Gasification in Detail
Gasifier Physical Chemistry
The chemistry of coal gasification is quite complex. For discussion purposes, it can be viewed as consisting of a few major reactions which can progress to different extents depending on the gasification conditions (like temperature and pressure) and the feedstock used. Combustion reactions take place in a gasification process, but, in comparison with conventional combustion which uses a stoichiometric excess of oxidant, gasification typically uses one-fifth to one-third of the theoretical oxidant. This only partially oxidizes the carbon feedstock. As a ”partial oxidation” process, the major combustible products of gasification are carbon monoxide (CO) and hydrogen, with only a minor portion of the carbon completely oxidized to carbon dioxide (CO2). The heat produced by the partial oxidation provides most of the energy required to drive the endothermic gasification reactions.
Within a gasification process, the major chemical reactions are those involving carbon, CO, CO2, hydrogen, water (steam) and methane, as follows:
The combustion reactions:
||C + ½ O2 → CO
||CO + ½ O2 → CO2
||H2 + ½ O2 → H2O
Other important gasification reactions include:
||C + H2O ↔ CO + H2
||“the Water-Gas Reaction”
||C + CO2 ↔ 2CO
||“the Boudouard Reaction”
||C + 2H2 ↔ CH4
||“the Methanation Reaction”
With the above, the combustion reactions are essentially carried out to completion under normal gasification operating conditions. And, under the condition of high carbon conversion, the three heterogeneous reactions (reactions 4 to 6) can be reduced to two homogeneous gas phase reactions of water-gas-shift and steam methane-reforming (reactions 7 and 8 below), which collectively play a key role in determining the final equilibrium synthesis gas (syngas) composition.
||CO + H2O ↔ CO2 + H2
|| CH4 + H2O ↔ CO2 + 3 H2
Under the sub-stoichiometric reducing conditions of gasification, most of the fuel’s sulfur is converted to hydrogen sulfide (H2S) and, to a lesser degree, carbonyl sulfide (COS). Nitrogen in the feed is converted to nitrogen (N2), with some ammonia (NH3) and a small amount of hydrogen cyanide (HCN). Chlorine in the feed is primarily converted to hydrogen chloride (HCl). In general, the quantities of sulfur, nitrogen, and chloride in the fuel are sufficiently small that they have a negligible effect on the main syngas components of hydrogen (H2) and CO.
Trace elements associated with both organic and inorganic components of the feedstock, such as mercury and other heavy metals, appear in various ash fractions as well as in gaseous emissions, which can be removed from the syngas prior to its final application.
Thermodynamics and Kinetics
The above cited 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 of equations 1 to 3 essentially go to completion (to the right). The thermodynamic equilibrium of the rest of the gasification reactions 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.
- Optimizating 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 coal-to-SNG section), 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 catalytic gasification section).
- 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.
- Gasification [Second Edition] (2008)
Christopher Higman and Maarten van der Burgt,
Gulf Professional Publishing, ISBN: 978-0-7506-8528-3