Lecture Objectives:

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