Comparative study of electric-heating torrefaction and solar-driven torrefaction of biomass: Characterization of property variation and energy usage with torrefaction severity

Abstract

Torrefaction is a promising pretreatment for upgrading biomass raw materials, which is traditionally performed using electric heating. In this study, torrefaction of three biomass materials (peanut shell, soybean straw, and pine wood) was performed using a new solar-driven torrefaction (Solar-T) in a temperature range of 200–300 °C for 15–45 min. Property variation and energy usage required for electric-heating torrefaction (Electric-T) of biomass was also analyzed for comparison. Results showed that the properties of torrefied biomass were greatly affected by both biomass species and torrefaction methods. Among the three biomass materials, soybean straw had the lowest solid yield (43.2% in Electric-T, 42.9% in Solar-T) and energy yield (50.8% in Electric-T, 51.3% in Solar-T) under the most severe torrefaction condition (300 °C, 45 min). A linear correlation with a high correlation coefficient was obtained between the biomass conversion (decarbonization, dehydrogenation, deoxygenation) and the torrefaction severity index. The deoxygenation efficiency of peanut shell, soybean straw, and pine wood in Solar-T (300 °C, 45 min) was 2.0%, 1.6%, and 1.2% higher than the corresponding value in Electric-T, respectively. Energy usage results showed that electrical energy consumed during Electric-T increased with increasing torrefaction severity, but the energy efficiency decreased. The remarkable advantage of Solar-T was energy savings, because no electrical energy was required for Solar-T. Considering the energy-mass co-benefit index, solar-torrefied biomass samples were more transportable than the samples torrefied using Electric-T. Therefore, Solar-T has the potential to replace Electric-T and improve the competitiveness of biomass-derived products.