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CHEMICAL TREATMENT PRIOR TO PHYSICAL COAL BENEFICIATIONA. B. Walters

Florida Power & Light CompanyP.O. Box 14000Juno Beach, Florida 33408

ABSTRACTConventional wisdom dictates that chemical treatment of coal shouldfollow physical beneficiation. This ordering is expected to achievereduced consumption and improved access of chemical reagents tothe smaller, partially demineralized coal particles which resultfrom physical cleaning. The advantages of reversing thisconventional order of treatment, particularly for coaldemineralization, are often overlooked. Chemical pretreatmentcan greatly improve the grindability of coal prior to the finalsize reduction required for physical beneficiation. The use ofinexpensive chemical reagents can greatly reduce the disadvantageof higher reagent consumption. Prior chemical treatment can alsoselectively remove inorganic components which adversely affectcoal ash behavior in combustion equipment. The effectivenessof subsequent physical coal demineralization can be improved bychemical pretreatment, especially for physical processes whichexploit surface chemistry. Examples describing the advantagesof chemical processing prior to physical coal beneficiation areincluded in this talk.

INTRODUCTIONCoal beneficiation research and development programs are commonlytargeted to the dual objectives of sulfur and gross mineral matterremoval. Many programs are primarily guided by environmentalnew source performance standards (NSPS) for coal-burning facilities.Attractive technical approaches to coal beneficiation which cannotmeet NSPS are given limited attention. Beneficiation objectivesin addition to sulfur and gross mineral matter removal, such asimproved combustion behavior, reduced slagging and fouling, improvedgrindability, and compatibility with emission control equipment,are often ignored.Earlier efforts to meet NSPS and restrictive oil-backoutrequirements concentrated on coal hydroliquefaction. These effortsyielded conceptual processes capable of producing highlydesulfurized and demineralized coal-based fuels suitable forreplacing petroleum-based fuels or feedstocks. High projectedprocess capital costs and modest overall energy yields havediscouraged the further development of these conceptual processes.Relatively high energy yields and relatively low capital andoperating costs are achievable with many commercially availablecoal beneficiation processes. Unfortunately, these processesare capable of only modest reductions in sulfur and gross mineralmatter. Newer, conceptual processes are again embarking on thepath of "super-clean" coal, the same path that led to

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n o n- c om p e ti t iv e p r o d u c t c o s t s i n t h e case o f h y d r o l i q u e f a c t i o n .O t h e r , l e s s a m b i t i o u s a p p r oa c h e s c a p a b l e o f i m pr ov in g t h ep e rf o rm a n ce o f c o a l - b u r n i n g e q u ip m e nt v i a co a l b e n e f i c i a t i o n m a yb e more p r a c t i c a l . I n p a r t i c u l a r , i n e x p en s i v e c he mi ca l t r e a t m e n tp r i o r t o n e a r- c o n ve n t io n a l p h y s i c a l c o a l b e n e f i c i a t i o n may a c h i ev emany c o a l b e n e f i c i a t i o n o b j e c t i v e s .COAL BENEPICIATION OBJECTIVES

Coal c an b e b e n e f i c i a t e d t o meet a v a r i e t y o f o b j e c t i v e s r e l a t e dt o i t s u s e a s a h e at - pr o du c in g f u e l . T he se o b j e c t i v e s i n c l u d e :

r e d u c i n g s u l f u r c o n t e n t f o r r e d u c ed SOx e m i s s i o n s :r e d u c in g g r o s s m i n e ra l m a t t e r c o n t e n t f o r re du ce d t o t a lp a r t i c u l a t e e m is s io n s and a s h p r o d u c t i o n ;r e d u c i n g m o i s t u r e l e v e l s f o r im prov ed e f f i c i e n c y ;i mp ro vi ng g r i n d a b i l i t y f o r r ed uc ed g r i n d i n g c o s t s :

O p r e s e r v i n g c om b us ti bl e v o l a t i l e s c o n t e n t f o r n eededco m b u s t i o n b eh av i o r ;i m pr ov in g c a r b o n b u rn o ut c h a r a c t e r i s t i c s f o r im pro vede f f i c i e n c y and c o m p a t i b i l i t y w i t h p a r t i c u l a t e c o n t r o lequ ipment ; andr e d u c i n g s e l e c t e d m i n e r a l c o mp on en ts t o r e d u c e s l a g g i n gand f o u l i n g c h a r a c t e r i s t i c s of t h e c o a l a sh .

Many o f t h e s e o b j e c t i v e s a r e i n c o n f l i c t . Reduced SO, i n f l u eg a s e s c an r e du c e t h e e f f i c i e n c y of e l e c t r o s t a t i c p r e c i p i t a t o r sd u e t o h i gh p a r t i c u l a t e e l e c t r i c a l r e s i s t i v i t i e s . Gro s s m i n e r a lmatter r e d u c t i o n c a n c h an ge t h e c o m p o si t i on of r e s u l t i n g c o a la s h l e a d i n g t o l o w e r c o a l a s h f u s i o n t e m p e r a t u r e s an d i n c r e as e ds l a g g i n g and f o u l i n g ; t h i s may i n some cases be t h e r e s u l t ofc o n ta m i na t io n by b e n e f i c i a t i o n a i d s , s u c h a s m a g n e t i t e f i n e sc a r r i e d - o v e r d u r i n g h e av y m ed ia s e p a r a t i o n s . S e v e r e c h em i ca lt r e a t m e n t s t o remove s u l f u r a nd m i n e ra l matter may s i g n i f i c a n t l yr ed uc e t h e c o m b u s ti b l e v o l a t i l e s c o n t e n t of t h e c o a l . Very l o wm i n e r a l m a t t e r l e v e l s c a n l e a d t o h i g h u n bu rn ed c a r b o n l e v e l si n f l y a s h p a r t i c u l a t e s w hic h may r e du c e t h e e f f i c i e n c y ofe l e c t r o s t a t i c p r e c i p i t a t o r s d u e t o low p a r t i c l e e l e c t r i c a lr e s i s t i v i t y .D e sp i t e t h e s e c o n f l i c t s , a few g e n e r a l o b j e c t i v e s c an be s e l e c t e df o r t o p p r i o r i t y . Co al p y r i t e s c o n t r i b u t e t o i n c r e a s e d SOx andp a r t i c u l a t e e m i s s i o n s and t o i n c r e a s e d s l a g g i n g a nd f o u l i n g duet o i n c re a s e d i r o n i n t h e c o a l a s h . P y r i t e re mova l i s a n i m p o r t a n to b j e c t i v e f o r c o a l b e n e f i c i a t i o n . Modern c o a l b u r n in g a p p l i c a t i o n st y p i c a l l y r e q u i r e f i n e coa l g r i n d i n g ( c a . ~ 2 0 0m esh ) . T h i s i st r u e f o r d r y , p u l v e r i z e d c oa l f i r i n g o r f o r c o a l- w at e r m i x tu r ef u e l s i n t e n d e d f o r o i l -b a c k o ut . Improved c o a l g r i n d a b i l i t y woulds i g n i f i c a n t l y r e d u c e t h e s ub se qu en t c o s t s of c o a l u t i l i z a t i o n .T h e p r e s e r v a t i o n of c om bu st ib le v o l a t i l e s i n t h e c o a l i s a l s oi m p o r t a n t . I t i s e s p e c i a l l y i m p o r ta n t f o r co a l /w a t e r m i x tu r ef u e l s due t o t h e h ig h m o is tu re l e v e l s ( c a . 3 0 - 4 0 w e i g h t p e r c e n t

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c

water) in these fuels. The preservation of combustible volatileslimits the severity, particularly temperatures, which can beemployed in coal beneficiation. Pyrite removal from many coalscan be improved by fine grinding. It can also be improved iffresh pyrite particle surfaces can be rendered more hydrophilicprior to froth flotation for mineral matter removal. The selectiveremoval of alkali and alkaline earth minerals would also bedesirable due to the resulting increase in coal ash fusiontemperatures which would reduce slagging and foaling. In thecase of dry, pulverized coal firing, the high costs of dewateringfine coal prior to burning might preclude the application of frothflotation following fine grinding.These refined coal beneficiation objectives point towards a chemicaltreatment aimed at coal mineral matter, particularly the interfacesbetween mineral matter and the carbonaceous coal macerals. Thistreatment, not so very distinct from developing chemical comminutiontechnologies should greatly improve subsequent grindability,significantly increase coal ash fusion temperatures, and preservecombustible volatiles. An additional objective of low costsrequires that inexpensive reagents and treatment equipment beemployed. An approach which may achieve these coal beneficiationobjectives is currently under investigation at the BrookhavenNational Laboratory (BNL). A pressurized carbon dioxide/watermixture is used as the chemical reagent for chemico-physical coalbeneficiationcl)

CAEMICO-PHYSICAL COAL BENEPICIATIONGaseous carbon dioxide is known to penetrate coal(2) and to improvecoal grindability(3). Unlike chemical comminution caused byammonia(4), water enhances the swelling and fracturing of coalcaused by carbon dioxide(1). The carbon dioxide/water mixtureis reported to have a synergy which enhances the carbon dioxidefracturing of coal while selectively removing alkali and alkalineearth minerals. Significant improvements in coal grindabilitywith very high retention of overall energy content and coalvolatiles are reported. Subsequent sink/float studies haveindicated selective fracturing of the interfaces of the mineralmatter and the carbonaceous coal macerals 5). Treatment effectson pyrite surface hydrophilicity for coal ground in the pressurizedcarbon dioxide/water mixture remain to be determined. Suchimprovements in the hydrophilic nature of fresh pyrite surfaceshave been reported for aqueous sodium carbonate treatment at theSame temperatures used in the BNL chemico-physical treatment (ca.8O"CI (6).A major drawback to this potentially effective coal beneficiationtreatment is the costs of achieving the treatment conditions (ca.750 psi, 80C). An investigation of the effects of treatmentconditions on coal beneficiation is being initiated to allow foran engineering study to evaluate the commercial potential of theBNL approach to coal beneficiation.

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CONCLUSIONS

The BNL carbon dioxide/water coal beneficiation treatment showspromise for meeting important coal beneficiation objectives. Thistreatment appears suitable for beneficiating coals for dry,pulverized coal firing without subsequent physical coalbeneficiation or for coal/water mixture fuels with subsequentphysical beneficiation, e.g. froth flotation. Froth flotationis already included in the processes for some of the proposedcommercial coal/water mixture preparation facilities. Theeffectiveness of the BNL treatment for enhancing pyrite surfacehydrophilicity needs further study. Also needed is further studyof the commercial potential of this coal beneficiation approach.Engineering studies await the results of more detailedinvestigations of the effects of varied treatment conditions.

REFERENCES1. Sapienza, R. S . , Slegeir, W. A., Butcher, T., and Healy, F.,I. Chem. E. Symp. Ser. No. 83, 85 (1983).2. Fuller, E . L., Jr. "Coal Structure" M. L. Gorbaty and K. Ouchi,Eds., Adv. Chem. Ser., 192, 293 (1981).3. Czaplinski, A . , and Holda, S., Fuel, 61 (12), 1281-2 (1982).4. Silveston, P. L., Hudgins, R. R., Spink, D. R., Smith, B.,and Mathieu, G, "Coal: Phoenix '80's" A. M. A1 Taweel, Ed.,Proc. 64th CIC Coal Symp., 1, 77-83 (1982).5 . Sapienza, R. S., personal communication.6. Patterson, E. C., Report No, IS-ICP-64, EMRRI, Iowa StateUniversity, Ames, Iowa, Sept. 1978.

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