Tangentially fired drum type controlled circulation boiler is one of the most widely used boilers. In this type 500 MWe steam generators are being used in India for power generation widely. The problems normally faced during the operation of these boilers are high super heater terminal temperatures, higher metal temperatures in super heaters and reheaters, high dry flue gas losses, high super heater and reheater spray requirements. The reasons for these problems are attributed to the coal quality variation, fluctuating loads and many other factors that need to be investigated and suitable remedies have to be sought after. An experimental investigation of the problems on a working boiler is very expensive, difficult and time consuming. These problems can be investigated by modeling and simulation of power plant steam. CFD offers a very fine tool to analyze different aspects of these problems. Most of the work on CFD simulation of large scale boilers is restricted to the 200 MWe steam generators or tower type 500 MWe steam generators, the results of which can not be attributed to work for the present steam generator considered.

This paper presents CFD modeling of a drum type, controlled circulation, dry bottom sub critical, balanced draught, radiant reheat type tangentially fired steam generator with burner tilt arrangement, using the FluentÒ software.

A geometric model of the boiler was developed in GambitÒ. A structured quadrilateral meshing scheme was used in the furnace section to align the grid with the flow direction, to minimize pseudo-diffusion. This is especially important in the furnace zone, where strong swirling flow occurs. The water walls are modeled with the no slip boundary condition. The heat exchangers along the main flow direction are modeled as double-sided constant temperature walls. Heat exchangers in the cross over pass and rear pass are modeled as porous medium, with constant pressure drop coefficients. Heat transfer to these exchangers is evaluated using the standard empirical correlations for flow across tube banks.

A staggered grid is used for pressure interpolation using the PRESTO! option in the FluentÒ. The realizable k-e model is used for the closure of turbulence variables, as the standard k-e model is reported to be over diffusive, especially in flows with sharp gradients. The realizable k-e model is reported to give better results in swirling flows by sensitizing the turbulent viscosity to the mean rate of strain. Radiation heat transfer is evaluated using the D-O model.

Coal particle trajectories are calculated in the Lagrangian framework. The particles are tracked through the computational domain and interaction between the particles and continuous phase is incorporated by an exchange of source term for mass, momentum and energy. Coal combustion is modeled using the double mixture fraction/PDF method. The effect of turbulence on combustion is taken into account using the b-PDF function for calculating the instantaneous mixture fraction from the mean mixture fraction and its variance. All the calculations are performed within the framework of FluentÒ.

The work modeled the steam generator at its design efficiency. The temperatures at various zone outlets are matched to design conditions. This work detected the steam temperature variations in the left and right parts of the furnace at the super heater zone. This work also identifies the swirling zones in the furnace which are responsible for gas temperature and steam temperature fluctuations. This work also predicted NOx composition variation when OFA inlets are used. This work can be success fully used to analyze the performance and gas distribution on different varying situations like coal composition variation, mill combination variation and air fuel ratio variations.