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

Authors: Ellison, Candice; ABDELSAYED, VICTOR - National Energy Technology Laboratory; SMITH, MARK - National Energy Technology Laboratory

Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2023
Publication Date: 7/29/2023
Citation: Ellison, C.R., Abdelsayed, V., Smith, M.W. 2023. Analysis of char structure and composition from microwave and conventional pyrolysis/gasification of low and middle rank coals. Fuel. https://doi.org/10.1016/j.fuel.2023.129301.
DOI: https://doi.org/10.1016/j.fuel.2023.129301

Interpretive Summary: Gasification is a process that converts solid hydrocarbon feedstocks (e.g. coal, biomass, wastes) into a valuable, energy-rich gas called syngas. Application of microwave heating to the gasification process can improve process efficiency and enables electrification, which can lower the carbon footprint if powered by renewable electricity. Improved syngas yield was previously demonstrated using a microwave gasifier instead of a conventionally heated gasifier, however the reason for the increased production is not well understood. By comparing the chemical and structural properties of the chars produced from microwave and conventional gasification this study aims to investigate the mechanisms of microwave selective heating during pyrolysis (the initial stage of gasification) and gasification. Major chemical and structural differences in the char porosity and carbon crystallinity were observed between the chars produced by the two heating methods. Chars from microwave gasification were more fully converted to syngas and there was evidence of porous structure collapse indicating deeper gasification as a result of microwave heating. These results along with previous reaction studies provide a better understanding of the microwave gasification reaction characteristics during carbonization processes, which is useful for design and optimization of a microwave gasification process for efficient, low-carbon syngas production.

Technical Abstract: 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 concentration. 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.