No abstract

Computational Fluid Dynamic (CFD) models give good predictions of coal combustion in utility boilers if the coal combustion kinetic parameters are known. We developed a three-step methodology to provide reliable prediction of the behavior of a coal in a utility boiler: (1) Obtaining the combustion kinetic model parameters from a series of experiments in a test facility, CFD codes and optimization algorithm. (2) Validation of the combustion kinetic parameters by comparison of different experimental data with simulation results obtained by the set of combustion kinetic parameters. (3) The extracted kinetic parameters are then used for simulations of full-scale boilers using the same CFD code. Three to four bituminous and sub-bituminous coals with known behavior in Israel Electric Corporation (IEC) 550MW opposite-wall (3 coals) and 575MW tangential-fired (4 coals) boilers were used to show the capability of the method. An unfamiliar bituminous coal was then examined prior of its firing in the utility boilers and prediction of its combustion behavior in the two boilers was carried out. This methodology was used to examine a Venezuelan coal that was found to yield high LOI.

We predict the combustion behavior and pollutant emissions of blends of a Colombian bituminous coal, Drummond, and an Indonesian sub-bituminous coal, Adaro, in pulverized-coal utility boilers. This work is based on full-scale numerical simulations with GLACIER, a powerful computational-fluid-dynamic (CFD) code that uses the two-mixture fraction approach which models two separate coal streams in the combustion chamber. By burning the coals and their blends in a pilot-scale test furnace, previously unknown information on the coal combustion, such as devolatilization and char oxidation kinetic parameters, was determined and the CFD model validated for the test furnace. The same set of parameters was used for the CFD model configured for an opposed-wall and a tangential fired utility boiler. Our results show good fits between numerical results and experimental data for gas temperature, CO2, O2, and NOx, both in the test furnace and in the utility boilers, for single coals and their blends. We believe that the tool we developed can help utility companies make rational decisions on the use of new coals or coal blends so as to lower pollutant emissions while maintaining the same combustion efficiency.

Slagging caused by deposit of molten fly ash on hot walls is a major concern in the operation of full-scale utility boilers. We carried out a comprehensive study, experimental and modeling, on slagging with various coals. Coal samples were taken prior and during combustion and analyzed by SEM (Scanning Electron Microscope). From the SEM analysis the coals could be divided into two types: (1) Coal with tiny particulates of the mineral matter deposited loosely on the surface of coal particles or between carbon particles (external ash) and (2) coal with the mineral matter encapsulated within the coal particles (internal ash). We found different slagging and char combustion characteristics directly related to the two coal types. It was observed that internal ash coals show higher slagging propensity and higher carbon content in the fly ash. Previous models to predict slagging did not distinguish between the two coal types and their impact on slagging and combustion behavior. We developed a model for high temperature ash deposition on the furnace walls for these two coal types. In this model, char combustion and carbon content in the fly ash are also considered. Comparison of experimental observation with calculation results from the ash deposition model show good agreement. Another conclusion from the model is that slagging propensity for internal ash coals increases with coal particle size. However, this conclusion has to be verified experimentally.