Triboelectrostatic benefiċjazzjoni ta ' l-art mimlija u Ponded Fly Ash

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Triboelectrostatic benefiċjazzjoni ta ' l-

Land Filled and Ponded Fly Ash

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

The American Coal Ash Association (ACAA) annual survey of production and use of coal fly ash reports that between 1966 u l- 2011, Fuq 2.3 billion short tons of fly ash have been produced by coal-fired utility boilers.1 Of this amount approximately 625 million tons have been beneficially used, mostly for cement and concrete production. Madankollu, 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%, Madwar 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.

Riċentement, 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

While some of this stored fly ash may be suitable for beneficial use as initially excavated, the vast majority will require some processing to meet quality standards for cement or concrete production. Since the material has been typically wetted to enable handling and compaction while avoiding airborne dust generation, 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. Madankollu, assuring the chemical composition of the ash meets specifications, most notably the carbon content measured as loss-on-ignition (LIĠI), is a greater challenge. As fly ash utilization has increased in the last 20+ Snin, most “in-spec” ash has been beneficially used, and the off-quality ash disposed. Għalhekk, 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 Tagħmir & 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: l-imponiment ta ' partiċella kien imdawwar minn dak ta ' l-irmied frisk mal-karbonju iċċarġjar negattivi fir-rigward tal-minerali.2 Riċerkaturi oħra ta ' separazzjoni elettrostatiċi ta ' l-irmied li jtir karbonju kienu wkoll osservati dan il-fenomenu.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. L-ebda modifikazzjonijiet speċjali lil-separatur disinn jew kontrolli huma meħtieġa biex jakkomodaw dan il-fenomenu.

TEKNOLOĠIJA ĦARSA ĠENERALI – IS-SEPARAZZJONI TAL-KARBONJU TAL-IRMIED LI JTIR

Fis-separatur tal-karbonju STET (Il-figura 1), il-materjal jiddaħħal fid-distakk irqiq bejn żewġ elettrodi planari paralleli. Il-partiċelli huma triboelectrically mitluba mill-kuntatt interparticle. The positively charged carbon and the negatively charged mineral (in freshly generated ash that has not been wetted and dried) huma attirati lejn elettrodi opposti. Il-partiċelli mbagħad jiġu mimsuħa minn ċinturin kontinwu li jiċċaqlaq u jitwasslu f'direzzjonijiet opposti. Iċ-ċintorin imexxi l-partiċelli maġenb kull elettrodu lejn truf opposti tas-separatur. Il-veloċità taċ-ċintorin għolja wkoll jippermetti produzzjoni għolja ħafna, sa 36 tunnellati fis-siegħa fuq separatur wieħed. Id-distakk żgħir, qasam ta 'vultaġġ għoli, fluss tal-kurrent tal-kontro, 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. Billi jikkontrollaw parametri varji ta ' proċess, bħalma huma l-veloċità taċ-ċintorin, punt ta ' l-għalf, u r-rata tal-għalf, il-proċess STET jipproduċi rmied li jtir LOI baxx f'kontenut ta' karbonju ta' inqas minn 1.5 biex 4.5% minn irmied li jtir fl-għalf li jvarja f'LOI minn 4% għal aktar 25%.

Fig. 1 STET Separator processing dried, landfilled fly ash

Id-disinn tas-separatur huwa relattivament sempliċi u kompatt. Magna disinjata biex tipproċessa 40 tons per hour is approximately 30 ft. (9 m.) twil, 5 ft. (1.5 m.) Wiesgħa, u l- 9 FT., m (2.75 m.) għoli. Il-belt u rombli assoċjati huma l-partijiet li jiċċaqalqu biss. L-elettrodi huma wieqfa u magħmula minn xi materjal dejjiemi kif jixraq. The belt is made of non-conductive plastic. The separator’s power consumption is about 1 kilowatt-sieg ħa għal kull tunnellata ta ' materjal ipproċessat bil-bi ċċa l-kbira tal-ener ġija kkunsmata minn żewġ muturi li jsuqu ċ-ċintorin.

Il-pro Ċess huwa kompletament niexef, ma jeħtieġ l-ebda materjal addizzjonali għajr l-irmied li jtir u ma jipproduċi l-ebda ilma mormi jew emissjonijiet tal-arja. Il-materjali rkuprati jikkonsistu f'irmied li jtir imnaqqas fil-kontenut tal-karbonju għal livelli adattati għall-użu bħala taħlita pozzolanika fil-konkrit, u frazzjoni għolja ta' karbonju utli bħala fjuwil. Użu ta ' żewġ nixxigħat prodott jipprovdi a 100% soluzzjoni għall-problemi ta ' rimi ta ' rmied.

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. Due to the intermingling of material in the landfills, no further information is available concerning specific coal source or combustion conditions.

The samples as received by STET contained between 15% u l- 27% water as is typical for landfilled material. The samples also contained varying amounts of large >1/8 inch (~3 mm) material. To prepare the samples for carbon separation, 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, sistema ta ' l-ipproċessar li joffri tnixxif u deagglomeration meħtieġa għal separazzjoni effettiva elettrostatiċi simultanju ta ' l-għalf. F'karta ta ' fluss tal-proċess ġenerali jkun ppreżentati fil-Figura 2.

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Il-figura 2: Process Flow Diagram

Il-propjetajiet tal-kampjuni preparati kienu ukoll fi ħdan il-medda ta ' l-irmied li jtir miksub direttament mill-utilità normali kaldaruni. Il-proprjetajiet aktar relevanti għall-separatur feeds u l-prodotti huma riassunti fit-tabella 2 flimkien ma ' prodott irkuprat.

IS-SEPARAZZJONI TAL-KARBONJU

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.

The properties of the low LOI fly ash recovered using the STET process for both freshly collected ash from the boiler and ash recovered from the landfill is summarized in Table 1. The results show that the product quality for ProAsh® produced from landfilled material is equivalent to product produced from fresh fly ash sources.

Tabella 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.

Tabella 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 (Ara t-Tabella 3 below) 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.

Tabella 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. Tabella 4 summarizes the fuel costs for operations in the USA for 15% u l- 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.

Tabella 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, marka bl-isem ProAsh®, il-proċess ta' separazzjoni tal-STET jirkupra wkoll karbonju mhux maħruq mod ieħor fil-forma ta' rmied li jtir b'kontenut ta' karbonju, EkoTherm tad-ditta™. Eotherm™ għandu valur sinifikanti tal-fjuwil u jista 'jiġi rritornat faċilment fl-impjant tal-enerġija elettrika billi juża l-STET EcoTherm™ Return system to reduce the coal use at the plant. Meta EkoTherm™ jinħaraq fil-bojler tal-utilità, l-enerġija mill-kombustjoni tiġi kkonvertita għal pressjoni għolja / fwar b'temperatura għolja u mbagħad għall-elettriku bl-istess effiċjenza bħall-faħam, Tipikament 35%. The conversion of the recovered thermal energy to electricity in ST Equipment & Technology LLC EcoTherm™ Is-sistema tar-ritorn hija darbtejn sa tliet darbiet ogħla minn dik tat-teknoloġija kompetittiva fejn l-enerġija tiġi rkuprata bħala sħana ta' grad baxx fil-forma ta' ilma sħun li jiġi ċċirkolat lis-sistema tal-ilma tal-għalf tal-bojler. Eotherm™ jintuża wkoll bħala sors ta' alumina fil-fran tas-siment, tneħħi l-boksajt l-aktar għali li ġeneralment jiġi ttrasportat fuq distanzi twal. Użu tal-EkoTherm b'karbonju għoli™ irmied jew f'impjant tal-enerġija jew f'forn tas-siment, timmassimizza l-irkupru tal-enerġija mill-faħam ikkunsinnat, it-tnaqqis tal-ħtieġa li l-minjieri u t-trasport ta' fjuwil addizzjonali għall-faċilitajiet.

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

FAĊILITAJIET GĦALL-IPPROĊESSAR TA' RMIED STET

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., Kanada, ir-Renju Unit, Il-Polonja, and Republic of Korea. ProAsh® irmied li jtir ġie approvat għall-użu minn aktar minn għoxrin Stat highway awtoritajiet, l-aġenziji speċifikazzjoni oħra kif ukoll kemm. ProAsh® ukoll ġiet ċertifikata taħt assoċjazzjoni Standards Kanadiżi u EN 450:2005 standards ta ' kwalità fl-Ewropa. Faċilitajiet ta ' ipproċessar irmied bl-użu ta ' teknoloġija ta ' l-STET huma elenkati fit-tabella 5.

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Tabella 5. Faċilitajiet ta ' ipproċessar ta ' l-irmied li jtir bl-użu tat-teknoloġija tas-separazzjoni tal-STET

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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. Addizzjonalment, 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.

REFERENZI

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

[2]ST internal report, awissu 1995.

[3]Li,T.X,. Schaefer, J.L., Ban, 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, jista ' 19 20, Pittsburgh, PA, 1998.

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

[5]Cangialosi, F., Notarnicola, 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 huwa l-Maniġer ta ' l-appoġġ tal-Ewropea tekniċi għal apparat ta ' ST & teknoloġija (STET) based in the United Kingdom

Abhishek Gupta hu-inġinier proċess ibbażat fil-separazzjoni teknoloġiji pilota tal-pjanti u lab faċilità, STET iċ-ċentru tekniku, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300

Dr. Stephen Gasiorowski, Ph.D. huwa Anzjan riċerka xjenzat għal apparat ta ' ST & teknoloġija (STET) ibbażata fil-New Hampshire.

Frank Hrach huwa il-Viċi President tal-proċess inġinerija bbażati fil-separazzjoni teknoloġiji pilota tal-pjanti u lab faċilità, STET iċ-ċentru tekniku, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300