Simultaneous Removal of SO₂ and Trace As₂O₃ from Flue Gas:  Mechanism, Kinetics Study, and Effect of Main Gases on Arsenic Capture

Abstract

Sulfur dioxide (SO2) and trace elements are pollutants derived from coal combustion. This study focuses on the simultaneous removal of S02 and trace arsenic oxide (As2O3) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range. Experiments have been performed on a thermogravimetric analyzer (TGA). The interaction mechanism between As2O3 and CaO is studied via XRD detection. Calcium arsenate [Ca3(AsO4)2] is found to be the reaction product in the range of 600-1000 degrees C. The ability of CaO to absorb As2O3 increases with the increasing temperature over the range of 400-1000 degrees C. Through kinetics analysis, it has been found that the rate constant of arsenate reaction is much higher than that of sulfate reaction. SO2 presence does not affect the trace arsenic capture either in the initial reaction stage when CaO conversion is relatively low or in the later stage when CaO conversion is very high. The product of sulfate reaction, CaS04, is proven to be able to absorb As2O3. The coexisting CO2 does not weaken the trace arsenic capture either.

Arsenic in Human History and Modern Societies

“…In bench-scale experiments with lime and calcium silicates, (Sterling and Helble (2003) found that the calcium compounds effectively remove As 4 O 6 ('As 2 O 3 ') vapors in either nitrogen gas (N 2 ) or air at 600-1000 • C. In other studies, Mahuli et al (1997) concluded that hydrated lime is most effective in removing volatile arsenic in flue gases at 600-1000 • C and Jadhav and Fan (2001) state that lime removed arsenic at 300-1000 • C (Chapter 7). In results that are similar to Sterling and Helble (2003) and Mahuli et al (1997), Li et al (2007) concluded that lime can sorb As 4 O 6 vapors and form calcium arsenates as Ca 3 (AsO 4 ) 2 at 600-1000 • C, as shown in the following reaction:…”

“…6CaO + As 4 O 6 (gas) + 2O 2 (gas) → 2Ca 3 (AsO 4 ) 2 (5.1) The ability of lime to sorb As 4 O 6 increases as temperatures increase between 400 and 1000 • C (Li et al, 2007). The presence of either sulfur dioxide (SO 2 ) or carbon dioxide (CO 2 ) gases did not substantially interfere with the ability of lime to capture As 4 O 6 .…”

“…The presence of either sulfur dioxide (SO 2 ) or carbon dioxide (CO 2 ) gases did not substantially interfere with the ability of lime to capture As 4 O 6 . CaSO 4 is also capable of sorbing at least some As 4 O 6 (Li et al, 2007).…”

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

“…(1) [18] reacts with SO 2 through Eq. (2) to form CaSO 4 [18,19] and with arsenic vapor through Eq. (3) to form .…”

“…(3) to form . Ca 3 (AsO 4 ) 2 [19]. CaCO 3 →CaO+CO 2 (Decomposition) (1) CaO+SO 2 +½O 2 →CaSO 4 (Sulfation) (2) 3CaO+As 2 O 3 +O 2 →Ca 3 (AsO 4 )…”