## 2 Materials and Methods  
### Apparatus and Experiment Setup  
As shown in Figure 1, a lab-scale biomass conversion system was designed, assembled, and developed by the Center for Advanced Energy Systems and Environmental Control Technologies (CAESECT) at Morgan State University. The system converted biomass fuels into useful energy for a small-size poultry farm (2-3 poultry houses). The lab-scale biomass conversion system included an advanced swirling fluidized bed combustion system, a Stirling engine, a fuel-feeding system, an air-supply system, a cyclone, and other instrumentation components. In this study, the lab-scale \begin{table}
\begin{tabular}{c c c} \hline \hline
**Fuel Types** & **Major Findings** & **Ref.** \\ \hline Chicken litter + coal & \begin{tabular}{c} Increasing of chicken litter mass fraction reduces SO\({}_{2}\), but \\ increases CO. \\ \end{tabular} & [6] \\ \hline Poultry litter + peat & \begin{tabular}{c} Secondary air (SA) in two stages reduces nitrogen oxide (NO\({}_{\text{x}}\)) \\ (about 160–220 ppm) in freeboard and also reduces CO emission \\ (about 1450–5820 ppm). \\ \end{tabular} & [9] \\ \hline Chicken litter + peat & \begin{tabular}{c} Concentrations (e.g., CO \& volatile organic compound) \\ decreased with the primary air (PA)/SA ratio is 0.4. SA decreases \\ N to NO\({}_{\text{x}}\) from 30\% to 15\%. \\ \end{tabular} & [16] \\ \hline Poultry-derived fuel (PDF) + coal & \
\begin{tabular}{c} Co-combustion emitted about 20\% higher NO\({}_{\text{x}}\) (in a range of \\ 265–280 ppm) than 100\% coal. \\ \end{tabular} & [17] \\ \hline Poultry wastes + coal &
\begin{tabular}{c} Excess air (EA) had a remarkable effect on CO \& CH\({}_{4}\). SA \\ reduces CO emissions, but NO\({}_{\text{x}}\) formation is still not very clear. \\ \end{tabular} & [18] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of co-combustion studies of poultry litter and coal. biomass conversion system was used to study electricity generation and emission characteristics during the poultry litter and natural gas co-combustion process. The cylinder combustion chamber of the lab-scale advanced swirling fluidized bed combustion system had a diameter of 304.8 mm and a height of 1500.0 mm. The chamber was fabricated with a 12\" Schedule 40 carbon steel pipe and was covered inside with 12.7 mm-thickness refractory ceramic. At the bottom of the chamber stood a PA distributor plate (with 36 holes) and its caps connected to the cylinder wind box (304.8 mm in diameter and 330.2 mm in height). PA was injected as fluidizing air to suspend the materials inside the chamber from a 9.8 Amps blower (HP33 ARP, Cadillac Products, Tory, MI, USA) through the wind box and the PA distributor plate. SA, also known as swirling air, was injected symmetrically from a 0.49 Amps blower (4C441A, Dayton, OH, USA) through four nozzles at each layer in the tangential direction. There were three different layers of SA supply into the chamber at heights of 650 mm, 850 mm, and 1100 mm. During the test, PA was controlled by using a variable voltage transformer (TDGC-2 KM, Circuit Specialists, Tempe, AZ, USA) while SA was controlled by a different voltage transformer (3 PN2210, Staco Energy, Miamisburg, OH, USA). A screw-type feeder (105X-G, Acrison, Moonachie, NJ, USA) was used to feed materials into the combustion chamber at a height of 228.6 mm. A speed motor controller was attached to the feeder and controlled the fuel-feeding rate of poultry litter. Natural gas was provided at a height of 120.0 mm for ignition and co-combustion with poultry litter. The 1 kW free piston Stirling engine (E1.4B-00001, Microgen Engine Corporation, The Netherlands) used pressurized helium as a working fluid. The Stirling engine was integrated into the existing lab-scale advanced swirling fluidized bed combustion system at a height of 406.4 mm in order to generate electricity and heat simultaneously during the poultry litter co-combustion process.