| Coal plays an important role in production of energy. However, in the coal combustion process, it releases a large amount of sulfur dioxide (SO{dollar}sb2{dollar}) into the air causing acid rain problem. Development of advanced, cost-effective coal desulfurization technologies has been topic of growing national and international importance. Although the widely used wet scrubbing is regarded as the best commercialized technology, its high capital and operating costs make it uneconomical for high-sulfur coal application.; The overall objective of this research was to study the feasibility of a precombustion scrubbing technology with lower cost and higher sorbent utilization--coal desulfurization by mild pyrolysis. A novel dual-screw coal feeder reactor was designed and constructed for this purpose. This feeder reactor consists mainly of two concentric tubes, the inner tube acting as a coal pyrolyzer and the outer tube acting as a desulfurizer with hot calcined limestone. Two basic concepts were involved in the development of this reactor: (1) sulfur release from coal in the form of H{dollar}sb2{dollar}S by mild pyrolysis, during which the total heating value of coal does not significantly decrease, and (2) the reaction of H{dollar}sb2{dollar}S with dry calcium-based sorbent, which can result in a higher sorbent utilization compared with the wet scrubbing technology.; An Ohio #8 coal, one of the high sulfur coals, was used in the experiments. A series of devolatilization and desulfurization tests were performed in the dual-screw feeder reactor within a temperature range of 400-475{dollar}spcirc{dollar}C. The experimental results showed that the increase of reaction temperature and residence time of the coal particle in the coal feeder reactor led to an increase in both devolatilization and desulfurization yields. At a temperature of 475{dollar}spcirc{dollar}C and a residence time of 6 minutes, 33.2% of the total sulfur in coal was removed in the screw feeder reactor. It indicated that a considerable number of sulfur atoms were bound with the mobile phase of coal and could be released by mild pyrolysis. The hydrogen sulfide concentration in the pyrolyzed gas (before reacted with CaO) increased with reaction temperature. At 400{dollar}spcirc{dollar}C, this concentration was 1.1% (volume) and at 475{dollar}spcirc{dollar}C, it reached 4.0% (volume). In respect to the sulfur removal efficiency, the tests showed that the lime pellets in the outer screw region had the ability to almost completely remove the hydrogen sulfide from volatiles by the reaction H{dollar}sb2{dollar}S + CaO {dollar}to{dollar} H{dollar}sb2{dollar}O + CaS. The heat transfer coefficient from the feeder surface to the coal particles was experimentally estimated to be 118.8 W/m {dollar}cdot{dollar} s {dollar}cdot{dollar} K indicating a good heat transfer performance of the dual-screw feeder reactor.; The analyses of reactor characteristics and the experimental data resulted in a mathematical model to simulate the pyrolysis and the desulfurization processes in the dual screw coal feeder reactor. In this part of work, the optimization method was used to obtain the kinetic parameters of the non-isothermal pyrolysis reaction. The temperature-residence time relationship was gained based on the optimized kinetic parameters. A thermodynamic loop method was used to estimate the heats of reaction. The devolatilization and the desulfurization activation energies for Ohio #8 coal under mild pyrolysis condition were also estimated to the 82,886 kJ/kmol and 91,431 kJ/kmol respectively. |
