## 3 Combustion of Non-Conventional Liquid, High-Moisture, and Slurry Fuels
### Synergism of Components at Ignition and Burnout of Composite Fuels  
The issue of choosing the ratio of the components in the fuel mixture is very relevant for researchers. This is important since even small changes in the component composition of the fuel can lead to both positive and negative consequences during combustion. When mixing components of different origins, synergies often arise. For example, [PERSON] et al. [65] studied the combustion of bio-oil/biochar slurry in a drop tube furnace at 1400 \({}^{\circ}\)C to identify synergistic effects in the formation of \(\mathrm{PM_{10}}\). For the preparation of slurries, bio-oil and biochar (pine pyrolysis products) were used. The proportion of biochar in the slurries was 5% and 10%. Mixed fuels, as well as individual components, were fed into a tubular drip furnace and burned at 1400 \({}^{\circ}\)C. Particle matters were collected and analyzed. The calculated values of the yield of \(\mathrm{PM_{0.1-1}}\) and \(\mathrm{PM_{1-10}}\) differed from those established experimentally for two-phase mixtures, which indicates the synergism of the components. In particular, the emissions of \(\mathrm{PM_{0.1-1}}\) were higher than the calculated ones, and the emissions of \(\mathrm{PM_{1-10}}\) were lower than the calculated ones. [PERSON] et al. [65] concluded that the two mechanisms operate simultaneously. The first mechanism is associated with the extraction of inorganic particles from biochar into bio-slurry; the second mechanism is due to the interaction of components during combustion (volatile oils contain reactive substances that activate the volatilization of inorganic particles of char). Synergistic interaction of components in slurry fuel during its combustion was also observed in the study [81]. Thermogravimetric analyzer STA409 PG was used for studying the combustion characteristics of slurries derived from biomass, coal, and char. Bio-oil was produced by pyrolysis of fruit trees and mixtures of rice and straw. The share of bio-oil in the slurry varied in the range of 34-40%. Characteristics were also obtained for samples of solid fuels--lignite, char, and long-flame coal. [PERSON] et al. [81] noted a synergistic effect between bio-oil and coal or coke, resulting in improvedcombustion performance [81]. Presumably, the main reason was the catalytic activity of alkaline and alkaline earth elements contained in bio-oil, as well as micro-explosions. The change in the activation energy had a different character when using bio-oil of different origins. Lignite-based fuels had the lowest activation energy [81]. [PERSON] et al. [81] concluded that the investigated slurries are applicable in the industry. Compared to coals and char, slurries based on coal and bio-oil have a lower combustion temperature and burnout time. Therefore, when they are burned, it is possible to reduce the requirements for the material of the inner surface of the boiler and use more compact combustion chambers [81]. Studies [41; 55; 56] have shown that the synergism of the components during the combustion of low-grade fuel blends is manifested under certain conditions (the temperature in the chamber has a dominant effect). For example, [PERSON] et al. [41] studied the ignition and combustion behavior of slurry droplets based on high-moisture coal waste, municipal solid waste, and waste oil when droplets are heated in a muffle furnace. Changes in the fuel composition (varying the proportion of municipal waste, adding oil) significantly affected the ignition characteristics in the temperature range 600-800 \({}^{\circ}\)C, but with an increase in the heating temperature (800-1000 \({}^{\circ}\)C), the ignition of fuels of different composition occurred with a very close time delay (the difference about 5% at 1000 \({}^{\circ}\)C) [41]. Co-combustion of coal and additional components also provides beneficial effects but on a limited scale. This was confirmed, for example, in the study [59] during the combustion of dewatered sewage sludge and coal sludge in a thermogravimetric analyzer.