BY MELINDA WILLIAMS
Clipper Staff Writer
WEST BOUNTIFUL — In less than a month since HollyFrontier installed a new Wet Gas Scrubber at its south Davis refinery, emissions of sulphur dioxide (SO2) and nitrogen oxide (NOx) have plummeted.
“We moved our continuous emission monitors over to the wet gas scrubber as soon as it was installed, and the SO2 numbers have dropped to practically nothing and the NOx is way down,” said Holly spokesman Mike Astin.
The Wet Gas Scrubber is a state-of-the-art pollution control device that uses multiple water sprays to “scrub” particulate matter from the Fluid Catalytic Cracking unit.
The particulate matter that is removed includes the very small PM2.5 that dirties the air during Utah’s common winter inversions.
In the new system, caustic is added to the water in the scrubber to remove the SO2 from the stack gas, according to a company statement.
The scrubber also includes a generator to create ozone, which reacts with and removes nitrogen oxide from the stack gas. The ozone is consumed in the reaction, so there are no significant ozone emissions, the statement explained.
The new scrubber is expected to remove approximately 100 tons of particulate matter, 450 tons of SO2 and 30 tons of NOx emissions yearly.
The refinery can no longer monitor the particulate matter, since what comes out of the scrubber is steam, Astin said.
Monitoring PM 2.5 requires an opacity monitor, which measures the dust that can be seen coming from the equipment, Astin said.
Since the Wet Gas Scrubber produces steam, there’s no way to see the dust. Even when the steam dries, Astin said, there is no visible dust.
“The benefit to the public is that when the scrubber is running, we will no longer be putting (fluid catalytic cracking unit) catalyst dust in the air,” said Holly’s local general manager Lynn Keddington.
The new scrubber is one of the tallest stacks at the refinery, and it can be identified by the plume of clean gas coming out of it, Astin said.
The steam is created as the water spray hits hot gas emitted by the catalytic cracking unit. That gas is treated by the scrubber, according to the release.