**3. Results**

The reaction ratio in the study was defined as the removal ratio of fixed carbon from the sample. A carbon combustion reaction can be described using the following chemical reaction, if CO gas formation is ignored:

$$\text{CO}(\text{s}) + \text{O}\_2(\text{g}) = \text{CO}\_2(\text{g}) \tag{1}$$

In the combustion experiment, the sample weight loss was attributed to the decrease in the amount of fixed carbon. Therefore, the reaction ratio (F) at a reaction time can be described by Equation (2):

$$F = \frac{\Delta w\_{\text{f}}}{W} \tag{2}$$

Fractional reaction curves at 1073 K are shown in Figure 4. The figure shows that combustion rates of charcoal were quicker than those of coke. This tendency was also observed at 1223 K, 1373 K and 1523 K.

**Figure 4.** Fractional reaction curves of coke and charcoal combustion at 1073 K.

To verify the reaction mechanism of the combustion reaction, samples with a reaction ratio of 50% were also prepared under 1073 K and 1523 K and cross-sectional and microscopic observations were made. Figures 5 and 6 show the cross-sectional observation and the microstructure at the reaction interfaces of each sample. It was clear that the combustion reaction was a topochemical reaction.

**Figure 5.** Cross-sectional view of the samples.

**Figure 6.** Microstructure of the samples at the reaction interface.
