A zonal model for radiation heat transfer in coal-fired boiler furnaces

Monnaemang, Whitney Ogalaletseng

A zonal model for radiation heat transfer in coal-fired boiler furnaces

Master Thesis

2015

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Problems associated with boilers are a major contributor to load losses in coal-fired power plants. The boiler furnace exit temperature is a key indicator of the combustion and heat transfer processes taking place and has a profound impact on the operation of the heat exchangers downstream of the furnace. Having a model that can predict the furnace exit temperature and heat flux distributions may enable furnace performance to be predicted without having to conduct extensive experimentation. Also, comparing the results with measurements taken on the plant may enable the identification of operating problems and potential sources of losses. Thermal radiation is the dominant mode of heat transfer in the boiler furnace. The primary objective of this study is to develop and implement a radiation heat transfer network solution methodology based on the zonal method that may be applied to boiler furnace modelling. The zonal method allows for the prediction of heat flux and temperature distributions on the walls, inside, and at the exit of the furnace. Direct exchange areas are the basis of the zonal method and are a function of the furnace geometry and radiative properties of the walls and the participating medium that fills the furnace volume. The evaluation of direct exchange areas is done by discrete numerical integration, after which it needs to be smoothed to satisfy energy conservation. After evaluating two different smoothing techniques, the least squares technique using Lagrange multipliers was selected for this study. Following this, the solution of the radiation heat transfer network was implemented for an emitting-absorbing-scattering participating medium for two different scenarios, namely (i) solving surface and volume heat fluxes for known surface and medium temperatures, and (ii) solving surface heat fluxes and medium temperature distributions for known surface temperatures and volume heat source terms. Intermediate verification and validation steps throughout the development process show good agreement with other numerical techniques and correlations available in literature. In order to illustrate the applicability of the final model, a number of case studies are conducted. These include an illustration of the effect of slagging on the furnace walls, of a faulty burner and of changes in the radiative properties of the participating medium. The results of the case studies show that the trends of the heat flux and temperature distributions obtained with the new model are in agreement with those found in literature.

Keywords

Mechanical Engineering

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