## 2 Materials and Methods  
### Description and Characteristics of the Gasification Plant  
The gasification plant consists of a downdraft fixed bed reactor, a producer gas cooling and cleaning system and a spark-ignition engine-generator set as a power generation unit. The choice of a downdraft fixed bed gasifier is perfectly justified for these main reasons:  
* Compared to fluidized bed and entrained flow gasifiers, fixed bed gasifiers are fairly simpler reactors and involve a considerably lower investment cost. * Fixed bed gasifiers are generally intended for small-scale power and distributed generation applications, since agrifood processing facilities usually require electric powers below 1 MW. * Fixed bed gasifiers are more suitable for coarse and non-fine particles (between 3 and 51 mm). This property is very important in the agrifood industry, since one of the most abundant residues, tree pruning, can be directly fed into this type of gasifiers after a mild crushing pretreatment. * As opposed to updraft gasifiers, a downdraft reactor configuration leads to a cleaner producer gas with considerably lower tar content (\(<\)3 g/Nm\({}^{3}\)) and thus more suitable for being supplied to internal combustion engines. Figure 3 shows a general layout of the experimental gasification plant. The residues from the olive oil and almond industries are loaded through the upper hopper into the downdraft gasifier (1), while the gasifying agent (air) is introduced through a small injector. In the upper part of the gasifier, the drying, pyrolysis and combustion stages occur; whereas the endothermic reduction reactions responsible for the formation of the producer gas take place in the lower part of the gasifier. The general chemical equation of the gasification process can be expressed as follows [5,13]  
\[ \begin{split}\text{CH}_{4}\text{O}_{\text{b}}\text{N}_{\text{c}} \text{S}_{d}\text{Ash}+w\text{ H}_{2}\text{O}+\text{$\dot{m}$}(\text{O}_{2}+3.76\text{ N}_{2})\longrightarrow y_{1}\text{ H}_{2}+y_{2}\text{ CO}+y_{3}\text{ CH}_{4}+y_{4}\text{ CO}_{2}\\ +y_{5}\text{ N}_{2}+y_{6}\text{ H}_{2}\text{O}+\text{Carbon}+\text{ Ash}+\text{Impurities}\end{split}  \tag{1}\]  
The first element symbolizes the chemical composition of the biomass feedstock based on the ultimate analysis (CHNS), where subscripts \(a,b,c,d\) are the number of hydrogen, oxygen, nitrogen and sulfur atoms, respectively. The second element represents the moisture content of the biomass feedstock and the third term is the airflow rate on a mass basis. On the products side, coefficients \(y_{1}\)-\(y_{6}\) stand for the chemical composition of the producer gas, which depends on the feedstock properties, such as moisture, ash, LHV and particle size. The producer gas at the outlet of the downdraft gasifier is at high temperature (\(>\)400 \({}^{\circ}\)C) and contains impurities (particles, dust, etc.) and tar (liquid particles with C\({}_{x}\)H\({}_{y}\) chemical composition). Under these conditions, the producer gas is not suitable for use in internal combustion engines, so it must be cooled and cleaned up to a certain extent according to the manufacturer's specifications. For this purpose, the gasification plant has a producer gas cleaning and cooling system with the following units: a cyclone (2) that separates a large part of the particulate matter; a Venturi scrubber (3) that drastically reduces the gas temperature and also removes the tars by spraying a pressurized water jet; moisture removal filters (units 4 and 5) and a safety filter (6) that ensures a thorough cleanliness of the producer gas before entering the power generation unit (10). Additionally, the gasification plant has a waste-water treatment unit (11) for recirculating the process water into the Venturi scrubber and several pressure and temperature gauges (12) to continuously monitor the gasification process. The characteristics of the gasification plant are summarized in Table 3 (information provided by the manufacturer for rice husk).