Effect of pyrolysis upgrading temperature on particulate matter emissions from lignite semi-char combustion

Highlights: • Pyrolysis upgrading temperature has an effect on particulate matter emissions. • Char produced at 350 °C has the best combustion reactivity. • Char produced at 250–350 °C has the lowest particulate matter emissions. -- Abstract: Low-temperature pyrolysis is one of the most promising methods for lignite upgrading and staged utilization, however, its effect on particulate matter emissions from the combustion of upgrad lignite has not been reported before. In this paper, Xilin Gol lignite was pyrolyzed in a fixed-bed reactor at 250–550 °C, the produced semi-char was characterized, and then burned in a drop tube furnace at 1300 °C. Particulate matter emissions from lignite semi-char combustion are collected and measured passing through a 13-stage impactor. Results show that the particulate matter emissions of lignite combustion can be significantly affected through the change of fuel properties by low-temperature pyrolysis upgrading. In the studied pyrolysis temperature range, the most drastic devolatilization, fragmentation, porous structure development, and removal of reactive functional group occur at about 350 °C, when the produced semi-char has the highest combustion reactivity. With the increase of pyrolysis temperature, both emissions of particulate matter <0.5 μm and particulate matter in the range of 0.5–10 μm first decrease and then increase. Pyrolysis upgrading significantly reduces the emissions of particulate matter <0.5 μm from lignite combustion, and the combustion of semi-char produced at 350 °C has the lowest emissions of particulate matter <0.5 μm. The lowest emissions of particulate matter in the range of 0.5–10 μm are obtained from the combustion of semi-char produced at 250 °C. In particulate matter emissions from semi-char combustion, the ultrafine particles <100 nm are rich in sulfur, magnesium, sodium, phosphorus and ferrum while the coarse ones are rich in silicon and aluminum. The influencing mechanism of pyrolysis temperature on particulate matter emissions is complicated and includes many factors, requiring further study. This study indicates that low-temperature pyrolysis upgrading is efficient to control particulate matter emissions from lignite combustion.