## 2 Materials and Methods
### Experimental Procedure  
In order to monitor the performance of the gasification plant, a series of temperature and pressure sensors, a portable gas chromatograph (Varian CP4900) and a gas flow meter were installed. Type K thermocouples were installed to measure the temperatures (up to 1000 \({}^{\circ}\)C) at different points of the gasifier: two thermocouples inside the reactor (drying and pyrolysis zone), one thermocouple just at the outlet of the reactor and another one after the Venturi scrubber. A pressure sensor was placed in the reactor bed to monitor the gasification process. The pressure difference between the bed pressure and the atmospheric pressure indicates the pressure drop in the reaction zone. This is an essential parameter to monitor the performance of the gasification process and, therefore, it must remain within the pressure limits established by the manufacturer (40-90 mmHgO). A Venturi flow meter, placed before the portable gas chromatograph (GC), and a differential pressure sensor (Omega PX653) were used to monitor the producer gas flow. The gas flow measurement was placed after the safety filter and before the main gas flare (between units 6 and 8 of Figure 3). The portable GC was used to determine the volumetric composition of the producer gas and was calibrated using the following gas mixture (vol. %): CO: 19%, H\({}_{2}\): 18%, CH\({}_{4}\): 3%, CO\({}_{2}\): 10%, O\({}_{2}\): 1%, N\({}_{2}\): 50%. The calibration mixture was injected into a Molsieve column (channel 1), using argon as the carrier gas. Channel 2 was equipped with a Cp-PoraPlot U column to detect CO\({}_{2}\) in the gas sample. The sampling point was located immediately after the safety filter, recording the gas composition every 25-30 min. Only H\({}_{2}\), CO, CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\) and O\({}_{2}\) were the gases considered. Data sampling began when the plant reached steady-state conditions (\(\sim\)20 min). Data were recorded in 3 test runs of 4 h each. The system start-up process begins with the weighing and loading of the feedstock (exhausted olive pomace pellets) into the gasifier through the upper hopper. In the first start-up, the gasifier requires external heat input, which is supplied with a blowtorch or a hand torch. Approximately 10 min after ignition, and with the inlet valve to the engine gas closed (unit 9, Figure 3), the producer gas begins to form and evacuate through the main gas flare (unit 8, Figure 3). At this time, using a small torch, the gas flare is lit. In steady-state conditions, the flare is colorless or may exhibit a blue hue if the ignition is performed at night. A colorless or blue flame indicates that the producer gas is clean and free of tars. After 5-10 min of flare ignition, the next step is the progressive opening of the inlet valve to the combustion engine and the gradual closing of the main gas flare valve. At this stage, the gas engine is started by opening the inlet valve until reaching the rated electric power of the gasification plant. Finally, in order to determine the electrical parameters of the gasification plant (active power, frequency, current injected into the power grid, voltage, etc.) a three-phase network analyzer (Chauvin Arnoux CA 8331) was used. This device was connected to the output of the engine-generator set, which in turn was connected to the power supply to the oil mill. The lower heating value of the producer gas (LHV) was determined by gas chromatography using the Equation (2), which takes into account the mass fraction (\(y_{i}\)) of each of the fuel components that make up the producer gas (H\({}_{2}\), CO and CH\({}_{4}\)) [5; 27; 28].