Controlling Stack Emissions in the Wood Products
Electrostatic precipitation has been a reliable technology since the early
1900's. Originally developed to abate serious smoke nuisances, the manufacturers
of zinc, copper, and lead quickly found electric gas cleaning a cost efficient
way to recover valuable product carried out of the stacks from furnace operations.
Today electrostatic precipitators are found mainly on large power plants,
cement plants, incinerators, and various boiler application.
In the wood products industry, the dry electrostatic precipitator preceded
by multi-clones is now normally considered the best available control technology
for wood fired boiler emissions.
Wet electrostatic precipitators have found renewed interest from OSB, particle
board, and plywood veneer manufactures for controlling dryer exhaust.
DESIGN AND OPERATION
A precipitator is a relatively simple device. The main
components are as follows:
- An insulated and lagged shell
- Collection plates or tubes
- Discharge electrodes
- Collection Plate Rappers/Electrode Vibrators
Dust laden gases are pushed or pulled through the box with the assistance
of a fan. The air flow is channeled into lanes formed by the collection plates
or tubes. Discharge electrodes are centered between each collection plate/tube
to provide a negative charge to the surrounding dust particles. The collection
plates/tubes are positively grounded and act as a magnet for the negatively
charged dust particles. The collected dust is transported down the collection
plates and electrode with the assistance of a rapper or vibrator system into
the collection hopper.
An electrostatic precipitator can consistently provide 99%+ removal reducing
emissions levels to 0.002 - 0.015 grains per dry standard cubic foot of exhaust
Precipitators are designed to handle gas flow form 10,000 cfm to 300,000
cfm and can operate at temperatures as high as 750 degrees F. Normal gas flow
through a precipitator is 2-5 feet per second, consequently, the pressure
drip is only 0.5" wc. When replacing existing scrubber systems the fan
horsepower to operated the precipitator can usually be decreased to one fourth
of the scrubber system, which may have a pressure drop as high as 20"
in order to deliver comparable efficiencies.
Insulated Steel Housing: The development of modular, factory built
units has significantly lowered the installed cost of precipitators. Dry precipitators
are normally fabricated from 3/16" thick steel plate, insulated and lagged
with aluminum. The electrodes are made of steel tubing and the collection
plates are made of rolled steel. Since no moving parts are in contact with
the gas stream, the housing can last 15-20 years. Wet precipitators are traditionally
fabricated of stainless steel for corrosion resistance.
Discharge Electrodes: The advancement of the discharge electrode
has solved many of the maintenance complaints associated with precipitators
in the past. Originally, the dust particles were charged by a series of small
diameter wires which were suspended from a ceiling rack and weighted at the
bottom. This maze of electrodes was subject to electric erosion. Replacement
was slow, cumbersome and required the unit to be off-line.
Today, discharge electrodes are rigid and constructed of 2" steel tubing,
securely fastened to the upper rack and guided at the bottom. Ten years of
continuous service is the expected norm.
Rappers and Vibrators: Heavy duty rappers are used by PPC in the
wood industry. They consist of 30 pound piston hammers designed to rap small
sections of collection plates. A timer periodically releases the rapper to
transfer the dust on the collection plates to the hopper.
Electric vibrators are placed on the electrode rack to transfer any collected
dust to the hopper and are operated by a timer.
Power: A typical precipitator will take 480 volt AC and with he assistance
of transformer/rectifier converts the power to operated the discharge electrode's
at 55-70 kv DC. This leads most inquirers to conclude they are huge electricity
consumers. In reality, the electrostatic precipitator is the lower power consumer
available to accomplish the job. Electrostatic forces are applied directly
to the particles and not the entire gas stream. Combining this feature with
the low pressure drop (0.5" wc) across the system results in power requirements
approximately 50% of comparable wet systems and 25% of equivalent bag filter
Power Consumption Chart
||Hourly Operating Cost
A dry electrostatic precipitator operates at temperatures above 700 F and
maintains the fly ash in its natural dry condition simplifying material handling.
Wet Scrubbers: A scrubber saturates the gas stream in order to remove
the dry fly ash. The wet ash has to separate from the water in settling ponds
or through a de-sludging unit which increases the annual labor and operating
It is not uncommon to see 150 to 300 hp fans on scrubber installations in
the wood industry. The energy necessary to separate the particulate from the
gas stream can require 15" - 20" wc of pressure drop through a typical
venturi. These are huge and wasteful power consumers, increasing the plant's
overall operating cost.
Wet scrubbers have to contend with freezing in northern climates and equipment
corrosion. Finally, regulatory authorities are moving towards zero water discharge
from operating plants.
Baghouses: The high temperatures and periodic cinders from the plant
boiler can cause fire problems with baghouses. Periodic bag replacement is
a definite operating cast consideration which affects the overall cost of
a baghouse installation.
Dryers: OSHA and state regulatory authorities are beginning to look
at dryer emissions. There is an inside emission problem because the fly ash
accumulates on top of the veneer. There is also an external emission problem
from the dryer vent stack.
A dry precipitator can be installed after the burner blend chamber to remove
the fly ash. The recirculation of the dryer air stream is then cleansed of
incoming ash. This arrangement produces cleaner veneer and eliminates dryer
Wet Units: Wet electrostatic precipitators have seen renewed interest
in the wood products industry as OSB, particle board, and veneer plants are
required to control VOC emissions form their dryer exhaust. The wet electrostatic
precipitator serves to remove particulate emissions and "condensable"
VOC's (pinenes, terpenes, cymene, toluene, etc.) from their dryer exhaust.
Depending upon the exhaust temperature and partial pressure considerations
of each component, the wet ESP can reduce the VOC emission by 20 - 40% while
solving all opacity problems. If additional VOC removal efficiencies are required,
the wet precipitator may be a necessary pre-treatment item for incinerators
or biofiltration systems which do not handle particulate concentrations very
The purpose for the "wet" electrostatic system is to mainly prevent
fires. The particulate carry over form an OSB dryer can represent large fiber
stands which can be ignited by the sparking inside a dry precipitator setting
off a hopper fire. The wet precipitators apply a water quench to the gas stream
before entering the collecting tubes. The collection tubes are also continuously
sprayed with water in order to wash the particulate off the tubes thereby
eliminating any chance of combustion.
Wet electrostatic precipitators are excellent particulate removal devices
for use ahead of RCO's, TRO's, and Biofilters. These VOC removal devices are
sensitive to particulate in the flue gas stream. Since wet electrostatic precipitators
can provide emission levels as low as 0.003 gr/dscf, they prevent fouling
of the VOC removal devices.
Thermal Oil Heater: Electrostatic precipitators are fast becoming
the device of choice for controlling emissions from thermal oil heaters. Several
of the recent Canadian OSB plants have selected PPC for the electrostatic
precipitator to control the particulate from combustion of wood. The electrostatic
precipitator's low power consumption combined with low maintenance make it
an excellent choice for plants with a limited number of operating personnel.
The boiler PLC can be configured with eh 10-12 channels required to completely
automate the operation of the electrostatic precipitator. The PLC provides
the plant with a written chart for submittal to the regulatory agencies that
require continuous emissions monitoring.
Electric Gas Cleaning is an old proven technology for removing particulate
emissions from exhaust stacks. Electrostatic precipitators have proven to
be reliable workhorses in the wood products industry as more companies focus
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