Alloy Selection for a Cofired Circulating Fluidized Bed Boiler Vortex

Oct 19, 2017 - X-ray diffraction and scanning electron microscopy (SEM) were used on a corroded industrial-scale circulating fluidized bed (CFB) boile...
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Alloy selection for a co-fired CFB boiler vortex finder application at 880 °C in a complex mixed mode corrosion environment Henrik Hagman, Mats Lundberg, and Dan Boström Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00984 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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Alloy selection for a co-fired CFB boiler vortex finder application at 880 °C in a complex mixed mode corrosion environment Henrik Hagman*a, Mats Lundbergb, Dan Boströma a

Thermochemical Energy Conversion Laboratory, Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden b

Division of Surface and Corrosion Science, Royal Institute of Technology, SE-901 87 Stockholm, Sweden

*Corresponding author [email protected]

XRD and SEM were used on corroded industrial-scale CFB vortex finder (VF) 253MA alloy plate material to identify the dominating corrosion products and to enable a qualified selection of candidate alloys for the long-term, full-scale exposure study. Alloys 253MA, 310S, 800H/HT, Alloy DS and Alloy 600 were chosen and the alloy plates were exposed to the CFB combustion atmosphere having an average temperature of around 880°C, consisting of a moist globally oxidizing gas, burning hydrocarbons, CO2, CO, SO2, HCl, NH3, N2, alkali species and erosive particles. The exposure times used in this study were 1 750, 8 000, 12 000 and 16 000 operating hours. After exposure, the alloy samples were cut, cross sections dry-polished, and analyzed with a SEM-BSD setup to quantify material loss and penetration depth of the corrosion attack. This work suggests two novel concepts; heavily affected depth (HAD) enabling quantitative evaluation of heavily degraded alloys; and remaining serviceable metal thickness (RSMT) enabling the use of long-term corrosion data from one alloy, to make rough service life estimations of other alloys exposed for significantly shorter periods. The findings of this work show that there is no simple correlation between the heavily affected depth of the alloy, and the nickel, chromium or iron content. Instead, there seem to be two successful alloy composition

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principles that work well for this application. Also, the work shows that major improvements can be made in terms of both technical life-span and the cost-effectiveness of the VF application if the most appropriate alloy is selected. In this study, a replacement of the frequently used Alloy 253MA with Alloy 310S, doubled the lifespan of full-scale VF’s, reducing the average VF maintenance cost to half. Keywords: Vortex finder, CFB, high temperature corrosion, hot corrosion, alloy selection, alloy performance, 253MA, 310S, 800H/HT, Alloy DS, Alloy 600, heavily affected depth, remaining serviceable metal thickness, corrosion rate, service life estimation, co-combustion, co-firing, biomass, waste-derived fuels, animal waste

1. Introduction Globally, the use of circulating fluidized bed boilers (CFB) is increasing. One of the reasons for this is the CFBs ability to efficiently combust challenging and diverse fuels, such as biomass, different types of waste fractions and low-rank coals. The performance of the CFB cyclones and loop-seals is decisive for the CFB functionality. For the cyclones, the condition of the often non-cooled alloy cyclone vortex finders (VF) is critical. For co- and waste-fired boilers, the maintenance cost make up for a substantial part of the total operational cost. For the studied 50MWth CFB boiler, the cost for frequently replacing the VFs amount to approximately 10 % of the total maintenance cost and approximately 3% of the total operational cost of the boiler. Previously, extensive studies have been made on alloy performance and mechanisms regarding superheater corrosion in biomass- and waste-fired boilers, including material temperatures up to typically 450-650 ºC

1-3

. In terms of the chemical composition of flue gas

and ash, these studies are relevant also for the VF application. However, the surface and alloy temperature in the VF application is significantly higher, close to 900 ºC, which places the

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VF’s in a different regime in terms of internal diffusion rates and thermochemical equilibrium reactions in alloy and deposit. Furthermore, the transport mechanisms between flue gas, ash, and construction material differ due to the absence of significant temperature gradients near the VF surface, in contrast to the superheater application. Also, combustible gases and solid burning particles are in direct contact with the VF’s, unlike many steam superheater applications. If one instead considers the work made on alloy performance and corrosion mechanisms in gas turbine- and petrochemical applications, comprehensive work can be found for the temperature regime (around 900 ºC)

4-6

that corresponds to the VF application.

However, these studies generally lack the complexity of the biomass and waste-fired CFB combustion process, where mixed mode corrosion environments often include burning fuel particles, gaseous components such as H2O, NH3, CO, SO2, HCl, KCl, NaCl, solid ash particles including e.g. potassium and sodium -sulfates and -chlorides, as well as erosive bed sand. Thus, erosion enhanced corrosion also needs to be considered in addition to the more complex corrosive environment. Tylczak 2013 7 recently summarized work relevant for hightemperature corrosion-erosion of boiler furnace walls in future high-performance coal boilers, and performed alloy erosion-corrosion tests in a simulated flue gas environment including alloy temperatures up to 700 ºC during the impact of 270 µm “silica particles”. The study shows how the impact of erosive particles can create cracks in the protective oxide layer, enabling rapid erosion-enhanced corrosion of the alloy. References

7-28

represent the most closely related work found in available literature. All

discuss the performance of austenitic or nickel base alloys in corrosive environments resembling the present study, or how process parameters relevant for the present study affect the alloy performance. But work available on issues regarding alloy selection in complex

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mixed mode corrosive combustion environments at material temperatures around 900 °C is limited. More specifically, no work has been found that address issues of alloy selection in an environment resembling the non-cooled waste-firing biomass boiler VF application. The aim of this work was to improve the CFB boiler availability and improve the costeffectiveness of the VF’s, making combustion of environmentally friendly and technically challenging fuels more competitive.

2. Experimental and methods In the present work, the performance and cost-effectiveness of several readily available hightemperature alloys (253MA, 310S, Alloy 800H/HT, Alloy DS and Alloy 600) were compared in a 50MWth co-fired CFB boiler VF application. This was done via a multi-step alloy exposure approach using different sample materials, exposure time and analytical techniques. These steps are summarized in During its first year of exposure, the 24 000 h sample material was installed in parallel to the 8 000 h samples.

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Table 1, in chronological order. The identification and discussion of alloy corrosion- and breakdown mechanisms are generally beyond the scope of this work.

A description of the overall alloy sample exposure-, sample preparation- and analysis is made below. A detailed description of the full-scale boiler process parameters defining the environment of the alloy exposures follows. The fuel mix, operation temperature, boiler load and levels of emission have been kept on the same principal levels during the years of exposure.

Alloy sample exposure-, sample preparation- and analysis 1. A failure mechanism screening was made on a heavily degraded sample of a 253MA 8 mm thick VF reinforcement shield after around 10 000 h of boiler operation, using SEM-BSD, SEM-EDS, and XRD. The results from the screening indicated that carburization and nitridation were key mechanisms in the alloy degradation. The XRD data collection during the screening was performed on ground surface corrosion products using a Bruker d8 Advance instrument in θ−θ mode. The optical configuration consists of a primary Göbel mirror, Cu Kα radiation, and a Våntec-1 detector. Continuous scans were applied. The PDF-2 databank, together with Bruker software was used. The SEM setup used in the screening consists of a Philips XL30 ESEM, an EDAX DX-4 EDS system using an EDAX CDU detector.

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Figure 1. Results from the failure screening of an Alloy 253MA VF reinforcement plate sample. SEM: SEM-BSD image of alloy plate cross-section, the exposed surface to the right (in the left image). XRD: Powder X-ray diffraction indicative analysis of ground surface corrosion products.

2. An alloy exposure pre-study was made based on candidate alloys with different concentrations of Ni and Cr (253MA, 310S and 600). In this pre-study, metal rings with an outer diameter of 14 mm and wall thickness 3 mm were mounted on a thermocouple protection tube protruding from the ceiling of the boiler furnace, in front of the cyclone inlets. In this position, the sample material rings were exposed to approximately the same environment as the actual VF’s. The exposure time was 1750 h. The pre-study showed that of the candidate alloys included so far, an increased Ni content also increased the corrosion resistance and service life of the alloys.

3. Based on the insights from the step 1 and 2, a wider selection of alloys was made for a more comprehensive exposure study. This exposure study (step 3) constitutes the main part of the work performed presented in this article. This exposure is therefore described in detail.

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Table 2 summarizes the selected alloys as well as their vendor analyzed elemental composition. All sample plates had the same width (280 mm) and height (120 mm). Alloy 253MA had a thickness of 4.13 mm, 310S 8.06 mm, 800H/HT 12.15 mm, DS 10.21mm, and 600 6.40 mm. The selection of sample plates with varying thickness was due to limitations in availability. All plates were bent to fit the outer radius of the VF, using the same method as for full-scale VF plates, and mounted to the VF reinforcement plate in the area most heavily exposed to wear. Continuous welds were used around the edges of the plates to ensure attack on the sample plate thickness from mainly one direction. The lower part of the sample plates 253MA, 310S and 600 were overlay-welded with Inconel 625 to evaluate its protective effect on the base materials. The effect of the overlay-weld is not covered in this article. The samples were exposed for approximately 8 000 h of “active” boiler operation. Additionally, the exposure included 7 boiler maintenance shutdowns, having a total duration of 760 h, where the VF’s were cooled from operating temperature to ambient temperature. The VF environment is dry and non-corrosive during shutdowns, resulting in no additional attack on the VFs. Thus, the 760 h of boiler shutdown is not included in the 8000 h exposure time. Except the 7 major thermal cycles resulting from shutdowns, frequent minor thermal cycling within the temperature interval 860 – 890 C continuously occur during operation. The VF exposure temperature was measured using a thermo-well positioned in the furnace roof in front of the cyclone inlets (see position TC in Figure 2 in the boiler description section). When the plates were dismounted, sample pieces unaffected by the welding were cut out. The sample plates were dry-cut with a band saw and ground in a rotary grinder with coarse zirconia-alumina 120 grit paper, followed by grinding with SiC grinding paper down to 800

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grit (average abrasive particle diameter of 22 µm). During sawing and grinding, the exposed side of the samples was directed towards the cutting direction of blade and paper, to avoid mechanical spalling of corrosion products. Photographs of the cross-section surfaces were taken, and the thickness of all reference plates and exposed plates was measured at 5 positions each, with a high precision sliding caliper. The caliper measurements include both the corrosion product layer and thin films of welladhering ash particles. The samples were cut down further to enable measurements and analysis with a SEM-BSD setup (Carl Zeiss Merlin field-emission SEM with Gemini II column and a four-quadrant solid state BSD diode). An acceleration voltage of 20kV, a beam current of 500-1000 pA, and a chamber pressure of 1.60×10^-6 mbar was used during the imaging.

4. A sample piece of an 8 mm thick 310S VF reinforcement shield that had been in service for 16 000 h was included in the analysis. This sample piece was cut out from an obsolete VF during the installation of new VFs together with the 8 000 h samples. The 16 000 h sample was stored in a desiccator during one year, and prepared during the same occasion as the 8 000 h samples, using the same method, and was also subject to the same analysis and measurements. See details in the description of step 3.

5. As the sample plates cover only 0.3 % of the outer surface of a VF, visual inspection of full-scale VF’s were carried out from the outside and/or inside of the VFs to ensure the fullscale-relevance of the exposure study (see Figure 2): The condition, specific damages, and VF maximum service life was recorded, and photos of damages and the degradation over time

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was documented. These full-scale follow-ups have been made for alloys 310S and 253MA (the two alloys used as full-scale VF construction materials in this specific CFB boiler). Details of how the VF’s start to deform, scale, and crack, as the degradation of the alloys develops, has been documented.

6. Heavily affected/degraded full-scale 310S VFs were cut out according to a cutout schedule after 24 000 h of operation, producing cross-sections to further our understanding of the mechanisms of breakdown in different regions of the VF’s. During its first year of exposure, the 24 000 h sample material was installed in parallel to the 8 000 h samples.

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Table 1. Experimental steps performed, samples evaluated and analytical techniques used. The analytical techniques (with respective data sets) highlighted with bold cursive text are used in this article. Experimental step 1.

2.

Failure mechanism screening analysis Pre-study

3.

Exposure study

4.

Degradation study 2-year exposure Full-scale VF visual inspection

5.

6.

Degradation study 3-year exposure

Sample material source

Alloy exposure time [h] 12 000

Heavily degraded 253MA VF reinforcement plate (VF, Figure 2)

Analytical techniques used Visual inspection, Photographs, SEM-BSD, SEM-EDS and XRD

Alloy rings (253MA, 310S and 600) mounted on the vertical thermocouple protective tubing (TC, Figure 2) Alloy sample plates 253MA, 310S, 800H/HT, DS, 600 (VF, Figure 2) 310S heavily affected reinforcement plate facing the furnace (VF, Figure 2)

1 740

Photographs, SEM-BSD

8 000

Photographs, SEM-BSD, SEM-EDS SEM-BSD, SEM-EDS

Visual inspection of 253MA and 310S fullscale VFs, including cut out sample pieces, (VF, Figure 2)

approx. 4 000 h intervals until VF failure (310S 24 000 h, 253MA 12 000 h) 24 000

Sample from 310S heavily affected reinforcement plate (VF, Figure 2)

16 000

Visual inspection, photographs

Photographs, cut out schedule, XRD

Table 2. Alloy designation, elemental composition, and market prices. Alloy designation

Composition [%wt]

Price

Alloy

EN / ASTM

Fe

Cr

Ni

Mn

Si

C

Other > 0.02

EUR/kg]

253MA

1.4835 / S30815

65

21

11

0.8

1.6

0.08

5.7

310S

1.4845 / S31008

53.89 24.42 19.30 1.71

0.47

0.05

800H/HT 1.4958, 1.4959 / N08810, N08811

46.50 20.70 30.30 0.70

0.40

0.08

DS

1.4862 / N08330

42.52 17.50 36.10 1.20

2.10

0.06

N 0.17, Ce 0.05, S 253MA ≈ 800H/HT > Alloy DS. The findings of this work show that major improvements can be made in terms of both the technical life-span and the cost-effectiveness of the vortex finder application, if the most appropriate alloy is selected, preferably via a long-term exposure study. For example, in this

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study where nitridation and carburization were identified as major mechanisms of alloy degradation, a replacement of VF’s built from the frequently used Alloy 253MA, with VF’s built from Alloy 310S doubled the lifespan of the VF’s, increasing the total replacement cost with only 3 %, while reducing the average maintenance cost of the VF’s with approx. 50 %. Using non-cooled VF’s working under glowing conditions in a highly erosive-corrosive environment is problematic, why cooled VF’s protected by masonry would be highly interesting from an owner and maintenance point of view. If non-cooled VF’s is used in a boiler with NH3 non-catalytic NOx reduction, it should be taken into consideration to increase the distance between the NH3 injection and the VF’s, or even better; to place the NH3 injection in an empty draft downstream the VF’s (upstream the boiler heat exchangers).

6. Acknowledgement The financial support given by the Swedish Research Council, Perstorp Specialty Chemicals AB, and the National (Swedish) Strategic Research Program Bio4Energy are gratefully acknowledged. The cooperation and support given by friends and colleagues at TEC-Lab, Umeå University, and Perstorp Specialty Chemicals AB, is deeply appreciated. The SEM studies in this work have been performed at Umeå Core Facility for Electron Microscopy (UCEM), Umeå University.

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