A high temperature impedance probe for local porosity measurements in a fluidized bed combustor

Chemiwll EngineeringScience Printed in Great Britain.

Vol. 40, No. 3. pp. 365-373.

A HIGH TEMPERATURE LOCAL POROSITY FLUIDIZED G. BOELENSt,

Laboratory

e

IMPEDANCE PROBE FOR MEASUREMENTS IN A BED COMBUSTOR

F. LIEFHEBBER*

and C. W. J. VAN

KOPPEN

of Thermal Power Engineering, Delft University of Technology, (Received

30 June

+ .oo

0009-25c9/85 53.00 1985 Pergamon Press Ltd.

1985

1982, accepted

10

January

The Netherlands

1984)

heat transfer and chemical reaction rates in a fluidized bed combustor can only be optimized, if the nature of the gas-particle flow is known sufliciently. Analysis of the local porosity fluctuations in the bed is of primary importance for an adequate modelling of these phenomena in bubbling fluidized systems. To serve this purpose, an electrical impedance probe for local porosity measurements suited for pressure and temperatures ranging up to 1 MPa and 1200 K respectively has been developed. To explore the probe performance, some results obtained from experiments in an atmospheric fluidized bed combustor are discussed. and solid residence time distribution measurements is The usefulness of the probe for bedheight indicated. Some preliminary experiments have been carried out in a pressurized fluidized bed combustor. Both spectral- and probability density of the probe signal due to porosity fluctuations have been estimated. A brief discussion on the results is given.

Abstract-The

INTRODUCTION

MEASUREMENT

The current development of pressurized fluidized bed combustors is severely limited by the lack of information on the characteristic gas-particle flow phenomena at high temperatures and elevated pressures. Several techniques suited for low temperature (300 K) experiments have been used by various investigators, mainly to discern classical characteristic two phase flow parameters as bubble velocity and -diameter. However, for temperatures up to 1200 K and elevated pressures there is a lack of measurement techniques, see, i.e. Dutta[l]. Wittmann [2] used a capacitance probe as developed earlier by Werther[3] for temperatures up to 475 K. Recently, Yoshida[4] developed an electric discharge probe. Both Mitmann [S] and Almstedt [6] used a capacitance probe for temperatures up to 1200 K. Toei[7] has shown that even small objects may disturb the gas-particle flow phenomena in a fluidized bed considerably. This has been confirmed by Rowe[8], who compared the experimental results of both an optical probe and X-rays. These X-ray techniques, however, only serve overall porosity measurements or observations on single well-defined bubbles. The objective high temperature impedance probe has been developed as a robust technique suitable for temperatures up to 1200 K and pressures of 1 MPa in order to study local porosity fluctuations in freely bubbling pressurized beds.

PRINCIPLE

The high temperature impedance probe essentially consists of two parallel needles, viz. Fig. 1, mounted on a water-cooled fin. Under the application of a voltage source to the probe needles, the presence of a medium other than vacuum generally gives rise to a characteristic electromagnetic field. From a macroscopic point of view the resulting field can be expressed in terms of the conductivity (a), permittivity (c) and permeability (,!I) of the medium. Following the quasi-static approximation for the time-varying electromagnetic field, an analytical expression for the electrical impedance between the needles in terms of the conductivity, permittivity and permeability properties of this medium on one hand, and the probe geometric parameters on the other hand, can be found[9]. Starting from the expression for two infinitely long parallel wires, the impedance between the needs generally consists of a conductance (G), a capacitance (C) and an inductance (L), according to[9]: (1) (2) (3) and

*Author to whom correspondenceshould be addressed. fPresent address Hoogovens B. V., IJmuiden, The Neth-

erlands.

365

k = In

(4)

G.

366

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J~OELENS et al.

cooxcables

and as a pure dielectric for: (7) For silica sand as bed material the following electric properties are relevant. Characteristic for silica sand is a conductivity of lOE-16 and lOE-3 (8 m)-’ at 300K and 1150K respectively and a relative permittivity of 3.9 with a negligible temperature dependency in this range [ lo]. Both independent of the gas composition and temperature, the relative permittivity for the combustion gases is stated to be 1.O[ 111. For the conductivity of air and flue gases Westphal[ 121 mentions 0.0 and 5 E-7 (52 m)-’ at 300 K and 1200 K respectively. Substitution of these data in eqns (1) and (2) leads to the conclusion that silica sand and air will be pure dielectrics at 300 K. However, at 1150 K a significant conductivity will appear. MEASURFMENT

Fig. 1. Photo and schematic drawing of the high temperature impedance probe construction. Dimensions are given in mm. The geometric parameters 1, d and r correspond to eqn (4). The needlesare embedded in ceramic and the water sealing consists of rubber.

SYSTEM

The electronic measurement system of the probe is based on a bridge driven by an oscillator voltage source in conjunction with the principle of coherent detection as commonly used in lock-in amplifiers, viz. Fig. 2. In agreement with experimental results, van Peppen [ 131 indicated that for an impedance consisting of a pure capacitance, a sensitivity limit of 0.03 aF can be reached. The availability of a simple analytical expression for the transfer function of the bridge, viz. Fig. 2, is served by the use of admittances, being the reciprocal impedances. For a relative probe admittance change (A Y) due to the presence of bedmaterial between the probe needles, the transfer function of the bridge can be derived as;

n+

A Y, ,

Y, + Y, + Y,

(8)

with: In the case of measurements in a fluidized bed combustor, the medium between the needles consists of a mixture of bed material and air or combustion gas. For non-magnetic bedmaterials, the relative permeability p = 1 .O and the relative permittivity will range from l-10. Equations (2) and (3) then indicate that the inductance component of the impedance will be negligible for:

For frequencies beyond the MHz range this inequality will be satisfied. Consequently, as follows from eqns (1) and (2), a mixture of silica sand bed material and air or combustion gases will act as a single conductor for:

Yd= Y, + Y,l+

Y**.

(9)

In practice, the admittance (Y,) mainly consists of the parasitical capacitance (Y,) of the probe-coax cables being about 50 pF/m. The overall probe admittance consists of the capacitance and conductance component corresponding to the electrical properties of the ceramic probe material used, viz. Fig. 3. The signal components corresponding to both the real and imaginary parts of the transfer function, can be measured simultaneously if an additional 7r/2 phase shifted detector is used. Signal processing has been performed on LSI 1 I /23 computersystem. POROSITY

MEASUREMENT

Khoe[14] gives a review of several correlations to derive the overall relative permittivity of gas-particle mixtures from the permittivities of the individual gases and particles.

367

A high temperature impedance probe for local porosity measurements

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I

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_______

Fig. 2. Schematic drawing of the electronic measurement system for the high temperature probe. The location of the probe admittances between the pins CIand b is given in Fig. 3. The logarithmic mixing rule appears to be a good approximation for non-spherical particles. For a mixture of n volume fractions x, of equal sized particles with permittivity ci, the overall permittivity is given as:

lnEi=

5 xilnei i-l

(10)

with

2

xi=

1.

i-1

(11)

For a binary mixture eqn (11) reduces to: In6 =(l

-x)lncS+xIn+

(12)

Assuming a dense phase porosity (xJ in a fluidized bed of 0.4, the overall relative permittivity for a mixture of silica sand and air or combustion gases, obtained via eqn (12) and the data from the previous section, becomes 2.3. Alternatively, assuming a particle free bubble phase, the overall relative permittivity equals the value for air, i.e. 1.0. The capacitance change corresponding to a porosity transition from 1 .O to 0.4, as follows from eqn (16) and eqn (2), is then found to be about 0.2 @F). The logarithmic mixing rule also applies to the conductivity[l5]. Therefore, the overall conductance change corresponding to the forementioned porosity transition can be derived similarly. At 1150 K the overall conductivities of the binary mixture consisting of silica sand and combustion gases then amounts to

4.8 E-S (am)-’ for the dense phase and 5 E-7 (am)-’ for the particle free bubble phase. The conductance change between the needles, as follows from eqn (I), is then found to be about 0.8 &a)-‘. SOLID

RESIDENCE

TIlclE DISTRIBUTIONS

The conductance change between the needles will be strongly dependent on the composition of the bedmaterial. This may be illustrated by the ratio of the conductance change of a bed with (dG,) or without (dG) a volume fraction (7) of the solid tracer. Applying the logarithmic mixing rule to the conductance change due to the transition from the dense phase to the particle free bubble phase porosity in both cases leads to the ratio:

AG, AG-

0

ut ’

;

(13)

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Fig. 3. Schematic drawing of the parasitical admittances of probe-fin and wax cables corresponding to the pins CIand b in Fig. 2.

368

G. BOELENS et al.

This indicates that even small amounts of a good conducting solid tracer will give a detectable probe signal. For a fluid&d bed the ratio of the conductivities of silica sand and non-homogeneously distributed coal-ash will be in the order of 10 E-4. In consequence of this, the presence of coal-ash particles in the fluidized bed may disturb porosity performed via measurements the conductance fluctuations. Conversely, a ferromagnetic solid tracer can be employed for residence time distribution measurements by exploring the inductance component of the probe impedance as given in eqn (3). However, this technique will he restricted to low temperatures due to the strong temperature dependence of the magnetic permeability [ 1O]. ATMOSPHERIC

-----probe capacitance

-_-.(PF)

___--------_--

--y------__

I

time

-

EXPERIMRNTS

In order to explore the usefullness of the high temperature impedance probe, some experiments have been carried out in an oil-fired atmospheric fluidized with a diameter of 0.25 (m) and silica sand of 780 (wm) serving as bedmaterial. The operating conditions have been set to a bed temperature of 1150 K and a fluidization velocity of 0.7 m/s. To verify the predicted theoretical capacitance and conductance change due to the presence of bedmaterial and gases at high temperatures, the probe signal components corresponding to these changes have been recorded. Typical sample records are given in Fig. 4. The capacitance fluctuations roughly agree with the theoretically predicted range of 0.2 pF. By traversing the probe vertically, the presence of gases with incidental packets of bedmaterial in the bed-freeboard zone has been verified to suit bedheight measurements. Furthermore, a 4% volume fraction of coal-ash particles has been added to the bed to verify the influence on the signal compared to the pure silica sand bed measurements, viz. Fig. 5. In agreement with the prediction from eqn (13) the results indicate a substantial influence on the amplitude of the signal due to the presence of coal-ash particles, expressed by a more peaky character. However, more extensive experiments should be necessary to suit solid density distribution measurement facilities. Nevertheless, these surveying experiments indicate the capability of the probe to register local porosity fluctuations in the bed.

fixcdbcd bed freeboard

(5)

Fig. 4. Probe signals recorded from experiments in an oil-fired atmospheric fluidized bed with a diameter of 0.25m. The operating conditions have been set to a fluidization velocity of 0.7 m/s, a temperature of 1120 K and silica sand bedmaterial with a mean diameter of 780 brn.

Analysis of the signals has been performed in both the time- and frequency domain by estimating probability- and spectral density functions. In recent years various methods to perform a transition from the time domain to the frequency domain have become available. Amongst these the Fast Fourier Transform (FFT) is widely used as a computationally efficient method for power spectral density estimation. This method, however, has a poor ability to resolve sharp spectral peaks. Extensive averaging techniques, implying long signal recordings, are required in order to obtain a satisfactory performance. Therefore, to discover the probable occurrence of a significant bubble frequency, Maximum Entropy (ME) spectral estimation has been performed on the data simultaneously using an algorithm first suggested by Burg[l9]. probe conductance

(LX-’

)

-_---

a b

I

SIGNAL ANALYSIS

Signal processing has been performed on a IS1 1 l/23 computersystem with a RT-11 operating system. For the analysis of the probe signals, the fluidized bed gas-particle flow phenomena have been considered to be second order ergodic random processes in a frequency range up to about 10 Hz.

I

I

1

0 -

Fig. 5.

I

2

time (51

Probe signals with (a) and without (b) a 4”/, volume fraction of coal-ash particles recorded from identical experimental conditions as given in Fig. 4.

A high temperature impedance probe for local porosity measurements

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Fig. 6. Schematic drawing of the THD-1 test rig. The inner vessel diameter is equal to 0.485 m.

369

370

G.

WELENS

The ME method is essentially based on a more general class of procedures collectively known as autoregression and linear prediction. It has been widely used during the last decade for engineering applications in the field of geophysics, neurophysics, speech processing and radar. The main advantage of these methods over the more conventional Blackman-Tuckey and FFT techniques is an improved frequency resolution when the data record is relatively short. This improved resolution makes these new techniques particularly valuable if the power spectrum contains narrow frequency peaks which have to be resolved. A judgement of the performance for the FFT and ME estimators may cover several aspects such as: consistency, variance, noise sensitivity and frequency resolution. A tutorial review on spectral analysis has recently been given by Kay [16] and Childers[l7]. For the objective signals, FFT spectral estimates have been obtained by averaging the separate FFT’s

et al.

of 16 subsequent segments of 128 datapoints, each zero padded with 128 zeros. Subsequently, a Hanning window was applied to this averaged estimate in order to reduce leakage effects. Order selection for the autoregressive model which is implied for the signal by the ME method, has been performed via the Akaike information criterion[l8]. The Burg algorithm has been applied to 256 point data records, sampled at 45 Hz. Probability density histograms have been obtained by cumulating the number of observations from 40 equal class intervals over records of 2048 data points, sampled at 45 Hz. The number of class intervals has been selected to satisfy a 5 percent level of significance to a Chi-square goodness of fit test[20].

PRESSURIZED EXPERIMENTS

Some preliminary experiments have been carried out in the THD-1 pressurized fluidized bed com-

(b)

___c

omplitude/st.deviation

Fig. 7. Probability densitiesof probe signals recorded from oil-fired experimentsin the THD-1 testrig. The operatingconditions have been set to a fluidizationvelocity of 1 m/s, a temperatureof 11 SO K, silica sandbedmaterial with a mean diameter of 780 pm and pressures of 0.4.0.6 and 0.9 MPa, viz. in Figs. (a), (b) and (c)

respectively. The

probe is situated at 0.3

m above the nozzle air distributor, see Fig. 6.

A high temperatureimpedance probe for local porosity measurements bustor, viz. Fig. 6. Boelens[21] gives the design and operating conditions of this testrig. To eliminate the influence of coal and ash particles on the probe signal, all experiments have been executed with oil as fuel. The operating conditions of the THD- 1 testrig have been set to a fluidization of 1.O m/s, a bed temperature of 1120 K and silica sand bed material of 780 pm dia. Typical probability density estimates for bed pressures of 0.4, 0.6 and 0.9 MPa are given in Fig. 7. The probability density functions indicate that the classical two phase assumption of a particle free bubble phase on one hand and a dense phase on the other hand to model the gas-particle flow phenomena does not hold for these conditions. This assumption would be useful only if the probability density is dominated by two distinct peaks corresponding to these phases. These measurements, however, indicate a more random distributed porosity range in the fluid bed. Both from FFT and ME spectral estimates, the most significant signal power is found in a frequency

I 0

I

range up to about 6 Hz, showing a tendency to consist of only one more or less flat peak, viz. Figs. 8 and 9. It should be mentioned that the shape of the ME estimate is strongly affected by the presence of sharp spectral peaks. The area under these peaks tends to be proportional to the square of the corresponding signal power, thereby remaining the total area equal to the power of the signal[ 17. The FFT estimate does not imply this mechanism.

CONCLUSIONS

The high temperature impedance probe construction, together with the used measurement system, proved to be a robust technique for the detection of local porosity fluctuations at operating conditions of 1200 K and pressures up to 0.9 MPa. The length of the probe coax cable-i.e. the parasitical capacities--is not a crucial limiting factor to the sensitivity of the measurement system. Temperature drift in the fluidized bed combustor operated at stationary process conditions, is damped

I

6

I

I

12

16

24

(b)

I

0

I

I

6

I

12

-

371

frequency

18

I

24

(Hz)

Fig. 8. FFT spectral densities of probe signals recorded from identical experiments as given in Fig. 7. Signal to noise ratio is abou: 5 dB.

372

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mELEN.

et al.

0

6

12

18

2L

0

6

12

18

24

-

frequency

I

(Hz)

Fig. 9. ME spectral densities of probe signals recorded from identical experimental conditions as given in Fig. 8.

sufficiently by the water-cooled fin construction to neglect the temperature dependence of the conductance and capacitance properties of the used ceramic probe-construction material. For silica sand bedmaterial, the capacitance component of the probe impedance is well suited both for the detection of porosity fluctuations and for the determination of the bedheight. Both the conductance and inductance components, however, show a strong temperature dependence. The presence of local temperature fluctuations in the bed may disturb porosity measurements via these components substantially. Therefore, the conductance and inductance components are less suitable for porosity measurements. Signal analysis on the porosity fluctuations suggests that the classical two phase flow modelling of the gas-particle flow phenomena does not hold for the pressurized fluidized bed. Further experiments on this subject are currently being executed in the THD-1 testrig.

Acknowledgement-The authors gratefully acknowledge the technical stag of the laboratory of Thermal Power Engineering for their contribution to the development of the probe. NOTATION

capacitance, F distance, m frequency, Hz conductance, S bridge transfer function height, m geometric parameter inductance, H length, m pressure, MPa diameter, m time, s fluidization velocity, m/s bridge input voltage, V

A high temperature impedance probe for local porosity measurements

v, X

x&i

Y Y

bridge output voltage, V porosity dense phase porosity admittance, S volume fraction

Greek Symbols perkittivity, F/m permittivity of combustion gases, F/m permittivity of silica sand, F/m permeability, H/m conductivity, (Qm)-’ conductivity of solid tracer, (62m)-’ radial frequency, Hz REFERENCES

Dutta S. and Wen C. Y., Can. J. Chem. Engng 1979 57 115. Wittmann K., Helmrich H. and Schugeri K., Chem. Engng. Sci. 1981 36(10) 1637. Werther J., Experimentelle Untersuchungen zur Hydrodynamik von Gas Feststof Wirbelschichten. Dissertation, Universitat Erlangen, 1972. Yoshida, K., Sakane J. I. and Shimizu F., Ind. Engng. Chem. Fundls 1982 21 83. Mittman M., Fonhat H., Horonid H. R., Kroger H. and Schugerl K., Chem. Engng Technik 1982 MS 996182. AlmsteJt A. E. and Olsson E., 7th ht. Conf. Fluid&d Bed Combustion. Philadelphia 1982.

373

Toei R., Kagaku Kogaku 1965 29 851. Rowe P. N. and Masson H., Chem. Engng Sci. 1980 35 1443. 191 Narayama Rao N., Basic Electromagnetics with Applications. Prentice-Hall, Englewood Cliffs, New Jersey 1972. [lOI Landolt Bomstein, Band 4, Vol. 8. Springer Verlag, Berlin 1957. illI Weast R. C., Handbook of Chemistry and Physics. 53rd Edn. The Chemical Rubber Corporation, Cleveland 1972. [I21 Westphal W., Handbuch der Physik, Band d 14. Elektrizitatsbewegung in Gasen. Springer Verlag, Berlin 1927. 1131 Van Peppen J. C. L. and Regtien P. P. L., Modern Electronic Measuring Systems, Delft University Press, 1978. 1141 Khoe G. K., Mechanics of spouted beds, Dissertation, Delft University of Technology. The Netherlands 1980. deel 1. WePSI Zwikker K., Fysische Mar&iaalkunde. tenschappelijke Uitgeverij N. V. Amsterdam, The Netherlands 1966. 1161 Kay S. and Marple S. L., Proc. IEEE 1981 69(11) 1380. 1171 Childers D. G.,( Ed.) IEEE Press selected Reprint Series, New York 1978. Akaike H., IEEE Trans. Aufo. Control 1974 19, 716. Andersen N., Geophysics 1974 39 69. Bendatt J. S. and Piersol A. G., Measurement and Analysis of Random Data. Wiley, New York 1966. PII Boelens G., Liefhebber F., De Groot M. C. and Van Koppen C. W. J., 2nd Colloquium on Pressurized FZuidized Bed Combustion. Delft University of Technology, The Netherlands, April 1983.

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