Ash deposition and slagging behavior of Chinese Xinjiang high-alkali coal in 3 MWth pilot-scale combustion test

Xiaojiang Wu, Xiang Zhang, Kai Yan, Nan Chen, Jianwen Zhang, Xueyuan Xu, Baiqian Dai, Jian Zhang, Lian Zhang

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Abstract

This paper aims to clarify ash deposition/slagging behavior of the newly found Xinjiang high-alkali coal in China, which is notorious for large amounts of sodium (Na2O, 4.0–10 wt%) and calcium (CaO, 20–40 wt%). A 12-h exposure test of ash deposition/slagging was conducted in a 3 MWth pilot plant, with a number of deposition probes and heat transfer tubes being inserted along the furnace to mimic the real pulverised coal-fired boilers. The samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Detector (EDX). Thermodynamic equilibrium calculations using FactSage 6.1 were also conducted to interpret the experimental observations. As has been confirmed, the Fe-rich species including magnetite and wollastonite are dominant in the fire-side deposit, due to the low melting temperature of Fe-/Ca-bearing minerals. Those particles were firstly melted and agglomerated to form large clusters, which subsequently attached to cold furnace wall by the inertial force. In convective section, the vaporisation of alkali metals, as well as their subsequent condensation and sulphation reaction to form Na2SO4 and CaSO4 are the major routes fouling the water tube. Compared to the coal samples studied in the literature, the ash deposit for Xinjiang coal grew quickly in the first 1.5 h, causing a significant decrease on heat transfer efficiency. Na2SO4, CaSO4 and Ca–Al–Si slags are the dominant phases in the growing layer of the ash deposit. The continuous interaction between liquid phase and newly captured particles led to the formation of eutectic compounds which embed anhydrite (CaSO4), thenardite (Na2SO4), gehlenite (Ca2Si2O7) and hematite (Fe2O3) inside. The formation of these eutectics accelerated the capture of more solid particles in ash deposit, which also turned sticky and harder to remove upon the increase on exposure temperature and time.
Original languageEnglish
Pages (from-to)1191-1202
Number of pages12
JournalFuel
Volume181
DOIs
Publication statusPublished - 1 Oct 2016

Keywords

  • Alkali coal
  • Ash deposition
  • Slagging
  • Coal combustion

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