Precipitator Operating Principle
Particles suspended in a gas enter the precipitator and pass through ionized zones around the high voltage discharge electrodes. The electrodes, through a corona effect, emit negatively charged ions into the gas and to the grounded collecting plates.
The ionized field around the discharge electrodes changes the particulate causing it to migrate to the positively charged surface of the collecting electrode.
The charged particles agglomerate on the grounded collecting plates and their charge bleeds off. Rappers dislodge the agglomerated particulate, which falls into the collection hoppers for removal.
Electrostatic Precipitator Sizing
Electrostatic precipitators have been used in many industries; several examples are cement, refinery and petrochemical, pulp and paper and power generation. Although the physical operation of a precipitator is simple and essentially the same for each industry, involving particle charging, collection, dislodging and disposal, the sizing of a precipitator is more complex combining both art and science.
The typical equation used in precipitator sizing is the modified Deutsch equation:
Where A is the collecting electrode surface area, V is the gas volume and w is the precipitation rate. The exponent y is a variable based on test data for each specific application.
Factors that influence precipitator sizing are:
- gas volume
- precipitator inlet loading
- precipitator outlet loading
- outlet opacity
- particulate resistivity
- particle size
Resistivity is a term used to describe the resistance of a medium to the flow of an electrical current. By definition, resistivity, which has units of ohm-cm, is the electrical resistance of a dust sample 1 cm2 in cross sectional area and 1 cm thick.
Resistivity levels are generally broken down into three categories:
- low; under 1x108 ohm-cm,
- medium; 1x108 to 2x1011 ohm-cm
- and high; above 2x1011 ohm-cm.
Particles in the medium resistivity range are the most acceptable for electrostatic precipitators. Particles in the low range are easily charged, however upon contact with the collecting electrodes, they rapidly lose their negative charge and are re-entrained back into the gas stream to either escape or to be recharged by the corona field. Particles in the high resistivity category may cause back corona which is a localized discharge at the collecting electrode due to the surface being coated by a layer of non-conductive material.
Resistivity is influenced by flue gas temperature and conditioning agents, such as flue gas moisture and ash chemistry. Conductive chemical species, such as sulfur and sodium will tend to reduce resistivity levels while insulating species, such as SiO2, AL2O3 and Ca will tend to increase resistivity. In those cases where high resistivity is encountered, such as the utility industry when low sulfur coal is being fired, flue gas conditioning with SO3 can reduce resistivity to a more optimum value thus reducing the size of the precipitator needed.
Particle size of the incoming particulate has a dramatic impact on the sizing of an electrostatic precipitator. Applications such as Fluid Catalytic Cracking Units and Recovery Boilers, which have particle resistivity in the medium range, exhibit very fine particulate. The size of the precipitator must be increased in these cases because the fine particulate is easily re-entrained into the gas stream. In the power industry, generally the higher the fuel ash content, the larger the ash particle size.
When are Electrostatic Precipitators not a suitable solution?
As the size of the required precipitator increases, other technologies become more cost effective. For low sulfur utility applications, fabric filters are an attractive alternative. As part of the overall precipitator/fabric filter cost evaluation, operating costs need to be included. Typically, the pressure drop across a flange to flange fabric filter will be in the 6 to 8" w.c. range whereas an electrostatic precipitator will have approximately a 1" w.c. pressure drop. This pressure drop penalty for a fabric filter will be somewhat offset by its lower power consumption which can run as high as 2.0 watts per square foot of collecting electrode area for a precipitator.
Another benefit of a fabric filter is high acid gas, SO2, chlorides, fluorides and Hg removal capability. When operating downstream of a spray dryer absorber, removal efficiencies of 90% or greater can be attained for some species when operating in conjunction with a fabric filter. The fabric filter dust layer acts as a fixed bed where high acid gas removal efficiency can take place. Since most of the particulate is removed from the collecting electrodes of a precipitator during normal operation, acid gas removal capability is much reduced.