coal gas

Syngas (from synthesis gas) is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal,^[1] biomass, and in some types of waste-to-energy gasification facilities. The name comes from their use as intermediates in creating synthetic natural gas (SNG)^[2] and for producing ammonia or methanol. Syngas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant via Fischer-Tropsch synthesis and previously the Mobil methanol to gasoline process.

Syngas consists primarily of hydrogen, carbon monoxide, and very often some carbon dioxide, and has less than half the energy density of natural gas. Syngas is combustible and often used as a fuel source or as an intermediate for the production of other chemicals.

Production

Syngas for use as a fuel is most often produced from coal, first by pyrolysis to coke (impure carbon), aka destructive distillation, followed by alternating blasts of steam and air, or from biomass or municipal waste mainly by the following paths:

C + H₂O → CO + H₂

C + O₂ → CO₂

CO₂ + C → 2CO

The first reaction, between incandescent coke and steam, is strongly endothermic, producing carbon monoxide,CO, and hydrogen ,H₂(water gas in older terminology).

When the coke bed has cooled to a temperature at which the endothermic reaction can no longer proceed, the steam is then replaced by a blast of air.

The second and third reactions then take place, producing an exothermic reaction - forming initially carbon dioxide - raising the

temperature of the coke bed - followed by the second endothermic reaction, in which the latter is converted to carbon monoxide, CO. The

overall reaction is exothermic, forming "producer gas" (older terminology). Steam can then be re-injected, then air etc, to give an endless

series of cycles until the coke is finally consumed. Producer gas has a much lower energy value, relative to water gas, due primarily to

dilution with atmospheric nitrogen. Pure oxygen can be substituted for air to avoid the dilution effect, producing gas of much higher

calorific value.

When used as an intermediate in the large-scale, industrial synthesis of hydrogen (principally used in the production of ammonia), it is also produced from natural gas (via the steam reforming reaction) as follows:

CH₄ + H₂O → CO + 3 H₂

In order to produce more hydrogen from this mixture, more steam is added and the water gas shift reaction is carried out:

CO + H₂O → CO₂ + H₂

The hydrogen must be separated from the CO₂ to be able to use it. This is primarily done by pressure swing adsorption (PSA), amine scrubbing and membrane reactors.

The syngas produced in large waste-to-energy gasification facilities can be used to generate electricity.

Coal gasification processes were used for many years to manufacture illuminating gas (coal gas) for gas lighting and to some extent, heating, before electric lighting and the natural gas infrastructure became widely available.

Post treatment

Syngas can be used in the Fischer-Tropsch process to produce diesel, or converted into methane and dimethyl ether in catalytic processes.

If the syngas is post treated by cryogenic processing, it should be taken into account that this technology has great difficulty in recovering pure carbon monoxide if relatively large volumes of nitrogen are present due to carbon monoxide and nitrogen having very similar boiling points which are -191.5 °C and -195.79 °C respectively. Certain process technology selectively removes carbon monoxide by complexation/decomplexation of carbon monoxide with cuprous aluminum chloride (CuAlCl₄), dissolved in an organic liquid such as toluene. The purified carbon monoxide can have a purity greater than 99%, which makes it a good feedstock for the chemical industry. The reject gas from the system can contain carbon dioxide, nitrogen, methane, ethane and hydrogen. The reject gas can be further processed on a pressure swing adsorption system to remove hydrogen and the hydrogen and carbon monoxide can be recombined in the proper ratio for catalytic methanol production, Fischer-Tropsch diesel etc. Cryogenic purification, being very energy intensive, is not well suited to simply making fuel, because of the greatly reduced net energy gain.^{[citation needed]}

See also

Energy portal

• Biochar
• Boudouard reaction
• Integrated gasification combined cycle
• Gasification
• Partial oxidation
• Producer gas
• Reformer sponge iron cycle
• Syngas fermentation
• Town gas
• Underground coal gasification
• Water gas
• Wood gas

References

External links

• Fischer Tropsch archive