Research On The Characteristics Of Coal Pyrolysis And Gasification At High Heating Rates And High Pressure

The pyrolysis and gasification kinetics of NM bituminous, SD sub-bituminous and YN lignite, were studied on a high pressure thermalgravimetric analyser (PTGA) and high pressure drop-tube furnace (PDTF). The objective of this thesis was to characterize the effects of high heating rate, temperature, pressure and complex gas compositions and particle size, which is close to the real condition in gasifier, on the pyrolysis and gasification process under the single particle level. The main results are presented as follows:1. The effects of pyrolysis conditions on the mass release of pyrolysis.Under ambient pressure, with the increase of heating rates, the mass release of pyrolysis was raised, but the effect was lower when the pressure was higher than1.0MPa. The increase of pyrolysis pressure could decrease the mass release, but it was weakened for low-rank lignite coal. The difference of particle sizes showed no important effect, especially under high pressures. Compared to the nitrogen conditions, the pyrolysis under steam atmosphere resulted in the small increase of mass release, and decreased the CH4concentration, lifted up the CO2and H2, through reforming and shift reactions.2. The effects of pyrolysis conditions on the char properties.YN lignite underwent an obvious shrinkage. NM bituminous swelled to1.6, and SD sub-bituminous fell in between. It was similar with different particle sizes. With the increase of pressure, the swelling ratio was increased first then decreased, the maximum was achieved under0.8～1.0MPa. YN lignite was less affected by pressure for its low plasticity. With the increase of particle size, the swelling ratios of three coals were all decreased and the fragmentation was observed with large particles, which could be restrained by high pressures. The drying pretreatment of coal could result in lower particle heating rate, and made the effect of pyrolysis temperature variation contrary to the raw coal with moisture. The particle sphericity was increased after pyrolysis, because of the softening and swelling. The sphericity would be higher with the increase of pressure and decrease of particle size, with a more narrow distribution between0.77-0.87after pressurized pyrolysis.Chars pyrolyzed under high heating rate, low pressure and short residence time under high temperature, could achieve a high reactivity. The reactivity was influenced by macro and meso pore surface areas, the residual volatile matter content, catalytic effect of ash and carbon deposition, but has no relation with microporosity. 3. The char combustion and CO2, H2Ogasification kinetics.The power exponent type and Langmuir-Hinshelwood type kinetics formulas were obtained with both isothermal and non-isothermal thermalgravimetric methods, which could be used in gasifier modeling. A non-isothermal thermalgravimetric method was also applied in the determination of L-H type kinetic formula, to reduce the experiments need, simply the process. Finally, the effects of reactant and product pressures on gasification rates were investigated.4. Complex gasifying agents and particle cluster effects.The experiments with H2O and CO2binary gasifying agents indicated that the H2O and CO2gasification shared active sites partially. With the addition of H2and CO, the share ratio was reduced. Particle cluster could reduce the reaction efficiency greatly, compared to single particle situation. The combustion efficiency was mostly reduced, because of the high intrinsic reaction rate to the mass transfer. Increasing the average solid concentration and carbon content resulted in lower efficiency. With the incease of particle size, the efficiency decreased first then increased, because of the change of cluster size, density of particle number in clusters and the particle reation rates. The lowest was achieved with about120μm particle.