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WASTE HEAT TO POWER WORKSHOP “Condensing Economizer Technology”Bill Carson Direct Contact Inc. Direct Contact, Inc. © 2003
Condensing Economizer Provides New Heat Recovery OpportunitiesVery High Heat Transfer Coefficient Extremely High Heat Transfer Surface Area Low Gas Side Pressure Drops Direct Contact, Inc. © 2003
DCI-Heat Recovery VesselsA single chamber vessel is used for clean gases combining adiabatic cooling and heat recovery in single chamber A dual chamber vessel is designed for dirty gases: conditioning and adiabatically cooler in its lower chamber and recovering heat in its upper chamber. Direct Contact, Inc. © 2003
Heat Content of Flue GasThe heat exhausted from a boiler is dependent on two factors: Flue Gas Temperature (boiler specific) Water Vapor Content (humidity) The excess combustion air and type of fuel burned determines the amount of water vapor (humidity) in the flue gas. hydrogen combustion generates only water vapor pure carbon combustion generates no water vapor Among hydrocarbons, methane combustion generates the most water vapor Direct Contact, Inc. © 2003
Combustion Stoichiometery(Basis = One Mole of Oxygen) Hydrogen moles O2 (G) + 2 H2 (G) 2 H2O (G) mass 36.03 Carbon moles O2 (G) + C (S) CO2 (G) mass 44.01 Methane moles O2 (G) + ½ CH4 (G) ½ CO2 (G) + H2O (G) mass Ethane moles O2 (G) + 2/7 C2H6 (G) 4/7 CO2 (G) + 4/7 H2O (G) mass Direct Contact, Inc. © 2003
Combustion Energy Direct Contact, Inc. © 2003
Psychometric Chart Direct Contact, Inc. © 2003
Simplified System Direct Contact, Inc. © 2003
Nomenclature Mass Heat Direction DescriptionWMW QMW in Boiler Make Up Water WDAV QDAV out Deaerating Air Vent WBD QBD out Boiler Blow Down WFG QFG out Boiler Flue Gas --- QBRH out Boiler Radiant & Convective Heat Loss WCA QCA in Combustion Air WNG QNG in Natural Gas --- QSRH out Steam Distribution Radiant & Convective Heat Loss WDWC QDWC out Drip Waste Condensate --- QPL out Process Heat Load (Client Heat) WWC QWC out Waste Condensate --- QCOMB generated Heat of Combustion Direct Contact, Inc. © 2003
Mass and Energy BalanceOVERALL ENERGY BALANCE (QMW + QCA + QNG)IN + (QCOMB)GEN = (QDAV + QBD + QFG+ QBRH+ QSRH+ QDWC+ QPL+ QWC)OUT OVERALL MASS BALANCE (WMW + WCA + WNG)IN = (WDAV + WBD + WFG+ WBRH+ WSRH+ WDWC+ WPL+ WWC)OUT SYSTEM EFFICIENCY eff = QPL/[QCOMB + (QMW + QCA + QNG)IN] Direct Contact, Inc. © 2003
Heat Flow Direct Contact, Inc. © 2003
Heat Return Paths Captured Heat can be returned to the Boiler system by Heating combustion air Heating water make-up Today’s discussion focuses on the latter and we will examine The heat removed above the Dew Point (via an Economizer) The heat removed below the Dew Point temperature (via a Condensing Economizer) Direct Contact, Inc. © 2003
Flue Gas Heat Recovery Direct Contact, Inc. © 2003
Assumptions Used in Mass and Energy BalanceBoiler produces 6,000 pounds per hour of steam at 100-psig saturated Combustion air, fuel and makeup water are all at inlet temperature 60ºF Combustion air dew point is at a temperature of 40oF Fuel is 100% methane, combustion air is 10% excess, and the combustion efficiency is 100% The boiler has 0% blow down and negligible heat is lost from the deaerating heater vent The boiler’s radiant and convective heat losses are at 2% and no heat is lost from the distribution piping Inefficiencies of rotating equipment are negligible Flue gas enters economizer at 530oF and leaves the economizer at 320oF 51% flue gas passes through the condensing economizer leaving at 113oF The condensing economizer gas exhaust and hot flue gas, directly from the economizer, combine to produce a stack exhaust temperature of 219.2oF Direct Contact, Inc. © 2003
Heat Available 1,742,160 BTU/hr 390,000 BTU/hrTotal Heat in Flue Gas leaving Boiler 1,742,160 BTU/hr 100% Heat captured in the Economizer 428,240 BTU/hr 24.6% Are numbers missing here? Heat captured in the Condensing Economizer 390,000 BTU/hr (Additional 22.4%) Condensing Economizer Only 444,000 BTU/hr 25.5% Direct Contact, Inc. © 2003
DCI Heat Recovery VesselChamber Note two chambers Conditioning Chamber Heat Recovery Chamber Conditioning Chamber Direct Contact, Inc. © 2003
Conditioning Chamber Incoming gas is flash cooled- Water vapor is evaporated - Gas is cooled and humidified Temperature of gas is driven to saturation (Wet Bulb) Large particulate is captured in excess water Majority of liquid water is separated from gas stream Direct Contact, Inc. © 2003
Heat Transfer Chamber Saturated gas enters this chamber and is contacted with a large number of small droplets. The gas is cooled to below its inlet dew point temperature, condensing all the water used to adiabatically cool it in the conditioning chamber, as well as some of the water vapor in the pre-quenched gas . Direct Contact, Inc. © 2003
Low Grade Heat A Heat Repository (or “Heat Sink”) is required to cool exhaust gas. Finding a “home” for the hot water you create can be challenging… …but if you can use this heat, and replace virgin energy, the project can turn into a net “money maker.” Good Examples: Boiler Feed Water, Domestic Hot Water, Process/Wash Water Direct Contact, Inc. © 2003
Direct Contact Inc. Heat Recovery SystemSeattle Steam Company ~14 MBTU/hr saved >9 million gallons/yr. water was recovered >9,000 tons/yr. less CO2 was generated Average fuel savings >$2000/day < 2-yr payback Direct Contact, Inc. © 2003
Direct Contact Inc. Heat Recovery SystemPactiv Corporation ~5 MBTU/hr saved >2 million gallons/yr. water was recovered >3,000 tons/yr. less CO2 was generated Average fuel savings >$500/day < 2-yr payback Direct Contact, Inc. © 2003
Direct Contact Inc. Heat Recovery SystemJames Hardie Gypsum; now BPB ~10 MBTU/hr saved >9 million gallons/yr. water was recovered >6,000 tons/yr. less CO2 was generated Average fuel savings >$1200/day < 2-yr payback Direct Contact, Inc. © 2003
Direct Contact Inc. Heat Recovery SystemNorthwest Groundwood Refiner 1st Unit of this design was installed in and is still performing 2nd unit was installed at the same mill in 1991 3rd system has been installed on a Hog Fuel Boiler Exhaust and is capturing >25 MBTU/hr Direct Contact, Inc. © 2003
Direct Contact Inc. Hog Fuel Boiler DAISHOWA AMERICAInstalled 2002 ~30 MBTU/hr thermal heat saved Average fuel savings >$150/hr Direct Contact, Inc. © 2003
Why lose all this heat? Consider a Heat Recovery SystemDirect Contact, Inc. © 2003
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