Researchers Demonstrate Liquid-Tin Anode SOFC Operating on Biodiesel
28 August 2009
| Schematic of the bread-board rig employed in the LTA-SOFC study. The top shows the LTA-SOFC and BOP (balance of plant). The bottom shows the JP-8 heater that transfers heat to the SOFC via a plate-style radiating heat exchanger. Credit: ACS. Click to enlarge. |
Researchers from CellTech Power and the University of Connecticut have demonstrated a liquid-tin anode solid-oxide fuel cell (LTA-SOFC) operating on pure biodiesel (B100) prepared via base-catalyzed transesterification of virgin and waste cooking oils. A paper on their work was published online 28 August in the ACS journal Energy & Fuels.
The LTA-SOFC was able to convert the biodiesel to electricity at commercially viable power densities, i.e., greater than 100 mW cm-2. The peak power for each cell was 3.5 W over an active area of 30 cm-2, which translates to a power density of 117 mW cm-2 and current density of 217 mA cm-2. The peak power densities correspond to 80% fuel use at the liquid-tin anode surface and overall cell efficiencies of >40%.
The research team separately has described the operation of the Gen 3.1 LTA-SOFC system used in the biodiesel study with JP-8 with a sulfur content of 1,400 ppm (McPhee et al. 2009b). Operation at a cell temperature of 1000 °C demonstrated power densities up to 120 mW cm-2 and cell efficiencies of up to 41% corresponding to a total output power of 2.3 W over an active area of 30 cm-2 for the single-cell design investigated.
These findings demonstrate the flexibility in operating a solid-oxide fuel cell capable of internal reforming from a blend of petroleum- and biomass-derived diesels for greater resource flexibility. Cells were operated for short times (4.5 h), owing to the experimental nature of the balance of plant. Results support future efforts in developing an efficient balance-of-plant system for demonstrating long-term (>1000 h) power generation from biodiesel using the LTA-SOFC design.
—McPhee et al. (2009)
The LTA-SOFC concept under development is for portable power applications in the 0.5 to 1 kW range. This would satisfy US Army interests in squad-portable generators (<20 kg and <40L) operating from JP-8 or biodiesel blends.
LTA-SOFC. The LTA-SOFC developed by CellTech builds upon conventional solid oxide fuel cell materials and components.
- Oxygen ions are extracted at the cathode and pass through the electrolyte to the anode (fuel) side where they combine with hydrogen or carbon monoxide from the fuel to form water or carbon dioxide.
- Electrons are released at the anode and travel through the load producing useful electrical energy on their way to the cathode.
- The cathode in the LTA-SOFC cell can be any one of a number of conventional cathode materials such as lanthanum manganite doped strontium (LSM).
- The LTA-SOFC electrolyte is also conventional ytria stabilized zirconia (YSZ).
| Basic concept of the LTA-SOFC. Source: CellTech. Click to enlarge. |
However, the anode is the point of differentiation: the LTA-SOFC anode is tin—which is molten at operating temperatures—held in place by a ceramic matrix. Electricity is produced by converting tin to tin oxide at the anode.
In a separate reaction, the tin is “recharged” by the fuel (carbon, plastic, JP-8, biodiesel). CellTech integrated the reforming reaction (the reduction of tin oxide to tin) into the fuel cell, but uncoupled that reaction from the electrical production reaction (the oxidation of tin to tin oxide), allowing each to proceed at a pace dictated by fuel quality/flow and electrical demand.
The first Liquid Tin Anode Solid Oxide Fuel Cell (LTA-SOFC) was tested in 1998. Since 1998 the LTA-SOFC has been operated on a range of carbonaceous fuels in the form of gases, liquids and solids. Over this time, several design revisions at the cell, stack and system levels resulted in increased volumetric and gravimetric power densities.
The Gen 3.0 cell and stack (2005-2006) achieved a power density of 40 mW cm-2 by direct JP-8 conversion to electricity. The Gen 3.1 cells/stack, used in this study, have a 4 times reduction in weight and volume over Gen 3.0.
The Gen 3.1 tubular cell is built from the cathode current collector (CCC) outward. The CCC is cylindrical in shape and runs the length of the cell. The material is a proprietary ceramic composite, with similar mechanical and electrical properties to typical current collectors employed in high-temperature SOFCs.
The cathode consists of a thin-walled (1 mm) closed-end tube of lanthanum strontium manganite (LSM) with an internal rib structure to provide electrical connection to the CCC. The electrolyte, 8% yttria-stabilized zirconia (8YSZ), is also a thinwall (200 µm nominal), closed-end tube that is bonded to the outside of the cathode.
This electrolyte/cathode assembly is placed inside a highly porous (65% nominal porosity) proprietary ceramic tube, with a wall thickness of <3mm. This ceramic tube has no catalytic or electrical function and does not chemically interact with the liquid tin.
This outer ceramic tube contains the liquid-tin anode by balancing tin surface tension against head pressure to form a 500 µm layer of tin around the electrolyte. The active area of the electrolyte is defined by the diameter of the electrolyte tube, 1.0 cm, and the length that is in contact with the cathode, 9.9 cm, resulting in an active area of 30.5 cm-2.
Resources
William A. G. McPhee, Matthew Boucher, James Stuart, Richard S. Parnas, Mark Koslowske, Thomas Tao, and Benjamin A. Wilhite (2009a) Demonstration of a Liquid-Tin Anode Solid-Oxide Fuel Cell (LTA-SOFC) Operating from Biodiesel Fuel. Energy Fuels. doi: 10.1021/ef9003413
McPhee, W. A. G.; Bateman, L.; Koslowske, M.; Slaney, M.; Uzep, Z.; Bentley, J.; Tao, T. (2009b) Direct JP-8 conversion using a liquid tin anode solid oxide fuel cell (LTA-SOFC) for military applications. J. Fuel Cell Sci. Technol. in press
Tao, T. T.; McPhee, W. A.; Koslowske, M. T.; Bateman, L. S.; Slaney, M. J.; Bentley, J. (2008) Advancement in liquid tin anode-solid oxide fuel cell technology. ECS Trans.12, 681 doi: 10.1149/1.2921593
Liquid Tin Anode Fuel Cell Direct Coal Power Generation (10th Annual SECA Workshop)
Sounds quite a promising technic to me, for soem reason I am still skeptical thta the cellulosic ethanol will really fly essentialy for being no really a ruuged process. This kind of approach looks mor robust and less selective with feedstock and also more easy to implement at small scale in the field or at the farm. The later being key for the scaling of a biofuel process that need to avoid the pitfall of carrying the biomass over longdistance
Posted by: Treehugger | 29 August 2009 at 04:02 AM
What are the emissions like from this process? At 1000 °C there could be NOx and black carbon formed, as occurs in a diesel engine.
If it is clean, how would it compete with the engines in locomotives, which are already used to generate electricity to turn the wheels?
Posted by: eak | 29 August 2009 at 11:14 AM
No NOx. These systems usually run at low pressure and only oxygen can go through the electrolyte. Sorry to say, but this doesn't look like it has much promise. Power densities are very low and this will always keep the system cost high. Running with 1000pp of H2S is nice, but H2S can be remove on the system level more cost effectively. Also, the 40% efficiency they quote has to be electrical efficiency. The system efficiency would be quite a bit lower. Don't get me wrong it's neat, but for the problems it tries to solve it appears to have it's own equally tough problems. SOFC's hold enormous potential but I don't see this fixing any of the problems currently holding the technology back.
Posted by: KDM | 29 August 2009 at 09:47 PM
Make a diesel engine with slow speed and a big piston and you can get better efficiency. INNAS NOAX can design such an engine to be used in a hydraulic hybrid. They would have to call the engine a hydraulic dynamic energy converter to have even a chance of selling such a car. Efficiency is not wanted by consumers; especially from a single piston automobile. No matter the performance. ..HG..
Posted by: Henry Gibson | 30 August 2009 at 02:22 PM
No potential? This has enormous potential in small-scale cogeneration applications and "housekeeping" APUs for heavy vehicles. If the LTA can operate with methane, any gas furnace or water heater can be converted to a cogenerator. The high operational temperature also has possibilities for industrial process heat.
Biodiesel is one thing, but if the LTA FC can run directly on pyrolysis oil that creates an entire fuel supply independent of fossil sources and a high-efficiency system at the end use. The potential for this needs to be checked out carefully, but it's huge.
Posted by: Engineer-Poet | 01 September 2009 at 04:11 PM
A conventional Ni-Zirconia anode can run on the same fuels. You just have to make the system a little more complex by adding a reformer and sulfur cleanup. The tubular design is a highly inefficient design. Even after 30 plus years and boat loads of money Westinghouse was never able to make that type of SOFC commercially viable. The only thing hold back SOFC's is cost. Increased lifetime, increased power density and lower fabrication cost are needed. This tech doesn't improve any of them. A reformer and sulfur clean up are way less expensive than a stack.
Posted by: KDM | 01 September 2009 at 07:57 PM
Tin melts at ~230°C, so the LTA appears to be adaptable to planar cell designs and metal-backed electrodes.
Eliminating external reformers and filters is a huge advance. Since the cell is not harmed by sulfur or carbon, it promises to radically simplify the system and make it cheaper and more reliable.
Posted by: Engineer-Poet | 02 September 2009 at 04:33 PM