Fuel Cell/ Direct Carbon 051209
Direct Carbon Fuel Cells,
Alternative to a Hydrogen Economy?
by Donald Georgi
A fuel cell - is not a fuel cell - is not a fuel cell. Electrochemists know this but the public - including most college graduates, business gurus and those who should care about cleaner air and energy independence are not sufficiently informed about fuel cell details. Fuel cells come in many sizes, have many different applications and have different electrochemistries. However, not all fuel cells will end up defeating the evils of combustion engine pollution and the giveaway of our country’s wealth with giant foreign oil trade imbalance. To date, some will understand that the hydrogen/PEM (Proton Exchange Membrane) fuel cell has garnered the lion’s share of government grant money to pursue automobile applications. Some may also understand that the DMFC (Direct Methanol Fuel Cell) may be the darling of the future electronic portable power market, and some know that SOFC (Solid Oxide), PAFC (Phosphoric Acid) and MCFC (Molten Carbonate) fuel cells may someday be commercialized for portable equipment power, auxiliary power units, distributed generation and large power generation applications.
The problem with this understood concept is that the term ‘Fuel Cell’ is inexorably linked to the word, hydrogen, so that when President Bush or “Ahnold’ used the term ‘Hydrogen Highway,’ the implicit connection is only between hydrogen as a fuel and the fuel cell as the energy converter to electricity. Such singular vision often leads to pursuit of goals which may leave out good solutions as in the case with radio, which first used semiconductor diodes and then diverted to vacuum tubes for 30 years before revisiting semiconductor technology, eventually making most vacuum tubes inefficiency and unreliability obsolete.
Hindsight is 20/20 but intelligence allows us to remember the words of the philosopher and poet George Santayana, who said, "Those who cannot remember the past are condemned to repeat it." Heeding the warning, we can expand our view of clean, home-grown energy methods beyond just hydrogen, to consider also Direct Carbon Fuel Cells (DCFCs) which, while not strongly funded by the government, may offer many features to alleviate the limitations of hydrogen-powered fuel cells.
First, one must understand the difference between a combustion reaction and the direct electrochemical production of electricity. Today’s world generates a large amount of electricity from combustion which is a high temperature chemical reaction of a fuel and oxidant, such as an internal combustion engine, space shuttle rocket or power plant coal burner. (Adiabatic combustion temperatures for coals are around 1500 0C , around 2000 0C for oil and 2200 0C for natural gas). Combustion produces heat which is converted into mechanical energy which is then used to expand gasses and thereafter turn a rotating machine (piston engine or turbine) which generates electricity. In the process of combustion, heat and pollutants such as carbon particulates, CO and CO2, are released to the atmosphere. Combustion-air converts nitrogen into undesirable oxides. Where the fuel source includes chemicals such as sulfur, the emissions also include harmful sulfur oxides.
The rosy alternative is to use a relatively low temperature method to create electricity. Here is where the fuel cell bursts on center stage with a glorious fanfare. Fuel cells employ an electrochemical process to generate the electricity at relatively low temperatures. Nitrous oxides are not created from a cathode bathed in room air, despite over 70% nitrogen content. If we limit the fuel to pure hydrogen, there is no sulfur to end up as airborne SOX, and CO2 and CO pollutants are eliminated from the fuel cell operation.
Not only does the fuel cell process energy in a cleaner way, but it also uses more of the stored energy, which means that a gallon or pound of any fuel produces more electricity. A part of this greater efficiency is another advantage of a fuel cell which produces electricity directly (no gas expansion, pistons or turbines). Eliminating the steam boilers and turbines results in more electricity per pound of fuel; also, the plant size is smaller and fewer parts means less chance for breakdown.
Is there any reason to ask why the political associations are being made to the hydrogen economy which implies replacing combustion engines with fuel cells? They are the ‘Daddy’s little darlings’, the ‘teacher’s pets’, the ‘precocious children’ of our hopes for better energy systems.
One slight hiccup in this picture is that clean, low cost, home grown, high density hydrogen is not here. It costs more than gasoline to produce and has significantly lower energy density. In the immediate future, fuel cells will use hydrocarbon fuels requiring reformers to obtain hydrogen. The reformers reduce efficiency, reduce reliability and add greatly to system cost and volume, while ejecting the carbon part of the hydrocarbon fuel into the air mostly as carbon dioxide. Even the ‘pure’ hydrogen running in California’s ‘hydrogen highway’ is mostly reformed with natural gas, purchased with painful balance of payment funds to other countries, while increasing the CO2 in the air.
Still, these hydrogen fuel cells are considered the major answer to our energy problems, with renewable fuels, solar and nuclear energy gleaning only honorable (or dishonorable) mention.
Attendees at the November 2005 Fuel Cell Seminar were again told that commercialization of hydrogen-based automotive fuel cells would not be significant until 2015, and other fuel cell types were still in need of great improvements in reliability and cost reductions. At that same seminar, an almost forgotten fuel cell technology was relegated to a ‘day-before’ workshop on Direct Carbon Conversion which most attendees did not bother to notice. But if we are to benefit from Santayana’s warning, we should hear what was presented here regarding DCFCs.
DCFC - Not a hydrogen-powered fuel cell
The basic element of DCFC technology demonstrated that like other fuel cells, carbon-based electrochemical fuel-air reactions occur at temperatures lower than combustion fuels. DCFCs do not use hydrogen as the fuel. Instead they use carbon found in basic graphite, coal, coke, biomass and other carbon sources. Using carbon is contrary to the popular conceptions of fuel cells because hydrogen-fueled cells using hydrocarbons see the carbon molecule as a pollutant rather than an energy source. Leftover carbon is ejected as CO2 and CO to the atmosphere. Conversely, the DCFC does not ‘burn’ hydrogen, instead using the otherwise-polluting-culprit carbon as the fuel. Suddenly, visions of eighteenth century smoke billowing from steam engines burst into our imagination with giant question marks.
Is this some sleight of hand technology such as the controllable fusion challenge or worse, a cold fusion dream? Actually, it is much less glamourous; this DCFC technology has a long history with significant recent real world proof of performance. We always consider the historical beginnings of fuel cells with Sir William Grove’s “gas Voltaic battery” in 1839 in which he converted hydrogen and oxygen into electricity. (See BD 46-16.) The fuel cell rested as a scientific curiosity until 1889 when Mond and Langer repeated the work and added a new electrolyte boosting the efficiency to 50%. In 1894, Oswald identified the electrochemical nature of the reaction but electrode kinetics were not well understood, and it continued to be a scientific curiosity.
Intertwined with this hydrogen fuel cell development is a historical milestone in 1896. Dr. William Jacques patented a “method of converting potential energy of carbon into electricity.” This could be considered the first DCFC, but much speculation over the actual performance and an inability of others to reproduce the claim put DCFC technology to rest for another 77 years. Before then, scientists ‘proved’ that the cell produced electricity not by electrochemical reaction, but by the thermoelectric (Seebeck) effect. This judgement will be revisited later.
Meanwhile, hydrogen fuel cells were progressing slowly as Dr. Harry Karl Ihrig built a working fuel cell-powered farm tractor in 1959. In the 1960s, Alkaline fuel cells powered the Apollo Program to the Moon and by 1970, Dr. Karl Kordesh (see story photo) was driving his Alkaline fuel cell-powered Austin A 40 on U.S. roads for four years.
In 1973, the Stanford Research Institute was successful in duplicating Dr. Jacques direct carbon fuel cell performance. Recall that outsiders had debunked Jacques patent for a carbon fuel cell and yet SRI was able to reproduce the work.
In the mid 90s, Scientific Applications & Research Associates, Inc. (SARA) began research on DCFCs, and since that time the company has allied itself with American Electric Power so the technology could be used to produce large (>100 MW) power plants.
With all the years of effort and funds directed to hydrogen fuel cells, alternates must have impressive credentials to play in the arena of future electrical power generation. It is the recent analytical understanding and experimental performance of DCFCs which give them the credentials to join the game.
One of the eyebrow-raising features is the high efficiency. Hydrogen fuel cells have nominal efficiency of 35 to 60%. From a theoretical standpoint, the DCFC has a maximum possible efficiency of 85-90%. This is for the electrical generation alone and does not count on cogeneration to boost that number. Greater efficiency has a positive effect on cost, all the way back to the source of fuel. Efficiency can also contribute to reduced plant size which again may lower costs and finally, it can effect component utilization, which again contributes to lower costs.
The next eyebrow raiser states that the fuel can be coal. Of all resources, other than sunlight resources in the U.S., coal has the greatest reserve, estimated presently at 250+ years. When we mine and transport a ton of coal, the cost is paid to people in the U.S. who churn it into goods and services within the country, putting additional people to work. Mix the local availability advantage with the efficiency and suddenly, the U.S. coal reserves change from a 250 year supply to a 500 year supply.
Still other features point to a future for DCFCs. As we hear the frustrating explanations of hydrogen fuel cell proponents regarding the high cost of platinum catalysts, its elimination in DCFCs, and replacement with simpler more commonly available materials such as titanium doped steel, offers the possibility of producing a fuel cell with reasonable plant cost per Watt-hour generated. Other elements of the DCFC similarly contribute to low cost such as cell structure of common welded steel and intrinsic operation of the electrolyte salt which protects the cathode from degradation by coal entrained solids. Similarly, its robust operational nature simplifies maintenance costs.
Utilization of the fuel itself may also be an advantage. While some preprocessing is required such as grinding of coal or other carbon fuel preparations, the simple insertion of the carbon-based fuel into the inlet hopper without reformer or complicated ‘balance of plant’ again provides operational economy. The function of the unit allows for very large plants to supply multi-100 MW power, so the future as a replacement for present grid power stations is possible.
Broader capabilities of the DCFC may include the generation of hydrogen and coal gas from fuel oils and coals as a by-product of fuel preparation. Such combinations might synergistically allow the DCFC to produce electricity while the other by-products power the PEM fuel cell vehicle.
As a side advantage, the possibility of use of the DCFC technology with biomass or organic waste offers heretofore unavailable possibilities for remote locations. In studies, remote areas using diesel-powered generators are proposed as sites for hydrogen fuel cells with higher efficiency merely because of the increased utilization of the hauled-in fuel. In reality, shipping fuel by any means to remote areas is very uneconomical. If on the other hand, renewable biomass in the remote area can be used to power a DCFC, costs may be comparable to those in present metropolitan areas. To think of third world countries, without hydrocarbon reserves, able to provide power from local energy supplies improves the future economy of many distressed nations. Despite the projection of large power plants, suppose the technology was scalable to large farms and specialized manufacturing plants where on-site by-products could power the DCFC providing electricity and other chemicals.
Operationally, coal, coke or carbon in the inlet melts into a slurry such that the process is not explosive in air. In the ideal system, exhaust CO2 and sulfur oxides are captured and sequestered leaving the process as environmentally constructive.
Certainly there have to be some problems with DCFCs or they would be under construction today. Obviously, the scale-up from laboratory tests to prototypes and then small scale systems yet needs to be done. The fuels such as coal need to be identified as optimal for transport and utilization within the cell.
Once in the cell, the form as it relates to interaction with electrodes and electrolytes must be optimized. Spallation limits of corrosion of metals used for current collection and construction need to be minimized.
Exhaust sequestration based on the fuel needs to be developed. Since the bulk by-product of the reactions fly ash, the entire utilization of that material needs to be developed.
There are many forms of the DCFC being pursued and patents granted. Presentations from the 2005 DCFC Workshop indicated a number of active participants including:
American Electric Power
Electric Power Research Institute
Lawrence Livermore National Laboratory with
Contained Energy, Inc,
University of Akron
Coal Clean Energy, LLC
A distinct attitude of competitively achievable outcomes prevails with this group. CellTech is a start-up with SBIR funding. SARA has DOE funding but projects development of an independent company within 5 years to pursue pilot plants. SARA has an association with American Electric Power (AEP), which is the nation’s largest electric power generator, and has much to be gained if DCFCs can increase fuel efficiency at lower costs while attacking the polluting aspects of coal use. Despite its nonprofit, R & D status, SRI International has an active role in developing the technology and projects a timetable as follows:
2011 Pre Production prototype 100 kW
2013 Pilot 500 kW distributed generator
2020 Present coal plants need replacement
While this technology is not a next-year choice, the potential advantages of DCFC technology and the attitude of commercialization as a function of a methodical development program certainly places it in the high-probability future. If we see real progression to prototypes in the next two to three years and additional work as pre-prods take their place, DCFC technology may become a power in the next generation of power generation.