Effects of hot water extraction pretreatment on pyrolysis of shrub willow

Authors: Tarves, Paul; Serapiglia, Michelle; Mullen, Charles; Boateng, Akwasi; VOLK, TIMOTHY - State University Of New York (SUNY)

Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2017
Publication Date: 11/7/2017
Publication URL: https://handle.nal.usda.gov/10113/5848746
Citation: Tarves, P.C., Serapiglia, M., Mullen, C.A., Boateng, A.A., Volk, T.A. 2017. Effects of hot water extraction pretreatment on pyrolysis of shrub willow. Biomass and Bioenergy. 107:299-304.

Interpretive Summary: Short rotational woody crops such as shrub willow are being considered as biomass sources to be converted to renewable fuels and petrochemical replacements. One process to convert the biomass to a dense liquid is fast pyrolysis, which is the rapid heating in the absence of oxygen. This liquid product, called bio-oil, has great promise as an intermediate to the production of renewable fuels but has some problems that make it difficult to further refine, most of which are attributable to a high concentration of reactive oxygenated compounds including acids. Various pretreatments of the biomass can alter the composition of the biomass, perhaps benefiting the properties of the bio-oil. One pretreatment method being considered is hot water extraction (HWE). The extraction reduces the amount of hemicellulose and ash (minerals) in the biomass. In this study, we tested the effect of HWE as a biomass pretreatment on the pyrolysis of shrub willow via both conventional heating and microwave induced heating. Using either heating method, pyrolysis of the HWE pretreated samples produced less acidic compounds; conversely when using the pretreated biomass there was an increased production of levoglucosan, a dehydrated sugar compound. The increase in levoglucsoan yield was particularly large (4 fold increase) for the conventionally heated experiments. Therefore, the HWE pretreatment appeared to have a larger effect on pyrolysis via conventional heating than on pyrolysis via microwave induced heating. This information will be valuable to those designing pyrolysis based biorefinery processes.

Technical Abstract: Treatment of biomass via hot water extraction (HWE) reduces the amount of hemicellulose and ash in the biomass resulting in a concentration of cellulose and lignin content. In this study, we tested the effect of HWE as a biomass pretreatment on the pyrolysis of shrub willow via both conventional heating (using a micropyrolyzer, Py-GC/MS) and microwave induced heating (using a laboratory scale microwave reactor). 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 ash content appeared to have a larger effect on conventional heating than in the microwave induced heating. The HWE pretreatment also increased the surface area of the biomass, leading to more rapid temperature increases during microwave pyrolysis experiments performed at 1000W. Therefore the 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 pyrolysis were all similar between the pretreated and raw willow. Deoxygenated aromatic hydrocarbons (e.g. benzene, toluene, xylenes) were only produced via microwave heating and were increased when HWE biomass was used. The overall yields and product distributions obtained from the microwave pyrolysis experiments suggest the HWE process has a greater effect on bio-oil composition than bio-oil yield.