The Influence of Microwave Drying on Biomass Pyrolysis

Microwave drying is a rapid, high-effect, and economical method to decrease the moisture content of biomass materials to produce higher quantity and better quality bio-oil during the fast pyrolysis process. In this paper, the influence of microwave drying on the behavior of biomass pyrolysis was investigated in a bench-scale fluidized-bed reactor in comparison to drying in an electrical oven at 105 °C. Three types of biomass were studied, comprising pine wood sawdust, peanut shell, and maize stalk. First, biomass samples were dried using a microwave oven at four different power levels, i.e., 200, 400, 600, and 800 W. It was found that the power capacity of the microwave showed great influence on the moisture removing property. The dehydration was very fast, and it was finished in 6 min at 600 W; however, it was quite slow with electrical oven drying (about 40 min). The specific surface area of biomass was found, being increased greatly after microwave drying. Then, the pyrolysis of biomass samples dried by two different methods was carried out in the bench-scale reactor at 500 °C. Samples dried at 600 W in 6 min were selected as representative for microwave drying. The yields and properties of pyrolysis products, including liquid oil, solid char, and uncondensable gas, were determined and analyzed. It was observed that the yields of solid char and liquid oil increased, while the yield of the gas product decreased by microwave oven drying compared to conventional electrical oven drying. A higher CO2 concentration in gas but a lower organic compound content in char was achieved using microwave drying. At the same time, the viscosity and heating value increased because the water content decreased and the average molecular weight increased in the bio-oil from the samples dried by the microwave. These results were all caused by the fact that microwave drying suppressed the second reaction of volatiles during biomass pyrolysis. A higher heating value and lower water content are beneficial to bio-oil use as a fuel, and more primary pyrolysis products make bio-oil produce high add-value chemical materials. Thus, microwave drying is a technically and economically feasible pretreatment method for biomass fast pyrolysis.