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Mineral matter transformations and ash deposition in pulverised coal combustion
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, 1992/pp. 1119-1126 INVITED TOPICAL REVIEW
M I N E R A L M A T T E R T R A N S F O R M A T I O N S A N D A S H D E P O S I T I O N IN PULVERISED COAL COMBUSTION T. F. WALL
Department of Chemical Engineering The University of Newcastle Callaghan, NSW 2308, Australia During combustion the inorganic constituents of coal are transformed into products (including vapors, aerosols and residual ash particles) which may transfer to furnace walls and form deposits which restrict heat transfer. Many of the mechanisms of these transformations have now been identified and are detailed by considering the elements, iron and sodium, which have been associated with slagging and fouling respectively. Developments in the scientific analysis of ash deposition, based on these mechanisms, include techniques for coal charaeterisation, control techniques, the development of engineering deposition parameters and mathematical modelling of the processes.
Background Power-plant technologists are confronted daily with the challenges associated with the inorganic matter in coal--the furnace aspects of slagging and fouling, metal wastage due to corrosion and erosion, ash collection and disposal as well as the control of emissions such as SO2 and fumes. The Marchwood Conference of 19631 was the first to involve researchers other than power-plant technologists in these issues. The state of understanding since that time has been detailed by Reid and others2-~ by Raask,5 several reviews6'7 together with the proceedings of the conference series arranged by the Engineering Foundations-12 and American Chemical Society. ta The present paper details recent research which has taken advantage of modern experimental and analytical techniques together with computational models to establish mechanisms by which coal minerals are transformed to ash during combustion. The ash character thus established allows the analysis of deposition rates to be made on a scientific basis.
Terminology: The result of ash deposition on walls is called
slagging when a liquid layer results (generally on furnace walls in proximity to the flame) and fouling when a dry, powdery deposit is formed (generally in the convection pass). The inorganic constituents of coal are primarily present as mineral matter with some coals (particularly lignites and brown coals) having a proportion of the elements which are or-
ganically associated. Pulverised coal (p.o.) covers the approximate size range 10-150 I~m. The mineral grains are (on average) finer, are dispersed unevenly within the coal particles, and may be partially separated during crushing prior to combustion. The mineral matter which is closely associated with pulverised coal (p.c.) and is not separated prior to combustion is called inherent, (or included) that which is loosely associated and which enters the furnace separate from organic matter (as separate particles) is called extraneous (or excluded).
Slagging and Fouling Factors: A recent survey 14 prepared for EPRI to evaluate the parameters presently used by boiler manufacturers concluded that most (>80%) deposition problems can be predicted by factors which account for ash chemistry and combustion intensity (or furnace temperature). The coal factors, which are given on Table I, are based on the ash analysis and on the ash constituents associated with observed deposition. Many indices have been proposed which correlate performance with particular coal types 2'15'16 with a majority associating the slagging of US bituminous ash coals with iron (as Fe2Oa, Fe2Oa/CaO, S) and the fouling of lignite-ash types with predominantly organically bound sodium and calcium (soluble Na20 and calcium, C1, S, etc). The review will detail research to identify mechanisms responsible for these associations.
Combustion Products and Mechanisms for Deposit Formation and Growth: The understanding of the association of coal constituents with deposits was initially based on the
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1120
INVITED TOPICAL REVIEW
TABLE I Recommended engineering indices for correlating ash composition with deposition, based on US coals Coal type
Slagging
Bituminous-ash/j~
(Fe203 + CaO + MgO + NazO + K20) (SiOz + A1203 + TiO2)
Fouling *S (dry coal)131
Lignitic-ash12/
Ash fluid temperature in reducing atmosphere
~/Fe203 > CaO /2/Fe203 < CaO /31The Attig and /41A variation of
+ MgO + MgO Duzy factor, (base/acid)* suflur, recommended in 1969~151. a factor again recommended in 1969aSI.
elements found to be preferentially enriched in deposits. Related work 17 revealed that mineral matter (as indicated by ash determinations) was unevenly distributed throughout size and density fractions prepared from p.c. This density fractionation technique was subsequently used as a tool for coal characterisation,18 and related to deposition, by the association of the distribution of mineral matter within the coal matrix with the enriched elements in deposits (e.g. iron in >2.9 specific gravity (SG) fraction, alkali in <1.7 SG fraction). Several studies in the late seventies e.g. 19 indicated that the size distribution of the final ash product is bimodal and this consists of submicron particles (-0.1 txm) resulting from the homogeneous condensation of flamevolatilized species, and large flyash particles formed from the products of the heating and oxidation of mineral matter (having a mean size of 10-20 txm). The formation of the ashy material associated with deposition, from the inherent and extraneous minerals established by extensions of these studies, led to the following three primary deposition mechanisms identified on Fig. 1.
Water soluble Na20 in coal, %14/
tion in the boundary layer and diffusion to the surface, or by direct condensation on tube surfaces and walls, or on a partically developed deposit so that it collects flyash. In addition sticky particles (Mechanism a) may be generated by heterogeneous condensation on flyash. Vaporized mineral matter can either condense (as an aerosol) homogeneously, or heterogeneously, on other aerosols or on residual ash depending on the gas cooling rate and the surface area for the condensation. A sequence, in time, is observed during the deposition process, with an initial layer generally formed by condensible submicron particles, fine silicates or by large sticky particles. The collection, following inertial impact, of sticky particles results in catastrophic uncontrolled deposits. 25 A mature deposit can require weeks or months to develop during which further reactions within the deposit and with furnace gases occur, (e.g., sintering, sulfation). Controlled experiments to monitor this total process are rare.
(a) Inertial impaction and retention of large sticky particles (>15 Ixm) unable to rebound on impact onto a dry surface, or other particles (>5 /xm) retained b~o the deposit glue. A model has been proposed to quantify this interaction whereby sticky particles are retained if the interfacial energy over which contact is established between dry and sticky surfaces is greater than the kinetic energy of the incoming particle. (b) Diffusion of fume (or aerosol) particles (which are formed by the evaporation of inherent mineral residues and elements as metals and suboxides, <1 /zm) by thermophoresis, 21'22 or the particle drift down the gas temperature gradient within the boundary layer adjacent to surfaces. (c) Condensation of salts,23 such as NaCl(s) and Na2SO4(s), subsequent to homogeneous condensa-
CaO + S
Iron and Slagging Iron occurs in bituminous coal primarily as minerals~ (pyrite-FeS2, siderite-FeCO3, with the possibility of associations with chlorite and illite). In pulverised coal these minerals can be extraneous or inherent. In low-rank coals, iron can also be chemically associated within the coal molecule. US bituminous coals, on which the parameters of Table I are based, have the pyrite mineral as the principal iron species. The weight percent of Fe203 in ash can range from 2 to 60%. Raask5 has identified the association of iron with deposition from four sources:
a) An oxide fume from inherent and organically associated iron.
MINERAL MA'ITER BEHAVIOR IN PULVERISED COAL COMBUSTION
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FI(,. 1. Schematic of the mechanism of ash formations and deposition. Large particles ~ , vapors and gases . . . . . . , fine particles - -
b) Large oxide ash particles and fume from dissociated FeCO3 (inherent and extraneous). c) Molten sulfide and oxide particles from partially oxidised pyrite (inherent and extraneous). d) Fused silicates with dissolved iron (inherent).
Extraneous Pyrite: The thermal decomposition and oxidation of extraneous pyrite has been extensively studied. ~'z6 Of interest, here, is the question of the duration of liquid particles (including molten FeS) which have been associated with the early stages of deposition on furnace walls. 2~'z8 Experimental studies on the combustion of pyrite particles which have been removed from coals zg'3~ have been used "~ to establish the mechanisms of the transfi)rmations, given on Fig. 2, with estimates of the associated rates. In summary, the transformations illustrated on Fig. 2 include decomposition of pyrite to pyrrhotite and its subsequent oxidation via a F e - - S - - O melt
to magnetite and hematite. The steps requiring the greatest time were predicted to be pyrrhotite oxidation and magnetite crystallisation, with a melt present for greater than 80% of the oxidation time. This pericnt is for the soft particles indicated on Fig. 2. The calculated time for fnll reaction to magnetite was 120 ms for a 45/xm particle. The time spent in the molten state was found to be approximately proportional to the square of the particle diameter, indicating coarse pyrite will be more troublesome. In 1978, Bryers recommended pyrite coarser than 7.5/xm as a slagging criterion ~ due to a recognition of the importance of this course pyrites. Magnetite was found to be the dominant oxidation product in experiments (and is also expected on thermodynamic grounds). The hematite found in deposits appears therefore to be due to magnetite which is oxidised after deposition. The times for transformations and oxidation of inherent pyrite, and the effect of the associated carbon in char, and the internal reducing atmosphere
1122
INVITED TOPICAl, REVIEW
l~a~,FoS2
Pln~lw'~., F~X
cRYrt~sA~ON~ SOFTpAR~CLES OXlDA'HON
M ~ n ~ , F ~ O 4,
Ho~, Fa203
FIG. 2. Schematic of the pyrite transformations in a combustion environment (after Srinivasachar and
Boni3J). indicated on Fig. 1, is presently not clear, and requires further research.
Iron Fume: Extraneous particles of pyrites and iron carbonates fracture and disintegrate when heated rapidly,~'3~ yielding fume particles, 0.1 to 1 Ixm in diameter. Experiments using synthetic chars doped with 4% inherent 1 to 5/xm pyrite, and coals with inherent pyrite, resulted in the formation of significant fume containing iron (-3% of Fe at particle temperatures of 1900~ C) yet little fume when extraneous pyrite from the same coals was used. 35 These experiments indicated that evaporation of inherent iron was the release mechanism and that this was strongly dependant on fine inherent pyrite inclusions. The iron vaporisation was matched by a mathematical model with the vapour pressure of atomic iron over the FeS melt from the following reaction: FeS(s = Fe(g) + 1/2 S2(g). Recent experiments36 have also reported the generation of a fume from inherent pyrite and associated the extent of fume (4-7% of Fe in coal) with the rapid disintegration of porous pyrrhotite formed from pyrite oxidation following devolatilization. A difference in the degree of oxidation of pyrite of the coal samples may explain this apparent inconsistancy.~'36
Iron and Soft Silicates: The dissolution of iron into clay-derived aluminosilicates and quartz reduces the melting point of the chemical mixture which constitutes an ash particle. The mixture composition depends on the
mineral content of the p.c. particle from which the ash particle is formed in that, disregarding the char fragmentation shown on Fig. 1, one flyash ash particle is formed due to coalescence of the products of the minerals in each coal particle. In practice, some fragmentation is likely to occur. In order to determine the composition of flyash particles the distribution of minerals within the p.c. particles must therefore be known. A mathematical approach to this problem is provided by the a technique which requires data provided by computer controlled scanning electron microscopy with automated image analysis (CCSEM) of the compositions and sizes of individual minerals inclusions in coal. The CCSEM technique was developed in the late seventies and was applied to coal characterisation by Huffman and Huggins, 3r to ash d e ~ i t i o n problems by the University of North Dakota and the modelling of the ash formation process by the MIT group. 39 For homogeneous particles, viscosity may be directly related to the ash composition established by these techniques. However, Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) analysis of ash formed from inherent pyrite-derived iron oxide and clay-derived aluminiosilicates has shown 4~ that the extent of chemical dissolution can be limited. For several coals, only 25% of iron was found to form glassy phases. The knowledge of the size and association of inherent iron minerals, which will determine the dissolution, must therefore be known.
Sodium and Fouling Sodium occurs in bituminous coals as halite (NaCI) or associated with illite-montromorillanite. Chlorine content indicates the fraction of sodium as halite. For low rank coals sodium is primarily associated with the organic matter as exchangeable ions (R--COONa) and salts. Na20 weight percent in ash is low (<2%) to high (>6%). The material identified with the association of sodium with deposition is of two types. 41 a) Sodium gases which condense or react to form the salts which are collected on cool walls and tubes, NaCI and NazSO4. b) As large silicaceous particles having a thin sticky exterior surface layer. As both of these materials are formed following the release of sodium from heating coal particles, the "active" sodium used as the fouling factor in Table I is used as an estimate of released sodium.
Sodium Release and Sodium-Ash Reactions During Combustion: Raask'5 suggested that 50% of the soluble sodium of the coal minerals vaporises to form salts. How-
MINERAL MATTER BEHAVIOR IN PULVERISED COAL COMBUSTION ~l~l(tmnts lOO
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FIG. 4. The mechanisms of the Na-ash reactions from the experiments of Lindner~ involving modified coal (demineralised coal, inherent 10/zm silica and NaC1 crystals).
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FIG. 3. Thermodynamic predictions of the distribution of sodium within the combustion products for a raw brown coal and the same coal treated to substitute A1 for some of the Na and CI. The raw coal is higher in sodium, so that changes in the absolute amounts of each species are evaluated by multiplying %Na (db) in the coal by the %total sodium plotted. Atomic ratios (Na/CI), (Na/S), (Na/Si), (Na/AI) are for Raw Coal: 1.71, 2.13, 1.87, 29.1 and for Treated Coal: 1.66, 0.67, 0.64, 1.44. Sticky particles - - , condensible salts . . . . . . . .
ever, recent work has shown that a high proportions of the water soluble sodium vaporises, (i.e. chlorides and as salts of carboxyl groups), but then reactions between the sodium containing gases and the silaceous fly ash reduces the concentration of these gases. Experiments can estimate these levels by collecting homogeneously condensed salts following combustion. Neville and Sarofim42 report sodium levels appearing in such salts of 19 to 43% of the original coal sodium with a dependence on the extent and size of the silicon containing minerals in coal. At flame temperatures the vaporised sodium exists in several forms. Equilibrium calculations ~3-45 predict the gas phase sodium species. Figure 3 gives the predicted distributions of sodium in a coal, and
the same coal treated with aluminium, to show that a significant proportion of chlorine exists as sodium chloride in the furnace gases for high chlorine systems. These predictions show qualitative agreement with measurements made4e-48 using analytical techniques such as flame photometry, mass spectrometry, laser-induced flourescence and laser-induced photodissociation. However, the predictions are based on full availability of elements, such as Si and AI, which are not fully free for reaction as they are contained in particles which react only on an external surface. The use of an availability coefficient (<1) can allow for this effect by limiting the involvement of the element. There is substantial evidence for the interaction of sodium with ash to form silicates and aluminium silicates, 4]'49-51 sodium being captured with silica rich grains in the residual ash. The studies 4z also indicate that the extent of formation of the condensible sodium salts decreases with an increase in combustion temperature, a result attributed to an increase in this interaction. Such behaviour is consistent with experiments5~ reporting the interaction of sodium with silica using a synthetic combustion gas containing sodium vapor. Later, experiments4] quantified the sodium affinity of silicon by using ash free coals impregnated with inherent 10 txm silica (SiOz) and kaolin (SiO2" AlzO3" 2H20), salt (NaCI), and sodium acetate (CH3COONa). Sulphur dioxide was added as a gas to simulate coal sulphur. The mechanisms thus established for sodium-ash interactions are given on Fig. 4. The interaction between the elements Na, Si, AI, CI, S in forming the silicates (Na2SiO3, NaA1SiO4) and the condensible salts has also been indicated by other studies. 5] These results were obtained for a brown coal treated with AI to substitute some of the Na and CI. Laboratory experiments support the thermodynamic predictions presented on Fig. 3 for the raw coal and treated coal, in that dramatically reduced deposits due to condensibles and sticky particles
INVITED TOPICAL REVIEW
1124
were observed with the treated coal. The formation of NaA1SiO4 from the treated coal, as suggested by Lindner5z together with a reduction in the formation of gaseous species containing sodium (which condense to form NaC1 and NazSO4) will reduce fouling. The extent to which the interactions occurs depends on the distribution and relative accessibility of mineral species within the coal particles and the sodium/ash yield ratios.
Condensation and Deposition: The equilibrium predictions of Fig. 3 suggest that the stable species at flame temperatures are NaCl(g), NaOH(g) and Na2SO4(g) whereas the stable condensible species at deposit temperatures are Na2SO4 and NaCI (in lesser proportion). The diffusion of the sodium containing gases to cool walls will therefore be accompanied with some sult:ation but the final Na2SO4 found in deposits may be also formed on furnace walls. Inspite of several investigations5z-~5 the mechanism and kinetics of Na2SO4 formation remain ambiguous.
Other Interactions Emphasis has been placed here on the mechanisms by which sodium and iron influence deposition. These mechanisms will be most important to some coal types, but other elements may clearly be important. For example, potassium will flux aluminosilicates more readily than iron if it is present in clays and fouling may be due to sulfated calcium and magnesium. 56 Other interactions are detailed elsewhere. 5
Development Based on Transformation Mechanisms The traditional analysis of coals in terms of their likely effect on furnace deposition has been based on the elemental analysis of 'whole' ash, the ASTM (or other standard) ash composition. From the mechanisms outlined above this is unlikely to be valid as 9 a small fraction of the mineral (or ash) mass, having a composition different to the average ash, may be the cause of deposits 9 elements may be present in different forms, as shown in Fig. i. This form has a dominant influence on the transformation of material associated with deposition. 9 elements may be 'reactive' or 'unreactive,' in that all of an element may not be associated with deposition. 9 the variability of the association of the minerals
(and elements) within individual coal particles influences deposition.
Analysis of Mineral Matter: The shortcomings noted above lead to the conclusion that knowledge of the sizes, types, abundance of the minerals in p.c. particles is essential to an analysis of deposition. The development of the CCSEM technique, together with other mineral and elemental techniques, to provide this information has underpinned the rapid progress made recently.
Coal Characterisation and Deposition Indices: The current expansion of the international trade in thermal coals necessitates a sophisticated approach to coal characterisation and testing. Again, iron and sodium will be considered here.
Iron mechanisms. Coarse extraneous pyrites should characterise slagging due to impaction of partially oxidised pyrite. Conversely, inherent pyrites will generate fume and softer particles if it is finely dispersed. The particle-to-particle variation of inherent pyrite and clays will also determine the range of viscosities of flyash particles. Sodium mechanisms. The formation of condensible salts depends on the sodium which is evaporated and yet does not react to form silicates. Thermodynamic calculations of the type given on Fig. 3 can be used as a guide when comparing coals or evaluating temperature effects. The stickiness of flyash particles can also be determined by assuming a diffusion limited model. 52 The association of the above analyses with observed fouling can be expected to lead to more sophisticated engineering indices, to complement those of Table I, once sufficient experience is available.
Coal Treatment and Additives: An understanding of the dominant mechanisms is clearly necessary when options to alleviate deposition problems are considered, for example, by the use of coal blends, washing to remove or add minerals or elements, or the use of additives to distort the ash chemistry.
Mathematical Modelling of Deposition, and Interpretation of Experiments: Apart from coal properties, furnace-related parameters clearly influence the thermal and aerodynamic conditions known to influence ash formation as well as deposition. Deposits also build and mature over long periods. Models, then, will be an essential tool in understanding furnace effects and interpreting and extrapolating short-term measurements. A possible experimental approach is to ac-
MINERAL MATI'ER BEHAVIOR IN PULVERISED COAL COMBUSTION celerate the deposition process by increasing temperature. Models can evaluate the validity of this approach, and the possibility of an associated change in mechanisms. The deposition rates due to the mechanisms of Fig. 1 may also be calculated if the concentration, size and composition of ashy material is known. Recently, contributions e.g. 57-60 have utilised the increased understanding of the formation processes in formulating initial mechanistic models.
Acknowledgements The author is indebted to many colleagues for their contributions to this review. Particular thanks are due to H. B. Vuthaluru, L. L. Baxter, S. A. Benson, R. W. Bryers, A. Conroy, G. Domazetis, A. F. Sarofim and S. Srinivasachar.
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49. 50.
namics of sodium in oxygen-rich flames, NASA Report No. NASA-Cr-169848 1982. CAMPESl, A. AND DOMAZETlS, G.: Experimental and modelling investigations of sodium and other ash forming inorganics in flames, SECV, R & D Dept. Report, No. N/90/088, 1990. HELBE, J. T., SRINIVASACHAR, S., BeNt, A. A., CHARON, O. AND MODESTINO, A.: Combust. Sei. Tech., 81, 193 (1992). Boow, J.: Fuel, 51, 170 (1972). WIBBERLEY, L. J. AND WALL, T. F.: Fuel, 61,
93 (1982). 51. VUTHALURU, H. B., GUPTA, R. P., WALL, T. F. AND DOMAZETIS, G.: Ref. 12, p 285. 52. LINDNER, E. R.: Ph.D. Thesis, Dept. of Chemical & Materials Engineering, The University of Newcastle, 1988. 53. ERICKSON, T. A., LUDLOW, D. K. AND BENSON, S. A.: Fuel, 71, 15 (1992). 54. HALSTEAD, W. n . AND RAASK, E.: J. Inst. F., 42, 344 (1969). 55. BISHOP, R. J.: j. Inst.: Fuel, 41, 325, 51 (1968). 56. BRYERS, R. W. : The Impact of Coal Characterisation on Boiler Design, in Compilation of Papers for Coal Characterisation and Boiler Design Seminar, Jakarta, Indonesia, Jan 28-30, 1992. 57. SRINIVASACHAR,S., SENIOR, C. L., HELBE, J. L. AND MOORE, J. W.: This symposium. 58. WALSH, P. M., SAYRE, A. N., LOCHDEN, D. O., MONROE, L. S., BEER, J. M. AND SAROFIM, A. F.: Prog. Energy Combust. Sci., 16, 327, (1990). 59. WALSH, P. M., SAROF1M, A. F. AND BEER, J. M. : Fouling of Convection Heat Exchangers by Lignitic Coal Ash, to be published in Energy and Fuels, 1992. 60. ZYGARLICKE, C. J., RAMANATHAN, M. AND ERICKSON, T. A.: Ref. 12, p 525.
M I N E R A L M A T T E R T R A N S F O R M A T I O N S A N D A S H D E P O S I T I O N IN PULVERISED COAL COMBUSTION T. F. WALL
Department of Chemical Engineering The University of Newcastle Callaghan, NSW 2308, Australia During combustion the inorganic constituents of coal are transformed into products (including vapors, aerosols and residual ash particles) which may transfer to furnace walls and form deposits which restrict heat transfer. Many of the mechanisms of these transformations have now been identified and are detailed by considering the elements, iron and sodium, which have been associated with slagging and fouling respectively. Developments in the scientific analysis of ash deposition, based on these mechanisms, include techniques for coal charaeterisation, control techniques, the development of engineering deposition parameters and mathematical modelling of the processes.
Background Power-plant technologists are confronted daily with the challenges associated with the inorganic matter in coal--the furnace aspects of slagging and fouling, metal wastage due to corrosion and erosion, ash collection and disposal as well as the control of emissions such as SO2 and fumes. The Marchwood Conference of 19631 was the first to involve researchers other than power-plant technologists in these issues. The state of understanding since that time has been detailed by Reid and others2-~ by Raask,5 several reviews6'7 together with the proceedings of the conference series arranged by the Engineering Foundations-12 and American Chemical Society. ta The present paper details recent research which has taken advantage of modern experimental and analytical techniques together with computational models to establish mechanisms by which coal minerals are transformed to ash during combustion. The ash character thus established allows the analysis of deposition rates to be made on a scientific basis.
Terminology: The result of ash deposition on walls is called
slagging when a liquid layer results (generally on furnace walls in proximity to the flame) and fouling when a dry, powdery deposit is formed (generally in the convection pass). The inorganic constituents of coal are primarily present as mineral matter with some coals (particularly lignites and brown coals) having a proportion of the elements which are or-
ganically associated. Pulverised coal (p.o.) covers the approximate size range 10-150 I~m. The mineral grains are (on average) finer, are dispersed unevenly within the coal particles, and may be partially separated during crushing prior to combustion. The mineral matter which is closely associated with pulverised coal (p.c.) and is not separated prior to combustion is called inherent, (or included) that which is loosely associated and which enters the furnace separate from organic matter (as separate particles) is called extraneous (or excluded).
Slagging and Fouling Factors: A recent survey 14 prepared for EPRI to evaluate the parameters presently used by boiler manufacturers concluded that most (>80%) deposition problems can be predicted by factors which account for ash chemistry and combustion intensity (or furnace temperature). The coal factors, which are given on Table I, are based on the ash analysis and on the ash constituents associated with observed deposition. Many indices have been proposed which correlate performance with particular coal types 2'15'16 with a majority associating the slagging of US bituminous ash coals with iron (as Fe2Oa, Fe2Oa/CaO, S) and the fouling of lignite-ash types with predominantly organically bound sodium and calcium (soluble Na20 and calcium, C1, S, etc). The review will detail research to identify mechanisms responsible for these associations.
Combustion Products and Mechanisms for Deposit Formation and Growth: The understanding of the association of coal constituents with deposits was initially based on the
1119
1120
INVITED TOPICAL REVIEW
TABLE I Recommended engineering indices for correlating ash composition with deposition, based on US coals Coal type
Slagging
Bituminous-ash/j~
(Fe203 + CaO + MgO + NazO + K20) (SiOz + A1203 + TiO2)
Fouling *S (dry coal)131
Lignitic-ash12/
Ash fluid temperature in reducing atmosphere
~/Fe203 > CaO /2/Fe203 < CaO /31The Attig and /41A variation of
+ MgO + MgO Duzy factor, (base/acid)* suflur, recommended in 1969~151. a factor again recommended in 1969aSI.
elements found to be preferentially enriched in deposits. Related work 17 revealed that mineral matter (as indicated by ash determinations) was unevenly distributed throughout size and density fractions prepared from p.c. This density fractionation technique was subsequently used as a tool for coal characterisation,18 and related to deposition, by the association of the distribution of mineral matter within the coal matrix with the enriched elements in deposits (e.g. iron in >2.9 specific gravity (SG) fraction, alkali in <1.7 SG fraction). Several studies in the late seventies e.g. 19 indicated that the size distribution of the final ash product is bimodal and this consists of submicron particles (-0.1 txm) resulting from the homogeneous condensation of flamevolatilized species, and large flyash particles formed from the products of the heating and oxidation of mineral matter (having a mean size of 10-20 txm). The formation of the ashy material associated with deposition, from the inherent and extraneous minerals established by extensions of these studies, led to the following three primary deposition mechanisms identified on Fig. 1.
Water soluble Na20 in coal, %14/
tion in the boundary layer and diffusion to the surface, or by direct condensation on tube surfaces and walls, or on a partically developed deposit so that it collects flyash. In addition sticky particles (Mechanism a) may be generated by heterogeneous condensation on flyash. Vaporized mineral matter can either condense (as an aerosol) homogeneously, or heterogeneously, on other aerosols or on residual ash depending on the gas cooling rate and the surface area for the condensation. A sequence, in time, is observed during the deposition process, with an initial layer generally formed by condensible submicron particles, fine silicates or by large sticky particles. The collection, following inertial impact, of sticky particles results in catastrophic uncontrolled deposits. 25 A mature deposit can require weeks or months to develop during which further reactions within the deposit and with furnace gases occur, (e.g., sintering, sulfation). Controlled experiments to monitor this total process are rare.
(a) Inertial impaction and retention of large sticky particles (>15 Ixm) unable to rebound on impact onto a dry surface, or other particles (>5 /xm) retained b~o the deposit glue. A model has been proposed to quantify this interaction whereby sticky particles are retained if the interfacial energy over which contact is established between dry and sticky surfaces is greater than the kinetic energy of the incoming particle. (b) Diffusion of fume (or aerosol) particles (which are formed by the evaporation of inherent mineral residues and elements as metals and suboxides, <1 /zm) by thermophoresis, 21'22 or the particle drift down the gas temperature gradient within the boundary layer adjacent to surfaces. (c) Condensation of salts,23 such as NaCl(s) and Na2SO4(s), subsequent to homogeneous condensa-
CaO + S
Iron and Slagging Iron occurs in bituminous coal primarily as minerals~ (pyrite-FeS2, siderite-FeCO3, with the possibility of associations with chlorite and illite). In pulverised coal these minerals can be extraneous or inherent. In low-rank coals, iron can also be chemically associated within the coal molecule. US bituminous coals, on which the parameters of Table I are based, have the pyrite mineral as the principal iron species. The weight percent of Fe203 in ash can range from 2 to 60%. Raask5 has identified the association of iron with deposition from four sources:
a) An oxide fume from inherent and organically associated iron.
MINERAL MA'ITER BEHAVIOR IN PULVERISED COAL COMBUSTION
Heattmt~sfer surfaces(val]s envimnmant
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-
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Mol~n sl~ inner layer Furnace heat transfer surfaces (walls)
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FI(,. 1. Schematic of the mechanism of ash formations and deposition. Large particles ~ , vapors and gases . . . . . . , fine particles - -
b) Large oxide ash particles and fume from dissociated FeCO3 (inherent and extraneous). c) Molten sulfide and oxide particles from partially oxidised pyrite (inherent and extraneous). d) Fused silicates with dissolved iron (inherent).
Extraneous Pyrite: The thermal decomposition and oxidation of extraneous pyrite has been extensively studied. ~'z6 Of interest, here, is the question of the duration of liquid particles (including molten FeS) which have been associated with the early stages of deposition on furnace walls. 2~'z8 Experimental studies on the combustion of pyrite particles which have been removed from coals zg'3~ have been used "~ to establish the mechanisms of the transfi)rmations, given on Fig. 2, with estimates of the associated rates. In summary, the transformations illustrated on Fig. 2 include decomposition of pyrite to pyrrhotite and its subsequent oxidation via a F e - - S - - O melt
to magnetite and hematite. The steps requiring the greatest time were predicted to be pyrrhotite oxidation and magnetite crystallisation, with a melt present for greater than 80% of the oxidation time. This pericnt is for the soft particles indicated on Fig. 2. The calculated time for fnll reaction to magnetite was 120 ms for a 45/xm particle. The time spent in the molten state was found to be approximately proportional to the square of the particle diameter, indicating coarse pyrite will be more troublesome. In 1978, Bryers recommended pyrite coarser than 7.5/xm as a slagging criterion ~ due to a recognition of the importance of this course pyrites. Magnetite was found to be the dominant oxidation product in experiments (and is also expected on thermodynamic grounds). The hematite found in deposits appears therefore to be due to magnetite which is oxidised after deposition. The times for transformations and oxidation of inherent pyrite, and the effect of the associated carbon in char, and the internal reducing atmosphere
1122
INVITED TOPICAl, REVIEW
l~a~,FoS2
Pln~lw'~., F~X
cRYrt~sA~ON~ SOFTpAR~CLES OXlDA'HON
M ~ n ~ , F ~ O 4,
Ho~, Fa203
FIG. 2. Schematic of the pyrite transformations in a combustion environment (after Srinivasachar and
Boni3J). indicated on Fig. 1, is presently not clear, and requires further research.
Iron Fume: Extraneous particles of pyrites and iron carbonates fracture and disintegrate when heated rapidly,~'3~ yielding fume particles, 0.1 to 1 Ixm in diameter. Experiments using synthetic chars doped with 4% inherent 1 to 5/xm pyrite, and coals with inherent pyrite, resulted in the formation of significant fume containing iron (-3% of Fe at particle temperatures of 1900~ C) yet little fume when extraneous pyrite from the same coals was used. 35 These experiments indicated that evaporation of inherent iron was the release mechanism and that this was strongly dependant on fine inherent pyrite inclusions. The iron vaporisation was matched by a mathematical model with the vapour pressure of atomic iron over the FeS melt from the following reaction: FeS(s = Fe(g) + 1/2 S2(g). Recent experiments36 have also reported the generation of a fume from inherent pyrite and associated the extent of fume (4-7% of Fe in coal) with the rapid disintegration of porous pyrrhotite formed from pyrite oxidation following devolatilization. A difference in the degree of oxidation of pyrite of the coal samples may explain this apparent inconsistancy.~'36
Iron and Soft Silicates: The dissolution of iron into clay-derived aluminosilicates and quartz reduces the melting point of the chemical mixture which constitutes an ash particle. The mixture composition depends on the
mineral content of the p.c. particle from which the ash particle is formed in that, disregarding the char fragmentation shown on Fig. 1, one flyash ash particle is formed due to coalescence of the products of the minerals in each coal particle. In practice, some fragmentation is likely to occur. In order to determine the composition of flyash particles the distribution of minerals within the p.c. particles must therefore be known. A mathematical approach to this problem is provided by the a technique which requires data provided by computer controlled scanning electron microscopy with automated image analysis (CCSEM) of the compositions and sizes of individual minerals inclusions in coal. The CCSEM technique was developed in the late seventies and was applied to coal characterisation by Huffman and Huggins, 3r to ash d e ~ i t i o n problems by the University of North Dakota and the modelling of the ash formation process by the MIT group. 39 For homogeneous particles, viscosity may be directly related to the ash composition established by these techniques. However, Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) analysis of ash formed from inherent pyrite-derived iron oxide and clay-derived aluminiosilicates has shown 4~ that the extent of chemical dissolution can be limited. For several coals, only 25% of iron was found to form glassy phases. The knowledge of the size and association of inherent iron minerals, which will determine the dissolution, must therefore be known.
Sodium and Fouling Sodium occurs in bituminous coals as halite (NaCI) or associated with illite-montromorillanite. Chlorine content indicates the fraction of sodium as halite. For low rank coals sodium is primarily associated with the organic matter as exchangeable ions (R--COONa) and salts. Na20 weight percent in ash is low (<2%) to high (>6%). The material identified with the association of sodium with deposition is of two types. 41 a) Sodium gases which condense or react to form the salts which are collected on cool walls and tubes, NaCI and NazSO4. b) As large silicaceous particles having a thin sticky exterior surface layer. As both of these materials are formed following the release of sodium from heating coal particles, the "active" sodium used as the fouling factor in Table I is used as an estimate of released sodium.
Sodium Release and Sodium-Ash Reactions During Combustion: Raask'5 suggested that 50% of the soluble sodium of the coal minerals vaporises to form salts. How-
MINERAL MATTER BEHAVIOR IN PULVERISED COAL COMBUSTION ~l~l(tmnts lOO
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1123 Ash
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Raw coal
NB2'S04
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= 2.5 OASES
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[~500~1
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[3~m)
Condensed slits of
NaL'I,Na'~Ot,
(O-hnl
FIG. 4. The mechanisms of the Na-ash reactions from the experiments of Lindner~ involving modified coal (demineralised coal, inherent 10/zm silica and NaC1 crystals).
% Na(d.b.) - 0.13
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~
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7Slam coatI ~ " Or'2 containing and NaCI cr)11als
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FIG. 3. Thermodynamic predictions of the distribution of sodium within the combustion products for a raw brown coal and the same coal treated to substitute A1 for some of the Na and CI. The raw coal is higher in sodium, so that changes in the absolute amounts of each species are evaluated by multiplying %Na (db) in the coal by the %total sodium plotted. Atomic ratios (Na/CI), (Na/S), (Na/Si), (Na/AI) are for Raw Coal: 1.71, 2.13, 1.87, 29.1 and for Treated Coal: 1.66, 0.67, 0.64, 1.44. Sticky particles - - , condensible salts . . . . . . . .
ever, recent work has shown that a high proportions of the water soluble sodium vaporises, (i.e. chlorides and as salts of carboxyl groups), but then reactions between the sodium containing gases and the silaceous fly ash reduces the concentration of these gases. Experiments can estimate these levels by collecting homogeneously condensed salts following combustion. Neville and Sarofim42 report sodium levels appearing in such salts of 19 to 43% of the original coal sodium with a dependence on the extent and size of the silicon containing minerals in coal. At flame temperatures the vaporised sodium exists in several forms. Equilibrium calculations ~3-45 predict the gas phase sodium species. Figure 3 gives the predicted distributions of sodium in a coal, and
the same coal treated with aluminium, to show that a significant proportion of chlorine exists as sodium chloride in the furnace gases for high chlorine systems. These predictions show qualitative agreement with measurements made4e-48 using analytical techniques such as flame photometry, mass spectrometry, laser-induced flourescence and laser-induced photodissociation. However, the predictions are based on full availability of elements, such as Si and AI, which are not fully free for reaction as they are contained in particles which react only on an external surface. The use of an availability coefficient (<1) can allow for this effect by limiting the involvement of the element. There is substantial evidence for the interaction of sodium with ash to form silicates and aluminium silicates, 4]'49-51 sodium being captured with silica rich grains in the residual ash. The studies 4z also indicate that the extent of formation of the condensible sodium salts decreases with an increase in combustion temperature, a result attributed to an increase in this interaction. Such behaviour is consistent with experiments5~ reporting the interaction of sodium with silica using a synthetic combustion gas containing sodium vapor. Later, experiments4] quantified the sodium affinity of silicon by using ash free coals impregnated with inherent 10 txm silica (SiOz) and kaolin (SiO2" AlzO3" 2H20), salt (NaCI), and sodium acetate (CH3COONa). Sulphur dioxide was added as a gas to simulate coal sulphur. The mechanisms thus established for sodium-ash interactions are given on Fig. 4. The interaction between the elements Na, Si, AI, CI, S in forming the silicates (Na2SiO3, NaA1SiO4) and the condensible salts has also been indicated by other studies. 5] These results were obtained for a brown coal treated with AI to substitute some of the Na and CI. Laboratory experiments support the thermodynamic predictions presented on Fig. 3 for the raw coal and treated coal, in that dramatically reduced deposits due to condensibles and sticky particles
INVITED TOPICAL REVIEW
1124
were observed with the treated coal. The formation of NaA1SiO4 from the treated coal, as suggested by Lindner5z together with a reduction in the formation of gaseous species containing sodium (which condense to form NaC1 and NazSO4) will reduce fouling. The extent to which the interactions occurs depends on the distribution and relative accessibility of mineral species within the coal particles and the sodium/ash yield ratios.
Condensation and Deposition: The equilibrium predictions of Fig. 3 suggest that the stable species at flame temperatures are NaCl(g), NaOH(g) and Na2SO4(g) whereas the stable condensible species at deposit temperatures are Na2SO4 and NaCI (in lesser proportion). The diffusion of the sodium containing gases to cool walls will therefore be accompanied with some sult:ation but the final Na2SO4 found in deposits may be also formed on furnace walls. Inspite of several investigations5z-~5 the mechanism and kinetics of Na2SO4 formation remain ambiguous.
Other Interactions Emphasis has been placed here on the mechanisms by which sodium and iron influence deposition. These mechanisms will be most important to some coal types, but other elements may clearly be important. For example, potassium will flux aluminosilicates more readily than iron if it is present in clays and fouling may be due to sulfated calcium and magnesium. 56 Other interactions are detailed elsewhere. 5
Development Based on Transformation Mechanisms The traditional analysis of coals in terms of their likely effect on furnace deposition has been based on the elemental analysis of 'whole' ash, the ASTM (or other standard) ash composition. From the mechanisms outlined above this is unlikely to be valid as 9 a small fraction of the mineral (or ash) mass, having a composition different to the average ash, may be the cause of deposits 9 elements may be present in different forms, as shown in Fig. i. This form has a dominant influence on the transformation of material associated with deposition. 9 elements may be 'reactive' or 'unreactive,' in that all of an element may not be associated with deposition. 9 the variability of the association of the minerals
(and elements) within individual coal particles influences deposition.
Analysis of Mineral Matter: The shortcomings noted above lead to the conclusion that knowledge of the sizes, types, abundance of the minerals in p.c. particles is essential to an analysis of deposition. The development of the CCSEM technique, together with other mineral and elemental techniques, to provide this information has underpinned the rapid progress made recently.
Coal Characterisation and Deposition Indices: The current expansion of the international trade in thermal coals necessitates a sophisticated approach to coal characterisation and testing. Again, iron and sodium will be considered here.
Iron mechanisms. Coarse extraneous pyrites should characterise slagging due to impaction of partially oxidised pyrite. Conversely, inherent pyrites will generate fume and softer particles if it is finely dispersed. The particle-to-particle variation of inherent pyrite and clays will also determine the range of viscosities of flyash particles. Sodium mechanisms. The formation of condensible salts depends on the sodium which is evaporated and yet does not react to form silicates. Thermodynamic calculations of the type given on Fig. 3 can be used as a guide when comparing coals or evaluating temperature effects. The stickiness of flyash particles can also be determined by assuming a diffusion limited model. 52 The association of the above analyses with observed fouling can be expected to lead to more sophisticated engineering indices, to complement those of Table I, once sufficient experience is available.
Coal Treatment and Additives: An understanding of the dominant mechanisms is clearly necessary when options to alleviate deposition problems are considered, for example, by the use of coal blends, washing to remove or add minerals or elements, or the use of additives to distort the ash chemistry.
Mathematical Modelling of Deposition, and Interpretation of Experiments: Apart from coal properties, furnace-related parameters clearly influence the thermal and aerodynamic conditions known to influence ash formation as well as deposition. Deposits also build and mature over long periods. Models, then, will be an essential tool in understanding furnace effects and interpreting and extrapolating short-term measurements. A possible experimental approach is to ac-
MINERAL MATI'ER BEHAVIOR IN PULVERISED COAL COMBUSTION celerate the deposition process by increasing temperature. Models can evaluate the validity of this approach, and the possibility of an associated change in mechanisms. The deposition rates due to the mechanisms of Fig. 1 may also be calculated if the concentration, size and composition of ashy material is known. Recently, contributions e.g. 57-60 have utilised the increased understanding of the formation processes in formulating initial mechanistic models.
Acknowledgements The author is indebted to many colleagues for their contributions to this review. Particular thanks are due to H. B. Vuthaluru, L. L. Baxter, S. A. Benson, R. W. Bryers, A. Conroy, G. Domazetis, A. F. Sarofim and S. Srinivasachar.
REFERENCES 1. JOHNSON, H. R. AND LI1-FLER, D. J., Eds.: The Mechanisms of Corrosion by Fuel Impurities, Butterworths Scientific Publications, London, 1963. 2. REID, W. T.: External Corrosion and Deposits: Boilers and Gas Turbines, Elsevier Publishing Company, New York, 1971. 3. STEWART, I. McC. AND WALL, T. F., Eds.: Combustion of Pulverised C o a l - - T h e Effect of Mineral Matter, Department of Chemical Engineering, University of Newcastle, NSW 1979. 4. REID, W. T.: Coal A s h - - I t s Effect on Combustion Systems in Chemistry of Coal Utilisation, Second Supp. Volume (Elliott, M. A., Ed.) Wiley-Interscience, p 1389, 1981. 5. RAASK, E.: Mineral Impurities in Coal Combustion, Behaviour Problems and Remedial Measures, Hemisphere Publishing, 1985. 6. SAROFIM, A. F.: Pollutent Formation and Destruction in Fundamentals of the Physical Chemistry of Pulverised Coal Combustion (Lahaye, J. and Prado, G., Eds.) Martinus Nijhoff, 245, 1987. 7. BONI, A. A., Ed.: Special issue on Ash Deposition, Prog. Energy Combust. Sci., 16, 193, 1990. 8. BRYERS, R. W., Ed.: Ash Deposits and Corrosion due to Impurities in Combustion Gases, Hemisphere Publishing, 1978. 9. BRYERS, R. W., Ed.: Fouling and Slagging Resulting from Impurities in Combustion Gases, United Engineering Trustees, 1983. 10. BARREI~i ", R. E. (Ed.): Slagging and Fouling due to Impurities in Combustion Gases, Engineering Foundation, New York, NY, 1989. 11. BRYERS, R. W. AND VORRES K. S. (Eds.): Mineral Matter and Ash Deposition from Coal.
1125
12. BENSON, S. A. (Ed.): Engineering Foundation Conference, Inorganic Transformations and Ash Deposition during Combustion, Engineering Foundation, New York, NY, 1992. 13. VORRES, K. S.: Mineral Matter and Ash in Coal, ACS Symposium Series 301, 1986. 14. BARRETT, R. E., TUCKERFIELD, R. C. AND THOMAS, R. E.: Slagging and Fouling in P.CFired Utility Boilers, Batelle Research Project 1891, 1987. 15. ATtic, R. C. AND DUZY, A. F.: Coal Ash Deposition Studies and Applications in Boiler Design, Proc. American Power Conference, p 290, 1969. 16. VANINE~rl, G. E. AND BVSCH, C. F.: Mineral Analysis of Ash D a t a - - A Utility Perspective, p 22, 1982. 17. L1TrLEJOHN, R. F. AND WATr, J. D.: Ref. 1, p 1-2-112. 18. BoRIo, R. W., GOETZ, G. J. AND LAVESSEUR, A. A.: Ref. 3, p W4.63, see also Borio, R. W. and Narciso, R. R. J. Eng. Power, 101, 500 (1979). 19. SAROFIM, A. F., HOWARD, J. B. AND PADIA, A. S.: Combust. Sci. Tech. 16, 187 (1977). 20. WALL, T. F. AND W1BBERLEY, L. J.: Sticky Particles and Deposit Formation, Ref. 9, p 493, 1983. 21. IM, K. H. AND CHANG, P. M.: AIChEJ 29, 498, 1983. 22. JAMALUDD1N,A. S. AND SMITH, P. J.: Ref. 11, p 565. 23. ROSNER, D. E. AND SLANG, B.: Chem. Eng. Comm. 64, 27, 1988. 24. MCNALLAN, M. J., YUNEK, G. J. AND ELLIOTT, J. F.: Combust. Flame 42, 45 (1981). 25. BRYERS, R. W.: Foster-Wheeler Development Corporation, Livingston, NJ (Private Communication), 1992. 26. WATr, J. D.: The Physical and Chemical Behaviour of Mineral Matter in Coal under Conditions Met in Combustion Plant, BCURA Literature Survey, National Coal Board, Chehenham, Vie., 1968. 27. HALSTEAD, W. D. AND RAASK, E.: J. Inst. Fuel 52, 173 (1969). 28. BRYERS, R. W.: J. Eng. Power 98, 517 (1976). 29. BORIO, R. W. AND NARC1SO, R. R.: Coal Ash Composition as Related to High Temperature Deposits, ASME paper 78 WA/CO-3, 1978. 30. GROVES, S. J. WILLIAMSON, J. AND SANYAL, A.: Fuel 66, 461, (1987). 31. ASAKI, Z., Morn, S., IKEDA, M. AND KONDO, Y.: Met. Trans. B. 16B, 627, 1985. 32. LEVESSEUR, A. A., GOETZ, G. J., LAO, T. C. AND PaTEL, K. L. : The Fundamental Role of Iron in Coal Ash Deposition, Internal Report, Combustion Engineering, CT, USA, 1989. 33. SRINIVASACHAR, S. AND BONI, A. A.: Fuel, 68, 829, (1989).
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