Ash formation during pulverized subbituminous coal combustion. 1

Apr 15, 1991 - of the inorganic matterin coals, two subbituminous coals were tested in a new down-fired combustor of 745 MJ/(h*m3) capacity...
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Energy & Fuels 1992,6, 47-58

which were less accessible to He atoms. The chars have substantially lower micropore surface areas than those of chars generated from lignite,24and the mesopore and macropore surface areas are lower than those of subbituminous coal chamz8 The amount of residual volatiles can be used as a measure of the reactivity of the char. Chars generated at 100 psig were the most reactive, compared to those generated at atmospheric or pressures >lo0 psig. The generation of active sites during pyrolysis resulted from movement of the carbon layers within the char particle, which in turn is pressure-dependent. At atmospheric pressure, the number of active sites generated by the


low-fluidity coal melt was low. As the pressure increased to 100 psig, the reactivity of chars increased because the number of active siteg generated by the higher fluidity melt increased. The decreased reactivity of the chars generated at 309 psig indicates that the number of active sites decreased as char structures produced from the high fluidity coal melt became more ordered.

Acknowledgment. We are pleased to acknowledge the financial support provided by the United States Department of Energy for this work. Technical support from Mr. Carl Martin and Mr. Ronald Wincek is also gratefully acknowledged.

Ash Formation During Pulverized Subbituminous Coal Combustion. 1. Characterization of Coals, and Inorganic Transformations during Early Stages of Burnout John P. Hurleyt and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received April 15, 1991. Revised Manuscript Received October 16, 1991 ~

As part of a broad effort to develop the capability to predict ash behavior based on detailed analyses of the inorganic matter in coals, two subbituminous coals were tested in a new down-fired combustor of 745 MJ/(h.m3) capacity. This paper deals with changes in the inorganic matter during the first 30-50% burnout of combustibles, corresponding to 0.07 s residence time. The inorganic components reached temperatures of 1210-1220 “C. Detailed analyses of coals and chars were performed by transmission and computer-controlled scanning electron microscopy and X-ray diffraction. Sodium, calcium, and magnesium decreased in concentration in the larger char particles, but increased in the smaller size particles, indicating volatilization of these elemenb at a rate more rapid than combustible burnout. The rate of sulfur release was the same as the rate of combustible burnout. There was little significant change in the overall size distributions of the mineral particles, chemical effects being limited to some interaction of calcium and iron with quartz and the aluminosilicates. Two subsequent papers in this series will examine the inorganic transformations during further stages of combustible burnout. There we will demonstrate that most volatilization and condensation processes are complete by 50% combustible burnout and are followed by coalescence interactions during later stages of combustion. Introduction Ash formed during combustion of pulverized low-rank coal in utility boilers causes a variety of problems, including impeding heat transfer by adhesion to heattransfer surfaces, erosion, and corrosion. Approximately 20-40% of the ash produced in a pulverized-coal-fired utility boiler deposits within the boiler, primarily in the hoppers at the bottom of the furnace, but also on interior surfaces. The remaining 60-80% of ash leaves the boiler with the flue gas to become a collection problem for particulate control devices. The efficiencies of these devices depend on the size distribution, and sometimes on the chemical composition, of the fly ash. To properly size such devices, the designer must have accurate predictions of the physical and chemical characteristics of the fly ash. It is common to deal with ash-related problems in an a priori fashion. The problems that might occur when burning a particular coal are predicted, and the boiler is ‘Present address: University of North Dakota Energy and Environmental Research Center, Grand Forks, ND 58202.

then designed specifically to ameliorate the problems. However, many western U.S.coals exhibit great variability in bulk composition from mine to mine and even within a given mine.’ Variations in coal composition during the active lifetime of the mine can result in ash-related problems not foreseen in the original boiler design. The great variety of equations being used to foretell the problems that may be encountered during the combustion of a particular coal2shows that the art of problem prediction is not well developed. Most empirical equations used to forecast ash-related problems only take into account bulk properties of the ash, implicitly assuming that “ash” is a homogeneous material. In fact, ash is not homogeneous on a particle-by-particle or aerodynamic size range basis. Since only certain size or composition fractions of the ash may cause problems, and since some fractions may cause (1) Hurley, J. P.; Steadman, E. N.; Kleesattel, D. R. U S . Dept. of Energy Report DOE/FE/60181; University of North Dakota Energy and Environmental Research Center: Grand Forks, ND, 1986. (2) Singer, J. G.,Ed.Combustion: Fossil Power S y s t e m ; Combustion Engineering: Windsor, CT, 1981.

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48 Energy & Fuels, Vol. 6, No. 1, 1992

different problems at different places in the boiler, equations that involve only bulk composition as an input are inherently inadequate. Instead, models must be developed that use the detailed analyses of the inorganic matter in a coal to predict the ash particle size and composition distributions. The inorganic constituents of western US. coals occur as discrete mineral particles and, in low-rank coals, as cations associated with organic acid groups or other organic complexation sites. Minerals abundant enough to form a significant portion of the ash include quartz, pyrite, gypsum, calcite, dolomite, and the clays kaolinite, illite, and m~ntmorillonite.~ During combustion, the inorganic constituents undergo a variety of physical and chemical changes that depend on their original mode of occurrence, their time-temperature history, and interactions among the constituents. Usually the mineralogy and size distribution of the ash is quite different from that of the inorganic particles originally in the coal. For example, X-ray diffraction of fly ash samples from utilities burning western US. low-rank coals identified 17 mineral species, only one of which, quartz, commonly exists in the The others formed through interactions among the inorganics during combustion. Tracing the reactions that lead to the formation of the different species in the ash is a difficult process. There are, however, two main reaction paths: that encountered by the inorganics present in the coal as discrete mineral particles, and that followed by the organically associated inorganics. Several types of changes can occur in the discrete mineral matter during combustion: decomposition, fragmentation, coalescence, vaporization, and condensation. Size reduction can occur during rapid heating via decomposition or fragmentation. Under rapid heating conditions pyrite may fragment upon partial oxidation to FeS and before the FeS melts.5 Significant portions of the calcite, siderite (FeCO,), and ankerite (CaFe (C03)J also fragment upon decomposition to form fume parti~les.~ In some cases, the overall particle size distribution of the ash particles is shifted toward smaller sizes than the distribution of the mineral matter in the coals.6 Coalescence also plays a major role in the conversion of inorganic matter into ash. Much of the coalescenceoccurs on the surface of burning char particles. During the combustion of coal in a drop-tube furnace, micrometer-sized molten ash particles are already present on the surface of burning char particles 0.1 s after introduction of the coal into the furnace.I With the exception of iron-rich particles, molten ash particles do not significantly wet the surface of the hot char particle. Instead, they remain in a nearly spherical form attached lightly to the surface. As the char surface recedes, the attached ash particles are brought closer together until they contact each other. If the ash particle temperatures are high enough, and if the particles axe miscible, they can unite to form a single, larger particle. Using synthetic char containing sodium silicate glass particles, it has been shown that this process can continue until only one ash particle forms per initial combustible particle.6 During coal combustion, however, the (3) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington, DC, 1985. (4) McCarthy, G. J.; Swanson, K. D.; Keller, L. P.; Blatter, W. C. Cement Concrete Res. 1984, 14, 471. (5) Raask, E. J.In&. Energy 1984, 231. (6) Physical Sciences Incorporated. US. Dept. of Energy Report PSI-1024/SR-385; Physical Sciences: Andover, MA, 1989. (7) Benson, S. A.; Sweeney, P. G.; Abrahamson, H. B.; Zygarlicke, C. J.; Puffe, W. H.; Maldonado, M. E. US. Dept. of Energy Report DEFC21-86MC-10637; University of North Dakota Energy and Environmental Research Center: Grand Forks, ND, 1988.

Hurley and Schobert

char particle usually fragments so that many ash particles are produced per initial coal particle.8 Also, some shedding of ash particles from the char may occur before coalescence. The factors affecting the relative importance of decomposition, fragmentation, coalescence, and shedding in determining the interactions of the discrete mineral grains are not well understood. The organically associated elements pass through a vapor phase at some point in the combustion process. The initial step in the release of alkali and alkaline earth metals from carboxylate groups is likely the decomposition of the carboxylate to form the metal carbonate.*I2 At higher temperatures decomposition of the carbonates occurs. The general route is believed to be from the carbonate to the oxide, followed by reduction of the oxide to metal or suboxide vapor by carbon monoxide or the char. The vapor diffuses through the porous char to the surface, where it is free to react with other gas species or existing ash particles. Gas-phase reactions may form an oxide particle attached to the char. Some vapor may escape the char particle. While diffusing through the boundary layer surrounding the burning char, the metal vapors encounter an increasing oxygen concentration and decreasing gas temperature. Both encourage condensation. The alkaline earth metals condense as oxides before the alkali metals. Thus 60% of the sodium, but only 20% of the magnesium, was vaporized from western US. low-rank coal chars in an entrained flow reactor.I3 Similarly, only a few percent of the calcium and less than 1% of the aluminum were vap0ri~ed.l~ The relatively low levels of calcium and magnesium that escape the burning char imply that the oxygen partial pressures at or near the surface of the char are high enough to promote oxidation and condensation of the vapor to relatively pure oxide particles at the char surface. Large numbers of micrometer-sized calcium and magnesium oxide particles were observed on the surface of quenched low-rank coal char parti~1es.l~ On further combustion, the oxide particles coalesce with other ash particles to form new particles with diameters of up to several tens of micrometers. However, as much as one-third of the ash formed after complete combustion of the coal was in the 1-8-pm size range. Quann and Sarofim state that ash particles in this size are formed predominantly through char fragmentation or particle shedding from the char.14 The more volatile elements may undergo significantly different reactions during combustion. Magnesium can oxidize and begin homogeneous nucleation in the boundary layer of gas surrounding the char.13 After nucleation, the particles continue to grow through heterogeneous condensation and coalescence even as they are being driven away from the hot char particle via thermophoresis. With constant thermophoretic transport out of the boundary layer, additional nucleates will continue to form as long as magnesium vapor is being produced.15 In the relatively cool, oxygen-rich region immediately outside the boundary (8) Sarofim, A. D.; Howard, J. B.; Padia, P. S.Combust.Sci. Technol. 1977, 16, 187. (9) Srinivasachar, S.; Helble, J. J.; Ham, B. 0.; Domazetis, G. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1989,34 (2), 391. (10) Huggins, F. E.; Shah, N.; Huffman, G. P.;Lytle, F. W.; Greegor, R. B.; Jenkins, R. G. Fuel 1988,67, 1662. (11) Stewart, G. W.; Stinespring, G. W.; Davidovita, P. Am. Prepr. Pap.-Chem. SOC., Diu. Fuel Chem. 1982,27 (l),138. (12) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983,62, 209. (13) Neville, M.; Quann, R. J . ; Haynes, B. S.; Sarofim, A. F. Symp. (Int.) Combust. [Proc.] 18 1981, 1267. (14) Quann, R. J.; Sarofim, A. F. Fuel 1986, 65, 40. (15) Neville, M.; Sarofm,A. F. Symp. (Int.) Combut. [Proc.]19 1982, 1441.

Ash Formation during Pulverized Coal Combustion Table I. Proximate, Ultimate, Btu, and Sulfur Data for the Test Coals (As-Burned Weight Percent) Eagle Butte Robinson


S 0 (diff) calorific value, MJ/kg

26.1 33.0 35.5 5.4 51.2 6.0 0.6 0.4 36.4 19.9

21.2 29.2 41.1 8.6 53.8 5.4 0.9 0.8 30.5 20.5

layer, less refractory compounds such as sodium oxide and sodium sulfate can condense homogeneously or on ash particles leading to a layered composition of the ~artic1es.l~ The surfaces of such particles may have low viscosities making them sticky even when the underlying bulk of the particles are not. Many research groups have shown that the formation of relatively low-melting alkali or alkaline earth metal aluminosilicates is a key step in the processes of ash deposition and deposit growth during low-rank coal combustion. These low-melting phases can form “sticky” surfaces, facilitating the growth of ash deposits by enhancing the capture of additional ash particles, and furthermore can act as the “glue” to provide the interparticle adhesion leading to large, strong deposits. Since the greater portion of the alkali and alkaline earth elements occur in association with the organic portion of the coal, some sequence of physical transport processes and chemical reactions is necessary for the formation of the aluminosilicates. A comprehensive study of the transformation of the inorganic components of the coal to ash particles must involve sampling, analysis, and characterization through the entire process of combustion, from a characterization of the inorganic components of the original coal through to the complete burnout of the combustible matter. We will report the results of our approach to this problem in a series of papers. The present paper focuses on characterization of the inorganic constituents in two subbituminous coals, and the changes that occur during combustion to a point a t which about 50% of the combustible material in the coal has been removed. The importance of investigating the inorganic transformations during this early stage of combustion is that, as we discuss in the material which follows, even at this point significant interaction of sodium with silicates and aluminosilicates has occurred, while little interaction with calcium or iron has taken place. Further, the shift in sodium concentration as a function of particle size will show that it is more reactive or volatile than the alkaline earth elements in early stages of combustion. Sodium has long been implicated as one of the principal causative agents in ash deposition during pulverized coal combustion of low-rank coals. A study of inorganic transformations during early burnout of combustibles shows that sodium has already undergone significant interaction with mineral species, thus setting the stage for other reactions that occur during later stages of combustion. Experimental Section Materials. Two subbituminous coals were used in the present study. One was from the Eagle Butte mine, Wyoming, and the other was from the Robinson seam, Sarpy Creek mine, Montana. The Eagle Butte coal was provided by Foster Wheeler Development Corp.; the Robinson seam sample was provided by the University of North Dakota Energy and Environmental Research Center (UNDEERC). Proximate, ultimate, heating value, and

Energy & Fuels, Vol. 6, No. 1, 1992 49 Table 11. ASTM Ash Compositions of the Eagle Butte and Robinson Coals (S03-Free Weight Percent) Eagle Butte Robinson 2.5 4.7 7.2 17.2 32.4 0.3 31.1