Computational fluid dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate ...

(CFD) are used to understand complex thermal flow regimes in

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...

s. The thermal power plant may be divided into different subsectors and the CFD analysis applied to critical equipment/components - mainly different types of

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...

s - which are of crucial significance for efficient and trouble free long-term operation of the plant.

Overview

The

thermal power station A thermal power station is a type of power station in which heat energy is converted to electrical energy. In a steam-generating cycle heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a stea ...

subsystem involves multiphase flow, phase transformation and complex chemical reaction associated with conjugate heat transfer. Typical thermal power plant layout

Methods

Finite difference method

Finite difference method In numerical analysis, finite-difference methods (FDM) are a class of numerical techniques for solving differential equations by approximating derivatives with finite differences. Both the spatial domain and time interval (if applicable) are ...

describes the unknowns of the flow problem by means of point samples at the node points of a grid co-ordinate lines.

Taylor series In mathematics, the Taylor series or Taylor expansion of a function is an infinite sum of terms that are expressed in terms of the function's derivatives at a single point. For most common functions, the function and the sum of its Taylor ser ...

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 In mathematics, an algebraic equation or polynomial equation is an equation of the form :P = 0 where ''P'' is a polynomial with coefficients in some field, often the field of the rational numbers. For many authors, the term ''algebraic equation'' ...

Finite element method

Finite element method The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat ...

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

Finite volume method The finite volume method (FVM) is a method for representing and evaluating partial differential equations in the form of algebraic equations. In the finite volume method, volume integrals in a partial differential equation that contain a divergenc ...

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.

Application of CFD in thermal power plants

Low NOx burner design

When fossil fuels are burned, Nitric oxide and

Nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...

are produced. These pollutants initiate reactions which result in production of

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

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.

CFD analysis of economiser

The

economiser Economizers (US and Oxford spelling), 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, po ...

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 In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...

/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.

CFD analysis of superheaters

Superheaters, which are generally placed in the radiant zone of the furnace, are used for increasing the temperature of dry

saturated steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

coming out from boiler drum and to maintain the required parameters before sending it to the steam turbine. The

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.

CFD analysis of pulverized coal combustion

In a thermal power plant combustion of fuel, especially

pulverized coal Coal dust is a fine powdered form of which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal. It is a form ...

, 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.

CFD application in other areas of thermal power plants

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. 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.Jairo Z. Souzaa, Leonardo P. Rangel" Numerical Simulation of Coal Boiler at Electric Thermal Plants Using Computational Fluid Dynamics" 10th International Symposium on Process Systems Engineering - PSE2009 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.