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Burnout Behaviour of Bituminous Coals in Air-Staged Combustion Conditions.

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Dev. Chem. Eng. Mineral Process., 9(3/4),pp.253-265,2001.
Burnout Behaviour of Bituminous Coals in
Air-Staged Combustion Conditions
F. Kluger*, H. Spliethoff and K.R.G. Hein
Institute of Process Engineering and Power Plants (WD), University of
Stuttgart, Pfagenwaldring 23, 70569 Stuttgart, Germany
In order to determine the influence on burnout by the combustion conditions and the
coal preparation, three bituminous coals sold on the world market, fiom three dtfferent locations, in Poland, South Africa, and Australia, were studied more closely. For
this purpose, the coals were ground in two diflerent particle size ranges, which,
besides the influence of the combustion conditions, such as temperature, residence
time, and stoichiometg? made it possible to also investigate the impact on burnout by
the coal preparation. The experiments were carried out in an electrically heated
entrained-flow reactor with a thermal input of 8.5 kW.
The parameters for the
experiments are wall temperature (1000 - 135OoC), air ratio (0.6
particle sizes (70%
-
1.15), and two
73pm, 90% c 75pm). The results show that in general, for
increasing temperatures, the burnout quality will improvel.
For the Australian
Illawara coal, another outcome is increased NO, emissions. Lowering the air ratio in
the reduction zone leads to less NO, emission but to increased unburnt matter in ash.
For the smaller particle size fraction, the analysis of the different particle sizes shows
an improvement of the burnout without a change in NO, emissions.
Introduction
Equipping power plants with NO,-lean primary combustion technology, i.e. by the
creation of reducing atmospheres, was a successful means to diminish the problem of
* Authorfor correspondence.
253
F. Huger, H.Spliethoff and K.R.G. Hein
nitrogen oxide emissions. On the other hand, the content of unburnt matter in the ash
increased.
Worse burnout leads to a higher carbon concentration in the fly ash and reduces
the overall efficiency of a power plant, involving financial losses for the utility company.
Furthermore, higher carbon in the fly ash has a negative effect as regards
utilization of the ash as a cement substitute in concrete. The profitable sales are thus
called into question and other, costly, methods of disposal have to be applied.
Coal is a heterogeneous substance - the single particles, which comprise the whole
grain size spectrum of pulverized-coal firing, vary considerably in terms of petrological properties, organic composition, pore structure, ash composition, and particle
morphology. This diversity of the individual particles results in great differences in
burnout rates and burnout times. As well as the structure and characteristics of the
particles, the burnout of the coal types depends on the combustion conditions (such as
temperature, heating rate, stoichiometry, residence time) in the power plant.
Detailed studies on combustion of coal particles, with respect to burnout at different test facilities and scale formation, leads to phenomena that describe the deactivating of the char during the combustion [I-61.
Samples and Experimental Procedures
This study considers the effect of coal preparation and combustion conditions on the
burnout and NO, emissions in the case of air-staged combustion. The coal preparation was investigated by two different particle size hctions. The influence of combustion conditions was investigated by varying the temperature and stoichiometry in
the reduction zone.
Three different coals fiom Poland (P), Australia (Illawara (I)) and South Afkica
(Middelburg (M)) are studied in this paper. Each coal is ground to two different particle size ranges (70% and 90% smaller than 75 pm). Table 1 lists the results of the
proximate and ultimate analyses as well as the calorific value of the different coals.
These coals are chosen with for their large differences in ash content and volatile
matter, which is additionally expressed in the fuel ratio (fixed carbon relative to the
volatiles content).
254
Burnout Behaviour of Bituminous Coals in Air-Staged Combustion
Table 1. Proximate and ultimate analyses of the investigated coals.
'[O] = 100 -([C] + [HI +[N] + [S]
The combustion process and the formation of pollutants in pulverized coal f ~ n g s
are the subject of extensive investigations carried out on experimental facilities of the
IVD. The experimental results discussed here were obtained from experiments using
an electrically heated tube reacI
tor [7,8,9].
r
The experimental furnace is
illustrated in Figure 1.
The
electrically heated ceramic tube
-bumar
c-
Om
of 200 mm.
Ti
-insolation
0-
9 :Pm-
e
is 2500 mm long with diameter
The electrical
heating is divided into five dif0,9m
-electrically heating
ferent zones in order to adjust
for constant wall temperature as
-burnout air probe
well as a temperature profile
-ceremic tube
0200mm
along the furnace.
The wall
temperature can be varied up to
2,5 m
1500°C. The burner to inject
the coal into the chamber is
I
filter
\ char sampling
-
situated on top.
The feeding
system consists of a gravimetric
Figure 1. Schematic of the test facility.
conveyor and a screw feeder.
255
F. Kluger, H.Spliethoff and K.R.G. Hein
The pulverized coal is supplied to the burner by carrier air. The coal feed rate depends on a thermal input of 8.5 kW,and corresponds to approximately 1 kgh for
bituminous coals and 1.5 k@
for lignite. The combustion air is injected through
annular clearances and is divided into primary and secondary air. This facility provides a good environment to investigate staged combustion conditions, since the
addition of burnout air can be done at several locations with a probe fkom below.
The investigation of the impact of combustion conditions and coal preparation
were carried out under unstaged and under air-staged conditions. During the experiments a constant wall temperature along the furnace was applied. Unstaged combustion takes place under excess air with an air ratio of 1.15. In the case of air staging an
air ratio of 1.15 (approximately 3% O2in the flue gas) in the burnout zone was chosen.
Table 2 lists the investigated combustion parameters.
Table 2. Investigated parameters.
256
Burnout Behaviour of Bituminous Coals in Air-Staged Combustion
Experimental Results
The reactor wall temperature impact on NO, emissions and burnout quality of the
Illawara coal is shown in Figure 2.
Along with rising reac100
99
-so
-
Airsuoinp
ooal: lllirmn
ac mb0: 0.8
res. time: 2 sac.
air nD0 overall: 1.15
97
1s
- 95
-94
.
E
:93
o/o,
,I”
-B- NO, mQs.wm5
-0- Bumoul
tor wall temperatures, the
/‘
-92
/’
:91
-a
5
:w m
/
189
950
1000
1RW
1100
1150
1203
1250
7363
of 1000°C, less than 400
mgm3 are emitted, the NO,
emissions increase up to
Ta6
740 mg/m3 at 135OOC.
85
14W
1350
’Whereas at a temperature
-M
- a7
m ! . 3 . ,, . , . , . , . , .
NO, emissions also rise.
The burnout quality of
Wall temperature [ia]
Illawara improves with
Figure 2. Influence of wall temperature on NOx
emissions and burnoutfor Illawara.
rising temperatures. The
values established at a
temperature of 1000°C
range
around
87.5%,
rising to a 96% burnout and
more at 1350°C.
Fi,we
3 shows the
impact on NO, emissions
and burnout by the air ratio
in the reduction zone and
by the milling fineness.
A smaller air ratio in
m-l
,
0.6
. ,
0.7
. ,
0.8
.
, .
0.9
!
1.0
.
,
1.1
.
, 185
12
Air ratio in the reductionzone [-]
Figure 3. influence of air ratio in the reduction zone
and particle size on NOx emissions and burnout for
Illawara.
the reduction zone reduces
the NO, emissions. At an
air ratio of 1.15 (in unstaged
combustion),
the
NO, concentrations emitted
amount
to
1300 mdm3
257
F. Kluger, H.Spliethofland K.R.G. Hein
-0"
s
1800
-
rsoo:
.
I .-./
&
14W-
5
(2W-
=
m
the Middelburg coal are
air m o 0 8
ras tima 2 ~ 0 ~
a i r m o o v ~1~15 1 p013
E
shown in Figure 4 for dif-
I92
-01
' 8o
2
ferent reactor wall tempera-
m
1m-
C
0 800-
-I
and the burnout values of
0
5
n .
6W-
p
4cQ:
/
0
-~-NO,@maWN
-0- Bumou!
-
89
-88
- 87
-86
a5
g
hues. The NO, emission
level is at 600 mg/m3, in-
m , . , . , . , . , . , . , . , dependent
. , . from the set wall
258
Burnout Behaviour of Bituminous Coals in Air-Staged Combustion
2OW
-
18W
-
r p .
1400-
,*'
Elom:
c
Q
-m
p
-65
: ~ r
t-es. bftlE: 2 rec
800-
'
400200
B- '
, . , . , .
I
-.
.
--.-NO,emiuims
-0- BMlout
I
.
I
.
-5
I
.
I
at 1000°C to 350 mg/m3
IgO rn
at 1300OC. The burnout
-
quality improves with
-8g
r
-
Figure 6. Influence of wall temperature on NOx
emissions and burnout for Polish coal.
decrease from 500mg/m3
8
-91
.
.
show that NO, emissions
I93
~ 9 2
mralbrrrab0: 1.15
600-
investigated, the results
-
,/' Pjrmging
/''
coal: Polish Coal
air ntio: 0.8
,I
/'
0
OI
1w
99
S8
97
-96
/o----O
0"1600
,E 1200-
-
-88
a7
-86
r 85
5
higher wall temperatures,
1000°C to 98.5% at wall
temperature of 1300°C.
Figure 7 demonstrates
how the air ratio in the reduction zone and also the milling fineness influence NO,
emissions and burnout quality. The results of the varied air ratio in the reduction zone
reveal a clear dependence of the NO, emissions. Together with a lower air ratio, NO,
emissions decrease - fkom about 1200 mg/m3 (air ratio 1.15) to a minimum of 350
mg/m3 (air ratio 0.8), increasing again, however, to 400 mg/m3with a further lowering
of the air ratio to 0.6. The burnout quality declines with lower air ratios. At an air
259
F. Huger, H. Spliethofland K.R.G.Hein
ratio of 1.15, in uustaged
zwo
-89
combustion, the
-98
97
-06
-
.:
-B-PoliYI
-0-Pdsh
1100
Colt 70 % c 75 ym
Coal 90 % c 75 ym
-84
-93
:oz
LSl
:w
z w ! ,
0.6
.
,
0.7
.
,
0.8
.
,
. , .
0.9
1.0
I
1.1
.
,
12
burnout
values achieved are over
=Ta
5
m
69
-68
87
86
8s
t
Air ratio in the reduction zone [-I
99%, as opposed to only
93% at an air ratio of 0.6.
A dependence on particle
size cannot be observed for
the NO, emissions, only for
the burnout. The more
finely ground fkaction, 90%
Figure 7. Influence of air ratio in the reduction zone
andparticle sue on NOx emissions and burnout for
Polish coal.
< 75pm, shows higher
burnout values at the measurement points than the
other fraction, 70% < 75pm.
Discussion
A rise of the reactor wall temperature has, for all the investigated coals, the effect of
increasing the burnout values.
The reactor wall temperature indirectly fixes the
combustion temperature. By means of the electrical wall heating it is possible to
establish the impact of temperature, independently from the other combustion parameters. The temperature exerts an influence on the heating rate of the coal particles.
With a higher heating rate, for instance, more volatile components of the coal are
released.
The bigger quantity of volatile matter makes an ignitable mixture form earlier, i.e.
the volatile matter ignites earlier. Therefore the particle temperature rises more
quickly so that ignition of the particles occurs earlier. This makes the burning out of
the coal faster and hence more complete.
Considering the temperature dependence of the NO, emissions shows a considerable influence of the type of coal. The low-volatile Australian coal (Illawara) shows
higher NO, emissions with increasing reactor wall temperatures, the NO, emissions
of the medium-volatile South African coal (Middelburg) are almost independent from
temperature, whereas the high-volatile Polish coal reveals lower NO, emissions with
260
B u m u t Behaviour of Bituminous Coals in Air-Staged Combustion
increasing temperatures.
In order to clarify these
Air Staging
1100
coal lllawara
effects, NO
Wall temperature
1000
air ratio in me red zone 0 8
1OOOr
res time in the red zone 2s
overall air ratio 1 15
-0- 1100r
-
tions in the furnace, for
1zoor
establishing
-0- 13002
-m400
/--
3w
+
0
-
100
burnout air addition
0
0
0
I
0
1
2
'
I
mation
13502
'
I
4
and
NO
for-
reduction
processes of the different
V
3
concentra-
coal types, were measured
at the investigation tem-
'
I
5
.
6
Residence time [s]
Figure 8. NO concentration process at diflerent
reactor wall temperatures,for Illawara.
peratures. The following
describes the results of
these measurements.
In Figure 8, the NO
concentration
profiles
fiom measurements taken in the furnace at different reactor wall temperatures are
described for the Illawara coal. A rise of the reactor wall temperature, within the
residence time range of only a few milliseconds, leads to an increase of the NO concentration. Furthermore, the higher temperatures help to achieve an acceleration of
the decomposition velocity, which can be seen by the increasing gradient of NO
breakdown in the reduction zone. The increase of the temperature has the additional
effect of reducing NO to an even greater extent. Though the temperature rise helps to
achieve a higher reduction rate, i.e. to reduce a higher quantity of NO, the NO concentrations developed in the start-up phase and increasing with rising temperatures
outweigh the NO reduction. The consequence is that the final section of the reduction
zone has high NO concentrations increasing with rising temperatures, hence developing a higher level of NOx emissions.
For the Middelburg coal, the NO concentration processes at different reactor wall
temperatures are described in Figure 9. The rise of the reactor wall temperature from
1000°C to 1300°C causes a rise of the NO peak in the residence time range of 0.5 sec.
The decomposition velocity of the NO concentration after the NO peak, and the
quantity of NO reduced increase with rising temperatures.
The temperature-
261
F. Huger, ff. Splierhof
and LR.G.ffein
dependent high NO concentration that formed initially gets diminished through
reduction to the extent that, for all the temperatures of investigation, the NO concenI
Alr staging
1100
coal Middelburg
1000
air ratio in the red
so0
zone
residence lime in the red
-
08
zone
25
,h\
6oo
400
200
300
500
100
o0
.
reduction
zone
reaches the same level
rooo,
+&
)
-0- 1 2 0 ~
-m-
overslairratio 1 15
700
the
Wall tempentun
after 2 seconds. The NO
730013
concentration in this final
part, and in consequence
the NOx emissions, do
not depend on tempera-
burnout air addibon
A2
I
3
4
5
I
ture.
6
For the Polish coal,
residence time [s]
Figure 9. NO concentrationprocess at diferent
reactor' wall temperatures*for Middelburg.
the influence of reactor
wall temperature on NO
concentration processes is
shown in Figure 10. The temperature rise from 1000°C to 13OO0Ccauses an increase
in the NO concentration within the residence time range of few milliseconds, just as
in the case of the two previously discussed coal types, Illawara and Middelburg.
Since NO reduction, and hence quantity of NO reduced, depend on temperature, a rise
in temperature accelerates the reduction process and increases the reduction quantity.
The temperature-dependent high NO concentration that developed in the initial
phase gets diminished by
reduction to the extent
that with increasing temperatures the concentration in the final section of
the reduction zone decreases after 2 seconds.
100
-
The NO concentration in
the final reduction part
and
Figure 10. NO concentrationprocess at diferent
reactor wall temperatures,for the Polish coal.
262
---_
consequently
the
Burnout Behaviour of Bituminous Coals in Air-Staged Combustion
NOx emissions depend on temperature and diminish with increasing temperatures.
In the investigation of the stoichiometric influence, the basis for comparison were
the emission values and the residual fuel analysis results after 1 sec. of residence time
in the burnout zone and after 2 sec. of residence time in the reduction zone. Due to
the lowered air ratio in the reduction zone, the fuel conversion in that zone is reduced
so that the conversion within one second of the residual fuel in the burnout zone falls
short of completion to a greater degree. This means that lowering the air ratio in the
reduction zone induces a worse burnout quality of the coals. For NO, reduction it is
an absolute necessity to create a zone with air deficiency (reduction zone). Various
investigations revealed an optimum air ratio in the range of 0.7 [lo]. More combustion air favours NO formation whereas with less air addition predominantly HCN and
NH3 are formed, substances that undergo conversion into NO, in the burnout zone.
Through the application of the air staging method, it was expected that there will
be slight differences in NO, emission reactions with a more finely ground coal grain
due to local changes in stoichiometry. With a more finely ground coal, the heating
rates achieved are higher, thus faster heating and ignition of the coal grain is attained.
Due to this faster heating to higher temperatures, a relatively higher fiaction of volatile matter from the coal is released, possibly leading to lower local air ratios and in
consequence to overall lower NO, emissions.
For the faster fuel conversion, the more finely ground coal grain, besides the faster
heating and earlier ignition, provides a larger surface for oxygen attack, so a higher
burnout value can be attained.
Conclusions
The objective of this investigation was to establish the impacts of combustion condi-
tions and coal preparation on burnout and NO, emissions. An additional aim is to
determine combustion conditions that are adequate to enhance the burnout quality
without raising the NO, emissions. The results of this investigation revealed a hi&
dependence of such combustion conditions on the coal type.
263
F.Kluger, H. Spliethoff and K.R.C. Hein
The investigation of the low-volatile coal, Illawara, brought about the result that a
rise of the reactor wall temperature does yield a better burnout quality, but makes the
NO, emissions increase. The lower air ratio in the reduction zone in tum leads to
reduced NO, emissions but likewise to a reduced burnout quality. Only finer milling
of this coal brings about the required result, i.e. an improvement of the burnout quality without an alteration in NO, emissions.
The result of the investigation of the medium-volatile bituminous coal, Middelburg, was that a rise of the reactor wall temperature improves the coal burnout quality
without remarkably altering the release of NO,.
Lowered air ratio and prolonged
residence time lead to a reduction of NO, emissions, as in the case of the Illawara coal,
but at the expense of the burnout quality. The coal preparation which focused on a
finer milling degree brought about an improvement of the burnout quality without a
change in NO, emissions.
The high-volatile Polish coal shows an improved burnout level with increasing
temperatures along with an improvement as regards NO, emission abatement. A
lower air ratio as well as a prolonged residence time in the reduction zone do yield a
better outcome regarding NO, emission reduction but deteriorate the burnout quality.
The combustion experiments with the more finely ground fuel did reveal an improved
burnout quality without a change in NO, emission release.
In general, finer milling yields an improved bumout quality with all three coals
without having a negative effect as regards NO, emissions. The temperature rise does
cause an enhanced burnout level throughout, but reveals a quite strong dependence of
the NO, emission behaviour on the type of coal. The tendency of both an improved
burnout and improved NO, emission reduction is contrary for the case of varied air
ratios and residence times in the reduction zone, i.e. the increase required for the
burnout is associated with an increase of the NO, emissions and vice versa.
Acknowledgments
The financial support provided by the European Coal and Steel Community, under
Grant No. 7220ED-085,is gratefully acknowledged.
264
References
1. Hurt, R., Gibbins, J. 1995. Residual Carbon from Pulverized Coal Fired Boilers: 1. Size Distribution
2.
3.
4.
5.
6.
7.
8.
9
101.
and Combustion Reactivity, Fuel 74(4):471480.
Hurt,R., Gibbins, J. 1995. Residual Carbon fiom Pulverized Coal Fired Boilers: 2. Morphology and
Physiochemical Properties, Fuel 74(4):1297-1306.
Hurt, R., Mitchell, R. 1992. Unified Hi$-Temperature Char Combustion Kinetics for a Suite of
Coals of Various Rank, Twenty Fourth Symposium (International) on Combustion, The Combustion
Institute, Pittsburgh, pp. 1243-1250.
Russel, N. V., Beeley, T. J., Gibbins, J. R., Man, C. K., Williamson, J. 1997. Char Deactivation at
Elevated Temperatures, 9th. Int. Conference on Coal Science, Essen Germany.
Beeley, T., Crelling, J., Gibbins, J., Hurt,R., Lunden, M., Man, C., Williamson, J., Yang, N. 1996.
Transient High-Temperature Thermal Deactivation of Monornaceral-Rich Coal Chars, Twenty Sixth
Symposium (International)on Cornbustion, The Combustion Institute, Pimburgh, pp. 3 103-31 10,
Lunden, M., Yang, N., Headley, T., Shaddxx, C. 1998. Mineral-Char Interactions during Char
Combustion of a High-Volatile Coal, Twenty Seventh Symposium (International) on Combustion, The
Combustion Institute, Pittsburgh, pp. 1695-1702.
Spliethoff, H.; Greul, U.; Riidiger, H., Hein, K.R.G. 1996. Basic Effects on NO, Emissions in Air
Staging and Reburning at a Bench Scale Test Facility, Fuel. Vol. 75(5), pp. 1123-1129.
Greul, U.; Kluger, F.; Peter, G.; Spliethoff, H.; Hein, K.R.G. 1996. Investigations into NO, Emissions
and Burnout for Coals with High Ash Content in a Bench Scale Test Facility, Int. Symp. On CoalScience Technology, Dhanbad, India.
Kluger, F.; Greul, U.; Spliethoff H., Hein K.R.G. 1997. Expenmentielle Untenuchungen zur
Freisetzung von C und N bei der Verbrennung von hoch- und rninelfliichbgen Steinkohlen”, 18.
Deutsch-Niederhdischer Flammentag Delft, Niederlande.
Greul, U. 1998. Experimentelle Untenuchung feuerungstechnischer NO,-Minderungsverfahren bei
der Kohlenstaubverbrennung,Dissertation, Universitat Stut,gart (1997), VDI-Fortschrittsbericht, Reihe
6, Nr 388, VDI-Verlag, Diisseldorf
265
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