Effects of hot water extraction pretreatment on pyrolysis of shrub willow

Abstract

In this study, we tested the effect of hot water extraction (HWE) as a biomass pretreatment on the pyrolysis of three cultivars of shrub willow via both conventional heating (using a micropyrolyzer, Py-GC/MS) and microwave-assisted heating (using a laboratory scale microwave reactor and activated charcoal as an added microwave absorber). The py-GC/MS experiments revealed that there was little difference in pyrolysis behavior among the cultivars for raw or HWE pretreated samples. Using either heating method, pyrolysis of the HWE pretreated samples produced less acetic acid and CO2 than did the untreated biomass; conversely there was an increase in levoglucosan yield with HWE pretreatment. The difference in levoglucosan yield was particularly large (4 fold increase) for the py-GC/MS experiments and was attributable in large part to the demineralization of the HWE samples. The decreased mineral content appeared to have a larger effect on conventional heating than in the microwave assisted heating. The pyrolysis of HWE pretreated biomass also led to more rapid temperature increases during microwave-assisted pyrolysis performed at 1000 W. Therefore the microwave-assisted pyrolysis of HWE was studied at two different microwave power settings to compare the effect of HWE on both processes at similar temperatures. At similar temperature conditions the yield of bio-oil, bio-char and non-condensable gases from microwave-assisted pyrolysis were similar between the pretreated and raw willow but the bio-oil contained higher concentrations of aromatic hydrocarbons and less acetic acid and levoglucosan. Overall, the HWE pretreatment had a greater effect on bio-oil composition than bio-oil yield.