Biogas is a renewable energy source that is produced through the process of anaerobic digestion. It is primarily composed of methane (CH4) and carbon dioxide (CO2), along with traces of other gases. However, nitrogen dioxide (NO2) is not a constituent of biogas.

Explanation:

- Biogas: Biogas is a type of gas that is generated through the anaerobic digestion of organic materials such as agricultural waste, livestock manure, and sewage sludge. It is a renewable energy source that can be used for various purposes, including heating, electricity generation, and as a vehicle fuel.
- Constituents of Biogas: Biogas mainly consists of methane (CH4) and carbon dioxide (CO2). These two gases make up the majority of the biogas composition, typically ranging from 50% to 75% methane and 25% to 50% carbon dioxide.
- Methane (CH4): Methane is the primary component of biogas, accounting for a significant proportion of its energy content. It is a potent greenhouse gas, but when captured and used as biogas, it can be harnessed as a renewable energy source, reducing greenhouse gas emissions.
- Carbon Dioxide (CO2): Carbon dioxide is another key component of biogas. It is a byproduct of the anaerobic digestion process and is released into the atmosphere during the decomposition of organic materials. While carbon dioxide is a greenhouse gas, it is considered carbon-neutral when produced from renewable sources like biogas.
- Hydrogen (H2): Hydrogen is not a significant constituent of biogas. Although trace amounts of hydrogen may be present in biogas, they are not a major component. Hydrogen gas is highly flammable and is usually produced through other processes such as electrolysis or reforming fossil fuels.
- Nitrogen Dioxide (NO2): Nitrogen dioxide is not a constituent of biogas. It is a different gas altogether and is not produced through the anaerobic digestion process. Nitrogen dioxide is a reddish-brown gas that is formed from the combustion of fossil fuels, particularly in vehicles and power plants. It is a harmful air pollutant and contributes to the formation of smog and respiratory issues.

In conclusion, while methane and carbon dioxide are the main constituents of biogas, nitrogen dioxide is not a part of its composition.