Failure analysis on abnormal corrosion of economizer tubes in a waste heat boiler

    Failure analysis on abnormal corrosion of economizer tubes in a waste heat boiler Qun Ding, Xiao-Feng Tang, Zhen-Guo Yang PII: DOI: Reference:

S1350-6307(16)30382-X doi:10.1016/j.engfailanal.2016.12.011 EFA 3002

To appear in: Received date: Revised date: Accepted date:

25 May 2016 24 December 2016 28 December 2016

Please cite this article as: Ding Qun, Tang Xiao-Feng, Yang Zhen-Guo, Failure analysis on abnormal corrosion of economizer tubes in a waste heat boiler, (2016), doi:10.1016/j.engfailanal.2016.12.011

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ACCEPTED MANUSCRIPT Failure analysis on abnormal corrosion of economizer tubes in a waste heat boiler

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Qun Ding, Xiao-Feng Tang, Zhen-Guo Yang*

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Department of Materials Science, Fudan University, Shanghai 200433, PR China

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Abstract: Repeated shell side corrosion occurred on economizer tubes of a waste heat boiler. In order to find out the root cause of the corrosion, a series of characterization methods were conducted. First of all, the tubes were proved to be qualified A106 Gr.A steel in chemical compositions and metallographic structures. Then, both macroscopic and microscopic characteristics of the failed tube were observed thoroughly. After that, phase compositions of the deposits

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collected from failed tubes were analyzed. Content of sulfur and nitrogen of deposits was determined precisely as well. The results revealed that repeated failures were primarily owing to sulfuric acid dew point corrosion and the sulfur mainly came from waste liquid of methyl methacrylate (MMA). Finally, mechanisms of the failure were discussed in

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detail and practical countermeasures were put forward.

Keywords: Waste heat boiler, Economizer, Dew point corrosion, Failure analysis

1. Introduction

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As the world has had a rude awakening from its abuse of exhaustible natural resources since the mid 20th century, implementation of waste heat boiler has played an increasingly significant role in industries. Nowadays, waste heat boilers are widely applied in chemical, petroleum, medicine and power industries to recover residual heat from flue gas and save companies millions of dollars. In a waste heat boiler, heat exchanger tubes are very critical

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components. Exposed to complicated operation conditions, such as high temperature, high pressure, corrosive flue gas, alkaline fly ash and so on, heat exchanger tubes of a waste heat boiler may often encounter severe failures. Common failure causes found in waste heat boilers can be classified into the following categories: material defects [1], high temperature corrosion [2, 3], stress corrosion cracking (SCC) [4, 5], dew point corrosion [6, 7], etc.

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In this paper, a famous German chemical company utilized a waste heat boiler to deal with the waste liquid of methyl methacrylate (MMA) from upstream factories, not only taking advantage of exhaust gas, but also ensuring that emissions were environmentally friendly. Appearance of the waste heat boiler is shown in Fig.1. It was put into service in 2009. Unfortunately, the boiler suffered successive leakage and had to be shut down for repair four times in next five years, resulting in huge economic losses. Fig.2 shows the industrial process of the waste-to-energy conversation. In detail, the liquid waste collected by S-1930, atomized and mixed with natural gas, is sprayed into the thermal oxidizer where the gas mixture burns with preheated air, producing high-temperature exhaust gas. Then the hot flue gas is neutralized by aqueous ammonia to get rid of hazardous nitrogen oxides and make it green. After that, the gas flows through the waste heat boiler, which is composed of evaporator X-1981, super-heater X-1984, evaporator X-1982, evaporator X-1983 and economizer X-1985, making feed water absorb latent enthalpy of the hot gas. Finally, the low-temperature flue gas can be discharged into atmosphere directly with minimal impacts on the environment and the high-pressure steam is transferred to downstream plants to be used.

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Corresponding author. Tel: +86-21-65642523; fax: +86-21-65103056.

E-mail address: [email protected] (Z.-G. Yang)

ACCEPTED MANUSCRIPT Leakage mainly took place in the economizer X-1985, constituted of horizontal shell-and-tube finned tubes and straight tubes. Technical parameters of the heat exchangers in economizer are listed in Table1. Feed water went on tube side and flue gas flowed on shell side. Temperature of the water inlet was 105ºC and it increased to 150ºC in outlet. Temperature of flue gas in shell side was 150ºC. In general, the economizer was operated in accordance with design

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requirements. However, repeated corrosion and leakage occurred on the economizer. The company considered nitric acid corrosion as the main cause which was questionable. Actions had been taken to prevent the failure but brought little

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effect. Therefore, comprehensive investigation and analysis of the failed economizer tubes were conducted and reported

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in this paper. Root cause of the corrosion was found and effective countermeasures were put forward as well.

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Fig.1. Appearance of the waste heat boiler.

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Fig.2. Industrial process of the waste-to-energy conversation. Table1 Technical parameters of the economizer.

Media

Operating

Operating

Design

Design

pressure

temperature

pressure

temperature

Tube side

Feed water

5.1MPa

105~150ºC

6.0MPa

260ºC

Shell side

Flue gas

1.0kPa

150ºC

1.0kPa

300ºC

2. Experiments and results 2.1 Visual inspection On-site inspection was carried out on heat exchanger tubes through the manhole of economizer header (Fig.3a). As seen in Fig.3b, the tubes suffered severe corrosion with large areas of deposits existed on outer surface and one finned tube had perforated. Samples of the perforated finned tube (Fig.4a) and corroded straight tubes (Fig.5a) were cut out for investigation. The original thickness and inner diameter of heat exchanger tubes were 4mm and 24mm, respectively. Residual thickness and inner diameter of the samples are given in Table2. Both the finned tube and the straight tube underwent considerable wall thinning and maximum thickness reduction of the latter incredibly reached 58%. In

ACCEPTED MANUSCRIPT contrast, inner diameter of the two samples had barely any change, suggesting that corrosion only took place on shell side. Both of the failed tubes were observed under a three-dimensional stereo microscope (3D SM). Fig.4 features morphology of the failed finned tube. It was corroded so heavily that welds between the tube and the fin got loose and

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the spiral fin could be taken down easily (Fig.4b). Meanwhile, the fin was covered with thick tan deposits. As shown in Fig.4c, screw threads on the finned tube got plain and lost their edges. Surface of the finned tube appeared dark

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indicating that it once experienced overheating. The rupture at the edge of the tube was irregular and step-like (Fig.4d). After measurement, wall thickness around the rupture was about 3mm. It was worth noting that two distinct sorts of

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morphology were discovered on the straight tube. As exhibited in Fig.5, on one side, there were hill-like convexes (Fig.5b) and on the other, continuous round concaves were formed in line (Fig.5c) which was strong evidence of

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localized corrosion.

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Fig.3. On-site inspection: (a) the manhole of economizer header and (b) leakage point on finned tubes.

Fig.4. Morphology of the failed finned tube: (a) sample of the perforated finned tube, (b) spiral fin, (c) screw threads and (d) rupture.

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Fig.5. Morphology of the failed straight tube: (a) sample of the corroded straight tube, (b) hill-like convexes and (c) continuous concaves in line.

Table2 Residual thickness and average inner diameter of the failed tubes. Residual

Original inner

Average inner

thickness/mm

thickness/mm

diameter/mm

diameter/mm

Finned tube Straight tube

4

2.74

24

23.44

4

1.68

24

23.72

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Original

Samples

2.2 Matrix material examination 2.2.1 Chemical compositions

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Chemical compositions of the failed tube and fin are listed in Table3 determined by optical emission spectrometer

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(OES). The tube was proved to be in accordance with the ASTM A106 Gr.A steel which was a kind of low-cost carbon steel for high-temperature service [8]. In addition, the failed tube contained excessive levels of vanadium and it helped to refine grains, enhanced the strength and toughness of steel [9]. The fin was confirmed to be the typical AISI 1008

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carbon steel that had excellent weldability and formability.

Elements

C

Tube Fin

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Table3 Chemical compositions of the tube and fin (wt. %).

AISI 1008

A106 Gr.A

Mn

P

S

Si

Cr

Cu

Mo

Ni

V

0.17

0.52

0.015

0.006

0.21

0.024

0.053

0.002

0.007

0.20

≤0.25

0.27~0.93

≤0.035

≤0.035

≥0.10

≤0.40

≤0.40

≤0.15

≤0.40

≤0.08

0.05

0.46

0.006

<0.005

-

-

-

-

-

-

≤0.10

0.30~0.50

≤0.040

≤0.050

-

-

-

-

-

-

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2.2.2 Metallographic structure

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After polished, specimen of the failed tube was etched with 4% nitric acid in ethanol. Fig.6 presents the metallographic structure of the tube. It had a typical carbon steel structure composed of continuous light ferrite and

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lamellar dark pearlite [10]. The size of grains was homogeneous and no visible inclusions were observed.

Fig.6 Metallographic structures of the failed tube.

2.3 Micro-zone analysis

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2.3.1 Micro-zone analysis of the finned tube

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In order to find out the root cause of the abnormal corrosion, scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) was employed to study the failed finned tube and straight tube thoroughly. Micro-morphology of the fin is illustrated in Fig.7. Fig.7a showed the edge of the fin with thick deposits on it. After

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partial magnification, long and narrow cracks were found on the surface of the fin surrounded by a lot of tubercles (Fig.7b). Further amplified, the tubercles turned out to be made up of spherical particles, seen in Fig.7c. EDS profiles showed that the spherical particles were mainly composed of iron oxides with a low content of sulfur (0.99% wt., Zone1, Table4). Under larger magnification, the structure of spinel was discovered around cracks, displayed in Fig.7d. EDS

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profiles showed that the spinel structure contained iron, oxygen and sulfur in high concentration (Fig.7e). It was such a surprise that the content of sulfur detected was as high as 16.21% wt. (Zone2, Table4). Hence, the spinel structure was supposed to be a mixture of oxides and sulfates.

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Fig.7. SEM images of the fin: (a) edge, (b) cracks, (c) spherical particles, (d) spinel structure around the crack and (e)

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EDS profiles of the spinel structure. Fig.8 exhibits the morphology of the finned tube. As shown in Fig.8a, surface of the finned tube was uneven with a

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lot of white granular particles spread on it. Moreover, some shallow pits were formed on the surface. Fig.8b reveals the morphology of a screw thread. It was full of bumps and hollows and covered with deposits where sulfur was also detected with the content of 1.02% wt. (Zone3, Table4). Unlike the morphology of finned tube seen before, the rupture of finned tube was sleek with few deposits (Fig.8c). Based on preliminary analysis, substances near the rupture were

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simply Fe2O3 with no sulfur which were common corrosion products of steel reacting with oxygen (Zone4, Table4).

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Fig.8. SEM images of the finned tube: (a) uneven surface with shallow pits, (b) screw thread (c) upside of the rupture

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and (d) EDS profiles of area near the rupture 2.3.2 Micro-zone analysis of the straight tube

Micro-morphology of the straight tube was similar to the finned tube since they experienced the same shell side

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environment, as shown in Fig.9. The surface was bumpy, distributed with white corrosion products (Fig.9a). When magnified, corrosion products appeared spinel structure as well, revealed in Fig9b. EDS analysis results showed that

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they were mainly iron oxides with a certain amount of sulfur (1.20% wt., Zone5, Table4).

Fig.9. SEM images of the straight tube: (a) corrosion products on the surface and (b) spinel structure.

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Table4 Chemical compositions of deposits on the failed tube and fin analyzed by EDS (wt. %).

O

S

Fe

Zone1 Zone2 Zone3 Zone4 Zone5

27.66 22.21 29.97 29.11 35.66

0.99 16.21 1.02 1.20

69.00 36.80 69.02 70.89 63.14

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2.4 Analysis of deposits 2.4.1 Determination of sulfur and nitrogen content

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Four samples of deposits collected from failed tubes and fins were tested by infrared carbon/sulfur analyzer and oxygen/nitrogen/hydrogen analyzer to precisely determine the content of sulfur and nitrogen respectively. Analysis results are listed in Table5. Sulfur was detected in the whole four samples, all above 0.1% and the highest one (sample3)

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reached 0.69%. Therefore, sulfur was validated to exist in deposits significantly. In contrast, the content of nitrogen in deposits was all below 0.1%. The trace amounts of nitrogen were derived from aqueous ammonia in the treatment of waste liquid. Compared the carbon/sulfur analyzer results with the EDS analysis results, it can be found that the former were a little smaller than the latter ones. That was because the carbon/sulfur analyzer results were average contents of a

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large area of mixtures while the EDS focused on micro-zones where corrosion products contained sulfur in higher

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concentration.

Table5 Sulfur and nitrogen content of deposits (wt. %). 1

2

3

4

Sulfur

0.319

0.295

0.690

0.158

Nitrogen

0.075

0.069

0.077

0.062

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Samples

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2.5.2 Phase compositions analysis Most of deposits were brown while a few of them were black. X-ray diffraction (XRD) was utilized to distinguish the components of deposits in two different colors. The XRD profiles, seen in Fig.10, shows that the brown deposits

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consisted of Fe2O3 with a small amount of Fe3O4 and the black ones were mainly Fe3O4. What’s more, the small peak at 28.15º caught our attention. It corresponded to Fe(OH)SO4·2H2O according to the standard powder diffraction file

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(PDF) card. It can be inferred that the hydrated alkaline ferric sulfate was the corrosion product of sulfuric acid reacting

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with matrix metals.

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Fig.10. Phase compositions of deposits in different colors: (a) brown and (b) black.

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3. Discussion

Based on the matrix material examination, the economizer heat exchanger tubes are qualified ASTM A106 Gr.A steel and should not be blamed for the abnormal corrosion. Considering that the dew point of nitric acid is usually

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below 60°C [11, 12], the operation temperature of economizer is not low enough for nitric acid to condense. In addition, little nitrogen was detected in deposits (Table5). Therefore, the effect of nitrous oxides in exhaust gas should be excluded. Given the fact that high sulfur content as well as the products of sulfuric acid corrosion (Fe(OH)SO 4·2H2O) was found in deposits, the failure of economic exchanger tubes should be ascribed to sulfuric acid dew point corrosion.

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Then, where did the sulfur come from and how was it introduced into the economizer? Attention was paid to the MMA waste liquid and co-fired natural gas. Samples of natural gas and waste liquid were sent to Societe Generate De Surveillance (SGS) for inspection of sulfur content. It turned out that the concentration of sulfur in natural gas fluctuated from 3 to 11 mg/m3 within the limit. Unfortunately, the sulfur content in waste liquid was 1062mg/kg. Assuring that the waste boiler worked 7688h per year and the average flow rate of waste liquid was 125kg/h, the total sulfur content of 1020kg was introduced to the boiler each year, posing a great threat to heat exchanger tubes. Mechanisms of the abnormal corrosion on economizer tubes can be summarized as follow. In the thermal oxidizer, sulfur of waste liquid oxidizes to form gaseous SO2 (eq.1). Then SO2 can be transformed into SO3 reversibly catalyzed by vanadium and other metal at 400°C which is the most appropriate temperature for both kinetics and thermodynamics (eq.2) [13]. After that, H2SO4 is produced by the combination of SO3 and H2O (eq.3). The sulfuric acid dew point is also known as the “cold end” corrosion [7]. On one hand, once the temperature of exhaust gas drops below the sulfuric acid dew point, gaseous H2SO4 will condense to form an acidic mist and be pushed onto shell side of tubes by flowing gas. On the other hand, even though the temperature of exhaust gas is high enough, condensation of H2SO4 can also happen on cold tube walls. Subsequently, the acidic droplets of high concentration can be diluted by moisture to form a thin liquid film on metallic surface which is even more corrosive. As a result, diluted H2SO4 reacts with iron and produces FeSO4 (eq.4) as corrosion products. The ferrous sulfates are porous and easy for H 2O and O2 to penetrate, making the eq.5 happen, producing Fe(OH)SO4·2H2O which has been detected in the XRD profiles of deposits [14].

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SO2 + 1/2O2 → SO3

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SO3 + H2O → H2SO4

(3)

H2SO4 + Fe → FeSO4 + H2

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S + O2 → SO2

4FeSO4 + O2+ 4H2O → 4Fe(OH)SO4·2H2O

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Thick deposits on tubes and fins will lower heat transfer efficiency and the temperature of tube wall, making it

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easier for gaseous H2SO4 to condense in return. When heated, corrosion products of sulfuric acid are apt to peel off owing to the different coefficient of thermal expansion (CTE) between them and matrix materials, leaving shallow pits on tubes. As the sulfuric acid dew point corrosion goes on, thickness of the tube wall reduce. When the strength of tubes is not strong enough to hold the inner water pressure, the tube will burst and a leakage occurs. Moreover, intermittent

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running of waste heat boiler due to shut down for maintenance also accelerates the corrosion of economizer tubes, because when the equipment stops operating, flue gas on the shell side cools down and gaseous acid will condense on shell side more rapidly.

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Then how does the finned tube in this case perforate? Is it due to dew point corrosion? The answer is negative. In case the perforation was caused by corrosion, wall thinning around the rupture would be obvious. However, wall thickness around the hole is 3mm with no significant reduction which means the shell side corrosion doesn’t play a leading role in the perforation. Furthermore, morphology around the rupture was smooth with few deposits (Fig.8c).

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Therefore it can be deduced that the perforation may be mainly produced by spray striking that comes from other leaked

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tubes. In addition, how is the peculiar appearance on straight tube formed (Fig.5)? As mentioned above, the exchanger tubes are arranged horizontal. Because of gravity, little droplets of acid tend to suspend on the downside of straight tubes and corrode metallic walls, leaving round concaves in line. While on the upside, thin liquid film covers walls and

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causes corrosion. Then corrosion products fall off unevenly and the left unreacted tube wall looks like convex. Many models and methods have been invented to estimate the dew point of flue gas [15-18]. A.G.Okkes proposed the correlation to predict sulfuric acid dew point based on several assumptions (eq.6) [16]. From the correlation, it can be concluded that the dew point increases when the partial pressure of SO3 and H2O gets higher. Meanwhile, the partial

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pressure of SO3 and H2O is mainly attributed to fuel composition, excess combustion air and gas temperature. Unfortunately, fuel composition and excess combustion air in this case are unknown. It is hardly possible to determine the sulfuric acid dew point accurately. Nevertheless, considering that concentration of sulfur in waste liquid is high, the water inlet temperature (105°C) of heat exchanger tubes is low enough to be under the sulfuric acid dew point of flue gas referring to reports in literature [19-20]. t=203.35+27.6logpH2O+10.83logpSO3+1.06(logpSO3+8)2.19 (°C)

(6)

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4. Conclusions and recommendations 1.

The failed tubes are qualified ASTM A106 Gr.A carbon steel in chemical compositions and metallographic structures. The sulfuric acid dew point corrosion as well as the low operating temperature is the root cause of failure on

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economizer exchanger tubes. In addition, effect of nitrogen oxides should be excluded.

It is the moisture in flue gas reacting with oxysulphide readily that makes the dew point corrosion possible, which

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is another important factor for the failure.

Based on the conclusions mentioned above, some practical recommendations are put forward to minimize the impact of sulfuric acid dew point corrosion on economizer. 1.

For the purpose of eliminating the possibility of sulfuric dew point corrosion, a desulfurizer is necessary to be

2.

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introduced to the waste heat boiler. Sulfur content in flue gas should be monitored and controlled as well. Raise the temperature of flue gas and tube wall moderately and maintain it high enough to avoid the dew point corrosion.

Taking control of the moisture in flue gas also help to retard the corrosion.

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When renovated, the heavily corroded economizer is recommended to use heat exchanger tubes made out of 304

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stainless steel with higher corrosion resistance.

The company accepted the suggestions. Fig.11a shows the lifting new economizer header made out of 304 stainless

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steel and the fixed new boiler is presented in Fig.11b. After the renovation, the economizer has been put into service for

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nearly 2 years until now with no severe corrosion or leakage.

Fig.11. Renovation: (a) the lifting new economizer header and (b) the fixed new boiler.

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[13] Brown CA, Hohne PA. Eliminating a sulfuric acid mist plume from a wet caustic scrubber on a petroleum coke calciner. Environmental Progress 2001; 20(3):182-186. [14] Gong Yi, Yang Zhen-Guo. Corrosion evaluation of one dry desulfurization equipment - Circulating fluidized bed boiler. Material & Design 2011; 32(2):671-681. [15] Verhoff FH, Banchero JT. Predicting dew points of gases. Chemical Engineering Progress 1974; 78(8):71-72. [16] OKKES AG. Get acid dew-point of flue-gas. Hydrocarbon Processing 1987; 66(7): 53-55. [17] ZareNezhad Bahman, Aminian Ali. A multi-layer feed forward neural network model for accurate prediction of flue gas sulfuric acid dew points in process industries. Applied Thermal Engineering 2010; 30(6-7):692-696. [18] Rosner Daniel E. Arias-Zugasti Manuel. Estimating transport-shifted acid dew-point surface temperatures and conditions for the avoidance of acid mists in energy recovery operations. Chemical Engineering Science 2012; 75: 243-249. [19] Bahadori Alireza. Estimation of combustion flue gas acid dew point during heat recovery and efficiency gain. Applied Thermal Engineering 2011; 31(8-9):1457-1462. [20] Moakhar Roozbeh Siavash, Mehdipour Mehrad, Ghorbani Mohammad. Investigations of the failure in boilers economizer tubes used in Power Plants. Journal of Materials Engineering and Performance 2013; 22(9): 2691-2697.

ACCEPTED MANUSCRIPT Highlights High content of sulfur was detected in deposits.

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Sulfuric acid dew point corrosion was proved to be the root cause.

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Mechanisms were proposed to explain the perforation and distinct morphology on tubes.

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It was necessary to raise the operation temperature and control sulfur content.

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