## 1 Introduction  
In recent decades, the role of renewable energy sources (RES) in the global energy sector has significantly increased [1, 2]. In the future, until 2050, their share in the energy balance will grow significantly faster than traditional energy sources [3]. This is due to public concern about climate change caused by carbon dioxide emissions from industry and energy sectors [4]. One option to reduce CO\({}_{2}\) emissions may be to replace fossil fuels with renewable alternatives [5]. This will be facilitated by programs to stimulating the use of renewable energy for electric and thermal energy production, adopted in more than a hundred countries [1]. Wood biomass has significant potential among renewable energy sources, along with widely developed hydropower, wind and solar energy. Development and implementation of technologies for biomass energy use has a positive effect on both the environmental situation and economic growth [6, 7]. Wood processing for energy production does not change the balance of carbon dioxide in the atmosphere and causes significantly fewer pollutants (SOx, NOx) than burning fossil fuels (coal and petroleum products) [8; 9]. At the same time, the object of energy use is not the forest itself, but the waste from the industrial harvesting of wood and its processing. It is necessary to utilize wood waste under controlled conditions rather than let it burn in forests without any treatment. Utilization of waste helps to avoid cluttering up territories, forest fires and pollution of surface and underground waters. This will positively affect the environmental situation in the areas of harvesting and processing of wood. Timber processing waste can be used in the energy industry both directly, in the form of wood chips, and in the form of fuel granules (pellets). It is worth noting that the economically justified radius of using wood chips in energy sources does not exceed 150 km. This fact is explained by the significant costs of their transportation [10]. Pellets have a higher bulk density and stable calorific value, which makes them attractive for energy use despite the high value-added production. Currently, pellets are widely used instead of coal (or together with it) in the economically developed countries of Europe, America, and Asia in the production of thermal and electric energy. For example, the International Energy Agency states that in Europe the consumption of pellets is about 20 million tons/year, of which 36% is used to generate electricity [11]. The largest consumer of pellets for electricity production is the Drax power plant in the UK, which has four 660 MW power units operating on biomass, and two 660 MW coal-fired power units [12]. About 64% of pellets consumed in Europe are used to generate thermal energy in local heating systems, which typically use small boilers up to 50 kW [11]. Direct burning of wood raw materials is the simplest and most common type of energy use but it has low thermal efficiency. Another promising area is the use of gasification technologies that are thermally self-sufficient and more efficient than direct burning [13]. Moreover, the use of gasification in electric power generation plants provides a higher overall electrical efficiency [14]. Electricity production using gasification technologies should be supplemented by the production and recovery of heat from exhaust gases, which will contribute to the combined heat and power (CHP) production. Such a pattern of energy production is the most energy-efficient. Combined heat and power plants can be used in both centralized and decentralized heating systems. Centralized heating systems employ large CHP plants that combine high efficiency and low operating costs. However, the acute issues of wood fuel availability and heat storage to cover peak loads [15] can arise. Small CHP plants are normally used in decentralized energy systems that use local resources. In this case, transportation costs are reduced and heat and electricity are supplied to consumers in a limited area [16].