Coal Gas

Coal gas is a combustible gaseous fuel produced by the destructive distillation of coal. It was one of the earliest manufactured gases used for heating, lighting, and cooking before the widespread adoption of natural gas and electricity. Comprising mainly hydrogen, methane, carbon monoxide, and small quantities of other hydrocarbons, coal gas played a crucial role in the industrial and urban development of the nineteenth and early twentieth centuries.

Definition and Composition

Coal gas is obtained by heating bituminous coal in the absence of air—a process known as pyrolysis or carbonisation. During this process, volatile components of coal are released as gas, while solid coke and liquid coal tar are left as by-products.
The typical composition of coal gas varies depending on the type of coal and production method but generally includes:

• Hydrogen (H₂): 40–50%
• Methane (CH₄): 25–30%
• Carbon monoxide (CO): 7–10%
• Carbon dioxide (CO₂): 2–5%
• Nitrogen (N₂): 3–15%
• Small amounts of ethane, ethene, ammonia, hydrogen sulphide, and water vapour

The gas is highly flammable, and its energy value is approximately 5000–6000 kcal per cubic metre. However, the presence of carbon monoxide makes it toxic and unsuitable for unventilated environments.

Historical Background

The production of coal gas began in the late eighteenth century. The Scottish engineer William Murdoch is credited with its first practical use around 1792, when he lit his house in Redruth, Cornwall, using gas produced from coal. By the early nineteenth century, gas lighting had spread across Europe and America.
The first public gasworks was established in London in 1812, and soon after, major cities worldwide built gas plants to supply homes, factories, and streets with lighting. Gas lamps illuminated urban centres long before electric bulbs became widespread.
By the mid-nineteenth century, coal gas had become a vital energy source for domestic and industrial use, driving the growth of the gas industry and laying the foundation for modern urban utilities.

Manufacturing Process

The production of coal gas involves several stages of heating, purification, and storage.
1. Carbonisation: Crushed bituminous coal is heated in retorts (closed, air-tight ovens) at temperatures between 1000°C and 1200°C. In the absence of air, the coal decomposes into:

• Coke: A solid carbon-rich residue used as a fuel and in metallurgy.
• Coal gas: A mixture of gaseous hydrocarbons.
• Coal tar and ammoniacal liquor: Liquid by-products containing valuable chemicals such as benzene, toluene, phenol, and ammonia.

2. Cooling and Condensation: The hot gas leaving the retort passes through coolers or condensers, where tar and water vapour condense out.
3. Purification: The gas is then cleaned to remove impurities such as hydrogen sulphide (H₂S), ammonia, and carbon dioxide (CO₂) using:

• Scrubbers: Towers where gas is washed with water or chemical solutions.
• Purifiers: Beds of hydrated ferric oxide that remove sulphur compounds.

4. Storage and Distribution: The purified gas is collected in gas holders—large cylindrical storage tanks—and distributed through underground pipelines for domestic and industrial use.

By-products and Their Uses

The production of coal gas yields several valuable by-products that contributed to the growth of chemical industries:

• Coke: Used in blast furnaces for iron and steel production, and as a domestic fuel.
• Coal tar: Source of aromatic hydrocarbons, dyes, phenols, naphthalene, and pharmaceuticals.
• Ammoniacal liquor: Used in the manufacture of ammonia and fertilisers.

These by-products made the gasification of coal economically attractive and formed the basis of the coal chemical industry in the nineteenth century.

Properties of Coal Gas

• Colour: Colourless
• Odour: Distinct, pungent smell due to impurities such as hydrogen sulphide
• Toxicity: Highly poisonous due to carbon monoxide content
• Flammability: Highly flammable; burns with a bright blue flame
• Calorific Value: Around 5000–6000 kcal/m³

Because of its carbon monoxide content, coal gas required careful handling and good ventilation when used for domestic heating or lighting.

Uses of Coal Gas

Historically, coal gas had multiple applications, including:

• Lighting: Used in street lamps, homes, and public buildings before electric lighting became prevalent.
• Heating and Cooking: Served as a domestic fuel in gas stoves and heaters.
• Industrial Use: Employed in furnaces, kilns, and glassmaking.
• Power Generation: Early internal combustion engines used coal gas as a fuel source.

Today, coal gas has largely been replaced by natural gas (methane) and electricity, which are cleaner and more efficient. However, the basic principles of coal gasification continue to influence modern energy technologies.

Environmental and Safety Concerns

The use of coal gas had several drawbacks:

• Toxicity: The presence of carbon monoxide posed a risk of poisoning.
• Pollution: Gasworks emitted smoke, tar, and wastewater that contaminated air and water sources.
• Explosion Hazard: The gas’s high flammability made leaks potentially dangerous.

These concerns, combined with the discovery of natural gas and advancements in electric power, led to the gradual phasing out of coal gas in most parts of the world by the mid-twentieth century.

Modern Relevance and Coal Gasification

Although traditional coal gas production has declined, the concept remains important in modern energy systems through coal gasification—a process that converts coal into syngas (synthesis gas), a mixture of carbon monoxide and hydrogen. Syngas serves as a feedstock for producing:

• Synthetic natural gas (SNG)
• Ammonia and methanol
• Hydrogen fuel
• Electric power through integrated gasification combined cycle (IGCC) plants

Coal gasification allows for cleaner and more efficient use of coal when coupled with carbon capture and storage (CCS) technologies.

Significance in Historical Context

The development of coal gas marked a major technological advance in energy history. It brought about:

• The first organised urban utility networks.
• The beginnings of chemical engineering and materials science.
• A foundation for modern gas-based and petrochemical industries.

Coal gas illuminated cities, powered industries, and transformed everyday life during the Industrial Revolution, bridging the transition from traditional fuels such as wood and charcoal to the modern era of hydrocarbons.