## 3 Results and Discussion  
### Poultry Litter as an Energy Resource for Electricity Production  
This study demonstrates that the Stirling engine was successfully installed to convert one of biomass fuels (i.e., poultry litter) into useful energy (i.e., electricity). Electricity outputs of the Stirling engine were determined with the difference between heat absorption from the co-combustion process and heat rejection through the cooling process. The engine head temperatures, also known as the head control temperatures, were an important indicator of the heat absorption during the co-combustion process while water flow rates in the Stirling engine were critical to heat rejection during the cooling process of working fluid. As shown in Figure 3, the electricity outputs were measured at water flow rates of 9.1, 10.4, 13.1, and 19.4 L/min, along with various engine head control temperatures ranging from 220 \({}^{\circ}\)C to 584 \({}^{\circ}\)C. Experimental results indicated that the electricity output linearly increased with the Stirling engine head control temperature. In fact, the engine head control temperature was directly affected by the surrounding chamber's temperature and external thermal energy during co-combustion. According to the increased heat input and chamber's temperature, more work was performed by the power piston to generate higher electricity. When the engine head control temperature was lower than 350 \({}^{\circ}\)C, the minimum threshold of electricity output was found at a water flow of 19.5 L/min and confirmed that heat rejection at this flow rate was more than the required cooling on the Stirling engine. However, when the engine head control temperature eclipsed at 400 \({}^{\circ}\)C, the minimum threshold of electricity output was found to be at a water flow of 9 L/min and showed that this flow rate was too low for enough engine cooling. These results inferred that neither the high (19.5 L/min) nor the low (9 L/min) water flow rate was enough to provide sufficient cooling and produce optimal electricity outputs during the poultry litter and natural gas co-combustion process. The lab-scale biomass conversion system was found to require a 13.1 L/min water flow rate to effectively cool pressurized helium gas in the Striling engine within the acceptable range of cooling temperature while preventing melting of engine parts and producing relatively high electricity outputs, and thereby improve the overall system performance. At a water flow rate of 13.1 L/min, the electricity output reached a maximum value of 905 W at a Stirling engine head control temperature of 584 \({}^{\circ}\)C.  
Figure 3: Power outputs at various head temperatures and water flow rates. Figure 4 illustrates the results of chamber temperature at a height of 381.0 mm near the Stirling engine and electricity output under various EAs during the poultry litter and natural gas co-combustion process. It was found that the chamber temperature increased from 838 \({}^{\circ}\)C to 933 \({}^{\circ}\)C when EA increased from 0.04 to 1.08, while chamber temperature started to decrease and finally dropped to 813 \({}^{\circ}\)C by further increasing EA from 1.04 to 1.54. The increasing chamber temperature can be explained by the complete combustion of the poultry litter and natural gas in the system by supplying oxygen in excess of the stoichiometric amount (up to 108%). However, if the EA is above 1.08, it may increase the gas velocity and reduce particle residence time, which reduce the flame and the chamber temperature. Results also indicated that the electricity output was one of the major performance factors for the Stirling engine and that the electricity output has a strong positive relationship with surrounding chamber temperatures. The electricity output had a similar trend to the chamber temperature as it increased from 856 W to 905 W while EA increased from 0.04 to 1.08. However, the electricity output decreased to 845 W when there was a great deal of EA injection.