Objectives- Discuss Final Project

- http://www.caee.utexas.edu/prof/Novoselac/classes/CE397b/Hw.html- Possible dates for the final project presentations: May 13 - 16

- Learn about - Power generation - Boilers

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

Here, we use thermal energy for heating and/or cooling

Gas powered turbine

http://www.youtube.com/watch?feature=player_embedded&v=rxps0sZ8T3Y

Combustion product gas powered turbines

• Limited to gas or oil as a major source of fuel

• Approximately 55 to 65% of the power produced by the turbine is used for compressor.

• Gas temperatures at the turbine inlet can be 1200ºC to 1400ºC

• Because of the power required to drive the compressor, energy conversion efficiency for a simple cycle gas turbine plant is ~ 30%

Combined Cycle(gas and steam)

http://www.youtube.com/watch?feature=player_embedded&v=D406Liwm1Jc

Steam powered turbine

Ideal Rankine Cycle

h1=hf saturated liquid

Wpump (ideal)=h2-h1=vf(Phigh-Plow)vf=specific volume of saturated liquid at low pressure

qin=h3-h2 heat added in boiler Usually either qin will be specified or else the high temperature and pressure (so you can find h3)

qout=h4-h1 heat removed from condenser)

wturbine=h3-h4 turbine work

Reheat Cycle

• It allows increase boiler pressure without problems of low quality at turbine exit

Regeneration• Preheats steam entering boiler using a feed-water

heater, improving efficiency

Further improvements

Analogy with cooling cycles

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!