## 6 Discussion of Results  
There are currently a lot of limitations to the creation and development of the technologies of industrial waste recovery as slurries of complex compositions. One of these limitations is a lack of knowledge about the characteristics and threshold conditions of fuel slurry combustion, as well as the absence of a complex evaluation and comparison with more widespread (traditional) fuel types. In this section, we aim to analyze the generalized efficiency indicators for bituminous coal, coal-water slurry, and three waste-derived slurries, using MCDA. Figure 2 illustrates the values of the integral efficiency indicator for the five fuels under study and the three calculation techniques. Figure 2 also shows the range that the fuel efficiency indicator can vary within, depending on the country that was analyzed. We can see that coal featured as the lowest indicator, and the slurries outrated it to a greater or lesser degree. This conclusion was true for all the countries and calculation methods. Coal differed from the other fuels in the following ranges: 10%-45% using the WSM (Figure 2a), 16%-70% using the WPM (Figure 2b), and 65%-95% using the AHP (Figure 2c). Despite the high priority of the energy parameter category in the WSM and the WPM (the weight of this category exceeded the weights of the other groups of parameters for all three countries), the good energy performance indicators of coal (Table 3) did not allow it to score highly overall. There were several reasons for the big discrepancy in the values of these criteria for coal and slurries. First, coal combustion products contain a higher amount of sulfur and nitrogen oxides than slurries (Table 7). The benefits of slurries in SO\({}_{\text{x}}\) and NO\({}_{\text{x}}\) emissions are explained not only by a lower temperature in the combustion zone, but also by the possibility of some chemical reactions involving water vapors, which prevent (or rather, reduce the rate of) the oxidation of nitrogen and sulfur [37; 38; 39; 40]. Second, coal is fire-hazardous and is characterized by dusting. The third reason is large-scale and long-term: using coal only cuts down its emergency stocks and increases inherent processing waste. The fourth reason is that coal is more valuable, and thus, more expensive than the waste components of slurries. The last two reasons, as well as additional storage difficulties (due to high lamination rate), were responsible for the rather low efficiency indicator of coal-water fuel (Figure 2). To illustrate the contribution of each indicator to the integral \(A_{n}\), in Figure 3, we present a diagram of the relative criteria when using the WSM and the WPM. Here, Russia is used as an example. The typical ignition and combustion indicators were taken from the experiments at a heating temperature of about 700 \({}^{\circ}\)C. The results are shown for coal and slurries with the highest and lowest efficiency. It is clear from Figure 3 that there was a significant difference in almost each particular criterion, which was especially distinct for coal and waste-derived slurries. According to the AHP calculations, coal and coal-water slurry have the lowest integral efficiency indicators (lower than in the WSM and the WPM). Moreover, the category of energy parameters obtained a low weight coefficient (Table 23) in the pairwise comparison of the AHP. This category characterized the overall advantages of coal and coal-water slurry (Table 5). Moreover, we put the fuel cost, which was the definitive factor for the final \(A_{n}\), at the zeroth (most important) level of the hierarchy. Therefore, we can see the greatest deviation of \(A_{n}\) for bituminous coal and coal-water slurry from that of cheaper fuels (Figure 2c). In particular, there was a 32-fold difference between filter cake and coal for China in the calculations using the AHP (Figure 2c). Undoubtedly, this approach can bring a low-cost fuel with poor environmental characteristics and a high fire hazard to the top of the ranking.