Effect of temperature on gasification performance and sodium transformation of Zhundong coal

The effect of temperature (between 850 degrees C and 1050 degrees C) on gasification performance and sodium transformation characteristics of a high-sodium Zhundong coal was investigated using a novel design of fluidized bed gasifier. The agglomeration tendency of Zhundong coal was predicted using the ternary phase diagram of Na2O-SiO2-Al2O3, and ways of inhibiting agglomeration were proposed. For the catalytic effect of sodium in coal ash, Zhundong coal shows good gasification performance; the concentration of CH4 is about 1% and changes slightly with an increase in temperature. Zhundong coal ash is located in the carnegieite (NaAlSiO4) region of the ternary phase diagram system, which is a high melting temperature zone where agglomeration does not easily occur. The sodium-containing compound detected in the 950 degrees C, 1000 degrees C, and 1050 degrees C bottom chars and fly ashes is NaAlSiO4. With increase in temperature, the sodium retention ratio first decreases but then increases. At higher temperature, more sodium is retained as NaAlSiO4 and therefore less is released. The enrichments of calcium and sodium on particle surface were observed by EDX analysis. The formed high melting point compounds of NaAlSiO4 and CaS may adhere to the surface of char particles and inhibit the release of sodium at higher temperature. Agglomeration and defluidization phenomena were not found in any experiment. The experimental results are in agreement with the prediction based on the ternary phase diagram analysis. Avoiding the use of silica as bed material, enhancing mixing and inducing the formation of the high melting point sodium-containing compound NaAlSiO4 are essential to inhibit agglomeration and defluidization. The experimental and thermodynamic calculation methods are useful in identifying conditions suitable for the successful use of high sodium coal during fluidized bed gasification. (C) 2016 Elsevier Ltd. All rights reserved.