## 3 Combustion of Non-Conventional Liquid, High-Moisture, and Slurry Fuels
### Combustion of Low-Grade Liquid, Wet, and Slurry Fuels in Large Experimental and Pilot-Scale Plants  
The experience of combustion of liquid, high-moisture and slurry fuels in large plants is relatively small today. The available research results [51; 53; 70; 71], as well as the experience of combustion of coal-water slurries [88; 89], allow us to conclude that vortex combustion chambers, boilers with burners and nozzles for fuel injection, grate, and fluidized bed boilers can used for industrial combustion of low-grade fuels with high moisture content (Figure 2). [PERSON] et al. [74] carried out studies at an operating grate-fired boiler at a thermal power plant (Enkoping in Sweden) with a capacity of 55 MW of thermal energy and 24 MW of electricity. The base fuel for the boiler was wood. In the experiment [74], a high-moisture sewage sludge (moisture content 76%) was burned together with wood chips from the demolition of buildings. The proportion of the sewage sludge was 35% and 45%. When testing new fuel, the boiler operated at 50% capacity. Compared to the combustion of wood chips, the mixture with sewage sludge produced more nitrogen oxides (a moderate increase relative to permissible levels) and sulfur oxides (a significant increase--at least by 8 times). [PERSON] et al. [74] concluded that, for the combustion of such mixtures in the existing plant, it is necessary to further improve the flue gas condensation and SO\({}_{2}\) removal. It was noted that the incineration of mixtures did not cause problems with slagging, contrary to expectations [74]. In the experiment, which lasted 12 h, it was possible to provide the required boiler power by increasing the fuel consumption. In general, [PERSON] et al. [74] assessed the experience gained as positive and concluded that the combustion of high-moisture mixtures with minimal preparation of sewage sludge can be considered successful in the existing boiler. [PERSON] et al. [75] obtained important results on the combustion of fuel slurries based on coal and liquid waste from petrochemical production. The fuels were burned in a pilot-scale furnace with a slurry flow rate of up to 100 kg/h. To warm up the furnace to at least 500 \({}^{\circ}\)C and improve the ignition of the slurry, diesel fuel was used, the supply of which was stopped after the combustion stabilized. Compared to the conventional coal slurry (base on water), the slurries based on waste liquids exhibited a faster reaction rate at ignition. [PERSON] et al. [75] concluded that this result may be due to the fact that the petrochemical waste liquids contain volatile reactive components and metal ions, which catalyze the release of volatiles and burnout of coal particles. The combustion temperatures of the waste-based slurries were generally 50-100 \({}^{\circ}\)C higher than that of the conventional coal-water slurry. Analysis of unburned residues showed that the carbon content in them during the combustion of the slurries based on liquid waste is less than that of the slurry based on water. However, this difference leveled out in certain areas of the furnace and the carbon content in the residues reached 10-25% for all fuels. The combustion efficiency of  
Figure 2: The main types of large plants used for the combustion of low-grade, high-moisture, and slurry fuels. waste-based slurry was higher than that of coal-water slurry and approached the efficiency of pulverized coal combustion. In this case, a negative effect was enhanced slagging due to the high content of alkaline earth metal ions in the liquid waste. However, at the same time, the catalytic retention effect of these ions can cause the observed decrease in SO\({}_{2}\) and NO\({}_{x}\) emissions compared to the slurry based on water [75]. [PERSON] et al. [57] used waste soot during combustion in the composition of a coal-water slurry based on coal. The soot concentration was 2.5%, 7.5%, or 10% [57].