The Lowtemperature Partialoxidation Reforming Of Fuels For Transportation Fuel Cell Systems

R. Kumar, S. Ahmed, and M. Krumpelt Electrochemical Technology Program Chemical Technology Division Argonne National Laboratory Argonne, IL 60439

Introduction

Passenger cars powered by fuel cell propulsion systems with high efficiency offer superior fuel economy, very low to zero pollutant emissions, and the option to operate on alternative and/or renewable fuels. Although the fuel cell operates on hydrogen, a liquid fuel such as methanol or gasoline is more attractive for automotive use because of the convenience in handling and vehicle refueling. Such a liquid fuel must be dynamically converted (reformed) to hydrogen on board the vehicle in real time to meet fluctuating power demands [1J. This paper describes the low-temperature Argonne partial-oxidation reformer (APOR) developed for this application. The APOR is a rapid-start, compact, lightweight, catalytic device that is efficient and dynamically responsive. The reformer is easily controlled by varying the feed rates of the fuel, water, and air to satisfy the rapidly changing system power demands during the vehicle's driving cycle. Reforming Processes

Hydrogen may be produced from fuels by either steam reforming or partial-oxidation reforming. In a steam reformer, the fuel (hydrocarbon, alcohol, etc.) is reacted with steam over a catalyst at a high temperature and pressure. The reaction is endothermic, and the heat of reaction is provided by the combustion of fuel and transferred to the process gas across a metal wall. Because of the indirect heat transfer, steam reformers are heavy, bulky, slow to start, and slow to respond to load changes. In a partial-oxidation reformer, part of the fuel is oxidized to provide the energy for the reforming reaction within the process gas. The direct heat transfer makes such a reformer compact, lightweight, and dynamically responsive. The addition of a suitable catalyst can be used to influence the product gas composition. The steam reformer is relatively complex, since it contains burners, extended heat transfer surfaces, and combustion air and exhaust duct work. The partial-oxidation reformer is mechanically simple due to the absence of these components. Fuel Cell Systems

Figure 1 shows greatly simplified schematic diagrams for two fuel cell systems, one with a steam reformer and one with an APOR [2], In the system with a steam reformer, the fuel and water are fed to the reformer, the temperature, humidity, and contaminant levels in the reformate are adjusted (not shown), and the fuel gas is then fed to the fuel cell stack, where 80-85% of the hydrogen is electrochemically oxidized to generate electricity. The exhaust fuel gas is recycled to the burner to provide the energy for fuel reforming. In the system with the APOR, the fuel, water, and air are fed to the reformer, and the reformate (after appropriate conditioning) is fed to the fuel cell stack; the spent fuel gas is not recycled to the reformer, although a catalytic burner (not shown) is used to avoid venting hydrogen to the environment.

The dynamic response of a steam-reformed, methanol-fueled, polymer electrolyte fuel cell system has been analyzed [3]. Different turn-down scenarios (from steady-state at the design point) were analyzed. In one, the flow rates of the fuel gas and air were ramped down while maintaining fuel utilization {uj> constant at 85%. For a 50% reduction in power level, the simulation showed that the reformer catalyst overheated within a few seconds. One solution to alleviating this problem is to inject additional water into the process gas just ahead of the reformer. However, if the fuel gas flow rate is not decreased in concert with the decrease in fuel cell power, combustion of the excess hydrogen in the spent fuel gas at the reformer burner rapidly leads to unacceptably high reformer catalyst temperatures. These scenarios are discussed in detail in reference [3].

The dynamic response of the APOR is excellent. Power transients are accommodated simply by varying the feed rates of fuel, water, and air to the APOR. The product gas flow rate responds almost instantaneously, while its composition remains essentially constant. The reactor temperatures (and, therefore, the reaction chemistries and kinetics) are not significantly affected by changes in the fuel processing rate. Thus, the process control for the APOR is analogous to that of the fuel injection systems used in today's cars.

The calculated steady-state efficiencies of the two systems (fueled with methanol) are shown in Fig. 2. At a uf of 85% or greater, the efficiency of the system with the APOR exceeds that of the steam reformer system. In automotive applications, the APOR system will be more efficient even at a lower Uj because the efficiency of the steam-reformer system decreases under fluctuating power demands [1]. The efficiency of the fuel cell system with the APOR is largely unaffected by power transients.

Argonne's Partial-Oxidation Reformer for Methanol

In the methanol APOR, hydrogen is generated by a combination of the exothermic partial-oxidation reaction, the endothermic decomposition and steam-reforming reactions, and the water-gas shift reaction:

CH3OH({) + 'AQ2

0 0

Post a comment