Scholarly article on topic 'Fireside Corrosion of Superheater Materials Under Oxy-coal Firing Conditions'

Fireside Corrosion of Superheater Materials Under Oxy-coal Firing Conditions Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Energy Procedia
OECD Field of science
Keywords
{"High-temperature corrosion" / "deposit-induced corrosion" / "gas phase oxidation" / "superheater alloys" / "oxy-coal combustion"}

Abstract of research paper on Materials engineering, author of scientific article — Roger Abang, Alexander Lisk, Hans Joachim Krautz

Abstract The present study investigates the fireside corrosion behavior of selected superheater materials, namely: T24, P92, VM12-SHC, A800HT, and 7RE10 under oxy-coal combustion atmospheres. Data on mass change, scale thickness, metal loss, surface morphology and micro-structural characteristics of corrosion products were obtained. The alloy specimens were analyzed by SEM-EDX, light microscopy and X-Ray diffraction techniques. The results after 2000hours of exposure at a metal surface temperature of 600°C indicate that metal wastage increased with decreasing Cr-content under oxy-coal conditions.

Academic research paper on topic "Fireside Corrosion of Superheater Materials Under Oxy-coal Firing Conditions"

Available online at www.sciencedirect.com

ScienceDirect

Energy Procedia 40 (2013) 304 - 311

European Geosciences Union General Assembly 2013, EGU Division Energy, Resources & the Environment, ERE

Fireside corrosion of superheater materials under oxy-coal

firing conditions

Roger Abanga*, Alexander Liska, Hans Joachim Krautza

_aBrandenburg University of Technology (BTU) Cottbus, Universitatsstrasse 22, 03046 Cottbus, Germany_

Abstract

The present study investigates the fireside corrosion behavior of selected superheater materials, namely: T24, P92, VM12-SHC, A800HT, and 7RE10 under oxy-coal combustion atmospheres. Data on mass change, scale thickness, metal loss, surface morphology and micro-structural characteristics of corrosion products were obtained. The alloy specimens were analyzed by SEM-EDX, light microscopy and X-Ray diffraction techniques. The results after 2000 hours of exposure at a metal surface temperature of 600 °C indicate that metal wastage increased with decreasing Cr-content under oxy-coal conditions.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the GFZ German Research Centre for Geosciences

Keywords: High-temperature corrosion; deposit-induced corrosion; gas phase oxidation; superheater alloys; oxy-coal combustion

1. Introduction

Oxyfuel technology (also known as O2/CO2 combustion) is the method of firing where fuel is burnt with oxygen of high purity (about 99.95 vol%) instead of air as in the conventional method of firing. In the oxyfuel process, nitrogen is first separated from air through cryogenic distillation in an air separation unit (ASU) to obtain pure oxygen which is used in a mixture of O2 and recycled flue gas as the oxidant gas for combustion. The resulting flue gas mainly consists of CO2 and H2O. The water is condensed out

* Corresponding author. Tel.: +49-355-694028; fax: +49-355-694011. E-mail address: roger.abang@tu-cottbus.de.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the GFZ German Research Centre for Geosciences doi:10.1016/j.egypro.2013.08.035

and the resulting high concentration of CO2 in the flue gas enables direct CO2 recovery [1]. With this method of firing, a negative (or zero) contribution of CO2 to the atmosphere is possible, if the emitted CO2 is captured and stored - Carbon Capture and Storage (CCS). For instance, the CO2 can be transported to suitable geological sites for long-term storage.

However, oxy-coal combustion also leads to an increase in the concentrations of corrosive gases such as SO2 and SO3. Several studies have shown that the flue gas composition and ash chemistry change significantly in oxy-coal combustion conditions compared to air combustion - oxy-coal yields high amounts of sulfates in the ash deposits than in air combustion [1, 2, 3]. These overall changes in gas and ash compositions can lead to an increased risk in fireside corrosion of vital plant components such as the heat exchanger materials (superheaters and reheaters). It is therefore important to investigate the effect of the increase in the concentration of corrosive products in oxy-coal atmospheres on the corrosion behavior of superheater materials.

The present study examines the corrosion characteristics of five selected superheater alloys: T24, P92, VM12-SHC, Alloy 800HT and Sandvik 7RE10, under oxy-coal combustion conditions for 2000 hours of exposure. The pre-corroded test specimens (100 hours of initial exposure under near realistic oxy-coal firing conditions) were exposed in a test tube furnace for another 1900 hours under the same oxyfuel atmosphere as in the initial corrosion test, at a metal surface temperature of 600 °C. The materials were then analyzed via optical microscopy (OM), scanning electron microscopy (SEM) together with energy dispersed x-ray spectroscopy (EDX), X-ray diffraction (XRD) as well as electron back-scattered diffraction (EBSD), to determine the morphology and micro-structural characteristics of the scale/oxides and products of corrosion. Thermodynamic modeling calculations (not presented in this paper) were also carried out in FactSage™ to study the thermodynamic properties of the oxy-coal flue gas and its interaction with the system. The result from mass change measurements and SEM analysis are presented

Nomenclature

A Area (cm2) Wf Final weight (g) Wo Initial weight (g)

1.1. Fireside corrosion in coal-fired boilers

High temperature fireside corrosion is a major problem in coal-fired boilers. Fireside metal wastage of superheaters and reheaters became a widespread problem in the 1940s with the increase of steam parameters to increase the overall thermal efficiency of coal power generation plants [4, 5]. As a result the steam conditions have been kept moderate over the last decades to reduce material damage from fireside corrosion. However, concerns to improve the thermal efficiency of power generation units to reduce carbon emissions have brought renewed interest to the topic.

1.1.1. Mechanisms of fireside corrosion

Fireside corrosion is caused either by gas-phase oxidation or liquid-phase (coal-ash) corrosion also known as hot temperature corrosion. Gas-phase oxidation can be minimized by using materials that are resistant to oxidation while liquid phase deposit-induced corrosion on the other hand poses serious problems due to its accelerated nature of attack resulting from a molten sulfate layer (between 593 to 760 °C) [6, 7]. It is worth noting that this type of corrosion attack is different from low temperature pyrosulfates attack and high temperature (815 °C) gaseous attack [4]. The mechanisms involved in liquid-phase deposit-induced corrosion in the superheater/reheater region of coal-fired boilers have been summarized as follows [8]:

• Alkali sulfates containing deposits build on the leading side of the tubes as a result of an increasing temperature gradient. This causes the outer surfaces of the materials to become sticky, thereby capturing any fly ash particles in the flue gas.

• Potassium sulfate and sodium sulfate concentrate on the section of the deposit with temperatures between 593 to 760 °C.

• The thermal dissociation of sulfur compounds contained in the ash then follows as well as the catalysis of SO2 by Fe2O3 in the flue gas to produce SO3. It is the reaction of SO3 with such oxide and alkali sulfates in the deposits which leads to formation of molten alkali-iron tri-sulfates at the metal interface as shown in Equation 1:

Fe2O3 +3(KNa)2SO4 +3SO3 ->2(KNa)3Fe(SO4)3

Finally, the molten alkali sulfates flux the protective oxide species (Cr, Ni, Mo, Fe) from the surface, thus leading to accelerated oxidation and sulfidation.

2. Experimental procedure

Five superheater alloys were selected for long-term exposure test. The material specimens (rings) received for the test were of varied ID and OD dimensions with wall thicknesses between 3.68 -4.20 mm. The compositions of the selected alloys are given in Table 1. Among those selected, Alloy800HT and 7RE10 are austenitic steels with high Cr and Ni contents. VM12-SHC is a ferritic-martensitic (9 - 12% Cr) steel with a scaling resistance of up to 650 °C and improved creep strength. P92 falls in the category of creep strength enhanced ferritic (CSEF) steels with a Cr-content of 8.9 wt%. T24 is a low Cr-alloy which is use as the reference material for comparison purposes for this study. The specimens were first introduced in a 400 kWth test boiler for 100 hours under near realistic oxy-coal combustion atmospheres to initiate corrosion. The flue gas composition and fly ash deposits from this test were obtained and used for the long term laboratory exposure test of 1900 hours. The materials were analyzed via microscopic techniques after the cumulative exposure sum of 2000 hours.

Table 1: Nominal composition (in wt%) of selected alloys for exposure test (in order of increasing Cr-content)

Mat. Des. Mat. Nr. Operating Temp. Fe Cr Ni Mo Co Mn Cu Al Ti C

T24 1.7378 <550 Bal 2.35 0.11 0.97 - 0.71 - - - 0.17

P92 - <630 Bal 8.9 0.23 0.38 - 0.43 - - - 0.07

VM12 1.4915 <650 Bal 12 0.28 0.23 1.3 0.2 - - - 0.11

A800HT 1.4959 >650 Bal 20.19 30.2 - 0.087 0.48 0.073 0.53 0.54 0.071

7RE10 1.4845 >650 Bal 24.28 20.19 0.26 - 1.3 0.17 - - 0.05

The long-term laboratory exposure test was performed by placing test specimens (rings) on a gas permeable Al2O3 plate coated with oxy-coal fly ash deposits, and then mounted on an air-cooled corrosion probe and placing the probe inside an insulated horizontal tubular furnace. A steady stream of synthetic oxy-coal flue gas is passed over the test specimens from below, see Figure 1. The test parameters were configured to simulate the entire range of environmental conditions that were used in the 100 hour pre-corrosion test which are expected to prevail in the superheater/reheater section of the boiler from the combustion of pulverized lignite coal from the Lusatia (Lausitz) region of Germany. The flue gas compositions and the gas flow rates regulated via a mass flow controller are given in Table 2. Corrosion testing was performed for 1900 hours at a temperature of 600°C. To ensure that fresh corrodent species were continuously in contact with the metal surfaces, the appropriate oxy-coal ash deposits were replenished twice after 200 hours and after 1000 hours. The specimens were weighed and without cleaning recoated with fresh ash and placed back into the furnace. Upon completion of the cumulative 2000-hour tests, the materials samples were weighed, inspected optically and under the light microscope to check the scale surface for any flatness, ripples or nodules or whether there is any form of excessive attack. Subsequent examination involved microscopic examination to measure remaining wall thickness thickness and to determine metal loss by subsurface penetration (e.g., sulfidation, carburization).

Fig. 1. Cross section of working tube

Table 2. Composition and flow rate of corrosive oxyfuel gas used

Gas species Conc. [vol%] Flow Rate

CO2 78.44 9.4 L/Min

O2 4.68 561.6 cm3/Min

N2 0.24 28.8 cm3/Min

SO2 0.04 4.8 cm3/Min

H2O 16.60 89.5 g/h

Sum 100

3. Results and discussions

3.1. Mass change

Results of the mass change measurements of the test samples after 100 hours, 200 hours, 1000 hours and at the end of the exposure test, 2000 hours are shown in Figure 2. The mass change data indicate greater amounts of corrosion of the ferritic steels T24, P92 and VM12-SHC with adhering ash deposits typical of the type of oxide fluxing in hot corrosion [9] as opposed to a minimal weight change in the high Cr-austenitic alloys Alloy800HT and 7RE10 showing a decrease in the weights due to spalling. The weight change was estimated from the following relationship [10]:

Mass change =

Wf-W0 A

where Wf is the final weight in grams, Wo is the initial weight in grams, and A is the original area in centimeters squared.

Fig. 2. Mass change of specimens after 2000 hours

3.2. Micro-structural examinations

Figures 3 (a) - (e) show the SEM photomicrographs of a cross section of the tested alloy after 2000 hours of exposure at 600°C, in a mixture of oxy-coal fly ash and synthetic oxy-coal flue gas composition. It is evident that the low Cr-alloy T24 was subjected to significant corrosion whereas the high Cr austenitic steels Alloy800HT and 7RE10 developed a thin oxide scale of approximately 1 - 2 ^m and experienced minimal corrosive attack. P92 and VM12-SHC (Figures 3 (c) and (d)) show duplex oxide scales which is typical of ferritic steels [8]. The oxide scales in VM12-SHC is somewhat porous with an ash reactive zone. Point analysis were carried out at various locations of the SEM cross sections to determine the product and penetration of corrosion. The results are presented in Figure 4.

100 iim

(e) SE MAG: 500 x HV: 15.0 kV WD: 9.6 mm I-

Fig. 3. SEM micrographs of tested alloys (a) 7RE10 (b) A800HT (c) VM12 (d) P92 (e) T24

Figures 4 (a) - (e) show the results from point analysis of the oxide scales of the exposed material specimens. As can be seen in Figures 4 (a), the alloy 7RE10 shows good properties with mainly Cr, Fe and O in its oxide scale indicating possible formation of protective Cr2O3 scales. Alloy 800HT shows similar characteristics to 7RE10 but with Ni in the oxide scale reflecting the high nickel content in the material. The high Cl-content observed in the scale of Alloy800HT is possibly from the embedding material from material preparation and not a product of corrosion. The material T24 (Figure 4 (e)) showed very high amounts of sulfur in the oxide scale indicating inward penetration of sulfur in the material.

123456789 10

(e) keV

Fig. 4. Results from point analysis of the oxide scales (a) 7RE10 (b) A800HT (c) VM12 (d) P92 (e) T24

4. Summary

The fireside corrosion behavior of selected superheater alloys under oxy-coal combustion atmosphere was presented in this paper. Long-term exposure tests of the materials' samples after 2000 hours at a temperature of 600 °C in simulated oxy-coal gases coated with oxy-coal fly ash deposits indicate, significant corrosion of the low Cr-alloy T24 with inward penetration of sulfur into the base material resulting from the high sulfur content in the oxy-coal atmospheres. The high austenitic alloys 7RE10 and Alloy 800HT both showed good corrosion performance probably due to the fact that these materials were tested below their respective creep and operating temperatures. It would be interesting to see how these alloys perform above 650 °C. The alloys VM12-SHC and P92 (9-12%-Cr) steels revealed typical characteristics of ferritic steels with duplex oxide scales and a reactive ash zone.

Although sulfur penetration of some alloys was observe (sulfidation), no uptake of carbon (carburization) by the materials was observed suggesting the high CO2 content in the oxy-coal flue gas did not result in an increase in corrosion.

Acknowledgements

This project was funded by the German Federal Ministry of Education and Research (BMBF) and coordinated by the German Research Centre for Geosciences (GFZ, Potsdam). The cooperating project partners are University of Potsdam, and Brandenburg University of Technology (BTU) Cottbus. Many thanks to Sandvik Sweden and VM-Tubes France for supplying the test materials. The materials analyses were carried out at BAM (German Federal Institute for Material Research and Testing).

References

[1] KaB H, Tappe S, Krautz HJ. The combustion of dry lignite under oxy-fuel process conditions in a 0.5 MWth test plant. In:Physics Procedia, 2008:1(1):p. 423-430.

[2] Bordenet B, Kluger F. Thermodynamic modelling of the corrosive deposits in oxy-fuel fired boilers. MaterialsScience Forum 2008, 595-98(1):261-269.

[3] Stromberg L, Lindgren G, Jacoby J, Giering R, Anheden M, Burchhardt U, Altmann H, Kluger F, Stamatelopoulos GN. Update on vattenfall's 30 mwth oxyfuel pilot plant in schwarze pumpe. Energy Procedia 2009, 1(1):581-589.

[4] Van Weele S, Blough JL. Literature search update: Fireside corrosion testing of candidate superheater tube alloys, coatings, and claddings. Technical Report Project N.:9-61-5329, Foster Wheeler Development Corporation, 1990.

[5] Blough JL. Fireside corrosion testing of candidate superheater tube alloys, coatings, and claddings - phase ii field testing. Technical report, Foster Wheeler Development Corporation, 12 Peach Tree Hill Road Livingston, New Jersey 07039, 1996.

[6] Roberge PR. Handbook of corrosion engineering. 2nd ed. New York: McGraw-Hill, 2000.

[7] Natesan K, Purohit A, Rink DL. Fireside corrosion of alloys for combustion power plants. Technical report, Argonne National Laboratory, Power Plant Chemistry, 2002.

[8] Gagliano MS, Hack H, Stanko G. Fireside corrosion resistance of proposed usc superheater and reheater materials: Laboratory and field test results. Presented at: The 2008 Clearwater Coal Conference, 33th International Technical Conference on Coal Utilization & Fuel Systems. Foster Wheeler North America Corp., 2008.

[9] Holcomb GR, Tylczak J, Meier GH, Yung KY, Mu N, Yanar N, Pettit F. Fireside corrosion in oxyfuel combustion of coal. National Energy Technology Laboratory, USA 2012

[10] Cramer SD, Covino-Jr BS. ASM Handbook Vol. 13A, Corrosion: Fundamentals, Testing, and Protection, volume 13 A. ASM International, 2003.