Fè chwa lang:
Asosyasyon Ash chabon Ameriken (ACAA) ankèt anyèl pwodiksyon ak itilize chabon vole ash te rapòte ke ant 1966 ak 2011, sou 2.3 milya tòn talè a vole te pwodwi pa te tire chabon itilite chodyè pou chofaj. Nan montan sa a, apeprè 625 milyon tòn te beneficially itilize, sitou pou pwodiksyon siman ak beton. Sepandan, ki te rete a 1.7+ milya tòn premyèman ap jwenn nan décharges oubyen te ranpli ponded impoundments.
Beneficiation Triboelectrostatic a Décharge ak Ponded vole
By Lewis Baker,Abhishek Gupta, Stephen Gasiorowski, and Frank Hrach
Asosyasyon Ash chabon Ameriken (ACAA) ankèt anyèl pwodiksyon ak itilize chabon vole ash te rapòte ke ant 1966 ak 2011, sou 2.3 billion short tons of fly ash were produced by coal-fired utility boilers.1 Of this amount, apeprè 625 milyon tòn te beneficially itilize, sitou pou pwodiksyon siman ak beton. Sepandan, ki te rete a 1.7+ milya tòn premyèman ap jwenn nan décharges oubyen te ranpli ponded impoundments. While use rates for freshly generated fly ash have increased considerably over recent years, ak yon pousantaj aktyèl tou pwe 45%, apeprè 40 milyon tòn a vole kontinye pou jete mò nan chak ane. While use rates in Europe have been much higher than in the United States, konsiderab volumes a vole te tou te sere nan décharges ak impoundments nan kèk peyi Ewopeyen yo
Depi kèk tan, enterè nan apwè dekouvèt sa a disposé materyèl yo kontinye ogmante, ò/demi akòz a demand lan pou bon kalite vole ash pou beton ak siman pwodiksyon pandan yon peryòd diminye pwodiksyon tankou chabon te tire pouvwa pitit an pitit te diminye an Ewòp ak Amerik di Nò. Kesyon sou yon tan ki long anviwònman enpak décharges konsa tou ap pouse peman pou sèvis piblik pou jwenn benefik itilize aplikasyon pou sa a mande pou lontan.
Pandan tout tan sa, gen kèk depo mande pou vole ka fè fèt nan Kiba pou itilize avantaje ke okòmansman an te defouye pyès, the vast majority will require some processing to meet quality standards for cement or concrete production. Because 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 because concrete producers will want to continue the practice of batching fly ash as a dry, fine powder. Sepandan, 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 use has increased in the last 20+ ane, most “in-spec” ash has been beneficially used, ak tout bon kalite debwi mò. Konsa, LOI reduction will be a requirement for using the vast majority of fly ash recoverable from utility impoundments.
While other researchers have used combustion techniques and flotation processes for LOI reduction of recovered landfilled and ponded fly ash, Sen ekipman & Teknoloji (STET) has found that its unique triboelectrostatic belt separation system, lontan te itilize pou beneficiation de prije généré mande pou vole, 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. Sa a mande récupérés pou separe trè menm pou mande pou prije te pwodwi ak yon diferans etone: 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 phenomenon.3-5 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 his phenomena
Nan rele STET séparateur kabòn (Pye fig. 1), materyèl nouri nan ti gap ant de paralèl) électrodes. Matyè yo triboelectrically yo akize interparticle kontak. Kabòn pozitivman accusé a ak a accusé négative ineral (nan prije généré mande sa yo ki pa te humide e secs) a pou opoze électrodes. Matyè yo lè sa a te baleye pa yon senti dwa kontinyèl, transmis nan direksyon yo bò. Kouwa a deplase matyè adjasan pou chak lektwòd anvè toupatou bò la séparation. The high belt speed also enables very high throughputs up to 36 ton pa edtan nan yon sel separateur. A ti gap, high-voltage field, counter—current flow, vigorous particle-particle agitation, and self-cleaning action of the belt on the electrodes are the critical features of the STET separator. Pa okipe kay yo plizyè karakteristik pwosesis, tankou senti vitès, bay manje pwen, ak manje bèt yo ki te konn, pwosesis la te rele STET pwodwi ba LOI vole ash kabòn sa ki nan liv la nan mwens ke 1.5 pou 4.5% nan manje vole sann ki sòti nan LOI de 4% pou fini 25%.
Plan séparateur relativman senp ak kontra. Yon machin ki fèt pou travay 40 ton pa edtan apwoksimativman 30 - (9 m) lontan, 5 - (1.5 m) lajè, ak 9 - (2.75 m) wote. Kouwa ak asosye woulèt se sèl kalite manm. Électrodes èstasyone ak reyalize yon materyèl ki fè a se sa dirab. The belt is made of nonconductive plastic. Pouvwa konsomasyon la séparation se osijè de 1 kilowat-edtan pou chak ton materyel trete ak pi fo nan pouvwa a boule pa de mote kondwi senti a.
Pwosè a chèch nèt, requires no additional materials other than the fly ash, and produces no waste water or air emissions. 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. Use of both product streams provides a 100% solisyon vole sann dispozisyon pwoblem.
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 ak 27% water, as is typical for landfilled material. The samples also contained varying amounts of large >1/8 Nan. (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 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 flowchart is presented in Fig. 2.
The properties of the prepared samples were well within the range of mande pou vole 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.
STET separator processing dried, landfilled fly ash
Process flow diagram
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 previously 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.
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 M 250 standards. Tab 2 summarizes the chemistry for samples from two of the sources showing the insignificant difference between the fresh and reclaimed material.
Strength development of a 20% substitution of the low-LOI fly ash in a mortar containing 600 lb/yd3 cementitious material (see Table 3) 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.
The availability of low-cost natural gas in the United States greatly enhances the economics of drying processes, including the drying of wetted fly ash from landfills. Tab 4 summarizes the fuel costs for operations in the United States for 15% ak 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.
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. Because the only additional air emission source to the standard STET process installation is a natural gas-fired dryer, permitting it would be relatively simple.
| Feed sample to separator | LOI, % | ProAsh LOI, % | ProAsh fineness, % +325 may |
ProAsh mass yield, % |
|---|---|---|---|---|
| Fresh A | 10.2 | 3.6 | 23 | 84 |
| Landfilled A | 11.1 | 3.6 | 20 | 80 |
| Fresh B | 5.3 | 2.0 | 13 | 86 |
| Landfilled B | 7.1 | 2.0 | 15 | 65 |
| Fresh C | 4.7 | 2.6 | 16 | 82 |
| Landfilled C | 5.7 | 2.5 | 23 | 72 |
| Landfilled D | 10.8 | 3.0 | 25 | 80 |
| Material source | Sio2 | Al2O3 | Fe2O3 | Kao | MgO | K2O | Na2O | SO3 |
|---|---|---|---|---|---|---|---|---|
| Fresh B | 51.60 | 24.70 | 9.9 | 2.22 | 0.85 | 2.19 | 0.28 | 0.09 |
| Landfilled B | 50.40 | 25.00 | 9.3 | 3.04 | 0.85 | 2.41 | 0.21 | 0.11 |
| Fresh C | 47.7 | 23.4 | 10.8 | 5.6 | 1.0 | 1.9 | 1.1 | 0.03 |
| Landfilled C | 48.5 | 26.5 | 11.5 | 1.8 | 0.86 | 2.39 | 1.18 | 0.02 |
| 7-day compressive strength, % of fresh ash control | 28-day compressive strength, % of fresh ash control | |
|---|---|---|
| Fresh B | 100 | 100 |
| Landfilled B | 107 | 113 |
| Fresh C | 100 | 100 |
| Landfilled C | 97 | 99 |
| Moisture content, % | Heat requirement KWhr/T wet basis | Drying cost/T dry basis (natural gas cost $3.45/mmBtu) |
| 15 | 165 | $ 2.28 |
| 20 | 217 | $ 3.19 |
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 has significant fuel value and can easily be returned to the electric power plant using the STET EcoTherm Return system to reduce the coal use at the plant. When EcoTherm is burned in the utility boiler, the energy from combustion is converted to high-pressure/high-temperature steam and then to electricity at the same efficiency as coal, typically 35%. The conversion of the recovered thermal energy to electricity in the STET 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. Using 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. Demen, 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’s separation process has been used commercially since 1995 for fly ash beneficiation and has generated over 20 million tons of high-quality fly ash for concrete production. Controlled low-LOI ProAsh is currently produced with STET’s technology at 12 power stations throughout the United States, Kanada, the United Kingdom, Poloy, and the Republic of Korea. ProAshfly ash has been approved for use by more than 20 state highway authorities, as well as many other specification agencies. ProAsh has also been certified under the Canadian Standards Association and EN 450:2005 quality standards in Europe. Ash processing facilities using STET technology are listed in Table 5.
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. Additionally, power plants that need to remove ash from landfills to meet changing environmental regulations will be able to use the process to alter a waste product liability into a valuable raw material for concrete producers. The STET separation process with feed preprocessing 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. ❖
1. American Coal Ash Coal Combustion Products and Use Statistics, http://www.acaausa.org/Publications/Production-Use-Reports.
2. ST Internal Report, Aug. 1995.
3. Bay manti, T. X.; Schaefer, J. L.; Ban, H. H.; Neathery, J. K. K.; and Stencel, J. madanm, “Dry Beneficiation Processing of Combustion Fly Ash,” Proceedings of the DOE Conference on Unburned Carbon on Utility Fly Ash, Pittsburgh, PA, Me 19-20, 1998.
4. Baltrus, J. P. P.; Diehl, J. R.; Soong, Yo.; and Sands, W. W., “Triboelectrostatic Separation of Fly Ash and Charge Reversal,” Fuel, v. 81, 2002, pp. 757-762.
5. Cangialosi, F.; Notarnicola, madanm; Liberti, L.; and Stencel, J., “The Role of Weathering on Fly Ash Charge Distribution during Triboelectrostatic Beneficiation,” Journal of Hazardous Materials, v. 164, 2009, pp. 683-688.
Lewis Baker is the European Technical Support Manager for ST Equipment & Teknoloji (STET) based in the United Kingdom
Abhishek Gupta is a Process Engineer based at the STET pilot plant and lab facility in Needham, GRAN.
Stephen Gasiorowski is a Senior Research Scientist for ST Equipment & Teknoloji (STET) based in New Hampshire.
Frank Hrach is Vice President of Process Engineering based at the STET pilot plant and lab facility in Needham, GRAN.
| Utility and power station | Kote yo ye | Start of commercial operations | Facility details |
|---|---|---|---|
| Duke Energy—Roxboro Station | North Carolina | Sept. 1997 | 2 separators |
| Talen Energy—Brandon Shores Station | Maryland | Avr. 1999 | 2 separators 35,000 ton storage dome Ecotherm Return 2008 |
| ScotAsh (Lafarge / Scottish Power Joint Venture)—Longannet Station | Scotland, RETIRE | Okt. 2002 | 1 separator |
| Jacksonville Electric Authority— St. John’s River Power Park, fl | Florida | Me 2003 | 2 separators Coal/petcoke blends Ammonia removal |
| South Mississippi Electric Power Authority R.D. Morrow Station | Mississippi | Jan. 2005 | 1 separator Ecotherm return |
| New Brunswick Power Company Belledune Station | New Brunswick, Kanada | Avr. 2005 | 1 separator Coal/petcoke blends Ecotherm return |
| RWE npower Didcot Station | England, U | Aug. 2005 | 1 separator Ecotherm return |
| Talen Energy Brunner Island Station | Pennsylvania | Desanm. 2006 | 2 separators 40,000 ton storage dome |
| Tampa Electric Co. Big Bend Station | Florida | Avr. 2008 | 3 separators, doub pase 25,000 ton storage dome Ammonia removal |
| RWE npower Aberthaw Station (Lafarge Cement UK) | Wales, RETIRE | Sept. 2008 | 1 separator Ammonia removal Ecotherm return |
| EDF Energy West Burton Station (Lafarge Cement UK, Cemex) | England, RETIRE | Okt. 2008 | 1 separator Ecotherm return |
| ZGP (Lafarge Siman Poloy / Ciech Janikosoda JV) | Poloy | Mar. 2010 | 1 separato |
| Korea South-East Power Yeongheung Units 5&6 | Kore di sid | Sept. 2014 | 1 separator Ecotherm return |
| PGNiG Termika-Siekierki | Poloy | Scheduled 2016 | 1 separator Ecotherm return |
| To Be Announced | Poloy | Scheduled 2016 | 1 separator Ecotherm return |
