Product Application: Furnace damper controls reduce NOx emissions

INDUSTRIAL

Because of recent government regulations, petroleum refineries and other industries throughout North America are forced to examine their process heating operations in order to reduce cumulative nitrogen oxides (NOx) emissions. While aging furnace burners are already being replaced with newer low NOx burners, attention is being directed toward the furnace and heater stack damper drives.

Changing the final damper control elements to more reliable and accurate damper drives greatly improves the combustion of air and flue gases and reduces tramp air from entering furnaces. The resulting efficiency, depending on the type of low NOx burner selected and on how tight the furnace is for leaks, can lower emissions as much as 55-80% when used in combination with other low NOx strategies. There is also potential to increase profit margins from 2-8% per furnace from fuel savings alone.

The province of Ontario is committed to reducing NOx emissions by 45% from 1990 levels by 2015, and is considering moving that date forward to 2010 (see discussion paper on Ontario’s Clean Air Plan for Industry, Ministry of the Environment for Ontario, December 2002). In the U.S., the Environmental Protection Agency has teamed up with the U.S. Department of Justice to force multi-million dollar emission-reduction deals with major oil producers.

Ian McLellan, P.Eng., a mechanical engineer with SNC-Lavalin in Sarnia, Ontario comments: “I’d say 90% of the drives in North America will need to be replaced because none of the old systems were designed with precise control in mind.” He adds, “Now we’re talking about fractions of a per cent control, so you can’t use the old stuff.”

Currently working on projects at oil refineries in Ontario, McLellan explains why damper control became an important element at one site.

“Plant management did an upgrade on a large furnace, but the furnace was never able to demonstrate a stable O2 level, partly because they didn’t have good repeatability control over the furnace air,” states McLellan. “Yet, we couldn’t manually fool around with the original system because it was controlled by some old, unreliable actuators.”

“The other problem was that all three traditional actuators were jamming when the towers would heat up,” continues McLellan. “The refractory was falling out, hitting the damper and stopping it. The old arm-connected pneumatic actuators couldn’t stand up to this. The damper shaft actuator arms were actually bending.”

The chief stationary engineer at the plant asked McLellan to specify Type K damper drives. Installation at this plant posed a particularly difficult challenge, as a special coupling was required.

“The damper drive had to attach to a duct that got up to 1500F,” says McLellan. “We needed a special drive coupling that would reach out into our existing damper coupling and not unduly end load the damper shaft as it expanded from the heat. So the engineers at Type K came up with a sliding coupling using a square key-way and a heat sink. This allows the shaft to expand sideways about 1 inch on each side as the air stream heats and cools. Pillow block bearings were installed to maintain support while still permitting the sliding action inside the shaft coupling. This also helped prevent the refractory from getting in the works and jamming the damper blade.”

Installation of the first two damper drives on McLellan’s project was completed by the middle of December 2001. McLellan and his staff immediately noticed an improvement in damper control.

“Before, the process used to wander around a lot,” reports McLellan. “Essentially, we run at approximately 7% excess air, so plus or minus 1.5% meant you could swing from 5.5 to 8.5%. But now, repeatability is in the order of 0.1 to 0.25% — a big improvement.”

Article supplied by Donnie Anderson, manager of business development, Type K Damper Drives, Dallas, a division of Controls International.