NOx and SO2 emission of oxygen carrier aided combustion in fluidized bed

Abstract

Oxygen carrier aided combustion (OCAC) has been established as an innovative concept for improving fuel combustion in fluidized bed (FB) combustors. OCAC uses the particles of oxygen carrier (OC) to partly or completely replace the inert bed materials within the FB, which can make the oxygen distribution in the combustor more uniform and subsequently enhance combustion efficiency. However, the fundamental study on SO2 and NOx emissions during the OCAC is still not sufficient and controversial. In this work, the OCAC of coal and petroleum coke has been evaluated under different operating conditions on a lab-scale FB combustor with a special focus on the SO2 and NOx emissions. The conversion ratios of volatile-N (XV-N) and char-N (XC-N) have been quantitatively decoupled, and the influence of operating parameters on XC-N and XV-N has been revealed. Results showed that the OCAC concept has significantly reduced CO emission, enhanced NOx emission, and increased nitrogen conversion (XN). However, the NOx emission decreased by reducing the ilmenite ore proportion in the bed materials. This was attributed to the presence of ilmenite ore, which reduced the reducing regions and weakened the NOx reduction. Under OCAC operation, the XV-N increased significantly with the increase of O2 concentration and primary air ratio (A); and the variation of A shows a most decisive effect on XV-N. However, the XC-N was insensitive to the change of O2 concentration and A, when the XC-N was in the range of 31.01–34.89 % for all OCAC operations. The significant reduction of the SO2 emission during OCAC is mainly due to the employment of the ilmenite ore as bed materials that not only can absorb S element, but also lead to more uniform distribution of both oxygen and bed temperature, thereby improving the self-desulfurization of ash and reducing the sulfate decomposition.