The removal of nitrogen oxides and mercury as condensates during the compression of oxyfuel flue gas

Research Doctorate - Doctor of Philosophy (PhD)Oxyfuel technology is a carbon capture and storage technology which burns pulverised coal in oxygen producing flue gas containing high partial pressure of carbon dioxide which can then be purified, compressed and stored. The impurities formed during the combustion of coal produces gaseous impurities in flue gas including sulphur oxides, nitrogen oxides and mercury. These impurities need to be treated to prevent damage to plant equipment and to comply with air pollution regulations. The Sour Gas Compression system proposed by Air Products utilises the compression circuit of an oxyfuel plant to simultaneously remove sulphur oxides, nitrogen oxides and mercury in the condensates. The extent of the removal and removal pathway of the impurities are uncertain. To answer these questions, experiments were done at different scales; a simple high-pressure setup and a piston compressor. The results obtained were compared to the data from the compression circuit of the Callide Oxyfuel Project. Using laboratory compression experiments, the study focused on the reaction mechanism of nitrogen oxides and elemental mercury removal from oxyfuel flue gas in condensates at pressures up to 30 bar gauge. It was found that nitrogen oxides readily react with oxygen under high pressure and form acids when water is added to the system. However, contrary to literature, mercury is not absorbed in the nitric acid formed due to low oxidation rate of gaseous mercury in the acid. Instead mercury reacts with nitrogen dioxide in the gas phase under high pressure which removes it from the gas phase. These gas phase reactions, from the oxidation of nitric oxide to nitrogen dioxide to the removal of mercury can be predicted using existing single step atmospheric kinetic equations. Identification of the product of reaction between mercury and nitrogen dioxide proved that it is not a known mercury compound. Experiments involving a piston compressor demonstrated the removal of nitrogen oxides and mercury from the flue gas and were comparable with the results produced by compressing a slipstream of the Callide Oxyfuel Project. The difficulty in closing the mass balance in the compressor experiments shows that products were likely to be trapped in the compressor. The nitrogen species in the liquid condensates were also shown to be partially volatile, forming gaseous nitrogen oxides upon depressurisation. This thesis has succeeded in demonstrating the viability of high-pressure removal of nitrogen oxides and mercury. It has shown the capability to meet removal targets relevant to the oxyfuel industry for sulphur oxides, nitrogen oxides and mercury. It also proved that the removal of mercury proceeded via a gas phase reaction with nitrogen dioxide, not via absorption into aqueous nitric acid as speculated in literature. It has identified potential future work, which is the stability of the nitrogen and mercury in the condensates, the identification of the mercury – nitrogen dioxide gas phase reaction product, the location of acid and mercury compound formation in an oxyfuel compression circuit and potential production of saleable waste products