| The effects of preoxidation and two-stage heating on the pyrolysis behavior of coal under conditions of rapid heating have been investigated using the heat loss compensation grid (HLCG) calorimeter. A subbituminous coal and preoxidized samples of coal (up to 0.7% oxygen added by wt.) were rapidly pyrolyzed to 900 K with a 400 K/s heating rate, and the enthalpy changes were measured.; The results show that the pyrolysis in the temperature range from 298 to 900 K is endothermic process. One-third of total energy supplied to pyrolyze coal is consumed by the pyrolysis reaction. The mild preoxidation reduces the endothermicity of subsequent pyrolysis. Resulting thermophysical properties of coal, such as pyroheat, specific pyroheat, and heats of pyrolysis, decrease as the degree of preoxidation increases. The volatile yields increase with the level of preoxidation. The volatile yields increase with the heating rate, and this effect is enhanced with the level of preoxidation. After the initial 5 hrs oxidation, the volatile yields of prexoidized coals under rapid heating condition exceed the ASTM proximate volatile matter. These preoxidation effects could be attributed to the introduced oxygen functional group, especially phenolic-OH, which has an activating effect on the bond-breaking reaction. The effective heats of pyrolysis of unoxidized coal and the coal preoxidized at 400 K for 24 hrs with air are 1086 J/g and 456 J/g at 900 K, respectively. The yields of volatiles for unoxidized and preoxidized coal are 29.1% and 53.1%, respectively. However, the yields of volatiles with complementary slow heating (2 K/s) for unoxidized coal and preoxidized coal are 22.5% and 35.4%, respectively.; The two-stage heating causes a reduction of the volatile yields as well as reduction of the heats of pyrolysis. Their degrees of reduction are different for each of the properties and each of the stages. The mass and energy balances of pyrolysis are strongly affected by the path being taken. The effective heats of pyrolysis for single- and two-stage heating are 1086 J/g and 913 J/g at 900 K, respectively. The volatile yields for single- and two-stage heating are 29.1% and 26.4%, respectively. (Abstract shortened with permission of author.)... |
