Investigation of NO conversion by different types of sewage sludge chars under low temperature

Different types of sludge chars, i.e. the original (S1), HNO₃ washed (S2), KOH activated (S3), and H₂ reduced chars, were prepared to investigate their performances for NO conversion under low temperature. Results indicated that the surface area of the sludge chars did not play key role on NO conversion. S1 showed the higher NO conversion performance than S2 and S3, due to the reducing effect of iron components in S1 (mainly FeS and Fe₂P). Both H₂ reduction at high temperature and the followed cooling atmosphere (H₂ or N₂) have great influences on the activity of S1. H₂ reduction can promote the reducing capacity of the iron components, thus the performance of S1 for NO conversion was also significantly enhanced. However, NO conversion over S1 decreased in the presence of O₂, because the iron components were more quickly oxidized in O₂ atmosphere. Results of duration tests showed that the activity of S1 decreased with time, but it can be recovered by calcination in N₂. The profiles of the temperature programmed desorption (TPD) of CO and CO₂ indicated that CO and CO₂ emissions are mainly from the decomposition of oxygen functional groups (OFGs), including anhydrides, phenols, esters, and carboxylic acid. Furthermore, mechanisms of NO adsorption and conversion by S1-S3 were explored by an in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was found that NO adsorbed on the sludge chars mainly transferred into NO₂ species at 50 °C, and then NO₂ species were gradually oxidized to nitrate species with the increase of temperature. Finally, the possible reaction routes for NO conversion were proposed based on experimental data.