Page 231 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

208 Biomass Gasification, Pyrolysis and Torrefaction

(Probstein and Hicks, 2006, p. 124). At higher temperatures (350 600 C)
Fe-based catalysts may be employed. Pressure exerts no appreciable effect
on the H 2 /CO ratio. Commercial shift conversions of CO uses the following
catalysts (Boerrigter and Rauch, 2005):

Copper-promoted catalyst, at about 300 510 C

Copper zinc aluminum oxide catalyst, at about 180 270 C.

7.3.3.5 Hydrogasification Reaction
This reaction involves the gasification of char in a hydrogen environment,
which leads to the production of methane.
C 1 2H 2 -CH 4 ðreaction R3 in Table 5:2Þ (7.17)
The rate of this reaction is much slower than that of the other reactions,
and so it is not discussed here. It is of importance only when the production
of synthetic natural gas is desired.

7.3.4 Char Combustion Reactions

Most gasification reactions are endothermic. To provide the required heat of
reaction as well as that required for heating, drying, and pyrolysis, a certain
amount of exothermic combustion reaction is allowed in a gasifier. Reaction
R5 (C 1 O 2 -CO 2 ) is the best reaction in this regard as it gives the highest
amount of heat (394 kJ) per mole of carbon consumed. The next best is R4
1
(C 1 /2O 2 -CO), which also produces the fuel gas CO, but produces only
111 kJ/mol of heat. Additionally, the speed of R4 is also relatively slow.
When carbon comes in contact with oxygen, both R4 and R5 can take
place, but their extent depends on temperature. A partition coefficient, β
may be defined to determine how oxygen will partition itself between the
two. R4 and R5 may be combined and written as:
(7.18)
βC 1 O 2 -2ðβ 2 1ÞCO 1 ð2 2 βÞCO 2
The value of the partition coefficient β lies between 1 and 2 and depends
on temperature. One of the commonly used expressions (Arthur, 1951) for
β is
½COŠ 6234
β 5 5 2400e 2ð T Þ (7.19)
½CO 2 Š
where T is the surface temperature of the char.
Combustion reactions are generally faster than gasification reactions
under similar conditions. Table 7.3 compares the rate of combustion and gas-

ification for a biomass char at a typical gasifier temperature of 900 C. The
combustion rates are at least one order of magnitude faster than the gasifica-
tion reaction rate. Owing to pore diffusion resistance, finer char particles’
combustion has a much higher reaction rate.