Nitrogen oxides (NOx) are gases produced by the oxidation of nitrogen during the combustion process and are one of the main substances causing the greenhouse effect, acid rain and photochemical reactions. The NOx emission limit requirements are becoming stricter around the world. Industrial boilers and power station boilers, as the most important source of NOx emissions, have received special attention. As an important aspect of biomass circulating fluidized bed boiler operation, the application of SNCR denitrification technology occupies a critical position.
Biomass circulating fluidized bed boiler
SNCR denitrification technology refers to spraying amino reductant into the furnace at 850-1100 C without catalyst. The reductant is rapidly pyrolyzed to NH3 and reacts with NOx in flue gas to form N2. There will be a certain amount of oxygen in the furnace. The injected reductant will react selectively with NOx instead of being oxidized by O2. Reducing agents generally use ammonia, ammonia water or urea.
1. Effect of Temperature Window
In SNCR process, the most important thing is the selection of injection point on furnace, that is, the selection of window temperature. For urea, the ideal temperature range is 800-1150°C, high temperature, reductant is oxidized to NOx, the content of NOx in flue gas does not decrease but increases; low temperature, inadequate reaction, resulting in loss of reductant, adverse impact on downstream equipment and even new pollution.
2. Residence Time
Any reaction takes time, so the reductant must have enough residence time with NOx in the appropriate temperature range to ensure the reduction rate of NOx in flue gas.
3. Effect of NH3/NOx molar ratio
The determination of molar ratio is determined by the desired reduction efficiency. According to the basic chemical reaction equation, reducing 2mol of NOx requires 1mol of urea or 2mol of ammonia. In practice, it is necessary to inject more reductant than theory to achieve the required reduction level.
4. Mixing of Flue Gas with Reducing Agent
Full mixing of the two is another key technology to ensure full reaction, and is an important link to ensure a high reduction rate of NOx at an appropriate NH3/NOx molar ratio.
With the increasing use of biomass boilers, strengthening the research and analysis of the application of SNCR denitrification technology in boilers is of great significance for achieving good practical results.