## 3 Results and Discussion
### Effect of Operating Conditions on Emissions  
#### 3.2.1 Effect of EA on Emissions  
Figure 5 shows the effect of EA ratios on the emissions in the range of EA between 0.04 and 1.54 at an MR of 2.23. Under this condition, the total heating input of 109,754 Btu/h was provided by a poultry litter-feeding rate of 15.6 lb/h and a natural gas-feeding rate of 34 ft3/h. Results indicated that the EA ratio had a marginal effect on SO2 emissions while the EA ratio played a major role in both CO and NOx emissions. A large CO emission reduction from 1539 ppm to 301 ppm occurred when increasing EA from 0.04 to 1.19. Additionally, a slight tendency of CO increment was observed when the EA ratio was greater than 1.19. CO reduction is probably due to the effect of additional oxygen in EA. A higher EA ratio may have provided sufficient oxygen for complete combustion and enhanced the rate of CO oxidation into CO2. However, a great deal of EA may increase the velocity of fluidizing air and reduce particle residence time. This may be due to the fact that CO does not have sufficient time to convert  
Figure 4: Effects of EA ratio on electricity outputs and chamber temperatures. into CO\({}_{2}\)[19]. A high amount of EA may also entrain more unburned small particles (i.e., CO) from incomplete combustion into atmosphere. In a previous study, [PERSON] et al. [20] had similar results that CO emissions are increased by the increasing EA ratio above 1.23 because any further increase in EA cools the combustor and causes CO emission to increase. NO\({}_{\text{x}}\) emissions during the biomass combustion mainly originate from the nitrogenous species (e.g., NH\({}_{3}\) and HCN). These compounds are released with the volatiles and the oxidation of nitrogen retained in the char may produce NO\({}_{\text{x}}\) emissions. Since biomass combustion has a relatively lower temperature than fossil fuels combustion, the formation of thermal NO is insignificant [19; 21]. In all test runs, the NO\({}_{2}\) concentrations at different points during the poultry litter co-combustion in the lab-scale biomass conversion system were negligible, ranging from 0 to 4 ppm. For this reason, NO would be expected to form the majority of NO\({}_{\text{x}}\) emissions and NO was originated from fuel-N during the poultry litter co-combustion. It was found that the emission of NO during the poultry litter co-combustion process increased significantly as the EA ratio increased from 0.04 to 0.79. These results are in accordance with other studies showing that the NO emission increases with the EA ratio [20; 22; 23; 24]. This phenomenon can be explained due to the increasing oxygen concentration as the EA increases, which may increase the chance of converting fuel-N components into NO. This is because the O\({}_{2}\) concentration is regarded as a significant factor in NO\({}_{\text{x}}\) formation [25]. In addition, increasing EA results in higher freeboard temperature, which then promotes the release of volatile nitrogen to enhance the conversion of nitrogen radicals into NO. Higher temperature also reduces the char and CO concentrations, which in turn decreases the reduction of NO to N\({}_{2}\) on the char surface and increases NO\({}_{\text{x}}\) formation. These observations suggest NO-forming nitrogen was released as nitrogen containing compounds within the volatiles and subsequently oxidized to NO in the gas phase [22]. NO\({}_{\text{x}}\) was started to decrease when EA was greater than 0.79. This is because the greater EA may suspend char-C within the freeboard and react with NO emission (i.e., 2 NO + 2C \(\rightarrow\) N\({}_{2}\) + 2 CO). The NO\({}_{\text{x}}\) emissions were further decreased when the EA was greater than 1.19. Additionally, NO reduction can be explained by the combination effect of decreased freeboard temperature to reduce a small amount of thermal NO\({}_{\text{x}}\) formation, which then increased CO emissions from 301 ppm to 600 ppm.