Ind. Eng. Chem. Res. 2006, 45, 5841-5858
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Autothermal Reforming of Gasoline for Fuel Cell Applications: A Transient Reactor Model Dennis Papadias* and Sheldon H. D. Lee Chemical Engineering DiVision, Argonne National Laboratory, Argonne, Illinois 60439
Donald J. Chmielewski Center for Electrochemical Science and Engineering, Department of Chemical and EnVironmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
A mathematical model for an autothermal reforming (ATR) fuel processor has been developed to study the transient response of the reactor for gasoline reforming. The model was developed with experimental data from temperature profiles along the reactor and product composition (CO2, CO, and H2 formation). Model versus experiment results were compared for three cases: partial oxidation, ATR using steam injection, and ATR using liquid water spray. In addition to analysis capabilities, the developed model is intended to help in the design of a feedback controller aimed at improving the dynamic response time of the system. 1. Introduction Fuel cells for transportation propulsion and auxiliary power applications have shown great promise, with respect to efficiency and environmental benefit. Unfortunately, in these mobile applications, the delivery of hydrogen fuel is a substantial hurdle to commercialization. Although the availability of pure hydrogen would yield a simple power-generating system with high efficiency and a fast response time, establishing the hydrogen production and distribution infrastructure will take considerable time and resources. In the meantime, high-energy-density fossil fuels have the advantage of the existing low-cost infrastructure for fuel storage and delivery. However, the conversion of these fuels to hydrogen adds substantial complexity to a system that is already subject to size, weight, and efficiency restrictions. As a result, the usual design metrics of efficiency, power density, and specific cost must be augmented to analyze the fuel processing unit, with respect to its dynamic characteristics. For a hydrocarbon fuel, the typical hydrogen production system consists of a reforming unit followed by several reactors designed to reduce the sulfur and CO content of the product stream (both of which are poisons to the low-temperature polymer-electrolyte fuel cells targeted for automotive applications). Of the many reforming options, endothermic steam reforming (SR) is, by far, the most common, because of the multitude of industrial-scale installations. However, the need for an external heat source, where the heat is transferred through a large surface area, leads to relatively poor dynamic performance. In contrast to SR, which is fed only fuel and steam, catalytic partial oxidation (CPOX) is fed a sub-stoichiometric mixture of air and fuel to generate heat directly at the reaction site. Although these compact reactors have a very fast response time, the hydrogen-to-carbon monoxide ratio at the exit is fairly low and, thus, will increase the size, weight, and response time of subsequent CO conversion/clean-up reactors. With the exception of steam being added to the feed, autothermal reforming (ATR) is almost identical to CPOX. During autothermal reforming, many reactions occur. These reactions commonly include total and partial oxidation, steam * To whom correspondence should be addressed. Tel.: +1-(630)252-3206. Fax: 1-(630)-972-4523. E-mail address: papadias@ cmt.anl.gov.
reforming, CO2 reforming, hydrocarbon cracking, water-gas shifting (WGS), and methanation.1 However, many authors have found it convenient to describe ATR reactions simply as a combination of exothermic total oxidation and endothermic steam reforming, followed by the equilibrium-limited WGS reaction.1-3
Total Oxidation:
Steam Reforming:
CmHn +
m+n n O2 f mCO2 + H2O 2 2 (1)
CmHn + mH2O T mCO +
Water Gas Shift:
m+n H2 2 (2)
CO + H2O T CO2 + H2
(3)
The additional steam in the ATR feed not only increases the rate of SR, but also reduces the operating temperature, both of which serve to increase the exit H2/CO ratio (because WGS favors CO2 at lower temperatures). Although ATR is almost as fast as CPOX, the operation of an ATR reactor is complicated by the need to select the feed rates of both air and steam.4 A literature review has revealed several modeling efforts aimed at the reforming process. Among the most cited is the modeling and reactor simulation work by Xu and Froment.5,6 Although that work focused on industrial-scale SR of methane over a Ni/MgAl2O4 catalyst, the proposed kinetic model structure has influenced almost all subsequent reforming models. In De Groote and Froment,7 the SR kinetic model of Xu and Froment5 was extended to methane-fed CPOX and ATR over a nickel catalyst. In addition to considering the influence of steam and carbon dioxide on the rate of coke formation, they sought to determine if the catalyst could accommodate combustion and reforming in parallel (i.e., occurring at the same reaction sites) or if they occur sequentially.8 This analysis was done by modifying the catalyst activity, with respect to SR, as a function of oxygen presence. The maximum temperatures determined in each case were then compared. In the automotive application, the desire to reduce reactor size has motivated the investigation of highly active catalysts based on noble metals. Regarding CPOX of methane, Hickman and Schmidt9 applied a 19-elementary-step model that included
10.1021/ie051291t CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
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Figure 1. Simplified schematic showing process streams in the fuel processor.
adsorption, desorption, and surface reaction rates. Application of this model to platinum and rhodium catalysts provided an activation-energy-based explanation of experimentally observed improvements in H2 selectivity when using a rhodium catalyst. In Wolf et al.,10 a methane-fed, adiabatic CPOX reactor with high space velocity was modeled. Although the form of the rate expressions was similar to that of De Groote and Froment,7 the parameters were fit with data from a Pt/MgO catalyst. Sensitivity analysis, with respect to radiation and heat conduction effects, indicated that the effective conductivity of the catalyst bed in the axial direction has a large role in the formation of hot spots within the CPOX reactor. To aid in the design of ATR systems, Hoang and Chan11 presented a transient two-dimensional (2D) model of a packedbed, methane-fed reactor using a nickel catalyst. They determined the inlet conditions (air-to-fuel and steam-to-fuel ratios) needed to maximize H2 yield and CH4 conversion. The simulations also showed that steady-state operation can be achieved within 3-5 min. In Pacheco et al.,12 an ATR model was developed for the reforming of liquid hydrocarbons over a Pt-CeO2 catalyst. The model assumes isooctane as a model molecule to represent a typical C8 naphtha or paraffinic gasoline. Direct oxidation and reforming were then proposed to develop a kinetic model. Despite these simplifications, the model compares well with experimental data over a wide range of operating conditions. In addition to focusing on gasoline-based reforming in an ATR with short contact time, Springmann et al.13 addressed the cold-start problem. Through one-dimensional (1D) simulations of the ATR (coupled with a heat exchange and WGS unit), they investigated the impact of time-dependent manipulations of the air/fuel ratio to minimize start time. The reforming reactor results presented in this paper are one component of a larger project aimed at identifying the start-up capabilities of an on-board fuel processing unit.14 This project (titled the Feasibility of Acceptable Start-Time Experimental Reformer (FASTER)) was charged with constructing a 10-kW(e) system and showing that it could be started in
