Molten nitrate salt torrefaction of switchgrass for producing nitrogen-enriched biochar

Torrefaction is a thermal pretreatment process that increases nutrient density and surface functionality in an organic feedstock. Biochar, a stable, carbon-rich solid material, has a history of use in agricultural applications, including hydrology, nutrient retention, and pH regulation. Molten salt torrefaction (MST) results in greater carbonization and lignification than traditional, inert gas torrefaction at the same temperature due to the salt's catalytic and heat transfer properties. The separation of biochar from salt used in the molten salt torrefaction process produces nitrate- and bio-oil-rich wash water, which also has potential as a soil nutrient amendment. Molten salt-torrefied biochar formulations were processed in binary and ternary blends of calcium, potassium and lithium nitrates. Biochars and soils were characterized by their mass yield, pH, chemical composition and electrical conductivity. Biochar nutrient content became more sensitive to changes in process temperature with higher concentrations of catalytic salts in the torrefaction process. MST biochar had up to 8x as much cation content as inert gas-torrefied samples, but there were no trends involving cation availability in treated soil. Molten nitrate salt-torrefied biochar contained up to 359% of the total nitrogen content of the original biomass and increased the nitrogen availability of treated soil by up to 83%. Wash water treated soil had 273% more bioavailable nitrogen than the control, along with a 135% increase in salinity. MST biochar application was demonstrated to increase soil pH levels by up to 1.15 pH units at 1.5% application by weight.