Case study: Choosing selective catalytic reduction as a preferred technology for the destruction of NOx
Authors: Rodenhausen Richard 
Published: April 2004

Choosing an environmental control technology for destroying oxides of nitrogen (NOx) became essential when Engelhard, a manufacturer of automotive catalysts, included inorganic nitrate compounds in their formulations. Heat treating catalysts inside a high temperature oven causes these compounds to decompose, producing NOx and other gaseous by ‐ products, including volatile organic compounds (VOCs). Local environmental regulations limit the maximum concentration of total NOx to 200 parts per million (ppmv). The NOx continuously emitted from the oven can reach concentrations of up to 10,000 ppmv with VOC levels reaching 500 ppmv. Therefore, an emissions control strategy needed to be implemented.

The exhaust temperature of this mixed gas stream operates at an average temperature of 230°C. The composition of the different oxidation states of NOx stays relatively constant at 30% nitrogen dioxide (NO2) and 70% nitric oxide (NO). Utilizing a scrubber column packed with high surface are a packing to affect intimate contact between the gas and absorbing liquid in a countercurrent flow pattern, resulted in a 60% maximum NOx destruction efficiency. The absorbing liquid was a dilute solution of sodium hydroxide. Since nitric oxide is insoluble, it must be oxidized to NO2 before diffusing across the mass transfer interface into the liquid phase in order to react with caustic soda and form sodium nitrate. A more sophisticated wet scrubbing process uses several columns arranged in series. First, it quenches the inlet gas stream, then oxidizes nitric oxide to nitrogen dioxide, after which the NO2 is absorbed into the scrubbing solution. Process control of scrubbing fluid concentrations and gas flows is vital to maintaining constant high levels of NOx destruction efficiency. The secondary wastewater stream may have to be treated prior to discharging it into the city's sewer system. In order to destroy VOCs, a separate pollution abatement process, in series with the NOx scrubbing columns, would have to be installed. The combined environmental control system is quite complex and requires a high capital investment.

The alternative NOx and VOC control technology uses a single skid mounted system, combining selective catalytic reduction (SCR) for the destruction of NOx and an oxidation catalyst for VOC removal. Most SCR catalyst systems used in the industry today abate NOx from stationary power generating sources with lower NOx concentrations that operate 24 hours per day and 365 days per year. However, SCR technology is slowly becoming a more popular option to reduce NOx from chemical manufacturing facilities because of its efficiency to destroy NOx and process flexibility to handle varying concentrations of NOx.

In 1988, Engelhard's automotive catalyst manufacturing plant located in Huntsville, Alabama designed and installed its first SCR catalyst system to reduce NOx to nitrogen and water from a high temperature furnace exhaust gas. Since then, several combination SCR/VOC catalyst systems have been installed at Engelhard's automotive catalyst manufacturing facilities around the world. These systems have proven successful. They are highly efficient, the equipment is easy to maintain and operate, they do not produce a secondary waste stream (low ammonia slip) and have relatively low capital cost compared to other available technologies.