Coal gas
Coal gas is a flammable gaseous fuel made from coal and supplied to the user via a piped distribution system originating from a gasworks. It is produced when coal is heated strongly in the absence of air. Town gas is a more general term referring to manufactured gaseous fuels produced for sale to consumers and municipalities.
The original coal gas was produced by the coal gasification reaction, and the burnable component consisted of a mixture of carbon monoxide and hydrogen in roughly equal quantities by volume. Thus, coal gas is highly toxic. Other compositions contain additional calorific gases such as methane, produced by the Fischer–Tropsch process, and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen.
Prior to the development of natural gas supply and transmission—during the 1940s and 1950s in the United States and during the late 1960s and 1970s in the United Kingdom and Australia—almost all gas for fuel and lighting was manufactured from coal. Town gas was supplied to households via municipally owned piped distribution systems. At the time, a frequent method of committing suicide was the inhalation of gas from an unlit oven. With the head and upper body placed inside the appliance, the concentrated carbon monoxide would kill quickly. Sylvia Plath famously ended her life with this method.
Originally created as a by-product of the coking process, its use developed during the 19th and early 20th centuries tracking the Industrial Revolution and urbanization. By-products from the production process included coal tars and ammonia, which were important raw materials for the dye and chemical industry with a wide range of artificial dyes being made from coal gas and coal tar. Facilities where the gas was produced were often known as a manufactured gas plant or a gasworks.
In the United Kingdom the discovery of large reserves of natural gas, or sea gas as it was known colloquially, in the Southern North Sea off the coasts of Norfolk and Yorkshire in 1965 led to the conversion or replacement of most of Britain's gas cookers and gas heaters from the late 1960s onwards, the process being completed by the late 1970s. Any residual gas lighting found in homes being converted was either capped off at the meter or, more usually, removed altogether. As of 2023, some gas street lighting still remains, mainly in central London and the Royal Parks.
The production process differs from other methods used to generate gaseous fuels known variously as manufactured gas, syngas, Dowson gas, and producer gas. These gases are made by partial combustion of a wide variety of feedstocks in some mixture of air, oxygen, or steam, to reduce the latter to hydrogen and carbon monoxide although some destructive distillation may also occur.
Manufacturing processes
Manufactured gas can be made by two processes: carbonization or gasification. Carbonization refers to the devolatilization of an organic feedstock to yield gas and char. Gasification is the process of subjecting a feedstock to chemical reactions that produce gas.The first process used was the carbonization and partial pyrolysis of coal. The off gases liberated in the high-temperature carbonization of coal in coke ovens were collected, scrubbed and used as fuel. Depending on the goal of the plant, the desired product was either a high quality coke for metallurgical use, with the gas being a side product, or the production of a high quality gas, with coke being the side product. Coke plants are typically associated with metallurgical facilities such as smelters or blast furnaces, while gas works typically served urban areas.
A facility used to manufacture coal gas, carburetted water gas, and oil gas is today generally referred to as a manufactured gas plant.
In the early years of MGP operations, the goal of a utility gas works was to produce the greatest amount of illuminating gas. The illuminating power of a gas was related to amount of soot-forming hydrocarbons dissolved in it. These hydrocarbons gave the gas flame its characteristic bright yellow color. Gas works would typically use oily bituminous coals as feedstock. These coals would give off large amounts of volatile hydrocarbons into the coal gas, but would leave behind a crumbly, low-quality coke not suitable for metallurgical processes.
Coal or coke oven gas typically had a calorific value between ; with values around being typical.
The advent of electric lighting forced utilities to search for other markets for manufactured gas. MGPs that once almost exclusively produced lighting gas shifted their efforts towards supplying gas for heating and cooking, and even refrigeration and cooling.
Gas for industrial use
Fuel gas for industrial use was made using producer gas technology. Producer gas is made by blowing air through an incandescent fuel bed in a gas producer. The reaction of fuel with insufficient air for total combustion produces carbon monoxide ; this reaction is exothermic and self-sustaining. It was discovered that adding steam to the input air of a gas producer would increase the calorific value of the fuel gas by enriching it with CO and hydrogen produced by water gas reactions. Producer gas has a very low calorific value of ; because the calorific gases CO/H2 are diluted with much inert nitrogen and carbon dioxide .The problem of nitrogen dilution was overcome by the blue water gas process, developed in the 1850s by Sir William Siemens. The incandescent fuel bed would be alternately blasted with air followed by steam. The air reactions during the blow cycle are exothermic, heating up the bed, while the steam reactions during the make cycle, are endothermic and cool down the bed. The products from the air cycle contain non-calorific nitrogen and are exhausted out the stack while the products of the steam cycle are kept as blue water gas. This gas is composed almost entirely of CO and H2, and burns with a pale blue flame similar to natural gas. BWG has a calorific value of.
Blue water gas lacked illuminants; it would not burn with a luminous flame in a simple fishtail gas jet as existed prior to the invention of the gas mantle in the 1890s. Various attempts were made to enrich BWG with illuminants from gas oil in the 1860s. Gas oil was the flammable waste product from kerosene refining, made from the lightest and most volatile fractions of crude oil.
In 1875 Thaddeus S. C. Lowe invented the carburetted water gas process. This process revolutionized the manufactured gas industry and was the standard technology until the end of the manufactured gas era. A CWG generating set consisted of three elements; a producer, carburettor and a super heater connected in series with gas pipes and valves.
During a make run, steam would be passed through the generator to make blue water gas. From the generator the hot water gas would pass into the top of the carburettor where light petroleum oils would be injected into the gas stream. The light oils would be thermocracked as they came in contact with the white hot checkerwork fire bricks inside the carburettor. The hot enriched gas would then flow into the superheater, where the gas would be further cracked by more hot fire bricks.
Gas in post-war Britain
New manufacturing processes
Following the Second World War the slow recovery of the British coal mining industry led to shortages of coal and high prices.| Year | Production, million tons | Production cost, £/ton |
| 1947 | 197 | 2.00 |
| 1950 | 216 | 2.40 |
| 1953 | 223 | 3.05 |
| 1956 | 222 | 3.85 |
| 1959 | 206 | 4.15 |
| 1961 | 191 | 4.55 |
| 1965 | 187 | 4.60 |
| 1967 | 172 | 4.95 |
The decline of coal as a feedstock for town gas production using carbonisation is demonstrated in this graph.
Coal-based town gas production, millions of therms
The rise of oil as a feedstock to manufacture town gas is shown on the graph below. The peak usage in 1968/9 and subsequent decline coincides with the availability of North Sea gas which, over the next few years, displaced town gas as a primary fuel and led to the decline of oil as a feedstock for gas making, as shown.
Oil-based town gas production, millions of therms