Molten salt biomass torrefaction – A sensitivity analysis of process conditions

Abstract

Biomass is an abundant renewable resource that can be upgraded via torrefaction. Molten salts catalyze the torrefaction reaction, creating enhanced products for fuel and soil amendment purposes at lower temperatures than inert gas torrefaction. The objective of this study is to elucidate the main effects of molten salt torrefaction process conditions on ponderosa pine (Pinus ponderosa) and cave in rock switchgrass (Panicum virgatum) in a binary salt blend of lithium nitrate and potassium nitrate using a Plackett-Burman screening analysis. The investigated process conditions include sweep gas, temperature, salt to biomass ratio (S-B ratio), residence time, and lithium content. The metrics used to evaluate torrefaction severity include mass yields, chemical composition (lignin, cellulose, hemicellulose, extractives), higher heating value (HHV), carbon and nitrogen content, pH, and water sorption. The results show that switchgrass is more severely torrefied through molten salt torrefaction than pine at the same process conditions. For example, switchgrass mass yields are on average 23.3 % lower than pine mass yields across the test conditions. For both feedstocks, the most impactful process conditions are temperature, time, and lithium content in that order with some exceptions. For instance, the effect of temperature, time and lithium content on HHV are, respectively, 3.4×, 2.3×, and 1.7× larger than the next largest process condition for pine, whereas for switchgrass, these values are 3.6×, 2.7×, and 1×. Particle size, sweep gas, and S-B ratio have minor effects depending on the metric, but are overall not significant compared to temperature. The data suggests that an inert gaseous environment need not be maintained to facilitate molten salt torrefaction. Additionally, molten salt torrefaction can produce torrefied biomass with slightly different characteristics than inert gas torrefaction.