Roles And Controlling Mechanisms Of Additives In Coal Oxidation At Low And Moderate Temperatures

Coal oxidation at low and moderate temperatures is the major phenomenon preventing a safe mining and effective utilization of coal. Aiming at furthering the understanding of coal oxidation process and its governing mechanisms at low and moderate temperatures, in the current work the mass change, heat evolution and gas as well as solid products of a sample were systematically studied at temperatures up to400℃with the combination of the TG/DTA (Themiogravimeric analysis/Differential thermal analysis), in-situ FTIR (Fourier transformed infrared spectrometry) and TG-FTIR techniques. The characteristics and influencing factors of the major reactions occurred at different temperature range were deeply studied; As for the phenomenon of mass increase observed by TG curves, an reaction model was proposed and the non-linear least-square fitting technique was used, and then the kinetic parameters of all the reactions occurring within the temperature range studied were determined by fitting the sets of TG and DTG data, which allowed the quantitative exhibition on the shifting mechanism on the governing reactions taking place at varying temperatures; The effects of various types of additives on coal oxidation were also explored using sophisticated techniques. The latest understanding lays important foundation for developing effective additives for inhibiting or accelerating the coal oxidation process.With the temperature increase, the coal oxidation process experiences4distinct stages, i.e. stage of water evaporation, stage of mass increase, stage accompanied by significant decomposition reactions and the stage dominated by direct’burn-off reaction. The typical variation showed in different stages is a comprehensive reflection of the complexity of coal oxidation phenomenon and the temperature effects. In engineering practice, the oxygen adsorption reaction provides initial energy for the self-heating of coal, and the significant occurrence of ’burn-off’ reaction is a sign of the spontaneous combustion of coal.A competitive reaction model was proposed for the coal oxidation at low and moderate temperatures, including water release, oxygen adsorption and subsequent decomposition of the oxygenated compounds, as well as the direct ’burn-off’ reaction, and the phenomenon of mass increase was successfully modeled using the non-linear least-square fitting technique. It is proved that the mass increase during coal oxidation at moderate temperatures is primarily contributed by the oxygen adsorption and the subsequent decomposition reactions, while influence of the direct’burn-off reaction on this phenomenon is relatively trivial. Enrichment of O2in the reaction medium speeds up the rates of oxygen adsorption reaction and the decomposition reaction, and the temperature corresponding to the maximum mass of the coal sample decreases accordingly. However, we did not observe the significant effect of the oxygen concentration on the direct ’burn-off’ reaction.A series of experiments confirmed that the addition of Na3PO4reduced the pyrolysis rate of the coal sample in an inert environment, and significantly inhibited the coal oxidation process. The addition of Na3PO4to the coal mainly influences the decomposition of hydroxyl by promoting its conversion into ether linkages. The formation of ether bonds not only improves the thermal stability of the coal, but also reduces the number of free radicals in the coal structure, which allow an inhibition of further oxidation and pyrolysis.Using CUCl2as an inducer, the important reaction of the coal oxidation phenomenon, i.e.’burn-off’ reaction was detailed studied. The presence of the additive has successfully decreased the initial temperature of the significantly occurrence of ’burn-off’ reaction, and simultaneously inhibited the occurrence of its parallel reaction sequence, i.e. oxygen adsorption reaction. The action mechanism of CuCl2is to seize hydrogen atoms from the coal structure in a suitable temperature range, and promote the formation of reactive free radicals. For the first time, it has been directly verified by measurements that the ’burn-off’ reaction is a heterogeneous reaction taking place between carbon centers in coal structure and O2, which generates gaseous products CO2, CO and H2O simultaneously. This reaction first occurs in the carbon radicals of the side chain, and then in the benzene ring structure. Compared to that by the oxygen adsorption sequence, the amount of heat released by ’burn-off’ reaction is a little bit higher.