Triboelectrostatic Beneficiation de terra ple i les cendres Ponded

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Triboelectrostatic Beneficiation of

Land Filled and Ponded Fly Ash

By Lewis Baker, Abhishek Gupta, Stephen Gasiorowski, and Frank Hrach

L'Associació Americana de carbó cendra (ACAA) enquesta anual de producció i ús de cendres de carbó que entre els informes 1966 i 2011, sobre 2.3 billion short tons of fly ash have been produced by coal-fired utility boilers.1 D'aquesta quantitat aproximadament 625 milions de tones s'han utilitzat junts, principalment per a la producció de ciment i formigó. No obstant això, the remaining 1.7+ billion tons are primarily found in landfills or filled ponded impoundments. While utilization rates for freshly generated fly ash have increased considerably over recent years, with current rates near 45%, aproximadament 40 million tons of fly ash continue to be disposed of annually. While utilization rates in Europe have been much higher than in the US, considerable volumes of fly ash have also been stored in landfills and impoundments in some European countries.

Últimament, interest in recovering this disposed material has increased, partially due to the demand for high-quality fly ash for concrete and cement production during a period of reduced production as coal-fired power generation has decreased in Europe and North America. Concerns about the long-term environmental impact of such landfills are also prompting utilities to find beneficial use applications for this stored ash.

LAND FILLED ASH QUALITY AND REQUIRED BENEFICIATION

Mentre que alguns d'aquest emmagatzema cendra volant pot ser adequat per a ús com inicialment excavat, la gran majoria serà necessari algun tractament per satisfer els estàndards de qualitat per a ciment o formigó. Ja que el material ha estat típicament wetted per permetre la manipulació i compactació, evitant la generació de pols en l'aire, drying and deagglomeration is a necessary requirement for use in concrete since concrete producers will want to continue the practice of batching fly ash as a dry, fine powder. No obstant això, assuring the chemical composition of the ash meets specifications, most notably the carbon content measured as loss-on-ignition (LOI), is a greater challenge. As fly ash utilization has increased in the last 20+ anys, most “in-spec” ash has been beneficially used, and the off-quality ash disposed. Thus, LOI reduction will be a requirement for utilizing the vast majority of fly ash recoverable from utility impoundments.

LOI REDUCTION BY TRIBOELECTRIC SEPARATION

While other researchers have used combustion techniques and flotation processes for LOI reduction of recovered landfilled and ponded fly ash, ST equips & Technologies (STET) has found that its unique triboelectrostatic belt separation system, long used for beneficiation of freshly generated fly ash, is also effective on recovered ash after suitable drying and deagglomeration.

STET researchers have tested the triboelectrostatic separation behavior of dried landfilled ash from several fly ash landfills in the Americas and Europe. This recovered ash separated very similarly to freshly generated ash with one surprising difference: the particle charging was reversed from that of fresh ash with the carbon charging negative in relation to the mineral.2 Other researchers of electrostatic separation of fly ash carbon have also observed this phenomena.3,4,5 The polarity of the STET triboelectrostatic separator can easily be adjusted to allow rejection of negatively charged carbon from dried landfilled fly ash sources. No special modifications to the separator design or controls are necessary to accommodate this phenomena.

TECHNOLOGY OVERVIEW – FLY ASH CARBON SEPARATION

In the STET carbon separator (Figura 1), material is fed into the thin gap between two parallel planar electrodes. Les partícules triboelectrically paguen per contacte interparticle. The positively charged carbon and the negatively charged mineral (in freshly generated ash that has not been wetted and dried) són atrets a davant elèctrodes. The particles are then swept up by a continuous moving belt and conveyed in opposite directions. El cinturó mou les partícules adjacents a cada elèctrode cap a extrems oposats del separador. L'alta velocitat del cinturó també permet rendiments molt alts, fins a 36 tones per hora en un únic separador. Del buit, high voltage field, counter current flow, vigorous particle-particle agitation and self-cleaning action of the belt on the electrodes are the critical features

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of the STET separator. Controlant diversos paràmetres del procés, Com la velocitat del cinturó, punt d'alimentació, i la taxa d'alimentació, el procés STET produeix cendres de mosca LOI baixes a un contingut de carboni inferior a 1.5 a 4.5% from feed fly ashes ranging in LOI from 4% per sobre 25%.

Figa. 1 STET Separator processing dried, landfilled fly ash

El disseny de separador és relativament senzilla i la compacta. Una màquina destinada a processar 40 tons per hour is approximately 30 M. (9 m.) llarg, 5 M. (1.5 m.) àmplia, i 9 peus., m (2.75 m.) alta. El cinturó i rodets associats són les úniques parts en moviment. Els elèctrodes són estacionaris i formada per un material durable adequadament. The belt is made of non-conductive plastic. Tracta de consum d'energia del separador 1 quilowatts-hora per tona de material processat amb la major part de la potència consumida per dos motors que condueixen el cinturó.

El procés és totalment sec, requereix cap material addicional que no sigui la cendra volant i produeix sense residus emissions aigua o aire. The recovered materials consist of fly ash reduced in carbon content to levels suitable for use as a pozzolanic admixture in concrete, and a high carbon fraction useful as fuel. La utilització d'ambdós fluxos de productes proporciona un 100% Solució als problemes d'eliminació de cendres volants.

PROASH® RECOVERED FROM LAND FILLS

Four sources of ash were obtained from landfills: sample A from a power plant located in the United Kingdom and samples B, C, and D from the United States. All these samples consisted of ash from the combustion of bituminous coal by large utility boilers. A causa de la barreja del material en els abocadors, cap més informació està disponible sobre les condicions d'origen o combustió carbó específics.

Les mostres CNMV STET contenia entre 15% i 27% aigua formades per landfilled material. Les mostres també conté quantitats variables de gran >1/8 polzada (~ 3 mm) material. Per preparar les mostres per a la separació de carboni, the large debris was removed by screening and the samples then dried and deagglomerated prior to carbon beneficiation. Several methods for drying/deagglomeration have been evaluated at the pilot-scale in order to optimize the overall process. STET has selected an industrially proven, feed processing system that offers simultaneous drying and deagglomeration necessary for effective electrostatic separation. A general process flow sheet is presented in Figure 2.

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Figura 2: Process Flow Diagram

The properties of the prepared samples were well within the range of fly ash obtained directly from normal utility boilers. The most relevant properties for both the separator feeds and products are summarized in Table 2 along with recovered product.

CARBON SEPARATION

Carbon reduction trials using the STET triboelectric belt separator resulted in very good recovery of low LOI products from all four landfill fly ash sources. The reverse charging of the carbon as discussed above did not degrade the separation in any way as compared to processing fresh ash.

Les propietats de la baixa cendres LOI recuperat utilitzant el procés STET per tant recollit acabat cendra de la caldera i recuperat de l'abocador de cendres es resumeix en la taula 1. The results show that the product quality for ProAsh® produced from landfilled material is equivalent to product produced from fresh fly ash sources.

Taula 1: Properties of feed and recovered ProAsh®.

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PERFORMANCE IN CONCRETE

The properties of the ProAsh® generated from the reclaimed landfill material were compared to that of ProAsh® produced from fresh fly ash generated by the utility boilers from the same location. The processed reclaimed ash meets all the specifications of ASTM C618 and AASHTO M250 standards. The following table summarizes the chemistry for samples from two of the sources showing the insignificant difference between the fresh and reclaimed material.

Taula 2: Ash Chemistry of low LOI ash.

Strength development of a 20% substitution of the low LOI fly ash in a mortar containing 600 lb cementitious/ yd3 (Veure taula 3 sota) showed the ProAsh® product derived from landfilled ash yielded mortars with strength comparable to mortars produced using ProAsh® from fresh fly ash produced at the same location. The end product of the beneficiated reclaimed ash would support high end uses in the concrete industry consistent with the highly valuable position ProAsh® enjoys in the markets it currently serves.

Taula 3: Compressive strength of mortar cylinders.

PROCESS ECONOMICS

The availability of low cost natural gas in the USA greatly enhances the economics of drying processes, including the drying of wetted fly ash from landfills. Taula 4 summarizes the fuel costs for operations in the USA for 15% i 20% moisture contents. Typical inefficiencies of drying are included in the calculated values. Costs are based on the mass of material after drying. The incremental costs for drying fly ash for STET triboelectrostatic separation processing are relatively low.

Taula 4: Drying costs on basis of dried mass.

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Even with the addition of feed drying costs, the STET separation process offers a low cost, industrially proven, process for LOI reduction of landfilled fly ash. The STET process for reclaimed fly ash is one-third to one-half of the capital cost compared to combustion based systems. The STET process for reclaimed fly ash also has significantly lower emissions to the environment compared to combustion or flotation based systems. Since the only additional air emission source to the standard STET process installation is a natural gas-fired dryer, permitting would be relatively simple.

RECOVERED FUEL VALUE OF HIGH-CARBON FLY ASH

In addition to the low carbon product for use in concrete, brand named ProAsh®, the STET separation process also recovers otherwise wasted unburned carbon in the form of carbon-rich fly ash, branded EcoTherm™. EcoTherm™ té valor significatiu de combustible i fàcilment pot tornar a la planta d'energia elèctrica utilitzant l'STET EcoTherm™ Sistema de devolució per reduir l'ús de carbó a la planta. Quan EcoTherm™ es crema a la caldera d'utilitat, l'energia de combustió és convertida a alta pressió / vapor a alta temperatura i després a l'electricitat a la mateixa eficàcia que carbó, típicament 35%. The conversion of the recovered thermal energy to electricity in ST Equipment & Technology LLC EcoTherm™ Return system is two to three times higher than that of the competitive technology where the energy is recovered as low-grade heat in the form of hot water which is circulated to the boiler feed water system. EcoTherm™ is also used as a source of alumina in cement kilns, displacing the more expensive bauxite which is usually transported long distances. Utilizing the high carbon EcoTherm™ ash either at a power plant or a cement kiln, maximizes the energy recovery from the delivered coal, reducing the need to mine and transport additional fuel to the facilities.

STET’s Talen Energy Brandon Shores, SMEPA R.D. Demà, NBP Belledune, RWEnpower Didcot, EDF Energy West Burton, RWEnpower Aberthaw, and the Korea South-East Power fly ash plants all include EcoTherm™ Return systems.

STET ASH PROCESSING FACILITIES

STET’s separation process has been used commercial since 1995 for fly ash beneficiation and has generated over 20 million tons of high quality fly ash for concrete production. Controlled low LOI fly ProAsh®, is currently produced with STET’s technology at eleven power stations throughout the U.S., Canadà, the U.K., Polònia, and Republic of Korea. ProAsh® fly ash has been approved for use by over twenty state highway authorities, Així com moltes altres agències d'especificació. ProAsh® has also been certified under Canadian Standards Association and EN 450:2005 Estàndards de qualitat a Europa. Ash processing facilities using STET technology are listed in Table 5.

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Taula 5. Fly Ash Processing facilities using STET separation technology

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CONCLUSIONS

After suitable scalping of large material, drying, and deagglomeration, fly ash recovered from utility plant landfills can be reduced in carbon content using the commercialized STET triboelectric belt separator. The quality of the fly ash product, ProAsh® using the STET system on reclaimed landfill material is equivalent to ProAsh® produced from fresh feed fly ash. The ProAsh® product is very well suited and proven in concrete production. The recovery and beneficiation of landfilled ash will provide a continuing supply of high quality ash for concrete producers in spite of the reduced production of “fresh” ash as coal-fired utilities reduce generation. A més, power plants that need to remove ash from landfills to meet changing environmental regulations will be able to utilize the process to alter a waste product liability into a valuable raw material for concrete producers. The STET separation process with feed pre-processing equipment for drying and deagglomerating landfilled fly ash is an attractive option for ash beneficiation with significantly lower cost and lower emissions compared to other combustion and flotation based systems.

REFERENCES

[1]American Coal Ash Coal Combustion products and Use Statistics: http://www.acaa- usa.org/Publications/Production-Use-Reports.

[2]ST internal report, Agost 1995.

[3]Li (A prop de,T.X,. Schaefer, J.L., Prohibició, H., Neathery, J.K., and Stencel, J.M. Dry Beneficiation Processing of Combustion Fly Ash, Proceedings of the DOE Conference on Unburned Carbon on Utility Fly Ash, Maig 19 20, Pittsburgh, PA, 1998.

[4]Baltrus, J.P., Diehl, J.R., Soong, I., Sands, W. Triboelectrostatic separation of fly ash and charge reversal, Fuel 81, (2002) pp.757-762.

[5]Cangialosi, F., Notarnicola, M. M., Liberti, L, Stencel, J. The role of weathering on fly ash charge distribution during triboelectrostatic beneficiation, Journal of Hazardous Materials, 164 (2009) pp.683-688.

AUTHORS

Lewis Baker is the European Technical Support Manager for ST Equipment & Tecnologia (STET) based in the United Kingdom

Abhishek Gupta is a Process Engineer based at the Separation Technologies pilot plant and lab facility, STET Technical Center, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300

Dr. Stephen Gasiorowski, Ph.D. is a Senior Research Scientist for ST Equipment & Tecnologia (STET) based in the New Hampshire.

Frank Hrach is Vice President of Process Engineering based at the Separation Technologies pilot plant and lab facility, STET Technical Center, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300