COAL DESULFURIZATION TECHNIQUES: THEIR DRAWBACKS AND COSTS: COAL DESULFURIZATION TECHNIQUES: THEIR DRAWBACKS AND COSTS Gülhan Özbayoğlu Mining Engineering Department Middle East technical University , Ankara-TURKEY


Slide2: COAL: One of the world’s most abundant fossil fuel resources Not a clean fuel, contains ash and sulfur SOx : most important pollutant as a real treat to both the ecosystem and human health The continued production and consumption of coal should be based on the environmentally cleaner and economically effective coal technologies


SULFUR TYPES IN COAL : SULFUR TYPES IN COAL 1) Inorganic sulfur: Pyritic –S ( ~0.1-4%), unusual 8% Sulfate-S (~<0.2%), unusual<1% Elemental –S (rare), unusual <0.15% 2) Organic sulfur : (~0.3-2%), unusual case up to 10%


Slide4: Sulfate-S: Occurs in combination with either Ca or Fe The hydrated calcium sulfate- gypsum Remains in the ash during combustion and does not contribute to SOx pollution The amount of Sulfate –S increases with weathering The oxidation of Fe-sulfides gives rise to Fe++ and Fe+++ sulfates Iron sulfate is water soluble, and readily removed during coal cleaning


Slide5: Pyritic-S: Refer to iron disulfide (FeS2) as pyrite , marcasite and melnikovite-pyrite Pyrite is cubic in crystal structure Marcasite is orthorhombic Of the two, pyrite is dominant mineral Pyrite occurs in coals: As narrow seams or veins (Epigenetic) As nodules (from a few to several hundred microns in diameter ) As discrete crystals (1-40, ~1-2 microns) (Syngenetic)


Slide6: Organic-S: Chemically bonded, can not be removed by physical cleaning methods Dibenzothiophene is the most difficult to desulfurize The amount of organic sulfur defines the theoretical lowest limit at which a coal can be cleaned by physical methods.


Slide7: DESULFURIZATION METHODS TO CONTROL SO2 EMISSIONS 1. Desulfurization of coal prior to combustion (physical, chemical, microbial) 2. The removal of sulfur oxides during the combustion 3. The removal of sulfur oxides after the combustion (FGD) 4. Conversion of coal to a clean fuel by gasification and liquefaction


Slide8: COAL PREPARATION (physical coal cleaning) prior to combustion Has a big role in the reduction of SO2 pollution A technology which could be applied rapidly, with a limited rate of expenditure Most of the coal destined for coking is cleaned, but only 15-20% of the coal used for power generation is so treated


Slide9: Most of the commercial coal cleaning processes employ difference in relative density (as density of FeS2~3x organic fraction of coal) (Jigs, shaking tables, spirals, cyclones, dense medium separators etc) Flotation, oil agglomeration, flocculation utilize the difference in surface phenomena(wettability) between coal and mineral matter. Magnetic susceptibility is employed in magnetic separation


Slide10: Coal cleaning methods are effective on the liberated particles. The degree of deashing and desulfurization of coal achievable increases with comminution. Coal needs to be comminuted to fine sizes to liberate ash and sulfur containing minerals The extent of comminution depends upon the amount


Slide11: In the current practices (by classical methods) : Coal is crushed only to control the top coarse size (as classical coal cleaning methods do not separate ultrafine particles very efficiently, where novel beneficiation techniques may be required. In addition, ultrafine particles are difficult to handle and dewater).


Slide12: In a classical coal washing plant: The crushed coal may be separated into 3 fractions: 1. Coarse coal (+18mm), cleaned by jigs (Baum and Batac types)or heavy medium drums,tanks, or baths 2. Fine coal (18x0.5mm), cleaned by heavy medium cyclones, tables and spirals 3. Slime (-0.5mm), cleaned by hydrocyclones or froth flotation Batac jig for desulfurization: Treats 1/2 in.x 0 raw coal, 40% sulfur reduction Kelsey centrifugal jig: For fine and ultrafine coal cleaning (-0.106x0.038 mm)


Slide13: Centrifugal Separators: Dense medium cyclones, Vorsyl, Dynawhirlpool, Larcodems and the Triflo Water only cyclones(autogenous DM-cyclones) for the 0.5x0.15mm size instead of flotation ( they show poor Ep in the 0.18 to 0.225) Autogenous drum separator: Used in China, in small and medium capacity CPP, since 1990.


Slide14: Autogenous medium: Materials 6mm) Adv: Elimination of magnetic medium and its regeneration circuit. Centrifugal tables: In China, to treat very fine particles (-40 microns) Remove 55 to 77 % pyritic sulfur from coal.


Slide15: Coal spirals: Coal size range: Between the fine coal fraction of heavy medium cyclone and the coarse feed of froth flotation Take place of water only cyclones in the treatment of 2mmx0.25mm Adv: low costs, simple installations, effective performances


Slide16: High gradient magnetic separation (HGMS): A novel fine particle beneficiation technique, no commercial coal preparation application . Can separate weakly magnetic materials In coal industry, can be used: 1.Beneficiation of fine coal for removal of mineral matter, including pyritic sulfur, 2. To recover micronized magnetite from dense medium circuits.


Slide17: Advanced novel coal beneficiation techniques have been proposed to meet certain requirements: 1. Treatment of very finely ground coal 2. Rejection of both ash and sulfur from the coal 3. Treatment in an environmetally accepted manner 4. Performane in a cost efffcetive manner


Slide18: The techniques which have been or are near being commercially deployed: 1. Advanced flotation and flocculation 2. Selective (oil) agglomeration 3. Advanced heavy medium cycloning


Slide19: Advanced froth flotation : Shortcomings : 1. Pyrites and most clays tend to float with the coal (poor separation) 2. Oxidized and lower rank coals do not respond well to froth flotation 3. Conventional froth flotation circuits are often inefficient when particle size less than 50 microns


Slide20: Factors contribute the poor rejection of pyritic sulfur: 1. Hydraulic entrainment 2. Incomplete liberation (physically locked) 3. Induced hydrophobicity; surface contamination. Factors affecting the selectivity of pyrite: 1. pH of the pulp 2. Nature of the metal ions 3. Non-polar oils


Slide21: Coal-derived pyrite is more floatable than mineral pyrite Oily collectors and rigorous conditions enhanced the rate of coal-derived pyrite. Inorganic depressants (lime, ferric sulfate and K-dichromate) have a preference on depressing mineral pyrite, while organic depressants (polyxanthates) can efficiently depress both coal-derived and mineral pyrite.


Slide22: Modifications of coal flotation: 1. Carrier (autogenous) flotation 2. Surface bacterial conditioning and flotation 3. Two-stage flotation Carrier (autogenous) flotation: Employed to enhance the surface properties of coal, containing larger amounts of fines, Use coarse coal concentrate (28x270 mesh) as reagentized carrier for ultrafine coal (-270 mesh) flotation. The test results: Coal can be cleaned to low ash (2%) and low sulfur (92%)


Slide23: Bioflotation process: Pyrite is depressed by preconditioning in the presence of Acidithiobacillus ferrooxidans prior to coal flotation. Bacterial conditioning at pH 2.0 and 30 C, for 2.5 min. up to 4 h. Coal is floated by kerosene and MIBC


Slide24: Pyrite removal for coal-pyrite artificial mixture: >80% after 2.5 min preconditioning For r.o.m. coal, pyrite removal up to 23% For Turkish r.o.m. coal, after 4 h preconditioning, 77% pyritic sulfur removal was achieved Disadv: Large quantities of bacteria is required coupled with the extended production times (500 h)


Slide25: Two stage flotation: First stage: Standard coal flotation (high ash refuse and some of the coarser pyrite are removed as tailing) Second stage: First stage coal concentrate is repulped, float the remaining pyrite. Use a coal deprassant ( Aero Deprassant 633) A pyrite collector ( K-amyl xanthate) A frother (MIBC)


Slide26: Results in pilot scale plant: Up to 90% pyrite rejection with high clean coal recovery The cost of adding coal-pyrite flotation to an existing plant which already has conventional froth flotation is estimated as 4500 USD/tph


Slide27: Much development in coal flotation has occurred on the equipment rather than reagents. Development of new coal reagents are slow or non-existent. Two flotation technologies: 1. Column flotation 2. Air-sparged hydrocyclone (not commercialized) To minimize the costs of fine grinding and subsequent advanced processing, the process is: -Conventional precleaning of coal to reject coarse pyrite, -Fine grinding of the precleaned coal, -Advanced column flotation


Slide28: Precleaning coal: Reject 57.5% of the pyritic sulfur, 4.5% loss of heating value, Decreased the amount of coal treated in advanced column flotation to 48% of feed Advanced column flotation: Reject an additional 24.2% of the raw coal pyrite Overall, an 81.8% pyritic sulfur rejection with 90.2% combustible matter recovery The cost of cleaning was estimated for a commercial scale advanced flotation plant , producing a clean coal at a rate of 475 tph, as 27 USD/t (not including the cost of coal)


Slide29: Air sparged Hydrocyclone: Flotation is accomplished in a centrifugal field which is generated by the fluid flow in air-sparged hydrocyclone. Air sparged hydrocyclone has ability to process the coal, via froth flotation at high capacity, at least 100 times that of the conventional flotation equipment BY the use of 6 inch. Diameter air sparged hydrocyclone, for treating -100 mesh classifying cyclone overflow , 86 % pyrite rejection at a greater than 80% heating value.


Slide30: Oil Agglomeration The benefits of oil agglomeration : 1. very high recoveries (up to 99% of the heating value) 2. ability to collect the ultrafine coal 3. excellent dewatering characteristics 4. applicability to oxidized coal Disadv: 1. higher oil prices 2. use of large quantities of oil 3. nonselective for pyrite


Slide31: In spherical agglomeration process: The reagent dosages is significantly reduced The product is highly compacted and spherical Dewatering becomes simplified. Finely ground coal in water is conducted with low levels of diesel oil (1-2% by weight) in high shear, and size enlargement is accomplished with low shear agitation with a viscous petroleum binder, such as asphalt. If heptane is used in high shear mixer and asphalt in low shear mixer, 70 to 90% pyritic-S removal with 80-98% combustible matter recovery was achieved Cost of an conceptual oil agglomeration plant (181tph): 22 to 25 USD/t of clean coal product (not include cost of coal)


Slide32: To improve selectivity of oil agglomeration for pyrite rejection: Preconditioning of coal by the use of bacteria to convert pyrite surface into hydrophilic, followed by oil agglomeration Result: 90% pyritic-S rejection due to bioadsorptipn mechanism


Slide33: Selective flocculation: Process based on: 1. Employment of a selective dispersant for the dispersion –removal of Ultrafine pyrite (e.i.Na-poly acrylate –acrylo-dithiocarbonate) 2. Employment of partially hydrophobic polymeric flocculant to flocculate selectively the hydrophobic coal particles 3. Recover of flocculated coal particles by gravitational settling or froth flotation


Slide34: Mycobacterium phlei is an excellent flocculant for fine coal , it is attached on coal particles and massive flocs are formed due to hydrophobic interaction and bridging mechanism, but not on pyrite. More than 80% pyrite rejection was achieved


Slide35: Advanced Heavy Medium Cycloning In current practice : Efficiency drops off sharply as particle size decreases : desliming The separation gravity rises dramatically The reasons are: 1. The medium is not homogenous with respect to fine size coal 2. The forces operating on fine particles inside the cyclone are too weak with respect to fluid resistance These problems can be eliminated by the use of heavy organic liquids or micronized magnetite In South Africa and USA, recent dense medium cyclones are used without desliming.


Slide36: Micronized magnetite is much finer which enables finer sized coal to be cleaned at higher efficiency. In South Africa, micronized magnetite (50% <10 microns) has been applied in commercial scale to treat 500x75 microns coal in heavy medium cyclone. Micro-Mag Process: Uses 70% passing 5 microns magnetite


Slide37: Carefree Fine Coal Cleaning Process: Utilizes 100% passing 5 microns magnetite The recovery of -5microns magnetite is possible by using a series of wet magnetic separators followed by a HGMS


Slide38: An alternative to micronized magnetite: Heavy Organic Liquids Organic liquids such as Freon-113 : Nontoxic, nonflammable, with a sp.gr.1.585 at 16 C Others: Methylene chloride, perchlorethylene solution Disadv: 1. Due to the losses of organic liquids via adsorption on the products have become a major environmental concern 2. There are various uncertainties, regarding to environmental acceptability and costs.


Slide39: Major problems involved in physical coal cleaning 1. Very fine grinding is required to liberate the fine pyrite inclusions. Fine grinding is the most energy intensive operation. 2. Only pyritic sulfur is removable. The pyrite rejection is dependent to the particle size distribution of pyrite. 3.Very small and highly disseminated pyrite particles are nearly impoosible to separate from coal. Desulfurization increases with grinding to finer sizes and with decreasing density of separating medium.


Slide40: 4. Pyrite removal causes certain loss in combustible matter. 5. Ultrafine particles are difficult to handle and dewater.


Slide41: COMPARISON OF DESULFURIZATION PROCESSES Advanced flotation 27 USD/t coal* or 275 USD / t SO2 removal Oil agglomeration 22-25 USD/t coal * or 323 USD/ t SO2 removal Flue gas desulfurization 275-1650 USD/t SO2 removal * Coal cost is not included