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Combustion analysis involves understanding the chemical reactions that occur when a fuel burns in the presence of oxygen. It helps in calculating the products formed and gauging the effectiveness of the combustion process. When dealing with hydrocarbon fuels like methane and hydrogen, we typically observe reactions such as complete and incomplete combustion. Complete combustion results in products like carbon dioxide (CO2) and water (H2O). However, when there is insufficient oxygen, incomplete combustion may occur, leading to the production of carbon monoxide (CO). In the given exercise, the stoichiometry of the reactions is crucial. For methane combustion, each mole requires two moles of O2, while hydrogen needs 0.5 moles of O2. This knowledge is used to compute how much oxygen is needed for the reactions. Stoichiometry ensures the correct proportions, which affects the amounts of CO, CO2, and water produced.

Excess air in combustion refers to the additional air supply beyond the stoichiometric requirement needed to ensure all the fuel burns completely. This practice is common to prevent the formation of carbon monoxide, a harmful pollutant. In the exercise, 25% excess air is introduced. This implies that 25% more air than the theoretical amount needed is supplied. It ensures complete combustion of the reactants by compensating for potential inefficiencies in air and fuel mixing, which can otherwise lead to incomplete burning and CO formation. However, using excess air comes with downsides. It can lead to energy losses as more air needs to be heated along with the fuel. The extra nitrogen from the air absorbs heat without contributing to combustion, which can reduce the system's thermal efficiency.

Fractional conversion is the fraction of reactant that is converted into products in a chemical reaction. It indicates the efficiency with which reactants are transformed into desired products during a combustion process. In this context, with a fractional conversion of 90% for methane and 85% for hydrogen, not all fuel is converted into the desired combustion products. Instead, a portion remains unreacted or converts into less desired forms, like carbon monoxide. This concept is vital as it affects the overall efficiency and the emissions from the combustion process. By improving fractional conversion, one can optimize fuel usage and reduce pollutant emissions, hence enhancing the overall combustion efficiency and reducing harmful environmental impacts.

Pollutant control in combustion involves strategies to minimize the release of harmful substances, such as carbon monoxide, into the atmosphere. Proper management is crucial for limiting environmental and health impacts. The exercise highlights that CO can be reduced by increasing excess air. However, it is important to balance this approach to avoid increased fuel costs and efficiency losses. Alternatives to increasing air include methods like catalytic converters or fuel restructuring, which can help mitigate CO emissions without significantly altering the combustion process. In addition to technological solutions, operational changes, such as maintaining optimal flame temperature and ensuring proper mixing of fuel and air, can effectively lower CO emissions and improve overall system performance.