The chemical reactions of gasification 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 (H2), water (steam) and methane (CH4), as follows:
The combustion reactions:
|1.||C + ½ O2 → CO||(-111 MJ/kmol)|
|2.||CO + ½ O2 → CO2||(-283 MJ/kmol)|
|3.||H2 + ½ O2 → H2O||(-242 MJ/kmol)|
Other important gasification reactions include:
|4.||C + H2O ↔ CO + H2||"the Water-Gas Reaction"
|5.||C + CO2 ↔ 2CO||"the Boudouard Reaction"
|6.||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.
|7.||CO + H2O ↔ CO2 + H2||"Water-Gas-Shift Reaction"
|8.||CH4 + H2O ↔ CO2 + 3 H2||"Steam-Methane-Reforming Reaction"
In the low-oxygen, reducing environment of the gasifier, most of the feedstock’s sulfur coverts to hydrogen sulfide (H2S), with a small amount forming carbonyl sulfide (COS). Nitrogen chemically bound in the feed generally converts to gaseous nitrogen (N2), with some ammonia (NH3), and a small amount forming hydrogen cyanide (HCN). Chlorine is primary 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 H2 and CO. Trace elements associated with both organic and inorganic components in the feed, such as mercury, arsenic and other heavy metals, appear in the various ash and slag fractions, as well as in gaseous emissions, and need to be removed from the syngas prior to further use.