Combustion of Coal Chars in O_2/CO_2 and O_2/N_2 Mixtures: A Comparative Study with Non-isothermal Thermogravimetric Analyzer (TGA) Tests

摘要

Two chars prepared from parent coals of a high-volatile bituminous coal and an anthracite coal were subjected to non-isothermal combustion tests in a thermogravimetric analyzer (TGA). The chars were burned in mixtures of O_2/CO_2 and O_2/N_2 with O_2 concentrations of 3, 6, 10, 21, and 30%. A range of non-isothermal combustion tests of each char were conducted with linear heating rates of 2.5, 5,7.5,10, and 12.5 K/min. Detailed comparisons of measured char combustion rates show that replacing the inert nitrogen gas in the oxidizer with CO_2 has very little influence on the measured combustion rates of coal chars at any char conversion level under the conditions of the experiments. Four different model-free isoconversion methods using the experimental data were applied to determine the activation energies of the combustion of chars in O_2/CO_2 mixtures. The results show that the activation energy of each char, determined by any of the four methods, decreases with the increase of the char conversion level. The effect of diffusion, which becomes more pronounced at higher char conversion levels, is believed to be the main reason for the above observation. At a low char conversion level (e.g., 20%), when the effect of diffusion can be neglected, the activation energy of each char determined by the most accurate method under the conditions of this study was found to be in good agreement with literature data: for the anthracite char, it was 138.03 kJ/mol, and for the bituminous coal char, it was 127.83 kJ/mol. The measured combustion rate of each char was found to be approximately first-order to the concentration of O_2 in the O_2/CO_2 mixtures.