Preliminary Research on the Effects of Coal Devolatilization and Char Combustion Processes on the Emission of Particulate Matter during Lignite Combustion under Air and Oxy-fuel Conditions

The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. The effects of these two processes have important influence on the emission characteristics of particulate matter. With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the apparent char–CO2 gasification reaction upon oxy-fuel combustion, the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. A typical Chinese lignite was devolatilized in a N2 or CO2 atmosphere to generate N2–char and CO2–char, respectively. Raw coal and the two chars were burned in a drop-tube furnace under both air and oxy-fuel conditions. Afterward, the yields and inorganic compositions of segregated PM10 were carefully analyzed. The results have shown that the devolatilization in the N2 condition promotes the emission of sub-micrometer particles (PM0.5) and coarse particles (PM1–10) but the devolatilization process in CO2 inhibits their emission. N2–char and CO2–char present entirely different combustion behavior and, thus, reflect the discrepant emission characteristics of PM10. The rough higher yields of PM0.5 and PM1–10 from N2–char combustion compared to raw coal combustion may be driven by the higher particle combustion rate and/or temperature of N2–char. Furthermore, the quite severe gasification reaction of CO2 with char during char preparation results in an inhibited combustion rate of CO2–char, which can explain the case in which much less PM0.5 and PM1–10 are generated from CO2–char combustion than those from raw coal and N2–char combustion. The apparent effects reveal that more detailed work can be continued to further explore this research.