Microwave Torrefaction of Corn Stover and Tech-Economic Analysis

Microwave torrefaction of corn stover with particle size of 4 mm was investigated and the effects of reaction temperature and time on the yields of volatile, bio-oil and torrefied biomass were determined. The response surface analysis of the central composite design (CCD) showed that the yields of volatile, bio-oil and torrefied biomass were significantly affected by the reaction temperature and time. Three linear models were developed to predict the yields of conversion products as a function of temperature and time. A first order reaction kinetics was also developed to model the corn stover torrefaction. Ph values of torrefaction bio-oils ranged from 2.3 to 2.76 which were similar to those of bio-oils from biomass pyrolysis. GC/MS analysis for torrefaction bio-oils showed that the organic acid was about 2.16% to 12.00%. The torrefaction bio-oils also contain valuable chemical compounds such as phenols, furan derivatives and aliphatic hydrocarbons determined by a GC/MS. There are no aromatic compounds and polycyclic aromatic hydrocarbons (PAHs) detected in the torrefaction bio-oils. The torrefaction biogas was mainly consisted of ch₄, c₂h₆, c₃h₈, which was about 56 wt% of the total bio-gas. The biogas can be used for chemical synthesis or electricity generation. The heating values of torrefied biomass were from 18.64–22.22 MJ/kg depending on the process conditions. The heating values of torrefied biomass were significantly greater than those of raw biomass and similar to those of coals. The energy yields of torrefied biomass from 87.03–97.87% implied that most energy was retained in the torrefied biomass. Economic analysis indicated that the biomass microwave torrefaction plant located in a farm is profitable.