Study on Migration Mechanism of Alkali Metals During Co-firing of Rice Straw and Coal

The migration mechanism of alkali metals during co-firing of rice straw and coal in a self-made vertical tube furnace at 850 ℃ was studied on the basis of thermodynamic equilibrium calculations.The X-ray powder diffraction(XRD) and scanning electron microscope(SEM) were performed on the selected bottom ashes.The results indicate that the amount of gaseous KCl,KOH,NaCl and NaOH increase with rice straw content increasing and the concentrations of gaseous alkali chlorides are more than gaseous alkali hydroxides.The volatile potassium from rice straw is reduced after co-firing rice straw with coal due to the capture by reactions with mineral compositions or elements in coal.While the release of sodium is independent on the proportion of rice straw and coal in the fuels.The formation of alkali metals in ashes is related to the composition of fuel.When the proportion of rice straw is 0% and 20%,potassium and sodium in ashes are in the form of K2O·Fe2O3 and Na2SO4 respectively.When the mixing ratio of rice straw is more than 50%,K2O·2SiO2 and Na2SiO3 increase with rice straw content increasing,which is in contrast to K2SO4 and Na2SO4.Meanwhile,some potassium in fuel can be captured in the form of KAlSiO4 and KAlSi3O8.