Humic acid (HA) was extracted by a hydrothermal method from Huolinhe lignite from Inner Mongolia. The effects of the alkali-to-carbon mass ratio, water-to-coal mass ratio, reaction temperature, and reaction time on the HA yield were investigated. The physicochemical characterization of the products was performed, and the reaction mechanism was explored. Raw coal, HA, and residual coal were characterized using Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–VIS), elemental composition, and X-ray diffraction (XRD) analyses and compared to each other. The maximum HA yield (90.2%) was obtained from the 0.250–0.180 mm size fraction of the coal sample at a reaction temperature and time of 190 °C and 7 h. Proximate analysis proved that the ash and sulfur of lignite can be removed by hydrothermal treatment. Elemental analysis showed that the O/C and H/C ratios were highest for HA, followed by those for residual coal and raw coal, indicating an increase in the oxygen and hydrogen content of HA. FTIR and UV–VIS analyses showed that hydrothermal extraction destroyed the macromolecular structure of lignite. Moreover, the organics were degraded and hydrolyzed during the reaction process.

Coal, as one of the most important sources of energy around the world due to the intensity of its exploiting and utilization, causes increasingly serious ecological problems, especially air pollution. Large amounts of pollutants are emitted during coal combustion, with NO_x and SO₂ being the principal compositions. Therefore, desulfurization and denitrification are extremely urgent. Many technologies have been developed and applied. Our paper introduces mature and new flue gas denitration and desulfurization technologies along with their processing principles, industrial status, merits, and demerits. The combination of SNCR and SCR technology avoids their respective shortcomings and improves denitrification efficiency and economy. Based on the present situation, future trends are put forward. Synergetic, effective, and cheap desulfurization and denitration technologies will be our focus.

Coal-based soild wastes (CBSWs) are industrial byproducts that can be harmful to the environment. The exploitation and utilization of CBSWs offer societal advantages such as resource conservation, pollution reduction, and cost-effective production. However, environmentally sustainable management remains a worldwide challenge due to the substantial production volume and limited disposal capacity of CBSWs. The physicochemical properties and utilization of CBSWs are summarized, including fly ash, coal gangue and coal gasification slag. It also presents the current global applications status of CBSWs resources and examines market supply and demand. Subsequently, the paper provides an overview of studies on ways to utilise CBSWs, highlighting the primary avenues of CBSWs resource utilization which are mainly from the fields of chemical materials, metallurgy and agriculture. Furthermore, a comparative evaluation of the various methods for CBSWs resource recovery is conducted, outlining their respective advantages and disadvantages. The future development of CBSWs recycling processes is also discussed. The review concludes that while there is a growing need for attention in CBSWs recycling, its utilization will involve a combination of both large-scale treatment and refinement processes. The paper aims to offer references and insights for the effective utilization and environmental protection of CBSWs. Future direction will focus on the collaborative utilization of CBSWs, emphasizing on the combination of large-scale and high-value utilization. In addition, there is a need to establish a comprehensive database based on on-site production practices, explore on-site solutions to reduce transportation costs, and improve physicochemical properties during the production process.