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Design and characterization of an apparatus for agglomeration testing during flash pyrolysis in entrained flow reactors
Design and characterization of an apparatus for agglomeration testing during flash pyrolysis in entrained flow reactors
David J. McCarthy CSI RO Division of Mineral Engineering, P.O. Box 3 12, Clayton, Victoria 3 168, Australia (Received 16 November 1979)
A small scale test apparatus is developed to measure the relative tendencies of different coals to agglomerate in dilute phase reactors with entrained flow and under conditions of flash pyrolysis. The important variables influencing agglomeration for a given coal are its feed rate and the temperature of pyrolysis. Changes in the method for collecting and measuring agglomerate were not significant at feed rates of dry coal greater than 2.5 g min-‘.
Agglomeration of coal particles is a problem in operating transport line pyrolysers with inert inlet gas 1-5, and thus a need exists for a simple small-scale apparatus to be used for measuring the relative tendencies of coals to agglomerate during flash pyrolysis in these types of reactors. The development of such an apparatus and its use for investigating methods for reducing or even preventing the agglomeration of coals either prior to or during flash pyrolysis is described. Three mechanisms can contribute to an increase of solids size in the type of pyrolyser considered here: (1) dilation of individual particles; (2) aggregation of particles due to collisions; and (3) deposition of particles as lumps on the pyrolyser wall. Dilation is characteristic of pyrolysis and in isolation does not contribute to agglomeration. Aggregation will occur even under laminar flow of polydisperse suspensions owing to variation of slip velocity with particle size6. However, such aggregation, which will not lead to reactor blockage in suitably designed reactors, can also be reduced by dilution of feed coal with char as in the ‘Oxy’ flash pyrolysis process’. The results of approximate calculations of particlewall collisions, based on the method of Hutchinson et al.‘, indicated that extensive deposition of particles would occur on the walls of the entry section of flash pyrolysers in which local turbulence is used to mix the coal particles with either a heat carrier or a transport gas. This prediction was confirmed by observations made on cold transparent models of such mixers and from measurements using a 100 mm diameter pyrolyserg. Consequently, it was decided that the test apparatus should be constructed to give a high probability for collision between the coal particles and heated internal surfaces of the pyrolyser. The test apparatus did not have the complication of using gaseous or solid heat carriers to supply process heat; instead the heat was supplied from sensible heat stored in the apparatus itself, however, such a design limits both the flow rates and the quantities of coal 0016-2361/80/080563%04$02.00 @ 1980 IPC Business Press
and gas which can be used while avoiding significant changes in temperature during a test. The limits for the apparatus were determined experimentally, and are described later. APPARATUS One of the five parallel sets of the apparatus is illustrated in Figure 1. The five pyrolyser tubes were mounted in a single stainless-steel heating block. The helix in each pyrolyser tube ensured that a high target efficiency for particles was obtained at internal surfaces. The helix also enhanced the heat transfer rate to the fluid phase. The feed rate of particulate coal was controlled by the intensity of vibration of the feeder tube and the opening of the discharge valve. EXPERIMENTAL Preliminary
non-pyrolytic
tests
A tube temperature of 550°C was used for these tests in which sand, sized to ~250 pm, was used as a clean feed. Charges of up to 8 g of sand, fed at rates up to 8 g min-’ were employed. The mean heating rate of the sand to a final temperature greater than 530°C after separation from the gas, was in excess of 500°C s-l. in all cases. This was assumed to be a sufficiently high mean-heating-rate to assume that flash pyrolysis conditions had been achieved in the apparatus. A decrease in temperature of 4°C was measured by a fine thermocouple embedded in the helix during tests in which 4 g of sand was fed at 8 g min- ‘. The magnitude of the temperature decrease was approximately proportional to the quantity of sand fed and its feed rate. As discussed later, the results indicate that this level of temperature decrease would be acceptable, and so the quantity of coal used in a test was limited to 4 g, with a feed rate always less than 8 g min-’
FUEL,
1980,
Vol
59, August
563
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy
the fraction of dry coal fed which was collected as agglomerate.
Vi brated feeder tube-glass \
RESULTS Insulation
Heating block 25/20 S.S.
The results of tests in which the quantity of Liddell coal fed, the feed rate of nitrogen and the helix pitch were varied are summarized in Figures 2, 3 and 4. The effect of varying the pyrolysis temperature at essentially constant coal feed rate, and in a feed-rate range where the effect of this variable is small, is shown in Figure 5.
lnsulat
ion
-Pyrolysis at measured
0T
Temperature point
t
measuring
tube
19mm o.d ‘lnconel ’
rate
Results obtained from runs in which all the coals listed in Tables 1 and 2 were tested, are shown in Figure 6 which also includes results for a set of four runs in which the 120 pm screen was not installed in the pyrolysis tube.
To catchpot and vent
DISCUSSION /
_
Collectively, the results in Figures 24 indicate that for coal of a fixed size distribution, pyrolysed at constant temperature, the only significant variable is the coal feed rate. Figure 5 shows that the degree of agglomeration is also dependent on the temperature of pyrolysis, and that when the pyrolysis temperature is in the region 500-550°C the slope of the curve is such
19mm tube
Block I
\_ Figure
1
Schematic
__A-, Tab/e 2 Size distribution
diagram of apparatus
for agglomeration
testing Wt % for coals noted Size range Liddell
Millmerran
Loy Yang
0 47.6 37.6 14.8
0 15.6 41.8 42.6
0 32.3 50.9 16.8
(crm) Table 1 Coal analysis Proximate analysis (wt % db)
Coal
Moisture (wt %as used)
Ash
VM
Fixed C
Crucible swelling numberlo
Liddell Millmerran Loy Yang
3.0 5.9 8.2
8.7 19.4 0.9
35.6 42.0 49.2
55.7 38.6 49.9
2-3 1 N.A.
N.A.:
Not applicable
-non
>106 75-l 53<
06 75 53
r
caking brown coal
L
Operating procedure -
pyrolysis runs
The proximate analyses and size distributions of the coals are as listed in Tables 1 and 2. The apparatus was heated to the test temperature with nitrogen flowing at the desired rate. When the desired temperature was reached and constant, a weighed sample of coal was placed in the feeder tube and the tube closed. The test was started by simultaneously opening the coal discharge valve, and starting the vibrator and clock. The coal feed line was vibrated during the coal feed and for approximately 10 s after the feed tube was emptied. The clock was stopped when the feed tube emptied. The pryolysis tube was withdrawn from the block and cooled by the flowing nitrogen. The char trapped above the 120 pm screen was carefully removed onto a sieve having an opening of 160 pm and gently hand screened. The char retained on the 160 pm screen was denoted as agglomerate. The results were expressed as
564
FUEL,
1980,
Vol 59, August
J
0' 0
I
I
I
I
I
1.0
2-o
3.0
4.0
5.0
Coal
feed
rate
fg
dry
I 6.0
coallmin)
Effects of coal feed rate and quantity of coal used in a Figure 2 test on the measured degree of agglomeration of the Liddell coal. Temperature, 600-615’C; helix pitch, 11.2 mm; transport gas rate, 1.87 I min-t at s.t.p.; 0, 1.9 g dry coal fed; A, 3.9 g dry coal fed
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy 0.6
were not critical to the significance of the results for the strongly agglomerating Liddell coal. However, for the moderately agglomerating Milmerran coal, some of the char collected as agglomerate in the test apparatus was found on the 120 pm screen in the pyrolysis tube. Obviously if such a mechanism accounted for essentially all the ‘agglomerate’ measured, the measured results would have little significance in the context of flash pyrolysis. Because of this possibility, four tests were done using Millmerran coal at the extreme condition in which the
0.6
1.0 Coal
Figure3 maration coal fed; val from at s.t.p.;
2-o feed
3.0
rate
(g
b.0 dry
5.0
6.0
1
coal/min
0.L
Effect of gas flow rate on the measured degree of aggioof the Liddell coal. Temperature, 600-620°C; 1.9 g dry helix pitch, 11.2 mm. Shaded area is the confidence interFigure 2 at a gas rate of 1.87 I min-t at s.t.p. 0,2.8 I min-’ 0, 3.74 I min-’ at s.t.p.
I
:pL
0.5
0
I
d
I
0 <) Q
0.3
0.2
0.6 0.1
0.5
I
0
1 500
I
I
I
550
600
650
Pyrolysis
0.L
-I 700
( "C 1
temperature
Figure 5 Effect of pyrolysis temperature on the measured degree of agglomeration of the Liddell coal. Dry coal fed 1.9 g; coal feed rate, 2.2-3.0 g min-‘; helix pitch 11.3 mm; transport gas rate, 1.87 I min-’ at s.t.p.
0.3
0.2
0.6
I
I
I
I
I
0.1
0.5 C
0
I
I
I
I
I
1.0
2.0
3.0
L.0
5.0
Coal
feed
rate
(g
dry
coal/min
6.0
0.4 from
)
Figure 4 Effect of helix pitch on the measured degree of agglomeration of the Liddell coal. Temperature, 600-62O’C; 1.9 g dry coal fed; transport gas rate, 1.87 I mine1 at s.t.p. Shaded area is the confidence interval from Figure 2 for a helix pitch of 11.2 mm. 0, helix pitch 7.6 mm; 0, helix pitch 22.4 mm
that temperature variations greater than 5°C have a significant effect. The quantities and flow rates of coal were chosen such that, when based on the evidence of the tests using sand, the temperature decrease during pyrolysis runs was always less than 5°C. The same evidence indicates that the data in Figure 5 were obtained with temperature decreases < 1°C. When Liddell coal was fed at rates greater than 2.5 g min-’ more than 90 wt % of the char formed by pyrolysis stuck to the helix and the pyrolyser tube in clusters with a size > 1 mm. Thus, the screen sizes
Figure
2
0.3
/ 0.2
0.1
0
. 0
la-l
01
1.0 Coal
II
2.0 feed
Figure 6 Measured degrees functions of coal feed rate. 11.2 mm; transport gas rate, 1.9-2.0 g A, Millmerran; 0, pyrolysis tube; 0, Loy Yang
3.0 rate
Ig
dry
I
I
A.0
5.0
coallmin
6.0
I
of agglomeration of three coals as Temperature, 600-620°C; helix pitch, 1.87 I min-’ at s.t.p.; dry coal fed, Millmerran without 120 pm screen in
FUEL,
1980,
Vol. 59, August
565
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy Tab/e 3
Comparison
of agglomeration Fraction
data from two pyrolysers
of dry coal collected
as agglomerate
Coal
100 mm pyrolyser9
Test apparatus
Liddel I Millmerran Lay Yang
0.50 0.21 0.02
0.50 0.15
a Agglomerate
was detected
from fig. 6
but was too small to measure
120 pm screen was removed. The results of these tests are in Figure 6 from which it will be noted that the effect of the screen is small for the higher coal flow rates. Complete screen analyses of chars from Millmerran coal were not made because the agglomerates were too soft. However, more than 50 wt y0 of the char had a size greater than 200 Frn. Again the form of the results and their significance would remain if the screen sizes had been changed over a relatively wide range. Agglomeration data were also measured for the coals described in Table 1 in a series of runs using a 100 mm diameter, entrained flow pyrolyser in which hot nitrogen was both the heat carrier and the major transport medium. In its fully developed state the flow was laminar, with mixing of the feed coal and heat carrier occurring in a conical inlet section. Agglomerate was defined in this case as all the char from the feed coal which was not transported pneumatically from the reactor’. A comparison of the data from the 100 mm pyrolyser with those from the test apparatus is shown in Table 3. The coal flow rate was constant for the runs in each pyrolyser but differed between pyrolysers. The flow rate chosen for comparative purposes was such that the fraction of Liddell coal denoted as agglomerate was the same from both reactors. The relative values of the measured extent of agglomeration of the coals were found to be similar in the test apparatus to those in the 100 mm pyrolyser. The differences in the results obtained for the three coals show that coal properties as well as the physical environment in a flash pyrolyser will contribute to the
566
FUEL, 1980, Vol 59, August
rate at which agglomerates form. This has been realised for many years and has been the basis for the pretreatment of agglomerating coals before gasification (see for example Forney et al.” and Curran et ~1.‘~). Use of the test apparatus to study the effects of char diuliton on the agglomeration is straightforward in principle. However, the limitations on the flow rate and the total quantity of solids may give a situation at high dilution ratios in which the relative error in the data is unacceptable owing to the small quantity of coal in the diluted feed. ACKNOWLEDGEMENT The author wishes to thank Dr 0. Sitnai for helpful discussions on this work as well as P. A. Casamento and H. F. Molony for their assistance with the experiments. REFERENCES 1 2
Sass, A., Finney, C., McCarthy, H. and Kaufman, P. US Par. 3,1X,233, May 29, 1913 Yavorski, P. M. Proceedings of Symposium on Clean Fuels from Coal, Institute of Gas Technology, September, 1973, pp. 209-218 Jones, J. F., Schmid, M. R. and Eddinger, R. T. Chem. Eng. Prog. 1964, 60, 69 Friedmand, L. D., Rau, E. and Eddinger, R. T. Fuel 1968,47, 149 Landers, W. S., Wagner, E. O., Games, H., Boley, C. C. and Goodman, J. B. US Bur. Mines, Report of Invest. 6608, 1965 Boothroyd, R. G. ‘Flowing Gas-Solids Suspensions’ Chapman and Hall, London, 1971 Durai-Swamy, K., Che, S. C., Green, N. W., Knell, E. W. and Zahrodvick, R. L. Am. Gem. Sot., Div. Fuel Chem. Preprints 1979,24, 177 Hutchinson, P., Hewitt, G. F. and Dukler, A. E. Chem. Eng. Sci. 1971, 26, 419 Sitnai, 0. and McCarthy, D. J. Unpublished results Joint Coal Board and Queensland Coal Board ‘Australian Black Coals’, September 1976 Forney, A. J., Kenny, R. F., Gasior, S. .I. and Field, J. U. Ind. Eng. Chem., Product Rex Dev. 1964, 3,48 Curran, G. P., Pasek, B., Pell, M. and Govin, E. Am. Chem. Sot. Dia. Fuel Chem. 1973, 18, 30
David J. McCarthy CSI RO Division of Mineral Engineering, P.O. Box 3 12, Clayton, Victoria 3 168, Australia (Received 16 November 1979)
A small scale test apparatus is developed to measure the relative tendencies of different coals to agglomerate in dilute phase reactors with entrained flow and under conditions of flash pyrolysis. The important variables influencing agglomeration for a given coal are its feed rate and the temperature of pyrolysis. Changes in the method for collecting and measuring agglomerate were not significant at feed rates of dry coal greater than 2.5 g min-‘.
Agglomeration of coal particles is a problem in operating transport line pyrolysers with inert inlet gas 1-5, and thus a need exists for a simple small-scale apparatus to be used for measuring the relative tendencies of coals to agglomerate during flash pyrolysis in these types of reactors. The development of such an apparatus and its use for investigating methods for reducing or even preventing the agglomeration of coals either prior to or during flash pyrolysis is described. Three mechanisms can contribute to an increase of solids size in the type of pyrolyser considered here: (1) dilation of individual particles; (2) aggregation of particles due to collisions; and (3) deposition of particles as lumps on the pyrolyser wall. Dilation is characteristic of pyrolysis and in isolation does not contribute to agglomeration. Aggregation will occur even under laminar flow of polydisperse suspensions owing to variation of slip velocity with particle size6. However, such aggregation, which will not lead to reactor blockage in suitably designed reactors, can also be reduced by dilution of feed coal with char as in the ‘Oxy’ flash pyrolysis process’. The results of approximate calculations of particlewall collisions, based on the method of Hutchinson et al.‘, indicated that extensive deposition of particles would occur on the walls of the entry section of flash pyrolysers in which local turbulence is used to mix the coal particles with either a heat carrier or a transport gas. This prediction was confirmed by observations made on cold transparent models of such mixers and from measurements using a 100 mm diameter pyrolyserg. Consequently, it was decided that the test apparatus should be constructed to give a high probability for collision between the coal particles and heated internal surfaces of the pyrolyser. The test apparatus did not have the complication of using gaseous or solid heat carriers to supply process heat; instead the heat was supplied from sensible heat stored in the apparatus itself, however, such a design limits both the flow rates and the quantities of coal 0016-2361/80/080563%04$02.00 @ 1980 IPC Business Press
and gas which can be used while avoiding significant changes in temperature during a test. The limits for the apparatus were determined experimentally, and are described later. APPARATUS One of the five parallel sets of the apparatus is illustrated in Figure 1. The five pyrolyser tubes were mounted in a single stainless-steel heating block. The helix in each pyrolyser tube ensured that a high target efficiency for particles was obtained at internal surfaces. The helix also enhanced the heat transfer rate to the fluid phase. The feed rate of particulate coal was controlled by the intensity of vibration of the feeder tube and the opening of the discharge valve. EXPERIMENTAL Preliminary
non-pyrolytic
tests
A tube temperature of 550°C was used for these tests in which sand, sized to ~250 pm, was used as a clean feed. Charges of up to 8 g of sand, fed at rates up to 8 g min-’ were employed. The mean heating rate of the sand to a final temperature greater than 530°C after separation from the gas, was in excess of 500°C s-l. in all cases. This was assumed to be a sufficiently high mean-heating-rate to assume that flash pyrolysis conditions had been achieved in the apparatus. A decrease in temperature of 4°C was measured by a fine thermocouple embedded in the helix during tests in which 4 g of sand was fed at 8 g min- ‘. The magnitude of the temperature decrease was approximately proportional to the quantity of sand fed and its feed rate. As discussed later, the results indicate that this level of temperature decrease would be acceptable, and so the quantity of coal used in a test was limited to 4 g, with a feed rate always less than 8 g min-’
FUEL,
1980,
Vol
59, August
563
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy
the fraction of dry coal fed which was collected as agglomerate.
Vi brated feeder tube-glass \
RESULTS Insulation
Heating block 25/20 S.S.
The results of tests in which the quantity of Liddell coal fed, the feed rate of nitrogen and the helix pitch were varied are summarized in Figures 2, 3 and 4. The effect of varying the pyrolysis temperature at essentially constant coal feed rate, and in a feed-rate range where the effect of this variable is small, is shown in Figure 5.
lnsulat
ion
-Pyrolysis at measured
0T
Temperature point
t
measuring
tube
19mm o.d ‘lnconel ’
rate
Results obtained from runs in which all the coals listed in Tables 1 and 2 were tested, are shown in Figure 6 which also includes results for a set of four runs in which the 120 pm screen was not installed in the pyrolysis tube.
To catchpot and vent
DISCUSSION /
_
Collectively, the results in Figures 24 indicate that for coal of a fixed size distribution, pyrolysed at constant temperature, the only significant variable is the coal feed rate. Figure 5 shows that the degree of agglomeration is also dependent on the temperature of pyrolysis, and that when the pyrolysis temperature is in the region 500-550°C the slope of the curve is such
19mm tube
Block I
\_ Figure
1
Schematic
__A-, Tab/e 2 Size distribution
diagram of apparatus
for agglomeration
testing Wt % for coals noted Size range Liddell
Millmerran
Loy Yang
0 47.6 37.6 14.8
0 15.6 41.8 42.6
0 32.3 50.9 16.8
(crm) Table 1 Coal analysis Proximate analysis (wt % db)
Coal
Moisture (wt %as used)
Ash
VM
Fixed C
Crucible swelling numberlo
Liddell Millmerran Loy Yang
3.0 5.9 8.2
8.7 19.4 0.9
35.6 42.0 49.2
55.7 38.6 49.9
2-3 1 N.A.
N.A.:
Not applicable
-non
>106 75-l 53<
06 75 53
r
caking brown coal
L
Operating procedure -
pyrolysis runs
The proximate analyses and size distributions of the coals are as listed in Tables 1 and 2. The apparatus was heated to the test temperature with nitrogen flowing at the desired rate. When the desired temperature was reached and constant, a weighed sample of coal was placed in the feeder tube and the tube closed. The test was started by simultaneously opening the coal discharge valve, and starting the vibrator and clock. The coal feed line was vibrated during the coal feed and for approximately 10 s after the feed tube was emptied. The clock was stopped when the feed tube emptied. The pryolysis tube was withdrawn from the block and cooled by the flowing nitrogen. The char trapped above the 120 pm screen was carefully removed onto a sieve having an opening of 160 pm and gently hand screened. The char retained on the 160 pm screen was denoted as agglomerate. The results were expressed as
564
FUEL,
1980,
Vol 59, August
J
0' 0
I
I
I
I
I
1.0
2-o
3.0
4.0
5.0
Coal
feed
rate
fg
dry
I 6.0
coallmin)
Effects of coal feed rate and quantity of coal used in a Figure 2 test on the measured degree of agglomeration of the Liddell coal. Temperature, 600-615’C; helix pitch, 11.2 mm; transport gas rate, 1.87 I min-t at s.t.p.; 0, 1.9 g dry coal fed; A, 3.9 g dry coal fed
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy 0.6
were not critical to the significance of the results for the strongly agglomerating Liddell coal. However, for the moderately agglomerating Milmerran coal, some of the char collected as agglomerate in the test apparatus was found on the 120 pm screen in the pyrolysis tube. Obviously if such a mechanism accounted for essentially all the ‘agglomerate’ measured, the measured results would have little significance in the context of flash pyrolysis. Because of this possibility, four tests were done using Millmerran coal at the extreme condition in which the
0.6
1.0 Coal
Figure3 maration coal fed; val from at s.t.p.;
2-o feed
3.0
rate
(g
b.0 dry
5.0
6.0
1
coal/min
0.L
Effect of gas flow rate on the measured degree of aggioof the Liddell coal. Temperature, 600-620°C; 1.9 g dry helix pitch, 11.2 mm. Shaded area is the confidence interFigure 2 at a gas rate of 1.87 I min-t at s.t.p. 0,2.8 I min-’ 0, 3.74 I min-’ at s.t.p.
I
:pL
0.5
0
I
d
I
0 <) Q
0.3
0.2
0.6 0.1
0.5
I
0
1 500
I
I
I
550
600
650
Pyrolysis
0.L
-I 700
( "C 1
temperature
Figure 5 Effect of pyrolysis temperature on the measured degree of agglomeration of the Liddell coal. Dry coal fed 1.9 g; coal feed rate, 2.2-3.0 g min-‘; helix pitch 11.3 mm; transport gas rate, 1.87 I min-’ at s.t.p.
0.3
0.2
0.6
I
I
I
I
I
0.1
0.5 C
0
I
I
I
I
I
1.0
2.0
3.0
L.0
5.0
Coal
feed
rate
(g
dry
coal/min
6.0
0.4 from
)
Figure 4 Effect of helix pitch on the measured degree of agglomeration of the Liddell coal. Temperature, 600-62O’C; 1.9 g dry coal fed; transport gas rate, 1.87 I mine1 at s.t.p. Shaded area is the confidence interval from Figure 2 for a helix pitch of 11.2 mm. 0, helix pitch 7.6 mm; 0, helix pitch 22.4 mm
that temperature variations greater than 5°C have a significant effect. The quantities and flow rates of coal were chosen such that, when based on the evidence of the tests using sand, the temperature decrease during pyrolysis runs was always less than 5°C. The same evidence indicates that the data in Figure 5 were obtained with temperature decreases < 1°C. When Liddell coal was fed at rates greater than 2.5 g min-’ more than 90 wt % of the char formed by pyrolysis stuck to the helix and the pyrolyser tube in clusters with a size > 1 mm. Thus, the screen sizes
Figure
2
0.3
/ 0.2
0.1
0
. 0
la-l
01
1.0 Coal
II
2.0 feed
Figure 6 Measured degrees functions of coal feed rate. 11.2 mm; transport gas rate, 1.9-2.0 g A, Millmerran; 0, pyrolysis tube; 0, Loy Yang
3.0 rate
Ig
dry
I
I
A.0
5.0
coallmin
6.0
I
of agglomeration of three coals as Temperature, 600-620°C; helix pitch, 1.87 I min-’ at s.t.p.; dry coal fed, Millmerran without 120 pm screen in
FUEL,
1980,
Vol. 59, August
565
Agglomeration testing during flash pyrolysis in entrained flow reactors: D. J. McCarthy Tab/e 3
Comparison
of agglomeration Fraction
data from two pyrolysers
of dry coal collected
as agglomerate
Coal
100 mm pyrolyser9
Test apparatus
Liddel I Millmerran Lay Yang
0.50 0.21 0.02
0.50 0.15
a Agglomerate
was detected
from fig. 6
but was too small to measure
120 pm screen was removed. The results of these tests are in Figure 6 from which it will be noted that the effect of the screen is small for the higher coal flow rates. Complete screen analyses of chars from Millmerran coal were not made because the agglomerates were too soft. However, more than 50 wt y0 of the char had a size greater than 200 Frn. Again the form of the results and their significance would remain if the screen sizes had been changed over a relatively wide range. Agglomeration data were also measured for the coals described in Table 1 in a series of runs using a 100 mm diameter, entrained flow pyrolyser in which hot nitrogen was both the heat carrier and the major transport medium. In its fully developed state the flow was laminar, with mixing of the feed coal and heat carrier occurring in a conical inlet section. Agglomerate was defined in this case as all the char from the feed coal which was not transported pneumatically from the reactor’. A comparison of the data from the 100 mm pyrolyser with those from the test apparatus is shown in Table 3. The coal flow rate was constant for the runs in each pyrolyser but differed between pyrolysers. The flow rate chosen for comparative purposes was such that the fraction of Liddell coal denoted as agglomerate was the same from both reactors. The relative values of the measured extent of agglomeration of the coals were found to be similar in the test apparatus to those in the 100 mm pyrolyser. The differences in the results obtained for the three coals show that coal properties as well as the physical environment in a flash pyrolyser will contribute to the
566
FUEL, 1980, Vol 59, August
rate at which agglomerates form. This has been realised for many years and has been the basis for the pretreatment of agglomerating coals before gasification (see for example Forney et al.” and Curran et ~1.‘~). Use of the test apparatus to study the effects of char diuliton on the agglomeration is straightforward in principle. However, the limitations on the flow rate and the total quantity of solids may give a situation at high dilution ratios in which the relative error in the data is unacceptable owing to the small quantity of coal in the diluted feed. ACKNOWLEDGEMENT The author wishes to thank Dr 0. Sitnai for helpful discussions on this work as well as P. A. Casamento and H. F. Molony for their assistance with the experiments. REFERENCES 1 2
Sass, A., Finney, C., McCarthy, H. and Kaufman, P. US Par. 3,1X,233, May 29, 1913 Yavorski, P. M. Proceedings of Symposium on Clean Fuels from Coal, Institute of Gas Technology, September, 1973, pp. 209-218 Jones, J. F., Schmid, M. R. and Eddinger, R. T. Chem. Eng. Prog. 1964, 60, 69 Friedmand, L. D., Rau, E. and Eddinger, R. T. Fuel 1968,47, 149 Landers, W. S., Wagner, E. O., Games, H., Boley, C. C. and Goodman, J. B. US Bur. Mines, Report of Invest. 6608, 1965 Boothroyd, R. G. ‘Flowing Gas-Solids Suspensions’ Chapman and Hall, London, 1971 Durai-Swamy, K., Che, S. C., Green, N. W., Knell, E. W. and Zahrodvick, R. L. Am. Gem. Sot., Div. Fuel Chem. Preprints 1979,24, 177 Hutchinson, P., Hewitt, G. F. and Dukler, A. E. Chem. Eng. Sci. 1971, 26, 419 Sitnai, 0. and McCarthy, D. J. Unpublished results Joint Coal Board and Queensland Coal Board ‘Australian Black Coals’, September 1976 Forney, A. J., Kenny, R. F., Gasior, S. .I. and Field, J. U. Ind. Eng. Chem., Product Rex Dev. 1964, 3,48 Curran, G. P., Pasek, B., Pell, M. and Govin, E. Am. Chem. Sot. Dia. Fuel Chem. 1973, 18, 30