Page 253 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 229

Solving for x, we get x 5 0.5. Thus, the mole fraction of CO 2 at
equilibrium 5 (1 2 x)/3 5 0.5/3 5 0.1667.
Part (c): To determine if this reaction is exothermic or endothermic, the
standard heats of formation of the individual components are taken
from the NIST-JANAF thermochemical tables (Chase, 1998).
0 0 0 0
ΔH 5 ðh Þ 1 ðh Þ 2 ½ðh Þ 1 ðh Þ Š
f CO 2 f H 2 f CO f H 2 O
ΔH 52393:52 kJ=mol 2 0kJ=mol 2 ½ 2110:53 kJ=mol 2 241:82 kJ=molŠ
ΔH 5241:17 kJ=mol
Since 41.17 kJ/mol of heat is given out, the reaction is exothermic.
Part (d): This reaction does not depend on pressure, as there is no volume
change. The equilibrium constant changes only with temperature, so the
equilibrium constant at 100 atm is the same as that at 1 atm for 1100 K.
The equilibrium constant is 0.9688 at 100 atm for 1100 K.

7.4.2 Char Reactivity
Reactivity, generally a property of a solid fuel, is the value of the reaction
rate under a well-defined condition of gasifying agent, temperature, and pres-
sure. Proper values or expressions of char reactivity are necessary for all
gasifier models. This topic has been studied extensively for more than
60 years, and a large body of information is available, especially for coal.
These studies unearthed important effects of char size, surface area, pore
size distribution, catalytic effect, and mineral content, pretreatment, and heat-
ing. The origin of the char and the extent of its conversion also exert some
influence on reactivity.
Char can originate from any hydrocarbon—coal, peat, biomass, and so
forth. An important difference between chars from biomass and those from
fossil fuels like coal or peat is that the reactivity of biomass chars increases
with conversion while that of coal or peat char decreases. Figure 7.3 plots
the reactivity for hardwood and peat against their conversion (Liliedahl and
Sjostrom, 1997). One can infer from here that while the conversion rate (at
conversion 0.8) of hardwood char in steam is 9% per minute, that of peat
char under similar conditions is only 1.5% per minute.

7.4.2.1 Effect of Pyrolysis Conditions
The pyrolysis condition under which the char is produced also affects the
reactivity of the char. For example, vanHeek and Muhlen (1990) noted that

the reactivity of char (in air) is much lower when produced above 1000 C

compared to that when produced at 700 C. High temperatures reduce the
number of active sites of reaction and the number of edge atoms. Longer res-
idence times at peak temperature during pyrolysis also reduce reactivity.