## 3 Combustion of Non-Conventional Liquid, High-Moisture, and Slurry Fuels
### Combustion of Wet Sludge, Process Fluids, and Waste Cooking Oils as Part of Fuel Mixtures
[60] investigated the joint and individual combustion of heavy fuel oil and pyrolysis oil obtained from sewage sludge. Thermogravimetric analysis of fuels was carried out, as well as their combustion in the form of single droplets. It was found that the addition of pyrolysis oil led to a decrease in the ignition temperature and an increase in temperature at the combustion stage. An increase in the proportion of oil (the range was 20-80%) intensified micro-explosions of mixed fuel droplets and increased the burning rate. The negative effect was an increase in the ignition delay time due to an increase in the total moisture content of the fuel. In addition, [PERSON] et al. [60] found that the proportion of pyrolysis oil has little effect at the final stage of the combustion of the composite fuel when heavy components are burned out. Waste cooking oils are one of the large categories of waste, typical for municipal areas. According to the report [84], in 2015, about 712 kt of waste cooking oils were collected in Europe (excluding data from some countries). Without proper disposal, waste cooking oils represent a dangerous source of environmental pollution. Typically, waste cooking oils are considered as feedstock for the production of biofuels (biodiesel, biogas, and hydrogen gas), but can also be used as fuel for boilers. [PERSON] et al. [85] investigated the spraying and combustion of waste cooking oil in a 500 kW industrial boiler. Compared to light diesel fuel, the cooking oil had a higher viscosity and a lower adiabatic flame temperature (the difference was 200-250 \({}^{\circ}\)C). CO and NOx emissions from burning cooking oil were several times less than from burning diesel fuel. The emissions of unburned hydrocarbons from cooking oil were 2-4 times higher than from diesel fuel. [PERSON] et al. [85] emphasized that many characteristics of both diesel fuel and waste cooking oil have been successfully improved through the use of additives. [PERSON] et al. [86] studied the combustion characteristics of mixtures of waste cooking oil and diesel fuel. The proportion of cooking oil was 20% and 40%. According to the results of the experiments, [PERSON] et al. [86] found that the emissions of dioxins, furans, and polychlorinated biphenyls during the combustion of these mixtures are 26% and 86% lower in comparison with petroleum diesel fuel. [PERSON] et al. [86] concluded that mixtures based on waste cooking oils are promising as an auxiliary fuel for a boiler that incinerates solid household waste. [PERSON] et al. [87] confirm that the energy recovery of waste cooking oils is one of the most beneficial strategies for their use. Ref. [87] considers three recovery scenarios: biogas production in an agricultural biogas plant, combustion in a cogeneration plant, and biodiesel production. All three approaches were recognized as environmentally promising since they reduce greenhouse gas emissions. [PERSON] et al. [87] concluded that the most efficient scenarios are the processing of waste cooking oils into biodiesel and use for the production of heat and electricity.