development of technology for high-accuracy …

Cement Science and Concrete Technology, Vol.71

571

ABSTRACT：Precise temperature control during clinker burning is important for reducing energy consumption during cement production. Although radiation thermometers are conventionally used for measuring the clinker burning temperature, they suffer from the problem of poor measurement accu-racy due to dust in the kiln. We therefore devised a method called the dust-canceling （DC） method for clinker temperature measurement that can eliminate the influence of dust in the kiln hood by using a radiation thermometer for measuring dust temperature in addition to the usual radiation thermometer for measuring clinker temperature. In this study, the validity of the DC method was confirmed with an experimental apparatus and applied to an existing production kiln. As a result, it was possible to obtain measured values closer to the theoretical clinker burning temperature compared with the conventional measurement method.

DEVELOPMENT OF TECHNOLOGY FOR HIGH-ACCURACY TEMPERATURE MEASUREMENT OF CLINKER IN KILNS：

PART 1 ─ THEORY OF THE DUST-CANCELING METHOD AND APPLICATION TO AN EXISTING KILN

Mitsuhiro YAMAMOTO*1, Yoshiaki TAKATA*1, Hirokazu SHIMA*1 and Yoshinori ITAYA*2

*1　 MITSUBISHI MATERIALS CO., Central Research Institute（1002-14, Mukohyama, Naka-shi, Ibaraki 311-0102, Japan）

*2　 GIFU UNIVERSITY, Graduate School of Engineering（1-1, Yanagido, Gifu-shi, Gifu 501-1193, Japan）

KEYWORDS：Temperature measurement, Clinker, Cement kiln, Low-temperature burning, Dust-canceling method, Radiation thermometer, Two-color temperature

1.　INTRODUCTION

　Energy conservation in the clinker burning process, which consumes the most energy in the cement manu-facturing process, is an important issue. To achieve energy savings, it is necessary to produce clinker using minimal amounts of coal and other energy resources. Clinker burning temperature is an important indicator that determines the amount of these resources that are consumed. This temperature is measured by using a radiation thermometer that can measure continuously and, for the purpose of durability, without contact.　However, this measurement method suffers from the problem of measurement accuracy deteriorating due to the cooler dust originating from the clinker. Because of the importance of product quality in clinker manufac-turing, clinker burning is carried out using excess coal and other energy recourses to account for temperature measurement errors. In low-temperature burning tech-niques using a mineralizer 1, 2）, there is concern that the measurement accuracy may become even lower due to further increase in the amount of dust. Even when the

burning temperature is reduced by applying low-tem-perature burning techniques, the operating tempera-ture cannot be kept significantly lower than the normal operating temperature and energy savings might not be achieved.　To solve this problem, we have devised a method that we call the dust-canceling （DC） method, which corrects the clinker temperature by using a radiation thermometer for measuring dust temperature in addi-tion to the usual radiation thermometer for measuring clinker temperature. In this paper, we explain the the-ory, report the validity verification results, and present the results of application to our production kiln. In ad-dition, to further improve the measurement accuracy, the location and target of the radiation thermometer for measuring dust was varied, and this is described in Part 2 of this work （“Improvement of measurement ac-curacy for practical application”）.


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where L'：Spectral radiance incident from surroundings

Ldus：Spectral radiance of dustεdus：Emissivity of dust crowda：Absorption coefficient of dust particlesσ：Scattering coefficient of dust particlesΦ：Phase function

　The first term on the right-hand side represents at-tenuation due to dust absorption and scattering, the second term represents the increase due to radiation from the dust, and the third term represents the in-crease due to scattering of ambient light by the dust. The absorption coefficient a and scattering coefficient σ in Eq. ［3］ are expressed by Eqs. ［4］ and ［5］, respec-tively.

a＝XaAN ［4］

σ＝XσAN ［5］

where Xa：Absorption efficiencyXσ：Scattering efficiencyA：Cross-sectional area of dustN：Number density of dust particles

　The phase function Φ is a function representing the directivity of scattering.2. 2　 Theory of the DC method

　In the DC method, two radiation thermometers are used and measurement is performed at two wave-lengths λ1 and λ2, respectively, as shown in Fig. 1. Ra-diation thermometer 1 measures the clinker tempera-ture, and radiation thermometer 2 measures the dust temperature by facing a quartz plate （Fig. 2）. Since the accuracy of clinker temperature measured by ra-diation thermometer 1 is affected by dust, the influence of dust needs to be eliminated. If radiation thermom-eter 2 faces a hot object behind the dust, the dust mea-surement will be affected by the temperature of that hot object. A quartz plate is therefore placed behind the dust so that the spectral radiance coming from be-hind the dust can be ignored. The two wavelengths of 0.90 and 1.55μm that are used for the measurement are mostly transmitted by the quartz plate （Table 1） such that only a small amount of light from the furnace is reflected back into the furnace. Furthermore, since the environment outside the quartz plate is at a suf-ficiently low temperature compared with the inside of the furnace, the spectral radiance incident from behind the quartz plate into the furnace is negligibly small.

2.　 THEORY OF TEMPERATURE MEASURE-

MENT

2. 1　 Measurement theory of radiation thermometers

　A radiation thermometer is a device that measures temperature by detecting the energy of the light emit-ted from the object to be measured. The energy （L：spectral radiance） of the light emitted from the object depends on the temperature of the object and is ex-pressed by Planck’s law （Eq. ［1］）.

［1］

where C1：First radiation constantC2：Second radiation constantT：Temperature of the objectλ：Wavelength used for measurement

　This can be rewritten as follows：

［1'］

　The radiation thermometer measures the tempera-ture by substituting the detected energy into Eq. ［1'］. Furthermore, the two-color method uses two wave-lengths （λ1, λ2） to measure the temperature, as calcu-lated with Eq. ［2］. The two-color method offers higher measurement accuracy than the single-wavelength method.

T＝［2］

　The two-color method improves the measurement accuracy because it eliminates the effects of the emis-sivity of the object, obstruction of the visual field, and dirt on the measurement window. The influence of dust can be removed if the dust temperature is sufficiently lower than the clinker temperature, but not if the dust temperature is high. In cement kilns, measurement by the two-color method at wavelengths of 0.90μm and 1.55μm is widely used.　The propagation of light from the object through dust to the radiation thermometer is described by the radiative transfer equation3）（Eq. ［3］）.

＝－（a＋σ）L＋aεdus Ldus＋ L'・ΦdΩ ［3］

2C1λ5

L＝ C2λT －1exp

1

C2λ

T＝ 2C1λ5L1＋ln

1

C2（1/λ2－1/λ1）ln（Lλ1/Lλ2）＋5ln（λ1/λ2）

dLds

σ4π


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equation is solved under the assumption that incident light not absorbed by dust travels straight ahead, the spectral radiances L1 and L2 incident on the radiation thermometers 1 and 2 are given by Eqs. ［6］ and ［7］, respectively.

L1＝τ1εcliLcli＋（1－τ1）Ldus ［6］

L2＝τ2εqLq＋（1－τ2）Ldus ［7］

where τ：Transmittance of dustε：Emissivity

　Here, the suffixes cli, dus, and q denote clinker, dust, and quartz plate, respectively, and τ is a function rep-resenting the cross-sectional area of the dust, number density of the dust, optical path length, and other fac-tors, and is expressed as follows：

τ＝exp（－XaANs）［8］

where Xa：Absorption efficiency of dustA：Cross-sectional area of dustN：Number density of dust particless：Optical path length

　Given that 1－τ is the ratio at which light does not pass through the dust, it can treated as the emissivity of the dust cloud. Considering that the temperature of the dust is more than 1,000℃ and that the quartz plate is in contact with the outside air and sufficiently

　Fig. 3 shows the results of measuring a sample in which the clinker was pulverized, classified, and dis-persed in liquid paraffin in order to investigate the wavelength dependency of the intensity ratio of di-rect light or scattered light to incident light by dust particles 4）. The results show that the forward and backward scattering components are smaller than the direct light component. If the radiative transfer

Table 1　Transmittance of the quartz plate

Wavelength（μm） 0.90 1.55Transmittance（-） 0.936 0.930

Fig. 1　Schematic of the positions of the radiation thermometers

Fig. 2　Quartz plate

Fig. 3　Optical properties of dust


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＝［11］

　Since Eq. ［11］ is the ratio of the spectral radiance of the clinker at two wavelengths, the clinker tempera-ture can be calculated from Eq. ［2］5）.

3.　 EXPERIMENTAL VALIDATION OF DC METH-

OD THEORY

　The validity of the DC method was verified using an experimental apparatus simulating the state inside the kiln.3. 1　 Experimental apparatus

　Fig. 4 shows the experimental apparatus for repro-ducing temperature measurement of clinker inside a kiln or the quartz plate behind the dust. A quartz ob-servation window is fitted to the front of the apparatus for measuring the inside of the apparatus. A hopper is installed so that dust can be discharged between the observation window and the clinker simulation area （dust simulation area）. A heater is attached to the hop-per for heating the dust. The object to be measured is placed in the clinker simulation area and a blackbody cavity is attached to the wall opposite the observation window （Fig. 5）. The blackbody cavity is a cavity in which the apparent emissivity is approximately 1. Since it is cooled with water from the outside, the spectral radiance from the blackbody cavity is much smaller than from the inside of the apparatus. A blackbody cav-ity is used to provide the same function as the quartz plate in a production kiln. To simulate a production kiln, the temperature of the clinker and the surround-ing walls needs to be about 1,450℃ and the tempera-ture of the dust needs to be about 1,200℃. However, at this dust temperature, it is difficult to discharge the dust from the hopper due to adhesion between the particles. Therefore, the temperature of the dust was set to 800℃. However, if this dust temperature is com-bined with a clinker temperature of 1,450℃, the spec-

L1,λ2－（1－τ1）Ldus,λ2τ1

L1,λ1－（1－τ1）Ldus,λ1τ1 Lcli,λ1

Lcli,λ2

cooled, Eq. ［7］ can be approximated as follows.

L2＝（1－τ2）Ldus ［7'］

　From Fig. 3, it is thought that the transmittance of dust is almost constant with respect to wavelength, and the ratio of the spectral radiance L2 of two wave-lengths can be given by Eq. ［9］.

＝＝［9］

　Because this is the ratio of the spectral radiance of dust at two wavelengths, the dust temperature can be obtained by using Eq. ［2］. In this study, it is assumed that the temperature and concentration of the dust are uniform along the optical paths of radiation thermom-eters 1 and 2. Since the spectral radiance Ldus of dust can be obtained from the dust temperature and Eq. ［1］, the transmittance τ1 of the measurement optical path of radiation thermometer 1 is given by Eqs. ［7'］, ［8］, and ［10］.

τ1＝exp ln 1－［10］

where s1, s2：Optical path lengths of radiation thermometers 1 and 2

　Equation ［6］ can be rewritten as follows.

＝εcliLcli ［6'］

　Equation ［11］ is obtained by calculating Eq. ［6'］ at two wavelengths and taking the ratio.

L2,λ1L2,λ2

（1－τ2,λ1）Ldus,λ1（1－τ2,λ2）Ldus,λ2

Ldus,λ1Ldus,λ2

s1s2

L2Ldus

L1－（1－τ1）Ldusτ1

Fig. 4　Experimental apparatus

Fig. 5　Blackbody cavity


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tral radiance of the dust is so much smaller than that of the clinker that it does not significantly affect the measurement. The clinker temperature was therefore also set lower at 1,000℃.3. 2　 Experimental method

　Two sets of experiments were conducted to repro-duce the measurements of radiation thermometers 1 and 2 in Fig. 1.　Radiation thermometer 1 measures the clinker through the dust dispersed inside the kiln. To simulate this, simulated clinker （1,000℃） was placed in the clinker simulation area, and dust （800℃） was dropped through the dust simulation area （Experiment 1）（Fig. 6）. The radiation thermometer measured the simulated clinker through the dust from the observa-tion window.　Radiation thermometer 2 measures the quartz plate placed in the kiln hood through the dust dispersed inside the kiln. To simulate this, a water-cooled black-body cavity was attached to the wall opposite the ob-servation window without the simulated clinker in the clinker simulation area （1,000℃）, and simulated dust （800℃） was dropped （Experiment 2）（Fig. 7）. The radiation thermometer measured the blackbody cavity through the dust from the observation window.

　In Experiment 1, the two-color temperature obtained from Eq. ［2］ when the dust is not dropping is the true value of the clinker temperature since there is no influ-ence of dust. The effectiveness of the DC method is examined by comparing the error between this true value and the two-color temperature obtained from the Eq. ［2］ when the dust is dropped in Experiment 1, and the error between the true value and the temperature using the DC method obtained from Experiments 1 and 2.3. 3　 Results

　Fig. 8 and Fig. 9 show the measurement results at 0.90μm in Experiments 1 and 2, respectively. In Ex-periment 1, since the temperature of dust is lower than that of simulated clinker, the temperature decreases compared to the case without dust. In Experiment 2, the temperature of dust is higher than that of the blackbody cavity, so the measured value rises while the dust is dropping. The measured value of the drop-ping dust increases over time because there is some temperature distribution within the dust in the hopper. The measured value of the blackbody cavity remains at about 600℃ before and after the dust is dropped. This is because the emitted light from the wall surface of the clinker simulation area slightly reflects inside

Fig. 6　Schematic of experiment 1 Fig. 7　Schematic of experiment 2

Fig. 8　Results of experiment 1 Fig. 9　Results of experiment 2


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the blackbody cavity and the radiation from the wall surface around the blackbody cavity （stray light） is in-cident on the radiation thermometer. In Experiment 1, although there are variations in the temperature of the dropping dust, the light emitted from the dust is weaker than that from the simulated clinker and does not affect the measured temperature.　Table 2 shows the measured temperature and the temperature obtained by the DC method. Considering that the dust supply is not stable immediately after the start and immediately before the end of the dust drop-ping, only data in the period between 10 and 60s after the start of dust dropping were used for the analysis in both Experiments 1 and 2. In Experiment 1, the true value of simulated clinker temperature before dust dropping was 1,063℃, while the temperature during dust dropping was 1,049℃, giving a measurement er-ror of －14℃. The dust temperature during dust drop-ping in Experiment 2 was 878℃, which is consider-ably higher than the 800℃ temperature setting of the hopper heater. This is attributed to heating by radiant heat from the clinker simulation area during dust drop-ping. The clinker temperature obtained by using the DC method based on the spectral radiance of the dust obtained from Eq. ［1］ and also using Eqs. ［10］, ［11］, and ［2］ was 1,059℃, and the measurement error was －4℃. From these results, the calculated value of the DC method had higher accuracy than the convention-ally used two-color method, and the effectiveness of the DC method was confirmed.

4.　 MEASUREMENT RESULTS AT OUR PLANT

　The DC method was applied at our production kiln （NSP type） to measure clinker temperature. The wavelengths used were 0.90 and 1.55μm. Fig. 10 shows the clinker temperature by the DC method, the two-color temperature （the conventional temperature of clinker） of radiation thermometer 1, the two-color temperature （the dust temperature） of radiation ther-mometer 2 and the transmittance obtained from Eq. ［10］. The clinker temperature by the DC method was 1,350 to 1,550℃, which was 60 to 160℃ higher than that by the two-color method. The dust temperature was 1,250 to 1,400℃, and the transmittance was 0.15 to 0.38. The transmittance represents the ratio at

which radiation from the clinker reaches the radiation thermometer, and decreases as the concentration of dust increases. As shown by Eq. ［8］, the transmittance decreases as the area blocked by dust increases. Also, the transmittance decreases as the particle size gets smaller, even if the dust concentration remains the same. Although the transmittance cannot be converted to dust concentration if the particle size is not known, transmittance is expected to be used in kiln control as an index that is dependent on dust concentration. It was confirmed that fluctuations in transmittance are consistent with the amount of dust observed visually. The dust temperature obtained by Eqs. ［9］ and ［2］ is also expected to be used for kiln control as an index corresponding to the temperature of secondary air used for combustion for the burner and recovered from the clinker cooler.　Although the burning temperature of the clinker is thought to be about 1,450℃, the conventional mea-surement method generally gives values as low as about 1,350℃. In contrast, the result of the DC meth-od is close to the theoretical value of about 1,450℃. Furthermore, the conventional method tends to give values that are about 50 to 150℃ lower than 1,450℃ over a four-day measurement period.　Considering that there is an approximate linear rela-tionship between temperature and spectral radiance in the range of 1,300 to 1,500℃, from the equation ［6］, it is found that the ratio of the difference between the clinker temperature by the two-color method and the dust temperature to the difference between the clinker temperature by the DC method and the dust tempera-ture is close to the value of the transmittance.　For transmittance and clinker temperature, the change in transmittance occurs slightly earlier than the change in clinker temperature. It is thought that when the clinker temperature in the kiln decreases, the granulating action of the clinker decreases and fine clinker is transported by cooling air from the cooler, resulting in a decrease in transmittance. However, it is difficult to explain the difference in timing between the changes, which suggests that complicated phenomena are occurring.　Fig. 11 shows the relationship between the trans-mittance and the difference in clinker temperature

Table 2　Application result of the DC method to experiment result

Without dust Dust fallingTwo-color temperature Dust Two-color temperature DC method

Temperature（℃） 1,063 878 1,049（－14） 1,059（－4）


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clinker and quartz are perpendicular.　The second method is to install the quartz plate so that the optical paths of radiation thermometer 1 and radiation thermometer 2 are close. While this kind of arrangement is difficult, a nose-ring plate installed on fixed refractories at the end of the kiln close to the clin-ker could be used instead of the quartz plate. Since the backside of the nose-ring plate is air-cooled, the sur-face temperature would be expected to be sufficiently lower than the clinker, and the spectral radiance could be neglected similar to that of the quartz plate. In Part 2 of this work （“Improvement of measurement accuracy for practical application”）6）, we will compare

between the DC and two-color methods. It can be understood that the difference tends to increase as the transmittance decreases. This is thought to be because the two-color method is strongly influenced by dust whereas the DC temperature is not strongly affected by dust. When the transmittance is high, the influence of dust is not large even in the two-color method, and so the temperature difference is small. This result sug-gests that the DC method can effectively remove the influence of dust.

5.　 ISSUES FOR IMPROVING MEASUREMENT

ACCURACY

　In the DC method, it is assumed that the dust con-centration and temperature of the measurement opti-cal path of radiation thermometer 2 that measures the quartz plate are the same as those of radiation ther-mometer 1, which measures the clinker. However, this assumption may not always be correct；the measure-ment accuracy could be reduced as a result. There are two possible solutions for this.　The first method is to set up multiple pairs of quartz plates and radiation thermometer 2 to obtain multiple dust temperatures and transmittances, and then cor-rect the clinker temperature. However, even if this method is used, it is difficult to estimate the dust con-centration and temperature on the optical path for measuring clinker since the optical paths for measuring

Fig. 10　Measurement results at our plant

Fig. 11　 Relationship between transmittance and difference between DC and two-color methods


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radiation thermometer measuring dust tempera-ture and the one measuring clinker temperature. So we proposed a method to solve this problem and verify its accuracy in Part 2.

ACKNOWLEGMENTS：　This paper is based on results obtained from a proj-ect subsidized by the Ministry of Economy, Trade and Industry and the New Energy and Industrial Technol-ogy Development Organization and conducted as a joint research with Chino Corporation. We express our sincere gratitude to them.

REFERENCES：1） T. Kamigouchi et al.：Low-Temperature Fir-ing Method and Cement Properties of Portland-Cement Clinker by Adding Mineralizer Vol. 23, pp. 151-155 （2016）（in Japanese）

2） M. Yamashita et al.：Low-Temperature Burnt Portland Cement Clinker Using Mineralizer, Ce-ment Science and Concrete Technology, Vol. 65, pp. 82-87 （2011）

3） Robert Siegel et al.：Thermal Radiation Heat Transfer, 3rd edition, p. 687 （1992）

4） Y. Itaya et al.：Non-Homogeneous Radiation Prop-erties of Slag Particle Cloud, International Sympo-sium on Transport Phenomena ISTP27-071, Pacific Center of Thermal Fluids Engineering （2016）

5） Y. Takata et al.：Development of Method for Temperature Measurement of Sintering Clinker in Rotary Kiln, THE ANNUAL MEETING OF CE-MENT AND CONCRETE ENGINEERING 2102, Japan Cement Association （2016）（in Japanese）

6） M. Yamamoto et al.：Development of Technology for High-Accuracy Temperature Measurement of Clinker Kilns：Part 2 ─ Improvement of Mea-surement Accuracy for Practical Application, Ce-ment Science and Concrete Technology, Vol. 71 （in press）

the accuracy of the DC method with and without the second method above.

6.　 CONCLUSION

　This research proposed the DC method for energy conservation in the clinker burning process. This method eliminates the influence of dust on clinker temperature measurement by using a radiation ther-mometer that measures the dust temperature in order to improve the measurement accuracy of the clinker temperature at the exit end of the kiln. The following findings were obtained.（1） When measurement by the DC method was repro-

duced in an experimental apparatus, the measure-ment errors of the conventional and DC method were －14℃ and －4℃, respectively, which veri-fied the effectiveness of the DC method.

（2） When the DC method was applied in our produc-tion kiln, the clinker temperature was 60 to 160℃ higher than that measured by the conventional method and close to the theoretical burning tem-perature of 1,450℃.

（3） From the measurement results at our production kiln, it was found that the lower the transmittance, the larger the temperature difference between the DC method and the conventional method. This suggests that the DC method can effectively elimi-nate the influence of dust.

（4） The transmittance related to dust concentration and the dust temperature related to secondary air temperature were obtained by the DC method. Further energy conservation can be expected by controlling the kiln operation using these new indi-ces.

（5） The DC method is considered more accurate than the conventional method. However, measurement error is still expected to occur since the distribu-tion of dust concentration and temperature is not completely uniform along the optical paths of the

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