DEVOLATILIZATION STUDIES ON COALS IN A FIXED BED REACTOR (COAL, TAR)

A laboratory scale fixed bed coal gasifier was designed to simulate the conditions existing in the devolatilization zone of an air blown Lurgi type fixed bed coal gasifier. Devolatilization behavior of a subbituminous and a bituminous coal was studied in the temperature range 350(DEGREES)C to 550(DEGREES)C and at pressures 30, 300 and 375 psig. Three feed coal particle sizes, (-2,+1), (-4,+3) and (-9,+6) mm, were studied. The feed gas was a synthetic mixture of composition similar to that leaving the gasification zone of a fixed bed gasifier and contained 30% by volume of steam. Devolatilization runs were conducted for 5, 10, 20 and 30 minute durations and the inlet and exit gas streams were analyzed. The gas evolution rates showed a peak at 5 minutes from the start of a run and then gradually tapered off to zero as the run progressed. The char and tar samples collected from each run were weighed. The tar samples were analyzed on a capillary gas chromatograph for individual components and thirty such components were quantified. The molecular weights of tar samples showed a maximum between 300 and 500. A first order kinetic model was applied to the total weight loss data and this yielded activation energies in the range 4 to 11 kcal/mol. For the subbituminous coal, differential equations for concentration profiles for tar and gas inside the coal particle were solved numerically. From these calculations it was found that the pressure build-up (due to evolution of tar and gas) inside the coal particle was higher for larger particles, at a given external pressure, but decreased with external pressure. The concentration of tar inside the particle did not appear to be sensitive to low pressures (around 1 atm), but increased in the higher range of pressure (above 20 atm) and also with particle size. The devolatilization of the bituminous coal resulted in more than double the amount of tar as compared to that from the subbituminous coal.