Study of the decomposition of kraft lignin impregnated with orthophosphoric acid

Abstract

The aim of this study was to analyze the pyrolysis of Kraft lignin impregnated with orthophosphoric acid by thermogravimetry (TG-DTG). We studied the effect of various parameters on both the char yield and the rate of mass loss: heat treatment temperature up to 650 °C, impregnation time, inclusion of isothermal periods, acid to lignin mass ratio (P/L) and gaseous atmosphere. Decomposition of pure lignin showed two maxima in the mass loss corresponding to evolution of moisture at 92 °C and to lignin decomposition in a broad temperature range from 150 to 650 °C, respectively. When orthophosphoric acid was added, lignin dehydration proceeded to a larger extent, decomposition occurred in a narrower temperature range and decomposition ended at lower temperatures with higher char yields. There was an optimum P/L at values between 0.8 and 1.0, and further increasing P/L had low influence on the decomposition mechanisms. Differential Thermal Analysis (DTA) showed that reactions occurring upon impregnation of lignin with orthophosphoric acid at room temperature are finished after only 1 h, which confirmed the TG-DTG results. Impregnation times longer than 1 h and inclusion of isothermal periods did not affect significantly the subsequent char yield. Concerning the gaseous atmosphere, identical char yield were obtained whether the samples be prepared in nitrogen or in air at 450 °C. However, decomposition in air at 650 °C produced a decrease in the char yield when compared to pyrolysis in nitrogen due to the evaporation of P 2O 5 and the subsequent oxidation of the unprotected carbon.