Source of information: http://ehp03.niehs.nih.gov/
Even as public debate rages over the question of whether coal should continue to provide the majority of U.S. electric power needs, the U.S. Energy Information Administration predicts in International Energy Outlook 2009 that, absent new policies to the contrary, the United States — along with China and India — is expected to account for 88% of the projected net increase in coal consumption between 2006 and 2030. Meanwhile, coal combustion waste (CCW) — the noncombustible remains from coal burning — continues to pile up at the rate of about 131 million tons per year in the United States alone, and electric utilities are looking to recycle a larger proportion of this material.
The American Coal Ash Association (ACAA) reported in its latest Coal Combustion Product Production & Use Survey Results that 43% of all CCW produced in the United States in 2007 was devoted to what are termed “beneficial uses.” The state-defined “beneficial use” designation means a waste product is used in the manufacture of or as a replacement for another product. Waste products granted a state “beneficial use” determination are exempt from solid waste regulations governing their disposal.
An estimated 23% of the total CCW produced in the United States each yea — more than 30 million tons — is used in construction products, primarily concrete and wallboard, but also clinker (raw material for making portland cement), roofing granules, saggregate for paving materials, and asphalt filler. In 2007, according to the ACAA, 14.5 million tons of CCW was used in concrete, another 5.0 million tons was used as clinker, and 8.3 million tons was used in gypsum wallboard, which is the standard interior wall material used in the United States. (In EHP’s exploration of the use of CCW in building products, the preponderance of studies and statistics focused on the situation in the United States.)
Other “beneficial uses” include applications that critics say are closer to unregulated dumping than to recycling. For example, about 6.7 million tons of CCW is used to fill abandoned mines, often as a measure to neutralize the acidic liquid that can drain from these sites into nearby waters. Another 10.6 million tons is used for structural fills and embankments. Under the “beneficial use” designation, these applications are exempt from safeguards that, depending on state law, may be required of disposed waste, such as the use of liners to prevent leaching of potentially toxic metals into ground and surface waters. This raises the potential for serious environmental consequences.
In Environmental Concerns and Impacts of Power Plant Waste Placement in Mines, a 2004 report on minefilling written for the U.S. Office of Surface Mining Reclamation and Enforcement, hydrogeologist Charles Norris reported that “ground waters and surface waters are being degraded by [minefilling]. Data from designated ash monitoring points show rises in the concentrations of total dissolved solids, sulfate, manganese, iron, boron, and a variety of trace heavy metals in these waters that significantly exceed baseline concentrations. . . . The data raise fundamental questions about the adequacy of safeguards in permits authorizing ash placement in coal mines and the assertions that alkaline coal ashes are inherently reliable and safe materials for preventing acid drainage or remediating abandoned mined lands.”
Given the potential for heavy metal contamination, questions have arisen about the advisability of adding CCW to construction products that will be used in roads, bridges, even homes. Do these types of “beneficial uses” also potentially expose people to toxic materials?
CCW in Construction Materials Top
Many of the construction-related uses of CCW involve materials traditionally made using energy-intensive processes that release large amounts of greenhouse gases. For instance, the cement industry creates about 5% of global carbon dioxide (CO2) emissions; in the United States, producing a metric ton of portland cement releases, on average, an estimated 0.95 metric ton of CO2 equivalent [for more information on cement manufacturing, see sidebar at left]. Craig Benson, Wisconsin Distinguished Professor of civil and environmental engineering and geological engineering at the University of Wisconsin–Madison and codirector of the University of New Hampshire–based Recycled Materials Resource Center, calculates that each year in the United States recycling CCW saves about 160 trillion BTUs of energy (about the amount of energy used by 1.7 million households), 11 million tons of CO2 equivalent (comparable to the average annual emissions of more than 1.8 million passenger vehicles), and 32 billion gallons of water.
About 71.1 million tons, or 55%, of the CCW produced each year is fly ash, a fine material that is captured after combustion in filters or electrostatic precipitators. Fly ash is composed of microscopic spheres containing largely silica, iron, aluminum, and calcium; the biggest current construction-related use of fly ash is to replace portland cement, which binds the sand and gravel in concrete. Fly ash has different characteristics depending on the chemical content of the coal from which it derives. Broadly speaking, lignite and subbituminous coals produce Class C fly ash, which has self-cementing properties, whereas anthracite and bituminous coals produce Class F fly ash, which typically must be mixed with water and a cementing agent in order to harden.
Nationally, 8–12% of the binder in concrete is fly ash, says John Sager, who coordinates the Coal Combustion Products Partnership, an initiative of the U.S. Environmental Protection Agency (EPA) that promotes recycling of CCW. David Goss, the former executive director of the ACAA, says concrete floors fortified with fly ash are “highly polished, attractive, have high wearability, and eliminate the need for tile.” Fly ash can reduce alkali silica reactivity, a chemical reaction that can cause extensive cracking of concrete made with certain types of aggregate, adds Steven Kosmatka, vice president of research and technology services at the Portland Cement Association (PCA).
“Often the reaction is ‘I don’t want garbage — waste — hidden in my concrete,’” says Kosmatka. “But when people hear that fly ash contributes to strength development, can help improve durability, help with economics, reduce heat generation during setting, reduce permeability to keep chlorides away from steel reinforcement, and prevent corrosion, suddenly people don’t think of it as waste anymore — they think of it as a substance with a positive impact on the ultimate product.”
Fly ash can also stabilize soil beneath a highway. Benson says fly ash mixed into the upper 300 cm of soil “sets up like lean concrete and creates a really good working platform” that can replace the 1-m layer of crushed rock that is typically used beneath major highways. “We avoid the other 700 cm of fill and eliminate all the energy and emissions associated with excavating and crushing this rock,” says Benson.
Fly ash constitutes 50–85% of a wood replacement called LifeTime Lumber, produced by LifeTime Composites of Carlsbad, California. The material, used for decking and fencing, is inedible to termites and does not support mold growth, unlike wood replacements made with sawdust, says company president Jim Mahler.
Fly ash also can be combined with water and pressed into bricks that harden without the use of clay, heat, or portland cement, says Henry Liu, president of the Freight Pipeline Company in Columbia, Missouri, who invented a process for producing such bricks. Freight Pipeline has licensed its Greenest Brick technology to companies in 11 countries. U.S. licensee CalStar Products of Newark, California, plans to start producing bricks at the end of 2009 near a Wisconsin coal-fired power plant run by We Energies. CalStar will be capable of making 40 million bricks a year, says chief operating officer Tom Pounds.
“People are coming to realize that when you build or renovate a building you are laying down a huge carbon footprint from the energy required to make the materials,” says Pounds. Largely due to the 1,100°C heating needed to convert clay into brick, he says, “The embodied energy in a single clay brick is about 6,000 BTUs, and we expect the fly ash bricks to be well under 1,000 BTUs.”
Another major application of CCW is the utilization of flue gas desulfurization (FGD) waste in wallboard. Power plants often remove sulfur oxides from their emissions by using “scrubbers” that spray powdered limestone into the coal smoke. A chemical reaction creates calcium sulfite, which can be oxidized into calcium sulfate — a synthetic counterpart to the gypsum rock used in wallboard. Approximately 33% of the gypsum that was used to make U.S. wallboard in 2008 was FGD gypsum, says Michael Gardner, executive director of the Gypsum Association, a trade group, who adds, “Only cutbacks in construction due to the recession have prevented the use of even more FGD gypsum.”
Playing It Safe: Construction Materials and Leaching Top
The heat of coal combustion eliminates compounds such as dioxins and polycyclic aromatic hydrocarbons that could form during combustion, according to “PAHs and Dioxins Not Present in Fly Ash at Levels of Concern,” a presentation by Lisa Bradley and colleagues at the 2009 World of Coal Ash meeting, a biennial conference organized by the ACAA and the University of Kentucky Center for Applied Energy Research. But CCW can contain concentrated amounts of many other toxics that occur naturally in coal, including arsenic, mercury, boron, cadmium, and chromium. Skeptics of CCW say these toxics may leach from many “beneficial uses,” such as minefills or embankments. Can they leach from construction materials as well?
For safety purposes, LifeTime Composites tests fly ash before using it in its LifeTime Lumber product. “We do not want to run the risk of having a product that exceeds limits [for heavy metals] in our system,” says Mahler. “Our process encapsulates the ash [in polyurethane] to the point where no heavy metals are released in any way to humans, pets, or plants.”
At the 2007 World of Coal Ash meeting Liu reported on a test simulation of heavy rain at a construction site where Greenest Bricks were stored. “We compared the water sample to the EPA standard for drinking water, and every item—lead, selenium, and so on — was 10, 100, or 1,000 times less than the standard,” he says.
Meanwhile, Pounds says CalStar has submitted its products to all relevant U.S. and California EPA tests for leaching and surface wipe tests. These tests were managed and reviewed by Massachusetts-based consultancy Gradient Corp., which concluded, “[T]he presence of [CCW] metals in newly manufactured CalStar bricks is not expected to result in any exposures of health concern via dermal contact with brick surfaces or via leaching.” Gradient’s report, including the test data, is available on CalStar’s website at http://www.calstarproducts.com/.
Like the natural rock, FGD gypsum contains heavy metals. In tables prepared to accompany its March 2008 brochure “Agricultural Uses for Flue Gas Desulfurization (FGD) Gypsum,” the EPA shows higher levels of antimony, arsenic, and mercury in FGD gypsum than in natural gypsum, although in every case except that of selenium and thallium, metal levels in natural and FGD gypsum overlapped or fell below national average background levels in soil. U.S.-made dry-wall containing FGD gypsum has been tested by time, says Gardner: “We have two decades of history that have shown no adverse effects.”
However, the high temperatures involved in the production of wallboard from FGD gypsum (as well as in cement manufacturing) can cause the release of mercury, according to an article by Constance L. Senior and colleagues published in the July 2009 issue of the Air & Waste Management Association’s EM magazine. “The wide variation in mercury loss (2 to 55%) from seven FGD gypsum samples [taken from five plants] was attributed to the different conditions under which each gypsum sample was generated,” the authors wrote. “Any remaining mercury in the finished FGD-wallboard could be released during use or subsequent disposal or recycling of the wallboard.” The authors noted that research is under way at the EPA to evaluate the fate of mercury and other metals through each stage of wallboard’s life cycle.
Heavy metals tend to stay put in conventional concrete, says Kosmatka, who cites a 2007 PCA-financed study of concrete that passed the EPA’s toxicity characteristic leaching procedure (TCLP) test despite containing cement carrying up to 0.1% lead, cadmium, and chromium. The study, titled Comparison of Mortar Leaching Methods, concluded that cement containing less than 500 mg/kg of these elements would even be usable in drinking water systems.