NETL Research Contributes to Efficient Operation of Fuel Cells

	This is the first version of the multi-cell array installed on the test skid at the Power Systems Development Facility at Wilsonville, AL.

NETL researchers recently returned from Alabama where they’ve been conducting experiments at the Power Systems Development Facility at Wilsonville, AL to learn the performance characteristics of solid oxide fuel cells when operated on synthesis gas, or syngas, coming directly from a gasification process.

The primary technology goal is to achieve highly efficient operation of a fuel cell with acceptably low degradation of the cell’s components when operating on syngas derived directly from coal gasification.  That means that researchers must identify all of the compounds exiting the gasifier that may react negatively with the fuel cell. 

It’s not an easy thing to accomplish. It requires deployment of a mobile test skid capable of interfacing a fuel cell with a gasifier host facility, and development of an on-line analytical device capable of detecting trace elements in the syngas in real time.

A fuel cell is an electrochemical device. The fuel – syngas in these tests – goes in on the anode side and an oxidant enters on the cathode site, and they react electrochemically to produce electricity.  Damage caused by interactions of trace materials with the fuel cell anodes leads to short operating times, and increases operating costs. The less degradation there is to the fuel cell, the longer and more efficiently the cell can run at a lower total cost.

Syngas is a mixture of gas that can contain carbon monoxide, hydrogen, water, carbon dioxide, and methane.  The syngas is generated from any carbon-containing fuel in a high-temperature (1800 – 2600 degrees Fahrenheit) process known as gasification.  The exact composition of syngas (including any trace elements) coming directly from the gasifier varies depending on  the type of carbon based solid material being fed, the gasifier operating conditions (temperature, pressure, etc.), and the process for cleaning the raw syngas as it exits the gasifier.

The carbonaceous feed materials could be coal or biomass, or a mixture of coal and biomass being gasified together. The syngas produced by the gasification system will contain all the contaminants that are in the feedstock including trace mercury, arsenic, cadmium and phosphorus. There could also be a bit of zinc, selenium, lead or antimony in addition to others.

The relative trace material proportions can vary significantly from one feedstock to another, or even for the same coal from hour to hour depending on changes in the gasifier operating conditions.

	Rich Pineault of NETL installs the multi-cell array at the test site in Alabama, with the gasifier visible in the background.

NETL staff and contractors designed and built a unique multi-cell array and test skid, which is transported by tractor-trailer to the gasifier site.  The NETL research is determining the effect of the trace materials on the fuel cell’s anode, looking at physical and chemical degradation.  The array can hold as many as 12 “button” cells.  A button cell is a small version of the solid oxide fuel cell, which is identical to a full size cell in composition and operation.

NETL runs tests on these smaller button cells, according to researcher Kirk Gerdes, because “smaller cells are much less expensive to make than larger cells. Also, they are a size that is standard in lab work for the type of basic studies we are doing here.”

Testing several cells at the same time is useful, Gerdes says, because, “We have a short window of opportunity at the PSDF to interface with the gasifier. Effectively the multi-cell arrangement gives us 12 times the number of test hours for the same duration of gasifier operating time.”

During the most recent tests, NETL researchers operated six fuel cells and associated testing devices continuously for 200 hours, the amount of time available for tests. “We obtained 1200 cell hours previously, and we plan to run longer when we return in fiscal year ’09,” Gerdes said.

Each of the 12 cells that can be tested in the array can be different.

“We can change the composition of the cell if we wish,” Gerdes said. “We can change many mechanical features of each individual cell, and can change the loading of each cell – for example, operate each at different current density.”

 They operated only six cells this particular test because they’re still in the process of shaking down the equipment, and truncated operation was more cost- and time-effective, according to Gerdes.

	NETL researcher Kirk Gerdes adjusts feed valves to the solid oxide fuel cell multi-cell array. The individual tubes each contain a single fuel cell and the assembly allows for parallel testing of up to 12 cells simultaneously.

The researchers didn’t attempt to control the syngas composition in the field test. Composition was determined by operation of the gasifier by PSDF personnel. The Wilsonville gasifier facility – a 10,000 pound-per-hour coal gasifier – has several features that are not standard for gasifiers; for instance, the operators can provide cleaned syngas without cooling the process gas below 250 degrees Celsius, which is useful for NETL research because warm gas cleanup is important for future technologies.

What sorts of things are the NETL researchers trying to learn?

“We have to know what contaminant load will degrade the cells at what rate,” Gerdes says. “Arsenic at concentrations of less than 1 part per million will interact with the anode and make nickel arsenide, which potentially degrades the anode performance.  However, it’s not realistic to clean up the arsenic to undetectable levels. What we’re doing is showing degradation as a function of exposure, which allows system designers and ultimately operators to optimize gas cleanup vs. degradation of the fuel cell. We can tell, for instance, that you must remove arsenic to some target concentration in order to reduce degradation rates to acceptable levels; these tests are among the very first reported that help determine the levels to which contaminants must be held when operating on direct syngas.”

The experiments at Wilsonville also provided information about NETL’s testing methods.

“The analytical systems we have deployed are doing an outstanding job,” according to Gerdes. “They are unique and among the best sensitivity available. We expect the sensitivity will be in sub parts-per-billion range for many elements.”

In fact, this was the first known test interfacing gas chromatography, inductively coupled plasma and mass spectrometry (GC-ICP/MS) for real-time testing of gasifier emissions. The GC-ICP/MS system was brought back to NETL for refining the testing methods and refurbishing the equipment, but is expected to be redeployed to Wilsonville for further testing in FY-09.