Fundamental mechanisms of coal pyrolysis and char combustion

[摘要] Coal pyrolysis and combustion have been systematically investigated at high temperatures where external and intraparticle transfer limitations become important. A thermogravimetric reactor equipped with in-situ video imaging capabilities provided the reaction rate measurements while its video microscopy system simultaneously allowed observation of the pyrolyzing or combusting coal particles. Video microscopy permitted direct observation of several transient phenomena occurring during combustion (particle ignition, macropore opening, particle fragmentation) or pyrolysis (particle swelling and bubbling) and these phenomena have been correlated with the combustion or devolatilization rate measurements. Particle ignition causes a sharp increase in the char combustion rates. The probability of particle ignition increases with increasing particle size, increasing porosity, increasing oxygen concentration and decreasing gas flow rate. Macropore opening also enhances char reactivity. During pyrolysis, the most vigorous bubbling of the particles occurred when the devolatilization rate was at its maximum. Pyrolysis conditions also affect char ignition behavior. Increasing pyrolysis heating rates result in chars with more open macropore structure and higher reactivity in the diffusion-limited regime. However, heating rates do not affect reactivity in the kinetic control regime. Chars pyrolyzed in 5% oxygen are more swollen and more porous than chars produced in pure nitrogen and are also more reactive in the diffusion-limited regime. Finally, increasing soak times and heat treatment temperatures deteriorate char reactivity in all regimes. A cellular automaton algorithm was developed to simulate combustion of chars with complex macropore structures in the diffusion-limited regime. This model accounts for diffusional limitations by assuming a finite penetration length of gas inside the porous solid and by treating the closed pores as inaccessible to the reactants. Computational grids were generated to model the structure of chars prepared at three different heating rates. Simulation results suggest that char reactivity depends strongly on macroporosity and macropore specific surface area. In agreement with our experimental reactivity measurements, the simulations show significant reactivity differences of the studied chars, even under isothermal conditions. The simulations do not detect significant particle fragmentation at conversions as high as 81%. Small fragments were produced though, at all conversions and their number reached a maximum at about 95% conversion.

[发布机构] Rice University