Xinjiang lignite ash slagging and flow under the weak reducing environment at 1300 °C – Release of sodium out of slag and its modelling from the mass transfer perspective

Hengsong Ji, Xiaojiang Wu, Baiqian Dai, Lian Zhang

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17 Citations (Scopus)

Abstract

This study examined the Na loss upon the slagging of five typical Xinjiang lignite ashes and their blends with clay at different ratios, under the simulated cyclone combustion conditions. The ash samples were laden on a 25° – inclined plate and exposed in 1% CO (nitrogen balanced) at 1000–1300 °C for maximum 2 h at each temperature. Apart from experimental approach, modelling based on the mass transfer mechanism was also attempted. The results show that, the Na loss ratios achieved are lower than that has been reported for the combustion of the same coals in the pulverised – coal fired plants, varying from 40 to 70% of the entire Na that largely depends on the basicity of ash or ash – clay blend. The reaction temperatures examined here exert a marginal influence on the Na loss. The addition of clay to raw coal ash and its dosage amount are crucial. At a mild combustion temperature of 1300 °C, the addition of 8–10wt% clay, on the ash mass basis to a basic lignite ash is the optimum, simultaneously accelerating both slag flow rate and Na retention in slag matrix. Additionally, the addition of clay alleviated the penetration and corrosion of Na vapour into the refractory corundum, thus extended its lifespan. The Na loss rate is dominated by the internal mass transfer rate of Na vapour inside slag film, in particular at the elevated temperatures from 1300 °C when ash is partially/mostly molten, as well as when the clay is added into the original Xinjiang ash. For the original ash which is rarely molten before 1200 °C, the gas-solid reaction prevailed, leading to the capture of Na vapour by Al and Fe – bearing solid species and even the refractory corundum plate. In contrast, at 1300 °C, the formation of slag film decreased the Na vapour mass transfer coefficient drastically. The resistance was further enlarged by the formation of more molten species and a denser slag film upon the addition of clay. The original Na content in ash has proven insignificant in the mass diffusion control regime.

Original languageEnglish
Pages (from-to)32-43
Number of pages12
JournalFuel Processing Technology
Volume170
DOIs
Publication statusPublished - 1 Feb 2018

Keywords

  • Ash slagging
  • Mass transfer coefficient
  • Sodium loss
  • Xinjiang lignite

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