Energy & Fuels 1992,6, 59-68
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Viscous Sintering of Coal Ashes. 2. Sintering Behavior at Short Residence Times in a Drop Tube Furnace Bongjin Jungt and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received May 8, 1991. Revised Manuscript Received October 15, 1991
Low-temperature viscous sintering of coal ashes is very important at the onset of the initial growth of sinter "necks" between ash particles. To determine the lowest temperature at which sintering would take place, it was first necessary to determine a temperature for ashing the coals low enough to avoid some inadvertent sintering during the ashing process. The appropriate ashing temperature for each coal was found empirically by examination of electron micrographs of ashes prepared at different temperatures; the ashing temperature was selected as the highest temperature at which sintering of ash particles was not observed in the electron micrographs of each ash. Sintering experiments were carried out by allowing ash particles to fall through an externally heated furnace. The sintering behavior of the ashes is governed by sodium and calcium content. Electron photomicrography and X-ray diffraction of each sintered ash showed that, as the furnace temperature increased, there was an increase of the amount of anhydrite corresponding with increase in the largest size of sintered particles. Comparing the sinter point determined by the traditional electrical resistance method with the minimum sintering temperature determined in the drop tube furnace showed higher sintering temperatures observed by the electrical resistance method than in the drop tube furnace. In part this is because laboratory ash samples prepared at 750 "C for the electrical resistance test have already experienced partial sintering. Results from this sintering study contribute to a better understanding of the sintering behavior involved in deposit formation in pulverized coal combustion and agglomerate formation in fluidized beds.
Introduction As pulverized coal is introduced into a boiler, the initial effect of heating is the loss of inherent water, below about 250 "C. The next significant change is the decomposition of organic matter to yield volatiles at temperatures above 250 "C. At a later stage of heating, reactions of the inorganic components of the coal begin to occur. Clays (e.g., kaolinite, illite, and montmorillonite) begin to lose water around 500 OC.I The clay structure partially collapses, retaining some degree of order and forming metakaolinite. The collapsed structure acta as a framework for the formation of several relatively low-melting aluminosilicates from interaction with sodium, potassium, calcium, iron, or magnesium.2 Pyrite thermally decomposes to pyrrhotite,' which is eventually oxidized to iron and sulfur oxides. The fluxing action of iron may play a role in the formation of low-melting point glass, by interaction with clays or quartz. Carbonate minerals decompose to calcium and magnesium oxides, which can rapidly flux other minerals, such as pyrrhotite or alkali metal silicate^.^ As the temperature increases further, quartz interacts with other components, such as alkalies, calcium oxide, or iron oxide, to form low-viscosity silicates.4 Gypsum transforms to anhydrite in the temperature range 4W500 OC.' The sintering of the ash is an important part of the overall process of ash deposition on heat transfer surfaces in boiler^.^^^ The formation of a liquid phase represents the onset of the kinds of ash behavior most likely to be troublesome in operating combustors: sintering, clinkering, agglomeration, and d e p ~ s i t i o n . The ~ ~ key feature linking these situations is the ability of the liquid phase to act as a glue to bond together particles that are still solid. An 'Present address: Department of Chemical Engineering, University of Delaware, Newark, D E 19716.
evaluation of ash sintering characteristics in the laboratory is useful in predicting whether a given coal ash will result in strongly bonded ash deposits during combustion. Several major parameters, such as composition and particle size distribution of the ash, sintering temperature and time, and the reducing or oxidizing nature of the atmosphere, will influence sintering behavior. Several laboratory methods have been developed to evaluate the sintering characteristics of coal ashes. Barnhart and Williams developed a test for determining the tendency of an ash to form hard, bonded deposits by measuring the compressive strength of sintered ash pellets.1° In this test, the sinter point was defined as the temperature at which an ash compact developed a measurable mechanical strength. A higher sintering temperature, longer sintering time, or both, will increase the strength of deposits, thereby increasing the difficulty of removing them. Dering and co-workers examined the sintering behavior of different size fractions of fly ash from brown coal." The finest size fraction (0-5 pm) sintered (1)Mitchell, R. S.;Gluskoter, H. J. Fuel 1976,55, 90. (2)Falcone, S.K.;Schobert, H. H.; Rindt, D. K.; Braun, S. Am. Chem. Soc., P r e p . Pap.-Diu. Fuel Chem. 1984,29 (4),76. (3)Jackson, P. J. In Pulverized Coal Firing: Mineral Matter and Its Effects;Stewart, I. M., Wall, T. F., Eds.; Hemisphere: Washington, DC, 1979. (4) Wibberly, L. J.; Wall, T. F. Fuel, 1982,61, 87. (5)Raask, E.Mineral Impurities in Coal Combustion; Hemisphere: Washington, DC, 1985. (6)Reid, W. T.In Chemistry of Coal Utilization, 2d. Suppl. Vol.; Elloit, M.A., Ed.; Wiley: New York, 1981;Chapter 21. (7)Tangsathitkulchai, M. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1986. (8) Benson, S. A. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1986. (9)Schobert, H. H.; Conn, R. E.; Jung, B. J. Proc. 4th Annu. Pittsburgh Coal Conf. 1987,423. (10)Barnhart, D.H.; Williams, P. C. Trans. AIME 1956,78, 1229. (11)Dering, I. S.;Dubrovskii, V. A.; Dik, E. E. Thermal Eng. 1972,19 (12),48.
0887-0624/92/2506-0059$03.00/00 1992 American Chemical Society
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60 Energy & Fuels, Vol. 6, No. 1, 1992
more strongly, and sintering began at lower temperatures than for fractions of coarser size. The results of this work were consistent with the tendencies of various coals to form a strong, bonded deposit in boilers. A dilatometric shrinkage technique has been used to determine the sinter point of ash, defined as the temperature at which a compacted ash sample begins to shrink as a result of particle-to-particle bonding.12 Raask developed a laboratory method for simultaneous measurement of electrical conductance and dilatometric ~hrinkage.'~Using soda glass as a model system, the onset of shrinkage and an increase in the conductance occurred at a temperature of 600 "C and viscosity of 1O'O Pas. The viscosity range of 106-1010 Pas was found to be relevant to formation sintered deposits. Strong sintered bonds can form in this viscosity range in times from a few minutes to several days, depending on the particle size of the ash being sintered. Electrical conductance methods are based on an assumption that the electrical resistance of solid ash is high, whereas that of molten ash is markedly lower due to ionic cond~ction.'~Benson's study of the growth of strength in a laboratory-produced deposit as a function of deposit height (i.e., distance away from its base) could be related to the amount of liquid available for sintering and to the viscosity of the liquid.8 In previous work in this laboratory, the present authors demonstrated that the sinter point decreased with decreasing particle size for four coal ashes, and that, at a given sintering temperature, the strength of the sintered ash was inversely proportional to the particle Chemical reactions accompanying sintering led to the formation of relatively low-melting calcium aluminosilicates, primarily by reaction of anhydrite with aluminosilicates present in the ash. These low-melting calcium compounds act as the liquid "glue" that helps to form strong, sintered ash deposits. In conjunction with that study, it was noted that ash prepared by traditional ASTM ashing methodsI6 at 750 "C showed extensive sintering when examined by scanning electron microscopy." The sintering of ash particles occurring at the relatively low temperature (Le.,
