## 3 Results and Discussion
### Physical Properties of Cenospheres
The plot has revealed that the shell thickness increased linearly with increasing the particle size, similar to those observed in the literature [14; 16; 50]. This clearly confirms that the increase in wall thickness strongly correlated with the increase in cenosphere particle size; the linear correlation coefficient is expressed in Figure 3a. The ratio of wall thickness to diameter was believed to strongly depend on the ash chemistry combustion condition, in terms of temperature-time history that the particles experienced in the furnace [16]. In relation to the particle diameter, the particle diameter/shell thickness ratio was also examined to elucidate the wall shell formation as a function of physical characteristics in terms of cenosphere particle. In Figure 3a, the particle diameter/shell thickness ratio for Mae Moh cenospheres obtained from lignite fly ash had a range from \(\sim\)10 to 37. This ratio varied in a range of 19 to 38 for the smaller particles (average diameters 25.9 and 46.9 \(\upmu\)m), and relatively constant at \(\sim\)10 for the larger particles (average diameters 93.7 and 229.2 \(\upmu\)m). A similar trend was observed for the cenospheres obtained from other fly ash sources; the particle diameter/shell thickness ratio was in the range from 20 to 30 for the smaller particles (diameters 50-140 \(\upmu\)m), and remained constant at \(\sim\)20 for the larger particles  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Item**} & \multicolumn{4}{c}{**Size Range**} \\ \cline{2-5}  & \textless{45} \(\upmu\)m & \textless{45-100} \(\upmu\)m & \textless{100-250} \(\upmu\)m & \textgreater{250} \(\upmu\)m \\ \hline Weight fraction (wt.\%) & 6.83 & 46.48 & 41.06 & 3.56 \\ Density (g/cm\({}^{3}\)) & 1.40 \(\pm\) 0.01 & 0. (diameters 140-340 \(\upmu\)m) [16; 37]. It is interesting to note that the cenospheres of similar sizes may have a wide range of shell thickness, and the wall thickness broadly increases with increasing the particle diameter [16]. The shell thickness observed for the cenospheres from different sources is about 1-18 \(\upmu\)m; the ratio of wall thickness to particle diameter of cenospheres is generally limited to the range of \(<\)10.3% [14; 16; 18; 50]. Agreeing well with those reported in the literature, the wall thickness-to-particle diameters of Mae Moh cenospheres were found in the ratio range of less than 10%; see Figure 4b. In Table 4, it can be seen that the majority (-87 wt.%) of the Mae Moh, lignite cenospheres have sizes between 45 and 250 \(\upmu\)m. Whereas the relatively smaller fractions, -7 wt.% and \(\sim\)3 wt.% of total cenospheres, have sizes of intervals \(<\)45 and \(>\)250 \(\upmu\)m, respectively. Comparatively reported by other studies, \(\sim\)60%-94% of cenospheres collected from Class F fly ash have particle sizes between 45 and 250 \(\upmu\)m, \(\sim\)15% is possibly in the range of 150-200 \(\upmu\)m, and \(\sim\)5% is in the range of 200-250 \(\upmu\)m [16; 44]. The particle size distribution of cenospheres was found primarily to depend on the solid fuel preparation or crushing process before combustion, rather than the factors of coal composition and the coal combustion conditions [42].