Occurrence and Fate of Some Trace Elements during Pyrolysis of Yima Coal, China

Modes of occurrence and transformation behaviors of selected trace elements during coal pyrolysis were studied. By considering the environmental effect, chemical features, and thermal immovability, six heavy-metal elements (V, Cr, Mn, Co, Ni, and Cu) and six rare earth elements (Y, Eu, Tb, Dy, Yb, and Sc) were selected for this study. The coal samples were collected from the Yima coal district, China. The pyrolysis experiment was carried out in a simulated quartz bed reactor at three temperatures: 500, 700, and 900 degrees C, with a heating rate of 20 degrees C/min and under a nitrogen atmosphere. The element content in coal and chars was determined by inductively coupled plasma-mass spectrometry (ICP-MS). The elemental occurrence in raw coal was analyzed through a sequential extraction procedure and determined by inductively coupled plasma-atomic absorption spectroscopy (ICP-AAS). The trace elements in Yima coal are separated into five fractions: water soluble, ion exchangeable, carbonate, organic material, and residue (mainly silicates and sulfides bound). The results are as follows: (1) All of the elements studied mainly exist in the residue fraction. With regard to heavy-metal elements studied, the residue fraction takes about 50-80% and the organic fraction takes about 15-40%; other chemical forms occurred at a very low level. Only Mn and Ni showed a sizable distribution of water-soluble, ion-exchangeable, and carbonate fraction. (2) During coal pyrolysis, part of the element became released from the coal body and some other parts became enriched in chars. All of the elements showed a increasing release ratio with the temperature ascending; at the same pyrolysis temperature, the release ratio (Rr) of heavy-metal elements is higher than that of the rare earth elements (REEs). At the temperature of 900 degrees C, the volatilizable fraction of the coal body is nearly depleted, the pyrolysis almost came to an end, and the release intensity (Ri) was approaching zero. (3) All of the elements studied showed a enrichment in chars, and the enrichment ratio (Er) increases with the temperature ascending. (4) The modes of occurrence of the element is a key role affecting the transformation during pyrolysis. With regard to REEs, there is a negative correlation between the elements left in chars and the residue fraction in raw coal. This dedicates that the element content left in chars for REEs is a result of the element distribution in the residue fraction in raw coal.