Effect Of Special Atmospheres On Transformation Of Alkali And Alkaline Earth Metals During Coal Oxy-fuel Combustion

Coal is primary fossil fuel of our country. The alkali and alkaline earthmetals (AAEM) in coal may release into gas phase during coal oxy-fuelcombustion. The release of AAEM may cause series problems, such as theformation of slags and ash deposits. The occurrence modes of alkali metalsand alkaline-earth metals (AAEMs) in coal relate to their release behavior and toash formation during combustion. The presence of SO2and H2O in thecombustion atmosphere will directly influence the release and transformation ofAAEM. The following paper investigated the release and transformation ofAAEM during oxy-fuel combustion in the presence of SO2and H2O.Transformation of AAEMs during oxy-fuel combustion in the presence ofSO2and H2O and load-Na in a drop-tube reactor was investigated through serialdissolution using H2O and HCl solutions. The content of Na, K, Mg, Ca in thedigested samples were analyzed by ion chromatography. The results show thatmost of the soluble Na and K species in raw coal volatilize in the gas phaseduring combustion, while amounts of three types of Na and K species in charsdecline with the increase in temperature. Transformation among occurrence modes of Na and K species may take place during combustion. The presence ofSO2increases the fractions of all types Na and K species in the chars because itpromotes the formation of sulfates, complex silicates (Na2O·3SiO2), andaluminosilicates. The presence of H2O promotes the release of all types of Naand K species, as well as the transformation reactions that can form stablealuminosilicates from water-soluble and acid-soluble Na and K species. Therelease rates of Mg and Ca of bituminous coal from Shenmu District (SM coal)are higher than those of other coals because of its higher soluble Mg and Cacontent. Fractions of Mg and Ca in chars may decrease and those of acid-solubleCa may increase at500~600°C via decomposition of water-soluble carboxylatesto CaCO3. At800°C, due to the insoluble Mg may be replaced by radicals toform water-soluble Mg, the fraction of water-soluble Mg increased slightly. Thefractions of other types Mg, Ca decreased with increasing temperature and thenkept stable when temperature reaches to800oC. The effects of SO2and H2O ontransformation of Mg and Ca species are the same as those on Na and K species,albeit small.The loaded-Na in coals accelerated the release of AAEM greatly in thetemperature range of600~800oC, and the effect was more obvious with theincrease of temperature. Because of the NaCl prefer to release directly and thepresence of NaCl is conducive to the release of potassium existing inaluminosilicate，the release rates of Na and K in NaCl-coal is increased, as wellas Mg and Ca, although the influence on Mg and Ca is smaller than on Na an K.Overall, the promption effect of NaAc is more obvious than that of NaCl.Furthemore, the results of simulation compounds experiments proved thatparts of NaCl and NaAc can react with SiO2or Al2O3to form silicates oraluminosilicates, and more NaAc was retained in this way.