## 4 Results and Discussion
### Process Optimisation (Carbon Energy)  
This section shows the relationship between the carbon content in grammes (g) of each charcoal sample after testing and the thermal energy (kWh) required to reach these carbon content values. First, the regression functions were obtained for the trend lines showing the amount of carbon energy (g_C/kWh), for both the L and C tests. A polynomial regression function of degree 3 was chosen, as the R\({}^{2}\) values were quite close to unity, as shown in Appendix A, Table A2 and Figure 7, as well as closely fitting the results obtained. This regression function is shown in Equation (5), and the values of its coefficients are shown in Appendix A, Table A2.  
\[\text{g\_C/kWh}=\text{a\_x}^{3}+\text{b\_x}^{2}+\text{c\_x}+\text{d} \tag{5}\]  
First, the optimisation was studied for the average amount of carbon obtained in the tests, which turned out to be 10.12 g per test. The average amount of energy consumed in the tests was also calculated, which is 0.55 kWh/test, resulting in an average of 18.54 g C per kWh. With these average values, an analysis of differences between the C and L tests could be carried out. This average value (horizontal line of 18.54 g C/kWh) is shown in Figure 7, and the results are separated according to the duration of the tests (2 h and 3 h). Analysing the type C tests, it can be seen that the function of the type C tests intercepted the average value almost at the 350 \({}^{\circ}\)C for 2 h test. On the other hand, for the case of the type L test function, this intersection occurred for the 400 \({}^{\circ}\)C for 2 h test. The same pattern was again shared for the three-hour duration results, where the average value of 18.54 g C/kWh was achieved at the 300 \({}^{\circ}\)C for 3 h point for the C type tests, while in the case of the L type tests the intersection point is between 300 \({}^{\circ}\)C at for 3 h and 350 \({}^{\circ}\)C at for 3 h. The first conclusion to be drawn from this analysis is that the same values of average carbon content per kWh used were reached earlier in the type C tests than in the type L tests, i.e., for lower temperature values. This means that the carbonisation process was most effective when carried out in the airtight conditions provided by the can.  
On the other hand, it should be remembered that a higher amount of fixed carbon means a better quality of energy content, higher stability, reduced emissions and less ash [36]. A carbon content of 70% or less in the sample results in a poor-quality charcoal. It is a corrosive material due to its composition of acid tars and will therefore burn without a smoky flame. Based on these considerations, it was decided to target a minimum desirable fixed carbon content of 75%, justified by the above, although it is true that at the industrial level, a variability of fixed carbon content ranging 60-80% had been identified, the average being 70% [37].  
Consequently, it had to be determined under which test conditions this fixed percentage of carbon content was achieved, depending on the temperature and the type of test  
Figure 7: Graph comparing g of carbon per kWh of energy used as a function of the type of test. (**a**) Tests lasting 2 h. (**b**) Tests lasting 3 h.  
(L, C). Looking at the chemical composition graphs (Figure 6), it could be seen that in the 400 \({}^{\circ}\)C for 2 h type L tests, the reference value of 75% was exceeded, with a value of 77.2% fixed carbon content, and that the reference value of 75% was almost reached in the 350 \({}^{\circ}\)C for 3 h test (74.2%). On the other hand, looking at Figure 7, we observed that in the 350 \({}^{\circ}\)C for 3 h tests of type C, 76% of the fixed carbon content was also reached. The results are very similar between both types of tests, which leads to us to surmise that in order to obtain a charcoal with an acceptable performance (75% fixed carbon content) the tests must be carried out at least at 350 \({}^{\circ}\)C for 3 h for the C type and 400 \({}^{\circ}\)C for 2 h for the L type.