July 1, 2008
Particulate emissions can be lowered through the use of the right operational practices, engineering controls, and equipment.
by Therese Dunphy, Editor-in-Chief
Producers looking to reduce dust in their operations have numerous options from which to choose. These options range from operational practices to the use of dry collection systems, says Mark Kestner, Ph.D., president of Mendham, N.J.-based National Environmental Service Co.
Typically, aggregates producers begin with low- or no-cost options and work through the spectrum of dust control measures based on conditions specific to each site and the area in which it operates.
“There are two outstanding operational practices to reduce dust,” Kestner says. “They are choke-feeding crushers and avoiding interruptions to process flow, both of which are good for production and dust control. The more rock you run through a crusher, the less air that passes through it and the less dust that is generated.” Conversely, interruptions in process flow allow machinery, particularly crushers, to windmill and generate more dust.
The next series of dust-reduction strategies involve engineering controls. One common practice is source enclosure. Examples include making sure transfer points are properly enclosed and covered and ensuring that crusher discharge is screened. “Good engineering control is designed to contain emissions,” he notes. “These procedures will obviously ratchet your emission rate down.”
A second engineering control involves landscaping and screening. “In-pit crushing has gotten very popular because if you’re down in the pit, people can’t see you,” he explains. “If you can’t go into the pit, you can shelter it from view with a berm, plants, screens – any way to keep (the public) from seeing you.”
The third category of dust controls includes spray systems. “The guideline there is to make sure you use high pressure – around 150 to 200 pounds per square inch – so that you are able to get the atomization and the power to penetrate material flow,” Kestner says. “You’re trying to do two things: you’re trying to knock down air-borne dust and you’re trying to add just a small amount of material to the process.”
Typically, a pump is sized so that it doesn’t add more than a half a percent by weight to the process. For example, a 20-gallon-per-minute pump would be suitable for a process flow of 1,000 tons per hour.
Dry collection is what Kestner describes as a dust-reduction strategy of “a last resort.” He says that a lot of states are implementing tougher standards and operations can’t achieve the necessary emission levels using water-based systems alone. Dry collection also is used if material is reactive, such as with cement.
Less than 5 percent of operations Kestner visits use all four levels of dust-control options, but that can depend on the area. He notes that it is fairly common to see 5 to 10 percent of capital costs of a new plant going into pollution controls, particularly in restrictive districts and in states such as New Mexico, Arizona, and California.
“We’re proceeding from control measures that cost the least, such as operating practices and engineering controls, all the way up to the more expensive systems, such as spray system and dry collection,” Kestner explains. “Fortunately, we’re in an industry where common sense works to control emissions.”
Bust the Dust
According to the Aggregates Manager Permit Survey, more than 48 percent of permits recently received by respondents contained restrictions related to air and emissions.