## 3 Results and Discussion
### _Furnace Space Particle Combustion_
#### 3.1.2 Particles Fragmentation during Combustion  
Destruction of fuel particles into smaller ones is referred to as fragmentation. This phenomenon occurs in the process of fuel combustion, gasification and pyrolysis [47; 48; 49]. There are two types of fragmentation. Primary fragmentation is destruction of particles at stages of fuel drying and devolatilization [50; 51; 52]. It is due to mechanical stresses between different points of the same particle with different temperatures, and due to the destructive effect of the pressure of gas-vapor formed during pyrolysis. Secondary fragmentation occurs at a stage of char residues conversion and is conditioned by the growth of the porosity of the particles [53].  
In a number of experiments, we observed the following mechanism of burning particle fragmentation. First, within several seconds after placing a sample into the furnace it  
Figure 4: Stages of particle combustion at a furnace temperature of 800 \({}^{\circ}\)C and air flow velocity of 20 m s\({}^{-1}\), type A holder. Temperatures indicated under the figures are readings of the thermocouple in the center of a particle. (a) 138 \({}^{\circ}\)C, (b) 340 \({}^{\circ}\)C, (c) 820 \({}^{\circ}\)C.  
Figure 5: Particle combustion at a furnace temperature of 800 \({}^{\circ}\)C and air flow velocity of 125 ms\({}^{-1}\), type A holder. Figures below the pictures are temperatures at the center of a particle. (a) 173 \({}^{\circ}\)C, (b) 202 \({}^{\circ}\)C, (c) 285 \({}^{\circ}\)C.  
carbonizes (Figure 6a). Then the outer layer of char starts reacting with the air flow and rapidly incandeses (Figure 6b). The incandesced char layer is separated from the particle surface (Figure 6c). The major share of separated fragments remains fixed to the particle and burns near its surface, but some fragments are separated and fly to the furnace space. In Figure 6d, the separated fragment can be seen in the top right-hand part of the photo. Particle fragmentation is probably due to the following two causes. At rapid heating of the surface layer of char, intensive pyrolysis commences in the underlying layers of a particle. The gaseous products formed put pressure on the surface layer. During heating this layer expands, which facilitates its separation from the particle. The existence of a similar mixed mechanism of fragmentation was previously reported in the literature [50].  
Fragmentation, to a certain extent, is a stochastic phenomenon, and the probability of its occurrence can conveniently be estimated by the fragmentation event rate [54]. This parameter is calculated as a ratio of fragmentation events to the total number of parallel samples tested under specified conditions. The number of parallel samples in the experiment ranged from three to five. From a statistical standpoint, this number is not large; therefore, estimation of fragmentation event rate was rather approximate.  
Fragmentation event rate to a great extent depends on the velocity of the air coming to the particle from a tuyere, and to a lesser extent, on the furnace temperature (Table 1). At the air flow velocity of up to 50 m s\({}^{-1}\) the fragmentation occurs rather rarely, and its event rate virtually vanishes. Starting from 60 m s\({}^{-1}\), the fragmentation event rate grows, and in the range of 95-125 m s\({}^{-1}\) it is observed in the major share of particles. Strong dependence of fragmentation event rate on the air velocity is consistent with the mechanism of process occurrence proposed above. Air flow reacts intensively with the char layer on the particle surface at high air velocity. Rapid heating of this layer accelerates particle pyrolysis and creates temperature stresses in the char layer.  
The fragmentation phenomenon is not desirable in the process of fuel combustion and gasification. It facilitates the formation of small char particles and their removal from the bed with ash, which reduces the efficiency of fuel conversion and aggravates the problems of solid residue utilization afterward [55].