A numerical study of a utility boiler tangentially-fired furnace under different operating conditions
Introduction
Simulations made with comprehensive combustion models and codes have led to significantly new physical insights into the behavior of complex flows [1], [2], offering great potential for use in optimizing the performance of energy conversion systems. A brief overview of comprehensive 3D CFD differential models of multiphase turbulent reactive flows in large-scale geometries is given in [3]. A number of CFD codes, being developed by specialized commercial companies, research organizations and individuals, are available worldwide. The codes provide qualitative, but not necessarily also quantitative characteristics of involving processes. For reliable application of the software packages with the possibility to control and get an insight into the calculation mechanism, a support (usually expensive) of the software owners is required most often. Getting insight into individual segments, as well as making single modifications of the software are not possible. For these reasons, we want, like many others, to provide our own mathematical models, numerical codes and software packages in trying to achieve an optimum usefulness of this powerful tool for solving technological problems of complex processes. For prediction of complex processes in two-phase turbulent reactive flows within large-scale boiler furnaces firing pulverized coal, a comprehensive 3D differential mathematical model and CFD computer code have been developed in-house. There has been also a practical motivation for development of the numerical model. It is to be applied to solve operation problems in 350 MWe. Kostolac B1 boiler furnace by engineering staff of “Electric Power Industry of Serbia”. An easy-to-use interface for introducing input data and grid generation is built within the code.
Within overview of mathematical models in the field [3], the present one has been compared with others. The merits of this model and computer code could be, e.g. the following: the model offers such a composition of submodels and modeling approaches so as to balance submodels sophistication with computational practicality; unlike in some comprehensive combustion models, the furnace geometry (especially in the burners region) is described in full details; application of diffusion model of particle dispersion provides better computational efficiency that the stochastic models widely used in this context, but requiring more particle trajectories for reliable solutions; in contrast to many other works, the effect of particles on gas turbulence is modeled as well. Moreover, a comparison between the temperature predictions obtained by our model and FLUENT (using PDF combustion model and P1 radiation model) is given in [4]. The comparison includes average temperature profile along the furnace height and horizontal local gas temperature profile close to the wall. Although there are certain discrepancies (probably to the inherent differences of the combustion and radiation models applied) the agreement is satisfactory. Additional merit of the developed code is a relatively simple and efficient user interface for pre- and post-processing.
A numerical study of a case-study furnace working under different operating conditions is presented in the paper, on the basis of the selected predictions obtained by the developed model. The case-study furnace is 350 MWe utility boiler dry-bottomed furnace tangentially-fired by pulverized lignite. The paper also describes the model and provides information on the model evaluation, with a grid refinement study and comparisons with comprehensive data. The results demonstrate the performance of the model and some possible applications of the corresponding computer code.
Section snippets
Mathematical model and computation procedure
The comprehensive model is described in more details in our previous references, e.g. [3], [5], [6], while general features and some specific problems are presented in this paper. Gaseous phase is described by steady state time-averaged Eulerian partial differential conservation equations for mass, momentum, energy, components concentrations of multicomponent gas mixture, turbulence kinetic energy and its rate of dissipation. In general index-notation, for general variable ϕ:
Results and discussion
To provide confidence in the solutions obtained from the developed model and the code, complex evaluation was performed. The code was verified by estimating the numerical error in conjunction with the grid refinement. The calculations done with the verified code were validated by means of comparisons with comprehensive measurements and the trend analysis (a study on the influence of selected operation parameters on the case-study furnace). The test-case unit was Kostolac-B1 power plant
Practical applications
Nowadays trends in the field of power plant coal-fired furnaces can be summarized as follows: a necessity for efficient and low-emission utilization of lower quality fuels and fuels with changing characteristics, need for utility boilers to operate with the same efficiency in a wide range of load changes, a growing effort to improve energy efficiency and minimize emission, as well as an increasing interest in combustion of coal blends and co-combustion of coal and biomass. The boiler furnace is
Conclusions
For prediction of complex processes in two-phase turbulent reactive flows within utility-scale pulverized coal-fired furnaces, a comprehensive differential mathematical model and 3D CFD code have been developed in-house, with an easy-to-use interface for introducing input data and grid generation. A complex evaluation of the model has been performed, with a grid refinement study and comparisons of selected predictions with comprehensive experimental data obtained during long-term steady
Acknowledgements
This work has been supported by the Republic of Serbia Ministry of Science and by the Public Enterprise “Electric Power Industry of Serbia”.
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