Modelling of thermal processes and optimization of energy-ecology characteristics of the modern boiler plants

The investigation accomplished in the framework of this work is concerned with the thermal processes in the furnaces of modern steam and hot-water boilers on fossil fuels. Aerodynamic and thermal conditions in the furnaces are described and models for separate processes and phenomena that occur there are presented. By using proper CFD technique, three-dimensional models of furnaces of coal-fired power boiler, hot-water boiler with circulating fluidized bed combustion and steam boiler on liquid/gaseous fuel are created. Graphical pre-processor is used for geometry creation and mesh generation of the investigated boiler plants. Mathematical model for the gas-solids mixture flow is based on Lagrange approach for the discrete phase simulations, in addition to the transport equations for the gas phase. A standard steady semi-empirical k-E model is employed for description of the turbulent flow. Coupling of velocity and pressure is achieved by the SIMPLEC method. Coal combustion is modelled as non-premixed kinetics/diffusion-limited process by the mixture fraction/probability density function approach for the reaction chemistry, with equilibrium assumption applied for description of the system chemistry. Radiation heat transfer is computed by means of the P-1 model, which is simplified variance of the P-N model, based on the expansion of the radiation intensity into an orthogonal series of spherical harmonics. Presence of discrete solid phase in the main gas stream is effectively taken into consideration through additional terms in the radiation energy transfer equation and in other model equations. Variable emissivity coefficient of the combustion products is modelled with the weighted-sum-of-grey gases-model. A model for NOx formation and reduction is included in the computations. Numerical simulations provide results concerning the boilers operation in several regimes. A methodology for optimisation of energetic-ecological characteristics of boiler plants is proposed. In order to verify the model, the obtained results are compared with the measurements data from the power boiler OB-380, as well as from other boiler plants. The results refer to the furnace temperature fields, heat flux to the walls, heat efficiency of the furnace walls, combustion efficiency and NOx emission. Accomplished comparative analysis shows qualitative agreement between the results of numerical experimentations and the measurements. On that basis, adequate conclusions are deducted, both on the model and the boiler plants operation. Recommendations on the future work in the investigated area are given.(Author)