Gasification reactions are reversible. The direction of the reaction and its conversion are subjected to the constraints of thermodynamic equilibrium and reaction kinetics. The combustion reactions
C + ½ O2 → CO | (-111 MJ/kmol) |
CO + ½ O2 → CO2 | (-283 MJ/kmol) |
H2 + ½ O2 → H2O | (-242 MJ/kmol) |
essentially go to completion (to the right). The thermodynamic equilibrium of the gasification reactions
CO + H2O ↔ CO2 + H2 | "Water-Gas-Shift Reaction" (-41 MJ/kmol) | |
CH4 + H2O ↔ CO2 + 3 H2 | "Steam-Methane-Reforming Reaction" (+206 MJ/kmol) |
are relatively well defined and collectively impose a strong influence on the thermal efficiency and the produced syngas composition of a gasification process. Thermodynamic modeling has been a useful tool for estimating key design parameters for a gasification process, for example:
Other deductions concerning gasification process design and operations can also be derived from the thermodynamic understanding of its reactions. Examples include:
Relative to the thermodynamic understanding of the gasification process, its kinetic behavior is more complex. Very little reliable kinetic information on coal gasification reactions exists, partly because it is highly depended on the process conditions and the nature of the coal feed, which can vary significantly with respect to composition, mineral impurities, and reactivity. Certain impurities, in fact, are known to have catalytic activity on some of the gasification reactions.
References/Further Reading
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Gasifier