Thermodynamic research on the effect of CaO on the transformation of arsenic and sulfur in coal in different pyrolysis atmospheres

Abstract

This study investigates the influence of CaO on the transformation of arsenic and sulfur in coal in different pyrolysis atmospheres by thermodynamic equilibrium calculation by HSC Chemistry. The effect of water vapor and reaction pressure on the release of arsenic and sulfur-related compounds was studied. The detailed As and S distribution between 0°C and 1600°C was explored. The results show that in reducing atmosphere and partly oxidative CO 2 environment, arsenic and sulfur containing compounds show very different trendy with increasing temperature. CaO addition helps arsenic compounds to form Ca(AsO 2 ) 2 and Ca 3 (AsO 4 ) 2 , and helps sulfur containing compounds to form CaSO 4 and CaSO 3 . The temperature has a significant effect on the distribution of Ca-As complexes and Ca-S complexes. Ca(AsO 2 ) 2 and Ca 3 (AsO 4 ) 2 are formed below 600°C in N 2 , while in the CO 2 atmosphere only Ca(AsO 2 ) 2 is found below 600°C. CaSO 4 and CaSO 3 exist below 600°C in N 2 , while they are formed above 600°C in CO 2 . Water vapor enhances the formation of all the Ca-As complexes and Ca-S complexes greatly both in N 2 . Organic sulfur compounds are hardly detected anymore in the N 2 atmosphere with H 2 O addition. In both of N 2 and CO 2 atmosphere, rising pressure enhances the formation of Ca-S complexes. For Ca-As complexes, elevated pressure is favorable for As in coal to form Ca(AsO 2 ) 2 .Arsenic is one of the harmful pollutants in coal combustion, which causes a serious problem in human health. Arsenic exists in coal in two forms, organic and inorganic. Pyrite is the main source of arsenic in coal. 1 In China, high arsenic sulfur is the main cause of arsenic release from power plants. Arsenic stays with a particular matter, which goes to the environment as ash. There are a few technologies to control its release, such as coal washing before combustion, arsenic capture sorbent, 2-4 and so forth. Besides, atmosphere and temperature have a great effect on arsenic distribution during the reaction. 5 During pyrolysis, the transformation of arsenic caused