Microwave-assisted Acid Pretreatment Of Biomass For Fast Pyrolysis

Lignocellulosic biomass is a promising alternative feedstock for bio-oil production.It includes agricultural residues, cellulosic wastes, municipal wastes, herbaceous andwoody crops. Lignin is a component of biomass that provides strength for the cell walland thus causes the recalcitrance of biomass to thermally conversion. Moreover, ligninproduces significant amounts of coke during thermal conversion, which would causedeactivation of catalysts. Removal of the lignin component via pretreatment has beenshown to be able to improve the conversion efficiency of biomass in thermal conversion.The main goal of this research is therefore to investigate microwave-assisted acidpretreatment for lignin removal and increase the conversion efficiency of biomass insubsequent fast pyrolysis and catalytic fast pyrolysis (CFP).This study evaluated microwave-assisted acid pretreatment and water bath heatedacid pretreatment of beech wood sawdust. In addition, two-stage acid (per acetic acid,PAA) and alkaline (NaOH) pretreatment was also studied. After the pretreatment,biomass was subject to fast pyrolysis and catalytic fast pyrolysis to produce bio-oils andaromatic hydrocarbons, respectively.Microwave-assisted formic acid (88%v/v) pretreatment effectively destructed thestructure of beech wood and increased its cellulose content and crystallinity byremoving85%hemicellulose and42%lignin component of beech wood. The charformation of pretreated beech wood was only9.5%, which was much lower than rawbeech wood. After microwave-assisted formic acid pretreatment for10min under800W and1200W irradiation, the yield of cellulose-derived levoglucosen (LG) increasedby933.8%and879.7%, respectively in fast pyrolysis, while the yield ofhemicellulose-derived acetic acid decreased by64.9%and72.2%. Lignin-derivedphenols also decreased by43.0%and59.5%. For CFP, the aromatic yield increasedfrom22.46%for raw beech wood to27.01%(carbon yield) and26.11%bymicrowave-assisted formic acid pretreatment for10min under800W and1200Wirradiation. The results showed that microwave-assisted formic acid pretreatmentincreased aromatics production and decreased the coke production.Microwave-assisted PAA (15%v/v) pretreatment also effectively destroyed thestructure of beech wood, and increased its cellulose crystallinity. The FTIR peaksoriginated from lignin decreased or disappeared, indicating that the pretreatment was effective at removing lignin. According to the TG analysis, the peak of hemicellulosedisappeared for the pretreated biomass, indicating the removal of hemicellulose by themicrowave-assisted pretreatment. Besides, the char yield of pretreated beech wood wasonly10.1%, which was much lower than raw beech wood. Therefore, the pretreatmentcan decrease catalyst deactivation by coke deposition, which a major obstacle in CFP ofbiomass.Two stage acids (PAA+NaOH) combination and water bath-assisted pretreatmentdestroyed the structure of beech wood, and increased its crystallinity largely. Besides,the FTIR peaks originated from both hemicellulose and lignin were diminished,indicating the two components were effectively removed by the pretreatment. TGanalysis further confirmed that hemicellulose and lignin were removed from thebiomass. The char yield was decreased to12.1%for the pretreated beech wood sample.The results indicate that microwave-assisted acid pretreatment can effectively improvethe conversion efficiency of biomass in thermal conversion.