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The thermal conductivity of pulverised-fuel ash cenospheres
THE T H E R M A L CONDUCTIVITY OF PULVERISED-FUEL ASH CENOSPHERES
M. A. JEWAD,I. E. SMITH and S. D. PROBERT
Centre for Thermal Insulation Studies, School of Mechanical Engineering, Cranfield Institute of Technology, Beds (Great Britain)
SUMMAR Y
The thermal conductivity of pulverised-fuel ash cenospheres has been determined using both a transient and a steady-state method. The effect of mechanical loading has been investigated, and the results show that the application of considerable loads is without appreciable effect on the insulating properties. Over the range 25 to 80°C, the presence of moisture adsorbed from the atmosphere gave rise to an increase of the effective thermal conductivity of the material, and this should be taken into account i f cenospheres are to be used for thermal insulation.
INTRODUCTION
Powdered materials combine the virtues of low density, low thermal conductivity, can readily fill the available space, irrespective of shape, and provide some mechanical support for the system. Hence they are often used as thermal insulants. Most powders, however, possess a poor resistance to compaction, and will not support large loads without permanent deformation. In addition, deformation leads to a reduction in the intergranular air spaces within the powder, which thereby becomes a less effective insulator. Furthermore, powders are susceptible to the absorption and transmission of water by capillary action, so leading to a considerable reduction in insulation performance. These problems have militated against the wider adoption of powder insulants. The poor load-bearing characteristics occur because most powders are irregularly shaped and inter-particle contact ensues via their surface asperities. As these deform or collapse, the crushing strength progressively increases. For a semi-mobile powder the load imposed may be sufficient to cause a gradual decrease of occupied volume with time, particularly if vibration is present, and the insulating value will slowly deteriorate. 67 Applied Energy (2) (1976)----O Applied Science Publishers Ltd, England, 1976 Printed in Great Britain
M. A. J E W A D , I. E. S M I T H , S. D . P R O B E R T
68
The second factor which may lead to a deterioration in powder insulant performance is moisture absorption, whereby the normally air-filled interstices become water-logged, and transmit heat by conduction. Even if complete water-logging does not occur, the presence o f water and water v a p o u r in small cavities can give rise to an evaporation/condensation mode of heat transfer (as in a heat pipe), for liquid pressures may be so reduced by surface tension forces acting over minute radii that local boiling ensues at temperatures well below 100°C. A material which has attracted the attention o f the present investigators as a high load-bearing thermal insulant is pulverised-fuel ash cenospheres 1 (see Table 1). TABLE 1 CHEMICAL COMPOSITION OF THE TESTED CENOSPHERES
Constituent
Per cent by weight
Silica (as SiO2) Alumina (as A1203) Alkali (as Na2. K20) Iron Oxide (as Fe203)
55 to 61 26 to 30 0.5 to 4 4
This material (which comprises about 4 per cent by weight of the total ash from pulverised-coal burning furnaces) consists o f hollow glass-like spheres which range in size from 20 to 300 g m with wall thicknesses approximately one-tenth of their diameters. Being spheres, their load-bearing characteristics are excellent, and, furthermore, they are hollow, with a high proportion o f their volume being occupied by relatively low-conductivity gas. A further incentive to their study is that approximately 2 × 10 6 tonnes per a n n u m are produced in the U K as waste from coalburning power stations, thereby creating a disposal problem. The cenospheres
The bulk density of the samples tested was determined to an accuracy o f 3 per cent by placing 100 g in a measuring cylinder and applying gentle vibration until a constant level was reached. The apparent porosity ( = 1 - Pbulk/Papp.... t) is calculated and also presented in Table 2. TABLE 2 BULK PROPERTIES OF THE TESTED SAMPLES
Sample designation
Nominal particle diameter
(/tm)
(kg m 3)
52/7 100/7 200/7 300/4
82 58 51 36
424 384 336 261
Bulk density
Relative density
Apparent porosity (per cent)
0'7 0.7 0.7 0'4
0"395 0.450 0"520 0.348
* The first number, i.e. 52 ; 100; 200 and 300 refers to the mesh size of powder particles; the second, e.g. 7, refers to apparent relative density.
CONDUCTIVITY OF PULVERISED-FUEL ASH CENOSPHERES
69
The transport properties of a powder are determined not only by the mean particle size but also by the size distribution, and so both were measured for each of the four samples tested with a Quantimet 720 Image Analysing Computer (see Table 3). TABLE 3 PARTICLE SIZE DISTRIBUTION T h e n u m b e r s in the tables represent the percentage o f the particles between the stated size a n d the next lower stated size. Each o f these is the average of three readings taken on different specimens o f the s a m e sample
•
(Particle diameter
Sample
"x~
,um)
34
50
65
85
100
120
135
150
170
Mean diameter 185 of particles (am)
52/7 100/7 200/7
17 38 37
17 25 29
16 16 18
15 9 9
12 6 5
9 2 1 "5
6 2 0.5
3"5 1 0
2 0 0
0'5 0 0
Particle diameter m)
Sample 300/4
18
27
36
45
54
63
72
81
90
99
~
82 58 51
Mean diameter of particles (am)
26
21
17
14
11
6
3
1 "5
0"5
0
36
Prior to the determination of the thermal conductivity no attempt was made to dry the powder which, after a long period of standing, was assumed to have a moisture Content in equilibrium with that of the laboratory where the relative humidity averaged 55 per cent during the period. This was deliberate since, in many installations where insulation is applied, the material has free contact with the atmosphere.
EXPERIMENTAL METHODS
The thermal conductivity of the packed cenospheres was measured by a flat-plate steady-state method and a transient probe method. The steady-state method employed a modification of the basic double-guard flat plate technique as specified in BS 874 and ASTM C 177. The hot surface of the apparatus (see Fig. l) was heated electrically and the cold surface was cooled by a steady flow of water through a manifold designed to maintain, as far as possible, a uniform temperature over the surface. Suitable controllers kept the guard ring heaters isothermal with respect to the main heater, and both hot and cold surfaces were blackened so as to have high
70
M. A. JEWAD, I. E. SM1TH~ S. D. PROBERT
emissivities and absorptivities. The load was applied directly to the upper surface of the heater, and the whole system was then gently vibrated in order to ensure that the loading was uniform throughout the specimen. The transient method employed has been described by Vos 2 and depends upon the temperature change that occurs at a line source of heat immersed in an infinite body of the material. After an initial perturbation period, the rate of rise in the centreline temperature is related directly to the thermal conductivity of the surrounding body and is independent of any other properties. The effective thermal c o n d u c t i v i t y kef f of the material equals: O--. ~d(ln t)'] 4re \ dT ] where O' is the rate of heat generation per unit length of the line source, t is the time and T the probe temperature.
LOAD
1
,\ \
\
\
SECONDARY ~GUARD HEATER
\
J
~ INSULATION ~ / / MAINHEATER
/// WATER
WATER
:
30 CM Fig. 1.
~]
Schematic diagram of steady-state apparatus.
A stainless steel probe 5 mm in diameter and 30 cm long carried within it a Nichrome heater and a chromel/alumel thermocouple (see Fig. 2). A thin ceramic insulator provided the electrical insulation. Because its effective thermal conductivity was much greater than that of the thermal insulant being studied, the temperature gradients within the probe itself could be considered negligible. In use this probe was inserted along the axis of a cylinder, 8 cm in diameter and
CONDUCTIVITY OF PULVER1SED-FUEL ASH CENOSPHERES
71
HEATER THERMOCOUPLE
LEADS
LEADS - m
L
----.-----------STEEL SHEATH ~HEATER
30 CM
~THERMOCOUPLE .........-.-.----.---CERAMIC FILLING
Fig. 2.
Schematic d i a g r a m of the thermal probe.
32 cm long, filled with the powder. In order to determine the local temperature of the powder and also, more importantly, to detect any temperature rise at the periphery of the sample, which would invalidate the infinite body assumption, a second thermocouple was immersed in the packed cenospheres near the outside of the vessel. Figure 3 depicts a typical temperature-time curve showing a linear relationship between the temperature and the logarithm of the time up to about 300 sec. Since the temperature rise on the centreline over this period of time was 32°C, this indicates that the thermal conductivity remained sensibly constant over this temperature range.
EXPERIMENTAL RESULTS
The packed cenospheres did not begin to yield until an applied load attained a value between 0.8 and 1.6 M N m - 2 according to the particular sample under t e s t a remarkably high figure when compared with most powdered materials. Although the absence of compaction would lead to the inference that the conductivity would not change appreciably, it was felt that only by carrying out measurements under load could this be established beyond doubt.
72
M. A. J E W A D , I. E. SMITH, S. D. PROBERT
j/ /X
120
/
/
X
/x
/" X
/
110
/
O I.U
/
I-rv.
ILl OILl t'--
/
i00
X
X
X
X
90
I
i0
i00
!
I000
TIME (SEC) FROM COMMENCEMENTOF HEATING Fig. 3. Typical time-temperature results used to determine kerr by transient method: sample 52/7. The conductivity under conditions o f mechanical loading was determined with the steady-state apparatus, and two loadings were selected, uiz: 0.034 and 0.25 M N m -2. The higher figure corresponds to the load that would arise under a height o f 60 m o f the powder, a figure which it was considered unlikely would be exceeded in practice. From inspection of the experimental results (Figs. 4 to 7), it is evident that: (a) the applied load does have a small effect on the conductivity and that this effect
CONDUCTIVITY
OF P U L V E R I S E D - F U E L
73
ASH CENOSPHERES
0,18 THERMAL CONDUCTI V I TY IV m-1 K- I
X
/
0,16
0,14
0,12
/ ,1
I
20
I
I
i
I
30
40
50
60
I
i
70
8O
TEMPERATURE Oc Fig. 4. Experimentaldata:sample52/7. becomes more apparent the smaller the mean size of the cenospheres, and (b) there appears to be no difference, as expected, between the results obtained by the 'low load' steady-state and the transient method, in both cases the load being due solely to the weight of the cenospheres in the test. The results agree qualitatively with those previously published by CEGB, 3 although the particle size range was unspecified in that investigation. The particle size does influence the thermal conductivity, in that the sample having the smallest average sized particles (300/4) showed the least conductivity, as might be expected. However, for the four samples, the conductivity appears to correlate well with the bulk density which, again, is not surprising. The effect of
M. A. J E W A D , 1. E. SMITH, S. D. PROBERT
74
relative density upon the conductivity is quite small, as may be seen by comparing Figs. 4 to 7 with Table 2. From these observations a fact emerges which requires further explanation, and that is the effect of the mean temperature upon the thermal conductivity. The results show that the temperature coefficient ranges from 1"2 to 1.8 per cent °C-1 at a temperature of 30°C, which is an order of magnitude greater than that recorded in
0,16
THERMAL CONDUCTIVITY W m-IK - I
0.14
0.04 MN m"2
0,12
O
0.25
X
TRANSIENT
MN m"2
0.1
X I
I
I
I
I
20
30
40
50
60
I
70
I
80
TEMPERATURE Oc, Fig. 5. Experimental data: sample 100/7. the literature* for powder insulants even under cryogenic conditions, and is well beyond the temperature variation for the conductivity of the material from which the cenospheres are formed. In seeking an explanation for this anomalous behaviour, the effect of humidity was considered, for, as has been mentioned, no particular effort was made to dry the samples prior to the conductivity measurements. The tests were therefore repeated for the 52/7 sample using the transient
CONDUCTIVITY
OF PULVERISED-FUEL
75
ASH CENOSPHERES
method with material which had been carefully dried in an oven. The results, whilst agreeing with those shown in Fig. 4 at 25°C, exhibited a temperature coefficient of only 0.03 per cent per degree. The significance of this finding is that although humidity exerts only a slight influence on the effective conductivity at normal ambient temperatures (say, 25°C and below), at higher temperatures
0.16 THERMAL CONDUCTIVITY W m- I
K- I
0,14
0.12
0,i0
/ 0.08
I
20
MN
-2
•
0,04
o
0,25 MN m
x
TRANSIENT
m -2
X I
i
i
J
,
30
40
50
60
70
80
TEMPERATURE Oc Fig. 6. Experimentaldata:sample200/7. it does lead to an increase in the conductivity which should be taken into account if this type of insulation is to be contemplated. Since the thermal conductivity of water vapour is not significantly different from that of air, there must be a more complex mechanism occurring in order to account
M. A. J E W A D , I. E. S M I T H , S. D. P R O B E R T
76
for the a n o m a l o u s l y high rate o f heat transfer across the interstices between the p a c k e d cenospheres, as shown by the experimental results. One possibility is that o w i n g to the presence o f a t e m p e r a t u r e g r a d i e n t t h r o u g h the material an e v a p o r a t i o n / c o n d e n s a t i o n process occurs, the return o f condensed moisture to the e v a p o r a t i o n zone arising t h r o u g h the influence o f capillary action.
0,14
THERMAL CONDUCTIVITY W m- z
K-z
0,12
/
0,i0
•
0,04
O
0,25 MN m
x
TRANSIENT
MN m -2 -2
0,08
/ [
20
X [
30
[
I
I
I
40
50
60
70
TEMPERATURE
I
80
OC
Fig. 7. Experimentaldata:sample300/4. H e a t transfer arising from v a p o u r diffusion has been shown s to lead to an effective conductivity : D kvap = R--'T
P
L
-' dT
where D is the diffusion coefficient, Pv the v a p o u r pressure o f water a n d L the
CONDUCTIVITY OF PULVERISED-FUELASH CENOSPHERES
77
latent heat of vaporisation. Insertion of appropriate values into this equation gives v a p o u r conductivities o f 0.20 W m - 1 K - 1 at 40°C, and 0"62 W m - 1K - 1 at 65°C. If the overall conductivity o f the packed cenospheres can be represented by a relationship having the f o r m : k . . . . . 11 = kdry + c . kvap the simultaneous solution o f this at two temperatures provides a figure for kdry of 0'11 W m - t K - ~ , which is close to the measured value, and a coefficient, c, of 0.085. This hypothesis provides an insight into the observed anomalous conduction in a range o f temperatures that is of practical interest. At temperatures below the freezing point and above the boiling point of water, such effects may be expected to disappear. CONCLUSIONS The results show that pulverised-fuel ash cenospheres have insdlating properties comparable with other commonly-used insulating materials in the 'good,' although not the 'best' category. However, they possess a load-bearing capability far superior to most lightweight insulants, and being an inert organic material the performance should not deteriorate following installation. Cenospheres in the smallest size range provide a better insulation than do larger sizes, but even an ungraded material has an attractively low therma ! conductivity, particularly when it is realised that it is at present regarded as a waste product. A further advantage is the fact that the material is not flammable, and indeed may be considered to be fire resistant. However, it does transmit water and water vapour readily. Thus it is desirable to use cenospheres in sealed packets for insulation applications at ambient temperatures. ACKNOWLEDGEMENTS The authors wish to thank Messrs Fillite (Runcorn) Ltd who provided the samples of cenospheres tested and the Electricity Council and the Building Research Establishment who encouraged the undertaking o f this project.
REFERENCES
1. E. RAASK, Cenospberes in pulverised fuel-ash, J. Inst. Fuel (September, 1969) pp. 339-44. 2. B. H. Vos, Measurements of thermal conductivity by a non-steady state method, Applied Science Research (Sect. A) 5 (1955) pp. 425-38. 3. PF,4 utilisation, CEGB, 1972. 4. 'A. W. PRATT, Heat transmission in low conductivity materials. In: Thermal conductivity (Ed. R. P. Tye), Academic Press, 1969. 5. O. KRISCHERand. H. EDSORN,Forsch. Geb. Ingwes, 22 0956) pp. 1-8.
M. A. JEWAD,I. E. SMITH and S. D. PROBERT
Centre for Thermal Insulation Studies, School of Mechanical Engineering, Cranfield Institute of Technology, Beds (Great Britain)
SUMMAR Y
The thermal conductivity of pulverised-fuel ash cenospheres has been determined using both a transient and a steady-state method. The effect of mechanical loading has been investigated, and the results show that the application of considerable loads is without appreciable effect on the insulating properties. Over the range 25 to 80°C, the presence of moisture adsorbed from the atmosphere gave rise to an increase of the effective thermal conductivity of the material, and this should be taken into account i f cenospheres are to be used for thermal insulation.
INTRODUCTION
Powdered materials combine the virtues of low density, low thermal conductivity, can readily fill the available space, irrespective of shape, and provide some mechanical support for the system. Hence they are often used as thermal insulants. Most powders, however, possess a poor resistance to compaction, and will not support large loads without permanent deformation. In addition, deformation leads to a reduction in the intergranular air spaces within the powder, which thereby becomes a less effective insulator. Furthermore, powders are susceptible to the absorption and transmission of water by capillary action, so leading to a considerable reduction in insulation performance. These problems have militated against the wider adoption of powder insulants. The poor load-bearing characteristics occur because most powders are irregularly shaped and inter-particle contact ensues via their surface asperities. As these deform or collapse, the crushing strength progressively increases. For a semi-mobile powder the load imposed may be sufficient to cause a gradual decrease of occupied volume with time, particularly if vibration is present, and the insulating value will slowly deteriorate. 67 Applied Energy (2) (1976)----O Applied Science Publishers Ltd, England, 1976 Printed in Great Britain
M. A. J E W A D , I. E. S M I T H , S. D . P R O B E R T
68
The second factor which may lead to a deterioration in powder insulant performance is moisture absorption, whereby the normally air-filled interstices become water-logged, and transmit heat by conduction. Even if complete water-logging does not occur, the presence o f water and water v a p o u r in small cavities can give rise to an evaporation/condensation mode of heat transfer (as in a heat pipe), for liquid pressures may be so reduced by surface tension forces acting over minute radii that local boiling ensues at temperatures well below 100°C. A material which has attracted the attention o f the present investigators as a high load-bearing thermal insulant is pulverised-fuel ash cenospheres 1 (see Table 1). TABLE 1 CHEMICAL COMPOSITION OF THE TESTED CENOSPHERES
Constituent
Per cent by weight
Silica (as SiO2) Alumina (as A1203) Alkali (as Na2. K20) Iron Oxide (as Fe203)
55 to 61 26 to 30 0.5 to 4 4
This material (which comprises about 4 per cent by weight of the total ash from pulverised-coal burning furnaces) consists o f hollow glass-like spheres which range in size from 20 to 300 g m with wall thicknesses approximately one-tenth of their diameters. Being spheres, their load-bearing characteristics are excellent, and, furthermore, they are hollow, with a high proportion o f their volume being occupied by relatively low-conductivity gas. A further incentive to their study is that approximately 2 × 10 6 tonnes per a n n u m are produced in the U K as waste from coalburning power stations, thereby creating a disposal problem. The cenospheres
The bulk density of the samples tested was determined to an accuracy o f 3 per cent by placing 100 g in a measuring cylinder and applying gentle vibration until a constant level was reached. The apparent porosity ( = 1 - Pbulk/Papp.... t) is calculated and also presented in Table 2. TABLE 2 BULK PROPERTIES OF THE TESTED SAMPLES
Sample designation
Nominal particle diameter
(/tm)
(kg m 3)
52/7 100/7 200/7 300/4
82 58 51 36
424 384 336 261
Bulk density
Relative density
Apparent porosity (per cent)
0'7 0.7 0.7 0'4
0"395 0.450 0"520 0.348
* The first number, i.e. 52 ; 100; 200 and 300 refers to the mesh size of powder particles; the second, e.g. 7, refers to apparent relative density.
CONDUCTIVITY OF PULVERISED-FUEL ASH CENOSPHERES
69
The transport properties of a powder are determined not only by the mean particle size but also by the size distribution, and so both were measured for each of the four samples tested with a Quantimet 720 Image Analysing Computer (see Table 3). TABLE 3 PARTICLE SIZE DISTRIBUTION T h e n u m b e r s in the tables represent the percentage o f the particles between the stated size a n d the next lower stated size. Each o f these is the average of three readings taken on different specimens o f the s a m e sample
•
(Particle diameter
Sample
"x~
,um)
34
50
65
85
100
120
135
150
170
Mean diameter 185 of particles (am)
52/7 100/7 200/7
17 38 37
17 25 29
16 16 18
15 9 9
12 6 5
9 2 1 "5
6 2 0.5
3"5 1 0
2 0 0
0'5 0 0
Particle diameter m)
Sample 300/4
18
27
36
45
54
63
72
81
90
99
~
82 58 51
Mean diameter of particles (am)
26
21
17
14
11
6
3
1 "5
0"5
0
36
Prior to the determination of the thermal conductivity no attempt was made to dry the powder which, after a long period of standing, was assumed to have a moisture Content in equilibrium with that of the laboratory where the relative humidity averaged 55 per cent during the period. This was deliberate since, in many installations where insulation is applied, the material has free contact with the atmosphere.
EXPERIMENTAL METHODS
The thermal conductivity of the packed cenospheres was measured by a flat-plate steady-state method and a transient probe method. The steady-state method employed a modification of the basic double-guard flat plate technique as specified in BS 874 and ASTM C 177. The hot surface of the apparatus (see Fig. l) was heated electrically and the cold surface was cooled by a steady flow of water through a manifold designed to maintain, as far as possible, a uniform temperature over the surface. Suitable controllers kept the guard ring heaters isothermal with respect to the main heater, and both hot and cold surfaces were blackened so as to have high
70
M. A. JEWAD, I. E. SM1TH~ S. D. PROBERT
emissivities and absorptivities. The load was applied directly to the upper surface of the heater, and the whole system was then gently vibrated in order to ensure that the loading was uniform throughout the specimen. The transient method employed has been described by Vos 2 and depends upon the temperature change that occurs at a line source of heat immersed in an infinite body of the material. After an initial perturbation period, the rate of rise in the centreline temperature is related directly to the thermal conductivity of the surrounding body and is independent of any other properties. The effective thermal c o n d u c t i v i t y kef f of the material equals: O--. ~d(ln t)'] 4re \ dT ] where O' is the rate of heat generation per unit length of the line source, t is the time and T the probe temperature.
LOAD
1
,\ \
\
\
SECONDARY ~GUARD HEATER
\
J
~ INSULATION ~ / / MAINHEATER
/// WATER
WATER
:
30 CM Fig. 1.
~]
Schematic diagram of steady-state apparatus.
A stainless steel probe 5 mm in diameter and 30 cm long carried within it a Nichrome heater and a chromel/alumel thermocouple (see Fig. 2). A thin ceramic insulator provided the electrical insulation. Because its effective thermal conductivity was much greater than that of the thermal insulant being studied, the temperature gradients within the probe itself could be considered negligible. In use this probe was inserted along the axis of a cylinder, 8 cm in diameter and
CONDUCTIVITY OF PULVER1SED-FUEL ASH CENOSPHERES
71
HEATER THERMOCOUPLE
LEADS
LEADS - m
L
----.-----------STEEL SHEATH ~HEATER
30 CM
~THERMOCOUPLE .........-.-.----.---CERAMIC FILLING
Fig. 2.
Schematic d i a g r a m of the thermal probe.
32 cm long, filled with the powder. In order to determine the local temperature of the powder and also, more importantly, to detect any temperature rise at the periphery of the sample, which would invalidate the infinite body assumption, a second thermocouple was immersed in the packed cenospheres near the outside of the vessel. Figure 3 depicts a typical temperature-time curve showing a linear relationship between the temperature and the logarithm of the time up to about 300 sec. Since the temperature rise on the centreline over this period of time was 32°C, this indicates that the thermal conductivity remained sensibly constant over this temperature range.
EXPERIMENTAL RESULTS
The packed cenospheres did not begin to yield until an applied load attained a value between 0.8 and 1.6 M N m - 2 according to the particular sample under t e s t a remarkably high figure when compared with most powdered materials. Although the absence of compaction would lead to the inference that the conductivity would not change appreciably, it was felt that only by carrying out measurements under load could this be established beyond doubt.
72
M. A. J E W A D , I. E. SMITH, S. D. PROBERT
j/ /X
120
/
/
X
/x
/" X
/
110
/
O I.U
/
I-rv.
ILl OILl t'--
/
i00
X
X
X
X
90
I
i0
i00
!
I000
TIME (SEC) FROM COMMENCEMENTOF HEATING Fig. 3. Typical time-temperature results used to determine kerr by transient method: sample 52/7. The conductivity under conditions o f mechanical loading was determined with the steady-state apparatus, and two loadings were selected, uiz: 0.034 and 0.25 M N m -2. The higher figure corresponds to the load that would arise under a height o f 60 m o f the powder, a figure which it was considered unlikely would be exceeded in practice. From inspection of the experimental results (Figs. 4 to 7), it is evident that: (a) the applied load does have a small effect on the conductivity and that this effect
CONDUCTIVITY
OF P U L V E R I S E D - F U E L
73
ASH CENOSPHERES
0,18 THERMAL CONDUCTI V I TY IV m-1 K- I
X
/
0,16
0,14
0,12
/ ,1
I
20
I
I
i
I
30
40
50
60
I
i
70
8O
TEMPERATURE Oc Fig. 4. Experimentaldata:sample52/7. becomes more apparent the smaller the mean size of the cenospheres, and (b) there appears to be no difference, as expected, between the results obtained by the 'low load' steady-state and the transient method, in both cases the load being due solely to the weight of the cenospheres in the test. The results agree qualitatively with those previously published by CEGB, 3 although the particle size range was unspecified in that investigation. The particle size does influence the thermal conductivity, in that the sample having the smallest average sized particles (300/4) showed the least conductivity, as might be expected. However, for the four samples, the conductivity appears to correlate well with the bulk density which, again, is not surprising. The effect of
M. A. J E W A D , 1. E. SMITH, S. D. PROBERT
74
relative density upon the conductivity is quite small, as may be seen by comparing Figs. 4 to 7 with Table 2. From these observations a fact emerges which requires further explanation, and that is the effect of the mean temperature upon the thermal conductivity. The results show that the temperature coefficient ranges from 1"2 to 1.8 per cent °C-1 at a temperature of 30°C, which is an order of magnitude greater than that recorded in
0,16
THERMAL CONDUCTIVITY W m-IK - I
0.14
0.04 MN m"2
0,12
O
0.25
X
TRANSIENT
MN m"2
0.1
X I
I
I
I
I
20
30
40
50
60
I
70
I
80
TEMPERATURE Oc, Fig. 5. Experimental data: sample 100/7. the literature* for powder insulants even under cryogenic conditions, and is well beyond the temperature variation for the conductivity of the material from which the cenospheres are formed. In seeking an explanation for this anomalous behaviour, the effect of humidity was considered, for, as has been mentioned, no particular effort was made to dry the samples prior to the conductivity measurements. The tests were therefore repeated for the 52/7 sample using the transient
CONDUCTIVITY
OF PULVERISED-FUEL
75
ASH CENOSPHERES
method with material which had been carefully dried in an oven. The results, whilst agreeing with those shown in Fig. 4 at 25°C, exhibited a temperature coefficient of only 0.03 per cent per degree. The significance of this finding is that although humidity exerts only a slight influence on the effective conductivity at normal ambient temperatures (say, 25°C and below), at higher temperatures
0.16 THERMAL CONDUCTIVITY W m- I
K- I
0,14
0.12
0,i0
/ 0.08
I
20
MN
-2
•
0,04
o
0,25 MN m
x
TRANSIENT
m -2
X I
i
i
J
,
30
40
50
60
70
80
TEMPERATURE Oc Fig. 6. Experimentaldata:sample200/7. it does lead to an increase in the conductivity which should be taken into account if this type of insulation is to be contemplated. Since the thermal conductivity of water vapour is not significantly different from that of air, there must be a more complex mechanism occurring in order to account
M. A. J E W A D , I. E. S M I T H , S. D. P R O B E R T
76
for the a n o m a l o u s l y high rate o f heat transfer across the interstices between the p a c k e d cenospheres, as shown by the experimental results. One possibility is that o w i n g to the presence o f a t e m p e r a t u r e g r a d i e n t t h r o u g h the material an e v a p o r a t i o n / c o n d e n s a t i o n process occurs, the return o f condensed moisture to the e v a p o r a t i o n zone arising t h r o u g h the influence o f capillary action.
0,14
THERMAL CONDUCTIVITY W m- z
K-z
0,12
/
0,i0
•
0,04
O
0,25 MN m
x
TRANSIENT
MN m -2 -2
0,08
/ [
20
X [
30
[
I
I
I
40
50
60
70
TEMPERATURE
I
80
OC
Fig. 7. Experimentaldata:sample300/4. H e a t transfer arising from v a p o u r diffusion has been shown s to lead to an effective conductivity : D kvap = R--'T
P
L
-' dT
where D is the diffusion coefficient, Pv the v a p o u r pressure o f water a n d L the
CONDUCTIVITY OF PULVERISED-FUELASH CENOSPHERES
77
latent heat of vaporisation. Insertion of appropriate values into this equation gives v a p o u r conductivities o f 0.20 W m - 1 K - 1 at 40°C, and 0"62 W m - 1K - 1 at 65°C. If the overall conductivity o f the packed cenospheres can be represented by a relationship having the f o r m : k . . . . . 11 = kdry + c . kvap the simultaneous solution o f this at two temperatures provides a figure for kdry of 0'11 W m - t K - ~ , which is close to the measured value, and a coefficient, c, of 0.085. This hypothesis provides an insight into the observed anomalous conduction in a range o f temperatures that is of practical interest. At temperatures below the freezing point and above the boiling point of water, such effects may be expected to disappear. CONCLUSIONS The results show that pulverised-fuel ash cenospheres have insdlating properties comparable with other commonly-used insulating materials in the 'good,' although not the 'best' category. However, they possess a load-bearing capability far superior to most lightweight insulants, and being an inert organic material the performance should not deteriorate following installation. Cenospheres in the smallest size range provide a better insulation than do larger sizes, but even an ungraded material has an attractively low therma ! conductivity, particularly when it is realised that it is at present regarded as a waste product. A further advantage is the fact that the material is not flammable, and indeed may be considered to be fire resistant. However, it does transmit water and water vapour readily. Thus it is desirable to use cenospheres in sealed packets for insulation applications at ambient temperatures. ACKNOWLEDGEMENTS The authors wish to thank Messrs Fillite (Runcorn) Ltd who provided the samples of cenospheres tested and the Electricity Council and the Building Research Establishment who encouraged the undertaking o f this project.
REFERENCES
1. E. RAASK, Cenospberes in pulverised fuel-ash, J. Inst. Fuel (September, 1969) pp. 339-44. 2. B. H. Vos, Measurements of thermal conductivity by a non-steady state method, Applied Science Research (Sect. A) 5 (1955) pp. 425-38. 3. PF,4 utilisation, CEGB, 1972. 4. 'A. W. PRATT, Heat transmission in low conductivity materials. In: Thermal conductivity (Ed. R. P. Tye), Academic Press, 1969. 5. O. KRISCHERand. H. EDSORN,Forsch. Geb. Ingwes, 22 0956) pp. 1-8.