Steady State Model of Gas-char Reactions in a Downdraft Biomass Gasifier

7/28/2019 Steady State Model of Gas-char Reactions in a Downdraft Biomass Gasifier

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Solar Energy 74 (2003) 8591

A steady state model of gas-char reactions in a downdraftbiomass gasifier

a b a ,*D.L. Giltrap , R. McKibbin , G.R.G. BarnesaInstitute of Fundamental Sciences, Massey University, Palmerston North, New Zealand

bInstitute of Information and Mathematical Sciences, Massey University, Auckland, New Zealand

Accepted 10 February 2003

Abstract

A phenomenological model of downdraft gasification under steady state operation is developed based on previously

published values for the reaction kinetics in the reduction zone. The model predicts a product gas with a composition similar

to that found experimentally, although the model over-predicts the methane concentration. The accuracy of the model is

limited by the availability of data on the initial conditions at the top of the reduction zone.

2003 Elsevier Science Ltd. All rights reserved.

Keywords: Downdraft gasification model

1. Introduction the requirement that input air be distributed evenly

throughout the entire cross-section of the bed limit the

Thermochemical gasification is a process for converting maximum diameter of downdraft gasifiers.

solid fuels into gaseous form. The chemical energy of the Beneath the flaming pyrolysis zone is the reduction zone

solid fuel is converted into both the thermal and chemical where CO and H O are reduced by the following2 2

energy of the gas. The chemical energy contained within endothermic reactions on the char surface:

the gas is a function of its chemical composition. Thus theReaction 1: C 1 CO 2CO

2chemical composition of the product gas determines its

quality as a fuel. High concentrations of combustible gases

such as H , CO and CH increase the combustion energy Reaction 2: C 1 H OCO 1 H2 4 2 2

of the product gas.Downdraft gasification is a comparatively cheap method

of gasification that can produce a product gas with very

low tar content. Fig. 1 shows the major features of a

downdraft gasifier.In the region nearest the air inlet, flaming pyrolysis

processes occur. Highly exothermic combustion reactions

provide the energy to pyrolyse (devolatilize) the solid fuel,

and these two processes can occur nearly simultaneously.

The temperature in the flaming pyrolysis region is

sufficiently high to thermally crack the pyrolysis products

into components of low molecular weight. In practice,

factors such as the flow properties of the solid biomass and

*Corresponding author. Tel.: 164-6-356-9099; fax: 164-6-

350-5682.

E-mail address: [email protected] (G.R.G. Barnes). Fig. 1. Schematic diagram of a downdraft gasifier.

0038-092X/03/$ see front matter 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0038-092X(03)00091-4
mailto:[email protected]:[email protected]


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86 D.L. Giltrap et al. / Solar Energy 74 (2003) 8591

Nomenclature

2

A Cross-sectional area of the gasifier bed in m

A Frequency factor for reaction ii

C Char Reactivity FactorRF 21 21c Molar heat capacity of the species x in J?mol Kx

D Average particle diameter in the bedp

21E Activation energy for reaction i in J?mol

i

f Pyrolysis fractionP

K Equilibrium constant for reaction ii

N Number of moles of species xx23

n Molar concentration of all gaseous species in mol?m23

n Molar concentration of species x in mol?mx

P Total pressure in Pa

P Partial pressure of species x in Pax

21 21R Gas constant in J?mol ?K

23 21

R Net rate of creation of species x by chemical reactions in mol?m ?sx

23 21r Rate of reaction i in mol?m ?siT Temperature in K

21

v Superficial gas velocity in m.s

Void fraction of the bed21 21

m Fluid viscosity in kg?m ?s23

r Mass density of the fluid in kg ?m

The chemical composition of a gasifier product gas is region where we assumed that all the oxygen from the air

often not equal to the equilibrium composition. Gumz inlet had been combusted to CO and that the pyrolysis2

(1950) used experimental data to calculate the degree of products had been completely cracked. Solid carbon in the

approach to equilibrium of the two reduction reactions form of char was assumed to be present throughout the

above and the methane formation reaction: reduction region.The following reactions were considered:

Reaction 3: C 1 2H CH2 4

Reaction 1: C 1 CO 2CO2The degree of approach to equilibrium of Reaction 3 was

difficult to determine due to the presence of methane fromReaction 2: C 1 H OCO 1 H

2 2pyrolysis of the fuel in experiments.

Wang and Kinoshita (1993) modelled the kinetics of theReaction 3: C 1 2H CH

2 4reactions occurring in the reduction zone assuming a given

gas residence time and temperature. This model is phys-Reaction 4: CH 1 H OCO 1 3Hically similar to a well-mixed fluidised bed; however in a 4 2 2

downdraft gasifier the gas travels down the bed as it reacts,This is the same reaction scheme used in Wang andso there is one-dimensional variation in the gas tempera-

Kinoshita (1993), where it was found that including theture and composition.watergas shift reaction (CO1H OCO 1H ) had littleWe used the reaction kinetics parameters found by Wang 2 2 2effect on the final result.and Kinoshita to build a model of the gas composition and

We assumed that there was always a plentiful supply oftemperature within a downdraft gasifier operating undercarbon from the fuel, so the chemical species that neededsteady-state conditions.to be tracked were CO , CO, CH , H O and H . N from

2 4 2 2 2

the inlet air was also tracked. Although N does not2

participate in any of the reactions considered, it does2. Chemical reactionsdilute the final product gas.

The reaction rates were all considered to have anThe pyrolysis and cracking reactions were not consid-Arrhenius-type temperature dependence and to be propor-ered in this model as the number of possible pyrolysistional to the difference between the actual reactant/productproducts along with all the possible reactions and inter-ratio and the corresponding equilibrium ratio. Thus wemediate products would make the model very complex.

obtained:Instead we started the model at the top of the reduction


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D.L. Giltrap et al. / Solar Energy 74 (2003) 8591 87

Table 122E P

1 CO Net rate of production of the different gaseous species by]] ]S Dr 5A exp ? P 2 (1)S D1 1 C O 2RT K

2 chemical reactions in terms of the rates of reactions 1 to 4

23 21Species R (mol?m ?s )P .P x2E CO H

2 2]] ]]S Dr 5A exp ? P 2 (2)S D2 2 H O

2 N 0RT K 23

CO 2 r2 1P CO 2r 1 r 1 r2E CH 1 2 43 42

]] ]]S Dr 5A exp ? P 2 (3)S D CH r 2 r3 3 H 4 3 42RT K4

H O 2 r 2 r2 2 4

H r 2 2r 1 3r32 2 3 4P P2E CO H

4 2]] ]]S Dr 5A exp ? P ? P 2 (4)S D Total number of gas molecules r 1 r 2 r 1 2r4 4 C H H O 1 2 3 44 2RT K

5

where A is the frequency factor for reaction i, E is thei i

21

activation energy of reaction i in J?mol , R is the gas21 21 d(n v)constant in J?mol ?K , T is the temperature in K, P is xx

]]5R (6)xthe partial pressure of gaseous species x, and K is the dzi

equilibrium constant of reaction i.or

The values for the activation energies for these reactionswere taken from Wang and Kinoshita (1993). However, we dn 1 dvx] ] ]incorporated several factors into our constant pre-multi- 5 SR 2 n D (7)

x xdz v dzpliers, such as char particle size and number of active

carbon sites, that were considered separately by Wang and Similarly, for steady-state operation, any change in theKinoshita. So to calculate our frequency factors we energy flow rate over distance Dz must be equal to the ratemultiplied the values found by Wang and Kinoshita by a of energy released by reactions within the volume minusChar Reactivity Factor (C ), that represents the relative

RF the rate of work done by the gas. This neglects changes inreactivity of different char types. That is, we used A 5

i the kinetic and gravitational potential energy of the gas9 9A ? C where A is the value found by Wang andi RF i which are relatively insignificant. The thermal energy ofN

Kinoshita. moles of gas species x is Nc T where c is the molar heatx x21 21

capacity of the gas in J?K ?mol . For a chemical

reaction the change in enthalpy DH in J /mol represents the3. Mass and energy balances change in the internal chemical energy of the system due

to the reaction. Thus the rate of energy released due toWe assumed a cylindrical gasifier bed of uniform cross- chemical reactions is 2o r ? DH where i ranges over alli i i

sectional area A with negligible radial variation in the the reactions being considered.properties of both the bed and the gas. The axial distance is So from the energy balance we obtained the followingz. The gases were assumed to behave ideally. We then relation:considered thin cross-sections of thickness Dz. For each

v A On c T 2 v A On c T 5 2Orchemical species in the gas, the change in the molar flow S D S Dz1Dz x x z x x ix z1Dz x z irate across Dz must equal the net rate of creation of species

? DH A ? Dz 2 D PAv (8)s dby chemical reactions within the volume. Therefore for ismall Dz we have:

where P is the total pressure (in Pa), the subscript x ranges

n (z)Av

(z)5

n (z1 D

z)Av

(z1 D

z)1

R AD

z (5)x x x over all chemical species and the subscript i ranges overall chemical reactions under consideration.23

where n is the molar density of species x in mol?m , v isx Dividing by A Dz and taking the limit as Dz 0 gives:21

the superficial gas velocity in m?s , and R is the net ratex

of formation of species x by chemical reactions in mol?d vOn c T23 21 S Dx xm ?s . d(Pv)x]]]] ]]5 2Or DH 2 (9)

i iEq. (5) can be used to generate expressions for the dz dzimolar concentrations of each of the six gaseous species

Rearranging we get:considered (CO , CO, H O, H , CH , N ). Similarly the2 2 2 4 2

total molar density of all gases, n, can be expressed as thedT 1 dP dvsum of the molar densities of each of the six component] ]]] ] ]5 2Or DH 2 v 2 P 2OR c TS Di i x xdz dz dzgases. Table 1 gives the net rates of formation of each gas xiv ?On c

x xxspecies in terms of the reaction rates of Reactions 14.

Rearranging Eq. (5) and taking the limit as Dz0 gives: (10)


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Eqs. (7) and (10) give a total of seven differential computer software. We used the ODE45 function in

equations when all six gaseous species are considered. MATLAB.

However, the state of the gasifier at any given point is

described by the concentration of each gas species, the

temperature, the superficial gas velocity and the total4. Initial valuespressure. So to complete our system of differential equa-

tions we still need expressions for dP/dz and dv/dz.The model can predict the state of the gasifier anywhereThe pressure gradient, dP/dz, is known to depend upon

within the reduction region given a complete set ofthe superficial gas velocity. The following empiricalmeasurements of the nine system variables at a singleformula relates the pressure gradient of a fluid flowingpoint. In practice, measurements are usually made at thethrough a bed of solid particles to the physical propertiesexit point of the gasifier. However, as a predictive tool theof the bed and the fluid (Ergun, 1952):model could be used to predict the gasifier output for a

2 given initial state at the top of the gasifier.dP 150m(1 2 ) 1.75r(1 2 )2

] ]]]] ]]]]2 5 v 1 v (11)2 3 3dz D D p p

4.1. Initial gas concentrations

where m is the fluid viscosity, is the void fraction of the

The initial position in this model is the top of thebed, D is the average particle diameter in the bed and r isp

reduction region. That is the point at which all the oxygenthe mass density of the fluid.2

from the air inlet has been consumed in combustionWe found coefficients for v and v empirically byreactions. It is also assumed that all the pyrolysis andmeasuring the pressure drop across a bed of pine chips ofcracking reactions have been completed by this point.fixed length at room temperature over a range of different

The gas at the top of the gasifier comes from twoair speeds. Of course, the bed of an operating downdraftdifferent sources: from the inlet gas (usually air, but puregasifier will vary in composition from fresh fuel at the top

oxygen, steam or CO are sometimes used) and fromto smaller char particles just above the grate.2

pyrolysis of the fuel. The exact proportion of each dependsTo get an expression for dv/dz we took the ideal gasupon the rate of air flowing into the gasifier relative to thelaw:rates of the combustion, pyrolysis and cracking reactions.

P 5 nRT (12) Rather than attempt to calculate this directly we introduced

a pyrolysis fraction variable (f ), which can vary from 0P

where n is the molar concentration of all gaseous species (no pyrolysis products) to 1 (pyrolysis products only).23

in mol?m , P is the total pressure in Pa, R is the gas The air flowing into the gasifier is a mixture of N and2

21

constant in J?mol , and T is the temperature in K. O . We assumed that all the O from the air inlet had been2 2

Differentiating both sides with respect to z gives: consumed by combustion reactions with the char (C 1

O CO ), while the N remains inert.2 2 2

dP dn dT The pyrolysis products were assumed to have been] ] ]5 RT 1 nR (13)dz dz dz cracked into an equivalent amount of CO, CH and H O.

4 2

We took the pyrolysis products of Douglas fir bark, withAn expression for dn/dz can be found by summing the the empirical formula CH O (Graboski and Bain,

3 .0 3 1 .1 7

expressions for dn /dz from Eq. (7) for each gaseousx 1981), as being typical of biomass. By an atomic balance

species x. This gives expressions for dn/dz and dT/dz that we found that each mole of pyrolysis gas thereforedepend upon dv/dz. Substituting Eqs. (7) and (10) into contains 0.341 mol CO, 0.277 mol CH and 0.382 mol

4

(13) and rearranging gives:H O. This assumes a dry feedstock. Additional moisture2content in the feedstock can be accounted for by increasing

Or DH the proportion of H O in the pyrolysis gas. Wet fuel willOn cOR i i 2x x xdv 1 x x i also lower the temperature within the gasifier as additionalS] ]]]] ]]] ]]]5 2dz n TOn c 1 nR energy is required to vaporise the water.x x

x

vOn cx xdP v 4.2. Initial gas temperaturexS D D] ] ]]2 1 2OR c (14)

x xdz T P xIn the combustion region energy is required to dry the

This gives us a set of nine coupled first-order ordinary incoming fuel, drive the pyrolysis/ cracking processes and

differential equations in the system variables n (where x to raise the temperature of the incoming gases. This energyxranges over the six different gas species considered), P, v is supplied by the exothermic combustion reactions. Thus

and T, that can be solved using commercially-available the steady-state temperature will be the temperature at


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which the rate of energy production matches the rate of 4.4. Pyrolysis fraction

energy consumption, and we expect lower temperatures for

fuels with high moisture contents or a large fraction of The pyrolysis fraction f is the effective fraction of theP

volatiles. initial gas that comes from pyrolysis and cracking re-

Senelwa (1997) found the average temperature just actions. The value off will depend upon the proportion ofP

beneath the air inlets when gasifying an oven-dried volatile matter in the fuel, the rate at which air enters the

biomass feedstock to be 955 8C (1228 K). We therefore system and the relative kinetics of the pyrolysis, cracking

used an initial temperature of 1200 K to test our model. and combustion reactions. Rather than attempt to calculate

this we examined the effect of different f values on theP

4.3. Char reactivity factor (C ) model output.RF

The air entering the gasifier is 79% N by volume. No2

The C for different chars were not known. However, N is produced by the reactions considered and it does notRF 2

the number of reactions occurring within a bed of fixed participate (significantly) in any reactions. So the N2

length L is related to L, the C , the superficial gas concentration can be used to infer f . Fig. 3 shows the NRF p 2

velocity v, the reaction rate r and the cross-sectional area concentration within the gasifier for different values off .i pof the bed A. Fig. 2 shows the temperature profiles as the Air-blown downdraft gasifiers typically produce gas con-

21

C value is varied using an f of 0.5, v of 1 m?s , A of taining |50% N (as a percentage of the dry gas volume).RF P 2

22 2

3.14310 m and a starting temperature of 1200 K. Therefore Fig. 3 shows that f values between 0.4 and 0.5P

As the C value increases the temperature within the produce results typical of downdraft gasification ofRFgasifier drops more rapidly as the gas undergoes more biomass.

endothermic reactions in the time it takes to travel the

length of the gasifier. The outlet temperature found in

Senelwa (1997) for a bed length of 0.275 m was 1034 K. 5. Comparison with experimental results

Comparing this with the results in Fig. 2, a C of theRF

order of 1000 produces a similar result. We tested our model against data collected from two

We have assumed that the C remains constant different downdraft gasifiers (Chee, 1987; Senelwa, 1997).RF

throughout the gasifier bed. Experimental results suggest Table 2 shows the parameters used in our model compared

that the reactivity of char depends upon the thermal history with those from the experiments. Fig. 4 shows the com-

and the degree of burnoff experienced by the char position of the dry product gas predicted by the model

(Graboski, 1981). This would result in differences in the along with those found experimentally.

C of the char along the length of the bed. So the C The model produced reasonable agreement with theR F RFused in this analysis should be considered an effective experimental results for all components with the exception

C . of CH . This over-prediction of CH is most likely theR F 4 4

Fig. 2. Temperature profile along gasifier bed as the C value varied.RF


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Fig. 3. N concentration along the gasifier as a percentage of the dry gas volume for different initial pyrolysis fractions.2

result of the assumption that the O in the air reacts only2Table 2

with the char. The pyrolysis products are cracked in aParameters used in experimental downdraft gasification and in theregion of high temperature and in the presence of O , so itmodel simulation 2is probable that some of the CH produced will undergo

4Parameter Chee (1987) Senelwa (1997) Modelcombustion with O (CH 12O CO 12H O).

2 4 2 2 2Data DataFig. 5 shows the results when the initial gas con-

Bed length not stated 0.275 m 0.275 m centration was altered under the assumption that all the O2

v not stated 0.699 m /s 1 m /sin the air reacted with the CH from the cracking of

4

T initial not stated 1228 K 1200 K pyrolysis gas. The CH concentration has dropped under4Moisture content 5 wt % oven dried 0

this new assumption. Similarly the CO level has also2C 1000

RF

dropped as CH combustion produces less CO per mole4 2f 0.5

P

of O consumed.2

Fig. 4. Composition of the dry product gas predicted by model compared with experimental results.


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Fig. 5. Composition of the dry product gas predicted by model compared with experimental results. Initial conditions of model assumed that

the CH produced by cracking of pyrolysis products reacted with O entering the gasifier from the air inlets.4 2

6. Discussion of approximations made. However, the model did sig-

nificantly over-predict the methane concentration in theThe model prediction of the gas composition was in product gas. The model could be improved with more data

reasonable agreement with the experimental results with on the initial gas concentrations at the top of the reductionthe exception that the model significantly over-predicted region, the relationship between the amount of pyrolysisthe CH concentration. The assumption that the CH products produced and temperature, and the variation of4 4produced by pyrolysis is rapidly combusted with the the char reactivity along the length of the gasifier bed.

oxygen at the air inlets reduced the amount of CH4predicted, but the prediction was still higher than the

experimental concentrations found.

The model output was quite sensitive to the initial Referencestemperature and gas composition assumed at the top of the

gasifier. We also assumed a constant C throughout the Chee, C.S., 1987. The air gasification of wood chips in aRFgasifier bed. If the assumed C actually decreased along downdraft gasifier. MSc Thesis, Kansas State University.RFthe bed length the effect would be approximately equiva- Ergun, S., 1952. Fluid flow through packed columns. Chem. Eng.

Prog. 48, 8994.lent to reducing the bed length as the major effect is toGraboski, M., Bain, R., 1981. Properties of biomass relevant todecrease the number of reactions that occur during the gas

gasification. In: Reed, T.B. (Ed.), Biomass Gasification Princi-residence time. However, if the actual C had a higherRF

ples and Technology. Noyes Data Corporation, New Jersey, pp.value over a small region at the top of the of the reduction4171.

region followed by a longer region of reduced reactivity,

Graboski, M., 1981. Kinetics of char gasification reactions. In:then the gas composition produced could be quite differentReed, T.B. (Ed.), Biomass Gasification Principles and Technol-

due to more reactions occurring at higher temperature. ogy. Noyes Data Corporation, New Jersey, pp. 154182.Gumz, W., 1950. In: Gas Producers and Blast Furnaces. John

Wiley and Sons, pp. 7695.7. Conclusion Senelwa, K., 1997. The air gasification of woody biomass from

short rotation forests. Ph.D. Thesis, Massey University, New

Zealand.The steady state model produced a reasonable predictionWang, Y., Kinoshita, C.M., 1993. Kinetic model of biomassof the output gas composition of a downdraft gasifier

gasification. Solar Energy 51, 1925.operating under steady-state conditions given the number

Steady State Model of Gas-char Reactions in a Downdraft Biomass Gasifier

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