Combustion can produce different products depending on the materials available. Clean combustion produces CO2, H2O, and energy, while incomplete combustion can produce CO, soot, nitrogen oxides, HCN, and NH3. Oxygen-free environments produce different products, including soot and CO. Nitrogen, halogens, sulfur, and phosphorus can also produce their own combustion products. The heat release potential and energy required to initiate the reaction can determine the tendency for clean or dirty combustion.
Combustion, defined as the rapid oxidation of a substance accompanied by a high temperature and usually a flame, can produce a number of different products, depending on the materials available in the reaction. The clean combustion products between a hydrocarbon and oxygen are carbon dioxide (CO2), water (H2O) and energy. Even incomplete, dirty or partial combustion can form carbon monoxide (CO), free carbon or soot, nitrogen oxides, hydrogen cyanide (HCN) and ammonia (NH3).
Combustion products of clean burning include CO2, H2O and energy. No other gases or solid particles are formed as combustion products in this type of reaction. The following balanced reaction is that of propane, which is an alkane, reacting with oxygen:
C3H8 + 5 O2 → 3 CO2 + 4 H2O
When a combustion reaction takes place in an oxygen-free environment, different combustion products can be produced. Free carbon, more commonly known as soot, and carbon monoxide are produced together with CO2, H2O and energy. The formation of soot as a product of combustion is why incomplete combustion is also known as dirty combustion. In the chemical industry, gasifiers burn flammable materials in oxygen-free environments to produce synthesis gas, consisting of hydrogen and carbon monoxide. Outside of chemical industries, incomplete combustion often occurs in internal combustion engines and poorly ventilated furnaces.
Oxygen from air is the most common source of oxygen for most combustion reactions. However, air is mainly composed of nitrogen, and during combustion, nitrogen is capable of producing a variety of combustion products. Nitric oxide gases, often referred to as NOX gas, can form in a combustion reaction. The most common NOX gas is toxic nitrogen dioxide (NO2). Ammonia (NH3) and the lethal hydrogen cyanide (HCN) can also be formed.
Halogens, sulfur and phosphorus can also produce their own combustion products. Halogens such as chlorine can react with radical hydrogen to form chemicals such as hydrochloric acid (HCl). Sulfur can produce toxic and smelly chemicals sulfur dioxide (SO2) and hydrogen sulfide (H2). When phosphorus is present in a combustion reaction, it produces phosphorus pentoxide (P2O5) as a solid white particulate.
The tendency of a hydrocarbon fuel to favor clean or dirty combustion products can be estimated by examining the heat release potential of the reaction and the energy required to initiate the reaction. Increasing the heat production potential increases the tendency of the fuel to undergo incomplete combustion. Propane, which doesn’t require much energy to start combustion, tends to burn cleanly. Aromatic compounds such as benzene and toluene, on the other hand, tend to produce a lot of soot when burned.
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