## 2 Materials and Methods
### Engines  
The plant has four internal combustion engines, namely spark ignition, Otto cycle, four strokes, cooled by a water circuit. The engines are Guascor type SFGLD360s one step turbocharging and intercooling, having a V two-rows of six-cylinder disposition and operating under lean-burn conditions--(\(\lambda\)) of 1.3-1.7--. The engines are of the square type with a stroke/bore ratio close to unity. Carburenci is electronic, blowing the lean mixture into the turbocharger and inlet manifolds to the cylinders. The biogas is sent from the gasometers or digester head to the engines--at 100.7 kPa and 30 \({}^{\circ}\)C--by four side channel blowers equipped with frequency converters. The blower nominal flow rate is 300 m\({}^{3}\)/h at 2800 rpm, raising the pressure to 115 kPa and the temperature to 55 \({}^{\circ}\)C, before being introduced into the biogas ramp and carburet, where it is mixed with the admission air, compressor and inlet manifolds before entering the combustion chamber. The operating principle of the carburet is a Venturi effect. Air flows through the carburet creating a vacuum proportional to the air velocity, causing biogas to enter and homogenously mix with air. The pressure in the carburet cannot be higher than atmospheric pressure 101 kPa, a restriction achieved by an electronic controller. According to the indications of the load/speed controller, a throttle valve at the outlet of the intercooler regulates the air-biogas ratio and the engine power. The intercooler lowers the temperature of the cylinder inlet air, thus preventing uncontrolled detonation and increasing its density. The compression ratio of turbocharging is usually 2-4. The exhaust gases enter the turbine radially and leave it axially, whereas, it is the other way around for the compressor--the air inlet is axial and the outlet radial--. The centripetal turbine is driven by the exhaust gases and is mechanically coupled at the same centrifugal compressor shaft. The engines are cooled by water in closed circuits using plate-type heat exchangers. The engine is a four-stroke and every two crankshaft turns (angle of rotation 2 \(\times\) 360\({}^{\circ}\)) causes one drive stroke in each of the cylinders. Therefore, there is a driving stroke in the cylinders each 120\({}^{\circ}\) (720\({}^{\circ}\)/6 = 120\({}^{\circ}\)), with a firing order of sequence 1-8-5-10-3-7-6-11-2-9-4-12, achieving good smoothness when running. The alternator converts mechanical energy into electrical energy by a synchronous generator of 753 kVA, 400 V. A pair of poles keep synchronism rotating at 1500 rpm, frequency 50 Hz and allows a power variation factor of 0.8-1. The residual thermal energy of the engine exhaust gases is used in the mixed recovery boiler. A fraction of the thermal energy from the main cooling circuits is used for preheating the boiler feed water. At the Burgos WWTP, the maximum degree of loading at which engines operate is 90% of the rated power as a preventive maintenance requirement to \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Biogas characteristics & - \\ LHV 1 (kJ/m\({}^{3}\)) & 21,240 \\ Density (kg/m\({}^{3}\)) & 1.15 \\ Specific heat capacity (c\({}_{\rm P}\), (kJ/kg \({}^{\circ}\)C)) & 1.56 \\ CH\({}_{4}\) (\%) & 62.4 \\ CO\({}_{2}\) (\%) & 34.1 \\ H\({}_{2}\) (\%) & 2.5 \\ H\({}_{2}\)O (\%) & 1.0 \\ WWTP operating conditions & - \\ Biogas production (kW) & 3590 \\ Biogas consumption (kW) & 2720 \\ Boiler biogas average consumption (kW) 2 & 870 \\ WWTP electrical power demand (kW) & 1885 \\ \hline \hline \end{tabular}  
* LHV: Lower heating value. 2 Values obtained from WWTP operating with a thermal hydrolysis treatment unit: Cambi THP B6.2 [20].  
\end{table} 
Table 1: Biogas parameters, operating point and electricity demand of the Burgos WWTP. Biogas reported at standard temperature and pressure (STP) conditions. extend engine service life and reduce the frequency and number of maintenance stops for spark plug replacements.