Air Pollution Control Innovations

Venturi scrubbers are commonly used in pollution control systems as particulate control devices. Particles are collected primarily according to their aerodynamic size through inertial mechanisms.  Good particle collection is achieved by maintaining a high differential velocity between particles in the gas stream and water droplets in the Venturi throat.  A high differential velocity is created by reducing the cross sectional area in the Venturi throat and thereby creating a pressure drop.  The reduction in area accelerates the particles relative to water that is injected into the throat perpendicular to the gas flow.  As particles collide with the water droplets they become entrained. The particle laden droplets are then collected in the Venturi sump and are purged in a blowdown stream.  

A key to Venturi performance is therefore maintaining a constant pressure drop across the throat. This is relatively straightforward if you have a process with a constant flow rate. However, many processes have variable flow rates.  An incinerator or kiln comes to mind where there are changing flow rates throughout the process cycle. In many cases the variation may be as high as 4:1 or 6:1 from the maximum to minimum flow rate.  This ratio is often called the turn-down ratio.  Three methods of maintaining a constant pressure drop for variable flow conditions are discussed below:

• Reflux Damper
• Variable Throat
• Manual Inserts

Reflux Damper - A reflux damper is often used on Venturi scrubber systems for solid waste combustors.  A solid waste combustor can be an incinerator, kiln, gasifier, or plasma reactor.  The Venturi is designed for the maximum flow condition.  When the gas flow decreases, ambient air is recycled to the Venturi inlet through a pneumatically actuated damper to make up the difference.  The ambient air is recycled from the downstream side (clean side) of an induced draft fan which is used to pull the gas through the system.  The damper modulates to maintain the combustor draft pressure based on a 4-20 mA control signal from a draft sensor mounted in the combustor chamber.

The flow rate is equal to the design gas velocity times the cross sectional area.  As the flow rate decreases the cross sectional area must be reduced to maintain the design gas velocity.  For this reason a reflux damper is particularly recommended for smaller gas flows because it is easier to modulate than for a variable throat. This is because the gas velocity of a reflux damper is about 1/6^th the gas velocity of a Venturi throat.  A reflux damper is therefore less sensitive to flow rate variation.  This makes it easier to tune and maintain the control loop.  Another advantage of a reflux damper is the recycled gas is clean because it has already passed through the Venturi. Therefore there is no potential for fouling the damper blade from particulate in the gas.

The adjacent photo shows a 400 lb/hr medical waste incinerator scrubber with a Venturi inlet flow rate of 1,200 scfm.  The reflux damper can be seen as the white horizontal duct from the ID fan outlet to the Venturi inlet on the right hand side of the rectangular condenser/absorber box.

Variable Throat - A variable throat Venturi is another common method of maintaining a constant pressure drop across a Venturi scrubber system.  A valve is integrated into the Venturi throat.  At maximum flow, the valve is fully open. As the flow decreases, the valve closes to reduce the cross sectional area accordingly.  The variable throat can be a damper blade, butterfly valve, plumb bob, or pinch valve.  As discussed above, variable throats are generally more suitable for larger gas flow processes.  Consideration should be given to the potential for fouling from particulate build up on the valve.  Particulate can accumulate and get stuck behind a butterfly valve, damper blade or on the shaft of a plumb bob. This can impede the ability to adjust or modulate the throat.  The potential for this type of fouling may depend on the nature of the particulate. Envitech often uses variable throat Venturi's on industrial dryer applications.  Variable throat Venturi's were discussed in a previous blog post, Venturi Scrubber: Adjustable Throats. 

Manual Inserts - A third approach for maintaining a constant pressure drop is the use of manual inserts.  This approach might be taken for a process that has distinct flow rates for long periods of time. It might also be used in situation where the design conditions are uncertain, say for a pilot or demonstration plant.   The use of manual inserts provides a way of designing flexibility into the equipment.

Please click on the icon below to view a video of a variable throat Venturi.

In my previous blog post I outlined new rules that were promulgated on September 15^th, 2009 for the hospital, medical, and infectious waste incinerator (HMIWI) maximum achievable control technology (MACT) standard.  Wet scrubbers are used on many of the existing medical and hazardous waste incinerators to meet this MACT standard.  The unfortunate news is that new control strategies are required to meet the more stringent standards.

The new emission limits present challenges for both existing and new systems.  These challenges  relate primarily to the following pollutants.

• Particulate
• Lead, Pd
• Cadmium, Cd

The particulate limits for new systems are reduced from 0.015 to 0.008 gr/dscf.  This is a 50% reduction.  The lead (Pd) emission limits are reduced from 1.2 to 0.036 mg/dscm for existing systems and from 0.07 to 0.00069 mg/dscm for new systems.  The cadmium (Cd) limits are reduced from 0.16 to 0.0092 mg/dscm for existing systems and from 0.04 to 0.00013 mg/dscm for new systems.  The new Pd and Cd limits for both existing and new systems are nearly a 100% reduction.

There are 4 keys to meeting these more stringent standards with wet scrubber systems.

• Add-on particulate polishing package
• Sub-cooling
• Venturi Scrubber
• Mist elimination

Add-on particulate polishing package

The new emission limits will exceed the design capability of most of the existing wet scrubber systems today.  This will require an add-on polishing control to meet the more stringent standards.  Envitech has had success achieving higher removal efficiencies by integrating an add-on particulate polishing package (PPP) into incinerator wet scrubber systems.  The PPP is comprised of a skid mounted package that provides slight reheat of the gas temperature to slightly above saturation in combination with a filter system.  The reheat eliminates the potential for condensation build-up in the filters.

This strategy has been used for both commercial and industrial waste incinerator scrubbers (CISWI) and low level radioactive waste incinerator scrubbers. Removal efficiencies of > 99.8% was achieved for both Pd and Cd at the outlet of the Venturi scrubber which already has a very low particulate load < 0.015 gr/dscf.  This has proven to be a cost effective strategy to meet the new standards.

Sub-Cooling - Sub-cooling the gas in a medical or hazardous waste incinerator scrubber provides several advantages.  It makes use of condensation effects to enhance particulate control in a downstream Venturi scrubber. The water vapor in the gas condenses onto the particulate and grows them in size.  A particulate that is 0.3 microns will grow to about 0.7 microns after condensing the water vapor. This makes it easier to collect in the downstream Venturi.  A second advantage of sub-cooling is condensing metals (i.e. Pd and Cd) as much as possible from a gas phase to a particulate. This allows them to be collected downstream in the scrubber.  The final advantage is steam plume suppression. Removing the water vapor eliminates a steam plume under most meteorological conditions.  This reduces the visibility of the system in the surrounding community.

Venturi Scrubber - The particulate capture efficiency of a wet scrubber system is determined by the pressure drop across the Venturi scrubber. Higher removal requires higher pressure drop.  Sub-cooling discussed above enhances the Venturi performance by growing the size of particulate.  Often times this reduces the power consumption by half for most medical and hazardous waste incinerator wet scrubbers.  The new HMIWI standards, however, exceed the practical capability of a Venturi scrubber. This can be overcome with an add-on particulate polishing package discussed previously.  It is recommended to optimize the Venturi scrubber performance to minimize the load on the PPP. This reduces the annual operating expense by increasing the life of the filter elements.

Entrainment Separator - An entrainment separator or mist eliminator is used after the Venturi scrubber to knock out water droplets in the gas stream.   Any water droplets that escape the mist eliminator will contain pollutants which can cause a stack test failure.  A horizontal, chevron style mist eliminator is commonly used in incinerator wet scrubber systems.   Effective mist elimination is important for the add-on particulate polishing package discussed previously.  Water droplets can lead to fouling of the add-on control.

As facilities get their arms around the new rules for the HMIWI MACT, they will need to consider all of the above items for complying with the new standard using a wet scrubber system.

Please read our paper on meeting the new HMIWI MACT standards by clicking the link below.

Venturi scrubber performance hinges on four key design factors: pressure drop, particle size, water flow, and entrainment separation.  When a Venturi scrubber is not performing properly, it is best to review these design factors one-by-one.

Pressure Drop

Perhaps the most important aspect of a Venturi scrubber is its pressure drop.  The pressure drop of the Venturi is directly correlated to the velocity of the gas passing through the throat.  The higher the pressure drop, the faster the gas.  The speed of the gas is important, as the success of a Venturi is due primarily to inertial impaction.  In inertial impaction, a fast moving particle in the gas strikes a relatively slower moving water drop.  The higher the velocity difference, the greater chance that particle is unable to "duck" into a slipstream around the water drop.

When the pressure drop of a Venturi decreases, the performance of the Venturi decreases.  A drop in pressure can occur for a variety of reasons, but by far the most common is a drop in overall air flow.  In a fixed throat system, the drop in air flow may be a result of a new or upset condition.  In a reflux system, it may be to due to a pressure drop increase in downstream equipment, or a failure with a reflux damper.  For a variable throat Venturi, that is designed to handle changes in air flow, a drop in pressure across the Venturi can signal a problem with the Venturi damper.  In any of these cases, the problem must be fixed to regain the Venturi performance.

Particle Size

In a Venturi scrubber, the particle size actually refers to the aerodynamic size of the particle, which is much more influenced by the mass of the particle than by the diameter of the particle.  Again, the reason is that the Venturi scrubber is an inertial impaction device, and thus the mass of the particle directly influences removal.

The best example of the effect of inertial impaction is a car windshield.  Large, heavy particles like rocks and insects slam into a car's windshield at high speed.  Plastic bags, though much larger, contain very little mass, and "slip" over the windshield.  Particles in a Venturi are captured similarly.  A denser particle with the same volume will be captured more efficiently in a Venturi scrubber than a corresponding lighter particle.

When the performance of a Venturi scrubber varies, it is important to look at upstream equipment to ensure that the particles themselves have not changed.  Smaller, lighter particles will reduce performance of the Venturi scrubber, and necessitate either a greater pressure drop or downstream particle removal equipment.

Water Flow

In inertial impaction, the particles must collide with water (or some other liquid medium) to be collected.  If there is not enough water to collect the particles, performance will degrade.

Water flow can be impacted by a host of common issues.  Since the water is pumped, a problem with a pump can lead to performance issue with the Venturi scrubber.  Plugging of the nozzles or of valves can also occur.  Both of these issues can be solved by performing regular preventive maintenance on the pump and the nozzles.

Entrainment Separator

The final step in a Venturi scrubber is to remove the particle-laden water drops from the gas stream.  Whichever way is selected to remove the water drops, it is important that it does perform well, or the drops will continue into downstream equipment (or even exit the stack).

Waveform entrainment separators, the predominant water separation method, work by causing a change in flow, forcing the water drops to hit the entrainment separator while letting the gas pass through.  The waveforms work as long as the gas travels through the waveforms at the right velocity.  At too high a velocity, the water drops re-entrain, and pass through the entrainment separator.  Particles can also stick to the entrainment separator, changing the waveform shape and reducing the area (thus increasing the velocity).  Waveforms must be designed properly upfront to ensure that flow is uniform through the waveforms, and waveforms must be regularly cleaned to ensure success.

To read more about a specific Venturi scrubber application, please download the white paper below.

Back in July I wrote a blog post for gasification syngas cleaning where I discussed two general approaches, 1) thermal tar destruction, and 2) tar removal scrubbers.  Both approaches require particulate removal. This blog post discusses several design considerations related to particulate control for syngas wet scrubber systems, including:

1. Performance
2. Capital Cost
3. Operating Cost
4. Safety

These considerations will be discussed in the context of two wet scrubber approaches for particulate control:

1. Venturi Scrubber
2. Wet Electrostatic Precipitator

 Performance - The distinguishing feature between a Venturi scrubberand a wet electrostatic precipitator (WESP) is the removal efficiency for sub-micron particulate.  This is shown in the above figure which compares the particle removal efficiency for a wet electrostatic precipitator (WESP) and a 50 inch water column (W.C.) pressure drop Venturi scrubber.   The figure illustrates that both the WESP and Venturi are highly efficient for removing particles greater than 1 micron.  The removal efficiency of a Venturi, however, begins to degrade for particles smaller than 1 micron.  The Venturi performance can be enhanced by sub-cooling the gas and taking advantage of condensation effects to grow the size of the particulate.  The effects of sub-cooling to improve Venturi performance is discussed in greater detail in the Envitech paper, "Wet Scrubbing Technology for controlling biomass gasification emissions" presented at the 2008 Joint Conference: International Thermal Treatment Technologies (IT3) & Hazardous Waste Combustors (HWC). 

In general, WESP's are used in applications where the sub-micron particulate concentration exceeds the capability of a Venturi to meet the performance requirements. It is therefore important to understand the following:

• Inlet particulate & tar concentration
• Inlet particle size distribution (PSD)
• Particulate tolerance of the downstream internal combustion engine (ICE)

A Venturi scrubber will give syngas cleaning performance similar to a WESP. The removal efficiency for particles greater than 1 um diameter will be equal to or greater than a WESP.  For particles smaller than 1 um diameter, a Venturi scrubber will be less efficient that a WESP.  However, many ICE engines will most likely tolerate these particles.  Understanding the tolerance of the engine is therefore a key aspect of deciding which approach is best suited for your application.

Capital Cost - It is broadly understood that a Venturi scrubber is much lower capital cost than a wet electrostatic precipitator.  Under most process conditions this cost difference can be as much as 3 to 4 times.  The trade-off for a lower capital cost Venturi scrubber is higher operating cost to provide the pressure drop. 

The Venturi scrubber capital cost is determined predominately by the size of the gas flow.  The WESP capital cost, however, is determined by both the size of the gas flow and the desired removal efficiency.  The desired removal efficiency can dramatically affect the size and cost of the system. The higher the removal efficiency, the higher the collection area, and consequently, the greater the number of collection tubes required.   The cost of a WESP is approximately exponentially related to the required removal efficiency.  It is important to define the performance requirements before budgeting for a WESP.

In addition to metal fabrication, there are other items contributing to the higher capital cost of a WESP, including the T/R set to provide a high voltage, electronics for a more sophisticated control system, and safety interlock system.

Operating Cost - Although a wet electrostatic precipitator is higher capital than a Venturi scrubber, part of that cost is offset by lower operating cost. The pressure drop of a WESP is in the range of a couple of inches W.C. compared to 30 to 50 inches W.C. for a Venturi scrubber.  The electricity cost for the fan horse power requirements is therefore considerably lower for a WESP than for a Venturi.  There are other WESP operating costs that need to be accounted for including the electricity for the T/R sets and for the heater and blowers for the insulator compartments.

Safety - The last design consideration discussed here for a syngas cleaning system is safety.  A key aspect for a syngas cleaning system is that it contains a combustible gas.  This carries a greater risk of fire than for other types of scrubber system.  If the system is located in a confined space, it is often required for instrumentation and motors to meet division I, class II (explosion proof) requirements.  A WESP can operate in sparking mode which can be an ignition source for the gas.  Care must be taken to ensure the WESP operates in a safe condition at all times.  A WESP has additional safety interlock requirements because it operates at a high voltage.  For these reasons, a WESP it is more costly to ensure safety in a WESP than a Venturi scrubber.

Summary

• A Venturi scrubber is lower capital cost than a WESP and in most cases is preferred if it can meet the performance requirements.
• A WESP is generally used in cases where the concentration of sub-micron particulate exceeds the capability of a Venturi scrubber to meet the peformance limits.
• Although a higher capital cost, a WESP has the advantage of lower operating cost. It will also achieve greater overall removal efficiency because it is more efficient for particles smaller than 1 micron.
• Because syngas is a combustible gas, there are safety considerations for both a Venturi scrubber and a WESP. Because a WESP uses a high voltage and can act as an ignition source, the cost to mitigate safety risks is generally considered to be higher than for a WESP.

 To learn more, please download our presentation on tar removal.

Envitech is often asked to make recommendations for particulate removal on a wide range of industrial applications.  This might entail deciding if a Venturi scrubber can do the job or if a wet electrostatic precipitator (WESP) is required.  Envitech uses proprietary modeling that accurately predicts Venturi performance under various conditions of pressure drop and sub-cooling.  Because Venturi performance is highly dependent on particulate size for particles less than 1 micron diameter, an essential piece of data for making a performance guarantee is the particle size distribution (PSD) for the particles in the inlet gas.

There are several test methods for determining the particle size distribution, however the test method must determine the aerodynamic particle size to adequately predict the Venturi performance.  The most common method for determining the aerodynamic particle size is to use a cascade impactor.  This can be a challenging test because the inlet gas stream may be at high temperature and contain a high particulate loading and moisture content.  It is important to select a testing company that has experience with this type of environment.  Below is a references from the California Air Resource Board (CARB) that describe acceptable test methods for this purpose.

• CARB Method 501 - Determination of Size of Distribution of Particulate Matter Emissions from Stationary Sources

A method that some companies use to to determine the PSD is to collect particles on a filter and analyze the dust with a particle size analyzer.  This test method will give the physical particle size.  However it can give an inaccurate estimate of the aerodynamic particle size.

The problems commonly associated with this method are outlined below:

• You may not get a representative sample. Different size particles end up in different layers on the filter. Agglomerates of particles can come apart. Individual particles can agglomerate and show up as one particle.
• This is a measure of the physical size and not the aerodynamic size. Venturi scrubbers primarily collect particles according to their aerodynamic size through inertial mechanisms. The aerodynamic size is a function of the density of the particle and the size of the particle. The greater the density, the greater the aerodynamic size. In some cases, particles may not be solid particles of inorganic material, but may be hollow glassy spheres, which will make them act more like soap bubbles rather than hard balls. Some combustion processes at extremely high temperatures can lead to hollow spheres.
• The size of the particle affects the aerodynamic size because, the smaller the particle, the easier it is to slip between gas molecules. As the particle size approaches the mean free path of the gas molecules this becomes a significant component of the aerodynamic size.
• To use data from a particle size analyzer, it is necessary to generate a log normal PSD curve of the physical size. This curve then needs to be corrected to provide an aerodynamic PSD curve. However, this requires making two assumptions: 1) particle density, and 2) particle shape (usually assumed to be a solid spherical shape). These assumptions may or not be valid and can lead to inaccurate Venturi performance predictions.

Before settling on a design path for particulate removal, Envitech recommends testing the process to generate good particle size distribution data.  This will help ensure meeting the performance requirements in the most economical way possible.

To read more about Envitech's Venturi scrubbers, download the free case study on a Venturi scrubber treating the exhaust of a dryer.

photo credit: azredheadedbrat

Concern for global climate change coupled with high oil prices has generated new interest in renewable energy sources.  One of these sources is waste to energy using gasification.  Gasification is a thermal destruction process which produces synthetic gas (syngas) as an end-result.  In one form, the syngas is then used as fuel in an internal combustion engine (ICE) to drive a generator, producing electricity.  Waste heat is recovered from the system to improve the overall plant efficiency.

During gasification, various pollutants may be produced depending on the make-up of the waste feedstock. The feedstock can vary by plant from biomass, municipal solid waste (MSW), or even hazardous waste.  The pollutants involved with these processes include sub-micron particulate matter, tars, ammonia, metals, dioxins and furans, and acid gases.  One of the primary challenges is cleaning the pollutants in the syngas to a level that is tolerated by the ICE.  There are many innovative companies working to commercialize waste-to-energy production using gasification.  Each application is unique and depends on the type of gasification process and feedstock material.  We've seen two general approaches regarding syngas cleaning:

1. Thermal Tar Destruction  
2. Tar Removal Scrubber

Thermal Tar Destruction - In this approach, the syngas passes out of the gasifier and through thermal process that destroys the tars at a high temperature.   This greatly simplifies the gas clean-up as it eliminates the need for a tar removal clean-up system.  The trade-off, however, is a lower energy content of the syngas.  The gas clean-up can be achieved with proven, reliable scrubbing technologies, similar to systems that have been used in conventional incineration scrubbing systems.

Tar Removal Scrubber - The tar removal scrubber approach has a lower outlet temperature and a higher energy content, but it contains tars that are more difficult to remove.  The main challenge of tar removal relates to the fouling that can occur in the initial stages of condensing and collecting the tars.  The source of the challenge is the formation of "tar balls" which are long-chained hydrocarbons that have a tendency to agglomerate and stick together, fouling equipment.  Tar removal processes also produce liquid wastes with higher organic compound concentrations, which increases the complexity of water treatment.

Although more complex, these problems can be overcome.  Envitech has developed a second generation syngas tar removal system that uses a clean liquid stream for condensing and collecting tars. The system utilizes an arrangement of conventional process equipment for solids/oil water separation that results in a clean discharge stream and return liquid to the scrubber. By returning a clean liquid stream to the cooling circuit and condensing section, problems associated with tar ball fouling is eliminated. In addition, the process mitigates the impact of organics in the liquid discharge.

In a future blog post I will discuss considerations involved with selecting a wet electrostatic precipitator versus a Venturi scrubber for particulate control for syngas cleaning systems.

Click on the icon below to download a white paper written about gasification emissions using wet scrubber technologies.

Photo - PRM Energy Gasifier

Venturi scrubbers are used for the removal of fine particulate.  Gas is accelerated at a high speed through a Venturi throat.  Water is injected perpendicular to the gas flow.  The large water drops injected into the gas stream collide with the fine particulate through a process called impaction. 

The efficiency of this process is dependent primarily on the size and velocity of the particulate.  Superfine, sub-micron particulate are able to follow a stream line around the water drops and are not collected.  Micron-size and larger particulate are not able to slip around the water drops fast enough due to inertial effects.  The exact "cut" of the Venturi depends on the velocity; smaller particles are captured at higher gas velocities.  Venturi scrubbers are excellent particulate control devices for particulate at or above a micron in size.

The performance of a Venturi though is dependent on maintaining the gas velocity at design conditions.  The above video details a Venturi scrubber with an adjustable throat.  In most industrial applications, the gas flow rate varies and with a fixed opening, the velocity of the gas through the Venturi would vary as well, impacting performance.  An adjustable throat offers one method for ensuring the Venturi works over a wide range of operating conditions.

The adjustable throat is a damper blade controlled by a positioner.  The blade is positioned to maintain a constant pressure drop across the Venturi.  The pressure drop across the Venturi is directly related to the gas velocity.  Essentially, the damper blade maintains the gas velocity in the Venturi even at much lower gas flow rates.  As stated above, gas velocity is critical to particulate removal in a Venturi.  An adjustable throat ensures that the gas velocity remains constant, so that particulate removal is unaffected by operation in a "real" environment.

I recently received a call from a small pellet mill operator inquiring about a visible stack emission problem at his plant.  The pellet mill is operating a rotary dryer and is small enough that they are not required to have pollution controls on the process.   However, they have started to receive complaints from neighbors about a visible blue haze being emitted from their stack.

The blue haze is a result of condensable organic compounds in the gas stream which condense in the duct and form very small, submicron particulate. This particulate is an aerosol mist. When the mist in the exhaust passes out the top of the stack, the light reflects through the mist and a blue haze appears.

This is a particulate control problem.  In general, there are three options to consider for particulate control.

1. Bag-house
2. Venturi scrubber
3. Electrostatic precipitator

A bag-house is a fabric filter that is generally used for higher temperature applications.  It would not be a suitable solution for this process because of the moisture content and the temperature of the exhaust gas.  The bags would plug up and have maintenance issues. The other two options to consider are discussed below:

Venturi Scrubber - A Venturi scrubber is a lower cost option, however, it also is not suitable for this application. The efficiency of a Venturi is dependent upon the particle size distribution (PSD) and drops off rapidly for particles less than 1 micron. Because this gas stream has a large concentration of sub-micron particulate (as evidenced by the blue haze), a Venturi will do very little to resolve the problem.

Wet Electrostatic Precipitator (WESP) - A Wet Electrostatic Precipitator is the most suitable technology for this problem.  A WESP has a higher capital cost than a Venturi system, but the removal efficiency is independent of the particle size.  It is most often used in situations where there is a high concentration of sub-micron particulates and the particulate are beyond the removal capability of a Venturi.

Although a WESP will resolve the blue haze caused by the aerosol mist, it will not do anything to remove gas phase volatile organic compounds (VOC).  Larger pellet mill operators that have permit limits for VOCs will likely need a WESP for particulate control, followed by a thermal oxidizer for VOC control.

To learn more about Envitech's wet electrostatic precipitators, download our brochure.

Wet electrostatic precipitators (WESP) are the preferred equipment for the removal of sub-micron particulate.  Sub-micron particulate control is a subset of particulate control, typically used for TSCA (Toxic Substances Control Act) particulate or where downstream equipment might be damaged by sub-micron particulate.

What is inside this efficient particulate removal equipment?  We have put together a short video that brings you inside the WESP and provides a high level overview of the internals of a wet electrostatic precipitator.

High voltage is brought into the insulator compartments, through the high voltage grid, and on to the electrodes.  The collector assemblies are grounded, leading to a high voltage differential between the electrodes and the collector.  Emitter discs on the electrodes promote electron migration from the electrode to the collector plate; particles are charged by colliding with the electrons.  The negative charge on the particle in turn attracts the particle to the collector.

Wet electrostatic precipitators are very efficient at the removal of sub-micron particulate, with single pass systems able to remove over 90% of sub-micron particulate.  We recommend electrostatic precipitators for most applications where efficient removal of sub-micron particulate is essential.