Upscaling and Evaluation of Absorption Based Systems for Co-removal of NOx and SOx from Flue Gases

Abstract: As long as combustion remains a vital part of the energy system, the removal of nitrogen oxides (NOx) will be a challenge that will need to be addressed. In addition, emissions of sulfur oxides (SOx) will be a problem as long as fuels such as coal and oil are used, as is envisaged in every road map suggested by the Intergovernmental Panel on Climate Change (IPPC). With stricter emission regulations being enforced across all fields, new technologies that are adapted to the circumstances will be required to keep costs down. A promising concept for lowering cost involves the removal of several impurities within the same unit, with the co-removal of NOx and SOx being one of the most intensively researched applications. This thesis covers three studies that are focused on the concept of co-removal of NOx and SOx in a wet scrubber unit, whereby the NO is oxidized to NO2 by the introduction of an oxidizing agent, ClO2, into the flue gas stream before the scrubber. The oxidation of NO to NO2 by ClO2 has been tested for a wide variety of flue gas compositions and temperatures, applying a total of three intermediate scales: 0.2 Nm3/h with synthetic flue gases, 100 Nm3/h with flue gases from a propane flame, and a ~400 Nm3/h slip stream from a waste-fired power plant. An efficient NO to NO2 oxidation was observed at all scales with complete conversion at a ClO2/NO ratio of ~0.5. No interaction between ClO2 and SO2 was observed. In terms of the absorption process, a number of liquid compositions has been tested at each scale, showing that the concentrations of SO2 and NO2 in the flue gas can both be reduced to a few ppm. However, the rate of NO2 removal is strongly dependent upon the presence of S(IV) in the absorbing liquid. The derived model gives good agreement with the experimental outcomes for the 100 Nm3/h and 400 Nm3/h setups, without any extensive parameter fitting. The most important discrepancy noted in relation to the understanding of the associated chemistries is the rate of S(IV) oxidation in the scrubber tower. The work of this thesis represents the first validation of the concept and methodology of scale-up of the co-removal process, and paves the way for commercialization.