Heat effects of pyrolysis of 15 acid washed coals in a DSC/TGA-MS system

Heat effects of coal pyrolysis are seldomly studied in the past and the few studies available in the literature differ from each other. The problem perhaps arises from the unclear definition of thermal steady state of differential scanning calorimetry (DSC), the different effects of minerals in coals, and the different mass bases used to quantify the heat flows. This work studies the heat effects during pyrolysis of 15 acid washed coals (D-coals), ranked from lignite to anthracite, in the temperature range of 117-900 degrees C in a system that couples DSC with a thermal gravimetric analyzer (TGA) and a mass spectrometer (MS). The steady state criteria of DSC are studied. The heat effects studied include the total pyrolysis heat (sensible heat + reaction heat) and the reaction heat. It is found that the time needed to reach the steady state for DSC is longer than that for TGA. The in situ char pyrolysis following the coal pyrolysis involves mass loss and reaction heat, so its profile should not be used to estimate the thermal capacity of coal. The reaction heat on DTG basis (q(r-DTG)) correlates better with C% of coals than that on the initial coal basis, both of them are endothermic in the temperature range of < 560 degrees C and turn to exothermic at higher temperatures. It is also found that the major endothermic q(r-DTG) occurs prior to the main DTG peak; the major exothermic q(r-DTG) occurs upon the completion of the main DTG peak, about 130 degrees C higher than the DTG peak temperature; the radical concentration of char increases with increasing temperature during the evolution of the main DTG peak, but starts to decrease at the completion of the main DTG peak, around 600 degrees C, and then diminishes at around 750 degrees C.