Structure characteristics and gasification activity of residual carbon from entrained-flow coal gasification slag

• Residual carbon from slags cannot be commonly considered as “unburnt carbon”.
• The micro-structures of residual carbon in slags were illustrated distinctly.
• The carbon activity differences between two slags were explained clearly.
• The activity of carbon from slags is quite higher than that from coal chars.
• The activity of carbon with sp2 bond is superior to that with mixed sp2–sp3 bond.

The structure characteristics and gasification activity of residual carbon from entrained-flow coal gasification slag were mainly investigated using a pore structure analyzer, a scanning electron microscopy, a Raman spectroscopy and a thermo-gravimetric analyzer. Simultaneously, a kind of residual carbon from coal chars was used for a reference. Results showed that the residual carbon which was usually considered as “the unburnt carbon” in gasification slags, was potentially originated from volatile matters (especially those with macromolecular structures) of the original coal pyrolysis, partly-gasified carbons and/or unreacted pyrolytic carbons. Compared to the residual carbon in coal chars, that in gasification slags had a higher pore surface area, a larger average pore size, a more ordered carbon crystalline structure and less total active sites, whereas that in both coarse and fine slags presented a distinctly higher gasification activity, mainly ascribed to its quite more abundant porosity. Compared to the residual carbon in the fine slag, that in the coarse slag had a lower pore surface area, a slightly more disordered carbon crystalline structure and more total active sites, but that in the coarse slag presented a higher gasification activity, especially after the carbon conversion of around 0.5. This was probably due to more total active sites in the residual carbon of the coarse slag, especially its more active sites with mixed sp2–sp3 bond form. Besides, it also could be inferred that the activity of active sites with sp2 bond form was superior to that with mixed sp2–sp3 bond form.