## 3 Results and Discussion  
### Gasification Performance Parameters  
The preliminary tests consisted of monitoring and recording various performance parameters every 30 min for 4 h of continuous operation. These tests were performed in triplicate after the plant had reached steady-state operation (\(\sim\)20 min) using exhausted olive pomace pellets as feedstock. Figure 4 shows the evolution of the pressure loss in the gasifier bed, the producer gas temperature at the gasifier outlet, the producer gas temperature at the engine inlet and the producer gas flow rate. Figure 4: Monitored operating parameters of the gasification plant. The pressure drop across the downdraft gasifier bed was 38-45 mmHg\({}_{2}\)O, which lies near the lower limit set by the manufacturer (40-90 mmHg\({}_{2}\)O). This relatively low-pressure drop indicates that the feedstock particle size was adequate, as well as the reactions originating in the reduction zone (lower part of the gasifier). The producer gas temperature at the gasifier outlet was around 550 \({}^{\circ}\)C, slightly above the temperature range recommended by the manufacturer (400-500 \({}^{\circ}\)C). This minor deviation did not have any significant impact on the operation of the gasification plant, only leading to a small increase in the producer gas temperature after the cleaning and cooling stage. However, the producer gas temperature at the engine inlet (cold gas temperature) was maintained at around 29 \({}^{\circ}\)C in accordance with the technical specifications of the combustion engine (<40 \({}^{\circ}\)C, Table 4). During the gasification process of exhausted olive pomace pellets, the producer gas volumetric flow was kept between 30-34 Nm\({}^{3}\)/h. This value is remarkably below the rated value set by the manufacturer (60 Nm\({}^{3}\)/h, Table 3). The main reason behind this is the difference in composition between rice husk and exhausted olive pomace pellets. Rice husk has an LHV equal to 12.0 MJ/kg [29]; whereas that of exhausted olive pomace pellets is 16.1 MJ/kg. Therefore, the amount of air necessary to reach the optimum gasification temperature (1000 \({}^{\circ}\)C) is much lower with exhausted olive pomace pellets as feedstock, and hence, the lower mass flow of producer gas generated. It is noteworthy that the gasification temperature was maintained at around 1000 \({}^{\circ}\)C in order to reduce the production of tars and promote their cracking. Tars are very damaging to the internal combustion engine and the different units and devices that make up the gasification plant, leading to severe corrosion problems and lifespan reduction [21]. Table 6 shows the average producer gas composition, heating values and other efficiency parameters of the three tests performed in the gasification plant. The performance of the gasification system was evaluated in terms of the producer gas composition, heating value, volumetric flow rate and cold gas efficiency. The gas chromatography results showed a very promising producer gas composition with a high hydrogen content (19.7%). In addition, the contents of methane (1.7%) and carbon monoxide (13.1%) are acceptable. The average LHV of the three tests was 4.39 MJ/m\({}^{3}\), which is included within the usual range for downdraft gasifiers (4-6 MJ/Nm\({}^{3}\)) [14; 30]. The average biomass consumption (13.3 kg/h), was lower than that set by the manufacturer (25-30 kg/h). This is mainly due to the higher energy density of exhausted olive pomace pellets compared to that of rice husk. Therefore, it follows that the higher the LHV, the lower the biomass feedstock consumption, the higher the energy content of the producer gas and the lower the volumetric flow generated by the gasifier. Another relevant parameter in the gasification process is the producer gas yield, which is defined as the ratio between the producer gas volumetric flow rate (Nm\({}^{3}\)/h) and the feedstock consumption