## 3 Combustion of Non-Conventional Liquid, High-Moisture, and Slurry Fuels
### Influence of the Composition of the Oxidizing Atmosphere on the Ignition and Combustion  
When burning low-grade mixed fuels, it is important to select the optimal ratio of components and the operating mode of technological equipment. It is known that ignition and combustion can be significantly improved by varying the composition of the oxidizing medium [76; 77]. For low-grade fuels with high moisture and ash content, this approach can be promising. This is confirmed by [PERSON] et al. in the study [55], where water-oil emulsions and coal-water slurries were burned in an environment enriched with oxygen (from 50% to 100%) at high temperatures in a chamber with industrial burners. These conditions are used to improve the combustibility and combustion efficiency of fuels with high water content. The slurry was prepared on the basis of water and pyrolytic carbon black (a tire pyrolysis product), which is difficult for direct disposal due to its high sulfur content [55]. [PERSON] et al. [55] found that it is important to use finely divided particles of pyrolytic carbon black for complete burnout of the slurry. The high share of water in the prepared mixtures (75% in the emulsion and 65% in the slurry) did not lead to combustion failure and contributed to a multiple decrease in NOx and CO emissions with a moderate decrease in the combustion temperature. [PERSON] et al. [55] concluded that, with a high O\({}_{2}\) content (50-100%), it is not necessary to additionally burn natural gas for the combustion of the slurry to be stable. It was also found that the oxygenated reagent can be used only as a spray gas, but in this case the thermal efficiency will be lower than when using oxy-fuel combustion. It is interesting to note that the O\({}_{2}\) proportion in the oxidizing atmosphere can have an ambiguous effect on the combustion efficiency of composite fuels. For example, [PERSON] et al. [78] performed a thermogravimetric analysis of the co-firing of lignite and corn straw while varying the proportion of biomass (28-40%) and oxygen (20-80%). Thermograms and the calculated combustion index showed that the proportion of straw has practically no effect on the combustion efficiency, in contrast to the oxygen content. At the same time, the oxygen saturation above 60% led to a decrease in the combustion index. The combustion index is often used by researchers when analyzing thermogravimetric results. Combustion index is a value calculated using thermogravimetric curves. The formula for calculating the combustion index can vary to a limited extent in different studies. However, the basic concept assumes the use of such quantities as the rate of weight loss during sample combustion (maximum and/or average), ignition temperature, and burnout temperature. In general terms, the combustion index is a fraction, in the numerator of which the product of the maximum and average rate of mass loss, in the denominator--the product of the ignition temperature squared and the burnout temperature. The higher this index, the more intensively the combustion of the sample proceeds. [PERSON] et al. [73] studied the combustion of woody biomass and the products of its pyrolysis (bio-oils and biochar). Fuels were burned in a tube furnace in a normal atmosphere and with an oxygen excess (40% and 60% O\({}_{2}\)). The bio-oils were pre-mixed with methanol (40 wt%) to lower the viscosity to facilitate atomization in the furnace. It was found that oxygen saturation had an insignificant effect on the emissions of inorganic substances during the combustion of bio-oils. The use of pyrolysis oil as a fuel makes it possible to reduce the number of solid particles in flue gases (up to 100 times), but the size of the resulting particles will be much larger than when burning the initial biomass. In general, [PERSON] et al. [73] recommended the liquid products of biomass pyrolysis for combustion in small boilers not loaded with specialized cleaning equipment, since such fuel will not cause major problems with slagging and ash removal. The solid residues