Effects of CaCO3 and CaO Loading on Pore Structure, Multifractal Evolution, and Product Transformation During CO2 Gasification of Low-Rank Coal

Underground coal gasification (UCG) is an important in situ conversion technology for the clean and efficient recovery of deep and difficult-to-mine coal resources. Reactions between high-temperature char and gasifying agents such as CO2 and H2O in the reduction zone directly control the generation of valuable gases, particularly CO and H2. Calcium-bearing minerals can catalyze char gasification and participate in high-temperature ash-phase transformation. However, the differential effects of CaCO3 and CaO on coal pyrolysis–gasification behavior and ash-phase evolution under a CO2 atmosphere remain unclear. In this study, a typical low-rank Jiangna (JN) coal from the Hami region of Xinjiang, China, was subjected to sequential HCl-HF leaching to prepare demineralized coal (DC), which was subsequently loaded with 5 or 10 wt% CaCO3 or CaO. TG-FTIR-MS, high-temperature in situ XRD, SEM, N2/CO2 adsorption, and multifractal analysis were employed to investigate thermal conversion, gaseous product release, pore structure, and Ca-bearing phase evolution. Demineralization decreased the mass loss at 600–1000 °C from 50.05% to 29.14%. The CaCO3-loaded samples showed their main high-temperature mass-loss and FTIR CO-band responses above 800 °C, whereas the CaO-loaded samples showed an earlier apparent high-temperature mass-loss response at 600–800 °C. At 600 °C, the CaO-loaded samples exhibited greater mesopore development. At 900 °C, the CaCO3-loaded samples exhibited pronounced micropore and mesopore development, with micropore volume increasing by 49.1–64.9%. The nonlinear mass scaling function τ(q) and the decrease in the generalized dimension D(q) with increasing q indicated heterogeneous multiscale distributions of pore volume. At 600 °C, the micropore spectrum width (Δα) of the CaCO3-loaded samples was 0.5537–0.6375, higher than that of the CaO-loaded samples (0.4093–0.4198). In contrast, the mesopore Δα of the CaO-loaded samples was 0.6688–0.9901, markedly higher than that of the CaCO3-loaded samples (0.1660–0.2126). At 900 °C, the CaCO3 and CaO-loaded samples exhibited contrasting micropore and mesopore multifractal responses. The CaO-loaded samples showed an earlier apparent high-temperature mass-loss response under the tested conditions. However, equal-mass additive loading prevented separation of the effects of precursor form and Ca input. Although HT-XRD detected crystalline phase changes, the active Ca species and extent of Ca incorporation into ash phases remain unresolved. These results document different temperature-dependent responses to CaCO3 and CaO addition under the tested conditions. These findings provide an experimental basis for selecting Ca-based additives, regulating valuable gas generation, and evaluating high-temperature ash stability in UCG.