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An exhaust gas is any gas that is emitted, typically through a combustion process of some type. It is most commonly thought of as a byproduct of the internal combustion engine in vehicles, but this is not the only place it is produced. This type of gas is also emitted through industrial processes that take place in factories and power plants. Though many exhaust gases are harmless, there are a few that have the potential to be very harmful, and are considered major pollutants.
In the typical automobile, the exhaust gas is actually a combination of many different gases. One of the most dangerous of these is carbon monoxide. This gas has the potential to kill individuals and animals if concentrations are high enough. Carbon dioxide, once thought to be the most desirable exhaust gas, is now considered a pollutant because of its suggested link to global warming. Nitrogen oxides are also released through the internal combustion process, and have been linked to acid rain and ozone.
Power plants fired by coal release these same gases, as well as a few others, with one of the most dangerous being sulfur dioxide. This gas that is partially responsible for haze and acid rain. Carbon dioxide is the main exhaust gas emitted from power plants. While technologies exist that can remove sulfur dioxide and other gases, equipment that can remove carbon dioxide from power plant emissions is still in the early stages of development, and not a financially viable option at this point.
In most cases, the exhaust gas that is produced must be filtered through some type of exhaust system. In factories, because the gas is usually lighter than air, tall smokestacks are often used to get the gas to a high enough point in the atmosphere where it poses less of a risk to things living nearby. Exhaust pipes and exhaust gas fans may be used in some places where the gases are created in a location where tall smokestacks are not feasible.
In some cases, exhaust gas is actually a beneficial product, and is sought out specifically. This is often true of water vapor, which takes the form of steam and is used to turn an exhaust gas turbine, which is then used to create electricity. In this case, the water or other liquid may be heated specifically for the gas that it produces. This produces a form of energy that is generally accepted to be clean and friendly to the environment. Some locations even tap into natural heat vents in the earth so that there is no need to artificially heat the liquids.