Formation and Deposition of Slag from Combustion of High Calcium Lignite on Pulverized Coal-fired Boiler

Abstract

Lignite from Mae Moh mine is the largest source of solid fuel for electricity generation in Thailand. It is used in pulverized coal-fired boilers of a 2400 MW thermal power plant. In a boiler chamber, the ash particles are adhere to the surface of the slag sinter layer, and then melt into liquid and transformed to solid slag on furnace walls. Slag from coal combustion for energy utilization results in many problems, such as loss of capacity, loss of heat, boiler equipment damage, loss of time and maintenance boiler costs. Slag is caused by many factors, such as coal organic properties, coal mineral matter properties, mineral transformation and decomposition. Generally, the blended coal from Mae Moh mine must have sufficiently high heating value, under limitation of sulfur content, and CaO (free SO3) in ash must not exceed 23%. Nevertheless, the CaO value of coal found in the remaining areas revealed that coal with up to 40% CaO may be available in the future. It is likely that more frequent and severe slagging problems may occur in the future. So, understanding of slag problem, and factors that influence formation and deposition of slag are required, especially for the case of high Ca lignite. Fluid dynamic inside the boiler furnace of Mae Moh coal fired power plant is also useful. The research methodology was separated into 2 parts. Firstly, the coal, ash, and slag were analyzed and the factors influencing the potential of slag were processed. Mae Moh slag samples with different appearances from the high Ca content from the Mae Moh power plant were collected. Characteristics and properties of the slag were investigated using SEM, EDS, XRF, and XRD. The microscopic surface morphologies were found to be similar. A majority of the crystal surfaces were agglomerates of particle-like clay and irregular granules stacked together. The elements found to be abundant were oxygen, aluminum, silica, and calcium. The major chemical constituents of the slag were CaO, SiO2, Al2O3, Fe2O3, and MgO, that correlated with the following minerals; anorthite, gehlenite, akermanite, diopside, and esseneite. Lignite C1 and SE were collected from the Mae Moh mine with low and high CaO (free SO3) contents in ash, respectively. Raw coals were analyzed using proximate analysis, ultimate analysis, bomb calorimeter, sulfur analyzer, SEM, EDS, XRF, and XRD. Nonisothermal thermogravimetric method was also used to study the thermal behavior of raw coals and to perform kinetics modeling. The properties of lignite C1 showed a better quality than SE. The lignites were blended for 7 types with different CaO (free SO3) content and analyzed by proximate analysis, ultimate analysis, bomb calorimeter, and sulfur analyzer, XRF, and AFT test. The coal properties were compared. Especially from AFT test, the lignite E (35.11% of CaO (freeSO3) in coal ash) was found to have the lowest ash melting point and IT-FT gap. From base to acid ratio, silica/alumina ratio, iron/calcium ratio, iron/dolomite ratio, the test of SEM, and EDS, the results indicated that sample E was the most likely to form slag in boiler furnace, and difficult to remove. Additionally, FactSage package and ANSYS Fluent program were used in the prediction of slag formation and deposition. The predicted formation of slag from FactSage, equilib and phase diagram models, were found to be in good agreement with the AFT test. Qualitatively, the predicted heat flux from ANSYS Fluent CFD program was in agreement with the measured data from a normal operation of the boiler. It was shown that the burner zone may have higher slag potential.