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Baghouses vs Precipitators
Baghouses vs Precipitators
by John W. Norton, P.E., B.C.E.E.
This article was first written by the Author for "Pollution engineering Magazine" in December, 1989. It is slightly updated here:
BAGHOUSES vs PRECIPITATORS
Precipitators have many operational advantages over baghouses. The net result may be that the environment is far better off with a precipitator than it is with a baghouse.
As operators of two large incineration plants with three large rotary kiln incinerators in each, Montgomery County, Ohio, staff and management operated large electrostatic precipitators for many years. Their engineers and managers visited similar waste combusting facilities across the nation and developed a distinct and well founded preference for the electrostatic precipitator.
Unfortunately, Federal Legislators and regulators spurred on by unscientific "environmentalists" and unsupportable evidence have, since 1989, passed legislation which has effectively eliminated the electrostatic precipitator as an air pollution control tool for waste combustion.
Consider the following observations:
Precipitators meeting the same emission standards require substantially less energy consumption during operation than do baghouses. Energy is, or should be, part of our total environment consideration, and it should not be wasted. A baghouse typically requires six times the energy to pull the flue gases through the fabric filters. This is very energy wasteful on a system that handles hundreds of thousands of cubic feet of gas per minute.
Baghouses are relatively more sensitive to temperature and humidity variations in the process gas stream. Incinerator gases are inherently variable in temperature and humidity due to the varying nature of trash. While electrostatic precipitators have some sensitivity to these variables as well, continuous static voltage regulators can accommodate the usual range of temperature and humidity without failure. Baghouses have been known to blind, crust over, melt, or burn under conditions which would not have harmed an electrostatic precipitator.
Baghouses can catch fire. During the operation of our existing electrostatic precipitators, we have observed particulate continuing combustion in the electrostatic precipitator enclosure, even after being carried through the wet conditioning system. You can imagine what this condition will do to a baghouse. The author has witnessed a brand new, large industrial baghouse on fire only two days after its startup.
Very fine particulate material is prone to "blind" the fabric of a baghouse. The nature of incineration particulate ("fly ash") is such that more than 50% of it is sub-micron in size. Blinded bags increase head loss across the filter (requiring the use of even more energy) and must be replaced.
Baghouses tend to "crust over." During the operation of the County incinerator precipitators, staff occasionally needed to wire brush crusty fly ash off the collector plates. Operators of baghouses acknowledge that they have similar crusty buildups on occasion on the bags. Mechanical removal from the bags (wire brushing?) is sometimes possible, but that causes damage to the filter bags themselves.
Baghouse performance deteriorates over a period of 15 months after new bag installation. Electrostatic precipitator performance, on the other hand, stays uniform for years after the precipitator has been started up without any deterioration. This has been observed in testing at a number of installations. Bags rubbing against their wire frame holders in the course of being cleaned can develop small holes.
Flue gases then zip through those holes without any cleaning action. Since the rest of the unit is slowly blinding and crusting over, the differential pressure may offset one another and sensing instruments on each side of the filter may not detect a net change in the back pressure. Thus, the baghouse may be continued in service long after its cleaning ability has substantially deteriorated.
Baghouse maintenance can be a threat to plant staff health. When baghouse bags need changing, plant staff is intimately exposed to the very materials we are trying to keep away from the ambient environment. Frequent changes requires frequent contact. While we can provide safety training and special clothing for careful bag changing, the fact remains that the contacts between staff and fly ash will be more frequent with a baghouse than with a precipitator.
All in all, County staff was well pleased with saving over $70,000 of electricity each year by using precipitators (this would be approximately $150,000 in 2007 energy costs). Mother Nature cannot long afford to squander energy on fruitless endeavors.
The nation needs to reassess its standards, base the regulations on human health requirements, and encourage engineers to select the most efficient control tools available and necessary to achieve healthful results.