Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Page 33 of 100

tion of the solvent concentration," Srinivasan notes. "Depending on the sol- vent, you will see different corrosion behaviors." The First Campaign The first monitoring campaign ran for ~149 days and comprised ~2,200 operat- ing hours. The campaign was divided into four periods. The same solvent was used for the first 128 days, then completely replaced for the remainder of the cam- paign. The flue gas contained 13 vol% CO 2 and 7 vol% O 2 for the first, third, and fourth operating periods, and 4 vol% CO 2 and 17 vol% O 2 during the second operat- ing period. Online, real-time corrosion monitoring was done during the third and fourth periods. Two days after start-up during the third period (Day 110), the corrosion rate peaked at ~800 µm/y. On Day 115, pure MEA was added to the unit as solvent make-up, and the corrosion rate value decreased. With continued operation, however, the corrosion rate increased to a peak of ~1,400 µm/y after several weeks. On Day 128 the unit was cleaned and the solvent completely replaced, and the cor- rosion rate decreased to <20 µm/y with only periodic low-level peaks during the rest of the campaign. The pitting factor values were also tracked during the campaign and varied between 0.2 and 0.4, which is within the range for pitting (0.1 to 1.0). The highest pitting factor values were recorded during periods when the general corrosion rate measurements were low; and periods with high corrosion rates showed very low pit- ting factor values. According to the pre- senters, these measurements suggest that the corrosion was likely general in nature and not related to pitting attack of the process unit. They also note that the peak corrosion rate measured would be consid- ered high for SS, but the time-averaged corrosion rate values indicated a maxi- mum metal thickness loss rate of 600 µm/y. While this corrosion rate would have produced a metal thickness loss of only 0.03 mm during the 20 days of the campaign when corrosion rates were high, the presenters point out that this corro- sion rate would have resulted in a thick- ness loss of 0.06 mm/y had the situation continued for a year, which is considered excessive for SS in amine service. The corrosion coupons placed in the hot lean solvent stream were lost in the system during this campaign, and corro- sion data from the online monitoring pro- cess could not be validated. The Second Campaign The second monitoring campaign ran for ~140 days for a total of ~1,700 operat- ing hours, and was divided into three peri- ods. The CO 2 capture plant operated intermittently during the first period and continuously during the second period; and then the solvent was completely replaced and the plant ran continuously during the third period. The flue gas dur- ing all three periods usually contained 13 vol% CO 2 and 7 vol% O 2 . Corrosion monitoring was performed over the entire duration of the campaign. The corrosion rate measurements var- ied during the intermittent operation, with peak corrosion rates ranging from 10 to 25 µm/y. During continuous operation, the corrosion rates typically ranged between 2 to 5 µm/y, but peaked at 30 µm/y prior to the solvent replacement on Day 65 and then remained stable at ~5 µm/y. Based on the corrosion coupon inserted during this campaign, the calcu- lated corrosion rate was 0.3 µm/y, which was comparably low to the much more sensitive online monitoring results. According to the presenters, the consis- tently low corrosion rate values for both the online and coupon readings are con- sidered acceptable for SS in a passive con- dition in amine service. Generally, the pitting factor values did not fall within the range of values that indicate pitting, which suggests very low rates of general corro- sion although pitting factor readings did reach ~1.0 in transient periods during intermittent operation. The presenters report that a major cor- rosion effect observed in both campaigns was an increase in the general corrosion rate over time during continuous opera- tion. The first campaign, however, showed higher corrosion rates than the second. After a supplemental analysis of the amine solvent for metal ions, they found that the metal ion concentration for the first moni- toring campaign was higher than it was for the second. The flue gas in the first cam- paign also had higher oxygen content. They noted these conditions are known to promote degradation of the amine solvent and formation of corrosive reaction prod- ucts. While in situ corrosion most likely contributed to the metal ions being pres- ent in the amine solvent, the presence of metal ions promoted oxidative degrada- tion of the solvent, which resulted in con- ditions that further degraded the solvent and led to increased corrosion in the unit. To reduce the corrosion rate, the solvent needed to be completely replaced. Srinivasan notes that the study demon- strated the efficacy and accuracy of the real-time, online monitoring system and its value in immediately identifying corro- sive operating modes during plant opera- tion, which will enable plant operators to take remedial actions to restore accept- able corrosion conditions on a real-time basis so corrosion damage can be avoided. "Corrosion is hard to see, it happens over a period of time," he says. "If we're able to monitor it in a reasonable time frame— real time—we're going to be able to catch it, fix it, or prevent it." More information on the real-time online monitoring technology and the monitoring campaigns at the Maasvlakte coal-fired power plant can be found in CORROSION 2015 paper no. 5954, "Plant Applications of Online Corrosion Monitor- ing: CO 2 Capture Amine Plant Case Study," by R.D. Kane, S. Srinivasan, P. Khakharia, E. Goetheer, and J. Mertens. References 1 "Carbon Dioxide Capture and Sequestra- tion," U.S. EPA, climatechange/ccs (April 13, 2015). 2 "Draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013," National Greenhouse Gas Emissions Data, U.S. EPA, February 2015, http://www.epa. gov/climatechange/ghgemissions/ usinventoryreport.html (April 13, 2015). 3 "Power plant CO 2 capture technologies," International Risk GovernanceCouncil, and-storage/power-plant-co2-capture- technologies (April 13, 2015). 31 MATERIALS PERFORMANCE MAY 2015 NACE INTERNATIONAL: VOL. 54, NO. 5 Tracking Corrosion in Real Time in a Carbon Dioxide Capture Plant

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