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Research Papers

Start-Up Optimization of a CCGT Power Station Using Model-Based Gas Turbine Control

[+] Author and Article Information
Alessandro Nannarone

Department of Process & Energy (P&E),
Delft University of Technology,
Leeghwaterstraat 39,
Delft 2628 CB, The Netherlands
e-mail: alessandro.nannarone1994@gmail.com

Sikke A. Klein

Department of Process & Energy (P&E),
Delft University of Technology,
Leeghwaterstraat 39,
Delft 2628 CB, The Netherlands
e-mail: s.a.klein@tudelft.nl

Manuscript received July 19, 2018; final manuscript received August 2, 2018; published online December 3, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(4), 041018 (Dec 03, 2018) (10 pages) Paper No: GTP-18-1508; doi: 10.1115/1.4041273 History: Received July 19, 2018; Revised August 02, 2018

The rapid growth of renewable generation and its intermittent nature has modified the role of combined cycle power stations in the energy industry, and the key feature for the operational excellence is now flexibility. Especially, the capability to start an installation quickly and efficiently after a shutdown period leads to lower operational cost and a higher capacity factor. However, most of existing thermal power stations worldwide are designed for continuous operation, with no special focus on an efficient start-up process. In most current start-up procedures, the gas turbine controls ensure maximum heat flow to the heat recovery steam generator, without feedback from the steam cycle. The steam cycle start-up controls work independently with as main control parameter the limitation of the thermal stresses in the steam turbine rotor. In this paper, a novel start-up procedure of an existing combined cycle power station is presented, and it uses a feedback loop between the steam turbine, the boiler and the gas turbine start-up controls. This feedback loop ensures that the steam turbine can be started up with a significant reduction in stresses. To devise and assess this start-up methodology, a flexible and accurate dynamic model was implemented in the Simulink environment. It contains >100 component blocks (heat exchangers, valves, meters and sensors, turbines, controls, etc.), and the mathematical component submodels are based on physical models and experimental correlations. This makes the model generally applicable to other power plant installations. The model was validated against process data related to the three start-up types (cold start, warm start, hot start). On this basis, the optimization model is implemented with feedback loops that control, for example, the exit temperature of the gas turbine based on the actual steam turbine housing temperature, resulting in a smoother heating up of the steam turbine. The optimization model was used to define the optimal inlet guide vanes position and gas turbine power output curves for the three types of start-up. These curves were used during real power station start-ups, leading to, for cold and warm starts, reductions in the start-up time of, respectively, 32.5% and 31.8%, and reductions in the fuel consumption of, respectively, 47.0% and 32.4%. A reduction of the thermal stress in the steam turbines is also achieved, thanks to the new start-up strategy.

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References

Shirakawa, M. , Nakamoto, M. , and Hosaka, S. , 2005, “ Dynamic Simulation and Optimization of Start-Up Processes in Combined Cycle Power Plants,” JSME Int. J., 48(1), pp. 122–128. [CrossRef]
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Tica, A. , Gueguen, H. , Dumur, D. , Faille, D. , and Davelaar, F. , 2012, “ Hierarchical Model Predictive Control Approach for Start-Up Optimization of a Combined Cycle Power Plant,” IFAC Proc. Vol., 45(21), pp. 301–306. [CrossRef]
Casella, F. , Farina, M. , Righetti, F. , Scattolini, R. , Faille, D. , Davelaar, F. , Tica, A. , Gueguen, H. , and Dumur, D. , 2011, “ An Optimization Procedure of the Start-Up of Combined Cycle Power Plants,” IFAC Proc. Vol., 44(1), pp. 7043–7048. [CrossRef]
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Pour-mohamadian, H. , 1982, “ Transient Inelastic Thermal Stresses in a Solid Cylinder With Temperature Dependent Properties,” Ph.D. thesis, The Louisiana State University and Agricultural and Mechanical College, Baton Rouge, LA. https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?referer=https://www.google.co.in/&httpsredir=1&article=4734&context=gradschool_disstheses

Figures

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Fig. 3

Digital control system and MANUAL control options compared

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Fig. 2

Trends of the main GT variables during start-up. The shaft speed is plotted on the left axis, the IGV angle, pressure ratio, and generator output on the right axis.

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Fig. 1

The energy flows linked to the flue gas (red/dashed), water (dark blue/dotted), steam (light blue/solid), and electrical power (yellow/long dash) are described as well

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Fig. 4

Valves opening trends in the first cold start period. This trend shows that the stop valves open at 70 min when the superheating of the pipe wall reaches 20C.

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Fig. 5

The red (dashed) and light blue (solid) arrows indicate, respectively, the flue gas flow and steam flows; the black (dotted) arrows indicate the control signals

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Fig. 6

Scheme of the gas turbine system model

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Fig. 7

The red (dashed) arrows indicate the flue gas flow; the dark (dotted) and light blue (solid) arrows indicate, respectively, the water and steam flows

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Fig. 10

P/Q ratio fitted curve for the HP steam turbine

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Fig. 9

Structure of the Steam turbines subsystem

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Fig. 14

MP steam temperature validation for a warm start

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Fig. 15

Steam turbines shaft speed validation for a warm start

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Fig. 8

Schematic representation of the Valves subsystem

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Fig. 11

HP steam superheating temperature validation for a cold start

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Fig. 12

MP steam flow validation for a cold start

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Fig. 13

HP turbine reference temperature validation for a cold start

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Fig. 18

Cold start-up time from 4:37 h to 3:07 h (end of start: closure of the bypass valve)

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Fig. 19

Warm start-up time from 1:15 h to 0:58 h (end of start: closure of the bypass valve)

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Fig. 20

Cold start-up maximum stress peak from 403.5 MPa to 362.9 MPa (HP steam turbine rotor)

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Fig. 16

Temperature profiles for flue gas temperature, HP steam temperature, and rotor temperature for a traditional cold start. The high-temperature difference at the beginning of the start lead to fast heating up and thereby high stresses.

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Fig. 17

Model temperature profiles for an optimized cold start. In the optimization, the temperature differences are optimized to reduce rotor stress.

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Fig. 21

Warm start-up maximum stress peak from 220.2 MPa to 129.0 MPa (HP steam turbine rotor)

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