Torrefaction of agriculture straws and its application on biomass pyrolysis poly-generation

•Torrefaction darkend the color and improved the grindability of agricultural straws.•Torrefaction had different effects on char, liquid oil and biogas during pyrolysis.•Char yield of pyrolysis torrefied straw increased and its fuel property not changed.•Liquid oil is upgraded and concentrated phenols with less water content below 40wt.%.•Biogas is concentrated H2 and CH4 with higher LHV up to 15MJ/nm3. This study investigated the properties of corn stalk and cotton stalk after torrefaction, and the effects of torrefaction on product properties obtained under the optimal condition of biomass pyrolysis polygeneration. The color of the torrefied biomass chars darkened, and the grindability was upgraded, with finer particles formed and grinding energy consumption reduced. The moisture and oxygen content significantly decreased whereas the carbon content increased considerably. It was found that torrefaction had different effects on the char, liquid oil and biogas from biomass pyrolysis polygeneration. Compared to raw straws, the output of chars from pyrolysis of torrefied straws increased and the quality of chars as a solid fuel had no significant change, while the output of liquid oil and biogas decreased. The liquid oil contained more concentrated phenols with less water content below 40wt.%, and the biogas contained more concentrated H2 and CH4 with higher LHV up to 15MJ/nm3.