Session: 12-04: Hydrogen Energy, Alternative Fuels, Bioenergy, and Biofuels

Paper Number: 131470

131470 - Investigation on the Catalytic Ability of Alkaline Earth Metal Cations and Nitrite Anions on Low Temperature Molten Salt Torrefaction 

Abstract: 

Biomass torrefaction is a thermochemical treatment that improves the fuel and soil amendment properties of lignocellulosic material. Typically, torrefaction occurs in an inert, gaseous environment. Recently, thermochemical treatment of biomass in molten salts has been explored as a way to improve heat transfer to the biomass and catalyze the reaction. Prior work investigated the catalytic effect of alkali metal nitrates with high lithium nitrate contents on solid products. High lithium nitrate compositions were investigated due to lithium’s catalytic ability. However, blends containing alkaline earth metal nitrates and low levels of lithium nitrate have not been studied as well. Additionally, molten salts containing nitrite anions have been studied for higher temperature pyrolysis and gasification processes to reveal their capacity to promote liquid and gaseous fuel production. However, there has not been any research on how nitrite anions affect low temperature torrefaction and solid yields. As such, the present study aims to investigate the catalytic ability of alkaline earth metal nitrates (calcium and magnesium nitrate), of low lithium-containing salt blends, and of nitrite anions on low temperature molten salt torrefaction emphasizing the impact on the solid products. To achieve this, a variety of binary, ternary, and quaternary salt blends are used to torrefy ponderosa pine and cave-in-rock switchgrass. The severity of torrefaction is determined by analyzing key metrics of the torrefied biomass such as mass yields and higher heating values. The relative catalytic ability of calcium and magnesium cations compared to lithium, sodium and potassium cations are assessed by comparing the torrefaction severity of different nitrate blends. The torrefaction severity of low-lithium containing blends are compared to high lithium, and no lithium containing blends. Lastly, similar methods are utilized to elucidate the reactive nature of nitrite anions on molten salt torrefaction which highlight the relative importance of sodium nitrite (NaNO2) compared to lithium nitrate (LiNO3) and sodium nitrate (NaNO3). The results of the study show that although lithium is the most catalytic cation, calcium appears to be more catalytic than potassium and shows similar catalytic ability to sodium. Torrefaction with magnesium nitrate produces a solid, non-water-soluble byproduct which results in undesirably low solid, torrefied lignocellulosic mass yields. Very low-lithium contents provide a notable increase in torrefaction severity showing comparable torrefaction to salts with moderately higher levels of lithium nitrate. The reactivity of nitrite anions is found to provide severe decomposition even at low temperatures which requires torrefaction to take place below 225°C to retain substantial solid product. Overall, the results indicate that salt compositions can be tailored using calcium nitrate and sodium nitrite to reduce and possibly eliminate lithium content for low temperature torrefaction while maintaining adequate torrefaction severity. Similarly, small amounts of lithium nitrate may be suitable for adequate torrefaction which would preventing extensive lithium nitrate use.

Presenting Author: Lee Kohlin North Dakota State University

Presenting Author Biography: Graduate student at North Dakota State University pursuing a masters degree in mechanical engineering. Currently researching molten salt torrefaction under the advise of Dr. Adam Gladen.

Authors:

Lee Kohlin North Dakota State University
Hayden Pritchard Montana State University
Behrooz Heidari Dehkordi North Dakota State University
Adam Gladen North Dakota State University
Dilpreet Bajwa Montana State University