Page 259 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 235

C, H, and O, or any other dominant elements. If other elements form a minor
part of the product gas, they are often neglected.
Let us take the example of 1 mol of biomass being gasified in d moles of
steam and e moles of air. The reaction of the biomass with air (3.76 moles
of nitrogen, 1 mol of oxygen) and steam may then be represented by:
CH a O b N c 1 dH 2 O 1 eðO 2 1 3:76N 2 Þ-n 1 C 1 n 2 H 2 1 n 3 CO
(7.65)
1 n 4 H 2 O 1 n 5 CO 2 1 n 6 CH 4 1 n 7 N 2
where n 1 , ..., n 7 are stoichiometric coefficients. Here, CH a O b N c is the chem-
ical representation of the biomass and a, b, and c are the mole ratios (H/C,
O/C, and N/C) determined from the ultimate analysis of the biomass. With d
and e as input parameters, the total number of unknowns is seven.
An atomic balance of carbon, hydrogen, oxygen, and nitrogen gives:
C: n 1 1 n 3 1 n 5 1 n 6 5 1 (7.66)
H: 2n 2 1 2n 4 1 4n 6 1 a 1 2d (7.67)

O: n 3 1 n 4 1 2n 5 5 b 1 d 1 2e (7.68)
N: n 7 5 c 1 7:52e (7.69)
During the gasification process, reactions R1, R2, R3, and R9 (see
Table 7.2) take place. The water gas shift reaction, R9, can be considered a
result of the subtraction of the steam gasification and Boudouard reactions,
so we consider the equilibrium of reactions R1, R2, and R3 alone. For a gas-
ifier pressure, P, the equilibrium constants for reactions R 1 ,R 2 , and R 3 are
given by:
y 2 P
5 CO R1 (7.70)
K e 1
y CO 2
P
5 R2 (7.71)
y CO y H 2
K e 2
y H 2 O
5 y CH 4 R3 (7.72)
K e 3 2
y P
H 2
where y i is the mole fraction for species i of CO, H 2 ,H 2 O, and CO 2 .
The two sets of equations (stoichiometric and equilibrium) may be solved
simultaneously to find the coefficients, (n 1 , ..., n 7 ), and hence the product
gas composition in an equilibrium state. Thus, by solving seven equations
(Eqs. (7.66) (7.72)) we can find seven unknowns (n 1 , ... ,n 7 ), which give
both the yield and the product of the gasification for a given air/steam-to-
biomass ratio. The approach is based on the simplified reaction path and the
chemical formula of the biomass.