Catalytic effects of potassium and magnesium on fast pyrolysis of yellow poplar and characteristics of pyrolytic products

Abstract

The goal of this study was to investigate the catalytic effects of inorganic metals, especially potassium and magnesium, on the pyrolysis of lignocellulosics and the physicochemical properties of the corresponding products. In this study, thermogravimetric analysis (TGA) and fast pyrolysis were performed with demineralized and inorganic metal-impregnated yellow poplar wood. The fast pyrolysis was carried out at 450°C, 500°C, and 550°C with a residence time of 1.3 s. TGA results suggested that the maximum decomposition temperature of the biomass was more influenced by magnesium than potassium as it decreased from 373.9°C for demineralized biomass (D-YP) to 359.0°C for biomass with 2.0 wt% of potassium-impregnation (K-2.0) and to 337.6°C for biomass with 2.0 wt% of magnesium-impregnation (Mg-2.0). During the fast pyrolysis, the char yield was greatly influenced by inorganic metals, especially potassium. The char yield of potassium-impregnated biomass was doubled regardless of the pyrolysis temperature as compared to demineralized yellow poplar. The presence of potassium and magnesium also affected the properties of the bio-oil. With increasing potassium concentrations, the water content increased from 14.4 wt% to 19.7 wt% and the viscosity decreased from 34 cSt to 16 cSt, while the pH of the bio-oil remained stable. Magnesium induced a dehydration reaction leading to an increase in the water content, while the viscosity increased from 45 cSt to 216 cSt and the pH decreased. Gas chromatography-mass spectroscopy (GC-MS) analysis revealed that potassium promoted thermochemical reactions, thus causing a decrease in the amount of levoglucosan and an increase in that of small molecules and lignin-derived phenols in the bio-oil. Magnesium, however, acted as a catalyst for the recombination reaction of levoglucosan and other small molecules to form large fractions such as oligomers and char particles in the bio-oil. Following the pyrolysis, most of the inorganic metals were distributed in the bio-char and the remaining amount depended on the temperature of the reaction. Additionally, various forms of aromatic hydrocarbons, probably derived from lignins, were detected in the non-condensed pyrolytic gas fractions.