## 3 Results and Discussion
### Morphologies of Cenospheres  
As seen in Figure 4a,b, the spherical particles were predominantly observed. Substantial differentiation of particle size is visible. Fly ash consists of spherical particles and some irregularities in size and shape. This is in good agreement with the result of particle size distribution varying broadly as shown in Table 2. The morphological structure of fly ash particles is controlled by the combustion temperature and the cooling rate of the boiler and its operating conditions [52]. During the combustion process, the heat causes the mineral matters to become fluid or volatile to react with oxygen. Whereas during the cooling process, the minerals may form crystalline solids, amorphous or condense spherical particles, or aggregates forming on the particle surface. In a boiler chamber of Mae Moh power plant, the organic materials in coal are combusted under high temperature and pressure, then the minerals transform into ash. The Mae Moh ash appeared to have crystal aggregates on the surface due to the slag occurrence in the boiler that is caused by quality of coal, mineral transformation and decomposition, and reaction chemistry of ash species due to heat [53]. Figure 3: (**a**) Average shell thickness and particle diameter/shell thickness ratio plotted against particle diameter. (**b**) A plot showing a relation between percent wall thickness-to-diameter and particle diameter. The SEM images of the collected nanospheres are shown in Figure 4c-h. The majority amount of cenospheres was found in the coarse grains. The size of cenospheres was obviously larger than fly ash. The smooth, dense, perforated, and vesicular spheres were commonly observed. Such dense and vesicular morphologies originate from fully and partly degasificated melts of various minerals during burning [54]. Most of the obtained cenospheres contains the intact cenospheres, while damaged spheres or the partially broken spherical particles (Figure 5a, b) were periodically observed in a slight amount, which can be visible in the low magnifications. The crushed spheres could affect the cenospheres recovery yield and their quality, as mentioned earlier in the Section 3.1. For a specific  
Figure 4: Scanning electron microscopy (SEM) images of (**a,b**) fly ash and (**c–h**) cenospheres collected by sink-float method, using a fly ash-to-water ratio of 1:10, a 2-h soaking period. purpose concerning the porosity and surface area characteristics, such broken spheres could probably lead to higher surface area than the other intact cenospheres fractions; hence, an additional step, such as sieving, may be applied prior to utilization. In addition, it can be noticed that there are agglomerates and irregularly shaped particles appearing on the cenospheres surface, as clearly seen in Figure 4f-h. This could probably be due to the mineral (e.g., quartz) melting around the inter-particle contact and rapid cooling [52]. The collected cenospheres are hollow spheres and have different structures: a single-ring structure, a porous shell structure (see the cross-sectional view in Figure 2). The single-ring structure was found for the small particles with the size \(<\)100 \(\upmu\)m, whereas the larger particles \(>\)100 \(\upmu\)m the porous, thick shell structure was observed. The presence of cenospheres was due to the relatively higher viscosity of the melt forming an appropriate envelope possibly retaining the gas bubbles for a longer period while hardening [54]. Spheroids were rarely noticed containing in the collected cenospheres fractions, as clearly seen in Figure 4d. The surface texture of spheroids is porous and vesicular similar to those observed fly ash and the typical cenospheres. The spheroid structure was believed to originate from the spontaneous formation while fast cooling of cenospheres in a liquid or plastic state [54].