PROPERTIES OF PULVERIZED COAL COMBUSTION IN HIGH
TEMPERATURE AIR/STEAM MIXTURE
ZHANG QINGLIN, SWIDERSKI ARTUR, YANG WEIHONG*, WLODZIMIERZ BLASIAK
Royal Institute of Technology (KTH), Division of Energy and Furnace Technology, email: weihong@kth.se,
telephone number: 0046 8790 8402
ABSTRACT
High temperature air combustion (HiTAC) is a promised combustion technology. It has
been proved that when the oxygen concentration in the preheated air is diluted, the NOx
emission for fossil fuels will be significantly reduced. When this idea is applied to
pulverized coal combustion, the same advantages can be expected. Water has high specific
heat, and can react with coal in high temperature. When steam is used to dilute preheated
air in HTAC of pulverized coal, better combustion properties may be achieved. However,
very little data can be found in this aspect, and further work should be done.
This paper presents the work on properties of pulverized coal combustion in high
temperature air/steam mixture. Experiments have been performed in a 2.5m long
combustion chamber with coal feed rates ranging of 5 kg/h and coal injection velocities
ranging from 9 to 30m/s. Coal nozzles with different diameters were used to consider the
influence of the transport factor. The combustion air was diluted by steam, whose
percentages varied from 0% to 44%. Steam and air were preheated by a high-temperature
gas generator, and the preheated oxidizer temperature could achieve 1150 oC. Numerical
simulations were also carried out to provide more information about fluid field.
The result shows that NOx emission reduces with the dilution by steam for all nozzle
diameters and injection velocities. Steam has a significant impact on flame structure. When
steam was added, the peak temperature would be remarkably reduced, and the visibility of
flame reduced significantly. Even small amount of steam in oxidizer would impact the
flame structure. It is also found that with the increase of coal injection velocity, the NOx
emission increases.
Key words: HiTAC; coal; steam; NOx; temperature;
INTRODUCTION
The thermal efficiency of industrial furnaces can be significant increased by preheating
the combustion air to a high temperature, using the heat exhaust gases. However, the NOx
emission is strongly promoted by elevated temperatures because of thermal NOx mechanics.
An effective way of solving this problem is to reduce the oxygen concentration in the
oxidizer. This technology is named High Temperature Air Combustion (HiTAC). Many
studies on HiTAC have been done in the recent years, and it has been proved that that
HTAC technology has many features that are superior to that in conventional combustion
(YANG and BLASIAK, 2004a,b; WEBER et al., 2000; HASEGAWA et al., 2002; TSUJI
et al., 2003; LILLE et al., 2005 ).
High temperature air combustion technology was first applied in gaseous fuel
combustion. When this idea is applied to combustion of pulverized coal, the same
advantages as for gaseous fuels can be expected, including the enhancement of combustion
stability and increase of combustion efficiency, the ability to use low-volatile coal like
anthracite, and also lower pollution emissions (KIGA et al., 2000; SUDA et al., 2002; HE
et al., 2004; HANNES et al., 2009). Kiga et al. presented the experimental results of
pulverized coal combustion in high-temperature and low-oxygen condition. The results
indicated that increasing air preheat results in increased combustion efficiency and reduced
NOx emission, whereas decreasing the oxygen content of the combustion air leads to large
reduction in combustion efficiency, accompanied with a slight decrease or increase in NOx
(KIGA et al., 2000). Suda et al. conducted experiments by injecting pulverized coal from a
nozzle co-axially placed at the preheated air inlet. The results showed that for both
bituminous and anthracite coals, release rate of volatile contents were remarkably enhanced
and flame front distances from coal nozzle became less when air temperature increased.
NOx emission also became less with temperature increasing (SUDA et al., 2002). The
corresponding numerical studies were performed later[11]. Numerical simulation results
were of great consistency with experimental results (HE et al., 2004). Hannes et al. carried
out a set of experiment to investigate the Nox emission of high temperature air combustion
under Ar/O2 as well as CO2/O2 atmospheres in order to quantify the ratio of fuel NOx to
thermal NOx. This investigation showed a high reduction of thermal NO as well as an
increase of fuel-NO which was primarily related to the decrease of the peak flame
temperature (HANNES et al., 2009).
When steam is induced into the combustion process, the reaction mechanics will be
changed. Studies of Kim et al showed that when steam is added into the oxidizer, the
concentration of OH radical will increase significantly, while the concentration of O and H
will decrease. The result also showed that the OH radicals will affect the formation rate of
thermal NO by the radical reaction step N OH NO H (KIM et al., 2002). Subsequent
work on the combustion behavior of steam-added mild combustion was done in 2004
(JEONG et al., 2004). The result showed that in mild combustion model, the OH
production via the reaction step O+ H 2O OH+ OH was enhanced by the increase of
oxidizer-side temperature, while chemical effects of added steam are influenced by the
competition between the reasctions O+ H 2O OH+ OH and OH+ H 2
H+ H 2O . It was
also found that in the whole ranges, steam addition suppresses NOx emission. When steam
is used to delute the oxidizer of solid fuels, like pulverized coal, the gasification reactions
of fixed carben should be taken into account, so the reaction mechanics would be more
complex, and different combusting properties should be observed. However, few works
have been found in the literature of high temperature air combustion of solid fuels, in steam
added conditions.
The aim of this work is to both experimentally and numerically study the combustion
behavior of pulverized coal in highly preheated air/ steam mixture. The temperature of
air/steam mixture was heated up to 1150 ºC, with the steam concentration ranging from 0%
to 44% of mass fraction. The flow rate of pulverized coal was 5 kg/h, and the injecting
velocity varied from 9m/s to 30 m/s. In order to investigate the influence of nozzle
diameters on combustion behavior, there nozzles with the diameters of 3, 5 and 6.5mm was
used. The influence of steam mass fraction on NOx emission and flame lift-off distance are
described. Additionally, the influences of transport factor and coal injection velocity on
NOx emission are also presented. In base of these results, some numerical simulations are
made to investigate detailed mechanics.
METHODS AND MATERIALS
Feedstock
The Helensburg coal was used in this work. Table1 shows the proximate and ultimate
properties of Helensburg coal. A Rosin-Ramler approach were assumed to represent the
Helensburg coal particle size distribution, as shown in Firure 1.
Unit
wet
dry
%
%
%
%
%
%
%
%
%
%
6.70
11.10
19.41
62.79
0.01
0.32
70.91
3.73
1.40
5.78
11.90
20.80
67.30
< 0.01
0.34
76.00
4.00
1.50
6.20
dry ash
free
23.61
76.39
0.01
0.39
86.27
4.54
1.70
7.04
MJ/kg
28.95
31.03
35.22
30.75
Q water is vapour MJ/kg
27.99
30.17
34.25
29.90
moisture
ash
volatiles
fixed carbon
chlorine
sulphur
carbon
hydrogen
nitrogen
oxygen
Q water is
condensed
air dried
0.90
11.79
20.61
66.69
0.01
0.34
75.32
3.96
1.49
6.14
Table 1 Physical and chemical properties of the coal used in the experiments
1
0.9
0.8
0.6
0.5
0.4
0.3
Mass fraction [-]
0.7
0.2
0.1
0
0.0003
0.00025
0.0002
0.00015
0.0001
0.00005
0
Particle diam e te r [m ]
Figure 1 Rosin-Ramler approach for pulverized coal dust
Test facility and experimental conditions
The test facility has been developed for several years. For the purpose of the coal
combustion, test facility was rebuilt, as shown in Figure 2.
Coal
Coal feeder
Transport air
flow meter
Combustion
air
Coal
scales
Transport gas
temperature
Natural
gas
Steam
flow meter
Preheater
Combustion
air
Combustion
chamber
Natural
gas
Process air
flow meter
Chamber
temperature
Safety Burner
Second
combustion
chamber
Fan
NOx ,O2d
Boiler
CO2 CO
Unburnt
particles
Fan
Natural
gas
Water
Figure 2 Schema of the test faculty
A cylindrical combustion chamber of 0.4 m diameter and 2.5m height was used, as
shown in Figure 3. Air was heated up to the temperature of 1150 °C by a compact preheater, and entered through the top of chamber, with the mass flow of 115 kg/h. The
pulverized coal is fed by a horizontal lance, and air was used as transport gas. Water cooled
coal lance was used to keep the temperature of transport gas. The volume of combustion
chamber was equal to 0.28m3, and surfeit 2.2m2. Two thermocouples were used to measure
temperature profile along combustion chamber and temperature in inlet and outlet.
Thermocouple tips were located in the middle of inlet and outlet channels. Tip of
thermocouples installed 200 mm from the wall. In flue gas channel, two iso-kinetics probes
were located for ash collecting and flue gas sampling. Flue gas samples were suck by these
probes, and then cooled down after suction probe. CO and CO2 components were measured
by Maihak analyzer type MULTOR 610 by using Non-Dispersive Infrared method. To
measure O2 component in sample gas, the M&C Analysentechnik analyzer type PMA 25
equipped with paramagnetic detector was used. Measure of NOx was taken by chemiluminescent analyzer made by ECO PHYSICS type CLD 700 EL ht.
Coal Injector
Cooling water
for coal gun
2 inlet:
pulverise coal
21
Hot Oxidizer
1 inlet:
Hot Air
Exhaust Gases
15
250
3 outlet:
Hot Air
40
a)
b)
Figure 3 View of the combustion chamber
Table 2 shows the experimental arrangement of tests. Before each test, experimental
facility was heated up for 10-12 hours, in order to achieve uniform temperature profile in
combusting chamber.
Velocity of transport gas in
the nozzle
6.5 mm
Diameter of coal nozzle
5 mm
3 mm
100% air
Composition of
oxidizer
85% air
15% steam
66% air
34% steam
56% air
44% steam
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
30 m/s
20 m/s
9 m/s
Table 1 Experimental arrangement
Numerical strategies
The air of the numerical simulations were to predict the flow and temperature field and
combustion behaviors inside the burner. CFD software FLUENT 6.3.26 was used as the
platform of simulations. In this work, three-dimensional governing equations were
employed to model the flow in the reactor due to the strong three-dimensional
characteristics. In order to simulate the turbulence flow, the standard k- model is selected.
The combustion process was simulated using the Finite Rate/ Eddy-Dissipation model, and
single-step devolatilization model was used to modeling the devolatilization process. In
order to take account of the gasification of char, a multiple char reaction model was used.
Six reactions were considered in this work:
mv _ coal 1.614O2
2.763H 2O CO
(R1)
C 0.5O2
CO
(R2)
C CO2
2CO
(R3)
C H 2O
CO H 2
(R4)
H 2 0.5O2
H 2O
(R5)
CO 0.5O2
CO2
(R6)
In order to simulate the NOx emission, the default NOx models of FLUENT was used,
and thermal NOx, prompt NOx, fuel NOx and N2 O intermediate models are all considered.
The kinetic properties of Helensburg coal were set as default values from Fluent, whilst
for char were computed on the basis of formulas taken from Orsina et al. 's work (ORSINA
et al., 2000). It is very likely that assumed kinetic data is not proper for given coal. It is
highly recommended to investigate coal itself to obtain this data (e.g. at drop tube furnaces)
in order to make modeling more accurate. However the laboratory investigation at the
present stage of work were not taken into account. So one has to be conscious that
predicted values of NOx, ignition distance and char burnout can differ from those in reality.
After all, when taking into account only comparison among different cases (technical issues
of burners etc.), this should have more minor effect than major.
RESULT AND DISCUSSION
The influence of steam mass fraction
The influence of steam mass fraction on NOx emission is shown in Figure 4. the NOx
concentration in flue gas has been standardized to 6% oxygen concentration. It could be
seen that the addition of steam has a significant impact on NOx emission. For all the coal
injucting velocities and nozzle diameters, the NOx emission decrease with the increasing of
steam in oxidizer. When the concentration of steam in oxidizer increases to 44%, the NOx
emission will reduce more than a half.
Figure 5 is the numerical modeling result of the temperature, NO concentration and O2
concentration profiles for several cases with different steam concentrations. The nozzle
diameter of these cases is 5mm, and the coal injecting velocity is 30m/s. With the
increasing of steam concentration in oxidizer, the peak temperature decrease significantly.
The temperature difference of peak temperature and oxidizer temperature at the inlet falls
from 1400K to about 600K. The impact of steam on NO emission was shown in Figure 5
(b). In case 1 where their is no steam addition, the peak NO concentration is about 1.5%,
but in case 2, when the steam mass fraction in oxidizer reaches 1.5%, the peak
concentration of NO drops to less than 0.3%. In case 3 and 4, where the steam mass
fractions are 34% and 44%, the peak NO concentration is less than 0.1%. This can be
explained by the decrease of thermal NO, which is strongly depend on the flame
temperature. Moreover, the low oxygen concentration can also suppress the formation of
NOx.
A most important characteristics of steam added combustion is that the concentration of
OH radicals in combustiong zone is enhanced significantly, which could underlines one of
the chain reactions: N OH NO H . In this aspect, the addition of steam may be
propitious to the formation of NOx. But form both experimental and numerical results, OH
radicals did not promote the formation of NOx significantly. The suppressant mechanism of
low peak temperature and oxygen concentration can overbalance the influence of increase
of OH radicals.
There exists a difference for NOx emission between numerical and experimental result.
The possible reason might be the property setting for pulverized coal in numerical
simulation.
Another impact of the steam addition is the visability. When there was no steam in the
oxidizer, the flame appeared an orange colour, but when steam was added, even in the 15%
steam cases, the flame became invisable.
3500
Coal mass flow 5 [kg/h]
c=9m/s d=6,5mm
3000
NOx [mg/Nm 3] @6%O2
c=9m/s d=5mm
c=9m/s d=3mm
2500
2000
1500
1000
500
0
100% air
85% air 15% steam
66% air 34% steam
56% air 44% steam
composition of oxidizer
(a) Coal injecting speed 9m/s
3500
Coal mass flow 5 [kg/h]
c=20m/s d=6,5mm
3000
NOx [mg/Nm 3] @6%O 2
c=20m/s d=5mm
c=20m/s d=3mm
2500
2000
1500
1000
500
0
100% air
85% air 15% steam
66% air 34% steam
composition of oxidizer
(b) Coal injecting speed 20m/s
56% air 44% steam
3500
Coal mass flow 5 [kg/h]
c=30m/s d=6,5mm
3000
NOx [mg/Nm 3] @6%O2.
c=30m/s d=5mm
2500
c=30m/s d=3mm
2000
1500
1000
500
0
100% air
85% air 15% steam
66% air 34% steam
56% air 44% steam
composition of oxidizer
(c) coal injecting speed 30m/s
Figure 1 Influence of oxidizer composition on NOx emission. Tests was made with three different nozzles 3,
5 and 6.5 mm.
(a) Temperature profiles
(b) NOx mass fractions profiles
(c) O2 mass fractions profiles
Figure 5 Temperature, NO mass fraction and O2 mass fraction profiles for different steam concentration(0%,
15%, 34% and 44%) with 5mm nozzle and 30m/s coal injecting velocity
The influence of coal injecting velocity
Impact of coal injecting velocity in nozzle on emission of NOx is presented on Figure 6.
It can be found that with the raise of coal injecting velocity, the NOx emission shows a
trend of increase. Shinji et al. have found that volatile nitrogen played an important role in
NOx formation, while more than 90% of the released nitrogen during devolatilization was
converted to HCN and NH3(SHINJI et al., 1994). According to the Feminore mechanism,
for equivalence ration less than 1.2, HCN and NH3 will be oxidized to form NO
(FENIMORE, 1970). For equivalence ration richer than 1.2, other routes open up and NO is
recycled to HCN, inhibiting NO production (MILLER et al., 1989).
In our system, pulverized coal was transported by air, so when the nozzle diameter was
fixed, higher coal injecting velocity means more transport air, which means less
equivalence ration in the devolatilization zone. At the same time, a high coal injecting
velocity may induce better turbulence, which means better mixing of coal and oxidizer
during the devolatilisation process. This also leads to less equivalence ration in the
devolatilization zone.
Figure 7 is the numerical modeling profiles of rate of fuel NO for several cases with
different coal injecting velocities. The nozzle diameter of these cases is 5mm, and the steam
mass fraction is 34%. It can be found that the fuel NO formation area increasing with the
coal injecting velocity.
3000
Coal mass flow 5 [kg/h]
100% air, d=6,5mm
85% air 15% steam, d=6,5mm
NOx [mg/Nm 3] @6%O 2
2500
66% air 34% steam, d=6,5mm
56% air 44% steam, d=6,5mm
2000
1500
1000
500
0
0
5
10
15
20
Velocity [m/s]
25
30
35
(a) Diameter of nozzle 6.5 mm
1800
1600
NOx [mg/Nm 3] @6%O 2
1400
1200
1000
800
600
Coal mass flow 5 [kg/h]
100% air, d=5mm
400
66% air 34% steam, d=5mm
200
56% air 44% steam, d=5mm
0
0
5
10
15
20
25
30
35
Velocity [m/s]
(b) Diameter of nozzle 5 mm
1800
1600
NOx [mg/Nm 3] @6%O 2
1400
1200
1000
800
Coal mass flow 5 [kg/h]
600
100% air, d=3mm
400
85% air 15% steam, d=3mm
66% air 34% steam, d=3mm
200
56% air 44% steam, d=3mm
0
0
5
10
15
20
25
30
35
40
Velocity [m/s]
(c) Diameter of nozzle 3 mm
Figure 6 Influence of coal injecting velocities on NOx emission
(a) Coal injecting velocity
equals to 30m/s
(b) Coal injecting velocity
equals to 20m/s
(c) Coal injecting velocity equals to 9m/s
Figure 7 Rate of fuel NO profiles for different coal injecting velocities(0%, 15%, 34% and 44%) with 5mm
nozzle and 34% steam mass fraction in oxidizer
CONCLUSION
Experimental and numerical studies have been done on NOx emission and combustion
properties of pulverized coal combustion in high-temperature steam diluted air, and the
following conclusions are attained.
The addition of steam into oxidizer will suppress the formation of NOx. The main
reason for that is the decreasing of peak temperature of combustion area, which restricts
the NOx formation by thermal NOx mechanism. The increasing of OH radicals due to
the addition of steam did not show significant impact on NOx formation. the flame
becomes invisible when steam is added into the oxidizer.
The coal injecting velocity also have impacts on NOx formation. NOx emission
increases with the coal injecting velocity. This is due to the change of transport factor
and the turbulent mixing of coal and air. High injecting velocity leads to lower
equivalence ration in the devolatilization zone, so that the fuel NOx is prompted.
ACKNOWLEDGMENTS
The authors gratefullu acknowledge the financial support from the LKAB, which
supported the funding of our experimental research. Zhang Qinglin also gratefully
acknowledges the China Scholarship Council which supported the scholarship for his
numerical work.
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