## 2 Materials and Methods
### Physicochemical Properties of Agrifood Industry Wastes  
The next step for validating the gasification technology is to determine the physicochemical properties of the different residues from the olive oil and almond industries. For this purpose, their proximate and ultimate analyses (CHNS), particle size, bulk density and heating value were determined and reported in Table 5.  
Exhausted olive pomace is one the most suitable by-products from the olive oil industry to be fed into the gasifier, mainly due to its massive rate of production in oil mills, for which it represents a huge economic, environmental and energy burden. Additionally, exhausted olive pomace has a high carbon and hydrogen content, low ash and moisture content and substantial heating value. Therefore, it was used as feedstock for the gasification tests. However, the exhausted olive pomace had to be pelletized in advance because its raw particle size is smaller than 3 mm and hence, not recommendable for direct load into the gasifier. Fine particles in downdraft fixed bed gasifiers favor the formation of ash clinkers [26], which disturb the optimal fuel and airflow in the gasifier bed (bottom), causing pressure drop problems, low temperatures in the reduction zone and unstable operation. One of the objectives of this research work was to ensure the correct operation of the downdraft gasifier fueled with exhausted olive pomace pellets. The results may represent \begin{table}
\begin{tabular}{l r r} \hline \hline \multicolumn{1}{c}{**Spark-Ignition Engine**} & \multicolumn{2}{c}{**Electric Generator**} \\ \hline Manufacturer & NPT & Rated electric power (natural gas) & 20 kW \\ Aspiration mode & Atmospheric & Rated voltage & 400/460 V \\ Compression ratio & 10.5 & Frequency & 50 Hz \\ Number of cylinders & 4 (inline) 47 & Power factor & \textgreater{}0.8 \\ Diameter \(\times\) stroke (mm) & 98 \(\times\) 115 & \\ Rotation speed (rpm) & 1500 & \\ Rated thermal power (natural gas) & 24 kW & \\ Fuel temperature & \textless{}40 \({}^{\circ}\)C & \\ Tar content & \textless{}20 mg/Nm\({}^{3}\) & \\ Lower heating value (LHV) & \textgreater{}4.2 M/Nm\({}^{3}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Technical specifications of the engine–generator set. \begin{table}
\begin{tabular}{l r r r r r r r} \hline \hline \multicolumn{1}{c}{} & \multicolumn{3}{c}{**Olive Oil Industry (2-Phase Process)**} & \multicolumn{3}{c}{**Almond Industry**} \\ \cline{2-7} \multicolumn{1}{c}{**By-Product**} & \multicolumn{1}{c}{**Pruning**} & \multicolumn{1}{c}{**Leaves Twigs**} & \multicolumn{1}{c}{**Pits**} & \multicolumn{1}{c}{**Exhausted Pomace**} & \multicolumn{1}{c}{**Pruning**} & \multicolumn{1}{c}{**Shells**} & \multicolumn{1}{c}{**Hulls**} \\ \hline \multicolumn{1}{c}{**Proximate analysis (wt. \%, dry basis)**} & & & & & & \\ \hline Moisture & 10–15 & 8.5 & 25–30 & 12.6 & 8–12 & 9.2 & 12.2 \\ Ash & 3–5 & 8.7 & 1–2 & 5–6 & 1–3 & 1–5 & 5–10 \\ Volatile matter & 78.2 & 71.4 & 76.4 & 73.1 & 76.8 & 75.1 & 71.2 \\ Fixed carbon & 17.1 & 19.9 & 21.6 & 20.9 & 17.3 & 20.4 & 19.8 \\ \hline \multicolumn{1}{c}{**Ultimate analysis (wt. \%, dry basis)**} & & & & & & \\ \hline Carbon & 47.10 & 45.08 & 50.08 & 51.31 & 44.62 & 47.37 & 45.16 \\ Hydrogen & 6.18 & 5.89 & 5.90 & 6.40 & 6.40 & 5.60 & 5.51 \\ Nitrogen & 0.55 & 0.52 & 0.64 & 2.00 & 0.36 & 0.50 & 0.98 \\ Sulfur & 0.10 & 0.09 & 0.02 & 0.26 & 0.04 & 0.22 & 0.20 \\ Oxygen (by difference) & 41.66 & 39.70 & 41.03 & 35.01 & 46.80 & 42.20 & 39.58 \\ \hline \multicolumn{1}{c}{**Other properties**} & & & & & & \\ \hline LHV (MJ/kg) & 16.3 & 12.3 & 17.9 & 16.1 & 16.3 & 16.1 & 15.6 \\ Ash melting point (\({}^{\circ}\)C) & \textgreater{}1200 & \textgreater{}1000 & \textgreater{}1200 & \textgreater{}1000 & \textgreater{}1200 & \textgreater{}1100 & \textgreater{}1000 \\ Bulk density (kg/m\({}^{3}\)) & 195 & 108 & 709 & 675 & 205 & 356 & 320 \\ Average particle size (mm) & 20–60 & 20–40 & 2–4 & 10–20 & 20–60 & 15-35 & 20–40 \\ \hline \hline \end{tabular}
\end{table}