by Hank Keiper, P.E.
The cover story of the November issue of this publication was “Technologies Address Coal’s Negative Impact.” The excellent article described in detail technologies to dewater the slurry generated by coal mining and processing as well as for carbon dioxide capture and sequestration. To keep coal a viable energy solution for many more years, a cradle to grave approach is absolutely essential.
Fly ash is the most plentiful of the residues from coal combustion at just under 50% of the total, and it is one of the most reusable.
At the “grave” end, carbon dioxide-laden flue gas is but one of many waste streams from the combustion of coal to produce electricity. Before the Clean Air Act of 1970, dusty flue gasses simply went up the stack and “flew” into the atmosphere and surrounding communities. This is the origin of the name “fly ash”. Since the Clean Air Act and other state regulations, power plants capture the fly ash from the flue gas through various technologies making fly ash the largest of the coal combustion byproducts. Others include bottom ash, boiler slag, iron pyrites, and flue gas desulfurization byproducts like synthetic gypsum.
The American Coal Ash Association reports that 134,699,739 tons of coal combustion byproducts were made in the U.S. in 2009. Much of that was disposed in landfills or surface impoundments (ash ponds). The good news is 55,614,563 tons or 41% was used beneficially (recycled) and diverted from disposal. That creates a great market opportunity for the other 59%! There’s very little argument that the residues from coal combustion may contain trace amounts of potentially harmful elements like arsenic and mercury. When millions of tons are lumped together with poor engineering controls, “trace amounts” can accumulate over time and possibly leach into the groundwater. Managing and beneficially reusing coal combustion byproducts is the second part (carbon dioxide is the first) to reducing the negative effects at the “grave” end of cradle to grave management.
As mentioned earlier, fly ash is the most plentiful of the residues at just under 50% of the total, and it is one of the most reusable. During coal combustion in a utility boiler, ground coal ignites and the carbon burns away. Coal, like any other ore dug from the ground, is not pure carbon. Minerals such as silica, iron, and aluminum are also in the coal but do not burn. They melt in the boiler and then reform as small spherical particles in the flue gas as the gas cools and exits the boiler. Fly ash, a fine, powdery material, is captured from the flue gas stream by bag houses or electrostatic precipitators.
The widest beneficial reuse for fly ash is as a mineral admixture in portland cement concrete. The physical and chemical properties of fly ash improve both the plastic and hardened properties of concrete. The ancient Greeks and Romans used their version of fly ash, volcanic ash, to construct roads, aquaducts, and buildings; many are still in service today.
Adding fly ash to concrete reduces the water required, improves pumpability, reduces segregation, yields higher ultimate strength, and is very effective at mitigating durability problems like alkali-silica reactivity and reinforcing steel corrosion. Class F fly ash, which comes from burning eastern bituminous coals found in our region, is actually mandatory in much of the concrete specified by the Corps of Engineers because of its ability to improve durability and extend the service life. But what of those trace amounts of metals found in the fly ash? Because fly ash participates in the chemical reactions in concrete, the trace metals are entombed in the resulting crystalline structure rendering them virtually unleachable. Even when the concrete is demolished, ground, or polished, the potentially harmful metals are extremely diluted and inert.
Also because fly ash participates in the chemical reactions in concrete, concrete mixtures with fly ash can use less portland cement. Using fly ash as a partial replacement for cement reduces overall cement demand which reduces the demand for virgin materials and the voluminous release of carbon dioxide during the manufacture of portland cement. So, in addition to reducing fly ash disposal and improving the durability of concrete, using fly ash in concrete reduces the production of carbon dioxide by nearly one pound per pound used! This is a triple win for the environment and for sustainable construction.
Something so good must come with a catch, and there are several catches. Not all fly ash is equal. Fly ashes from softer anthracite and subbituminous coals (Class C) contain more calcium oxide. Although they may gain strength well, they may not have the same excellent durability benefits, and they often contribute to unpredictable concrete set times. Some utilities add sorbents to the coal before or after combustion so the resulting ash may contain excess amounts of sodium, gypsum, activated carbon, sulfur and other compounds that are harmful to concrete. Ammonia is widely used by utilities for pollution prevention, and some of the unreacted ammonia, or slip, may be absorbed on the ash. When fly ash with ammonia is mixed in water at a high pH (exactly what happens in a concrete truck), the ammonia is liberated. Although there is no danger to workers or to the concrete, the odor may be less than pleasant.
The biggest concern for use of fly ash in concrete is the quantity and form of unburned carbon. As tons of powdered coal are blown through a utility boiler every second, not all of the carbon burns completely. Many of the unburned carbon particles are very small yielding large surface areas. One of the most effective ways for utilities to lower their emissions of nitrous oxides or NOx is to lower the combustion temperature in the boiler. Lowering the temperature means less complete combustion and more unburned carbon in the fly ash. When mixed in concrete, the unburned carbon may adsorb the chemical surfactant concrete producers use to entrain millions of microscopic air bubbles. The air bubbles help the concrete withstand repeated cycles of freezing and thawing. Very high amounts of unburned carbon, measured by Loss on Ignition – LOI; or fluctuating amounts make it difficult to produce consistent air-entrained concrete.
The coal ash industry responded to the challenge of high or variable amounts of unburned carbon with several technologies. It boils down to either removing the carbon, or rendering the carbon passive, or both. Chemicals can be sprayed on the fly ash to make the carbon less thirsty for the air-entraining surfactant. Carbon can also be passivated with heat to oxidize the particle surface making it less absorbent. There are three techniques to remove carbon from fly ash: mechanical, electrostatic, and thermal. Mechanical methods usually take advantage of the large mass difference between carbon and minerals to float or spin out the carbon particles. Electrostatic separation uses opposite, electrostatic charges resident on the carbon and minerals. As the ash passes through oppositely charged electrodes, the carbon is attracted to one and the minerals to the opposite.
Thermal beneficiation is the third and very effective strategy to remove the carbon from fly ash. The basic idea is to reburn the fly ash with the unburned carbon providing enough fuel to sustain combustion in a small, controlled setting. Even if all of the organic carbon is not removed, the first carbon to burn are the very fine particles, the same particles that cause the most headaches for entraining air bubbles in concrete. The surfaces of the remaining carbon particles are passivated by the heat further reducing the deleterious effect on air entraining. The bonus benefit to thermal processing is that all of the ammonia is decomposed. Flue gasses from thermal treatment are either diverted to the utility for cleansing, or best available technologies to scrub contaminants from the new flue gases are employed on a small scale with pinpoint accuracy. The heat generated by thermal beneficiation can be piped back to the utility for use in many parts of power generation. In effect, nearly all of the carbon that is mined, hauled, stockpiled, crushed, and blown into the boiler is used to produce electricity. A unit using Carbon Burn Out technology operates at Dominion Power’s Chesapeake Station, and a unit using STAR or Staged Turbulent Air Reactor technology is under construction at GenOn’s Morgantown station in Newburg, Maryland just across the Potomac River from Dahlgren, Virginia.
Regardless of one’s personal feelings about burning coal for electricity, the facts are coal in Virginia is cheap, plentiful, and local. We are likely to continue to rely on it for power for many years into the future until something else is cheap, plentiful, and local. The beneficial use of fly ash and other coal combustion products, when done in an engineered manner, is one of America’s great recycling success stories. Specifically, using fly ash as a mineral admixture in concrete is a triple win because it reduces disposal and the potential for leaching, it improves the lifespan of concrete by fighting off insidious durability attacks, and it lowers carbon dioxide emissions by reducing the world-wide demand for portland cement.
ABOUT THE AUTHOR
Hank Keiper is a licensed civil engineer based in Mechanicsville, VA and the Mid-Atlantic area manager for The SEFA Group, a coal combustions byproducts marketing and manufacturing firm. Mr. Keiper is a graduate of the U. S. Air Force Academy and the University of Florida School of Building Construction. For the last 15 years he has specialized in the production and quality assurance of concrete and fly ash. He is a member of the National Ready Mix Concrete Association’s Research, Engineering, and Standards committee, both Virginia and Maryland Ready Mix Concrete Associations’ technical committees, and he is a member of the American Concrete Institute (ACI) Technical Committees 232 and 229. Mr. Keiper can be reached at firstname.lastname@example.org, or via telephone at 804.380.8078.