Coal gas (also called town gas and illumination gas) is a flammable gaseous fuel made by the destructive distillation of coal and contains a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen. Combined heat and power production (CHP) systems, or simply “cogeneration systems” that use coal gas to generate power in the form of electricity and heat helps prevent emissions to atmosphere and provides a useful form of on-site power.
Studies have determined that 30-40% of all coal mines produce gas that can be effectively used for power generation with gas engines. It is also possible to gasify coal deposits in situ to provide synthetic gases (syngas) for use in power generation.
The main component of the primary coal seam gas is methane in a concentration of 90-95 %. The gas develops during the geochemical conversion of organic substances to coal (carbonization). Coal seam gas can be found in fissures and faults as liberated gas and on the inner surface of the coal and neighboring rock as adsorbed gas.
Coal gas types
There are four types of gases that come from coal:
Coal Seam Methane (CSM), or Coal Bed Methane (CBM)
Coal Mine Methane (CMM), or Working Mine Methane (WMM)
Abandoned Mine Methane (AMM)
Syngas from Underground Coal Gasification (UCG)
Coal Bed Methane (CBM) or Coal Seam Methane (CSM) is primary coal seam gas extracted from coal beds that have not been mined. These coal seams are drilled into the coal beds, releasing the associated gas which is extracted. CBM consists of over 90% methane and can be harvested independently of coal mining in some locations. This gas can be fed directly into the natural gas network or a gas engine, as it normally has stable gas composition.
Coal mine methane (CMM) is a type of gas found in active, working mine sites. This gas is extracted from the air in the coal mine helping improve safety and preventing uncontrolled release of methane to atmosphere. CMM consists of methane & air released during the process of coal mining and must be vented for safety reasons. Capturing it and using it in gas engines is of significant importance for the environment, as Methane has significant effects as a greenhouse gas being 21 times higher than that of carbon dioxide. The methane content of CMM ranges from 25-60% and it typically has an oxygen content of 5-12%. However, its use in gas engines can be complicating as the methane/air proportion can change suddenly.
Abandoned mine methane (AMM) is the coal mine gas that continues to be released even after coal mines are shut down. Coal mine gas from abandoned mines typically contains no oxygen, and its composition changes slowly. The methane content ranges from 60-80%.
Physical coal can be processed industrially to be gasified into a product called Syngas. This synthetic gas is produced through the process of Underground Coal Gasification (UCG) which takes place on site.