Fate of alkali elements during pyrolysis and combustion of Chinese coals

The distribution of alkali elements in coal is of the utmost importance for determining their transformation during combustion, however, little is known of it because of their lower concentration in coal. From this viewpoint, several advanced techniques were used in this study to investigate the evolution of alkali elements during combustion of two Chinese coals at 1200degreesC. The contents of alkali elements were analyzed using Inductively Coupled Plasma (ICP) spectroscopy; their distribution in raw coals and the combustion residues were analyzed by computer controlled SEM (CCSEM); moreover, the particulate matters in the emission gas were collected by a Low Pressure Impactor (LPI) to study the vaporization of these elements in combustion. The results indicate that the organically bound alkali-based compounds totally vaporized during coal pyrolysis, meanwhile, the included ones fragmented into ultra-fine particles with a size of about 1.0 mum, a portion of which entered into a gas atmosphere and changed into particulate matters. On the other hand, the excluded alkali elements have bimodal distribution in raw coals, in which the fine particles fragmented and changed into particulate matters, the large portion having a size of about 20.0 mum kept unchanged till the carbon had burnt out. For the vaporized alkali elements in the particulate matters, they had a bimodal sized-distribution; fine particles of about 0.4 mum were formed from combination of fine Na/Al-Si, NaCl, Na2SO4 and Na3PO4 less than 0.1 mum in size. They could capture the trace elements, too, and a portion of them coagulated into large particles in the particulate matters. The different vaporization behavior of sodium and potassium was also addressed using XPS analysis.