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5.1.4. Thermodynamics and Kinetics

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: 

  • Calculating of the relative amounts of oxygen and/or steam required per unit of coal feed.
  • Estimating the composition of the produced syngas.
  • Optimizing the process efficiency at various operating conditions.

Other deductions concerning gasification process design and operations can also be derived from the thermodynamic understanding of its reactions. Examples include:

  • To produce a syngas with a low methane content, a high temperature and substantial amount of steam in excess of the stoichiometric requirement are required.
  • Gasification at very high temperature, on the other hand, will increase oxygen consumption and decrease the overall process efficiency.
  • To produce a syngas with a high methane content (see discussion of synthetic natural gas production), gasification needs to be operated at low temperature (~700°C), but the methanation reaction kinetics will be poor without the presence of a catalyst (see discussion of catalytic gasification).
  • There is considerable advantage to carry out gasification under pressure. At a typical entrained flow gasifier operation temperature of ~2,700°F (1,500°C), the syngas composition shows very little change as a function of operating pressure (Higman, 2008), but significant savings in compression energy and cost reduction from using smaller equipment can be realized.

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
  • Gasification [Second Edition] (2008)
    Christopher Higman and Maarten van der Burgt, Gulf Professional Publishing, ISBN: 978-0-7506-8528-3



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