Study of GAS POWER PLANT OPERATION & CONTROL

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Study of GAS POWER PLANT OPERATION & CONTROL

INTRODUCTION:

1.Gas

This page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas disambiguation.

Gas phase particles atoms, molecules, or ions move around freely.

A gas is a state of matter, consisting of a collection of particles molecules, atoms, ions, electrons, etc. without a definite shape or volume that are in more or less random motion

1.1Physical characteristics

Due to the electronic nature of the aforementioned particles, a “force field” is present throughout the space around them. Interactions between these “force fields” from one particle to the next give rise to the term intermolecular forces. Dependent on distance, these intermolecular forces influence the motion of these particles and hence their thermodynamic properties. It must be noted that at the temperatures and pressures characteristic of many applications, these particles are normally greatly separated. This separation corresponds to a very weak attractive force. As a result, for many applications, this intermolecular force becomes negligible.

A gas also exhibits the following characteristics:

Relatively low density and viscosity compared to the solid and liquid states of matter.
Will expand and contract greatly with changes in temperature or pressure, thus the term “compressible”.
Will diffuse readily, spreading apart in order to homogeneously distribute itself throughout any container.

1.1.1 Macroscopic

When analyzing a system, it is typical to specify a length scale. A larger length scale may correspond to a macroscopic view of the system, while a smaller length scale corresponds to a microscopic view.

On a macroscopic scale, the quantities measured are in terms of the large scale effects that a gas has on a system or its surroundings such as its velocity, pressure, or temperature. Mathematical equations, such as the extended hydrodynamic equations, Nervier-Stokes equations and the Euler equations have been developed to attempt to model the relations of the pressure, density, temperature, and velocity of a moving gas.

1.1.2 Pressure

The pressure exerted by a gas uniformly across the surface of a container can be described by simple kinetic theory. The particles of a gas are constantly moving in random directions and frequently collide with the walls of the container and/or each other. These particles all exhibit the physical properties of mass, momentum, and energy, which all must be conserved. In classical mechanics, Momentum, by definition, is the product of mass and velocity. <href=”#Newtonian_kinetic_energy” title=”Kinetic energy”>Kinetic energy is one half the mass multiplied by the square of the velocity.

The sum of all the normal components of force exerted by the particles impacting the walls of the container divided by the area of the wall is defined to be the pressure. The pressure can then be said to be the average linear momentum of these moving particles. A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define this quantity

1.1.3 Temperature

The temperature of any physical system is the result of the motions of the molecules and atoms which make up the system. In statistical mechanics, temperature is the measure of the average kinetic energy stored in a particle. The methods of storing this energy are dictated by the degrees of freedom of the particle itself (<href=”#Molecules” title=”Energy level”>energy modes). These particles have a range of different velocities, and the velocity of any single particle constantly changes due to collisions with other particles. The range in speed is usually described by the Maxwell-Boltzmann distribution.

1.1.4 Specific Volume

When performing a thermodynamic analysis, it is typical to speak of intensive and extensive properties. Properties which depend on the amount of gas are called extensive properties, while properties that do not depend on the amount of gas are called intensive properties. Specific volume is an example of an intensive property because it is the volume occupied by a unit of mass of a material, meaning we have divided through by the mass in order to obtain a quantity in terms of, for example, Notice that the difference between volume and specific volume differ in that the specific quantity is mass independent.

1.1.5 Density

Because the molecules are free to move about in a gas, the mass of the gas is normally characterized by its density. Density is the mass per volume of a substance or simply, the inverse of specific volume. For gases, the density can vary over a wide range because the molecules are free to move. Macroscopically, density is a state variable of a gas and the change in density during any process is governed by the laws of thermodynamics. Given that there are many particles in completely random motion, for a static gas, the density is the same throughout the entire container. Density is therefore a scalar quantity; it is a simple physical quantity that has a magnitude but no direction associated with it. It can be shown by kinetic theory that the density is proportional to the size of the container in which a fixed mass of gas is confined.

1.1.6 Microscopic

On the microscopic scale, the quantities measured are at the molecular level. Different theories and mathematical models have been created to describe molecular or particle motion. A few of the gas-related models are listed below

1.2 Kinetic theory

Kinetic theory attempts to explain macroscopic properties of gases by considering their molecular composition and motion.

1.3 Brownian motion

Brownian motion is the mathematical model used to describe the random movement of particles suspended in a fluid often called particle theory.

Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.

1.4 Intermolecular forces

“.As discussed earlier, momentary attractions (or repulsions) between particles have an effect on gas dynamics. In physical chemistry, the name given to these “intermolecular forces” is the “Vander Waals force

1.4.1 Simplified models

An equation of state (for gases) is a mathematical model used to roughly describe or predict the state of a gas. At present, there is no single equation of state that accurately predicts the properties of all gases under all conditions. Therefore, a number of much more accurate equations of state have been developed for gases under a given set of assumptions. The “gas models” that are most widely discussed are “Real Gas”, “Ideal Gas” and “Perfect Gas”. Each of these models have their own set of assumptions to, basically, make our lives easier when we want to analyze a given thermodynamic system.

1.5 Real gas

Real gas effects refer to an assumption base where the following are taken into account:

Compressibility effects
Variable heat capacity
Vander Waal forces
Non-equilibrium thermodynamic effects
Issues with molecular dissociation and elementary reactions with variable composition.

For most applications, such a detailed analysis is excessive. An example where “Real Gas effects” would have a significant impact would be on the Space Shuttle re-entry where extremely high temperatures and pressures are present

1.6 Ideal gas

An “ideal gas” is a simplified “real gas” with the assumption that the compressibility factor Z is set to 1. So the state variables follow the law. This approximation is more suitable for applications in engineering although simpler models can be used to produce a “ball-park” range as to where the real solution should lie. An example where the “ideal gas approximation” would be suitable would be inside a <href=”#Combustors” title=”Jet engine”>combustion chamber of a jet engine. It may also be useful to keep the elementary reactions and chemical dissociations for calculating emissions.

1.7 Perfect gas

By definition, A perfect gas is one in which intermolecular forces are neglected. So, along with the assumptions of an Ideal Gas, the following assumptions are added

By neglecting these forces, the equation of state for a perfect gas can be simply derived from kinetic theory or statistical mechanics.

This type of assumption is useful for making calculations very simple and easy to do. With this assumption we can apply the Ideal gas law without restriction and neglect many complications that may arise from the Vander Waals forces.

Along with the definition of a perfect gas, there are also two more simplifications that can be made although various textbooks either omit or combine the following simplifications into a general “perfect gas” definition. For sake of clarity, these simplifications are defined separately.

1.8Natural gas

Natural gas is a gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, propane, butane, and pentane—heavier hydrocarbons removed prior to use as a consumer fuel —as well as carbon dioxide, nitrogen, helium and hydrogen sulfide. It is found in oil fields (associated) either dissolved or isolated in natural gas fields (non associated), and in coal beds (as coaled methane). When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, marshes, and landfills (see landfill gas), as well as sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation particularly in cattle.

Since natural gas is not a pure product, when non associated gas is extracted from a field under supercritical (pressure/temperature) conditions, it may partially condense upon isothermal depressurizing–an effect called retrograde condensation. The liquids thus formed may get trapped by depositing in the pores of the gas reservoir. One method to deal with this problem is to reinvest dried gas free of condensate to maintain the underground pressure and to allow evaporation and extraction of condensates.

Natural gas is often informally referred to as simply gas, especially when compared to other energy sources such as electricity. Before natural gas can be used as a fuel, it must undergo extensive processing to remove almost all materials other than methane. The by-products of that processing include ethane, propane, butanes, pentanes and higher molecular weight hydrocarbons, elemental sulfur, and sometimes helium and nitrogen.

1.8.1 Chemical composition

The primary component of natural gas is methane (C H4), the shortest and lightest hydrocarbon molecule. It often also contains heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8) and butane (C4H10), as well as other sulfur containing gases, in varying amounts, see also natural gas condensate. Natural gas that contains hydrocarbons other than methane is called wet natural gas. Natural gas consisting only of methane is called dry natural gas.

Typical wt. %

70-90

5-15

< 5balance

Component
Methane (CH4)	Ethane (C2H6)	Propane (C3H8) and Butane (C4H10)	CO2, N2, H2S, etc.

Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and odorants can also be present Natural gas also contains and is the primary market source of helium. Mercury is also present in small amounts in natural gas extracted from some fields The exact composition of natural gas varies between gas fields.

Organ sulfur compounds and hydrogen sulfide are common contaminants which must be removed prior to most uses. Gas with a significant amount of sulfur impurities, such as hydrogen sulfide, is termed sour gas; gas with sulfur or carbon dioxide impurities is acid gas. Processed natural gas that is available to end-users is tasteless and odorless, however, before gas is distributed to end-users, it is odorized by adding small amounts of odorants (mixtures of t-butyl merchantman, isopropyl mercaptanthiol, tetrahydrothiophene, diethyl sulfide and other sulfur compounds), to assist in leak detection. Processed natural gas is, in itself, harmless to the human body, however, natural gas is a simple asphyxiate and can kill if it displaces air to the point where the oxygen content will not support life.

Natural gas can also be hazardous to life and property through an explosion. Natural gas is lighter than air, and so tends to escape into the atmosphere. But when natural gas is confined, such as within a house, gas concentrations can reach explosive mixtures and, if ignited, result in blasts that could destroy buildings. Methane has a lower explosive limit of 5% in air, and an upper explosive limit of 15%. Explosive concerns with compressed natural gas used in vehicles are almost non-existent, due to the escaping nature of the gas, and the need to maintain concentrations between 5% and 15% to trigger explosions.

GAS POWER PLANT

2.Combined Cycle Power Plan

The combined-cycle unit combines the Rankin (steam turbine) and Breton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure “Combined-Cycle Cogeneration Unit”. Process steam can be also provided for industrial purposes.

A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). The remaining heat from combustion is generally wasted. Combining two or more “cycles” such as the Brayton cycle and Rankine cycle results in improved overall efficiency.

In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. Most new gas power plants in North America and Europe are of this type. In a thermal power plant, high-temperature heat as input to the power plant, usually from burning of fuel, is converted to electricity as one of the outputs and low-temperature heat as another output. As a rule, in order to achieve high efficiency, the temperature difference between the input and output heat levels should be as high as possible (see Carnot efficiency). This is achieved by combining the Rankine (steam) and Brayton (gas) thermodynamic cycles. Such an arrangement used for marine propulsion is called Combined Gas (turbine)And Steam (turbine) (COGAS).

2.1 Working principle of a combined cycle power plant

In a thermal power station water is the working medium. High pressure steam requires strong, bulky components. High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive steel. These alloys limit practical steam temperatures to 655 °C while the lower temperature of a steam plant is fixed by the boiling point of water. With these limits, a steam plant has a fixed upper efficiency of 35 to 42%.

An open circuit gas turbine cycle has a compressor, a combustor and a turbine. For gas turbines the amount of metal that must withstand the high temperatures and pressures is small, and less expensive materials can be used. In this type of cycle, the input temperature to the turbine (the firing temperature), is relatively high (900 to 1,350 °C). The output temperature of the flue gas is also high (450 to 650 °C). This is therefore high enough to provide heat for a second cycle which uses steam as the working fluid; (a Rankine cycle).

In a combined cycle power plant, the heat of the gas turbine’s exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG) with a live steam temperature between 420 and 580 °C. The condenser of the Rankine cycle is usually cooled by water from a lake, river, sea or cooling towers. This temperature can be as low as 35 °C

Figure: 2.1.1 combined cycle power plant

Fossil fuel-fired (central) power plants use either steam or combustion turbines to provide the mechanical power to electrical generators. Pressurized high temperature steam or gas expands through various stages of a turbine, transferring energy to the rotating turbine blades. The turbine is mechanically coupled to a generator, which produces electricity.

2.1.1 Steam Turbine Power Plants:

Steam turbine power plants operate on a Rankin cycle. The steam is created by a boiler, where pure water passes through a series of tubes to capture heat from the firebox and then boils under high pressure to become superheated steam. The heat in the firebox is normally provided by burning fossil fuel (e.g. coal, fuel oil or natural gas). However, the heat can also be provided by biomass, solar energy or nuclear fuel. The superheated steam leaving the boiler then enters the steam turbine throttle, where it powers the turbine and connected generator to make electricity. After the steam expands through the turbine, it exits the back end of the turbine, where it is cooled and condensed back to water in the surface condenser. This condensate is then returned to the boiler through high-pressure feed pumps for reuse. Heat from the condensing steam is normally rejected from the condenser to a body of water, such as a river or cooling tower.

Steam turbine plants generally have a history of achieving up to 95% availability and can operate for more than a year between shutdowns for maintenance and inspections. Their unplanned or forced outage rates are typically less than 2% or less than one week per year.

Modern large steam turbine plants (over 500 MW) have efficiencies approaching 40-45%. These plants have installed costs between $800 and$2000/kW, depending on environmental permitting requirements.

Figure: 2.1.2 Combustion (Gas) Turbines

Combustion turbine plants operate on the Brayton cycle. They use a compressor to compress the inlet air upstream of a combustion chamber. Then the fuel is introduced and ignited to produce a high temperature, high-pressure gas that enters and expands through the turbine section. The turbine section powers both the generator and compressor. Combustion turbines are also able to burn a wide range of liquid and gaseous fuels from crude oil to natural gas.

The combustion turbine’s energy conversion typically ranges between 25% to 35% efficiency as a simple cycle. The simple cycle efficiency can be increased by installing a recuperate or waste heat boiler onto the turbine’s exhaust. A recuperate captures waste heat in the turbine exhaust stream to preheat the compressor discharge air before it enters the combustion chamber. A waste heat boiler generates steam by capturing heat form the turbine exhaust. These boilers are known as heat recovery steam generators (HRSG). They can provide steam for heating or industrial processes, which is called cogeneration. High-pressure steam from these boilers can also generate power with steam turbines, which is called a combined cycle (steam and combustion turbine operation). Recuperates and HRSGs can increase the combustion turbine’s overall energy cycle efficiency up to 80%.

Figure: 2.1.3 Gas turbine with Regeneration system

Combustion (natural gas) turbine development increased in the 1930’s as a means of jet aircraft propulsion. In the early 1980’s, the efficiency and reliability of gas turbines had progressed sufficiently to be widely adopted for stationary power applications. Gas turbines range in size from 30 kW (micro-turbines) to 250 MW (industrial frames). Industrial gas turbines have efficiencies approaching 40% and 60% for simple and combined cycles respectively.

The gas turbine share of the world power generation market has climbed from 20 % to 40 % of capacity additions over the past 20 years with this technology seeing increased use for base load power generation. Much of this growth can be accredited to large (>500 MW) combined cycle power plants that exhibit low capital cost (less than $550/kW) and high thermal efficiency.

The capital cost of a gas turbine power plant can vary between $35000-$950/kW with the lower end applying to large industrial frame turbines in combined cycle configurations. Availability of natural gas-fired plants can exceed 95%. In Canada, there are 28 natural gas-fired combined cycle and cogeneration plants with an average efficiency of 48 %. The average power output for each plant was 236 MW with an installed cost of around $ 500/kW.

2.1.2 Simple Cycle Power Plants (Open Cycle)

The modern power gas turbine is a high-technology package that is comprised of a compressor, combustor, power turbine, and generator, as shown in the figure “Simple-Cycle Gas Turbine”.

Figure: 2.1.4 Simple-Cycle Gas Turbine

In a gas turbine, large volumes of air are compressed to high pressure in a multistage compressor for distribution to one or more combustion gases from the combustion chambers power an axial turbine that drives the compressor and the generator before exhausting to atmosphere. In this way, the combustion gases in a gas turbine power the turbine directly, rather than requiring heat transfer to a water/steam cycle to power a steam turbine, as in the steam plant. The latest gas turbine designs use turbine inlet temperatures of 1,500C (2,730F) and compression ratios as high as 30:1 (for aero derivatives) giving thermal efficiencies of 35 percent or more for a simple-cycle gas turbine.

2.2 Brayton cycle

The Brayton cycle is a constant-pressu that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton (1830–1892), the American engineer who developed it, although it was originally proposed by Barber in 1791. It is also sometimes known as the Joule cycle. The Ericsson cycle is also similar but uses external heat and incorporates the use of a regenerator Like other internal combustion power cycles, The Brayton cycle is an open system, though for thermodynamic analysis it is conventionally assumed that the exhaust gases are reused in the intake, enabling analysis as a closed system.

Figure: 2.1.5 Idealized Braytpn Cycle

Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies. In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.

Here are two plots, Figure 1 and Figure 2, for the ideal Brayton cycle. One plot indicates how the cycle efficiency changes with an increase in pressure ratio, while the other indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values.

Figure:2.1.6 Brayton cycle efficiency

Figure: 2.1.7 Brayton Cycle Specific Power Output

A Brayton-type engine consists of three components:

A gas compressor

A mixing chamber
An expander

In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process. The compressed air then runs through a mixing chamber where fuel is added, a constant-pressure isobaric process. The heated (by compression), pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another theoretically isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.

2.2.1 Ideal Brayton cycle:

Isentropic process – Ambient air is drawn into the compressor, where it is pressurized.

Isobaric process – The compressed air then runs through a combustion chamber, where fuel is burned, Heating that air—a constant-pressure process, since the chamber is open to flow in and out.
Isentropic process – The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
Isobaric process – Heat Rejection (in the atmosphere).

2.2.2 Actual Brayton cycle:

Adiabatic process – Compression.

Isobaric process – Heat Addition.
Adiabatic process – Expansion.
Isobaric process – Heat Rejection.

In 2002 a hybrid open solar Brayton cycle was operated for the first time consistently and effectively with relevant papers published, in the frame of the EU SOLGATE program. The air was heated from 570 K to over 1000 K into the combustor chamber.

2.2.3 Methods to increase power

The power output of a Brayton engine can be improved in the following manners:

Reheat, wherein the working fluid—in most cases air—expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines. This has the advantage of increasing the power output possible for a given compression ratio without exceeding any metallurgical constraints (typically about 1000°C). The use of an afterburner for jet aircraft engines can also be referred to as reheat, it is a different process in that the reheated air is expanded through a thrust nozzle rather than a turbine. The metallurgical constraints are somewhat alleviated enabling much higher reheat temperatures (about 2000°C). The use of reheat is most often used to improve the specific power (per through put of air) and is usually associated with a reduction in efficiency, this is most pronounced with the use of after burners due to the extreme amounts of extra fuel used.

2.2.4 Methods to improve efficiency

The efficiency of a Brayton engine can be improved in the following manners:

Intercooling, wherein the working fluid passes through a first stage of compressors, then a cooler, then a second stage of compressors before entering the combustion chamber. While this requires an increase in the fuel consumption of the combustion chamber, this allows for a reduction in the specific volume of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall. There is also an increase in the maximum feasible pressure ratio due to reduced compressor discharge temperature for a given amount of compression, improving overall efficiency.

Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to pre-heat the fluid just entering the combustion chamber. This directly offsets fuel consumption for the same operating conditions improving efficiency. It allows also results in less power lost as waste heat.

A Brayton engine also forms half of the combined cycle system, which combines with a Rankine engine to further increase overall efficiency.

Cogeneration systems make use of the waste heat from Brayton engines, typically for hot water production or space heating.

2.2.5 Reverse Brayton cycle

A Brayton cycle that is driven in reverse, via net work input, and when air is the working fluid, is the air refrigeration cycle or Bell Coleman cycle. Its purpose is to move heat, rather than produce work. This air cooling technique is used widely in jet aircraft.

2.3SimpleCyclePowePlants

Simple Cycle Power Plants (Open Cycle)

The modern power gas turbine is a high-technology package that is comprised of a compressor, combustor, power turbine, and generator, as shown in the figure “Simple-Cycle Gas Turbine”.

Figure: 2.1.8 Simple-Cycle Gas Turbine

2.4 Combined Cycle Power Plants

The combined-cycle unit combines the Rankin (steam turbine) and Brayton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure “Combined-Cycle Cogeneration Unit”. Process steam can be also provided for industrial purposes.

Figure: 2.1.9 combined-cycle power plant

2.5 Steam Turbine Power Plants:

Steam turbine power plants operate on a Rankine cycle. The steam is created by a boiler, where pure water passes through a series of tubes to capture heat from the firebox and then boils under high pressure to become superheated steam. The heat in the firebox is normally provided by burning fossil fuel (e.g. coal, fuel oil or natural gas). However, the heat can also be provided by biomass, solar energy or nuclear fuel. The superheated steam leaving the boiler then enters the steam turbine throttle, where it powers the turbine and connected generator to make electricity. After the steam expands through the turbine, it exits the back end of the turbine, where it is cooled and condensed back to water in the surface condenser. This condensate is then returned to the boiler through high-pressure feedpumps for reuse. Heat from the condensing steam is normally rejected from the condenser to a body of water, such as a river or cooling tower.