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An Experimental Study of Combustion Characteristics of Petroleum Coke.

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Do.Chem. Eng. Mineral Process., 10(5/6), pp.601-614, 2002.
An Experimental Study of Combustion
Characteristics of Petroleum Coke
T. Mi*, Z.S. Wu, B.X. Shen, H.P. Chen and D.C. Liu
National Laboratory of Coal Combustion, Huazhong University of
Science and Technology, Wuhan 430074, P R. China
The combustion characteristics ofpetroleum coke have been analyzed by the TGA
method. The same tests have also been performed to determine the combustion
characteristics of 3 types of coals for comparison. The combustion tests and
measurement of the pollutant emissions for petroleum coke and a mixture of
petroleum coke and coal have been performed in a bench-scale CF3 rig by
limestone addition. The pollutant emissions have been measured by an on-line
analyzer The results show that the index of combustion characteristics of the
petroleum coke is higher than that of anthracite, and lower than that of sop coal,
and close to lean coal. During the 72-hour combustion tests, agglomeration and
slagging were not observed. The combustion is stable and the pollutant emissions
are within the environmental standards.
Introduction
Petroleum coke is a by-product of the petrochemical industry. It has high fixed
carbon, low ash content, high sulfur content and high heating value. Therefore, a
good option for the utilization of petroleum coke containing high-sulfur content is
to use it as fuel in CFB boilers [l, 21. In recent years, more and more thermal
power plants worldwide have been using petroleum coke, especially with a
high-sulfur content, as a fuel to produce steam for electricity generation and also
as the heating source in circulating fluidized bed boilers [3,4]. Coal combustion in
* Authorfor correspondence (mitiel 999@163.~om).
601
T. Mi, Z.S. Wu,B.X Shen, H.P.Chen and D.C. Liu
CFB boilers has been extensively studied, however, studies on combustion
characteristics of petroleum coke and their pollutant emissions in CFB boilers are
not available. Because the physical properties of petroleum coke are very
different from that of coal, it is essential to investigate combustion characteristics
of petroleum coke and the pollutant emissions in CFB boilers.
Conn [5] has studied agglomeration characteristics of petroleum coke in
CFB boilers. Bryers [6] performed studies on combustion characteristics of
different petroleum cokes and mixture of petroleum coke with coal. Chang et al.
[7] investigated combustion, heat transfer and pollutant emissions of petroleum
coke and waste tires in a pilot CFB boiler. In this paper, the combustion
characteristics of petroleum coke have been analyzed by the TGA method. The
same tests have also been done for combustion characteristics of 3 kinds of coals
for comparison. The combustion tests and the pollutant emissions of petroleum
coke, and for the mixtures of petroleum coke and coal, have been performed in a
bench-scale CFB rig with limestone addition.
Study of Combustion Characteristics of Petroleum Coke by TGA
(i) TGA test apparatus and experimental preparation
The thermogravimetric analysis was performed with a thermal analyzer
(NETZSCH STA-409), which is shown schematically in Figure 1. The system
consists of a dynamic mechanical analyzer, a temperature controller, a driving
head with a controlled temperature environment, and a computer that serves to
control the measurements, to collect the real-time data, and to carry out the data
analysis. The samples of petroleum coke are crushed and sieved. The sample
(0.1 mm-0.154 mm, 10.2 mg) was placed in the thermal analyzer with an air flow
rate of 100 mumin, and heating rate of 2O0C/min.The same tests have also been
performed for 3 types of coals for comparison. Proximate and ultimate analyses
for these samples are presented in Table 1. The experimental results are presented
in Table 2. According to Chen Jianyuan's suggestion [S], we used the volatile
characteristics index (D) and combustion characteristics index (S) to evaluate the
combustion characteristics of petroleum coke.
602
An Experimental Study of Combustion Characteristics of Petroleum Coke
(ii) Index of volatile characteristicsof petroleum coke
The index for volatile characteristics of petroleum coke (D) is used to describe the
release of volatile matter in petroleum coke. The bigger the D value, then the
faster (and more concentrated) the volatile materials of the petroleum coke release,
and the more favorably the petroleum coke ignites. The index is defined by:
D=
(dwldt) k
TI, VT,I 3
The research results show that the index of volatile matter for petroleum coke
is 1 . 5 9 1~O4 which is less than that of coal, such as Wuxiang coal or Mixuan coal.
This means that the volatile matter of petroleum coke is low.
Figure 1. Thermogravimebic apparatus STA-409.
(iii) Combustion characteristics index of petroleum coke
The combustion characteristics index (S) is a comprehensive index for evaluating
ignition and burnout of petroleum coke. The higher the value of S,then the better
the combustion characteristics. The index is defined by:
603
7:Mi, ZS.Wu,B.X Shen, HJ?Chen and D.C. Liu
The combustion characteristics indices of the petroleum coke, soft coal and
anthracite are shown in Table 2. The experimental results in Table 2 show that the
combustion characteristics index of petroleum coke is between the values for soft
coal and anthracite. This means that the petroleum coke is not difficult to ignite
and bum out.
(iv) Ignition index of petroleum coke
The carbon, nitrogen and sulfur content and calorific value of petroleum coke are
higher than for other fuels. According to proximate analysis of petroleum coke,
the ignition index (Fz) developed by Fu weibiao, can be used to judge ignition
characteristics of petroleum coke where: F, = (Va d +Ma d ) * c, d X104. The
research results show that the ignition index of petroleum coke is 1.37, and the
ignition indices of the other coals, such as Wuxiang coal and Xinzhen coal, are
1.668 and 1.102 respectively.
Table 1. Proximate and ultimate analysesfor the samples.
Petroleum
Wuxiang
Xinzhen
Miruan
coke
coal
coal
coal
coal
Volatile matter
12.34
12.81
10.51
15.81
22.32
Fixed carbon
8 1.08
72.69
67.95
69.67
39.48
Moisture
0.6
2.34
2.23
2.67
2.1
Ash
1.24
12.06
19.314
11.85
36.10
C
88-69
8 1.75
80.3
81.85
64.47
H
4.12
3.68
3.42
3.28
4.39
N
2.50
1.22
1.17
1.21
1.29
S
1.84
0.35
0.32
0.78
0.47
Soft
i#
Proximate analysis
(wt%.
ad)
Ultimate analysis (wt%
33624
LHV (w/kg)
24952
29017
27420
30835
Note: Petroleum cokefiom Jinlingpetrochemicul plant, Nanjing in China; # is coal for FBCB
experiment.
604
An Experimental Study of Combustion Characteristics of Petroleum Coke
The higher the ignition index, then the better the ignition characteristics [8].
The research results show that the ignition characteristic of petroleum coke is
between the ignition characteristics of coals with higher and lower volatile matter
content. Therefore, petroleum coke as a fuel is not difficult to ignite, its ignition
point is between the ignition point of soft coal and the ignition point of anthracite.
(v)
Eflect ofpetroleum coke particle size and heating rate on the ignition point
The experimental results are shown as Figures 2 and 3. As shown in Figure 2,
when the particle size of petroleum coke increases, the ignition point of petroleum
coke increases. As particle size increases, the specific surface area, pore area and
pore volume decrease. In addition, the volatile matter in petroleum coke is lower,
so the ignition temperature increases. From Figure 3, it can be inferred that
heating rate has hardly any effect on ignition temperature due to the low volatile
content.
Table 2. Comparisonfor Fz, S, D of coal and petroleum coke.
Coal type
FZ
SXlU9
Dxlod
Wuxiang coal (lean coal)
1.668
4.664
7.15
Xinzhen coal (anthracite)
1.102
3.097
1.75
Mixuan coal (lean coal)
2.26
4.806
6.52
Petroleum coke
1.37
3.790
1.54
Figure 2. The efect of the particle
diameter on the ignition temperature.
Figure 3. The efect of the heating
rate on the ignition temperature.
605
T Mi,Z.S. Wu,B.X. Shen, H.R Chen and D.C.Liu
Combustion Experiments on Petroleum Coke in a Bench-scale CFB
Boiler
The combustion experiments on petroleum coke were carried out in a bench-scale
fluidized bed boiler, shown schematically in Figure 4, at the National Laboratory
of Coal Combustion, Huazhong University of Science and Technology (China).
The characteristic parameters of the boiler are as follows: steam flow: 1 t/h; steam
pressure: 1.25 MPa; steam temperature: 194OC; feed water temperature: 2OOC;
stack gas temperature: 160°C.
On--line
Y
Plenum
Combustor
U-shaped burnout chamber
Convective bank
Multiple cyclone
Ash circulating combustion
system
Economizer
Figure 4. Schematic diagram of the circulatingfluidized bed boiler
(a) Experimental details
The fuel size requirement for combustion in CFB required the petroleum coke,
limestone and coal to be crushed. The size distributions of the crushed petroleum
coke and coal, with average size of 2.8 mm and 3.9 mm respectively, are shown
in Table 3. The crushed limestone, petroleum coke and coal are fully mixed in a
definite weight ratio (limestone : petroleum coke : coal = 1 : 2.3 : 3.5; and
limestone : petroleum coke = 1 : 1.7). Concentrations of 02,SO2 and NO, in the
flue gas were measured by an on-line analyzer.
606
An Experimental Study of Combustion Characteristics of Petroleum Coke
Table 3. The size distribution ofpetroleum coke and so3 coal.
Particle size
>8
8 4
6 . 5
S-4
4-2
2-1
1-4.6
0.60.4
c0.4
5.0
7.0
7.6
8.1
27.7
7.6
11.6
11.3
14.1
fmm)
.
I
I"(% Wt)
2'(% Wt)
5.8
14.5
12.8 13.7
33.3
3.7
6.6
4.6
5.0
Note: 1' s for petroleum coke from Jinling petrochemical plant, Nanjing in China; 2' is for coal
from FBCB experiment.
(b) Results and dikcussion of combustion experiments
The combustion experiments were carried out in three stages. The first stage was
start-up and temperature rise stage. After successful start-up when the bed
temperature reaches the desired level, the second stage begins as the mixture of
petroleum, limestone and coal are fed into the CFB for co-firing combustion
experiments. The experimental results in the second stage show that the bed
temperature has no significant fluctuation (as shown in Figure 5), the static
pressure of the plenum chamber is almost constant (as shown in Figure 6), and the
operational process is stable and is no different from coal combustion.
Figure 5. The curve of the bed
temperature.
Figure 6. The curve of the static
pressure of plenum.
The third stage is petroleum coke combustion. It was carried out after 24 hours
of second stage operation. During petroleum coke combustion, we found that the
feeding quantity increased and operational air flux decreased, compared with
60 7
2: Mi,Z.S. Wu,B.X. Shen, H.P Chen andD.C, Liu
co-firing of petroleum coke with coal
(as shown in Figure 7), but the
combustion was stable and the bed
temperature showed no evident
fluctuation, and agglomeration and
slagging were not observed. The
operational static pressure of the
plenum was lower because of the
higher carbon content and lower ash
content in petroleum coke. Therefore,
0
2
4
ma-
6
8
10
Figure 7. The curve of rotary speed
in the experiments it is difficult to
__ .
ojjeeder.
maintain the height of bed material in
the bed. However, for co-firing of petroleum coke with coal, then coal ash can be
used as bed material and maintain a definite bed material height. In the
experiment, the carbon content of fly ash collected from the U-shaped chamber,
multiple cyclone and the economizer were analyzed and the results are shown in
Table 4. As shown in the table, the carbon content of fly ash collected in
petroleum coke combustion is higher than for fly ash collected in co-firing of
petroleum coke with coal. The reason is that the smaller petroleum coke particle
was not fully combusted and was blown away from the combustion chamber.
Therefore, the fly ash collected from flue gas has higher carbon content (as shown
in Table 4) and lower combustion efficiency, owing to the petroleum coke
characteristics (low volatile content and difficult burnout). Furthermore, the
smaller boiler capacity, a finite combustion chamber, and lower fly ash recycle
ratio also have an influence on the higher carbon content of fly ash. The bottom
ash from co-firing combustion of petroleum coke with coal in the experiments
was collected and sieved. The results for the distribution of the bottom ash
particle size are shown in Table 5.
(c) The analyses of SO2 and NOX emissions
The effects of bed temperature and excess oxygen content on SO2 and NO,
emissions were determined for petroleum coke combustion and co-firing
combustion of petroleum coke with coal. Figure 8 shows SO2 emission increases
608
An Experimental Study of Combustion Characteristics of Petroleum Coke
Table 4. The carbon content offly ash.
Fly ashfrom multiple
Source
Of
sample
Fly
ash from
chamber %(wt)
cyclone % (wt)
economizer (wt)
11.56
24.11
22.69
30.91
29.29
Petroleum coke
and coal
Petroleum coke
21.93
~~-
~
~-
~
~
Table 5. The distribution of bottom ash particle size.
Particlesize (mm) >8
8-6
6-5
5 4 4-2
2-1
14.6
0.60.4
<0.4
0
3.6
4.4
5.3
8.3
21.8
19.7
11.5
% (Wt)
25.5
with increasing bed temperature. It can be seen that at 84O-86O0C, the SO2
emission slightly decreases. The result is consistent with literature sources [9,
lo]. Many references consider that the range of optimal desulfurizing temperature
in CFB is at 835-850OC. When bed temperature exceeds this range, then with
increasing bed temperature, the sintering of CaO surface area and blocking of the
CaO pore branching lead to a weaker reaction of CaO with S02.
Figure 9 shows that with increasing bed temperature, NO, emission
increases. At constant excess oxygen content in the bed, the higher the bed
temperature then the faster the burning rate of carbon particles. Therefore, the
concentration of the carbon-containing species in flue gas and char in fly ash
(which plays an important role in heterogeneous destruction of NO3 is low, and
the final fraction of fuel nitrogen conversion to NO, is high. Alternatively, the
reaction rate for thermal decomposition of N20 by hydrogen rapidly increases as
the burning temperature increases. At bed temperatures above 9 1O'C, NO,
emission decreases. The relative importance of the oxidation and reduction
consequently determines the resultant nitrogen oxide emissions. Most of these
reactions are catalytic. In addition to coal, soot and coke particles, coal ash and
other bed materials have a catalytic effect [ I l , 121. Ralf et al. [I31 in their
research also found that at low combustion temperatures (TC650"C) the NO,
formation reactions, and at higher combustion temperatures (P7OO'C) the NO,
609
T.Mi, Z.S. Wu, B.X. Shen, H.P Chen andD.C. Liu
reductions, are catalytically accelerated by lignite ash. For hard coal at bed
temperatures above 850"C, the ash preferentially favors the NO, reduction. For
petroleum coke, it appears to follow the same trend. The sole difference is when
the bed temperature is above 910°C and NO, reduction occurs.
Figure 10 shows that with increases in excess oxygen, the SO2 emission of
the two combustion modes decreases. The decrease of the SO2 emission is
attributed to the following reasons. First, in the higher excess air condition, the
sulfation reactions taking place at oxidizing conditions are [14]:
- -
CaO+CO2
CaC03(s)
CaO +1/2 0 2 + SO2
CaS04
or
CaCO3 (s) +1/2
0 2
+ SO2
-
CaS04(s)+ C02
calcination
sulfation
(2)
direct sulfation.
(3 1
(1)
When the excess oxygen is increased, more SO2 will convert to CaS04, and the
emission of SO2 reduces. Second, there are many heavy metals in petroleum coke,
especially vanadium [3, 151, which have a catalytic effect on the conversion of
SO2 to S03. The SO2 conversion reaction is more favorable in the high
temperature and pressure condition. Burdett et al. [17] have investigated the
reaction kinetics and reported the kinetics constant (k; cm3/mol s) in the range
900-1350°C [16, 171 for k = 2 . 6 ~ 1 0 exp
' ~ (-23OOO/T). They calculated the SO3
equilibrium concentration for 5% and 10% oxygen content as 8 ppm and 20 ppm
respectively. This result shows that during desulfurization, SO3 plays an important
role. Third, perhaps the higher excess air dilutes the SO2 emission concentration.
e
h
Figure 8. The relation of bed
temperature with SO, emission.
610
Figure 9. The relation of bed
temperature with NOxemission.
An Experimental Study of Combustion Characteristics of Petroleum Coke
Figure 11 shows that with the increasing excess oxygen content, NO,
emission for the two combustion modes increases. The increase of NO, emission
with excess oxygen content is attributed to the higher oxygen concentration in the
fluidized bed combustor. In the higher oxygen concentration condition, oxidation
of N-containing intermediate components (NH and CN) is more favorable. In
addition, the higher oxygen concentration results in lower char and CO
concentrations throughout the combustor, thus minimizing the heterogeneous
reduction of NO, on the char surface. It can be interpreted using the following
reaction equations:
HCN
-
NCO-NO
It can be seen from Figures 8 to 11 that for either petroleum coke
combustion or co-firing of petroleum coke with coal, both the SO2 emission and
NO, emission are low and meet the environmental standards.
1-
-
,
. ,
,
.
,
.
.
,
,
.
I
Figure 10. The relation of excess oxygen
content with SO2 emission.
Figure 11. The relation of excess
oxygen content with NO, emission.
T Mi, ZS. Wu,B.X Shen,H.l?Chen and D.C.Liu
Conclusions
The volatile matter content of petroleum coke is low, but the petroleum coke is
not difficult to ignite. The ignition temperature of petroleum coke is between that
of soft coal and anthracite. The combustion characteristics of petroleum coke are
close to that of lean coal.
As a fuel for circulating fluidized bed boilers, the combustion process of
petroleum coke is as stable as the coal combustion. In the experiments, whether
for co-firing of petroleum coke with coal or petroleum coke combustion,
agglomeration and slagging of the bed material are not observed. Therefore,
petroleum coke can be substituted for coal in circulating fluidized bed
combustion.
With increasing bed temperature, the sintering of CaO surface area and
blocking of CaO pore branching lead to weakened reaction of CaO with S 0 2 . The
SO2 emission increases with bed temperature. The relative importance of
oxidation and reduction consequently determine the resultant nitrogen oxide
emission. For petroleum coke combustion, at lower combustion temperature
(T<880°C) the NO, emission increases with bed temperature, the NO, formation
is predominant. At combustion temperatures above 91O'C, NO, emission
decreases with increasing bed temperature, therefore the NO, reduction is
predominant.
The SO2 emission decreases with increasing excess oxygen content. When
excess oxygen is increased, more SO2 converts to CaS04, therefore the emission
of SO2 reduces.
The NO, emission increases with excess oxygen content. Higher oxygen
concentration results in lower char and CO concentrations throughout the
combustor
Controlling bed temperature and adding limestone to the fuel can reduce SO2
and NO, emission, and comply with environmental emission requirements.
In the experiments, agglomeration of petroleum coke in fluidized bed
combustion mentioned by other researchers was not studied. It requires further
investigation.
612
An Experimental Study of Combustion Characteristics of Petroleum Coke
Nomenclature
Maximal combustion velocity (mg/min)
Maximal release velocity of volatile matter (mg/min)
Average combustion velocity (mg/min)
Temperature range corresponding to (dwldt);, = 1 / 3
Temperature corresponding to maximal release velocity of
volatile matter (“C)
Burnout temperature (OC)
Ignition temperature (OC)
Carbon content of petroleum coke (ad: air dry)
Moisture in petroleum coke
Volatile matter content of petroleum coke
1.
Abdulally, L.F., and Voyles, R.W. 1992. Multiple fuel firing experience in a circulating fluidized bed
boiler [A]. Proceedings of the American Power Conference, Proceedings of 54* Annual Meeting of
American Power Conference, April 1992; Chicago, Illinois, USA; Published by Illinois Institute of
Technology, 54(1), pp.756-764.
2.
Abdulally, I.F., and Reed, K. 1995. Experience update of firing waste fuels in Foster Wheeler’s
circulating fluidized bed boiler [A]. Proceedings of 13“ International Conference on Fluidized Bed
Combustion. Part 2 (of 2); 7-10 May 1995; Orlando Florida, USA; Sponsored by ASME, V01.2,
pp. 753-766.
3.
Somechuer, B., Flour, I., Maissa, P., and Bruyet, B. 1996. Kinetic study of char combustion in a
fluidized bed, Fuel, 75(5), 536-544.
4.
Brereton, C.M.H., Lim, C.J., and Grace, J.R. 1995. Pitch and coke combustion in a circulating
fluidized bed. Fuel, 74(10), 1415-1423.
5.
Conn, R.E. 1995. Laboratory techniques for evaluating ash agglomeration potential in petroleum coke
fired circulating fluidized bed combustors. Fuel Processing Technology, 44( 1-3), 95-103.
6.
Bryers, R.W.1995. Utilization of petroleum coke and petroleurn cokdcoal blends as means of steam
raising, Fuel ProcessingTechnology.
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T Mi,Z.S. Wu, B.X. Shen, H.P Chen andD.C. Liu
7.
Chang, Yu-min, and Chen, Mer-yen. 1993. Industrial waste to energy by circulation fluidized bed
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Chen, Jianyuan. 1989. The Investigation of Ignition Process and Ignition Model of Coal. Doctoral
Thesis (Chinese), Huazhong University of Science and Technology, Wuhan, P.R. China.
9.
Couturier, M.F. 1986. Sulphur Dioxide Removal in Fluidized Bed Combustors. Technical Report
QFBC. TR. 86.1.
10. Hanson, P.F. 1991. Sulphur Capture in Fluidized Bed Combustors. Doctoral Thesis, Technical
University of Denmark.
11. Gavin, D., and Dorrington, M. 1993. Factor in the conversion of fuel nitrogen to nitric and nitrous
oxides during fluidized bed combustion, Fuel, 72,381-388.
12. Johnson, J. 1994. Formation and reduction of nitrogen oxides in fluidized bed combustion, Fuel, 73,
1398-1415.
13. Kopsel R.F.W.,
and Halang, S. 1997. Catalytic influence of ash elements on NO, formation in char
combustion under fluidizcd bed conditions,Fuel, 76,345-351.
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combustion gases. Chem. Eng. Sci., 45(5), 1175-1187.
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D.M.,and Voyles, R.W. 1993. NISCO cogenerations facility. Proceedings of 12"
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Received: 26 April 2001; Accepted after revision: 1 February 2002
614
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