Eeffects of occurrence mode of Cd in low rank coal on its volability during pyrolysis process

In order to reveal the release mechanism of heavy metal cadmium ( Cd) in low rank coal during pyrolysis process,two low rank coals from Xinjiang and Neimeng (Baori) province was pyrolyzed at different temperature from 400 to 800 ℃ in an aluminum crucible reactor to investigate the migration behavior of Cd during pyrolysis process. The technical route of “coal pyrolysis behavior-the occurrence mode of Cd in coal-the release behavior of Cd-FactSage sim- ulation” was proposed. Firstly,the pyrolysis characteristics of two low rank coals were investigated in the TGA reactor, the occurrence mode of Cd was studied by stepwise chemical extraction and scanning electron microscopy. Then,the release behavior of Cd in coal pyrolysis process was studied in a fixed aluminum crucible reactor. Finally,the FactSage thermodynamic simulation calculation was used to systematically reveal the release mechanism of Cd during pyrolysis process. The results showed that the occurrence mode of Cd was crucial for its volatility during coal pyrolysis process. The main occurrence mode for these two rank coals was organic bound state Cd,which could be released with the de- composition of organic matter in coals at low temperature. Baori coal contained more iron-manganese bound Cd,and most of them were distributed on the surface of coal particles,which was easily decomposed by heat and cause it to vol- atilize. At high temperature,the volatility of Cd in Baori coal decreased due to its more silicate,which can react with gaseous cadmium. For Xinjiang coal,it contained more alkali and alkaline earth metal,which can form low-temperature eutectic with other minerals,leading to the release of mineral-rated Cd. In addition,the results of the FactSage chemi- cal thermodynamic simulation were consistent with the experimental phenomena. The calculation showed that Cd was a volatile element. At low temperature,almost all Cd in coal was volatilized in the form of gaseous elemental cadmium. Cadmium does not react with elements such as Si,Al,Cl,Fe and alkali metals in coal,but these minerals would adsorb or encapsulate Cd which affected the release of cadmium in pyrolysis process.