A mechanistic description of ash deposition during pulverized coal combustion: predictions compared with observations

A mechanistic description of ash deposition during pulverized coal combustion: predictions compared with observations* Larry L. Baxter

and Richard

W. DeSollart

Combustion Research Facility, Sandia National PCentral Illinois Public Service Co., Springfield, (Received 24 March 1992)

Laboratories, Livermore, IL 62701, USA

CA 94550,

USA

A mechanistic model of ash deposition is based on the transformations of mineral species in coal during transport of particles through an arbitrary combustion environment. Quantitative predictions include the elemental composition of boiler ash deposits as a function of location, operating conditions and coal type. Qualitative predictions relating to practical aspects of boiler operation are also included. Model predictions are compared with experimental results at pilot and utility scales. A three-week test burn of a Wyoming coal in a power plant boiler designed for midwestern and eastern coal is described. Data reported include deposit accumulation rate, strength, morphology, removability, emissivity and elemental composition. Similar data are also reported for the Sandia multifuel combustor, a pilot-scale facility. Deposits from the Wyoming coal accumulated at about the same rate as those from the fuel used previously. The deposits from the Wyoming coal were granular and friable. They were easily removed from boiler heat transfer surfaces by normal soot blowing practices. They were light-coloured and highly reflective. All these qualitative trends are consistent with model predictions. The measured elemental composition of the ash deposits from the Wyoming coal is within - 5% (absolute) of the predicted composition. (Keywords: ash; deposition; combustion)

Among the issues that determine the design and operation

of coal utilization equipment, ash deposition on heat transfer surfaces plays a significant, in many cases dominant, role. A brief survey’ of the development of coal conversion technologies identified the historical role of ash deposition as a motivation for major new technologies and concluded that ‘it is . . . inevitable that there will be always some problems with ash, whatever the system of coal combustion’. Nevertheless, the fate of inorganic material associated with coal remains less well understood than the behaviour of the organic material during coal combustion. Although there are several indices of ash behaviour, there is no complete model that describes ash deposition in a comprehensive way. This paper describes progress towards development and validation of such a model. Significant experimental and theoretical work has been directed at developing a better understanding of ash deposition and the resulting deposit properties. Quantitative data relating deposit properties to coal properties, location within an experimental facility and operating conditions have been published by several investigator@. However, there are fewer published ash deposition data from facilities larger than pilot scale. Fly ash formation models are in various states of development by several researchers. Investigators at the MIT Energy Laboratory’ described a model for the generation of fly ash capable of predicting the size and

*Presented at the International Conference on Environmental Control of Combustion Processes, 7-10 October 1991, Honolulu, Hawaii 001&2361/93~10/1411~)8 (“ 1993 Butterworth-Heinemann Ltd.

composition distributions from detailed descriptions of coal mineral matter. Similar models are under development by other investigators’*“, building on published fundamental results5.’ ‘.12. The output of these models is a description of the size and elemental composition distributions of the entrained particulate phase resulting from the combustion of pulverized coal. A great deal of information is available on rates and mechanisms of ash deposition. This paper considers four major mechanisms of deposition, or mass transport to a surface: (1) inertial transport including impaction and sticking; (2) thermophoresis; (3) condensation; and (4) chemical reaction. In general, the rates of inertial impaction on cylinders in cross-flow are well established. Rates on walls with parallel flows are less well established. The capture efficiency, a measure of the propensity of material to stick to a surface after impaction, is far less well established. The rates of thermophoretic deposition on heat transfer surfaces are reasonably well established when local temperature gradients and the functional form of the thermophoretic force on the particle (or the thermophoretic velocity) are known. Condensation rates can be predicted reasonably well, given accurate vapour pressure and concentration data. The accuracy to which rates of chemical reaction are known is often inadequate, especially those involving sulfation and alkali adsorption in silicates. The results discussed in this paper rely in part on an engineering model that predicts relevant aspects of ash deposition in pulverized coal boilersL3. This model, called ADLVIC (Ash Deposit Local Viscosity, Index of refraction, and Composition), is based on both first-principle

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A mechanistic description of ash deposition: L. L. Baxter and R. W. DeSollar

derivations and a series of experimental results, supplementary to those indicated above, that allow specification of critical parameters in the mechanisms of ash deposition. Predictions from this model have been compared with experimental results from combustion systems of several different sizes and coals of many different ranksi3*i4. This paper presents the results of a joint project between Central Illinois Public Service Co. (CIPS) and Sandia to anticipate the deposition-related consequences of switching from midwestermeastern coals to a western coal in a 600 MW, utility boiler. These results, based on a three-week test burn, represent the first application of ADLVIC to a utility boiler. OVERVIEW OF MODEL The outputs of the model used in this investigation are: (1) quantitative predictions of the elemental composition of ash deposits formed in pulverized coal combustors as a function of combustor operating conditions, coal (ash) type and amount, and location within the boiler; and (2) quantitative estimates of the rates of several deposition mechanisms. From these outputs, qualitative indications of deposit strength, morphology, removability and emissivity can be derived. Required input includes: (1) a description of the boiler in terms of both dimensions and operating conditions; and (2) a description of the coal and mineral matter, including chemical species information about selected mineral components. The model is reviewed in detail elsewhere l3 . A conceptual overview is presented here. ADLVIC is distinguished from traditional industrial indices for fouling and slagging and from other deposition models described in the literature in the following ways: (1) explicit prediction of the effects of boiler operating conditions on deposition behaviour; (2) explicit prediction of variations of deposit properties with location within a boiler; (3) explicit dependence on the total amount of inorganic material flowing through the boiler (essentially all industrial indices are based on the elemental composition, independent of the total amount); and (4) a predictive framework based on the specific mineralogy of the inorganic material in the coal, rather than on its elemental composition. It is significant that a mineralogical description of the coal is required, as opposed to an ASTM ash analysis. ASTM procedures can be used to generate much of the required information. For example, pyritic sulfur can be used to estimate quantitatively the fraction of pyritic iron in the coal, and free silicon (silicon in the form of silica) can be estimated from the ratio of silicon to aluminium in the ash. Other coal mineralogies cannot be easily estimated from ASTM procedures. Principal among these are calcitic calcium, atomically dispersed species of any type, and the precise composition of silicates. Given these inputs, the model predicts the path of the particle clouds through the boiler and the response of the particles to the changing environment encountered along this path. These are cast in the form of a series of coupled ordinary differential equations, the solutions to which indicate particle temperature, velocity and position as a function of particle residence time. Also included in these equations is the rate of accumulation of ash on heat transfer surfaces. The solutions to the differential equations are used to predict deposit elemental composition and other properties.

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The four deposition processes that are treated by ADLVIC are: (1) inertial impaction (and particle capture); (2) thermophoresis; (3) condensation; and (4) heterogeneous reaction. All heat transfer surfaces within the boiler are involved in these processes. ADLVIC currently uses flat walls in turbulent flows as an approximation to waterwalls, and single tubes in crossflow as an approximation to convective pass tubes. The four processes of ash deposition are assumed to have additive influences on deposit composition. That is, the rate of deposition of ash at particle residence time t is given by dm, x = Zi(r, r)Gi(r, t) + q(t, t) + Ci(r, t) + Ri(r, t) (1) In this equation, m, represents the mass of component i in the deposit. The factor Ii represents the rate of inertial impaction, Gi the particle capture efficiency, T the rate of thermophoretic deposition, Ci the rate of condensation, and Ri the rate of chemical reaction. The subscript i refers to each of the mineral components in the coal. These include pyritic iron, other forms of iron, silica, silicates, calcite, atomically dispersed species (sodium, calcium, potassium, magnesium, and titanium) and ‘other’. The variable t is a material time-scale, designating particle residence time relative to the time of injection of the particles. It typically varies between 0 and 3 s. The variable r designates elapsed or laboratory time, i.e. time relative to an arbitrary time of day, independent of the particle residence time. It represents for example the time between soot blowing cycles, and typically varies between 0 and 20h. In a steady-state or stationary system, the only relevant time-scale is t, and ash deposition rates, composition and all other characteristics of the process would have the same mean values at all times at a given location. However, ash deposition is clearly a nonstationary processi5*16. Therefore both the material and elapsed time-scales must be addressed. Equation (1) can be thought of as an ordinary differential equation parametrized by the variable r. Practical illustrations can be used to clarify the differences between the two time-scales. Changes in deposit composition from one location to another in a boiler indicate variation of one or more of the terms in Equation (1) with particle residence time (t). For example, commercial- and pilot-scale observations indicate that ash deposits formed from eastern and midwestern pyrite-bearing coals are enriched in iron. This enrichment is most pronounced when the deposits are sampled near the burners, with typical enrichments of 60%. Deposits sampled midway between the burners and the furnace exit are slightly less enriched in iron. A typical enrichment of 40% may be observed in this area. Near the furnace exit, iron enrichment in the deposit drops to l&20%. This change in deposit composition with location is a reflection of the residence-time (t) dependence of the inertial impaction and particle capture efficiency terms in Equation (1). Changes in deposit composition as a function of deposit thickness indicate variation of one of the terms in Equation (1) with clock time (r). For example, deposits formed in the convection pass of boilers typically show pronounced variation in composition between the heat exchanger surface and the outside of the deposit. These composition changes are often associated with variation in the condensation rate with 5. As the deposit accumu-

A mechanistic

description

Iates, its surface temperature increases and the rate of condensation decreases. Each of the major mechanisms of ash deposition indicated in Equation (1) is conceptually reviewed below. A cylinder in cross-flow is used to illustrate several of the mechanisms, although the same mechanistic processes describe deposition on both cylinders and waterwalls. Inertial

impaction,

of ash deposition:

Inertial impaction is most often the process by which the bulk of the ash deposit is transported to the heat transfer surface. Particles depositing on a surface by inertial impaction have sufficient inertia to traverse the gas streamlines and impact on the surface. The particle capture efficiency describes the propensity of these particles to stay on the surface once they hit. The rate of inertial impaction depends almost exclusively on target geometry, particle size and density, and gas flow properties. The capture efficiency depends strongly on these factors and on particle composition and viscosity”. It also depends on deposit surface composition, morphology and viscosity’8. The relative magnitudes of the characteristic times and dimensions of particle and fluid relaxation processes control the rate of inertial impaction. Specifically, inertial impaction occurs when the distance a particle travels before it fully adjusts to changes in the fluid velocity is larger than the length-scale of an object, or target, submerged in the fluid. The particle Stokes number is defined as the ratio of these length-scales. Inertial impaction is illustrated schematically in Figure 1 for the case of a cylinder in cross-flow. Two particles are illustrated as they approach the cylinder. Both respond to the gas flow field around the cylinder by beginning to move around the cylinder on approach. The inertia of both particles overwhelms the aerodynamic drag forces, and they hit the cylinder. One is shown rebounding and the other sticking to the surface. Gas streamlines, including recirculation zones, are shown in lighter shading. This process is most important for large particles (3 1O-1 5 pm) and results in a coarse-grained deposit. The impaction rates are highest at the cylinder stagnation point, decreasing rather rapidly with angular position along the surface as measured from this stagnation point. At angular displacements larger than - 50” (as measured

Particle Impaction

. m

Figure 1 Conceptual illustration of inertial impaction mechanism a cylinder in cross-flow. One rebounding and one sticking particle also illustrated

on are

and R. W. DeSollar

0.6

3 E ._

g w

5 5

Z(r, z)

L. L. Baxter

0.6

0.4

f 0.2

1 Stokes Number Figure

2

cross-flow designated

Correlation of particle impaction efficiency on a tube in as a function of Stokes number. Points and functions R are from refs 14 and I9

from the forward stagnation point), the rate of inertial impaction drops to essentially zero under conditions typical of boiler operation. The impaction efficiency is indicated in Figure 2 and is defined as the ratio of the number of particles that hit the tube surface to the number that are directed at the tube in the free stream. Predictions of the impaction efficiency as a function of particle, gas and tube properties have been published, at various levels of approximation, by numerous investigators 19-24. Further investigation of the influence of the boundary layer, thermophoresis and turbulence on this impaction efficiency is discussed elsewhere14. Figure 2 includes predictions from a fundamental modelI and several correlations. The details of the equations are given elsewhere’ 4,19. The particle capture efficiencies are estimated from global empirical correlations based on particle residence time and composition. The product of the capture and impaction efficiencies yields the collection efficiency. Therefore the deposition rate of each mineral component on the heat transfer surface is directly proportional to its capture efficiency. There are wide variations in capture efficiency between different chemical components. Thermophoresis,

T( r, z)

Thermophoresis is a process of particle transport in a gas due to local temperature gradients. Thermophoretic forces on a particle may be induced either by the temperature gradient in the gas in which the particle is suspended or as a consequence of a temperature gradient in the particle itself 25--30. In general these forces act in the direction opposite to that of the temperature gradient, although they can act in the direction of the gradient under certain conditions of particle surface temperature. An illustration of thermophoretic deposition is presented in Figure 3. Thermophoretic deposits are finergrained and more evenly distributed around the tube surface than deposits formed by inertial impaction, as indicated. With increasing deposit accumulation on the tube surface, there is a decrease in the temperature gradient in the thermal boundary layer, decreasing the rate of thermophoresis. The authors have adapted a functional form for the thermophoretic force that should apply over a broad range of Knudsen number (ratio of the gas mean free path to the particle diameter). It is based on an

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A mechanistic

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of ash deposition:

L. L. Baxter

Thermophoresis

and R. W. DeSollar

Chemical reactions, such as sulfation of alkali species and combustion of residual carbon in the ash, are similar to condensation in their mathematical treatment14. Both condensation and chemical reactions are strongly temperature-dependent and give rise to spatial variation in ash deposit composition. Chemical reaction terms are not fully incorporated in the model and were not used in the predictions discussed below. EXPERIMENTAL

Figure 3 Schematic illustration of thermophoretic tube in cross-flow

deposition on a

The Sandia Multifuel Combustor (MFC) was used to collect fundamental data under controlled conditions to validate the model at small scale under well-controlled and characterized conditions. The CIPS 600 MW, utility boiler was used to validate the model at large scale under typical industrial conditions. Multifuel

integration of particle-gas momentum exchange over the surface of the particle3i and has been used by other investigators with some success32*33.The thermophoretic force, as used in this model, is given by F, = -

67c&f(Kn)VT,

(2)

where f(Kn) depends on particle diameter, Knudsen number and several material-specific properties, Further discussion about thermophoresis is to be found elsewhere14. Condensation,

C(t, z), and chemical

reaction,

ci

1

Fuel 1993 Volume 72 Number 10

The MFC is vertically orientated, with gas flowing from top to bottom (Figure 5). A methane-air flame is stabilized in a gas burner at the top of the MFC. This flame is used to preheat the air stream and supply a vitiated flow for a series of seven heated/insulated modules. Each of these modules provides access ports for fuel lances and thermocouples. The first six modules

Condensation

Figure 4

Schematic illustration of condensation on a tube in cross-flow

(3)

where ii is the condensation efficiency, 8 is a blowing factor (very near unity for this application), k, is a mass transfer coefficient with a value that depends in known ways on geometry, Reynolds number and fluid properties, and x represents the mole fraction of species i in the bulk gas (subscript b) and at the tube surface (subscript s). The second term on the right-hand side of the equation represents convective transport to the surface.

1414

(Sandia)

R(t, z)

Condensation is the mechanism by which vapours are collected on cooled heat transfer surfaces. All vapours that enter the thermal boundary layer around a heat transfer surface and are subsequently deposited on the surface are treated within the condensation term of the model. At least three mechanisms for this process are available: (1) vapours may traverse the boundary layer and heterogeneously condense on the heat transfer surface; (2) vapours may homogeneously nucleate to form a fume and subsequently deposit by thermophoresis on the surface; and (3) vapours may heterogeneously condense on other particles in the boundary layer and arrive at the heat transfer surface by thermophoresis3k36. An illustration of deposition by condensation on a tube in cross-flow is presented in Figure 4. Condensation deposits have no granularity and are more uniformly deposited on the tube than either thermophoretically or inertially deposited material. The deposits are tacky and have a strong influence on the surface capture efficiency. Condensation deposits also contact the metal surface more efficiently than inertially impacted deposits, increasing the tenacity with which the deposit clings to the surface. The condensation flux is described by the equation Ci=iiekm(Xi,b-Xs)+Xi,bC

combustor

10

Figure 5

Exhaust

Schematic diagram of the Sandia Multifuel Combustor

A mechanistic

description

contain silicon carbide heating elements between refractory layers to control heat loss from the vitiated flow. A more detailed discussion of the MFC is available in the literature”. The MFC is used to simulate the local particle environment at various locations in a utility boiler. The gas temperature and composition experienced by a particle passing through the combustor are controlled to match the conditions in a utility boiler. The overall particle residence time in the MFC can be varied from N 40 ms to > 3 s, covering the range of relevant residence times in a utility boiler. Ash deposits are grown on air-cooled surfaces simulating waterwalls and convection tubes, and advanced diagnostics are used to characterize the particle-laden flow and the deposits, both in situ and by collecting solid samples. Experiments typically last 3 h. Utility boiler (CIPS)

The utility boiler used in this study is a 600 MW, unit operated by Central Illinois Public Service Co. (CIPS) with no slagging and/or fouling load restrictions when burning the traditional coal (Figure 6). It is a tangentially fired unit with a maximum continuous rating (MCR) coal feed rate of 215 t h- ‘. The boiler is 18.6m wide, 15.8 m deep and 59.4 m high with 9290m2 of heat transfer surface and a total furnace volume of 14 600 m3. Six levels of adjustable burners feed coal from each of the corners of the furnace box. The economizer is constructed of spiral-loop finned tubes. This unit was designed to burn a local coal. However, the results discussed in this paper are from a three-week test burn of a Wyoming (Hanna Basin) coal during which _ 72000 t of coal were consumed. Additional details about the unit are available elsewhere14.

Platen Superheater

of ash deposition:

L. L. Baxter and R. W. DeSollar

Model predictions of ash composition were prepared before the test burn. These are discussed below. During the test burn, ash deposits were sampled at the boiler nose for comparison with model predictions. Ash deposits at other locations in the boiler were not accessible for on-line sampling. At the conclusion of the test burn, the boiler was inspected and samples of deposit were collected throughout the boiler, as indicated by the circles in Figure 6. Ash deposits at the same locations were photographed and several bulk deposits from the boiler were collected to study their morphology. The elemental composition of each of the samples was determined for comparison with model predictions. Variations in deposit composition with distance from the heat transfer surface were not determined. The predicted properties depend on both the composition of the material being deposited and on the relative rates of the various mechanisms by which they are deposited, as discussed below. COMPARISON

OF EXPERIMENTS

AND MODEL

Comparisons of anticipated and observed deposition behaviour and deposit properties are discussed below. These include: (1) qualitative comparisons of anticipated and observed deposit properties in the utility boiler when fired with the Wyoming coal; (2) quantitative comparisons of predicted and observed deposit composition in the utility boiler when fired with the Wyoming coal; and (3) quantitative comparisons of the predicted and observed deposit composition in the MFC when fired with an eastern coal. There is a distinction between predicted and anticipated properties. Predicted properties are direct outputs from the model and are limited to elemental composition. Anticipated properties are inferred from predicted rates of, for example, inertial impaction versus condensation and are not quantitatively predicted. Anticipated properties include morphology, strength, emissivity and removability.

Flnal Superheater

Qualitative

predictions

and observations

Rate of deposition. Experimental

results indicate that char particle fragmentation is more substantial for the cenosphere-forming bituminous coals than for subbituminous or lignite coals3’. Therefore the rate of impaction of particles on the tubes for the Wyoming coal is predicted to be higher than for the bituminous coal normally used in the boiler, owing to the size distribution of the fly ash. However, the subbituminous coal fly ash contains a higher proportion of constituents with low capture efficiencies than the bituminous coal fly ash. On balance, the rate of accumulation was qualitatively predicted to be about the same for the Wyoming coal as for the coals previously used in the boiler. This result, as with most of the results discussed here, is specific to this coal under these operating conditions and is not a general property of western subbituminous fuels (see for example ref. 38). The observed rate of deposition during the test burn is consistent with the predictions discussed above. The rate of deposit accumulation was about the same, perhaps slightly slower, than that experienced with previous bituminous coals in this boiler under similar operating conditions.

Furnace Rear Wall

Furnace Side Wall Furnace Front Wall

Scale : Feet

Figure 6 Cross-sectional view of the CIPS 6OOMW, indicate locations from which ash deposits were sampled burn

unit. Circles after the test

Morphology and strength. The Wyoming coal contains a relatively high proportion of free silica compared with other coals of similar rank, ash chemistry and geo-

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L. L. Baxter

graphical origin. The free silica particles are slow to fuse or sinter in the deposit, and contribute to both the granular nature and the lack of strength of the deposit. This high percentage of free silica is also largely responsible for the high ash fusion temperatures of this coal compared with other similar coals. A second major contribution comes from the calcite. Calcium that originates as calcite (in contrast to organically bound or siliceous calcium) has an effect similar to that of free silica on the deposit with respect to granularity and strength, although less pronounced. This anticipated deposit strength is specific to this coal and is in fact the opposite of typical utility experience with coals similar in geographic origin and organic composition38. The observed morphology and strength of the deposits is consistent with the predicted results. Specifically, the deposits were granular and friable, and showed no indication of significant sintering. Removability. Sintering and fluid formation in these coals is often associated with (a) the incorporation of alkali material in silica to form low-melting-point silicates and (b) sulfation of sodium or calcium on the relatively cool heat transfer surface to form their respective sulfates. A lack of sintering or fluid formation was expected, based on experience with other coals in MFC tests and the mineralogy of the Wyoming coal. Deposits generated from Wyoming-type coals that sinter in the MFC are typically associated with coals that have relatively high sodium and/or high sulfur contents relative to the ash content. The Hanna Basin coal (on which combustion tests have yet to be performed in the MFC) has a modest sodium content and a low ratio of sulfur to total ash compared with other western coals. Western coals often form sintered deposits in utility boilers38. During the three-week test burn, deposits were easily removed by standard maintenance procedures, consistent with the anticipated behaviour. There was no indication of fluid or condensed phases next to heat transfer surfaces.

and R. W. DeSollar I”“““““““““““““‘1

- (4

- (b)

O-O\I’.o ’ ’02I ’ ’ ’04 : ’ ’ ‘ok

’ ‘018’ ’ Yo

’ ‘112’ ’ ‘114.’

Residence Time (s) Figure 7 Comparison of sensitivity of data (symbols) and predictions (lines) to particle residence time for Pittsburgh No. 8 coal. The dominant dependence in this case is the iron concentration. Predictions were performed before experiments.

~“““~‘l”“I”“I”“l’~ \

4

-Experimental

Emissivity. It was expected that the deposits would have a high reflectivity, based on the relatively high deposition rates of calcium and the low deposition rates of iron. This is consistent with the observations during the test burn. During the test burn, the furnace exit gas temperature was - 100 K above its typical value when burning bituminous coals, indicating higher ash reflectivity in the radiant section of the boiler. Quantitative

predictions

and experimental

results

predicted and measured ash deposit elemental composition as a function of particle residence time24. These data were collected in the MFC, with particle temperature histories that are representative of a commercial boiler, and where particle velocities are measured. Replicate experiments indicate that the coefficient of variation (standard deviation divided by the mean) for these data is 67%. For a typical pulverized coal boiler operating at maximum continuous rating (MCR), the data would represent conditions near the burners at 0.15 s and near the furnace arch at 1.1 s residence time. As indicated in the figure, the ash chemistry changes rapidly early in the particle residence period. Later, changes in ash chemistry become less pronounced. These data are in qualitative and in many cases quantitative Figure

1416

7 illustrates

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100 1”’ 1200

‘I”’ 1250

”

”

‘I”’

j “I’

1300 1350 1450 1400 Deposit Temperature [K]

”

‘1 1500

Figure 8 Deposit viscosity predicted from measured, predicted, and coal ash composition for Pittsburgh No. 8 coal at 0.2 s residence time in the MFC. There is a notable improvement in prediction compared with values calculated from coal ash

agreement with the predictions, including those elements of < 2 wt% concentration in the ash (Figure 7b).They are also in agreement with general utility experience, in that iron is both predicted and measured to be enriched in the deposits at early residence time relative to later residence time. Most currently used indices of fouling and slagging are based on properties of the raw coal. Such indices do not provide for the prediction of different behaviour in

A mechanistic

description

different regions of the furnace or of the convective pass. The data in Figure 7 illustrate how the deposit composition changes continuously through the boiler. These changes effect similar changes in deposit properties. This is illustrated in Figure 8, where deposit viscosity is shown as a function of temperature, based on a published deposit viscosity model 39. Results are shown based on the composition of the raw coal, the predicted deposit composition and the measured deposit composition. The results for the raw coal correspond to currently used indices of ash behaviour (which are based on coal rather than deposit composition). The figure illustrates both the agreement between predicted and measured values and the difference between them and the corresponding values for the raw coal. Similar plots are used for other comparisons below. The predictions and measurements in Figures 7 and 8 benefit from being related to a facility with well-known and tightly controlled operating conditions. Utility boilers do not typically have such well-controlled or characterized conditions. Comparison of predicted and observed deposit compositions for the CIPS boiler provides some indication of the practical usefulness of the model. Results from two of the regions indicated by the circles in Figure 6 are presented below. Figures Y and 10 illustrate the predicted composition of deposits collected from the nose and economizer regions of the boiler. Particle residence times are estimated at 1.2 s and 1.9 s respectively. The data in Figure 8, which represent the average of four replicate samples, indicate that the calcium composition is underpredicted while the aluminium and silicon compositions are slightly

of ash deposition:

--t

1200

CaO

lOOA-103A)

MgO

Na20

K20

Ti02

1

I Economizer

Al203

Fe203

CaO

1500

- + - Experimental

I

1250

1300

1350

1400

1450

1500

12 Comparison

MgO

Na20

K20

of in the economizer

deposit viscosity as a function of the 600 MW, CIPS boiler

of

overpredicted. The 95% confidence interval for these data 1s - II 8% (relative) of the mean. Therefore some of the differences between the predicted and observed concentrations are greater than the uncertainty in the data. However, the data in both figures are in reasonable agreement and, for the most part, show consistent trends. The viscosity of the deposits in the first superheater tube bank and in the economizer (both predicted as tubes in cross-flow) is illustrated in Figures II and 12. These figures indicate quantitatively that deposit properties cannot be reliably based on coal properties. The improvement in prediction using the means discussed above is also indicated. CONCLUSIONS

n Predicted Observed

I

SiO2

1450

--D-Predicted

Figure

Figure 9 Comparison of predicted and measured deposit compositions near the boiler nose in the CIPS 6OOMW, utility boiler. Replicate experiments indicate a standard deviation of < 5% (rel.) for all oxide species

50

1400

Deposit Temperature [K] (Samples

0 Fe203

1350

Comparison of deposit viscosity as a function of in the first superheater tube bank over the nose in the 600 MW, CIPS boiler

temperature

Al203

1300

Figure 11 temperature

l Predicted

Si02

1250

Deposit Temperature [K]

1200

Observed

- Experimental

- + - Coal Ash

50

E

L. L. Baxter and R. W. DeSollar

Ti02

Figure 10 Comparison of predicted and measured deposit compositions on economizer tubes in the CIPS 600 MW, utility boiler

The deposition of mineral matter during coal combustion can be described as a process involving four general mechanisms, each with two time-scales. The mechanisms are inertial deposition, thermophoresis, condensation and chemical reaction. Both particle residence time and clock time are important in each mechanism. Coal mineralogy, as opposed to the elemental composition of the ASTM coal ash, determines many important deposition mechanisms and deposit properties. Other

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A mechanistic description of ash deposition: L. L. Baxter and R. W. DeSollar

important variables determining deposit properties include operating conditions and boiler geometry. Means of incorporating all of these variables in a tractable engineering model for ash deposition have been demonstrated. Deposit properties can be anticipated with reasonable accuracy, based on elemental composition and the predicted rates of various deposition mechanisms. Deposit composition, morphology, removability and emissivity have been reasonably well anticipated, based on the mechanistic details of the engineering model. Although not all properties are currently predicted or anticipated to within the precision of measurement, the results of this approach to anticipating deposit properties are encouraging.

10

11 12 13

14

15 I6

ACKNOWLEDGEMENTS This research was supported in part by the US Department of Energy through the Pittsburgh Energy Technology Center’s Direct Utilization Advanced Research and Technology Development Program. Major contributions were also made by Consolidation Coal Company and Central Illinois Public Service Co. Several visitors at the Sandia Combustion Research Facility (CRF) and the MFC laboratory contributed to this work. The contributions of Krishnan Padmanabhan in performing some of the initial impaction and thermophoresis predictions is gratefully acknowledged. The assistance of Eric Harwood, Joe Stieve, and James Brandt in performing experiments and maintenance of the MFC are gratefully acknowledged. The assistance of Alan Salmi and Ephraim Arquitola, both from Sandia, in maintaining the MFC is also gratefully acknowledged.

17

18

19 20

21 22 23

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REFERENCES 21 Raask, E. ‘Mineral Impurities in Coal Combustion’, Hemisphere, New York, 1985 Chow, 0. K. and Lexa, G. F. ‘Combustion Characterization of the Kentuckv No. 9 Cleaned Coals’. EPRI Final Reuort (X-4994, Electric Power Research Institute, Palo Alto, CA, lb87 Griffith, B. F., Lexa, G. F. and Teigen, B. C. ‘Pilot-Scale Combustion Characterization of Two Illinois Coals’, EPRI Final Report CS-6009, Electric Power Research Institute, Palo Alto, CA, 1988 Durant, J. F., Kwasnik, A. F. and Lexa, G. F. ‘Impacts of Cleaning Texas Lignite on Boiler Performance and Economics’, EPRI Final Report GS-6517, Electric Power Research Institute, Palo Alto, CA, 1989 Loehden, D., Walsh, P. M., Sayre, A. N., Beer, J. M. and Sarofim, A. F. J. Inst. Energy 1989, 62, 119 Harding, N. S. and Mai, M. C. In ‘Mineral Matter and Ash Deposition from Coal’ (Eds R. W. Bryers and K. S. Vorres), United Engineering Trustees, Santa Barbara, CA, 1990, pp. 375-399 Srinivasachar, S., Helble, J. J. and Boni, A. A. In ‘Twenty-third Symposium (International) on Combustion’, The Combustion Institute, Pittsburgh, 1991. DD. 130551312 Baxter, L. L. In ‘Proceedings of the EPRI Conference on the Effects of Coal Quality on Power Plants, St Louis, 19-21 September, 1990’, EPRI Report No. GS-76361, pp. 5.59-5.74 Boni, A. A., Beer, J. M., Bryers, R. W., Flagan, R. C., Helble,

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J. J., Huffman, G. P., Huggins, F. E., Peterson, T. W., Sarofim, A. F., Srinivasachar, S. and Wendt, J. 0. L. ‘Transformations of Inorganic Coal Constituents in Combustion Systems’, Phase I Draft Final Renort. US DOE Contract No. CE-AC2286PC90751, March 1990 Zygarlicke, C. J., Benson, S. A., Toman, D. L., Steadman, E. N., Brekke, D. W., Erickson, T. A. and Hurley, J. P. ‘Combustion Inorganic Transformations’, Annual Technical Progress Report, US DOE/PETC Cooperative Agreement No. DE-FC21-86MC10637, October 1990 Friedlander, S. K. and Johnstone, H. F. Ind. Eng. Chem. 1957, 49,115l Rosner, D. E. and Tassopoulos, M. AIChE J. 1989,35, 1497 Baxter, L. L. In ‘Coal Combustion Science: Quarterly Progress Report’ (Ed. D. R. Hardesty), Sandia Report SAND-8217, Livermore, CA, 1991, pp. 3.18-3.48 Baxter, L. L. In ‘Coal Combustion Science: Quarterly Progress Report’ (Ed. D. R. Hardesty), Sandia Report SAND-8247, Livermore, CA, 1990, pp. 3.413.53 Baxter. L. L. and Mitchell. R. E. Combu.r(. Flame 1992. 88. 1 Baxter; L. L., Harding, N. S. and Hencken, K. R. In ‘Proceedings of the Twenty-third Symposium (International) on Combustion’, The Combustion Institute, Pittsburgh, 1991, pp. 993-999 Srinivasachar, S., Helble, J. J. and Boni, A. A. In ‘Proceedings of the Twenty-third Symposium (international) on Combustion’, The Combustion Institute, Pittsburgh, 1991, pp. 1305-1312 Smouse, S. M. and Wagoner, C. L. In ‘Proceedings of the Engineering Foundation Conference on Ash Deposition, Palm Beach, FL, l991’, ASME, New York, pp. 625-637 Israel, R. and Rosner, D. E. Aerosol Sci. Technol. 1983, 2, 45 Beer, J. M., Monroe, L. S., Barta, L. E. and Sarofim, A. F. In ‘Proceedings of the Seventh International Pittsburgh Coal Conference’, The Combustion Institute, Pittsburgh, 1990, pp. 30-31 Laitone, J. A. J. Appl. Mech. 1981, 48, 465 Rosner, D. E. AIChE J. 1989, 35, 164 Walsh, P. M., Beer, J. M. and Sarofim, A. F. In ‘EPRI Conference on Effects ofCoal Quality on Power Plants, Atlanta, 13-15 October 1987’, in press Baxter, L. L., Abbott, M. F. and Douglas, R. E. In ‘Proceedings of the Engineering Foundation Conference on Ash Deposition, Palm Beach, FL, 1991’, pp. 679-698 Byers, R. L. and Calvert, S. hd. Eng. Chem. Fundam. 1969, 8, 646 Castillo, J. L., Mackowski, D. and Rosner, D. E. Bog. Energy Cornbust. Sci. 1990, 16, 253 Fuchs, N. A. ‘The Mechanics of Aerosols’ (trans. R. E. Daisley and M. Fuchs), Dover, New York, 1964 Giikoglu, S. A. and Rosner, D. E. AIAA J. 1986,24, 172 Gokoilu. S. A. and Rosner. D. E. Ind. Enu. Chem. Fundam. 1985, u24,‘208 Gokoglu, S. A. and Rosner, D. E. Int. J. Heat Mass Transfer 1984, 27, 639 Jacobsen, S. and Brock, J. R. J. Co/laid Sci. 1965, 20, 544 Im, K. H. and Chung, P. M. AIChE J. 1983, 29,498 Jamaluddin, A. S. and Smith, P. J. In ‘Mineral Matter and Ash Deposition from Coal’ (Eds R. W. Bryers and K. S. Vorres), United Engineering Trustees, Santa Barbara, CA, 1990, pp. 565-516 Castillo, J. L. and Rosner, D. E. Int. J. Multiphase Flow 1988, 14.99 Helble, J., Neville, M. and Saroftm, A. F. In ‘Proceedings of the Twenty-First Symposium (International) on Combustion’, The Combustion Institute, Pittsburgh, 1987, pp. 411-417 Rosner, D. E. and Ganarajan, R. Chem. Eng. Sci. 1985,40,177 Baxter, L. L. Cornbust. Flame 1992, 88, 174 Ziesmer, B. C., Barna, L. J. and Bull, D. L. In ‘Proceedings of the EPRI Conference on the Effects of Coal Quality on Power Plants, St Louis, 19-21 September 1990’, pp. 4.39-4.56 Kalmanovitch, D. P. and-Frank, M. In ‘Mineral Matter and Ash Deuosition from Coal’ (Eds R. W. Brvers and K. S. Vorresl. United’ Engineering Trustees, Santa Barbara, CA, 1996; pp. 899101