Experimental Investigation of Rice Straw and Model Compound Oxidative Pyrolysis by in Situ Diffuse Reflectance Infrared Fourier Transform and Coupled Thermogravimetry–Differential Scanning Calorimetry/Mass Spectrometry Method

Abstract

The oxidative pyrolysis properties of cellulose, xylan, lignin, and rice straw were studied by thermogravimetric analysis–differential scanning calorimetry (TGA–DSC) coupled with mass spectrometry. The mass loss, reaction heat, and volatile release properties were analyzed to reveal the role of oxygen in the biomass thermal degradation process. Differential thermogravimetry (DTG) results show that the primary mass loss peak was brought forward with the increase of the oxygen concentration for all samples as well as the peak value. Oxygen improved the degradation rate of lignocellulose. The oxidative pyrolysis processes of all four types of material were accompanied by energy consumption or release, generally divided into three stages: moisture release stage, primary pyrolysis stage, and char evolution or oxidation stage. The primary degradation of cellulose under inert and 1% O2 atmospheres was distinctly endothermic. With the increase of the oxygen concentration, the endothermic peak decreased, while an exothermic peak dominated the oxidative process. Xylan and lignin showed an exothermic primary degradation peak even under an inert atmosphere at the primary pyrolysis stage, and with the increase of the oxygen concentration, the reaction heat released at the primary and char oxidation stage increased. Rice straw showed weak endothermic properties in the primary stage. Volatile compound analysis of oxidative pyrolysis indicated that oxygen promoted the yields of water and permanent gas compounds, such as CO2, CO, and CH4. The yield of condensable compounds, such as benzene, reached a maximum at a mediate oxygen concentration, and too much oxygen would lead to being combusted out completely. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of three model compounds and rice straw under inert and oxidative atmospheres indicated that oxygen played a less important role at a low-temperature stage, especially for cellulose, which was kind of a uniform structure with less active function groups. Heterogenous oxidation at a relatively high temperature (>400 °C) would lead to the degradation of some weak bonds and benefited the formation of an aromatic ring.