Power generation

Introduction

The availability of electricity and its per capita consumption is often regarded as an index of national standard of living in the present day civilization. The amount of power generated is a sign of growing gross national products which reflects prosperity of the people. Energy goes in tandem with progress. The lack of it and inadequate measure can throttle the entire economic activity and well being of the country. Therefore, energy is considered as the most basic input for any country for keeping the wheels of its economy moving.

Electricity is an easy form of energy which can be produced easily, transported easily, can be used easily and also controlled easily. Power plants are used to produce electricity in bulk quantities.

However, in a developing country like India, the demand for power is increasing at a very rapid rate. As a result the use of renewable forms of energy is being increased and at the same time actions are being taken to improve the overall efficiency of the existing power plants.

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Combined cycle power plant couples a steam and a gas power plant in order to improve the overall efficiency to 70%. This combined cycle recovers much of the exhaust energy and uses it further to drive a steam or gas turbine or a district heating plant. As a result, there is an increase in the power produced and at the same time it reduces additional cost and the generating cost.

There are many other benefits of a combined cycle power plants. Benefits like high efficiency and low environment impact are the most important. In today’s world, emission levels of all kinds of fuels must meet stringent regulations acceptable to the country’s government. It is therefore important for power producers to invest in plants which have low emissions level. Risk mitigation and public acceptance are paramount. Combined cycle plants especially those which use natural gas or other renewable resources are a good choice for low emissions. Carbon dioxide emissions and other gases produced in a combined cycle power plant are much lower than other fossil fuel technologies.

Power Generation System

Simple Power Plant Cycle

A simple power plant generation cycle utilizes only a single thermodynamic cycle at a time. It converts the energy stored in fossil fuels into shaft work and ultimately into electricity. It uses water which is generally in the liquid state and sometimes even in the vapor state, as the working medium. The energy which is released by the burning the fuel is used to heat water and convert it to steam which is then further used to run a turbine to generate electricity. The steam which leaves the turbine is sent to a condenser such that the water can be re used. However, the efficiency of a simple cycle is still less and large amount of exhaust is emitted at the end of the cycle.

The efficiency of a simple power plant is given by:

ή(cycle) =W(net)/Q(1)=W(T)-W(P)/Q(1)=Q(1)-Q(2)/Q(1)

=1-Q (2)/Q (1)

The major components of a simple power plant cycle are as follows:

(i). Compressor

(ii). Combustor

(iii). Power turbine

(iv). Generator

In a simple power plant as seen, ambient air is filtered and then compressed to a pressure of 14 to 30 bar (190 to 420 psig). As the air is compressed its pressure increases which in turn are used to burn the fuel producing hot gases with a temperature generally higher than 1,000 degree Celsius. This then expands in a turbine driving the compressor and generator. The expanded hot gases leave the turbine at ambient pressure and at a temperature between 450 to 650 degree Celsius depending on the turbine efficiency, pressure ratio, and turbine inlet temperature. Such a power plant has 35% efficiency only.

Gas Compressor

It is a mechanical device which is used for increasing the pressure of a gas by decreasing its volume. Generally air is used in a power plant for compression, however, oxygen, nitrogen and other gases are also compressed. There are three general types of compressors, namely, positive displacement, axial and centrifugal.

Positive displacement compressors may be reciprocating piston type, in which the gas is taken in during the suction stroke of the piston, and is compressed by decreasing the volume of the gas by moving the piston in the opposite direction.

Centrifugal compressors use high speed impeller to increase the energy of the gas which is then converted into pressure in the diffuser. They are used to compress large volume of gas to moderate pressures.

In Axial compressors gas is made to flow parallel to the axis of rotation of the rotor which in turn contains a number of rows of blades. As the gas passes through the blades its energy increases which is converted into pressure. This type of compressor is used for jet aircraft engines and gas turbines.

Combustor

It is a part of turbine where the combustion takes place. In a gas turbine, air enters the first combustion chamber after the compressor. Here, fuel is mixed with the compressed air and the exhaust is then used to drive the turbine to obtain work.

A combustor should fulfill the four basic conditions:

(i). Supply enough air for complete combustion of air.

(ii). Secure enough turbulence for thorough mixing of fuel and air.

(iii). Maintain a furnace temperature high enough to ignite the incoming fuel air mixture.

(iv). Provide a furnace volume large enough to allow time for combustion to be completed.

It is important to determine the right amount of oxygen that should enter the combustor. Amounts of Carbon-di-oxide and oxygen are indicators of excess air. If the amount of oxygen is more in the combustor than the required amount, it will burn fuel more rapidly and the efficiency would reduce.

A combustor has three main components, namely, the outer casing which is the high pressure container, the combustion chamber, and the fuel injection system. [1][4]

Turbine

A steam turbine is a prime mover which continuously converts the energy of high-pressure, high temperature steam supplied by a steam generator into shaft work with the low temperature steam exhausted to a condenser. This energy conversion essentially occurs in two steps:

(i). The high pressure, high temperature steam first expands in nozzles and comes out at high velocity.

(ii). The high velocity jets of steam coming out of the nozzles, impinge on the blades mounted on a wheel, get deflected by an angle and suffer a loss of momentum which is absorbed by the rotating wheel in producing torque.

A steam turbine is basically an assemblage of nozzles and blades. The fig

of a Turbine is given below.

Many types of turbines are used in Power Plants, namely:

1. Steam turbines are used for electricity generation in thermal power plants, i.e. plants using coal and fuel.

2. Gas turbines are also called turbine engines.

Every turbine is provided with an emergency control lever which can immediately shut down the running of turbines in case a catastrophy occurs. Each power plant has a fixed turbine rotation rate. These rates at which the turbines rotate are different for different process. In a power plant the turbines fail to operate if:

1. Shaft speed exceeds 3300 rpm

2. The lubrication system fails

3. Turbine balancing is not proper

4. Temperature of plant increases

5. Cooling mechanisms not working properly

Control And Supervisory Instruments:

Many control and supervisory instruments are provided for the safe and effective operation of a turbine. They are as follows:

1. Pressure gauges are used to monitor the pressure of main steam at various valves.

2. Thermometers are used to record temperatures at every valve and in the combustor valve.

3. A Speed recorder is used to monitor the turbine rpm all the time and in case its speed increases beyond a set value then it is deactivated.

4. Watt meters and voltmeters are used to determine the steam and heat rates at various points in the process.

5. A trip control lever is provided in case of an emergency.

Generator

It is a device that converts mechanical energy into electrical energy. A steam generator generates steam at the desired pressure and temperature by burning fuel in its furnace. A generator forces electric charges to be in motion through an exterior electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings.

As it can be seen from Fig 4, heat is produced in a generator because of losses caused by current flow in the stator and field windings. This affects the temperature in the generator. By using cooling mechanisms such an event can be avoided. Generators are usually cooled using hydrogen at very high pressures say at 3 bars. Hydrogen is used since its specific heat is the highest and its molecular weight is the least.

Combined Cycle Power Generation

The Carnot efficiency is the efficiency of an ideal thermal process. Generally the efficiencies of some processes are less since there are a large amount of losses involved. Thus, a distinction between energetic and exergetic losses is drawn. Energetic losses are mainly losses incurred due to heat, and are thus the energy lost in the process. Exergetic losses are internal losses caused by irreversible processes in accordance with the second law of thermodynamics.

The process efficiency can be improved by raising the maximum temperature in the cycle, releasing the waste water at a lower temperature or by improving the process to minimize the internal exergetic losses.

The interest in combined cycles arises particularly from these considerations. Although, no single cycle can make all the improvements. It thus seems reasonable to combine two cycles to one.

Supplementary firing may raise the exhaust temperature to around 900 degree Celsius. Also, high gas temperature raises the condition of steam which in turn improves the efficiency of a power plant.

The two thermodynamic cycles generally used in a combined cycle power

generation are Brayton cycle and Rankine cycle.

Brayton Cycle

In Brayton cycle the air is first compressed and then at constant pressure reversibly heat is added to it. Due to this, expansion of air takes place in the turbine. This leads to emission of hot gases at a constant pressure which helps in bringing it to the initial state. The Brayton cycle consists of 4 cycles in total, out of which two are reversibly isentropic and the other two are reversible adiabatic. Also a Brayton cycle is called as the air standard cycle for the gas turbine power plant. [1]

Cycle Processes:

A. 1-2 Isentropic Compression (q = 0)

B. 2-3 Isobaric Heat Addition (w = 0)

C. 3-4 Isentropic Expansion (q = 0)

D. 4-1 Isobaric Heat Rejection (w = 0)

Rankine Cycle

We can use a hypothetical value for every process in the vapor power cycle which represents its basic intended operation and something which does not produce any extraneous effect. From the steam boiler, it is used as a constant pressure heating process to convert water to form steam, for the turbine as an ideal reversible adiabatic expansion of steam, for the condenser it is a reversible constant pressure heat rejection as the steam condenses till it becomes saturated liquid, and for the pump, the ideal process is the reversible adiabatic compression of this liquid ending at the initial pressure.

When all these four processes are ideal, the cycle is called a Rankine cycle.

There are four basic steps in a Rankine cycle:

Step1-2: working fluid is pumped to high pressure from its initially low pressure state. This process requires low energy.

Step2-3: the high pressure working fluid then enters a boiler where it is heated continuously such that it becomes dry saturated vapour.

Step 3-4: the dry saturated vapour is then used to turn the blades of a turbine to generate power. The temperature and pressure of the working fluid decreases.

Step 4-1: the wet vapour then enters a condenser where it is condensed to become a saturated liquid.

District Heating

A district heating system uses hot water to bring heat to towns and communities rather than using electrical power. This technology is quite old. The oldest district heating system which is still in operation was used to warm a French village from geothermal hot springs in the fourteenth century. Later on, US were the first country to use the process of district heating in plants. A steam district heating system has been in use by the US Naval Academy since 1852. The first commercial system began in Denver in as early as 1880. As of today, nearly 30,000 district heating plants are working in the US and there are a thousand more around the world.

A district heating plant contains insulated pipes which carry hot water from the plant to various sites. Also these pipes are interconnected between various buildings through a junction point as can be seen from Fig 7. From these junction points, hot water is taken from the mains to a heat exchanger which is also called a heat sub-station and present inside each building. Because of this the heating circuit which is present inside each building can be isolated from the main heating system. A temperature sensor is present on the heat substation which monitors the temperature of water at all times. Also a meter is attached, which calculates the amount of water consumed by each apartment or building and are charged accordingly. To ensure safe and smooth running of the plant, human intervention is reduced by running the plant automatically.

Advantages Of Combined Cycle Power generation

The worldwide demand for combined cycle power plants is growing dramatically because of its large advantages, namely:

(i). High overall plant efficiency: up to 70% can be obtained.

(ii). Low investment costs: up to 30% lower than that required for a conventional steam power plant.

(iii). Small amount of water required: amount of cooling water required is only about 40 to 50% as much as for a steam plant.

(iv). Great operating flexibility: the simple steam cycle makes it possible to start up and shut down the plants quickly which also affects the efficiency.

(v). Phased installation: because the gas turbines can go into operation much sooner than the steam plant, installation in stages is possible. The gas turbine can keep on generating power as the steam plant is under construction. This makes it possible to adjust the growth in demand for energy in a grid.

(vi). Simplicity of operation: combined power plants are fully automatic and are suitable for operating where the operating staff is less experienced.

(vii). Low environmental impact: because of their low emission levels and high efficiency, such plants are suitable for use in a heavily populated region. [1][5]

Components Of A Combined Cycle Power Plant

The major components of a combined cycle power plant are listed below:

i. Fuel: Different types of fuel may be used, namely, natural gas, coal, oil, petrol, diesel or any other conventional source of energy. Generally, natural gas is used as primary fuel. Also, experiments are being conducted to use renewable sources of energy like solar energy, wind energy, geothermal energy, nuclear energy etc as the main source of fuel. Using renewable source if energy would decrease the cost of running the plant tremendously and increase the efficiency as well. [2]

ii. HRSG (Heat Recovery Steam Generator): HRSG is a heat exchanger. It is the link between the gas turbine and the steam turbine process. It is used to recover heat from a hot gas stream. It produces steam that can be used in a process. HRSG are commonly used in a combined-cycle power station, in which the exhaust is fed to the HRSG to generate steam which in-turn drives a steam turbine.There are three main types of HRSG’s, namely:

1. HRSG without supplementary firing.

2. HRSG with supplementary firing.

3. Steam generators with maximum supplementary firing.

HRSG without supplementary firing are the most common. Its main function is to convert the exhaust energy of gas turbine into steam. [2]

iii. Deaerator: the main function of a Deaerator is to remove air and other dissolved gases from the water or steam in gas/steam cycle. This is necessary as the high oxygen content present in the steam/water corrodes the components of a plant along with the pipes. Generally, oxygen content of more than 7 to 10 parts per billion(ppb) is highly dangerous. Deaeration should be done continually since small leakages of air at air flanges and pump seals in the part of the cycle vacuum cannot be avoided. The solubility of the gases increase at a higher temperature and low pressure. A Deaerator removes these gases by heating the feed water to the saturation temperature. This feed water then passes through a heat exchanger after which it is sprayed from the top. By coming in contact dissolved oxygen and carbon di oxide get released. The Deaerator is usually kept in between the feed water system such that the total pressure difference between the boiler and the condenser is shared equally between the condenser pump and the boiler feed pump. The Deaerator is not used water cooled nuclear power plants as there is a danger of radioactive substance release.

There are two types of Deaerators:

a. Tray type Deaerator

b. Spray type Deaerator

iv. Control System: In today’s world almost all cycles are automated in order to improve the efficiency and at the same time reducing human intervention. Control system is to a power plant what a brain is for a human body. Supervision, control, performing operations in a safe and reliable environment, continuously checking for leakages and faults is all done by the Control System. For this reason, the control and automation systems of a combined cycle power plant form a relatively complex system even though the thermal process is fairly simple. Fully electronic control systems are employed in today’s modern combined cycle plants.

The main features of the control system for a combined cycle power plants are:

1. Truly distributed architecture

2. Complete range of functions for process control

3. Communication capability due to several bus levels

4. Compliance with standard communication protocol

5. Openness for third party applications

6. On-line programmability with easy creation/editing of the programs

v. Cooling system: An engine that is not cooled will soon reach a temperature which will seriously harm its functioning. If the heat is too intense it can cause the lubricant to burn up, leading to souring of cylinders, burning of pistons and bearings or any of variety of other mechanical troubles. Especially in a power plant where combustion of fuel continuously takes place for power generation, excess heat is like an enemy which needs to remove as soon as it is generated, least it would be dangerous.

Two types of cooling systems are used, Air cooling and Liquid cooling.

vi. Cogeneration: It is the simultaneous generation of electricity and useful heat. It is basically a recycling process. In conventional power plants, the waste heat is released into the atmosphere. Cogeneration captures this heat and re uses it for industrial heating purposes. In processes like district heating temperatures up to 130°C can be reached. Manhattan is the biggest steam district in the world. Many European countries also make use of this technology.

Using cogeneration a bottoming plant captures the byproduct heat for domestic and industrial purposes thus increasing the overall efficiency of a plant.

Paper and textile mills, chemical factories, sugar factories etc are few of the many industries which use saturated steam as the desired temperature for many purposes like heating and drying. For constant heating or drying steam is used. Also, the industry also needs power to drive its various machines. For this purpose combined cycle plants which use cogeneration are used since it serves both the purposes.

District Heating Through Cogeneration

Cogeneration involves the production of both electricity and thermal energy simultaneously from a common fuel source. The rejected or exhaust heat is used here for the process of district heating. Other applications included Desalination & cooling. District heating is the latest technology for heating of homes and buildings in cold places like Europe. Heat which is produced in the thermal section in form of hot water is transported to houses and other areas using insulated pipes, such that the temperature doesn’t change and a separate boiler is not needed at the receiving end.

Vapor Absorption System

A vapor absorption system can be used in the process of district heating. This system uses Ammonia which has a relative lower boiling point than water. Thus, less heat is needed to heat the water. Later ammonia can be removed from the water-ammonia mixture and reused again.

The flow chart for such a system is as follows:

Advantages Of District Heating

There are many advantages of using this technology. A few of them are as follows:

1. Heat is transported to the urban areas using a heat exchanger which has a longer life.

2. Water is the main carrier. It can be easily procured from sea (desalination) or underground water.

3. Heating equipment takes less space and can be easily installed.

4. It can be used all year along without ant breaks.

5. Efficiency is more.

6. Distribution system is controlled using Computer which automatically increases or decreases the amount of water being delivered, depending upon the needs.

Operation Of A Combined Cycle Power Plant

Usually a combined cycle power plants are operated automatically. There are switches available which make it possible to activate the starting or shutting down of the equipment from a central control room. The commands may be given by operating staff or from a high level starting program which runs automatically. The start and stop are decided on several parameters which are pre defined in the program.

Because the plants have a shorter start up time and an even quicker load change capability, the combined cycle power plants are often called to be dynamic in its behaviour. It is also quick in reacting, thus it is capable of following up quick changes. [2][6]

Modern CCPP in the 50-400 MW range can be started within the following times:

For a combined cycle power plant, Start up procedure is divided into three stages:

1. HRSG purging

2. Speeding up and synchronization of Gas turbine

3. Speeding up and synchronization of Steam turbine

Purging of the boilers is a very important process. It prevents any explosion from unburned hydrocarbons by running the gas turbine at a high ignition speed of about more than 30% of normal speed, which helps in blowing of air through the HRSG. Purging then depends on the volume of gas left behind in the gas turbine. After the purging has been done, the gas turbines run at nominal speed, synchronized and loaded to the desired level. [7]

Programmable Logic Controller

Plc Definition

A programmable Logic Controller, which is generally called as PLC, is a state of the art, digital industrial computer. A programmable logic controller is an industrial computer in which control devices such as limit switches, push buttons, temperature sensors or pressure sensors provide incoming control signals into the unit. An incoming control signal is called an Input.

Incoming control signals interact with instructions specified in the user ladder program, which tells the PLC how to react to the incoming signals. The user program also directs the PLC on how to control the field devices like starter lights. A signal going out of the PLC to control a field device is called an Output. [5]

Advantages Of Plc

The main advantages of using PLC in the field are as follows:

(i). Gain complete control of the manufacturing process

(ii). Achieve consistency in automation

(iii). Improve quality and accuracy

(iv). Work in difficult hazardous condition

(v). Shorten time to market

(vi). Lower the cost of quality, scrap and rework

(vii). Offer greater product variety

(viii). Control inventory[5]

How Does PLC Work

Microprocessor is the main working brain of all the computers. The computer’s microprocessor, which is technically known as the central processing unit (CPU), supervises many controls and instructions as defined by the user. The microprocessor responds to the input signals and follows the instruction that it has been programmed to do. When the PLC is running and is following the programmer’s instructions it is called as solving the user problem.

PLC’s follow the instructions that are stored inside their memory. Also they may store programs for future use. Each instruction that has been entered will be placed inside the memory in the increasing order. These lists of instructions are called as a Ladder diagram.

A basic PLC and its components are shown below.

The instructions that are required to be carried out are transferred to the memory of the controller using a computer. The ladder diagram is made by the user using various kinds of latches, timers, counters and other accessories available with the software. After the ladder diagram has been verified and corrected, we download the program into the processor’s memory. Downloading basically means transferring the program from a personal computer’s memory to the logic controller’s memory.

Before downloading a user program, the processor must be in the program mode. After downloading the entire program all the wires have to be connected properly unless the required outputs would not be shown. Also, one can download the program as many number of times as it may be required. The continual running of the program in the PLC is useful in continuously determining if any input is being changed by the user or not. This process is called scanning. According to the commands given and the kind of timers or counters used, the output would be shown which is basically turning of the output light, which is present on the right hand side of the controller.

A PLC interfaced with a computer is shown below.

The PLC has come a long way since the first time it was used for industry applications. In these years, the PLC’s usage has been increased drastically. It has been designed to withstand the harsh temperature. Since mostly PLC’s are employed in industries where the temperature is usually on a higher side, they have been made resistant to heat. It is for this reason it is called as industrially hardened device. Also PLC’s are small and easy to store. Also they require minimum of space. Also PLC gives the user the ability to try new things.

The PLC can easily be programmed for any number of times. The programs are developed from the ladder diagrams. As an industrial computer, the PLC can easily replace functions of timers or counters. Also any type of sequence can be tried on the software. Also these days’ functions like arithmetic and data manipulation or shift registers have been made available along with the software which has increased its range of operation.

The main benefit of using PLC is easy troubleshooting. In industries ladder diagrams may span for many pages. As a result it becomes very difficult to identify the errors. However, PLC software comes with an inbuilt troubleshooting device which tells the user where errors have occurred, if any. This helps in easy identifying of the problem and saves precious time. PLC, thus, is a very important tool in industry usage.

RS LOGIX 500

RSLogix is a used to run a particular or a full segment in an industry. RSLogix is a product of Allen-Bradley, which provide the best industry controllers. This software offers unbeatable productivity and is widely used in industries. A simpler version called RSLogix 500 was made by Allen-Bradley for laboratory purposes. I have used this software for interfacing different cycles in a combined cycle power plant. RSLogix 500 offers [5]:

(i). Flexible easy to use editors

(ii). Diagnostics

(iii). Troubleshooting tools

(iv). Time saving features and functionality

Simulation On Rs Logix 500

The project was divided into three parts:

1. Brayton Cycle

2. Rankine Cycle

3. District Heating

Ladder diagrams for each were constructed after thoroughly understanding each process in detail. Use of timers, counters and switches were made in the ladder diagrams to illustrate limit switches. Temperature and pressure values have been assumed and are not based on actual standards.

RSLogix 500 provides with a large variety of instruction palettes, like normally open & closed switches, timers, counters etc. Values of temperature and pressure sensors were assumed and are not based on factual data. Also, various kinds of up-counters and down-counters along with many types of timers were used in designing of the ladder diagrams. I started with designing of ladder diagram for Brayton cycle. Then I designed a Rankine cycle which uses the waste of Brayton cycle as the input and lastly I designed a District Heating plant which uses the wasteful outputs of both Brayton and Rankine cycles. A Combined Cycle Power Plant increases the efficiency of a power plant by almost double. As a result, its usage is increasing globally as more and more simple plants are being converted to combined cycle power plants.

Given below is the figure of an instruction palette as seen in the software.

After the Ladder logic is entered, the whole file or project is verified without errors and is given access for download.

The figure for the same is given below:

Brayton Cycle

The ladder diagram is as shown below. It has a lower efficiency than a steam cycle.

The above figure shows the ladder diagram for the Brayton Cycle. The first rung illustrates the fuel which is supplied. I have taken Natural gas as the main source of fuel and Diesel for emergencies. The fuel mixes with Air. The ratio of fuel: air is about 1:30. A timer which acts like a pressure sensor is attached in parallel with the compressor. Since this timer is attached to a done bit, it cuts off the fuel and air supply after 15 seconds. This is so done since it is assumed that after, say 15 seconds, the pressure in the compressor reaches 200 Bar. If the compressor is opened for more time, then the compressor might blow off because of extreme pressure. This pressure I have assumed is totally hypothetical and varies with the type of material used in the making of the compressor. After the air and fuel gets compressed i.e. the pressure increases and the volume decreases (according to Boyle’s law), then the compressor valve opens. The compressor mixture then enters a combustor where the combustion takes place. I have attached a up-counter in parallel with the combustor. The up-counter does the same function