Combustion Process

The generation of energy for human use comes largely from a process called combustion, and this situation is likely to continue well into the next century. The process involves a chemical transformation between a substance or substances which we describe as fuels (mainly fossil fuels, such as coal and oil) and an oxidising agent (atmospheric oxygen). Combustion is the rapid oxidation (combination with oxygen) of a fuel resulting in the release of usuable heat and, usually, the production of a visible flame. In addition to fuel and oxidizer, an ignition source to start the process is usually needed. For example, in one of the earliest forms of natural combustion, the forest fire, wood is the source of fuel, the air is the oxidizer and the ignition source can be a spark (lightning). Combustion has a wide variety of uses. It is used for electric power generation, domestic comfort, internal combustion engines, etc. Combustion is also used as a heat source for chemical processing, general heating and drying operations. It is useful for waste disposal (incineration), melting operations and welding. It is involved in explosions for both useful and hostile purposes.

The combustion process is also a major source of air pollution (carbon monoxide, nitrogen oxides, unburned hydrocarbons, sulfur dioxide and particles) as well as "greenhouse" gases such as carbon monoxide.

Understanding combustion in detail is a complex subject which requires knowledge and understanding of a number of physical and chemical processes. However we can say that most fuels contain carbon and hydrogen (and sometimes sulfur) and that common combustion consists of the following three processes:

The three final compunds listed above are called combustion products. The most important aspect of these reactions is that all release heat. Perfect combustion is an ideal process obtained by mixing and burning exactly the right proportions of fuel and oxygen so that nothing is left over.

If too much oxygen is available, we describe the mixture as "lean" and that the fire is "oxidising". The excess oxygen plays no part in the process. For example, if four atoms of oxygen (instead of two) were mixed with an atom of carbon, two oxygen atoms would be left over.

If too much fuel (or not enough oxygen) is supplied we say that the mixture is "rich" and that the fire is "reducing". This is usually called incomplete combustion because all of the fuel particles will attempt to combine with oxygen, but may not get enough to burn completely. For example, if we mix two atoms of carbon (instead of one) with two atoms of oxygen, the carbon atoms may share the available oxygen, but neither has enough to become carbon dioxide. Instead they may form carbon monoxide (CO), a compound which will convert to carbon dioxide if more oxygen becomes available.

Pollutants from Combustion Processes

The principal pollutants which can be generated by combustion processes are carbon monoxide, nitrogen oxides, hydrocarbons, sulfur dioxide and particulate matter.

Carbon monoxide and hydrocarbons are the products of an incomplete combustion and could be formed when we burn a fuel rich mixture; when the fuel and oxidizer are not in full contact; when there is not enough time for the combustion process to be completed; or when the temperature in the combustion chamber is not high enough.

Sulfur dioxide is emitted as a consequence of burning fuels that contain sulfur compounds. This is true of standard diesel in Mexico, which is used as fuel by many trucks and buses in the country.

Nitrogen oxides (NOx) can be formed in three different ways. So called Thermal-NOx is produced when combustion occurs at high temperatures (typically above 1000^oC) because the nitrogen contained in the combustion air will also react with the available oxygen. Fuel-NOx is produced when the fuel contains nitrogen compounds which react with the available oxygen. Prompt-NOx is the way that smaller amounts of NOx can be produced, and is characterized by the rapid formation of NOx during the first stages of the combustion process beginning with nitrogen and oxygen from the air.

The formation of particulate matter can be the consequence of a high ash content in the fuel (in the case of fuel oils), or poor presentation of fuel producing incomplete burning (for solid and liquid fuels).