## 2 Materials and Methods
### Ostwald Diagram  
In this section, the calculation of the Ostwald triangle is shown in detail (providing, as an example, the case of poplar pellet combustion, with the rest of the fuels showing equivalent behavior) as a complementary tool, in addition to the equipment used for the combustion system. Thus, the complete combustion line, the air line (n = 1), the air lines at different \(\eta\) values (1.5, 2, etc. ), and the lines at constant CO values are explained. Regarding the complete combustion line, Table 5 shows the points obtained in the case of poplar pellet combustion. As observed, \(\rm V_{CO_{2}}\) values are obtained from Equation (9). The slope determination of complete combustion line (\(\rm m_{ccl}\)) is obtained by using a line passing through two selected points (\(\rm X_{1}\), \(\rm Y_{1}\)) and (\(\rm X_{2}\), \(\rm Y_{2}\)). For that purpose, a and b parameters are determined, where a is the slope of the line and b is the independent term (see Equations (12) and (13)). \[\rm m_{ccl}=a=\frac{Y_{1}-Y_{2}}{X_{1}-X_{2}} \tag{12}\]  
\[\rm b=Y_{1}-\left(\frac{Y_{1}-Y_{2}}{X_{1}-X_{2}}\right)X_{1} \tag{13}\]  
In this case, two points from Table 5 were selected--(0.00, 0.16) and (0.21, 0.00)--and used to obtain the corresponding values for \(\rm m_{ccl}\) (that is, a)--(\(-0.765\)) and \(b\) (0.1607)--according to the abovementioned equations. Figure 6 shows the resulting complete combustion line for poplar pellets. \begin{table}
\begin{tabular}{c c c} \hline
**Raw Material** & **Position or Level Selected** & **Pellet Feed** \\ \hline Pine & & Normal (100\%) \\ Poplar & 1, 2, 3, 4 and 5 & Extra (125\%) \\ Plum tree & & \\ \hline \end{tabular}
\end{table} 
Table 4: The main conditions selected for pellet combustion. \begin{table}
\begin{tabular}{c c c c} \hline
**V\({}_{\rm O_{2}}\), \% (X)** & **V\({}_{\rm CO_{2}}\), \% (Y)** & **V\({}_{\rm O_{2}}\), \% (X)** & **V\({}_{\rm CO_{2}}\), \% (Y)** \\ \hline
0.00 & 0.161 & 0.11 & 0.077 \\
0.01 & 0.153 & 0.12 & 0.069 \\
0.02 & 0.145 & 0.13 & 0.061 \\
0.03 & 0.138 & 0.14 & 0.054 \\
0.04 & 0.130 & 0.15 & 0.046 \\
0.05 & 0.122 & 0.16 & 0.038 \\
0.06 & 0.115 & 0.17 & 0.031 \\
0.07 & 0.107 & 0.18 & 0.023 \\
0.08 & 0.099 & 0.19 & 0.015 \\
0.09 & 0.092 & 0. Regarding the air line (n = 1), the burnt gas ratio (x) was obtained using Equation (6). The selected points used to build the air line were as follows:  
* \(\rm V_{CO_{2}max}\), \(\rm V_{O_{2}}=0\), with n = 1 and x = 1. \(\rm V_{CO_{2}max}\) was obtained from Equation (10), resulting in a value of 0.161. * \(\rm V_{CO_{2}}=0\), \(\rm V_{O_{2}max}\), with n = 1 and x = 1. \(\rm V_{O_{2}max}\) was obtained from Equation (11), resulting in a value of 0.074. From these points, (0.000, 0.161) and (0.074, 0.000), the equation of the air line can be obtained, that is, a = \(-\)2.16 (in this case, the slope when n = 1, \(\rm m_{n=1}\)) and b = 0.161. The resulting line is shown in Figure 7.  
Concerning different values of n (for instance, n = 1.5) for the air line, the elements used included the slope obtained for the air line, with n = 1.0 (that is, \(\rm m_{n=1}\)); the point \(\rm V_{O_{2}max}\); \(\rm V_{CO_{2}=0}\); and the conditions (n = 1.5 and x = 0). The following points were obtained: (0.119, 0.000) and (0.000, 0.257). Thus, the resulting line is shown in Figure 8, as follows:  
Figure 6: The complete combustion line for poplar pellets. Figure 7: The air line for poplar pellet combustion. Concerning constant CO lines, they were obtained when a fixed value of CO was selected. For instance, in the case of CO = 0.02, the intersection point between the air line (n = 1) and the line where O\({}_{2}\) is constant was obtained, and then the line resulting from the abovementioned point and the slope of complete combustion was obtained. Figure 9 shows all of the lines obtained at constant values of CO (0.2, 0.4, 0.6, etc.).