3. NOx Characteristics and Reduction
As shown in Figure 2, there are more options for NOx removal and reduction within the CO2 capture processes compared to conventional solutions for utility boilers. The understanding of NOx behavior in oxyfuel combustion and gas processing unit (GPU) is an important task for the development of flue gas cleaning. NOx behaviour in the CO2 capture chain has been characterised at the OxPP, which includes NOx formation/reduction in boiler and conversion, absorption and adsorption in the flue gas path, as well as FGD, FGC and GPU. The comprehensive investigation provides a scientific basis for the handling of NOx issues, and also for the optimisation of NOx removal
and reduction within the CO capture chain. 2
Although the optimisation of primary NOx reduction is still an ongoing task in the oxyfuel combustion pilot, there was a significant reduction of NOx formation in the oxyfuel combustion in comparison to air-firing. The conversion of NO to NO2 from the outlet of the furnace to the end of FGC was not very significant. Therefore, the possibilities for the removal of NOx are limited by using the conventional wet scrubbing process. Generally, NOx has no significant impacts on the boiler operation, except for emission related issues. Therefore, they have less influence on the flue gas cleaning specification I. Instead, the requirements of the flue gas cleaning specification II