In emission control of hydrogen-rich gas boilers, reducing nitrogen oxide (NOx) generation requires a synergistic approach involving combustion process optimization, fuel characteristic adjustment, and end-of-pipe treatment technologies. NOx generation primarily originates from three pathways: thermal, rapid, and fuel-related. Thermal NOx generation is closely related to combustion temperature and oxygen concentration, rapid NOx generation is influenced by hydrocarbons, and fuel-related NOx generation depends on the nitrogen content of the fuel itself. Given the characteristics of hydrogen-rich fuels, multi-dimensional technological approaches are needed to suppress NOx generation pathways.
Optimizing the combustion process is the core element in reducing NOx generation. Hydrogen-rich gas boilers can employ staged combustion technology, dividing the combustion process into a main combustion zone and a recombustion zone. By controlling the fuel-air mixing ratio, the oxygen concentration in the main combustion zone is reduced, thereby suppressing the generation of thermal NOx. The recombustion zone completes fuel combustion by supplementing air, ensuring thermal efficiency while reducing NOx emissions. Furthermore, flue gas recirculation technology can return some low-temperature flue gas to the combustion zone, reducing local temperature peaks and oxygen concentrations, further reducing thermal NOx generation, especially suitable for high-load operating conditions.
Adjusting fuel characteristics directly impacts nitrogen oxide (NOx) formation. Hydrogen-rich fuels, with their higher hydrogen content, burn rapidly and generate high flame temperatures, easily leading to localized high-temperature zones and increasing the risk of thermal NOx formation. Therefore, blending with low-NOx fuels or adjusting the hydrogen-to-natural gas mixing ratio can optimize fuel calorific value distribution and prevent excessively high combustion temperatures. Simultaneously, fuel pretreatment technologies such as denitrification can reduce the nitrogen content in the fuel, reducing fuel-derived NOx formation at the source.
The application of low-NOx burners is crucial for controlling NOx formation. Optimizing flame morphology through technologies like fully premixed combustion and multi-stage air distribution can suppress the formation of high-temperature zones and reduce thermal NOx formation. Fully premixed burners premix fuel and air, resulting in more complete combustion, a more uniform flame temperature distribution, and prevention of localized overheating. Multi-stage air distribution technology creates lean and rich combustion zones by supplying air in stages, further reducing NOx formation conditions.
End-of-pipe treatment technologies can remove already generated NOx, ensuring emissions meet standards. Selective catalytic reduction (SCR) technology uses a catalyst to reduce nitrogen oxides (NOx) into nitrogen and water vapor, achieving high denitrification efficiency and making it suitable for large boilers. Selective non-catalytic reduction (SNCR) technology directly reduces NOx by injecting a reducing agent into a high-temperature zone, making it suitable for small and medium-sized boilers or scenarios with limited retrofitting options. Polymer denitrification technology utilizes amino compound powders for denitrification under low-temperature conditions, suitable for biomass boilers or scenarios with low-temperature flue gas characteristics.
Precise control of combustion temperature and oxygen concentration is crucial for balancing thermal efficiency and environmental requirements. Adjusting fuel supply, excess air coefficient, and combustion temperature can prevent the formation of localized high-temperature zones. For example, reducing boiler load or employing a staged load adjustment strategy can keep the operating point within a range with lower NOx generation. Simultaneously, optimizing the staged air supply method within the furnace creates a reducing atmosphere, further suppressing NOx generation.
Optimization of boiler operating parameters requires consideration of fuel characteristics and equipment conditions. Hydrogen-rich gas boilers require adjusting burner air distribution according to the hydrogen ratio to ensure uniform fuel-air mixing and avoid incomplete combustion or localized overheating. In addition, regularly cleaning boiler ash and inspecting burner wear can prevent the formation of localized high-temperature zones and maintain efficient and stable equipment operation. Installing an online monitoring system to track nitrogen oxide emission concentrations in real time provides data support for adjusting operating parameters.
Reducing nitrogen oxide generation in hydrogen-rich gas boilers requires a multi-dimensional approach, including combustion process optimization, fuel characteristic adjustment, application of low-NOx burners, synergistic use of end-of-pipe treatment technologies, and optimization of operating parameters. Combining source reduction with end-of-pipe treatment can systematically reduce nitrogen oxide emissions, meet ultra-low emission standards, and promote the sustainable development of hydrogen-rich gas boilers towards higher efficiency and cleaner operation.
