Fuel Cells in the Automotive Industry

W H I T E P A P E R O N C H E M I C A L E N G I N E E R I N G Fuel Cells in the Automotive Industry by Ed Fontes EvaNilsson © COPYRIGHT 1994 - 2001 by COMSOL AB. All rights reserved. C H E M I C A L E N G I N E E R I N G I N T H E A U T O M O T I V E I N D U S T R Y Fuel Cells in the Automotive Industry The design of catalyst reactors has made the car engine substantially cleaner during the last two decades. Car manufacturers have applied large efforts in reducing the emissions of hazardous gases from the combustion engine. However, it has still proven to be difficult to eliminate NOx and SOx emissions from this process. The chemical energy of gasoline is converted to mechanical energy via the production of heat in the combustion process of conventional car engines. The efficiency of this process is limited by the efficiency formula for the Carnot cycle. A more efficient process, and one of the main candidates for power production in future cars, is the new generation of fuel cells. These work principally as batteries do, yet while batteries can be considered as batch reactors, fuel cells are continuous reactors. In a fuel cell-powered engine, the chemical energy in the fuel is converted to electrical energy, and then to mechanical energy by an electric motor. The process by-passes the limitations of the Carnot cycle, and the theoretical efficiency is substantially higher than that for the combustion engine. This implies that a fuel cell-powered car will be able to run for longer distances using the same amount of fuel compared to a conventional car. Carbon dioxide emissions are consequently lowered, since smaller amounts of fuel are consumed for the same distance traveled. The low temperatures in the process practically eliminate the production of NOx and SOx. The development of fuel cell powered-vehicles has accelerated during the last five years. Competition between the different players is growing and the fight for a share of a potentially huge market has already started. Technological development is one of the most important weapons at this early stage, and small companies with technical skills in the field of fuel cell processes have become important partners to the large automotive companies. Mathematical modeling is one important tool in the development of fuel cell systems. A combination of modeling and experiments has shown to lower costs and accelerate the pace of building prototypes and understanding of these new systems. The optimization of the fuel cell, in combination with the auxiliary equipment and the operation of the electrical motor, requires a lot of mathematical puzzling. Advancement in the area of computing has implied that simulations that required super computers just a few years ago can today be run on workstations or even PCs. This has made computer simulations available to a much larger number of engineers. In this paper, we will look at the fuel cell system through a gallery of mathematical models, and particularly at models of the electrochemistry in the fuel cell itself. We will 3 C H E M I C A L E N G I N E E R I N G I N T H E A U T O M O T I V E I N D U S T R Y look at the processes as they take place in the heart of the fuel cell system, i.e. in the electrodes and electrolyte in the fuel cell stack. These processes are studied at a micro level, where single catalyst agglomerates are modeled, as well as on the level of a unit cell consisting of an anode, a cathode and the electrolyte in between them. We will also look at reactor models of the reformer and the catalytic burner in the fuel processor. Finally, we will study the design of the bipolar plates and their influence on the ohmic losses in the fuel cell stack. All of the models shown in the figures throughout the paper have been produced by the finite element package, Femlab. One of Femlab’s strongest features is that we can define arbitrary nonlinear systems of partial differential equations and fully couple them. This makes Femlab extremely powerful in handling the nonlinearities that arise when we model reactors and when we treat the kinetics at the fuel cell electrodes. The fuel cell system The fuel cell system can be simply structured into the following components: a fuel processor, an air system, a fuel cell stack and a water and heat management system. Figure 1 shows a simplified flow chart of the system. Figure 1. Simplified flow chart of the fuel cell system. 4 C H E M I C A L E N G I N E E R I N G I N T H E A U T O M O T I V E I N D U S T R Y The fuel, methanol in the case of figure 1, enters the fuel processor where it is converted into hydrogen. The produced hydrogen reformate is cleaned from by-products that are hazardous for fuel cell catalysts, like carbon monoxide, in a clean-up system. The cleaned and moisturized hydrogen-rich reformate is run to the fuel cell’s anode chambers where the hydrogen is oxidized while oxygen is reduced at the cathode. Water is used to moisturize hydrogen, since water is transported from the anode to the cathode by electro-osmosis. Air is supplied to the cell via a compressor. The compressed cathode chamber exhaust is run through an expander, in order to win back some of the energy from the compression step. Air is also supplied to the fuel cell processor. The DC-current produced in the process is transferred to a power conditioner before it is supp-lied to the electric motor. The spent fuel which still contains some hydrogen, is fed to a catalytic burner and the heat produced in this combustion process is used in the fuel processor. The Fuel Processor and Auxiliary System The optimal fuel in a fuel cell, from the environmental point of view, is hydrogen produced by means of renewable sources, such as solar power. However, hydrogen is still difficult to store in an efficient way, despite extensive research being put into using metal hydrides and nano fibers. The storage of hydrogen in alcohols and hydrocarbons is the most effective storage available today. For automotive applications, hydrogen can be stored efficiently in methanol. Methanol can be reformed into hydrogen in an external reformer, which is in essence a tubular reactor. This reformation can be obtained through steam reforming or partial oxidation Partial oxidation offers quick start up and, since it is an exothermic reaction, requires heat dissipation. Steam reforming has a higher rate of conversion, but is a slower process and, since it is an endothermic reaction, requires heat being supplied to the system. A combination of both reactions is obtained in the auto-thermal reactor, in which the reformation reaction gets its heat from the partial oxidation reaction. The design of the reformer is important for the performance and efficiency of the total system. It should be able to work at low and high loads, and at high sudden outputs, e.g., when the car is accelerated. The weight and space taken up by the reactors should be minimized, and the heat management system optimized for different operating conditions. 5 C H E M I C A L E N G I N E E R I N G I N T H E A U T O M O T I V E I N D U S T R Y Figure 2 shows the temperature distribution in a tubular reactor for reformation of methanol to hydrogen through the steam reforming reaction. We can see from this figure that a jacket heats the reformer while the reactions in the reactor core consume heat. The curved surfaces in the core represent isothermal surfaces. Different color scales are used for the heating jacket and for the core, since temperature differences are significantly larger in the core. The heat is exchanged from the heating channels in the jacket, through the highly conductive jacket material, and into the core. We can calculate the temperature profile by defining a heat balance in the reactor, assuming that heat transfer takes place by conduction and convection. Figure 2. Simulated temperature profile in the heating jacket and in the core of a steam reforming reactor. The hydrogen-rich reformate is supplied to the fuel cell where hydrogen is consumed. However, to avoid build up of by-products from the processor, and to optimize the operation of the cell, a surplus of fuel is usually fed to the cell. The exhaust from the cell is therefore supplied to a catalytic burner and the heat produced in the combustion process is subsequently used in the fuel processor. The advantage of using a catalytic burner is that the combustion process takes place at a low temperature thus minimizing the production of NOx. The catalytic burner might consist of a packed bed of sintered palladium catalysts. Figure 3 shows the simulated reaction distribution in a catalytic burner. We can obtain the flow distribution by combining the mass balance with Darcy’s law for flow through porous media. In this case, we assume that one of the burner walls accidentally became too thin in the manufacturing process, which results in a non-uniform flow distribution through the porous catalyst and a non-uniform combustion. In figure 3, the red color signifies areas of higher combustion rate due to a larger convective flow of fuel. This might eventually lead to a non-uniform temperature distribution, and eventually ignition of the gas stream. 6 ... - tailieumienphi.vn