Diesel Exhaust Aftertreatment with Scrubber Process: NO _x Destruction

Oxides of nitrogen (NO_x) emissions, produced by engines that burn fuels with atmospheric air, are known to cause negative health and environmental effects. Increasingly stringent emissions regulations for marine engines have caused newer engines to be developed with inherent NO_x reduction technologies. Older marine engines typically have a useful life of over 20 years and produce a disproportionate amount of NO_x emissions when compared with their newer counterparts. Wet scrubbing as an aftertreatment method for emissions reduction was applied to ocean-going marine vessels for the reduction of sulfur oxides (SO_x) and particulate matter (PM) emissions. The gaseous absorption process was explored in the laboratory as an option for reducing NO_x emissions from older diesel engines of harbor craft operating in ports of Houston and Galveston. A scrubber system was designed, constructed, and evaluated to provide the basis for a real-world design. The details of this research have been presented in previously reported work. The main byproduct of the scrubber process was a weak nitric acid solution (≺10 wt%). This paper presents an exploration of a method developed to destroy the NO_x emissions captured and stored during the scrubbing process. This method utilized a stainless steel acid boiler vessel and catalyst for the NO_x destruction process. The acid boiler vessel allowed for the injection of weak nitric acid (≺10 wt%) and diesel fuel into a chamber for evaporation. The thermal decomposition of the nitric acid produced a nitrogen dioxide rich stream that then reacted with metered diesel fuel over two automotive-style, platinum based, three way catalysts (TWCs). In the laboratory, heat energy for acid evaporation and reduction was provided by electric resistance heaters. Real-world applications would use waste heat from the engine and scrubber process for acid evaporation and reduction. The system was evaluated with a fixed acid injection rate and various fuel flow rates to validate the thermal decomposition of nitric acid and overall NO_x reductions. Exit stack emissions were measured on a "semi-raw" and dilute basis. Acid to fuel ratios of 49.8:1 and 58.9:1 yielded NO_x reductions of 99.7% and 98%, respectively. The fuel utilized for the destruction process added a 1% fuel penalty to the complete scrubber system. The scrubber component of the whole system was capable of reducing NO_x emissions by 42% and 62% over a high flow and low flow cycle, respectively. The scrubber section of the overall system was therefore the primary limitation to the level of NO_x reduction.