Effect of silica additive on the high-temperature fireside tube corrosion during the air-firing and oxy-firing of lignite (Xinjiang coal): Characteristics of bulk and cross-sectional surfaces for the tubes

Iman Ja'baz, Juan Chen, Barbara Etschmann, Yoshihiko Ninomiya, Lian Zhang

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Detailed speciation analysis has been conducted on the corroded tubes coated with two different ash deposits collected from the combustion of a lignite, namely Xinjiang coal mixed with and without external silica in both air- and oxy-firing modes. The exposure conditions are of temperature of 650 °C, 50 h and the use of different flue gases (i.e. pure CO2, air, air-firing flue gas and oxy-firing flue gas). Apart from the lab-based XRD used for the characterisation of the top surface, synchrotron XANES was employed to determine the oxidation states of Fe, S and Cr on both top and cross-sectional surfaces of the corroded tubes. The results indicate that, irrespective of the Cr content, the tubes exposed to pure CO2 underwent oxidation, which is most likely due to the reaction CO2 + M = CO + MO where M stands for Fe or Cr. Cr in the Cr-bearing tubes were oxidised more rapidly than Fe in the pure CO2. The combined use of silica additive and Cr-bearing tubes with a minimal Cr content of 2 wt% (i.e. T23) is beneficial in inhibiting the tube surface oxidation as well the penetration of oxygen and sulphur. Except alkali sulphates, the free oxides in ash deposit can also trigger the reactions for the formation of new species such as augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) on the carbon steel tube surface. The oxidation states of sulphur on the tube surface is highly tube-specific and dependent on ash deposit composition as well, with the abundance of pyrite (FeS2) on the carbon steel tube coated with the raw coal ash deposits, whilst the enrichment of troilite (FeS) upon the coating of ash deposits derived from the combustion of coal mixed with silica additive, due to the shortage of free sulphur in ash deposit and a preferential scavenging of Fe into chromite. For the Cr-bearing tubes, sulphate is the only S-bearing species due to the inhibited inward diffusion of S by the Cr-oxide layer, irrespective of ash deposit type. In addition, spatial organisation of the oxidation state of Cr suggests a slightly accelerated outward diffusion of Cr by the ash coating, thereby leading to a dense Cr-O layer that is highly protective on the tube top outer surface.

Original languageEnglish
Pages (from-to)68-83
Number of pages16
JournalFuel
Volume187
DOIs
Publication statusPublished - 1 Jan 2017

Keywords

  • Synchrotron XANES
  • Oxy-fuel combustion
  • Tube corrosion
  • Chromium
  • Sulphide

Cite this

@article{80b260de9dd743259a9b6e3b0288532c,
title = "Effect of silica additive on the high-temperature fireside tube corrosion during the air-firing and oxy-firing of lignite (Xinjiang coal): Characteristics of bulk and cross-sectional surfaces for the tubes",
abstract = "Detailed speciation analysis has been conducted on the corroded tubes coated with two different ash deposits collected from the combustion of a lignite, namely Xinjiang coal mixed with and without external silica in both air- and oxy-firing modes. The exposure conditions are of temperature of 650 °C, 50 h and the use of different flue gases (i.e. pure CO2, air, air-firing flue gas and oxy-firing flue gas). Apart from the lab-based XRD used for the characterisation of the top surface, synchrotron XANES was employed to determine the oxidation states of Fe, S and Cr on both top and cross-sectional surfaces of the corroded tubes. The results indicate that, irrespective of the Cr content, the tubes exposed to pure CO2 underwent oxidation, which is most likely due to the reaction CO2 + M = CO + MO where M stands for Fe or Cr. Cr in the Cr-bearing tubes were oxidised more rapidly than Fe in the pure CO2. The combined use of silica additive and Cr-bearing tubes with a minimal Cr content of 2 wt{\%} (i.e. T23) is beneficial in inhibiting the tube surface oxidation as well the penetration of oxygen and sulphur. Except alkali sulphates, the free oxides in ash deposit can also trigger the reactions for the formation of new species such as augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) on the carbon steel tube surface. The oxidation states of sulphur on the tube surface is highly tube-specific and dependent on ash deposit composition as well, with the abundance of pyrite (FeS2) on the carbon steel tube coated with the raw coal ash deposits, whilst the enrichment of troilite (FeS) upon the coating of ash deposits derived from the combustion of coal mixed with silica additive, due to the shortage of free sulphur in ash deposit and a preferential scavenging of Fe into chromite. For the Cr-bearing tubes, sulphate is the only S-bearing species due to the inhibited inward diffusion of S by the Cr-oxide layer, irrespective of ash deposit type. In addition, spatial organisation of the oxidation state of Cr suggests a slightly accelerated outward diffusion of Cr by the ash coating, thereby leading to a dense Cr-O layer that is highly protective on the tube top outer surface.",
keywords = "Synchrotron XANES, Oxy-fuel combustion, Tube corrosion, Chromium, Sulphide",
author = "Iman Ja'baz and Juan Chen and Barbara Etschmann and Yoshihiko Ninomiya and Lian Zhang",
year = "2017",
month = "1",
day = "1",
doi = "10.1016/j.fuel.2016.09.040",
language = "English",
volume = "187",
pages = "68--83",
journal = "Fuel",
issn = "0016-2361",
publisher = "Elsevier",

}

Effect of silica additive on the high-temperature fireside tube corrosion during the air-firing and oxy-firing of lignite (Xinjiang coal) : Characteristics of bulk and cross-sectional surfaces for the tubes. / Ja'baz, Iman; Chen, Juan; Etschmann, Barbara; Ninomiya, Yoshihiko; Zhang, Lian.

In: Fuel, Vol. 187, 01.01.2017, p. 68-83.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Effect of silica additive on the high-temperature fireside tube corrosion during the air-firing and oxy-firing of lignite (Xinjiang coal)

T2 - Characteristics of bulk and cross-sectional surfaces for the tubes

AU - Ja'baz, Iman

AU - Chen, Juan

AU - Etschmann, Barbara

AU - Ninomiya, Yoshihiko

AU - Zhang, Lian

PY - 2017/1/1

Y1 - 2017/1/1

N2 - Detailed speciation analysis has been conducted on the corroded tubes coated with two different ash deposits collected from the combustion of a lignite, namely Xinjiang coal mixed with and without external silica in both air- and oxy-firing modes. The exposure conditions are of temperature of 650 °C, 50 h and the use of different flue gases (i.e. pure CO2, air, air-firing flue gas and oxy-firing flue gas). Apart from the lab-based XRD used for the characterisation of the top surface, synchrotron XANES was employed to determine the oxidation states of Fe, S and Cr on both top and cross-sectional surfaces of the corroded tubes. The results indicate that, irrespective of the Cr content, the tubes exposed to pure CO2 underwent oxidation, which is most likely due to the reaction CO2 + M = CO + MO where M stands for Fe or Cr. Cr in the Cr-bearing tubes were oxidised more rapidly than Fe in the pure CO2. The combined use of silica additive and Cr-bearing tubes with a minimal Cr content of 2 wt% (i.e. T23) is beneficial in inhibiting the tube surface oxidation as well the penetration of oxygen and sulphur. Except alkali sulphates, the free oxides in ash deposit can also trigger the reactions for the formation of new species such as augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) on the carbon steel tube surface. The oxidation states of sulphur on the tube surface is highly tube-specific and dependent on ash deposit composition as well, with the abundance of pyrite (FeS2) on the carbon steel tube coated with the raw coal ash deposits, whilst the enrichment of troilite (FeS) upon the coating of ash deposits derived from the combustion of coal mixed with silica additive, due to the shortage of free sulphur in ash deposit and a preferential scavenging of Fe into chromite. For the Cr-bearing tubes, sulphate is the only S-bearing species due to the inhibited inward diffusion of S by the Cr-oxide layer, irrespective of ash deposit type. In addition, spatial organisation of the oxidation state of Cr suggests a slightly accelerated outward diffusion of Cr by the ash coating, thereby leading to a dense Cr-O layer that is highly protective on the tube top outer surface.

AB - Detailed speciation analysis has been conducted on the corroded tubes coated with two different ash deposits collected from the combustion of a lignite, namely Xinjiang coal mixed with and without external silica in both air- and oxy-firing modes. The exposure conditions are of temperature of 650 °C, 50 h and the use of different flue gases (i.e. pure CO2, air, air-firing flue gas and oxy-firing flue gas). Apart from the lab-based XRD used for the characterisation of the top surface, synchrotron XANES was employed to determine the oxidation states of Fe, S and Cr on both top and cross-sectional surfaces of the corroded tubes. The results indicate that, irrespective of the Cr content, the tubes exposed to pure CO2 underwent oxidation, which is most likely due to the reaction CO2 + M = CO + MO where M stands for Fe or Cr. Cr in the Cr-bearing tubes were oxidised more rapidly than Fe in the pure CO2. The combined use of silica additive and Cr-bearing tubes with a minimal Cr content of 2 wt% (i.e. T23) is beneficial in inhibiting the tube surface oxidation as well the penetration of oxygen and sulphur. Except alkali sulphates, the free oxides in ash deposit can also trigger the reactions for the formation of new species such as augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) on the carbon steel tube surface. The oxidation states of sulphur on the tube surface is highly tube-specific and dependent on ash deposit composition as well, with the abundance of pyrite (FeS2) on the carbon steel tube coated with the raw coal ash deposits, whilst the enrichment of troilite (FeS) upon the coating of ash deposits derived from the combustion of coal mixed with silica additive, due to the shortage of free sulphur in ash deposit and a preferential scavenging of Fe into chromite. For the Cr-bearing tubes, sulphate is the only S-bearing species due to the inhibited inward diffusion of S by the Cr-oxide layer, irrespective of ash deposit type. In addition, spatial organisation of the oxidation state of Cr suggests a slightly accelerated outward diffusion of Cr by the ash coating, thereby leading to a dense Cr-O layer that is highly protective on the tube top outer surface.

KW - Synchrotron XANES

KW - Oxy-fuel combustion

KW - Tube corrosion

KW - Chromium

KW - Sulphide

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DO - 10.1016/j.fuel.2016.09.040

M3 - Article

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SN - 0016-2361

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