Emission characteristics of non-methane hydrocarbons and typical benzene compounds under ultra-low emission constraints in biomass direct combustion power plants

To achieve ultra-low NO_x emissions, biomass direct-fired power plants typically employ low excess air coefficient combustion methods to control NO_x emissions synergistically. However, incomplete combustion can lead to a significant increase in the emissions of non-methane hydrocarbons (NMHC) and benzene series compounds, which are atypical pollutants that have not been extensively studied or reported. This study conducted tests on NMHC and benzene series compounds in a 30 MW biomass grate furnace operating under ultra-low emission conditions. The results indicated that the emission concentrations of NMHC were as high as 3557.4−11219.7 mg/m³, and the total concentrations of six typical benzene series compounds ranged from 49.2−134.9 mg/m³. The most toxic compounds, benzene, toluene, and styrene—accounted for 52.6%−60.5% of the total, significantly exceeding emission standards. When the primary air volume increased from 59 t/h to 74 t/h, the NMHC concentration decreased from 11351.02 mg/m³ to 3631.88 mg/m³. When the boiler load increased from 40 t/h to 55 t/h, the NMHC emission concentration rose from 5989.01 mg/m³ to 8238.50 mg/m³. Increasing the primary air volume and reducing the boiler load were found to promote more complete combustion and elevate the oxygen concentration at the boiler tail outlet, thereby significantly reducing emissions of NMHC, benzene series compounds, CO, and CH₄. In summary, while the low excess air coefficient combustion method effectively reduces NO_x emissions in biomass boilers, it also significantly increases the emissions of NMHC and benzene series compounds. This study provides a valuable reference for optimizing combustion processes and reducing pollutant emissions in biomass power plants. For biomass direct-fired boilers, achieving ultra-low emissions of typical pollutants must also consider the emissions of NMHC and benzene series compounds. The goal of ultra-low emissions should not come at the cost of increased emissions of more toxic pollutants.