High pressure conversion of NOₓ and Hg and their capture as aqueous condensates in a laboratory piston-compressor simulating oxy-fuel CO₂ compression

Oxy-fuel technology for CO₂ capture has largely focused on combustion characteristics as a driver towards demonstration. Impurity removal studies typically centre on the how current environmental control units (FGD, SCR, activated carbon beds) operate in oxy-fuel firing. However, it is expected that some removal of NOₓ and SOₓ may occur during compression of flue gas through the lead chamber process. Some commercial systems link the capture of mercury to the formation of acid condensates (as a soluble mercury salt). Mercury in compressed flue gas represents a potential corrosion risk in the processing of CO₂ from oxy-fuel combustion processes. Gas phase elemental mercury (Hg⁰) is difficult to remove from the flue gas and the level of cleaning required to prevent corrosion of cryogenic brazed aluminium heat exchangers is uncertain. This work has investigated the behaviour of gaseous Hg⁰ in pressurised oxy-fuel systems in terms of the potential capture in acidic condensates, interaction with NOₓ gases and liquid stability on de-pressurisation. The work was undertaken on an adapted laboratory scale three stage axial-piston compressor with gas and liquid sampling at pressures up to 30 bar. The main finding was that gaseous Hg⁰ reacts readily with NO₂ formed from NO oxidation at high pressure. This reaction occurred without the presence of water, either water vapour or liquid water, contrary to speculation in the literature. Without NO₂, no capture of Hg⁰ was observed in the compression system. Overall, the capture of mercury during compression occurred as a consequence of high pressure, longer residence time and concentration of NO₂. Capture rates of 100% Hg and 75–83% NOₓ were measured from the compressor exit at 30 bar g