Converting corncob to caking materials via hydro-modification in a subcritical water and carbon monoxide (H2O–CO) system

China faces a long-term shortage of high-quality coking coal. Low-sulfur, low-ash biomass resources from agricultural and forestry waste are potential substitutes due to their low cost and environmental friendliness. This study applies a subcritical water-carbon monoxide (H₂O-CO) treatment for this purpose, finding that transforming corncob leads to a significant increase in its caking index, aligning with the standards of premium coking coal. This paper analyzes the main structural changes and removal patterns during corncob modification using elemental analysis, ¹H NMR, ¹³C NMR, and XPS. The results show that the modified corncob products mainly contain three elements: carbon (C), hydrogen (H), and oxygen (O). The C/H and H/O ratios are positively correlated with the caking index. The modified corncob products are rich in polycyclic aromatic hydrocarbon compounds with aliphatic side chain structures; these mainly drive the caking property transformation of the biomass. The active hydrogen produced by the water-gas shift reaction is an important hydrogen source for the modification reaction, it more easily combines with the CO single bonds in corncob. This facilitates the material transformation into substances such as xylo-oligosaccharides and gluco-oligosaccharides in the modified liquid products. Synchronizing the active hydrogen provision with the cleavage rate of chemical bonds in the corncob’s structural units is key to enhancing the modification process and elevating the caking properties. This research broadens the list of effective ways to utilize biomass resources, enriches the basic theory of biomass hydrogenation conversion process, and provides a reference for the processing and utilization of other carbon-based materials.