Lecture Objectives:

• Continue with power generation

• Learn basics about boilers and furnaces

Ideal Rankine Cycle

1-2 isentropic pump 2-3 constant pressure heat addition

3-4 isentropic turbine 4-1 constant pressure heat rejection

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

Improvements

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

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

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

Other method for CHPHere, we use mechanical energy for poweringvapor compressioncooling systems

Boilers are integral part of power generation system

It is much more complex system than presented in this graph

Coal Boilers

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

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

NOON 222

2221 NOONO

221

2 OCOCO

Formation of NOx and CO in Combustion

Coal-fired power plant filtersHigher the temperature of combustion mean more NOx

•Chemistry for NOx reduction:

• Large boilers typically use chemistry that produce N2 and H2O. Example is addition of Ammonia (NH3)

• Use of catalysts for NOx reduction:

•Often in combination with NH3

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

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, too-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!