Hetero-/homogeneous combustion of syngas mixtures over platinum at fuel-rich stoichiometries and pressures up to 14 bar (original) (raw)
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The influence of a catalytic surface on the gas-phase combustion of H2 + O2
Combustion and Flame, 1999
The OH concentration outside a Pt catalyst at 1300 K, in a stagnation flow of 90% O 2 and 10% H 2 , has been studied by planar laser-induced fluorescence (PLIF), and compared to measurements outside a heated glass surface. The total pressure in the system was varied from 0.2 to 120 torr. At low pressure, surface reactions were observed for the Pt surface, but not for the glass. At higher pressure, gas-phase ignition occurred for both systems, but not at the same pressure: ignition occurred at a lower pressure outside the inert glass surface. Computer modeling using CHEMKIN confirmed these results. The difference in gas-phase ignition is also seen in the modeling results, and it is due to the removal of atomic O and H from the gas by adsorption and reaction on the catalytic surface. The catalytic reaction mechanism on the surface plays an important role as it enhances the removal of radicals, compared to a surface where only radical recombination back to reactants is allowed.
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.
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 Science and Technology, 2007
The impact of large exhaust gas dilution (up to 59.5% H 2 O and 30.3% CO 2 per vol.) on the heterogeneous (catalytic) and homogeneous (gas-phase) steady combustion of fuel-lean CH 4 =O 2 =N 2 mixtures over platinum has been investigated experimentally and numerically at pressures of 5 to 14 bar. In situ, one-dimensional Raman measurements of major gas-phase species concentrations and planar laser induced fluorescence (LIF) of the OH radical were used to assess the heterogeneous and homogeneous combustion processes, respectively. Comparisons between measurements and
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.
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, 2012
This paper addresses the interactions between homogeneous and heterogeneous reactions for different hydrocarbons namely, compressed natural gas (CNG), liquefied petroleum gas (LPG), butane and dimethyl ether (DME) over platinum. Experiments are performed to study the effects of varying the temperature of the incoming mixture (T jet ), its equivalence ratio (Ø) and the Reynolds number (Re), on the reactivity limits. Computational fluid dynamic (CFD) calculations using detailed chemical kinetics for both the platinum surface and gas phase are completed for a range of methane-air mixtures to resolve the impact of varying T jet , Ø and Re on the compositional structure of the flow. Comparison between numerical and experimental results is performed where relevant.
Effect of Oxygen enrichment on Acid Gas Combustion under Claus Conditions
11th International Energy Conversion Engineering Conference, 2013
The effect of oxygen enrichment (using a mixture of oxygen and nitrogen) in the oxidant on the combustion of acid gas (H 2 S and CO 2 ) in hydrogen fuelled flames is examined under Claus condition (at equivalence ratio of Φ=3). The acid gas composition examined was 50% H 2 S/50% CO 2 at different percentages of oxygen enrichment (0%, 19.5% and 69.3%) in the oxygen/nitrogen mixtures. Combustion of acid gas formed SO 2 with increase in O 2 mole fraction. The results showed that an increase in oxygen enrichment to combustion air results in reduced formation of elemental sulfur while simultaneously increasing the amounts of SO 2 that is less favorable. This is attributed to the release of oxygen from CO 2 at high temperatures into the reaction pool. Moreover, the mole fraction of carbon monoxide increased at increased oxygen enrichment which is an indicator of increased release of oxygen into the reaction pool. Other sulfurous-carbonaceous compounds formed (primarily COS and CS 2 ) are due to the presence of carbon monoxide. The results showed mitigation of CS 2 formation with oxygen enrichment in the combustion process and increased rate of COS formation from the increased availability of CO due to the dissociation CO 2 at higher temperatures. The reduction in CS 2 formation helps to improve efficiency of sulfur capture in Claus thermal reactor.
Combustion and Flame, 1997
Ignition behavior of premixed methane-oxygen mixtures in stagnation flow near a heated inert surface was examined using the GRI reaction mechanism [l]. The effects of pressure (l-100 atm), preheat (298-773 K), and residence time (3-250 ms) on ignition temperature were investigated for the full range of fuel-to-oxygen ratios. A minimum in ignition temperature with composition occurred at about a 15% methane in oxygen feed (4 = 0.3) and was not affected strongly by pressure, preheat, or residence time. At atmospheric pressure, thermal feedback from the heat of reaction was a prerequisite for ignition at all fuel-to-oxygen ratios. However, at 50 atm, thermal feedback was only necessary for ignitions of mixtures leaner than 15% methane. For mixtures richer than 15% methane at 50 atm, ignitions due to chain branching preceded thermal ignitions. These chain-branching ignitions were unaffected when the thermal feedback was computationally turned off. Moreover, the fuel-rich ignitability limit increased from 55% methane at atmospheric pressure to 66% methane at 50 atm, while the fuel lean ignitability limit at 7% methane was not affected significantly by pressure. Reaction path analysis before ignition showed that for high-temperature ignitions, methane consumption for fuelJean mixtures was by OH and 0 radicals, while for fuel-rich mixtures, it was by H and OH radicals. The main source of OH radicals for these ignitions was by the reaction of H and 0,. For low-temperature ignitions, methane was predominantly consumed by OH radicals before ignition regardless of feed composition, and the main source of OH radicals was by reactions involving HO, and H,Oz.