The Characteristics Of Low Temperature Oxidation Process Of Biomass Chars And The Impact Of Inorganic Matter

Low temperature oxidation is the main mechanism of self-heating and spontaneous combustion of biomass char during storage, while the inorganic matter of the biomass may have the impact on the reactivity of the char low temperature oxidation. Therefore, understanding the characteristics of low temperature oxidation process is so important and necessary to describe the process of the self-heating, evaluate of self-heating and spontaneous combustion tendency, and prevent the spontaneous combustion during the production, transport, and storage of biomas chars. For these purposes, two sets of the samples (cornstalk and water-leached cornstalk, cellulose and KCl-doped cellulose) were pyrolyzed at different temperature to generate chars. Two thermal analysis methods, i.e., simultaneous thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC) and isothermal calorimetry, were applied to examine low temperature oxidation and its heat evolution characteristics of the chars. Based on the structure analysis, in-situ diffuse reflectance infrared Fourier Transform spectroseopy (in-situ DRIFTS) was employed to monitor the evolution of char surface functional groups under non-isothermal and isothermal heating conditions. Through the systematical comparison between various chars, the influences of inorganic matter and heat treatment temperature on the characteristics of low temperature oxidation of biomass chars were investigated.The results of TGA-DSC and isothermal calorimetry of the 500? chars showed that, at the low temperature oxidation prior to ignition, the reactivity of the cornstalk chars is stronger than that of the water-leached cornstalk char, and the reactivity of KCl-doped cellulose char is slightly stronger than that of the cellulose char. The inorganic matter particularly K in the biomass apparently increases the reactivity of low temperature oxidation of biomass chars. The treatment of water leaching can reduce the heat evolution reactivity of the cornstalk char close to that of the cellulose char. It implied that the method of water leaching biomass to remove the active inorganic matter is able to reduce the self-heating and spontaneous combustion tendency of the resulting char.The results of TGA-DSC and isothermal calorimetry of the cornstalk and water-leached cornstalk chars generated at different temperatures showed that the heat treatment temperature affects the biomass thermal conversion process and the resulting chars properties, and consequently affects the characteristic of char low temperature oxidation. The chars generated at 400? and 500? have a stronger power of heat release, while 600? and 250? chars have a weaker power. When the corustalk and water-leached formed at the same pyrolysis temperature were compared, the heat release intensities of the cornstalk chars are higher than those of the water-leached cornstalk chars. It means that the active inorganic matter has a catalytic effect on the low temperature oxidation of the cornstalk chars, which evidences that the treatment of water leaching is an effective method to reduce the self-heating and spontaneous combustion tendency of biomass chars.The analysis of in-situ DRIFTS showed that the evolution of surface functional groups is mainly presented as the increase of the C=O groups and the decrease of C-H and O-H groups during the chars exposed to air under low temperature oxidation. The heating temperature and time have a significant effect on the total amounts of C=O and C-H groups. The higher heating temperature and the longer heating time, the stronger the total integrated area intensities of their bands. Under the same heating conditions, the chars generated at 400? and 500? have more formation of C=O groups and reduction of C-H groups, indicating their stronger reactivity of low temperature oxidation. Reaction temperature has a greater impact on the formation of carboxylic mode at 1600-1500 cm-1 and ester modes at 1735 cm-1 and 1775-1765cm-1, while has a weaker impact on the C-H groups. The inorganic matter may promote the decomposition and desorption of C=O groups on char surface and provide active sites overtaking aliphatic C-H and O-H sites for O2-char reactions at low temperatures. The inorganic matter affects the formation behavior of total C=O and C-H groups but may not have siginificant influences on the formation behavior of the individual C=O groups and reduction behavior of the individual C-H groups.