Microstructural properties, thermal conversion, kinetics, and thermodynamic properties of pressurized coal gasification slag

The Texaco coal gasification process (TCGP) is an entrained flow bed gasification system that generates coal gasification fine slag (CGFS_TCGP) as solid waste. The present study demonstrates a combustion-based approach to energy recovery and reuse of this material. The structural characteristics of CGFS_TCGP generated in a 110,000 Nm³/h pressurized water coal slurry gasifier and the combustion properties of CGFS_TCGP under O₂/N₂ and O₂/CO₂ atmospheres were assessed. The CGFS_TCGP was found to have a more developed pore structure and greater heterogeneity compared with circulating fluidized bed (CFB) and dry coal powder (DCP) gasification slags. A specific surface area of 305 m²/g and a pore volume of 0.31 cm³/g were obtained. The CGFS_TCGP exhibited a disordered carbon layer structure resulting from defects or heteroatoms together with irregular graphitic crystalline and amorphous morphologies incorporating various functional groups. This material contained fewer active sites than those generated in DCP gasifiers. The numerous functional groups and chemical bonds in the CGFS_TCGP provide more opportunities for the detachment of small molecules and for collisions with free radicals during combustion under either O₂/N₂ or O₂/CO₂. Thermodynamic calculations established that the combustion performance of this slag was superior to those of CFB and DCP gasification slags. The highly porous structure and unstable chemical bonds in the CGFS_TCGP evidently promoted combustion. At an oxygen concentration of 21 %, the comprehensive combustion index under O₂/N₂ was superior to that under O₂/CO₂ but with no significant difference in the combustion kinetics. Diffusion and contraction kernel models accurately described these kinetics. Under an 21 %O₂/79 %N₂ atmosphere, the CGFS_TCGP was chemically unstable in terms of activation energy and pre-factor, indicating the direct combustion of CGFS_TCGP in air atmosphere. However, combustion under O₂/CO₂ requires an increased oxygen concentration to enhance performance.