View with images and charts 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 1.1
Physical 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. 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 (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 MaxwellBoltzmann 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 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. 1.8
Natural 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 (CH4), 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. Component
Typical wt. %
Methane (CH4)
70-90
Ethane (C2H6)
5-15
Propane (C3H8) and Butane (C4H10) < 5 CO2, N2, H2S, etc.
balance
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
2.3
2.Combined Cycle Power Plan SimpleCyclePowePlants The combined-cycle unit combines the Rankin (steam turbine) and Breton (gas Simple Cyclethermodynamic Power Plants (Open turbine) cycles Cycle) 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 Theinmodern power"Combined-Cycle gas turbine is a high-technology package that is steam comprised compressor, combustor, the figure Cogeneration Unit". Process can of bea also power turbine, generator, as shown in the figure "Simple-Cycle Gas Turbine". provided forand 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 Figure: 2.1.8 Gas Turbine should beSimple-Cycle as high as possible (see Carnot efficiency). This is achieved by combining the Rankine (steam) and Brayton (gas) thermodynamic cycles. Such an arrangement In aused gas for turbine, volumesis of air are compressed to high pressure in a(turbine) multistage compressor for marinelarge propulsion called Combined Gas (turbine) And Steam distribution to one or more combustion gases from the combustion chambers power an axial turbine that (COGAS). drives the compressor and the generator before exhausting to atmosphere. In this way, the combustion gases in a2.1 gasWorking turbine power the of turbine directly, rather thanplant requiring heat transfer to a water/steam cycle to power principle a combined cycle 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 water as high (for aero derivatives) giving thermal In a thermal power station is as the30:1 working medium. High pressure steam efficiencies of 35 percent or more for a simple-cycle gas turbine. requires strong, bulky components. High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive steel. These alloys limit practical steam 2.4 Combined Cycle Power Plantsthe lower temperature of a steam plant is fixed by the temperatures to 655 °C while boiling point of water. With these limits, a steam plant has a fixed upper efficiency of The35 combined-cycle unit combines the Rankin (steam turbine) and Brayton (gas turbine) thermodynamic to 42%. cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production supplygas a steam in the figure "Combined-Cycle Cogeneration Unit". Process An opentocircuit turbineturbine cycle as hasshown a compressor, a combustor and a turbine. For steam can be also provided for industrial purposes. 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: 2.1.9 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