0_ThSci2013 05_JANIC_DEDOVIC_11_3_2014

8/11/2019 0_ThSci2013 05_JANIC_DEDOVIC_11_3_2014

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THERMAL POWER OF SMALL SCALE MANUALLY FED BOILER

by

Todor V. JANICa*, Sasa M. IGIC

b, Nebojsa M. DEDOVI C

a, Dari jan B. PAVLOVI C

a, Jan J.

TURANa, Aleksandar D. SEDLARaaUniversity of Novi Sad, Faculty of Agriculture, Novi Sad, Serbia

bUniversity Business Academy, Faculty for Economics and Engineering Management, Novi Sad,

Serbia

This study reviews test results of the combustion of prismatic soybean straw

bales used as fuel in manually fed boiler with nominal thermal power of 120

kW. The influence of the of the volume flow rate (150, 220, 290, 360 and 430

m3 h

-1) of the inlet air and share of the flue gas recirculation (0%, 16.5%

and 33%) fed to the boiler furnace was continuously monitored. Directmethod was used for determination of the boiler thermal power.

Correlation between boiler thermal power and bale residence time was

developed and simple empirical correlation has been derived. General

conclusions are as follows: the increase of the flow rate of inlet air fed to the

boiler furnace has as a consequence decrease of the bale burning time and

increase of the boiler thermal power. Share of the flue gas recirculation of

16.5% increases bale residence time and decreases average boiler thermal

power in all regimes except in the regime with inlet air flow rate of 220 m3h

-

1

. In regime with 0% flue gas recirculation boiler thermal power was greaterthan nominal in regimes with 360 and 430 m

3 h

-1 inlet air flow rates. In

regimes having inlet air mass flow rate of 290 m3h

-1boiler thermal power is

equal to the nominal power of 120 kW.

Key words: boiler,thermal power, soybean straw combustion, inlet air

1. Introduction

Korienje biomase kao energenta u Srbijinije novost.Pored drvne biomase u

poljoprivrednim regionima za produkciju energije od uvek su korieni ostaci poljoprivredne

proizvodnje. Najee se za produkciju energije koriste biljni ostaci ratarske poljoprivredne

proizvodnje i to: wheat, rye, barley, oats, soybeen and rapeseed straw and cornstalk [1 - Brki, Jani i

Igi, Thermal science]. It is a cheap and available fuel, but its utilization is linked to the problems of

its collection, form preparations, transportation and storage [2 - Oka monografija].

U zavisnosti od naina energetske valorizacije, biomasa ostatakaratarskih kultura u Srbiji se

najee priprema u formi malih i velikih etvrtastih ili rol bala, poto je to najjeftiniji nain

pripremanja biomase koja se koristi za produkciju energije (3 - Dodi, 2012, 4 - Zeki).

Balirana biomasa se u Srbiji uglavnom sagoreva u toplovodnim kotlovima, pri emu su kao

najbrojniji i najrasprostranjeniji posebno interesantni kotlovi nominalnih toplotnih snaga od 50 do 120

kW sa ravnim nepokretnim reetkama za sagorevanje.Navedeni kotlovi se najee koriste u

sistemima za zagrevanje stambenih, proizvodnih i poslovnih objekata, u industriji i dr. i touglavnom u


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ruralnim podrujima(5 - Repi, Thermal science), tj.na mestima gde se raspolae veim prostorom za

skladitenje biomase, kao biogoriva. Ne postoje precizni podaci o broju takvih kotlova koji se koriste

u Srbiji, poto je pored fabrike proizvodnje veliki broj kotlova proizveden od lokalnih majstora. Na

osnovu grubih istraivanja autora pretpostavka je da se u Srbiji u eksploataciji nalazi preko 5.000

kotlova u kojima se kao energent koristi biomasa, ukupno instalisane snage od preko 300 MW.Iako se u razvijenijim zemljama koriste kotlovi veih energetskih efikasnosti, tj.standardi

koriene opreme i korienog biogoriva su znatno napredniji, za prostor Srbije unapreenje

konstrukcije i naina rada toplovodnih kotlova sa ravnom nepokretnom reetkom toplotnih snaga do

120 kW je od sutinske vanosti. Rasprostranjene konstrukcije kotlova sa ravnom nepokretnom

reetkom retko kada prelaze toplotne snage od 500 kW i bez obzira na snagu imaju znaajnih

problema u radu (6 - Jani SPT). Meu najizraenijim problemima u radunavedenih kotlova se mogu

navesti velika variranja u intenzitetu produkovane toplotne snage, esta termikapreoptereenja

loinog prostora, nagla pothlaivanja loita (prilikom ubacivanja bala biomase), emisijom dimnih

gasova sa velikim sadrajem nesagorelih jedinjanja (CO, H2, CH4i dr) koji povremeno imaju visoke

temperature (preko 650oC), posebno kada se eli postii to vea izlazna toplotna snaga kotlovskog

postrojenja. Pored navedenog poseban problem pri radu kotlovskih postrojenja u kojima se sagoreva

biomasa nastala u procesima poljoprivredne proizvodnje predstavlja topljenje pepela na relativno

niim temperaturama, to dovodi do zaepljavanja otvora za dovoenje primarnog vazduha i samim

tim do nepotpunog ili ak nemogueg sagorevanja biomase (7 - Turanjanin, Thermal science )

Toliko loih pokazatelja u radu kotlova ima za posledicu smanjenja veka eksploatacije

postrojenja, znaajno smanjenje energetske efikasnosti rada, tj.znaajno vei utroak biogoriva,

emitovanje dimnih produkata sagorevanja sa nedozvoljeno propisanim koncentracijama CO, H2, CH4,

NOx, a i dr. (8 - Obaidullah ). Time se ee javlja potreba za novim investicijama pri kupovini

novog kotla, eksploatacija postrojenja je znaajno skuplja, odravanje tee, a zagaivanje radne i

ivotne sredine mnogo izraenije.

Svesni toga da e se u Srbiji jo dugi niz godina i dalje koristiti kotlovska postrojenja sa

ravnom nepokretnom reetkom snaga do 120 kW izvrena su ispitivanja sa ciljlem da se u to veoj

meri utvrde zavisnosti i meuzavisnosti uticajnih faktora na proces sagorevanja balirane biomase i

donesu zakljuci kojim bi se unapredio rad navedenih postrojenja.

Istraivanja u ovom radu su usmerena na definisanju korelacije izmeu izlazne toplotne

snage i koliine i sastava vazduha za sagorevanje balirane sojine slame kod njihovog sagorevanja u

toplovodnom kotlovskom postrojenju sa ravnom, nepokretnom i vodom hlaenom reetkom.

Dobijeni rezultati se mogu koristiti za poboljanje tehnologije rada kotlovskog postrojenja(naina i dinamike loenja i dodavanja vazduha za sagorevanje), uvoenje automatizacije u radu

takvih postrojenja i poboljanje njihovih konstrukcionih reenja.

2. Boilerplant and measuring points

Ispitivanje uticaja koliine i sastava vazduha za sagorevanje na izlazne toplotne snage je

vreno na runo loenom toplovodnom kotlu nominalne snage od 120 kW. Kotao je prikazan na slici 1

zajedno sa najvanijom merno-regulacionom opremom sa kojom su vreni eksperimenti.

Kotao (3) je kutijasti, izraen od kotlovskog lima (bez amotnog ozida)koji je obloen sa

termo izolacijom debljine od 50 mm. Posmatrano po vertikali kotao ima etiri funkcionalne celine


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koje se meusobno nadovezujui to: pepeljaru, loite, komoru za sekundarno sagorevanje (volatila) i

dimne cevi za konvektivnu razmenu toplote izmeu dimnih produkata sagorevanja i vodenog plata

kotla. Pepeljara je dimenzionisana tako da omoguuje celodnevni rad kotla u nominalnom

kapacitetu.U pepeljaru je postavljena perforirana cev (2) za dovod vazduha za sagorevanje u loite u

nadpritisku centrifugalnim ventilatorom (1).Loite je dimenzija 600x800x1100 mm. Biomasa sesagoreva na ravnoj, nepokretnoj i vodom hlaenoj reetki izraenoj od cevi prenika 60,3 mm , koja se

nalazi po celom preseku loita. Gornji svod loita je u vodenom platu, a nazadnjem delu ispod

otvora za ulaz dimnih produkata sagorevanja u komoru za dogorevanje volatila su popreno

postavljene tri cevi kroz koje protie voda. Cevi su postavljene da bi se ojaala konstrukcija kotla i

pospeilo meanje dimnih produkata sagorevanja sa preostalim vazduhom, tj. kiseonikom. U komori

za dogorevanje se razvija plamen i postiu vie temperature kojima se ne izlae pepeo sagorevane

slame i smanjuje opasnost njegovog topljenja. U veim radnim optereenjima plameni front ulazi do

oko 1/4 dimnih cevi i vri njihovo dobro ienje od pepela. U dimne cevi gasoviti produkti

sagorevanja ulaze iz prednje skretne komore i u njima se vri konvektivna razmena toplote izmeu

dimnih produkata sagorevanja i vodenog plata kotla. Izlaskom iz dimnih cevi dimni produkti

sagorevanja dolaze u sabirnu komoru na ijem kraju se nalazi odvodna cev za recirkulaciju dimnih

produkata sagorevanja. Nakon toga dimni produkti sagorevanja prolaze kroz dimnjau, dimnjak i

odlaze u atmosferu.

Figure 1:Boiler plant of manufacturer "Ekoprodukt" from Novi Sad with the measuring and

regulation equipment

(1fan of air for combustion, 2- tube of air for combustion, 3-hot-water boiler, 4- chimneys, 5-

chimney, 6- tube of racirculation flue gas, 7- valve of racirculation flue gas, 8- standard orifice

flow meter, 9- differential pressure transmitters,10- probe for temperature measuring,11- sensor

for pressure measuring, 12-ultrasonic water flowmeter, 13- flue gas analyzer)

Recirkulacija dimnih produkata sagorevanja se realizuje preko dimne cevi (6), ventila za

regulaciju obima recirkulacije (7) i blende za merenje protoka recirkulisanih dimnih produkata

sagorevanja (8). Dimna cev (6) se zavrava na ulazu u ventilator vazduha za sagorevanje (1)


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ostavlljajui dovoljno mesta za ulaz i sveeg - okolnog vazduha. Meanje i delom predgrevanje

okolnog vazduha i recirkulisanih dimnih gasova se obavlja u radnom kolu ventilatora.

Na elu kotla nalaze se vrata pepeljare, loita i gornje komore za sagorevanje i razmenu

toplote (slika 2), a na zaelju kotla dimnjaa sa klapnom za priguivanje protoka dimnih gasova .

Rashlaena voda ulazi na zadnjem vodenom platu kotla, prolazi kroz reetku za sagorevanje, bonimi zadnjim kanalima i izlazi zagrejana na prednjem gornjem delu kotla.

Eksploatacija kotla se zasniva na runom ubacivanju bala biomase ili nekog drugog vrstog

goriva u kotao sa uestalou kojom bi se obezbedila eljena temperatura u grejnom sistemu. Kotao je

predvien da radi u sistemu prirodne promaje (za ta je potrebno imati odgovarajui dimnjak) ili da se

vazduh za sagorevanje ubacuje ventilatorom kroz dovodnu perforiranu cev koja prolazi kroz vrata

pepeljare i nalazi se ispod reetke za sagorevanje, kao to je u ovim ispitivanjima uraeno.

Fig 2: The hot water boiler at the time of research with functional parts:ashtray, furnace,

secondary combustion chamberand the flue gas tube

Navedena konstrukcija kotla je predviena da se u njoj sagorevaju male etvrtaste bale

biomase, preporuene vlanosti do 25%. Takoe, navedeni kotao bi trebao da radi u grejnom sistemu,

maksimalnih temperatura do 110oCsa akumulatorom toplote koji bi omoguio da se biomasa u kotlu

sagoreva maksimalnim intenzitetom.

Prilikom sagorevanja bala biomase postoji vie faktora koji odreuju mogunost postizanjaeljene izlazne termike snage i nain opsluivanja kotlovskog postrojenja. Meu najuticajnijim

faktorima se mogu navesti: konstrukcione karakteristike kotla i dodatne opreme, vrsta, vlanost i

sabijenost bala sagorevane biomase, koliina, sastav i nain raspodele vazduha za sagorevanje, kao i

koliina i "snaga" ara od prethodno sagorevane bale.

Poto balirana biomasa prikupljlena sa njiva nije gorivo standardizovanih karakteristika za

pravilnu eksploataciju kotla neophodno je kotao konstrukciono primeriti oekivanim zahtevima, a

njegovo opsluivanje vriti na nain kojim e se u to veoj meri umanjiti negativni uticaji sagorevane

biomase. Naprimer, ako se sagorevaju bale biomase poveanih vlanosti nove bale se u loite moraju

ubacivati kada u loitu ima dovoljno ara koji e omoguiti prvo suenje, a zatim i potpalu ubaene

bale, pri emu je kod vlanijih bala velikih gustina neophodno bar jednom rastresti balu posle nekoliko


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minuta od njenog ubacivanja u loite kotla. Kod bala biomase manjih vlanosti i gustina navedeno

nije potrebno initi. U svakom sluaju opsluivanje kotla je arno, to uslovljava itav niz

negativnosti, kao to su: neravnomerno sagorevanje, razliita potreba za koliinom vazduha za

sagorevanje (koeficijentom vika vazduha), neravnomerno produkovanje toplotne snage, izuzetno

neujednaen sastav dimnih produkata sagorevanja sa veim koncentracijama ai, CO i drugihnesagorelih produkata i dr.

Zbog svega navedenog, a u elji da se eliminiu mnogi faktori koji bi doveli do pogrenih

zakljuaka, eksperimenti u ovim istraivanjima su raeni u pojedinanom reimu u kojem su mnogi

uticajni faktori eliminisani ili su odravani na konstantnom nivou.

U cilju utvrivanja uticaja koliine i sastava vazduha za sagorevanje na produkovanu

toplotnu snagu kotlovskog postrojenja pored ostalog je mereno: protok, temperatura i pritisak vazduha

za sagorevanje, temperatura izlaznih dimnih produkata sagorevanja i njihov sastav, protok i

temperatura recirkulisanih gasova, kao i protok, temperatura i pritisak ulazne i izlazne vode grejnog

sistema. Mesta za postavljanje mernih sondi su prikazana na slici 1.

3. Materials and methods

3.1. Soybean straw characteristics

Kao materijal u radu su odabrane male etvrtastebale sojine slame. Sojina slama se u Srbiji

znaajno koristi kao biogorivo, poto se soja gaji na povrinama od preko 130.000 ha. Forma malih

etvrtastih bala slame je odabrana iz razloga, to se moe direktno ubacivati u predvieni tip

eksperimentalnog postrojenja.

Slama je balirana odmah posle etvei na njivi je izabrano 250 bala indentinih dimenzija i

mase, a samim tim i stepena sabijenosti, tj. gustine (9- Igi PhD). They were stored in a warehouse

near the boiler plant so they were not exposed to additional climate effects, which helped in preserving

their quality and prevented from deterioration of the content, shape and the level of compactness.

U fiziko-hemijskom pogledu sojina slama zadovoljavaju veinu zahteva za primenu kao

biogorivo. Poto su bale sojine slame prikupljane sa iste parcele gde je gajena ista sorta, na zemljitu

slinih karakteristika i pri uzgoju su koriene indentine agrotehnike mere (norme ubrenja,

hemijskog tretiranja i dr.) smatralo se da e se sa etiri uzorka postii reprezantativni pokazatelji

elementarne i tehnike analize sojine slamekoja je korienja za realizaciju eksperimenata. Prosene

vrednostiizabranih pokazatelja su prikazane u tabeli 1.

Table 1: Characteristics of soybean as a fuel

Component Unit Value

Podaci o balama slame

Dimenzije bala (m) 0.35 x 0.5 x 0.8

Masa bala (kg) 11

Gustina bala (kg/m) 78.57

Elemental chemical analysis

Carbon (%) 42.32Hydrogen (%) 5.57


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Sulphur (%) 0.17

Nitrogen (%) 0.34

Oxygen (%) 37.24

Chlorine (%) 0.76

Ash (%) 4.1Moisture (%) 9,5

Technical analysis

Gornja toplotna mo (kJ/kg) 15.757

Donja toplotna mo (kJ/kg) 14.029

Volatile matters (%) 71,73

Navedeni pokazatelji su odreivani u Laboratoriji za termotehniku i procesnu tehniku na

Poljoprivrednom fakultetu u Novom Sadu, Laboratoriji Naunog instituta za ratarstvo i povrtarstvo iz

Novog Sada i laboratoriji Termoelektrane-toplane Novi Sad.

3.2. Metod istraivanja

Primenjeni nauni metod istraivanja se baziraona naunom metodu matematiko-statistike

analize na osnovu kojegje mogue utvrditi uticaj koliine i sastava vazduha za sagorevanje na proces

sagorevanja malih etvrtastih bala sojine slame (disperzionom analizom), analitiki oblik tih uticaja

(regresionom analizom), kao i znaaj tih uticaja na posmatrani proces (koeficijentima korelacije i

determinacije) (10- Jani PhD).

U radu je predvieno praenje koliine i sastava ulaznog vazduha za sagorevanje i stepena

recirkulacije, kao veoma znaajnih faktora koji utiu na proces sagoranja balirane sojine slame, tj.

produkovanu termiku snagu eksperimentalnog postrojenja. Ostali faktori, iji je uticaj izraen u

procesu sagorevanja, su se odravali na konstantnom nivou.

Izabranom metodom istraivanja se u to veoj meri teilo pribliavanju stvarnim,

eksploatacionim procesima sagorevanja da bi dobijeni rezultati imali to veu primenljivost u praksi.

Svesni ogranienih mogunosti izbora faktora i opsega u kojima se mogu analizirati

odreeno je da sepromena protoka vazduha za sagorevanje posmatra na 5 nivoa (150, 220, 290, 360 i

430 m3/h), a stepen recirkulacije dimnih produkata sagorevanja na 3 nivoa (0, 16,5 i 33%).

Procentualni udeli dimnih produkata sagorevanja se odnose na ukupnu zapreminu vazduha za

sagorevanje (smee okolnog vazduha i recirkulisanih dimnih gasova).

Prema planu istraivanja eksperimenti su izvoeni u minimalno tri ponavljanja. Nakon

ubacivanja nove bale slame ukljuivan je sistem za automatsko prikupljanje podataka sa postavljenih

sondi. Svi podaci su beleeni svakih 5 sekundi. Jedino je gasoanalizator ukljuivan 30 sek posle

poetka sagorevanja bala slame zbog toga to je na poetku sagorevanja bala uvek bila izrazito visoka

koncentracija CO koja je blokirala rad gasoanalizatora. Prikupljeni podaci su odmah prikazivani

dijagramski i u sluaju veeg odstupanja eksperimenti su izvoeni i vei broj puta, dok se nisu dobili

rezultati sa ijim su se prosenim vrednostima dalje vrile disperzione i regresione analize.

I pored kontinualnog loenja eksperimenti u kojem su bale sagorevane izvoeni su

pojedinano. To je bilo neophodno da bi se koliina i "snaga" ara, kao jednog od najuticajnijihfaktora na dimamiku sagorevanja pokuao u to veoj meri odravati na konstantnom nivou. Takoe,


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pojedinanim obavljanjem eksperimenata omogueno je adekvatno definisanje vrednosti poetnog

koeficijenata vika vazduha. Pored toga, praenjem sagorevanja jedne bale omogueno jepraenje

opadanja produkovane toplotne snage kotla do relevantnih vrednosti koje su bile ograniene

vrednostima sadraja kiseonika u produktima sagorevanja, tj. maksimalno postavljenim koeficijentom

vika vazduha. Pretpostavka je da je tada u loitu kotla preostala slina nesagorela masa bale.Preklapanjem dobijenih dijagrama mogue je odrediti optimalnu uestalost loenja bala u razliitim

reimima rada kotlovskih postrojenja, rad kotlovskih postrojenja pri niim koeficijentima vika

vazduha, to e poveati efikasnost postrojenja.

Measuring methods are in accordance to SRPS EN 303-5:2007 standards and DIN 4702 in

boiler thermal power determination. Boiler thermal power was determined using a direct method [19],

i.e. by measuring volume of water supply and by measuring the water temperature at boiler entrance

and exit points. A detailed overview (measuring codes, bale dimensions, instrument names, measuring

ratio) is also given in [21,22]. During the experiments, boiler thermal power was determined based on

Equation (1) [23]

(1)where: P - is the boiler thermal power[kWth],w- is mass flow of water through the boiler [kg h

-1], cp-

is specific heat of water [kJ/kg K], tow- is boiler outlet water temperature [K]and tiw- is boiler inlet

water temperature [K].

Iako recirkulacija gasova nije predviena u osnovnoj konstrukciji kotlovskog postrojenja u

radu se pored uobiajenog rada kotla pokuao sagledati i njegov rad kada se sa vazduhom za

sagorevanje ubacuje i deo recirkulisanih gasova. Takav nain radaje interesantan sa vie aspekata, kao

to su: predgrevanje vazduha za sagorevanje, sniavanje temperature sagorevanja, smanjenje emisije

NOx, ali je osnovna hipoteza za ispitivanje bila da se sa uvoenjem recirkulisanih gasova u proces

sagorevanja moe uticati na izlaznu termiku snagu kotlovskog postrojenja i njegovu efikasnost rada.

3.3. Statistical analysis

Experimental data have been processed using nonlinear regression analysis (software

packages Mathematica 6 and Statistika 10 were used [19-21].The compatibility of analytical

formulation for presenting measured data is checked using the following tests: (1) t-test examines the

regression coefficients significance; (2) checking of the level of reliability of dependable variables,

shown by the coefficient of reliabilityR2for the chosen independent variables and the compatibility of

the regression line to the results [17].

This paper presents the analytical approach and expression for correlation between boiler

thermal power and bale residence time, for all regimes tested and all values of the flue gas

recirculation ratios (0%, 16.5% and 33%). Average values of three times repeated experiments

uraenih u razliitom vremenufor each regime tested are used for analysis.

4. Results and discussion


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The average times necessary for bales total burnout, as well as the average values of boiler

thermal power during the whole combustion process are presented in tab. 3. Range of standard

statistical error of average boiler thermal power is presented, also.

Table 1: Values of the measured bale burn out time and average boiler thermal power

Degree ofrecirculation

Boiler operating regime [m3h

-1]

150 220 290 360 430

Average time when

bale totally burnt (s)with standard error (%)

0% 1258 713 670 453 400

16.0% 1335 642 763 532 427

33% 943 667 587 480 348

Average boiler therm.power (kW) with range

of standard error (%)

0% 75.66 92.24 113.12 96.63 112.94

16.0% 69.33 85.06 88.98 97.72 111.21

33% 72.32 77.48 95.01 97.49 114.94

General conclusions are: 1) due to increase of inlet air flow rate to the boiler firebox bale

combustion time decreases and boiler thermal power increases, 2) for all regimes (except for the

regime of 220 m3

h-1

), flue gas recirculation of 16.5% gives higher bale residence time and smaller

average boiler thermal power, 3) at recirculation ratio 0%, average boiler thermal power is the largest

in regime 290 m3 h-1, 4) comparing recirculation of 16.5 and 33%, it can be seen that last one provides

higher boiler thermal power, but almost certainly this is achieves by the reduced energy efficiency.

Implementation of the flue gas recirculation not only reduce of nitrogen oxides emission

(although this is partially achieved), but also makes bales burnout time longer, and affects boiler

thermal power (and energy efficiency). As it can be seen from (tab. 3), these effects are only partlyachieved.

4.1 Boiler thermal powerexperimental data and analytical approach

Our first task was to express boiler thermal power as a function of air flow rate and bale

residence time. According to fig. 3, for higher air flow (360 and 430 m3 h1), experimental data should

be approximated by exponential function Exp((xa)2) which is symmetric with respect to line x = a

and its graphical interpretation suits to normal distribution (parameter a is time when maximum

thermal is achieved). Also, according to Fig. 3: a) higher volume ( or mass?) air flow rate v implies

higher maximum boiler thermal power P (so the function Exp((xa)2) should be multiplied by air

flow rate v and regression coefficientband we obtain bvExp((xa)2); b) also, higher volume air

flow v makes total burnout time krshorter and graph becomes narrower. Finally it is proposed that

function bvExp((c(xa)/kr)2) can properly approximate experimental data at regimes with larger

volume air flow rate. Regression coefficients a, band care useful for graph corrections according to

experimental data. For smaller volume air flow rates (220 and 290 m3

h-1

), experimental data can be

approximated by function vsin((xb)/kr), with the same arguments as stated above. Finally,

analytical form of the empirical correlation between boiler thermal power and bale residence time for

different operating regimes can be, according to detailed analysis given in [5], formulated:


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( ) ( ) (2)

Here, b1, b2,..., b7are regression coefficients given in tab. 4, 5 and 6, is the bale residence

time,P- is the boiler thermal power[kWth], v - is volume air flow rate [m3h

-1] and kr- is the time when

thebales are totally burnt[s].

Table 4: Estimation of empirical regression coefficients and analysis of variance of equation (2), norecirculation

Level of confidence: 95%, Determination coefficient:R2= 90.58%

Estimate t-test Confidence interval

b1* 44.7503 54.55 (43.139 , 46.362)

b2* 0.3392 58.09 (0.328 , 0.351)

b3* -2.3073 -57.02 (-2.387 , -2.228)

b4* 264.8410 60.81 (256.286 , 273.394)

b5* 0.1272 34.11 (0.120 , 0.135)

b6* 3.7176 40.96 (3.539 , 3.896)

b7* 411.0314 45.24 (393.190 , 428.873)

*significant coefficients at significance level of P


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b1* 47.2398 60,33 (45.701 , 48.778)

b2* -0.0575 -14,68 (-0.0652 , -0.050)

b3* -4.0827 -12,62 (-4.718, -3.447)

b4* 179.1803 33,65 (168.718 , 189.643)

b5*

0.2129 43,29 (0.203 , 0.223)b6

* 2.0180 50,60 (1.940 , 2.096)

b7* -77.9528 -12,62 (-90.084 , -65.822)

*significant coefficients at significance level of P


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Fig. 5: Correlation between boiler thermal power

and bale residence time, experimental data,

recirculation 16.5%


and bale residence time, model (1), recirculation

16.5%


and bale residence time, experimental data,

recirculation 33%

Fig 8: Correlation between boiler thermal

power and bale residence time, model (1),

recirculation 33%

Experimental data and empirical correlation (2) for two other recirculation values can

be seen in fig. 5, 6, 7 and 8. If the flue gas recirculation of 16.5% (or 33%) is used, total bale

burnout time is longer, but average thermal boiler power is smaller comparing to regimes with

zero recirculation (tab. 3).

4.2. Maximum boiler thermal power and energy efficiency

To determine compliance between analyzed regimes and nominal boiler power, it is

necessary to find out at what time, starting from the moment of bale feeding, the maximum boiler

thermal power is attained. Maximum boiler thermal power and corresponding time necessary to

achieve maximum power are presented in tab. 7 for all regimes that have been investigated.

Table 7: Time of reaching the maximum thermal power

Degree of Boiler operating regime [m3h

1]


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recirculation 150 220 290 360 430

Time necessary toreach maximum

thermal power (s)

0% 435 415 520 385 325

16.5% 880 630 485 505 375

33% 840 465 505 465 275

Maximumthermal power

Pmax(kW)

0% 94.05 108.12 120.02 126.42 138.6116.5% 74.57 78.55 103.24 101.91 127.01

33% 71.66 98.56 110.49 116.28 122.56

Boiler attains maximum power in the first half of the time period necessary for total burnout

of the straw bale. In regimes with air flow rates 150 and 220 m3 h

1 and with zero recirculation

maximum power is smaller than the nominal one (120 kW for 290 m 3h-1). In regime with 290 m3h-1

air flow rate maximum thermal power of boiler is identical with nominal power of 120 kW. In regime

360 and 430 m3h-1, maximum boiler thermal power is greater than the nominal one. It follows that

those regimes are not appropriate for this specific boiler design, since thermal power greater than

nominal one can cause damage of the boiler in exploitation.

In regimes with low air flow rates, implementation of the recirculation give significantly

worse conditions for combustion of soybean straw bales, leading to the diminishing of the boiler

thermal power. Recirculation of 16.5%, for high air flow rates has positive impact on the conditions or

straw combustion, due to the preheating of the inlet air. Favourable combustion conditions enable

higher maximum boiler thermal power and higher boiler efficiency comparing to the case when there

is no recirculation. Extremely high recirculation of 33%, did not give positive results, as it was

expected. For this level of recirculation better parameters of bale burning are obtained at higher air

flow, also. It is most probable that such results can be explained by the positive influence of air

preheating on combustion of soybean straw bales. Generally, it can be concluded that recirculation of

33% is not adequate and should be avoided. Using higher air flow regimes are desirable when

recirculation is 16.5%.

5. Conclusion

The results obtained by detailed testing of the boiler in real exploitation and presented in the

paper show that the optimal operating regimes are obtained when the air flow rate through the firebox

reaches 220 and 290 m3 h

-1respectively, and with zero recirculation.Recirculation of

33%hasdeterioratedcharacteristicsof the boiler.

Average boiler thermal power ranges between 75.66 and 113.12 kW if there is no flue gas

recirculation. Boiler thermal power ranges between 69.33 and 111.21 kW with 16.5% recirculation

and from 72.32 to 114.94 kW with 33% recirculation.

The presented empirical correlation can be considered as valid for the given boiler plant and

the conditions under which the experiment was conducted. The analytical approach applied and

obtained empirical correlation can be used for different engineering purposes. Apart from giving the

basic information on soya straw bale combustion that can be used to obtain higher boiler thermal

power and plant energy efficiency in real operating conditions, results can be applied in the course of

the design of future plants, as well as for the reconstruction and for automatization of the existing

power plants.


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Acknowledgement

This paper was financed by the Ministry of Science and Technology, the Republic of Serbia, Grant no.

III 42011, 2011-2014.

References

[1] Brki, M., Jani, T., Igi, S., Assessment of species and quantity of biomass in serbia and

guidelines of usage, Thermal Science, 16(2012)1, 79-86

[2] Oka, S., Jovanovi, Lj., Combustion Cycle and Combustion Lines of Biomass, in: Biomassa

Renewable Energy Source (Eds. S. Oka, Lj. Jovanovi), Monograph (in Serbian), Yugoslav Society

of Thermal EngineersVina Institute of Nuclear Sciences, Belgrade, Serbia and Montenegro,

[3]Dodi, S., Zeki, V., Rodi, V., Tica, N., Dodi, J., Popov, S.,The economic effects of energetic

exploitation of straw in Vojvodina,Renewable and Sustainable Energy Reviews,

16(2012)1, pp. 397403

[4] Zeki, V., Rodi, V., Jovanovi, M., Potentials and economic viability of small grain residue use as

a source of energy in Serbia, Biomass and bioenergy, 34(2010)12, pp. 17891795;

[5] Repi, B., Daki, D., Paprika, M., Mladenovi, R., Eri, A., Soya straw bales combustion in high-

efficient boiler, Thermal Science, 12(2008)4, 51-60

[6] Jani, T., Brki, M., Igi, S., Dedovi, N, Lazi, R., Upravljanje sagorevanjem balirane biomase u

toplovodnim kotlovskim postrojenjima , Savremena poljoprivredna tehnika, 36(2010)4, pp. 366-

372

[7] Turanjanin, V., urovi, D., Daki, D., Eri, A., Repi, B., Development of the boiler for

combustion of agricultural biomass by products, Thermal Science, 14(2010)3, 707-714[8]M. Obaidullah, M., Bram, S., Verma, V., K., De Ruyck, J., A Review on Particle Emissions from

Small Scale Biomass Combustion, International Journal of Renewable Energy Research- IJRER,

2(2012)1, pp. 147-159

[9] Igi, S., Impact of Sort and Condition of Baled Straw, Incoming Air Quantity and Composition on

Boiler Energy Efficiency, Ph. D. thesis., University of Novi Sad, Novi Sad, Serbia, 2008

[10] Jani, T., Combustion kinetics of wheat straw, Ph. D. thesis., University of Novi Sad, Novi Sad,

Serbia, 2001
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