## 3 Results and Discussion
### Effect of Operating Conditions on Emissions
#### 3.2.2 Effects of SA Ratio and SA Height on Emissions  
As shown in Figure 6, results showed that NO\({}_{x}\) emissions decreased by increasing the SA/TA ratio because the bed at lower parts of the combustor formed a reduction zone and the freeboard became an oxidizing zone when the SA was applied as the staged combustion [21]. When the SA was introduced into the combustion combustor, the amount of PA was reduced in the fuel bed, and a more reducing atmosphere dominated in the dense phase. Therefore, with an increasing portion of SA in the freeboard region, NO was more likely to be reduced by species such as CO, CH\({}_{4}\), and char in the fuel bed region [21]. In addition, SA injection enabled the adequate mixing of combustion air with combustible gases formed by devolatilization and achieved the complete burnout, which encouraged the formation of N\({}_{2}\) rather than NO emissions [26].  
Unlike the researches that investigated the staged combustion as primary measures to reduce NO\({}_{x}\) in solid fuel combustion, some studies found that air staging has a relatively weak role in determining CO emissions. Moreover, the effect of SA on CO emission is still not very clear [22; 27; 28]. Many authors strongly suggested that SA was mainly responsible for the reduction of CO emissions [23; 29]. In this study, a similar phenomenon was noticed that CO emissions decreased with an increase of SA/total air (TA) from 0.22 to 0.44. First, it is plausible that the combustion fraction in the bed zone was smaller than those of the bed surface and freeboard zone due to its lower density and lower amounts of fixed carbon in the poultry litter. Thus, SA was required to be near the freeboard zone to achieve the complete combustion of suspended volatiles. Second, the accumulated poultry litter and its unburned char were carried back to the bed surface, which prolonged the residence time of particles  
Figure 6: Effect of SA/total air (TA) ratios on gaseous emissions.  
in the chamber and achieved the complete combustion. However, CO emissions became worse with an additional increase in SA/TA ratio from 0.44 to 0.78. This is because significantly high SA ratios may have caused the temperature to decrease at the cyclone outlet while some unburned particles were entrained from the combustion chamber to the cyclone before completing combustion. Therefore, there was an increasing trend of CO emission when the SA/TA ratio was greater than 44%.  
In previous studies, it was found that the different SA locations at the freeboard zone [27] and splash zone [28] have an influence on emissions. In this study, the effect of SA injection locations on the CO and NOx were further investigated to identify optimal SA injection location during the poultry litter and natural gas co-combustion process in the lab-scale biomass conversion system. SA was injected into the freeboard region of the lab-scale combustion chamber at heights of 650 mm, 850 mm, and 1100 mm. It was observed that CO and NOx emissions at a height of 850 mm were relatively smaller than at heights of 650 mm and 1100 mm. The SA location was very low (i.e., at 650 mm), resulting in large quantities of CO emission emitted because the lower SA location may have created an extra resistance on the PA coming through the PA distributor plate, which then may affect hydrodynamics of the dense phase. The dense phase also became denser with large particle numbers and increased CO production. On the contrary, results indicated that there was a high chance of removing unburned particles before completing combustion of solid fuels. Thus, there was an increase in CO emissions if the location of SA was very high (i.e. at 1100 mm). The optimal location of SA injection at a height of 850 mm during the poultry litter co-combustion process may reduce CO emission levels by promoting oxygen distribution and swirling effects, and increasing residence time. Nevertheless, the relationship between NOx emission and SA injection height is still not very clear and therefore further investigation is needed.