the former group operated their reactor at ambient pressure and diluted the feed gas with 77% argon, the latter experiments were conducted at elevated pressures (1.3–2.4 bar) and varying dilution of 40–80% helium. In Heitnes Hofstad’s experiments the platinum gauze was sandwiched between two ceramic monoliths, whereas in the other setups the gauze was positioned in the free gas-phase.
In all reports the question was addressed whether homogeneous and heterogeneous reactions compete and transport-limitation occur. It is known that there are different regimes for homogeneous-heterogeneous dynamics [41, 43, 44]. Apart from cases in which either surface or phase reactions dominate, there can be cases in which gas-phase chemistry is sustained by heterogeneous processes or conversely, the surface represents a sink for radical species thus terminating the homogeneous reaction chan-nel. Davis et al. note that the homogeneous and heterogeneous reactions appeared to be spatially decoupled in their lean combustion experiments. Apart from using high inert gas dilution, in the reactor from Heitnes Hofstad gas-phase reactions before and after the gauze are presumably suppressed by radical quenching in the monolith. De Smet et al. state that they used low reactant partial pressures to suppress homogeneous chemistry and argued that gas-phase reactions can be neglected because homogeneous plug-flow reactor simulations showed very low conversion.
One of the main hindrance in answering the question of homogeneous-heterogeneous dynamics is the lack of experimental techniques to recover the intrinsic kinetics of the process. This report presents an approach to attain the kinetic information by measur-ing species profiles over a platinum gauze. The experiments were conducted at two stoichiometries, namelyφ= 0.5 and 2.0, in a dedicated flow reactor [92]. In this re-actor, stable product species as well as gas-phase radicals can be measured along the reactor centerline using a sampling technique in combination with a novel fiber-optic laser-induced fluorescence method allowing detection of OH radicals [93].
6.2. Experimental
Reactor walls
Pt gauze
Sampling capillary
Optical probe
Inlet
652 µm
Sampling process
z
Figure 6.1: Experimental setup. The magnification shows the sampling capillary as well as the optical-fiber probe.
The sampling volume of that probe is of the size of the orifice which was ascertained by CFD simulations of the sampling process. Since, species profiles taken with either sampling capillary showed congruent results a spatial resolution on the order of 100µm is generally assumed for the presented data.
The Pt gauze was prepared and activated in a similar manner as described by de Smet et al. [118]. It was first reduced in a flow of 10 % H2 in Ar while heating the reactor up to 700◦C. Then, the catalyst was activated using a C2H6/O2/Ar = 3/2/5 mixture. The actual experiments were conducted with a CH4/O2 mixture diluted in 80 % Argon. The flow rates for the lean (oxygen-rich) conditions, i.e.φ=0.5, were 120 mln/min CH4, 480 mln/min O2, and 2400 mln/min Ar, while in the fuel-rich case, i.e.
φ= 2.0, flow rates were 150 mln/min CH4, 150 mln/min O2, and 1200 mln/min Ar.
The furnace temperature was controlled at 700◦C in both experiments.
6.2.2 Fiber-optic LIF
Gas-phase methane oxidation involves radical chain reactions. Unlike stable reaction species, radicals are quenched throughout the sampling process and can therefore not be detected in that way. This common problem can be overcome by introducing laser-spectroscopic methods such as laser-induced fluorescence (LIF) spectroscopy, at the expense of providing optical access to the system. However, this is not always possible in practical systems. On this account a fiber-optic probe has been developed which is conveniently accommodated inside the sampling capillary. The presented fiber assem-bly is an advancement of the optical probe reported previously (cf. [93] or Chapter 4). It consists of a multi-mode fiber (core-diameter 100µm) for delivering the excita-tion laser-pulse, while the surrounding light guiding capillary collects the fluorescence signal (Figure 6.2). In the former experiments, a single fiber was used for both, laser delivery and fluorescence collection. The disadvantage of the latter excitation-detection geometry is that the transmission of the laser pulse leads to detrimental scattering inter-ferences and hence low signal-to-background ratios for the desired OH fluorescence.
Even tough the overall collection efficiency is smaller for the new fiber-capillary
as-Figure 6.2: Microscope top-view image of the sampling capillary and the polished optical-fiber probe. The image was taken after use. The outer diameters for the capil-lary and the optical-fiber probe are 652µm and 323µm, respectively. The optical-fiber probe consists of the excitation fiber in the center (core diameter 100µm) surrounded
by the collection light-guiding capillary.
sembly, it achieves significantly better signal-to-background ratios compared to the single fiber probe.
It is evident that the LIF signal is collected over a finite volume. Based on geo-metrical considerations an instrumental functionγ(z)can be derived. Generally, the LIF signal originating from a certain point in spacexis proportional to the excitation intensityI(x)and the collection solid angleΩ(x). Following the same considerations as outlined in [93], the instrumental functionγ(z), as a function of the distance from the probe tipz, may be found:
γ(z) = Z 2π
0
dϕ Z ∞
0
r dr I(x)Ω(x)
4π (6.1)
Figure 6.3 showsΩ(x)(top) andI(x)(middle), and the resulting instrumental function γ(z)(bottom). Also shown in the plot is experimental data, showing good agreement with the analytically determined instrumental function. This experimental data cor-responds to the scattering signal originating from the platinum gauze when the fiber probe approaches the gauze from the bottom and was acquired in the same experiment as described hereafter. The analytical instrumental function will be applied in what follows.
A frequency-doubled dye laser (Sirah, Cobra-Strech), which is pumped by the sec-ond harmonic of a Q-switched Nd:YAG laser (Spectra-Physics, Quanta-Ray) was used for excitation. Dye-laser pulses are characterized by a pulse length of∼8 ns and a nominal fundamental linewidth of 0.06 cm−1. Pulses of around 0.1 mJ were coupled into the excitation fiber, the LIF signal was guide on a metal-package photomultiplier (Hamamatsu, R9980U) and the voltage was acquired with a high-bandwidth digitizer.
Every acquisition constitutes an excitation scan of the OH A2Σ- X2Π(0→1) band head at∼281 nm, while detecting the 1→0 and 1→1 bands (306 - 330 nm). The LIF signal depends on temperature through the Boltzmann population distribution of the
6.2. Experimental
I / P (in m−2)
r (mm)
z (mm)
0 0.5 1 1.5 2 2.5 3 3.5 4
0 0.2 0.4 0.6 0.8 1
2 4 6 8 10 12 x 107
Ω
z (mm)
r (mm)
0 0.5 1 1.5 2 2.5 3 3.5 4
0 0.2 0.4 0.6 0.8
1 0
0.05 0.1 0.15
0 0.5 1 1.5 2 2.5 3 3.5 4
0 2 4 6
x 10−3
z (mm)
γ(z)
Instr. funct.
Exp.
Figure 6.3: Irradiance fieldI(x)normalized by the laser powerP (top), and effec-tive collection solid angle distributionΩ(x)(middle) in cylindrical coordinates (radial coordinatedrand axial coordinated z). The excitation fiber has a core diameter of 100µm, while the collection light-guiding capillary has an outer diameter of 300µm.
Both have a numerical aperture NA = 0.22. The bottom graph shows the instrumental functionγ(z)of the fiber probe, according to Equation (6.1). Also shown is experi-mental data which shows good agreement to the analytically determined functionγ(z)
.
rotational states. In order to alleviates the temperature-dependence, the integral area under each spectrum was defined as the LIF signal. A one-point calibration with re-spect to the LIF signal was used to assign a OH number density to the signal. For the calibration a stoichiometric Bunsen-type flame as described in reference [93] was used.