Objectives- Discuss Exam

- Finish with - Boilers

- Discuss low temperature energy systems

Coal Boiler

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

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)

Oil – Gas based boilers

Gas circulate through tubes water is in-between

Water tube boiler

Furnaces

For homes

Roof tops and induct heaters

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:

Stoichiometric combustion

+Q combustion

Depends on the fuel: - 5 to10% for natural gas - ~ 40 for coal

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!

Stoichiometric air/fuel ratio for selected gases

Energy densities of fuels

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!

Condensing vs. noncondensing boilers

Example is for a small residential gas powered boiler - wall mount

fan coils, or baseboard hearts

Condensing vs. noncondensing boilers

~86%(depends on fuel)

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

…..

Boiler and Furnace Efficiency Definitions

Example (for large coal based boilers)

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!

Combined heat and power(cogeneration CHP or three generation CCHP)

Boiler(or heat generationsystem )

Low temperature energy systems

• Waist heat is part of almost every CHP• Many new micro CHP system use Stirling engine

http://www.viessmann.com

Stirling cycleClosed Cycle Engine

No phase change Working fluid: air, hydrogen, or helium

Stirling engine

Theoretical efficiency of up to 25%(for 500ºC)

Real efficiency in residential application between 6-8%