What Is Coal Gas?

Coal gas is a combustible gas mixture produced by the thermal decomposition or partial oxidation of coal, consisting primarily of hydrogen, carbon monoxide, methane, and smaller quantities of carbon dioxide, water vapor, and hydrocarbon vapors. Historically, it was manufactured by heating coal in the absence of air, a process called carbonization, and piped through urban distribution networks for street lighting and domestic heating under the name town gas. In modern engineering, coal gas produced by high-temperature reaction of coal with steam and oxygen is more commonly called synthesis gas or syngas, and it serves as a feedstock for chemical synthesis and as fuel for combined cycle power generation. The term encompasses both the historical product that shaped early industrial cities and the contemporary gasification-derived gas that is part of clean coal and hydrogen production strategies.

Coal gas draws its technical foundations from thermochemistry, combustion engineering, and process engineering. Its composition varies with the coal rank, temperature, pressure, and gasifying agent used, giving engineers control over the ratio of combustible components and the calorific value of the product.

Production by Gasification

Gasification converts coal into gas by reacting it with a controlled mixture of steam and oxygen or air at temperatures typically between 700 and 1500 degrees Celsius. In partial oxidation, oxygen reacts with carbon in the coal to produce carbon monoxide and heat, which drives the endothermic steam-carbon reaction that generates additional hydrogen and carbon monoxide. The gasification chemistry proceeds through several coupled reactions: the Boudouard reaction, the water-gas reaction, and the methanation reaction, whose relative contributions depend on temperature and pressure. Fixed-bed gasifiers, such as the Lurgi dry-ash design first developed in the 1930s, operate at moderate pressures and produce a gas with higher methane content. Entrained-flow gasifiers, operating at high temperatures and pressures, favor carbon monoxide and hydrogen over methane and achieve near-complete carbon conversion. The U.S. Department of Energy's National Energy Technology Laboratory gasification program documents reactor types, syngas compositions, and integrated gasification combined cycle configurations in detail.

Composition and Properties

The composition of coal gas depends on the production route. Historical town gas made by carbonization had a typical composition of roughly 50 percent hydrogen, 30 percent methane, and 10 percent carbon monoxide by volume, with a heating value of around 20 megajoules per cubic meter. Modern entrained-flow syngas from oxygen-blown gasifiers contains primarily carbon monoxide (40 to 60 percent) and hydrogen (25 to 35 percent), with methane below 1 percent and a higher calorific value per unit volume after cleanup. Carbon dioxide and hydrogen sulfide are removed in downstream acid gas treatment steps before the syngas is used. When the hydrogen-to-carbon monoxide ratio is adjusted by the water-gas shift reaction, syngas becomes a precursor for methanol synthesis, Fischer-Tropsch liquid fuels, and ammonia production. The IEA's coal energy system analysis places coal gasification within the broader context of coal's role in global energy supply and low-carbon transition scenarios.

Historical Development

Coal gas was produced commercially for the first time in the early nineteenth century, with the London and Westminster Gas Light and Coke Company distributing gas for street lighting in London by 1812. Through the nineteenth and early twentieth centuries, manufactured coal gas lit factories, homes, and city streets across Europe and North America before being displaced by natural gas pipeline distribution. The transition from coal gas to natural gas in most markets occurred between the 1950s and 1970s. The historical record of coal gas chemistry and distribution technology is preserved in the U.S. Energy Information Administration's fossil fuel explanations.

Applications

Coal gas has applications across energy and industrial sectors, including:

• Electricity generation in integrated gasification combined cycle plants, where syngas fuels high-efficiency gas turbines
• Production of hydrogen for petroleum refining, ammonia synthesis, and fuel cell applications through water-gas shift and pressure-swing adsorption
• Synthetic liquid fuels and chemical feedstocks via Fischer-Tropsch synthesis from carbon monoxide and hydrogen
• Historical and niche applications in municipal gas distribution where natural gas is unavailable or expensive
• Carbon capture pilot projects, where pre-combustion removal of CO2 from syngas is more efficient than post-combustion capture