WHITE PAPER ON CHEMICAL ENGINEERING
Fuel Cells in the
Automotive Industry
©

COPYRIGHT 1994 - 2001
by
COMSOL AB
. All rights reserved.
by
Ed Fontes
EvaNilsson
CHEMICAL ENGINEERING IN THE AUTOMOTIVE INDUSTRY
3
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

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.
CHEMICAL ENGINEERING IN THE AUTOMOTIVE INDUSTRY
5
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

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.