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Effect of the Air-Preheated Temperature on Sodium Transformation during Zhundong Coal Gasification in a Circulating Fluidized Bed

Song, Guoliang, Song, Weijian, Qi, Xiaobin, Yang, Shaobo
Energy & Fuels 2017 v.31 no.4 pp. 4461-4468
air, bottom ash, carbon, coal, fluidized beds, fly ash, gasification, silica, sodium, temperature
Preheated air could improve the coal gas heating value and gas quality but, meanwhile, may cause slagging in the gasifier as a result of the enrichment of sodium in the ash. In this work, the migration and transformation of sodium during the Zhundong coal circulating fluidized bed gasification with different air-preheated temperatures were investigated. During the Zhundong coal gasification at 940 °C, with the air-preheated temperature increased from 20 to 600 °C, the air equivalent ratio decreased from 0.52 to 0.38 and the sodium content in the bottom ash increased 62.99%, 425.58% for circulating ash, and 44.02% for fly ash as a result of the effect of carbon inhibition. Accordingly, the release fraction of gaseous sodium decreased 42.82%. The condensation of gaseous sodium started at 800 °C of the coal gas temperature and reached a maximum at about 700 °C. The turning point of the temperature increased with the increase of the air-preheated temperature. The air-preheated temperature had little influence on the sodium occurrence of bottom ash. With the increase of the air-preheated temperature, the fraction of H₂O-soluble sodium in fly ash and circulating ash increased and the ash fusion temperature of both fly ash and bottom ash dropped. Slagging blocks appeared during the gasification when the air-preheated temperature was up to 600 °C. Slagging blocks mainly contain SiO₂ and Na₂O. Sodium in the slagging blocks mainly exists as Na₂Si₂O₅, insoluble sodium. The air-preheated temperature should be less than 600 °C to ensure the stable operation when the content of Na₂O in the Zhundong coal ash is up to above 7.28%.