## 3 Results and Discussion
### Pine Pellet Combustion  
Table 8 and Figures 11 and 12 show the main results obtained for pine pellet combustion in our commercial stove when different levels were selected. From these data, it can be inferred that a decrease in the excess air coefficient (n) was found; additionally, the total flue gas volume was reduced, which implies a decrease in losses due to sensitive enthalpy of exhaust gas (q\({}_{a}\)), increasing thermal efficiency (n). This behavior is in accordance with the posited theory. On the other hand, when the excess air coefficient decreased, the percentage of carbon monoxide was reduced, increasing the gas ratio that was burned (x), as well as the percentage of carbon dioxide, which implies a decrease in loss due to unburned gas, while equally increasing the thermal efficiency of the process (n). It should be pointed out that CO emissions from biomass combustion could be influenced by three factors: gas temperature, oxygen concentration, and fuel-oxygen mixing. More oxygen would result in  
Figure 11: Ostwald diagram for pine pellets, and experimental results for each level used in the combustion stove (standard feed). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **P1** & & **P5** \\ \hline
**Parameter** & **Average** & **Average Deviation** & **Average** & **Average Deviation** \\ \hline O\({}_{2}\), \% & 18 & 0.01 & 14.8 & 0.006 \\ Air supply temperature, \({}^{\circ}\)C & 46.7 & 1.9 & 68.4 & 0.4 \\ Flue gas temperature, \({}^{\circ}\)C & 74.7 & 0.5 & 107 & 0.3 \\ Ambient temperature, \({}^{\circ}\)C & 20.9 & 0.6 & 20.5 & 0.2 \\ CO, ppm & 700.1 & 214.1 & 445.2 & 126.4 \\ CO, \% & 0.070 & 0.021 & 0.045 & 0.013 \\ y, \% & 2.27 & 0.46 & 4.75 & 0.44 \\ CO\({}_{2}\), \% & 2.93 & 0.58 & 6.12 & 0.56 \\ \(\Lambda\) & 7.19 & 1.38 & 3.36 & 0.29 \\ n, \% & 85.82 & 3.00 & 89.74 & 0.78 \\ q\({}_{A}\), \% & 14.18 & 3.00 & 10.26 & 0.78 \\ NO, ppm & 17.9 & 6.7 & 43.2 & 6.0 \\ Calculated n & 6.94 & 1.33 & 3.26 & 0.28 \\ X (x = \(\mathrm{\Gamma}\)V\({}_{\mathrm{CO_{2}}}\)/V\({}_{\mathrm{CO_{2}}}\) + & 0.966 & 0.016 & 0.991 & 0.002 \\ V\({}_{\mathrm{CO}}\))]) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison between P1 and P5 for pine pellet combustion. either a reduced gas temperature or additional residence time for a well-mixed combustion. Consequently, the design of the stove plays an important role in providing enough air during the combustion process [43]. Previous studies pointed out the requirement of a suitable air supply, while trying to avoid high airflow rates to reduce CO emissions [42]. Results showed that, if x is increased, combustion is more complete, thus decreasing loss due to unburned gas, but it is contradictory compared to the yield-loss graph and an excess air coefficient, where low values of n would imply an increase in losses due to unburned gas. Similar trends were observed with air distribution in a wood stove (with an increase in CO emissions with higher air supply ratios) [44]. The outlet temperature (flue gas temperature, Figure 12), considerably increased from Position 1 to Position 5, which indicates an increase in losses due to sensitive enthalpy, as observed in previous studies for increasing input levels or positions (from 6 to 12 kW) [34]. Consequently, the yield of the process was reduced. Nevertheless, in this case, the yield increased, due to the reduction in the excess air coefficient, decreasing the total volume of flue gas and the losses due to sensitive enthalpy. In this way, the yield was improved. In other words, there are two opposite effects, whereby the decrease in flue gas volume is more important than the increase in flue gas temperature when it comes to yield determination. The room temperature was kept practically constant during the different experiments, and there was an increase in NO content with power, as observed in Table 8, even though this increase could be considered negligeable due to the low concentration of this pollutant.