Page 232 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 209

TABLE 7.3 Comparison of the Effect of Pore Diffusion on Char
Gasification and Combustion Rates
Particle Combustion Rate Gasification Rate Combustion Rate/
21
21
Size (μm) (min ) (min ) Gasification Rate (2)
6350 0.648 0.042 15.4
841 5.04 0.317 15.9
74 55.9 0.975 57.3
Source: Adapted from Reed (2002), pp. II 189.

Another important difference between char gasification and combustion
reactions in a fluidized bed is that during gasification the temperature of the
char particle is nearly the same as the bed temperature because of simulta-
neous exothermic and endothermic reactions on it (Gomez-Barea et al.,
2008). In combustion, the char particle temperature can be much hotter than
the bed temperature (Basu, 1977).
The availability of relative amounts of fuel, oxidant (air or oxygen), and
steam (if used) govern the fraction of carbon or oxygen that enters R5 or R4
(Table 7.2). The presence of any more oxidant than that needed for the endo-
thermic reaction will increase the gasifier temperature unnecessarily as well
as reduce the quality of the product by diluting it with carbon dioxide.
Example 7.1 illustrates how the heat balance works out in a gasifier.

Example 7.1
In an updraft gasifier, the water gas gasification reaction (C 1 H 2 O-
CO 1 H 2 1 131 kJ/mol) is to be carried out. Assume that drying and other losses
in the system need 50% additional heat. Find a means to adjust the extent of the
combustion reaction by controlling the supply of oxygen and carbon such that
this need is met.
Solution
The reaction needs 131 kJ of heat for gasification of each mole of carbon. In
oxygen-deficient or substoichiometric conditions like that present in a gasifier,
the exothermic combustion reaction (C 1 /2O 2 -CO 2 111 kJ/mol) is more likely
1
to take place than the more complete combustion reaction (C 1 O 2 -
CO 2 2 394 kJ/mol). If we adjust the feedstock such that for every mole of carbon
gasified, only p moles of carbon will be partially oxidized using p/2 mol of oxy-
gen, the heat released by the combustion reaction will exactly balance the heat
needed by the gasification reaction. In that case the reaction is
C 1 H 2 O-CO 1 H 2 1 131 kJ=mol (i)