Arsenic Leachability of Coal Fly Ashes from Different Types of Coal Fired Power Plants

The leaching behavior of arsenic (As) in coal fly ash collected from two different types of coal fired power plants (unit 1 and unit 2: 600 MWe) has been investigated to understand their behavior during combustion and effect of different boiler types on arsenic leachability. To determine dominant factors on arsenic leaching from coal fly ash, the change of arsenic chemical during coal combustion was predicted from the perspective of thermodynamic equilibrium and leaching test under alkaline condition. It found that, arsenic leaching fractions in unit 1 were higher than that of unit 2, it is associated with the amount of reactive CaO (calcium oxide) containing in coal fly ash from unit 1 was lower than that from unit 2. As2O3(gas) formed in the boiler reacts with CaO in the fly ash to form calcium arsenate Ca3(AsO4)2 which is thermodynamically stable calcium-arsenic compound. Hence the coal fly ash from unit 2 having higher CaO/Ash ratios generates more Ca3(AsO4)2 and has lower As leaching fraction than that from unit 1. CaO/Ash ratios were a promising index to reduce arsenic leachability from fly ash.