# Control System Modification of a 5-kW Pellet Stove: Study and Analysis
## 3 Results and Discussion
### Biomass Characterization
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Holm Oak** & **Pear Tree** & **Poplar** & **Sugarcane Bagasse** \\ \hline HHV, kJ\(\cdot\)kg\({}^{-1}\) & 15,893 & 16,443 & 17,125 & 16,216 \\ Ash, \% & 3.17 & 2.41 & 2.17 & 3.87 \\ Moisture, \% & 7.34 & 7.03 & 3.08 & 3.38 \\ Volatile matter, \% & 74.23 & 76.24 & 81.64 & 77.11 \\ Fixed carbon, \% & 15.26 & 14.32 & 15.26 & 15.63 \\ C, \% & 47.3 & 47.1 & 47.4 & 45.9 \\ H, \% & 5.77 & 5.95 & 5.99 & 5.91 \\ N, \% & 0.46 & 0.36 & 0.44 & 0.41 \\ S, \% & 0.031 & 0.045 & 0.048 & 0.171 \\ Bulk density, kg\(\cdot\)m\({}^{3}\) & 604.86 & 624.88 & 606.91 & 699.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Characterization of different raw materials for combustion in the commercial stove. Thus, for each case, a specific linear equation was found (with high determination coefficients, from 0.9905 for sugarcane bagasse to 0.9988 for poplar pellets), which can be used to obtain the thermal capacity for the different biomass samples. This way, following the example of poplar pellets (as the rest of the samples would follow the same reasoning), Equation (1) was obtained:  
\[P=54.401\cdot t \tag{1}\]  
where \"\(P\)\" is the weight of biomass (expressed in kg) and \"\(t\)\" is the time, in min or h. Considering this equation for 1 min, \(P=54.401\) g\(\cdot\)min\({}^{-1}\) = 9.06\(\cdot\)10\({}^{-4}\) kg\(\cdot\)s\({}^{-1}\). According to the working times of the screw conveyor established for the operating stage, 8.57 s per minute, the mass flow rate (expressed in kg\(\cdot\)min\({}^{-1}\)) can be obtained (Equation (2)):  
\[\dot{m}=9.06\cdot 10^{-4}\text{kg}\cdot\text{s}^{-1}\times 8.57\text{ s}\cdot\text{min}^{-1}=7.77\text{ kg}\cdot\text{min}^{-1} \tag{2}\]  
Finally, the thermal capacity (expressed in kW) of the stove is given by Equation (3):  
\[\dot{Q}=\dot{m}\times HHV \tag{3}\]  
Thus, a thermal capacity of 2.22 kW was obtained for poplar pellets. According to this reasoning, Table 5 shows the main results for all the pellets included in this work. The thermal capacity of the stove for the different biomass samples is included in Table 6.  
On the other hand, one of the ways to increase the yield in a domestic stove is the improvement of its combustibility. For this purpose, the percentage of oxygen supplied for combustion should be modified, which can be easily carried out by changing the rotation speed of the flue gas fan. In this fan, a potentiometer was installed in order to vary the power of this device. Thus, according to the different rotation speeds, flue gas composition was analyzed, representing these results in the Ostwald diagram, whose interpretation would give useful steps to improve the combustion performance in this case. Figure 17: Feeding calibration for different biomass pellets: (**a**) holm oak; (**b**) pear tree; (**c**) poplar; (**d**) sugarcane bagasse. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Holm Oak** & **Pear Tree** & **Poplar** & **Sugarcane Bagasse** \\ \hline Power, kW & 1.43 & 1.60 & 2.22 & \(1.83\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Thermal capacity of the stove according to different pellets. Regarding the different rotation speeds of the flue gas fan, the main results are included in Figure 18. Thus, according to O\({}_{2}\) evolution with rotation speed, its percentage decreased with the corresponding decrease in rotation speed (finding oxygen content, similar to other studies [23]), whereas the opposite was observed in the case of CO\({}_{2}\) percentage, with a slight increasing trend when the speed was reduced. Finally, CO and NO content did not show a clear trend with fan speed (although a slight increase was observed at lower rpm), which could be due to the low concentrations found for each compound near the quantification limit of the analyzer.