Effects of fuel/air distribution on the NH3/CH4 flame stability limit and NOx emission in a dual-annular burner
Yipeng Li, Jinguo Sun, Qian Huang, Reinhold Kneer, Shuiqing Li
Abstract
Fuel-air mixing strategy always plays an important role in NO x mitigation during ammonia/methane (NH 3 /CH 4 ) co-combustion. In this paper, a novel dual-annular bluff-body burner is employed to investigate the effects of fuel/air distribution on the flame stability and NO x emission of NH 3 /CH 4 flames. Both straight and swirl flames are studied over a wide range of four key operating parameters, i.e., the overall NH 3 thermal ratio ( E N H 3 , o v e ), the NH 3 thermal ratio ( E N H 3 , p r i ) and equivalence ratio (Φ pri ) in the primary stream, and the swirl number ( S w ) in the secondary stream. First, flame stability over these operating parameters is investigated, where E N H 3 , o v e shows nonmonotonic impacts on the lean and rich blowout limits represented by Φ pri . Subsequently, an analysis of the Damköhler number ( Da ) helps reveal the underlying mechanism. The reaction and flow time scales are adjusted while operating the distribution of CH 4 and air in primary or secondary streams, thus altering the flame stability. Secondly, NO x emissions can be reduced by increasing E N H 3 , p r i and Φ pri . A minimize value of ∼400 ppm is obtained at relatively high Φ pri values (>1.4) for E N H 3 , o v e = 0.1 of both straight and swirl flames. Furthermore, OH*/NH 2 * chemiluminescence is measured to understand the kinetics of NO x formation. In straight flame, two NO x reduction mechanisms are revealed. While increasing operation parameters Φ pri and E N H 3 , p r i , OH inhibition mechanism is shown by lower OH* in the flame zone, and the NH 2 -induced reduction mechanism is marked by increasing NH 2 * content downstream. Subsequent chemical reactor network (CRN) analysis confirms that OH decrement and NH 2 increment contribute to NO reduction. For swirl flames, both OH* and NH 2 * distribute in the flame zone and decrease with Φ pri . When increasing Φ pri , a continuous decrease of NO x emission indicates that the effect of NH 2 on NO x reduction is suppressed compared to that of OH*. These findings suggest the feasibility of an enhanced fuel-nitrogen/hydrocarbon separation to further achieve effective NO x mitigation.