Cast Studies: VOC, HAPs and Particulate Controls
The Problem . . .
The Solution . . .
The exhaust temperature of this mixed gas stream operates at an average of 230 degrees C. The composition of the different oxidation states of NOx stays relatively constant at 30% nitrogen dioxide (NO2) 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, resulting 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 NO2 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 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 Durr 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 Result . . .
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.