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SUMMARYA three-dimensional computational fluid dynamics code was used to analyse the performance of 550 MW pulverized coal combustion opposite a wall-fired boiler (of IEC) at different operation modes. The main objective of this study was to prove that connecting plant measurements with three-dimensional furnace modelling is a cost-effective method for design, optimization and problem solving in power plant operation. Heat flux results from calculations were compared with measurements in the boiler and showed good agreement. Consequently, the code was used to study hydrodynamic aspects of air-flue gases mixing in the upper part of the boiler. It was demonstrated that effective mixing between flue gases and overfire air is of essential importance for CO reburning. From our complementary experimental-numerical effort, IEC considers a possibility to improve the boiler performance by replacing the existing OFA nozzles by those with higher penetration depth of the air jets, with the aim to ensure proper mixing to achieve better CO reburning.

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Over the past years experience has been gained in employing changing types of imported coal. Apart from the proximate analysis this led to development of evaluation criteria regarding the operation of coals. These are criteria numbers obtained from operational experience and criteria numbers used for the characterization of specific operational properties on the basis of special laboratory analyses. The study evaluates the effect of the characteristics of pulverized coal on the furnace fouling and radiation heat transfer. The aim of the study was to access whether fouling and radiation heat transfer could be predicted from coal characteristics. The paper presents the experimental results on the fouling propensity of fifteen coals tested in a 575 MW combustion engineering tangential firing boiler. The results showed that no coals produced a strong molten deposit. In order to rank the fouling propensity and radiation heat transfer properties numerically, we measured the profile of incident heat fluxes, defined furnace exit flue gas temperature and absorbed heat fluxes. The basic molar ratio correlates the fouling propensity. Besides that increasing of SiO2 and Al2O3 content in the ash strongly reduces water wall absorptivity factor. The present work is also concerned with the effect of different bituminous coal on their flame emissivity. Using the radiation properties of flue gases derived from the full scale experiments, we run computational fluid dynamics (CFD) on the combustion process. The known fouling and radiation heat transfer properties enable the prediction of the effect of coal quality on the performance of a specific boiler.

Present regulatory requirements enforces the modification of the firing modes of existing coal-fired utility boilers and the use of coals different from those originally designed for these boilers. The reduction of SO2 and NOx emissions were the primary motivation for these changes. However, economic considerations played a major role too. Using sub-bituminous coals has become an important solution for emissions compliance due to their unique constituents and combustion characteristics; these coals are often referred to as enviro coals. Powder River Basin (PRB) Coals are classified as sub-bituminous ranked coals. Unlike higher ranked bituminous coals, which have tight pore structures that limit the amount of moisture they could hold, low rank coals, such as PRB coals, have looser pore structure and additional moisture retention capacity. PRB coals differ in many properties from those of the commonly burned bituminous coals, including low heating value, low fusion temperature, and high moisture content. However, PRB coals have low sulfur content and are relatively low cost. They can also lower NOx emission from power plants due to higher volatile content. When power plants switch from the designed coal to a PRB coal, operational challenges, including transportation, handling, storage, and combustion, were encountered. A major problem faced when using PRB coals is severe slagging and excess fouling on the heating surface. Not only is there an insulating effect from deposit, but there is a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction of heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGT), especially with opposite wall burners and with single backpass. Higher FEGT usually result in higher stack gas temperature and increasing in the reheater spray flow and therefore decreasing the boiler efficiency with higher heat rate of the unit. The modification of an existing unit for firing of PRB coals is confined to — and constrained by — existing equipment. All successful conversions happen when in the design phase of a project the following parameters are evaluated: (1) capacities or limitations of furnace size, (2) firing system type and arrangement, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to predict the behavior of three PRB coals fired in a 575MW T-fired boiler.