Tar-rich coal has the potential to substitute the supply of oil-gas resources, which is abundant in China. The effective conversion of tar-rich coal into oil-gas products can promote coal utilization, reduce resource wastage, alleviate environmental pollution, and benefit carbon neutrality. Nevertheless, less work, if any, has been performed on the pyrolysis and mild oxidation behaviors of tar-rich coal in Northwestern China. The influences of limited oxygen addition and an extremely low heating rate on the micromorphology of the residual semi-coke are yet to be fully understood. Here, an experimental study on the pyrolysis and mild oxidation characteristics of tar-rich coal was conducted by the thermogravimetric analysis method, with further elucidation of the physical–chemical properties of the residual semi-coke. Experimental results show that an increase in the ultimate temperature of pyrolysis leads to a decline in the residue mass, while the mass loss from 500 to 550 °C presents the maximum elevation. Volatile matter is inclined to discharge from a certain direction, and the pores formed in various directions hold different possibilities. The organic components undergo both pyrolysis and slow oxidation with limited oxygen in the heating medium. Compared with an inert atmosphere, the mass loss under conditions of a small amount of O 2 is brought forward but prolonged. Compared with a N 2 atmosphere, the oxidation reactions of tar-rich coal are weakened in the presence of CO 2 . A large decrease in the heating rate exerts an unfavorable effect on the production of total volatiles. An extremely low heating rate possibly brings about a change in the mechanism of chemical bond cracking during pyrolysis. More pores can be yielded in tar-rich coal with an increase in the heating rate, and the morphology of the residual semi-coke after pyrolysis is susceptible to the heating rate. The present study offers an improved understanding of the pyrolysis characteristics of tar-rich coal as well as insights into the efficient utilization of tar-rich coal.
With the development of low-rank coal chemical industry, the production of low-volatile carbon-based solid fuels, such as pyrolyzed and gasified semi-cokes, is ever-increasing. However, it is difficult to use such fuels efficiently because of their poor burnout performance and high NO x generation during the combustion process. Co-combustion of semi-cokes with bituminous coal is a promising approach for large-scale utilization of semi-cokes with ultra-low volatile content. Nevertheless, the NO x generation and burnout characteristics of co-combustion of multiple carbon-based solid fuels are yet to be fully understood. In this paper, the cocombustion characteristics of semicokes and bituminous coal were investigated using a deep-staged co-combustion system. The experimental results indicate that NO x generation displays an increasing trend with the mass fraction of gasified semi-coke. The unburned carbon (UBC) generation in fly ash increases with the proportions of gasified and/or pyrolyzed semi-cokes. The NO x generated from char-N of semi-cokes is reduced by CO and CH i generated from volatile-N of bituminous coal. Compared to air combustion, the increase of oxygen concentration of the O 2 /CO 2 atmosphere is useful for the blended fuels to reduce NO x generation and improve burnout performance because of the prolonged residence time under oxygen enrichment conditions. The conversion of fuel nitrogen to NO x (N-NO x ) and the nitrogen left in ash (N-ash) in O 2 /CO 2 and O 2 /Ar atmospheres decrease with increasing oxygen concentration, with more fuel nitrogen converted into N 2 . When the oxygen concentration of over-fire air varies from 21 to 100%, the NO x generation is increased by 23% and the UBC is decreased by 45%. The present study can offer better understanding on large-scale clean utilization of pyrolyzed and gasified semi-cokes in the utility coal-fired power plants.
Summary The co‐firing of bituminous coal under oxy‐fuel conditions is a feasible measure to improve the ignition and combustion performances of ultra‐low volatile carbon‐based fuels and effectively control pollutants. The present study aimed at the co‐combustion performances and kinetics of bituminous coal and ultra‐low volatile carbon‐based fuels under oxy‐fuel condition by means of thermogravimetric experiments. The results indicate that the volatile content in fuels is likely to affect the ignition temperature and the oxy‐fuel co‐firing of bituminous coal accelerates the ignition and advances combustion of ultra‐low volatile fuels. The combustion behaviors of individual fuel are retained in the co‐firing process to a large extent, and there is a slight synergy at the 350 to 950°C temperature range. Enhancing the oxygen concentration is effective to improve the combustion performances of ultra‐low volatile fuels in oxy‐fuel atmosphere. The apparent activation energy of residual carbon from coal‐water‐slurry gasification in oxy‐fuel atmosphere exceeds that from fluidized bed gasification, which is highly related to gasification technology and raw fuel properties. The apparent activation energy under condition of co‐firing bituminous coal and residual carbon from coal‐water‐slurry varies considerably, changed from 100 to 46 kJ/mol. Therefore, the addition of bituminous coal significantly reduces the activation energy of ultra‐low volatile fuels to improve the co‐firing performances. Based on the present investigation, a better understanding on the oxy‐fuel co‐firing of various carbon‐based solid fuels is obtained, thereby promoting the efficient and clean utilization of solid waste in the coal chemical industry.
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