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Coal is vital for the energy system of China. As the digital economy develops rapidly in China and in the context of carbon peaking and carbon neutrality, promoting the digital development of the coal industry becomes crucial for the industrial upgrade and high-quality development of the industry. This study defines the digitalization of the coal industry, clarifies the digital economy in the coal industry, and for the first time calculates the scale of digital economy in China's coal industry. The basic conditions for the digital development of the coal industry are sorted out, and the urgent issues that constrain the digital development of the industry, such as concept understanding, infrastructure, talent teams, and core technologies, are summarized. On this basis, strategic goals are proposed, and a strategic path is elucidated from the aspects of digital infrastructure, skilled personnel, key capabilities, and development environment. The key development directions, technologies for the entire industry chain, and key technologies for the digital development of the coal industry are emphasized. Furthermore, the following policy recommendations are proposed to build a solid support for the digital development of the coal industry: (1) establishing data management institutions and platforms, (2) establishing laboratories for developing coal-related digital technologies, (3) enhancing the data sharing and value mining capabilities of the industry, and (4) building training bases to cultivate skilled personnel for the digital development of the coal industry.

The rock bolting construction equipment is an important part of the complete set of bolt support technology in coal roadway, which directly affects the speed and quality of rock bolting. After a long period of development, China's coal mines have developed various forms of rock bolting equipment, including single bolter, mobile bolter, roadheader bolter, bolter miner, bolter conveyor and straddle bolter. This paper systematically analyzes the development history, technical performance, technical parameters, applicable conditions and latest development of six kinds of rock bolting construction equipment. The single bolter is light, flexible and mature in technology, forming a series of products, which are widely used. The onboard bolter can be integrated on different platforms to form a variety of integrated construction equipment. The mobile bolter has formed a variety of models from two arms to nine arms. It can closely follow the heading equipment, work alternately with the heading equipment, and complete the rock bolting construction with multi-arm cooperation and multi-arm parallel operation. The onboard bolter is equipped on the cantilever roadheader, and various types of roadheader bolters are developed, realizing two functions of heading and bolt construction. The bolter miner has realized localization. The bolter miner with forward moveable bolting platform, shield type bolter miner and bolter miner with the integration of drilling and bolting are developed. The bolter conveyor from two arms to seven arms and the ten-arm straddle bolter are developed. The rock bolting, heading and transportation have been carried out in different zones. This paper introduces the technology and equipment of rock bolting with the integration of drilling and bolting. They include bolt with the integration of drilling and bolting, pump-injected anchoring agent, onboard bolter with the integration of drilling and bolting, drill box, rock bolt bin, anchoring agent pump-injection system and intelligent control system. The traditional six-step rock bolt construction process is simplified into one continuous process. It realizes the automatic operation of 'one key rock bolting', significantly improving the speed and automation degree of rock bolting. The paper points out the development trend of rock bolting construction equipment in coal roadway. The trend includes following aspects: the digital intelligent bolt onboard bolter, onboard bolter positioning, pose self-adjustment and multi-onboard bolter collaborative control technology; the optimization of bolt (cable) structure and materials, upgrading of anchor grouting materials and improvement of construction equipment; the environmental dynamic sensing and stability discrimination of surrounding rock with heading; the fault diagnosis technology of rock bolting construction equipment and the online monitoring of roadway surrounding rock deformation and bolt load; establishing a big data platform to realize automation and intelligenitization of rock bolting.

The application of hydraulic fracturing technology on rock fracturing for hard and complete roof, and destressing for roadways with high stresses is increasingly widespread. Based on a top coal caving working face with extra-thick coal seam and complete roof layers in the Caojiatan coal mine in Shaanxi Province, the roof rock geomechanical measurements, compressibility tests, theory of hydraulic fracture propagation, and 3D numerical simulation were carried out, and the determination approaches for fracturing rock horizon above a working face, fracturing borehole layout and parameters, and fracturing technology were put forward. The underground industrial trial and a comprehensive surface microseismic monitoring with actual time were conducted, and the spatial distribution characteristics of hydraulic fractures in roof were obtained. Meanwhile, shield resistance, periodic weighting step distance and duration distance, weighting dynamic load coefficient, and roof breaking energy were monitored and analyzed, and the hydraulic fracturing effect was synthetically evaluated. A complete set of technology for hydraulic fracturing with directional boreholes in underground coal mines were preliminarily established, which includes determination of fracturing rock horizon and parameter design, fracturing technology and equipment with directional boreholes, monitoring of hydraulic fracture spatial distribution, and comprehensive evaluation of hydraulic fracturing effect. The underground trial pointed out that: on the in-situ stress regime in the Caojiatan coal mine, that is the minimum principal stress is vertical stress, the hydraulic fractures were mainly horizontal fractures with a propagation distance of about 80 m on both sides of boreholes, the hard and complete roofs above the working face were effectively weakened, and regional roof reconstruction was achieved. The intensive ground pressure appearance of the working face was obviously reduced in the fracturing area, and production safety was ensured. Finally, the existing issues associated with hydraulic fracturing were discussed, and technical developments were envisaged.

As the reconstruction of global industrial chain and energy supply chain accelerates and variables in China’s energy supply security increase, it becomes inevitable for China to maximize the guarantee function of coal. Considering the characteristics of the accelerated reconstruction of the two chains, this study analyzes the impacts of the reconstruction on China's energy sector: large fluctuation of energy demand structure, restricted local production capacity, increased import risks, and increased pressure on lowcarbon transition. A fluctuation prediction model is established to predict the fluctuation range of coal demand considering these scenarios: fluctuation of fossil energy imports, output fluctuation of renewable energies, and consistent growth of energy consumption. The results indicate that coal demand will fluctuate by ±14.1% in 2025, ±16.5% in 2030, and ±18.2% in 2035, respectively. Therefore, we propose that a coal technology innovation chain should be constructed by extending and improving the coal industrial and supply chains. Moreover, future key tasks should include intelligent construction of coal mines, green development and clean transformation of coal, research and development of carbon sequestration and utilization technology in coal mine areas, and collaborative development of coal and new energies. To promote the long-term high-quality development of the coal industry in China, it is necessary to encourage the fine exploration of coal resources, strengthen the construction of coal reserves and emergency transportation capacities, improve the flexible supply of coal, enhance the support capacity for coal-to-oil and coal-to-gas production, and formulate support policies for coal industry development.

As China develops toward a high-quality stage and the carbon peak and carbon neutral targets are proposed, a new green and low-carbon development pattern is urgently required. Coal is the primary energy of China and it provides a solid guarantee for the stable operation of the economy; however, the coal industry in China is unbalanced and insufficient in terms of efficiency, technology, market, safety, environment, and other aspects and it can no longer satisfy the new requirements for high-quality energy development and active response to climate change. In this article, we analyze the three imbalances and six insufficiencies that coal industry development faces, and define high-quality development of the coal industry considering the new requirements of energy development.Subsequently, we establish a three-level evaluation index system from three aspects of power, quality, and efficiency reforms. The system is also comprised of five dimensions—innovation-driven, intelligence and high efficiency, diversification and economy, safety and health, green and low carbon—and 23 indexes. Moreover, we evaluate the development level of the coal industry in China using the comprehensive evaluation method of GRA-TOPSIS. The development of an innovation-driven green and intelligent development system, a collaborative and integrated green and low-carbon development system, a market system with orderly competition, a comprehensive open system with win-win cooperation, and a high-quality talent team system has become the key task for the highquality development of the coal industry. Furthermore, we propose suggestions from the aspects of top-level institutional arrangement, carbon reduction in coal industry, and coal production and demand balance.

In order to make clear the pressure relief mechanism and application effect of directional hydraulic fracturing for gob-side entry retaining, the directional hydraulic fracturing was carried out by 400 m in haulage gateway remaining along the goaf in 50108 working face of Hejiata Coal Mine. Taking this as the engineering background, a mechanical model of roof cutting was established and the pressure relief mechanism was clarified. The theoretical research shows that it is the moments of gravity FG of the curved triangular roof plate at the face end, the pressure q of the overlying soft rock, and the transverse force TCB in the “voussoir beam” structure to the left endpoint of the triangular block, that is, MFG, Mq, and MTCB, which determines the roadside supporting resistance. Hydraulic fracturing can reduce the lateral cantilever length of the basic roof, thus greatly reducing the values of MFG, MTCB, and Mq, and significantly reduce the roadside supporting resistance. The field test shows that the directional hydraulic fracturing technology can effectively improve the stress environment of the face end and reduce the deformation of the roadway, and it has a good application effect on the gob-side entry retaining.

Roof disaster is an important hidden danger affecting the safety of coalmines. This paper summarized the types, locations, causes and developing trend of roof accidents in the past decade, and studied the characteristics of coalmine roof accidents in China. On the basis of previous analysis, the paper introduced the controlling techniques for dealing with coalmine roof disasters, including the geo-mechanical test and evaluation of surrounding rock in mining area and surrounding rock control technology of roadway and working face; in the aspects of roadway surrounding rock control, there are coal and rock geological mechanics testing technology and equipment, bolt support technology, grouting reinforcement technology, deep high-stress and strong mining roadway pressure relief technology, and complex section support technology; working face surrounding rock control technology includes hydraulic support, hard and difficult-to-span roof weakening technology, broken roof reinforcement and comprehensive control technology. Although the number of roof accidents and the number of deaths in China’s coal mines have decreased significantly in the past 10 years, roof accidents are still the most frequent accidents in China’s coal mines. Since 2017, roof accidents have rebounded, and there is still a long way to go in the prevention and control of roof disasters.

Firmly, the bearing capacity test of 1:1 equal ratio pillar under different constraint forms and different filling medium conditions was carried out. The results show that the binding pillar-forming effect is relatively good. The constraint ability of unconstrained, metal mesh, polyester mesh, hooked iron flat-hoop bushing, bellows, and spiral iron pipe is enhanced, in turn, and the carrying capacity is improved successfully. The homogeneity of high-water materials is better than concrete, and they have better compressibility, but their carrying capacity is relatively weak. The carrying capacity of concrete pillars is generously higher than that of high-water materials, but the compressibility is poor. Second, the migration characteristics of the surrounding rock structure of the gob-side entry retaining and the rule of side support are analyzed, the requirements of the side support are pointed out, and the side-support technology of the binding pillar is proposed. Taking Hijiata Mine's 50108 working face gob-side entry retaining as an example, the bellows pump-filled concrete pillar is used as the side support body, supplemented by handling steel mesh and air-duct cloth, and toughness material is sprayed between the pillars to seal the goaf, meeting the requirements of side support and road stability. The pillar has the characteristics of high early strength, strong final consolidation carrying capacity, good crimping effect, high mechanism degree, fast construction speed, less concrete consumption, low comprehensive cost, etc., and it has a good application prospect in the gob-side entry retaining or rapid advanced working face. [ABSTRACT FROM AUTHOR]

The electro-hydrostatic actuator (EHA) is a highly integrated and reliable actuation system which has been widely used in aerospace and construction machinery. In addition, the direct load sensitive EHA (DLS–EHA) has been investigated by introducing load-sensitive techniques which can well match the system output and external loads. More importantly, the matching design of motor–pump parameters is an important stage in the pre-design phase of DLS–EHA. For this reason, a multi-objective optimization design method for DLS–EHA motor–pump parameters is proposed in this paper. Firstly, the motor–pump mass and energy loss models are constructed based on similarity criteria. Secondly, the multi-objective particle swarm optimization algorithm (MOPSO) is used to optimize the design of the motor-pump parameters, and the Pareto frontier solutions of the optimized parameters are obtained. Further, the optimal design parameters are selected on the basis of meeting the actual requirements. Finally, the DLS–EHA prototype is designed based on the optimized design parameters and experimental verification is completed. The results of the study show that the optimized design parameters can meet the design requirements. The design method proposed in this paper provides an important theoretical basis for the optimized design of DLS–EHA. [ABSTRACT FROM AUTHOR]