Less than 1 Percent Adds Up....
The Advanced Hybrid Particle Collector
Improving particulate matter collection by less than 1 percent over conventional electrostatic precipitators, which, on average, trap about 99 percent of a plant's particles, is more significant than it may seem. If, for example, a plant produces 100 pounds of fly ash in the boiler flue gas, a 99 percent efficient ESP would capture 99 pounds, with 1 pound entering the atmosphere. A particle collector operating at 99.99 percent efficiency would trap 99.99 pounds, with only 0.01 pounds exiting the stack.
When enough tiny particles are emitted into the air, visibility and human health can be impaired. With average coal plants handling tons of fly ash per day, quite a bit of soot and fine dirt particles can escape from a collector, a problem that is intensified due to existing precipitators losing efficiency through age.
Electrostatic precipitators and baghouses are the most common devices used to control the emissions of fine-particulate from coal-fired power plants. There are approximately 700 of these precipitators (cold side, the type of precipitator used at the Big Stone plant) and 100 baghouses cleaning the flue gas from over 260,000 megawatts of coal-fired power plants in the United States (236,000 megawatts use precipitators and 25,000 megawatts use baghouses).
Seventy-five percent of those precipitators operate at marginal performance, preventing a number of plants from running at peak load when the demand for energy is at its highest. This, in turn, limits the types of fuel a plant uses. Inexpensive, low-sulfur coals from the Powder River Basin in Wyoming, for example, can burn cleaner, but particulates from these fuels are much harder for any electrostatic precipitator to collect. That problem is exacerbated by an inefficient precipitator, which would allow more particles to escape into the air and perhaps exceed a plant's allowed emissions levels.
The Advanced Hybrid Particulate Collector (AHPC) combines the best features of baghouses (also called fabric filters) and precipitators in an entirely novel design. About one-third the size of a standard electrostatic precipitator, the AHPC unit is definitely a space-saver, reducing its "environmental footprint" as well as making it suitable for space-limited retrofits and new installations.
An average coal plant uses an precipitator or a baghouse to capture fly ash and smaller particles before the flue gas exits the stack. An electrostatic precipitator is comprised of a series of plates and electrodes that create an electrical field that charges particles in the gas stream. The charged particles are collected on the plates, which use a rapper—a mechanical device containing several hammer-like instruments—to dislodge the particles that then fall into a collection bin or hopper. A baghouse uses a series of fabric filters 8 to 26 feet long that operate like a sieve, trapping particles on the outside and allowing gas to pass through. The bags are cleaned when air is pulsed through the inside of the bags, causing particles on the outside to fall off. Stress from this cleaning technique causes wear and tear on the bags, which are typically replaced every three to five years.
The AHPC enhances both processes into an upgraded two-step system. The unit is composed of metal plates containing numerous openings that resemble circles and are laid end to end, running the length of the collector. Behind the plates are durable filter bags. In between the plates are metal rods that emit an electrical charge. When flue gas enters the AHPC, particles in the gas are electrically charged and adhere to the plates much like an ESP. More than 90 percent of the particles are captured this way. Particles eluding the plates flow throughout the openings in the plates and are trapped by the filter bags, which are especially efficient at removing extremely small particles. Electrostatic and fabric filter technologies combined in this way reduce the amount of filtration fabric needed by up to 4 times when compared to a conventional baghouse, thus allowing the use of better filtration materials at a competitive cost.
Like an ESP, the plates are cleaned by rappers. Dust on the plates forms a cake that falls into a hopper. Loose particles on the plates that are not collected have another chance to be trapped by the filter bags when the plates are rapped. Similarly, when the filter bags are cleaned, fine particles that are not heavy enough to fall into a hopper can be trapped by the plates, keeping the bags cleaner longer and prolonging their life.
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