Triboelectrostatic Beneficiation zemljišta ispunjen i letećeg pepela u jezero

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Triboelectrostatic Beneficiation of

Land Filled and Ponded Fly Ash

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

Američki ugljena pepela Udruga (ACAA) godišnji pregled proizvodnje i korištenja ugljena pepela izvještava da je između 1966 i 2011, preko 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. Međutim, 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%, približno 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.

Nedavno, 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. Međutim, 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+ godine, 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 oprema & 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

U STET separatoru ugljika (Slika 1), materijal se hrani u tanki jaz između dvije paralelne planarne elektrode. Čestice su utrobno optužene od strane međučestica.. The positively charged carbon and the negatively charged mineral (in freshly generated ash that has not been wetted and dried) privlače ih suprotne elektrode.. Čestice su onda zahvatio neprekinutom pojasu kreće i prenio u suprotnim smjerovima. Pojas potezi uz svake elektrode prema suprotnim stranama separator čestica. Visoki pojas brzinom omogućuje vrlo visoku propusnost, do 36 tona na sat na jednom razdjelnika. Mali razmak, polja visokog napona, brojač protoka, 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. Kontroliranjem raznih parametara procesa, kao što je brzina pojasa, feed za izabrati, i brzina hranjenja, STET proces proizvodi niski LOI leteći pepeo pri sadržaju ugljika manjem od 1.5 da 4.5% od feed muha pepeo u rasponu u LOI od 4% na više 25%.

Smokvu. 1 STET Separator processing dried, landfilled fly ash

Razdjelnik dizajn je relativno jednostavna i kompaktna. Mašina dizajnirana za obradu 40 tona na sat je približno 30 FT. (9 m.) Dugo, 5 FT. (1.5 m.) Široko, i 9 FT., m (2.75 m.) Visoko. Jedini pokretni dijelovi su pojas i povezan Valjci. Elektrode su stacionarni i sastoji se od odgovarajuće izdržljiv materijal. The belt is made of non-conductive plastic. The separator’s power consumption is about 1 Kilowatt-sat po tonu materijala obrađen sa većinom energije koju su konzumirali dva motora koja voze remen..

Proces je potpuno suh., ne zahtijeva dodatne materijale osim mušice pepeo i ne proizvodi otpadne vode ili emisije zraka. Pronađeni materijali se sačinjavaju od mušice pepeo smanjenog u sadržaju ugljika na nivoe pogodan za upotrebu kao pozzolanski admiksture u betonu, i visok ugljicni razlomak koristan kao gorivo. Utočište oba toka proizvoda pruža 100% rješenje za letenje problema s odlaganjem pepela.

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% i 27% water as is typical for landfilled material. The samples also contained varying amounts of large >1/8 cm (~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, 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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Slika 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.

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.

Tablica 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.

Tablica 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 (Vidi stol 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.

Tablica 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. Tablica 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.

Tablica 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, brend pod imenom ProAsh®, odvajanje STET obraditi i obnova inače izgubiti neizgorenu ugljik u obliku ugljik-bogat letećeg pepela, potpuno novo EcoTherm™. EcoTherm™ ima značajne goriva vrijednost i mogu lako vratiti do električne centrale pomoću STET EcoTherm™ Return system to reduce the coal use at the plant. Kada EcoTherm™ je izgorjela u korisnost kotla, energija izgaranja se pretvara u visokim tlakom / Visoka temperatura parne i onda na struju na istu učinkovitost kao ugljen, Obično 35%. The conversion of the recovered thermal energy to electricity in ST Equipment & Technology LLC EcoTherm™ Povratni sistem je dva do tri puta veći od konkurentne tehnologije gdje se energija oporavlja kao nisko-razredna toplota u obliku tople vode koja cirkulira u sistem vode za kotlovsku hranu. EcoTherm™ također se koristi kao izvor alumine u cementnim kilama, rasklapanje skupljeg boksita koji se obično prevozi na velike udaljenosti. Utilizing the high carbon EcoTherm™ pepeo ili u elektrani ili cementna kila, maksimizira energetski oporavak od isporučene ugljena, smanjenje potrebe za moje i prijevoz dodatno gorivo od sadržaja.

STET’s Talen Energy Brandon Shores, IRENA SMEPA. Sutra, NBP Belledune, RWEnpower Didcot, EDF energije zapadni Burton, RWEnpower Aberthaw, and the Korea South-East Power fly ash plants all include EcoTherm™ Povratak sustavi.

OBJEKTI ZA OBRADU STET PEPELA

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, U.K., Poljska, and Republic of Korea. ProAsh® pepela je odobren za upotrebu preko dvadeset državnih autocesta vlasti, kao i mnoge druge agencije specifikacije. ProAsh® također certificirani pod kanadski standarda udruge i EN 450:2005 standardi kvalitete u Europi. Ash processing facilities using STET technology are listed in Table 5.

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Tablica 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. Dodatno, 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.

REFERENCE

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

[2]ST internal report, August 1995.

[3]Lija,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, Svibanj 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 is the European Technical Support Manager for ST Equipment & Tehnologija (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 & Tehnologija (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