Molten salt torrefied biomass as a soil nutrient amendment

Abstract

Torrefaction is a thermal pretreatment process that decreases moisture content and increases energy density in an organic feedstock. This is useful for decreasing the transportation cost of biomass resources while increasing their shelf life. Molten salt torrefaction (MST) results in greater carbonization and lignification than traditional, inert gas torrefaction at the same temperature due to salt's catalytic and heat transfer properties. Biochar, a stable, carbon-rich solid material, can be produced using torrefaction. The charcoal-like characteristics of biochar are valuable not only for fuel use but also for soil amendment. Biochar has a history of use in agricultural applications, including hydrology, nutrient retention, and pH regulation. The separation of biochar from salt used in the molten salt torrefaction process produces nitrate-rich wash water, which also has potential as a soil nutrient amendment. The present study aims to compare the influence of traditional biochar, molten salt-torrefied biochar and wash water on soil nutrient availability and plant responses. Molten salt torrefied biochar formulations were processed in binary and ternary blends of Calcium, Potassium and Lithium Nitrates. Biochar was characterized by its mass yield, lignocellulosic composition, pH, water uptake, chemical composition and electrical conductivity. Processing conditions of sweep gas, temperature, time, lithium content, salt to biomass ratio and particle size were evaluated for their effects on the biochar's characteristics. Biochar composition was found to be most sensitive to temperature, time and lithium content processing conditions. Soil responses to biochar treatment included increased soil pH and nutrient availability. Substantial amounts of lithium and nitrates from the molten salt torrefaction process were retained in both the treated biochar and soil. Durum wheat (Triticum turgidum ssp. durum) was potted with medium-texture soil and switchgrass biochar (Panicum virgatum), applied at 1.5% by weight. The harvested wheat was measured for leaf and root mass yield, plant height and germination. Biochar treatments did not elucidate statistically significant effects in the potted plants. However, plants treated with the wash water had earlier growth and greater root mass, albeit a decreased germination rate. The average root mass nearly doubled with the wash water treatment (64.4mg) compared to the control treatment (35.7mg).