## 2 Materials and Methods of Experimental Research  
Typical coals for the energy sector were selected as the basic components of the studied mixed solid fuels: lean (T-grade coal) (Alardinskoye field, Russia) and metalignitous (D-grade coal) (Listvyazhnoye field, Russia). These coals have a relatively low sulfur content and a high calorific value, which is why they are used at large thermal power plants. Their analogues in composition and main characteristics are coals of the Hongyang, Handan, and Huaibei grades, Northern China, and San Juan in the United States [52,53]. The biomass used was crushed wood--waste from pine processing (LLC Dzerzhinskiy LPK, Tomsk, Russia). The experiments were carried out under standard conditions in the laboratory (at an air temperature of 293 K and pressure of 101 Pa). A method for the preparation of mixed fuels has been developed. Fuel preparation was carried out before the start of experimental studies. Lumpy coal was crushed in a two-stage grinding system, consisting of a jaw crusher and disintegrator, grinding to a fraction of less than 500 microns. The milled coal was sifted through a sieve system. The particle fraction selected for the experiments ranged from 80 microns to 200 microns. Wood processing waste was previously cleaned of bark and garbage, dried for one day to remove excess moisture at a temperature of 293 K, and then also sifted to isolate particles with a fraction of 80 to 200 microns. Proximate analysis of the initial fuel components and mixtures based on them was performed (calorific value, ash content, moisture, and volatile yield were determined) according to the methods of GOST 147-2013 (ISO 1928-2009), GOST 11022-95, GOST 27314-91(ISO 589-81), and GOST 6382-2001. Ultimate analysis of the initial fuel components was performed. The results of the analyses are shown in Table 1.  
The results shown in Table 1 allow us to conclude that the mixing of particles of such types of coal with wood using a percentage (by weight) ratio (coal/wood of 90%/10%, 75%/25%, and 50%/50%) leads to a significant reduction in the total mass of the ash residue. This reduction is not additive in relation to the calculated ash content of the base components and mixtures formed on their basis. The actual ash content values of the mixtures of the two studied coals and wood exceed the theoretical values. For example, if we consider the processes of pyrolysis and combustion of the coal and wood mixture independently, the theoretical ash content of such fuels, for example, based on T-grade coal (with 50% share of sawdust), should be 9.34%, and based on D-grade coal with the same share of sawdust in the mixture, should be 8.03%. According to the results of the experimental studies, the ash content of such mixtures was 11.08% and 10.44%, respectively [51]. These results give grounds for the conclusion about the chemical reaction of the gaseous and solid products of the pyrolysis of coal and wood with formation of metal salts that are part of coal (calcium, aluminum) and wood (calcium). Regularities in the formation processes of sulfur, nitrogen, and carbon oxides in the gaseous products of the thermal decomposition of the mixed fuels based on D and T coals and wood processing waste (pine sawdust) were experimentally investigated. Analysis of the gaseous pyrolysis products of the mixed fuels was performed using a TEST-1 gas analyzer manufactured by Boner LLC, Novosibirsk, which is part of the experimental stand. The layout of the stand is shown in Figure 1.  
The experiments were carried out according to the following scheme. A fuel sample (1) weighing 5 g (discreteness of the analytical balance is 10\({}^{-4}\) g) was placed in the cylindrical reactor (2). The reactor, in turn, was installed in a cylindrical heat-insulated cavity fixed at the \"hot\" end of a temperature-controlled air heater (3) of the LHS 61L PREMIUM brand (LEISTER, Kegiswile, Switzerland) with a capacity of 16 kW. Air was supplied to the heater using an AIRPACK supercharger (LEISTER, Kegiswile, Switzerland).