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Fluidized bed combustion of wet brown coal
Fluidized bed combustion of wet brown coal Kujong Jung and Brian R. Stanmore Department of Chemical Engineering, University of Melbourne, Melbourne, Australia (Received 12 April 1979)
The behaviour of very wet Victorian brown coal was examined in a bed of sand fluidized, at temperatures around 1000 K, with either air or nitrogen. Small batches of coal with a narrow particle size range were added to the 76 mm diameter bed and the times required for devolatilization and total combustion were recorded. Changes in particle water content, volatiles level and particle size distribution were also measured. All the particles tested, up to 8.4 mm in diameter, dried rapidly and remained substantially intact throughout carbonization and combustion. Devolatilization was complete after about 60 s but extensive freeboard combustion of volatiles was evident. The water content of the coal had very little influence on burnout time. Char combustion dominated the overall combustion process and took place under kinetic control with significant pore burning.
The fluid bed combustion of coal is now a well-established technique with large programmes underway in Europe and the USA. However, little information has appeared on the fluid bed combustion of wet brown coals, although other low-grade materials are being burned by this method’. In addition to the usual advantages cited, for example see Reference 2, the adoption of fluid bed systems offers solutions to two major problems encountered in the p.f. firing of very wet (60-66s water) Victorian brown coals: (1) flame instability, both during startup and during operation at high turndown ratios; (2) the fouling of tubes with the sticky ash. A number of basic questions need to be answered before the fluid bed combustion of these coals can be adopted. For example, the high water and volatiles levels may cause large particles to shatter on addition to a high temperature bed. The rapid release of large quantities of volatiles raised doubts as to whether freeboard combustion will be excessive. The char formed is very porous and reactive and previous work carried out on its combustion 3fl indicates that it may behave differently from other chars. The initial investigation of the topic described in this paper was experimental and aimed at answering the questions above. With the data collected it is hoped to set up a mathematical model to describe the behaviour of the particles fed into a fluid bed combustor. Comparatively large particles up to 8.5 mm in diameter were used in the tests because drying and devolatilization produce char particles which are only about 20% of the volume of the original wet coal. The investigation was divided into three parts: (1) the drying and devolatilization step; (2) the combustion of char particles; and (3) the overall combustion process. In order to trace the progress of the drying and devolatilization stages, good mass balances were required so the procedure of Badzioch and Hawksley’ involving the use of ash as a tie element was adopted. The mass balances were repeated with nitrogen replacing air as the fluidizing medium in order to distinguish the effect of oxygen on these preliminary processes. EXPERIMENTAL The bed consisted of 400 g of -36 +52 B.S. mesh river sand (360 pm mean particle size) which formed a 200 mm deep bed in a length of 76 mm i.d. inconel tube. The fluidizing 0016-2361/80/020074-0782.00 0 1980 IPC Business Press
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FUEL, 1980, Vol 59, February
gas was introduced through a S.S. plate distributor with 100 1 mm diameter holes drilled in it and covered with a layer of woven S.S. mesh wire of 150 pm aperture to retain the sand. The minimum fluidizing velocity was 59 mm s- 1 (measured at ambient conditions) and at operating velocities of around 200 mm s-l, the pressure drop across the distributor was about 1% of that across the bed. The bed was heated externally by two 2 kW electric resistance heaters clamped to the outside of the tube, with Variac controllers. Temperature indication was provided by a series of four thermocouples in 3.2 mm o.d. sheaths projecting horizontally into the bed at various levels. Air or nitrogen was supplied through a rotameter and sampling of the offgases could be carried out through a suction nozzle equipped with a series of sample flasks. Gas analysis was by means of a g.c. fitted with a dual column suitable for 02, CO2 and CO, generally similar to the technique described in Reference 6. The test programme involved a series of batch (transient) tests in which a charge of coal (generally 2 g dry i.e. 5 g wet) of a narrow particle size range was added to the top of the bed. This was maintained at a steady initial temperature around 970-1070 K by the heaters, and as their input was held constant, the heat released by combustion raised the bed temperature. In the study of drying and devolatilization, the coal particles were removed from the bed for testing at various intervals after addition. This was accomplished by means of a sampler which was inserted into the bed before the test to contain the sample. It consisted of a length of 50 mm i.d. tube closed at the bottom by a S.S. woven gauze of aperture 2.2 mm. When the coal or char being tested was added to the hot bed, it burned within the confines of the sampler and could be removed after a set reaction time by withdrawing the sampler. This was held above the bed to allow sand and fine coal particles to drain from it, leaving only residues larger than 2.2 mm, which were tipped into a container of dry ice to quench further reaction. They were then collected and weighed, screened to give a particle size distribution and analysed for water* and proximate volatile matter?. *To B.S. 1016,
?The proximate
Part 3
volatile matter was determined by a variation of B.S. 1016, Part 3, developed by the State Electricity Commission of Victoria. It entails heating for 7 min at 673 K in one furnace, followed immediately by 7 min at 1173 K in another
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore Table 7 Analysis of Loy Yang coal Moisture
content
(wet basis) (dry basis) (dry basis) Ash Proximate volatile matter (dry basis) Ultimate analysis (dry basis) C H
Table 2 Conditions
57.7 (wt %) 1.36 g water/g coal 1.2 (wt %) 50.8 (wt %) 64.3 (wt %) 4.79 (wt %)
for testing
Particle water content Particle size ranges
Bed temperatures Fluidizing media Fluidizing velocities Treatment times
.
(at ambient
conditions)
57.7 (wt %I -8.4 + 6.4 mm -6.4 + 5.6 mm -5.6 + 5.0 mm 993,1043 K air, nitrogen 169.191 mm s-1 15.35.55s
Finally the residues were ashed at 1073 K. The presence of the sampler did not appear to alter the quality of fluidization in the bed as the results were interchangeable with those in the larger bed. The bed temperatures used in the study were 993 K and 1043 K, somewhat lower than normally used for combustion. This was necessary to keep below the ashing temperature of 1073 K to preserve the mass balances. It should be noted that brown coal char is very reactive and since particle temperatures can be considerably higher than bed temperatures’, the temperature of the reacting char is uncertain. Combustion rates were adequate but higher temperatures favour volatiles burning and this may be a controlling factor. The coal used throughout came from S.E.C. trial bore No. 1195 in the Loy Yang field in Victoria, and its analysis is listed in Table 1. The particles were dry-screened to prepare sized fractions but the nature of the material meant that many of the particles were markedly non-spherical. The char used was from a sample of Yallourn briquettes commercially carbonized at 1073 K by Australian Char Pty. Ltd. at Morwell. This was crushed and screened and from the product, nearly spherical particles were selected. These particles were then weighed and their dimensions checked with a travelling microscope. One particle was then placed into the sampler with the bed held at 1043 K while fluidized with nitrogen. After 30 s the gas was switched to air and after a further 60 s the particle was removed. It was quenched then weighed and its dimensions checked, after which it was returned to the bed. It was then removed every 60 s and retested until its dimensions were too small to enable it to be retained in the sampler. Initial qualitative tests were carried out in the bed to characterize the stages of the process visually. Coal particles added to the bed stayed basically intact during the early stages when drying and devolatilization occurred. They could be momentarily viewed when they appeared at the top of the bed. Immediately after addition, from O-5 s the particles appeared as black specks in the dull red bed, although small fragments were burning brightly. The particles then turned a dull orange-red and a yellow-orange ‘halo’ of burning volatiles formed around them, growing in size to be l-2 times the diameter of the particle at maximum thickness. The halo then shrank and disappeared after 60-70 s, sometimes exhibiting a final bluish tinge. Throughout this
period some decrepitation apparently occurred as a crackling sound could be heard. The colour of the particles then changed to a bright red as char burning commenced, and this colour persisted until the particles were finally consumed after a total burnout time, tb, of between 200 and 400 s. The small amount of ash present in the coal was blown from the bed as a fine grey powder. For the purposes of this work the drying and devolatilization step which is regarded as terminating at time, TV,when the particle colour intensified is designated as the ‘early stage’. RESULTS i%hrly stage behaviour of wet coal The range of process variables tested in this phase of the investigation are listed in Table 2. All determinations were carried out 12 times in order to check the result and to produce a sufficient sample for analysis. Only two sets of results will be reported, those for the extreme cases of -8.4 + 6.4 mm particles at 993 K and -5.60 + 5.00 mm particles at 1043 K. The other results fall into an intermediate location as expected. Water removal
The progress of water removal is depicted in Figure 1, in which the water contents are expressed on the basis of the mass of dry coal originally present. The actual water contents of the coal residues were higher than those plotted
1.4
0 a
I
I
I
Tlrne
I
I
1
Is)
Figure 7 Water and volatiles levels as functions of time. 0, air, 993 K, -8.4 + 6.4 mm; 0, air, 1043 K, -6.4 + 5.6 mm; n, nitrogen, 993 K, -8.4 + 6.4 mm; 0, nitrogen, 1043 K, -6.4 + 5.6 mm; dried coal run ----.
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Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
in Figure 1 because some coal matter had been lost by devolatilization. To recalculate the data as shown to give an idea of actual water removal, the ash balance procedure was required. It can be seen that most of the water present has been removed after 15 s. The types of water present in brown coal have been classified by Stewart and Evans’, and the ranges of these types are marked on the Figure. Monolayer water which constitutes the most strongly held material at concentrations below 0.08 g water/g dry coal is driven off very slowly and will be present well into the char burning stage. This was confirmed by air-drying a sample to 0.05 g water/g and repeating the test. Very little water was lost from this sample over a period of 55 s as shown in Figure 1. Evans estimates’ that this water could be in pores of radius less than 3 nm, from which it would be very difficult to remove. The best data available for coal drying under these conditions was obtained by McIntosh” who exposed wet coal spheres of diameter 6 and 12 mm to a stream of nitrogen at 873 K. He found that the relation of water content to time could be expressed by water content = exp(-jt) initial water content
(1)
where j, is a function of the heat transfer rate, and t, is time. He set up a shrinking core model to describe the evaporative process, and assumed that heat transfer is the rate-limiting step because vapour diffusion occurs readily in the porous exterior. The present data fit a similar expression when plotted on logarithmic co-ordinates. The rate of water removal in nitrogen is distinctly slower than in air under identical conditions. This suggests that the combustion of volatiles in the ‘halo’ which begins immediately the coal is fed into the bed, transmits heat back into the particle. The temperature of its interior will be the wetbulb value” but the region outside the evaporation zone will quickly rise to a value at or above bed temperature. This is supported by the shape of the volatiles level curve Figure 1 which shows that rate of volatiles evolution is at a maximum from the 15 s mark. It can be concluded that drying follows the model proposed by McIntosh but, as will be seen from the next section, his decision to ignore the loss of volatiles is questionable.
sists up to 55 s. This is consistent with high particle temperatures being attained later in the run as a result of volatiles combustion; indeed the final volatile value is similar to those found for 1043 K runs. The ratio of volatiles yield (i.e. mass loss on a dry basis) to the proximate volatile matter of the original coal was calculated for all runs. In general the ratio was greater than unity (1 .O to 1.35) suggesting that part of the fixed carbon was being converted to volatiles under the influence of rapid heating. This is accounted for by the fact that during slow heating tarry volatiles have sufficient time to be thermally cracked on the pore walls during their expulsion. Thus, carbon remains in the solid and smaller gas molecules escape. During rapid heating in the fluid bed, the shorter residence time would permit more volatiles to escape untracked and the solid residue mass would be lower. In the extreme case, -5.6 + 5.0 mm particles at 1043 K, the residual solid mass was 40% of the original dry coal mass, whereas the proximate volatile figure suggests that the minimum mass of residue possible would be 49.2%. Total mass loss The change in mass of a wet particle with time is depicted in Figure 2. It can be seen that after 55 s between 20 and 30% of the mass remains if nitrogen is the fluidizing gas and 18 to 22% if air is used. After 15 s the mass fractions for the two runs with air differ markedly but converge with time as volatile combustion occurs around the larger particles as noted previously. With nitrogen an initial divergence occurs owing to the effect of temperature and particle size and this difference is maintained throughout. It is remarkable that at these high rates of mass loss most of the particles remain intact. As a check on the reliability of the ash tie element approach, the ratio of the actual mass of residue recovered
Evolution of volatiles The history of the volatiles value expressed as proximate volatile matter based on the mass of dry coal originally present in the feed is depicted in Figure 1. It can be seen that the 50 K difference in bed temperature produces a bigger difference in devolatilization rates than in drying rates. The drying process takes place at wet-bulb temperature whatever the bed temperature, but devolatilization requires temperatures above 700 K before substantial rates occur, and the rate of heat transfer then becomes critical. The slow initial rate up to 15 s represents the delay brought about by drying and devolatilization is virtually complete after 55 s. When the data are plotted on logarithmic co-ordinates using the 15 s points as the origin, the values fall on a straight line indicating that the residual volatiles could be expressed by a relation of the type given by equation (1). The larger particles heated in air at 993 K behave somewhat anomalously as a high rate of volatiles evolution per-
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FUEL, 1980, Vol 59, February
F!gure 2 Particle mass and collection efficiency as functions of time. Legend as Figure 7 except: - - - -, collection efficiency
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
1
i
1
greater than 2% of the residue mass and the gradients of the plots, being similar to that of the feed coal, indicate that particles are shrinking uniformly. If the mean diameter of a fraction is taken to be the size at 50% cumulative fraction, the change of diameter with time can be estimated (Figure 4). The rapid shrinkage immediately after addition can be seen, and if recalculated on a cubic (volume) basis, the residue particles occupy only about 2% of their original volumes. The larger -8.4 + 6.4 mm particles displayed a tendency for greater initial shrinkage than the -6.4 + 5.6 mm ones, but this trend was reversed after 15 s. When the volume data are combined with the mass losses in Figure 2, values for apparent particle density can be obtained. The density of the coal, which is initially 1.10 g cmP3 falls to around 1.05 g cm- 3 after 15 s and then rises to about 1.30 g crnmd3after 35 s. This slowly falls to 1.25 g cm--3 after 55 s. and it thus appears as if shrinkage during drying is not as great as when devolatilization is at its peak between 15 and 35 s. Shrinkage effectively ceases in the latter stages of devolatilization and char burning commences. Similar effects were found with lignites by Nsakala et al. ‘I. Char combustion
0
1
2 Particle
L
3 sfize
5
6
7
(mm )
Figure 3 Particle size distributions as functions of time. Fluidizing velocity, 191 mm s-1, 1043 K; 0, air; 0, nitrogen; - - - - -, feed sample; -,15s;--------,35s;---,55s
from the sampler to the ideal recovery, calculated from the ash balance, was calculated for each run. The ratio, called the collection efficiency, is also plotted against time in Figure 2. Although scatter occurs there is a systematic decrease in efficiency consistent with small particles being abraded or thermally shattered from the parent particles. The efficiency becomes effectively constant at 55 s once the particles are completely carbonized. Figure 2 substantiates the tie element approach and confirms the initial impression that the particles remain substantially intact. Further confirmation of this is provided by the analysis of particle size distribution for the residue as described in the next section. Particle size distribution of the residue The solid residues removed from the sampler were screened to establish their particle size distribution, although very little material below 2.2 mm was present in view of the aperture size of the screen. Typical data obtained are plotted in Figure 3 in this case for -6.4 + 5.6 mm particles treated at 1043 K in both air and nitrogen. The mean aperture size between a pair of screens was used to construct a cumulative undersize mass distribution. A measure of estimation was required in constructing the upper particle size range, where most of the mass appears, as there are few data points. Very little fine material, less than lo%, has been generated up to 15 s although most of the drying has already occurred. It must be noted however that only 9% recovery was effected at this point (Figure 2) and the real quantity of fines may be up to double this value. It would be expected that the fmes concentration would be less in the case of the air medium as they would tend to be incinerated, but this was true only at 55 s. The quantity of fines was never
The residual char although amounting to only about 20% of the mass of the feed coal dominates the overall burning process because of the period required for its combustion. The chief areas of interest in quantifying its behaviour centre on the combustion mechanism, i.e. the intermediate steps within the overall C + 02 -+ CO2 reaction, and on the interaction between kinetic and mass transfer effects. No attempt was made to elucidate reaction mechanism and only carbon removal rates were measured. Because of the comparatively low temperatures involved it was anticipated4 that kinetic effects would dominate. If kinetic control applies throughout combustion: tb = kldp
(2)
where kl, is a constant incorporating the reaction rate constant, and d,, is the particle diameter. The specific reaction rate (g cmd2 s-1) is, however, independent of diameter.
Z .o
0.6-
', IY 05
0
I
I
I
!
I
10
20
30
LO
50
Time Figure 4 Figure 1
Particle diameters
60
(s I
as functions
of time.
Legend as
FUEL, 1980, Vol 59, February
77
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
measured for batches of 0.5 to 2.0 g of sized char at two fluidizing velocities. It was found that increasing the velocity from 169 to 191 mm s-r led to a decrease in burnout time of only 57%. Similarly, increasing the mass of char from 0.5 to 2.0 g increased the burnout time for a single particle by only 19 to 32%. When these results are compared with the effects predicted by equation (3) it underlines the small effect being exerted by mass diffusion. The concentration of carbon dioxide in the off-gas during these tests never exceeded 2% so that the particulate phase oxygen concentration would have remained high. Burnout time for wet coal particles
/
I
I
I
I
1.0
2.0
3.0
4.0
5.0
Particle
size
I 60
7.0
Cmm)
Specific combustion rate of char particles. -, range of kinetic control; y, limit of diffusion control, LSh) = 5.0; ______ , limit of diffusion control, (Sh) = 1.0
If bulk diffusion control applies, Avedesian and Davidson showed” (in their equation 14): tb = kyn + k3dp2
(3)
where k2 and k3, are constants involving mass transfer components, and m, is the mass of carbon charged. In this case the specific reaction rate is inversely proportional to particle diameter. The specific combustion rates were calculated for four particles from their mass loss and diameter change, and are plotted on Figure 5. The fact that only a slight influence of particle diameter is detected indicates that the control is by pore diffusion/kinetic effects with only a small influence from bulk mass transfer limitations at the higher diameters. The expression proposed by Smith4 to summarize combustion rates for chars identical to the one used here was used to compare his rates with ours. Using an activation energy of 69 MJ mol-1 based on the external area, the range calculated encompasses the values found here (Figure 5). It is not surprizing that the reaction rates measured, fall at the high end of the range as the actual particle temperatures would have been somewhat higher than the bed temperature which was used in the kinetic expression. For comparison, the diffusion-limited region as predicted by bulk diffusion using a Sherwood number (Sh) of 2 in conjunction with a voidage factor of 0.5 to account for the surrounding sand particles12 is shown in Figure 5. The rates of combustion actually measured are unattainable using this assumption and it appears that higher values of (S/z) apply. The rates predicted by a value of 5.0 are also shown on Figure 5 and this fits the data well. Evidence is coming to hand that values of (Sh) in fluid beds should be greater than 2 and this is especially likely in view of the large char particles involved in these tests. The density of the char particles during the tests was calculated from their measured diameters and masses. Although the data scattered widely as a result of the crude volume analysis used, the densities remained constant as the particles burned away. Since the particle diameters decreased regularly, the reactive gas species must have been consumed before it penetrated far into the pores and most of the reaction was near the surface. This is to be expected in view of the very small pore diameters involved. To test the effect of a larger mass of char particles in the bed without the sampler, burnout times at 1043 K were
78
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1980,
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59, February
The tests in this section involved the charging of batches of wet coal into the hot bed under the range of conditions outlined in Table 2. In each case a large excess of oxygen was present and the time for char ignition, t,, and the total burnout time, tb, were measured. The effect of bed temperature over the range 943 to 1043 K is depicted in Figure 6a for three particle sizes at 191 mm. ssl Huidizing velocity. There is a decrease in tb mainly as a result of more rapid char combustion as t, remains relatively constant. Smith’s kinetic expression predicts a decrease of 22% in char combustion time ([b-t”) over this range which is consistent with these data. At the same time the oxygen diffusion coefficient as calculated from Field13 rises from 161 mm* s-t at lOOOK to 188 mm2 ssl at 1100 K, approximately the same order of magnitude. However, any diffusion effects limiting combustion would occur in the pores where Knudson diffusion would apply. The effects of mass of charge and fluidizing velocity are only very slight as would be expected from the behaviour of the char described in the previous section. The initial diameter of the coal particle is a much more significant variable and is of importance in any industrial application. A 50% increase in burnout time occurred as the particle size was increased from -5.6 + 5.0 mm to
ul
I
b I
900
I
I
950
1000
Bed temperature
I
I
1050 5.0
IK)
lnitlol
I
I
6.0
7.0
particle
8.0
51ze (mm)
Figure 6 Effect of bed temperature and particle diameter on burnout times. (a) n, -8.4 + 6.4 mm; 0, -6.4 + 5.6 mm; 0, -5.6 + 5.0 mm; (b) n, 943 K; 0,993 K; 0, 1043 K
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore Table 3
Burnout
times for wet coal as a function Burnout
of water
COntent
Time (s.1
Water Content (g water/gdry 0.16 0.75 1.36
coal)
Early Stage, t,
Overall,
42 70 77
300 300 322
tb
Char, tb-tv 258 230 245
-8.4 + 6.4 mm, Figure 6b. This is consistent with equation (2), the kinetic-control model as the char combustion time, tb- t,, is roughly proportional to particle diameter. The most interesting result to emerge from these tests was the effect of water content. Three coal samples were prepared with water contents of 0.16,0.75 and 1.36 g water/g, and tested under identical conditions. To take account of the effects of shrinkage the samples were taken from the same bulk sample of -8.4 + 6.4 mm coal and two were dried for different periods in an air oven. Thus the particle diameters of the low water content samples were smaller than that of the wet material, but the char samples formed from them at the commencement of burning should have been the same provided the different drying techniques produced identical shrinkages. The results of this test are included in Table 3. The total burnout time changed very little over this wide range of water content, and the char from the wetter coals burned faster than that from the nearly dry material. The combustion of bed-moist coal will occur at the same net rate as that of dried coal of the same initial size range, and this has important commercial implications. The reason for the high char combustion rate of wet coals is not clear. One possibility is that the porosity of the char formed by rapid drying in the bed may be greater than that produced by slow air-drying, and this may result in better access for oxygen during pore combustion. Another is that the particle shrinkage brought about in the bed is greater than that in slow air drying, and a smaller char particle may result. Alternatively, the coal dried in the fluid bed has been shielded from oxygen during drying and devolatilization so that a greater concentration of active sites may be present on the char surface. The catalytic effect of monolayer water on the gasification reactions should not be responsible as it will be uniformly present in all samples.
Heat dense rates The combustion energy released during a test causes changes in the bed temperature as the test proceeds. Typical temperature-time traces obtained for 5 g of wet coal of water content 1.36 g/g are reproduced in Figure 7. One is for particle sizes -8.4 + 6.4 mm at 993 K, and the other for -5.6 + 5.0 mm particles at 1043 K. The bed was initially stable at this temperature but in each case the evaporation of water during the first 15 s caused an 11 K drop in bed temperature. The high volatile-release rate then caused the temperature to rise rapidly for 50 s, then more slowly until a maximum was reached about 150 s after addition. Since thermal equilibrium was at 993 or 1043 K, the temperatures began to fall towards these values even though char combustion was continuing. The char burnout times are marked on the traces. The high volatile-release rates mean that freeboard burning was extensive and it could be observed as a blue flame in the tube during the early stage. The temperature traces
of Figure 7 can be manipulated to give the fraction of heat release occurring within the bed, by means of technique summarized below. The total heat release from a coal charge, Qc, is given by Q, = m,AH where m,, is the coal mass charged, and AH, is its specific energy of combustion, corrected for product formation at bed temperature. The electric heaters supply sufficient heat to raise the incoming air to the initial bed temperature, To, and to replace losses. It can be assumed that the bed is isothermal, the combustion gases leave at bed temperature and that the increase in temperature during a test results in a negligible increase in losses. Taking an energy balance over the bed itself with system boundaries at the bed surface and the outside of the insulation: rate of heat release in the bed by combustion = rate of heat accumulation in the bed + rate of absorption by the air. The combustion gases are essentially air as this was run in excess and the inconel tube can be treated as an equivalent mass of sand. The relation becomes: dT ci = m,CPS $ +%lCpll(T
- To)
(4)
where 4, instantaneous rate of heat release in the bed (W); m,, mass of sand (g); C #, specific heat of sand (J ggl K-l); T, bed temperature (K P; To, initial bed temperature (K); m,, air mass flow rate (g s-l); CPa, specific heat of air (J g-l K-l). The total heat release during combustion is found by integrating equation (4) from time t = 0 to t = tb. Assuming that the specific heat remains constant with temperature Tt=tb
tb
I
(4W = msCps i 0
tb
dT + tiaCpa s
(T ~ To)dt
(5)
0
Tr=o
heat release in the bed, F, is then given by
The fractional fb (ir)dt s 0 F =
n
0”
-15’
Figure 7 water/g,
QC
I 0
I 50
I 100 Time (5)
Bed temperatures as functions -5.6 + 5.0 mm. -,993K;--,1043K
FUEt,
I
I
150
200
of time.
250
5 g coal, 1.36 g
1980, Vol 59, February
79
Fluidized
of wet brown coal: K. Jung and B. R. Stanmore
bed combustion
Table 4 Fraction of heat released in the bed. rn,, 5.0 g; water, 1.36 g/g; fluidizing velocity 191 mm s-1 Conditions Bed Temperature
(K)
of Test Particle Diameter
-8.4 -6.4 -5.6 -8.4 -6.4 -5.6
993 ,I II 1043 ,, I,
ACKNOWLEDGEMENT
f •t + •t + +
6.4 5.6 5.0 6.4 5.6 5.0
(mm)
Fraction
F
0.40 0.41 0.45 0.51 0.56 0.57
The authors wish to thank the State Electricity Commission of Victoria for the use of analytical facilities at their Hermann Laboratory.
REFERENCES 1
The temperature traces can be integrated numerically to evaluate the two terms in equation (5) and the specific energy of the coal samples was available. The results obtained for the conditions tested are listed in Table 4. As anticipated only about half the heat release took place in the bed. Since the 50 K increase in bed temperature led to a significant increase in the heat release fraction, F, higher temperatures should be examined in future work. Larger particles were less desirable, possibly because local oxygen deficiencies were produced and F fell. Alternatively, particle temperatures may be higher in the smaller particles during devolatilization, giving rise to higher ‘halo’ temperatures and more rapid combustion of volatiles. An estimate of the specific energy of the volatiles component indicates that it represents about l/2 of the energy in the coal and this suggests that the bypassing by volatiles is considerable.
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1980, Vol 59, February
2 3 4 5 6
Hodgkinson, N. and Thurlow, G. G. A.1.Ch.E. Svmo. . . Series 1976,73,108 Skinner, D. G. ‘The Fluidized Combustion of Coal’, Mills and Boon Monograph CE/3, London, 1971, l-59 Hamor, R. Jy, Smith, 1. W. and Tyler, R. J. Comb. Name 1973,21,153 Smith, 1. W., and Tyler, R. J. Comb. Sci. Tech. 1974,9, 87 Badzioch, S. B. and Hawksley, P. G. W.ind. Eng. Chem. Proc. Des. Dev. 1970, 9, 521 Bailey, J. B. W., Brown, N. E. and Phillips, C. V. The Analysf.
1971,96,447 I 8 9 10 11 12 13
Basu, P. Fuel 1977,56, 390 Stewart, R. and Evans, D. G. Fuel 1967,46,263 Evans, D. G. Fuel 1973,52,186 McIntosh, M. J. Fuel 1976,55,47 Nsakala, N. Y., Essenhigh, R. H., and Walker, P. L. Jr. Fuel 1978.10.605 Avedesian, M. M. and Davidson, J. F. Trans. Inst. Chem. Eng. 1973,51,121 Field, M. A. et al. ‘Combustion of Pulverized Coal’ BCURA, Leatherhead, 1967, 346
The behaviour of very wet Victorian brown coal was examined in a bed of sand fluidized, at temperatures around 1000 K, with either air or nitrogen. Small batches of coal with a narrow particle size range were added to the 76 mm diameter bed and the times required for devolatilization and total combustion were recorded. Changes in particle water content, volatiles level and particle size distribution were also measured. All the particles tested, up to 8.4 mm in diameter, dried rapidly and remained substantially intact throughout carbonization and combustion. Devolatilization was complete after about 60 s but extensive freeboard combustion of volatiles was evident. The water content of the coal had very little influence on burnout time. Char combustion dominated the overall combustion process and took place under kinetic control with significant pore burning.
The fluid bed combustion of coal is now a well-established technique with large programmes underway in Europe and the USA. However, little information has appeared on the fluid bed combustion of wet brown coals, although other low-grade materials are being burned by this method’. In addition to the usual advantages cited, for example see Reference 2, the adoption of fluid bed systems offers solutions to two major problems encountered in the p.f. firing of very wet (60-66s water) Victorian brown coals: (1) flame instability, both during startup and during operation at high turndown ratios; (2) the fouling of tubes with the sticky ash. A number of basic questions need to be answered before the fluid bed combustion of these coals can be adopted. For example, the high water and volatiles levels may cause large particles to shatter on addition to a high temperature bed. The rapid release of large quantities of volatiles raised doubts as to whether freeboard combustion will be excessive. The char formed is very porous and reactive and previous work carried out on its combustion 3fl indicates that it may behave differently from other chars. The initial investigation of the topic described in this paper was experimental and aimed at answering the questions above. With the data collected it is hoped to set up a mathematical model to describe the behaviour of the particles fed into a fluid bed combustor. Comparatively large particles up to 8.5 mm in diameter were used in the tests because drying and devolatilization produce char particles which are only about 20% of the volume of the original wet coal. The investigation was divided into three parts: (1) the drying and devolatilization step; (2) the combustion of char particles; and (3) the overall combustion process. In order to trace the progress of the drying and devolatilization stages, good mass balances were required so the procedure of Badzioch and Hawksley’ involving the use of ash as a tie element was adopted. The mass balances were repeated with nitrogen replacing air as the fluidizing medium in order to distinguish the effect of oxygen on these preliminary processes. EXPERIMENTAL The bed consisted of 400 g of -36 +52 B.S. mesh river sand (360 pm mean particle size) which formed a 200 mm deep bed in a length of 76 mm i.d. inconel tube. The fluidizing 0016-2361/80/020074-0782.00 0 1980 IPC Business Press
74
FUEL, 1980, Vol 59, February
gas was introduced through a S.S. plate distributor with 100 1 mm diameter holes drilled in it and covered with a layer of woven S.S. mesh wire of 150 pm aperture to retain the sand. The minimum fluidizing velocity was 59 mm s- 1 (measured at ambient conditions) and at operating velocities of around 200 mm s-l, the pressure drop across the distributor was about 1% of that across the bed. The bed was heated externally by two 2 kW electric resistance heaters clamped to the outside of the tube, with Variac controllers. Temperature indication was provided by a series of four thermocouples in 3.2 mm o.d. sheaths projecting horizontally into the bed at various levels. Air or nitrogen was supplied through a rotameter and sampling of the offgases could be carried out through a suction nozzle equipped with a series of sample flasks. Gas analysis was by means of a g.c. fitted with a dual column suitable for 02, CO2 and CO, generally similar to the technique described in Reference 6. The test programme involved a series of batch (transient) tests in which a charge of coal (generally 2 g dry i.e. 5 g wet) of a narrow particle size range was added to the top of the bed. This was maintained at a steady initial temperature around 970-1070 K by the heaters, and as their input was held constant, the heat released by combustion raised the bed temperature. In the study of drying and devolatilization, the coal particles were removed from the bed for testing at various intervals after addition. This was accomplished by means of a sampler which was inserted into the bed before the test to contain the sample. It consisted of a length of 50 mm i.d. tube closed at the bottom by a S.S. woven gauze of aperture 2.2 mm. When the coal or char being tested was added to the hot bed, it burned within the confines of the sampler and could be removed after a set reaction time by withdrawing the sampler. This was held above the bed to allow sand and fine coal particles to drain from it, leaving only residues larger than 2.2 mm, which were tipped into a container of dry ice to quench further reaction. They were then collected and weighed, screened to give a particle size distribution and analysed for water* and proximate volatile matter?. *To B.S. 1016,
?The proximate
Part 3
volatile matter was determined by a variation of B.S. 1016, Part 3, developed by the State Electricity Commission of Victoria. It entails heating for 7 min at 673 K in one furnace, followed immediately by 7 min at 1173 K in another
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore Table 7 Analysis of Loy Yang coal Moisture
content
(wet basis) (dry basis) (dry basis) Ash Proximate volatile matter (dry basis) Ultimate analysis (dry basis) C H
Table 2 Conditions
57.7 (wt %) 1.36 g water/g coal 1.2 (wt %) 50.8 (wt %) 64.3 (wt %) 4.79 (wt %)
for testing
Particle water content Particle size ranges
Bed temperatures Fluidizing media Fluidizing velocities Treatment times
.
(at ambient
conditions)
57.7 (wt %I -8.4 + 6.4 mm -6.4 + 5.6 mm -5.6 + 5.0 mm 993,1043 K air, nitrogen 169.191 mm s-1 15.35.55s
Finally the residues were ashed at 1073 K. The presence of the sampler did not appear to alter the quality of fluidization in the bed as the results were interchangeable with those in the larger bed. The bed temperatures used in the study were 993 K and 1043 K, somewhat lower than normally used for combustion. This was necessary to keep below the ashing temperature of 1073 K to preserve the mass balances. It should be noted that brown coal char is very reactive and since particle temperatures can be considerably higher than bed temperatures’, the temperature of the reacting char is uncertain. Combustion rates were adequate but higher temperatures favour volatiles burning and this may be a controlling factor. The coal used throughout came from S.E.C. trial bore No. 1195 in the Loy Yang field in Victoria, and its analysis is listed in Table 1. The particles were dry-screened to prepare sized fractions but the nature of the material meant that many of the particles were markedly non-spherical. The char used was from a sample of Yallourn briquettes commercially carbonized at 1073 K by Australian Char Pty. Ltd. at Morwell. This was crushed and screened and from the product, nearly spherical particles were selected. These particles were then weighed and their dimensions checked with a travelling microscope. One particle was then placed into the sampler with the bed held at 1043 K while fluidized with nitrogen. After 30 s the gas was switched to air and after a further 60 s the particle was removed. It was quenched then weighed and its dimensions checked, after which it was returned to the bed. It was then removed every 60 s and retested until its dimensions were too small to enable it to be retained in the sampler. Initial qualitative tests were carried out in the bed to characterize the stages of the process visually. Coal particles added to the bed stayed basically intact during the early stages when drying and devolatilization occurred. They could be momentarily viewed when they appeared at the top of the bed. Immediately after addition, from O-5 s the particles appeared as black specks in the dull red bed, although small fragments were burning brightly. The particles then turned a dull orange-red and a yellow-orange ‘halo’ of burning volatiles formed around them, growing in size to be l-2 times the diameter of the particle at maximum thickness. The halo then shrank and disappeared after 60-70 s, sometimes exhibiting a final bluish tinge. Throughout this
period some decrepitation apparently occurred as a crackling sound could be heard. The colour of the particles then changed to a bright red as char burning commenced, and this colour persisted until the particles were finally consumed after a total burnout time, tb, of between 200 and 400 s. The small amount of ash present in the coal was blown from the bed as a fine grey powder. For the purposes of this work the drying and devolatilization step which is regarded as terminating at time, TV,when the particle colour intensified is designated as the ‘early stage’. RESULTS i%hrly stage behaviour of wet coal The range of process variables tested in this phase of the investigation are listed in Table 2. All determinations were carried out 12 times in order to check the result and to produce a sufficient sample for analysis. Only two sets of results will be reported, those for the extreme cases of -8.4 + 6.4 mm particles at 993 K and -5.60 + 5.00 mm particles at 1043 K. The other results fall into an intermediate location as expected. Water removal
The progress of water removal is depicted in Figure 1, in which the water contents are expressed on the basis of the mass of dry coal originally present. The actual water contents of the coal residues were higher than those plotted
1.4
0 a
I
I
I
Tlrne
I
I
1
Is)
Figure 7 Water and volatiles levels as functions of time. 0, air, 993 K, -8.4 + 6.4 mm; 0, air, 1043 K, -6.4 + 5.6 mm; n, nitrogen, 993 K, -8.4 + 6.4 mm; 0, nitrogen, 1043 K, -6.4 + 5.6 mm; dried coal run ----.
FUEL,
1980,
Vol 59, February
75
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
in Figure 1 because some coal matter had been lost by devolatilization. To recalculate the data as shown to give an idea of actual water removal, the ash balance procedure was required. It can be seen that most of the water present has been removed after 15 s. The types of water present in brown coal have been classified by Stewart and Evans’, and the ranges of these types are marked on the Figure. Monolayer water which constitutes the most strongly held material at concentrations below 0.08 g water/g dry coal is driven off very slowly and will be present well into the char burning stage. This was confirmed by air-drying a sample to 0.05 g water/g and repeating the test. Very little water was lost from this sample over a period of 55 s as shown in Figure 1. Evans estimates’ that this water could be in pores of radius less than 3 nm, from which it would be very difficult to remove. The best data available for coal drying under these conditions was obtained by McIntosh” who exposed wet coal spheres of diameter 6 and 12 mm to a stream of nitrogen at 873 K. He found that the relation of water content to time could be expressed by water content = exp(-jt) initial water content
(1)
where j, is a function of the heat transfer rate, and t, is time. He set up a shrinking core model to describe the evaporative process, and assumed that heat transfer is the rate-limiting step because vapour diffusion occurs readily in the porous exterior. The present data fit a similar expression when plotted on logarithmic co-ordinates. The rate of water removal in nitrogen is distinctly slower than in air under identical conditions. This suggests that the combustion of volatiles in the ‘halo’ which begins immediately the coal is fed into the bed, transmits heat back into the particle. The temperature of its interior will be the wetbulb value” but the region outside the evaporation zone will quickly rise to a value at or above bed temperature. This is supported by the shape of the volatiles level curve Figure 1 which shows that rate of volatiles evolution is at a maximum from the 15 s mark. It can be concluded that drying follows the model proposed by McIntosh but, as will be seen from the next section, his decision to ignore the loss of volatiles is questionable.
sists up to 55 s. This is consistent with high particle temperatures being attained later in the run as a result of volatiles combustion; indeed the final volatile value is similar to those found for 1043 K runs. The ratio of volatiles yield (i.e. mass loss on a dry basis) to the proximate volatile matter of the original coal was calculated for all runs. In general the ratio was greater than unity (1 .O to 1.35) suggesting that part of the fixed carbon was being converted to volatiles under the influence of rapid heating. This is accounted for by the fact that during slow heating tarry volatiles have sufficient time to be thermally cracked on the pore walls during their expulsion. Thus, carbon remains in the solid and smaller gas molecules escape. During rapid heating in the fluid bed, the shorter residence time would permit more volatiles to escape untracked and the solid residue mass would be lower. In the extreme case, -5.6 + 5.0 mm particles at 1043 K, the residual solid mass was 40% of the original dry coal mass, whereas the proximate volatile figure suggests that the minimum mass of residue possible would be 49.2%. Total mass loss The change in mass of a wet particle with time is depicted in Figure 2. It can be seen that after 55 s between 20 and 30% of the mass remains if nitrogen is the fluidizing gas and 18 to 22% if air is used. After 15 s the mass fractions for the two runs with air differ markedly but converge with time as volatile combustion occurs around the larger particles as noted previously. With nitrogen an initial divergence occurs owing to the effect of temperature and particle size and this difference is maintained throughout. It is remarkable that at these high rates of mass loss most of the particles remain intact. As a check on the reliability of the ash tie element approach, the ratio of the actual mass of residue recovered
Evolution of volatiles The history of the volatiles value expressed as proximate volatile matter based on the mass of dry coal originally present in the feed is depicted in Figure 1. It can be seen that the 50 K difference in bed temperature produces a bigger difference in devolatilization rates than in drying rates. The drying process takes place at wet-bulb temperature whatever the bed temperature, but devolatilization requires temperatures above 700 K before substantial rates occur, and the rate of heat transfer then becomes critical. The slow initial rate up to 15 s represents the delay brought about by drying and devolatilization is virtually complete after 55 s. When the data are plotted on logarithmic co-ordinates using the 15 s points as the origin, the values fall on a straight line indicating that the residual volatiles could be expressed by a relation of the type given by equation (1). The larger particles heated in air at 993 K behave somewhat anomalously as a high rate of volatiles evolution per-
76
FUEL, 1980, Vol 59, February
F!gure 2 Particle mass and collection efficiency as functions of time. Legend as Figure 7 except: - - - -, collection efficiency
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
1
i
1
greater than 2% of the residue mass and the gradients of the plots, being similar to that of the feed coal, indicate that particles are shrinking uniformly. If the mean diameter of a fraction is taken to be the size at 50% cumulative fraction, the change of diameter with time can be estimated (Figure 4). The rapid shrinkage immediately after addition can be seen, and if recalculated on a cubic (volume) basis, the residue particles occupy only about 2% of their original volumes. The larger -8.4 + 6.4 mm particles displayed a tendency for greater initial shrinkage than the -6.4 + 5.6 mm ones, but this trend was reversed after 15 s. When the volume data are combined with the mass losses in Figure 2, values for apparent particle density can be obtained. The density of the coal, which is initially 1.10 g cmP3 falls to around 1.05 g cm- 3 after 15 s and then rises to about 1.30 g crnmd3after 35 s. This slowly falls to 1.25 g cm--3 after 55 s. and it thus appears as if shrinkage during drying is not as great as when devolatilization is at its peak between 15 and 35 s. Shrinkage effectively ceases in the latter stages of devolatilization and char burning commences. Similar effects were found with lignites by Nsakala et al. ‘I. Char combustion
0
1
2 Particle
L
3 sfize
5
6
7
(mm )
Figure 3 Particle size distributions as functions of time. Fluidizing velocity, 191 mm s-1, 1043 K; 0, air; 0, nitrogen; - - - - -, feed sample; -,15s;--------,35s;---,55s
from the sampler to the ideal recovery, calculated from the ash balance, was calculated for each run. The ratio, called the collection efficiency, is also plotted against time in Figure 2. Although scatter occurs there is a systematic decrease in efficiency consistent with small particles being abraded or thermally shattered from the parent particles. The efficiency becomes effectively constant at 55 s once the particles are completely carbonized. Figure 2 substantiates the tie element approach and confirms the initial impression that the particles remain substantially intact. Further confirmation of this is provided by the analysis of particle size distribution for the residue as described in the next section. Particle size distribution of the residue The solid residues removed from the sampler were screened to establish their particle size distribution, although very little material below 2.2 mm was present in view of the aperture size of the screen. Typical data obtained are plotted in Figure 3 in this case for -6.4 + 5.6 mm particles treated at 1043 K in both air and nitrogen. The mean aperture size between a pair of screens was used to construct a cumulative undersize mass distribution. A measure of estimation was required in constructing the upper particle size range, where most of the mass appears, as there are few data points. Very little fine material, less than lo%, has been generated up to 15 s although most of the drying has already occurred. It must be noted however that only 9% recovery was effected at this point (Figure 2) and the real quantity of fines may be up to double this value. It would be expected that the fmes concentration would be less in the case of the air medium as they would tend to be incinerated, but this was true only at 55 s. The quantity of fines was never
The residual char although amounting to only about 20% of the mass of the feed coal dominates the overall burning process because of the period required for its combustion. The chief areas of interest in quantifying its behaviour centre on the combustion mechanism, i.e. the intermediate steps within the overall C + 02 -+ CO2 reaction, and on the interaction between kinetic and mass transfer effects. No attempt was made to elucidate reaction mechanism and only carbon removal rates were measured. Because of the comparatively low temperatures involved it was anticipated4 that kinetic effects would dominate. If kinetic control applies throughout combustion: tb = kldp
(2)
where kl, is a constant incorporating the reaction rate constant, and d,, is the particle diameter. The specific reaction rate (g cmd2 s-1) is, however, independent of diameter.
Z .o
0.6-
', IY 05
0
I
I
I
!
I
10
20
30
LO
50
Time Figure 4 Figure 1
Particle diameters
60
(s I
as functions
of time.
Legend as
FUEL, 1980, Vol 59, February
77
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore
measured for batches of 0.5 to 2.0 g of sized char at two fluidizing velocities. It was found that increasing the velocity from 169 to 191 mm s-r led to a decrease in burnout time of only 57%. Similarly, increasing the mass of char from 0.5 to 2.0 g increased the burnout time for a single particle by only 19 to 32%. When these results are compared with the effects predicted by equation (3) it underlines the small effect being exerted by mass diffusion. The concentration of carbon dioxide in the off-gas during these tests never exceeded 2% so that the particulate phase oxygen concentration would have remained high. Burnout time for wet coal particles
/
I
I
I
I
1.0
2.0
3.0
4.0
5.0
Particle
size
I 60
7.0
Cmm)
Specific combustion rate of char particles. -, range of kinetic control; y, limit of diffusion control, LSh) = 5.0; ______ , limit of diffusion control, (Sh) = 1.0
If bulk diffusion control applies, Avedesian and Davidson showed” (in their equation 14): tb = kyn + k3dp2
(3)
where k2 and k3, are constants involving mass transfer components, and m, is the mass of carbon charged. In this case the specific reaction rate is inversely proportional to particle diameter. The specific combustion rates were calculated for four particles from their mass loss and diameter change, and are plotted on Figure 5. The fact that only a slight influence of particle diameter is detected indicates that the control is by pore diffusion/kinetic effects with only a small influence from bulk mass transfer limitations at the higher diameters. The expression proposed by Smith4 to summarize combustion rates for chars identical to the one used here was used to compare his rates with ours. Using an activation energy of 69 MJ mol-1 based on the external area, the range calculated encompasses the values found here (Figure 5). It is not surprizing that the reaction rates measured, fall at the high end of the range as the actual particle temperatures would have been somewhat higher than the bed temperature which was used in the kinetic expression. For comparison, the diffusion-limited region as predicted by bulk diffusion using a Sherwood number (Sh) of 2 in conjunction with a voidage factor of 0.5 to account for the surrounding sand particles12 is shown in Figure 5. The rates of combustion actually measured are unattainable using this assumption and it appears that higher values of (S/z) apply. The rates predicted by a value of 5.0 are also shown on Figure 5 and this fits the data well. Evidence is coming to hand that values of (Sh) in fluid beds should be greater than 2 and this is especially likely in view of the large char particles involved in these tests. The density of the char particles during the tests was calculated from their measured diameters and masses. Although the data scattered widely as a result of the crude volume analysis used, the densities remained constant as the particles burned away. Since the particle diameters decreased regularly, the reactive gas species must have been consumed before it penetrated far into the pores and most of the reaction was near the surface. This is to be expected in view of the very small pore diameters involved. To test the effect of a larger mass of char particles in the bed without the sampler, burnout times at 1043 K were
78
FUEL,
1980,
Vol
59, February
The tests in this section involved the charging of batches of wet coal into the hot bed under the range of conditions outlined in Table 2. In each case a large excess of oxygen was present and the time for char ignition, t,, and the total burnout time, tb, were measured. The effect of bed temperature over the range 943 to 1043 K is depicted in Figure 6a for three particle sizes at 191 mm. ssl Huidizing velocity. There is a decrease in tb mainly as a result of more rapid char combustion as t, remains relatively constant. Smith’s kinetic expression predicts a decrease of 22% in char combustion time ([b-t”) over this range which is consistent with these data. At the same time the oxygen diffusion coefficient as calculated from Field13 rises from 161 mm* s-t at lOOOK to 188 mm2 ssl at 1100 K, approximately the same order of magnitude. However, any diffusion effects limiting combustion would occur in the pores where Knudson diffusion would apply. The effects of mass of charge and fluidizing velocity are only very slight as would be expected from the behaviour of the char described in the previous section. The initial diameter of the coal particle is a much more significant variable and is of importance in any industrial application. A 50% increase in burnout time occurred as the particle size was increased from -5.6 + 5.0 mm to
ul
I
b I
900
I
I
950
1000
Bed temperature
I
I
1050 5.0
IK)
lnitlol
I
I
6.0
7.0
particle
8.0
51ze (mm)
Figure 6 Effect of bed temperature and particle diameter on burnout times. (a) n, -8.4 + 6.4 mm; 0, -6.4 + 5.6 mm; 0, -5.6 + 5.0 mm; (b) n, 943 K; 0,993 K; 0, 1043 K
Fluidized bed combustion of wet brown coal: K. Jung and B. R. Stanmore Table 3
Burnout
times for wet coal as a function Burnout
of water
COntent
Time (s.1
Water Content (g water/gdry 0.16 0.75 1.36
coal)
Early Stage, t,
Overall,
42 70 77
300 300 322
tb
Char, tb-tv 258 230 245
-8.4 + 6.4 mm, Figure 6b. This is consistent with equation (2), the kinetic-control model as the char combustion time, tb- t,, is roughly proportional to particle diameter. The most interesting result to emerge from these tests was the effect of water content. Three coal samples were prepared with water contents of 0.16,0.75 and 1.36 g water/g, and tested under identical conditions. To take account of the effects of shrinkage the samples were taken from the same bulk sample of -8.4 + 6.4 mm coal and two were dried for different periods in an air oven. Thus the particle diameters of the low water content samples were smaller than that of the wet material, but the char samples formed from them at the commencement of burning should have been the same provided the different drying techniques produced identical shrinkages. The results of this test are included in Table 3. The total burnout time changed very little over this wide range of water content, and the char from the wetter coals burned faster than that from the nearly dry material. The combustion of bed-moist coal will occur at the same net rate as that of dried coal of the same initial size range, and this has important commercial implications. The reason for the high char combustion rate of wet coals is not clear. One possibility is that the porosity of the char formed by rapid drying in the bed may be greater than that produced by slow air-drying, and this may result in better access for oxygen during pore combustion. Another is that the particle shrinkage brought about in the bed is greater than that in slow air drying, and a smaller char particle may result. Alternatively, the coal dried in the fluid bed has been shielded from oxygen during drying and devolatilization so that a greater concentration of active sites may be present on the char surface. The catalytic effect of monolayer water on the gasification reactions should not be responsible as it will be uniformly present in all samples.
Heat dense rates The combustion energy released during a test causes changes in the bed temperature as the test proceeds. Typical temperature-time traces obtained for 5 g of wet coal of water content 1.36 g/g are reproduced in Figure 7. One is for particle sizes -8.4 + 6.4 mm at 993 K, and the other for -5.6 + 5.0 mm particles at 1043 K. The bed was initially stable at this temperature but in each case the evaporation of water during the first 15 s caused an 11 K drop in bed temperature. The high volatile-release rate then caused the temperature to rise rapidly for 50 s, then more slowly until a maximum was reached about 150 s after addition. Since thermal equilibrium was at 993 or 1043 K, the temperatures began to fall towards these values even though char combustion was continuing. The char burnout times are marked on the traces. The high volatile-release rates mean that freeboard burning was extensive and it could be observed as a blue flame in the tube during the early stage. The temperature traces
of Figure 7 can be manipulated to give the fraction of heat release occurring within the bed, by means of technique summarized below. The total heat release from a coal charge, Qc, is given by Q, = m,AH where m,, is the coal mass charged, and AH, is its specific energy of combustion, corrected for product formation at bed temperature. The electric heaters supply sufficient heat to raise the incoming air to the initial bed temperature, To, and to replace losses. It can be assumed that the bed is isothermal, the combustion gases leave at bed temperature and that the increase in temperature during a test results in a negligible increase in losses. Taking an energy balance over the bed itself with system boundaries at the bed surface and the outside of the insulation: rate of heat release in the bed by combustion = rate of heat accumulation in the bed + rate of absorption by the air. The combustion gases are essentially air as this was run in excess and the inconel tube can be treated as an equivalent mass of sand. The relation becomes: dT ci = m,CPS $ +%lCpll(T
- To)
(4)
where 4, instantaneous rate of heat release in the bed (W); m,, mass of sand (g); C #, specific heat of sand (J ggl K-l); T, bed temperature (K P; To, initial bed temperature (K); m,, air mass flow rate (g s-l); CPa, specific heat of air (J g-l K-l). The total heat release during combustion is found by integrating equation (4) from time t = 0 to t = tb. Assuming that the specific heat remains constant with temperature Tt=tb
tb
I
(4W = msCps i 0
tb
dT + tiaCpa s
(T ~ To)dt
(5)
0
Tr=o
heat release in the bed, F, is then given by
The fractional fb (ir)dt s 0 F =
n
0”
-15’
Figure 7 water/g,
QC
I 0
I 50
I 100 Time (5)
Bed temperatures as functions -5.6 + 5.0 mm. -,993K;--,1043K
FUEt,
I
I
150
200
of time.
250
5 g coal, 1.36 g
1980, Vol 59, February
79
Fluidized
of wet brown coal: K. Jung and B. R. Stanmore
bed combustion
Table 4 Fraction of heat released in the bed. rn,, 5.0 g; water, 1.36 g/g; fluidizing velocity 191 mm s-1 Conditions Bed Temperature
(K)
of Test Particle Diameter
-8.4 -6.4 -5.6 -8.4 -6.4 -5.6
993 ,I II 1043 ,, I,
ACKNOWLEDGEMENT
f •t + •t + +
6.4 5.6 5.0 6.4 5.6 5.0
(mm)
Fraction
F
0.40 0.41 0.45 0.51 0.56 0.57
The authors wish to thank the State Electricity Commission of Victoria for the use of analytical facilities at their Hermann Laboratory.
REFERENCES 1
The temperature traces can be integrated numerically to evaluate the two terms in equation (5) and the specific energy of the coal samples was available. The results obtained for the conditions tested are listed in Table 4. As anticipated only about half the heat release took place in the bed. Since the 50 K increase in bed temperature led to a significant increase in the heat release fraction, F, higher temperatures should be examined in future work. Larger particles were less desirable, possibly because local oxygen deficiencies were produced and F fell. Alternatively, particle temperatures may be higher in the smaller particles during devolatilization, giving rise to higher ‘halo’ temperatures and more rapid combustion of volatiles. An estimate of the specific energy of the volatiles component indicates that it represents about l/2 of the energy in the coal and this suggests that the bypassing by volatiles is considerable.
80
FUEL,
1980, Vol 59, February
2 3 4 5 6
Hodgkinson, N. and Thurlow, G. G. A.1.Ch.E. Svmo. . . Series 1976,73,108 Skinner, D. G. ‘The Fluidized Combustion of Coal’, Mills and Boon Monograph CE/3, London, 1971, l-59 Hamor, R. Jy, Smith, 1. W. and Tyler, R. J. Comb. Name 1973,21,153 Smith, 1. W., and Tyler, R. J. Comb. Sci. Tech. 1974,9, 87 Badzioch, S. B. and Hawksley, P. G. W.ind. Eng. Chem. Proc. Des. Dev. 1970, 9, 521 Bailey, J. B. W., Brown, N. E. and Phillips, C. V. The Analysf.
1971,96,447 I 8 9 10 11 12 13
Basu, P. Fuel 1977,56, 390 Stewart, R. and Evans, D. G. Fuel 1967,46,263 Evans, D. G. Fuel 1973,52,186 McIntosh, M. J. Fuel 1976,55,47 Nsakala, N. Y., Essenhigh, R. H., and Walker, P. L. Jr. Fuel 1978.10.605 Avedesian, M. M. and Davidson, J. F. Trans. Inst. Chem. Eng. 1973,51,121 Field, M. A. et al. ‘Combustion of Pulverized Coal’ BCURA, Leatherhead, 1967, 346