Arun Sheth - Director, Mechwell Industries
In a thermal power plant hot flue gases along-with ash containing quartz and mullite (abrasive) particles leave through Economizer (ECO) outlet, pass through the Regenerative Air Pre heater (RAPH), flue gas duct, Electrostatic Precipitator (ESP), Induced Draught (ID) Fan and Stack. The flue gas distribution along the duct cross sectional area is usually non-uniform. Hence proper study of the flue gas path is required. CFD technique has emerged to become one of the most economic methods to analyze various equipments in power plant. CFD results can be used to validate various design parameters.
Issues in flue gas ducting - air pre-heater outlet to induced draft fan inlet
- Uneven flow distribution through the ducts and ESP
- High auxiliary power consumption
- Very high turbulent flow regimes
- Recirculation zones
- Ash accumulations
- High pressure drop across ducts
- Erosion effects and other various parameters
The CFD technique is best explained by a real case analysis. Accordingly the following case is presented to showcase the usefulness of CFD as a tool.
Real Case Analysis
One of the Jindal Power Ltd´s, Raigarh unit with load of 250 MW power plant was facing all the above mentioned generalized issues. Various input data were collected from the client for problematic areas including existing operating parameters. As a first step of CFD, based on the data a 3D model was developed in Ansys Work bench Fig 1.
Confirmation of model was done mutually between plant and CFD Experts. A mesh (model was divided into millions of minute elements) was developed in ANSYS ICEM CFD and numerical algorithm representing various Mathematical Differential Equations related to flow conditions were applied to meshed model. Boundary Conditions were provided as input to these meshes to stimulate ´AS IS CONDITION (Existing)´ of the running pant in CFX/Fluent. A base case analysis was done to confirm to all the existing problematic areas.
Once the problems were identified, further Iterative analysis was done to optimize the possible solutions. Accordingly number of suggestions with their benefits were given to the client and to check the site feasibility. The general modification covered inclusion of diverter plates to uniformly distribute the flow. Removal of sharp corners was also proposed to avoid direct impact on duct surface and thus reduce Erosion. After the modifications were accepted by the client, Engineering Drawings related to modifications were submitted to the client. On approval of the modifications, the supply and erection of these modifications were carried out. The post-installation benefits were evolved by measuring various parameters like pressure drop reduction, I.D. fan current reduction, duct erosion reduction etc.
Following inference can be drawn that by CFD analysis in Jindal power plant, the problems of existing duct like unequal flow as well flow concentration inside the duct, causing the high erosion of the duct at particular locations, high turbulence, flow separation and re-circulation zone which caused high pressure drop in the ducting were resolved.
By designing the Optimized guide vanes using CFD, the flow was nearly uniform with optimum turbulence & completely avoiding re-circulation zones. After Modification, the Total Pressure drop reduction From AH Outlet to ID fan Inlet was 30 mm WC which reduced the Load on ID Fan. It was also observed that at the ESP Inlet Duct, Flow had been balanced in all pass of the ESP which resulted in increased efficiency of ESP.
- Improved duct design to ensure proper flow distribution across the duct
- Reduction in high velocity and turbulence will reduce erosion of ducts
- Velocity of flue gases is within the specified range
- Reduced draft loss
- Reduced pressure drop
- Lower turbulence level (smooth flow)
- Decreased amount of ash settling in the duct
- Minimised erosion effect of ash
- High auxiliary power saving of Id fans
Paper presented by CAE -R&D Center, Mechwell Industries Nashik, India