Hydrogasification of lignite semicoke to produce methane and the textural properties of coke residues

The hydrogasification of Inner Mongolia lignite semicoke to produce methane was investigated in a high-temperature and high-pressure (1 000 ℃, 12 MPa) fixed bed reactor; the specific surface area, pore structure and surface feature of the coke residues were characterized by nitrogen physisorption and scanning electron microscope (SEM). The results showed that the hydrogasification process of semicoke can be divided into three stages, viz., hydropyrolysis, rapid hydrogenation and slow hydrogenation, in which the hydrogenation of oxygen-containing functional group and alkyl side chain, the hydrogenation of aromatic structure, and the hydrogenation of hydrogen depleted carbon skeleton structure take place successively. The optimum reaction temperature and pressure for the semicoke hydrogasification are 800 ℃ and 3.0~4.0 MPa, respectively. An increase of heating rate may shorten the reaction time of earlier hydrogasification stage with a carbon conversion below 46%, but has little effect on the later stage with a carbon conversion above 46%. The nitrogen physisorption of the coke residues exhibits a reversed S-shape isotherm with a H3 hysteresis loop. Along with the progress of hydrogasification, the average pore size of semicoke decreases first and then increases, the total pore volume and mesopore volume increase gradually, whereas the micropore volume and specific surface area increase first and then decrease.