Abstract
Ammonia (NH3) is a promising hydrogen carrier and can be used as carbon-free alternative fuel. However, NH3 combustion faces critical challenges, such as narrower flammability range and high NOx emission, which limits its applications. To extend the knowledge on NH3 combustion characteristics, intensive previous studies have been carried out, but most of them were concerned on premixed swirl flames. This work aims to experimentally provide a thorough comparison between premixed and non-premixed NH3/CH4/air swirl flames in terms of their flame structure, chemiluminescence spectrum, and pollutant emission characteristics at various NH3 volumetric blending ratios (XNH3) and global equivalence ratios (φ). The results show that, at a fixed φ of 0.85, the premixed combustion features a closely attached flame profile at any XNH3 of interest, while the non-premixed combustion exbibits a lifted flame appearance for all XNH3 conditions, hence a higher flammability limit of XNH3 is attained by the premixed combustion mode. At a fixed XNH3 of 50%, the non-premixed combustion tends to move the whole flammability range towards the rich direction slightly. Considering the monotonic relationship between chemiluminescence intensity of different excited radicals and operating parameters, OH∗/CH∗ intensity ratio is suggested to be used for sensing the change of XNH3 in both premixed and non-premixed combustion modes, while NH∗/CH∗ intensity ratio is recommended for sensing the change of φ. Comparing to premixed combustion, the non-premixed combustion produces a lower NO emission in the whole ranges of considered XNH3 and φ. Moreover, the NO emission profiles suggest the necessity of employing rich-lean technology for achieving acceptably low NO emission for both premixed and non-premixed combustion modes, while the latter shows a possibility to be operated in relatively leaner condition in the first stage.
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