An updated short chemical‐kinetic nitrogen mechanism for carbon‐free combustion applications

Abstract

Chemical-kinetic combustion mechanisms for hydrogen-oxygen-nitrogen systems, motivated originally by concerns about NO x emissions during hydrogen burning, have recently acquired renewed interest as a result of the possibility of employing ammonia-hydrogen mixtures in gas turbines and reciprocating engines as drop-in fuel to replace the use of natural gas. Specifically, this is of relevance to the implementation of engineering approaches for economical power generation with carbon sequestration or to large-scale energy-storage schemes, based on hydrogen or efficient hydrogen carriers such as ammonia. Because computational investigations are facilitated by short mechanisms (since the use of large mechanisms is often prohibitively expensive in reactive flow simulations), in response to the original concerns, a short nitrogen mechanism was developed in San Diego in the 1990s (not updated since 2004), without consideration of ammonia combustion. In view of the renewed interest in this topic, that mechanism has now been expanded to encompass 60 elementary steps among 19 reactive chemical species, including ammonia burning and NO x production, as reported herein, greatly improving predictions. With particular attention to high reactant temperatures and high-pressure conditions, relevant to industrial applications, it is shown that the present short mechanism retains satisfactory accuracy, exhibiting deviations that in most cases are within acceptable bounds (±20%). The revisions maintain the shortness of the original mechanism, adding only one more reactive species and six more elementary steps (while updating values of rate parameters of nine other steps, on the basis of newly available information). In addition, the short mechanism is applied herein to the analysis of fundamental combustion properties of ammonia/hydrogen/nitrogen-air laminar premixed flames, at unstrained and strained conditions, for comparison with methane-air flames as a reference gas-turbine fuel. It is found by comparing carbon-free and hydrocarbon laminar flames that these reactive mixtures, even if characterized by nearly identical adiabatic flame temperature and laminar flame speeds, nevertheless exhibit substantially different resistance to strain, with the ammonia/hydrogen flames exceeding the strain limit of methane flames by a factor of 5.

Review on Ammonia as a Potential Fuel: From Synthesis to Economics

“…To validate model performances for neat ammonia ignition delay times under rapid compression machine conditions, two sets of IDT measurements were adopted: the temperature-dependent IDT measurements from He et al 71 for equivalence ratios of 0.5, 1.0, and 1.5 at pressures of 4.0 and 6.0 MPa (as shown in Figure 9) and the pressuredependent measurement from Dai et al 76 for the equivalence ratio from 0.5 to 3.0 at 1140 and 1200 K (as shown in Figure 10). As seen in both figures, the mechanisms Jiang-2019, 138 Li-2019, 137 and Mei-2019 91 underestimate the ignition delay time with a factor of 1.5−4 for most conditions, except the case at φ = 3 and T c = 1200 K, where they underestimate the IDT with a factor larger than 5. Dai et al 76 found that Jiang-2019 predicted well the ignition delay times of a lean NH 3 mixture with 70% argon dilution but failed to reproduce the IDTs at other conditions satisfactorily.…”

“…By updating several reaction rate coefficients of Li-2019, its performance for fuel-rich mixtures was further improved. 67 Since Jiang-2019 138 performed well for the shock tube measurements but not satisfactorily for RCM conditions, it is worth finding out the difference between the key reactions for these two conditions. The sensitivity analyses for Jiang-2019 and Li-2019 are compared in the left panel of Figure 18 for the shock tube conditions.…”

“…Calculations were conducted using the Jiang-2019 mechanism. 138 under fuel-rich conditions, a substantial increase of laminar burning velocities with hydrogen substitution can be observed as a result of the increased amount of hydrogen atoms, suggesting the potential of hydrogen as a promoter for improving the burning performance with low NO x and N 2 O emissions in fuelrich flames. 78 Actually, in the tests of practical combustion devices, it was demonstrated that a fuel composition of 80% NH 3 and 20% H 2 produces a comparable magnitude of NO emissions to that of gasoline as a result of lower flame temperatures.…”

Experimental and modeling study on ignition delay of ammonia/methane fuels

“…In terms of utilizing ammonia in combustion equipment, pertinent studies have tested and applied ammonia in internal combustion engines, 10‐12 gas turbines, 13‐15 etc. Meanwhile, in order to support the application and have better understanding of ammonia combustion, fundamental studies also have been performed using ammonia fuels 16‐19 . Generally, the ammonia combustion studies have proved the feasibility for potential combustion applications.…”

“…Meanwhile, in order to support the application and have better understanding of ammonia combustion, fundamental studies also have been performed using ammonia fuels. [16][17][18][19] Generally, the ammonia combustion studies have proved the feasibility for potential combustion applications. Although previous work has made a good progress in the utilization and understanding of ammonia combustion, there are still lots of issues which need to be better settled.…”