Cycles and the Second Law In 1823 Carnot Said: Max Efficiency = ≈ 65% for typical rankine cycles T0 = Heat Sink Temperature T1 = Temperature at which heat is added • Increase T1 to improve efficiency • Primary limiting factor is cost and availability of materials On the Bo iler Side
Efficiency is a Function of: • Gas Temp Leaving the air heater • Ambient Temp • Excess Air • Unburned Combustibles • Fuel Properties
A i r H e at e r P er f o r m a n c e Affects every air pollution control and combustion device in the plant
EMISSIONS
BURNERS
COMBUSTION
A i r H e at e r P er f o r m a n c e Poorly maintained Air Heaters could degrade plant heat rate by 0.7 to 0.9%.
A i r H e at e r P er f o r m a n c e Minimize Boiler Setting Air In-leakage Penthouse Roof Seals Access and Observation doors Expansion Joints Furnace Hopper Seal Setting Leakage • Degrades Air Heater Performance 3% air leakage ≈ +10F ≈ - 0.25% Eff
•
Degrades Combustion System performance – Increases UBCL and some emissions
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Requires operation at higher total excess air – Increases stack losses and ID/FD fan power consumption
A i r H ea t er P e r f o r m a n c e Operation and Maintenance of Boiler Cleaning Equipment • Maintain boiler cleanliness to minimize exit gas temperature and stack losses A 30F Reduction in boiler exit gas temperature ≈ 0.25% Heat Rate • Implement Intelligent sootblower control to optimize absorption distribution and heat rate
A i r H ea t er P e r f o r m a n c e Operation and Maintenance of Coal Pulverizers • Proper maintenance of pulverizer wear parts will increase fineness and decrease drive motor power consumption. Increased fineness reduces unburned carbon loss (UBCL) and possibly emissions • Upgrading to a dynamic classifier will improve coal fineness and reduce UBCL • Upgrading to an auto-loading system optimizes primary air fan and pulverizer motor power consumption, and coal fineness
E n s u r e Pr o p e r O2 M ea s u r e m e n t a n d C o n t r o l
• Due to O2 Stratification at normal measurement locations, multiple instruments should be installed in a grid arrangement • Improper O2 measurement and control lead to off-design excess air, emissions excursions, slagging and fouling, absorption maldistribution, and other problems that degrade boiler and emissions performance, and heat rate
T u r b i n e S t e am P a th U p g r a d e s ≈ 4 % i m p r o v e m e n t i n N P H R
• Incorporate peak generating load increase • Requires boiler heating surface modifications to match the boiler to the revised turbine conditions
Heat rate degrades as load is reduced
• Split / Sliding Pressure Operation • Allows the furnace to operate at full pressure with turbine throttle valves wide open - full steam temperature to the 1st stage - at all loads
Once Through Boiler
• Permits increased load change rate capability • Can be retrofitted onto drum or oncethrough boilers • Extends RH steam temperature control range (better low load heat rate)
Drum Boiler
• Variable Frequency Drives for Large Fans and Pumps • In a typical modern coal fired power plant, air and gas fans consume 2-3% of gross generator electric output • VFDs allow fans to operate more efficiently over the range of ambient conditions and fuel variations • Most significant efficiency gains realized during reduced load operation
Reduce Boiler Exit Gas Temperature • Economizer resurfacing / heating surface addition • Air Heater Basket Upgrades
Not a Viable Option for all Units • Lower economizer exit temperature reduces SCR control range • Air heater exit gas temperature may already be at the dew point limit
Condensing Heat Exchanger • Water vapor formed during the combustion process results in a large stack heat loss ≈4% for a typical coal fired unit – about 1/3 of the total losses ≈10% for a typical Nat. Gas fired unit – about 2/3 of the total losses
• Most of the lost energy is due to latent heat of vaporization
Opportunity • Condensing heat exchanges could be used to reclaim a large percentage of this lost energy Why it isn’t already a routine practice • Heat exchangers are large and expensive • Corrosion is a problem to address • What to do with the low grade energy
Combustion Efficiency • Burners • Overfire Air Systems • Pulverizer Upgrades Opportunities New burners and OFA systems optimized with CFD • Reduce total excess air: 5% reduction ≈0.2% NPHR • Reduce UBCL • Maintain or reduce NOx and CO emissions
Su b c r it ic al ) h W k / g ( n o i t p m u s n o C l a o C c i f i c e p S
Su p er c r it ic al (B o t h at 1000/1000F
2950
2900
2850
2800
2750 2400 psig
3600 psig
Subcritical
Supercritical
Steam pressure @ Turbine Inlet (psig)
Source: Siemens, KWU FTP2/Ka/Gs 30.6.1997
Data based on: 2 x 660 MW units 6500 hr/a LHV = 25MJ/kg
16% better heat rate and lower CO2 emissions @ nominal 600 MWNET Average heat rate 8858 Btu/kWh in 2013 US Fleet Average 10,555 Btu/kWh
+11% reduction in fuel consumption and CO2 emissions vs. 600C plant heat rate +29% reduction vs. the current fleet average heat rate and CO 2 emissions – could replace existing units with new A-USC plants and meet EPA CO2 goal without carbon capture • • • •
Lower flue gas handling equipment size and fan power Lower plant fuel handling Lower fuel transportation system impact Lower water consumption and condenser heat duty
Lower CO2 emitted and auxiliary power consumption for capture
• Fireside Corrosion and Coatings • Steam Side Oxidation • Welding and Manufacturing Development • Conceptual Design Studies • Header Design 600C and 700+C (B&W projects)
+$15.2 million by B&W in previous 12 years for A-USC
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Opportunities to improve efficiency of existing fleet without significant capital investment are incremental and unless the unit is ill -maintained, will not result in large improvements to NPHR (<1.0 - 1.5%)
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Selected older units with original equipment -- turbine steam path upgrades combined with boiler heating surface modifications are the most likely opportunity to improve heat rate by multiple percentage points (≈4%)
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Opportunities are available to improve reduced load heat rate and load cycling capability
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New ultra supercritical or advanced ultra-supercritical units offer the most significant heat rate improvement opportunities (16-29%)
