## 3 Results and Discussion
### CO and NO\({}_{x}\) Emission  
The Testo 335 was adopted to measure the CO of flue gas, but the accurate range of the CO sensor was 0-2000 ppm. Hence, CO emission under fuel-lean combustion was only tested and compared, as the results show in Figure 7a. It is shown that as the pressure decreases from 97.75 kPa to 61.66 kPa, CO emission decreases from 219.34 mg\(\cdot\)m\({}^{-3}\) to 81.17 mg\(\cdot\)m\({}^{-3}\) when \(\alpha=1.1\), from 63.83 mg\(\cdot\)m\({}^{-3}\) to 3.75 mg\(\cdot\)m\({}^{-3}\) when \(\alpha=1.2\), and from 11.55 mg\(\cdot\)m\({}^{-3}\) to 0 mg\(\cdot\)m\({}^{-3}\) when \(\alpha=1.3\). This trend of CO emission is similar to that reported by [PERSON] [15]. The lower CO emission under smaller pressure can be because, with a fixed air mass flow rate, a smaller pressure produces a higher jet flow velocity, and therefore a larger amount of ambient air is entrained into the flame to support combustion.  
Figure 5: Variations of flame length with pressures and \(\alpha\) (The pressure within the furnace varied from 97.75 to 61.66 kPa, and \(\alpha\) varied from 0.8 to 1.3).  
Meanwhile, the trend of CO emission indicates that the reduced pressure promotes more complete combustion. However, [PERSON] et al. [29] found that CO emission increases with decreasing pressure when \(\alpha=1.25\). It may be caused by the poor flame stability under sub-atmospheric pressure. In addition, in Figure 7a, it is also shown that the variation of CO emission with decreasing \(\alpha\) generally follows the same trend for all pressures.  
Besides CO, the NO emission under sub-atmospheric pressure is also taken into consideration because, generally, a significantly higher level of NO emission for utilization equipment of natural gas was found when operating at high altitudes [2; 5]. Figure 6b shows the dependence of NO emission on pressure. As the pressure decreases from 97.75 kPa to 61.66 kPa, NO emission decreases under fuel-lean combustion. In Figures 3 and 4,  
Figure 6: Variations of smoke point with pressures and \(\alpha\): (**a**) the fuel flow rate of the smoke point; (**b**) the smoke point flame length; (**c**) the residence time.  
Figure 7: Variations of CO and NO emission with pressures and \(\alpha\): (**a**) CO emission; (**b**) NO emission.  
the flame length and temperature at the front furnace decrease with decreasing pressure, which is unfavorable for NO\({}_{x}\) formation via the thermal mechanism. [PERSON] et al. [30] have observed the same change pattern for methane premixed flames. However, the NO emission appears opposite trend under stoichiometric and fuel-rich combustion. On the one hand, the flame length increased with decreasing pressure under stoichiometric and fuel-rich combustion, which promotes NO generation. On the other hand, the reduced pressure leads to improved fuel-air mixing, which promotes active groups' (OH, O and H et al.) formation. The combined effect of the above two aspects causes the NO emission to increase with decreasing pressure under stoichiometric and fuel-rich combustion. Therefore, for those low-NO\({}_{x}\) burners designed for low altitudes, operation at high altitudes cannot probably comply with the integrated emission standard of air pollutants. When designing low-NO\({}_{x}\) burners, we should fully consider the influence of altitude and air distribution.