# Combined Cycle Theory Dalton Plant Ouachita Plant.

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Combined Cycle Theory Dalton Plant Ouachita Plant

What is a “Combined Cycle” Power Plant?
Uses 2 thermodynamic cycles to generate electricity… Brayton - Gas Turbine & Rankine - Steam Plant ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ The Brayton cycle gets its heat from burning the fuel. The Rankine cycle uses heat rejected from the Brayton cycle.

Combined Cycle Designation
S207FA Steam and Gas (STAG) The Number of CT’s (and HRSG’s) per Steam Turbine Model Series MS6001FA MS7001FA MS9001FA MS7001FB MS9001FB MS9001H MS7001H ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Brayton Cycle ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Ideal Brayton Cycle 3 4 1 2 Compressor Turbine 1 2 3 4 Fresh Air Exhaust Combustion Fuel 1 2 3 4 Entropy S Temperature q in q out Const. p Generation Heat Rejected 1 2 3 4 Pressure Specific Volume v q in q out Const. s Work Out A Brayton Cycle is an all gas cycle where combustion and exhaust take place at different, but constant pressures. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Ideal Brayton Cycle Efficiency
The theoretical maximum Brayton Cycle thermal efficiency is a function of: 1 2 3 4 Entropy S Temperature q in q out Const. p Generation Heat Rejected The Pressure ratio - Ratio of Combustion to Atmospheric Pressure The maximum temperature achieved or the maximum temperature that the machine can withstand Ambient temperature Expressed as: th = rp (1-k)/k Where: rp = Pressure Ratio k = specific heat ratio of the working fluid ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Typically 30% - 40%

Perryville Simple Cycle GT
Inlet Transformer Generator Gas Turbine Exhaust Stack

Rankine Cycle ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Rankine Cycle (Steam Side)
Entropy (Btu/#/degF) Boiling Condensation Pump FW Heating LPT Reheating HPT Absolute Temperature (degF) Heat Rejected to Condenser Electrical Generation LPT Exhaust enters condenser A Rankine Cycle is a condensing cycle. In our case, steam is condensed. Heat is added independent of pressure. Superheating ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Ideal Rankine Cycle Efficiency
Entropy (Btu/#/degF) Boiling Condensation Pump FW Heating LPT Reheating HPT Absolute Temperature (degF) Heat Rejected to Condenser Electrical Generation LPT Exhaust enters condenser Superheating The theoretical maximum Rankine Cycle thermal efficiency is a function of: Boiler pressure The max. temperature achieved (the max. temperature the machine can withstand) Ambient temperature Expressed as: th = (Wout - Win) / Qin Where: Wout = work done by the system, Win = work done on the system, and Qin = heat added ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Typically 30% - 40%

Combined Cycle ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Combined Cycle Brayton Rankine
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The Concept Generator Intake Fuel Combustion Turbine Heat Recovery Steam Generator Steam Turbine Cooling Tower A Combined Cycle Power Plant has One or more Gas Turbines with Electrical Generators and One or more Steam Turbines with Electrical Generators A Combined Cycle Power Plant produces electric power from fossil fuel using: Gas Turbine, supplying turbine exhaust heat to Heat Recovery Steam Generator, supplying steam to Steam Turbine ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Perryville Combined Cycle Plant

For any system……. Q – E = W Heat In – Energy Change = Work Out
Combined Cycle For any system……. Q – E = W Heat In – Energy Change = Work Out WORK AND ENERGY ARE INTERCHANGEABLE ! To a power plant this means: Heat in from fuel BTU/Hr Electrical Generation MW Heat rejected MW - = ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

So why use a Combined Cycle?
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Because it increases overall efficiency!
Fuel Energy In GT Gen. GT Waste Heat (HRSG Energy In) ST Gen. ST Waste Heat Total Gen. Out the Stack CT Efficiency = ~ 35% CT Gen. Fuel Energy In ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ST Efficiency = ~ 35% ST Gen. HRSG Energy In Overall Efficiency = ~ 58% Total Gen. Total energy In OA = CT + ST - CT ST

Why use a Combined Cycle?
Let’s put in numbers… 907 MW (3 Billion BTU/Hr) Fuel 330 MW 577 MW CT 190 MW 349 MW ST 520 MW Total 38 MW CT Efficiency = ~ 36% 330 MW 907 MW ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ST Efficiency = ~ 33% 190 MW 577 MW Overall Efficiency = ~ 57% 520 MW 907 MW OA = x .33 = .57 = 57%

Combined Cycle 349 MW 38 MW 190 MW 907 MW 330 MW 577 MW
____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 330 MW 577 MW