Analysis of low emission characteristics of NH3/H2/air mixtures under low temperature combustion conditions

Abstract

The complex dynamics of NH3/H2/air mixtures have been numerically investigated in an idealized reactor environment of fast fuel/air mixing represented by perfectly stirred reactors (PSR). The complete steady state “S-curve” solutions are obtained with eigenvalue analysis for quantifying the stable/unstable characteristics of different branches. With four different chemical mechanisms, stable cool flame branches are observed for NH3/H2/air mixtures. The reactions involving species NH2, H2NO and NNH are found important for both fuel consumption and NOx formation on the cool flame branch. The parametric study further shows that with less than 5 % hydrogen addition in ammonia (in volume), stable cool flames exist over a wide range of inlet temperature, equivalence ratio and pressure. For the states in the cool flame branch, although hydrogen is almost consumed completely, only portion of the ammonia is consumed resulting in low NOx emission due to the reduction effects of remaining ammonia and high N2O emission resulting from low temperature. Inspired by the observed stable cool flame characteristics, a multi-staged low-temperature model system is formulated for low NOx emission of NH3/H2/air combustion, in which small amount of hydrogen is sequentially injected to homogeneous reactors of lean NH3/H2/air mixture. With the representative high inlet temperature of 850 K, pressure of 20 atm, fuel lean of ϕ=0.7 and critical fuel efficiency of 99.7 %, results with the 28-species Li reduced mechanism show that less than 75 ppm of NOx and 98 ppm of N2O can be achieved with two-staged homogenous combustion operating at low temperature of 1300 K and 2 % H2 addition in each stage. With three-staged homogenous combustion, less than 70 ppm of NOx and 10 ppm of N2O can be achieved at 1300 K. NOx emission is lower than those of direct NH3/H2/air combustion without aftertreatment, demonstrating the opportunity of utilizing low temperature combustion to achieve high fuel conversion, low NOx and N2O formation for lean NH3/H2/air mixtures.