“…For entry 16, the updated rate parameters shown have produced excellent agreement with experimental measurements 28,29 ; the new rate constant is slightly higher than that of the previous mechanism. This step affords significant additional chain branching under those conditions.…”
Section: Updated Nitrogen Chemistrysupporting
confidence: 56%
“…This step affords significant additional chain branching under those conditions. For entry 16, the updated rate parameters shown have produced excellent agreement with experimental measurements 28,29 ; the new rate constant is slightly higher than that of the previous mechanism. Entry 22, of importance because of its competition with the dissociation reaction in entry 23, was not previously present and therefore now is added.…”
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.
“…For entry 16, the updated rate parameters shown have produced excellent agreement with experimental measurements 28,29 ; the new rate constant is slightly higher than that of the previous mechanism. This step affords significant additional chain branching under those conditions.…”
Section: Updated Nitrogen Chemistrysupporting
confidence: 56%
“…This step affords significant additional chain branching under those conditions. For entry 16, the updated rate parameters shown have produced excellent agreement with experimental measurements 28,29 ; the new rate constant is slightly higher than that of the previous mechanism. Entry 22, of importance because of its competition with the dissociation reaction in entry 23, was not previously present and therefore now is added.…”
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.
“…The CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p) energies are summarized in Table and shown in Figure relative to that of the H 2 + NO limit 4 with appropriate zero-point vibrational energy corrections using UMP2 frequencies. The values given in parentheses represent the G2 values obtained by Wolf et al The unscaled harmonic vibrational frequencies of the isomers and of the various saddle points are listed in Table , as they are of interest to the modeling kineticists. Energetically, H 2 NO 1 was found to be more stable than trans -HNOH 2 by 7 kcal/mol.…”
Section: Resultsmentioning
confidence: 99%
“… 1 Overall profile of the potential energy surface for the [H 2 NO] system calculated at CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p)+ZPE level of theory. Values in parentheses correspond to the G2 level of theory taken from ref . 2 CCSD(T)/6-311++G(d,p) optimized geometries (in Å and deg) of the stationary points 1 , 2 , 3 , 1/2 , 2 / 3 , 1 / 4 , 1 / 8 , 2 / 8 , 3 / 7 , 4 / 8 in the [H 2 NO] system. Values in parentheses correspond to the UMP2/6-31G(d,p) level of theory.…”
Mechanisms of the reactions occurring on the
multichannel lowest doublet potential energy surface of
the
[H2NO] system containing H2NO,
HNOH, and NOH2 intermediates and including the
H2 + NO, NH2 + O,
NH + OH, and H + HNO entry channels have been probed using
unrestricted coupled-cluster formalism.
Also, the transition structure on the doublet energy surface for
the direct hydrogen abstraction from H2 + NO
yielding H + HNO has been identified. The energetic and
molecular parameters derived from coupled cluster
singles and doubles with triples correction (CCSD(T)) calculations
using the 6-311++G(3df,3pd) basis set,
based on their respective optimized geometries obtained with a
6-311++G(d,p) basis set, have then been
utilized to compute the apparent rate constants of the different
competitive channels in the [H2NO]
system
within the framework of a quantum version of Rice−Ramsperger−Kassel
theory (QRRK). The H2 + NO
reaction is found to be a rather slow reaction, and it occurs via
bimolecular hydrogen abstraction. The calculated
total rate constant for this reaction at 2000 K and 1 atm is 2.35 ×
108 cm3 mol-1
s-1. Stabilization of
NH2O
or NHOH intermediates plays a minor role in the kinetics of
NH2 + O reaction which is dominated by the
dissociation products, HNO + H, NH + OH and H2 + NO.
The H + HNO reaction kinetics has only 10%
contribution from addition/isomerization/dissociation paths at 2000 K
and 1 atm. H2 + NO and H + HNO
are expected to be the competitive products in the reaction of OH with
nitrene. The calculated standard
heats of formation are
ΔH
f
298(NH2O) =
17.5 ± 2 and ΔH
f
298(HNOH) =
23.7 ± 2 kcal/mol.
“…The reactions of NH 2 with both NO and NO 2 are of great interest because of their importance to atmospheric and combustion chemistry . For the reaction of NH 2 with NO, there is a systematic disagreement between rate constants measured via different experiments. − A reaction rate of 1.8 × 10 –11 cm 3 /(molecule·s) was obtained using flash photolysis, but the rate measured using the discharge flow technique was only 0.9 × 10 –11 cm 3 /(molecule·s), or half of the flash photolysis result. A similar disagreement has been found for the reaction of NH 2 with NO 2 .…”
The
effects of water, formic acid, and nitric acid on N2 and
N2O generation from NH2NO and NH2NO2 are studied using high-level quantum-chemical calculations.
It is shown that the reaction barriers from isolated NH2NO and NH2NO2 are 35.5 and 38.1 kcal/mol, respectively.
When the NH2NO and NH2NO2 reactions
are examined in the presence of water, formic acid, or nitric acid,
the energy barriers are different. The mechanisms of these reactions
are revealed through reaction pathway calculations. The isomerization
of intermediate HNNOH and HNNOOH molecules, which follows a group
rotation mechanism, plays a key role in reactions of isolated NH2NO and NH2NO2. However, the presence
of water, formic acid, or nitric acid changes the isomerization mechanism
substantially. HNNOH or HNNOOH and the catalyzing molecule form a
doubly hydrogen-bonded prereactive complex, which, in turn, facilities
hydrogen atom migration (this is denoted as the hydrogen atom migration
mechanism). This study demonstrates the feasibility of N2 or N2O generation from NH2NO and NH2NO2 in the presence of water, formic acid, or nitric acid.
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