Characteristics of iron and sulphur in high-ash lignite (Pakistani lignite) and their influence on long-term T23 tube corrosion under super-critical coal-fired boiler conditions

Baiqian Dai, Xiaojiang Wu, Jianwen Zhang, Yoshihiko Ninomiya, Dunxi Yu, Lian Zhang

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8 Citations (Scopus)

Abstract

This paper aims to understand the characteristics of iron (Fe) and sulfur (S) in a high-ash lignite, namely Pakistani lignite that is unique and notorious for its extraordinarily high contents of ash. To date, the properties of such a unique ash and its influence on pulverised-coal fired boiler remain blank. Both bench-scale facilities and synchrotron-based X-ray adsorption spectroscopy were used to characterise these two elements. Subsequently, the high-temperature tube corrosion tests on T23 alloy coated with the Pakistani ashes were conducted under an air-firing atmosphere at 650 °C for up to 200 h. For comparison, four super-heater ash deposits collected from a 30 MWth PC boiler burning Xinjiang (XJ) lignite were also selected for the corrosion tests. The results show that, the Pakistani lignite has very high contents of Fe (up to ~28 wt% in its most stable oxide form) and S (up to ~43 wt%) in its ash, where most of these two elements are organically bounds. However, such a high-S ash caused a less mass gain per unit area of the T23 specimen than the Xinjiang ash deposits did. Compared to S, the alkalis (sodium and potassium) that are rich in Xinjiang ash deposits are more influential on tube corrosion. For both two types of ash samples, the entire tube corrosion rate is controlled by both phase boundary reaction and diffusion, where the ash-induced corrosion starts with a phase boundary control in the interface at the initial stage. The presence of ash deposit on the tube surface promotes the diffusion when a parabolic law applies subsequently. Due to the lower contents of alkalis, the Pakistani ash is less corrosive with relatively smaller oxidation kinetic rate constants than the XJ ash deposits. Additionally, with regard to the ash deposit induced corrosion kinetic constants, and the magnitude is in proportion to the content of S with XJ ash deposits, whilst has a closer correlation with the content of Fe in Pakistani ashes, which could be due to the catalytic role of Fe on the oxidation of SO2 in flue gas into SO3 that is more corrosive than the solid sulfates.

Original languageEnglish
Article number116855
Number of pages12
JournalFuel
Volume264
DOIs
Publication statusPublished - 15 Mar 2020

Keywords

  • Characteristics of sulphur and iron
  • High-ash Pakistani lignite
  • Oxidation kinetics
  • T23 tube corrosion

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