Absorption Based Systems for Co-removal of Nitrogen Oxides and Sulfur Oxides from Flue Gases

Doctoral thesis, 2022

Enhanced control of nitrogen and sulfur emissions, found in many of industrial processes, is anticipated for decades to come. Stricter emission regulations require new technologies adapted to the new conditions at reasonable cost. Multi-pollution control is an efficient way to lower costs, although difficult to implement, and the co-removal of NOx and SOx is therefore presently achieving much attention.
     This thesis covers five 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 a gaseous oxidizing agent, ClO2, into the flue gas stream before the scrubber. The gas phase 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-to-heat 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 or other flue gas impurities was observed, which is important to minimize ClO2 consumption.
     In terms of the absorption process, a number of liquid compositions has been tested in four setups, showing that the concentrations of SO2 and NO2 in the flue gas can both be reduced to a few ppm. The rate of NO2 removal is strongly dependent upon the presence of SO32- and HSO3- in the absorbing liquid. To study this interaction a new method for on-line analysis of the liquid composition in the system is developed. The SO32- and HSO3- oxidation caused by NO2 absorption and O2 is investigated and concluded to be following a radical initiated chain. It is also confirmed that this chain is efficiently interrupted by S2O32-. The derived model gives good agreement with the experimental outcomes for the 100 Nm3/h and 400 Nm3/h setups. A design for full scale implementation at a 20 MWth waste-to-heat plant is proposed together with cost estimation for installation and operation. 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.