Experimental Investigation into the Interactive Effects of Methanol and Potassium Carbonate on Supercritical Water Gasification of Coal

Supercritical water gasification (SCWG) is a promising technology for the efficient and potentially lower-pollutant conversion of coal. However, char formation caused by the polycondensation of aromatic structures hinders the popularization and application of this technology. Promoting the depolymerization of aromatic structures is the key to suppressing char formation and achieving efficient coal gasification. Both methanol and potassium carbonate (K2CO3) can facilitate the cleavage of aromatic structures. To promote aromatic depolymerization and realize efficient coal gasification, this study systematically investigated the effects of different methanol concentrations and K2CO3–methanol coupling systems on the SCWG of Zhundong coal. The results showed that neither methanol alone nor its coupling with K2CO3 enhanced the gasification performance of Zhundong coal, as judged by carbon gasification efficiency (CE), hydrogen gasification efficiency (HE), and gas yield, but they altered the distribution of gaseous and liquid products. The co-gasification of methanol and Zhundong coal promoted the generation of CH4 and C2H6 at an appropriate methanol concentration. Specifically, 2.5 wt% methanol accelerated the cleavage of aromatic structures to produce naphthalene and its derivatives, whereas high-concentration methanol mainly promoted the formation of naphthalene and phenolic compounds. In the K2CO3–methanol coupling system, K2CO3 preferentially catalyzes methanol reforming, thereby weakening the direct catalytic effect of K2CO3 on coal gasification. Nevertheless, compared with the theoretical yields calculated assuming no methanol–coal interaction, the CO and CH4 yields under the condition of 5 wt% methanol coupled with K2CO3 reached 2.24-fold and 3.14-fold of the corresponding theoretical values, respectively. Based on the complementary functions of methanol and K2CO3, a two-stage regulation strategy was proposed. Specifically, 2.5 wt% methanol was adopted for the aromatic structure pre-cleavage, and 5.0 wt% K2CO3 was subsequently added for catalytic gasification. This strategy provided new insights into overcoming the char formation bottleneck during coal SCWG.