## 2 Methodology
### Simulation of the Downdraft Gasifier  
In order to determine the biomass consumption and the volumetric composition of the producer gas, which is a key aspect in sizing the downdraft gasifier for a given the power output of the engine-generator set, a model was developed in Cycle-Tempo(r)[12; 13; 17; 18]. Olive cake pellets and olive pruning chips were evaluated as biomass feedstock, with moisture contents ranging from 5% to 20% (wet basis). The reference ambient temperature and pressure conditions for simulation of the biomass gasification plant were 25 \({}^{\circ}\)C and 1.013 bar, respectively. The model of the downdraft gasifier assumed that the residence time of the reactants was sufficiently high to assume that thermodynamic equilibrium was reached during the gasification process. About 7% of input energy was considered to be losses in the gasifier, with 2% corresponding to the pyrolysis stage and 5% to the oxidation-reduction stage [17]. The gasifier-generation system operated in a steady state, meaning that transient behavior was not considered, and the maximum temperature reached during the drying-pyrolysis stage was about 600-650 \({}^{\circ}\)C, while the temperature of the oxidation-reduction stage was set at 1000 \({}^{\circ}\)C [35]. To ensure \begin{table}
\begin{tabular}{l r r} \hline
**Proximate analysis (wt.\%)** & **Olive cake** & **Olive pruning** \\ \hline Moisture content (as received) & 45–50 & 10–15 \\ Ash content (dry basis) & 6.3 & 4.3 \\ Volatile matter (dry basis) & 76.4 & 79.4 \\ Fixed carbon (dry basis) & 17.3 & 16.3 \\ \hline
**Ultimate analysis (wt.\%, dry basis)** & **Olive cake** & **Olive pruning** \\ \hline C & 50.4 & 47.4 \\ H & 6.2 & 5.8 \\ N & 0.9 & 0.6 \\ S & 0.1 & \(<\)0.1 \\ O & 36.1 & 41.9 \\ \hline
**Other properties** & **Olive cake** & **Olive pruning** \\ \hline LHV (MJ/kg, dry basis) & 19.3 & 17.2 \\ Ash melting point (\({}^{\circ}\)C) & \(>\)1200 & \(>\)1200 \\ Bulk density (kg/m\({}^{3}\)) & 715.8 & 347.9 \\ Average article size (mm) & 1–10 & 20-50 \\ Required pretreatment & Drying and pelletizing & Chipping \\ \hline \end{tabular}
\end{table} 
Table 1: Physicochemical properties of the biomass wastes generated in the olive oil mill. complete cracking of the tars, a gasification temperature above 1000 \({}^{\circ}\)C was set. The maximum moisture content of the biomass feedstock for this type of gasifier is 20%, and about 5% of the carbon contained in the biomass is not transformed into producer gas [12; 36]. Of this carbon, 4% is lost through the ash deposit and 1% leaves with the producer gas in the form of soot [37]. The transient period of a downdraft gasifier, which includes start-up and stabilization, lasts for about 30-60 min [20; 21]. The presence of impurities in the producer gas can lead to severe fouling and corrosion issues to all mechanical equipment, which in turn involve a decreased overall efficiency and increased maintenance costs [8]. Therefore, including a producer gas conditioning unit was essential to limit the presence of impurities such as dust and tars in the producer gas before being supplied to the engine-generator set [20]. The producer gas conditioning unit aims to prevent any damage to the generator and minimize emissions of pollutants to the environment. As shown in Figure 4, the simulation was divided into two distinct parts: the downdraft gasifier and the gas conditioning unit. The downdraft gasifier, located in the top left, models the chemical reactions that lead to the producer gas formation. The thermal losses in each section were taken into account, and air was used as a gasifying agent. The second part modeled the producer gas conditioning process. At the gasifier outlet, the gas encounters a cyclone, which separates the heaviest particles. Next, a wet scrubber cools the gas and removes tars. Thereafter, a heat exchanger finishes cooling the producer gas, which then passes through a series of fine filters before reaching the flare or the engine.