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Coal Combustion a literature study

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  1. Coal Combustiona literature study • Phenomena in the blast furnace • The coal combustion process • The effects of • Particle motion • Type of coal –particle size • O2 concentration • Additives • Coal combustion in a blast furnace

  2. Phenomena in the blast furnace at a high PCR

  3. Phenomena in the blast furnace at a high PCR - coal combustion • Enrichment of coke fines in the dead man • Unburnt coal fines are consumed before coke fines • Increased generation of coke fines

  4. Phenomena in the blast furnace at a high PCR -coke quality • Smaller depth of raceway and greater spreading of birds’ nests for B and C than for A • The portion of raceway with a lower share of voids increases in the order B, A, C • CSR 60-50-45 /reduced coke size to 21,7-17,6-14,4

  5. Phenomena in the blast furnace at a high PCRcoke utilization • Increased residence time • Increased share of coke is used for • Solution loss • Carburization

  6. Phenomena in the blast furnace at a high PCR - ”soot” formation • ”soot” is formed from unburnt volatiles • reduced formation with improved O2 supply to the coal particles

  7. Phenomena in the blast furnace at a high PCR • Reduced radial transport of ash generated due to adhesion to dripping material and coke • Tuyere slag reacts with approx. 15-20 % of the dripping highly basic slag between the tuyeres • Unburnt char and coke fines are enriched in the deadman 157 kg/thm 190 kg/thm

  8. Processes at coal combustion • The carbon particle is preheated – heat radiation, heat conduction • Pyrolysis (T > 400° C) • Ignition of volatiles, temperature 1000-1700 °C • Coal particles (char) are ignited

  9. Processes during coal combustion-Structural change of coal particles during ignition • <100 m rapid heating, evaporation of volatiles and homogeneous ignition Figur 6 Strukturell förändring hos kol-partiklar under antändning.

  10. Modelling of coal combustion • Limiting for coal combustion • Diffusion of reactants and products • Pore diffusion • Chemical reaction rate • Sequence division • Preheating zone • Flame zone • Post-flame zone • Important factors • Emission of volatiles • Ignition • Char combustion • Particle spreading • Heat transfer

  11. Particle size - burn-out time Figur 6 Strukturell förändring hos kol-partiklar under antändning.

  12. Effect of particle motion/density

  13. The Effect of Coal type-an example Weight% PR coal K - 9 coal Volatiles 39,2 18,8 Fixed coal 63,5 70,2 Ash 2,5 10,4 Moisture 4,8 0,6

  14. The Effect of Coal typean example; 3D simulationcoal type, O2 enrichment, particle size and particle size distribution • Three coal types • Warndt VM 36,9% • Guasare VM 36,1% • Jellinbah VM 15,1% • The content of volatiles had the greatest effect on the combustion efficiency • Particle size • <48m • 86-162 m • >181m • Char combustion

  15. Coal typescoal blend • Ijmuiden • 1992: 212kg coal/thm, 271 kg of coke/thm • 1998 tests 250 kg of coal/thm • Blend of UHV coal och LV coal • Thyssen • ash content, content of volatiles, heat value, elementary analysis • coal types lignite-anthracite • best result blend of a high-volatile and a low-volatile coal (ignition and heat requirements)

  16. O2 enrichment • Coal combustion is limited by the O2 diffusion at high temperature • Rate of combustion – burn-out time • Thyssen • Critical O/C ratio was not reached at 140-150 kg of coal/thm • Coaxial lance, two separate lances • Ijmuiden • Soot generation • ”mixing” chamber at lance tip • Addition via blower/lance/ to heated coal

  17. Additives • CaCO3, KMnO4 • Iron oxide • Dust • BOF slag • Plastics

  18. Additives-one example • Fe2O3 can supply O2 for use in the combustion • Fe3O4/FeO can react with • coal char or coke fines • SiFe • The reaction behaviour is dependent on the coal type

  19. Temperature and gas distribution in raceway

  20. Reactions in raceway

  21. Combustion focus- effects

  22. Conclusions High combustion efficiency and high coke replacement ratio can be reached through early ignition, good char combustion, minimized soot formation and controlled heat load on the BF wall • Coal type • VM • Particle size distribution • Ash • Coal structure • Blast parameters • Lance design • Coke quality