Abatement system cost-effectively controls NOx and VOCs
By Gordon Harbison
Business Development Manager, Dürr Environmental
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 its formulations. Heat treating catalysts inside a high-temperature oven causes these compounds to decompose, producing NOx and other gaseous byproducts, including volatile organic compounds (VOC’s).
Local environmental regulations now limit the maximum concentration of total NOx. The NOx continuously emitted from the oven can reach concentrations of more than 20,000 ppm with VOC levels reaching 500 ppm. Therefore, an emissions control strategy needed to be implemented.
The system, known as a combination SCR (Selective Catalytic Reduction) system from Dürr Environmental, was chosen for Englehard’s Huntsville, Alabama plant due to the conversions of NOx required. A scrubber was considered, however the reduction in NOx would have been unacceptable given that the company required 99.9% NOx reductions. A scrubber would be limited to about 90 to95% reduction at best. A SNCR (Selective Non Catalytic Reduction) process was another option but the reductions would be limited to about 75% and the equipment would be quite a bit more expensive.
“We really didn’t look at other technologies,” said Richard Rodenhausen, senior project engineer for Engelhard. “The SCR was the best application in terms of reducing NOx for this situation.”
The exhaust temperature of this mixed gas stream operates at an average of 230 2 ) and 70% nitric oxide (NO). Previous experience with a packed tower scrubber used a scrubber column packed with high surface area 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 (NO) is insoluble, it must be oxidized to NO 2 before diffusing across the mass transfer interface into the liquid phase in order to react with caustic soda and form sodium nitrate.
The combination NOx and VOC control technology implemented by Dürr Environmental used a single skid-mounted system, combining selective catalytic reduction (SCR) for the reduction of NOx and an oxidation catalyst for VOC removal. Most SCR catalyst systems used in the industry today reduce 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 rapidly becoming a more popular option to reduce NOx from more complex chemical manufacturing processes because of its ability to reduce NOx to extremely low levels in the presence of other compounds and process flexibility to handle varying concentrations of NOx.
In a Dürr combination system, the gas stream first passes through an oxidation catalyst to convert VOCs to carbon dioxide and water. Then it passes through a control system that modulates the ammonia flow added to the polluted gas stream. Ammonia regulated through a control valve mixes with ambient air pumped through a distribution manifold for uniform mixing with the NOx-containing gas before entering the SCR catalyst bed. Ensuring an even distribution of ammonia helped achieve optimal NOx destruction.
“This is the first time I know of that both VOC and NOx were treated in the same system to the high degree that this required,” said Jim Griffin, regional sales manager, Durr Environmental. “Prior systems sometimes treated the VOC or CO to about 50% reduction. Here we went to 99%+ reduction.”
The first of the two SCR/VOC systems installed at Engelhard continues to destroy NOx generated during the high-temperature treatment of automotive catalysts. The second system has been installed on another high temperature furnace. Both systems continue to maintain high NOx reduction capabilities with no major mechanical problems .
These abatement systems have allowed the catalyst manufacturer to stay within VOC and NOx emissions limits while proving to be easy to maintain and operate. They do not produce a secondary waste stream (low ammonia slip) and have lower capital costs compared to other available technologies considered.
“Any system will have some maintenance,” says Engelhard’s Rodenhausen. “Overall, the SCR system requires less maintenance because its operation is a lot simpler than other technologies.”
Gordon Harbison is business development manager at Dürr Environmental,
31285 Durr Drive, Wixom, MI 48393-0459. He can be reached at mailto:firstname.lastname@example.org or