(71a) Adsorption of Trace Elements and Sulfur Dioxide on Calcium-Based Sorbents

With the reduction of natural gas and petroleum sources and subsequent increases in their costs, energy from coal is becoming increasingly popular. However, coal is not a clean technology, so with this demand comes a demand to make this energy source more environmentally friendly. Trace elements, such as mercury, arsenic, and selenium, are highly volatile and are known to escape into the atmosphere from coal combustion flue gas.

Ab initio quantum mechanical tools were used to explain the adsorption mechanism of trace elements on a calcium oxide surface in the gas phase. Density functional theory was used to calculate binding energies of elemental mercury, oxidized mercury, selenium dioxide and sulfur dioxide molecules with a calcium oxide sorbent using the software programs, Gaussian 03 and Vienna Ab initio Simulation Package (VASP). Super cells with periodic boundary conditions versus cluster approaches were compared to illustrate the potential mechanisms of adsorption. Further, effects of hydrogen chloride on the adsorption of oxidized mercury are investigated to determine how hydrogen chloride plays a role in activating the calcium oxide surface to increase potential trace element adsorption.

Our predictions calculated in the gas phase indicate SO2, HgCl2 and SeO2 molecules are capable of adsorbing onto calcium oxide and hydrated calcium oxide surfaces. However, elemental mercury does not adsorb onto calcium oxide unless it is first oxidized. Moreover, HCl inhibits the adsorption of HgCl2 on the CaO surface. These results are in good agreement with the current data in the literature.