Biogas is a type of renewable energy that is formed through a process known as anaerobic digestion, where organic materials such as cow dung or plant waste are broken down by bacteria. The given statement states that biogas is a non-renewable source of energy, which is incorrect.

Option 1 states that biogas is mainly composed of methane gas, which is correct. Methane is the primary component of biogas and contributes to its flammability and energy content.

Option 3 states that biogas is formed by the decomposition of cow dung in the absence of oxygen, which is also correct. This anaerobic decomposition process releases methane, carbon dioxide, and other gases that make up biogas.

Option 4 states that biogas burns without smoke and leaves no residue, which is generally true. Biogas combustion is cleaner compared to other fossil fuels and produces lower levels of air pollutants. However, a small amount of residue may be left behind in the form of ash or impurities depending on the quality of the biogas.

Therefore, the incorrect statement is Option 2, which states that biogas is a non-renewable source of energy. In reality, biogas is a renewable source of energy