Research progress and new thoughts for high-value utilization of low-rank coal according to the future “dual-carbon” policy

There are abundant reserves of low-rank coals in China, which are primarily used for combustion applications, but they have low-added value. Thermal conversion methods, such as pyrolysis and gasification, have achieved some degree of clean utilization; they do not fundamentally alter the energy utilization properties of coal. The preparation of high-value carbon nanomaterials such as carbon dots, nanodiamonds, and graphene from coal can enable the high-value, clean, and efficient utilization of coal. However, these high-value materials are typically produced using high-rank coal as the raw material and strong-acid-oxidation methods that generate secondary pollution and waste valuable residual coal resources. Following the transformation of national energy reserves and the advancement of the “dual-carbon” policy, development of mild-conversion methods based on the structural characteristics of low-rank coal, aiming to utilize its non-energy properties, can enable the clean and high-value utilization of low-rank coal. This approach focuses on the preparation of new high-value carbon nanomaterials, and represents a major innovative strategy for the high-value utilization of low-rank coal. This review focuses on the latest progress and basic principles of mild-oxidation conversion, utilization of low-rank coal, and the preparation of carbon nanomaterials, including vapor deposition, arc discharge, microwave, laser, and various chemical oxidation methods. Regarding the chemical oxidation method, the basic principles and progress of oxidation with different oxidizing media (such as air, nitric acid, H2O2, and oxidative salts) are introduced. Among the different oxidation methods, H2O2 has been evaluated for its environmental friendliness and high efficiency, and can be studied further as a key technology for the preparation of low-rank coal materials. This study compared and analyzed the preparation techniques of materials using different coal types via chemical oxidation methods, demonstrating the product, technical feasibility, and economic viability of mild-oxidation methods to prepare carbon nanomaterials from low-rank coal. The authors recommend that future research on the preparation of carbon nanomaterials from low-rank coal should focus on four aspects: (1) accurate analysis of coal structure, and understanding the evolution of coal structure and its derived carbon at the molecular level; (2) efficient depolymerization and transformation mechanism; by utilizing the activity of low-rank coal and the structure of oxygen-containing organic functional groups, we can achieve efficient conversion of low-rank coal and improve yield and economy by developing reasonable processes, increasing in-situ characterization or calculation combinations; (3) analysis of heteroatom occurrence forms and functions, study the genesis characteristics of complex minerals and heteroatoms in low-rank coal and their influences on the structures and properties of coal-based carbon materials, and develop functional carbon-based composite materials with unique structures; (4) expansion of potential application areas, conduct research on the preparation and application of coal-based carbon materials, focus on the development of cost-effective and scalable synthesis technologies, and construct multifunctional, high-performance, coal-based functional carbon materials. It is hoped that the proposed work and its findings will provide a reference for the coal conversion industry and the high-value utilization of carbon nanomaterials.