coal gas

 

gas obtained in the destructive distillation of soft coal, as a byproduct in the preparation of coke. Its composition varies, but in general it is made up largely of hydrogen and methane with small amounts of other hydrocarbons, carbon monoxide (a poisonous gas), carbon dioxide, and nitrogen. It is used as a fuel and illuminant.

See also: Wood gas

Syngas (from synthesis gas) is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen generated by the gasification of a carbon containing fuel to a gaseous product with a heating value. Examples include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal^[1] 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 carbon monoxide, carbon dioxide and hydrogen, 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. Syngas for use as a fuel is most often produced by gasification of coal or municipal waste mainly by the following paths:

C + O₂ → CO₂

CO₂ + C → 2CO

C + H₂O → CO + H₂

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

CH₄ + H₂O → CO + 3 H₂

The syngas produced in large waste-to-energy gasification facilities is used as fuel to generate electricity.^[3]

Coal gasification processes are reasonably efficient and were used for many years to manufacture illuminating gas (coal gas) for gas lighting, before electric lighting became widely available.

When syngas contains a significant amount of nitrogen, the nitrogen must be removed. Cryogenic processing has great difficulty in recovering pure carbon monoxide when 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 absorption system to remove hydrogen and the hydrogen and carbon dioxide can be recombined in the proper ratio for methanol production, fischer-tropsch diesel etc. Howvever, the total energy efficiency is not very high, if the gas is used to make fuel, meaning that the purification processes are very energy intensive.

See also

• Integrated Gasification Combined Cycle (IGCC)
• Gasification
• Town gas
• Wood gas
• Boudouard reaction
• Partial oxidation

External links

• Fischer Tropsch archive
• COPure^SM process for carbon monoxide purification

References

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