## 3 Results and Discussion
### Gasification Performance Parameters
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Measured Parameters** & **Unit** & **Test 1** & **Test 2** & **Test 3** & **Average** & **Std. Dev.** \\ \hline H\({}_{2}\) & vol. \% & 19.97 & 19.99 & 19.24 & 19.73 & 0.43 \\ O\({}_{2}\) & vol. \% & 0.12 & 0.07 & 0.13 & 0.11 & 0.03 \\ N\({}_{2}\) & vol. \% & 47.51 & 47.83 & 46.72 & 47.35 & 0.57 \\ CH\({}_{4}\) & vol. \% & 1.71 & 1.66 & 1.85 & 1.74 & 0.10 \\ CO\({}_{2}\) & vol. \% & 13.01 & 13.28 & 12.88 & 13.06 & 0.20 \\ CO\({}_{2}\) & vol. \% & 17.68 & 17.17 & 19.18 & 18.01 & 1.04 \\ Lower heating value (LHV) & MJ/Nm\({}^{3}\) & 4.40 & 4.41 & 4.35 & 4.39 & 0.03 \\ Higher heating value (HHV) & MJ/Nm\({}^{3}\) & 4.85 & 4.87 & 4.80 & 4.84 & 0.04 \\ Exhausted pomace pellet consumption & kg/h & 13.33 & 13.12 & 13.42 & 13.29 & 0.15 \\ Producer gas volumetric flow & Nm\({}^{3}\)/h & 30.7 & 30.2 & 31.1 & 30.7 & 0.45 \\ Cold gas efficiency & \% & 62.86 & 63.09 & 62.61 & 62.86 & 0.24 \\ Biochar (dry basis) & kg/h & 1.71 & 1.78 & 1.74 & 1.74 & 0.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Composition of the producer gas and efficiency parameters of the gasification plant. rate (kg/h). The average producer gas yield was 2.3 Nm\({}^{3}\)/kg, which agrees with the typical range of producer gas yield values reported for wood gasification in downdraft gasifiers (2-3 Nm\({}^{3}\)/kg) [30]. The gasification process is governed by the equivalence ratio (ER), which is the amount of air supplied to the downdraft gasifier in relation to the amount of air required for stoichiometric combustion [5]. Atmospheric air with an oxygen concentration of roughly 21% by volume was supplied to the gasification plant with a blower (unit 7, Figure 3). The volumetric flow rate of air supplied to the downdraft gasifier was not measured onsite. However, as the equivalence ratio (ER) in fixed bed gasifiers is directly related to the producer gas yield [30], it was estimated at about 0.30-0.35 as shown in [31]. This means that the volumetric airflow rate supplied to the downdraft gasifier was about 30-35% with respect to that necessary for stoichiometric combustion [30]. This parameter lies within the usual range in downdraft gasifiers (ER = 0.2-0.4) [30] and is in accordance with the equivalence ratios reported for olive pit (ER = 0.31) and olive tree pruning (ER = 0.32) in a 70 kW\({}_{\text{e}}\) downdraft gasification plant [5]. On analysis about the overall performance of the gasification process, the average cold gas efficiency (\(\eta_{cg}\)) achieved by the plant was 62.9%. This parameter represents the available energy in the cold producer gas related to the LHV of the feedstock supplied to the gasifier through the upper hopper, as shown earlier in Equation (3). In general, the efficiency of a gasification process is greater than that of other thermochemical conversion processes, such as direct combustion [18]. For biomass gasification in downdraft gasifiers, the cold gas efficiency typically ranges between 50-70% [18; 30]. The average cold gas efficiency lies within this range and is in accordance with the value indicated by the manufacturer (\(\sim\)65%), demonstrating a correct operation of the gasification plant with exhausted olive pomace pellets as feedstock. Figure 5 presents the electrical parameters of the engine-generator set connected to an olive oil mill's low voltage electrical network in self-consumption mode. The objective of these measurements was to demonstrate the stable operation of the generator coupled to the power grid. Figure 5 shows the electrical variables measured in one of the tests during 4 h of operation. The two remaining tests showed similar values. The three-phase output voltage of the engine-generator set is perfectly coupled to the voltage of the olive oil mill, synchronizing with the network correctly (408 V). The current generated by each phase (equal to the line current in star-connected generators) ranged between 15.0 and 16.4 A. The electrical frequency of the engine-generator set was kept constant during all the tests and equal to 50 Hz.