## 3 Results and Discussion
### Heat Transfer through the Tube Wall of the Heat Exchanger When Slagging  
Figure 9 illustrates the change in the thickness of a slag layer on the surface of the heat exchanger tube during combustion of different fuels. When burning coal, the limiting value of the ash deposit thickness of 6.3 mm is reached in 7 min (areas in the boiler furnace subject to the most intense slagging, see Figure 6). When CWS and CWSP are burned, the formation of an ash layer in similar areas (Figures 7 and 8) occurs at a slower rate (a possible maximum is reached in 90 min). When burning coal, the growth rate of the slag layer thickness is 0.90 mm/min. When burning CWS and CWSP, this parameter is 0.077 mm/min and 0.072 mm/min, respectively. Figure 9: Typical change in the thickness of the slag layer on the surface of the heat exchanger tube when burning different fuels. Figure 10 shows the trends in the surface temperature of the slag layer at different temperatures of the combustion products. An intense temperature increase is most typical at the stages of formation and growth of a slag layer (Figure 10) on the heat exchanger surface. A constant surface temperature is reached after the completion of the slag layer formation. The time delay (about 6 min) between the moments of stabilization of the surface temperature and the thickness of the slag layer is because the thermal conductivity of the latter increases with an increase in the degree of ash sintering under the heating by flue gas [59; 60]. Figure 10: Ash surface temperature trends when burning different fuels: (**a**) coal, (**b**) CWS, (**c**) CWSP. When burning coal, the calculated rate of increase in the slag surface temperature was about 140 K/min (Figure 10a). During the combustion of CWS and CWSP (Figure 10b,c), the increase in the surface temperature of the ash layer was non-monotonic. At the beginning of the process, the average temperature growth rate was about 70 K/min. After 5-7 min and until the values stabilized, the average rate of temperature increase in the ash layer was about 5 K/min. In this case, the higher the temperature of the surrounding gas, the higher the intensity of heating of the ash layer surface. After stabilization of the thickness of the ash layer, the temperature of its surface no longer reaches the temperature of the surrounding high-temperature gases. Temperature losses during slagging of heating surfaces are 13-17% when coal is burned, 10-15% when CWS is burned, and 11-16% when CWSP is burned. Thus, the maximum difference between the temperature of the gases and the surface of the ash layer can be expected during the combustion of coal dust. For slurry fuels, this difference is expressed to a lesser extent. The results obtained suggest that, in terms of the ash layer thermal diffusivity, the combustion of CWS or CWSP is more efficient than coal combustion. Figure 11 shows the time change of the heat flux to a heat transfer fluid when burning different fuels at several temperatures of the gases. As the thickness of the ash layer increases, the heat flux decreases significantly and stabilizes after the formation of the layer, similarly to the ash surface temperature. The heat flow is reduced due to insulation of heat exchanger tubes during their slagging by 33-50% when coal is burned, and by 39-60% and 35-55%, when CWS and CWSP are burned, respectively. The intensity of heat flux decrease is at its maximum in the first 7 min of the process. When coal dust is burned (Figure 11a), the values of this parameter vary from 9.14 to 24.57 kW/min. Further, the value of the heat flux takes a steady value and does not change (Figure 11a). If CWS or CWSP are burned (Figure 11b,c), the decrease in heat flux is much slower (0.67-1.87 kW/min).