Pyrolysis of spruce wood, wheat straw, switchgrass, miscanthus and swine manure using catalysts containing alkali and alkaline earth metals: thermogravimetric analysis and kinetic modeling

Abstract

This paper examines the pyrolysis of 5 biomass types (spruce wood, wheat straw, switchgrass, miscanthus and swine manure) when being catalyzed by additives (NaCl, KCl, CaCl2 and MgCl2) containing alkali and alkaline earth metals (AAEMs). Thermogravimetric analyses (TGA) were carried out with raw samples and with catalyzed ones prepared by wet impregnation. 4 different heating rates (5, 10, 15 and 30 K min−1) were used, and the associated experimental data were modeled by implementing 3 isoconversional approaches (Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (OFW) and Friedman). The inferred rate constant parameters were then used to compute the variations of the conversion degree of the fuels versus the temperature while considering different reaction mechanisms commonly employed in the literature, including order-based, diffusion, geometrical, nucleation and power law models. As highlights, the results obtained revealed that AAEM catalysts shift the decomposition process to lower temperatures. Besides, conversion profiles computed using each tested modeling approach were found to properly reproduce the TGA results as long as order-based models were selected. An analysis of the kinetic parameters estimated when implementing the three above-listed isoconversional methods showed that the rate constants tend to increase when adding catalysts, with CaCl2 and MgCl2 being found to exhibit a stronger capacity to increase pyrolysis rates as compared to NaCl and KCl. Finally, a sensitivity analysis focusing on the impact of the catalyst load on the pyrolysis kinetics revealed a so-called saturation phenomenon indicating that the AAEMs tend to promote the conversion of biomass more effectively when the concentration of metal cation in the impregnating solution is relatively low (not higher than ⁓0.15 mol L−1 in the present work).