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2011 | Book

Introduction Read chapter Read first chapter

Catalytic Microreactors for Portable Power Generation

Author: Symeon Karagiannidis

Publisher: Springer Berlin Heidelberg

Book Series : Springer Theses

Part of: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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About this book

"Catalytic Microreactors for Portable Power Generation” addresses a problem of high relevance and increased complexity in energy technology. This thesis outlines an investigation into catalytic and gas-phase combustion characteristics in channel-flow, platinum-coated microreactors. The emphasis of the study is on microreactor/microturbine concepts for portable power generation and the fuels of interest are methane and propane. The author carefully describes numerical and experimental techniques, providing a new insight into the complex interactions between chemical kinetics and molecular transport processes, as well as giving the first detailed report of hetero-/homogeneous chemical reaction mechanisms for catalytic propane combustion. The outcome of this work will be widely applied to the industrial design of micro- and mesoscale combustors.

Table of Contents

Frontmatter
Chapter 1. Introduction
Abstract
In the last two decades significant advances have been achieved in the miniaturization and compacting of various electromechanical and electronic consumer devices such as laptop computers, mobile phones or even personal transport vehicles. Moreover, new needs have appeared in such diverse areas as medicine (e.g. artificial organs) and military applications (e.g. unmanned aerial vehicles) subsequently giving rise to new technological challenges [1‐3]. The push towards even greater portability, integration and autonomous operation of such devices has given rise to new and promising research fields, such as the field of micro electro-mechanical systems (MEMS) [4]. Once the major constrains of precision fabrication in the microscale had been removed [5], attention was drawn to the main factor bottlenecking further advancements in scaling down such devices: the lack of efficient and reliable power sources.
Symeon Karagiannidis
Chapter 2. Experimental Setup
Abstract
Visual inspection of the subscale unit revealed an overall good cross-section uniformity for the catalytic channels (see  Fig. 2.4). The unit tested in this study differed from the proposed mesoscale catalytic combustor only in its inner diameter (all other geometric parameters were kept constant), with the former unit having a radial dimension ~42% smaller than the latter. With the number of catalyst-coated channels being proportional to the honeycomb cross sectional area, the power output of the subscale unit was reduced nearly threefold compared to the mesoscale unit; heat losses from the outer combustor surface were accordingly impacted due to the reduced surface area of the subscale combustor.
Symeon Karagiannidis
Chapter 3. Numerical Models
Abstract
By choosing a time step for the transient solid temperature equation long enough for gas-phase equilibration, the discretized, time-independent set of the Navier–Stokes equations under the boundary layer approximation can be solved for the flow field inside the catalytic channels [12] (quasisteady assumption for the gas-phase) using the CRESLAF package [13]. The applicability of the boundary layer approach in catalytic combustion at sufficiently large Reynolds numbers (Re ≥ 20) has already been demonstrated [14]. The simplified equations thus become:
Symeon Karagiannidis
Chapter 4. Experimental and Numerical Investigation of the Hetero-/Homogeneous Combustion of Lean Propane/Air Mixtures Over Platinum
Abstract
In the present chapter a combined experimental and numerical investigation is presented on the catalytic combustion of fuel-lean C3H8/air mixtures over platinum at pressures 1 ≤ p ≤ 7 bar, range of interest to microreactors and small-scale industrial turbines. Experiments have been carried out in the optically accessible, channel-flow catalytic reactor at propane-to-air equivalence ratios ranging between 0.26 and 0.43. The heterogeneous reactivity is assessed with Raman spectroscopy of major gas-phase species concentrations across the channel boundary layer, and the onset of homogeneous ignition is monitored with planar laser induced fluorescence (LIF) of the OH radical. Simulations are performed with the full-elliptic 2-D CFD code. The main objectives are to assess the catalytic reactivity of propane and its pressure dependence by constructing an appropriate global reaction step and then to couple this step with a detailed homogeneous reaction mechanism so as to reproduce the measured homogeneous ignition characteristics.
Symeon Karagiannidis
Chapter 5. Experimental and Numerical Investigation of a Propane-Fueled, Catalytic, Mesoscale Combustor
Abstract
The present work undertakes a combined experimental and numerical investigation of a propane-fueled, platinum-coated, mesoscale combustor, which is candidate for portable power generation applications. As a first step, the heterogeneous reactivity of propane on platinum has been assessed with gas-phase Raman spectroscopy and an appropriate global reaction step for lean propane/air combustion was constructed, valid at pressures 1 ≤ p ≤ 7 bar, a range of interest to microreactors and small-scale industrial turbines. In a second step, a detailed numerical parametric study in a single catalytic channel has been performed, with inlet conditions and combustor material properties as the parameters of main interest, aimed at defining regimes of optimal power output and fuel conversion. Detailed numerical treatment was provided for the heat transfer mechanisms inside the channel, which included heat conduction in the solid and surface radiation heat exchange between the reactor elements themselves and between the reactor and its surroundings. Based on the findings of the aforementioned parametric study, the third and final step involved the construction and testing of a mesoscale catalytic combustor for a range of operational parameters relevant to micro- and mesoscale power generation applications. A continuum model for the entire combustor monolith was developed to complement the experiments, thus allowing for the simulation of the temperature 2-D distribution inside the combustor. This numerical tool facilitated the detailed modeling of the heat losses from the catalytic combustor, evident in the experiments, and allowed for further design improvements.
Symeon Karagiannidis
Chapter 6. Hetero-/Homogeneous Combustion and Stability Maps in Methane-Fueled Catalytic Microreactors
Abstract
In this chapter, a numerical investigation is undertaken to study the coupled catalytic and gas-phase combustion processes in a methane-fueled microreactor with catalytically active Pt walls. Simulations were carried out with the 2-D full elliptic model for both the gas and solid phases. Elementary hetero-/homogeneous chemical reaction schemes were included along with heat conduction in the walls, surface radiation heat transfer, and external heat losses. The main objectives were to investigate the interplay of hetero-/homogeneous combustion, transport, and heat transfer mechanisms in the microreactor and to delineate combustion stability maps in terms of the underlying parameters.
Symeon Karagiannidis
Chapter 7. Stability of Hetero-/Homogeneous Combustion in Propane- and Methane-Fueled Catalytic Microreactors: Channel Confinement and Molecular Transport Effects
Abstract
A detailed parametric study is undertaken in this chapter, using a full elliptic 2-D CFD code for both gas-phase and solid domains, in order to delineate the stable combustion regimes of propane-fueled catalytic microreactors at pressures 1 and 5 bar (pressures up to 5 bar are of interest to recuperated microturbine systems [1‐3]), channel confinements 1.0 and 0.3 mm and wall thermal conductivities 2 and 16 W/mK. Methane simulations are also included, so as to exemplify the significant differences in both chemical and transport properties on microcombustor stability. The main objectives are to assess the effect of high pressure operation, molecular transport and gas-phase chemistry on the stability of propane-fueled catalytic microreactors and to study the impact of increased geometrical confinement and high wall thermal conductivity on the non-adiabatic reactor operation. Particular objectives were to qualify the differences between the two fuels in terms of reactor stability and performance.
Symeon Karagiannidis
Chapter 8. Numerical Investigation on the Start-Up of Methane-Fueled, Catalytic Microreactors
Abstract
In the present work, a numerical study is undertaken to investigate the start-up of methane-fueled, catalytic microreactors under conditions pertinent to microturbine-based power generation devices. A full elliptic, transient in the solid and quasisteady in the gas numerical code is used to simulate the reacting flow in a catalytic plane channel configuration with a gap of 1 mm and a length of 10 mm, with this setup effectively representing a single channel of a catalytic honeycomb combustor structure. Detailed catalytic and gas-phase reaction mechanisms for the total oxidation of methane on platinum are used. The main objectives are to study the ignition characteristics for microreactors of a nominal geometry by independently varying the pressure, reactor wall material (including wall thermal conductivity and heat capacity), inlet mixture velocity, radiation properties of the solid, and fuel-to-air air equivalence ratio. The role of gas-phase chemistry in the start-up process is also investigated. Particular objectives are to assess the impact of the aforementioned parameters on the elapsed time required for reactor ignition and subsequent attainment of steady state.
Symeon Karagiannidis
Chapter 9. Conclusions Summary: Outlook
Abstract
The present work investigated experimentally and numerically the lean hetero-/homogeneous combustion of propane on platinum as well as numerically the steady-state and transient performance of methane- and propane-fueled catalytic microreactors. The ultimate goal was the in-depth study of catalytic microscale combustion for portable power applications, with particular focus on conditions pertinent to micro-gas-turbines. The following constitute the major findings and conclusions of this work.
Symeon Karagiannidis
Backmatter
Metadata
Title
Catalytic Microreactors for Portable Power Generation
Author
Symeon Karagiannidis
Copyright Year
2011
Publisher
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-17668-5
Print ISBN
978-3-642-17667-8
DOI
https://doi.org/10.1007/978-3-642-17668-5

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