Coal Gas: Processes, Uses, and Significance

Coal gas is a gaseous mixture, mainly composed of methane, hydrogen, and carbon monoxide that are formed by the destructive distillation (it means heating in the absence of air) of bituminous coal and is also used as a fuel. At times, coal steam is added to react with the hot coke, therefore increasing the yield of gas. Coke and Coal tar are obtained as the by-products of coal.

Coal Bed Methane - CBM is a natural gas form that is recovered from coal seams or coal deposits.

Development or Production of Coal Oil and Gas

Prior to the advent of natural gas supply and transmission, nearly all gas for lighting and fuel was produced from coal itself in the United States during the 1940s and 1950s, and in Australia and the United Kingdom during the late 1960s and 1970s. Town gas was supplied to the households via municipally-owned pipeline distribution systems.

Originally, coal gas is created as a by-product of the cooking process, and its use developed during the 19th and early 20th centuries, tracking the urbanization and industrial revolution. Ammonia and coal tars were important chemical feedstock for the chemical and dye industries, with a wide variety of artificial dyes produced from coal tar and coal gas as by-products of the manufacturing process. Facilities where the gas was produced were often called gasworks or Manufactured Gas Plants (MGP).

The discovery of the large reserves of natural gas in 1965 in the North Sea off the English coast has led to the replacement or expensive conversion of most of the gas cookers and gas heaters of the UK, except in Northern Ireland, from the late 1960s onwards.

And, the production process is distinct, both chemically and physically, from that used to create a gaseous fuels range known variously as syngas, manufactured gas, hygas, producer gas, and Dowson gas. Partially combusting a variety of feedstocks in a combination of oxygen, air, or steam to reduce the latter to carbon dioxide and hydrogen, with a few destructive distillations thrown in for good measure.

Manufacturing Process

Manufactured gas is made by two processes: gasification or carbonization. Gasification is defined as the process of subjecting a feedstock to chemical reactions, which produce gas. On the other hand, carbonization refers to the de-volatilization of an organic feedstock to yield the char and gas.

The first process that was used is partial pyrolysis and carbonization of coal. The off-gases liberated in the high-temperature carbonization (it means coking) of coal in coke ovens were scrubbed, collected, and also used as fuel. Based on the goal of the plant, the desired product was either a high-quality coke, the production of a high-quality gas, with the coke being the side product or for the metallurgical use. Coke plants are usually associated with metallurgical facilities such as blast furnaces or smelters, whereas gas works are typically associated with urban areas.

A facility that is used to manufacture coal gas is the carburetted water gas (CWG), and today, oil gas is generally referred to as the Manufactured Gas Plant (MGP).

Industrial Use of Coal Gas

Fuel gas for industrial use was made using the technology of producer gas. Producer gas can be made by blowing air through an incandescent fuel bed (commonly coal or coke) in the producer of gas. The fuel reaction with insufficient air for the total combustion produces carbon monoxide (CO); this reaction is self-sustaining and exothermic.

It was also discovered that adding steam to a gas producer's input air increased the calorific value of the fuel gas by enriching it with hydrogen (H2) and carbon monoxide (CO) formed by water gas reactions. Producer gas contains a very low calorific value of 3.7 to 5.6 MJ/m³ (it means 99 to 150 Btu/cu ft); because the calorific gases, either H2 or CO, are diluted with much carbon dioxide (CO2) (from combustion) and inert nitrogen (from the air).

The nitrogen dilution problem was overcome by the blue water gas (BWG) process, which was developed by Sir William Siemens in the 1850s. Alternately, the incandescent fuel bed would be blasted with the air, followed by steam. The air reactions during the blow cycle are exothermic, which heats up the bed, while the steam reactions during the make cycle are endothermic and cool down the bed.

The products which are from the air cycle have non-calorific nitrogen and are exhausted out the stack, while the steam cycle products are kept as the blue water gas. This particular gas is composed almost entirely of H2 and CO and also burns with a pale blue flame, the same as coal petroleum and natural gas. BWG holds a calorific value of 11 MJ/m³ (300 BTU/cu ft).

Domestic Heating

The post-war house building program put the gas at a disadvantage. Whereas the electricity had long developed a national distribution grid that has enabled the supplies to reach even the new small housing developments, still, the gas was distributed locally only. Several new housing estates were beyond the reach of the stringent Treasury rules, and gas main about Return on Investment (RoI) made the extension of mains uneconomic. With night storage heaters and underfloor heating that use cheap off-peak energy supplies, electricity has made inroads into the home heating industry.