MARAMA Webinar August 7, 2014

Angelos KokkinosChief Technology OfficerBabcock Power, Inc.

Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid.

Typical power steam generation cycle has four main components:◦ Boiler◦ Turbine◦ Condenser◦ Feed Pump

Process Steps◦ 1-2: Increase pressure of

condensate, then increase temperature through economizer

◦ 2-3: Increase energy by adding heat to get water to steam and superheated steam

◦ 3-4: Expand steam through HP turbine

◦ 4-5: Reheat steam through Reheater

◦ 5-6: Expand steam through IP and LP turbine

◦ 6-1: Condense steam in condenser

Thamir K. Ibrahim , M. M. Rahman , "Effect of Compression Ratio on Performance of Combined Cycle Gas Turbine", International Journal of Energy Engineering, Vol. 2 No. 1, 2012, pp. 9-14. doi: 10.5923/j.ijee.20120201.02

Combines a combustion turbine with a steam generator

Combines Brayton and Rankine cycles

30 – 50% typical efficiency improvement over Rankine steam generation

Definition◦ Measures the combined performance of the turbine

cycle, boiler cycle and associated power auxiliaries◦ The amount of energy input (consumed) to generate

electricity◦ Btu/kWh

Formula◦ Heat Rate = (Fuel consumed x Fuel Heating Value)/

Power output

Plant efficiency rate◦ As the fuel input increases, for the same plant output,

the heat rate increases therefore the system efficiency decreases

Controllable losses are those that are impacted by plant operation. Operating load Steam conditions◦ Temperatures◦ Pressures

Condenser pressure Final feedwater temperature Steam attemperation flows Auxiliary steam and power consumption Boiler exit gas temperature and excess oxygen

Controllable Loses

Impact on Heat Rate – 10,000 Btu/kWh plant

Parameter Change Heat Rate Change, %

Main Steam Temperature -10 °F 0.17

Hot Reheat Temperature -10 °F 0.16

Main Steam Pressure -1% 0.06

Condenser Pressure +0.5 in Hg 0.6

Feedwater Temperature -10 °F 0.27

Superheater Spray Flow +1% of steam flow 0.025

Reheat Spray Flow +2% of steam flow 0.4

Auxiliary Steam Flow + 0.5% of Cold Reheat 0.35

Excess O2 - 1% 0.2

Auxiliary Power 1 MW 0.2

APH Exit Temperature + 10 °F 0.25

Typical boiler efficiency is 85 – 90%◦ Typical losses (coal) :

Dry gas 4.5

Hydrogen and Water in fuel 5.7

Unburned combustibles 0.1

Moisture in air 0.1

Radiation 0.15

Unaccounted 1.5

TOTAL 12.05%

1% in boiler efficiency loss is approximately 1 % increase in heat rate

Regulatory operating constrains

Dry gas loss◦ Quantity and temperature of flue gas◦ Excess air ◦ Maintain proper exit gas temperature

Hydrogen and water in fuel◦ High hydrogen fuels◦ High moisture fuels

Unburned carbon

Boiler degradation◦ Surface fouling◦ Air preheater leakage◦ Pulverizer performance◦ Off design fuel

Heating value Moisture content Hydrogen content Slagging/fouling characteristics Sulfur content

Typical power plant turbine efficiencies:◦ High pressure: 78 - 84%

◦ Inter. pressure: 87 - 92%

◦ Low pressure: 86 - 91%

Turbine cycle heat rate:◦ Similar calculation to the net heat plant rate

Turbine heat rate = energy input/power output

◦ Net plant heat rate = turbine rate/boiler efficiency

Turbine degradation◦ Deposition

◦ Erosion

◦ Mechanical damage

◦ Internal leakage

Variable pressure operation◦ Sliding pressure for low load

Partial arc admission

Converts steam to water so it can be pumped back to the boiler

Reduces pressure at the turbine outlet to below atmosphere increasing available energy to the turbine

About 60% of the energy from the steam generated is transferred to the condenser so it is lost.◦ Largest single loss of the energy to generate electricity

Important that condenser operates in sync with turbine due to its impact on heat rate

Any of the areas below causes the heat rejection to be less efficient causing an increase in condenser pressure:◦ Cooling water inlet temperature

◦ Heat load

◦ Circulating water flow

◦ Tube fouling

◦ Air in-leakage

◦ Condenser degradation

Feedwater heaters◦ Feedwater heaters out of service

◦ Tube leaks

Cooling tower◦ Water distribution

◦ Fill

◦ Air flow

Maintaining current equipment to the “as designed” condition is advisable

Achieving “as designed” efficiency on a daily basis is difficult due to:◦ Fuel quality variations◦ Equipment malfunction(s)◦ Weather conditions◦ Equipment maintenance

Load dispatch impacts heat rate◦ Ramp rate◦ Peaking vs baseloaded

Fuel costs being the major operating cost force operators to operate at optimum heat rate

Achieving the proposed 6% heat rate improvement would require major equipment modifications