## 4 Conclusions  
Co-combustion studies of poultry litter and natural gas were performed in the lab-scale biomass conversion system with the Stirling engine to analyze electricity production, gaseous emissions (e.g., CO, NO\({}_{\text{x}}\), and SO\({}_{2}\)), fly ash composition, and PM emissions. The effects of operating conditions on the system performance were also investigated, namely EA ratio (0.04-1.54), SA/PA ratio (0.22-0.78), SA heights (650 mm, 850 mm, and 1100 mm), and mixing ratio (1.89, 2.29, 2.61, 4.51, and 7.42). Results indicated that increasing EA ratio from 0.04 to 0.79 may significantly reduce the amounts of CO and SO\({}_{2}\) emissions with an increase in NO\({}_{\text{x}}\) production. The further increment of EA ratio above 0.79 caused NO\({}_{\text{x}}\) reduction, but increased CO and SO\({}_{2}\) emissions. EA was suggested to be 0.79 to 1.08 for emission reduction and produced 905 W of electricity from the co-combustion of poultry litter and natural gas co-combustion process. An optimal SA/TA ratio was found to be 0.44 and the ideal SA height was 850 mm, in order to minimize the emissions. This is because these conditions promoted oxygen distribution and swirling effects, while increasing residence time. It was found that increasing the content of poultry litter and the MR to 4.51 may have potential to reduce both SO\({}_{2}\) and NO\({}_{\text{x}}\) emissions. In addition, collected PM emissions were all below Maryland emissions standards, ranging from 0.002 lb/MMBtu to 0.1538 lb/MMBtu. Moreover, high amounts of phosphate (close to 10%) and potassium (close to 6%) in fly ash confirmed the fact that poultry litter ash can be recycled as a soil amendment. This study showed that the co-combustion of poultry litter and natural gas in the lab-scale biomass conversion system can be used as a cleaner alternative to current practices for converting energy sources (e.g., poultry litter and natural gas) into useful energy (i.e., electricity) while lowering emissions.  
[PERSON] and [PERSON] coordinated projects and received grants from the Abell Foundation. [PERSON] reviewed published literature reviews and found the research gaps during the poultry litter co-combustion process. [PERSON] and [PERSON] conceived and designed approach to setting up testing facility and performed the combustion test. [PERSON], [PERSON], [PERSON] and [PERSON] collected poultry litter samples from a poultry farm. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] collected and analyzed experimental results in this study. [PERSON] and [PERSON] wrote the draft manuscript. [PERSON] and [PERSON] reviewed this manuscript and provided their constructive comments and suggestions to improve the quality of article.  
This research was funded by the Abell Foundation and the APC was partially funded by the School of Graduate Studies at Morgan State University.  
The author ([PERSON]) would like to acknowledge the Abell Foundation and Office of Technology Transfer at Morgan State University for providing a partial financial support for this study. In addition, authors would like to appreciate kind support of research facilities from the Microgen Engine Corporation and Center for Advanced Energy Systems and Environmental Control Technologies (CAESECT).  
The authors declare no conflicts of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **As Received (Raw) Basis (\%)** & **Dry Basis (\%)** & **Method Reference** \\ \hline Available phosphate (P\({}_{2}\)O\({}_{5}\)) & 8.8 & 9.0 & AOAC 993.31 \\ Total phosphate (P\({}_{2}\)O\({}_{5}\)) & 9.77 & 9.98 & AOAC 965.09/ICP \\ Soluble potash (K\({}_{2}\)O) & 4.5 & 4.6 & AOAC 969.04 \\ Total potash (K\({}_{2}\)O) & 5.58 & 5.70 & AOAC 965.09/ICP \\ Total nitrogen & 2 & 2 & Combustion \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of fly ash composition analysis.