Nitrogen/NO conversion characteristics of coal chars prepared using different pyrolysis procedures under combustion conditions

In actual combustion facilities, coal chars are often generated using a variety of pyrolysis processes, such as secondary pyrolysis, which is characterized by a long residence time in high temperature zone. The effects of such processes on the conversion of char N to NO during combustion have seldom been explored. In this study, the releases of NO during the combustion of coal chars obtained from different pyrolysis processes in a drop tube and in a fixed bed reactor were investigated. In addition, the extent of char N/NO conversion was studied in relation to the char reactivity, pore surface structure and carbon conversion in a horizontal tube furnace. The results show that, compared with chars generated by a single pyrolysis, chars treated by a subsequent secondary pyrolysis process exhibit larger pore surface areas but less reactivity because of the thermal annealing resulting from a longer thermal history. Chars with higher intrinsic reactivity were also found to release a lower amount of NO. However, a weak correlation was identified between the apparent reactivity and char N/NO conversion, indicating that intrinsic reactivity is more important and directly determines the NO reduction process under combustion conditions. Moreover, char N/NO conversion was significantly affected by the coal rank, and a greater extent of conversion of char N to NO was observed in the case of high-rank coal chars. At a high combustion temperature (1373 K), variations in the bulk O-2 concentration had little effect on the char N/NO conversion, and an apparent correlation was found between the extents of char N/NO conversion and the accessible pore surface area. These results indicate that at high temperatures, the char N/NO conversion is directly determined by the accessible pore surface area due to transportation limitations. The NO/(CO + CO2) ratio increased with increasing burn-off in the latter stages of char conversion, which can be attributed to decreases in both the BET surface area and accessible pore surface area available for NO reduction during combustion.