Heterogeneous Hydrocarbon Oxidation - ACS Publications - American

Chapter 4

Ignition and Extinction of Hydrogen—Air and Methane—Air Mixtures over Platinum and Palladium Downloaded by UNIV OF NEW ENGLAND on February 8, 2017 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch004

F. Behrendt, O. Deutschmann, R. Schmidt, and J. Warnatz Interdisciplinary Center for Scientific Computing, Universität Heidelberg, Im Neuenheimer Feld 368, D-69120 Heidelberg, Germany Ignition and extinction of hydrogen-oxygen as well as methane -oxygen mixtures flowing towards resistively heated platinum or pal ladium foils are experimentally investigated. The ignition temperatures and the hysteresis observed between ignition and extinction are compared with time-dependent simulations. The simulation is per formed for the coupled system of surface and gas phase utilizing ele mentary reaction mechanisms for both phases. Comparison of experimental and numerical results for ignition and extinction at platinum shows a very good agreement with respect to temperature as well as its temporal development during ignition. Due to deficiencies regarding kinetic data for the surface reactions on Pd, the results here do not compare as well as for Pt.

Extensive experimental and theoretical attention has been given to catalytic combu stion in the past decade. The potential of heterogeneous processes for reducing emis sion of pollutants, improved ignition, and enhanced stability of flames has been reco gnized. Here, catalytic ignition as a sudden transition from a kinetically controlled system to one controlled by mass transport is of special interest (1-4). Its description needs a detailed knowledge of both the elementary reaction steps at the gas-surface interface and the gas phase as well of the transport processes between surface and gas phase. In the present work, mixtures of hydrogen or methane with oxygen (for some of the measurements diluted by nitrogen) at atmospheric pressure are utilized. These mixtures flow slowly through a tube towards a platinum or palladium foil, which is heated resistively. When the foil is heated to a sufficiently high temperature, heterogeneous reactions start and the reactants are consumed at the metal surface. To simulate the catalytic ignition under the given conditions the above mentioned transition from kinetic to transport control requires a closely coupled solution of the governing equation for the gas phase and the surface processes (9,15). The set of differential equations describing this coupled system is discretized and solved using a time exact solver (10,11). This numerical code, originally developed for the simulation of gas phase combustion processes (8), applies a detailed description of the elementary chemical processes occurring at the gas-surface interface, and couples them to the reactive flow in the surrounding gas phase.

0097-6156/96/0638-0048$15.00/0 © 1996 American Chemical Society Warren and Oyama; Heterogeneous Hydrocarbon Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

4. BEHRENDT ET AL.

Ignition & Extinction ofH-Air & CH ~Air Mixtures 4

The ignition temperature is calculated as a function of the fuel concentration, and is compared with the experimental results. The surface chemistry is described by a detailed reaction mechanism including adsorption, desorption, and surface reactions. Experiment The experimental setup is shown in Figure 1. The catalyst is mounted in a rectangular tube with a cross section of 28 x 38 m m and a length of 16 cm. This catalyst is a polycrystalline foil of either platinum or palladium with a purity of 99.95 % for Pt and 99.9 % for Pd. The thickness of the Pt foil is 0.027 mm and that of the Pd foil 0.025 mm, the outer dimensions of both are 25 by 5 mm. Before each experiment the foil is cleaned following a procedure described by Keck et al. (16). After several ignition/extinction cycles a irreversible deactivation of the foil is observed, and it has to be replaced.

Downloaded by UNIV OF NEW ENGLAND on February 8, 2017 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch004

2

Constant current generator

i

Ii

1

Gas inlet ~(H2, O2, N2)

Figure 1. Experimental Setup The foils are mounted on two copper rods so that the flow is directed towards the foils (stagnation point flow configuration). Through these rods the electrical current used to heat the foil is supplied. Additionally, two thin platinum wires are attached to the foil to measure the temperature dependent electrical resistivity of the foil, and thus, the temperature. Compared to the application of a spot-welded thermocouple this method is simpler to use, it is very responsive even to fast changes of the foil

Warren and Oyama; Heterogeneous Hydrocarbon Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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HETEROGENEOUS HYDROCARBON OXIDATION

temperature, and does not introduce additional catalytically active material to the system. After passing through a mixing chamber filled with glass spheres the gas mixture flows towards the foil. The fuel content of the mixture is defined as a = Pfue/(Pfuei Poxyffen) * * i d between 0.25 < a < 0.75 for hydrogen and 0.28 < a < D.96 for metnane. The combined partial pressure of fuel and oxygen for mixtures diluted by nitrogen is introduced as 8 = (Pf i P ygen^Ptotar ^ ^ l° ity * 8 cm/s. +

a n c

sv a r

e

n e

+

ue

o

w

v e

c

s

OX


Model and Simulation The experiment is simulated using a one-dimensional stagnation point flow model with time and distance from the foil as independent variables. By confining the at tention to the centre of the surface, edge effects can be neglected. Details of this ap proach are discussed by Warnatz et al. (4). The dependent variables (density, momentum, temperature, and mass fraction of each gas phase species) depend only on the distance from the foil. The system is clo sed by the ideal gas law. The boundary-value problem that has to be solved has been stated by Evans and Greif (5) and by Kee et al. (6). This set of governing equations is analogous to that used for simulation of laminar counterflow diffusion-flames (7,8). The solution of the gas-phase problem is coupled to the surface properties and the surface reaction mechanism. Therefore, the surface boundary-conditions become more complex compared with the pure gas-phase problem. In a time-dependent for mulation the mass fraction of a gas-phase species at the surface is determined by the diffusive and convective processes as well as the creation or depletion rate of that species by surface reactions. The temperature boundary-condition is derived from energy contributions at the interface. Included are the conductive, convective, and diffusive energy transport in the gas phase, the chemical heat release at the surface, radiation, and the resistive heating of the foil. Furthermore, the heat capacity of the platinum foil and the dependence of the specific resistance of the foil on temperature are taken into account. Details on the governing equations and boundary conditions can be found in Behrendt et al. (9,15). To solve this stagnation flow problem numerically, a computer code originally developed to simulate counterflow diffusion-flames is adapted. This code computes species mass fraction, temperature, and velocity profiles, and fluxes at the gas-surface interface as a function of time. The program accounts for finite-rate gas-phase and surface chemical kinetics. A simplified multicomponent molecular transport model is used. The Navier-Stokes equations describing the gas phase together with the boundary conditions represent a differential-algebraic equation system. Discretization bases on a finite-difference scheme using a statically adapted non-equidistant grid. The resulting systems of algebraic and ordinary differential equations is solved by the extrapolation solver L I M E X (10,11). The chemistry is modelled by a set of elementary reactions in the gas phase as well as on the surface. The gas-phase reaction data are taken directly from modelling work on flame chemistry (12,13). Its validity has been established through extensive studies of various combustion systems, and is applied here without modification. The surface rate data can be found in (9). This set of detailed reaction steps including O and H atoms, O H , H 0 , C O , and C H (i = 0, 3) molecules is based on several publications (17-18). The rate coefficients for the reverse reactions as well as the reaction enthalpies of the surface species are calculated from the thermochemical data for the H / 0 -system as stated in (4) extended for CO and CH. (i = 0, 3). The only adjustable parameter to the simulation is the heat loss to the supporting copper rods. This loss is determined experimentally using inert gas and accounted for in the simulation. 2

2


4. BEHRENDT ET AL.

Ignition & Extinction ofH-Air & CHj-Air Mixtures

In a previous paper (75) experimental data on the ignition of methane-air mixtures published by Schmidt et al. (17) have been used to evalute the recation mechanism and the numerical code used here. Results Ignition of H2-O2 Mixtures. In Fig. 2 the surface temperature is plotted versus the current supplied to the foil.


2a

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j -

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Figure 2. Experimental (top, 2a) and numerical (bottom, 2b) ignition and extinction curve for hydrogen and oxygen on platinum (a = 0.5) Before ignition, the increase of foil temperature is due to this resistive heating, while during and after ignition the heat released by surface reactions contributes significantly to the foil temperature.The point of ignition is clearly marked by the sudden rise of the foil temperature. Figure 2a shows a typical experimental igni tion/extinction curve for a hydrogen-oxygen mixture on platinum.


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Before ignition, the foil is covered by hydrogen keeping oxygen from adsorption, and no surface reactions take place (see Fig.3). Elevating the foil temperature by electrical heating above a certain value leads to a shift of the adsorption/ desorption equilibrium of hydrogen towards desorption, thus liberating surface sites for dissociative oxygen adsorption. The oxygen atoms react with hydrogen atoms forming OH radicals and finally water. The heat released by the water formation leads to a further increase of the surface temperature, which in turn enhances the desorption of hydrogen. This self-accelerated process leads to ignition. During ignition the system undergoes a transition from kinetically controlled surface reactions to diffusion control, where the transport of reactants from the surrounding gas phase to the surface limits the reaction rate. After ignition, the current supplied to the foil is reduced until finally extinction occurs for a current much lower than that one needed for ignition. The heat released at the foil maintains enough free surface sites for the continued adsorption of either reactant until finally the surface is fully covered by hydrogen again. Figure 2b shows the results of calculations using the same parameters as in the measurement. The ignition temperature in both cases is 355 K . In the calculation no extinction is observed for these parameters, while in the experiment the surface reactions extin guish again. The polycrystalline foil undergoes recrystallization during this heating and cooling cycle, an effect which cannot be accounted for in the simulation. Extinction is observed for calculations using smaller a.

550

500

450 3 400

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300 40.0

60.0

100.0

Time [s]

Figure 3. Time-resolved change of surface temperature and coverage for hydrogen oxidation on platinum (a = 0.5) The detailed time-resolved development of the surface temperature (experimental and calculated) and surface coverage (calculated only) is presented in Figure 3. Time zero here is the moment of last increase of the current through the foil. After ignition, most of the platinum sites are empty with about 15 % H atoms and 1 % water remaining.


4. BEHRENDT ET AL.

Ignition & Extinction ofH-Air & CH ~Air Mixtures 4

In Figures 4 (platinum) and 5 (palladium) experimental results for the ignition of nitrogen-diluted mixtures of hydrogen and oxygen are compared with simulations. For all cases 8 is kept at 0.059. In general, for a given value of a, the ignition tempe rature on platinum is higher than on palladium.


400

a =

P 2 (Po2 PH ) H

/

+

2

Figure 4. Measured and calculated ignition temperatures for hydrogen and oxygen diluted by nitrogen on Pt (8 = 0.059) (for a and b see text) The difference between both catalysts is more pronounced at the lower ignition limit. Below a = 0.3 for platinum and a = 0.26 for palladium significant differences are observed in experiment and simulation depending on the initial coverage of the metal surface, i.e., bistability is observed. Starting the calculation with a surface covered by hydrogen atoms (case a;using an initially pure hydrogen flow), ignition starts without external supply of heat for a smaller than 0.12 for platinum and smaller than 0.23 for palladium. Even at room temperature the desorption of hydrogen leads to a sufficient number of free sites allowing oxygen to adsorb, causing the onset of reactions at the surface. At low a, the lower sticking coefficient of oxygen is compensated for by the much higher concentration of oxygen in the gas phase. Above a = 0.12 and 0.23, respectively, hydrogen sticking becomes so efficient that external heating is required to create enough free sites, so that adsorption of oxygen remains competitive. The sticking coefficients for both oxygen and hydrogen are more than an order of magnitude larger for palladium than for platinum, thus extending the range of ignition without heating to higher values of a. When the surface is initially covered by oxygen atoms (case b; gas flow of pure oxygen), ignition without external heating is observed below a = 0.32 for both catalysts. Hydrogen has a higher sticking coefficient compared to oxygen and, at the same time, is more mobile on the surface. Consequently, on an oxygen-covered surface, even at room temperature, hydrogen will be able to adsorb and react to form water. This releases some heat, leading to more free sites and finally to ignition.


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The discussion above conclusively shows the platinum and palladium surface to be covered initially by hydrogen for sufficient high a, i.e., a larger than 0.32 for the present conditions.

400


5f

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'

r

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a=P /(P +P ) H 2

0 2

H 2

Figure 5. Measured and calculated ignition temperatures for hydrogen and oxygen diluted by nitrogen on Pd (5 = 0.059) (for a and b see text) Ignition of CH4-O2 Mixtures. Experimental ignition temperatures for mixtures of methane and oxygen diluted by nitrogen on platinum as a function of a are shown in Figure 6. The relative partial pressure of methane a = p ^ / (P han^Poxy en) varied between 0.28 and 0.96, while the dilution by nitrogen for all a is given by 0 = (Pmetiiane Poo^ygen^Ptotal 0-059. . The ignition temperatures for methane-oxygen mixtures are much higher com pared with hydrogen-oxygen mixtures. Remarkable is the reversed dependence of ignition temperature on a. While for hydrogen-oxygen mixtures the ignition tempe rature increases with increasing a, here a decrease of the ignition temperature is ob served. In contrast to the hydrogen-oxygen system, the platinum surface is initially covered by oxygen instead by the fuel. So, the reaction is initiated by the desorption of oxygen, offering free sites for the adsorption of methane. For small a, i.e., a high concentration of oxygen in the gas phase, adsorption of oxygen on these free sites is more likely than of methane. For higher temperatures more free sites are generated, resulting in a higher probability of adsorption of methane leading to ignition. With increasing a, the larger mole fraction of methane in the gas phase increases the chance of methane to adsorb, thus lowering the temperature needed for ignition. Figure 7 shows a complete calculated ignition-extinction cycle for a methaneoxygen mixture at a = 0.5. Compared to the same kind of cycle for a hydrogenoxygen mixture as shown in Figure 2, the difference between ignition and extinction temperature is much larger. Additionally, for the oxidation of methane a reduction of electrical power supplied to the foil after ignition causes the surface temperature to decrease about 300 K before the system extinguishes. is

m e t

+

n e

met

=


g

4. BEHRENDT ET AL.

Ignition & Extinction of H—Air & CHj-Air Mixtures l

1

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E

Heterogeneous Hydrocarbon Oxidation - ACS Publications - American

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