Synergistic Effect of Alkali Metals in Coal and Introduced CaO during Steam Gasification

The steam gasification kinetics of Zhundong raw coal and the washed coal by deionized water or hydrochloric acid with/without addition of CaO were tested by thermogravimetric analyzer(TGA) at medium temperatures(650°C to 800°C). The cation contents of potassium and sodium in samples were determined by Inductively Coupled Plasma Optical Emission Spectrometer(ICP-OES). The Brunauer-Emmett-Teller(BET) surface area of the samples was tested by N2adsorption, and the morphologies of the samples were characterized by scanning electron microscopy(SEM). Experimental results showed that the organic sodium was the main catalyst for the gasification of the pyrolysis char, and the gasification rate of the char could be enhanced further with the introduction of CaO. The inherent alkali metals in coal and the introduced CaO showed a synergistic effect that occurred obviously above 735°C. The char conversion curves with or without CaO were fitted by the modified volumetric model(MVM). The calculated results indicated that the addition of CaO increased the pre-exponential factor obviously, but made little changes on the activation energy. It was proposed that the synergistic effect was resulted from the co-melting of the sodium and CaO, which facilitated the migration of the catalyst ions and the generation of C(O) intermediates for the gasification.