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

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) yer_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 CHP Here, we use mechanical energy for powering vapor compression cooling 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: 2) Powdered coal:

Thermal NOx -Oxidation of atmospheric N 2 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 CO 2 at high temperature Formation of NOx and CO in Combustion

Coal-fired power plant filters Higher the temperature of combustion mean more NO x Chemistry for NO x reduction: Large boilers typically use chemistry that produce N 2 and H 2 O. Example is addition of Ammonia (NH 3 ) Use of catalysts for NO x reduction: Often in combination with NH 3

Coal-fired power plant filters For Particulate Maters Electrostatic precipitator Filter bags Scrubber for SO 2 (to prevent formation of Sulfuric acid – H 2 SO 4 ) Grinded Limestone in water (slurry) sprayed into the gas fluid stream SO 2 + 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 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!