EFFECT OF O2 AND CO2 CONTENT ON PARTICLE SURFACE
TEMPERATURE AND SIZE OF COAL CHAR DURING
COMBUSTION
MILENA RODRIGUEZ and RISTO RAIKO
Tampere University of Technology
Department of Energy and Process Engineering (EPR)
P.O. Box 589
33101 Tampere, Finland
Tel. + 358 3 3115 11
Email: milena.rodriguez@tut.fi
ABSTRACT
The effect of oxygen and carbon dioxide on the combustion of coal char particle was
studied in a drop-tube furnace. Char particle size fractions of 100–125 and 180–200 µm
were combusted at the temperatures of 973, 1123 and 1273 K in two different mixtures,
i.e., oxygen in nitrogen and oxygen in carbon dioxide. The oxygen concentrations in each
mixture were 3, 6, 12, 25, 35, 50 vol-%. Particle surface temperature, size and velocity
were measured with a two-colour pyrometer. Particle size and velocity were measured
also with a high-speed camera. Increasing the oxygen content raised the average char
surface temperature, which was higher with oxygen in nitrogen than with oxygen in carbon dioxide. Particle size decreased with increasing oxygen concentration due to particle
fragmentation during combustion. The effect of the oxygen content on particle surface
temperature was stronger than that of the atmosphere gas temperature.
KEYWORDS:
Coal char, Oxygen, Carbon dioxide, Particle surface temperature.
1
1
INTRODUCTION
Today, solid fuel combustion plays an important role in energy conversion. This situation
will continue in the future. Reducing fuel consumption and harmful emissions is the main
objective in developing new conversion and combustion technology. Before new technology can be put into operation, laboratory studies are required to elucidate the behavior of
different fuels and combustion processes.
So-called oxy-fuel combustion has been considered one of the most promising techniques for pulverized coal. Its benefits include energy efficiency, flame stability and increased productivity, i.e. the material processing rate through the combustion chamber is
increased because oxygen-enhanced combustion increases the radiation from the flame to
the load and this increases the heat transfer to the load with reduced exhaust gas volume
and pollutant emissions (Baukal, 1998).
Different laboratory-scale analysis methods have been used for the characterization of
solid fuel and for coal combustion experiments. The drop-tube furnace (DTF) is probably
becoming the most common type used due to high heating rate, control of high temperatures and control of atmosphere combustion. Therefore, it was selected for the present
investigation. Some researchers, such as Chen (2007), used the DTF to study coal reaction in the environment of partial oxidation. In studying coal combustion, Monsona
and Germane (1993) used a DTF with pressures ranging from 1 to 15 atm, with oxygen
concentration from 0 to 21 vol-% and with gas temperature of 1000–1700 K. Murphy
and Shaddix (2006) investigated high-volatile bituminous coal with oxygen concentration
ranging from 6 to 36% and with a gas temperature of 1320–1800 K in an entrained flow
reactor. Kajitani et al. (2001) used a pressurized drop-tube furnace (PDTF) for gasification rate analysis of coal char, whereas Joutsenoja et al. (1998) used it to measure the
temperature and the size of the pulverised coal. On the other hand, Aho et al. (1995) used
PDTF to determine the effects of pressure, oxygen partial pressure and temperature on
the formation of N2 O, NO and NO2 from pulverized coal. They found that NO formation
decreased sharply with pressure and increased, although not as strongly, with temperature
and partial pressure of oxygen.
The surface temperature and the size of the burning fuel particles affect both the combustion rate and the characteristics of the combustion product. Particle temperature may
have a significant effect on some technical problems in the operation of boilers, such as
on slugging and fouling, as well as on fly-ash formation and depositions.
The temperature measurement of small moving particles is difficult to perform by
physical contact because the contact with the thermocouple changes the combustion and
heat transfer conditions, the free movement of the particle and only a certain amount of
particles can be measured. Therefore, other methods such as optical two-colour pyrometry must be used. This method has been used for large particles (3–15 mm) (Lu et al.,
1999) and in a recent study with small particles (45–53 µm) (Bejarano and Levendis,
2007). The two-colour pyrometry metody is based on non-contact measurement of par2
ticle surface temperature, which is based on the detection of thermal radiation emitted
by the particle and on Planck’s law of radiation. With this method it is also possible to
measure particle size.
Char is the substance from the solid fuel particle remaining after pyrolysis. The composition and characteristics of the char may vary because the composition and quantity
of volatiles depend on the type of solid fuel, heating rate and maximum temperature at
which the decomposition of the hydrocarbons takes place. It is necessary to study the
char combustion process for every solid fuel independently and record the conditions under which the char was formed.
Fundamental understanding of the effect of operating conditions and solid fuel properties in the char combustion process is essential for the development of efficient combustion devices and advanced clean fuel technologies. The use of numerical codes is of great
importance in the modelling of the combustion system. The economy of the combustion
process depends heavily on the extent to which the loss of unburnt fuel can be minimized
(Barranco et al., 2003). Experimental research and mathematical modelling of char combustion are needed to determine the ignition temperature, combustion rate, burnout time
and the temperature of char particles.
This paper provides experimental results regarding the effect of oxygen and carbon
dioxide during char combustion. It covers the experimental device, procedure, conditions
and results, discussion and finally conclusions.
2
EXPERIMENT
In the present study, a drop-tube furnace has been designed and constructed for use in
both pyrolysis and char combustion. The DTF was connected with a pyrometer and a
high-speed camera. They were used to investigate reactions of coal char in oxy-fuel combustion. Temperature, size and velocity of the particle were measured in situ. In this
section, details of the fuel samples and a description of the reactor and experimental conditions will be given.
2.1
Fuel samples
The tests were carried out with bituminous coal. Prior to pyrolysis, the samples were
crushed in the laboratory and screened into two particle size fractions, i.e. 100–125 and
180–200 µm. Then, they were dried in an oven at 105 °C for 24 h to ensure continuous
and stable feeding. These fractions are called parent coal. Table 1 shows the proximate
and ultimate analysis of the coal. The char was produced from the parent coal in a 60
cm-long heated tube in nitrogen at 973, 1123 and 1273 K in flow gases of 1.8, 1.6 and 1.4
l/min, respectively. The devolatilization residence time was calculated to be of the order
of 1 s.
3
Proximate Analysis
Coal
Moisture (as received)
2.1 wt-%
Volatile (TG)
30.8 wt-% dry
Fixed carbon (TG)
50.2 wt-% dry
Ash (815 C)
8.1 wt-% dry
Ash (TG)
7.3 wt-% dry
CO2 (TG)
5.9 wt-% dry
C (TG)
1.6 wt-% dry
Ultimate Analysis
C
66.6 wt-% dry
H
4.2 wt-% dry
N
1.0 wt-% dry
S
0.5 wt-% dry
Cl
0.0013 wt-% dry
O (Calculated)
19.5 wt-% dry
Table 1: Proximate and ultimate analysis of coal.
2.2
Reactor description and procedure
Char combustion was carried out in a DTF shown schematically in Figure 1. The reactor
is an austenitic Cr-Ni stainless steel tube with the inner diameter of 26.3 mm, and it was
heated electrically by a Kanthal AF wire forming four separate 15 cm heating elements.
The reactor zone can be heated up to 1373 K continuously. Each heating was controlled
independently by maintaining a constant temperature over the reactor height. The temperature of the furnace was measured by a K-type thermocouple. The char particles were
carried by small gas flows, i.e. 1.8, 1.6 and 1.4 l/min, through a counter-flow heat exchanger by means of a feeding device consisting of a glass tube with a vibrating system
to ensure a constant feeding rate. Afterwards, the fuel went into the hot heating zone with
the mixture gases flowing downwards through a vertical furnace tube at a Reynolds number low enough to ensure a laminar flow. After that, the particles were quenched for the
second counter-flow heat exchanger. Finally, the ash was collected by a microfiber filter
with a metallic cover. In the middle of the lowest heating section a quartz glass window
with the diameter of 2 cm was placed in order to measure the particle surface temperature
and diameter through a two-colour pyrometer, see Figure 1. The particle velocity and
diameter were also measured using the IPX-2M30-G high-speed camera.
4
Figure 1: Sketch of drop-tube furnace.
2.3
Experimental conditions
The flow rate, the temperature and the species concentrations in the atmosphere were
varied in the experiments with a given sample. The volume flow was varied to 1.8, 1.6
5
and 1.4 l/min with the temperatures of 973, 1123 and 1273 K, respectively, in order to
keep the gas velocity in the heating section constant. The oxygen concentrations used
were 3, 6, 12, 25, 35 and 50 vol-% both in N2 and in CO2 . The lowest of the four heating
elements was used for the measurements, see Figure 1.
2.4
Temperature and size measurement
The two-colour pyrometer was used to measure at the same time the temperature, size and
velocity of individual burning char particles at a selected height (7.5 cm) in the middle
of the lowest heating element. The method for measuring the surface temperature Tp of
combusting fuel particles is exclusively based on the detection of the thermal radiation
emitted by the particle and Planck’s law of radiation. The object’s radiation is measured
at two narrow wavelength bands and the temperature is detected from the ratio of these
measurement results. The ratio of the signals is (Joutsenoja et al., 1998)
(R1 − R01 )/(R2 − R02 ) = [F1 (Tp ) − R01 ] / [F2 (Tp ) − R02 ] ,
(1)
where R1 is the signal with wavelength λi when a particle is in the field of view (FOV);
R1 − R01 is the pulse height at wavelength band λi (i = 1, 2); R01 is the system response
when no particles are in the FOV, in others words the background signal with wavelength
λi and Fi (T ) is the system response calibrated against a blackbody radiator at temperature
T . Here, the particles have been assumed to be grey bodies.
After particle temperature has been measured, the only unknown variables are particle
size and emissivity. Using the proportionality between the pyrometric response signal and
the particle cross-section area in the FOV, the cross-section area of the particle A p can be
solved from
ε p A p /A0 = (Ri − R0i )/ [F1 (Tp ) − R0i ] ,
(2)
where ε p is the emissivity of the particle, setting it at 0.9 (Davis, 1978), and A0 is
the cross-sectional area of the FOV in the focal plane. The diameter D p of an equivalent
spherical particle is calculated from A p (Joutsenoja, 1998).
3
RESULTS AND DISCUSSION
The purpose of the present study is to investigate the effect of oxygen and carbon dioxide
on coal char reactions in a DTF and to evaluate the reaction performances of the coal
char. Experiments were made with bituminous coal char. The parent coal size fractions
are 100–125 and 180–200 µm. The gas temperature was fixed in the experiments at 923,
1123 and 1273 K at atmospheric pressure.
6
3.1
The effect of oxygen in char combustion
Each particle showed different behaviour. This is due to the shape, size, structure and morphology of particles. Figure 2 exhibits the average temperatures of particles at different
oxygen concentrations. The particle temperature of the coal char increased significantly
as the oxygen concentration was raised, see Figure 2. This has also been demonstrated by
Smoot (1991), Joutsenoja et al. (1998) and Bejarano and Levendis (2007). The gas temperature had little effect on particle temperature. The temperature for both particle size
fractions was quite similar. At lower oxygen concentrations from 0 vol-% to 12 vol-% the
gas temperature altered the temperature of particles about ±90 degrees for small particles
and ±100 degrees for large particles. However, at oxygen concentrations greater than 25
vol-% the difference in particle temperature is less than 50 degrees.
(a) 100–125 µm
(b) 180–200 µm
Figure 2: Average surface particle temperature Tp of individual coal char at different oxygen concentrations.
(a) 100–125 µm
(b) 180–200 µm
Figure 3: Average particle size D p of individual coal char at different oxygen concentrations.
The results regarding particle temperature and particle size results shown in Figures
2 and 3 are average values for all particles measured, see Appendix A. Variation between
different measurements was rather small. For example, the standard temperature deviation of a bituminous coal char particle of size 100–125 µm at gas temperature 1123 K
7
varied from 22–164 K, while for the particle size of 180–200 µm the standard temperature
deviation varied from 77–204 K. It is smallest with an oxygen concentration of 0 vol-%
and greatest at a concentration of 50 vol-%.
Figure 4: Particle shape of individual coal char 100–125 µm.
Figure 3 shows the mean value of particle size as detected by the two-colour pyrometer at different oxygen concentrations. At lower O2 concentration from 0 vol-% to 12
vol-% and with parent coal size 100-125 µm, the combusting particles are slightly bigger
than the initial size. Possible explanations for this phenomenon are the swelling of particles during pyrolysis or the initial stage of combustion and the fact that the initial particle
shape was elongated rather than spherical.
Figure 4 shows different char particles during combustion at 3 vol-% O2 and 1123 K
as obteined by the high-speed camera. In the picture, the particles are elongated and thus
it is possible that the two-colour pyrometer detects the longest dimension of the particle
instead of the shortest. To elucidate why a particle seems to grow, microscopic measurements of particle shape and size are needed. However, studies of the swelling behaviour
of individual coal particles have been done by Yu et al. (2003) and Strezov et al. (2005).
They concluded that temporary swelling ratios decreased clearly with increasing coal density. Also the manceral content and the rank of the coal affect the swelling behaviour of a
particle. The plastic properties of the different coals and their associations will determine
the char morphology, i.e. increased diameter due to swelling, sphericity and porous wall
thickness (Alvarez et al., 1998). These properties could have a major influence on the
efficiency of char combustion. Char reactivity is an important consideration in relation to
aspects such as incomplete combustion leading to carbon in fly ash. The extent to which
this unburnt carbon is minimized will significantly affect the economy of the whole coal
combustion process, see Barranco et al. (2003).
8
Figure 5: Particle temperature Tp and diameter D p of individual coal char at different oxygen content.
Detection limit of pyrometer device (solid curve) and estimated gas temperature (dashed line) are included.
(D p = 100–125 µm).
Figure 6: Velocity distribution of coal char measured by two-colour pyrometer and CCD. (D p = 100–125
µm, Tp = 1123 K and O2 = 3 vol-% in CO2 ).
The individual particles of the coal char at different oxygen content are plotted in the
D p -T p plane in Figure 5 .The detection limit of size measurement and gas temperature
9
are also indicated. The measurements were made by comparing the velocity of particles
during combustion using the two-colour pyrometer and the high-speed camera measurements and the results can be seen in Figure 6. The probability density of the velocity for
both methods is very similar. Most of the particles have a velocity that fluctuates from 35
to 45 cm/s, even though the gas velocity is merely 50% of that, i.e. 20 cm/s.
3.2
The effect of carbon dioxide in char combustion
The effect of carbon dioxide is significant and gives information for future study regarding
recycling gases. Figure 7 illustrates the cooling effect of carbon dioxide on particle temperature, which is more significant with high oxygen concentration from 25 to 50 vol-%.
Similar results were presented by Molina and Shaddix (2005). At lower O2 content from
0 to 12 vol-%, particle temperature barely differs from when nitrogen or carbon dioxide
are used as dilution gas. The table in Appendix A shows all the measurements done at
different oxygen concentration.
Figure 7: Effect of CO2 and O2 on particle temperature. Gas temperature is 1273 K and particle size is
180–200 µm.
4
CONCLUSION
The effect of oxygen and carbon dioxide on a coal char particle was studied in a drop-tube
furnace at temperatures from 923–1273 K. Temperature was predicted using a two-colour
pyrometer where the signal was converted to temperature using the Planck’s radiation law.
During the measurements it was observed that the oxygen in the furnace increased particle
temperature and shortened the burnout duration. The carbon dioxide decreased particle
temperature during combustion, especially at an oxygen concentration of more than 25
vol-%.
10
References
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partiall pressure and temperature on the formation of N2 O, NO and NO2 from pulverised coal. Combustion and Flame. Vol.102, pp. 387–400.
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Barranco, R., Cloke, M. and Lester, E. (2003). Prediction of the burnout performance of
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structure of char particles. Second International Conference on CFD in the Mineral
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oxygen-enhanced and oxygen/carbon dioxide pulverized coal combustion. Combustion
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Smoot, L. (1991). Fossil Fuel Combustion: A Source Book. John Wiley & Sons, Inc..
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Strezov, V., Lucas, J. A. and Wall, T. (2005). Effect of pressure on the swelling of density
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Appendix A
Drop-Tube Furnace
Reactor diameter: 26.3 mm
Length of heating section:150mm
Fuel: Coal char
Gas: Oxygen in nitrogen
Tg : Gas temperature
T p : Particle temperature
vg : Gas velocity
v p : Particle velocity
D p : Particle diameter
STD: Standard Deviation. Estimates standard deviation based on a sample. The standard
deviation is a measure of how widely values are dispersed
q from the average value (the
mean). It was calculated using the following formula:
mean and n is the sample size.
O2 [%]
0
3
6
12
25
35
50
O2 [%]
0
3
6
12
25
35
50
∑(x−x̄)2
(n−1) ,
where x is the sample
vg = 20 cm/s, Tg = 973 K, parent coal size = 100–125 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
137
131
110
111
112
112
1088
1243
1578
2038
2253
2502
52
72
117
67
78
132
218
189
153
101
104
100
46
46
49
21
27
22
36
37
41
47
53
60
vg = 20 cm/s, Tg = 973 K, parent coal size = 180–200 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
12
54
102
138
117
112
1093
1219
1518
1953
2158
2432
49
103
131
106
134
170
201
194
171
150
152
149
12
40
46
50
37
34
36
36
39
42
49
52
57
STD [cm/s]
9
8
7
11
24
18
STD [cm/s]
10
8
9
8
11
13
O2 [%]
0
3
6
12
25
35
50
O2 [%]
0
3
6
12
25
35
50
O2 [%]
0
3
6
12
25
35
50
O2 [%]
0
3
6
12
25
35
50
vg = 20 cm/s, Tg = 1123 K, parent coal size = 100–125 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
105
1068
22
190
50
43
103
1163
63
168
35
42
144
1353
86
156
39
40
120
1697
100
115
32
43
124
2070
122
98
22
49
99
2293
111
96
21
52
78
2516
165
95
27
65
STD [cm/s]
10
13
7
6
12
29
49
vg = 20 cm/s, Tg = 1123 K, parent coal size = 180–200 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
STD [cm/s]
15
67
117
100
114
111
1140
1294
1637
1994
2199
2418
77
117
138
130
153
204
207
185
154
157
156
162
46
49
44
38
38
35
56
46
47
53
54
56
14
9
8
6
8
8
vg = 20 cm/s, Tg = 1223 K, parent coal size = 100–125 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
178
1165
40
129
28
50
125
1278
58
173
42
49
116
1427
122
146
38
47
101
1759
86
127
36
47
121
2020
64
96
22
50
116
2330
126
98
24
53
STD [cm/s]
10
7
7
6
6
7
vg = 20 cm/s, Tg = 1223 K, parent coal size = 180–200 µm
Samples T p [K] STD [K] D p [µm] STD [µm] v p [cm/s]
117
1149
38
190
39
66
145
1309
65
207
41
53
152
1461
88
191
57
52
130
1724
126
170
41
54
113
2061
102
163
37
53
171
2258
146
159
36
55
132
2534
172
159
39
55
STD [cm/s]
12
9
9
8
8
10
9
13