The optical properties of fly ash in coal fired furnaces

COMBUSTION

AND FLAME

61:145-151

(1985)

145

The Optical Properties of Fly Ash in Coal Fired Furnaces R. P. GUPTA and T. F. WALL Department of Chemical Engineering, The University of Newcastle, 2300, Australia

One of the most important parameters characterizing the radiative behavior of a fly ash cloud in p.f. furnaces has been found to be the optical properties of the ash as determined by the complex refractive index. The available data on this property are limited and inconsistent and there has been previously only one in situ measurement on a large boiler. The value of the absorption index thus obtained was not only different from other values quoted in the literature, but ais? was an order of magnitude higher when compared with those of pure silica and alumina mixtures, the major constituents of the fly ash particles. To eliminate this confusion the in situ measurements were repeated with greiier precision and laboratory measurements (infrared absorption spectra) were made on fly ashes from different coals. These measurements suggest that high reported values of the absorption index of fly ash particles may be due to unburned char present at the furnace exit where measurements were made or where ash is sampled.

1. INTRODUCTION fly ash carried in the combustion gases of furnaces fired with pulverized coal (p.c.) can have a dominant influence on radiative transfer [ 11. The radiative behavior of a fly ash particle is determined by the ratio of its circumference to the wavelength of thermal radiation, called the particle size parameter, a = zss/h, and the complex refractive index, m = n - in I. For a = 1.0, both absorption and scattering are important, scatter predominantly forward with significant scatter sideways and backward. For ashes, about 90% of the mass lies betwee? 2 and 100 pm. For temperatures z;und in fur_ naces, most of the the-. : _.c~rnal radiation spectrum lies between nd ’lower wavelength of about 1 pm and P41~ upper wavelength of about 10 pm.

The fine

Scatter can therefore be significant [2]. The radiative effect of a fly ash cloud can be predicted from a knowledge of the size distribution of the cloud, the ash concentration, and the radiative properties defined in terms of the complex refractive index of the particles. The

complex refractive index contains the (real) refractive index, n, for which experimental data exist for many ashlike materials, and the absorption index, n’, for which there are comparatively little data [ 1, 21, 2. I$ADIATIVE ASti CLOUD

PROPtiftTLES

OF A FLY

The spectral absorption coefficient, Ka,h, for a Particle cloud is defined as the fraction of radiation absorbed from a beam per unit length. The Spectral scattering coefficient, ~~,h, is defined in a similar manner, and the extinction coefficient, Ke,x, is defined as the sum of the absorption and scattering coefficients. In one dimension the emissivity of an isothermal cloud is defined considering absorption only : (1) Ed= 1 - exp( - K,,k). For a dust loading B, the absorption coefficient is given by (2) &,A= Qa.44

Copyright@ 1985 by The Combustion Institute by Eisevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, IVY 10017 Published

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R. P. GUPTA and T. F. WALL

146 where &, is the cloud absorption efficiency, and A, the projected surface area of the cloud, defined as

(4) wheref(s) is the mass frequency function for the particle size distribution, and the limits on the integrals are the extremes of the measured particle size distribution. The single particle absorption, scatter, and extinction efficiencies and phase function may be calculated from the Mie theory [3-51. These efficiencies are complicated functions of the particle size parameter-and therefore of the wavelength of radiation-and the complex refractive index of the particles. Individual fly ash particles are not homogeneous in shape, size, or chemical composition [6], but in order to extend the theoretical treament to fly ash, it may be assumed that all particles ate spherical and of the same chemical composition-an assumption for any experimental estimate of the complex refractive index for a particle cloud in the laboratory or field which has been made many times previously [7]. The total radiative properties of the particle cloud at a particular temparatuuudcan he obtained from the (Planck) w@hted average of the spectral property, For example, the total cloud absorption efficiency is given by

vary significantly with wavelength but when integrated over the spectrum according to Eq. (5) they are fairly constant over the temperature range of interest for furnaces. To allow the cloud efficiency to vary with temperatures complicates calculations of furnace heat transfer a great deal, and Fig. 1 indicates that efficiencies at an average furnace temperature might be used rather than allowing them to vary with temperature. The dust loading is temperature dependent (B cc TV *),so that the absorption coefficient remains temperature dependent. Details of furnace models using these simplifications are given in Ref. [2]. 3. PREVIOUS ESTIMATES OF THE OPTICAL PROPERTIES OF FLY ASH A comparison of the literature values for the refractive index n and the absorption index n’ for fly ash, silica, alumina, dusts, and glasses chemically similar to fly ash indicates that [2] a. n for fly ash is similar to that of its major constituents and varies between 1.4 to 1.7; an average value of 1.5 may be assigned. b. n 1 for fly ash can be very much greater from that of its major constituting oxides, for SiOZ and Al203 n 1 = 0.005, for fly ash values from 0.005 to 0.6 have been reported. A recent reevaluation [7] of the values given by Willis IS] contracts this range from 0.005 to 0.05, the extremities used in Fig. 1. This range gives an unacceptable error in predicting heat transfer [I]. 4. MEASUREMENTS

where EBX is the Planck function for the Ippeetral distribution of blackbody radiation. Spectral cloud efficiencies have been culculated at wavelengths from 1 to 10 pm, and total cloud efficiencies from 1200 to 18OOKfor three fly ash size distributions and refractive indices of 1.5-0.005i and 1.5-0.05i (see Fig. 1). Figure 1 indicates that the spectral cloud absorption and extinction (absorption and scatter) efficiencies

IN P.F. BOILERS

The enly pvlor- ‘**lQestimates of n1 from in situ meascitim~fit~ in ft: f: b&; ,‘yl assigned an y of 1.3 and eetimab$ afre&rai n * va2.Y ranging from bJX& to 0;03 at wavdengtho from :. 2 to 4.2 j.im for ah% ffw Bnumber of coals. ‘The& estimates requited a eorreetion for furnace radiation reflected into a pyrometer beam which reduced the signal corresponding to radiation from fly ash in the beam by up to 60%. The previous instrument [9] was therefore

147

OPTICAL PROPERTIES OF FLY ASH

CLONI MTlNlXON EFFlClfNCV

0

2

L WAVELENGTH,

6

a

IO

am

Bm

cm

1100

TEMPERATURE,

m

wm

17ol

lam

K

Fig. 1. Calculated spectral cloud efficiencies (left) and total cloud efficiencies (right) for coarse (c), average (a) and fine (4 fly ash size distributors.

modified as shown on Fig. 2, replacing the thermister detector (with a response of 25 V/W) with a Phillips photosensitive detector RPY-87 (with a response of 500 V/W). The fast response of this detector allowed the use of a chopper and an ac amplifier to reduce drift, a problem with the previous arrangement. The narrow angle filter probe (NAFP) was therefore sighted through a purged water cooled probe, through a 1 m depth of furnace gas and onto a water cooled target. To minimize reflections from the target, a surface was constructed from a number of sintered steel cones (to increase emissivity) through which purging air was blown to reduce the deposition of a reflecting fly ash layer. The surface was further placed in a cylindrical well,

so that the “effective” target reflectivity was 0.07. Measurements were made along the 1 m beam with measured gas temperature (by suction pyrometer or venturi pneumatic pyrometer, and equated to the temperature of the fly ash), dust burden (by a sampling probe), and spectral intensities (from the NAFP calibrated against a blackbody). The particle size distribution of the ash was determined by a Malvem PSA 2000, which uses a diffraction technique and has been compared favorably [lo] with other techniques. As illustrated in Fig. 3, the spectral radiation detected 1~ is the sum of the radiation of that due to the fly ash cloud and the radiation reflected from the target and transmitted through the cloud. That is, Ix = E&x + 7i\Iox.

Fig. 2. The modified (NAFP).

narrow

angle

filter

pyrometer

(6)

The emission from the cooled target can be neglected; lox is therefore the reflected component of the furnace radiation bathing the target, which may be estimated from the hemispherical emission of a fly ash cloud as seen by the target with the properties and temperature of the cloud in the pyrometer beam. A spectral absorption index of the fly ash is therefore obtained by an iterative procedure. A first estimate of oh is obtained from Eq. (6), after setting IOto zero. A first estimate of n l can then

R. P. GUPTA and T. F. WALL

148

Fig. 3. Arrangement of the pyrometer, sighting tube, and target (top) with (bottom) the radiative beams considered in the analysis of the furnace measurements.

be estimated from generalized charts [ll], a knowledge of the product of the dust burden and projected area, and Eqs. (1) and (2). lox is then estimated from the emission of the cloud in the given furnace geometry, and a revised estimate of n L can be obtained from Eq. (6). Small changes in n’ between interations yields the converged value. Filters having peak transmission of 1.215, 1.625, 2.218, 3.300 and 3.970 pm were used to cover the thermal spectrum while avoiding the COz and Hz0 emission bands. However, the slight overlap of the 3.300 pm filter and a water vapor band necessitated a correction in the above calculation. 5. INFRARED ABSORPTION

SPECTRA

The technique used on fly ash by Volz [12] has also been used to determine the absorption index of ashes and mixtures of silica, alumina, and iron oxides. The absorption spectra of pellets of KBr with 2-3 mg of samples was obtained in a Nicolet MX- 1 machine. The technique gave n ’ in the wavelength range by n’

Absorption x wavelength

p=

(sample mass/pellet area)

(7)

6. RESULTS The in situ tests are detailed in Table I. Measurements were made at the furnace exit of two

power stations burning the three coals detailed in Table II. The ash analyses are given in Table III. The estimates of .’ from the in situ measurements are given on Fig. 4a and compared with the previous in situ measurements of Lowe [9]. Typical results given in Table IV for Liddel coal ash show that the background intensity ZOh is from 18 to 25% of the detected intensity IX. Referring to Eq. (6), with a beam transmissivity of about 0.9, the correction of the detected signal to account for the background intensity which is transmitted through the gas beam is generally less than 20% of the signal. This is substantially less than the previous connection

PI. A comparison of Figs. 4a and 4b shows that both measurement techniques give an increase of n 1 with wavelength above 1.5 pm as expected of dielectric materials [14]. However, the rate of increase of the furnace measurements is greater than expected. This may be attributed to assumptions in the estimation of lox [ll]. The high values of .* at low wavelengths may be caused by small concentrations of soot in the beam [ll]. Infrared spectra were obtained for the fly ash sampled isokinetically during the in situ measurements; the same ash burned in a muffle furnace at 1lOOK to burn out residual carbon and also ash prepared by the standard ASTM procedure by heating coal in a crucible. The n l estimates for these three ash groups, given in Fig. 4b, show that the unburned carbon has a substantial effect on n l, with the carbon free fly ash being characterized by a lower n ’ which is similar to that for the ASTM ash. In order to establish whether carbon was present as char or soot in the ash, two size fractions (< 63 pm and > 125 pm) were compared. The large fraction had the higher unburned carbon and higher 12I estimates. An attempt to separate soot by immersion of the fly ash in water as recommended by Volz yielded little soot. Examination of the samples under the electron microscope also indicated little soot was present, with most carbonaceous material being unburned char.

149

OPTICAL PROPERTIES OF FLY ASH 1

TABLE

Summaryof in Situ Measurements

Power Station

Test No.

Gas Temperature Measured by

Coal

Dust Burden (B) Estimated by

K

Solids sampling probe

1539 Liddell Venturi pneumatic pyrometer

Liddell

1423

g/cm’ at Gas Temperatures

Projected Surface Area, Ar

(m*km)

5.14

56.0

5.36

56.0

2.8

63.7

4.6

86.0

(500 MW) 1423

Buchanan

Calculation Wangi

Suction pyrometer

Wangi

(60 MW)

TABLE Proximate

II

Analysis of the Coals Used in in Situ Measurements

_____ Coal Type ___Moisture V.M. Ash Fixed carbon Ash density, gmkm 3

Liddell

Buchanan

Wangi

3.0 24.9 28.0 44.1

2.8 31.1 15.1 51.0

2.6 27.0 25.8 44.6

1.97

2.05

(L)

0%

Cw)

30.25 1.29 6.89 0.79 0.90 57.09 1.26 1.53

18.51 0.52 2.35 0.57 0.37 76.06 0.79 0.83

29.43 0.69 4.07 1.22 0.42 62.16 1.15 0.86

7.0

10.1

3.5

1.78

TABLE

1353

An indication of the nonhomogeneity of ash is given in Fig. 5, which presents ir results for the fine end of the ash (< 53 pm) and the coarse end (> 125 pm). From the sampled ash, which contains unburned char, n ’ is seen to depend primarily on the proportion of unburned carbon. On burning out the unburned char n 1 decreases

O-01

III

Ash Analysis”

Oxides ___ A1203

CaO Fe@, K20

MgO SiOr TiOr Other oxides Carbon in ash ____

a Liddell (L), Buchanan

(B), Wangi (W).

Fig. 4. Estimates of the spectral absorption index from the furnace measurements (a, left) and the laboratory ir measurements (b, right) for the ash from the following coals: Liddell, (L), Buchanan (B), Wangi (W), and Wallarawang (WI).

150

R. P. GUPTA and T. F. WALL TABLE

IV

Summary of Calculations for n’ for Test No. 1 .’ Neglecting Estimated Target Spectral Target Wavelength Radiation Reflection Reflection Corrected I, (W/mz) (I,, = 0) Iok (W/m*) fr’ X (am) 1.215 1.625 2.218 3.300 3.970

6459 5556 9119 8765 6602

0.015 0.005 0.01 0.02 0.04

1310 1443 1997 1728 1253

0.009 0.0035 0.007 0.012 0.025

but n ’ for the coarse fractions remains greater than n ’ for the fine fractions. This may be due to the higher proportion of iron in the coarse fractions [ 141. 7. DISCUSSION Given the uncertainties of the furnace measurements, the furnace estimates of ni of Fig. 4 show acceptable agreement with the laboratory n 1 estimates for the fly ash and unburned char. Apart from the estimate at the lowest wavelength which we have previously speculated may be due to soot, the furnace measurements show a greater rate of increase of n ’ with X. The correction for reflection from the background [10x in Eq. (6)] assumed the background was bathed in radiation by an isothermal hemispherical slab 3 m thick between the background and the furnace wall at the beam temperature as shown in Fig. 3. In fact, the gas temperature will decrease near the wall [ 151 and so IOhwill be overestimated, especially at low wavelengths where blackbody emission changes at a greater rate with temperature. Also, for Fez03 containing mixed oxides, n l is expected to increase with temperature [ 161, associated with a widening of the absorption bands [ 17, 181, especially at wavelengths less than 4 pm [14]. However, the furnace measurements (at temperatures exceeding 1000°C) are not obviously higher than the ir measurements (at room temperature).

The greatest error in the in situ determination to the correction for the radiation reflected from the target which, from Table IV, may be 30% of the signal. With ash size distribution and sampling errors, the in situ estimates of the absorption index may be in error by + 25 % . The observation previously made by Sarofim [ 191 that the n1 values reported by Lowe [91 appeared to be large relative to the values for glasses with compositions not too different from some ashes is therefore valid. The estimated values had been calculated on the basis of a homogeneous ash sampled at the furnace exit, whereas, in fact, some unburned char was also in the furnace gases. This may also explain the range of values reported by Wyatt [6] and Volz is due

WI. 8. CONCLUSIONS Previous laboratory measurements of the optical

MO y

t

W/ 0005 /

/ -FLvAsi&cwi ASH

_--FU

t

I

2

3

1

I

I

5

6

X m Fig. 5. Estimates of the spectral absorption index from sampled furnace ash before ashing (full lines) and after ashing in a muffle furnace (dash lines). The bracketed numbers refer to the loss on ignition of the collected ash.

OPTICAL PROPERTIES OF FLY ASH

151

properties of fly ash collected in electrostatic precipitators or from furnace measurements where burn-out has been assumed to be complete are, in fact, influenced by a small amount of unburned carbon (char) in the ash. On burning out this char the absorption index (n ‘) of the ash is decreased. n l can be estimated from Mie theory with the ash represented as homogeneous spheres. For char free ash from subbituminous coal, n 1 can be expected to range from 0.005 to 0.01, increasing in the wavelength range of importance in p.f. boilers (1.5-5 pm).

Council grant. The authors acknowledge the support from the Electricity Commission of New South Wales, in particular from Dr. A. Lowe in arranging the experimental work on POwer Station Boilers. REFERENCES 1.

2. 3.

NOTATION

4

B E Bh I

K L m n n1

Q Q s T c: P x TX

4.

projected surface area, m2/kg dust burden, kg/m3 Planck function for blackbody spectral energy, W/m3 intensity, W/m* absorption coefficient, m- 1 beam length, m complex refractive index, m = n - in ’ the real part of n, often called the refractive index the imaginary part of. m, often called the absorption index cloud efficiency single particle efficiency particle size, m temperature, K emissivity density, kg/m3 wavelength, m or pm transmissivity (1 - Ed), spectral

5. 6. 7. 8. 9. 10.

11. 12. 13. 14.

15. 16.

Subscripts 17.

a, e, s absorption, extinction, scatter wavelength (or monochromatic x m or pm 0 background value (target)

value),

The work was carried out under a National Energy Development and Demonstration

18. 19.

Wall, T. F., Lowe, A., Wibberley, L. J., Mai-Viet, T., and Gupta, R. P., Comb. Sci. Technol. 26:107 (1981). Gupta, R. P., Wall, T. F., and Truelove, J. S., Int. J. Heat and Mass Transfer 26: 1649 (1983). Hottel, H. C., and Sarofim, A. F., Radiative Transfer, McGraw-Hill, New York, 1967. Van de Hulst, H. C., Light Scattering by Small Particles, Wiley, New York, 1957. Kerker, M., Scattering of Light and Electromagnetic Radiation, Academic Press, London, 1970. Wyatt, P. J., Appl. Opt. 19:975 (1980). Gupta, R. P., and Wall, T.F., J. Phys. D14:L95 (1981). Willis, C., J. Phys. D3: 1944 (1970). Lowe, A., Wall, T.F., and Stewart, I. McC., 17th (Int) Symp. Comb., 1980, p. 105. Seville, G. P. K., Coury, J. R., Gbadiri, M., Raper, J. A., and Cliff, R., Australian Combustion Science Symposium, The Australian Section of the Combustion Institute, 1983, p. 69. Gupta, R. P., Ph.D. Thesis, University of Newcastle, 1983. Volz, F. I., J. Geophys. Res. 77:1017 (1972) and Appl. Phys. 12:564 (1973). Wall, T. F., and Stewart, I. McC., 14th Symp. (Int.) on Combustion, 1972, p. 689. Wall, T. F., and Becker, H. B., Fouling and Slagging from Impurities in Combustion Gases (R. W. Bruyers, Ed.), Engineering Foundation, New York, 1983, p. 211. Wall, T. F., and Stewart, I. McC., J. Inst. F. 44:234 (1971). Field, M. A., et al., Combustion of Pulverised Coal, BCURA, Cheney & Sons, 1967. Wickersheim, K. A., and Lefever, R. A., J. Chem. Phys. 36:844 (1962). Marusck, L. A., Messier, R., and White, W. B., J. Chem. Phys. Solids 41:981 (1980). Sarofim, A. F., 17th (Int.) Symp. Comb., 1980, p. 113.

Received 25 April 1984; revised 22 February I985