CNG is:
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highly polluting
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less polluting
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not at all polluting
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none of these
Compressed Natural gas is the cleanest of all the fossil fuels, composed primarily of methane. The main products of the combustion of natural gas are carbon dioxide & water vapor, which are the same compounds we exhale when we breathe. Coal & oil are composed of much more complex molecules, with a higher carbon ratio & higher nitrogen & sulfur contents. This means that when combusted, coal & oil release higher levels of harmful emissions, including a higher ratio of carbon emissions, nitrogen oxides & sulfur dioxide. Coal & fuel oil also release ash particles into the environment, substances that do not burn but instead are carried into the atmosphere & contribute to pollution.
The combustion of natural gas, on the other hand, releases very small amounts of sulfur dioxide & nitrogen oxides, virtually no ash or particulate matter & lower levels of carbon dioxide, carbon monoxide & other reactive hydrocarbons