Combustion is a high-temperature chemical reaction that serves as the basis for nearly all energy generation, from internal combustion engines to power plant boilers. The process typically involves a rapid reaction between a fuel, usually a hydrocarbon compound, and an oxidant, most commonly the oxygen found in the air. This reaction releases a significant amount of energy, primarily in the form of heat, and creates new chemical substances referred to as byproducts. The type and quantity of these byproducts depend heavily on the efficiency of the reaction, which is determined by factors like oxygen availability and temperature.
Products of Complete Combustion
Complete combustion represents the theoretical ideal outcome of the reaction, occurring when there is a perfectly sufficient supply of oxygen for the fuel to fully react. In this highly efficient scenario, the hydrocarbon fuel’s carbon and hydrogen atoms are fully oxidized.
The two main byproducts of complete combustion of hydrocarbon fuels are carbon dioxide (CO2) and water vapor (H2O). The hydrogen atoms in the fuel combine with oxygen to form water vapor. Simultaneously, every carbon atom in the fuel reacts with oxygen to produce carbon dioxide.
While carbon dioxide is a known greenhouse gas, its production indicates that the combustion process itself was highly efficient. This ideal state, where only CO2 and H2O are created, is rarely achieved in real-world applications. Most combustion processes produce a mixture of complete and incomplete byproducts.
Products of Incomplete Combustion
Incomplete combustion occurs when the oxygen supply is restricted, or the temperature is too low, preventing the fuel from fully oxidizing. This failure to reach the ideal reaction state results in the production of byproducts that pose more immediate health and environmental concerns. This process is common in older engines, home heating systems, and industrial processes.
One of the most dangerous byproducts of incomplete burning is carbon monoxide (CO), a colorless and odorless gas. It forms when a carbon atom only has enough oxygen to form a single oxygen bond instead of the two required for carbon dioxide. This gas is highly toxic because it binds to the iron in hemoglobin in the bloodstream much more readily than oxygen, effectively suffocating the body.
In addition to carbon monoxide, incomplete combustion releases unburnt hydrocarbons (UHCs), which are fuel molecules that either did not react at all or only partially decomposed. These include a complex mixture of organic compounds, often referred to as volatile organic compounds (VOCs). These unburnt fragments contribute to smog formation.
Another significant byproduct is soot, which consists of fine, solid carbon particles resulting from highly inefficient burning. These particles, also known as particulate matter (PM), are categorized by their size, with PM10 (smaller than 10 micrometers) and PM2.5 (less than 2.5 micrometers). The small size of PM2.5 allows them to penetrate deeply into the lungs and even enter the bloodstream, leading to serious respiratory and cardiovascular impacts.
Pollutants Derived from Impurities and Air
Some of the most problematic combustion byproducts do not originate from the fuel’s hydrocarbon structure but rather from contaminants within the fuel or from the surrounding air used in the reaction. These substances are formed through side reactions that occur under the extreme conditions of the combustion chamber.
Nitrogen oxides (NOx), a collective term for nitric oxide (NO) and nitrogen dioxide (NO2), are a major category of these pollutants. They form when the atmospheric nitrogen (N2) and oxygen (O2) naturally present in the air react with each other at the very high temperatures found in combustion chambers, a process known as thermal NOx formation. This reaction typically begins at temperatures above 1300°C.
Once released, NOx plays a significant role in the formation of photochemical smog, and it also contributes to acid rain when it reacts with water vapor in the atmosphere. The other main category is sulfur oxides (SOx), principally sulfur dioxide (SO2), which results from the oxidation of sulfur impurities found in many fossil fuels, particularly coal and heavy oils.
Sulfur dioxide is a colorless gas with a pungent odor that can irritate the human respiratory system and contribute to breathing difficulties. In the atmosphere, SO2 can react further to form sulfur trioxide (SO3), which combines with water to create sulfuric acid, a primary component of acid rain. Additionally, trace metals like mercury and lead, which may be present in the original fuel, are released as combustion byproducts, often attaching themselves to particulate matter.