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Objectives

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This discussion explores the different methods of coal burning in boilers, such as fluidized bed and powdered coal, and the efficiency of low temperature energy systems. Topics include air-fuel ratios, combustion pollutants, energy densities of fuels, and the efficiency of condensing and non-condensing boilers.

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Objectives

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  1. Discuss Exam Finish with Boilers Discuss low temperature energy systems Objectives

  2. Coal Boiler

  3. Coal burning method Long time ago: Today: 1) Fluidized bed: http://www.youtube.com/watch?v=8n78CDI3GoU 2) Powdered coal: http://www.youtube.com/watch?v=g1ojza-nbqs

  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 wallboard)

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

  6. Furnaces For homes Roof tops and induct heaters

  7. 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

  8. 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!

  9. Stoichiometric air/fuel ratio for selected gases

  10. Energy densities of fuels

  11. 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!

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

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

  14. 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 …..

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

  16. 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!

  17. Combined heat and power(cogeneration CHP or three generation CCHP) Boiler (or heat generation system )

  18. Low temperature energy systems • Waist heat is part of almost every CHP • Many new micro CHP system use Stirling engine http://www.viessmann.com

  19. Stirling cycle Closed Cycle Engine No phase change Working fluid: air, hydrogen, or helium

  20. Stirling engine Theoretical efficiency of up to 25% (for 500ºC) Real efficiency in residential application between 6-8%

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