(371b) Kinetic Investigation of Secondary Reactions in Agglomerating Coal Pyrolysis
Final product quality and yield from industrial coke ovens are directly related to secondary reactions of tar and gaseous products. A technique for quantifying extents of secondary reactions has been developed using a Netzsch Luxx Simultaneous Thermal Analyzer 409PC closed coupled to an Inficon 3000 micro-gas chromatograph (GC). Two confinements of different geometric shapes were utilized to elucidate devolatilization and secondary reactions. Identical coal pellets were used in each confinement. A confinement with an aspect ratio of 0.11:1 (14.3 mm in diameter and 1.6 mm in height), designated as “pan,” was designed to favor devolatilization of decomposition products leading to minimal secondary reactions. The confinement with a larger aspect ratio of 2:1 (6.5 mm in diameter and 13 mm in height), designated as “cup,” creates a stagnant region in the gas flow profile above the sample. The stagnation above the sample created the opportunity for secondary reactions to affect the product distribution. This space above the sample pellet in the cup confinement was 10 ± 0.5 millimeters. Multiple agglomerating coals were studied to understand the specific nature of secondary reactions in agglomerating coal pyrolysis. The tests in a given confinement were repeated in triplicate for each coal to obtain representative statistical values which were typically within 1.9 % variation. Coal pyrolysis was studied by this method between the temperature of 298 and 1373 K using sample size of 30 mg. A vertically oriented sweep gas of research grade purity argon was maintained constant at 30 mL/min inside a furnace of one inch inside diameter. The effluent of coal pyrolysis in the TGA was connected to a quarter-inch silicon coated stainless steel tube heated to 548 K constituting an inert, heated sample transfer. The heated transfer line routed the effluent to a series of impingers which served as a condensation unit operating at 273 K and 1.0 bar. Uncondensed gaseous effluent was analyzed by the GC. Gas sampling from the effluent stream by GC was performed at a frequency of nominally every 3 minutes. Gas species measured were hydrogen, oxygen, methane, carbon monoxide, carbon dioxide, ethylene and ethane. The mass balances for pyrolysis of three agglomerating coals were closed by measurement within 0.8%. Average product distributions between solid, tar, and gas were obtained. By varying the height to diameter of the reaction confinement between aspect ratios of 0.11:1 and 2:1 a variation up to 3% in residual solid mass from pyrolysis resulted. This proved stagnation of products above the sample in the confinement of a higher aspect ratio promotes secondary reactions which increase solid yield. A similar effect was observed by varying heating rate between 1, 3 and 10 K/min. Secondary reactions such as tar recombination and addition of radicals which contribute to solid product are proved to commence at 773 K. Hydrolysis of tar species and fate of methyl radicals play a decisive roles in product distributions when varying heating rate and confinement. The metaplast acts as a primary radical pool, and stagnated gases above the sample may form a second radical pool where tar products become saturated and/or recombine. Temperature shifts from 1000 to 1100 K for methane evolution and 1200 to 1373 K for hydrogen evolution are observed at 10 K/min between each confinement. These shifts in gas product evolution are governed by onset temperature of carbonization reactions in the radical pools.