Recent progress on low rank coal conversion to dimethyl ether as clean fuel: A critical review

Dimethyl ether (DME) is a greener fuel, which is derived from low-rank coal. Integrating synthetic gas (syngas) as an intermediate and DME production is greener route for low-rank coal processing. Syngas could be fabricated by catalytic gasification over K₂CO₃ or Ca(OH)₂ exhibited lower energy consumption than conventional gasification. Then, syngas product is transformed to DME via indirect or direct route. The indirect route depends on methanol production over metallic oxide, which prone to catalyst deactivation. In addition, acidity of the catalyst is also crucial for methanol dehydration to DME with high water tolerant. A hybrid catalyst is required for direct DME production, which challenging for coke deposition. In indirect route of coal to DME, the metallic oxide should be dispersed homogenously which could be achieved via hydrothermal assisted with ultrasonic or controlled sol-gel with surfactant. Meanwhile, direct route preferred introduction of dopant to increase oxygen vacancy or noble metal to reduce the pressure. The catalyst stability to water must be received more concern in the future since water could block the active site. There are still few studies emphasizing to the effect of CO₂ as co-feed to syngas in DME synthesis. Besides that, the technoeconomic analysis and optimum condition for CO₂ addition are crucial to be explored in upcoming years. Lastly, future research should be expanded to LPG fraction production directly from syngas. Multiple functional catalyst is required in this process to work synergistically in methanol production, two subsequent dehydration reaction to DME and olefins, then hydrogenation to alkane fractions.