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Reduction of NO x and SO x from Coal Combustion

Reduction of NO x and SO x from Coal Combustion. Ezra Bar-Ziv Department of Mechanical Engineering and Institutes for Applied Research Ben-Gurion University of the Negev. Can Coal Combustion be Clean?. Increase in coal combustion, will double by 2030 Severe environmental impact, include:

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Reduction of NO x and SO x from Coal Combustion

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  1. Reduction of NOx and SOx from Coal Combustion Ezra Bar-Ziv Department of Mechanical Engineering and Institutes for Applied Research Ben-Gurion University of the Negev

  2. Can Coal Combustion be Clean? • Increase in coal combustion, will double by 2030 • Severe environmental impact, include: 1. High level of CO2 2. Particulate emission (soot, small fly ash) 3. SOx, NOx 4. Volatile metals 5. PAH 6. Fly ash

  3. Coal Combustion in Utility Boilers • Coal combustion in utility boilers is strongly coupled with boiler geometry, flow conditions, local stoichiometries, and temperature • Two phase flow complicates even further coal combustion in utility boilers • Impossible to predict behavior unless system behavior and characteristics is well known • Further complications due to changes in boiler walls

  4. Raw Coal • Mineral matrix • Carbonaceous infrastructure • Volatile matters • Moisture Above depend strongly on: coal age, source, packing conditions, etc.

  5. Combustion of Pulverized Coal Coal particle devolatilization + highly porous char particle (1) Volatiles+O2CO2, H2O, CO, NOx, SOx, etc. (2) Highly porous Char particle +O2CO2, CO, NOx, SOx, etc. (3) Involves: heat & mass transfer and gas- phase and heterogeneous reactions

  6. Combustion of char • Chars are highly porous • Mechanism for combustion through adsorption-desorption: reacting sites • Reacting sites responsible for -N reaction as well • Reacting sites depend on parent coal and carbon structure within char

  7. In this Presentation: Emphasis on NOx & SOx Control 1. Introduction: various pollutant emissions 2. Effect of various emissions and control 3. Fate of Fuel-Nitrogen (N) 4. Fate of volatile-N & volatile-S 5. Fate of char-N & char-S 6. Conversion to N2 7. New concepts

  8. Impact of Emissions • CO2: the green house effect, imagine when third world (4/5 of world population) will start to approach Western consumption of fuels • NOx and SOx: major hazard to vegetation by being acid rain precursors • Soot and PAH are generally carcinogenic • Fly ash: if above 5% carbon content - gain if bellow - loss • Small particulate: lung diseases

  9. CO2 Reduction • In general: increase conversion efficiency from heat to electricity • Combined cycle • High pressure combustion • Pinpoint heat release to certain zones • Solution: better boilers based on CFD simulations

  10. SOx Reduction • No benign gaseous sulfur species, hence chemistry will not help • SOx must be cleaned up post combustion • Sorbent injection • Scrubbing • Low sulfur coal

  11. Poly-Aromatic Hydrocarbons (PAH) and Soot • Small poly-aromatic molecules • PAH precursors to soot • Produced and terminates in gas phase • PAH adsorbs in soot and fly ash • Can control concentration if mechanism known -- control chemistry • Modeling

  12. NOx Reduction • Can be converted to benign gas N2 during combustion • Need to know right conditions • Mechanism is essential knowledge • Experiments were done at various conditions: 1. Gaseous flames 2. Coal and char in gaseous flames 3. Combustion of coal/char in pc reactor 4. Combustion of coal/char in fb reactor

  13. Fate of Fuel- Nitrogen (N) Determined by a variety of factors • coal rank (C/H ratio) • nitrogen content in fuel • volatile content • particle size • temperature • local stoichiometry

  14. Fate of Fuel-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH3 (3) Volatile + O2 NOx + … (4) Char-N + O2 NOx + … (5) HCN + O2 NOx + … (6) HCN + NOx  N2 + … (7) HCN + Char  N2 + … (8)

  15. Effect of: Nitrogen & Volatile Content in Fuel • No correlation was found with nitrogen content in fuel • No correlation was found to total -N content, but on -N functionality

  16. Effect of Coal Particle Size • Indirect effect of particle size on conversion to NOx • Size affects strongly both devolatilization and char oxidation, can vary from chemically controlled to diffusion controlled • Consequent reactions depends strongly on reaction regime via reacting sites

  17. Effect of Temperature • Formation of NOx from coal depends weakly on temperature due to competing effects: increase of generation of NOx and N2 with temperature

  18. Effect of Stoichiometry • Strong effect of stoichiometry on NOx formation • Monotonic decrease of NOx with fuel/O2 ratio • Extreme importance of volatile-N/O2 ratio to NOx formation -- same as for fuel

  19. Fate of Fuel-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH3 (3) Volatile + O2 NOx + … (4) Char-N + O2 NOx + … (5) HCN + O2 NOx + … (6) HCN + NOx  N2 + … (7) HCN + Char  N2 + … (8)

  20. Conclusion for Fuel-N Fate • Depends strongly on coal-N fate • Depends strongly on volatile-N fate • Stoichiometry • Coal rank -- reactivity • Particle size

  21. Fate of Volatile-N • Most of NOx emission arises from volatile-N • Rate of release of NOx seems kinetically controlled, indicative to gas-phase reaction • Release of NOx follows devolatilization rate • There are still many contradictions, arising from coal rank (type), variability (probably due to catalysts in coal)

  22. Fate of Char-N • The two main products of char-N oxidation are: NO and N2O • Occur via homogeneous formation/destruction heterogeneous formation/destruction of HCN

  23. How is NOx Formed? • Heterogeneous through adsorption of O2 that interacts with -N site then desorption via thermal process to NO or N2O • Heterogeneous reactions are very sensitive to evolution of porous structure • Indications that at high temperature, heterogeneous reaction is controlled by diffusion

  24. Fate of Fuel-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH3 (3) Volatile + O2 NOx + … (4) Char-N + O2 NOx + … (5) HCN + O2 NOx + … (6) HCN + NOx  N2 + … (7) HCN + Char  N2 + … (8)

  25. How is NOx Formed? • Indication that for heterogeneous reactions NO is generated at reacting sites and N2O is produced within pores • Strong correlations between reacting sites and formation of NO, N2O, HCN (formed always at surface) • Homogeneous through oxidation of HCN • Still homogeneous pathways are likely to be strongly involved in NO, N2O formation

  26. Homogeneous Reactions • If HCN released, oxidation to NO, N2O occurs homogeneously through NCO • NCO will react with O2 or OH to form NO or N2 • No time for homogeneous reactions to occur within particle, must be outside

  27. Heterogeneous Reactions • Some evidence that NO is also formed heterogeneously • NO can be reduced to N2 or/and N2O either heterogeneously or homogeneously • Heterogeneous reduction of NO was found to strongly depend on: CO, surface area, and temperature

  28. Reduction of NOx by External Agents NH3 + NO  N2 + H2O (HOCN)3 + NO  N2 + H2O (cynuric acid) N2H4 + NO  N2 + H2O (hydrazin) CO(NH2)2 + NO  N2 + H2O (urea)

  29. Reduction of NOx by External Agents NH3 + OH  NH2+H2O + NO  N2 (NH3)2CO NH3+HNCO NH2+CO (HNCO)3 HNCO  + OH NCO+H2O +NO N2O +OH,M,H

  30. Catalytic Reduction of NOx from Flue Gas Selective Catalytic Reduction: NH3 + NO  N2 + H2O Metals: Pt, Pd Oxides: Ru/Al2O3, Fe2O3/Cr2O3, V2O5/TiO2, V2O5/MoO3/WO3/Al2O3 Zeolites (AlxSiyOz/M)

  31. Forms of Sulfur in Coal • Organic-S compounds (thiophenes, sulfides, thiols) • Pyritic sulfur (FeS) • Sulfates (Ca/FeSO4) Significant chemical changes of sulfur occur during coal devolatilization and combustion

  32. Transformation of Coal-S Coal-S  (CS, S2, S, SH)  SO  SO2  SO3 -SO4 O2, M char COS, CS2 H2S

  33. Sulfur Pollutant Reduction No benign sulfur gas compounds Reduction of sulfur pollutant • Pre-combustion coal cleaning • In-situ cleaning • Post-combustion cleaning: Solidification to sulfur salt compounds

  34. Sulfur Pollutant Reduction: Pre-Combustion 1. Differences in density removes 30-50% of FeS 2. Leaching by sodium/potassium bases R-C-SH + NaOH  NaS-C-R + H2O 3. Biological cleaning by bacteria or fungi with high affinity to sulfur By leaching and biological techniques 90% can be removed

  35. Sulfur Pollutant Reduction: In-Situ Cleaning Addition of sorbents: Ca/Mg/Zn/Fe/Ti oxides In furnace conditions CaCO3  CaO + CO2 Ca(OH)2  CaO + H2O 2CaO + O2 + 2SO2  2CaSO4 CaO + H2S  CaS + H2O CaO + SO3  CaSO4

  36. Models for Sulfur Capture by Sorbents • Sorbents are porous spheres • Known properties of porous structure • Rapid heating of sorbent -- CaCO3, Ca(OH)2 • Decomposition sorbent to oxide & CO2 Diffusion of CO2 through CaO to outer surface • Mass transfer of CO2 from surface to bulk gas

  37. Models for Sulfur Capture by Sorbents/continues • Diffusion of SO2 and O2 from bulk gas to surface • Diffusion of SO2 and O2 from outer surface to inner pores • Reaction of SO2 and O2 with CaO to form CaSO4

  38. Models for Sulfur Capture by Sorbents/continues CO2 O2 SO2 CaCO3 CaO CaSO4

  39. Sulfur Pollutant Reduction: Postcombustion Lime or limestone scrubbers CaCO3  CaO + CO2 2CaO + O2 + 2SO2  2CaSO4 CaO + SO3  CaSO4 (Gypsum)

  40. Summary: NOx and SOx Reduction • NOx can be reduced during combustion, with right conditions • SOx should be reduced pre-combustion

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