(277b) Engineering Next Generation Biomass Feedstocks: Effect of Torrefaction On Gasification Products

Growing energy demand worldwide requires a solution that is both domestic and stable, regardless of the region. Renewable energy sources have been implemented and proven in affluent nations in Europe and North America, but it remains that these technologies only satisfy a fraction of the energy requirement globally. Biomass-derived energy sources are abundant, carbonaceous, diverse and domestic. Low thermal efficiencies and uneconomical process operation have limited the implementation of these sources. However, research for cost reduction and process improvement over the last 30 years has demonstrated that economical biomass-derived energy is achievable. In this work, we gasified several traditional and advanced feedstocks at 800°C and 1 atm in a 10.2 cm i.d. gasifier with a continuous solid feeder, hot char removal, and a dodecane scrubber for vapor condensation and separation. Gas chromatography (GC), non-dispersive infrared spectroscopy (NDIR), and a diode laser were used to speciate and quantify non-condensable gases, hydrogen sulfide, and ammonia in the raw syngas stream. Molecular beam mass spectrometry (MBMS) was used to measure and quantify tar species in the hot vapor product stream (~425°C). Idaho National Laboratory (INL) provided all feedstocks including: torrefied “clean” and “whole” southern pine, non-pretreated clean and whole southern pine, oak, and switch grass. Bark, branches, and needles were removed from “clean” southern pine while “whole” pine comprises the whole tree. Torrefaction (thermal pretreatment) was performed at 270°C in a fluidized sand bed, and the resultant solid sample was sieved for feeding to the gasifier. Non-condensable gas quality (e.g. CO, H₂, CO₂, CH₄, C₂H₄) did not vary greatly between the feedstocks as has been shown in recent work in the literature.¹ The torrefied feedstocks had markedly lower tar concentrations: 17.4% for untreated whole pine and 10.2% for torrefied whole pine. Recent studies have demonstrated that torrefaction deacetylates hemicellulose in the biomass thereby breaking down a main plant constituent without degrading the majority of the biomass (> 90% mass retention).²Based on work in the literature and results from this study, torrefaction reduced the tar content by unraveling the hemicellulose and cellulose polymers in the native biomass. Additionally, oxygen reduction during torrefaction increased the energy density of the biomass feedstock (17 MJ/kg to 19.6 MJ/kg). This densification makes the feedstock easier to transport, grind and pelletize. Future work will focus on using thermal pretreatment to homogenize blended biomass feedstocks to increase the pool of feedstocks resources to include more waste materials.

1.            Couhert, C.; Salvador, S.; Commandre, J.-M., Impact of torrefaction on syngas production from wood. Fuel 2009, 88, 2286-2290.

2.            (a) Melkior, T.; Jacob, S.; Gerbaud, G.; Hediger, S.; Le Pape, L.; Bonnefois, L.; Bardet, M., NMR analysis of the transformation of wood constituents by torrefaction. Fuel 2012, 92, 271-280; (b) Tumuluru, J. S.; Sokhansanj, S.; Hess, J. R.; Wright, C. T.; Boardman, R. D., A review on biomass torrefaction process and product properties for energy applications. Industrial Biotechnology 2011, 7 (5), 384-701.