Analysis of char structure and composition from microwave and conventional pyrolysis/gasification of low and middle rank coals

•Characteristics of microwave and conventionally generated chars are compared.•There is evidence of deeper gasification from microwave heating.•Microwave gasification results in rapid pore development followed by pore collapse.•Microwave gasified chars have greater ordering, especially for low ash coals.•The results are useful for microwave gasifier development and coal char utilization. The structure and composition of residual coal char after pyrolysis dictates its reactivity towards gasification. The nature of the char is a result of the parent coal composition and structure as well as the reaction conditions during pyrolysis. Further, due to the unique characteristics of selective dielectric heating, char development during microwave CO2 gasification may differ from conventional thermal gasification, and little is known about the effect of microwave heating on the char structure development. In this work the heating method used is compared (microwave vs conventional thermal) to generate char samples from four different coal types. These coal chars generated by pyrolysis and gasification were characterized by ultimate analysis, surface area analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and dielectric characterization to investigate the chemical and structural differences between microwave- and conventionally-generated chars. The results reveal notable chemical and structural differences between the chars from microwave and conventional pyrolysis and gasification. In general, the conventional chars had greater pore structure development as seen by higher specific surface areas compared to microwave chars, possibly due to rapid thermal collapse of coal pore structure. Microwave pyrolysis chars had greater ordering of the carbon crystal structure for low-ash coals, while graphitization was inhibited for coals with high ash percentage yield. The observations from this study highlight the differences in char characteristics from microwave and conventional pyrolysis and gasification, which can aid microwave gasifier design, implementation, and integration for efficient syngas production and coal ash utilization.