Equilibrium Model For Biomass Gasification

EQUILIBRIUM MODEL FOR BIOMASS GASIFICATION Anil Khadse1, Prasad Parulekar2, Preeti Aghalayam1 and Anuradda Ganesh2* 1,

Department of Chemical Engineering, IIT Bombay, Powai, Mumbai-400076 2, Energy Systems Engineering, IIT Bombay, Powai, Mumbai-400076 * Corresponding author, Energy Systems Engineering, IIT Bombay, Powai, Mumbai-400076 Phone: 022-25767886, Fax: 022-25726875, e-mail:[email protected]

Abstract The utilization of biomass will provide sufficient energy which can be used for electricity generation, engine applications, etc. Gasification is the process in which biomass is converted into clean and combustible gas in the presence of steam and air. To model gasification process in detail, knowledge of chemical reaction kinetics is required which is not available in open literature. Thermodynamic equilibrium composition prediction is the important step in modeling the gasification process. Here the biomass is represented as CHxOy. The biomass is reacted with steam and air to give product gases viz. CO, CO2, H2 and CH4. The model assumes that the principle reactions are at thermodynamic equilibrium. The model equations containing four atom balances (C, O, H and N) and three equilibrium relations are solved for gas compositions using MATLAB. The four types of biomasses from India are used for prediction of equilibrium gas compositions. These four samples are compared based on gross calorific values. Keywords: Biomass, Thermodynamic equilibrium, gasification.

1. Introduction Biomass is a non conventional energy source. The main biomass sources are wood, saw dust, agricultural residues, aquatic and marine biomass and wastes (Bridgwater, 1994). The utilization of biomass will provide sufficient energy which can be used for electricity generation, engine applications, etc. Gasification is the process in which biomass is converted into clean and combustible gas in the presence of steam and air. There are different gasifiers like updraft, downdraft, cross draft and fluidized bed gasifiers (Khummongkol and Arrunlaksadamrong, 1990). The main aim of this study is to produce producer gas from biomass and use it in engine for power generation. The experimental set up is shown in Figure 1. As shown in the Figure 1, the biomass is fed from top and is gasified to produce producer gas.

BIOMASS IN

TAR CRACKING UNIT

GAS COOLER + GAS 800 O C STEAM GENERATOR

UPDRAFT GASIFIER STEAM + AIR ASH

S. I. ENGINE AIR BLOWER

Figure 1: Experimental set up

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The tar in the outlet gases is cracked thermally in the tar cracking unit. The gases coming out of the tar cracking unit is at 800 oC and hence need to be cooled. A heat exchanger is provided for the same and then this cooled gas is used for engine applications. The steam generated out of the heat exchange between hot gas and cooling water is mixed with input air to the gasifier. The addition of steam will enhance the hydrogen percentage in the product gas and its calorific value thereby. We assumed that updraft gasifier is at thermodynamic equilibrium. The review of various thermodynamic equilibrium models and its applications is given by Channiwala (1992). The gasification reactions occur are at equilibrium and thermodynamic equilibrium compositions are predicted for four biomasses. The effect of air input, steam/air ratio is studied for each biomass. They are compared based on their calorific value.

2. Thermodynamic Equilibrium Model 2.1 Assumptions 1. 2. 3. 4.

Biomass is represented by the general formula C Hx Oy The gasification products contain CO2, CO, H2, CH4, N2, H2O and un-burnt carbon The reactions are at thermodynamic equilibrium The reactions proceed adiabatically

Based on the above assumptions, the general reaction of biomass with air and steam is written as C Hx Oy + z (p O2 + (1-p) N2)+ k H2O = a CO2 + b CO + c H2+ d CH4 + e N2 + f H2O+ g C

(1)

Where, x and y are the H/C and O/C mole ratio, respectively. The moisture content of the biomass is neglected and the product quality depends on the x and y. The above reaction represents an overall reaction but a number of competing intermediate reactions take place during the process. These are: 1) Oxidation 2) Steam gasification 3) Bouduard reaction 4) Methanation reaction 5) Water gas-shift reaction

C+ O2=CO2

(2)

C+H2O=CO+H2

(3)

C+ CO2=2CO

(4)

C+2H2=CH4

(5)

CO+ H2O=CO2+ H2

(6)

Out of these only four reactions are independent reactions, which are chosen as oxidation, steam gasification, bouduard reaction, and the methanation reaction. The water gas shift reaction can be considered as the subtraction of the steam gasification and bouduard reactions. Oxidation reaction is typically assumed to be very fast and goes to completion (Von Fredersdorff and Elliott, 1963) and three reactions namely bouduard reaction, steam gasification and methanation are in equilibrium.

2.2. Model Equations Taking atom balances on carbon, oxygen, hydrogen and nitrogen, we obtain, Carbon 1=a+b+d+g Oxygen y + 2pz + k = 2a + b + f Hydrogen x + 2 k = 2c + 4d + 2f Nitrogen z (1-p) = e

(7) (8) (9) (10)

The three equilibrium relations for the three reactions (other than oxidation) are 1) Bouduard reaction K e1 =

y co2 Pt y co2

(11)

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2) Steam gasification reaction K e2 =

yco y H 2 Pt

(12)

y H 2O

3) Methanation reaction K e3 =

yCH 4 y H2 2 Pt

(13)

ΔG 0 RT

(14)

The equilibrium constants are given by ln K e = −

Where, ΔG0 is the Gibb’s free energy (kJ/mol), T is the temperature in K and R is the universal gas constant in consistent units. The energy balance can be considered as follows The heat of the overall gasification reaction (1) is given by (15)

T

n

ΔH (T ) = ΔH (T ) + ∑ n ∫ C dT R

R

R

i =1

i

TR

pi

At adiabatic condition, this heat of reaction is zero. Heat of reaction at the reference temperature is calculated using heats of combustions of the species.

ΔH (T ) = ΔH R

R

− ΔH

ccoal

− ΔH

cCO

cH 2

− ΔH

(16) cCH 4

T

n

0 = ΔH (T ) + ∑ n ∫ C dT

Thus,

R

R

i =1

i

TR

(17)

pi

where ΔHR is the heat of reaction, ΔHci is the heat of combustion of species ‘i’, ni is the moles of species ‘i’, Cpi is the specific heat capacity of species ‘i’. The eight non-linear algebraic equations (7-13 and 17) are solved simultaneously in order to determine a, b, c, d, e, f, g (which determine the product gas composition) and the adiabatic temperature, at various pressures using the solver FSOLVE in MATLAB. The model is simulated without energy balance. This gives us the expected exit gas compositions at each temperature and pressure. Inclusion of energy balance will give the adiabatic temperature along with gas compositions. Gross calorific value is calculated using the following formula GCV=(b ΔHcCO + c ΔHcH2 + c ΔHcCH4) / (a + b + c + d+ e)

(18)

The ultimate analysis of four biomasses used in this study (Iyer et al., 2002) are tabulated in Table 1. Table 1: Ultimate analysis of different biomass samples (Iyer et al., 2002) Sr.No. 1. 2. 3. 4.

Biomass sample Saw dust Bagasse (A.P.) Subabul Rice husk

C%

H%

N%

O%

Ash%

52.28 47 42.76 36.42

5.2 6.5 5.68 4.91

0.47 0.00 1.07 0.59

40.85 42.5 49.29 35.88

1.2 4 1.2 22.2

Calorific value kcal/kg 3795 4200 3980 3000

3. Results and Discussions 3.1. The Effect of Temperature Figure 2 shows the product gas compositions and gross calorific value for Sawdust gasification without steam. The CO and H2 are increases with increase in temperature. CO2, H2O and CH4 are decrease with increase in temperature. Figure 3 shows the product gas compositions and gross calorific value for Sawdust with air and

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steam (steam/air=1). It is clear that steam introduction in gasification process increases CO and H2 and gross calorific value of the product gas. Similar results are obtained for other three biomasses. The trends are similar for all other three biomasses. The gross calorific value (GCV) of four biomasses with temperature at air=0.1 and steam/air ratio=8 is compared in Figure 4. The air is 0.1 and steam/air ratio is 8. These inputs are chosen based on the maximum gross calorific value of the product gas. GCV is increased with increase in the temperature for all samples. The GCV of bagasse is the maximum and subabul is the lowest at lower temperatures. At higher temperatures above 1100 K, the GCV is constant and there is no significant difference in value of four biomasses. 0.45

55

CO2 CO H2 CH4 H2O

0.4 0.35

50 45 40

0.3

35 Gross calorific value (kcal/mol)

Mole fractions

0.25 0.2 0.15 0.1 0.05 0 600

700

800

900 1000 Temperature (K)

1100

30 25 20 15 10 5 600

1200

700

800

900 1000 Temperature (K)

1100

1200

(a) (b) Figure 2: Sawdust without steam (air=0.55) a) product gas composition b) gross calorific value 0.4 0.35

55

CO2 CO H2 CH4 H2O

50 Gross calorific value (kcal/mol)

0.45

0.25 0.2 0.15 0.1

0

45

40

35

30

25

0.05

700

800

900

1000 1100 Temperature (K)

1200

1300

1400

20

700

800

900

1000 1100 Temperature (K)

1200

1300

1400

(a) (b) Figure 3: Sawdust with steam (steam/air=1) a) product gas composition b) gross calorific value 70 65 GCV (kcal/mol)

Mole fractions

0.3

air=0.1; steam/air=8

60 55 50 45

sawdust

bagasse

subabul

rice husk

40 700

900

1100 Temperature (K)

1300

1500

Figure 4: Comparisons of the gross calorific value (GCV) of four biomasses.

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3.2. The Effect of Air Input The effect of air input is studied for each sample using four different air inputs. Figure 5 shows the GCV of sawdust for four air inputs when steam in input is zero. As air input is decreased the GCV increases with increase in temperature. After 1100 K, GCV remains constant. This is observed for all four biomasses.

70

Effect of inlet air

GCV (kcal/mol)

60 50 40

air=0.8

30

air=0.55 air=0.3

20

air=0.1

10 0 700

900

1100

1300

1500

Temperature (K)

Figure 5: Effect of inlet air on GCV for Sawdust

3.3. The Effect of Steam/Air Ratio The effect of steam/air ratio for sawdust is shown in Figure 6. As steam/air ratio increases at fixed air inlet, the GCV increases. This trend is observed for all biomasses. Thus with increase in steam/air ratio favours steam gasification reaction and H2 and CO increase in product gas increasing GCV. At higher temperature (>1100K), GCV remains constant. 70

GCV (kcal/mol)

65

air=0.1

60 55 50 air=0.1; steam=0 steam/air=2 steam/air=4

45 40 35 700

900

1100 Temperature (K)

steam/air=1 steam/air=3 steam/air=8

1300

1500

Figure 6: Effect of inlet steam/air ratio on GCV for Sawdust

3.4. Determination of the Adiabatic Temperature The inclusion of energy balance equation will give adiabatic gasification temperature. The compositions and adiabatic temperature are obtained for four biomass samples with different steam to air ratios. Figure 7 shows adiabatic temperature, GCV and carbon conversions of four biomasses with different steam/ air ratios. Adiabatic temperature decreases with increase in steam/air ratio except for rice husk. GCV decreases with increase in steam/air ratio except for rice husk. This is reverse trend than the trends we get without energy balance. Carbon conversion increased with increase in steam/air ratio. The results are compared with experimental results from literature and simulation. The results are not directly comparable since we didn’t account for heat losses and moisture content of biomass. Our model predicts results

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comparable with the experiments (Shashikantha, 1988) and simulation by Channiwala (1992). The results are compared in Table 2. Table 2: Comparison of results with literature CO2 12.17 12.85 18.72

Experiment (Shashikantha,1988) Model Channiwala(1992) Present model

CO 18.24 22.72 10.46

H2 27.65 27.91 30.90

CH4 2.06 1.58 5.93

The biomass used is Subabul wood x=1.451, y=0.697 and heating value=4780 kcal/kg. Present model predicts more CO2, H2 and CH4 .CO is less predicted by present model. This is due to heating losses and moisture present in biomass is not included in present model. In future model will be modified to take care of there parameters. 24 Sawdust

700

Bagasse

650

Subabul

GCV (kcal/mol)

Adiabatic temperature (K)

750

Rice husk

600 550 500

Sawdust

22

bagasse

20

Subabul Rice husk

18 16 14 12

450

10

400 0

0.5

1 steam/air

1.5

2

0

0.5

1

(a)

Carbon conversion

1.5

2

steam/air

(b) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Sawdust bagasse Subabul Rice husk

0

0.5

1 steam/air

1.5

2

(c) Figure 7: Effect of inlet steam/air ratio on a) adiabatic temperature and b) GCV c) carbon conversion for four biomasses

4. Conclusion The equilibrium calculations are performed for four biomasses from India. The GCV variation with temperature suggests that as temperature increase GCV increase. At temperature greater than 1100K, GCV remains constant for all biomasses. At air 0.1 moles and steam/air=8, all samples gives maximum GCV. The GCV of bagasse is the highest and that of Subabul is the lowest at lower temperature (<1000K). At higher temperature is GCV remains constant. The adiabatic temperature and GCV decrease with increase in steam/air ratio except for rice husk. The GCV is the main criteria for engine applications. Knowing the GCV variation with various parameters the model can be further extended for whole system including engine with inclusion of heat losses and moisture in biomass.

References 1.

Bridgwater A. V., 1994, Thermochemical Processing of Biomass, Butterworths, England.

112 2. 3. 4. 5. 6. 7.

Advances in Energy Research (AER – 2006) Channiwala S. A., 1992, On Biomass Gasification Process and Technology Development -some Analytical and Experimental Investigations, Ph.D. thesis, IIT Bombay, Mumbai. Iyer P.V.R., T. R. Rao and P.D. Grover, 2002, Biomass Thermo-Chemical Characterization, IIT Delhi, India. Khummongkol D., and W. Arrunlaksadamrong, 1990, Performance of an Updraft Mangrove – Wood Gasifier, Energy vol. 15, No. 9, 781-784. Shashikantha, 1988, Performance evaluation of Gasifier-Engine system operating on dual fuel mode, M.Tech. Dissertation, C.E.S.E., IIT Bombay. Von Fredersdorff C. C., and M.A. Elliott, 1963, Coal Gasification, in Chemistry of Coal Utilization, H.H. Lowry, ed., Wiley, New York.