Experimental investigation on carbon microstructure for coal gasification in supercritical water

•The carbon microstructure evolution of different ranks of coal was analyzed.•The evolution law of carbon microstructure for three coal samples was similar.•There were three stages of gasification process of coal in supercritical water.•The condensation of aromatic structures was the main side reaction.•Carbon structure parameters were closely related to carbon gasification efficiency. Supercritical water gasification of coal (SCWGC) has great potential for converting coal into CO2 and H2 effectively and environmentally. For a better technological competitiveness, the conversion mechanism of carbon structure needs to be investigated in depth to achieve 100% carbon conversion for SCWGC under mild conditions. Ningxia anthracite (NX), Hongliulin bituminous coal (HLL) and Zhundong lignite (ZD) were selected as feedstock to investigate the carbon microstructure evolution for different ranks of coal gasification in supercritical water (SCW). Raman and XRD spectra were employed to investigate the carbon microstructure information of solid residues. The experimental results illustrated that coal rank had slight effect on the evolution law of carbon microstructure in coal. The process of SCWGC included three stages depending on the characteristic reactions. Inhibiting the condensation of aromatic rings under 550–750 °C in the third stage was the key to effectively control the ordered conversion and achieve complete conversion of carbon in the coal. Besides, the Raman and XRD spectral parameters had close relationship with carbon gasification efficiency, respectively.