How does excess air adjustment impact NOx formation in a stoker-fired boiler?

Excess air adjustment in a stoker-fired boiler directly influences the formation of nitrogen oxides (NOx), which are regulated pollutants. NOx formation occurs through two primary mechanisms: thermal NOx and fuel NOx. Thermal NOx is generated when nitrogen in the combustion air reacts with oxygen at high temperatures, typically above 2500°F (1371°C). Fuel NOx, on the other hand, is formed from the nitrogen chemically bound within the fuel itself. Increasing excess air, meaning supplying more air than theoretically required for complete combustion, tends to lower the overall combustion temperature. This occurs because the excess air absorbs some of the heat released during combustion. Lower combustion temperatures reduce the formation of thermal NOx. However, increasing excess air also provides more oxygen, which can increase the oxidation of nitrogen in the fuel, leading to higher fuel NOx emissions if not managed carefully. Finding the optimal excess air level is crucial. Too little excess air results in incomplete combustion, producing carbon monoxide (CO) and unburned hydrocarbons, while too much excess air can lead to increased fuel NOx and reduced boiler efficiency due to increased heat loss in the exhaust gases. Therefore, excess air must be carefully controlled to minimize both thermal and fuel NOx formation while maintaining efficient combustion.