Detailed Analysis of Residual Volatiles in Chars from the Pyrolysis of Biomass and Lignite

Pyrolysis of biomass or coal, if operated at temperature high enough to complete tar evolution, is expected to give resulting char a tar-free nature. Use of such tar-free char instead of the original fuel in gasification would reduce or even completely eliminate the need for use of complex devices/mechanisms for tar/soot removal. Increasing pyrolysis temperature may not only decrease the content of residual tar in the char but also reduce its reactivity in the gasification. There is thus a range of optimum pyrolysis temperatures depending on the original fuel and type/mode of gasification, which yields char of suitable quality. In this work, a variety of char samples were prepared by pyrolysis (first pyrolysis) of three different woody biomass feedstock and a Victorian lignite with heating rate and peak temperature of 10 degrees C/min and 450-750 degrees C, respectively, and were further subjected to flash pyrolysis (second pyrolysis) at 920 degrees C. A gas-chromatography/ mass-spectrometry (GC/MS) detected more than 200 compounds in the volatile products from the second pyrolysis and quantified 58 aromatic compounds over a range from benzene to coronene, which accounted for more than 85% of the compounds detected based on peak area on the total ion chromatogram. Total emission of tar, defined as the aromatics except monoaromatic hydrocarbons, from the biomass chars was 0.03-0.08 wt %-char even at the first pyrolysis temperature of 450 degrees C and further decreased to a level around 0.01 wt % by raising the temperature to 600 degrees C. It was also found that despite containing as much as 20-27 wt % of residual volatile matter, chars produced by pyrolysis at 500 degrees C contained less than 0.1 wt % of residual tar.