## 2 Materials and Methods
Filter cake is a slurry consisting of coal dust, water, and surfactants. Coal particles have a size of 80-140 \(\upmu\)m, therefore, no additional pulverization was necessary. Fuel mixtures were produced by stirring all the slurry components for 5-10 minutes, using a homogenizer. The composition of all fuel mixtures consisted of 90% filter cake (in the wet state) and 10% biomass or vegetable oil. The mass fraction of the main component and additives, when preparing composite fuels, is selected based on the region-specific distribution of resources. The concentration of additives in fuels can normally vary from 5% to 20%. Such ratio is quite typical when recovering oil production waste [8], residual wood [44], sunflower hulls [45], and rapeseed oil [46] by co-firing them with coal fuels. When burning coal-water fuels based on coal processing waste, which is abundant at industrial landfills, adding a relatively small amount of other combustible components with different generation rates makes it possible to recover a group of different kinds of waste by co-firing them. The mass concentration we chose (10%) is also universal in terms of combustion technologies. Adding a lot of solid plant waste significantly increases the viscosity of the mixture, making it unfit for pipeline transportation and flame combustion in boiler furnaces. One should also keep in mind that power generation facilities must not depend entirely on the scale of crop farming, which is, in turn, too dependent on subjective factors (weather conditions in the region and natural hazards). Additional components should be chosen based on their availability, generation scale, and presence of alternatives. Thus, for instance, various kinds of wood (pine, birch, oak), forest fuels (leaves of different trees, pine needles, grass), hull of different crops (rice, wheat, cotton), and waste of plant raw material processing to produce foodstuffs (sunflower and rapeseed press cake, palm kernel waste, cocoa shells, sugar cane bagasse) are interchangeable. It can be therefore concluded that for this research we have chosen typical biomass components, having alternatives in other regions, and their mass concentration of 10% is an appropriate value for composite fuels. Table 2 presents the properties of coal and other components, used in this research to produce slurries. Thus, in the present study, the following types of fuels (with relative mass fractions) were considered, the list of which is presented in Table 3.  
In present research, it was recorded the following main parameters: ignition delay times \(\tau_{\text{d}}\), threshold temperatures of combustion initiation Tgmin, maximum combustion temperatures \(T_{\text{b}}\,^{\text{max}}\), and the most hazardous gaseous anthropogenic emissions (NO\({}_{\text{x}}\), SO\({}_{\text{x}}\)). It was also calculated the ash content and combustion heat of slurry fuels with regard to the characteristics and mass concentration of the components (Table 2). The parameter values obtained were then used to calculate the relative parameters showing the advantages of slurry fuels vs. coal. The ignition and combustion characteristics of slurry fuels with biomass were determined using an experimental setup presented in Figure 1[15; 18]. A high-temperature air medium was created with a tube muffle furnace R 50/250/13 (Nabether therm GmbH, Lilienthal, Germany). The temperature in the experiments was varied from 700 to 900 \({}^{\circ}\)C. This range covers the most typical temperatures of the combustion of slurry fuels with biomass, for which the typical ignition and combustion stages are usually studied [43; 47]. After the furnace was heated, the fuel on the holder was placed into it, using the sample feeder. A Phantom v411 high-speed color camera (Vision Research Inc, Wayne, NJ, USA) was used to determine the ignition delay times of the experimental samples. The Tema Automotive Video software (Image Systems AB, Linkoping, Sweden) was used to analyze the video recordings of the experiments.