Free-radical behaviors of co-pyrolysis of low-rank coal and different solid hydrogen-rich donors: A critical review

Low-cost and accessible “hydrogen source” are the key to reducing the cost of coal hydropyrolysis technology. The mechanism of hydrogen supply is a crucial scientific issue in this technology. Solid “hydrogen-rich” donors, such as biomass, waste plastics, oil shale, and coal liquefaction residue, possess favorable characteristics such as affordability, abundant availability, high volatility and a high H/C ratio, which are considered ideal solid “hydrogen sources” for coal hydropyrolysis. The co-pyrolysis reactions between low-rank coal and solid “hydrogen donor” follow the mechanism of free radical reactions. Moreover, a clear investigation of the free-radical behaviors and synergistic interactions between them can clarify the hydrogen supply mechanism in the co-pyrolysis process of solid “hydrogen-rich” donors, resulting in the controlled production of high-quality oil and gas products. This review systematically summarizes the free-radical behaviors during the co-pyrolysis of low-rank coal and solid “hydrogen-rich” donors: Three detection methods of free radicals and their applications are first introduced. Then, the factors influencing the free-radical behaviors during the co-pyrolysis are discussed, and the influences of the free-radical behaviors on the products obtained by the co-pyrolysis are analyzed. Finally, the results and challenges of free-radical behaviors are outlined. This work is intended to provide a comprehensive and scientific basis for completing the mechanism of free-radical reactions in the co-pyrolysis of low-rank coal and solid “hydrogen-rich” donors.