## 1 Introduction  
The current development of the global economy leads to a steady growth of energy consumption [1], which is conditioned not only by an increase in the population, but also by the annual growth of energy consumption per capita (from 72 to 76 GJ per capita during the last 10 years). Solid fossil fuel is one of the main energy resources, accounting for more than 35% of primary energy in the structure of global energy generation [2]. The extensive usage of coal is promoted by its rather low and stable cost,abundance of deposits, and vast stocks in many regions of the world. According to expert estimates, the global stocks of coal were over 1054 billion tons by the end of 2018 [1]. The global consumption of coal has risen by 68% in the last 30 years [2; 3]. This is caused not only by a greater amount of energy, generated by coal-fired thermal power plants, and energy consumption by different industries, but also by a wide use of coal as a raw material in the metallurgical, chemical, and construction industries [4; 5; 6]. One of the negative implications of large-scale coal mining and consumption is that a large amount of coal processing waste is accumulated in the landfills of coal washing plants; it has not been reclaimed for industrial application up until recently [7]. Such waste is referred to as a filter cake (FC). It is a slurry of fine coal dust (80-140 um) and water (40-50%). Coal processing waste is normally stored at open-air disposal sites, thus, being exposed to strong impact of natural factors (wind, sun, and air). Storing it this way causes moisture to evaporate, while wind scatters coal dust onto the surrounding areas, polluting the land and water bodies, suitable for agricultural purposes, with acid-forming substances and heavy metals [5]. Moreover, dry fine coal dust can ignite spontaneously [8; 9; 10], therefore, the industrial landfills of FC are fire hazardous for coal washing plants and nearby populated areas. One way to diminish the impact of such waste on the environment and to satisfy the growing needs of thermal power engineering for energy resources is to involve low-grade solid fuels and coal processing waste as resources into the energy industry [11; 12]. At the initial stage of solving this problem, it is a promising approach to develop composite compositions based on FC with the addition (if necessary) of some amount of solid or liquid components to enhance energy performance [13; 14], and environmental [15; 16; 17; 18], characteristics, as well as for the production of fuel briquettes and pellets for grate-firing. Using briquettes and pellets reduces the fire hazard of the fuel (as compared with coal dust). Moreover, it cuts transportation and storage costs, as well as decreases the amount of fuel falling through the grate in the boiler furnace. However, the extensive use of such solid fuel briquettes in real practice involves the problem of long and energy-consuming combustion initiation, relatively low (as compared with high-grade coals) heat of combustion, possible significant underburning, and poor physical and mechanical characteristics. However, a distinct advantage of such pellets as a fuel for local heat supply sources is their very low cost as compared with fuel oil [19], which is a widespread energy resource of the local power industry. Even considering the ash removal costs (when using FC-based pellets as fuel), the cost of a unit of energy, produced by a local heat supply system, will be significantly lower than when using fuel oil. Moreover, oil prices and, thus, petroleum product prices are unpredictable and influenced by many factors. However, the cost of coal processing waste is made up of trans-shipment and storage expenses, while the fuel pellet production cycle (per a thermal unit) is less expensive than the fuel oil production technology [19]. Another factor to be considered is that the FC-based pellet price is independent of the global political situation. That is why, in the medium term, this energy resource looks much more attractive than fuel oil.