Town gas

Figure 1. The To Kwa Wan plant in Hong Kong that produces town gas from oil.

Town gas, or coal gas, refers to a gaseous fuel mixture produced by heating bituminous coal in the absence of air. More recently, some power plants have created town gas using heavy oil and naphtha as feedstocks instead of coal. Town gas was introduced as a fuel for lighting and cooking in the early 19th century. Town gas became popular for cooking because gas cookers could be turned on and off quickly, unlike coal stoves. Town gas is generally considered a relatively safe fuel because it rises and will dissipate into the air if a leak occurs. Additionally, odorants are added to the town gas so that leaks can be easily detected. However, most distribution networks have been converted to supply natural gas rather than town gas, so its use is no longer widespread.

Town Gas from Coal

When coal is heated in the absence of air, volatile compounds are released when undergoing combustion where there is no oxygen present, producing a gas suitable for lighting and leaving behind a solid residue known as coke. The coke was then commonly used to fuel the furnaces or other industrial processes that burn coal. The high temperatures also break down much of the coal's tarry, bituminous material. The resulting impure gas, composed primarily of carbon monoxide and hydrogen, is then collected through a series of pipes or tubes. The gas also contains sulfur compounds, particularly toxic hydrogen sulfide, as well as ammonia and other impurities. In order for this gas to be usable, it must be cleaned. The gas is first bubbled through water to remove ammonia and residual oils or tars vapours. Hydrogen sulfide is removed by passing the gas over iron oxide. The gas may then undergo a final scrubbing process to remove aromatic compounds, such as benzene.

Town Gas from Oil

In the 1960s, a new process was developed that allowed town gas to be produced from naphtha, a light distillates obtained during the fractional distillation of crude oil. Naphtha generally consists of hydrocarbons containing between 5 and 12 carbon atoms. Using naphtha creates a cleaner-burning gas than coal and results in a more efficient fuel. During steam reforming, naphtha is treated with high-pressure steam at high temperatures, breaking the carbon-carbon bonds within the hydrocarbon molecules. This process, known as steam reforming, produces a gas mixture containing carbon monoxide, hydrogen, and a small amount of methane. After the gas is formed, part of the mixture is converted into methane and carbon dioxide through methanation, a process that generates methane from carbon monoxide and hydrogen. The final result is a sulfur-free alternative to coal-based town gas that contains hydrogen, methane, carbon dioxide, and relatively small amounts of nitrogen.

Environmental Issues

Because town gas is a manufactured gaseous fuel with properties similar to natural gas, it poses many of the same environmental concerns. Although cleaner burning than coal or oil, there are still emissions associated with the production and use of town gas. Natural gas emits approximately 53 kilograms (117 pounds) of carbon dioxide per million BTU (293.07 kWh of energy produced, which is significantly less than many other fossil fuels. Because town gas has a similar composition, its carbon dioxide emissions are generally comparable. For comparison, anthracite coal emits 103.7 kilograms (228.6 pounds), gasoline emits 71.3 kilograms (157.2 pounds) of carbon dioxide per million BTU. Additionally, small amounts of carbon monoxide, sulfur compounds, and other impurities may be released when town gas is used, contributing to air pollution and acid rain.

For Further Reading

• Bituminous coal
• Naphtha
• Combustion
• Steam reforming
• Anthracite
• or explore a random page