In direct-fired furnaces, nitrogen oxide (NOx) formation primarily stems from the oxidation of nitrogen in the fuel and the reaction of nitrogen with oxygen in the air at high temperatures. To reduce NOx formation, a multi-pronged approach is needed, encompassing combustion process control, process optimization, and the synergy of auxiliary technologies. This involves precisely controlling combustion conditions and introducing advanced technologies to achieve emission reduction targets.
Temperature control during combustion is crucial for suppressing NOx formation. High temperatures accelerate the formation rate of thermal NOxes; therefore, optimizing burner design or adjusting fuel supply methods is necessary to lower peak flame temperatures. For example, staged combustion technology can divide the combustion process into a main combustion zone and a secondary combustion zone: the main combustion zone controls the fuel-air mixing ratio, creating a locally oxygen-deficient environment to suppress NOx formation; the secondary combustion zone supplements air to ensure complete combustion. This zoned combustion method reduces residence time in the high-temperature zone and avoids incomplete combustion due to oxygen deficiency.
Fuel characteristics directly impact NOx formation. Prioritizing low-NOx fuels is a fundamental measure for emission reduction; for example, natural gas or denitrified biomass fuels have significantly lower nitrogen content than coal or heavy oil. For fuels with high nitrogen content, pretreatment technologies can reduce their nitrogen content, such as denitrification of biomass fuels or blending of fuel with low-nitrogen substances. Furthermore, adjusting fuel particle size and distribution uniformity can optimize combustion efficiency and prevent nitrogen oxide surges caused by localized high temperatures.
Flue gas recirculation technology, by introducing low-temperature flue gas to dilute the oxygen concentration in the combustion zone, is an effective means of suppressing nitrogen oxide formation. This technology reintroduces a portion of the purified flue gas into the combustion chamber, utilizing inert gases such as carbon dioxide and water vapor in the flue gas to lower the flame temperature while reducing the contact between oxygen and nitrogen. In practical applications, the recirculated flue gas ratio needs to be adjusted according to the operating conditions of the direct-fired furnace to avoid decreased combustion efficiency or excessive carbon monoxide emissions due to insufficient oxygen.
The application of low-NOx burners can further optimize the combustion process. These burners improve the mixing method of air and fuel, forming a more uniform flame distribution and reducing the formation of localized high-temperature zones. For example, fully premixed combustion technology premixes fuel and air to an optimal ratio, resulting in a more stable combustion process and a more uniform temperature distribution; multi-stage air distribution technology controls combustion intensity and temperature gradient by supplying air in stages. These designs effectively suppress the formation of nitrogen oxides (NOx).
The synergistic application of end-of-pipe treatment technologies can achieve deep removal of NOx. Selective catalytic reduction (SCR) technology injects ammonia or urea into the flue gas, reducing NOx to nitrogen and water under the action of a catalyst, achieving high denitrification efficiency. For direct-fired furnaces, the appropriate catalyst type must be selected based on the flue gas temperature; for example, low-temperature catalysts are suitable for biomass boilers. Non-catalytic reduction technology injects a reducing agent directly at high temperatures, suitable for small and medium-sized boilers or scenarios with minimal operational fluctuations.
Intelligent upgrading of the combustion control system is key to improving emission reduction. By monitoring parameters such as combustion temperature, oxygen concentration, and NOx emission concentration in real time, and automatically adjusting fuel supply, air ratio, and flue gas recirculation rate, dynamic optimization of the combustion process can be achieved. For example, appropriately reducing the excess air coefficient under low-load conditions can reduce nitrogen oxide (NOx) generation while avoiding a decrease in combustion efficiency.
Reducing NOx generation in direct-fired furnaces requires a comprehensive approach, utilizing technologies such as combustion process control, fuel optimization, flue gas recirculation, low-NOx burners, end-of-pipe treatment, and intelligent control. Through the synergistic effect of source suppression and end-of-pipe treatment, NOx emission concentrations can be significantly reduced, meeting increasingly stringent environmental protection requirements.
