4.9.3.1,4.9.3.2 - Air pollutants from fuels

Combustion and pollutant formation

The elements in a fuel and the conditions of burning determine which products can enter the atmosphere.

Most fuels, including coal, contain carbon and/or hydrogen, and some contain sulfur. With sufficient oxygen, carbon is oxidised to carbon dioxide and hydrogen to water. A hydrocarbon such as methane burns completely according to:

Water may leave a hot system as vapour. Carbon dioxide is a greenhouse gas, so complete combustion can still have environmental consequences. “Complete” means the carbon is fully oxidised under these conditions; it does not mean the process is pollution-free.

With insufficient oxygen, incomplete combustion can form carbon monoxide and solid carbon particles, called soot, as well as water and some carbon dioxide. For example, a possible balanced equation is:

Actual exhaust may be a mixture because oxygen supply and mixing vary through the flame. Unburned hydrocarbons and solid particles can also be released and contribute to airborne particulates.

Sulfur in the fuel can burn to sulfur dioxide. At high temperatures, nitrogen and oxygen from the air can react to form oxides of nitrogen, so these pollutants do not require nitrogen to be part of the fuel. A sulfur-free pure hydrocarbon cannot produce sulfur dioxide from its own atoms, but it can still lead to nitrogen oxides when burned in hot air.

To predict products, first identify the elements in the fuel, then check oxygen supply and temperature. Do not infer all products solely from the fuel's name or assume that every listed pollutant must be produced in every combustion process.

Carbon monoxide and particulates

Incomplete combustion can create a toxic gas and airborne particles whose effects differ from the greenhouse effect of carbon dioxide.

Carbon monoxide is colourless and odourless, so it is not easily detected by sight or smell. It is toxic. It interferes with the blood's ability to carry oxygen by binding to haemoglobin, so body tissues receive less oxygen. Exposure can be dangerous even without an obvious smoky appearance. Carbon monoxide is $\ce{CO}$; carbon dioxide is $\ce{CO2}$. The different formulae identify different substances with different properties.

Poor oxygen supply or mixing can increase carbon monoxide formation. In a heater or other combustion device, correct operation and suitable ventilation help supply oxygen and remove combustion gases. Never use smell as a test for carbon monoxide. Detecting its presence requires appropriate equipment, rather than deciding that an odourless room must be safe.

Particulates are tiny airborne solid particles or droplets. Fuel combustion can release soot, which consists of carbon particles, and unburned hydrocarbons that contribute to particulate matter. Breathing particles can damage the respiratory system and cause health problems; their small size lets them enter the airways.

Particulates can also cause global dimming. They scatter or reflect some sunlight so less reaches Earth's surface. This concerns incoming radiation and differs from greenhouse gases absorbing outgoing infrared radiation. Global dimming does not make particulate pollution desirable: the particles still harm health and change the environment. Nor does it mean greenhouse warming has stopped; the effects can act together.

Sulfur dioxide and nitrogen oxides

Sulfur dioxide and oxides of nitrogen are formed by different routes but can both harm breathing and contribute to acid rain.

If sulfur is present in the fuel, it reacts with oxygen during combustion: