NETL Targets Clean, More Efficient Power Generation

One of the missions of NETL's Office of Research and Development is to conduct fuel cell system validation tests that support the SECA Fuel Cell Program. Here researcher Randall Gemmen prepares the DOE Fuel Cell Test Facility for unit testing.

The typical power plant converts fossil fuel into electricity inefficiently, essentially throwing away over 65 percent of every unit of fuel burned. In 1999, the U.S. Department of Energy established a program called the Solid State Energy Conversion Alliance (SECA), which has the goal of reversing this wasteful practice by developing power plant systems with efficiencies of over 60 percent. This will be achieved using high performance fuel cells that can be hybridized with a heat engine (e.g., a gas turbine) in order to convert as much energy from fossil fuels into electric energy as possible.

Fuel cells are one of the cleanest and most efficient technologies for generating electricity from fossil fuel. They operate much like a battery, turning oxygen and hydrogen into electricity in the presence of an electrically conductive material called an electrolyte. Unlike a battery, however, fuel cells never lose their charge. As long as there is a constant source of fuel — usually natural gas or a synthetic version of natural gas made from coal, called syngas, for the hydrogen, and air for the oxygen — fuel cells will generate electricity.

A solid oxide fuel cell (SOFC) is flexible in its fuel requirement, capable of being operated with natural gas, coal, or bio-mass fuel. NETL’s SOFC Research Team is investigating several important ways to advance current SOFC technology so that it can achieve its promise of high efficiency, low-cost power. Specifically, research is being conducted on: 1) new cathode materials and designs that can reduce electrical losses that result when oxygen ions are formed on the cathode surface and transported into the electrode; 2) understanding anode contaminant reactions that reduce the performance of the fuel cell, and finding new anode designs that reduce the impact of fuel contaminants on the catalytically active anode materials; and 3) advanced fuel cell system development whereby heat engines (such as turbines) are safely combined (i.e., hybridized) with a fuel cell system so that efficiencies of over 60 percent can be achieved.

A well-designed hybrid of a SOFC with a heat engine could greatly decrease the amount of fuel required to satisfy our electric energy demands, which would in turn reduce our fuel costs and raise the standard of living for everyone. It would also reduce pollutant emissions by 50 percent and provide other benefits, all related to CO₂ management: the amount of CO₂ produced and emitted would be greatly reduced, which would in turn would reduce the amount of CO₂ that would have to be sequestered; and the capture of CO₂ would be greatly simplified since the nature of these systems keeps fuel from mixing with air, thereby enabling the capture of nearly pure CO₂. [For news on carbon capture research, click here.]

Improving SOFC Cathode Performance

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	SEM image of an infiltrated cathode showing newly formed nanoscale material that enhances oxygen reduction.

NETL researchers are enhancing cathode performance by applying very tiny (almost nanometer size) particles and films to the backbone of conventional cathode electrode material. To envision what needs to be done, imagine yourself as a cave explorer finding your way through a cave (which represents the inside of a cathode electrode) with hundreds of thousands, if not millions, of passageways. The scanning electron microscope image of a cathode shown here illustrates this perspective. Your job as an explorer is to throw small stones against all the walls of the cave and get them to stick to the walls; this will provide improved conditions for hosting bats (which are meant to represent oxygen molecules in this analogy)! What must be done is to find a type of stone (which represents the active oxygen catalyst) that can be applied to the cave walls and remain active for many years. The oxygen molecule, once attached to the walls, must dissociate, pick up two electrons, and thereby create two charged oxygen ions. These oxygen ions then enter the electrode material where they can migrate to the electrolyte separation membrane and then on to the anode, where they react with the fuel to create electrical energy. Research at NETL is showing how to create these nanoscale films and particles and thereby improve cell performance. Results (see chart) show that by using a lanthanum strontium cobalt catalyst material, the voltage drop can be reduced by almost 20 percent. This increases the fuel cell efficiency by nearly 10 percent, which means that less fuel is used and less CO₂ is generated!

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Chart of test data showing favorable reduction in losses of a fuel cell due to different amounts of infiltrated material.		Chart showing X-ray diffraction data of material used for cathode infiltration under different synthesis conditions.

Using NETL's Multi-cell Array (MCA) to Improve SOFC Operation

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	Photo of the multi-cell array unit designed by NETL to rapidly test SOFC cells.

NETL has developed a unique test platform, called the multi-cell array (MCA), to rapidly test multiple fuel cells and determine how they degrade when contaminants exist in the fuel stream, such as might occur when using syngas from a coal gasifier. Coal contaminants, such as arsenic, phosphorous, sulfur, etc., attack the anode electrode and either reduce the electrical conductivity of the anode, or reduce its electrochemical activity, thereby causing the fuel cell output voltage to decay gradually, which reduces the efficiency. The goal of NETL’s tests is to identify the maximum acceptable contaminant levels that will allow viable operation of a fuel cell on coal syngas. The MCA platform, and other similar cell test facilities at NETL, have revealed that low (parts-per-billion) levels of contaminant material may be needed in order to achieve lifetimes of 4-5 years for a given fuel cell stack. Shorter lifetimes will result in unacceptable replacement costs for the fuel cell system. Achieving these low level contaminant levels is possible with available technology, but will require additional research to further improve system efficiency and provide a unit at low cost.

Photo of the multi-cell array unit installed at the National Carbon Capture Center test site.

Recent laboratory tests conducted at NETL have focused on quantifying exposure thresholds for hydrocarbons (benzene and naphthalene) and process chemicals. Through carefully controlled 500-hour tests where standard SOFCs were exposed to simulated synthesis gas environments containing precise trace material concentrations, the maximum exposure limit for benzene was determined to be 150 ppm, and the maximum acceptable exposure to naphthalene was 110 ppm. Collected electrochemical impedance data indicate that at higher concentrations of benzene and naphthalene, resistances associated with lower frequency processes (such as mass transfer) increased, which implies that the fuel gas was encountering resistance as it traveled to the reaction sites within the anode. Post-operational microscopy and spectroscopy were performed on the cells exposed to benzene and naphthalene to search for physical degradation. Evidence of anode particle growth in hydrocarbon exposed samples (especially for naphthalene) has been detected, but no signs of carbon deposition were recorded.  Selexol, which is a promising material for CO₂ removal from coal plants, has not been observed to accelerate degradation at exposures of 3.5 ppm, and therefore remains a viable option for SOFC applications as well.

Fuel Cell System Development

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	Dynamic behavior of fuel cell temperature during a test run at NETL.

To advance fuel cell system development, NETL researchers are conducting ‘hardware-in-the-loop’ experiments. An accurate hardware representation of the hybrid system is constructed using a gas turbine configured with a heat exchanger/ recuperator, a representative fuel cell cathode volume, and a combustor. In our research, the individual component and complete system dynamic characteristics are closely studied to determine the proper and safe method of operating such a system as it works to deliver power to meet instantaneous load requirements. Multiple fuel cells are usually operated together, in a collective unit known as a ‘stack’. The approach taken by NETL researchers employs a simulated fuel cell stack within the above hardware environment. This removes the risk of costly cell/stack failure. As an example, one of the most severe events that can occur (if the system is not properly controlled) is called a surge and stall of the gas turbine compressor. When this event occurs, the entire hardware system can shake violently as the gases in the system suddenly exit out from the compressor (as in an “engine back-fire”). With the hardware-in-the-loop approach described here, information on important fuel cell operating parameters, such as temperature distributions and current, as indicated by the colors in the figure) is readily available during critical events, such as transient start-up. Without this unique hybrid test facility, costly tests using large-scale stacks would be required to increase performance efficiency, and given the high costs of failure, progress would be slow due to the need to take extreme precautions against events that could cause stack failure.

NETL researchers have achieved simultaneous startup of the fuel cell and turbine without compressor surge and without excessive temperature gradients in the fuel cell. A nine-step process for system startup was identified that included normal gas turbine startup accelerations to avoid stall, proper sequencing of a hot and cold bypass valve to avoid high-temperature gradients in the fuel cell stack, and electric load management of the gas turbine and the stack. The results showed that the proposed control strategy could maintain desired temperature dynamics within the fuel cell, thereby providing confidence that this new hybrid system can be safely operated without damage to either the fuel cell or the gas turbine.

Recently, the facility capability was expanded to include the dynamic effects of a coal gasifier. Results showed, for example, that variable speed gas turbine operations cause minimal variations in the fuel cell inlet temperature as a consequence of the almost-constant recuperator outlet temperature. In contrast, constant speed causes thermal stress at the inlet, while mitigating temperature oscillations at the outlet. This unexpected result can be useful for discerning which control system to adopt for the gas turbine side, depending on the most important thermal stress to be minimized within the fuel cell stack.