(514a) Water-Gas Shift Reactor for Fuel Cell Systems: Stable Operation for 5000 Hours

Water-gas shift reactor for fuel cell
systems: Stable operation for 5000 hours

Joachim
Pasel*¹, Remzi Can Samsun¹, Andreas Tschauder¹,
Ralf Peters¹, Detlef Stolten^1,2

¹Forschungszentrum
Jülich GmbH, Institute of Energy and Climate Research, IEK-3: Electrochemical
Process Engineering, 52425 Jülich, Germany

²Chair for
Fuel Cells, RWTH Aachen University, Germany

The scientific work in the fuel
processing and systems group at Forschungszentrum Juelich has the strategic aim
of developing high-temperature polymer electrolyte fuel cell (HT-PEFC) systems
based on autothermal reforming of diesel fuel and kerosene in the power class
of 5 kW_e to 10 kW_e. One important component of the
system besides the HT-PEFC stack and the autothermal reformer is the water-gas
shift reactor (WGS). Its function is to significantly reduce the concentration
of CO in the product gas flow leaving the autothermal reformer from
approximately 7-10 vol% to the range of 1.0-1.5 vol%. CO poisons the
anode reaction in the HT-PEFC working between 160 °C and 180 °C due
to preferential adsorption and blocking of the active sites.

In this contribution the development
of reactors for the water-gas shift reaction conducted in recent years at
Jülich is described with a focus on technical advances in actual reactor
generation denoted as ‘WGS 6’. In addition, experimental results from a
long-term experiment for 5000 hours on stream with WGS 6 are given and discussed
with respect to the temporal trends of product gas composition and reactor
temperatures within this timespan. For WGS 6, a specific layout was developed
to increase the values for the power density and specific power. The layout of
the WGS 6 is shown in Figure 1. The reformate emanating from the upstream
autothermal reformer flows into the high-temperature shift (HTS) monolith,
whose catalytically-active precious metal coating promotes the reduction of the
CO concentration in the reformate. According to the design, the temperature
level is ideally approximately 400-450 °C. After leaving the HTS monolith, the
hot reformate molecules hit the underside of a curved deflection surface (hemisphere),
onto whose topside a pressure swirl nozzle directs a fine spray of cold (20 °C)
water droplets. Via the deflecting surface, the enthalpy flow of the hot
reformate molecules is transferred to the cold water droplets hitting the
topside of the hemisphere. Thereby, one minor part of the water droplets
directly evaporates when it comes into contact with the hot surface. The
majority of the water droplets, however, form a film on the deflecting surface.
This film is then moved into the annular gap between the HTS and low-temperature
shift (LTS) stages. Steam produced during initial contact with the hot
deflecting surface then serves as carrier gas. At the end of the annular gap,
the reformate flow is again deflected by 180 ° and enters the monolith of the
LTS stage at a temperature of approximately 300 °C. Following the LTS stage,
the reformate leaves the WGS 6 with a temperature of about 330 °C and can be
fed into the anode of the HT-PEFC. The improved layout of the WGS 6 leads to a
strongly increased power density of 12.2 kW l^-1 and a specific power
of 11.2 kW kg^-1. Thereby, WGS 6 can significantly contribute to
decreasing the weight and volume of the fuel processing unit of an HT-PEFC fuel
cell system.

Figure 1          Basic layout of the
Jülich water-gas shift reactor WGS 6

Figure 2 displays the dry
concentrations of H₂, CO₂ and CO as a function of time on
stream during the long-term experiment. Under the reaction conditions defined
by the outlet gas composition of the autothermal reformer and its temperature
(340 °C), the dry concentrations of H₂, CO₂ and CO
at the thermodynamic equilibrium amount to 40.7, 19.8 and 1.0 vol%,
respectively. It is obvious that the concentrations of H₂ are fairly
stable during the entire long-term experiment and amount to approximately 40
vol%. The concentrations of CO₂ are approximately 19.7 vol% at the
beginning of the experiment and then slightly and continuously decrease to
values close to 19.0 vol%. This difference of 0.7 vol% can be identified in the
concentration trend for CO in a reversed way according to the stoichiometrics
of the water-gas shift reaction. The concentration of CO amounts to 0.8 vol% at
the start of the long-term run and increases to approximately 1.5 vol% by the
end. It is noteworthy, that the concentrations of H₂, CO₂
and CO at the beginning of the experiment are very close to their values at
thermodynamic equilibrium. During the entire long-term experiment with WGS 6,
CO concentrations are within or below the advisable range of 1.0-1.5 vol%.

Figure 2          Dry concentrations
of H₂, CO₂ and CO as a function of time on stream