BiomassAn overview

G.V. FracastoroEnergy Department

011-090-4438

Dictionary• Auger = coclea• Bark = corteccia• Beech = faggio• Chipwood = cippato• Digestate = digestato• Feedstock = materia prima• Fir = abete• Fixed grate = griglia fissa• Fluidized bed = letto fluido• Fuzz = pula• Grind milling = macina a

mulino• Grate = griglia• Hopper = tramoggia

• Landfill = discarica• Limestone = calcare• Livestock = bestiame• Log = tronchetto• Manure = concime• Pits or stones = noccioli• Productive chain = filiera• Rice husk = lolla di riso• Scrubber = torre di lavaggio• Shells = gusci• Straw = paglia• Tar = catrame

Classification• There are three possible classifications according to

– nature of raw material• Forest residues• Agricultural waste• Livestock and civil (exhaust oil) or industrial sludge• Ad hoc energy crops (starch or sugar, ligno-cellulosic, oily,… )• Municipal (MSW) and Industrial solid waste

– final energy use• Heat• Electricity• Transport

– state of biomass as input to the process• Solid biomass (from vegetation or dry waste)• Gaseous biomass (sludge transformation and ad hoc crop digestion)• Liquid biomass (oil, alcohol, etc.)

Glossary• BIOMASS: all organic material produced by plants or any process involving

life is called biomass. Biomass is labelled as green house gas emission neutral energy source since it has a short carbon cycle, i.e. Emission of carbon from plant to atmosphere and absorbed by plant during growth only takes between one to some tens of years, whereas fossil carbon exists for millions of years.

• BIOMASS COMBUSTION: Traditional combustion process where biomassis used as a fuel and where the hot exhaust gases are used to produce heat or steam that is processed through a steam turbine to generate electricity.

• BIOMASS CO-COMBUSTION: Combination of fossil (for example, coal) and bio-energy fuels in conventional power plants.

• BIOMASS GASIFICATION : Oxygen –starved combustion process where biomass is combusted in order to obtain CO and gasses as end products (so called producer gas or synthesis gas). In comparison to solid biomass the electricity produced by feeding gas into a gas turbine or internal combustion engine guarantees higher efficiency (35-40%, against 25-30% of traditional combustion).

Glossary• BIOGAS: a fuel gas produced from biomass feedstock through a low

temperature biochemical process named anaerobic digestion.• CO-DIGESTION: when the biomass involved in the digestion

process is a mix of manure and organic waste or/and agriculturalwaste. By adding organic waste sources to the manure higher yields of biogas is obtained.

• DIGESTATE: semi-solid sub-product of anaerobic digestion stabilized in odours, in pathogenic bacteria and in carbon percentage, but with a still too high concentration of nitrates.

• PELLET: The term pellet identifies the sub-product realized through mechanical mincing of wood and agro-industrial residuals. That material has the right dimension and humidity to assure an optimal combustion performance of every kind of combustor, from big biomass combustion plants to the domestic boilers.

Biomass for commercial energyuses

• Agricultural and forest biomass– Wood from forest– Short Rotation Forestry (SRF)– Agricultural food products waste– Energy crop grown plants

• Biogas, produced from – waste from livestock and agriculture– organic fraction of Municipal Solid Waste

• Municipal Solid Waste (MSW)• Biofuels

Overview of agri-forest biomass• Main European consumers of Agri-Forest Biomass (AFB) are Germany,

France, Sweden and Finland, followed by Italy with 5.2 Mtoe (2.7% of total energy consumption) and 20% of all renewable electricity

• AFB is used for producing heat and electricity, or both (CHP plants)– Heat production in Italy

• Residences (< 1 MWt): 1 million pellets units, 800 M€ of turnover (2009)• District heating systems (1-20 MWt): installed capacity 400 MWt almost only in

Northern Italy, 100 M€ turnover in 2009– Electricity production in Italy: 100 installations (500 MWe, 1000 M€ in 2009),

Calabria is #1 in Italy. Combustor + Boiler +Power cycle + Flue gas treatment. Power cycle consists in

• Organic Rankine Cycle (ORC) (0.3-1.5 MWe)• Steam Rankine cycle (>4-5 MWe)• Cost of a “standard” 20 MWe plant is around 60 M€ (3000 €/kW)

• New technologies: pyrolisis and gasification (industrially irrelevant)• An Italian value chain made by 350 companies (97% operated and 95%

supply chain by Italian companies)• For electric installations smaller than 1 MWe incentive is 0.28 €/kWh

Overview of biogas• Sources: anaerobic digestion of waste waters, MSW organic

fraction, waste from agro-food industry, livestock• Germany (10 TWh) and UK (7.5 TWh) are the largest biogas users,

followewd by Italy with 2.9 TWh produced by 450 MW installed capacity, about 500 installations (70% from MSW organic fractionand 30% from agricultural waste).

• Biogas is used to produce electricity through a digester and an internal combustion engine (ICE), with size varying between 100 kW and 1 MW or more (for MSW) and cost between 1400 €/kW and 800 €/kW for large engines. Whole installations costs range between 2500 and 4500 €/kW

• Load factor can exceed 80% (7000 hrs), and electricity cost is between 0.18 and 0.22 €/kWh

• Incentives are the same as for AFB

Overview of MSW• In the EU 53 Mt/yr (22% of total) are used for energy production. Germany has the

largest capacity (59 kt/day), followed by France (46 kt/d) and Italy (21 kt/d) • Energy recovery from MSW is regulated in Italy by DLgs 11/5/2005 n. 133

implementing directives 1999/31/EC and 2000/76/EC • DLgs 133 establishes measures and procedures finalized at preventing atmospheric,

water, and soil pollution and protecting human health. All installations require an Environmental Integral Authorisation (a complex and lengthy procedure)

• The main components of an MSW incineration plant are the same as for an AFB installation, but the cost will be higher. A 20 MW plant will use about 200.000 tons of MSW per year and have a cost of about 6000 €/kWe

• Total MSW in Italy is about 33 Mt (550 kg/person), 14% is used for energy recovery (4.6 Mt, or 886 ktoe), 33 % is recycled (10%) or recovered in other ways (23%), 53% is sent to landfill (garbage dump)

• In Italy there are 53 plants for energy recovery from MSW with a total capacity of 730 MWe. The average size is 120.000 t/yr. They privilege electricity production, with a few exceptions, like district heating systems of Milano and Brescia (30.000 families).

• Turnover of this sector in Italy is about 1000 M€, with 168 companies operating in the design, construction and operation of plants, waste collection and treatment. 40% of the upstream activities are made by foreign companies

Overview of biofuels• Biodiesel and bioethanol are the two main biofuels.• Biodiesel is a mixture of methyl-esters and fat acids derived from vegetal oil

or animal fat. • Bioethanol is a fuel obtained from sugar (sugar cane, beet) or starch

(amido). Before being mixed with gasoline it may be transformed into ETBE (ethyl-ter-buthyl-ether). Second generation bioethanol is made from ligno-cellulosic (non food) biomass. Third generation is algae fuels

• Directive 2003/30/EC has introduced “reference values” of biofuel share on the market (from 2% in 2005 to 5.75% in 2010, now postponed to 2015 by Directive 2009/28/EC).

• In Italy these levels are 3.5% (2010), 4% (2011) and 4.5% (2012).– There are 15 biodiesel production plants with a capacity of about 2.5 Mt/yr, but

only 0.9 Mt (30% imported) have been consumed in 2009. This is due to incentive schemes based on capacity rather than production.

– Bioethanol and ETBE consumption amounted to 0.1 Mt in 2009, 90% of which comes from imported biomass treated in Italy

Biomass consumption in Italy

Biomass consumption in Italy in 2009 (7917 ktoe)

831

1000

5200

886

BiogasBiofuelsAgro-forestal BiomassMSW

Data in ktoe (4% of total energy consumption)

• Renewable or better regenerable source• Price not linked to oil price• Limited “life cycle” CO2 emissions (only during

crop growing and transportation)• Minimum sulphur content in flue gases• May improve hydrogeologic territory conditions

and socio-economical contest• Economically already viable, for thermal use, even

if with higher installation and maintenance costs

Biomass advantages

Primary energy factors (EN 15603)

Productive chain

traction

Thermal production

cogeneration

electrical production

Grind-milling

esterification

Anaerobic digestion

Forestry management

Energy crops

Agriculture waste

wood

biodiesel

biogas

bio-oil

Biomass energy conversionsystems

• Thermochemical C/N > 30 and humidity < 30%: wood, straw, rice husk, fuzz, shells, pits or stones– Combustion – Carbonization – Gasification – Pyrolysis

• Biochemical (animal waste, biomass from MSW)– Anaerobic digestion– Aerobic digestion– Alcoholic fermentation– Esterification of vegetal oils

Direct combustion

• Combustion reactions

• Feedstock Preparation & Handling– Most biomass-to-energy technologies need the feedstock to meet certain specifications in regards to

size, moisture content, ash content, contamination. Fuel preparation, by size reduction, screening, and drying are processes most biomass has to go through before being turned into energy.

– Moreover, in most cases the feedstock needs to be delivered to the plant, generally by conveyors or trucks. Loading or unloading a truck can be time consuming and can result in spills that might be acceptable to a sawmill operation, but not a public facility, such as a hospital.

– Finally the supply of a certain feedstock might be seasonal or intermittent, requiring intermediary storage or long-term storage on site.

• Combustion technologies:– Pile burner– Fixed or mobile (travelling) grate– Fluidized bed

( ) kg/MJ5.17OHn5COn6On6nOHC 2225106 ++→+

CombustorExhaust gas

treatmentPower cycleBoiler

Pile burner (fixed grate)• Pile burners typically consist of a two-stage

combustion chamber with a separate furnace and boiler located above the secondary combustion chamber.

• Biomass fuel burns on a grate in the lower chamber, releasing volatile gases. The gases burn in the upper (secondary) combustion chamber.

• Wood is piled deep on a grate using an auger in the bottom section and combustion air is fed upwards through the grate and inwards from the walls; combustion is completed in a secondary combustion zone using overfire air.

• Ash is removed by isolating the combustion chamber from the furnace and manually dumping the ash from the grate after the ash is cooled.

• Pile burners typically have low efficiencies (50% to 60%), have cyclic operating characteristics because of the ash removal, and have combustion cycles that are erratic and difficult to control.

• Although capable of handling high-moisture fuels and fuels mixed with dirt, pile burners have become obsolete with the development of more efficient combustion designs with automated ash removal systems. www.biomassenergyresearch.ca

Fixed or travelling grate• In a stationary or

travelling gratecombustor, an automatic feeder distributes the fuel onto a grate, where the fuel burns. Combustion air enters from below the grate.

• In the stationary grate design, ashes fall into a pit for collection.

• In contrast, a travelling grate system has a moving grate that drops the ash into a hopper.

Fluidized-bed combustor

– A fluidized-bed combustor is a furnace chamber whose floor is slotted, perforated, or fitted with nozzles.

– Air is forced through the floor and upward through the chamber. – The chamber is partially filled with particles of either reactive or

inert material (sand), which will fluidize at an appropriate air flow rate.

– When the gas velocity is high enough, fluidization takes place, solid particles are widely separated and circulate freely, creating a “fluidized-bed” that looks like a boiling liquid and has the physical properties of a fluid

– As air velocity increases, the particles mix more violently, andthe surface of the bed takes on the appearance of a boiling liquid.

Main features of FBC combustors

• There are two reasons for the rapid increase of fluidized bed combustion (FBC) in combustors.

– possibility of using fuels which are difficult to burn using other technologies– possibility of achieving, during combustion, a low emission of nitric oxides and

the possibility of removing sulphur in a simple manner by using limestone as bed material (increasingly important)

• FBC is a combustion process able to control pollutant emissions without external emission controls (such as scrubbers* for flue gas desulfurization). The technology burns fuel at temperatures of 750-900 °C, well below the threshold where NOx (at approximately 1400 °C).

• The mixing action of the fluidized bed brings the flue gases into contact with a sulphur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulphur pollutants in coal can be captured inside the boiler by the sorbent.

• A cyclone is used to either return fines to the bed or to remove ash-rich fines from the system.

* See following page

Wet scrubbers• Inertial separators had the

drawback of dust particles not accumulating properly on the collecting surface, resulting in finer particles being swept back into the air stream.

• Wet scrubbers , use water to absorb the finer dust particles (scrubbed). Water comes into contact with the dust filled air stream and captures dust in water droplets.

• The dirty water droplets accumulate, and are collected and removed. The cleaned air in air washers contains fine water droplets forming a mist, which can be removed from the cleaned air by a mist eliminator

Scheme of FBC

Carbonization

• Carbonization: transforming wood into vegetal coal• Process:

– Preheating @ 165 °C without air with emission of C O2, water vapour and heat

– Heating @ 250 °C

• Carbonization is a complex process in which many reactions take place concurrently such as dehydrogenation, condensation, hydrogen transfer and isomerisation*.

• The final temperature applied controls the degree of carbonization and the residual content of foreign elements

*Isomerisation is the process by which one molecule is transformed into another molecule whichhas exactly the same atoms, but the atoms are rearranged

Gasification• Converts biomass into a gaseous fuel easily employable in more

advanced energy production systems (internal combustion engines, gas turbines).

• Consists in partial oxidation @ low oxygen rates and @ high temperature (900-1000 °C) of lignocellulosic produ cts.

• Yields a combustible synthesis gas, formed by a mixture of hydrocarbons CxHy, CO, H2, CO2 and H2O.

• Typical lower calorific value– 4 MJ/Nm3 air gasificators ⇒ gasogen gas– 10 MJ/Nm3 steam gasificators – 12 MJ/Nm3 oxygen gasificators ⇒synthesis gas

• Problems:• technology still in the experimental phase, especial ly on

small size • low calorific value• impurities in flue gases (particles and soot)

Pyrolysis (dry distillation )• Converts biomass in liquid • Pyrolysis – or liquefaction of biomass – converts solid biomass into a liquid fuel.

Gases and solids are generated as a by-product only and are partly used to fuel the conversion process and drying of the feedstock.

• Most pyrolysis technologies generate two or more liquid biofuels of different viscosity that can be physically separated and are usually refined further before use in applications such as boilers, turbines, or combustion engines. The energy content of these liquids are roughly half of the calorific value of diesel.

• A key advantage of pyrolysis is that a rather crude, bulky feedstock ‘biomass’ is converted into a higher value, dense energy product that is easily transportable.

• The concept of pyrolysis has historically been used by utilities for the gasification of coal and until to date in the (petro-)chemical industry; only recently has the technology been expanded to recover carbon contained in waste material, including municipal solid waste

• Pyrolysis is realised heating ligno-cellulosic materials at temperatures between 400 °C and 800 °C without any oxygen

• Yields high calorific value gas (presence of tar vapours) and char (agglomerate made of C, ashes, CmHn)

Calorific value of products frompyrolisis and gasification

process temperature products LCV (MJ/Nm3)pyrolisis 300-500 char,bio-oil e gas >15fast pyrolisis 500 bio-oil e gas 16.5-18.5gasification pyrolisis 500 gas 18gasification in air 900-1000 gas 5.5-7.5gasification in oxygen 900-1000 gas 11gasification in steam <800 gas 10

Anaerobic digestion• Anaerobic digestion is a series

of processes in which pathogenic bacteria (Salmonella, Escherichia coli,..) break down biodegradable material in the absence of oxygen.

• As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.

• The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases.

Anaerobic digestion• The digestion process begins with

– bacterial hydrolysis of the input materials to break down insoluble organic polymers, such as carbohydrates, and make them available for other bacteria.

– Acidogenic bacteria convert the sugars and amino acids into CO2, H2, NH3, and organic acids.

– Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbondioxide.

– Finally, methanogens convert these products to methane and carbon dioxide.

• This biogas can be used directly as cooking fuel, in CHP gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

• Yields biogas: methane (50-80%) + CO2

• Calorific value H = xCH4*36 MJ/Nm3

• CO2 may be afterwards eliminated

Aerobic digestion• Aerobic digestion is a bacterial process occurring in the presence of

oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide (metabolization).

• Once there is a lack of organic matter, bacteria die and are used as food by other bacteria. This stage of the process is known as endogenous respiration. Solids reduction occurs in this phase.

• Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of energy costs for aeration needed to add oxygen to the process.

• AD is an exogenous process suitable for sludge depuration installations in which complex substances are converted into simpler ones with CO2 and H2O emission

• Composting is an example of aerobic process

Alcoholic fermentation

• Applied to sugar materials (sugar cane, beet (bietola), sugary sorghum (sorgo zuccherino)) and starch plants (wheat, barley (orzo), maize, potato)

• Transforms vegetal glucides (carbohydrates) into ethylic alcohol (C2H5OH) or bioethanol

• Reaction• May also be produced from lignocellulosic biomass

(straw, maize waste) from previous hydrolization• Bioethanol may be used as gasoline additive (up

to 15% in weight)

2526126 CO2OHHC2OHC +→

Vegetal oil esterification

• squeezing of oleaginous seeds (sunflower, colza-seeds, soya, palm, cocoa nut) yields vegetal oil (triglyceride)

• Esterification = reaction of oil with excess methylic alcohol

• methylic ester is called biodiesel• Typically

1000 kg of oil + 100 kg of methanol = = 1000 kg of biodiesel + 100 kg of glycerine

oil me thanol glycerine methylic ester+ ↔ +

Pollutant emissions for differenttechnologies

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SOx NOx CO PM-10

emis

sion

s (g

/kW

h)

waste wood combustion in boilers

fluidized bed combustion

poplar tree gasification

MSW combustion

biogas combustion

coal installations

bituminous coal combustion

coal powder combustion

fluidized bed combustion

natural gas installations

internal combustion engines

gas turbines

Emissions from biomass and otherfossil fuels installations

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waste woodcombustion

in boilers

fluidized bedcombustion

poplar treegasification

MSWcombustion

biogascombustion

bituminouscoal

combustion

coal powdercombustion

fluidized bedcombustion

internalcombustion

engines

gas turbines

SOx

NOx

CO

PM-10

Some data on possible usesof biomass

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12

lignocellulosic from hydrosolubleglucides

oleaginous “short-rotationforestry”

forest management

resa

(te

p/ha

)

min max

Yearly solar energy on the horizontalcorresponds to about 1000 toe/ha

min maxlignocellulosic cane, whole maize, gramineous 5 10from hydrosoluble glucides beetrave, sorghum, sugar cane 1 2oleeaginous colza, soya, sunflower 0.5 1short rotation forestry locust tree, willow, poplar 2 5forest management oak, chestnut, conifer, beech 0.2 0.3

yield (toe/ha)type of biomass examples

Wood biomass

Availability of wood biomassPiemonte

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

forest surface [875.000 ha]

available wood [153 Mm3] withdrawable wood mass(820 kt/yr)

yearly withdrawable energy[200 ktoe/yr]

Oakwood, Locust tree (robinia)

Shrub (arbusti)

Pine

New woods and underwood

Beech (Faggio)

Other conifers

Chestnut

Hygrophile plants

Present situation of wood heatingappliances

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70

80

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100

0 200 400 600 800 1000

Potenza massima al focolare (kW)

Minimum efficiencyaccording to DPR 412/93

Pnlog667 +=η

• Small (20-100 kW): firewood, logs or pellets• low efficiency (15 - 45 %): open fireplaces, stoves, wood ranges or ovens

(70-80% of users)• High efficiency boilers (70-80%): 20-30% of users

• Average (100-350 kW): wood chips or pellets (efficiency 75-80%)• Large (> 350 kW): wood chips (efficiency 80-85%)

Maximum power at the burner (kW)

Effi

cien

cy(%

)

Wood is normally not dry, but has a variable moisture content. This may be expressed through moisture on dry wood Udry or as moisture on wet wood (“tal quale”) Uw, defined as

Mw = mass of wet woodMdry = mass of dry wood

Moisture on wet wood is usually between 35% and 50%, depending on the type of wood, time passed since biomass production (new biomass is obviously more humid) and type of storage.

Moisture

dry

drywdry M

MMU

−=

w

dryww M

MMU

−=

Calorific value (H) of wood

wetdrywater mmm =+ wetwaterdrydry mHmrmH =−

( ) wetwaterwaterwetdry mHmrmmH =−−

( ) ( )rHUHm

rHmmHH drywdry

wet

drywaterwetdry +−=+−

=

MJ/kg

19.2Hdry (calorific value of dry matter)

2.4r (vaporization heat)

[ ]19 2 21 6 wH . . U MJ / kg= −

0

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25

0% 10% 20% 30% 40% 50% 60%

Uwet (%)

Cal

orifi

c va

lue

(MJ/

kg)

0%

20%

40%

60%

80%

100%

Ud

ry (

%)

Calorific value of wood

chipwood

logs

Seasoned woodUdry

The calorific value (H, sometimes called energy content) of a wooden species depends on its content of lignin (25 MJ/kg), of cellulose (16.7 MJ/kg) and resin. Also bark percentage modifies calorific value, because it has low calorific value and high ash content. Typical average values of Hi are reported in the table below for broadleaves and conifers. Lower calorific value of wood U dry = 12-15%

Energy content of wood

type of wood kcal/kg MJ/kg kWh/kgconifers 3800 15.9 4.42broad-leaved 3600 15.1 4.19general 3700 15.5 4.30

calorific value

Evolution of humidity loss in stacks

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0 5 10 15 20 25 30

mesi

umid

ità s

ul ta

l qua

le (

Utq

)

cataste aria aperta

c.s. dopo 3 mesi al riparo

c.s. dopo 3 mesi tagliati in pezzi da 23 cm

tagliati in pezzi da 23 cm e accatastata al riparo

Logs left on cutting bench in the open air for 6 months and afterwards stacked

logs stored under shed after three months lying on the cutting bench,

Small (33 cm) logs stored under shed after three months lying on cutting bench,

Small (33 cm) logs stored under shed immediately after cutting

Evolution of wooden mass with time

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0 5 10 15 20 25 30

anni

peso

a s

ecco

(kg

)

pioppoacerofrassinoolmoabetepino

poplarmapleash-treeelmfirpine

Mass-diameter ratio

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18000

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diametro (cm)

peso

(kg

)

frassinofaggioolmoacerocastagnopino

ash-treebeechelmmaplechestnutpine

Volume of 100 kg of fuel (lt)

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1400

gasoil hard wood soft (dolce)wood

chipwood sawdust wood-shaving

Volume of 100 kg of fuel (litres)

Wood logs boilers (20-100 kW)

• Wood logs boilers are fire tubes boilers, using water as a heat carrier fluid.

• Typical models are: • Inverse combustion boilers

• Horizontal combustion boilers

• Inverse combustion with fan for secondary combustion

• With flue gas suction

Typical features of wood boilers

featureinverse

combustion

Horizontal combustion

Inverse combustion with fan for secondary

combustion pressurizedoptimal power range (kW) 20-100 20-150 10-100 10-100Power control range 20-100% 20-100% 20-100% 15-100%Moisture on wet (%) 10-35 10-40 20-30 20-40ash content (%) 10 10 10 10efficiency (%) 75-85 70-85 75-85 70-80

Inversion flame boiler

Three turns fire tube boiler

Heat storage tank

For wood boiler it is advisable to have a heat storage tank for a number of reasons:

• possibility to dissipate the heat when the user demand is lower than the power produced by the boiler without interrupting the wood load combustion

• Limitation of number of boiler loadings • safety (high thermal inertia of the system)• DHW production in summer months

As a direct consequence:• Better efficiency • Longer boiler lifetime• More homogeneous heat production

Wood installation with heat storage(accumulator)

( )[ ]min/3.0115 PQPTV HNBsp −=

( )[ ]min/3.0160 PQmV Hsp −=

Heat storage size

• Vsp = storage tank capacity (litres)• PN = nominal thermal power (kW)• TB = combustion time (h)• QH = average heating load of the building (kW), equal, as a

first approximation, to half the design thermal load Qp• Pmin = the minimum thermal output of the boiler (kW)Assuming Hi = 4kWh/kg, the formula above may be rewritten as

in which m is the maximum load of the boiler, in kg.

0

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0 50 100 150 200 250 300

Prodotto potenza-ore di funzionamento (kWh)

Vol

ume

accu

mul

o (li

tri)

Q/Pmin = 0.8

Q/Pmin = 1

Q/Pmin = 1.2

Q/Pmin = 1.5

Q/Pmin = 2

Q/Pmin = 2.5

Storage volume

Size of timber store-house

• Size of store-house influenced by many factors:– Required wood demand, – wood type, – Reliability of distribution– Available space– Size of transport medium, etc.

• No more than 70% of the available space may be used• Ensuring complete unload of a truck without waiting that

the store-house is completely empty is important (stock breaking should be avoided)

• useful volume of fuel store-house should be :– > 50% maximum truck load– = two weeks fuel need

Boiler room and store-room have always to be separated for fire protectionreasons. Adequate room shoul be provided for around the differentcomponents of the installation in order to allow maintenance, loading and repair.

deposito

Example of 30kW boiler room and log store-room

Wood chip boilers (average capacity)

• Lower feed boiler for dry fuel

• Lower feed boiler for moist fuel

• Fixed grille boiler• Mobile grille

boiler

Lower feed boiler

1. oxygen residual sensor in flue gases2. sensor of flue gas temperature3. Exhaust gas blower4. Exhaust gas dust remover 5. combustor6. sensor for char (braci) control7. primary air introduction8. primary air introduction9. secondary air introduction10. Combustor hatch (portello)11. Furnace temperature12. Smoke tube heat exchanger13. sensor for boiler temperature

The best equipment for wood factories, especially for low-medium capacities. The special combustion chamber structure reduces slag (scorie) formation even for very dry material and wood shavings.

Moveable grid boiler

Power range: 100 kW - 3000 kWPower control range 30-100%Moisture of fuel 9-60%Ash content up to 10%Nominal efficiency from 80 a 85%

Suitable for any sort of woodenfuel, directly from the forest, withhigh moisture content and bark. The moveable pull-push grille iseasy to maintain.Used for large installations also fordry wood combustion.

Combustion control

• Control may be exerted on power yield, and combustion parameters (excess air, incombustibles).

• The best chip wood boilers can control power and combustion using the so-called LAMBDA probe (control of residual O2 in flue gases) in case integrated by CO control.

Pollutant abatement

NOx• NOx abatement is normally realised through postponed

introduction of combustion air. With this system NOx values may be reduced by 40-80%, depending on the biomass type.

Solid pollutant abatement• Sleeve filter (for dry flue gases) • Multicyclone• Other systems (electrostatic, washing, condensation, etc.)

Wood chip boiler

Wood chip boiler

Most important design parameters for sizing the bin volume are:

E = daily maximum useful energy [GJ/day]ηu,m = average efficiency during the period N = number of days between supplies [days]Hv = fuel calorific value referred to volume [GJ/m3]V = minimum bin volume [m3]α = stock factor increase

muvH

NEV

,ηα=

Size of the bin

• Ash quantity depends on biomass typology: beech wood, (faggio) almost without bark, will produce 0,2% ashes, while ash-tree (frassino) will produce 7%.

• When burning branches, bark, dirty (waste) wood and packing the frequency of removal of ash container will increase

• Ash extraction systems also include particulate extraction from cyclone flue gas filters and from combustion chamber. They can be

• manual for small installations• automatic for large installations. E.g., scroll or

auger or pneumatic extractor.

Extraction of ashes

Wood chip transportation

• Chip wood quality (shape, size, oversized pieces, etc.) affect normal operation of chip wood boilers and especially transport systems from the bin to the combustion chamber.

• Extraction systems of chip wood from bin – internal rake (a rastrelli interni), for particularly moist fuel – Scroll (a coclea), only for homogenous and small size chip wood,– funnel and mobile screw (a imbuto e vite mobile)

• Feed systems to combustion chamber– Scroll (a coclea),– conveyor belt (a nastro trasportatore), for low quality chip wood (e.g.,

for long pieces) or bark, with final hydraulic pusher (spintore idraulico)– hopper (benna, tramoggia) driven by “overhead travelling crane”, for

very large bins