## 3 Results and Discussion  
### Pellet Characterization and Experiments Selected for Combustion Tests  
The main results found for the pine, poplar, and plum tree pellets are included in Table 6, as follows:  
These results are consistent with those of other wood pellets found in the literature, which were in the same order of magnitude [22,42]. Consequently, it can be concluded that the quality of these pellets, according to both the proximate and ultimate analyses, is standard compared to the literature. Specifically, it can be seen that the highest ash content was found in poplar pellets, the percentage of which was within the normal range found in the literature for pellets obtained from similar species. On the contrary, pine pellets offered the lowest ash content, a property which is desirable in combustion chambers. The HHVs for pine, poplar, and plum tree pellets were 4771, 4714, and 467 \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Proximate Analysis (Dry Basis)**} \\ \hline Sample & Ash, \% & Moisture, \% & Volatile matter, \% & Fixed carbon, \% \\ Pine & 0.50 & 6.42 & 84.01 & 15.49 \\ Poplar & 1.79 & 6.61 & 78.24 & 13.36 \\ Plum tree & 0.83 & 6.90 & 78.37 & 13.90 \\ \hline \hline  & \multicolumn{4}{c}{Ultimate analysis (Dry Basis)} \\ \hline Sample & C, \% & H, \% & N, \% & S, \% & O, \% \\ Pine & 47.70 & 6.12 & 0.33 & 0.004 & 45.85 \\ Poplar & 46.40 & 6.01 & 0.52 & 0.028 & 47.04 \\ Plum tree & 47.50 & 6.23 & 0.38 & 0.093 & 45.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Pellet characterization. Figure 10: Ostwald diagram for poplar pellet combustion. For each fuel and level included in the stove (from 1 to 5), a test was carried out. Each test included 20 analyses, took 5 min long, and was conducted using the Testo 335 gas analyzer and other two temperature probes. Regarding the feeding rate, two different experiments were carried out, one with a normal feed and the other with 25% extra feed, by reducing the excess air coefficient and obtaining different results for each fuel and level. The standard biomass is labeled as \"biomass I\", and the increased feeding rate is referred to as \"biomass II\". As a result, for each experiment with a specific level and feed, the samples were labeled as follows: P (number of level) (number of feed). For instance, P2I would correspond to an experiment with the second level at a standard feeding. For the discussion of the results, the tests with the same kind of fuel were prepared, as each fuel type showed different results regarding proximate and ultimate analyses, and, therefore, the Ostwald diagram will be different for each case (see Table 7). The aim of these comparisons was to assess the trends observed for these tests when level and feed are varied. Also, there are some parameters that were theoretically calculated. Thus, the theoretical values, \(n_{th}\) (Equation (14)), \(x_{th}\) (Equation (15)), and \(y_{th}\) (Equation (16)) differed from those obtained with the analyzer, in the fact that the former considered data from the studied fuel, whereas those obtained with the Testo analyzer did not regard the kind of fuel utilized. The following equation was used to express the excess air coefficient calculated from the data obtained from the tests and the data measured with the analyzer for a specific fuel:  
\[n_{th}=\frac{-21\big{(}-2P_{C}-6P_{H}+P_{C}V_{CO}+6P_{H}V_{O_{2}}}{(P_{C}+3P_ {H})\big{(}42+79V_{CO}-200V_{O_{2}}\big{)}} \tag{14}\]  
In the same way, the burned gas ratio is expressed as follows:  
\[x_{th}=\frac{-\big{(}-42P_{C}+121P_{C}V_{CO}+474P_{H}V_{CO}+200P_{C}V_{O_{2}} }{P_{C}\big{(}42+79V_{CO}-200V_{O_{2}}\big{)}} \tag{15}\]  
These expressions were determined through the simplification of Equations (11) and (12), used to obtain the air line (n = 1). To determine \(y_{th}\), that is, % \(V_{CO_{2}}\), similar steps included in the obtention of lines when CO was constant were followed.