## 3 Results and Discussion
### Gasification Performance Parameters
The three-phase active electric power was determined according to Equation (5), ranging from 10.3 kW to 11.3 kW, being the average value equal to 10.6 kW. These results confirm that the electrical energy production of the gasification plant is slightly higher than that set by the manufacturer with rice husks as feedstock (limited to a rated electric power of 10 kW). Finally, the spark-ignition-engine generator set was able to reach a relatively high efficiency of energy conversion into electricity (\(\eta_{genset}=28.3\%\)). Table 7 summarizes the most important performance parameters of the gasification plant operating with residues from the olive oil industry (exhausted olive pomace pellets) and their comparison with those provided by the manufacturer. Gasification of exhausted olive pomace pellets improved nearly all performance parameters compared to gasification of rice husk. Two parameters stand out, the lower feedstock consumption of the plant (13.3 kg/h) and the higher electric power generation (10.6 kW). All this is due to the excellent energy properties of the exhausted olive pomace pellets in comparison with those of rice husk. The electrical efficiency of the gasification plant increased by 4.7 percentage points up to 17.8%. The main drawback observed during the experimental validation of the plant was the lower production of biochar as a result of the lower feedstock consumption. Gasification processes for power generation are environmentally friendlier than direct combustion processes, since not only do they perform more efficiently, but also lead to lower emissions of greenhouse and other polluting gases into the atmosphere. This is due to the fact that combustion of gaseous fuels is inherently more efficient than that of solid \begin{table}
\begin{tabular}{l r r} \hline
**Gasification Parameter** & **Olive Oil Industry** & **Manufacturer Specs.** \\ \hline Feedback & Exhausted olive pomace pellets & Rice husk \\ Biomass consumption (kg/h) & 13.3 & 25–30 \\ Producer gas volumetric flow (Nm\({}^{3}\)/h) & 30.7 & 60 \\ Producer gas LHV (MJ/Nm\({}^{3}\)) & 4.4 & \textgreater{}4.4 \\ Producer gas output temperature (\({}^{\circ}\)C) & 553 & 400–500 \\ Producer gas temperature at engine inlet (\({}^{\circ}\)C) & 29.0 & \textless{}40 \\ Cold gas efficiency (\%) & 62.9 & \textless{}65 \\ Electric power (kW) & 10.6 & 10.0 \\ Electrical efficiency (\%) & 17.8 & 12.1 \\ Biochar production (wt. \%, wet basis) & 21.3 & \textgreater{}25 \\ \hline
**Producer gas composition (vol. \%)** & & \\ \hline H\({}_{2}\) & 19.7 & 11–17 \\ O\({}_{2}\) & 0.1 & \textless{}1 \\ N\({}_{2}\) & 47.4 & 45–50 \\ CH\({}_{4}\) & 1.7 & \textless{}3 \\ CO & 13.1 & 15–19 \\ CO\({}_{2}\) & 18.0 & 11–17 \\ \hline \end{tabular}
\end{table} 
Table 7: Gasification performance parameters with residues from the olive industry and manufacturer specifications for rice husk. Figure 5: Electrical parameters measured at the output of the engine–generator set. fuels and also because the producer gas from biomass gasification is generally purified before being combusted in spark-ignition engines [13]. Table 8 provides the exhaust gas composition from the 10 kW\({}_{\text{e}}\) engine-generator set running on the producer gas from gasification of exhausted olive pomace pellets. The European Directive (EU) 2015/2193 lays down rules to control or monitor emissions of sulfur dioxide (SO\({}_{2}\)), nitrogen oxides (NO\({}_{\text{X}}\)), dust and carbon monoxide (CO) into the air from combustion plants with a total rated thermal power between 1 MW\({}_{\text{th}}\) and 50 MW\({}_{\text{th}}\), irrespective of the type of fuel they use [32]. The content of this European Directive was transposed into Spanish law by means of Royal Decree 1042/2017 [33]. As stated therein, the emission limit value of nitrogen oxides (NO\({}_{\text{X}}\)) for new engines running on gaseous fuels other than natural gas is 190 mg/Nm\({}^{3}\). Emissions of carbon monoxide (CO) must be monitored, but so far no specific emission limit values have been set for them.