## 3 Materials and Methods
### Pilot Facility Description  
The main characteristics of the designed pilot downdraft fixed-bed gasification power production facility (DGPP), implemented at the Polytechnic University of Valencia (UPV), are shown in Table 2. The experimental facility consists of a reactor, a cooling and cleaning system, auxiliary and control systems, and an internal combustion engine coupled to an electric generator for power production. Moreover, to perform the experimental tests, the DGPP is equipped with an acquisition and measurement system by which the required variables are monitored. In addition, there is an automatic control system and a gas analyzer through which it is possible to determine the gas composition and the low heating value (LHV). The experimental plant is equipped with a biomass deposit of 0.22 m\({}^{3}\), equivalent to 45 kg of chips with a bulk density 200 kg/m\({}^{3}\) which provides the gasifier with operational autonomy for approximately 4.5 h. Future studies will be focused on the implementation of a continuous feed system introducing biomass via the top of the reactor, using two valves with the installation of an intermediate tank to allow for the regulation of the biomass's entrance into the reactor. The aim of this analysis, based on data gathered in tests, is to reach a stable functioning of the DGPP while achieving the expected electrical power of 5 kW. The initial configuration of the experimental facility showed some flaws during its functioning, mainly due to the instable and high pressure drop recorded in the bed. Previous studies carried out by the authors [10,20] demonstrated that the amendments they implemented in the DGPP, listed below, succeeded in stabilizing the pressure drop to within the optimum range (<12.5 mbar):  
* Increase in the diameter of the throat reactor from 7 cm to 10 cm (see Figure 2); \begin{table}
\begin{tabular}{l c} \hline
**Fuel** & **Pellets** \\ \hline Biomass diameter & 0.5–5 cm \\ Biomass length & 2–5 cm \\ Fuel consumption & 5–10 kg/h (depending on size, moisture properties of the biomass, and the air/fuel relation) \\ Biomass deposit volume & 0.23 m\({}^{3}\) \\ Storage capacity & 40–90 kg (depending on biomass apparent density) \\ Autonomy & 5–10 h \\ Motor–generator & PRAMAC Honda GX630 two-cylinder engine \\ Gas cooling medium & Water and wet scrubber. \\ Gas cleaning & Scrubber, tar separator pump, filter chips, filter fabric, and filter cotton \\ Water flow & 1 m\({}^{3}\)/h \\ \hline \end{tabular}
\end{table} 
Table 2: DGGP characteristics. Figure 1: Process of pellet manufacturing used in the experimental plant. Source: own elaboration. * Increase in the diameter of the grid from 220 mm up to 280 mm by adding an additional ring;
* Reduction in the height of the grid underneath the reducer cone by 1.5 cm (see Figure 2). The distance between the grid and the cone of the reactor influences the pressure drop in the bed, in this case reducing it. To vary this parameter, an auxiliary piece to support the grid was mounted;
* Installation of two automatic solenoid valves at the beginning of the air inlet pipe and the exhaust outlet pipe of the gasifier and at the beginning of the pipes of the torch and the motor. This study aims to evaluate the effects of these performed amendments on maintaining the most critical operating parameters of the facility and validate the stable functioning of the 10 cm throat gasifier. Previous studies [10; 20] have demonstrated that these amendments succeeded in stabilizing the pressure drop to within the optimum range for the biomass power production facility's proper functioning. Moreover, these modifications have the following effects:  
* Reduces the total amount of biomass unburnt in the ashes deposit;
* Avoids the formation of holes;
* Reduces the ingress of unburnt air that could result in undesired combustion in the ash deposit. Finally, the residues of biomass in the ash deposit are used as carbon fuel in the next test, so it is not considered as waste.