2, CO and methanol be-gins. The decomposition of cellulose and lignin happens much slower and crucially less intense when compared to hemicellulose, since in general both substances are of a much more stable molecular structure. Consequently the mass amount of degraded hemicellu-lose at roughly 300 °C is several times higher than the cellulose and lignin together. In contrast to cellulose, the lignin composition and also the hemicellulose molecular struc-ture are different for hardwood and softwood, leading to distinguishable kinetic parame-ters of decomposition for the two basic wood types. According to BOCKHORN et al., 2003 coniferous lignin is expected to feature a greater thermal stability than deciduous lignin. In the case of hemicellulose, deciduous wood is essentially more reactive and thus, devo-latilization and degassing occurs earlier and significantly more intense than for coniferous wood.21However, cellulose is generally the most thermally stable wood component of the three lignocellulose groups, especially in the temperature range of torrefaction. The exact pyrolytic decomposition reactions for the three main components of wood are not fully understood yet, though BERGMAN et al. (2005) categorized the main reaction pathways to five main reaction regimes, shown in the picture below.

21WAGENFÜHR et al.: Holzatlas, 1974

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Picture 10 Main physic-chemical phenomena during heating of lignocellulose fractions at torrefaction relevant temperature range, based on BERGMAN: Combined Torrefac-tion for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005

Except the glass transition and softening that is exclusively reserved for lignin, all main components run through the same reactions, though the temperature levels are shifted. The glass transition of lignin is an advantageous feature when it comes to densification of the biomass, since the softened lignin may cause the possibility to replace external bind-ers. The depolymerization and recondensation step means that the polymers begin to break up and the shortened polymeric chains condense within the solid structure. It must be clarified incidentally that the graphic above can only be a vague illustration to convey an impression of the different reactions in combination with the distinguishable main components of woody biomass. The transition to the next reaction regime cannot be de-fined as sharp as it is visualized in the figure. Especially for lignin and cellulose the transi-tions occur over a wider temperature range, mainly dependent on the exact biomass type. Furthermore the terms “limited” and “extensive” as used in the graphic are only valid and proportional in the framework of each polymer group individually. Furthermore it must be kept in mind that also the biomass type can have a large influence on the actu-al position and width of transition. Summarizing all that conclusively, Picture 10 can only answer the purpose to show an insight towards the coherences of the different reaction

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regimes and the lignocellulose fractions. For determined cases there are several factors to be considered when a more accurate result is targeted. Therefore it is necessary not to exceed the optimal temperature range of torrefaction.

2.5 Fuel Refinement

The following chapters shall show the improvements of the torrefied wood against un-treated biomass, when it comes to the energetic utilization as a fuel. Furthermore it shall present the composition of the process products.

2.5.1 Van-Krevelen Diagram

For a first impression of what happens during torrefaction, the Van-Reveled diagram is one useful tool that allows an assessment of complex organic mixtures such as oil, coal or biomass. It shows the relative quantity of the three most important elements of combus-tibles, as they are C, H and O, in form of the H/C ratio on the ordinate and the O/C ratio on the abscissa. Taking coal as the target figure, it has the lowest of both H/C- and O/C-ratio whereas the untreated woody biomass has much higher values. In BERGMAN et al., 2005 the data of several experiments were brought together to see how the elemental composition of wood reacts during torrefaction and to compare it with other fuels, pre-sented in picture 11Virhe. Viitteen lähdettä ei löytynyt.. It turned out that an increase of temperature brings the two ratios closer to the ones of coal and the same is true for a raise of the residence time of the material in the torrefaction reactor. The higher the rise of temperature or residence time, the more intense are the ratios’ movements towards coal. Another conclusion found was that the composition of torrefied wood lies between the ones from coal and wood – corresponding with the statement above, the values of torrefied wood at lower temperatures are closer to the ones of untreated wood and - in contrast to this – those of torrefied wood at higher temperatures are nearer to coal. However, the torrefied material still has huge differences when compared to charcoal, that requires higher temperatures and loses much more mass, but also energy during the heating if its production. As stated in PRINS, M.J., 2005 the achieved properties of wood, by means of the torrefaction process, depend strongly – apart from temperature and res-idence time – from the wood type or, in more general, from the biomass feedstock.

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Picture 11 Van-Krevelen diagram for coal, charcoal, peat, torrefied wood [TW = temper-ature that was kept constant for about 30 min.] and untreated wood, based on BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”

As can be seen from the diagram above, the biomass’s relative loss of H and O is signifi-cantly higher than of C. The reason for this is the degassing of mainly low calorific com-pounds such as the two dominantly calcinated substances CO2 and H2O. Nevertheless, also combustive compounds like CO and CH4 are released during the process.22 One con-sequence of this is the decrease of volatile components of the torrefied wood, as com-pared to the untreated wood the volatile content is reduced from 80 % to 60 %.23 Due to the reasons above there are many torrefaction technologies, whose benefits and disadvantages can hardly be guessed in practical reality. Still, the technical differences are quite obvious. The unit operations and process arrangements are predominantly the same, although the performance conditions vary strongly. 2.5.2 Products of torrefaction

The reaction products of torrefaction can be classified according to the phase, they would have at atmospheric pressure and room temperature. Thus, the first class would be the remaining solids, whereas the second and third would contain all the volatile compounds, that are released during the process. The volatiles consist of a condensable or liquid and a non-condensable or permanently gaseous fraction. The yield depends highly on the pro-cess conditions like temperature and residence time.24 GAUR et al.; 1998 stated the hemicellulose decomposition consists mainly of two reaction steps, the first being the formation of light volatiles and the second being the catalytic

22

DHUNGANA, A.: Torrefaction of Biomass 2011 23

PRINS, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005 24

PRINS, M.J. et al.: Torrefaction of wood part 2 – Analysis of products; 2006

25

degradation, resulting in the formation of predominantly CO and CO2. Carboxyl groups under heat cleavage can lead to the formation of acids, which in turn can catalyze dehy-dration and together with other heat influenced reactions this may result in carbonyls like methanol, propionaldehyde and other short hydrocarbons.25 In general, under heat influence the biomass molecules may react to new molecules or remain modified or unchanged. For example when hydrogen bonds get broken and the polar forces are lost with the escape of the water molecules, so that the polymers may rearrange in new orders. Or when hydroxyl groups react with hydrogen to water that evaporates afterwards from the solid material and the hydrocarbon chain is left unsatu-rated. However, the reactions that take place during torrefaction are complex and not fully understood, so only the well comprehended and validated observations shall be ex-plained here. Furthermore the wood type is remarkably important, concerning the substances that are produced during the process, since the hemicellulose composition differs between conif-erous and deciduous wood.26 Table 3 Hemicellulose composition, based on WAGENFÜHR et al.: Holzatlas; 1974

Hemicellulose substance Deciduous Coniferous

4-O methyl glucuronoxylan 80-90 5-15

4-O methyl glucuronoarabinoxylan >1 15-30

Glucomannan 1-5 60-70

Galactoglucomannan <1 1-5

Arabinogalactan <1 15-30

Other galactose polysaccharides <1 >1

Pectin 1-5 1-5

To be more accurate, crucial reasons for the difference is the coniferous wood’s low per-centage of the most reactive hemicellulose substance xylan and its property of containing bigger amounts of glucomannan, being a hemicellulose component of reduced reactivi-ty.27Other values than in the table above were stated in the publication by Prins et al., 2006. According to their results, the hemicellulose composition strongly affects the weight loss kinetics of biomass during pyrolytic decomposition and so is an important factor of the torrefaction performance. Nevertheless, the liquid fraction of torrefaction can be divided into the three subclasses of reaction water, organic compounds and lipids. Depending on the biomass type, the distribution of the exact substances varies. The organics are predominantly products of carbonization and devolatilization, whereas the lipid fraction consists of compounds that all can be detected in the original biomass, which implies that they are only driven out of the solid material during torrefaction but not real reaction products. Liquid substances that are confirmed by several research findings are lactic acid, formic acid, furfural, hy-

25

GAUR, S.; REED, T.B.: Thermal Data for Natural and Synthetic Materials; 1998 26

PRINS, M.J. et al.: Torrefaction of wood part 2 – Analysis of products; 2006 27

CIOLKOSZ, D. et al.: A review of torrefaction for bioenergy feedstock production; 2011

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droxyl acetone, methanol but also phenols. However, acetic acid and water, meaning the water formed during the torrefaction transformation reactions in addition to the free water that remains un-evaporated after the upstream drying, are the substances that can be expected to be the main liquid products for any torrefied biomass. The solid phase consists of various compounds and remains with an unordered and multi-tudinous structure of original sugars and modified or entirely new polymers. The occur-rence of aromatic rings is possible, as well as the appearance of coal-like carbon skeletons and first ash formations.28 The permanent gas, also referred to as torrgas or torrefaction gas, includes mainly CO and CO2, whereby the oxygen is released from the original biomass compounds, and also trac-es of molecular hydrogen and methane. As described in BERGMAN et al., 2005, the for-mation of CO2 may be caused by decarboxylation of the wood’s acid groups, whereas the creation of CO cannot be a result of decarboxylation nor dehydration. One possible ex-planation for the large outcome of CO is reported in DIETENBERGER et al., 2001, stating that the reaction of CO2 and steam in the ambiance of porous char produces CO with increas-ing temperature. However, the gas fraction includes also carcinogenic, light aromatic compounds like benzene and toluene. Picture 12 summarizes the distribution of products and the suggested classification method.

Picture 12 Torrefaction product analysis, taken from BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005

2.5.3 Combustibility of the torrefaction gas

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PRINS, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005

27

It is important to keep in mind, that the liquid fraction, as classified above, is usually con-tained in the torrgas under the real conditions of the torrefaction process. Since this gas mixture is supposed to be burned to feed the reactor with process heat and thereby en-hance the energy balance of the entire process, it is necessary to know the quantitative composition of the compounds in the gas mixture and how supportive they are for the combustion step. As water hinders any incineration, it is necessary for an efficient combustion to condense the water upstream to the furnace. Furthermore CO2 is another substance obstructing the incineration, as it cannot be oxidized any further, for the final oxidation stage is already reached. From an absolute view on energy bound to the permanent gases, CO contains the largest amount of energy, since the hydrocarbons in this fraction are only present in traces. The lipids contain the most energy from all the volatile compounds, followed by the organics, though the measurements were only done for willow. Picture 13shows the mass and energy percentages of the three main fractions, although the liquid section is illustrated with its single substance categories. Furthermore it shows the composition of the permanent gas.

Picture 13 Torrefaction main fractions and permanent gas composition, taken from BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005

Picture 13 shows the detailed composition of the organics, where quantitative measure-ment results were achieved. Other published research results related to similar results.29

29

Prins, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005

28

Picture 14 Detailed composition of organics, taken from BERGMAN: Combined Torrefac-tion for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005

BERGMAN et al., 2005 concluded that, even when completely dry biomass is torrefied, the wet torrgas still possess roughly 50 % water and 10 % CO2, so both incombustible sub-stances occur in huge amounts. Nevertheless, oxidative compounds exist in the torrgas as well and the real composition must be considered for each individual case, since the pro-cess conditions and the input material play key roles for the gas mixture’s composition. Also several developers claim to feed the reactor exclusively with the heat of the torrefac-tion gas combustion. Another aspect that is to be considered in case of torrgas combustion is the adiabatic flame temperature. To oxidize even the most difficult but still combustible compounds in the gas mixture, the adiabatic flame temperature needs to be significantly higher than the auto-ignition temperature of those substances. A difference of 400 °C is a typical value to secure a stable combustion. Furthermore the analysis in this work revealed that CO and phenol are the most challenging compounds for combustion, each with the point of auto-ignition of approximately 600 °C. As a result, an adiabatic flame temperature of roughly 1000 °C is required to secure the combustion of all oxidative components of the torrefac-tion gas. In simulations of different torrefaction conditions they further figured out a cor-relation between reaction time, respectively torrefaction temperature and the adiabatic flame temperature. This can easily explained with the increasing concentration of com-bustible compounds that follows a rise of the residence time or torrefaction temperature. So permanent gases with a higher calorific value are produced, leading to the knowledge that an adiabatic flame temperature of 1000 °C is achievable in most cases, but the mar-gin may shift, depending on the individual case and its parameters.30

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BERGMAN et al..: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”; 2005

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3 ALTERNATIVE PROCESS CONCEPTS AND SPECIFICATIONS

3.1 Autothermal operation

The ideal case of torrefaction processing is an autothermal operation, which basically means that there is a balance between the chemical energy that is bound in the released volatiles during the process and the return of this energy to the reactor and to the drying kiln in the form of heat after combusting the torrefaction gas. Hence, there is a circulation of energy flows and no necessity of introducing other energy carriers to the process. To achieve this, the process parameters must be optimized in terms of residence time and torrefaction temperature in the context of the input biomass and the process design. In the case of insufficient energy content in the torrefaction gas, it is necessary to add an-other fuel like natural gas, which then causes increased operating costs. However, it is also possible to burn some feedstock biomass to supply the process with heat, though the water content of the biomass may decrease the adiabatic flame temperature. In contrast to this, another, yet undesired scenario may occur, when the process is operated above the point of autothermal operation. This may lead to major energy losses, since the torre-faction gas contains too much energy and the remaining biocoal’s calorific value is de-creased too strongly and the product depreciates.

3.2 On-site and off-site

To find the best location for a torrefaction plant there are three basic systems to be dif-fered. The classical categories are on-site- and off-site torrefaction. The on-site torrefaction re-fers to the physical connection of the torrefaction system and the power plant, where the torrefied product is utilized for electricity and possibly additional heat generation. The integration of the torrefaction system into the power plant can be realized in versatile ways.

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Picture 15 On-site torrefaction

To state some possibilities, the power generation process could deliver its waste heat to the torrefaction reactor and drying chamber. BERGMAN et al., 2005 stated that most likely all existing power plants need to be modified, and may have remarkable negative effects on the plants steam capacity and soon its electricity efficiency. Furthermore, both torre-faction and electricity generation processes could become highly dependent to each oth-er and problems with one process might deteriorate the other one’s operation. If the tor-refaction process produced exhaust gases that need to be cleaned (for example VOC’s from the drying step and the torrefaction step itself, or flue gases, when the torrgas or additional biomasses were combusted for heat supply of the process) the power plant’s exhaust gas cleaning systems could be used to treat those gas flows. From an emission-wise view, the combustion of torrgas would be easy to handle, as long as the integration of the torrefaction gases to the power plants flue gas pipes is possible. Still, the piping of an existing plant needs to be analyzed in the individual case, but always means an addi-tional effort. The biggest disadvantage of this system, however, is the loss of one main torrefaction benefit: The decreased logistics costs after densification of the torrefied ma-terial, leading to a higher specific, volumetric energy content. All in all, the eventual ef-fects of an integration of torrefaction into an existing power station are not expected to be recommendable. Locations for off-site torrefaction require a good infrastructure and huge biomass sources in the surrounding area. The process’s required heat must be produced by the torrefac-tion system itself and may lead to additional feedstock biomass combustion or torrgas utilization.

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Picture 16 Off-site torrefaction

Still, the torrefied product may profit from the decreased transportation costs of feed-stock biomass to the off-site location, due to its surrounding forest areas, and of the tor-refied pellets to the client power plants, because of the higher, volumetric energy density. The idea of unproblematic outside storage of torrefied biomass must also be considered to be cost beneficial.

The third one, being a very special system, is mobile torrefaction, in form of a moveable torrefaction plant, for instance located in a truck. The main idea and positive aspect of mobile torrefaction is that the process can be operated at any location, which is tempo-rarily rich of (waste) biomass and can afterwards be moved to the next place where it makes sense to torrefy biomass residues. Most likely the mobile system is not able to per-form with such a high efficiency as a static one and furthermore the capacity is expected to be highly limited. However, the company Renewable Fuel Technology LLC from Ne-braska, USA is working on such a torrefaction plant and the worldwide interest seems to be quite big. Especially in regions with problematic infrastructure but high biomass out-come, mobile torrefaction could be a great opportunity to produce a quality biofuel in-stead out of waste materials.

3.3 Comparison of direct- and indirect heating

Another basic aspect of torrefaction is the concept of how the required heat is brought into the reactor to the biomass particles, so that pyrolytic decomposition can take place. Firstly, the reactor design determines what kind of heating strategy can be applied and this choice decides also the further process arrangement. Today the differentiation of two main reactor categories gained acceptance, being direct heating and in-direct heating.

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3.3.1 Process flow sheet: Direct heating

Picture 17 Exemplary process flow sheet: Direct heating

This concept is characterized by direct contact of the biomass in the reactor and a heat carrying fluid, in all known cases in the form of flue gas of torrgas combustion or circulat-ing torrefaction gas itself. In the illustration above the torrgas is directed to a combustion chamber, which is additionally supported by natural gas (it could further be fed with input biomass). A partial torrgas flow is guided to a heat exchanger before it reaches the com-bustion chamber. From the incineration step the hot flue gas streams through the heat exchanger. The flue gas heat is transferred to the partial torrgas flow, which is guided into the reactor once more. The cooled, yet still warm flue gas can then be brought to the dry-ing kiln, before it is released into the atmosphere or, possibly together with the volatile compounds from the drying, treated in an exhaust gas cleaning system. Usually, the flue gas cannot be used as the direct heat carrier, because the combustion is operated under air excess, so oxygen would be brought into the reactor, disturbing the process operation.

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3.3.2 Process flow sheet: In-direct heating

Picture 18 Exemplary process flow sheet: In-direct heating

In contrast to direct heating, in this concept the heat is transferred through a solid wall into the reactor. That means that the torrefaction reactor has some analogous tasks as a heat exchanger and so its design must also feature some conformities. The torrgas is completely guided to the combustion chamber and the flue gas gives off its heat to a fluid that circulates between the torrefaction reactor and the heat exchanger. The transmis-sion medium is normally suggested to be superheated steam or some thermo-oil. A se-cond heating circle may be established for drying purposes. In-direct heating means more effort than direct-heating – on the other hand, the separation of the heat transfer medi-um and the treated biomass offers no possibility of negative impacts from the heat supply on the product.

3.4 Future trends

There are, besides alternating process arrangements, even more possibilities in how the industrial scale torrefaction could look like, if the detailed options in the reactor design are considered. Possible reactor technologies are in general:

Rotary drum reactor

Screw conveyor reactor

Microwave reactor

Compact moving bed

Oscillating belt conveyor

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However, there are some special reactors that have been designed especially for pyrolytic applications, like torrefaction.

Torbed reactor31

Multiple Hearth Furnace32

WyssmontTurbo-Dryer®33

4 CO-FIRING OF UNTREATED BIOMASS AND TORREFIED WOOD IN COAL POWER PLANTS

4.1 Biomass co-firing in general

As the most eligible field of application for the torrefaction’s product most publications refer to co-firing of the torrefied material in existing coal power plants. This chapter will show the decisive aspects of this area of implementation in general and state the benefits of torrefied material towards conventional biomass for co-firing purposes. Furthermore it will relate these aspects to the two most important technologies that are intended for co-firing, as they are pulverized coal firing and circulating fluidized bed combustion. More precisely about 90% of the world’s coal firing power plants mill the fuel to powder and inject it into the combustion chamber, thus called pulverized coal firing. The predominant number of the remaining 10% of power plants uses modifications of fluidized bed firing, which means basically a bulk of coal particles that is flown through from below by com-bustion air.34 In general there are two ways of co-firing, namely direct and indirect co-firing. In the case of direct co-firing the biomass is brought into the coal input-flow at a certain point of the plant system, so the boiler is fed with a biomass-coal mixture. Since the biomass proper-ties can be from such a difference to the coal processing, it is usually necessary to modify the plant components, as will be described later. Indirect co-firing means that a second pre-treatment and processing line is constructed, which deals exclusively with the bio-mass feedstock. The biomass is then combusted separately from the coal, and only the steam of both processing is brought together afterwards to rotate the turbines, driving the connected generator. This leads first of all to an immense increase of capital invest-ment costs, but may also result in higher operating costs. In most cases the expenses for direct co-firing are lower, so this way is generally preferred.35 However, other references divide the co-firing of biomass into other categories, like EVANS, 2008 did.36

31http://www.torftech.com/applications/biomass_processing.html, 10.01.2012 32

http://www.cmigroupe.com/en/p/multiple-hearth-furnace-m-h-f-and-shaft-kiln-s-k, 10.01.2012 33

http://www.wyssmont.com/lib/images/pdf/torrefaction-newsletter.pdf, 10.01.2012 34

LÖSCHEL, A.: Die Zukunft der Kohle in der Stromerzeugung in Deutschland; 2009 35

MACIEJEWESKA et al.: Co-firing of biomass with coal – constraints and role of biomass pre-treatment; 2006 36

EVANS, Dr.G.: Techno-Economic Assessment of Biomass “Densification” Technologies; 2008

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The utilization of biomass in coal fired boilers is limited to a certain percentage beside the used fossil fuel due to several issues. If greater amounts of biomass are to be used, some plant components need major modifications that are linked with greater costs. Another limiting factor is the cost-effective supply of biomass at the individual plant’s location. In fact, with a plant’s capacity rises also its efficiency, thus it would be the best to use bio-mass material exclusively in plants with a total output of several 100 MW. However, in reality co-firing takes also place in smaller power plants since at most locations of power plants the availability of the required biomass resources in the area of catchment is se-verely limited. In order to secure the plant ‘s supply at higher co-firing amounts with feedstock biomass, nonetheless, it would be indispensable to bear the additional huge costs for logistics, that would turn the total economics in all likelihood for the worse. That is why, at least for the western European countries, the upper limit for the supply of bio-mass fuel performance for a single power plant is in the range between 50 and 100 MW. However, considering the large installed electrical output of high-performance power plants, even co-firing percentages of only 10 % for such plants, result in immense biomass amounts. Furthermore, since one benefit of torrefaction is the increase of energy density and the reduction of transport costs at the same time, it is possible to extend the borders of a high-performance plant’s catchment area. This enables a higher percentage of bio-mass input for co-firing purposes and in the case of co-firing in high-capacity power plants a more efficient utilization of the renewable resource of torrefied wood. Another positive aspect to be considered is the easy replacement of biomass by coal in the case of supply shortage due to seasonal or other unavailability. Furthermore torrefac-tion reduces the chance of such situations for the simple reason that its product is not as degradable as untreated material and can be produced in advance to store it effortlessly for later use. Aside from that, torrefied biomass tops conventional biomass feedstock in terms of modification requirements, since it has properties more similar to coal, allowing it to be added to the existing coal volume flows without greater challenges. Especially in Europe the legal framework for the combustion of fossil fuels becomes more and more complex, strict and expensive. Substituting those by renewable secondary en-ergy carriers can lead to financial gains for power plant operators and make it unneces-sary to purchase additional CO2-certificates. Furthermore it can help in terms of market-ing and promotion, seeing that environmental protection is effectuated.

4.2 Pretreatment

In general, the pretreatment requirements of co-firing applications are dependent on the furnace and the type of biomass. Basically it is necessary to liberate the biomass material from any foreign matters to protect downstream process devices like grinding apparatus-es like crusher and mills or dosing units like nozzles. This demand is not applied for torre-fied material, since the raw biomass has already been freed from extraneous matter. The same is true for the drying pretreatment, for the torrefied biomass has a maximum up-take of moisture of 3 %. That allows an improved total energy balance for the entire pro-cess. In contrast to strongly polluted materials like sewage sludge or chemically treated wood, torrefied material ordinarily requires no upstream scrubbing, since no special emissions are to be expected.

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4.3 Milling

To achieve a complete combustion, especially in case of pulverized coal fired boilers, it is necessary to mill the coal and added woody biomass to powder of 2 to 4 mm average size. Nevertheless, in power stations with higher thermal capacity of several 1,000 MW, the longer residence times in the combustion chamber allow to feed the incineration with bigger particles as well. The comminuting can be done by upstream cutting-, bowl- and hammer mills. Unfortunately, untreated biomass can cause malfunctions of these com-ponents, due to its fibrous structure that confers mechanical strength and tenacity. Dur-ing the process the bundled cellulose fibrils may not be broken, so they wrap around the milling tools, with a chance of stalling them. In general, the higher the biomass’s water content, the larger is a mill’s power consumption to grind it. In case of pulverized bed firing it is for most cases sufficient to feed the combustion with wood chips. The power consumption of the mills rises also with decreasing particle size. In case of the hammer mill, being the most energy-saving variant, still about 1% of the biomass’s energy is need-ed for the grinding operation.

An upstream torrefaction can eliminate this set of problems, since the tenacious nature of biomass is replaced by brittleness and the absorption and content of water is minimized. The result referring to the milling process is crucial, as Bergman et al., 2005 found out. The diagrams below show the outcomes of these investigations.

Picture 19 Left: Size reduction of coal, biomass and various torrefied biomasses. [Cod-ing: biomass type (C = Woodcuttings; D = Demolition wood; W = Willow) (torrefaction temperature, reaction time)]; Right: Mill capacity correlation to average particle size [same coding], taken from BERGMAN: Combined torrefaction and palletisation – The TOP Process; 2005

According to the left diagram there are great differences between woodcuttings with rel-atively low moisture contents of roughly 15% and torrefied wood. The reduction of the mill’s power consumption lies in the range of 70% to 90% and depends in detail on the torrefaction’s process parameters and the feedstock material. Generally, the higher the

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temperatures, the better the improvement for the milling step. For reasons of compari-son there are also data points for bituminous coal entered in the diagram and this fossil fuel’s curve is similar to the torrefied material. In some cases the torrefied products even exceed the well positioned values of coal. This means, however, that the input of bio-mass, when torrefied, is no longer limited in consequence of grinding issues. Torrefied material can easily be brought to a particle size that can be conveyed and handled like coal, so there are no needs for modifications. Instead, a mill’s capacity can increase sig-nificantly by factors ranging from 7.5 to 15 and dependent on the process conditions. Thus, the target capacity can be achieved with smaller comminuting tools, leading to de-creased investment costs for new plants and, in respect to the enhanced energy efficien-cy, diminished operational costs in general.

4.4 Expected performance of torrefied wood in coal power plants

By co-firing biomass in coal power plants the mass- and volume flows of the fuel, but also of the exhaust gas can change significantly. Also the conveying and storage systems need sufficient capacity. The exhaust gas may consist of a higher water percentage when bio-mass is added to the combustion, which could make it necessary to change the fuel mix-ture’s residence time in the incineration chamber on the one hand and could lead to al-tered heat transfer behavior at the steam generator. The following picture shows a com-prehensive view on the possible effects of average biomass co-firing on the components of a pulverized coal firing power plant.

Picture 20 Possible co-firing induced effects and affected components of a pulverized coal power plant, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

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Bringing biomass into the processing system of a coal fire plant increases the input flows mainly due to the distinctly lower volumetric energy density. The type of coal primarily burned, decides over the plant’s design, concerning for instant the conveying- and stor-age capacity or the combustion air supply system. Considering this, it is clear that co-firing can much easier be obtained when the secondary fuel is similar to the fossil coal. Since torrefied material has a higher calorific value than the untreated biomass, it can solve the problem to a certain scale, for the heightening of mass- and volume flows is not as in-tense as before. The diagram below illustrates the change of fuel volume flows when certain amounts of untreated biomass are co-fired. The alteration by co-firing torrefied wood is most likely not so strong in the combination with bituminous coal, for their calorific values lie closer together.

Picture 21 Increase of fuel volume flow at goods inward state [bulk densities: Bitumi-nous coal 870 kg/m3, brown coal 740 kg/m3, wood chips 250 kg/m3 (w = 30 %), straw 150 kg/m3 (w = 15 %)], based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

According to KALTSCHMITT et al.; 2009, it is indispensable to consider the different water contents of the primary and secondary fuels, since the new exhaust gas moisture can cause immense volume extensions.

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Picture 22 Change of humid exhaust gas flow at biomass co-firing, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009

Naturally this is accompanied with increased flow velocities, modified heat transfer and raised pressure losses. That is another argument why torrefied and thereby dry wood is only to be co-fired with bituminous coal, whereas humid biomass would better be com-bined with raw brown coal. After all, one problem of co-firing untreated biomass is expected to be overcome com-pletely: The homogeneity of the supporting bio fuel. Whatever origin the torrefied bio-mass comes from, after torrefaction the physical and chemical properties between the different feedstock biomasses are more similar than before. After co-firing tests with tor-refied biomass in a power plant (400MWelpulverized coal power plant in Borssele, the Netherlands) WESTSTEYN, 2004 concluded a potential for higher co-firing ratios than those applied in the investigation, where the torrefied wood was progressively mixed with coal up to 9 % content on an energy basis. Another outcome of this experiment were, that the pulverized coal boiler did not deliver any measurable negative effects, so torrefied wood was assigned to be possibly a viable option for direct co-firing.37

4.4.1 Slug formation, ash accumulation and pollution

With decreasing ash melting points of the feedstock biomass, the pollutions and slugging at the convective heating walls increase. The ash composition is solely related to the feedstock, since the torrefaction process itself does neither change the ash properties nor the melting points. Alkali metals (Li, Na, K, Rb, Cs and Fr) are the elements that cause the decrease of the melting points, which certainly is not a problem, as long as wood serves as the only biomass type to be co-fired, because of its low alkali content.

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WESTSTEYN A.: First Torrefied Wood Successfully Co-Fired; 2004

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The ash percentage of wood or stalk-type biomass is significantly lower against coal, and so unburdens the dedusting systems of the plant, since the total amount is always de-creased when such biomass is co-fired. Nevertheless, the ash utilization gets more com-plicated, whenever coal and solid biofuels are combusted together, for the properties, especially the chemical composition, differ between coal ash and biomass ash. The mix-ture ash, that remains when both are burned together, is not utilizable, as it would be possible for the ashes each solely of coal or biomass. Half the amount of available ashes from100 % coal combustion are used in the construction industry and underground min-ing, whereas the other half is utilized for restoration of open cast mines, pits and quarries. Those ashes, especially fly ashes, can be highly contaminated with ecologically damaging substances like heavy metals. That is also the reason why parts of the ash must be dis-posed and is not adapted for further utilization. In contrast to this, the biomass ashes contain usually significantly fewer pollutants, since the inorganic elements that remain as ashes from the biomass have originally been extracted from the environment where it was grown. So restoring those substances to these locations can be sustainable, for this supports and helps closing the mineral cycle of the soils and the ecosystem. Finland’s leg-islation allows the recycling of wood ashes in the fields of forestry. If it is not advisable to return the ash to the natural environment, due to possible contaminations, high pollutant contents or insufficient combustion, DE NIE, et al., 2005 suggest to use it as a raw material for fertilizer, as a building material or re-using them as a fuel on condition that the ash is rich on carbon with a high calorific value, which normally is only applicable to fly ashes from fluidized bed gasification of biomass.38 When indirect co-firing is applied and the combustion of coal and biomass takes place in individual chambers, the ashes occur also separately and can be used each for the optimal purposes as described above. However, since the normal way of co-firing is the direct one, the remaining ash is a mixture of coal and biomass ashes. The mixture is neither ap-propriate for the utilization ways of exclusive coal ash nor of exclusive biomass ash, so new strategies are to be found to use this mixture as best as possible. As those utilization strategies are strongly dependent on the coal and biomass quality, the optimal treatment can differ between individual cases, so no universal advice can be given at this point. Nevertheless it is generally accepted that wood is more suitable for co-firing with coal in respect to the ash formation. Since the ash composition of wood and torrefied wood is not changed significantly, there are no notable differences expected from this point of view.

4.4.2 Corrosion and erosion

Another nature of certain biomass types that prevents unlimited input of biomass in coal fired boilers is that may promote corrosion. While generally stalk-type biomass like straw has an increased chlorine-content, when compared to coal, and also contains potassium chloride (KCl) that intensifies corrosion effects, woody biomass does not have such prop-erties. The effect of high chlorine amounts in the feedstock is an intense high-

38MACIEJEWSKA, A. et al.: Co-firing of biomass with coal – constraints and role of biomass pre-treatment; 2006

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temperature corrosion at the convective heating walls, where the steam and exhaust gas reach their temperature maxima. Hence, the input of such biomass is limited to low per-centages of roughly 10 %, while wood can usually be brought into the boiler at higher rates without hesitation. The plant components, which are flown through by dust-laden gases, can be under the influence of erosion, whenever it comes to friction and collisions between particles and the component surfaces, resulting in material abrasion. Since, in contrast to coal, biomass types that are in discussion for co-firing have normally very low amounts of ash, erosion problems, especially in the exhaust gas pipes, may be mitigated. However, effects on the corrosion and erosion behavior by torrefaction are not expectable, but as long as wood serves as feedstock material, there are no limitations for the input amount anyway.

4.4.3 Emissions and exhaust gas cleaning

The co-firing of both woody and stalk biomass results in decreased occurrences of the most harmful gases in the raw gas and in the exhaust gas cleaning. Biomass has naturally lower amounts of sulphur but also partly binds the sulphur chemically in its ashes during combustion, so less sulphur dioxide can be formed. So usually the load of the flue gas desulphurization plant is reduced by co-firing biomass. Furthermore, different other sub-stances like mercury, arsenic, plumbum and more heavy metals are removed beside sul-phur oxides in the flue gas desulphurization plant, which affects the denox performance as explained below. Nevertheless, in particular cases, the change of the exhaust gas com-position and new substances can lead to the burden of such plants, especially when too much chlorine is contained in the gas flow. As described in the section of corrosion, this is not likely for wood as feedstock material, since the chlorine content is similar to the coal’s one. Also positive effects on the emission behavior of nitrous gases maybe observed, due to advantageous combustion kinetics of biomass, although the state of knowledge is not yet clear in respect to those emissions, since the observations are irregular. In some cases also enhancements in the emission balance of nitrous gases are possible, especially when wood serves as feedstock biomass. Related to the denox-plants in the case of pulverized coal firing, the possible effects are significantly different for low-dust, respectively high-dust arrangements. For the case of low-dust arrangement, the dedusting system and the flue gas desulphurization are arranged upstream to the denox plant, which decreases the risk of negative effects on the denitration system, since substances that are hazardous for the catalyst are removed to the greatest extent. High-dust arrangement can result in plugging of the catalyst’s active cells (alkaline metals and alkaline earth metals) or deacti-vation of the catalyst itself (reaction of the catalyst with e.g. K, Na, P and As), though this is also dependent on the combination of the coal and biomass input and the applied co-firing technology. That is, however, why in this respect the low-dust arrangement is supe-rior to the high-dust system, even though high-dust process arrangement still can be fea-sible for particular cases. Again, wood is superior against straw and other stalk-type biomass, since wood contains or produces generally much less of the problematic substances. Furthermore, experi-

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ments have shown that nitrous oxide (N2O) can be reduced by biomass co-firing. Especial-ly for pulverized coal firing, there are no particular requirements on the nitrogen content of the biomass, since even higher amounts of nitrogen can be controlled by adjusting the combustion conditions, like temperature, fuel feed and excess air ratio. The emissions of carbon monoxide are likewise not expected to increase, provided that the biomass is ground to a sufficient size in the case of pulverized coal firing. Co-firing straw increases the emission of hydrogen chloride (HCl) extremely, whereas the use of wood results usually in decreased emission values. The same applies for PCDD/PCDF emissions. All this considered, the emission balance of co-firing wood does not deteriorate against exclusive coal combustion, neither in pulverized coal- nor in fluidized bed firing. In some points the biomass feeding can even be advantageous. However, the type of biomass is the crucial parameter, when it comes to the assessment of the emissions and in each case, wood is the superior feedstock, when compared to stalky biomass or even sewage sludge, which is the most problematic one.39 Torrefied wood has the potential to substitute huge amounts of coal for electricity gener-ation in existing but also in power plants yet to come. Vogel et al.; 2011 speak of possible co-firing rates of torrefied pellets of 50 %, but state also that more experiences with the new fuels in terms of technical and logistical aspects are required. In comparison, accord-ing to Flyktman et al. (2011) the co-firing rate of the wood pellets is only 15 – 20 %. All in all, the direct substitution of coal had a tremendous potential to decrease the world’s greenhouse problems.

5 CONCLUSION

In 2009 the global economic recession caused an enormous downfall of the demand for forest industry projects.40 To stop the trend and to protect the industry from such hap-penings in the future, several projects were started to find new fields of expertise and to secure the existing business environment. When it comes to the energetic utilization of wood, the technology of torrefaction has many advantages against existing processes. Especially in Finland, with its huge outcome of woody biomass and great amounts of forestry waste residues, the idea of refining the natural resources can be realized much more easily than in other countries in the world, but especially in contrast to Central Europe, where there simply are not such forestry are-as left. It is quite expectable that the big European energy suppliers have a great interest in biofuel products, as these substances can be used in existing plants without the need of greater modification investments. At least in theory torrefied material fulfills all require-ments that would allow co-firing it in coal power plants.

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KALTSCHMITT et al.: Energie aus Biomasse; 2009 40

Aarne et al.: Finnish Statistical Yearbook of Forestry 2010; 2010

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Furthermore it is generally accepted that biomass is CO2-neutral, so there may occur fi-nancial benefits related to emission trading. In contrast to conventional biomass prod-ucts, torrefied wood has several advantages due to its new gained properties of hydro-phobicity, effortless grindability and its remarkable calorific value. Considering that densi-fication is applied additionally, the torrefied product (with its increased volumetric energy density) has even more great advantages, when it comes to long distance transportation. In this case, the densification would have additional benefits, since great amounts of greenhouse gases are emitted by coal and other solid fuels during transportation, inde-pendently from the final utilization in a power plant. The general trend is: Long distances deteriorate the specific emission values of woody biomass.41 Since conventional wood pellets perform remarkably better than wood chips (due to higher energy density and decreased dusting behavior), torrefied pellets should be able to enhance those values and to minimize the problem. This is why secondary energy carriers are preferred for long-distance transportation.42 There are, however, still several open questions about torrefaction. First of all it is the process handling and the process design. How exactly does residence time, heating rate, particle size, heating strategy, etc. influence the product quality, the decomposition kinet-ics and the process conditions itself? What exactly happens from a thermo-chemical view with the biomass during the process, what reactions take place and how does the bio-mass’ chemical composition influence these reactions? And one very important issue is the scale-up of the process. Now that first torrefaction plants come nearer to realization, it will be interesting how large-scale torrefaction will perform and whether there will be any process designs that gain a general acceptance and eliminate others from the screen. And of course the economics of the process, which have not been analyzed at all in this report, are crucial to the technology’s future success. The logistics and net efficiency of the process, but also subsidies seem to play key roles in this context. Nevertheless, the process of torrefaction has the potential to enhance the sustainability character of the energy industry and to reduce environmental pollutions emitted by coal power plants. Over the last years, the percentage of biomass derived electricity genera-tion has increased and as the global interest for this new technology is rocketing, the de-mand for a fuel that process developers claim the torrefied product to be, is growing as well. Considering this, joining this innovative business field is from an economic view on the one hand and from an ecological view on the other, a reasonable decision.

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EDEL, M. et al.: Die Mitverbrennung holzartiger Biomasse in Kohlekraftwerken; 2011 42

FAAIJ, A.: Export of torrefied and non torrefied biomass, comparison of technical and economic perfor-mance; 2011

PICTURE LIST

Picture 1 Torrefaction development activities in Europe ...................................................... 3

Picture 2 Torrefaction development activities in the USA and Canada ................................ 4

Picture 3 Basic exemplary operation arrangement for biomass torrefaction....................... 8

Picture 4 Linear correlation between water content and calorific value ............................ 13

Picture 5 Illustration of adhesive and capillary water ......................................................... 17

Picture 6 Phases of torrefaction .......................................................................................... 18

Picture 7 Stages of oxidation ............................................................................................... 19

Picture 8 Exemplary thermal gravimetric analysis of wood ................................................ 20

Picture 9 Thermal decomposition of lignocellulose fractions and wood ............................ 21

Picture 10 Main physic-chemical phenomena during heating of lignocellulose fractions at torrefaction relevant temperature range ............................................................................ 22

Picture 11 Van-Krevelen diagram for coal, charcoal, peat, torrefied wood and untreated wood .................................................................................................................................... 24

Picture 12 Torrefaction product analysis ............................................................................. 26

Picture 13 Torrefaction main fractions and permanent gas composition .......................... 27

Picture 14 Detailed composition of organics ....................................................................... 28

Picture 15 On-site torrefaction ............................................................................................ 30

Picture 16 Off-site torrefaction............................................................................................ 31

Picture 17 Exemplary process flow sheet: Direct heating ................................................... 32

Picture 18 Exemplary process flow sheet: In-direct heating ............................................... 33

Picture 19 Left: Size reduction of coal, biomass and various torrefied biomasses. Right: Mill capacity correlation to average particle size ................................................................ 36

Picture 20 Possible co-firing induced effects and affected components of a pulverized coal power plant .......................................................................................................................... 37

Picture 21 Increase of fuel volume flow at goods inward state .......................................... 38

Picture 22 Change of humid exhaust gas flow at biomass co-firing .................................... 39

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