Dust control, prevention
Sept. 17, 2013
Grain dust is an inevitable part of grain handling and storage that must be addressed with best management practices in order to prevent safety and health concerns.
Dust from grain handling operations can be a mixture of grain particles, soil, plant material, fungi, bacteria, agricultural chemical residues, and waste from insects, rodents and/or birds. The mixture can vary depending on the type of grain, the growing conditions, how the grain was harvested, stored and processed, according to Kansas State University (KSU). Rotten grain contains large amounts of dust and bacteria.
Grain dust is highly combustible and can explode if enough becomes airborne or accumulates on a surface. A grain dust explosion includes four basic elements: the fuel (dust particles); oxygen; confinement (grain bins, bin deck galleries, silos, downspouts, enclosed drag conveyors); and an ignition source (short circuits, static electricity, lighters, overheated bearings, friction, cutting torches or welding devices, grinder sparks, etc.).
The explosive nature of dust varies based on the type of grain and the dust’s minimum explosive concentration (MEC), a measurement of particle size and energy nature. The MEC vary depending on the type of grain, but a critical MEC level can be reached in a very short time. Keeping dust levels below the MEC reduces the risk of explosion.
In addition to the obvious safety concerns, grain dust can cause serious health problems for those exposed. If inhaled, grain dust can cause lung-related problems such as coughing, shortness of breath, wheezing, asthma, chronic bronchitis, farmer’s lung and allergies. Gastrointestinal problems can also occur, along with skin rashes, nasal irritation, grain fever and inflammation of the membrane lining the eyelids.
Some people may be extremely sensitive to dust, while others may be able to withstand several exposures before showing any sensitivity, according to KSU.
Three general types of practices can be used to reduce dust accumulation and emissions from grain handling and processing, including process modifications, capture/collection systems and oil-suppression systems. Controlling grain dust emissions will provide a cleaner and safer work environment for elevator employees as well as the surrounding community.
Modifying or changing the process of grain handling and storage is an affordable solution, but can seem harder, because it means changing the way it’s always been done.
Communication is an important part of grain handling, particularly with neighbors. Grain elevator operators should provide their community with contact information should they have a complaint. Notify neighbors when harvest is about to start and what hours grain will be loaded and unloaded. If a crop is particularly wet or large, let the community know the dryer will be running more than usual.
Let neighbors know if you will be conducting particularly noisy operations and how long you expect it to last.
Consider the wind speed and direction, and limit grain handling when the wind would cause neighbors to receive excessive amounts of dust or chaff. Winds are stronger and windy days more common, in the spring and fall — the same times of year when more grain is handled.
One of the primary ways dust is generated is when a falling stream of grain hits the receiving pit. A substantial reduction in emissions can be achieved by reducing the grain free-fall distances and grain velocities. Some methods for this include:
Enclosing the receiving area, preferably with doors at both ends of a receiving shed. A receiving shed with no doors (or open doors) may create a “wind tunnel” effect that is worse than no shed at all. Another possibility is curtains or a shroud around the receiving pit.
Prevent grain from falling as possible at loadout with dead boxes, socks, drop-down spouts or sleeves that extend at least six inches below the sides of the receiving container to minimize the free-fall distance.
Specifying dust-tight cleaning and processing equipment.
Using lip-type shaft seals at bearings on conveyors and other equipment housings.
Using flanged inlets and outlets on all spouting, transitions and miscellaneous hoppers.
Fully enclosing and sealing all areas in contact with products handled.
Restricting the flow of the grain when the receiving container is empty or only partly full.
Using aeration fans as little as possible when loading grain into storage bins.
Controlling the speed of grain handling equipment, keeping conveyor belts at the minimum speed necessary.
Locating and configuring equipment so that the building helps block windblown dust.
If grain is being stored in outdoor piles, store it for the shortest amount of time possible and keep the piles covered with a tarp.
Capture and collection systems control emissions after they occur. Typical devices used in grain handling and processing are cyclones (mechanical collectors) and fabric filters. Both devices must be properly operated and maintained to do an effective job. Malfunctions can actually cause increased emissions until repairs are made, according to KSU.
Cyclones tend to collect a higher percentage of larger particles, which can be returned to the grain stream. Smaller particles typically pass through the cyclone to the filter, where they are collected for disposal in some other manner.
Use of cyclones before a fabric filter can lower the dust loading on the filter. However, the filter is less efficient because the larger particles captured by the cyclone would otherwise contribute to the collection efficiency of the mat of dust that builds up on the filter fabric, according to the National Research Council. The lower dust loading and lower efficiency tend to balance each other.
Air pollution control devices should be operated whenever there is grain present. Check the equipment daily for proper functioning, and fix any malfunctions quickly to minimize the amount and duration of excess dust.
Schedule regular maintenance of control equipment, and keep equipment logs with information such as dates of inspections, maintenance procedures and replacements.
Oil suppression systems
Oil suppression systems have been developed to prevent elevator explosions as well as control emissions. White mineral oil, soybean oil or some other vegetable oil is typically used.
In an oil suppression system, dust adheres to the oil added to the surface of the grain. Physical forces can cause dust particles to detach from the surface of the grain. Suppression of smaller dust particles is more prevalent because the adhesion force for smaller particles is greater relative to the acting physical forces, according to the U.S. Environmental Protection Agency (EPA).
Effectiveness of oil suppression systems depends heavily on the point of application, the EPA said. It should be added as early as possible, preferably before the grain is first elevated.
To ensure the oil is dispersed well, it can be applied as a top dressing before the grain enters the bucket elevator or at other grain transfer points, or from below the grain stream at a grain transfer point, using one or more spray nozzles.
Oil can also be added in the boot of the bucket elevator leg, at the discharge point from a receiving pit onto a belt or other type conveyor and in a screw conveyor, according to KSU.
The amount of oil needed for dust suppression varies depending on the type of grain handled and the amount of dust present. Adding more oil will not substantially increase the amount of dust suppressed, the EPA said.
As little as a half gallon of oil per thousand bushels of grain can reduce dust by up to 80%. Effective dust control can be achieved for up to six months or for six to eight handlings of the grain after the oil is applied.
Once preventive processes are in place, maintaining equipment is one of the best ways to ensure dust emissions are controlled. Make sure equipment is operated according to manufacturer’s instructions. This is especially true for grain dryers, which can be the source of many complaints from neighbors.
Inspect dryer screens before each start-up, and do not exceed the recommended capacity for the amount of grain passing through the dryer. Enclose grain inlets and outlets to the dryer, and enclose as much of the dryer as is practical. Double check the screen perforations for column dryers and the screen filter size for rack dryers.
In other areas of the grain elevator, be sure to perform regular inspections on all equipment and make needed repairs immediately.
Clean up indoor and outdoor areas such as floors, roofs and decks to prevent accumulation of chaff and dust that can blow around. In addition, clean up the yard, ditches and curbs as needed to prevent accumulation of grain, chaff and grain dust.
Any grain spills on driveways or access roads should be cleaned up, and unpaved roadways or traffic areas can be watered to minimize the amount of dust in the air.