Lignite upgrading by multi-stage fluidized bed pyrolysis

This study is devoted to demonstrating experimentally the technical advantages of the multi-stage fluidized bed pyrolysis for upgrading lignite. A Chinese lignite was pyrolyzed and partially gasified in a three-stage laboratory-scale fluidized bed, with an overflow standpipe between its neighboring stages, to clarify the improvement on the pyrolysis product quality by increasing the number of the stages. While the bottom stage had the highest temperature of about 900 °C for fuel gasification, the upper stage had temperatures of 550–650 °C for coal pyrolysis. The multi-stage fluidized bed was operated with a continuous feed in the modes with one to three stages. The resulting yields of gas and tar were higher, whereas the yield of char was lower for the operations with multiple stages. The produced CO, H2 and CH4 in the two- and three-stage modes were more than that in the single-stage mode, having thus the higher gas heating value as well. The tar from the three-stage fluidized bed pyrolysis contained more light oil, and it plus phenol oil reached 99.5 wt.% of the tar. The char produced in the multi-stage pyrolysis showed the higher thermal stability in terms of its higher ignition temperature and suppressed spontaneous combustion propensity.