# Nozzles for Spraying Coal-Water Fuels
## 3 Generalization of Information  
The review of modern CWF spraying devices allows for an evaluation of their efficiency in terms of several criteria: simplicity of design (A), predisposition to clogging of the fuel channel (B), durability in terms of erosion wear of the nozzle channel (C), dispersion of the jet--the average size of CWF droplets in the jet (D). Table 1 shows the indicators of the analyzed CWF spraying devices according to the above criteria. The analysis of the considered coal-water fuel nozzles [7; 11; 17; 18; 19; 20; 21; 22] allows the conclusion that they all meet the requirement--the formation of a fine-dispersed CWF jet. Simple design and durability from the point of view of erosive wear of the nozzle channel is typical of the nozzle in [17]. A sufficiently fine-dispersed jet is formed during its operation, but the presence of large CWF drops is not excluded. The pressure of the CWF and SA is approximately equal due to the mixing inside the nozzle which leads to fine atomization of the fuel. However, the instability of the spraying characteristics is typical for injectors with internal mixing. As a result, large drops of fuel are present in the jet during their operation. The fuel channel is possible to be clogged with large coal particles or fuel agglomerates (with characteristic size of more than 1 mm) when spraying viscous CWF. Thus, the CWF nozzle in [17] meets criteria A and C.  
Figure 10: CWF spraying nozzle [22]. The spraying devices in [7; 18] exclude the presence of large fuel droplets in the jet. First of all, this happens due to the high consumption of SA. A large amount of SA at high speed breaks the CWF jet and large drops occur even at a considerable distance from the nozzle. The fuel flow rate for such injectors is insignificant, for example, no more than 1000 kg/h. At the same time, the injectors in [7; 18] have a rather complex design and a small diameter of the fuel channel. The latter can cause clogging with large coal particles or fuel agglomerates. The geometric characteristics of the fuel channel may cause the relatively short operation of the nozzle in combination with the erosion wear. Thus, the CWF spraying devices in [7; 18] correspond only to criterion D.  
The spraying device with the internal mixing of CWF and SA in [19] has a rather complex design, while geometrically the characteristics of the nozzle indicate that it is not protected against clogging of the fuel channel (diameter of the fuel channel is no more than 4 mm) for the same reasons characteristic of the injectors in [7; 17; 18], and erosion wear. Large fuel droplets are present in the jet despite the presence of small-sized CWF droplets. A well-homogenized coal-water fuel prepared with very fine coal is necessary when using such a nozzle. Thus, the CWF spraying device in [19] does not meet any of the criteria. Injectors with external [20] and internal [11] mixing of CWF and SA have a fairly simple design (there is no complex geometry of the nozzle channel). At the same time, they also allow for the exclusion of large drops of CWF in the jet. However, the relatively small diameter of the fuel channel may also cause its clogging and erosion wear. The issue of fuel flow rate remains unsolved. Thus, the coal-water fuel nozzles in [11; 20] meet criteria A and D.  
The positive qualities of the nozzle in [21] certainly include a large diameter of the nozzle channel. The clogging of the fuel channel is possible to be eliminated due to this. The high probability of erosion wear remains. At the same time, this nozzle has a fairly simple design, which is very important for practical application. A sufficiently fine jet is formed due to the internal mixing of CWF and SA. This type of CWF nozzle is characterized by instability of the spraying characteristics, as in the case of [17]. As a result, large drops of fuel are present in the jet during their operation. Considering the above, the nozzle for CWF spraying in [21] meets criteria A and B. The nozzle for CWF spraying in [22] is of the internal mixing type.