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Per capita energy consumption is yard stick to measure the prosperity and growth of any society. For any infrastructural development the key is continuous supply of electricity .At world level in totality there is a huge gap between demand and supply of power . This gap can be bridged by proper operation and maintenance of existing thermal power plant so that they can run continuously without any failure apart from ,the more number of new installations of new thermal power plant . Now a days design modifications, troubleshooting and performance enhancement of thermal power plant with the help of CFD technique is rapidly picking up.

Introduction

The thermal power plant may be divided into different subsectors and the CFD analysis of critical equipments/components mainly different type of heat exchangers which are of crucial significance for efficient and trouble free long-term operation of thermal power plant .

• Boiler Section
  • Coal Burners
  • Economiser
  • Super heaters and Re heaters
  • Air Pre heaters
• Turbine Section
  • Turbine blade profile
  • Feed water Heaters
  • Boiler feed pumps
  • Condenser
• Flue Gas System
  • ID and FD fan profile analysis
  • Electrostatic precipitator performance ,as it critical to address environmental issues

The Thermal power plant subsystem involves multiphase flow ,phase transformation and complex chemical reaction associated with conjugate heat transfer.

Typical layout of thermal power plant

Steps involved in CFD for problem solving

Application of the CFD to analyze a fluid problem requires the following steps. First, a set of mathematical partial differential equations describing the fluid flow are written. Then These equations are discredited to produce a numerical analogue of the equations. The domain is then divided into small grids or elements. Finally, the initial conditions and the boundary conditions of the specific problem are applied to solve these equations. The solution method can be direct or iterative. In addition, certain control parameters are used to control the convergence, stability, and accuracy of the method. All CFD codes contain three main elements: (1) A pre-processor, which is used to input the problem geometry, generate the grid, and define the flow parameter and the boundary conditions to the code. (2) A flow solver, which is used to solve the governing equations of the flow subject to the conditions provided. (3) A post-processor, which is used to massage the data and show the results in graphical and easy to read format.

The equations governing the fluid motion are the three fundamental principles of mass, momentum, and energy conservation.

Continuity Equation

${\partial \rho \over \partial t}+\nabla \cdot (\rho \mathbf {u} )=0$

where

• ρ is fluid density,
• t is time,
• u is the flow velocity vector field.

Momentum Equation

${\begin{aligned}{\partial (\rho {\mathbf {u} }) \over \partial t}+\nabla \cdot (\mathbf {u} \otimes (\rho \mathbf {u} ))+\nabla p=\mathbf {0} \\[1.2ex]\end{aligned}}$

• ρ is the fluid mass density,
• u is the fluid velocity vector, with components u, v, and w,
• p is the pressure,
• $\otimes$ denotes the tensor product, and
• 0 is the zero vector.

Energy Equation

${\begin{aligned}{\partial E \over \partial t}+\nabla \cdot (\mathbf {u} (E+p))=0,\end{aligned}}$

where

• u is the fluid velocity vector, with components u, v, and w,
• E = ρ e + ½ ρ ( u² + v² + w² ) is the total energy per unit volume, with e being the internal energy per unit mass for the fluid,
• p is the pressure, and
• 0 is the zero vector.

The governing equations shown above are partial differential equations (PDEs). Since digital computers can only recognize and manipulate numerical data, these equations cannot be solved directly. Therefore, the PDEs must be transformed into numerical equations containing only numbers and no derivatives. This process of producing a numerical analogue to the PDEs is called ‘numerical discretization’. Various techniques used for discretization are the finite difference method, the finite element method, and the finite volume method.

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 different 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.

Various application of CFD in thermal power Plant

Low NOx Burner Design

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 majority of the 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) flame in turn reducing the NOx formation .The role of CFD analysis is very vital for designing and analysis of such low NOx burners. Many available CFD tools 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 environment due to NOx formation during combustion[1].

CFD analysis of Economiser

Economiser is very crucial component for efficient performance of thermal power plant. It is non steaming type of heat exchanger which is placed in convective zone of furnace . It takes the heat energy of flue gas for heating of feed water before it enters to boiler drum. The thermal efficiency/boiler efficiency largely depends on performance of economiser. CFD analysis helps in optimizing the thermal performance of economiser by analysing the pressure, velocity & temperature distribution and to identify the critical areas for further improvement with the result obtained by CFD analysis.

CFD analysis of Superheter

Superheaters which are generally placed in radiant zone of furnace are used for increasing the quality of dry saturated steam coming out from boiler drum and to maintain the required parameters before sending it to Turbine. The thermal efficiency of thermal power plant depends on the performance of superheter. The CFD analysis of superheters are done at design stage and later at the troubleshooting and performance evaluation during the operation of plant. The CFD results obtained can be useful for the maintenance engineer to make suitable prediction of the tube life and make suitable arrangement for the high temperature zone to reduce the erosion of tube coil and restricting the tube leakage problem. CFD analysis consists of modelling super heater and doing analysis to study the velocity, pressure and temperature distribution of the steam inside the super heater. The uneven temperature distribution of steam in the tube leads 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 always not possible to take shut down and analyse the problem.CFD helps in this .

CFD analysis of Pulverized Coal Combustion

In thermal power plant combustion of fuel specially pulverized coal combustion is of significant importance . Proper and complete combustion with required supply of air of fuel is required for total energy transfer to water for steam generation and 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 ,coal through burners . The results obtained from CFD analyses gives insight to identify the potential areas for improvement[4].

The CFD application in other areas of Thermal power plant

There are some other areas of importance where The CFD can play 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 a 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. [5]

The CFD is also used to obtain the numerical solution to address the problem of water wall erosion of furnace of thermal power plant. This is caused due to 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 result shows velocity profiles, pressure profiles, streamlines and other data that is helpful to understand the fluid flow phenomena inside the equipment[6] It is clearly evident from above examples that how crucial is the application of CFD in addressing the bottlenecks in thermal power plant , improving power plant efficiency and assisting in maintenance decisions.

References

1. P. J Stopford, “Recent Application of CFD Modeling in the Power Generation and Combustion Industries”, Applied Mathematical Modeling, Vol 26, Pg. No. 351-374, 2002.
2. Krunal .P Mudafle , Hemant S. Farkade "CFD analysis of economizer in a tengential fired boiler", International Journal of Mechanical and Industrial Engineering (IJMIE) ISSN No. 2231 –6477, Vol-2, Iss-4, 2012.
3. Ajay N. Ingale, Vivek C. Pathade, Dr. Vivek H. Tatwawadi" CFD Analysis of Superheater in View of Boiler Tube Leakage" International Journal of Engineering and Innovative Technology (IJEIT) Volume 1, Issue 3, March 2012
4. Cristiano V. da Silva, Maria Luiza S. Indrusiak, Arthur B. Beskow " CFD Analysis of the Pulverized Coal Combustion Processes in a 160 MWe Tangentially-Fired-Boiler of a Thermal Power Plant" Journal of the Braz. Soc. of Mech. Sci. & Eng. October-December 2010, Vol. XXXII, No. 4.
5. SowjanyaVijiapurapua , JieCuia, SastryMunukutla "CFD application for coal/air balancing in power plants" AppliedMathematicalModelling30(2006)854–866
6. 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
7. H.Versteg ,W.malalasekra " An Introduction to Computational Fluid Dynamics" Second edition,Pearson Publications.