## 6 Discussion of Results
This point needs to be taken into account at the stage involving the pairwise comparison of criteria. Despite this, the potential of AHP is that we can place a parameter of interest at the zeroth level of the hierarchy and obtain an indicator that shows the fuel efficiency with a focus on the set priority. Figure 2: Integral efficiency indicators of fuels for China, Japan, and Russia that were calculated using the weighted sum method (WSM) (**a**), the weighted product method (WPM) (**b**) and the analytic hierarchy process (AHP) (**c**) (the numbers of columns in the diagrams correspond to fuels from Table 4; \(T_{g}\approx 800\)\({}^{\circ}\)C). It is clear from Figure 3 that \(A_{n}\) was not the same for all countries. In this case, the dominant reasons were the ones associated with the availability and cost of the fuel or its components in different regions, as well as the difference in the priorities given to certain categories of parameters linked to the specific aspects of the current state of the environment and the economy in the countries under study (Section 4). Notably, in each of the MCDA methods, fuels retained their overall ranking, irrespective of the country, i.e., variations in the role of single factors (cost, availability) and weight coefficients of a group of parameters for different countries did not significantly change \(A_{n}\). Depending on the country, \(A_{n}\) for the same fuel could vary in the following ranges: 3%-15% using the WSM, 3%-25% using the WPM, and 10%-60% using the AHP. The WSM and WPM calculations have a common concept. Therefore, the relative rating we composed for the fuels was identical, despite quantitative differences (Figure 2a,b). The slurry consisting of 40% filter cake (dry), 50% water, and 10% waste turbine oil had the highest coefficient. The rating of the fuels was rearranged when AHP was used. In this case, the leading role belonged to coal processing waste without additives and slurry with peat (Figure 2c, compositions 1 and 4). These fuels have environmental and social significance, as well as the benefit of low cost, which played a crucial role in the calculation of the integral efficiency indicator using AHP. The influence of heating temperature on the integral efficiency indicator is shown in Figure 4. The curves presented are for waste-derived slurry with peat (calculated for Japan). The analysis of Figure 4 and the results in Table 12, Table 13, and Table 23 indicates that there was no obvious increase or decrease in the fuel efficiency indicator when varying the temperature. This conclusion was also true for the position of the extremum of \(A_{n}\) at various temperatures with different fuels and calculation techniques. The obtained result was explained by the fact that the parameters that depend on temperature (e.g., SO\({}_{\rm x}\) and NOx emissions, ignition delay time) were normalized separately at each temperature value. Thus, even if the absolute values of a parameter did change with an increase of temperature, there was no guarantee that a normalized parameter would retain the same pattern of changes (Tables 7 and 9). Consequently, it was reasonable to perform a detailed analysis of the fuel efficiency that involved taking into account the absolute values of the constituents whose values depend on temperature. The relative change of \(A_{n}\) with a temperature increase (700-900 \({}^{\circ}\)C) had the following maximum values for different estimation methods: 15% for filter cake, \(\approx\)7% for coal-water  
Figure 3: Relative criteria (economic, environmental, social, energy, and safety) for coal, coal–water fuel, and waste-derived slurry. Criteria typical of Russia are shown at \(T_{g}\approx\) 700 \({}^{\circ}\)C.  
slurry and waste-derived slurry with turbine oil, 10% for waste-derived slurry with peat, and 20% for coal. Figure 5 shows a change in the efficiency indicator that was dependant on a set of conditions and factors (fuel composition, heating temperature, method of calculation), as illustrated by China.