Capture of Gas-Phase Arsenic Oxide by Lime:  Kinetic and Mechanistic Studies

Abstract

Trace metal emission from coal combustion is a major concern for coal-burning utilities. Toxic compounds such as arsenic species are difficult to control because of their high volatility. Mineral sorbents such as lime and hydrated lime have been shown to be effective in capturing arsenic from the gas phase over a wide temperature range. In this study, the mechanism of interaction between arsenic oxide (As2O3) and lime (CaO) is studied over the range of 300-1000 degrees C. The interaction between these two components is found to depend on the temperature; tricalcium orthoarsenate (Ca3As2O8) is found to be the product of the reaction below 600 degrees C, whereas dicalcium pyroarsenate (Ca2As2O7) is found to be the reaction product in the range of 700-900 degrees C. Maximum capture of arsenic oxide is found to occur in the range of 500-600 degrees C. At 500 degrees C, a high reactivity calcium carbonate is found to capture arsenic oxide by a combination of physical and chemical adsorption. Intrinsic kinetics of the reaction between calcium oxide and arsenic oxide in the medium-temperature range of 300-500 degrees C is studied in a differential bed flow-through reactor. Using the shrinking core model, the order of reaction with respect to arsenic oxide concentration is found to be about 1, and the activation energy is calculated to be 5.1 kcal/mol. The effect of initial surface area of CaO sorbent is studied over a range of 2.7-45 m2/g using the grain model. The effect of other major acidic flue gas species (SO2 and HCl) on arsenic capture is found to be minimal under the conditions of the experiment.

Toxic Substances From Coal Combustion-a Comprehensive Assessment

“…Table 3-41 shows the kinetic parameters associated with these experiments. It must be noted that Jadhav et al [124] showed that for the Ca-As reaction using calcium oxide as solid reactant, under oxidizing conditions and over a temperature range of 300 to 500 o C the activation energy was 21.3 kJ/mol for a vapor concentration of 14 ppm.…”

Arsenic Chemistry

“…Due to its semivolatility, arsenic in smelting ores and combusting coal mostly concentrates in flue gases. Gaseous arsenic in coal combustion and smelting operations primarily exists as arsenic(III) oxide (As 4 O 6 ) (Cotton et al, 1999), 400; (Jadhav and Fan, 2001), 794; (Shih and Lin, 2003), although AsCl 3 may be prominent from the combustion of chlorine-rich coals at about 530-930 • C (Urban and Wilcox, 2006). As the flue gases cool after leaving the combustion zone, volatile arsenic largely condenses onto any high surface area, fine-grained fly ash particles, especially in coal combustion systems ( (Hower et al, 1999;Mukherjee and Kikuchi, 1999), 64; (Guo, Yang and Liu, 2004;Llorens, Fernández-Turiel and Querol, 2001;Galbreath and Zygarlicke, 2004); Chapters 5 and 7).…”

“…Potentially effective sorbents for flue gas arsenic include hydrated lime (Ca(OH) 2 , portlandite), lime (CaO), calcium carbonate, limestone, fly ash, and sometimes activated carbon ((Jadhav and Fan, 2001;López-Antón et al, 2007;Helsen and Van den Bulck, 2004), 287, 289;(Taerakul et al, 2006;Gupta et al, 2007); Chapters 5 and 7). The injection of hydrated lime is especially effective and probably removes volatile As 4 O 6 through the formation of calcium arsenates at least over a temperature range of 600-1000 • C ( (Mahuli et al, 1997); Chapter 5).…”

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:…”