Approximately 40 percent of the coal combustion by-products (CCBs) generated in the United States is used. The majority of the uses are related to concrete. Although the amounts used are increasing, the availability of high quality CCBs is decreasing as utilities strive to meet more stringent emission regulations. For example, in order to meet lower NOx emission standards, many utilities are converting existing burners to low-NOx burners, which frequently produce a coarser ash with higher levels of unburned carbon. Both of these quality considerations limit, or even prohibit, the use of this ash in traditional concrete markets. If traditional markets are to be preserved and new ones established, it is increasingly necessary to "beneficiate," (i.e., improve) CCBs to meet end-use specifications. To address this decrease in ash quality, CAER has developed several beneficiation technologies that can produce high-quality, consistent products from ash that is both poor quality and inconsistent with respect to grade.
Figures 1 and 2
One example of technology development is the FastFloatTM process, which incorporates beneficiation technologies commonly applied in coal preparation and mineral processing. The FastFloatTM process provides the opportunity for total utilization of CCBs and is being implemented in stages in order to expedite commercial development.
The first stage deals with bottom ash and incorporates sizing and density separation. Bottom ash is recovered from ponds or from boilers and is screened to remove oversize (>3/8 inch) material. The coarse ash is then hydraulically classified to efficiently reject fines (<100 mesh or 150 µm). The ash is then separated by density using concentrating spirals to produce a lightweight aggregate suitable for use in the manufacturing of concrete masonry units and a variety of other applications. This technology was first demonstrated in 1997 and is in commercial operation at three plants in the U.S., producing approximately 200,000 tons per year (tpy) of lightweight aggregate (Figures 1 and 2).
A fourth plant (Figure 3) is also in operation where the bulk density of the bottom ash precludes use as lightweight aggregate. In this application, construction fill sand is being produced. While construction fill sand is not as high-valued as lightweight aggregate, over 250,000 tons have been marketed to date, reducing disposal costs incurred by the host utility and extending the life or their ash storage facility.
An innovative new market for lightweight aggregate derived from bottom ash produced from this process is being developed by Charah, Inc, a Louisville based company . Charah, Inc. has constructed a facility in Emporia, VA that produces pre-packaged lightweight concrete mix under the trade name PROJECT MIX as well as a variety of similar products. These products are available at twenty Home Depot® stores in Virginia. This innovation dramatically increases the potential use of bottom ash and fly ash produced by coal-burning utilities.
The second stage of the FastFloatTM process adds selective density separation to recover coarse carbon (>100 mesh) and froth flotation to recover fine carbon (<100 mesh) from fly ash to produce a supplemental fuel. A mobile, pilot-scale processing plant to demonstrate this phase of the technology was constructed and operated by CAER (Figures 4, 5 and 6). The test site was Western Kentucky Energy's Coleman Station located in Hawesville, KY as part of a US DOE-sponsored research program. The demonstration produced over 500 pounds of coarse carbon with a heating value of 7300 Btu/lb and an additional 500 pounds of fine carbon with a heating value of 4360 Btu/lb. Combustion testing and economic analyses confirmed the technical and economic viability of recovering these products as supplemental fuel.
The third stage is to incorporate thickening and filtration of the carbon-free ash rejected after flotation to produce a high-quality pozzolan. This phase has been selected by the US DOE as part of the Clean Coal Power Initiative and is being implemented at Kentucky Utility's Ghent Station in Ghent, KY in collaboration with Cemex, Inc. Pilot-scale evaluations (Figure 7) are underway at a feed rate of 2.5 tph and larger quanitities (several tons) of high-quality pozzolan are being generated for concrete testing. It is envisioned that the demonstration of this phase of the technology will result in the construction of a processing plant that will generate approximately 200,000 tpy of processed CCBs.
The fourth and final phase of the FastFloatTM technology will be to process a slip-stream of the flotation tailings through a secondary classifier to produce a mineral-grade filler suitable for use in plastic resins. Typical product size specifications are an average particle size of 2 to 3 µm. In addition to the plastic filler market, this ultra-fine processed ash is suitable for use in high-performance concrete. Testing is already in progress at both the laboratory-pilot (Figure 8) and demonstration scale (Figures 9, 10 and 11). Large batches (200 to 500 pounds) of ultra-fine ash (average particle size of 3.5 to 5 µm) have been produced and are being evaluated.
Contact: Jack Groppo
Gasification is the conversion of carbonaceous solids (e.g., coal, char, etc.) into a combustible gaseous product in a reactive gaseous atmosphere. Coal gasification differs significantly from the more widely applied coal utilization technology: combustion. During combustion, coal is directly heated, first resulting in the evolution and ignition of volatile components followed by combustion of fixed carbon. With either technology, a residual ash component is generated. Not surprisingly, the nature of the ash generated by these two utilization technologies is as different as the technologies are.
A slag by-product is produced from gasification as molten ash flows from the bottom of a gasifier where it is quenched. The coarse component of the slag is essentially a vitreous, high density, abrasive solid that is low in carbon content. The physical shape of the coarse slag particles ranges from rod or needle-like structures to irregularly-shaped jagged pieces. The coarse slag by-product has unique physical and chemical properties and can be used for many applications such as:
- abrasive media (i.e. sand blasting grit)
- polishing media
- roofing granules
- cement kiln feed
- athletic-track surfaces
- road surface coating
In addition, the slag by-product can also be ground and used as a cement additive to improve the properties of concrete.
The finer slag is comprised of char or unburned carbon particles containing varying amounts of siliceous ash. The carbonaceous phase of the ash is irregularly-shaped particles with a highly developed pore structure. This material has been utilized as a supplemental pulverized coal combustion fuel and has shown excellent potential as both an adsorbent and as a precursor for activated carbon.
In order to utilize gasification slag, it is necessary to beneficiate the slag to separate the mixture of particles into useful components with acceptable quality for a specific end use market. A beneficiation process was developed and implemented by Charah Environmental, Inc. working in collaboration with the CAER. The process was developed specifically to beneficiate the gasification slag produced at Tampa Electric Company's Polk Station in Mulberry, FL, but is applicable for beneficiation of gasification slag from any source.
The slag beneficiation plant at Polk Station has been in operation since autumn, 2000 and has processed over 140,000 tons of stockpiled by-product, enabling over 80 % of this material to be utilized, rather than being landfilled as waste.
Photograph of Charah Environmental's Slag Beneficiation
Plant at Polk Station
Contact: Jack Groppo
By far, the majority of the beneficial uses for CCBs are related to cement, concrete and concrete products. Not surprisingly, changes in ash chemistry can have a pronounced effect on cement and concrete quality. In order to improve the fundamental understanding of the role of CCBs for these uses, CAER has developed an intensive research program focused on CCB chemistry and end-use quality considerations.
One example of this area of research is the role of ammonia in concrete. Many utilities are installing Selective Catalytic Reduction (SCR) technologies to meet increasingly more stringent NOx emission requirements. Most SCR systems require the addition of ammonia into the flue gas to facilitate NOx reduction. Inevitably, small amount of ammonia is adsorbed onto the ash. The questions to be addressed are whether the ammonia has any effect on concrete properties and at what concentration does the ammonia present health and safety concerns.
Research at CAER in collaboration with several ash marketers and utilities has demonstrated that ammonia in ash does not adversely affect concrete performance. These results have been confirmed at several scales of testing from laboratory cubes to 5 yd3 pours. Ammonia concentrations in the air were monitored closely during this program to assess any potential health and safety concerns. The results quantified the kinetics of ammonia evolution and identified the ranges of ammonia on ash that would be acceptable for worker safety without any additional precautions. This area of research presently focuses on the development of a simple, accurate field test to quantify the amount of ammonia on ash to avoid any potential hazards.
CAER has installed a pozzolan laboratory with capabilities to address most areas of cement and concrete quality. Facilities are available to address quality issues such as air entrainment, workability, flow, strength time of set, consistency, density and fineness. Aggregate testing capabilities include gradation, density, staining and absorption as well as 8" test block production. Lastly, CAER has purchased and modified an 8 yd3 concrete truck for large-scale testing. With these capabilities as well as the extensive analytical capabilities available at CAER, detailed evaluations can be made on cement, mortar, admixtures, concrete and concrete products at a variety of scales.
CAER's Concrete Truck
Contact: Bob Jewell
One of the goals of the ECT program at CAER is to monitor changes in the CCBs produced in the Commonwealth of Kentucky. Surveys of ash quality and quantity in Kentucky have been conducted every five years since 1992, with the next survey planned for 2007. The surveys are a good supplement to the national survey conducted by the American Coal Ash Association. The Kentucky Coal Association has used the survey results in their Kentucky Coal Facts books. Coal and ash chemical data is supplied to the individual utilities.
Each plant is visited and sampled comprehensively, including coal feed and rejects from each boiler, all bottom ash streams, fly ash collection hoppers as well as scrubber effluents. Samples are thoroughly analyzed petrographically and chemically and archived.
Selected samples of stored and freshly produced CUBs are subjected to batch and continuous leaching studies to ensure that any potential utilization is carried out in a sound environmental manner. Trace-element concentrations are closely monitored throughout all leaching studies and standard EPA procedures are used for both the leaching studies and elemental analyses.
Additional research is conducted on the fate of mercury during combustion. Detailed thermal analyses quantify mercury evolution and speciation from combustion sources as well as the kinetics and collection efficiency of mercury sorbents.
Contact: Jim Hower
Mercury is widely distributed in the environment, and sometimes in concentrations that threaten humans and wildlife. Natural and artificial sources of mercury include volcanoes, forest fires, some coal-fired power plants, crematoria, mineral deposits, seawater and chloralkali plants. In nature, mercury may be dissolved or adsorbed onto particles in water; occur as vapor or on particles in air; be found within organisms; or it may exist as solids, liquids or vapors in soils, sediments, and rocks.
Methylmercury (H3CHg+) is an especially serious health threat to humans and wildlife. Once mercury is methylated by bacteria or inorganic processes in organisms, soils, sediments, or water, it may bioaccumulate to dangerous concentrations in fish and fish-eating organisms, including people. Mass poisonings involving methylmercury and other mercury compounds have occurred in Minamata, Japan; Pakistan; Italy; Iraq; Guatemala and during the recent "gold rush" in Brazil. Many American states currently have mercury health advisories against eating fish in specific lakes and marine areas.
CAER personnel recognize that multidisciplinary field and laboratory research is required to develop technologies for reducing mercury emissions and finding economical solutions for contaminated sites. A thorough understanding of the role of mercury in commercial operations and its behavior in the environment will assist in achieving our research goals.
Contact: Kevin Henke
Another area of mercury research is looking at the potential for using PCC (Pulverized Coal Combustion) and IGCC (Intergrated Gasification Combined Cycle) by-products as cheap sorbents for Hg and NOx capture from the flue gas of electric power plants. The CAER has the capability to test a variety of materials for both Hg and NOx adsorption capabilities. Hg adsorption potential is done using a system involving a mercury vapor generator, a fixed bed reactor, an on-line Hg vapor, an automated gas sampling system and a Jerome 431X Hg vapor analyzer. NOx adsorption isotherms are measured using a thermal analyzer - mass spectrometer. Refer to this factsheet - Value-Added Carbon Products from PCC and IGCC Byproducts. Other work involves a study of the effect of flue gases such as NOx and SO2, on the ability of IGCC by-products to adsorb Hg. Attempts are being made to increase the adsorption capacities of IGCC chars through chemical and thermal activation of the chars.
Contact: Bob Jewell