## 3 Results and Discussion
### Poplar Pellet Combustion  
Regarding Poplar pellets and their combustion performance in the domestic stove, the main results for different power levels are included in Table 12 and Figures 17 and 18. As observed, when power was increased, the excess air coefficient (\(\eta\)) decreased. However, unlike pine and plum tree pellets, the burned gas ratio (x) was kept practically constant regardless the level or position selected, thus increasing the flue gas temperature and, subsequently, keeping the yield of the process constant. These results are consistent, as \(\eta\) did not decrease as much as it did in the two previous cases, not considerably decreasing the exhaust gas total volume. In contrast, the exhaust gas temperature increased in this case (see Figure 18). This fact indicates that losses due to sensible heat are practically constant and, considering the constant trend observed for x, this implied a constant yield  
When extreme conditions were compared (see Table 13), the same trends were observed, as in the case of pine and plum tree pellets, leading to the conclusion that a decrease in excess air implied a more optimal combustion condition. Higher carbon dioxide emissions are related to higher temperatures in combustion processes, requiring temperature control according to previous studies, in which analyses of the combustion emissions of sugarcane bagasse were carried out in a burner pilot [47]. Figure 17: Ostwald diagram for poplar pellets, and experimental results for each level used in the combustion stove. Figure 18: Evolution of the main parameters for poplar pellet combustion. Finally, according to Figure 19, increasing the feeding rate showed similar results compared to the previously tested pellets, although a constant CO percentage, along with a lower increase in exhaust gas temperature when the pellet feed was at 125%, implied a higher yield for poplar pellet combustion. It should be noted that the behaviors of different kinds of biomass could be different in some aspects, such as carbon monoxide or carbon dioxide emissions, depending on their composition and disposition (raw, briquette, or pellet) [22; 28; 48]. If the different biomass products are compared at their optimum conditions (that is, P = 5 and +125% biomass for pine; P = 4 and +125% biomass for plum tree; and P = 3 \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Test Parameter**} & **Poplar Pellet** & **Poplar Pellet** & \multirow{2}{*}{**Comparison**} \\  & **(P = 3, 100\% Feed)** & **(P = 5, 125\% Feed)** & \\ \hline O\({}_{2}\), \% & 17.1 & 12.5 & \(\downarrow\) \\ Air supply temperature, \({}^{\circ}\)C & 60.7 & 77 & \(\uparrow\) \\ Flue gas temperature, \({}^{\circ}\)C & 78.8 & 115.6 & \(\uparrow\)\(\uparrow\) \\ Ambient temperature, \({}^{\circ}\)C & 14.2 & 21 & \(\uparrow\) \\ CO, ppm & 806.7 & 1058.6 & Constant \\ CO, \% & 0.081 & 0.106 & Constant \\ \%, \% & 2.90 & 6.40 & \(\uparrow\)\(\uparrow\) \\ CO\({}_{2}\), \% & 3.76 & 8.28 & \(\uparrow\)\(\uparrow\) \\ \(\lambda\) & 5.76 & 2.45 & \(\downarrow\)\(\downarrow\)\(\downarrow\)\(\downarrow\) \\ \(\eta\), \% & 85.62 & 90.53 & \(\uparrow\) \\ qA, \% & 14.38 & 9.47 & \(\downarrow\)\(\downarrow\)\(\downarrow\) \\ NO, ppm & 73.7 & 165.4 & Negligible \\ Calculated n & 5.56 & 2.39 & \(\downarrow\) \\ x (x = [V\({}_{\rm CO_{2}}\)/(V\({}_{\rm CO_{2}}+V_{\rm CO}\))]) & 0.969 & 0.984 & \(\uparrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Comparison of different levels and feeding ratios for poplar pellets (example for P = 3, 100% feed, and P = 5, 125% feed). Down and up arrows indicate a decrease or increase in a certain parameter when P and feed were increased. Figure 19: Comparison between different feeding rates in the case of poplar pellets.