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Emissions from a 30 kW rotating retort combustor burning coking coals
Emissions from a 30 kW rotating retort combustor burning coking coals C. R. Howarth,
A. A. Malik
and D. Pehlivan*
University of Newcastle upon Tyne, Newcastle upon *Firat University, Elazig, Turkey (Received 16 April 1992; revised 30 July 1992)
Tyne NE7 7RU
UK
A 30 kW pilot-scale rotating retort combustor was designed to burn a range of coking coals and to be capable of operating with different primary and secondary air distributions. By improving the air supply arrangements it was possible to increase the peak combustion efficiency from 80 to 95%. The NO, emission was in the range 61-258ppm and was subject to the well-known temperature effect. Total hydrocarbon emission levels (C,-C,) were significantly reduced to < 230 ppm with the improved air supply arrangements. Investigations on the effect of tilt and bed depth also showed a reduction in the CL-C, emission to < 15 ppm at the highest bed depth. Smoke levels were below Ringelmann 2 for all operating conditions. (Keywords: emissions: retorting; coking coal)
Fixed-bed retort stokers are still widely used for space heating because of their simplicity and ease of operation. They can also have environmental advantages; for instance, NO, emissions from fixed-grate appliances tend to be low because a degree of natural staging is often included as part of the combustion process. The low hydrocarbon emissions from fixed-bed combustors burning singles also make emission control relatively easy’. In recent years, the development of self-de-ashing underfeed stokers has been a major advance in the introduction of low-cost, fully automatic coal-fired boilers for both the domestic and commercial markets, although unfortunately the range of coals that can be handled by such systems is limited. For example, coking coals tend to cake and swell, and complete combustion is impracticable in the shallow bed, which ultimately becomes completely unreactive2.3. Such problems can be overcome by constant agitation of the firebed. This may be achieved by a variety of techniques, including simple manual raking to break the bed and expose fresh surface area to the air supply. In general this is not a practicable solution, and as an alternative a rotary combustor has been proposed. In this device, coal is continuously fed from a hopper to a tilted rotating retort by means of a screw feeder. The combustion chamber is continuously slowly rotated about its axis so that the coal bed is constantly agitated and mixed with the air supply. This geometry allows the ash to flow from the open end of the drum into a suitable receptacle. A 30 kW prototype has been designed and constructed, and as part of the performance study for this combustor4, the NO,, hydrocarbon ((Z-C,) and smoke emissions have been measured and the effects of air/fuel ratio, coal bed depth and coal type have been studied. Initial trials Presented at ‘Environmental Aspects of Coal Utilization and Carbon Science Conference’. upon Tyne. UK
31 March-2
001~2361/93/030315-06 (’ 1993 Butterworth-Heinemann
April 1992, University
Ltd
of Newcastle
using moderately swelling coals were carried out to assess the combustion efficiency as a function of coal throughput and air/fuel ratio at two different air slot settings. These trials showed that an improved air distribution could lead to increased peak combustion efficiency5. This paper reports the monitoring of NO,, hydrocarbon and smoke levels and also the combustion efficiencies for some of the conditions examined. EXPERIMENTAL Rotating
retort.
Mk-II
As shown in Figure I, the retort consists of three main parts, as follows. Coal ,freder. Coal is continuously fed from a sealed hopper to a tilted rotating combustion chamber by means of a screw. The rate of feed depends on the amount of combustibles in ash and is adjusted by varying the speed of a d.c. motor to achieve minimum combustible loss. Combustion c-lumber. The chamber consists of three cylindrical steel sections, each separated by an air distribution slot. The combustion chamber is continuously rotated about its axis so that the coal bed is constantly agitated and mixed with the air supply and ash flows from the open end of the drum into a suitable receptacle. Air distribution. The air is supplied via the plenum chamber completely surrounding the rotating combustion chamber. Coarse air adjustment can be achieved by varying the width of air slots between the bottom and middle combustion chamber cylinders, middle and top cylinders, and top cylinder and stationary front flange. Fine adjustment uses air control bands which reduce the flow of air into the slots. As the coal bed rotates, air passes through the part of the slot covered by the bed
FUEL, 1993, Vol 72, March
315
Emissions from a 30kW
rotating retort combustor: Air
‘:=
plenum
C. R. Howarth et al.
chamber
pp~\zz%+& /\
Rotating
combustor -330
Figure
1
Mk II prototype
T--t
Coal
rotating
h’ rcL5-J A
n
n
,x
+?9M ,
Drum
retor;
Jll
hopper
L-Screw
drive
diive
mm------r
combustor
Flue
gas
to atmosphere Gas sample \
Insulated hood with chimney Water
Figure 3
- Sealed
Water
cooled
gas
device
Rotating retor; combustor
Arrangement
of sealed ducting
for flue gas sampling
(primary air) and also through the slot region not covered by the bed (secondary air). The operational
variables are as follows:
1. The retort can be tilted to any suitable angle to achieve
2. 3. 4.
of Rue gas sampling
probe (not to scale)
ducttng
sampling
Figure 2
Diagram
out
a range of bed depths, based on the requirement for the coal type. The speed of rotation of the retort can be varied from 0 to 5revmiK*. Individual air slot widths can be altered to vary the air distribution along the firebed. Fine control of air can also be attained using the flexible bands around the slots. The coal flow rate can be varied by adjusting the speed of the screw.
of a 90” angled section of ducting approximately the same diameter as the retort, installed at the front end of the combustor as outlined in Figure 2. This arrangement enabled the ash to fail into the collecting bin and simultaneously allowed gas to be sampled at the outlet of the retort. To insert the gas sampling probe, a hole was drilled 1Ocm from the outlet end of the angled ducting. This system was introduced only to study the performance of the combustor and would not be necessary in an operational unit. For permanent gases other than NO,, a stainless steel water-cooled gas sampling probe (Figure 3) was connected to the front section of the sealed system. The water flow rate was high, to quench the flue ga@. A vacuum pump was used to draw the combustion gases at 5 1min- ’ into a collection bottle, from which small samples were withdrawn by syringe and analysed. The sampling rate was low compared with the total flue gas flow rate of 15CL2001min-‘. For NO, sampling, the probe used was a separate quartz tube (3 mm i.d.). This avoided the moisture condensation inevitable in a water-cooled probe. The sampling rate was 5 1min- ‘. Flue gas analysis
Flue gas sampling
The flue gas was analysed for CO,, CO, O,, NO, and higher hydrocarbons. Gas chromatography was used to detect the permanent gases and hydrocarbons levels, and a calorimetric method for NO, determination. Smoke was monitored with a smoke meter calibrated against the Ringelmann scale according to BS2740.
Early attempts at gas analysis produced inaccurate CO, results because air leaked into the free-standing insulated stack chamber, shown in Figure 2. To avoid air infiltration during Rue gas sampling, an internal sealed duct was connected directly to the retort. This consisted
Permanent gases. The gas chromatograph used was a Pye Model 44 with a katharometer and two separation columns, both 3m x 3mm, operated at 150°C with helium as carrier gas. For 0, and CO the first column
5.
316
FUEL, 1993, Vol 72, March
Emissions from a 30kW Table 1
Properties
of coals” Gedling singles (A)
Proximate analysis (wt% as received) 14.1 Moisture 4.5 Ash 32.1 Volatile matter 48.7 Fixed carbon Ultimate analysis (wt% dmmf) Carbon 81.4 5.5 Hydrogen Nitrogen I.9 1.5 Sulphur Chlorine 0.43 9.2 Oxygen Calorific value 27 260 (kJkg_‘l: BS swelling no. 2 Gray-King coke type C “Supplied
Lynemouth
Dawdon
(B)
(Cl
11.9 6.0 31.0 51.1
9.1 6.1 30.1 54. I
84.1 5.4 2.0 1.2 0.04 7.3 28 I20
85.6 5.1 1.8 0.8 0.13 6.5 29 200
2 E
7 G3
by British Coal
was packed with molecular sieve and for CO, the second column was packed with carbon.
Hvdrocarhons. The gas chromatograph used was a Periin Elmer Sigma 3 with flame ionization detector and a 2m x 3 mm separation column packed with Chromosorb-108, with nitrogen as carrier gas. The column temperature was programmed from 100 to 200°C. Standard calibration gases comprising methane, ethane, acetylene, ethylene, propane, propylene, n-butane, isobutane, isobutene, n-butylene, n-pentane, isopentene and isohexene from 43 ppmv to 1.04 vol.% in air were used to calibrate the chromatograph. The chromatograms of the flue gases showed the presence of all these gases.
NO,. The Saltzman calorimetric method was used’. A 50ml syringe was filled first with lOm1 of Saltzman reagent (a mixture of sulphanilic acid, N-(l-naphthyl)ethylenediamine dihydrochloride and acetic acid) and then with the flue gas sample. After vigorous absorption of the sample, the absorbance was measured at 550 nm. Coals
Two medium-swelling coals and one highly swelling coal were used. Their properties are shown in Table I. RESULTS AND DISCUSSION All combustors and boilers should ideally combine good combustion efficiencies with low pollution emission. For this rotary combustor, NO, and hydrocarbon emissions and smoke levels were recorded for various conditions of bed depth, air/fuel ratio and coal type. As a first stage in characterization of the Mk II unit, cold tests were carried out using coal singles4. These tests had a dual purpose: (1) to establish the physical parameters of average residence time, bed depth and bed surface area; (2) to ensure correct distribution of air supply when coal was present. The physical parameters were determined as functions of coal flow rate, angle of tilt of combustor and speed of rotation. The bed was found to be essentially in plug flow and the speed of rotation had only a small effect on residence time and bed surface area. However, the effects of tilt and coal
rotating retort combustor:
C. R. Howarth et al.
feed rate were more pronounced. The residence time (defined as bed volume divided by feed rate) increased with an increase in the angle of tilt. The bed depth (defined as the average of two heights of the truncated right circular cylinder) also increased with tilt, as expected. The combined effect of increasing tilt and coal feed rate was at first to increase the bed surface area and then decrease it, suggesting an optimum value. In order to investigate the air distribution system, air velocities across the bed were measured with and without coal. As can be seen in Figure 1. air is supplied via slots located at various stages under the firebed. Ensuring that these slots supplied air effectively was a primary concern of this work, and measurements were carried out at tw@ air settings: Setting I. The slots were set at almost equal width when cold; however, longitudinal expansion of the retort against the bolted front flange in the hot condition reduced the width of the front slot. The ratio of the crosssectional areas under hot conditions became 23:40:37 (front to back), so the volumetric distribution should be close to this ratio. Setting 2. The slot widths were adjusted to be closely proportional to the bed volume over each slot at 30’ angle of tilt, to produce a ratio of 14:3 155. This ratio also allowed for contraction of the front slot during the hot run. Combustion performance
Since no boiler is attached to this unit, performance was measured by evaluating the combustion efficiency, based on the ratio of the amount of combustibles consumed to the combustibles in the coal feed. The combustibles consumed were determined from the total hydrocarbons and CO in the combustion gases and the carbon in ash. Tests were carried out to assess the combustion efficiency when burning coal A (mediumswelling, of rank 802) as a function of coal throughput and air/fuel ratio. Typical results in Figure 4 show the effect of the two air distributions. Matching the air to the immediate bed volume (air setting 2) produced an improvement in efficiency; combustion efficiencies > 90% were obtained with 35-40% excess air. Further work using coal B (also medium-swelling, of higher rank) produced combustion efficiencies < 90%; however, an increase in bed depth improved this to >95%, as can clearly be seen in Figure 5. Eflect of improved air distribution on h~~drocarborz emissions
Tests on medium-swelling coal B showed that improved air distribution led to a significant reduction in total hydrocarbon emissions from > 1000 to < 250 ppm at 30’ tilt, as shown in Figure 6. This was qualitatively supported by the observation4 that air slot setting 2 produced a homogeneous glowing surface of the bed, in contrast to the existence of three distinctive dark zones between the air slots at setting 1. The emissions of individual components for this coal at setting 2 were, in ppm: C,, 4-165; C,, 3-19; C,, nil; C,, 2-36; C,, 3-59: C,, l-93; C,, nil. Efect
of bed depth on hydrocarbon emissions
Hydrocarbon emission is related to the rate of volatiles emission, which decreases at lower temperatures4.
FUEL,
1993,
Vol
72, March
317
Emissions
from a 30kW
rotating
retort combustor:
II
H
GI Air-Fuel
*.
Ralio
* .*
A = 117 B = 12.6 c = 14.4 D= 15.8 E= 172 F= 11.7 G= 14.4 n = 17.2
et al.
C. R. Howarth
1000~
E2
I
Dotted
I
Solid
lines: lines:
Air slot Air slot
setting setting
1 2
ij,800 II t t : : 600 I
F
400:
50 ! Solid
lines:
Dotted
Air Slot Setting
2
lines: Air Slot Setting
1 1
40
0
I
I
I
3
6
9
Throughput
12
4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9
15
(kg/hr)
Figure 4 Effect of coal throughput on different air/fuel ratios (kg/kg) for coal A
combustion
l_.____ __ !
I
0’
Throughput efficiency
at
(kgihr)
emission Figure 6 Effect of air slot setting on total hydrocarbon air/fuel ratio 14.4 and bed depth 0.165m (30~ tilt) for coal B
Angle
Air
Fuel
ratio
of
Solid lines: Dotled lines:
-11.7
at
Bed depth
,,I,
30 40
018Sm 0.247 ml
c -14.4 "17.2
‘-11.7 L
-:‘- 14.4 O- 17.2
1.
.‘. ....._.
@.
‘..,
4oLL 4
I
I
5
6
I
7
Throughput
8
9
10
(kg/hr)
Figure 5 Effect of angle of tilt on combustion air for coal B
318
1
FUEL, 1993, Vol 72, March
efficiency at 60% excess
4
I
I
5
6 Throughput
0
. .. .. ;; ._.. ‘, :.~i
. .
7
9
8
(kg/hr)
Figure 7 Effect of bed depth on total hydrocarbon different air/fuel ratios (kg/kg) for coal B
emission
at
Emissions
from
Generally an increase in bed depth leads to a lower average temperature of the bed, so increasing the depth is one possible way of reducing pollution and also, in beds of cross-feed pattern, allows the evolved volatile matter, moisture and combustion air to pass through a layer of burning fuel l. Figure 7 shows the effect of air/ fuel ratio and bed depth (i.e. angle of tilt) on total hydrocarbon emission for coal B. At lower bed depths, the hydrocarbon levels were between 50 and 225 ppm. A considerable drop to negligible emission can be seen to result from the increase in bed depth; this low emission remained almost unchanged for all excess air levels. This coincided with the observation of improved combustion efficiency at higher depths as discussed above. For comparison, the effect of coal type on the total hydrocarbon emission at one air/fuel ratio (14.4) at the lowest bed depth is shown in Figure 8. The three coals emitted different hydrocarbon levels, and there appears to be no direct relation to coal type; however, it is clear that the combustor is capable of burning the highly swelling coal C with low emission levels. It is probable that different coal types perform differently, depending on the bed depth, and the angle of tilt will have to be optimized for a range of coal types. l$ecr oj‘bed depth and excess air on NO, emission As bed depth is related to temperature, the NO, emission for two bed depths (two angles of tilt), different excess air levels and air slot setting 2 were monitored. Figure 9 shows the effects of excess air and bed depth on the emission of NO, for coal B. It can be seen that for a 30” tilt the NO, levels are at a maximum at 60% excess air (air/fuel ratio 14.4). As the flue gas temperatures were found to be highest at 60% excess air4, this result appears
_____
T
Coal Type ‘Gedling ‘, Lynemouth + Dawden
- A (SI = 2)
t
- c (SI = 7)
,’
1
retort
combustor:
et a I.
C. R. Howarth
300 Solid lines: Bed depth of 0 247 m (40 degree
tilt)
Dotted lines’ Bed depth of 0.165 m (30 degree
tilt1
B
2501
\ e. : : :
. .
:o.,
*
‘. ‘.
: .
.
. .
;.’ E
‘. . .
.
‘_
‘.
:,:,:
‘. $1. ‘.
::
Air Fuel Ratio
50t
A=117
: ., .
B=l44
:
c=172 D = 11.7
:
E=144
O-
4
__ _ _Lpp
5
F
h
F=172
I ._.__L
._.~~~_
6
7
Throughput
8
D
9
10
(kgihr)
Figure 9 Effect of air/fuel ratio (kg/kg) and bed depth on NOZ emission for coal B
to confirm the well-known effect of temperature on NO, levels. A slight increase in NO, with bed depth was also observed. This could be due to the higher average temperature of the bed surface, thus enhancing the thermal NO,. This is in agreement with the findings of other workers’. Smoke levels
During all the runs no smoke was visible and no permanent percentage obscuration was recorded by the smoke meter. Once steady-state operation had been reached, typical chart recordings gave zero smoke levels. However. at start-up small peaks were obtained, but these never exceeded Ringelmann 2.
.’
- B (SI = 2)
a 30 k W rotating
<+
CONCLUSIONS This combustor allows more degrees of freedom of operation than conventional fixed bed units. The interrelation between speed of rotation, angle of tilt (related to bed depth), air distribution and the coal type is complex, but the following general conclusions can be drawn. --n
> 4
5 Throughput
Figure 8
ratio 14.4
6
7 (kg/hr)
Effect of coal type on total hydrocarbon emission at air/fuel
Combustion of coking coal of swelling index up to 7 can be achieved by modest agitation produced by slow retort rotation. This ensures that the whole bed is in motion and uniform combustion of all the coal particles can be achieved. Bed geometry is related to retort tilt. To provide uniform air distribution for uniform combustion over the bed surface, the width of the air supply slot should be set according to the bed volume. This produces more uniform combustion and considerably improves
FUEL,
1993,
Vol 72,
March
319
Emissions from a 30kW
3.
4.
5. 6.
7.
rotating
retort combustor:
C. R. Howarth
the efficiency, from typically 7CL80% to >95% at a throughput of -7.5 kgh-‘. Turndown for overnight operation can be successfully achieved by simply stopping the rotation and feed and increasing the angle of tilt to give a deeper bed. For medium-swelling coal B at least, changes in air/fuel ratio from 11.7 to 17.2 have only a marginal effect on hydrocarbon emission, but a more uniform supply of air, related to the bed volumes above the supply ports, significantly reduces the emission from > 1200 to < 200 ppm. Hydrocarbon emission decreases with an increase in bed depth (tilt angle). Coal type has an effect on hydrocarbon emission, but it has not been possible to quantify this for the one bed depth used. It is clear that optimization of the relation between depth, air supply and coal type is required in order to produce a design specification for a range of coal types. Higher temperatures in the flue gas lead to high levels of NO, emission. However, NO, emission does not
320
FUEL, 1993, Vol 72, March
et al.
exceed 260ppm for all operating conditions and is thus within the range for non-agitated fixed-bed combustors. 8. After initial firing no smoke is visible, and smoke emission is below Ringelmann 2 for all operating conditions. REFERENCES Elliott. M. A. (Ed.). ‘Chemistry of Coal Utilization’. Second supplementary volume. Wiley-Interscience, New York, 1981. Ch. 21 Dunningham, A. C. and Grumell. E. S. Fuel 1938, 17, 324 Kavarana. B. J. Ph.D. Thesis. Universitv of Sheffield. 1963 Malik, A. A. Ph.D. Thesis, University of: Newcastle upon Tyne, 1991 Malik, A. A., Crowther, M. E., Harker, J. H., Holtham, R. D. and Howarth. C. R. J. Inst. Enerav 1992, 65, 90 Chedaille, J. and Braud, Y. ‘Industrial Flames’, Vol. 1, Edward Arnold. London, 1972 Saltzman. B. E. Anal. Chem. 1954. 26, 1949 Cooke, M. J. and Ford, N. Institution of Chemical Engineers, Enairon. Bull. 1990. (OO9), 3
A. A. Malik
and D. Pehlivan*
University of Newcastle upon Tyne, Newcastle upon *Firat University, Elazig, Turkey (Received 16 April 1992; revised 30 July 1992)
Tyne NE7 7RU
UK
A 30 kW pilot-scale rotating retort combustor was designed to burn a range of coking coals and to be capable of operating with different primary and secondary air distributions. By improving the air supply arrangements it was possible to increase the peak combustion efficiency from 80 to 95%. The NO, emission was in the range 61-258ppm and was subject to the well-known temperature effect. Total hydrocarbon emission levels (C,-C,) were significantly reduced to < 230 ppm with the improved air supply arrangements. Investigations on the effect of tilt and bed depth also showed a reduction in the CL-C, emission to < 15 ppm at the highest bed depth. Smoke levels were below Ringelmann 2 for all operating conditions. (Keywords: emissions: retorting; coking coal)
Fixed-bed retort stokers are still widely used for space heating because of their simplicity and ease of operation. They can also have environmental advantages; for instance, NO, emissions from fixed-grate appliances tend to be low because a degree of natural staging is often included as part of the combustion process. The low hydrocarbon emissions from fixed-bed combustors burning singles also make emission control relatively easy’. In recent years, the development of self-de-ashing underfeed stokers has been a major advance in the introduction of low-cost, fully automatic coal-fired boilers for both the domestic and commercial markets, although unfortunately the range of coals that can be handled by such systems is limited. For example, coking coals tend to cake and swell, and complete combustion is impracticable in the shallow bed, which ultimately becomes completely unreactive2.3. Such problems can be overcome by constant agitation of the firebed. This may be achieved by a variety of techniques, including simple manual raking to break the bed and expose fresh surface area to the air supply. In general this is not a practicable solution, and as an alternative a rotary combustor has been proposed. In this device, coal is continuously fed from a hopper to a tilted rotating retort by means of a screw feeder. The combustion chamber is continuously slowly rotated about its axis so that the coal bed is constantly agitated and mixed with the air supply. This geometry allows the ash to flow from the open end of the drum into a suitable receptacle. A 30 kW prototype has been designed and constructed, and as part of the performance study for this combustor4, the NO,, hydrocarbon ((Z-C,) and smoke emissions have been measured and the effects of air/fuel ratio, coal bed depth and coal type have been studied. Initial trials Presented at ‘Environmental Aspects of Coal Utilization and Carbon Science Conference’. upon Tyne. UK
31 March-2
001~2361/93/030315-06 (’ 1993 Butterworth-Heinemann
April 1992, University
Ltd
of Newcastle
using moderately swelling coals were carried out to assess the combustion efficiency as a function of coal throughput and air/fuel ratio at two different air slot settings. These trials showed that an improved air distribution could lead to increased peak combustion efficiency5. This paper reports the monitoring of NO,, hydrocarbon and smoke levels and also the combustion efficiencies for some of the conditions examined. EXPERIMENTAL Rotating
retort.
Mk-II
As shown in Figure I, the retort consists of three main parts, as follows. Coal ,freder. Coal is continuously fed from a sealed hopper to a tilted rotating combustion chamber by means of a screw. The rate of feed depends on the amount of combustibles in ash and is adjusted by varying the speed of a d.c. motor to achieve minimum combustible loss. Combustion c-lumber. The chamber consists of three cylindrical steel sections, each separated by an air distribution slot. The combustion chamber is continuously rotated about its axis so that the coal bed is constantly agitated and mixed with the air supply and ash flows from the open end of the drum into a suitable receptacle. Air distribution. The air is supplied via the plenum chamber completely surrounding the rotating combustion chamber. Coarse air adjustment can be achieved by varying the width of air slots between the bottom and middle combustion chamber cylinders, middle and top cylinders, and top cylinder and stationary front flange. Fine adjustment uses air control bands which reduce the flow of air into the slots. As the coal bed rotates, air passes through the part of the slot covered by the bed
FUEL, 1993, Vol 72, March
315
Emissions from a 30kW
rotating retort combustor: Air
‘:=
plenum
C. R. Howarth et al.
chamber
pp~\zz%+& /\
Rotating
combustor -330
Figure
1
Mk II prototype
T--t
Coal
rotating
h’ rcL5-J A
n
n
,x
+?9M ,
Drum
retor;
Jll
hopper
L-Screw
drive
diive
mm------r
combustor
Flue
gas
to atmosphere Gas sample \
Insulated hood with chimney Water
Figure 3
- Sealed
Water
cooled
gas
device
Rotating retor; combustor
Arrangement
of sealed ducting
for flue gas sampling
(primary air) and also through the slot region not covered by the bed (secondary air). The operational
variables are as follows:
1. The retort can be tilted to any suitable angle to achieve
2. 3. 4.
of Rue gas sampling
probe (not to scale)
ducttng
sampling
Figure 2
Diagram
out
a range of bed depths, based on the requirement for the coal type. The speed of rotation of the retort can be varied from 0 to 5revmiK*. Individual air slot widths can be altered to vary the air distribution along the firebed. Fine control of air can also be attained using the flexible bands around the slots. The coal flow rate can be varied by adjusting the speed of the screw.
of a 90” angled section of ducting approximately the same diameter as the retort, installed at the front end of the combustor as outlined in Figure 2. This arrangement enabled the ash to fail into the collecting bin and simultaneously allowed gas to be sampled at the outlet of the retort. To insert the gas sampling probe, a hole was drilled 1Ocm from the outlet end of the angled ducting. This system was introduced only to study the performance of the combustor and would not be necessary in an operational unit. For permanent gases other than NO,, a stainless steel water-cooled gas sampling probe (Figure 3) was connected to the front section of the sealed system. The water flow rate was high, to quench the flue ga@. A vacuum pump was used to draw the combustion gases at 5 1min- ’ into a collection bottle, from which small samples were withdrawn by syringe and analysed. The sampling rate was low compared with the total flue gas flow rate of 15CL2001min-‘. For NO, sampling, the probe used was a separate quartz tube (3 mm i.d.). This avoided the moisture condensation inevitable in a water-cooled probe. The sampling rate was 5 1min- ‘. Flue gas analysis
Flue gas sampling
The flue gas was analysed for CO,, CO, O,, NO, and higher hydrocarbons. Gas chromatography was used to detect the permanent gases and hydrocarbons levels, and a calorimetric method for NO, determination. Smoke was monitored with a smoke meter calibrated against the Ringelmann scale according to BS2740.
Early attempts at gas analysis produced inaccurate CO, results because air leaked into the free-standing insulated stack chamber, shown in Figure 2. To avoid air infiltration during Rue gas sampling, an internal sealed duct was connected directly to the retort. This consisted
Permanent gases. The gas chromatograph used was a Pye Model 44 with a katharometer and two separation columns, both 3m x 3mm, operated at 150°C with helium as carrier gas. For 0, and CO the first column
5.
316
FUEL, 1993, Vol 72, March
Emissions from a 30kW Table 1
Properties
of coals” Gedling singles (A)
Proximate analysis (wt% as received) 14.1 Moisture 4.5 Ash 32.1 Volatile matter 48.7 Fixed carbon Ultimate analysis (wt% dmmf) Carbon 81.4 5.5 Hydrogen Nitrogen I.9 1.5 Sulphur Chlorine 0.43 9.2 Oxygen Calorific value 27 260 (kJkg_‘l: BS swelling no. 2 Gray-King coke type C “Supplied
Lynemouth
Dawdon
(B)
(Cl
11.9 6.0 31.0 51.1
9.1 6.1 30.1 54. I
84.1 5.4 2.0 1.2 0.04 7.3 28 I20
85.6 5.1 1.8 0.8 0.13 6.5 29 200
2 E
7 G3
by British Coal
was packed with molecular sieve and for CO, the second column was packed with carbon.
Hvdrocarhons. The gas chromatograph used was a Periin Elmer Sigma 3 with flame ionization detector and a 2m x 3 mm separation column packed with Chromosorb-108, with nitrogen as carrier gas. The column temperature was programmed from 100 to 200°C. Standard calibration gases comprising methane, ethane, acetylene, ethylene, propane, propylene, n-butane, isobutane, isobutene, n-butylene, n-pentane, isopentene and isohexene from 43 ppmv to 1.04 vol.% in air were used to calibrate the chromatograph. The chromatograms of the flue gases showed the presence of all these gases.
NO,. The Saltzman calorimetric method was used’. A 50ml syringe was filled first with lOm1 of Saltzman reagent (a mixture of sulphanilic acid, N-(l-naphthyl)ethylenediamine dihydrochloride and acetic acid) and then with the flue gas sample. After vigorous absorption of the sample, the absorbance was measured at 550 nm. Coals
Two medium-swelling coals and one highly swelling coal were used. Their properties are shown in Table I. RESULTS AND DISCUSSION All combustors and boilers should ideally combine good combustion efficiencies with low pollution emission. For this rotary combustor, NO, and hydrocarbon emissions and smoke levels were recorded for various conditions of bed depth, air/fuel ratio and coal type. As a first stage in characterization of the Mk II unit, cold tests were carried out using coal singles4. These tests had a dual purpose: (1) to establish the physical parameters of average residence time, bed depth and bed surface area; (2) to ensure correct distribution of air supply when coal was present. The physical parameters were determined as functions of coal flow rate, angle of tilt of combustor and speed of rotation. The bed was found to be essentially in plug flow and the speed of rotation had only a small effect on residence time and bed surface area. However, the effects of tilt and coal
rotating retort combustor:
C. R. Howarth et al.
feed rate were more pronounced. The residence time (defined as bed volume divided by feed rate) increased with an increase in the angle of tilt. The bed depth (defined as the average of two heights of the truncated right circular cylinder) also increased with tilt, as expected. The combined effect of increasing tilt and coal feed rate was at first to increase the bed surface area and then decrease it, suggesting an optimum value. In order to investigate the air distribution system, air velocities across the bed were measured with and without coal. As can be seen in Figure 1. air is supplied via slots located at various stages under the firebed. Ensuring that these slots supplied air effectively was a primary concern of this work, and measurements were carried out at tw@ air settings: Setting I. The slots were set at almost equal width when cold; however, longitudinal expansion of the retort against the bolted front flange in the hot condition reduced the width of the front slot. The ratio of the crosssectional areas under hot conditions became 23:40:37 (front to back), so the volumetric distribution should be close to this ratio. Setting 2. The slot widths were adjusted to be closely proportional to the bed volume over each slot at 30’ angle of tilt, to produce a ratio of 14:3 155. This ratio also allowed for contraction of the front slot during the hot run. Combustion performance
Since no boiler is attached to this unit, performance was measured by evaluating the combustion efficiency, based on the ratio of the amount of combustibles consumed to the combustibles in the coal feed. The combustibles consumed were determined from the total hydrocarbons and CO in the combustion gases and the carbon in ash. Tests were carried out to assess the combustion efficiency when burning coal A (mediumswelling, of rank 802) as a function of coal throughput and air/fuel ratio. Typical results in Figure 4 show the effect of the two air distributions. Matching the air to the immediate bed volume (air setting 2) produced an improvement in efficiency; combustion efficiencies > 90% were obtained with 35-40% excess air. Further work using coal B (also medium-swelling, of higher rank) produced combustion efficiencies < 90%; however, an increase in bed depth improved this to >95%, as can clearly be seen in Figure 5. Eflect of improved air distribution on h~~drocarborz emissions
Tests on medium-swelling coal B showed that improved air distribution led to a significant reduction in total hydrocarbon emissions from > 1000 to < 250 ppm at 30’ tilt, as shown in Figure 6. This was qualitatively supported by the observation4 that air slot setting 2 produced a homogeneous glowing surface of the bed, in contrast to the existence of three distinctive dark zones between the air slots at setting 1. The emissions of individual components for this coal at setting 2 were, in ppm: C,, 4-165; C,, 3-19; C,, nil; C,, 2-36; C,, 3-59: C,, l-93; C,, nil. Efect
of bed depth on hydrocarbon emissions
Hydrocarbon emission is related to the rate of volatiles emission, which decreases at lower temperatures4.
FUEL,
1993,
Vol
72, March
317
Emissions
from a 30kW
rotating
retort combustor:
II
H
GI Air-Fuel
*.
Ralio
* .*
A = 117 B = 12.6 c = 14.4 D= 15.8 E= 172 F= 11.7 G= 14.4 n = 17.2
et al.
C. R. Howarth
1000~
E2
I
Dotted
I
Solid
lines: lines:
Air slot Air slot
setting setting
1 2
ij,800 II t t : : 600 I
F
400:
50 ! Solid
lines:
Dotted
Air Slot Setting
2
lines: Air Slot Setting
1 1
40
0
I
I
I
3
6
9
Throughput
12
4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9
15
(kg/hr)
Figure 4 Effect of coal throughput on different air/fuel ratios (kg/kg) for coal A
combustion
l_.____ __ !
I
0’
Throughput efficiency
at
(kgihr)
emission Figure 6 Effect of air slot setting on total hydrocarbon air/fuel ratio 14.4 and bed depth 0.165m (30~ tilt) for coal B
Angle
Air
Fuel
ratio
of
Solid lines: Dotled lines:
-11.7
at
Bed depth
,,I,
30 40
018Sm 0.247 ml
c -14.4 "17.2
‘-11.7 L
-:‘- 14.4 O- 17.2
1.
.‘. ....._.
@.
‘..,
4oLL 4
I
I
5
6
I
7
Throughput
8
9
10
(kg/hr)
Figure 5 Effect of angle of tilt on combustion air for coal B
318
1
FUEL, 1993, Vol 72, March
efficiency at 60% excess
4
I
I
5
6 Throughput
0
. .. .. ;; ._.. ‘, :.~i
. .
7
9
8
(kg/hr)
Figure 7 Effect of bed depth on total hydrocarbon different air/fuel ratios (kg/kg) for coal B
emission
at
Emissions
from
Generally an increase in bed depth leads to a lower average temperature of the bed, so increasing the depth is one possible way of reducing pollution and also, in beds of cross-feed pattern, allows the evolved volatile matter, moisture and combustion air to pass through a layer of burning fuel l. Figure 7 shows the effect of air/ fuel ratio and bed depth (i.e. angle of tilt) on total hydrocarbon emission for coal B. At lower bed depths, the hydrocarbon levels were between 50 and 225 ppm. A considerable drop to negligible emission can be seen to result from the increase in bed depth; this low emission remained almost unchanged for all excess air levels. This coincided with the observation of improved combustion efficiency at higher depths as discussed above. For comparison, the effect of coal type on the total hydrocarbon emission at one air/fuel ratio (14.4) at the lowest bed depth is shown in Figure 8. The three coals emitted different hydrocarbon levels, and there appears to be no direct relation to coal type; however, it is clear that the combustor is capable of burning the highly swelling coal C with low emission levels. It is probable that different coal types perform differently, depending on the bed depth, and the angle of tilt will have to be optimized for a range of coal types. l$ecr oj‘bed depth and excess air on NO, emission As bed depth is related to temperature, the NO, emission for two bed depths (two angles of tilt), different excess air levels and air slot setting 2 were monitored. Figure 9 shows the effects of excess air and bed depth on the emission of NO, for coal B. It can be seen that for a 30” tilt the NO, levels are at a maximum at 60% excess air (air/fuel ratio 14.4). As the flue gas temperatures were found to be highest at 60% excess air4, this result appears
_____
T
Coal Type ‘Gedling ‘, Lynemouth + Dawden
- A (SI = 2)
t
- c (SI = 7)
,’
1
retort
combustor:
et a I.
C. R. Howarth
300 Solid lines: Bed depth of 0 247 m (40 degree
tilt)
Dotted lines’ Bed depth of 0.165 m (30 degree
tilt1
B
2501
\ e. : : :
. .
:o.,
*
‘. ‘.
: .
.
. .
;.’ E
‘. . .
.
‘_
‘.
:,:,:
‘. $1. ‘.
::
Air Fuel Ratio
50t
A=117
: ., .
B=l44
:
c=172 D = 11.7
:
E=144
O-
4
__ _ _Lpp
5
F
h
F=172
I ._.__L
._.~~~_
6
7
Throughput
8
D
9
10
(kgihr)
Figure 9 Effect of air/fuel ratio (kg/kg) and bed depth on NOZ emission for coal B
to confirm the well-known effect of temperature on NO, levels. A slight increase in NO, with bed depth was also observed. This could be due to the higher average temperature of the bed surface, thus enhancing the thermal NO,. This is in agreement with the findings of other workers’. Smoke levels
During all the runs no smoke was visible and no permanent percentage obscuration was recorded by the smoke meter. Once steady-state operation had been reached, typical chart recordings gave zero smoke levels. However. at start-up small peaks were obtained, but these never exceeded Ringelmann 2.
.’
- B (SI = 2)
a 30 k W rotating
<+
CONCLUSIONS This combustor allows more degrees of freedom of operation than conventional fixed bed units. The interrelation between speed of rotation, angle of tilt (related to bed depth), air distribution and the coal type is complex, but the following general conclusions can be drawn. --n
> 4
5 Throughput
Figure 8
ratio 14.4
6
7 (kg/hr)
Effect of coal type on total hydrocarbon emission at air/fuel
Combustion of coking coal of swelling index up to 7 can be achieved by modest agitation produced by slow retort rotation. This ensures that the whole bed is in motion and uniform combustion of all the coal particles can be achieved. Bed geometry is related to retort tilt. To provide uniform air distribution for uniform combustion over the bed surface, the width of the air supply slot should be set according to the bed volume. This produces more uniform combustion and considerably improves
FUEL,
1993,
Vol 72,
March
319
Emissions from a 30kW
3.
4.
5. 6.
7.
rotating
retort combustor:
C. R. Howarth
the efficiency, from typically 7CL80% to >95% at a throughput of -7.5 kgh-‘. Turndown for overnight operation can be successfully achieved by simply stopping the rotation and feed and increasing the angle of tilt to give a deeper bed. For medium-swelling coal B at least, changes in air/fuel ratio from 11.7 to 17.2 have only a marginal effect on hydrocarbon emission, but a more uniform supply of air, related to the bed volumes above the supply ports, significantly reduces the emission from > 1200 to < 200 ppm. Hydrocarbon emission decreases with an increase in bed depth (tilt angle). Coal type has an effect on hydrocarbon emission, but it has not been possible to quantify this for the one bed depth used. It is clear that optimization of the relation between depth, air supply and coal type is required in order to produce a design specification for a range of coal types. Higher temperatures in the flue gas lead to high levels of NO, emission. However, NO, emission does not
320
FUEL, 1993, Vol 72, March
et al.
exceed 260ppm for all operating conditions and is thus within the range for non-agitated fixed-bed combustors. 8. After initial firing no smoke is visible, and smoke emission is below Ringelmann 2 for all operating conditions. REFERENCES Elliott. M. A. (Ed.). ‘Chemistry of Coal Utilization’. Second supplementary volume. Wiley-Interscience, New York, 1981. Ch. 21 Dunningham, A. C. and Grumell. E. S. Fuel 1938, 17, 324 Kavarana. B. J. Ph.D. Thesis. Universitv of Sheffield. 1963 Malik, A. A. Ph.D. Thesis, University of: Newcastle upon Tyne, 1991 Malik, A. A., Crowther, M. E., Harker, J. H., Holtham, R. D. and Howarth. C. R. J. Inst. Enerav 1992, 65, 90 Chedaille, J. and Braud, Y. ‘Industrial Flames’, Vol. 1, Edward Arnold. London, 1972 Saltzman. B. E. Anal. Chem. 1954. 26, 1949 Cooke, M. J. and Ford, N. Institution of Chemical Engineers, Enairon. Bull. 1990. (OO9), 3