Furfural production from microwave catalytic torrefaction of Douglas fir sawdust

The torrefaction bio-oil is composed of a number of chemical compounds with low concentrations, which results in a low efficiency of its application. To improve the concentrations of high-value chemicals such as furfural in the torrefaction bio-oil, four metal chlorides and hydrochloric acid were investigated as catalysts in the torrefaction of Douglas fir by microwave heating. The number of chemical compounds in the attained bio-oil was significantly reduced with introducing MgCl₂, FeCl₃, and AlCl₃ being into the process of torrefaction. The bio-oil obtained with these three catalysts were dominated by furfural with the concentration over 20 mg/g with the torrefaction temperature, time, and catalyst/reactant ratio of 250 °C, 10 min, and 8 wt.%, respectively. The effects of torrefaction conditions on product yield were investigated by selecting MgCl₂ as the catalyst, and the results indicated that the yield of torrefied biomass decreased significantly with the increase of reaction temperature, time, and catalyst loading while the yield of bio-oil collected simultaneously was increased. Further analysis for the bio-oil suggested that low temperature, long reaction time, and high catalyst loading favored furfural production. The analysis for non-condensable gas suggested that high reaction temperature and catalyst loading promoted the methane and carbon monoxide production during torrefaction. According to bio-oil and non-condensable gas analysis, a reaction pathway was proposed to explain biomass catalytic torrefaction using MgCl₂.