## 1 Introduction
However, most of the U.S. poultry farms often clear sheds completely every 1-2 years. Different farming practices, cleaning periods, and bedding materials in the U.S. result in low fixed carbon, high ash, and heating values of poultry litter between 6.78 and 27.90 MJ/kg with an average of 14.08 MJ/kg [2]. These characteristics result in problems on maintaining steady and complete combustion of poultry litter alone [1; 2; 6]. In addition, decentralized poultry farms and a large volume of poultry litter generation per farm make it difficult to transport and process poultry litter in a centralized location within a large-scale combustion system. Therefore, co-combustion of poultry litter with fossil fuels (i.e., coal) has been considered as an alternative to increasing the heating value of solid fuels. This alternative will also help to solve technical challenges in the small-scale combustion system. Table 1 provides a summary of major findings associated with poultry wastes and coal combustion studies in the last two decades. When compared with coal, natural gas possesses several advantages and is usually considered as a \"bridge fuel\" to aid in transitions to renewable and near-zero-emission energy resources. As a result, there is an increase in the utilization of natural gas, and thereby makes up roughly a quarter of the total U.S. energy consumption [12]. The shifts from coal to natural gas in recent decades have been due to very small amounts of CO\({}_{2}\), NOx\({}_{x}\), SO\({}_{2}\), CO, and other reactive hydrocarbons in its emissions. In addition, virtually no particulate matter (PM) emissions are found during the combustion process [13]. Despite possessing several environmental benefits, natural gas has rarely been used over coal during the poultry litter and fossil fuel co-combustion processes. To this end, [PERSON] and [PERSON] [14] conducted a co-combustion study of poultry litter and natural gas within a lab-scale advanced swirling fluidized bed combustion system to mainly evaluate system performance and predict carbon combustion efficiency. The highest carbon combustion efficiency (89%) was found at around 30% excess air (EA) and 20% secondary air (SA) at a height of 0.85 m during the co-combustion process. Results showed that the emissions of NO\({}_{\text{x}}\) increase significantly, and the CO concentration decreases with increasing EA ratios within a range of 0-40%. In a further study, [PERSON] et al. [15] applied an artificial neural network to predict carbon combustion efficiency and found relatively high carbon combustion efficiencies (over 84%) within the following ranges: moisture contents of 11%-14%, litter ratios of 0.05-0.1, EA of 0.22-0.45, and SA of 0.18-0.27. Previous studies have focused on the combustion performance (e.g., combustion efficiency and temperature distribution) and gas emissions (e.g., CO, NO\({}_{\text{x}}\), and SO\({}_{2}\)) during the poultry litter and natural gas co-combustion process. So far, electricity generation from poultry litter and natural gas co-combustion has not been investigated. In addition, PM emissions and fly ash analysis during the poultry litter and natural gas co-combustion process has not been studied in the previous research work. The main objective of this study is to explore the feasibility of electricity production with low emissions during the poultry litter and natural gas co-combustion process within a lab-scale advanced swirling fluidized bed combustion system. During the electricity generation and production process, the effects of operating conditions, namely EA ratio (0.04-1.54), SA/ primary air (PA) ratio (0.22-0.78), SA height (650 mm, 850 mm, and 1100 mm), and mixing ratio (MR) (1.89, 2.29, 2.61, 4.51, and 7.42) on gas emissions (e.g., CO, NO\({}_{\text{x}}\), and SO\({}_{2}\)) were also investigated.