Course 0101 Combined Cycle Power Plant Fundamentals

Transcription

1 Course 0101 Combined Cycle Power Plant Fundamentals

2 Fossil Training 0101 CC Power Plant Fundamentals All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical reviews and certain other noncommercial uses permitted by copyright law. For permission requests, write to the publisher, addressed Attention: Permissions Coordinator, at the address below. Technical Training Professionals 9401 Indian Creek Parkway Suite 1170 Overland Park, KS Ordering Information: Quantity sales. Special discounts are available on quantity purchases by corporations, associations, and others. For details, contact the publisher at the address above. Orders by U.S. trade bookstores and wholesalers. Please contact Technical Training Professionals: Tel: (913) ; Fax: (316) or visit Printed in the United States of America

3 Fossil Training 0101 CC Power Plant Fundamentals Page 1 Combined Cycle Power Plant Fundamentals Course The combined cycle power plant fundamentals course will present the basic information regarding the design and operation of the major components of a combined cycle power plant. The course is divided into six sections, each covering a general topic, starting at the gas turbine and building towards a complete plant. This course will provide the trainee with a solid foundation of knowledge from which experience can be built upon Gas Turbines 0401 Heat Recovery Steam Generators 0501 Steam Turbines 0601 Generators 0701 Combined Cycle Plant Operation 1001 Gas Turbine Routine Maintenance

4 Fossil Training 0101 CC Power Plant Fundamentals Page 2 101_B5_Ch1 Combined Cycle Power Plant Overview 1. Introduction a. Welcome to the Combined Cycle Power Plant Fundamentals overview video. b. In this course, the trainee will learn about the basic fundamentals and equipment operating principles to understand the concepts that allow these machines to create electrical power. c. At the end of the program, the trainee will comprehend the operation of the combined cycle plant equipment and have the confidence to apply this knowledge to their daily routines.

5 Fossil Training 0101 CC Power Plant Fundamentals Page 3 2. Courses a. The combined cycle power plant fundamentals course contains the following sections: Section 0301, reviews the operating principles of each section of a typical gas turbine Section 0401 explains the function of the heat recovery steam generator Section 0501 reviews the operation of a typical steam turbine Section 0601 reviews the design and operation of a generator Section 0701 reviews the layout and operation of a combined cycle power plant, and Section 1001 covers routine maintenance of a gas turbine

6 Fossil Training 0101 CC Power Plant Fundamentals Page 4 3. Combined Cycle a. A combined cycle facility is an assembly of gas and steam turbines that work in tandem from the same source of heat. b. This heat is initially provided by the combustion of natural gas. The heat provided by natural gas is converted directly into mechanical energy in the gas turbines where it is used to turn an electric generator. c. The waste heat from the gas turbine is used to create steam in the heat recovery steam generator. The steam is then used to rotate the steam turbine. d. By combining these multiple streams of work, the overall net efficiency of producing electric power increases to as much as 60% as compared to a simple cycle plant which may be typically around 30%. e. If a plant does not utilize the heat recovery steam generators and steam turbines, it is called simple cycle. In a simple cycle gas turbine, the exhaust heat of combustion is generally released out the exhaust to the atmosphere.

7 Fossil Training 0101 CC Power Plant Fundamentals Page 5 4. Gas Turbine a. The first cycle of a combined cycle power plant is the gas turbine. Here outside air is drawn into the gas turbine through the inlet air house. The air travels through the compressor section of the gas turbine where its pressure and temperature will rise. b. This air is forced through the fuel combustion section where it mixes, ignites, and expands hot gases through the turbine section. 5. Rotation

8 Fossil Training 0101 CC Power Plant Fundamentals Page 6 a. The hot gases rapidly move and expand through the turbine blades toward the heat recovery steam generator causing the rotor to spin at approximately 3600 rpm when the coupled generator is synchronized with the electrical grid. b. As the gas turbine shaft rotates, it also rotates an electro magnet inside the three-phase electrical generator where it induces electrical current into the stationary phases of the generator. The electrical current flows from the generator through three individual conductors called isophase conductors that are connected to a step-up transformer. 6. Exhaust a. Hot exhaust gas exiting the gas turbine is still very hot, typically at temperatures approximately 1,150 degf. b. Within the heat recovery steam generator, or HRSG, the hot gases flow over finned boiler tubes. As hot gases travel over the finned tubes, the heat is transferred to the water traveling on the inside of the tubes. c. The heat energy gained by the water in the tubes eventually causes the creation of high pressure steam. d. Steam produced in the HRSG high pressure superheater joins with the steam from other HRSGs if they are available and is piped to the steam turbine.

9 Fossil Training 0101 CC Power Plant Fundamentals Page 7 7. Steam a. The steam flows through the high pressure section of the steam turbine where its thermal energy will be converted to mechanical energy. The steam at the HP turbine exhaust will return to the HRSG for reheating and back to the IP turbine section and the low pressure turbine. b. The mechanical energy produced by the steam is converted to electrical power by the coupled generator which will supply the local grid.

10 Page 8 Course 0301 Gas Turbine Fundamentals

11 Page 9 Gas Turbine Fundamentals Course Learning Objectives, Chapter: 1. Simple Cycle In this course, we will review the operation of standalone gas turbine. The Brayton cycle is explained and the advantages of a combined cycle power block are introduced. 2. Air Path This chapter will review the operation of the axial flow compressor found on all gas turbines. Although similar in construction each compressor is designed differently to match the purpose of the specific gas turbine. 3. Hot Gas Path This chapter will cover the design and operation of a typical gas turbine combustion system that includes the combustors, control or power production, and combustion. 4. Turbine Section This chapter covers the operation of the turbine section of the machine which converts thermal energy into mechanical torque. 5. Auxiliary Equipment This chapter reviews the gas turbine systems that make operation of the machine possible. These systems include lubricating oil system, hydraulic oil system, lift oil system, and other control type equipment.

12 Page _B5_Ch1 Simple Cycle 1. Gas Turbine a. The gas turbine, also referred to as a combustion turbine, is an internal combustion engine where a very lean mixture of fuel and air is burnt. b. The resulting hot pressurized gas expands through a series of stationary and moving blades in the turbine section, causing rotation of the shaft. Classroom Discussion Point: 1. Gas turbines produce power at a relatively higher cost when compared to other sources of power, but they can produce large amounts of power and are only limited by the supply of fuel. A clear advantage over hydroelectric plants is the high power production and availability; over coal plants is the post combustion byproducts; and nuclear plants, environmental concerns, regulation, and byproduct disposal. 2. Based on a 2014 FERC Form 1 report, Nuclear power plants produce power at 2.68 cents per kilowatt-hour, Coal power plants at 3.9 cents per kilowatt-hour, hydro-electric plants at 1.19 cents per kilowatt-hour, and gas turbines at 4.26 cents per kilowatt-hour. 3. Some of the more desirable aspects of gas turbines is the relatively short installation time and the footprint is generally smaller.

13 Page Process a. On a basic gas turbine, there are four processes that take place, compression in the compressor, heat addition in the combustion chambers, expansion through the turbine, and exhaust to atmosphere where heat is lost. Reference Image: See Reference 1 for an overview of a simple cycle gas turbine configuration.

14 Page 12 Reference 1 Gas turbine simple cycle configuration Copyright 2016 by Technical Training Professionals

15 Page Compression a. The first process in the cycle is the drawing of air from the atmosphere through an air filter and into the compressor. b. As the air passes through the compressor, its pressure and temperature will rise. c. The increase of the pressure is referred to as pressure ratio and varies by gas turbine design. For example, if the pressure ratio is 10 to 1, then the air pressure at the compressor exhaust is 10 times that of atmosphere, for this example, 147 psi absolute. Classroom Discussion Point: 1. Higher compression results in more efficient compressor operation. Manufacturers must balance the efficiency that can be reached with the amount of power it will consume attempting to reach it since so much power produced by the turbine is used to turn the compressor. 2. Air inlet houses condition the air for use in the compressor. See Reference 2 for a cutaway of a typical air inlet house.

16 Page 14 Reference 2 Gas turbine inlet house Copyright 2016 by Technical Training Professionals

17 Page Turbine a. As the gas passes through the turbine, its temperature and pressure drop. The gas has given up heat energy and converted it to mechanical energy. b. The gas is finally exhausted to the atmosphere. This is a simple cycle gas turbine. Classroom Discussion Point: 1. The hot exhaust gas is at high pressure at the inlet of the turbine section, as opposed to air drawn through the compressor, as the gas expands, its temperature and pressure will be reduced. This expansion occurs as the gas passes through the turbine section, making it spin. 2. Turbine stage blade size increases toward the exhaust of the machine. As the gas pressure is reduced by each stage of blades the increased blade area continues to extract energy.

18 Page Designs a. Although compressors are designed differently to meet the purpose for the turbine, they all perform the same function. b. Here we can see the compressor of a 7FA gas turbine, here is a 501FD2, here is an SGT-800, and here is an example of an aero-derivative LM6000 gas turbine. Classroom Discussion Point: 1. Many new, modern large gas turbine designs include variable guide vanes on several stages of compression. This multiple vane design is borrowed from smaller aero-derivative machine designs. 2. Aero-derivative gas turbines will typically use multiple compressor sections to achieve much higher pressure ratios. These compressor sections will rotate at different speeds. In the following picture, the exhaust gas drives two separate turbines, each connected to a compressor.

19 Page Ignition a. The compressed air enters the combustion section where it is mixed with incoming fuel. b. At startup, a spark plug produces the initial ignition of the fuel and air mixture. c. Due to the combustion, the temperature of the fuel-air mixture will be around 1800 degf; higher for larger machines. d. The temperature of combustion will increase as the amount of fuel is increased to accommodate the megawatt load increase.

20 Page 18 Classroom Discussion Point: 1. Spark plugs are relatively simple devices, utilizing a small transformer to increase the voltage to a level which will cause a spark. 2. Turbines with multiple combustion chambers have the chambers connected in such a way that only one sparkplug is required to ignite all of the chambers. Any additional sparkplugs are for backup.

21 Page Expansion a. As the gas expands through the turbine, its temperature and pressure fall as it converts heat energy into mechanical work before exhausting to the atmosphere at low pressure and a temperature of around 1000 degf. b. The turbine shaft is connected to a generator. The rotation of the generator produces power. Classroom Discussion Point: Heat is energy; the high temperature of the exhaust will be dissipated into the atmosphere in simple cycle configurations.

22 Page Brayton Cycle a. This simple cycle gas turbine operation is called the Brayton cycle and is represented on a chart. The chart compares temperature to extraction or addition of heat. b. The chart depicts that from point A to point B, air drawn into the compressor will experience an increase in temperature with no heat addition from an external source. c. At point B, fuel is added and combustion takes place, the temperature rises as heat is added. d. At point C, the hot gas enters the turbine and expands, converting the heat energy into mechanical energy. e. At point D, the gas exits the machine and out to the atmosphere, where the remaining heat is lost. The cycle repeats as air at ambient temperature is drawn into the compressor. Classroom Discussion Point: The Brayton cycle curve is typically used by engineers during the design phase of a gas turbine. The data plot can be used to track the changes in gas turbine performance and each section of the machine.

23 Page Efficiency a. The efficiency of a gas turbine s Brayton cycle depends on the pressure ratio achieved by the compressor. b. On this chart, at a pressure ratio of 10 to 1, efficiency is around 50%. c. On a typical gas turbine, the compressor requires 50 to 60 percent of the power produced by the turbine. d. Smaller gas turbines utilize a dual shaft arrangement, where the compressor is driven by a separate turbine spinning at much higher speed, achieving higher pressure ratio and efficiency. Classroom Discussion Point: Although aero-derivative gas turbines have higher compression ratios, the volume of air they process is much lower than larger frame type gas turbines. The efficiency of compression still applies to all gas turbines.

24 Page Heat a. Because the exhaust contains a lot of heat, a heat recovery steam generator is installed to capture the energy. In the newest designs, the exhaust temperature at the stack can be as low as 185 degf. b. The heat recovery steam generator will produce steam at 100 to 1900 psi for use in industrial purposes such as district heating or other heating requirements. c. In a combined cycle facility, the steam is used to drive a steam turbine generator. Classroom Discussion Point: 1. The combined cycle exhaust temperature is very important, too low may cause damage to the HRSG rear tube metals, much higher than 170 to 185 degf means wasted energy. 2. The temperature spread from the inlet to the outlet of a HRSG requires that construction materials be selected to withstand the temperatures achieved in each section. High pressure superheater tubes will be made from more exotic metals than LP economizer tubes.

25 Page Combined a. It is typical to use two gas turbines and heat recovery steam generators to produce steam and drive a steam turbine which produces as much power as one of the gas turbines. b. The advantage of this arrangement is that the overall system efficiency is higher since the heat in the gas turbine exhaust is not wasted. c. Combined cycle plants will be covered in a separate training course. Classroom Discussion Point: 1. Capturing the energy of the exhaust does not stop there, combined cycle designs extract as much energy as possible while reducing wear and tear, and operating costs. 2. The biggest draw-back of a combined cycle plant is the long startup time. Even a hot start will take no less than 2 hours.

26 Page Operation a. Gas turbines used in simple cycle operations vary from smaller and rare 3 megawatt turbines up to larger and close to 100 megawatts. b. The smaller turbines are typically used for black start capability, where their output is enough to power the loads required to start a larger turbine. c. These smaller gas turbines are usually aero-derivative type, meaning their design is based on aircraft engines. d. The larger gas turbines in simple cycle operations are used for peaking capability. Due to their lower efficiency, they are only operated when the financial return exceeds the cost of operation.

27 Page 25 Classroom Discussion Point: 1. Black start capability refers to a plant that can start power production without backfeed from the grid. It is equipped with an independent power source, typically diesel generators or a small gas turbine. These generators can provide sufficient power to supply the start system for one main gas turbine. Without black start capability, the plant will remain in a shutdown condition. 2. An example of the power requirement is approximately 8 megawatts for a solid state starting device such an SFC (static frequency controller) or LCI (load commutated inverter). 3. Gas turbine generators are rated much higher than the gas turbine. This is because gas turbine output varies with ambient temperatures; in cold temperatures, gas turbines can produce much higher outputs than the guaranteed rated output. 4. Aero-derivative gas turbines can be at full load within 10 to 15 minutes for the most modern designs. Large gas turbines used in simple cycle (or peaking) service will take around 30 to 45 minutes to reach full load. Although slower, the larger gas turbines provide lower emissions at base load compared to their aero-derivative counter parts. They also require significantly less support equipment for operation (no water injection or external cooling systems).

28 Page 26 Test 1) Combustion A gas turbine is a machine that burns fuel and air to allow it to expand through stationary and rotating blades to rotate a shaft. A. True B. False 2) Cycle The basic thermodynamic cycle which describes the operation of a gas turbine is which of the following? A. Rankine cycle B. Carnot cycle C. Brayton cycle D. Vapor-compression cycle

29 Page 27 3) Brayton For which of the following reasons does the Brayton cycle indicate the inefficiency of a simple cycle gas turbine? A. The ratio of expansion through the turbine section is higher B. The large difference in temperature between exhaust and inlet C. The increase in mass through the turbine section D. The rise in pressure at the compressor section is minimal 4) Power Of all the power extracted from the fuel through the turbine section, approximately 60% is used to turn the compressor. A. True B. False

30 Page 28 5) Exhaust The high temperature of gas turbine exhaust gas can be used for which of the following? A. No use for it B. Fuel heating C. Steam production if equipped with a HRSG 6) Simple Due to the efficiency of a simple cycle gas turbine, they are typically used for full load, base-load operation. A. True B. False

31 Page 29 7) Stages The number of stages of a compressor is the multiple for the increase in pressure. A. True B. False 8) Compression As air is compressed in the compressor section of a gas turbine, which of the following parameters also increases? A. Temperature B. Gas cycle C. Mass D. Velocity

32 Page 30 Test Answers 1) True 2) C 3) B 4) True 5) C 6) False 7) False 8) A