Energy & Fuels 1993, 7, 542-553
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Ash Formation during Pulverized Subbituminous Coal Combustion. 2. Inorganic Transformations during Middle and Late 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 December 4,1992.Revised Manuscript Received April 30,1993
Two subbituminous coals were burned in a down-fired combustor of 745 MJ/h.m3 capacity. Transmission electron microscopy, computer-controlled scanning electron microscopy, and X-ray diffraction were used to study changes of inorganic matter in aerodynamically sized char samples collected at different degrees of burnout. The previous paper in this series' showed that the coals underwent 33 % -50 % burnout of combustible matter between introduction to the combustor and the first sampling port. During this stage, inorganic interactions consisted primarily of vaporization and condensation of inorganics that were originally associated with the organic portion of the coals. The present paper deals with subsequent changes that occur in two stages up to 99.9% combustible burnout. Between approximately 50 % and 90 % burnout, mineral and ash particle decomposition, fragmentation, and coalescence are the primary interactions, dominated by coalescence to form new calcium-rich phases. Submicron mineral matter that was not exposed at a burning char surface underwent no change in morphology up to these residence times. As burnout increased to 99.9%, the changes in the supermicron inorganic particles were similar but less dramatic, although some sulfation of the smallest particles occurred, and sodium interacted with aluminosilicates because of the lower temperatures. However, because of the longer residence times, the submicron mineral matter in the remaining char particles flowed through char micropores to form coalesced inorganic globules and rivulets within the pores.
Introduction One of the most important aspects of coal-fired power plant design is predicting the problems related to the formation and behavior of ash during combustion of the coal in the boiler system. As the ash is carried through the boiler, a portion separates from the flow of gas and impacts boiler surfaces. If the ash does not stick, the impaction can cause erosion. Ash that does deposit on boiler surfaces can corrode them and will reduce the transfer of heat to the working fluid. The propensity of ash to impact and stick to boiler surfaces is a function of the size and composition of the ash particles. To remove the deposits, as much as 1%of the generated steam may be diverted to on-line cleaning devices such as soot blowers. In some cases, unscheduled boiler shutdown may be necessary in order to remove the deposits, leading to losses of up to $100 000 per days2Especially severe and persistent problems may ultimately lead to derating of the boiler by up to 20%. The ash that does not deposit in the boiler must be collected by particulate control devices. The collection efficiency of many of these devices is also a function of the size and composition of the ash particles. Once collected, the ash must be disposed in an environmentally acceptable manner. The simplest methods for predicting ash-related problems involve applying data from coal or ash analyses to estimate the ash behavior or the magnitude of the problem. This approach is complicated by the complex behavior of t Present address: University of North Dakota Energy and Environmental Research Center, Grand Forks, ND 58202. (1) Hurley, J. P.; Schobert, H. H. Energy Fuels 1992, 6,47.
inorganic matter during combustion. Such behavior is dependent on the size, composition, and associations of the inorganic matter in the coal, as well as on boiler operation. One especially complex facet is the variety of reactions that change the size and composition distributions of the material. To delineate the types and sequence of interactions of the inorganic species in subbituminous coals of the western United States, two such coals were burned in the Penn State down-fired combustor. Samples of char and ash collected a t three stages of burnout of combustible matter were intensively characterized to determine the types of interactions that had occurred at each stage of burnout. In the first paper of this series, it was shown that, during the first 33 % -50% of combustible burnout, the dominant reactions were vaporization and condensation of inorganic material that was associated with the organic portion of the coals.' The present paper deals with subsequent reactions in two stages up to 99.9 % burnout. During combustion, the inorganics undergo a variety of interactions, depending on their association in the coal. Organically associated material is vaporized during the early stages of burnout, as shown in the first paper in this series.' It may then interact as a vapor with existing ash particles or condense homogeneously to form small particles of relatively pure material, which may themselves interact with existing ash particles. The mineral grains can undergo a large variety of interactions, including oxidation, decomposition, coalescence,fragmentation, and vaporization, leading to the formation of ash particles with (2) Buscheck, T. E.; Smith, R. T.; Burr, M. W. US. Department of Energy Report DOE/FC/10232-T1; 1981.
0887-0624/93/2507-0542$04.00/00 1993 American Chemical Society
Inorganic Transformations during Burnout
size and composition distributions very different from those of the mineral matter in the coal. The type of reaction undergone by a coal mineral during combustion depends on both its chemical composition and its physical morphology. The complexity in determining the interactions that will occur on the basis of analyses of the minerals in coal is illustrated by the interactions of pyrite during combustion. Pyrite oxidizes to produce gaseous SO2 and iron oxide particles. The effect of this reaction on the size of the pyrite particles is not well understood. The type of physical change is likely dependent on the morphology and association of the pyrite in the coal. Pyrite is usually present in two main morphologies, massive and framboidal. It is possible, though not proven, that framboidal pyrite will undergo fragmentation to produce large numbers of small iron oxide particles whereas the massive pyrite will form one large iron oxide particle. Pyrite is also present in many eastern United States coals in direct contact with aluminosilicate clay minerals. As the coal burns, the clay and pyrite can interact to produce an iron aluminosilicate glass or may dissociate to form separate iron oxide and aluminosilicate ash particles. Relatively new analytical techniques, such as computer-controlled scanning electron microscopy,3can give important information about the size and composition distribution of the minerals in the coal but do not delineate the morphology and associations of the minerals well (although this ability is being de~eloped).~ Most other minerals are not present in such a variety of forms, so their reaction pathways are simpler to determine. Like pyrite, carbonates undergo a chemical reaction (calcination) during combustion. This decomposition can sometimes lead to particle fragmentation: especially if the mineral was adventitious, or excluded, from the coal. Excluded mineral particles usually undergo little interaction withother ash particles in low-turbulence systems? On the other hand, minerals that are included within coal particles may undergo extensive coalescence with other grains in the coal particle. Coalescence of ash particles increases the overall size distribution of the ash but, more dramatically, alters the compositions of individual ash particles from those of the mineral particles originally in the coal. Coalescence of minerals can continue until only one ash particle forms per initial coal particle, although the char particle usually fragments so that several ash particles are produced per initial coal p a r t i ~ l e .The ~ degree of fragmentation depends on the combustion behavior of the coal particle. If the particle burns out in a region of the combustor where the ash can still coalesce, and if shedding of ash particles is not significant, then only one ash particle will form from one coal particle. If the temperature is low enough so that the ash particles on the surface of the char do not coalesce on contact, a large number of small ash particles may form from one particle.8 Large numbers of ash particles can (3) Strazheim, W. D.; Yousling, J. G.;Younkin, K. A.; Markuszewski, R. Fuel 1988,62, 1042. (4)Galbreath,K. C.;Brekke, D. W.; Folkedahl, B. C. R e p r . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1992,37, 1170. (6)Raaak, E. Erosion Wear in Coal Utilization; Hemisphere: Washington, D.C., 1988. (6) Sarofii, A. D.; Howard, J. B.; Padia, P. S. Combust. Sei. Technol. 1977, 16, 187. (7) Boni, A.A.; Helble, J. J.; Srinivasachar, S.Elect. Power Research Inst. Report EPRI-GS-7361; 1990. (8) Shiboaka, M.; Ramsden, A. R. In Ash Deposits and Corrosion Due to Impurities in Combustion Gases; Bryers, R., Ed.; Hemisphere: Washington, D.C., 1978; pp 67 ff.
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also form from one coal particle through the bursting of hollow, thin-walled ash spheres (cenospheres). A related mechanism involves the formation of plerospheres, which essentially are ash cenospheres that contain a large number of smaller ash particles. The smaller particles are prevented from coalescingwith the cenosphere by thin layers of carbon. The interior ash particles are released when the plerosphere bursts. The contribution of cenosphere and plerosphere bursting to the creation of large numbers of small ash particles from a single coal particle is under debate.”11 Volatilization of the mineral matter, followed by condensation as a submicron fume, is also a route to the formation of large numbers of small ash particles. Raask has described the volatilization of suboxides of silicon and aluminum produced from the reduction of silicate and aluminosilicate minerals at high temperature^.^ At lower temperatures the vapor condenses and oxidizes to silica and alumina. Some volatilization of potassium from clay minerals such as illite also occurs? as a result of segregation of potassium at the surface of the illite, followed by vaporization.12 The potassium is believed to condense in cooler regions of the furnace as potassium sulfate. In general, however, vaporization of discrete mineral matter is believed to play a minor role in the production of submicron fume during the combustion of pulverized lowrank coals in utility boilers.
Experimental Section Materials.The changes in inorganic matter during combustion were studied in two subbituminous coals. One was from the Eagle Butte mine, Wyoming, and the other was from the Robinson seam of the Sarpy Creek mine, Montana. The characterization of these coals was reported in detail in the previous paper of this series.’ Equipment. The coals were burned in the Penn State downfired combustor system a t a rate corresponding to an energy input to the combustor of 210 MJ/h (58kW). The combustor, which was described in detail elsewhere,lJs is designed for selfsustained combustion of pulverized coal in a premixed flame without recirculation or swirl. The combustor was also designed for easy access for sampling a t all stages of combustion. In the present paper we report results from the characterization of samples obtained at port 2, which is located 36 cm below the burner, and at port 10,the lowest sampling port in the combustor, 2.5 m below the burner. The particulate-laden gas samples were passed through an Anderson Samplers three-stage multicyclone system. The multicyclone system was followed by a polypropylene fiber filter to separate particulates from the gas. Gas temperatures during combustion of the Robinson coal were determined with a suction pyrometer. The pyrometer incorporates a type S thermocouple, fabricated from 0.51 mm diameter wire and shielded from radiation loss to the walls by cemented concentric mullite tubes. Combustion gas was pulled through the tip and over the thermocouple bead at average velocities approximating 120 m/s. Gassamples were collected using a water-cooled stainless steel probe. The temperature of the gas was kept above the dew point to prevent condensation of water which could possibly dissolve SOz. The gas flow was split into two streams. One was refrigerated to remove moisture and then analyzed using a (9) Raaak, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington, D.C., 1986. (10) Smith, R. D.;Campbell, J. A.; Nielson, K. K. Atmos. Enuiron. 1979,13, 607. (11) Wibberly, L.J.; Wall, T. F. Combust. Sci. Technol. 1986,48,177. (12) Stinespring, C. D.; Stewart, G . W. Atmos. Enuiron. 1981,15307. (13) Hurley, J. P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990.
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Beckmann Model 755 02 meter and Beckmann Model 864 infrared detectors for CO and COz. The other gas stream was diverted to a Thermo Electron Model 900 gas conditioner for dilution to lower the dew point, followed by analysisusing a Thermo Electron Model 40 pulsed fluorescent SOnmeter and a Thermo Electron Model 10 chemiluminescent NO, meter. Analyses of Products. The analytical methods have been discussed in detail elsewhere.lJ3 Briefly, the inorganic elemental analyses were performed by LiBO3 fusion, dissolution in 4% HN03,and analysis with a SpectrometricsSpectroscan 3 direct
current plasma spectrometer. X-ray diffractionanalysis (XRD) was performed with a Phillips XRG-3100 diffractometer using Cu K a radiation; peak identifications were made with Phillips SANDMAN software. Computer-controlled scanning electron microscopy (CCSEM)was performed at the University of North Dakota Energy and Environmental Research Center following the method of Zygarlicke and Steadman.” Errors in this method are usually 10, Ca + Mg 1 8 0 S < 15, Mg < 10, Fe > 20, Ca > 20, Ca + Mg + Fe 2 80 A1 + Si 1 80,0.8 < Si/Al < 1.5, Fe < 5, K < 5, Ca 9 5, Na < 5 Al+ Si 1 80,1.5 < SYAl < 2.5, Fe < 5, K < 5, Ca 5 5 , Na < 5 Na I 5, Ca I 5, Fe I 5, K 2 5, Si > 20, A1 1 15, K + A1 + Si 1 80 Fe 1 5, A1 1 15, Si > 20, S I 5, Ca I5, K I5, Na I5, Fe + Al+ Si 1 80 S I 5, K I 5, Fe I 5, Na I 5, Ca 1 5, A1 1 15, Si 1 20, Ca + Al + Si 1 80 S I 5, K I 5, Fe I 5, Ca I 5, Na 1 5, Al115, Si > 20, Na + A1 + Si 1 80 K I 5, Ca I5, Fe d 5, Na I 5, Si > 20, Al > 20, Si + A l 1 80 Na < 10, Fe < 10, Ca < 10, K < 10, S I 5, Si > 20, A1 > 20, Si + A1 + Fe + Ca + K + Na 1 80 Fe > 10, Na I 5, K I 5, Ca I 5, Al I 5, S I 5, Si > 20, Fe + Si 1 80 N a I 5, K I5, Fe 5 5 , Al I5, S 5 5 , Ca > 10, Si > 20, Ca + Si 2 80 P 5 5 , S 5 5 , Si 5 5 , Al> 15, Ca > 20, Ca + A1 1 80 Ca < 10, Fe > 20, S > 40, Fe + S 1 80, Fe/S < 0.7, Ba < 5 Fe > 20, Fe + S 1 80, S > 20,0.5 < Fe/S < 1.5, Ca < 10, Ba < 5 Ba < 5, Ca < 10, Fe > 40, S > 5, Fe/S > 1.5, Fe + S > 80 Ti < 10, Ba < 10, Si < 10, S > 20, Ca > 20, Ca + S 1 80 Fe < 10, Ca I 5, S > 20, Ba + Ti > 20, Ba + S +Ti 2 80 P > 20, Ca > 20, A1 < 5, S < 5, Ca + P L 80 A1 > 10, P > 10, Ca > 10, S I 5, Si I5, Al+ Ca + P 1 80 K 1 30, C 1 1 30, K + C 1 2 80 Fe I 5, Ca 1 5, Ba 1 5, Ti 1 5, S > 20, Ca + Ba + S + Ti 1 8 0 A1 1 5, Si 1 5, S 1 5, Ca 2 5, Ca+ A l + Si+ S 1 8 0 65 < Si < 80 65 < Ca < 80, A1 < 15 Si + Ca > 80, Ca > 20, Si > 20 all other compositions
sampling port in the furnace. This is valid because the complete combustion of carbon creates no net change in the number of moles of gas; the measured concentration of CO is negligible, and the volume of gas created by HzO vaporization and formation from hydrogen combustion is only about 2 % of the total gas flow. The greatest increase in gas volume, relative to the volume of combustion air, is due to the evaporation of moisture from the coal, which is complete by the time the particles reach the first port. It was also assumed that the pressure in the combustor does not vary with position and that the gas is ideal. The calculation of residence times in the combustor follows standard concepts; details have been published elsewhere.13 Figure 2 shows the time-temperature history of a 20 pm diameter extraneous quartz particle as it passes from port 1 to port 10 during a 210 MJ/h test of the
Inorganic Transformations during Burnout
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99.9 % burnout. The total residence time in the refractory-lined portion of the combustor was approximately 2.4 s. The equilibrium temperature of an extraneous inorganic particle at the position of the sampling probe was approximately 1140 "C. The elemental contents of the ashes of the Robinson particulates collected at port 10 are listed in Table VIII. The only important change in the distribution of the elements among the different cyclone samples between
Figure 10. TEM photomicrograph of another area of the char particle illustrated in Figure 9, showing melting of the small, high-contrast inclusions.
Table VIII. Inorganic Elemental Composition of Robinson Port 10 Cyclone Samples (SOs-Free Weight Percent) elemental oxide
wt%oftotal
elemental composition (% ' ) of cyclone 1 2 3 4.2 9.1 10.5 4.2 3.8 3.1 24.1 26.7 17.0 32.5 33.0 43.1 0.2 0.2 0.2 25.1 22.3 20.3 0.5 0.4 0.6 co.1
