In a thermal power plant, hot flue gases along with ash containing quartz and mullite (abrasive) particles leave through the Economizer (ECO) outlet, pass through the Regenerative Air Pre-heater (RAPH), flue gas duct, Electrostatic Precipitator (ESP), Induced Draught (ID) Fan and Stack. Flue gas distribution along the duct cross sectional area is usually non-uniform. Hence, requiring proper study of the flue gas path. CFD technique has emerged as one of the most economic methods to analyse various equipments in power plants and can be used to validate various design parameters.
Issues in flue gas ducting - Air Pre-Heater outlet to induced draft fan inlet
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 generalised issues. Various input data was 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 (Refer 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 this 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 optimise the possible solutions. Accordingly, a 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, and removal of sharp corners was also proposed to avoid direct impact on duct surface and thus reduce erosion. Engineering drawings related to modifications were submitted to the client after they accepted the modifications. The supply and erection of these modifications were carried out on approval. The post-installation benefits were evolved by measuring various parameters like pressure drop reduction, ID fan current reduction, duct erosion reduction etc.
The inference that can be drawn from the CFD analysis done in Jindal power plant, is that the problems of existing duct like unequal flow andl 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 optimised guide vanes using CFD, the flow was nearly uniform with optimum turbulence and 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 the 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.
Paper presented by CAE - R&D Center, Mechwell Industries Nashik, India
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