Oxyfuel CO2 compression: The gas phase reaction of elemental mercury and NOx at high pressure and absorption into nitric acid

Abstract

Oxyfuel combustion is a technology which combusts coal in oxygen and recycled flue gas, producing a carbon dioxide rich flue gas for sequestration. Oxyfuel flue gas contains trace amounts of elemental mercury, which may corrode brazed aluminium heat exchangers used in the carbon dioxide purification system. International gas vendors have tested the use of the compression system to remove other flue gas impurities such as NOx; however, the reaction mechanism of mercury and its reaction products with NOx and nitric acid formed with condensed water vapour are unclear. This study used lab scale experiments to study the absorption of gaseous elemental mercury into nitric acid and the gas phase reaction between mercury and nitrogen dioxide formed from oxidised NO at pressures up to 25bar. It was observed that mercury has limited absorption into nitric acid and may partially desorb out of solution after depressurisation. On the other hand, mercury reacted readily with nitrogen dioxide (formed from nitric oxide oxidation at high pressure) in the gas phase. These gas phase reactions from the oxidation of nitric oxide to nitrogen dioxide to the subsequent oxidation of elemental mercury by nitrogen dioxide were predicted using existing global kinetic equations. The limited absorption of gaseous elemental mercury in nitric acid and significant oxidation of gaseous elemental mercury by nitrogen dioxide suggests that the primary removal step for elemental mercury is through the gas phase reaction. Oxyfuel compression circuits should therefore allow sufficient residence time for this gas phase reaction to occur.