Enforcement of air pollution regulations or codes is usually done by the local pollution agency. However, since the dust control system is integrated with plant operation, the Technician is required to at least be aware of the controls and equipment necessary to meet these standards. The Technician is required to also be aware of how this equipment may affect asphalt mixture properties. Mixing plant manufacturers recognize the problem of air pollution and have developed equipment that restricts the escape of pollutants from the plants. Even so, during the operation of a plant, some gaseous and particulate pollutants may escape into the air. These pollutants are required to be limited to meet established clean air regulations. The Producer is required to be familiar with the state and local laws concerning air pollution. More definitive standards are based on the quantity of particulates coming from the stack. The most common requirement sets an upper limit on the mass of the particles being released as compared to the volume of gas released with them. Other standards relate the quantity of particulates emitted to the mass of the material being produced. A major air pollution concern at a plant is the combustion unit. Dirty, clogged burners and improper air-fuel mixtures result in excessive smoke and other undesirable combustion products. Continual close attention to the cleanliness and adjustment of the burners and accessory equipment is important. Another source of air pollution at a plant is aggregate dust. Dust emissions are greatest from the plant rotary dryer. Dust collectors commonly are used in the plant to meet anti-air pollution requirements. The types of dust collectors that are commonly used to capture the dust from the dryer are the centrifugal dust collectors, wet scrubbers, drop boxes, and baghouses (fabric filters). When the aggregate is especially dusty, two or more of these devices may need to be used in sequence. If the dust system returns the material to the plant, the return system is required to be calibrated. Some of the dust emitted from a plant is fugitive dust. This is dust escaping from parts of the plant other than the primary dust collectors. A scheduled maintenance program is required to keep fugitive dust to a minimum.
Centrifugal Dust Collectors Centrifugal dust collectors (cyclone type collectors) operate on the principle of centrifugal separation. The exhaust from the top of the dryer draws the smoke and fine materials into the cyclone where they are spiraled within the centrifuge (Figure 3-5). Larger particles hit the outside wall and drop to the bottom of the cyclone, and dust and smoke are discharged through the top of the collector. The fines at the bottom of the cyclone are collected by a dustreturn auger and may be returned to the plant or wasted.
Centrifugal dust collectors have been the most common type used, especially in rural areas. However, under today's more stringent pollution laws, the centrifugal dust collectors are usually used in combination with either a wet scrubber or a baghouse. Wet Scrubbers The purpose of a wet scrubber (Figure 3-6) is to entrap dust particles in water droplets and remove the particles from the exhaust gases. This is done by breaking up the water into small droplets and bringing those droplets into direct contact with the dust-laden gases. As the figure illustrates, gases from the dryer are introduced into a chamber through one inlet, while water is sprayed into the chamber from nozzles around the periphery.
Wet scrubbers are relatively efficient devices, but have certain drawbacks. First, the dust entrapped in the water is not recoverable. Second, the waste water containing the dust is required to be properly handled to prevent another source of pollution, since more than approximately 300 gallons per minute may be used. Most wet scrubbers are used in combination with a cyclone collector. The cyclone collects coarser materials and the wet scrubber removes the finer particles. Due to the drawbacks, wet scrubbers are no longer in common use. Baghouses (Fabric Filters) A baghouse (Figure 3-7) is a large metal housing containing hundreds of synthetic, heat-resistant fabric bags for collecting fines. The fabric bags are usually silicone-treated to increase their ability to collect very fine particles of dust. A baghouse functions much the same way as a vacuum cleaner. A large vacuum fan creates a suction within the housing, which draws in dirty air and filters the air though the fabric of the bags. To handle the huge volume of exhaust gases from the aggregate dryer, a very large number of bags (a typical unit may contain as many as 800) are required.
A baghouse is divided into a dirty gas chamber and a clean gas chamber. The filter bags are contained in the dirty gas chamber, into which the air from the dryer enters. The flow of air carrying the dust particles passes through the fabric of the filter bags, depositing the dust on the surface of the bag. The air then continues to the clean gas chamber. During the operation, the fabric filter traps large quantities of dust. Eventually, the dust accumulates into a "dust cake", that is required to be removed before the dust reduces or stops the flow of gas through the filter. There are many ways of cleaning the bags in a collector, but the most common methods are to flex the bags, back-flush the bags with clean air, or both flex and back-flush. Dust removed from the bags drops into an auger at the bottom of the baghouse and is transferred to a storage silo. The dust may then be returned to the plant or wasted