- Email: [email protected]
Experimental study and mechanism analysis on low temperature corrosion of coal fired boiler heating surface
Accepted Manuscript Experimental Study and Mechanism Analysis on Low Temperature Corrosion of Coal Fired Boiler Heating Surface Zhi-min Li, Feng-zhong Sun, Yue-tao Shi, Fei Li, Lei Ma PII:
S1359-4311(15)00111-8
DOI:
10.1016/j.applthermaleng.2015.02.003
Reference:
ATE 6353
To appear in:
Applied Thermal Engineering
Received Date: 5 April 2014 Revised Date:
29 January 2015
Accepted Date: 3 February 2015
Please cite this article as: Z.-m. Li, F.-z. Sun, Y.-t. Shi, F. Li, L. Ma, Experimental Study and Mechanism Analysis on Low Temperature Corrosion of Coal Fired Boiler Heating Surface, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.02.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT 1
Experimental Study and Mechanism Analysis on Low Temperature Corrosion of Coal Fired
2
Boiler Heating Surface
3
Zhi-min LIa,b, Feng-zhong SUNa, Yue-tao SHIa , Fei LIa , Lei MAa
4
a
5
b
6
Corresponding author information:
7
Corresponding author name: Feng-zhong SUN
8
Affiliation: Shandong University
9
Postal Address: 17923 Jingshi Road ,Jinan 250061, China
School of Energy and Power Engineering, Shandong University, 17923 Jingshi road, Jinan 250061,China
M AN U
SC
RI PT
College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao
Tel/Fax: +86 0531 88395691
11
E-mail: [email protected]
12
Highlights
13
The phenomena and mechanism of the low temperature corrosion was analyzed.
14
The SEM and XRD methods were used to analysis ash samples.
15
The ash particles would gather together in the course of deep-cooling flue gas.
16
The experiment tube occurred severe sulfuric acid corrosion in the wall temperature range from 65 to 47 .
EP
41
to 71
and
AC C
17
TE D
10
18
ABSTRACT: The low temperature corrosion experiment was made to investigate the phenomena and mechanism in
19
the process of the flue gas deep-cooling. The experiment tube was installed in the flue gas channel between the air
20
preheater and the electrostatic precipitator in the power plant. The scanning electron microscope (SEM), energy
21
spectrum analysis and X-ray diffraction (XRD) experiment were used to the analysis of the ash samples obtained from
22
the outer wall of experiment tube under the different tube wall temperature. By contrasting energy spectrum figures and
23
XRD figures among different ash samples, it has been found that the sulfuric acid corrosion occurred on the surface of 1
ACCEPTED MANUSCRIPT
24
the experiment tube under the temperature ranging from 65
25
vapor under the temperature ranging from 41
26
range was proposed which low temperature heating surface would be capable of long-term operation for the experiment
27
unit. The analysis also showed that the ash particles would gather together under the impact of physical chemistry in the
28
course of deep-cooling flue gas for heat recovery and efficiency gain.
29
KEY WORDS: low temperature corrosion
30
corrosion;
31
Introduction
and the corrosion was caused by SO2 and water
X-ray diffraction; the sulfuric acid
SC
scanning electron microscope
RI PT
in the certain types of coal conditions. The wall temperature
M AN U
32
to 47
to 71
Exhaust losses counted for the largest share of about 70% to 80% of the total pulverized coal furnace heat loss[1,2]. [3,4]
Boiler efficiency will decline by 1% once the exhaust temperature rise 15
34
is an effective way to improve the efficiency of the boiler. The design exhaust temperature of power plant boiler is 120
35
to 140
36
temperature of wet desulfurization tower is about 80
37
Generally the inlet temperature is reduced by injecting water before the desulfurization or increasing the amount of
38
desulfurization seriflux. This method not only increases the cost also increases water consumption, aggravated chimney
39
corrosion, reducing economy and security performance. Our research group installs flue gas waste heat recovery system
40
before desulfurization tower to reduce the flue gas temperature and reach the purpose of recycling of waste heat.
41
Generally the inlet flue gas temperature is 120
42
this case the tube temperature is close to the dew point and facing the risk of corrosion.
TE D
33
while the actual operation exhaust temperature is 120 [5,6]
. Reducing flue gas temperature
in China. The best inlet flue gas working
. There is the temperature difference of 40
to 60
.
AC C
EP
to 85
to 150
to 20
to 140
in this system and the air was heated from 20
to 60
. In
43
Many domestic and foreign scholars have studied in the field of low temperature corrosion. Early in 1959,
44
Moskovits[7] found that the corrosion speed was affected by acid condensation rate. Through the experiment Aki
2
ACCEPTED MANUSCRIPT
Sebastian Ruhl[8] discovered the main influence factors of the low temperature corrosion were the temperature and the
46
humidity of flue gas. Osakabe[9] experimentally studied thermal-hydraulic behavior of heat exchanger and discussed the
47
dew point of the flue gas. From 2010 to 2012, Alireza Bahadori[10] built a model to predict the acid dew point of
48
combustion flue gas. H Han[11] used a numerical model to simulation the condensation phenomenon of the
49
non-condensable gas and to predict the rate of sulfuric acid and water vapor. A. Kranzmann Humidity[12] explained CO2,
50
O2, and SO2 contents were the important factors determining corrosion under the oxyfuel combustion conditions and
51
discussed the temperature affection. Based on mass and energy balances, Kwangkook Jeong[13,14] established the
52
governing equations to predict sulfuric acid condensation rates and developed the analytical model to predict
53
condensation rate of water in the flue gas. Qin-xin Zhao[15] analyzed the low temperature corrosion products which were
54
produced in simulated atmospheric conditions and drew the conclusion that the corrosion resistance of the six test
55
materials was as following: GR2> ND >316L > Corten> 20G > 20# steel. Yet in-depth analysis on the mechanism of
56
the low temperature corrosion has not carried out in this paper. In order to found the temperature impact on corrosion, H.
57
Tallermo[16] did experiments on the air-preheater tubes of the oil shale CFB boilers. Yun-Gang Wang[17] researched on
58
the formation mechanism of corrosion under the coupling effect of dew point corrosion and ash deposition in power
59
plant flue gas atmosphere. The composition of ash was analyzed by X-ray diffraction (XRD) which has certain
60
reference value. But the paper focused on the corrosion phenomenon when inlet water temperature was 30
61
does not belong to the temperature range of waste heat recovery working. At 30
62
the water dew point, and mass water vapor condensation and the corrosion process is significantly different from the
63
corrosion happening near the dew point. Therefore the conclusion in this paper does not have the universal significance.
64
Ming-lei Xu[18] focused on studying high temperature corrosion of garbage incinerator by adopting the method of phase
65
analysis and chromatographic analysis. Fe´lix Barreras[19] analyzed low temperature corrosion in the rotary
66
continuous-regenerative air heaters of heavy fuel oil boiler.
AC C
EP
TE D
M AN U
SC
RI PT
45
3
which
the tube wall temperature was below
ACCEPTED MANUSCRIPT
The low temperature corrosion is affected by many factors, such as the coal composition, combustion and heat
68
transfer in the furnace, air leakage of heat exchangers, boiler load and so on. Therefore corrosion mechanism research is
69
complicated because it combines with the principles of boiler, heat and mass transfer, fluid mechanics, materials science
70
etc. There has been no satisfactory explanation which greatly restricted the utilization efficiency of heat recovery. This
71
article is aimed at the mechanism research of low temperature corrosion occurred in the temperature range, hence, the
72
conclusion has more practical value for system design and optimization.
73
1. Equipment and methods
74
1.1 Experiment system
M AN U
SC
RI PT
67
In order to investigate the mechanism of low temperature corrosion, this experiment was done on 300 MW
76
supercritical coal-fired boiler units. The experiment point in the flue gas channel was between the air preheater and the
77
electrostatic precipitator, as shown in Fig. 1. Main operating parameter of the boiler is shown in table 1 and industrial
78
analysis data of coal, ash composition are shown in table 2.
79
AC C
EP
TE D
75
80
Fig. 1 Schematic structure of the experiment
81
Table 1. Main operating parameter of the boiler Items
Quantity
units
Evaporating Capacity
954.6
t/h
Evaporating Pressure
20.75
MPa
Steam Temperature
565.5
4
ACCEPTED MANUSCRIPT Flue gas Temperature
131.1
SO2 Content before Desulfurization
1840
mg/m3
Conversion Rate of SO2 to SO3
2
%
SO3 Concentration (corresponding to A cross section)
12.45
ppm
Water Vapor Concentration (corresponding to A cross section)
8.3
%
82
Total Moisture
12.74
Air-dried Moisture
6.07
Air-dried Ash
17.73
Air-dried Volatile Content
27.28
Fixed Carbon Content
48.69
Higher Heating Value
24856
Net Calorific Value
22138
Total Sulfur
0.795
RI PT
Quantity
units %
% % %
SC
Items
M AN U
83
Table 2. Industrial analysis data of coal and ash composition
1.2 Experiment methods
% KJ/Kg KJ/Kg %
The experimental system is shown in Fig 1.The experiment tube was installed in the flue gas channel between the
85
air preheater and the electrostatic precipitator. The wall temperature of experiment tube (TW) could be changed by adjust
86
the cooling water temperature. During the experiment，the first step was to adjust the temperature controller to stabilize
87
the cooling water temperature. Then the valve on the water supply pipe was open to supply water for experiment tube.
88
Flow increased to experiment condition by adjusting the valve and kept the state for 20 ~ 30 minutes to make a stable
89
flow field. The relevant data can be collected when the import and export water temperature was stable. Then kept
90
cooling water system run stable for 12 hours and collected ash samples outer of the experiment tube. Thus an
91
experiment condition was completed. After that, it’s needed to adjust the cooling water temperature and repeat the
92
above experiment steps to obtain the ash samples of different wall temperature.
AC C
EP
TE D
84
93
The ash formation, ash color, ash thickness and ash composition changed with temperature reduction. The ash
94
outside the heating surface under different wall temperature was collected in sealed bag for composition analysis.
95
Measured parameter, measuring device and measuring accuracy are shown in table 3.
96
Table 3 Measured parameter and measuring device
5
ACCEPTED MANUSCRIPT Measuring Device Thermocouple thermometer
Measuring Accuracy
Cooling Water Temperature
Thermocouple thermometer
Cooling Water Flow
Electromagnetic flow meter
0.01L/min ±2.5%
0.1 0.1
Ash Sample
TH-880F dust sampling instrument
SO2 Concentration
KM9106 flue gas analyzer
±5%
Quality of Ash Sample
BSM-220.3electronic balance
0.001g
Microscopic Morphology of Ash Sample
SU-70 thermal field emission scanning electron microscopy
Chemical Composition of Ash Sample
Japan's D/Max-B X-ray diffract meter
— —
1.3 SEM and XRD experiment
RI PT
97
Measured Parameter Flue Gas Temperature
After the completion of the field test experiment the microscopic morphology and energy spectrum of ash samples
99
were obtained by using thermal field emission scanning electron microscopy SU-70(SEM). In order to further
100
determine the ash chemical composition, the Japan's D/Max-B X-ray diffract meter was used to test the ash
101
after grinding and sieving ash by 200 mesh sieve, and Jade software was used to analysis the XRD results.
102
2. Experimental results and analysis
M AN U
Significant change has been found in ash deposition outer wall of tube in the temperature range from 60
104
and from 40
105
and 41
to 50
. The SEM diagrams of ash magnified 200times under the wall temperature of 71
are shown in Fig. 2.
TE D
103
SC
98
to 70 ,65
,47
Fig.2a shows that ash sample is composed of large quantities of single particles while Fig.2b and c contain some
107
large diameter particles. Contrasted to the Fig.2a, there are many large diameter particles in Fig.2d. It can be analyzed
108
that many single particles gather together under the impact of physical chemistry in the course of the temperature
109
reduction.
AC C
EP
106
6
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
110 111
TE D
112
Fig. 2 Ash magnified 200times SEM
The SEM and energy spectrum diagrams of the ash particles under the wall temperature of 71
are shown in Fig.
3. It can be seen from the diagrams that most of the particles are independent and the diameter is under 10µm. The
114
major elements are O, Si, Al, Ca and K according to the decrease of the mass fraction by further energy spectrum
115
analysis on the particle surface. The mass balance equation of these elements is shown in Equation (1) and mass fraction
116
is shown in table 4. It can be concluded that the main components of the ash are SiO2 and Al6Si2O13 and little S element
117
content according to the XRD analysis results in Fig. 4. The mass balance equation of major components is shown in
118
Equation (2) and mass fraction is shown in table 5. The above analysis shows that experiment tube has not occurred
119
serious sulfuric acid corrosion when the wall temperature is 71
AC C
EP
113
.
120
qo + qSi + q Al + qCa + qK = 1
1
121
QSiO2 + QAl6 Si2O13 = 1
2 7
RI PT
ACCEPTED MANUSCRIPT
SC
122 123
TE D
M AN U
Fig. 3 The ash particles SEM and energy spectrum of wall temperature 71
124 125
EP
Fig. 4 The ash XRD diagram of wall temperature 71
126 TW 71
65
AC C
Table 4 Major elements and mass fraction of the ash
Major elements and mass fraction (%)
O
Si
Al
Ca
K
-
-
52.1
44
1.95
1.26
0.68
-
-
O
Si
Al
Fe
Ca
K
S
61.64
18.33
16.21
1.55
1.2
0.91
0.16
O
Si
Al
Ca
S
Fe
K
57.16
10.97
10.54
8.51
6.81
1.69
0.76
Fe
O
Cl
S
Si
Al
Ca
37.54
30.92
7.11
5.98
5.06
2.78
0.8
47
41
127
Table 5 Major components and mass fraction of the ash TW
Main components and mass fraction (%) SiO2
Al6Si2O13
CaAl2Si2O8.4H2O
-
-
75.03
17.24
7.73
-
-
71
8
ACCEPTED MANUSCRIPT SiO2
KFe3(SO4)2(OH)6,
Al2SiO5,
Al6Si2O13
Ca2SiO4
29.28
28.58
20.05
11.55
10.54
CaO+Al2O3
FeSO4·7H2O
K2Si4O9,
KFe3(SO4)2(OH)6
Al6Si2O13,
27.21
25.9
25.71
11.12
10.06
FeSO4·7H2O
CaO+Al2O3,
CaSO4·2H2O
Al2SiO5
Fe2Al2Si5O18
37.84
26.63
20.31
13.35
1.87
65
47
128
RI PT
41
Fig.5 shows the SEM and energy spectrum diagrams of the ash particles under the wall temperature of 65
. As
shown in Fig.5a, the ash samples contain not only small independent particles but also some reunion larger particles.
130
Through energy spectrum analysis on the reunion larger particle surface, a conclusion can be gained as shown in Fig.5b
131
that the mass fraction of the major elements is as following: O>Si>Al>Fe>Ca>K>S. The mass balance equation of these
132
elements is shown in Equation (3) and mass fraction is shown in table 4. The Fe and S elements appear compared with
133
the composition of 71
134
components of the ash are SiO2 , KFe3(SO4)2(OH)6, Al2SiO5 and Al6Si2O13 according to the XRD analysis results in
135
Fig.6. The mass balance equation of major components is shown in Equation (4) and mass fraction is shown in table 5.
136
The mass fraction of KFe3(SO4)2(OH)6 is 28.58% which can indicate the experiment tube has occurred sulfuric acid
137
corrosion at the wall temperature of 65
M AN U
SC
129
TE D
while the content of Fe element is 1.55%, S is 0.16%. It can be determined that the main
.
qo + qSi + q Al + qFe + qCa + qK + qS = 1
139
QSiO2 + QKFe3 ( SO4 )
EP
138
(OH )6
+ QAl2 SiO5 + QAl6 Si2O13 = 1
AC C
2
140 9
3 4
ACCEPTED MANUSCRIPT
Fig. 5 The ash particles SEM and energy spectrum of wall temperature 65
SC
RI PT
141
M AN U
142 143
144
Fig. 6 The ash XRD diagram of wall temperature 65
The ash layer on outer wall of the experiment tube gradually thickened in the process of temperature reduction.
145
Fig.7 shows the SEM and energy spectrum diagrams of the ash particles under the wall temperature of 47
146
reunion particles are obviously more and bigger than before which can be found in Fig.7a. The major elements are O, Si,
147
Al, Ca, S, Fe and K according to the decrease of the mass fraction by energy spectrum analysis on reunion particle
148
surface material. The mass balance equation of these elements is shown in Equation (5) and mass fraction is shown in
149
table 4. The content of S element is 6.81%, Fe is 1.69% and the element types are the same as that in the case of 65
150
while the S element content increased obviously from 0.16% to 6.81%. The increasing S element shows sulfuric acid
151
vapor condensation quantity increases with the temperature reduction. According to the XRD analysis results in Fig.8,
152
the main components of the ash are FeSO4·7H2O CaO+Al2O3 K2Si4O9 KFe3(SO4)2(OH)6 and Al6Si2O13, among which
153
the mass fraction of FeSO4·7H2O is 25.9% and the KFe3(SO4)2(OH)6 is 11.12%. All that indicates the chemical
154
reactions of sulfuric acid solution with the alkaline substances of the ash and experiment tube base material continue.
155
The mass balance equation of major components is shown in Equation (6) and mass fraction is shown in table 5. It can
156
be concluded from the obviously increasing FeSO4·7H2O that the neutralization reaction of sulfuric acid with ash is
AC C
EP
TE D
. The
10
ACCEPTED MANUSCRIPT
close to saturation meanwhile the major chemical reactions are sulfuric acid solution with experiment tube base material.
158
The obvious pitting corrosion phenomenon appears on the pipe surface which shows the experiment tube has occurred
159
severe acid corrosion.
160
qo + qSi + q Al + qCa + qS + qFe + qK = 1
161
QFeSO4 ·7 H 2O + QCaO + Al2O3 + QK2 Si4O9 + QKFe3 ( SO4 )
RI PT
157
5
( OH )6
+ QAl6 Si2O13 = 1
6
M AN U
SC
2
Fig. 7 The ash particles SEM and energy spectrum of wall temperature 47
164 165
166 167
AC C
EP
163
TE D
162
Fig. 8 The ash XRD diagram of wall temperature 47
The ash particles SEM and energy spectrum diagrams under the wall temperature of 41
are shown in Fig.9 and
Fig.10.It can be seen that single particle number decreased significantly and a large number of particles gather together 11
ACCEPTED MANUSCRIPT
to big particles as shown in the Fig.9a. The energy spectrum analysis on reunion particle surface material indicates that
169
the major elements are Fe, O, Cl, S, Si, Al and Ca according to the decrease of the mass fraction. The mass balance
170
equation of these elements is shown in Equation (7) and mass fraction is shown in table 4. The Fe element content
171
reaches 37.54% and the S element reaches 5.98%. From the change of the element content and a surge in Fe elements
172
can be determined the experiment tube substrate suffered serious corrosion. Combined with XRD analysis results in
173
Fig.10, the main components of the ash are FeSO4·7H2O, CaO+Al2O3, CaSO4·2H2O and Al2SiO5 while the mass
174
fraction of FeSO4·7H2O is 37.84%. The mass balance equation of major components is shown in Equation (8) and mass
175
fraction is shown in table 5.
M AN U
SC
RI PT
168
qFe + qo + qCl + qS + qSi + q Al + qCa = 1
7
177
QFeSO4 ·7 H 2O + QCaO + Al2O3 + QCaSO4 ·2 H 2O , + QAl2 SiO5 = 1
8
178 179
AC C
EP
TE D
176
Fig. 9 The ash particles SEM and energy spectrum of wall temperature 41
12
RI PT
ACCEPTED MANUSCRIPT
181
Fig. 10 The ash XRD diagram of wall temperature 41
It can be seen that when the tube wall temperature is 65
the ash micro-morphology and composition outside the
M AN U
182
SC
180
183
heating surface are obviously different from that in 41
184
vapor with the temperature change are different. Flue gas is the mixture with various components. When the flue gas
185
temperature reaches the sulfuric acid saturation temperature under partial pressure, sulfuric acid vapor will be condense
186
and the temperature is defined as thermodynamics acid dew point. The thermodynamics acid dew point was obtained
187
based on the theory of gas-liquid equilibrium and calculation according to coal quality data without considering the
188
influence of ash. For this experiment the total pressure of the flue gas and concentration of water vapor, H2SO4, SO2 in
189
flue gas are shown in table 6. Different dew points of the flue gas are shown in table 7. It is observed that
190
thermodynamics acid dew point is 132.2
TE D
EP
AC C
191
because the characteristics of sulfuric acid vapor and water
and the water dew point is 42.3
.
Table 6 Main parameter of the flue gas
Items
Quantity
units
SO2 Concentration
1840
mg/m3
Water Vapor Concentration
8.3
%
H2SO4 Concentration
12.45
ppm
Flue Gas Pressure
0.1
MPa
Flue gas Temperature
131.1
192
Table 7. Different dew points of the flue gas Items
Quantity
thermodynamics acid dew point(td)
132.2
13
units
ACCEPTED MANUSCRIPT water dew point(tw)
42.3
The sulfuric acid vapor condenses gradually when the temperature reduces to the thermodynamics acid dew point.
194
Due to the strong water imbibition of sulfuric acid, even if not reach the water dew point, part of the water vapor also
195
condenses with sulfuric acid. The concentration of the sulfuric acid solution is a function of water vapor content,
196
sulfuric acid vapor content and temperature. Condensed sulfuric acid solution at this time have the effect of the binder
197
which make fly ash particles gather together to form larger particles under the impact of physical chemistry.
In this experiment, the severe corrosion occurred when the tube wall temperature reduced down to about 65
SC
198
RI PT
193
199
while no obvious corrosion phenomenon appeared over 70
200
instead of gradient process and was typical of sulfuric acid corrosion according to the corrosion product analysis. A
201
large amount of water vapor condenses and absorbs SO2 to form sulfurous acid solution once the tube wall temperature
202
reduced down to about 41
203
acid solution under oxidizing atmosphere. Generally, the concentration of water vapor in flue gas is 6% to 15%, SO2 is
204
0.01%~0.2% while sulfuric acid vapor is 5ppm to 20 ppm[20]. At this time the SO2 content is much higher than the SO3,
205
so the corrosion was caused by SO2 and water vapor.
M AN U
TE D
which below the water dew point. Sulfurous acid solution was oxidized to form sulfuric
EP
206
. It indicated that the corrosion was heightened mutation
It can be seen from the above analysis that the ash deposits first appeared and then the sulfuric acid corrosion occurred in the temperature range from 65
208
temperature range from 41
209
In this system design of deep cooling flue gas, the recommended wall temperature of heat exchanger is higher than 70
210
and left a certain margin for security reasons.
211 212
AC C
207
to 47
to 71
. Finally SO2 and water vapor. caused severe corrosion in the
on outer wall of the experiment tube in the process of the temperature reduction.
In this article the analysis of the phenomena and mechanism is for a specific experiment of coal and has the certain reference value for other boiler unit.
14
ACCEPTED MANUSCRIPT
213
214
3
Conclusions
This section summarizes the conclusions of the investigation. 3.1 The microscopic morphology and chemical composition of ash samples were obtained by using electron microscope
216
scanning(SEM), energy spectrum analysis combined with XRD analysis.
217
3.2 Analysis showed that the ash particles would gather together under the impact of physical chemistry in the course of
218
deep-cooling flue gas.
219
3.3 It has been found that the experiment tube occurred severe sulfuric acid corrosion in the wall temperature range
220
from 65
221
3.4 Through the analysis
222
to 47
223
Acknowledgements
224 225
This work is supported by the National Basic Research Program of China (973 Program) (2011CB710702) and the
229 230 231 232 233 234
SC
it can be found that the corrosion was caused by SO2 and water vapor in the range from 41
TE D
Science & Technology Development Projects of Shandong Province, China (2012GGE27012).
EP
References
[1] Y.Q.Bao et al. Design and practice of flue gas deep cooling device for energy saving and emission reduction: ICMREE[C]. Beijing: 2012,1495-1501.
AC C
228
. The wall temperature of heating surface for long-term safety work should be higher than this range.
during the experiment.
226 227
M AN U
to 71
RI PT
215
[2] Chaojun Wang et al. Application of a low pressure economizer for waste heat recovery from the exhaust flue gas in a 600 MW power plant[J]. Energy. 2012, 48(1): 196-202. [3] I Z Aronov. Fuel Saving by thorough Cooling of Flue Gases in Contact Economizers[J]. Chem.Pet.Eng. 1981, 17(11): 580-583. [4] R Saidur et al. Energy, exergy and economic analysis of industrial boilers[J]. Energy Policy. 2010, 38(5): 2188-2197. 15
ACCEPTED MANUSCRIPT
235
[5] C F You et al. Coal combustion and its pollution control in China[J]. Energy. 2010, 35(11): 4467-4472.
236
[6] Liyun Yan et al. Research on sulfur recovery from the byproducts of magnesia wet flue gas desulfurization[J]. Appl.
240 241 242
RI PT
239
[7] Moskovits P D. Low-temperature boiler corrosion and deposits—a literature review[J]. Ind. Eng. Chem. 1959, 51(10): 1305-1312.
[8] Aki Sebastian Ruhl et al. Corrosion behavior of various steels in a continuous flow of carbon dioxide containing
SC
238
Therm. Eng. 2014, 65(1-2): 487-494.
impurities[J]. Int. J. Greenh. Gas Con. 2012, 9: 85-90.
[9] Osakabe M. Latent heat recovery from oxygen-combustion flue gas[J]. Energy Conversion Engineering
M AN U
237
Conference and Exhibit2. 2000: 804-812.
244
[10] Alireza Bahadori. Estimation of combustion flue gas acid dew point during heat recovery and efficiency gain[J].
245
Appl. Therm. Eng. 2011, 31(8-9): 1457-1462.
246
[11] H Han et al. A numerical study of the deposition characteristics of sulfuric acid vapor on heat exchanger surfaces[J].
247
Chem. Eng. Sci. 2013, 101: 620-630.
248
[12] A Kranzmann et al. The challenge in understanding the corrosion mechanisms under oxyfuel combustion
249
conditions[J]. Int. J. GreenH. Gas Con. 2011, 5: S168-S178.
250
[13] Kwangkook Jeong et al. Theoretical prediction of sulfuric acid condensation rates in boiler flue gas[J]. I Int. J. Heat
251
Mass Transfer. 2012, 55(25-26): 8010-8019.
252
[14] Kwangkook Jeong et al. Analytical modeling of water condensation in condensing heat exchanger[J]. Int. J. Heat
253
Mass Transfer. 2010, 53(11-12): 2361-2368.
254
[15] Qin-Xin Zhao et al. Experimental Study on Sulfuric Acid Dewpoint Corrosion in Simulated Atmospheric
255
Conditions[J]. Power Engineering. 2012, 32(5): 420-424.
256
[16] H Tallermo et al. Corrosion of Air Preheater Tubes of Oil Shale CFB Boiler. Part II.Laboratory Investigation of
AC C
EP
TE D
243
16
ACCEPTED MANUSCRIPT
Temperature Impact[J]. Oil Shale. 2009, 26(1): 13-18.
258
[17] Yun-Gang Wang et al. Mechanism research on coupling effect between dew point corrosion and ash deposition[J].
259
Appl. Therm. Eng. 2013, 54(1): 102-110.
260
[18] Ming-Lei Xu et al. Mechanism Analysis of Ash Deposits Corrosion in Waste Incinerator[J]. Proceedings of the
261
CSEE. 2007, 27(23): 32-37.
262
[19] Fe´lix Barreras et al. Behavior of a high-capacity steam boiler using heavy fuel oil Part II: Cold-end corrosion[J].
263
Fuel Process.Technol. 2004, 86(2): 107-121.
264
[20]J.M.Blanco et al. Increase in the boiler’s performance in terms of the acid dew point temperature: Environmental
265
advantages of replacing fuels[J]. Appl. Therm. Eng. 2008, 28(7): 777-784.
266
Nomenclature
267
q = mass fraction of the elements (%)
268
Q = mass fraction of the components (%)
269
TW = wall temperature of the experiment tube( )
270
td = thermodynamics acid dew point( )
271
tW = water dew point( )
AC C
EP
TE D
M AN U
SC
RI PT
257
17
S1359-4311(15)00111-8
DOI:
10.1016/j.applthermaleng.2015.02.003
Reference:
ATE 6353
To appear in:
Applied Thermal Engineering
Received Date: 5 April 2014 Revised Date:
29 January 2015
Accepted Date: 3 February 2015
Please cite this article as: Z.-m. Li, F.-z. Sun, Y.-t. Shi, F. Li, L. Ma, Experimental Study and Mechanism Analysis on Low Temperature Corrosion of Coal Fired Boiler Heating Surface, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.02.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT 1
Experimental Study and Mechanism Analysis on Low Temperature Corrosion of Coal Fired
2
Boiler Heating Surface
3
Zhi-min LIa,b, Feng-zhong SUNa, Yue-tao SHIa , Fei LIa , Lei MAa
4
a
5
b
6
Corresponding author information:
7
Corresponding author name: Feng-zhong SUN
8
Affiliation: Shandong University
9
Postal Address: 17923 Jingshi Road ,Jinan 250061, China
School of Energy and Power Engineering, Shandong University, 17923 Jingshi road, Jinan 250061,China
M AN U
SC
RI PT
College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao
Tel/Fax: +86 0531 88395691
11
E-mail: [email protected]
12
Highlights
13
The phenomena and mechanism of the low temperature corrosion was analyzed.
14
The SEM and XRD methods were used to analysis ash samples.
15
The ash particles would gather together in the course of deep-cooling flue gas.
16
The experiment tube occurred severe sulfuric acid corrosion in the wall temperature range from 65 to 47 .
EP
41
to 71
and
AC C
17
TE D
10
18
ABSTRACT: The low temperature corrosion experiment was made to investigate the phenomena and mechanism in
19
the process of the flue gas deep-cooling. The experiment tube was installed in the flue gas channel between the air
20
preheater and the electrostatic precipitator in the power plant. The scanning electron microscope (SEM), energy
21
spectrum analysis and X-ray diffraction (XRD) experiment were used to the analysis of the ash samples obtained from
22
the outer wall of experiment tube under the different tube wall temperature. By contrasting energy spectrum figures and
23
XRD figures among different ash samples, it has been found that the sulfuric acid corrosion occurred on the surface of 1
ACCEPTED MANUSCRIPT
24
the experiment tube under the temperature ranging from 65
25
vapor under the temperature ranging from 41
26
range was proposed which low temperature heating surface would be capable of long-term operation for the experiment
27
unit. The analysis also showed that the ash particles would gather together under the impact of physical chemistry in the
28
course of deep-cooling flue gas for heat recovery and efficiency gain.
29
KEY WORDS: low temperature corrosion
30
corrosion;
31
Introduction
and the corrosion was caused by SO2 and water
X-ray diffraction; the sulfuric acid
SC
scanning electron microscope
RI PT
in the certain types of coal conditions. The wall temperature
M AN U
32
to 47
to 71
Exhaust losses counted for the largest share of about 70% to 80% of the total pulverized coal furnace heat loss[1,2]. [3,4]
Boiler efficiency will decline by 1% once the exhaust temperature rise 15
34
is an effective way to improve the efficiency of the boiler. The design exhaust temperature of power plant boiler is 120
35
to 140
36
temperature of wet desulfurization tower is about 80
37
Generally the inlet temperature is reduced by injecting water before the desulfurization or increasing the amount of
38
desulfurization seriflux. This method not only increases the cost also increases water consumption, aggravated chimney
39
corrosion, reducing economy and security performance. Our research group installs flue gas waste heat recovery system
40
before desulfurization tower to reduce the flue gas temperature and reach the purpose of recycling of waste heat.
41
Generally the inlet flue gas temperature is 120
42
this case the tube temperature is close to the dew point and facing the risk of corrosion.
TE D
33
while the actual operation exhaust temperature is 120 [5,6]
. Reducing flue gas temperature
in China. The best inlet flue gas working
. There is the temperature difference of 40
to 60
.
AC C
EP
to 85
to 150
to 20
to 140
in this system and the air was heated from 20
to 60
. In
43
Many domestic and foreign scholars have studied in the field of low temperature corrosion. Early in 1959,
44
Moskovits[7] found that the corrosion speed was affected by acid condensation rate. Through the experiment Aki
2
ACCEPTED MANUSCRIPT
Sebastian Ruhl[8] discovered the main influence factors of the low temperature corrosion were the temperature and the
46
humidity of flue gas. Osakabe[9] experimentally studied thermal-hydraulic behavior of heat exchanger and discussed the
47
dew point of the flue gas. From 2010 to 2012, Alireza Bahadori[10] built a model to predict the acid dew point of
48
combustion flue gas. H Han[11] used a numerical model to simulation the condensation phenomenon of the
49
non-condensable gas and to predict the rate of sulfuric acid and water vapor. A. Kranzmann Humidity[12] explained CO2,
50
O2, and SO2 contents were the important factors determining corrosion under the oxyfuel combustion conditions and
51
discussed the temperature affection. Based on mass and energy balances, Kwangkook Jeong[13,14] established the
52
governing equations to predict sulfuric acid condensation rates and developed the analytical model to predict
53
condensation rate of water in the flue gas. Qin-xin Zhao[15] analyzed the low temperature corrosion products which were
54
produced in simulated atmospheric conditions and drew the conclusion that the corrosion resistance of the six test
55
materials was as following: GR2> ND >316L > Corten> 20G > 20# steel. Yet in-depth analysis on the mechanism of
56
the low temperature corrosion has not carried out in this paper. In order to found the temperature impact on corrosion, H.
57
Tallermo[16] did experiments on the air-preheater tubes of the oil shale CFB boilers. Yun-Gang Wang[17] researched on
58
the formation mechanism of corrosion under the coupling effect of dew point corrosion and ash deposition in power
59
plant flue gas atmosphere. The composition of ash was analyzed by X-ray diffraction (XRD) which has certain
60
reference value. But the paper focused on the corrosion phenomenon when inlet water temperature was 30
61
does not belong to the temperature range of waste heat recovery working. At 30
62
the water dew point, and mass water vapor condensation and the corrosion process is significantly different from the
63
corrosion happening near the dew point. Therefore the conclusion in this paper does not have the universal significance.
64
Ming-lei Xu[18] focused on studying high temperature corrosion of garbage incinerator by adopting the method of phase
65
analysis and chromatographic analysis. Fe´lix Barreras[19] analyzed low temperature corrosion in the rotary
66
continuous-regenerative air heaters of heavy fuel oil boiler.
AC C
EP
TE D
M AN U
SC
RI PT
45
3
which
the tube wall temperature was below
ACCEPTED MANUSCRIPT
The low temperature corrosion is affected by many factors, such as the coal composition, combustion and heat
68
transfer in the furnace, air leakage of heat exchangers, boiler load and so on. Therefore corrosion mechanism research is
69
complicated because it combines with the principles of boiler, heat and mass transfer, fluid mechanics, materials science
70
etc. There has been no satisfactory explanation which greatly restricted the utilization efficiency of heat recovery. This
71
article is aimed at the mechanism research of low temperature corrosion occurred in the temperature range, hence, the
72
conclusion has more practical value for system design and optimization.
73
1. Equipment and methods
74
1.1 Experiment system
M AN U
SC
RI PT
67
In order to investigate the mechanism of low temperature corrosion, this experiment was done on 300 MW
76
supercritical coal-fired boiler units. The experiment point in the flue gas channel was between the air preheater and the
77
electrostatic precipitator, as shown in Fig. 1. Main operating parameter of the boiler is shown in table 1 and industrial
78
analysis data of coal, ash composition are shown in table 2.
79
AC C
EP
TE D
75
80
Fig. 1 Schematic structure of the experiment
81
Table 1. Main operating parameter of the boiler Items
Quantity
units
Evaporating Capacity
954.6
t/h
Evaporating Pressure
20.75
MPa
Steam Temperature
565.5
4
ACCEPTED MANUSCRIPT Flue gas Temperature
131.1
SO2 Content before Desulfurization
1840
mg/m3
Conversion Rate of SO2 to SO3
2
%
SO3 Concentration (corresponding to A cross section)
12.45
ppm
Water Vapor Concentration (corresponding to A cross section)
8.3
%
82
Total Moisture
12.74
Air-dried Moisture
6.07
Air-dried Ash
17.73
Air-dried Volatile Content
27.28
Fixed Carbon Content
48.69
Higher Heating Value
24856
Net Calorific Value
22138
Total Sulfur
0.795
RI PT
Quantity
units %
% % %
SC
Items
M AN U
83
Table 2. Industrial analysis data of coal and ash composition
1.2 Experiment methods
% KJ/Kg KJ/Kg %
The experimental system is shown in Fig 1.The experiment tube was installed in the flue gas channel between the
85
air preheater and the electrostatic precipitator. The wall temperature of experiment tube (TW) could be changed by adjust
86
the cooling water temperature. During the experiment，the first step was to adjust the temperature controller to stabilize
87
the cooling water temperature. Then the valve on the water supply pipe was open to supply water for experiment tube.
88
Flow increased to experiment condition by adjusting the valve and kept the state for 20 ~ 30 minutes to make a stable
89
flow field. The relevant data can be collected when the import and export water temperature was stable. Then kept
90
cooling water system run stable for 12 hours and collected ash samples outer of the experiment tube. Thus an
91
experiment condition was completed. After that, it’s needed to adjust the cooling water temperature and repeat the
92
above experiment steps to obtain the ash samples of different wall temperature.
AC C
EP
TE D
84
93
The ash formation, ash color, ash thickness and ash composition changed with temperature reduction. The ash
94
outside the heating surface under different wall temperature was collected in sealed bag for composition analysis.
95
Measured parameter, measuring device and measuring accuracy are shown in table 3.
96
Table 3 Measured parameter and measuring device
5
ACCEPTED MANUSCRIPT Measuring Device Thermocouple thermometer
Measuring Accuracy
Cooling Water Temperature
Thermocouple thermometer
Cooling Water Flow
Electromagnetic flow meter
0.01L/min ±2.5%
0.1 0.1
Ash Sample
TH-880F dust sampling instrument
SO2 Concentration
KM9106 flue gas analyzer
±5%
Quality of Ash Sample
BSM-220.3electronic balance
0.001g
Microscopic Morphology of Ash Sample
SU-70 thermal field emission scanning electron microscopy
Chemical Composition of Ash Sample
Japan's D/Max-B X-ray diffract meter
— —
1.3 SEM and XRD experiment
RI PT
97
Measured Parameter Flue Gas Temperature
After the completion of the field test experiment the microscopic morphology and energy spectrum of ash samples
99
were obtained by using thermal field emission scanning electron microscopy SU-70(SEM). In order to further
100
determine the ash chemical composition, the Japan's D/Max-B X-ray diffract meter was used to test the ash
101
after grinding and sieving ash by 200 mesh sieve, and Jade software was used to analysis the XRD results.
102
2. Experimental results and analysis
M AN U
Significant change has been found in ash deposition outer wall of tube in the temperature range from 60
104
and from 40
105
and 41
to 50
. The SEM diagrams of ash magnified 200times under the wall temperature of 71
are shown in Fig. 2.
TE D
103
SC
98
to 70 ,65
,47
Fig.2a shows that ash sample is composed of large quantities of single particles while Fig.2b and c contain some
107
large diameter particles. Contrasted to the Fig.2a, there are many large diameter particles in Fig.2d. It can be analyzed
108
that many single particles gather together under the impact of physical chemistry in the course of the temperature
109
reduction.
AC C
EP
106
6
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
110 111
TE D
112
Fig. 2 Ash magnified 200times SEM
The SEM and energy spectrum diagrams of the ash particles under the wall temperature of 71
are shown in Fig.
3. It can be seen from the diagrams that most of the particles are independent and the diameter is under 10µm. The
114
major elements are O, Si, Al, Ca and K according to the decrease of the mass fraction by further energy spectrum
115
analysis on the particle surface. The mass balance equation of these elements is shown in Equation (1) and mass fraction
116
is shown in table 4. It can be concluded that the main components of the ash are SiO2 and Al6Si2O13 and little S element
117
content according to the XRD analysis results in Fig. 4. The mass balance equation of major components is shown in
118
Equation (2) and mass fraction is shown in table 5. The above analysis shows that experiment tube has not occurred
119
serious sulfuric acid corrosion when the wall temperature is 71
AC C
EP
113
.
120
qo + qSi + q Al + qCa + qK = 1
1
121
QSiO2 + QAl6 Si2O13 = 1
2 7
RI PT
ACCEPTED MANUSCRIPT
SC
122 123
TE D
M AN U
Fig. 3 The ash particles SEM and energy spectrum of wall temperature 71
124 125
EP
Fig. 4 The ash XRD diagram of wall temperature 71
126 TW 71
65
AC C
Table 4 Major elements and mass fraction of the ash
Major elements and mass fraction (%)
O
Si
Al
Ca
K
-
-
52.1
44
1.95
1.26
0.68
-
-
O
Si
Al
Fe
Ca
K
S
61.64
18.33
16.21
1.55
1.2
0.91
0.16
O
Si
Al
Ca
S
Fe
K
57.16
10.97
10.54
8.51
6.81
1.69
0.76
Fe
O
Cl
S
Si
Al
Ca
37.54
30.92
7.11
5.98
5.06
2.78
0.8
47
41
127
Table 5 Major components and mass fraction of the ash TW
Main components and mass fraction (%) SiO2
Al6Si2O13
CaAl2Si2O8.4H2O
-
-
75.03
17.24
7.73
-
-
71
8
ACCEPTED MANUSCRIPT SiO2
KFe3(SO4)2(OH)6,
Al2SiO5,
Al6Si2O13
Ca2SiO4
29.28
28.58
20.05
11.55
10.54
CaO+Al2O3
FeSO4·7H2O
K2Si4O9,
KFe3(SO4)2(OH)6
Al6Si2O13,
27.21
25.9
25.71
11.12
10.06
FeSO4·7H2O
CaO+Al2O3,
CaSO4·2H2O
Al2SiO5
Fe2Al2Si5O18
37.84
26.63
20.31
13.35
1.87
65
47
128
RI PT
41
Fig.5 shows the SEM and energy spectrum diagrams of the ash particles under the wall temperature of 65
. As
shown in Fig.5a, the ash samples contain not only small independent particles but also some reunion larger particles.
130
Through energy spectrum analysis on the reunion larger particle surface, a conclusion can be gained as shown in Fig.5b
131
that the mass fraction of the major elements is as following: O>Si>Al>Fe>Ca>K>S. The mass balance equation of these
132
elements is shown in Equation (3) and mass fraction is shown in table 4. The Fe and S elements appear compared with
133
the composition of 71
134
components of the ash are SiO2 , KFe3(SO4)2(OH)6, Al2SiO5 and Al6Si2O13 according to the XRD analysis results in
135
Fig.6. The mass balance equation of major components is shown in Equation (4) and mass fraction is shown in table 5.
136
The mass fraction of KFe3(SO4)2(OH)6 is 28.58% which can indicate the experiment tube has occurred sulfuric acid
137
corrosion at the wall temperature of 65
M AN U
SC
129
TE D
while the content of Fe element is 1.55%, S is 0.16%. It can be determined that the main
.
qo + qSi + q Al + qFe + qCa + qK + qS = 1
139
QSiO2 + QKFe3 ( SO4 )
EP
138
(OH )6
+ QAl2 SiO5 + QAl6 Si2O13 = 1
AC C
2
140 9
3 4
ACCEPTED MANUSCRIPT
Fig. 5 The ash particles SEM and energy spectrum of wall temperature 65
SC
RI PT
141
M AN U
142 143
144
Fig. 6 The ash XRD diagram of wall temperature 65
The ash layer on outer wall of the experiment tube gradually thickened in the process of temperature reduction.
145
Fig.7 shows the SEM and energy spectrum diagrams of the ash particles under the wall temperature of 47
146
reunion particles are obviously more and bigger than before which can be found in Fig.7a. The major elements are O, Si,
147
Al, Ca, S, Fe and K according to the decrease of the mass fraction by energy spectrum analysis on reunion particle
148
surface material. The mass balance equation of these elements is shown in Equation (5) and mass fraction is shown in
149
table 4. The content of S element is 6.81%, Fe is 1.69% and the element types are the same as that in the case of 65
150
while the S element content increased obviously from 0.16% to 6.81%. The increasing S element shows sulfuric acid
151
vapor condensation quantity increases with the temperature reduction. According to the XRD analysis results in Fig.8,
152
the main components of the ash are FeSO4·7H2O CaO+Al2O3 K2Si4O9 KFe3(SO4)2(OH)6 and Al6Si2O13, among which
153
the mass fraction of FeSO4·7H2O is 25.9% and the KFe3(SO4)2(OH)6 is 11.12%. All that indicates the chemical
154
reactions of sulfuric acid solution with the alkaline substances of the ash and experiment tube base material continue.
155
The mass balance equation of major components is shown in Equation (6) and mass fraction is shown in table 5. It can
156
be concluded from the obviously increasing FeSO4·7H2O that the neutralization reaction of sulfuric acid with ash is
AC C
EP
TE D
. The
10
ACCEPTED MANUSCRIPT
close to saturation meanwhile the major chemical reactions are sulfuric acid solution with experiment tube base material.
158
The obvious pitting corrosion phenomenon appears on the pipe surface which shows the experiment tube has occurred
159
severe acid corrosion.
160
qo + qSi + q Al + qCa + qS + qFe + qK = 1
161
QFeSO4 ·7 H 2O + QCaO + Al2O3 + QK2 Si4O9 + QKFe3 ( SO4 )
RI PT
157
5
( OH )6
+ QAl6 Si2O13 = 1
6
M AN U
SC
2
Fig. 7 The ash particles SEM and energy spectrum of wall temperature 47
164 165
166 167
AC C
EP
163
TE D
162
Fig. 8 The ash XRD diagram of wall temperature 47
The ash particles SEM and energy spectrum diagrams under the wall temperature of 41
are shown in Fig.9 and
Fig.10.It can be seen that single particle number decreased significantly and a large number of particles gather together 11
ACCEPTED MANUSCRIPT
to big particles as shown in the Fig.9a. The energy spectrum analysis on reunion particle surface material indicates that
169
the major elements are Fe, O, Cl, S, Si, Al and Ca according to the decrease of the mass fraction. The mass balance
170
equation of these elements is shown in Equation (7) and mass fraction is shown in table 4. The Fe element content
171
reaches 37.54% and the S element reaches 5.98%. From the change of the element content and a surge in Fe elements
172
can be determined the experiment tube substrate suffered serious corrosion. Combined with XRD analysis results in
173
Fig.10, the main components of the ash are FeSO4·7H2O, CaO+Al2O3, CaSO4·2H2O and Al2SiO5 while the mass
174
fraction of FeSO4·7H2O is 37.84%. The mass balance equation of major components is shown in Equation (8) and mass
175
fraction is shown in table 5.
M AN U
SC
RI PT
168
qFe + qo + qCl + qS + qSi + q Al + qCa = 1
7
177
QFeSO4 ·7 H 2O + QCaO + Al2O3 + QCaSO4 ·2 H 2O , + QAl2 SiO5 = 1
8
178 179
AC C
EP
TE D
176
Fig. 9 The ash particles SEM and energy spectrum of wall temperature 41
12
RI PT
ACCEPTED MANUSCRIPT
181
Fig. 10 The ash XRD diagram of wall temperature 41
It can be seen that when the tube wall temperature is 65
the ash micro-morphology and composition outside the
M AN U
182
SC
180
183
heating surface are obviously different from that in 41
184
vapor with the temperature change are different. Flue gas is the mixture with various components. When the flue gas
185
temperature reaches the sulfuric acid saturation temperature under partial pressure, sulfuric acid vapor will be condense
186
and the temperature is defined as thermodynamics acid dew point. The thermodynamics acid dew point was obtained
187
based on the theory of gas-liquid equilibrium and calculation according to coal quality data without considering the
188
influence of ash. For this experiment the total pressure of the flue gas and concentration of water vapor, H2SO4, SO2 in
189
flue gas are shown in table 6. Different dew points of the flue gas are shown in table 7. It is observed that
190
thermodynamics acid dew point is 132.2
TE D
EP
AC C
191
because the characteristics of sulfuric acid vapor and water
and the water dew point is 42.3
.
Table 6 Main parameter of the flue gas
Items
Quantity
units
SO2 Concentration
1840
mg/m3
Water Vapor Concentration
8.3
%
H2SO4 Concentration
12.45
ppm
Flue Gas Pressure
0.1
MPa
Flue gas Temperature
131.1
192
Table 7. Different dew points of the flue gas Items
Quantity
thermodynamics acid dew point(td)
132.2
13
units
ACCEPTED MANUSCRIPT water dew point(tw)
42.3
The sulfuric acid vapor condenses gradually when the temperature reduces to the thermodynamics acid dew point.
194
Due to the strong water imbibition of sulfuric acid, even if not reach the water dew point, part of the water vapor also
195
condenses with sulfuric acid. The concentration of the sulfuric acid solution is a function of water vapor content,
196
sulfuric acid vapor content and temperature. Condensed sulfuric acid solution at this time have the effect of the binder
197
which make fly ash particles gather together to form larger particles under the impact of physical chemistry.
In this experiment, the severe corrosion occurred when the tube wall temperature reduced down to about 65
SC
198
RI PT
193
199
while no obvious corrosion phenomenon appeared over 70
200
instead of gradient process and was typical of sulfuric acid corrosion according to the corrosion product analysis. A
201
large amount of water vapor condenses and absorbs SO2 to form sulfurous acid solution once the tube wall temperature
202
reduced down to about 41
203
acid solution under oxidizing atmosphere. Generally, the concentration of water vapor in flue gas is 6% to 15%, SO2 is
204
0.01%~0.2% while sulfuric acid vapor is 5ppm to 20 ppm[20]. At this time the SO2 content is much higher than the SO3,
205
so the corrosion was caused by SO2 and water vapor.
M AN U
TE D
which below the water dew point. Sulfurous acid solution was oxidized to form sulfuric
EP
206
. It indicated that the corrosion was heightened mutation
It can be seen from the above analysis that the ash deposits first appeared and then the sulfuric acid corrosion occurred in the temperature range from 65
208
temperature range from 41
209
In this system design of deep cooling flue gas, the recommended wall temperature of heat exchanger is higher than 70
210
and left a certain margin for security reasons.
211 212
AC C
207
to 47
to 71
. Finally SO2 and water vapor. caused severe corrosion in the
on outer wall of the experiment tube in the process of the temperature reduction.
In this article the analysis of the phenomena and mechanism is for a specific experiment of coal and has the certain reference value for other boiler unit.
14
ACCEPTED MANUSCRIPT
213
214
3
Conclusions
This section summarizes the conclusions of the investigation. 3.1 The microscopic morphology and chemical composition of ash samples were obtained by using electron microscope
216
scanning(SEM), energy spectrum analysis combined with XRD analysis.
217
3.2 Analysis showed that the ash particles would gather together under the impact of physical chemistry in the course of
218
deep-cooling flue gas.
219
3.3 It has been found that the experiment tube occurred severe sulfuric acid corrosion in the wall temperature range
220
from 65
221
3.4 Through the analysis
222
to 47
223
Acknowledgements
224 225
This work is supported by the National Basic Research Program of China (973 Program) (2011CB710702) and the
229 230 231 232 233 234
SC
it can be found that the corrosion was caused by SO2 and water vapor in the range from 41
TE D
Science & Technology Development Projects of Shandong Province, China (2012GGE27012).
EP
References
[1] Y.Q.Bao et al. Design and practice of flue gas deep cooling device for energy saving and emission reduction: ICMREE[C]. Beijing: 2012,1495-1501.
AC C
228
. The wall temperature of heating surface for long-term safety work should be higher than this range.
during the experiment.
226 227
M AN U
to 71
RI PT
215
[2] Chaojun Wang et al. Application of a low pressure economizer for waste heat recovery from the exhaust flue gas in a 600 MW power plant[J]. Energy. 2012, 48(1): 196-202. [3] I Z Aronov. Fuel Saving by thorough Cooling of Flue Gases in Contact Economizers[J]. Chem.Pet.Eng. 1981, 17(11): 580-583. [4] R Saidur et al. Energy, exergy and economic analysis of industrial boilers[J]. Energy Policy. 2010, 38(5): 2188-2197. 15
ACCEPTED MANUSCRIPT
235
[5] C F You et al. Coal combustion and its pollution control in China[J]. Energy. 2010, 35(11): 4467-4472.
236
[6] Liyun Yan et al. Research on sulfur recovery from the byproducts of magnesia wet flue gas desulfurization[J]. Appl.
240 241 242
RI PT
239
[7] Moskovits P D. Low-temperature boiler corrosion and deposits—a literature review[J]. Ind. Eng. Chem. 1959, 51(10): 1305-1312.
[8] Aki Sebastian Ruhl et al. Corrosion behavior of various steels in a continuous flow of carbon dioxide containing
SC
238
Therm. Eng. 2014, 65(1-2): 487-494.
impurities[J]. Int. J. Greenh. Gas Con. 2012, 9: 85-90.
[9] Osakabe M. Latent heat recovery from oxygen-combustion flue gas[J]. Energy Conversion Engineering
M AN U
237
Conference and Exhibit2. 2000: 804-812.
244
[10] Alireza Bahadori. Estimation of combustion flue gas acid dew point during heat recovery and efficiency gain[J].
245
Appl. Therm. Eng. 2011, 31(8-9): 1457-1462.
246
[11] H Han et al. A numerical study of the deposition characteristics of sulfuric acid vapor on heat exchanger surfaces[J].
247
Chem. Eng. Sci. 2013, 101: 620-630.
248
[12] A Kranzmann et al. The challenge in understanding the corrosion mechanisms under oxyfuel combustion
249
conditions[J]. Int. J. GreenH. Gas Con. 2011, 5: S168-S178.
250
[13] Kwangkook Jeong et al. Theoretical prediction of sulfuric acid condensation rates in boiler flue gas[J]. I Int. J. Heat
251
Mass Transfer. 2012, 55(25-26): 8010-8019.
252
[14] Kwangkook Jeong et al. Analytical modeling of water condensation in condensing heat exchanger[J]. Int. J. Heat
253
Mass Transfer. 2010, 53(11-12): 2361-2368.
254
[15] Qin-Xin Zhao et al. Experimental Study on Sulfuric Acid Dewpoint Corrosion in Simulated Atmospheric
255
Conditions[J]. Power Engineering. 2012, 32(5): 420-424.
256
[16] H Tallermo et al. Corrosion of Air Preheater Tubes of Oil Shale CFB Boiler. Part II.Laboratory Investigation of
AC C
EP
TE D
243
16
ACCEPTED MANUSCRIPT
Temperature Impact[J]. Oil Shale. 2009, 26(1): 13-18.
258
[17] Yun-Gang Wang et al. Mechanism research on coupling effect between dew point corrosion and ash deposition[J].
259
Appl. Therm. Eng. 2013, 54(1): 102-110.
260
[18] Ming-Lei Xu et al. Mechanism Analysis of Ash Deposits Corrosion in Waste Incinerator[J]. Proceedings of the
261
CSEE. 2007, 27(23): 32-37.
262
[19] Fe´lix Barreras et al. Behavior of a high-capacity steam boiler using heavy fuel oil Part II: Cold-end corrosion[J].
263
Fuel Process.Technol. 2004, 86(2): 107-121.
264
[20]J.M.Blanco et al. Increase in the boiler’s performance in terms of the acid dew point temperature: Environmental
265
advantages of replacing fuels[J]. Appl. Therm. Eng. 2008, 28(7): 777-784.
266
Nomenclature
267
q = mass fraction of the elements (%)
268
Q = mass fraction of the components (%)
269
TW = wall temperature of the experiment tube( )
270
td = thermodynamics acid dew point( )
271
tW = water dew point( )
AC C
EP
TE D
M AN U
SC
RI PT
257
17