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Computational fluid dynamics (CFD) are used to understand complex thermal flow regimes in power plants. The thermal power plant may be divided into different subsectors and the CFD analysis applied to critical equipment/components - mainly different types of heat exchangers - which are of crucial significance for efficient and trouble free long-term operation of the plant.
The thermal power station subsystem involves multiphase flow, phase transformation and complex chemical reaction associated with conjugate heat transfer.
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Finite difference method describes the unknowns of the flow problem by means of point samples at the node points of a grid co-ordinate lines. Taylor series expansions are used to generate finite difference approximations of derivatives in terms of point samples at each grid point and its immediate neighbours. Those derivatives appearing in the governing equations are replaced by finite differences yielding an algebraic equation.
Finite element method uses piece wise functions valid on elements to describe the local variations of unknown flow variables. Here also a set of algebraic equations are generated to determine unknown co-efficients.
Finite volume method is probably the most popular method used for numerical discretization in CFD. This method is similar in some ways to the finite difference method. This approach involves the discretization of the spatial domain into finite control volumes. The governing equations in their differential form are integrated over each control volume. The resulting integral conservation laws are exactly satisfied for each control volume and for the entire domain, which is a distinct advantage of the finite volume method. Each integral term is then converted into a discrete form, thus yielding discretised equations at the centroids, or nodal points, of the control volumes.
When fossil fuels are burned, Nitric oxide and Nitrogen dioxide are produced. These pollutants initiate reactions which result in production of ozone and acid rain. NOx formation takes place due to (1) High temperature combustion i.e. thermal NOx and (2)Nitrogen bound to fuel i.e. fuel NOx and which is insignificant. In the majority of cases the level of thermal NOx can be reduced by lowering flame temperature. This can be done by modifying the burner to create a larger (hence lower temperature) flame, in turn reducing the NOx formation. The role of CFD analysis is vital for design and analysis of such low NOx burners. Many available CFD tools, such as CFX, Fluent, Star CCM++ with different models as RNG k-ε turbulence models with hybrid and CONDIF upwind differencing schemes has been used for analysis purpose and the data obtained with these analysis helped in modifying the burner design in turn lowering the adverse effect on the environment due to NOx formation during combustion. [1]
The economiser is a crucial component for efficient performance of a thermal power plant. It is a non-steaming type of heat exchanger which is placed in the convective zone of the furnace. It takes the heat energy of the flue gases for heating the feed water before it enters the boiler drum. The thermal efficiency/boiler efficiency largely depends on the performance of the economiser. CFD analysis helps in optimizing the thermal performance of the economiser by analysing the pressure, velocity and temperature distribution, and to identify the critical areas for further improvement with the result obtained by CFD analysis.
Superheaters, which are generally placed in the radiant zone of the furnace, are used for increasing the temperature of dry saturated steam coming out from boiler drum and to maintain the required parameters before sending it to the steam turbine. The thermal efficiency of a thermal power plant depends on the performance of the superheater. The CFD analysis of superheaters is done at design stage and later at the troubleshooting and performance evaluation during the operation of the plant. The CFD results obtained can be useful for the maintenance engineer to make suitable predictions of the tube life and make suitable arrangements for the high temperature zone to reduce the erosion of the tube coil and restricting the tube leakage problem. CFD analysis consists of modelling the superheater and doing analysis to study the velocity, pressure and temperature distribution of the steam inside the superheater. The uneven temperature distribution of steam in the tube leads to boiler tube leakage. CFD also helps to study the effect of the operating parameters on the tube erosion rate. Thermal power plants operates round the year and it is not always possible to shut down and analyse the problem. CFD helps in this.
In a thermal power plant combustion of fuel, especially pulverized coal, is of significant importance. Proper and complete combustion, with the required proportions of air and fuel, is required for total energy transfer to water for steam generation and to reduce pollutants. CFD models based on fundamental conservation equations of mass, energy, chemical species and momentum can be used to simulate the flow of air and coal through the burners. The results obtained from CFD analyses give insight to identify the potential areas for improvement. [2]
There are some other areas of importance where CFD can play a significant role in performance and efficiency improvement. The unbalanced coal/air flow in the pipe systems of coal fired power plants leads to non-uniform combustion in the furnace, and hence an overall lower efficiency of the boiler. A common solution to this problem is to put orifices in the pipe systems to balance the flow. If the orifices are sized to balance clean airflow to individual burners connected to a pulverizer, the coal/airflow would still be unbalanced and vice versa. The CFD with standard k–e two-phase flow model can be used to calculate pressure drop coefficients for the coal/air as well as the clean air flow. [3]
The CFD is also used to obtain the numerical solution to address the problem of water wall erosion of the furnace of a thermal power plant. This is caused by flame misalignment, thermal attack and erosion due to the contact with chemicals. The flame misalignment occurs because of alteration in fluid dynamics factors due to burner geometry. CFD results show velocity profiles, pressure profiles, streamlines and other data that is helpful in understanding the fluid flow phenomena inside the equipment. [4] It is clearly evident from above examples how crucial is the application of CFD in addressing the bottlenecks in thermal power plants, improving power plant efficiency and assisting in maintenance decisions.
A boiler is a closed vessel in which fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels.
A fire-tube boiler is a type of boiler invented in 1828 by Mark Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.
A high pressure watertube boiler is a type of boiler in which water circulates in tubes heated externally by fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.
A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet.
A magnetohydrodynamic generator is a magnetohydrodynamic converter that transforms thermal energy and kinetic energy directly into electricity. An MHD generator, like a conventional generator, relies on moving a conductor through a magnetic field to generate electric current. The MHD generator uses hot conductive ionized gas as the moving conductor. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this.
A heat recovery steam generator (HRSG) is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process (cogeneration) or used to drive a steam turbine.
A cyclone furnace is a type of coal combustor commonly used in large industrial boilers.
A thermal power station, also known as a thermal power plant, is a type of power station in which the heat energy generated from various fuel sources is converted to electrical energy. The heat from the source is converted into mechanical energy using a thermodynamic power cycle. The most common cycle involves a working fluid heated and boiled under high pressure in a pressure vessel to produce high-pressure steam. This high pressure-steam is then directed to a turbine, where it rotates the turbine's blades. The rotating turbine is mechanically connected to an electric generator which converts rotary motion into electricity. Fuels such as natural gas or oil can also be burnt directly in gas turbines, skipping the steam generation step. These plants can be of the open cycle or the more efficient combined cycle type.
An air preheater is any device designed to heat air before another process, with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.
A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal that is blown into the firebox.
Economizers, or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) may all use economizers. In simple terms, an economizer is a heat exchanger.
A pulverizer or grinder or flour mill is a mechanical device for the grinding of many different types of materials. For example, a pulverizer mill is used to pulverize coal for combustion in the steam-generating furnaces of coal power plants.
A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combusted in an industrial furnace, a power plant's steam-generating boiler, or other large combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water vapor, as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 420 metres (1,380 ft), to increase the stack effect and dispersion of pollutants.
Recovery boiler is the part of kraft process of pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed wood. The black liquor is burned, generating heat, which is usually used in the process of making electricity, much as in a conventional steam power plant. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.
A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure but, at pressures above this, it is more usual to speak of a steam generator.
Staged combustion is a method used to reduce the emission of nitrogen oxides (NOx) during combustion. There are two methods for staged combustion: air staged supply and fuel staged supply. Applications of staged combustion include boilers and rocket engines.
High Marnham Power Station was a coal fuelled power station in Nottinghamshire, to the west of the River Trent, approximately 0.5 miles (0.8 km) north of the village of High Marnham. Construction site clearance began in November 1955, No. 1 Unit power generation commenced in October 1959, and the station became fully operational in June 1962. The plant operated until 2003 when it was decommissioned, though the cooling towers weren't demolished until 2012.
Three-drum boilers are a class of water-tube boiler used to generate steam, typically to power ships. They are compact and of high evaporative power, factors that encourage this use. Other boiler designs may be more efficient, although bulkier, and so the three-drum pattern was rare as a land-based stationary boiler.
A package boiler is a factory-made boiler. Package boilers are available in a range of standard designs. Package boilers are used for heating and act as a steam generator for small power purposes such as self-powered industrial plants. Package boilers are low pressure designs. A low pressure means low temperature water in the heat exchanger. The large difference between the flame temperature and the heat exchanger discards most of the available entropy. Discarding most of the entropy caps the thermodynamic efficiency below the range needed to make a low pressure boiler suitable for a co-generation plants even when the available capacity is adequate for the application. Advantages of package boilers are that they can be delivered and installed as a complete insulated assembly that doesn’t require a large exclusion zone around itself. The required steam, water, fuel, and electrical connections can be made rapidly. These boilers are inexpensive to operate because their automatic burner management system doesn’t require continuous supervision and they have low scheduled maintenance costs.