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Single droplet combustion of coal-methanol slurry
Single droplet slurry Takeshi
Sakai,
combustion
Masayoshi
Sadakata
of coallnethanol
and Masahiro
Saito
Department of Chemical Engineering, Gunma University, Kiryu, Gunma 376, Japan (Received 5 July 1983; revised 26 June 7984)
Coal-methanol slurry (CMS) has attracted special interest as a new coal slurry fuel. In this work, the combustion characteristics of CMS have been investigated experimentally by single droplet combustion with the following results. The scattering of pulverized coal particles was observed during the gas-phase combustion period. The combustion behaviour of CMS was very similar to that of pulverized coal. The overall burning rate coefficient for CMS was apparently increased compared to other coal slurry fuels, using residual heavy fuel oil as the dispersion medium. It was concluded that CMS has excellent properties which can mask the defects of pulverized coal and other coal slurry fuels for combustion, as well as in transportation and handling. (Keywords: coal; coal-methanol
slurry; single droplet combustion)
After the oil crisis of 1973, coal slurry fuel technology has been developed as part of coal utilization technology. Coal slurry fuel has the advantages of handling, storing and transportation without the danger of a coal dust explosion. Various types of coal slurry fuels such as coaloil mixture (COM), SRC-oil slurry and tar-coal mixture (TCM) have been developed so far. However, the combustion, atomization and transportation characteristics of those fuels do not improve on those of residual heavy fuel oil’ -4. In this work, single droplet combusion of coalmethanol slurry (CMS), which consists of methanol and pulverized coal, has been examined. Studies concerned with CMS combustion are limited5. CMS has a great advantage in handling and transportation in a cold district because the freezing point of methanol is - 94°C. The possibility of transportation of CMS in a long pipeline of x 1000 km has been verified from a feasibility study. Furthermore, coal of inferior quality could be put to practical uses. It is well known that emissions of smoke, NO, and SO, from combustion of methanol are quite low. Accordingly, it is expected that the equipment for prevention of environmental pollution in a power station might be less than for pulverized coal combustion, with economic advantage. Table 1
EXPERIMENTAL The experimental apparatus used in this work has been reported previously4. The experimental conditions were as follows. The ambient air temperature was varied, stepwise, in the range 8OO-1000°C. A fuel droplet, suspended at the tip of a fine thermocouple, was forcibly ignited by passing through a small pilot flame located at the sidewall entrance of the combustion tube en route to the centre of the tube. In the case of combustion of a coal particle, this was retained in a small basket made of platinum wire of 0.1 mm diameter because the coal particle could not be suspended at the tip of the thermocouple. The characteristic initial particle diameters, based on the specific surface areas, were between 1.3 and 2.5 mm. The sample fuels used in these experiments were commercial methanol, coals, and mixtures of methanol and coals (CMS). The proximate analyses and the physical properties of the sample fuels are shown in Table 1. The values for CMS were calculated from the mass blending ratio of the components, which was 1 :l in all cases. C coal is bituminous, while M coal and K coal are subbituminous. The grain size of the pulverized coal used in CMS was -200 mesh (-74 pm), 70%. The ultimate analyses of the three coals are shown in Table 2.
Details of sample materials
Ash fwt%)
Volatile matter (wt%)
Fixed carbon
Sample
Moisture (wt%)
Methanol CMS (C coal, 50%) CMS (M coal, 50%) CMS (K coal, 50%) C coal M coal K coal
0 3.5 a.7 7.1 7.0 17.3 14.1
0 5.1 3.4 0.9 10.1 6.8 1.7
100 66.9 66.3 70.0 33.8 32.6 39.9
0016-2361/85/020163~$3.06 @ 1985 Butterworth & Co. (Publishers)
Ltd.
fwt%)
Sulphur fwt%)
Calorific value (kJ kp’ f
Density (g crns3
0 24.5 21.6 22.0 49.1 43.3 44.3
0 0.1 0.4 0.4 0.2 0.8 0.7
22320 _ 26000 22990 25040
0.796 1.075 1.055 1.041 1.548 1.604 1.503
FUEL,
1985,
Vol 64, February
1
163
Single
droplet
combustion:
T. Sakai
Table 2
Ultimate
analysis of methanol
et al.
and coals Iwt%, daf)
Sample
C
H
N
S
0 (by diff .I
Methanol
37.5
12.5
0
0
50.0
C coal M coal K coal
79.7 76.4 74.4
5.5 5.6 5.6
1 .l 1 .o 1.4
0.1 0.6 0.7
13.6 16.4 17.9
RESULTS AND DISCUSSION Burning behaviour of a CMS droplet
From a commonsense standpoint, it may be presumed that in combustion of CMS methanol would be evaporated and burned at first and then devolatilization of coal would follow. However, just after ignition of a CMS droplet, the blue flame of methanol was not observed by eye or by colour films taken with a Bolex 16 mm motion picture camera at 64 fps. Therefore, within an accuracy of l/64 s, it is considered that the gas-phase combustion of methanol occurs simultaneously with that of the volatile matter of coal because the bright yellow flame of the latter appears at the same ignition time. In this Paper, gas-phase combustion refers to the simultaneous combustion of both methanol and coal volatile matter. From the experimental results, the combustion of a CMS droplet progressed in a sequential two-stage process, gas-phase combustion of the volatile matter, followed by solid-phase combustion of the residual carbon. Figure I shows the burning sequence of a CMS droplet during the gas-phase combustion period. The scattering of pulverized coal which is shown by flight trajectories was observed during the gas-phase combustion in the case of any CMS mixture; this scattering combustion is a remarkable feature, which was not observed during previous work2-4 on the combustion of COM, SRC-il slurry and TCM. The scattering phenomena of a CMS droplet differ from the disruption of pulverized coal particles themselves. In the case of CMS combustion, the pulverized coal particles, scattered along the wake behind the CMS droplet, burned in the hot ambient air. From observation of the bright trajectories of the scattered coal particles, however, no evidence could be found of disruption of the particles themselves. Also, the scattering phenomena differed from the microexplosion of emulsified fuel such as an oil-water mixture reported earlier6. The fragments from a coaloil/methanol/water emulsified fuel droplet were scattered in all radial directions, the microexplosion being more violent than the scattering phenomena of a CMS droplet. The scattering phenomenon of CMS droplets occurs when they bum with relative velocity between the droplet and the ambient hot air. Within a practical furnace, atomized CMS droplets should have relative velocity and would bum with scattering. Because of their much smaller size, the scattered particles would burn out quickly compared with an agglomerate supported on the thermocouple. As is mentioned later, the burning rate coefficient of a CMS droplet defined in this Paper means the apparent burning rate coefficient. Diameter change and temperature history of CMS droplet Figure 2 shows the diameter change and the tempera-
ture history of a CMS droplet. The simultaneous com-
164
FUEL, 1985, Vol 64, February
Figure 1 Scattering combustion of a CMS (C coal, 50%) droplet during gas-phase combustion period
Burn-out
k I 1
I 2.0
OW
0
I
Time Figure
2
Diameter
I
4.0
change
6.0
IO
8.0
(5)
and temperature
history of a CMS
(C coal, 50%, -74 pm) droplet. T,=85OT. 0, Gas-phase combustion period; 0, solid-phase combustion period; -, particle temperature
bustion of methanol and coal volatile matter occurred as soon as the droplet ignited. At the same time the droplet temperature rose rapidly. Though the diameter decreased temporarily by volatilization just after ignition, the droplet swelled again due to thermal cracking of the coal at ~400°C. Following the gas-phase combustion, solidphase combustion occurred. During this latter period, the particle temperature recorded a peak value and then gradually decreased to the ambient air temperature, while the droplet diameter decreased linearly. After completion of combustion, a small amount of ash remained on the tip of the thermocouple. Initial diameter of solid residue
It is important to emphasise the difference between COM and CMS combustion. In the case of a COM
Single droplet combustion: T. Sakai et al.
the coal used, that for M coal being by far the smallest. It is presumed that this arose as a result of the greater scattering of pulverized coal during the gas-phase combustion of M coal CMS. < 2 a
0.2-
Effect of ambient air temperature on overall burning rate coeficient
0 l-
0.4-
0.3-
2
0.2-
> a 0.1 -
Oh-U
0
dL 0.5
1.0
1.5
1
The apparent burning rate coefficient for overall combustion, k,, based on the square of initial diameter was defined as described previously4. Figure 4 shows the effect of the ambient air temperature, T,, on k, for CMS, methanol and the sample coals. In the case of combustion of a methanol droplet, &,increased with ambient air temperature because the rate of vaporizing combustion was controlled by the heat transfer rate. Although the temperature dependencies of&for CMS mixture and the coals were very small, large differences in k, were recognized between the different fuel samples. The values of k, for the combustion of coal particles were lower than those for the other fuels because the solidphase combustion time of the coal particles was very long. In contrast to the combustion of a coal particle, the solid-phase combustion time of a CMS droplet became shorter because only small agglomerates of solid residue remained after the gas-phase combustion, due to the scattering of pulverized coal. Consequently, the apparent overall mean burning rate coefficient was increased by the scattering combustion.
hl
Relation between k, and (R,p/lOO)
In previous workZW4, it was observed that a solid residual agglomerate was formed after gas-phase com2
i Q/Do
1.
-
1.5
Figure 3 Diameter distribution of solid residue agglomerates formed just after gas-phase combustion. (a), CMS (C coal 50%. -74pm), T,=800-1000°C. x=0.81, 0=0.218; (b), CMS (M coal 50%, -74pm). T,=800-1000°C; X=0.58, a=0.136; (c), CMS (K coal 50%. -74pm). T,=800-1000°C; x=1.01, u=O.160
droplet, pulverized coal particles suspended within oil will become carbonized substance after gas-phase combustion and the oil will change to viscous substances of higher molecular weight. These substances act as an adhesive for the solid carbonaceous particles formed from the coal, and a COM droplet thus yields a large residual carbon particle, a so-called agglomerate of a size nearly equal to the initial droplet diameter, because disruption or scattering does not occur during the gas-phase combustion period. During combustion of a CMS droplet, part of the pulverized coal particles are scattered out of the droplet because of the weak adhesive property of methanol. The ratio of D,, the initial diameter of the solid residual agglomerate immediately after gas-phase combustion, to D,, the initial diameter of the fuel droplet before ignition, varied between experiments, and the distribution was analysed statistically. Figure 3 shows the relation between number distribution AN/N and DJDO for combustion of three kinds of CMS droplets. In each case DJDO has a wide distribution. The mean value of DJD, varied with
1.5t
7 ul N$ ’
.o -
A
9
0 0
0 l-l
0 .5-
O-
n
0 0
n
cl
I 800
0
I
I
900 Tr, (“Cl
I
I
1000
Figure 4 Effect of ambient air temperature on mean overall burning rate coefficient. x, Methanol; 0, CMS (C coal, 50%); A, CMS (M coal, 50%); 0. CMS (K coal, 50%); 0, C coal; A M coal; W, K coal
FUEL, 1985, Vol 64, February
165
Single droplet combustion: T. Sakai et al. -
SRC -oil slurry
3.
o-
2.
o-
: .
8001000
C coal
M coal K coal
\ 1 o‘i In
coefficient k, was related to the produce of the fuel density, p, and the residual carbon content (w/w) within the slurry before combustion, Rc, for non-scattering combustion. The quantity R,-p accordingly represents the residual carbon content (w/v) within a unit volume of slurry fuel. Figure 5 is a log-log plot of G0uerws R&100. It is seen that the values of k, for non-scattering combustion nearly coincide with the single straight line derived from Essenhigh’s’ theoretical equation by assuming film diffusion rate control. For comparison, the data for CMS have been plotted in the same figure. It is seen that the k, values for CMS were generally higher than those for non-scattering coal slurry fuels. In these experiments, differences in the values of k, were recognized among the three kinds of CMS; k, for M coal CMS was far higher than the predicted line and &for C coal CMS was also high. However, k, could be explained by the degree of scattering, which might decrease in the order CMS of M coal, C coal, K coal, since the particle size of a solid residual agglomerate formed after scattering combustion became statistically larger in the same order, as was shown in Figure 3. Judging from the experimental results, CMS will be promising as a new coal slurry fuel since its combustion characteristics are superior to those of the other coal slurry fuels.
I Is” 0
5-
CONCLUSIONS
0. 40 3-
0. 2-
0 .l 0. 1
I 0.2
I 0.3
I
I 0.5
IIIII 1.0
R,p/100(gcm-3)
The following conclusions were drawn from the experiments on the single droplet combustion of coal-methanol slurry. The combustion of CMS is a sequential two-stage process, consisting of gas-phase combustion followed by solid-phase combustion. Scattering of pulverized coal occurred during the gas-phase combustion period. The effect of ambient air temperature on the apparent overall burning rate coefficient of CMS droplets was small. The apparent solid-phase combustion time of CMS droplets is shorter than that of COM, TCM, etc. under the same conditions.
Figure 5 Relation between mean overall burning rate coefficient, k,, and residual carbon content within a unit volume of fuel, Rc p. -, Essenhigh’s theoretical value
REFERENCES bustion, since scattering did not occur with combustion of COM, SRCoil slurry and TCM, etc. When residual oil or tar was used as the dispersion medium for those slurry fuels, the pitch produced by thermal cracking was likely to act as a binder of pulverized coal particles. Accordingly, the combustion of the solid residue agglomerate is considered to control the overall combustion rate of such a slurry fuel droplet. Therefore, the overall burning rate
166
FUEL, 1985, Vol 64, February
Carmi, S. and Ghassemzadeh, M. R. Fuel 1981,60, 529 Sakai, T. and Saito, M. J. Fuel Sot. Jpn. 1980,59, 642, 822 Sakai, T. and Saito, M. J. Fuel Sot. Jpn. 1981,60, 647, 183 Sakai, T. and Saito, M. Combustion and Flame 1983, 51, 141 Pan, Y. S., Bellas, G. T., Snedden, R. B., Wieczenski, D. E. and Joubert, J. I. ‘Fourth International Symposium on Coal Slurry Combustion’, Pittsburgh Energy Technology Center, Florida, 1982 Saito, M., Sadakata, M. and Sakai, T. Fuel 1983,62, 1481 Essenhigh, R. H. J. Inst. Fuel 1961,34,245, 239
Sakai,
combustion
Masayoshi
Sadakata
of coallnethanol
and Masahiro
Saito
Department of Chemical Engineering, Gunma University, Kiryu, Gunma 376, Japan (Received 5 July 1983; revised 26 June 7984)
Coal-methanol slurry (CMS) has attracted special interest as a new coal slurry fuel. In this work, the combustion characteristics of CMS have been investigated experimentally by single droplet combustion with the following results. The scattering of pulverized coal particles was observed during the gas-phase combustion period. The combustion behaviour of CMS was very similar to that of pulverized coal. The overall burning rate coefficient for CMS was apparently increased compared to other coal slurry fuels, using residual heavy fuel oil as the dispersion medium. It was concluded that CMS has excellent properties which can mask the defects of pulverized coal and other coal slurry fuels for combustion, as well as in transportation and handling. (Keywords: coal; coal-methanol
slurry; single droplet combustion)
After the oil crisis of 1973, coal slurry fuel technology has been developed as part of coal utilization technology. Coal slurry fuel has the advantages of handling, storing and transportation without the danger of a coal dust explosion. Various types of coal slurry fuels such as coaloil mixture (COM), SRC-oil slurry and tar-coal mixture (TCM) have been developed so far. However, the combustion, atomization and transportation characteristics of those fuels do not improve on those of residual heavy fuel oil’ -4. In this work, single droplet combusion of coalmethanol slurry (CMS), which consists of methanol and pulverized coal, has been examined. Studies concerned with CMS combustion are limited5. CMS has a great advantage in handling and transportation in a cold district because the freezing point of methanol is - 94°C. The possibility of transportation of CMS in a long pipeline of x 1000 km has been verified from a feasibility study. Furthermore, coal of inferior quality could be put to practical uses. It is well known that emissions of smoke, NO, and SO, from combustion of methanol are quite low. Accordingly, it is expected that the equipment for prevention of environmental pollution in a power station might be less than for pulverized coal combustion, with economic advantage. Table 1
EXPERIMENTAL The experimental apparatus used in this work has been reported previously4. The experimental conditions were as follows. The ambient air temperature was varied, stepwise, in the range 8OO-1000°C. A fuel droplet, suspended at the tip of a fine thermocouple, was forcibly ignited by passing through a small pilot flame located at the sidewall entrance of the combustion tube en route to the centre of the tube. In the case of combustion of a coal particle, this was retained in a small basket made of platinum wire of 0.1 mm diameter because the coal particle could not be suspended at the tip of the thermocouple. The characteristic initial particle diameters, based on the specific surface areas, were between 1.3 and 2.5 mm. The sample fuels used in these experiments were commercial methanol, coals, and mixtures of methanol and coals (CMS). The proximate analyses and the physical properties of the sample fuels are shown in Table 1. The values for CMS were calculated from the mass blending ratio of the components, which was 1 :l in all cases. C coal is bituminous, while M coal and K coal are subbituminous. The grain size of the pulverized coal used in CMS was -200 mesh (-74 pm), 70%. The ultimate analyses of the three coals are shown in Table 2.
Details of sample materials
Ash fwt%)
Volatile matter (wt%)
Fixed carbon
Sample
Moisture (wt%)
Methanol CMS (C coal, 50%) CMS (M coal, 50%) CMS (K coal, 50%) C coal M coal K coal
0 3.5 a.7 7.1 7.0 17.3 14.1
0 5.1 3.4 0.9 10.1 6.8 1.7
100 66.9 66.3 70.0 33.8 32.6 39.9
0016-2361/85/020163~$3.06 @ 1985 Butterworth & Co. (Publishers)
Ltd.
fwt%)
Sulphur fwt%)
Calorific value (kJ kp’ f
Density (g crns3
0 24.5 21.6 22.0 49.1 43.3 44.3
0 0.1 0.4 0.4 0.2 0.8 0.7
22320 _ 26000 22990 25040
0.796 1.075 1.055 1.041 1.548 1.604 1.503
FUEL,
1985,
Vol 64, February
1
163
Single
droplet
combustion:
T. Sakai
Table 2
Ultimate
analysis of methanol
et al.
and coals Iwt%, daf)
Sample
C
H
N
S
0 (by diff .I
Methanol
37.5
12.5
0
0
50.0
C coal M coal K coal
79.7 76.4 74.4
5.5 5.6 5.6
1 .l 1 .o 1.4
0.1 0.6 0.7
13.6 16.4 17.9
RESULTS AND DISCUSSION Burning behaviour of a CMS droplet
From a commonsense standpoint, it may be presumed that in combustion of CMS methanol would be evaporated and burned at first and then devolatilization of coal would follow. However, just after ignition of a CMS droplet, the blue flame of methanol was not observed by eye or by colour films taken with a Bolex 16 mm motion picture camera at 64 fps. Therefore, within an accuracy of l/64 s, it is considered that the gas-phase combustion of methanol occurs simultaneously with that of the volatile matter of coal because the bright yellow flame of the latter appears at the same ignition time. In this Paper, gas-phase combustion refers to the simultaneous combustion of both methanol and coal volatile matter. From the experimental results, the combustion of a CMS droplet progressed in a sequential two-stage process, gas-phase combustion of the volatile matter, followed by solid-phase combustion of the residual carbon. Figure I shows the burning sequence of a CMS droplet during the gas-phase combustion period. The scattering of pulverized coal which is shown by flight trajectories was observed during the gas-phase combustion in the case of any CMS mixture; this scattering combustion is a remarkable feature, which was not observed during previous work2-4 on the combustion of COM, SRC-il slurry and TCM. The scattering phenomena of a CMS droplet differ from the disruption of pulverized coal particles themselves. In the case of CMS combustion, the pulverized coal particles, scattered along the wake behind the CMS droplet, burned in the hot ambient air. From observation of the bright trajectories of the scattered coal particles, however, no evidence could be found of disruption of the particles themselves. Also, the scattering phenomena differed from the microexplosion of emulsified fuel such as an oil-water mixture reported earlier6. The fragments from a coaloil/methanol/water emulsified fuel droplet were scattered in all radial directions, the microexplosion being more violent than the scattering phenomena of a CMS droplet. The scattering phenomenon of CMS droplets occurs when they bum with relative velocity between the droplet and the ambient hot air. Within a practical furnace, atomized CMS droplets should have relative velocity and would bum with scattering. Because of their much smaller size, the scattered particles would burn out quickly compared with an agglomerate supported on the thermocouple. As is mentioned later, the burning rate coefficient of a CMS droplet defined in this Paper means the apparent burning rate coefficient. Diameter change and temperature history of CMS droplet Figure 2 shows the diameter change and the tempera-
ture history of a CMS droplet. The simultaneous com-
164
FUEL, 1985, Vol 64, February
Figure 1 Scattering combustion of a CMS (C coal, 50%) droplet during gas-phase combustion period
Burn-out
k I 1
I 2.0
OW
0
I
Time Figure
2
Diameter
I
4.0
change
6.0
IO
8.0
(5)
and temperature
history of a CMS
(C coal, 50%, -74 pm) droplet. T,=85OT. 0, Gas-phase combustion period; 0, solid-phase combustion period; -, particle temperature
bustion of methanol and coal volatile matter occurred as soon as the droplet ignited. At the same time the droplet temperature rose rapidly. Though the diameter decreased temporarily by volatilization just after ignition, the droplet swelled again due to thermal cracking of the coal at ~400°C. Following the gas-phase combustion, solidphase combustion occurred. During this latter period, the particle temperature recorded a peak value and then gradually decreased to the ambient air temperature, while the droplet diameter decreased linearly. After completion of combustion, a small amount of ash remained on the tip of the thermocouple. Initial diameter of solid residue
It is important to emphasise the difference between COM and CMS combustion. In the case of a COM
Single droplet combustion: T. Sakai et al.
the coal used, that for M coal being by far the smallest. It is presumed that this arose as a result of the greater scattering of pulverized coal during the gas-phase combustion of M coal CMS. < 2 a
0.2-
Effect of ambient air temperature on overall burning rate coeficient
0 l-
0.4-
0.3-
2
0.2-
> a 0.1 -
Oh-U
0
dL 0.5
1.0
1.5
1
The apparent burning rate coefficient for overall combustion, k,, based on the square of initial diameter was defined as described previously4. Figure 4 shows the effect of the ambient air temperature, T,, on k, for CMS, methanol and the sample coals. In the case of combustion of a methanol droplet, &,increased with ambient air temperature because the rate of vaporizing combustion was controlled by the heat transfer rate. Although the temperature dependencies of&for CMS mixture and the coals were very small, large differences in k, were recognized between the different fuel samples. The values of k, for the combustion of coal particles were lower than those for the other fuels because the solidphase combustion time of the coal particles was very long. In contrast to the combustion of a coal particle, the solid-phase combustion time of a CMS droplet became shorter because only small agglomerates of solid residue remained after the gas-phase combustion, due to the scattering of pulverized coal. Consequently, the apparent overall mean burning rate coefficient was increased by the scattering combustion.
hl
Relation between k, and (R,p/lOO)
In previous workZW4, it was observed that a solid residual agglomerate was formed after gas-phase com2
i Q/Do
1.
-
1.5
Figure 3 Diameter distribution of solid residue agglomerates formed just after gas-phase combustion. (a), CMS (C coal 50%. -74pm), T,=800-1000°C. x=0.81, 0=0.218; (b), CMS (M coal 50%, -74pm). T,=800-1000°C; X=0.58, a=0.136; (c), CMS (K coal 50%. -74pm). T,=800-1000°C; x=1.01, u=O.160
droplet, pulverized coal particles suspended within oil will become carbonized substance after gas-phase combustion and the oil will change to viscous substances of higher molecular weight. These substances act as an adhesive for the solid carbonaceous particles formed from the coal, and a COM droplet thus yields a large residual carbon particle, a so-called agglomerate of a size nearly equal to the initial droplet diameter, because disruption or scattering does not occur during the gas-phase combustion period. During combustion of a CMS droplet, part of the pulverized coal particles are scattered out of the droplet because of the weak adhesive property of methanol. The ratio of D,, the initial diameter of the solid residual agglomerate immediately after gas-phase combustion, to D,, the initial diameter of the fuel droplet before ignition, varied between experiments, and the distribution was analysed statistically. Figure 3 shows the relation between number distribution AN/N and DJDO for combustion of three kinds of CMS droplets. In each case DJDO has a wide distribution. The mean value of DJD, varied with
1.5t
7 ul N$ ’
.o -
A
9
0 0
0 l-l
0 .5-
O-
n
0 0
n
cl
I 800
0
I
I
900 Tr, (“Cl
I
I
1000
Figure 4 Effect of ambient air temperature on mean overall burning rate coefficient. x, Methanol; 0, CMS (C coal, 50%); A, CMS (M coal, 50%); 0. CMS (K coal, 50%); 0, C coal; A M coal; W, K coal
FUEL, 1985, Vol 64, February
165
Single droplet combustion: T. Sakai et al. -
SRC -oil slurry
3.
o-
2.
o-
: .
8001000
C coal
M coal K coal
\ 1 o‘i In
coefficient k, was related to the produce of the fuel density, p, and the residual carbon content (w/w) within the slurry before combustion, Rc, for non-scattering combustion. The quantity R,-p accordingly represents the residual carbon content (w/v) within a unit volume of slurry fuel. Figure 5 is a log-log plot of G0uerws R&100. It is seen that the values of k, for non-scattering combustion nearly coincide with the single straight line derived from Essenhigh’s’ theoretical equation by assuming film diffusion rate control. For comparison, the data for CMS have been plotted in the same figure. It is seen that the k, values for CMS were generally higher than those for non-scattering coal slurry fuels. In these experiments, differences in the values of k, were recognized among the three kinds of CMS; k, for M coal CMS was far higher than the predicted line and &for C coal CMS was also high. However, k, could be explained by the degree of scattering, which might decrease in the order CMS of M coal, C coal, K coal, since the particle size of a solid residual agglomerate formed after scattering combustion became statistically larger in the same order, as was shown in Figure 3. Judging from the experimental results, CMS will be promising as a new coal slurry fuel since its combustion characteristics are superior to those of the other coal slurry fuels.
I Is” 0
5-
CONCLUSIONS
0. 40 3-
0. 2-
0 .l 0. 1
I 0.2
I 0.3
I
I 0.5
IIIII 1.0
R,p/100(gcm-3)
The following conclusions were drawn from the experiments on the single droplet combustion of coal-methanol slurry. The combustion of CMS is a sequential two-stage process, consisting of gas-phase combustion followed by solid-phase combustion. Scattering of pulverized coal occurred during the gas-phase combustion period. The effect of ambient air temperature on the apparent overall burning rate coefficient of CMS droplets was small. The apparent solid-phase combustion time of CMS droplets is shorter than that of COM, TCM, etc. under the same conditions.
Figure 5 Relation between mean overall burning rate coefficient, k,, and residual carbon content within a unit volume of fuel, Rc p. -, Essenhigh’s theoretical value
REFERENCES bustion, since scattering did not occur with combustion of COM, SRCoil slurry and TCM, etc. When residual oil or tar was used as the dispersion medium for those slurry fuels, the pitch produced by thermal cracking was likely to act as a binder of pulverized coal particles. Accordingly, the combustion of the solid residue agglomerate is considered to control the overall combustion rate of such a slurry fuel droplet. Therefore, the overall burning rate
166
FUEL, 1985, Vol 64, February
Carmi, S. and Ghassemzadeh, M. R. Fuel 1981,60, 529 Sakai, T. and Saito, M. J. Fuel Sot. Jpn. 1980,59, 642, 822 Sakai, T. and Saito, M. J. Fuel Sot. Jpn. 1981,60, 647, 183 Sakai, T. and Saito, M. Combustion and Flame 1983, 51, 141 Pan, Y. S., Bellas, G. T., Snedden, R. B., Wieczenski, D. E. and Joubert, J. I. ‘Fourth International Symposium on Coal Slurry Combustion’, Pittsburgh Energy Technology Center, Florida, 1982 Saito, M., Sadakata, M. and Sakai, T. Fuel 1983,62, 1481 Essenhigh, R. H. J. Inst. Fuel 1961,34,245, 239