Jouni Mahanen, Kyösti Vänskä,

Edgardo Coda Zabetta

Amec Foster Wheeler

Varkaus, Finland

Presented at

22nd international conference on

fluidized bedconversion

Turku

Finland

16.6.2015

Corrosion monitoring in commercial CFBs

© Amec Foster Wheeler 2015.

Corrosion monitoring in commercial CFBs

Jouni Mahanen1, Kyösti Vänskä

1, Edgardo Coda Zabetta

1*

1Amec Foster Wheeler, Power Systems and Technology, R&D Department, Relanderinkatu 2, FI-

78201, Varkaus, Finland

*) Corresponding author: phone: +358 (0)400 250 712, e-mail address: edgardo.coda@amecfw.com

Abstract

Hot corrosion is a common challenge in boilers when firing aggressive fuels such as recycled wood

and waste. Mitigating the losses caused by corrosion requires an optimal balance between i) pricy

preventive actions, ii) time-consuming scheduled maintenance, and iii) the risk of unscheduled failures

causing loss of production and repairs. Such balance is often based on the understanding of corrosion

phenomena and on the operational experience, which set somewhat conservative limits on process

temperatures and the use of corrosion-resistant materials or protections. A less conservative approach

consists in monitoring the progress of corrosion in real time with online corrosion measurements and

in parallel tuning the process conditions in order to minimize corrosion. However, online corrosion

measurements are not trivial, and available methods are not fully validated and accepted.

This paper describes the validation efforts conducted on an online corrosion monitoring technique in

several CFB boilers. The method – originally developed for mildly aggressive recycled wood – was

deployed in boilers co-firing a variety of fuels, including coal, woody biomass, agricultural residues,

and waste-derived fuels. Validation of the online method was conducted against metal wastage

measured in real boiler components, in test pieces temporarily installed in the boilers, and in test

probes. These three types of measurements provided full understanding of the online monitor

capabilities and its validation for long-, mid-, and short-term corrosion. The online method proved to be

suitable for real time corrosion monitoring, although with somewhat varying accuracy depending on

the co-fired fuels. With minor procedural changes, the same monitoring equipment also proved to be

suitable for online fuel quality monitoring when co-firing erratic shares of several fuels.

Keywords: CFB boilers, online corrosion monitoring

1. Introduction

Electrochemical online corrosion measurement methods are widely used in monitoring the

corrosion of critical components in the chemical process industry. The measurement is usually

based on linear polarization resistance (LPR) method, which is shortly introduced in chapter 2.

The method is suitable also for monitoring the corrosion of convective heat exchangers in boilers,

since the ash deposit that forms on top of the corrosion probe acts as an electrolyte. It must be

kept in mind that the composition and melt behavior of the deposit greatly affects the

measurement. This paper illustrates cases from several plant measurements, ranging from a

boiler firing coal with moderate share of agro biomass to a boiler firing waste-derived fuels. Online

measurement results were compared to results obtained from calibration rings (weight loss) which

were installed in the same probe, and wall thickness measurements of boiler tubes. Online

measurement results were also compared to changes in fuel quality, when applicable.

2. Test methods

Electrochemical online corrosion measurements are based on determining the corrosion current

within a confined system. Once the corrosion current is known, corrosion rate can be calculated

according to Faraday’s law, stating that the amount of material that is dissolved to an electrolyte in

electrochemical reactions is directly proportional to the current between anode and cathode. In

this case the system comprises a corrosion probe apparatus and the deposit forming on it when

exposed in a boiler. When the corrosion current through the system is known, a corrosion rate can

be calculated. In LPR measurements, the corrosion current is determined indirectly by measuring

the corrosion potential and polarization resistance between a reference electrode and a work

electrode. The polarization resistance is derived from inducing a small deflecting voltage (typically

tens of millivolts) to the system, while measuring the current that is required to induce this

deflecting voltage. Based on LPR measurement, Amec Foster Wheeler developed the MECO

online corrosion monitoring system. This system has been deployed in nearly 30 boilers to monitor

the corrosion of convective heat exchangers during guarantee period, especially in units where

challenging fuels are fired. The MECO system consists of a measuring probe which is installed in

the flue gas duct, and a control unit which controls the probe cooling and executes the necessary

measurements, and based on the measurements calculates the projected corrosion rate in µm/a.

MECO can be connected to power plants data collecting system, which allows monitoring

corrosion from the control room. The probe can be cooled with water and pressurized air, or only

with pressurized air. A conceptual figure of the probe is shown in Figure 1. Two different materials

can be installed in the probe at the same time, thus allowing monitoring corrosion of two materials

simultaneously. The measuring circuit consists of three test pieces – work electrode 1, reference

electrode and work electrode 2. In addition to the measuring circuit, there are two test pieces from

which the actual corrosion rate can be determined based on their weight loss after exposure.

Corrosion rate based on the weight loss test pieces is performed by weighing the test pieces

before the exposure, and again after exposure after removing all deposit, oxide and corrosion

products from the weight loss test pieces. These results can be further validated to metal

corrosion rates obtained from ultrasonic tube wall thickness measurements, which are common

practice in the industry and are performed during maintenance outages. In order to obtain

meaningful statistics and detailed local corrosion data, ultrasonic tube wall thickness

measurements are performed on a very extensive number of points onto the heat exchangers, in

order to follow up the corrosion rate, these measurements are performed in the very same points,

providing extensive local information. Statistical methods are then apply to determine a

representative yearly corrosion rate.[1]

Figure 1. Conceptual figure of the MECO corrosion monitoring probe.

3. Plant measurements

The following sections present selected cases from the nearly 30 references where the MECO

online corrosion monitoring system was deployed. This selection is intended to demonstrate the

validity of the method in boilers firing a broad variety of fuels and fuel mixtures. The selected

cases also highlight side features of the system, which allow other than corrosion monitoring.

3.1. CFB boiler, coal and up to 20w-% agro biomass

Online corrosion monitoring was conducted for approximately one year in a CFB boiler firing coal

and up to 20 weight% biomass. The monitor’s probe temperature was set to match the reference

temperature in the convective superheaters. An austenitic steel with 25% chromium (1) and a

ferritic-martensitic steel with 11% chromium (2) were selected as probe materials. The online

measurement showed no corrosion for most of the monitoring period (~1 year). Figure 2 shows an

extract of such measurements, stretching for 8 continuous months. The corrosion rate in the figure

is normalized to a standardized reference rate, which is not disclosed in this paper. For

comparison, corrosion rates obtained from weight loss test pieces (WL) and from ultrasonic tube

wall thickness measurements (WTM) that were performed at the beginning and end of the

monitoring period are also shown in the same plot. Corrosion rates determined by these three

techniques are comparable to each other, confirming essentially no corrosion.

Figure 2. Corrosion probe temperature and normalized corrosion rates determined from

online measurement, weight loss test pieces (WL) and tube wall thickness measurements

(WTM).

Appearance of the corrosion probe removed after exposure is shown in Figure 3. The deposit

formed on the probe was very hard, and according to SEM EDS analyzes (Figure 4), it consisted

mainly of Ca and S, which is typical for coal combustion when limestone is used to control SOx

emissions. No chlorine was found from the deposit nor from the oxide scale, which confirms that

the deposit was not corrosive in nature.

Figure 3. Appearance of the corrosion probe after exposure.

Figure 4. Analyzed compositions (SEM EDS) of deposit and oxide layers formed on test

materials.

3.2. CFB boiler, coal and up to 40w-% biomass, including agro

Online corrosion monitoring was conducted for approximately 3 years in a CFB boiler firing coal

and up to 40 weight% biomass. The probe temperature was set to match the reference metal

temperature in the convective superheaters. The probe temperature was periodically increased in

order to i) verify the sensitivity of the corrosion monitor to temperature, and ii) to quantify the

corrosion sensitivity of tested materials to temperarure. A ferritic steel with 0,5% chromium (1) and

an austenitic steel with 18% chromium (2) were selected as probe materials. During the

monitoring period three separate but identical probes were exposed. Normalized corrosion

measurements alongside daily average shares of fired biomass are shown in Figure 5. For

comparison, normalized corrosion rates obtained from probe weight loss test pieces (WL) and

from ultrasonic tube wall thickness measurements (WTM) which were performed on yearly basis

during the 3 years testing period are also shown in the same plot. Measurements show an

increase in corrosion as the share of biomass increased, although a direct correlation between the

corrosion rate and biomass share is not evident because of the large variety of fired biomass,

ranging between clean wood chips to challenging agricultural residues.

Figure 5. Share of combusted biomass and normalized corrosion rates determined from

online measurement, weight loss test pieces (WL) and tube wall thickness measurements

(WTM).

The appearance of all three corrosion probes after exposure is shown in Figure 6. The deposit

formed on the probe was very hard, and according to SEM EDS analyzes (Figure 7), consisted

mainly of Ca and S, which is typical for coal combustion when limestone is used to control SOx

emissions. However, in this case, chlorine was found beneath the deposit layer, indicating that

chlorine induced corrosion took place during the monitoring period.

Probe I

Probe II

Probe III

Figure 6. Appearance of the corrosion probes after exposure.

Figure 7. Analyzed compositions (SEM EDS) of deposit and oxide layers formed on test

materials

Results from one of the periodical changes of probe temperature are shown in Figure 8.

Increasing the probe temperature clearly increased the measured corrosion, and decreasing the

probe temperature back to its original value brought the corrosion back to its initial value, with only

minimal (if any) memory effect of increased chlorine corrosion that occurred at higher temperature.

The best correlation between probe temperature and measured corrosion was obtained from the

material (2) i.e. the austenitic 18%Cr steel. Such result demonstrated that - with the fuels and

corrosion mechanisms involved in this case - the MECO corrosion monitoring probe can be used

effectively as a fuel quality monitoring system.

Figure 8. Effect of probe temperature on measured corrosion.

3.3. CFB boiler, recycled wood

Online corrosion monitoring was conducted for approximately two years in a CFB boiler firing

recycled wood. The probe temperature was set to match the reference metal temperature in the

convective superheaters. An austenitic steel with 25% chromium (1) and a nickel based alloy with

21% chromium (2) were selected as probe materials. Normalized corrosion rate measurement

results, probe temperature and normalized corrosion rates determined from probe weight loss

rings (WL) are shown in Figure 9. In this case, ultrasonic wall thickness were not performed during

the monitoring period, so the comparison of corrosion rates can be done only between online

measurement and probe weight loss rings Contrary to the previous cases, here the probe

temperature suffered higher fluctuations due to accidental disruptions in coolant supply. Although

unplanned, these fluctuations helped to confirm once again the correlation between temperature

and measured corrosion, and demonstrated the dynamic behavior of the probe in case of

temperature upsets. Boiler tubes wall thickness measurements were not performed during the

monitoring period, so the online measurement result can be compared to the corrosion rates

determined from probe weight loss rings only. The correlation between the measured and actual

corrosion rate for the austenitic 25% chromium steel (material 1) is good.

Figure 9. Corrosion probe temperature and normalized corrosion rates determined from online measurement and weight loss test pieces (WL)

Figure 10. Appearance of the corrosion probe after exposure.

Appearance of the corrosion probe after exposure is shown in Figure 10. The deposit layer on it

was gray and brittle. According to SEM EDS analyzes, Figure 11, the deposit was rich in Ca and

S, but also Na, K, Ti and Cl were detected in the deposit. Considerably high content of chlorine

was also found beneath the deposit layer, indicating that chlorine induced corrosion took place

during the monitoring period.

Figure 11. Analyzed compositions (SEM EDS) of deposit and oxide layers formed on

test materials

3.4. CFB boiler, forest residue, recycled wood, RDF pellets

Online corrosion monitoring was conducted for approximately two years in a CFB boiler firing

forest residue, recycled wood and RDF (refuse derived fuel) pellets. Probe temperature was set to

match the reference metal temperature in the convective superheaters. An austenitic steel with

18% chromium (1) and a ferritic-martensitic steel with 11% chromium (2) were selected as probe

materials. During the monitoring period two probes were exposed. Normalized corrosion rate

measurement results, probe temperature and normalized corrosion rates determined from probe

weight loss rings (WL) and tube wall thickness measurements (WTM) are shown in Figure 12.

Similarly to the previous case, also here the probe temperature fluctuated during the monitoring

period. Once again, the temperature variations highlight the effect of temperature on corrosion and

the dynamic behavior of the probe. Additionally, here large variations in corrosion rate also

resulted from changes in the fuel quality (mixture) during the measuring period. Once again, the

corrosion rates determined with the online measurement are well in line with the corrosion rates

determined from probe weight loss rings and tube wall thickness measurements.

Figure 12. Corrosion probe temperature and normalized corrosion rates determined from

online measurement, weight loss test pieces (WL) and tube wall thickness measurements

(WTM).

Probe I

Probe II

Figure 13. Appearance of the probes after exposure.

The appearance of corrosion probes after exposure is shown in Figure 13. Deposit layer was gray

and brittle. According to SEM EDS analyzes, Figure 14, deposit was rich in K, S, and Na. Chlorine

was also found beneath the deposit layer, indicating that chlorine induced corrosion has taken

place during monitoring period.

Figure 14. Analyzed compositions (SEM EDS) of deposit and oxide layers formed on test

materials

4. Discussion and conclusions

During the past two decades several publications have reported effective online corrosion

measurements based on the LPR method in boilers. Few monitoring systems have been

developed based on such technique by different organizations, each demonstrating the potential

of this technique but also arising drawbacks, the most critical being: i) the difficulty to adapt the

same system to different corrosion mechanisms (i.e. in boilers firing different fuels or widely

changing fuel diet), and ii) the unavailability of boiler data such as tube wall material losses to

validate the online measurement.

This paper illustrates the successful use of one and the same system (MECO) to monitor online

the corrosion rate of superheater tubes in boilers that fired fuels ranging from coal to biomass and

waste, as well as their mixtures. Only four cases were selected here to represent the extensive

experience collected from nearly 30 boilers in over 15 years. Such extensive experience not only

provided the bases for a qualitative knowhow of corrosion monitoring, but also provided concrete

validation data for its calibration.

Not surprisingly, when co-combusting moderate shares of biomass with coal, LPR measurements

showed no corrosion, which was confirmed by probe calibration test pieces and tube wall

thickness measurements. The deposit (electrolyte) that forms on the probe in these conditions is

not corrosive in nature. With higher shares of biomass, the deposit composition and properties

change, and the LPR measurements detect this change and warn of increased corrosion.

Additionally, testing activities confirmed other two applications for the MECO system, which can

be adopted when lagging phenomena such as the chlorine memory effect are avoided. Firstly, the

system can be used to monitor fuel quality in real time. Secondly, the probe can scan the

sensitivity of corrosion to temperature and find corrosion limits for given combinations of fuels and

materials.

Based on the above, the MECO is best applied:

- in plants that fire aggressive fuels to help estimating the lifetime of convective superheaters

and to test the effectiveness of changing tube materials or process temperatures, or. - by in cofiring plants to monitor the corrosivity of fuel mixtures and help avoiding too aggressive

combinations.

Acknowledgements

This work is partly conducted in projects supported by the Finnish funding agency for innovation

(TEKES/CLIFF).

References

[1] Kunnossapitoyhdistys ry, Korroosiokäsikirja, 2nd

ed. Hamina: Oy Kotkan kirjapaino Ab; 2004.

ISBN 951-97101-7-5.