Experimental Thermal and Fluid Science 33 (2009) 1133–1141

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Experimental Thermal and Fluid Science

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Spectroscopic research on infrared emittance of coal ash deposits

Aleksandar Saljnikov a,*, Biljana Vucicevic c, Mirko Komatina a, Milan Gojak a,Darko Goricanec b, Zoran Stevanovic d

a Department of Thermomechanics, Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11120 Belgrade 35, Serbiab Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, Maribor 2000, Sloveniac Laboratory for Thermal Engineering, Institute of Nuclear Sciences VINCA, P.O. Box 522, Belgrade 11001, Serbiad Faculty of Mining and Geology, University of Belgrade, Ðušina 7, 11120 Belgrade 35, Serbia

a r t i c l e i n f o

Article history:Received 24 January 2009Received in revised form 14 May 2009Accepted 4 July 2009

Keywords:Heat radiationSpectroscopyEmittanceAsh depositBoiler furnace

0894-1777/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.expthermflusci.2009.07.002

* Corresponding author. Tel.: +381 63 8849 189; faE-mail address: asaljnikov@mas.bg.ac.yu (A. Saljni

a b s t r a c t

This paper deals with thermal radiation characteristics of ash deposits on a pulverized coal combustionboiler of an electric power plant. Normal emittance spectra in the near to medium infrared (2.5–25 lm)region and total normal emittances were measured on four kinds of ground ash deposits. Measurementswere conducted in the 570–1460 K temperature range which is common for boiler furnaces, by bothheating and cooling the ash samples, with the aim to study the effect of their thermal history. Depen-dence of emittance on wavelength, temperature and chemical composition was studied, too. Sampleswere tested for transparency (opacity) to verify the accuracy of results. It was determined that the thick-nesses used for the ash powders are opaque for infrared radiation for thicknesses in the order of a mil-limeter. Tests have shown that spectral emittance increases with an increase of wavelength with acharacteristic pattern common for all samples. Spectral normal emittance increases strongly with tem-perature at shorter wavelengths and remains high and unchanged at longer ones. Emittance spectraare not very sensitive to chemical composition of ashes especially beyond k � 5 lm. With an increaseof temperature, total emittance of the powdered sample decreases to a minimum value around1200 K. Further temperature rise induces an increase of total emittance due to sintering in the ash. Oncooling, the emittance increases monotonically following the hysteresis. Quantitative directions for eval-uating thermal radiation characteristics of ash deposits for the merits of the safety design of boiler fur-naces were proposed. That comprises correlating the experimentally obtained emittance spectra withcurves of simple analytical form, i.e., a continuous function of minimum emittance vs. wavelength. Theproposed method can be extended to other specimens from the same furnace and used to determine cor-relations for thermal calculation of old and design of new furnaces – with similar geometry and combust-ing similar coal. The method is potentially applicable to completely different boiler furnaces combustingdifferent coal, and the authors recommend running the tests with new deposit samples. The data willthen be applicable to the thermal design of a whole new class of furnaces, having similar geometryand combusting similar coal. This is expected to greatly enhance the accuracy and precision of thermalcalculation as well as the efficiency of thermal design of steam boilers.

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1. Introduction

Within the framework of thermal calculation of the existing andthermal design of the new power plant boilers, in which pulverizedlignite is used as the fuel, knowledge of thermal radiation charac-teristics of the ash is important since ash deposited on the boilerwalls decreases the boiler performance by being the leading obsta-cle to heat transfer.

Ash deposits are a non-metallic material. Thermal radiationcharacteristics of such a material greatly differ from those of the

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x: +381 11 3370 364.kov).

metallic walls of the furnace, i.e., the surfaces of the heat exchangertubes in the furnace. Because the real surfaces differ from the idealsurfaces by their non-homogeneity and structure, it is difficult toquantify the influence that the deposits exert upon the heat trans-fer by using the analytical tools. Therefore, this problem is predom-inantly solved by the use of the experimental methods in order todetermine the thermal radiation characteristics of deposits bymeasuring their emittance and sometimes their absorptance, aspointed out by Wall et al. in their review of research conductedin this field [1] as well as by Zbogar et al. [2].

Before beginning a review of the extensive work in this fielddone within decades worldwide, there is a need to discuss the ad-verse effects of an intrusive analysis as is the one performed in the

Nomenclature

A(k), B(k) system constants for spectral emittance measurements,lV

Atotal, Btotal system constants for total emittance measurements,lV K�4

C2 Planck’s second radiation constant, =14 388 lm Kd diameter of particle, mf weight fraction, %p slope of the inclined part of the fitting linet thickness of specimen layer, mT temperature, K or �C

Greek symbolse radiation emittanceemax, emin emittance asymptotes of the fitting linek wavelength of radiation, lmkm wavelength of the mid-slope of the fitting line, lm

v output of the instruments measuring spectral and totalradiation intensity, lV

qp heaping mass density of specimen layer, kg/m3

Subscriptsap apparentB blackbodyH hemisphericalN normalroom surroundings in laboratoryS specimen

Superscripts2.5–25 lm spectrally-integrated over k = 2.5–25 lm regiontotal over the whole spectrum

Fig. 1. Total hemispherical emittance of ash deposit vs. temperature [3].

1134 A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141

present work and in a vast majority of experimental investigations.These include, but are not limited to, the inability to represent sur-face morphology of the actual ash deposits, layers of differentchemical composition or surface morphologies that might havebeen formed during the firing of guaranteed fuels or during theoperation under different load conditions. Determination of emis-sivity based on offsite, laboratory measurements or calculationsbased on chemical composition are sometimes a necessary com-promise to the more accurate but expensive and tedious in situdiagnostic methods.

In this work the deposits in the form of rocks were ground andcrushed by hand to powder as it was done by other earlier authors.Not to lose the morphology would require running the in situexperiments that are complicated to organise and conduct. Certainlaboratory tests in which the deposit rocks remain untouchedwould thus be appropriate, too. However, the tests would sufferfrom technical drawbacks like difficult control of: temperature,specimen positioning, etc.

What remains are the laboratory tests with ash powder speci-mens each placed in a vessel and temperature controlled and theiremission optically measured. Good news on these tests is thatmost rough non-metallic surfaces (as are of our specimens) emitradiation diffusely into the hemisphere (according to Lambert’s co-sine law) and absorb radiation in a same manner. This case is muchdifferent from the same phenomenon with (polished) metalswhere directional distribution is pronounced depending a lot onangle of incident radiation on the surface. It is obvious that thegeometry of the radiation heat transfer and the state of the surfacedo not exert a strong influence upon it.

Lastly, one could argue from the dimensional viewpoint. Boilerfurnaces are usually 50–70 m high and 20–30 m wide with a rect-angular cross-section. The inhomogeneities are of an order of cen-timeters. In the thermal calculation of a furnace (e.g., in ‘‘zonalmethod”) it is divided into several zones, usually 10–20. The inho-mogeneities are 2–3 orders of magnitude smaller than the zonesize so their dimensions are not decisive upon the heat flux inthe prescribed section. This means that same results are obtainedwhen the somewhat averaged data are used, i.e., as the value ofemittance from the present experiments as opposed to untouchedash tests.

Mulcahy et al. [3,4] and Boow et al. [5] described their researchfor determining the emittance of deposits formed in lignite com-bustion at temperatures up to 1450 K. The measurement of totalhemispherical emittance was conducted in the 550–1450 K tem-perature interval, in the 0.13–9.5 lm wavelength range. It was

concluded, and later confirmed by several authors [6–8], that theanalyzed quantities strongly depend on temperature and on thethermal history of the material. It was observed (see Fig. 1) thatetotal decreases from about 0.8 (at 550 K) down to about 0.55 (at1200–1300 K). The following temperature increase (1300–1450 K) leads to the sintering of ash deposits (accomplished by fu-sion of certain constituents); etotal starts to increase and reaches�0.7 (at �1450 K). In subsequent cooling etotal rises continuallyto about 0.85 (at �560 K).

Mitor and Konopel’ko [8] conducted their experiments in orderto determine the total normal emittance of powdered ash depositstaken from furnaces which were fired with different Russian coalsin the 470–1270 K interval during sample heating. The emissionintensity of a sample was compared with that of a blackbody. Tem-perature of the sample surface was monitored by thermocouplesand above 1070 K it was controlled by optic pyrometry. For thinash layers possibility of radiation transparency was considered.To get an insight, the authors chose two types of vessels to placethe samples in: pyro-resistant steel and ceramics. The obtainedplots, i.e., experimental values of emittance, with different vessels,for ash thickness 0.8–1.0 mm, agree well so it had been concludedthat layer of such thickness is opaque for thermal radiation. Theexperimental results have shown that heating of an ash sample re-duces its total emittance from roughly 0.8 at 570 K to about 0.7 attemperature about 1220 K.

Brajuškovic et al. [6] had investigated total normal as well as to-tal directional emittance for five types of Serbian lignite ash depos-its. The sample was powdered deposit taken from the furnace. The

A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141 1135

values of total normal emittance were determined for several com-mercial powders of certain chemical compounds that are found inthe composition of real deposit: ferric oxide (Fe2O3), calcium sul-phate (CaSO4), sodium sulphate (Na2SO4), silica (SiO2), alumina(Al2O3) as well as of their certain mixtures that were formed in lay-ers 1, 3 and 5 mm thick, within the 570–1270 K temperature inter-val. The directional emittance was measured in a vertical planewithin �60� to +60� from the vertical at 773 K by a total radiationfluxmeter. The authors have observed a significant drop in the val-ues of total normal emittance as the temperature increased up to1000 K. Further temperature increase resulted in emittance rise,particularly for the samples 1 and 3 mm thick, caused by sinteringof the deposit material. All these tested samples demonstrated apronounced hysteresis.

Wall et al. [1] reviewed research in this field with intention torelate the radiative properties of deposits with physical and chem-ical properties easily observed or measured. Goodwin [9] analyzedthe relation between the theory and real data for the first time bymeasuring the spectral values of real indices of refraction andabsorption at room temperature for four synthetic materials ofcompositions similar to real slag. Spectral absorptance and totalemittance of ash particles were determined from experimentaldata by Mie theory.

Richards et al. [7] measured the spectral emittance of ashdeposits by the FTIR spectroscopy on ashes of two different typesof American coals in a pilot set-up during the initial 3 h of opera-tion starting from completely clean tubes. It was concluded thatthe measured emittances strongly depend on wavelength as afunction of physical properties and chemical composition of depos-its. At wavelengths above 7 lm, emittance reaches a constant va-lue caused by thicker deposit and increased size of its particles.At shorter wavelengths deposit ceases to be opaque. The value ofemittance is affected by the conditions at the metal surface ofthe tube at which the deposit is formed and by properties of parti-cles. Measurement data reveal the influence of triatomic moleculesof carbon-dioxide that strongly absorb the radiation in spectralbands 4.2–4.7 and 14.4–15.2 lm and water-vapor which absorbin spectral bands 2.9–3.0 and 5.5–7.5 lm.

Bhattacharya et al. [10] have presented their own experimentaland analytical results showing the influence that the dependent ef-fects (scattering and absorption are called dependent effects ifthese phenomena of a particle in a medium are affected by theneighboring particles) exert upon the determination of the ash de-posit emittance. In that paper an original model is outlined – fordetermining the values of spectral emittance (directional, normaland hemispherical) of certain semi-transparent and opaque depos-its – that deliberately does not consider the dependent effects. Thepredictions that were performed by using this model duly illus-trate the influence that particle size and chemical composition ex-ert upon the emittance of coal ash deposits.

Also, that was the first time ever that the properties like spec-tral hemispherical transmittance and spectral hemisphericalreflectance were experimentally determined on semi-transparentand opaque deposits in the wavelength range that is typical forthe pulverized coal furnaces, and then therefrom the values ofthe spectral emittance were determined. In this series of experi-ments, the measurement object was/were the artificial depositsof various chemical composition and various particle sizes thatwere made to be similar to the actual deposits that are formedon the furnace surfaces during combustion. The difference inspectral emittance obtained by the model and experiments foran opaque deposit, is too large below 7 lm. It can be attributedto dependent effects.

While the measured and the predicted values of total hemi-spherical emittance of deposits are apart by roughly the same va-lue, due to difference chemical composition, the offset between the

measured total emittance values and – the predicted ones (by‘‘non-dependent theory”) was pronounced for the more transpar-ent materials. In these diagrams there is no ‘‘hysteresis” character-istic for the experiments performed on these phenomena (e.g., in[3, 6–8]). For simplicity, the authors considered the deposit in itsalready sintered and fused state which is realistic for hotterfurnaces.

Considering the preceding works on thermal radiation charac-teristics of ashes, the following two points deserve to be focusedonto. First, the ashes are compounds whose chemical/physicalcomposition is not homogeneous and it can change in the subse-quent combustion. In order to investigate the characteristics ofsuch an uncertain material for engineering application, an ideafor the safety design of the boiler furnaces is taken as the startingpoint. That is, experimental research should be done to obtain asmany numbers of data as possible on an enough thickness of thelayer and to estimate the maximum and minimum energy fluxespossible on beat transfer surfaces. Second, in order to deal withthe strong temperature dependence of the radiation characteristicsfor engineering applications, investigation of spectral characteris-tics is important. Furthermore, opacity vs. transparency of theash deposit layers to the radiation heat transfer in the infraredspectral domain should be investigated.

Mistakes in boiler furnace design calculations can lead to exces-sive temperatures of flue gases at the furnace exit. That inducesexcessive thermal stress upon the material of heat exchangers,leads to their strong fouling and increases the amount of waterneeded for regulation of superheated vapor temperature. Such aboiler will suffer of shorter life that is followed by unexpected out-ages and necessity for reconstructing the superheater section. Inorder to accurately evaluate the radiation energy transfer in a boi-ler system where there is a strong temperature distribution, aspectral analysis which uses the data of emittance spectra of theash layers is more adequate than a traditional analysis which usesthose of total emittance. But, as it is known, emittance spectra ofash layers depend on the chemical composition, temperature andthermal history of ash. In order to accomplish a safety design ofa boiler, under the condition that the chemical/physical states ofthe ash deposited on the boiler walls are uncertain, it is importantto evaluate the maximum and the minimum thermal energy fluxespossible that arrive onto the boiler walls.

The maximum flux can be calculated by the spectra of theblackbody. On the other hand, the minimum flux means minimumabsorption of flame heat radiation by water/steam in boiler tubes,and consequently extremely high temperatures of flue gases at fur-nace exit. This is a serious threat to boiler safety as explainedabove. The minimum flux should be evaluated on a basis of knowl-edge on spectra of the minimum emittance possible. So, evaluationof spectra of the minimum emittance is important for thermalengineering applications.

Data on emittance of Serbian (and other as well) lignites andtheir ashes are scarce. These data are considered among the mainparameters in steam boiler design, so the present authors under-took a research on thermal radiation properties of Serbian coalash. An experimental set-up was designed and constructed bythe present authors. Spectral normal emittances in 2.5–25 lmrange and total normal emittances were measured on four kindsof ash layers of a millimeter order thickness, at 570–1460 K inheating and cooling. Dependences on wavelength, temperature,sintering, and the impact of chemical composition were studied.The authors propose, and explain by an example, correlating theexperimentally obtained emittance spectra by simple curves. Therelevance of safety (minimum and maximum fluxes) approach toboiler design was discussed above. The method can be extendedto other specimens from the same furnace; correlations are readyfor thermal testing of old and design of new furnaces – with similar

Table 2Chemical composition of ash specimens (wt.%).

No. 1 No. 2 No. 3 No. 4

CaSO4 48.2 25.9 53.4 42.5CaO 4.7 18.9 4.5 9.9Al2O3 3.1 3.4 2.6 4.2FeO 29.4 35.3 25.7 28.4SiO2 7.4 9.5 7.4 8.3Others 7.2 7.0 6.4 6.7

Fig. 2. Particle size distribution of powdered ash specimens.

1136 A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141

geometry and combusting similar coal. The proposed method ispotentially applicable to different boiler furnaces combusting dif-ferent coal; experiments should be run on new deposit samples.The correlations will then be applicable to thermal design of awhole new class of furnaces, having similar geometry and com-busting similar coal as in these new experiments. The authors be-lieve that the methodology given in this article may greatlysimplify the safety thermal design of existing and new steam boilerfurnaces.

2. Experimental research

2.1. Materials of specimens

Materials of the specimens are broken pieces of fly ash depositson a boiler furnace heat exchange surfaces of a power plant in Ser-bia where pulverized lignite is combusted. These pieces were sam-pled at four positions along the furnace height. The materials areinorganic compounds that are solidified like rocks with the inho-mogeneities of an order of millimeters. The pieces were powderedmanually and layers of the sampled pieces were prepared for theexperiments.

Table 1 shows the specification of the ash specimens where qp

is mass density of the ash powder layers. Table 2 shows the chem-ical composition of the specimens. Finally, Fig. 2 shows the sizedistribution of particles in the powdered specimens where f isthe weight fraction of the ash powder. Specimen no. 2 contains19% of CaO and is dark brown. Specimen no. 3 is similar to speci-men no. 1 by its composition but less dense due to their differentthermal history at different locations in the furnace. Specimen no.4 is brick red-colored. Its grain size is somewhat smaller than thatof other specimens.

2.2. Experimental apparatus and procedure

Fig. 3 is the schematic diagram of the experimental set-up. Pow-dered ash specimens are set in vessels /number 1 in set-up/ ofstainless steel JIS-SUS304 to form specimen layers /2/ (1, 3 or5 mm thick). Surfaces of the vessels were oxidized enough in ad-vance so that the surface states do not change in the course of mea-surements. A K-type thermocouple /3/ of 100 lm in diameter iswelded on the rear surface of the vessel for the temperature con-trol of the specimen layer. The hot-junction of the thermocoupleis set at a position of 0–0.5 mm depth in the layer. The specimenlayer is radiation-heated from the back by silicon carbide rod heat-ers /4/. A water-cooled copper radiation shield /5/ is set at sur-roundings of the specimen surface so that the radiation emittedby the hot surroundings of the specimen surface is not being mul-tiply reflected as well as it does not possibly reach the optical mea-suring system itself.

The specimen layer is heated up to �570 K by a rate of 2 K/min(common for similar experiments) and a measurement is made.

Table 1Specification of ash specimens.

Height in furnace Color qp (kg/m3)

Bulk Powder

No. 1 10 m, inclined wall Brown Pink 1378Dark red

No. 2 30 m, rear wall Brown Brown 1469Black

No. 3 46 m, super heater Violet Pink 898Dark red

No. 4 10 m, inclined wall Violet Brick red 1199Dark red

Heating is continued at the same rate and a similar measurementis repeated at every 100 K up to �1460 K. This value of maximumtemperature was chosen since it is known since the experiments ofMulcahy et al. [3–5] that ash deposit of that chemical compositionand granulation is already sintered before reaching it. (This coin-cides with observations of present authors on cooled-down speci-mens.) After reaching the maximum temperature, specimen iscooled down with a rate of 2 K/min and a similar optical and elec-tronic measurement is repeated at every 100 K down to �570 K.

Measurement of spectral and total emissive power is performedas follows. At every 100 K, movable mirror /number 6 in set-up/ isdisplaced out of the observation area of the spectrophotometer /8/which accepts the specimen radiation flux within the solid angledefined by a zenith angle of ±10� and the spectral emittance mea-surement is done. Then the movable mirror is slided above thespecimen to position for the total normal emittance measurementby redirecting the same flux to the total radiation fluxmeter /9/.

Emittance of specimen ash layers is measured by a comparisonmethod in which the radiation emitted from the specimen surfaceis compared with that emitted from the reference blackbody /7/.The spectrophotometer that is utilized for the infrared spectralmeasurement is of a Fourier transformation type (Shimadzu Co.,Kyoto, Japan, FTIR-4200 [11]) and monitors the spectrum of 1868wavelength points in the region of wavelengths k = 2.5–25 lmwhere more than 95% and 75% of radiation energy of the blackbodyat temperatures of 570 and 1460 K are covered, respectively. Spec-tral resolution of the FTIR spectrophotometer is 0.012 lm atk = 12.3 lm which is rather too fine for the thermal engineering re-search. This resolution is approximately an average in terms ofwavelengths since the data are equispaced in wavenumbers notin wavelengths. Total radiation fluxmeter for the total emittancemeasurement is a modified radiation type pyrometer (MinoltaCo., 505S [12]) which senses the infrared radiation by a thermo-electric detector via two non-coated aluminum mirrors and aKRS-5 window. The spectral absorption characteristics of the mir-rors and the window are checked to be gray enough over k = 1–40 lm region.

9

7

6 6

1

25

1

6

8

4

3

1. vessel

2. specimen

3. thermocouple

4. heater

5. radiation shield

6. mirror

7. black- body

8. spectrophotometer

9. fluxmeter

Fig. 3. Schematic of the experimental set-up.

A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141 1137

The reference blackbody is cylindrical in shape. Temperatures ofthe interior surface of the blackbody are measured by using a radi-ation pyrometer that was calibrated using the standard black-bodies at National Laboratory of Metrology, Tokyo, Japan. Temper-ature of the reference blackbody was controlled by utilizing themeasured values of a thermocouple installed in the blackbody.

2.3. Processing of experimental data

The intensity vB(k, TB) of blackbody radiation at temperature TB

that is measured by the spectrophotometer at each wavelength k isdescribed by a relation given in standard textbooks (e.g., [13]) as

mBðk; TBÞ ¼AðkÞ

eC2kTB � 1

þ BðkÞ

eC2

kTB;room � 1; ð1Þ

where the second term compensates the influence of ambientradiation at the room temperature. A(k) and B(k) are the measuringsystem calibration constants that depend on wavelength. TB,room isthe room temperature at the time of system calibration by usingthe blackbody. Intensity vB(k, TB) at each of the 1868 wavelengthpoints k was measured in advance of the tests on specimens atseveral temperatures TB to obtain the constants A(k) and B(k). Then,spectral normal emittance of the specimen at temperature TS iscalculated

eNðk; TSÞ ¼mSðk; TSÞ � BðkÞ

e

C2kTS;room�1

AðkÞ

eC2kTS�1

; ð2Þ

where vS(k, TS) stands for measured radiation intensity of the spec-imen layer into the fixed solid angle. TS,room is the mean room tem-perature at the time of the optical measurement. Measurement ateach temperature finished within several minutes. Temperaturesof the blackbody, specimen and room did not change much duringthis time, so TB,room � TS,room.

Total intensity mtotalB ðTBÞ measured by the total fluxmeter is de-

scribed as

mtotalB ðTBÞ ¼ AtotalT4

B þ BtotalT4B;room; ð3Þ

where Atotal and Btotal are the calibration constants of the total emit-tance measurement system. Intensity mtotal

B ðTBÞwas measured in ad-vance of the tests at several temperatures TB to determine thecalibration constants Atotal and Btotal. Total emittance of the speci-men at temperature TS can be computed by using the following ana-lytical expression

etotalN ðTSÞ ¼

mtotalS ðTSÞ � BtotalT4

S;room

AtotalT4S

; ð4Þ

where mtotalS ðTSÞ is the measured total radiation intensity of the spec-

imen layer.

2.4. Experimental error

Total normal emittance and spectral normal emittance weremeasured by an open measurement system so that reflection ofthe irradiation from the surroundings of the specimen was not in-cluded into the emission of the specimen, as it is in furnaces. Thespecimen was heated from the back. Since thermal conductivityof the inorganic materials is not so high as that of the metals, tem-perature distribution is formed in the specimen layer, which raisesthe following problem. Thermocouple for measuring the surfacetemperature was not set at certain point in depth direction, andthermal contact with the powdered specimen was not ideal.

Thence, it was not an obviously solvable task to estimate the er-ror of emittance determination caused by the error of the temper-ature measurement. Therefore, based on several repeatedexperiments with the specimen no. 1 (5 mm) at T = 1000 K thestandard deviation was computed by the commonly used relation:

SD ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðe1 � �eÞ2 þ ðe2 � �eÞ2 þ � � � þ ðen � �eÞ2

n� 1

s� 100%

�e; ð5Þ

where e1; e2; . . . ; en are experimentally determined emittances; �e isarithmetic mean of these emittances and n is number of repeatedexperiments.

Standard deviation of the measured eN values from the averagevalue lies within ±4.2 and ±2.2% at k = 3.75 and 12.5 lm, respec-tively. On the other hand, standard deviation of the measuredetotal

N values from the average value is ±4.7%.

3. Experimental results and discussion

3.1. Emission spectra of ash specimens

Fig. 4 shows the series of isothermal spectra of emissive powerof thermal radiation flux of sample 1 (5 mm thick) emerging fromthe specimen within a narrow solid angle defined by the zenith an-gle of ±10�. In fact, output of the spectrophotometer is in micro-volts. However, this value is directly proportional to spectralvalue of thermal radiation emissive power, i.e., flux. This is a sen-sitive issue since it is possible to gather data within different solidangles. Larger solid angle conveys larger amount of thermal powerand results in a stronger signal. On the other hand, it is averagedover the solid angle which means less precise directional value.

Fig. 4. IR emissive power spectra of an ash layer in heating mode.

Fig. 5. Spectra of ash deposit emittance in heating mode.

1138 A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141

The solution is to find the optimum between these two oppositerequirements (signal strength and precision). The spectra shownhave been measured during the heating process in the 570–1460 K temperature range and represent an example of raw mea-surement data. Similar series of isothermal spectra have been gath-ered during the cooling part of the complete process, i.e., in the1460–570 K interval. Both (heating and cooling) series of isother-mal spectra were measured in accordance with the methodologyset in Section 2.2.

For clarity of the graphical representation, a common datareduction method has been used. The arithmetic mean of the emis-sive power values was determined, for each of the 1868 wave-length values, out of 11 (five at left, one central and five at right)neighboring data points. After this data processing is performed,the plot consists only of every 11th so-determined consecutive val-ues. In that manner, for a specimen at a certain temperature, only187 data points are plotted instead of the multitude of 1868 datathat are measured in the present experiments by thespectrophotometer.

In the plot, one clearly observes the influence of triatomic gasespresent in the air above the heated specimen which absorb theradiation in certain intervals of wavelength (the absorption bands)in the infrared spectral region. The overlapping carbon-dioxide andwater-vapor absorption bands are pronounced in the k = 2.6–2.7 lm range, absorption by only CO2 in the bands aboutk = 4.3 lm and about k = 15 lm and absorption by only H2O inthe band about k = 6.3 lm. Absorption by CO2 exerts a most criticaldistortive effect on the measurement results in the k = 4–5 lm re-gion with high absorption peaks that are easily observed in Fig. 4.

Series of isothermal spectral emittances of the four tested ashdeposit layers, in the direction perpendicular to specimen surface(within the solid angle defined by zenith angle of ±10�), duringtheir heating in the 570–1460 K range, in the course of a completeexperiment (i.e., 570–1460–570 K) are shown in Fig. 5 for speci-mens (nos. 1–4) of equal thickness (t = 5 mm). The presented val-ues were determined by using the Eqs. (1) and (2), i.e., byfollowing the methodological approach described in detail inSection 2.3.

One can observe that emittance continually increases with thewavelength in each spectrum. The increase is steep within the cen-tral part of each spectra (k = 4–8 lm). To the left and to the rightfrom this interval, the emittance is practically constant, i.e., it isnot sensitive to the change in wavelength. However, for k lowerthan 4 lm increase of emittance with temperature is strong whilein 4–7 lm range the same dependence weakens with wavelength.

Also, above 7 lm – isothermal lines of spectral emittance of ashdeposits practically coincide.

A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141 1139

3.2. Transparency and opacity of ash specimens

In the specimen system of the present experiment, the temper-ature distribution is formed in the depth direction. Since tempera-ture dependence of emittance is not strong, for most of theemittance spectrum (as mentioned above), it can be concluded thatthe emittance does not vary significantly in the depth direction. If apowder ash deposit specimen layer were semi-transparent for theinfrared radiation, radiation emitted at the part below the surface,or at the specimen vessel, would be included in the measured flux.Thence, the measured emittance eN would only be the apparentemittance eN,ap higher than the true emittance and therefore notrepresentative of the genuine ash deposit.

Dependence of measured spectral emittances, at one low-emit-tance wavelength and at one high-emittance wavelength, i.e., atk = 3.75 and 12.5 lm, of powdered ash deposit layers on theirthickness is shown in Fig. 6. The data presented in the plot arethose obtained from the tests conducted on all specimens (ashesnos. 1–4) all three thicknesses (t = 1, 3 and 5 mm) at �1000 K dur-ing heating and cooling. It was not possible to form adequatesmooth specimen layers thinner than 1 mm from the availablecoal powder because of its harsh granulation (see Fig. 2) and tostill preserve the same physical and chemical properties. It isobvious that the emittance eN does not depend substantially onthickness t of the specimen layer in each specimen and at theshorter and longer wavelengths (of k = 3.75 and 12.5 lm). It isnearly equal for all three thicknesses, being slightly higher forthinner layers. This does not jeopardize their opacity becausehigher emittance means higher absorptance (Kirchhoff’s law).That means lower transparence which excludes the possibility ofthinner layers being more transparent then the thicker ones. Itis concluded that even the layers of thickness t = 1 mm are opaqueto spectrum of radiation dealt with in this research. The differ-ences are attributed to measurement error. Finally, the measuredemittances eN of the layers are considered to be the true emit-tances that are duly representative property values of the consid-ered ash deposit material.

1 2 3 4 50.000.160.310.460.620.780.93

0.310.460.620.780.930.310.460.620.780.93

T=1000KNo.4

heating (λ=3.75μm, cooling heating (λ=12.5μm), cooling

t mm

T=1000KNo.3

T=1000KNo.2

0.310.460.620.780.93

T=1000KNo.1

ε N

Fig. 6. Thickness dependence of ash deposit spectral emittances.

3.3. Dependence of emittance on chemical composition

Fig. 7 is composed of a selected portion of data given in Fig. 5.Fig. 7 compares the dependence of emittance spectra of ash spec-imens no. 1 through 4 (all t = 5 mm thick) at about 1000 K duringheating upon their chemical composition. Obviously the variationis strong at wavelengths below 5 lm. The emittance eN of speci-men no. 2, which contains 35% of FeO and is dark brown colored,is about 10% higher than that of the other three specimens atk = 3–4 lm. This wavelength region is important for thermal engi-neering systems around the 1000 K. However, in the longer wave-length region in the infrared, emittance spectrum depends weaklyon chemical composition and temperature of ash.

3.4. Total emittance of ash specimens

Temperature dependence of total normal emittance of testedash layers has been analyzed along with the thermal history ofthe material. All four specimens (nos. 1–4 of t = 5 mm) were testedat 570–1460–570 K (during heating and cooling, Section 2.2). Twotypes of total emittance were compared: etotal

N , measured by the to-tal fluxmeter, was compared to e2:5—25lm

N , computed from spectra ofemittance eN (measured over k = 2.5–25 lm in the present experi-ment) as proposed by Vucicevic [14]:

e2:5—25lmN ¼

R 25lm2:5lm eNðk; TSÞ � mBðk; TSÞdkR 25lm

2:5lm mBðk; TSÞdk: ð6Þ

This integral equation is being approximated by an arithmeticequation, Eq. (7). This is performed by replacing integrals withsums by using the common ‘‘trapezoidal rule” of numericalintegration:

e2:5—25lmN ¼

P1867i¼1

eN kiþ1 ; TSð Þ�mB kiþ1 ; TSð ÞþeN ki ; TSð Þ�mB ki ; TSð Þ2 � kiþ1 � kið ÞP1867

i¼1mB kiþ1 ; TSð ÞþmB ki ; TSð Þ

2 � kiþ1 � kið Þ: ð7Þ

Since more than 95% of Planckian blackbody emission at 570 K(more than 75% at 1460 K) lie in this wavelength region, emit-tances e2:5—25lm

N and etotalN are comparable. Temperature dependenc-

es of both emittances (specimen no. 1) are shown in Fig. 8. Thebottom line and data points were collected during heating of thespecimen. The top ones were gathered during cooling, right aftersintering which is the cause of hysteresis.

It is obvious that the temperature dependences of e2:5—25lmN and

etotalN comply well. This fact says that the two methods produce con-

sistent results with each other. It adds to the validity of both simul-taneously run measurements: (a) using FTIR spectrophotometerand (b) using total radiation fluxmeter. Curves have a common

3 4 5 6 7 8 9 10 200.1

0.2

0.3

0.4

0.50.60.70.80.9

1

25152.5

No.1 1011K No.2 1000K No.3 993K No.4 992K

ε N

λ μm

Fig. 7. Emittance spectra of ash deposits with different chemical composition.

500 600 700 800 900 1000 1100 1200 1300 1400 15000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

No.1t=5mm

tota

l nor

mal

em

ittan

ce,

εtot

(-)

temperature, T(K)

εNtot

2.5-25μεN

N

m

Fig. 8. Temperature dependence of total emittances of an ash deposit.

1 2 3 4 5 6 7 8 910 20 30 40 500.1

0.2

0.3

0.4

0.50.60.70.80.9

1

No.1

non-linear curve fit

584K909K

ε N

λ μm

Fig. 9. Graphical representation of proposed correlation for safety design.

1140 A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141

‘‘ship like” form that is the general consequence of the ‘‘hysteresis”.This clearly illustrates a permanent change (increase) of emittancedue to an irreversible phenomenon – sintering. Also, comparison ofFigs. 8 and 1, i.e., data from the present experiment and data ofother authors, that obviously do comply well, additionally confirmsthis validity.

Table 3Parameters for evaluation of emittance spectra of ash specimens.

emin (–) emax (–) km (lm) p (–)No. 1 0.225 0.821 6.15 6.69No. 2 0.261 0.855 6.13 6.23No. 3 0.212 0.842 5.99 7.48No. 4 0.217 0.763 5.19 7.55

3.5. Correlating data for safety design

Mistakes in boiler furnace design calculations can lead to exces-sive temperatures of flue gases at the furnace exit. That inducesexcessive thermal stress upon the material of heat exchangers,leads to their strong fouling and increases the amount of waterneeded for regulation of superheated vapor temperature. Such aboiler will suffer of shorter life that is followed by unexpected out-ages and necessity for reconstructing the superheater section. Inorder to accurately evaluate the radiation energy transfer in a boi-ler system where there is a strong temperature distribution, aspectral analysis which uses the data of emittance spectra of theash layers is more adequate than a traditional analysis which usesthose of total emittance. But, as is suggested in Figs. 5–8, emittancespectra of ash layers depend on the chemical composition, temper-ature and thermal history of ash. In order to accomplish a safetydesign of a boiler, under the condition that the chemical/physicalstates of the ash deposited on the boiler walls are uncertain, it isimportant to evaluate the maximum and the minimum energyfluxes possible on the boiler walls.

The maximum flux can be calculated by the spectra of theblackbody. However, it is not critical because it yields minimumtemperatures of flue gases and that poses no threat to boiler safety.On the other hand, the minimum flux means minimum absorptionof flame heat radiation by water/steam in boiler tubes, and conse-quently extremely high temperatures of flue gases at furnace exit.This is a serious threat to boiler safety as explained above. Theminimum flux should be evaluated on a basis of knowledge onspectra of the minimum emittance possible. So, evaluation of spec-tra of the minimum emittance is important for thermal engineer-ing applications. It is also true that the emittance used in classicradiation energy exchange calculation when using configurationfactors is hemispherical emittance eH. It is not equal to normalemittance eN of the present measurement but it is equal 0.98eN

(rough non-metallic surface) systematically throughout this andsimilar research from Boow and Goard [5] to Itaya et al. [15].

We choose the lowest value of emittance at each wavelengthfrom the data of emittance spectra of each specimen layer in thetemperature region of 570–1460 K dealt in this study to draw aspectrum of the lowest emittance. For example discrete experi-mental points in Fig. 9 are obtained for specimen no. 1. This spec-

trum consists of mainly the points in the spectra at 584 and 909 K.For the actual engineering applications, it is desirable that thespectrum is described in a form of a continuous function of wave-length k of radiation. The processing contributes also to the extrap-olation of the wavelength region and to the comfortable simplicityof the computer calculation. We propose the following function tocorrelate the experimental points

eHðkÞ ffi eNðkÞ ¼emin þ emax

kkm

� �p

1þ kkm

� �p ; ð8Þ

where emin and emax are emittance asymptotes of the fitting line, km

is wavelength of the slope at the middle of linear portion of the S-shaped fitting curve and p is slope of the linear part of the fittingline. Experimental points in Fig. 9 are obtained by a best-fit tech-nique. Table 3 shows the determined values for the four specimenlayers.

The line is extrapolated beyond the wavelength region of theexperiment; let us justify it. That is, in the far-infrared region upto several 10 lm in wavelength out of the experimental region,various lattice vibration modes in the complex inorganic crystalsin the ash are distributed over the region, and the strong absorp-tion with high emittance is still preserved there assuming black-body emission, according to Siegel and Howell [13]. In the near-infrared region out of the experiment, the shorter wavelength tailof the spectral blackbody emissive power is low enough so thecrude assumption of the emittance in this wavelength region doesnot affect the calculated energy flux based on it much. Parametersfor specimens 2–4 are given in Table 3. The obtained emittancesare applicable to the safety design of the furnaces with similargeometry and combusting similar type of coal. The methodologyoutlined above including the form of Eq. (8) offers simplicity aswell as the physical meaning of the parameters. The two horizontalasymptotes emin and emax are emittances at low and high wave-lengths of the spectra, km is wavelength of the slope at the middleof linear portion of the fitting curve and p is the slope itself.

A. Saljnikov et al. / Experimental Thermal and Fluid Science 33 (2009) 1133–1141 1141

This technique can be applied to specimens from other zones ofthe same furnace; the correlations are applicable to design of newfurnaces – with similar geometry and combusting similar coal – asthe existing ones.

The proposed routine is applicable even more widely; with dif-ferent furnaces burning different coal; the tests should be con-ducted on new ash samples. The correlations will then be readyfor the thermal design of new furnaces, with similar geometryand using similar coal.

4. Conclusion

In order to facilitate thermal design, i.e., computation of energytransfer in coal combustion boilers of electric power plants withstrong fouling (ash depositions), an investigation was done oninfrared radiation characteristics of coal ash. The obtained resultswere analyzed from a practical thermal engineering point of view.

This research was focused on experimental determination of to-tal and spectral normal emittance of lignite ash deposits formed onheat exchange surfaces in a boiler furnace of a Serbian thermalpower plant. The results are presented as dependent on tempera-ture and wavelength for deposits taken at four different locationsin the furnace. An experimental set-up was designed, constructedand tested/calibrated by the present authors. It permits measuringradiation flux emitted from a specimen at temperature from 570 to1460 K, in a wavelength range from 2.5 to 25 lm and for specimenthickness of 1, 3 and 5 mm. All samples were powdered manuallyand analyzed for chemical composition and grain size.

The concluding remarks of the undertaken research have beensummarized:

– ash deposit layers are opaque for infrared radiation; it meansthat the emittance values obtained by the used methodologyare indeed the true properties of deposit surfaces;

– spectral normal emittance continually increases with increase ofwavelength; at shorter waves emittance increases with temper-ature; impact of chemical composition is weak at longer andpronounced at shorter wavelengths;

– above 1200 K ash layers are sintered and higher density layersare formed that cannot possibly reach their initial state by anycooling;

– total emittance decreases with the increase of temperature tothe onset of sintering, then increases till the melting point. Inthe cooling mode a strong ‘‘hysteresis” is present;

– for safety design of steam boilers, authors propose correlatingthe measured emittance spectra by a continuous function ofminimum emittance vs. wavelength; the method can beextended to other specimens from the same furnace; data areready for thermal testing of old and design of new furnaces –with similar geometry and combusting similar coal;

– the proposed method is potentially applicable even more gener-ally; with different boiler furnaces combusting different coal;

experiments should be run on new deposit samples; these datawill then be applicable to thermal design of a whole new class offurnaces, having similar geometry and combusting similar coalas in these new experiments.

Acknowledgments

This research was partly supported by Grant-in-Aid for JSPS Fel-low (01200058) of Ministry of Education, Science and Technologyof Japan. Authors are grateful to Sensei, Prof. Dr. Toshiro Makino,Kyoto Daigaku, for invaluable guidance and support and to Prof.Dr. Hidenobu Wakabayashi, Kyoto Daigaku for the importantassistance.

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