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Lecture Objectives:

This lecture discusses the formation of NOx and CO in combustion processes, as well as methods for reducing their emissions. It also explores the use of thermal storage systems in boilers and furnaces.

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Lecture Objectives:

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  1. Lecture Objectives: • Finish boilers and furnaces • Start with thermal storage systems

  2. Formation of NOx and CO in Combustion Thermal NOx • Oxidation of atmospheric N2 at high temperatures • Formation of thermal NOx is at higher temperature Fuel NOx • Oxidation of nitrogen compounds contained in the fuel Formation of CO • Incomplete Combustion • Dissociation of CO2 at high temperature

  3. Coal-fired power plant filters Higher the temperature of combustion mean more NOx • Chemistry for NOxreduction: • Large boilers typically use chemistry that produce N2 and H2O. Example is addition of Ammonia (NH3) • Use of catalysts for NOxreduction: • Often in combination with NH3

  4. Coal-fired power plant filters • For Particulate Maters • Electrostatic precipitator • Filter bags • Scrubber for SO2 (to prevent formation of Sulfuric acid – H2SO4) • Grinded Limestone in water (slurry) sprayed into the gas fluid stream SO2 + Limestone slurry → Gypsum (used for wallboards)

  5. What happened with other waist ? ~55 % Ash dump ~45 % reused - substitute for cement - soil and waste stabilization - asphalt - bricks - ….. Fly ash captured at the electrostatic precipitators Bottom ash captured at the electrostatic precipitators http://www.acaa-usa.org/Portals/9/Files/PDFs/revisedFINAL2012CCPSurveyReport.pdf http://www.epa.gov/radiation/tenorm/coalandcoalash.html

  6. Oil or Gas based boilers Gas circulate through tubes water is in-between Water tube boiler

  7. Furnaces For homes Roof tops and induct heaters

  8. Fuel combustion - Stoichiometry • Boiler efficiency as a function of excessive air • Stoichiometry • Chemistry of reactants, products and energy in chemical reactions • A stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion: +Q combustion Stoichiometric combustion Depends on the fuel: - 5 to10% for natural gas - ~ 40 for coal

  9. Air Pollutants from Combustion Air-Fuel Ratio - Rich mixture - more fuel than necessary (AF) mixture < (AF)stoich - Lean mixture - more air than necessary (AF) mixture > (AF)stoich Most combustion systems operate under lean conditions! However, lean mixture results in Nox products!

  10. Stoichiometric air/fuel ratio for selected gases

  11. Energy densities of fuels

  12. Higher heating value (HHV) vs. Lower heating value (HHV) • HHV is the heat of combustion of the fuel when the water product is at liquid state (water vapor from the product are condensed) • LHV is the heat of combustion of the fuel when the combustion product contain water vapor For methane ~10% difference!

  13. Condensing vs. noncondensing boilers Example is for a small residential gas powered boiler - wall mount fan coils, or baseboard hearts

  14. Condensing vs. noncondensing boilers ~86% (depends on fuel)

  15. Boiler Efficiency Definitions • ASHRAE Standard 90.1-2004 describes the minimum acceptable ratings for new boilers • Combustion Efficiency % = ((Fuel Input – Stack Losses) / Fuel Input) x 100 • Thermal Efficiency % = (Output / Input) x 100 • Annual or Seasonal Efficiency …..

  16. Boiler and Furnace Efficiency Definitions Example (for large coal based boilers)

  17. Typical boiler and furnace efficiency (based on the higher heating value) • Condensing boilers manufacturers claim up to 98% (be careful with this number; check for which conditions) • Older conventional boilers 70%-80% • Typical new models around 90% • New gas furnaces are in the rage of 80-90% • These numbers are for well maintained and tuned boilers & furnaces. Also, Seasonal Efficiency can be significantly smaller!

  18. Thermal storage for adjustment production to consumption We need a thermal storage somewhere in this system !

  19. Thermal storage • Store heat • Many issues (∆T, pressure, losses,…. ) • Store cooling energy • Chilled water • For cooling condenser • For use in AHU (cooling coils) • Ice storage • Compact but… • Other materials (PCMs) that change phase the temperature we need in cooling coils • Many advantages, but disadvantages too!

  20. On-Peak and Off-Peak Periods This profile depends on the type of building(s) !

  21. Chilled water tank Use of stored cooling energy Store Use

  22. Which one is better ? • Depends on what you • want to achieve: • Peak electric power reduction • Capacity reduction • …..

  23. Downsizing the Chiller • Lower utility costs • Lower on-peak electrical consumption(kWh) • Lower on-peak electrical demand (kW) • Smaller equipment size • Smaller chiller • Smaller electrical service (A) • Reduced installed cost • May qualify for utility rebates or other incentives

  24. Sizing storage system (use Annual Cooling-Load Profile) How often you need to use it? What are the cost-benefit curves ? What is the optimum size ?

  25. Ice Storage Tank Many issues ! …. As freezing progresses progress the ice becomes thicker and significantly impedes heat transfer

  26. Open Ice Storage Tank Also issues ! …..

  27. Fluid Flow Rate and Freeze ΔT

  28. Impact on Chiller Efficiency

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