for NOx should be compromised with the downstream specifications for CO2 processing as shown in Figure 2 and described below.
Significant conversion of NO to NO2 as well as absorption and adsorption of NOx have been observed in the CO2 compression process [8]. These provide more opportunities for NOx removal and reduction, but it also can generate new operational issues for the handling of NOx components during the processes. Therefore, NOx in oxyfuel combustion CO2 capture processes should be handled as both environmental (emissions and by-products) and operational issues.
The optimisation of NOx reduction/removal is a more complicated task in comparison with those for conventional coal-fired power plants, because of more available options and specific operational issues. Figure 5 shows an estimation of NOx reduction requirements based on the non-optimised NOx levels generated from the oxyfuel combustion pilot, as well as current and future NOx emission regulations (200 and 100 mg NOx (as NO2)/Nm3 dry for plants > 300 MWth), which are converted to fuel basis in Figure 5). It reveals that more reduction of NOx is expected when the combustion is optimised for NOx reduction; a concentration of about 200 mg/Nm3 should be achievable, but this is yet to be proven. A comparison of the gas volume flow rates at various potential locations for the NOx reduction is shown in Figure 5 (right) as well. As shown in the figure, less than 60% of NOx reduction may be required for future strict air emission limits (corresponding to 100 mg NOx/Nm3 dry) based on a non-optimised NOx generation level (which can be further reduced). It has been observed that more than 60% of the the conversion of NO to NO2 followed by the removal of NO2 by absorption in condensate is not very difficult in pressurised CO2 processing processes. There are more options for the removal of NOx in the GPU, which could be applied to easily achieve the air emission requirements in comparison to traditional SCR. SCR has disadvantages while handling large volumes of gas that have high dust contents (as shown on the right of Figure 5). Using SCR for NOx reduction also presents a loss of the advantage of oxyfuel with the primary de-NOx function (re-burning recycled NOx). A simple solution for air emission is less important than the development of compression and dehydration processes for the easily handled NOx issues in combination with the reduction of NOx. While developing the GPU NOx removal options, it is important to consider additional issues such as corrosion potential and by-product handling.