Fossil Fuels Will Be Around for a Long, Long Time
August / September 2004
Volume 2 Number 4
What if we could nearly double the amount of electric energy we could produce from a ton of coal—€”and do it in a way that largely avoids the air pollution and climate change problems associated with fossil fuel combustion?
Direct carbon conversion, an advanced energy technology now being tested at Lawrence Livermore National Laboratory, holds promise of doing just that. If proven successful and adopted on a large scale, the process could conserve precious fossil resources by allowing more power to be harnessed from the same amount of fuel, while cutting the amount of carbon dioxide produced per kilowatt almost in half.
Ninety percent of the earth's electric energy comes from the burning of fossil fuels. Coal constitutes half of our fossil-fuel resources, and 80 percent of the coal belongs to the United States and Canada, the former Soviet Union, and China. Coal-fired plants produce 55 percent of U.S. electricity —€”as well as large amounts of pollutants. As a result, the vast energy reserves of coal remain underused. Thus direct carbon conversion has the potential to be the long-sought "clean coal" technology.
The conversion efficiency of today's coal-fired power plants is between 35 and 40 percent. Direct carbon conversion, an electrochemical process that converts carbon particles obtained from different fossil fuels directly into electricity without the need for such traditional equipment as steam-reforming reactors, boilers and turbines, would be roughly twice as efficient as today's technologies.
The LLNL method, the result of a four-year study funded by the Laboratory Directed Research and Development Program, would push the efficiency of using fossil fuels for generating electricity much closer to theoretical limits than ever before. It would also improve the environment by substantially decreasing the amount of pollutants emitted into the atmosphere per kilowatt-hour of electrical energy generated. Perhaps most important, it would reduce carbon dioxide emissions, which are largely responsible for global warming, by producing a pure carbon dioxide byproduct that could be sequestered or used in industry at no additional cost of separation or concentration. [1,2]
Direct carbon conversion requires a unique kind of fuel cell. However, instead of using gaseous fuels, as is typically done, the new technology uses particles of elemental carbon (or thermally decomposed fossil fuel molecules called "chars") that are highly disordered on an atomic scale. These particles are wetted by a mixture of molten lithium-sodium and potassium carbonate at a temperature of 750 to 800°C. The overall cell reaction is carbon and oxygen (from ambient air) forming carbon dioxide and electricity.
The reaction yields 80 percent of the carbon-oxygen combustion energy as electricity. It provides about 1 kilowatt of power per square meter of cell surface area—€”a rate sufficiently high for practical applications. Yet no burning of the carbon takes place.
Direct carbon conversion is a simpler, more efficient, and more environmentally friendly process that obtains the greatest possible fraction of energy from the starting fossil fuel with little waste heat.
The total conversion efficiency of the direct carbon conversion cell, based on the "higher heating value" of the carbon, exceeds the 70 percent requirement of the next-generation fuel cell envisioned by the Department of Energy. (Higher heating value, or HHV, is the total amount of heat released when a fuel is burned completely and the products are returned to their natural, room-temperature states.) The best combined-cycle pilot plants that burn natural gas in multistage turbines now operate at 57 percent efficiency, based on the fuel's HHV. High-temperature fuel cell hybrid systems are expected to operate on natural gas at 60 percent HHV. But natural gas is in short supply and its price is unstable and unpredictable, while our major fossil resource in the United States is coal.
Direct carbon conversion can use fuel derived from many different sources, including coal, lignite, petroleum, natural gas, and even biomass (peat, rice hulls, corn husks).
The rate at which the direct carbon conversion cell produces power at high efficiency depends on atomic disorder and electrical conductivity, but not on purity. Still, the ash normally present in raw coal must be minimized to prevent fouling of the electrolyte, and the particles of coal must be treated to prevent tarring and agglomeration as they enter the hot cell. We have experimented with samples of a chemically de-ashed, non-agglomerating bituminous coal now being evaluated as a particulate fuel for turbines. After charring, we found it to yield energy at 80 percent efficiency based on higher heating value of the char, at about 1 kilowatt per square meter. The cost ($3 per gigajoule) is lower than that of natural gas. [3,4] In our tests, we first rapidly pyrolyze (thermally decompose) the raw cleaned coal, driving off a high-BTU gas in just a few minutes. The chars remain in the fuel cell and are more slowly converted to electric power. When the gases are used to produce electric power by the most efficient methods (fuel cells), the total conversion efficiency of the coal input should be about 75 percent. More important, the offgas is a source of valuable hydrogen gas, which has many uses in the "hydrogen economy" beyond its possible use in utility fuel cells.
The cost of an advanced fuel cell design that first pyrolyzes the fuel particles and then converts the chars to electric power (as a single unit process) is being carefully determined by industrial suppliers of the components. At this stage, the total cost of the fuel cell is estimated to be about $500 per kilowatt (at 1 kilowatt per square meter). While the efficiency is very high, the most attractive attributes of this dual function cell are low cost and simplicity.
The carbon—€“air fuel cell gives off a pure stream of carbon dioxide that can be captured without incurring additional costs of collection and separation from smokestack exhausts. The stream of carbon dioxide, already only a fraction of current processes, can be sequestered in geological formations or the ocean or used for oil and gas recovery through existing pipelines.
The LLNL approach circumvents the historic barriers to a coal fuel cell by using fine, virtually ash-free, "turbostratic" carbon particles that have a high degree of structural disorder. The Livermore team has found that such "turbostratic" carbon particles, when mixed with molten carbonate to form a semi-dry paste, operate like rigid electrodes when the mixture is brought into contact with an inert metallic screen such as nickel.
The technology has been demonstrated in a number of small, experimental cells with reaction areas of about 3 to 60 square centimeters. In repeated tests, the cells deliver up to 0.1 watt continuously per square centimeter and are 80 percent efficient at 100 milliwatts per square centimeter. Certain other more exotic (but more expensive) forms of carbon yield such efficiencies at nearly three times the rate. The team has operated cells for days, simply by adding more carbon fuel.
The cell's fundamental thermodynamic properties mean almost no waste heat and full fuel consumption. In addition, cell components and fuel are nontoxic and relatively hazard-free. In particular, because the slurry does not explode if inadvertently brought into contact with air, no explosion-prevention safeguards need to be engineered into the cells.
The most direct potential applications of this power generating technology are industrial generators for users of DC power (such as aluminum and chlor-alkali plants) or distributed utilities on 5-50 megawatt scales. Here the carbon fuel cell has advantages of scale-down potential and avoidance of transmission, distribution and (in DC applications) power inverter costs. [5] For each kilowatt of power about 20 liters of cell volume is required (1.5 kilowatt per cubic foot).
John F. Cooper, Ph.D., is a senior scientist, energy systems, Materials Science and Technology Division, Lawrence Livermore National Laboratory
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