Catalytic Combustion of Hydrogen-Air Mixtures Over Platinum: Validation of Hetero/Homogeneous Chemical Reaction Schemes (original) (raw)
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Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum at pressures up to 10 bar
Proceedings of the Combustion Institute, 2009
The hetero-/homogeneous combustion of fuel-lean hydrogen/air premixtures over platinum was investigated experimentally and numerically in the pressure range 1 bar 6 p 6 10 bar. Experiments were carried out in an optically accessible channel-flow catalytic reactor and included planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition, and 1-D Raman measurements of major gas-phase species concentrations for the evaluation of the heterogeneous (catalytic) processes. Simulations were performed with a full-elliptic 2-D model that included detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduced the measured catalytic hydrogen consumption, the onset of homogeneous ignition at pressures of up to 3 bar and the diminishing gas-phase combustion at p P 4 bar. The suppression of gaseous combustion at elevated pressures bears the combined effects of the intrinsic homogeneous hydrogen kinetics and of the hetero/homogeneous chemistry coupling via the catalytically produced water over the gaseous induction zone. Transport effects, associated with the large diffusivity of hydrogen, have a smaller impact on the limiting pressure above which gaseous combustion is suppressed. It is shown that for practical reactor geometrical confinements, homogeneous combustion is still largely suppressed at p P 4 bar even for inlet and wall temperatures as high as 723 and 1250 K, respectively. The lack of appreciable gaseous combustion at elevated pressures is of concern for the reactor thermal management since homogeneous combustion moderates the superadiabatic surface temperatures attained during the heterogeneous combustion of hydrogen.
Hetero-/homogeneous combustion of ethane/air mixtures over platinum at pressures up to 14 bar
Proceedings of the Combustion Institute, 2013
The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated experimentally and numerically at pressures of 1-14 bar, equivalence ratios of 0.1-0.5, and surface temperatures ranging from 700 to 1300 K. Experiments were carried out in an optically accessible channel-flow reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a 2-D CFD code with detailed hetero-/homogeneous C 2 kinetic mechanisms and transport. An appropriately amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous reactions were significant and moreover, provided good agreement to the measured homogeneous ignition locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to 1000 K. Implications for high-pressure catalytic combustion of natural gas were finally drawn.
Combustion and Flame, 2011
The gas-phase combustion of H 2 /O 2 /N 2 mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gasphase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p P 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.
Combustion and Flame, 2005
The gas-phase combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in a laminar channel-flow catalytic reactor at pressures 1 bar p 16 bar. In situ, spatially resolved one-dimensional Raman and planar laser induced fluorescence (LIF) measurements over the catalyst boundary layer were used to assess the concentrations of major species and of the OH radical, respectively. Comparisons between measured and predicted homogeneous (gaseous) ignition distances have led to the assessment of the validity of various elementary gas-phase reaction mechanisms. At low temperatures (900 K T 1400 K) and fuel-to-air equivalence ratios (0.05 ϕ 0.50) typical to catalytic combustion systems, there were substantial differences in the performance of the gaseous reaction mechanisms originating from the relative contribution of the low-and the high-temperature oxidation routes of methane. Sensitivity analysis has identified the significance of the chain-branching reaction CHO+M = CO+H+M on homogeneous ignition, particularly at lower pressures. It was additionally shown that C2 chemistry could not be neglected even at the very fuel-lean conditions pertinent to catalytic combustion systems. A gas-phase reaction mechanism validated at 6 bar p 16 bar has been extended to 1 bar p 16 bar, thus encompassing all catalytic combustion applications. A reduced gas-phase mechanism was further derived, which when used in conjunction with a reduced heterogeneous (catalytic) scheme reproduced the key catalytic and gaseous combustion characteristics of the full hetero/homogeneous reaction schemes.
Combustion and Flame, 2017
Hydrogen combustion over platinum at rich stoichiometries Impact of pressure on homogeneous ignition In situ Raman measurements Hot-O 2 and OH planar laser induced fluorescence (LIF) Catalytic-rich/gaseous-lean combustion concept a b s t r a c t The hetero-/homogeneous combustion of fuel-rich H 2 /O 2 /N 2 mixtures (equivalence ratios ϕ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14 bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5 bar and of hot-O 2 at pressures above ∼5 bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O 2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/ p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14 bar compared to 1 bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition.
Proceedings of the Combustion Institute, 2002
The gas-phase ignition of fuel-lean methane/air premixtures over Pt was investigated experimentally and numerically in laminar channel-flow configurations at pressures of up to 10 bar. Experiments were performed in an optically accessible catalytic channel reactor established by two Pt-coated ceramic plates, 300 mm long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser-induced fluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor the onset of homogeneous (gas-phase) ignition, and thermocouples embedded beneath the catalyst provided the surface temperature distribution. Computations were carried out with a two-dimensional elliptic numerical code, which included the elementary heterogeneous (catalytic) reaction scheme for methane on Pt from Deutschmann and two different elementary homogeneous reaction schemes, Warnatz and GRI-3.0. Following homogeneous ignition, very stable V-shaped flames were established in the reactor. At pressures of up to 6 bar, the measured and predicted (Deutschmann/Warnatz schemes) flame sweep angles and OH levels were in good agreement with each other, while the homogeneous ignition distances were predicted within 10%. However, at pressures greater than or equal to 8 bar, a marked overprediction of the homogeneous ignition distances was evident (Ͼ25%). The Deutschmann/GRI-3.0 schemes yielded much shorter (ϳ55%-65%) homogeneous ignition distances at all pressures. Sensitivity analysis indicated that the latter discrepancies were ascribed to the homogeneous reaction pathway. GRI-3.0 yielded a much faster radical pool buildup than the scheme of Warnatz, clearly showing its inapplicability under catalytically stabilized combustion (CST) relevant conditions. The heterogeneous reactivity was enhanced with increasing pressure. Although the increase in pressure inhibited the adsorption of methane due to the resulting higher oxygen surface coverage, this effect was overtaken by the corresponding increase of the methane gas-phase concentration.
Symposium (International) on Combustion, 1998
The homogeneous ignition of lean methane-air mixtures was investigated numerically and experimentally in a laminar plane channel flow configuration established by two externally heated catalytically active (Ptcoated) ceramic plates, 250 mm long by 100 mm wide, placed 7 mm apart. Preheated fuel-air mixtures with equivalence ratios of 0.31 and 0.37 and uniform velocities of 1 and 2 m/s were examined, resulting in incoming Reynolds numbers ranging from 190 to 380. Planar laser-induced fluorescence (PLIF) was used to map the OH concentration field along the streamwise direction and thermocouples to monitor both catalyst plate temperatures. The numerical predictions included a two-dimensional elliptic model with detailed heterogeneous and homogeneous chemical reactions. The homogeneous ignition location strongly depends on the incoming velocity and mildly on the equivalence ratio. Following homogeneous ignition, a very stable V-shaped flame is formed in all cases. Measured and predicted flame sweep angles, OH levels, and the post-flame OH relaxation are in good agreement with each other, while the homogeneous ignition distance is predicted within 9% in all cases. The homogeneous ignition location is shown to be better identified with changes of averaged (over the channel cross section) quantities rather than with changes in local wall gradients. The overall model performance suggests that the employed surface scheme is capable of capturing the coupling between surface and gaseous chemistries leading to homogeneous ignition. Experiments and predictions were also carried out with noncatalytic plates. The resulting flame is unstable and asymmetric, clearly showing the stability advantages of catalytically assisted combustion.
Combustion and Flame, 2004
The catalytic combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in the pressure range 4 to 16 bar. Experiments were performed in an optically accessible, laminar channel-flow catalytic reactor. In situ, spatially resolved Raman measurements of major species and temperature over the reactor boundary layer were used to assess the heterogeneous (catalytic) reactivity and planar laser-induced fluorescence (LIF) of the OH radical confirmed the absence of homogeneous (gas-phase) ignition. Numerical predictions were carried out with a two-dimensional elliptic code that included elementary heterogeneous and homogeneous chemical reaction schemes. Comparisons between measurements and numerical predictions have led to the assessment of the high-pressure validity of two different elementary heterogeneous chemical reaction schemes for the complete oxidation of methane over platinum. It was shown that the catalytic reactivity increased with increasing pressure and that crucial in the performance of the heterogeneous reaction schemes was the capture of the decrease in surface free-site availability with increasing pressure. Even in the absence of homogeneous ignition, the contribution of the gaseous reaction pathway to the conversion of methane could not be ignored at high pressures. The delineation of the regimes of significance for both heterogeneous and homogeneous pathways has exemplified the importance of the preignition gaseous chemistry in many practical high-pressure catalytic combustion systems. Sensitivity and reaction flux analyses were carried out on a validated elementary heterogeneous reaction scheme and led to the construction of reduced catalytic reaction schemes capable of reproducing accurately the catalytic methane conversion in the channel-flow configuration as well as in a surface perfectly stirred reactor (SPSR) and, when coupled to a homogeneous reaction scheme, the combined heterogeneous and homogeneous methane conversions. A global catalytic step could not reproduce the measured catalytic reactivity over the entire pressure range 4 to 16 bar.
Hydrogen assisted catalytic combustion of methane on platinum
Catalysis Today, 2000
The objective of this paper is to study hydrogen assisted catalytic combustion of methane on platinum experimentally and numerically. In the experiment, we measure the exit temperatures of methane/hydrogen/air mixtures flowing at atmospheric pressure through platinum coated honeycomb channels. A single channel of this monolith is investigated numerically by a two-dimensional Navier-Stokes simulation including an elementary-step surface reaction mechanism. Furthermore, a one-dimensional time-dependent simulation of a stagnation flow configuration is performed to elucidate the elementary processes occurring during catalytic ignition in the mixtures studied. The dependence of the hydrogen assisted light-off of methane on hydrogen and on methane concentrations is discussed. The light-off is primarily determined by the catalyst temperature that is a result of the heat release due to catalytic hydrogen oxidation. Increasing hydrogen addition ensures light-off, decreasing hydrogen addition requires an increasing methane feed for light-off.