Enabling large-scale enhanced hydrogen production in deep underground coal gasification in the context of a hydrogen economy

• A large-scale deep coal seam UCG model with real-world properties was constructed; • The mechanisms for large-scale enhanced hydrogen production were comprehensively investigated; • Water injection schemes were analyzed for optimal hydrogen production; • Proposed a novel technique for prolonged continuous hydrogen yield; • Revealed the advantages of the new technique in cavity control with environmental merits; Underground coal gasification (UCG) is an emerging clean energy technology with significant potential for enhanced hydrogen production, especially when coupled with water injection. Previous lab-scale studies have explored this potential, but the mechanisms driving water-assisted hydrogen enhancement in large-scale, deep UCG settings remain unclear. This study addresses this gap using numerical simulations of a large-scale deep coal model designed for hydrogen-oriented UCG. We investigated single-point and multipoint water injection strategies to optimize hydrogen production. Additionally, we developed a retractable water injection technique to ensure sustained hydrogen output and effective cavity control. Our results indicate that the water–gas shift reaction is crucial for increasing hydrogen production. Multipoint injection has been proven to be more effective than single-point injection, increasing hydrogen production by 11% with an equal amount of steam. The introduction of retractable injection allows for continuous and efficient hydrogen generation, with daily hydrogen production rates of approximately five times that of a conventional injection scheme, and an increase in cumulative hydrogen production of approximately 105% over the same time period. Importantly, the multipoint injection method also helped limit vertical cavity growth, mitigating the risk of aquifer contamination. These findings support the potential of UCG as a low-carbon energy source in the transition to a hydrogen economy.