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Drying of Biomass

Biomass can be considered as a source of charcoal and water

Harvested wood has greater weight of water than the weight of the wooditself

The total amount of water in a given piece of biomass is called its moisturecontent (MC)

Intrinsic moisture (bounded Water)moisture content of the material without influence of theweather

Extrinsic moisture (free water)

Influence of prevailing weather conditions during harvest on theoverall biomass moisture content

The MC percent of wood can be greater than 100%

)(6.0 22.06.04.1 OHCHOCH =

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Water Content Versus Moisture Content

The general range of moisture content for green (un-dried) hardwood lumber

can range between 45% and 150%.

)(6.022.06.04.1OHCHOCH =

FM53.047+=FC

C MM

M100

)100( +=

WeightFuelWet

MoistureChemicalMoistureFuelMT

+=

WeightFuel

MoistureChemicalMC = %47%)100(]16)1(2[6.0)1(2.012

]16)1(2[6.0=

+++

+=

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Moisture Content of Some Biomass

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r 2r

Mass v.s. Surface area

radius= 2

Surface = 50

Volume = 33.5

radius = 1

Surface = 12.57

Volume = 4.2

Surface / Volume = 3 Surface/ Volume = 1.5

Particle Size and ShapeParticle Size and Shape

Same volume, different shapes

Surface = 600

Volume = 1000

Surface = 4090

Volume = 1000

10

Surface / volume = 0.6 Surface/volume = 4.1

50

400.5

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Shape and Bulk of Biomass Energy

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Drying Time

Energy balance on a small particle

Mass transport-off-gasing

Radiative andconvective

heattransfer

Heat conductionin particle

Moisture transportinside the particleand away fromthe surface

fgwiiwi hmTCpmdH += )373(

)()( 44 pgppbpe TThATTA

dt

dH+=

+

+==

)()(

)373(

44pgppbp

fgwiiwi

e

dry

TThATTA

hmTCpm

dH

dHt

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Exhaust Gas Drying

Continues drying by Stack gases

Hot air or stack gases contact biomassas it enters the boiler

The stack gas (exhaust gas) contain 15wt% moisture at almost 250 C

Limited amount of moisture could beremoved

= fuel flow [kg/s]

Hcd = higher calorific heat value [kJ/kg],(18940 kJ/kg)

P = Effect [kW]

f f = moisture content [%]h = Latent heat of combustion water

[kJ/kg], (2260 kJ/kg)]

CleavinggasdryingofetemperaturT

CenteringgasdryingofetemperaturT

hkgevaporatedwaterM

hkgweightgasdryingWG

TTMWG

i

i

o

o

o

o

.....

.....

/..

/...

/)2940(

=

=

=

=

=

))*())1(*((*)(

.

fff fhfHcdmfP=

Power as Function of MC.

m

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Exercise

13. Calculate the moisture content if the fuel has 300 g water per kg gram of thefuel and it has a chemical formula of C6H7O(OH)5 .

14. Calculate the available power and power lost as function of moisture contentfor a fuel.

15. In a biomass fired boiler, the stack gases contain 15 wt% moisture and thatthe temperature 250C, calculate the additional moisture which can beremoved before the gas become fully saturated.

16. Comment on whether ash or moisture would help or hinder the combustion of

wood in a furnace.

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Ultimate Analysis

Proximity Analysis

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Direct Combustion

The principal categories of biomass conversion technologies for power andheat production are direct-firedand gasification systems. The two mostcommonly used types of boilers for biomass firing are stoker boilers andfluidized bed boilers.

Biomass power systems are typically below 50MW in size (coal-fired: 100-to 1,000-MW range).

Cofiring involves substituting biomass for a portion of the coal in an

existing power plant boiler. It is the most economic near-term option forintroducing new biomass power generation.

Because much of the existing power plant equipment can be used withoutmajor modifications, cofiring is far less expensive than building a new

biomass power plant. Compared to the coal it replaces, biomass reducesSO2, NOx, CO2, and other air emissions.

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Direct Fired Systems, Stoker (Fixed Bed)

Stoker boilers employ direct fire combustion of solid fuels with excess air,producing hot flue gases, which then produce steam in the heat exchangesection of the boiler.

Modern mechanical stokers consist of four elements,1) a fuel admission system,

2) a stationary or moving grate assembly

that supports the burning fuel and

provides a pathway for the primary combustion air,3) an overfire air system that supplies

additional air to complete combustion

and minimize atmospheric emissions, and

4) an ash discharge system.

There are two general types of

systemsunderfeed and overfeed.

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Direct Fired Systems, Stoker

Underfeed stokers supply both the fuel and air from under the grate, whileoverfeed stokers supply fuel from above the grate and air from below.

Overfeed stokers are further divided into two types: mass feed andspreader. In the mass feed stoker, fuel is continuously fed onto one end of

the grate surface and travels horizontally across the grate as it burns.

Cross Section of Underfeed, Side-Ashv

Discharge Stoker

Cross Section of Overfeed, Water-Cooled,

Vibrating-Grate, Mass-Feed Stoker

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Fluidized Bed Boilers

In this method of combustion, fuel is burned in a bed of hot inert, orincombustible, particles suspended by an upward flow of combustionair that is injected from the bottom of the combustor to keep the bed

in a floating or fluidized state.

The fluidized bed combustion process provides a means for efficientlymixing fuel with air for combustion. When fuel is introduced to thebed, it is quickly heated above its ignition temperature, ignites, andbecomes part of the burning mass

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Packed Bed Combustion Processes

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Solid Fuel Combustion Processes Overfeed Bed

Thin bed - primary product CO2

Thick bed - ( > 8 particles thick) primary product CO - essentially agasifier

Volatiles almost always burn above bed with secondary air

Stoichiometry of an overfeed bed is determined ONLY by bedthickness. Changing air flow only changes combustion rate, not

stoichiometry!

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Solid Fuel Combustion Processes, Underfeed Bed

Stoichiometry of bed changes sharply with air flow (unlike overfeedbed)

Smaller particles operating diagram shifts to higher combustionrates while roughly preserving stoichiometry

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Co-Firing Concepts

Energy production in coal-fired power plants by partial substitutionof coal, as the main fuel, with biomass feedstock is called co-firing.

Three basic types co-firing in power plants can be identified

direct co-firing, In this option biomass (a secondary fuel) enters theboiler together with coal (primary fuel).

indirect co-firing, In this option biomass is gasified (or combusted)separately and the produced gas is injected and burned in the coalboiler

parallel co-firing, In this option biomass is combusted in a separate(from coal) boiler, supplying steam to a common header.

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Co-firing

A coal fired boiler is going to be revamp to be a coring boiler. To begin 5% of the

coal will be replaced by wood calculate the Percentage output from wood.

assumed efficiencies of the two fuels, 33% and 36% for wood and coal

respectively.

FUEL 1: Wood 5% by weight: 15 GJ/t

FUEL 2: Coal 95% by weight: 25 GJ/t

Electrical energy output from wood = 0.05 x 15 x 0.33 = 0.2475 GJ/t fuel

Electrical energy output from coal = 0.95 x 25 x 0.36 = 8.550 GJ/t fuel

Total electrical output = 0.2475 + 8.550 = 8.7975 GJ/t fuel

Percentage output from coal = 8.550 / 8.7975 = 97.2%

Percentage output from wood = 0.2475 / 8.7975 = 2.8%

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Exercise

A coal fired boiler is going to be revamp to be a coring boiler. To begin,5% of the coal will be replaced by sewage sludge. Calculate the

percentage output from sewage sludge. Assumed efficiencies of the two

fuels, 33% and 36% for sewage sludge and coal respectively.

Sewage sludge 5% by weight

Gross heating value 4.0 GJ/t

Moisture content 60% as received

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Pyrolysis

Pyrolysis is a form of incineration that chemicallydecomposes organic materials by heat in the absenceof oxygen. Pyrolysis typically occurs under pressureand at operating temperatures above 430 C

Organic materials are transformed into gases, smallquantities of liquid, and a solid residue containingcarbon and ash.

Several types of pyrolysis units are available,including the rotary kiln, rotary hearth furnace, orfluidized bed furnace. These units are similar toincinerators except that they operate at lowertemperatures and with less air supply.

Kinetic Models relying on three parallel reactions,represented by below

Pyrolysis can be controlled by chemical reactions,heat transfer, and/or mass transfer

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Kinetic Scheme of Biomass Pyrolysis

1.08E+07121000kC

2.00E+08133000kV

1.30E+08140 000kG

Pre exponential factor(1/s)

Activation energy(J /mol)

kinetic Parameter(1/s)

)/exp( RTEAk =

The chemical rate constants are supposed toobey Arrhenius-type laws:

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Dimensionless Number

Internal heat transfer by conduction

Internal mass transfer by diffusion

Internal mass transfer by convection

External heat transfer

a is 1.84 s/mm

n is about 1.5

dP is particle diameter

fdm /DLt2=

/LCt phc2=

fffcm K/PLt2=

L / hCt pht =

n

pv adt =

Devolatilization Time Correlation

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Gasification

Gasification is a term that describes a chemical process by which

carbonaceous (hydrocarbon) materials (coal, petroleum coke, biomass,

etc.) are converted to a synthesis gas (syngas) by means of partial

oxidation with air, oxygen, and/or steam.

Modern gasification technologies generally operate as follows:

A hydrocarbon feedstock is fed into a high-pressure, high-temperature chemical reactor (gasifier) containing steam and alimited amount of oxygen.

Under these reducing conditions, the chemical bonds in thefeedstock are severed by the extreme heat and pressure and a

syngas is formed. This syngas is primarily a mixture of hydrogen andcarbon monoxide.

The syngas is then cleansed using commercially available andproven systems that remove particulates, sulfur, and trace metals(e.g. mercury).

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Syngas

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Gasification Technology

Basic Gasifier Types

The future improvements to gasifiers will be based on a better understanding of the basic processes,

improved measurements of gasifier behavior and

better regulation of fuel properties

Diagram of downdraft gasification lmbert (nozzle and constricted hearth) gasifierDiagram of updraft gasification

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Efficiency of Gasification Exercise

Example: 1 kg of wood produces 1.5 m3 of gas with averagecalorific value of 5.4 MJ/m3. Average calorific value of wood(dry) is 19.8 MJ/kg. Calculate the efficiency of the gasifire.

%100*

.

..

.

fuelfuel

gasgas

cg

mLHV

VLHV=

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Suspension mod

Fluidized beds more than 40 GJ(thermal)/h and smaller particlefeedstock sizes. Above the bed itself the vessel increases indiameter, lowering the gas velocity and causing particles torecirculate. The recirculation results in high heat and mass transfer

between particle and gas stream

Suspended particle gasifiers move a suspension of biomassparticles through a hot furnace, causing pyrolysis, combustion, and

reduction to give producer gas.

Neither fluidized bed nor suspended particle gasifiers have beendeveloped for small-scale engine use.

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The Crossdraft Gasifier

It is the simplest and lightest gasifier.

Air enters at high velocity through a singlenozzle, induces substantial circulation, andflows across the bed of fuel and char. Thisproduces very high temperatures in a verysmall volume and results in production of alow-tar gas, permitting rapid adjustment toengine load changes.

The fuel and ash serve as insulation for thewalls of the gasifier

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The Crossdraft Gasifier

Air-cooled or water-cooled nozzles are often required. The hightemperatures reached require a low-ash fuel to prevent slagging

The crossdraft gasifier is generally considered suitable only for low-tar

fuels.

Some success has been observed with unpyrolyzed biomass, but thenozzle-to-grate spacing is critical (Das 1986).

Unscreened fuels that do not feed into the gasifier freely are prone tobridging and channeling, and the collapse of bridges fills the

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Description of the Downdraft (Imbert) Gasifier

About one-third of the way up fromthe bottom, there is a set of radiallydirected air nozzles

the incoming air burns andpyrolyzes some of the wood, mostof the tars and oils, and some of thecharcoal that fills the gasifier below

the nozzles

The diameter of the pyrolysis zoneat the air nozzles is typically abouttwice that at the throat

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Description of the Downdraft (Imbert) Gasifier

After the combustion/pyrolysis of wood and hot char at the nozzlelevel, the resulting hot combustion gases (C02 and H20) pass intothis hot char where they are partially reduced to the fuel gases COand H2

Tars that have escaped combustion at the nozzle may crackfurther in the hot char although tar cracking is now thought tooccur only above about 850C

High-grade charcoal is an attractive fuel for gasifiers becauseproducer gas from charcoal, which contains very little tar andcondensate, is the simplest gas to clean.

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Superficial Velocity, Hearth Load, and Gasifier Sizing

Superficial velocity, Vs," of the gas calculated where it passesthrough the narrowest part of the gasification zone

Although the units of Vs are length/ time (e.g., m/s), one shouldthink of the superficial velocity as gas production expressed interms of gas volume/cross-sectional area-time [m3/m2-s), aspecific gas production rate.

the maximum hearth load, Bh, expressed in gas volume/ heartharea, h

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Superficial Velocity, Hearth Load, and Gasifier Sizing

Turndown ratio" the ratio of the highestpractical gas generation rate to the lowestpractical rate

Vehicle operation requires turndown ratios ofat least 8:1, making the need for insulationand proper sizing in high-turndownapplications apparent

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Disadvantages of the lmbert Design

The hearth constriction seriously limits the range of biomass fuelshapes that can be successfully gasified without expensive cubingor pelletizing pretreatment

It requires a high-grade, usually hardwood, fuel, generally at least 2cm along the smallest dimension with no more than 20% moisture

The Imbert design cannot be scaled-up to larger sizes because the

air enters at the sides and is incapableof penetrating a large-diameter fuel bed

unless the fuel size is increased

proportionally

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The Stratified Downdraft Gasifier

"open-top or "topless gsifire with a hearth on the bottom

Air reacts with pyrolyzing biomass in the second zone, andmost of the volatile wood oil is burned to supply heat forthis pyrolysis, "flaming pyrolysis

Adiabatic char gasification

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Tar-Cracking Gasifiers

The cost of the gas cleanup system needed for engine use generallyexceeds the cost of the gasifier

Combustion of Tars

The DeLaCotte tar-recycling gasifierDowndraft center nozzle gas producer

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Thermal Tar Cracking

Temperatures above 800C rapidly crack the primary pyrolysisoils to olefins and aromatic compounds

While high temperatures (above 800C) can destroy tarsrapidly, these same high temperatures also promote reactionwith char, which in turn rapidly quenches the gas to 800C

Therefore, the time available for tar cracking

in a bed of hot charcoal is very short

Transparent gasifier and tar reformer

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Producer Gas for Transportation

Transportation applications system must be small,lightweight, and compact; operate at widely varying loadconditions; have fast response times; be low in tar; be lowin cost; be safe; and be convenient to use and service

Updraft and fluidized-bed gasifiers have the slowestresponse times of the gasifier types and cannot beexpected to follow changing loads with favorable results

The gas from both updraft and fluidized-bed gasifiers alsocontains large quantities oftars, making these gasifiertypes unsuitable for engine

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Sizing the Gas Producer to the engine

The average vehicle engine power required from a gasifier may befigured from the gasoline mileage at cruising speeds:

Power (hp) = Cruising speed (km/h) x Conversion (hp-h/liter)

/Gasoline mileage at cruising speed (km/liter)

Gasifiers typically require about 3 kg of wood or 1.3 kg of charcoal toreplace one liter of gasoline. Thus, it is possible to calculate wood

consumption rates

d f i

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Producer Gas for Transportation

The fastest response time is obtained from crossdraft gasproducers, but they are suitable only for low-tar fuels suchas charcoal

Downdraft gas producers provide a low-tar gas productfrom biomass and also have a rapid response time, so theyare suited for powering engines with either varying orfixed loads.

A common problem among gasifiers is the use of anoversized gasifier. An oversized gasifier producesexcessive tars because lower flow rates do not develop thehigh temperatures necessary for good tar destruction.

An undersized gasifier has excessive pressure drop, weakgas, and excessive raw gas temperature, and may beprone to burning out the grate.

G ifi ti

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Gasification

On an average 1 kg of biomass produces about 2.5 m3 of producer gas.

In this process it consumes about 1.5 m3 of air for combustion.

For complete combustion of wood about 4.5 m3 of air is required.

Thus biomass gasification consumes about 33% of theoretical stoichiometericratio for wood burning.

The average energy conversion efficiency of wood gasifiers is about 60-70% ais defined as:

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Devolatilisation

Removal of bound moisture and some volatiles

Breakdown of hemi-cellulose: emission of CO CO2 Exothermic reaction causing the wood temperature to rise from 250 to 350C;

emission of methane and ethane

External energy is required to continue the process

The rate of mass loss during devolatilisation

)/exp( RTEAmdt

dm vv =

mv= mass of volitiles remaining (kg)

A = Arrhenius constantE= Activation energy (kj/kmol)

R= universal gas constant (kj/kmol-K)

T= Temperature (K)

E i

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Exercise

18. The ultimate and proximate analysis of the biomass in question isgiven below. Assume that gas consist of CH4, H2, and CO and thatall O is consumed during devolatilization process.

85%100%90%0.2%0.9%32.5%11.4%55%1.4%Biomass

Release ofO duringDevolatilization %

Release ofH duringDevolatilization %

Release ofC duringDevolatilization %S%N%O%H%C%Ash%Name

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Exercise

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Exercise

A stock of biomass (waste wood) is found to have a sulfur contentof 0.1 wt% (dry basis) and a heating value on dry basis of 2.4

MJ/kg. This fuel will be used to replace a subbitumus coal which has

sulfur content of 1.0 % (dry basis) but a heating value (dry basis) of

38 MJ/kg. By how much will the emission of sulfur dioxide in kg perMJ, be lowered when the coal is replaced by the biomass.

Exercise

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Exercise

A small wood particle has a temperature of 800 K. Find the timerequired to devolatilise 90% of the volatile mass, assuming that itfollows a first order reaction and with Arrhenius constant(A)=7.E+7 and activation energy (E)=125kJ/mole.

Using the equation for a first order reaction

Integrating

=v2

v1

2

1

m

m

t

tv

v dtRT)exp(-m

dm

/EA

)/( RTexpmdt

dmv

v EA =

12v1

v2

tt

/RT)exp(-

m

mln

=

EA

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Since, mv2 = 0.1 mv total & mv1 = mv total

Substituting

Thus:time (t2) required is 4.78 sec

0t00)00/8.314/8exp(-125,0

m

m0.1ln

2

totalv

totalv

=

7107x

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