## 3 Results and Discussion
### Effect of Operating Conditions on Emissions
#### 3.2.3 Effect of MR on Emissions  
The effect of MR on the pollutant emissions, which included CO, NOx, and SO2, were investigated during the electricity production process in the lab-scale biomass conversion system. In order to ensure similar combustion conditions, the total heating value input of the fuel was maintained constant (110,000 Btu/h) while regulating the poultry litter- and natural gas-feeding rates under the suggested operating conditions--EA = 1.08 and SA = 0.44 at an SA height of 850 mm from the previous two sections (Sections 3.2.1 and 3.2.2). The poultry litter shares in natural gas mixture were determined to be 1.89, 2.29, 2.61, 4.51, and 7.42 by thermal fractions. As shown in Figure 7, increased poultry litter shares from 1.89 to 7.42 resulted in an increase of the CO concentrations from 360 ppm to 456 ppm in the flue gas. There are three possible reasons that can be invoked as a way to account for this CO increment: (1) high volatile content of poultry litter attributed to the cooling effect and unburned of the upper part of the combustor, (2) higher poultry litter shares at the dense zone, which created a reducing atmosphere with lower temperature, and (3) added moisture from poultry litter, which may have caused convective cooling in the bed region and inhibit conversion of CO into CO2.  
It was observed that NO emissions followed an increasing trend from 1.89 to 4.51 while a decreasing trend was observed when the MR was greater than 4.51. The increase in NO, despite a drop in the bed temperature, could be due to a number of reasons. First, as the poultry litter fraction increased, more NH3 would be released and subsequently converted to NO. Second, increased moisture on biomass fuels contributed to more O and OH radicals, which would react with the NH species to form NO [21]. However, further increasing the MR of poultry litter from 4.51 to 7.42 reduced NO emissions. A possible reason for that is the low percentage of poultry litter, where the volatile matter released in the freeboard could not create a strong reducing atmosphere to suppress the NOx formation while it increased the content of organic N fraction in the poultry litter, which is similar to the de-NOx effect of adding NH3 (or urea). Thus, with the increasing poultry litter share up to a certain extent, volatiles (i.e., NH3) evolved from fuel produced an instantaneous fuel-rich condition in the freeboard region, which led to NOx reduction via reactions (4 NH3 + 5O2 \(\leftrightarrow\) 4 NO + 6H2O and 2 NO + 4 NH3 + 2O2 \(\leftrightarrow\) 3N2 + 6H2O) to form N2 instead of NO emission during the co-combustion process. In addition,the large amounts of CO generated in the dense zone at an MR of 4.51 may have acted with NO and therefore inhibited the formation of NO\({}_{\rm x}\) emissions.  
Based on the ultimate analyses results of poultry litter and natural gas, S content of poultry litter was higher than that of natural gas. However, experimental results indicated that SO\({}_{2}\) emission was reduced by the addition of poultry litter as a result of poultry litter ash derived from natural desulfurization. Since poultry litter contains considerable amounts of CaO and K\({}_{2}\)O, it may have taken part in the retention reactions [30]. In addition, poultry litter co-combustion has an optimal temperature (close to 800 \({}^{\circ}\)C) for retaining SO\({}_{2}\) as CaCO\({}_{4}\)[30]. As poultry litter share increases, the chamber temperature gets lower at the bottom of the riser because the poultry litter has a lower heating value than natural gas. As a result, this promoted the retention reaction of SO\({}_{2}\) at optimal temperature. Results indicated that SO\({}_{2}\) emission decreased from 34 ppm to 5 ppm as MR of poultry litter increased from 1.89 to 7.42.