Air Pollution Control Innovations

There are several industrial processes that face the challenge of meeting increasingly aggressive metals emission standards.  In many cases these standards exceed the capability of existing air pollution control equipment which can include bag-houses and packed bed absorbers. Some of these processes include:

• Secondary lead smelters
• Lead refining
• Refinery sludge incinerators
• Geothermal energy plants

Some of the metals of concern can include mercury, arsenic, lead, cadmium, nickel and others, depending on the process.  To achieve more stringent metals emission standards, three features should be designed into the air pollution control system. 

• Removal of the bulk particulate load
• Sub-cooling the gas
• Wet Electrostatic Precipitator (WESP) for polishing

Removal of the bulk particulate load - Removal of the bulk particulate load may be required if there is a high particulate concentration from the upstream process.  This will be the case for secondary lead smelters, lead refining, and refinery sludge incinerators.   Geothermal energy plants will not have this requirement.  A bag-house will be used for secondary lead smelters and lead refining.  A wet Venturi scrubber system can be used for refinery sludge incinerators   Removal of the bulk particulate minimizes space-charge effects inside the WESP. Space-charge effects occur when particles interact and repel each other. This reduces WESP performance because it interferes with the migration of charged particle to the tube wall for collection.

Sub-cooling the gas - Some of the volatile metals may be in the vapor phase when they pass through a bag-house at higher temperature.  In this case they will pass right through the bag-house and will not be collected.   Sub-cooling the gas is done after the bag-house and uses a condenser/absorber to cool the gas below the saturation temperature.  Sub-cooling is beneficial because it condenses as much of the volatile metals as possible so they can later be removed as particulate.  In the case where a Venturi is used to remove bulk particulate, like for a refinery sludge incinerator, it has the added benefit of condensing water onto the particulate and condensed metals.  This increases their diameter making them easier to remove in a Venturi scrubber.  Sub-cooling also reduces the gas volume, which helps to reduce the size and cost of a downstream WESP.

Wet Electrostatic Precipitator (WESP) for polishing - A wet electrostatic precipitator is used as a final particulate polishing stage.  The performance is relatively independent of the particle size so it is highly effective at sub-micron particulate control.  In some cases, a condenser/absorber (C/A) for sub-cooling can be integrated into the conditioning section of an upflow WESP.  This was successfully done at a secondary lead smelter downstream of bag-house.  The C/A was also used to neutralize SO2 in the gas stream. The integrated C/A and WESP achieved > 98% removal of arsenic and > 92% removal of lead and other condensed metals after the bag-house.  This substantially reduced the plants cancer risk index and helped to meet more stringent fence line lead emission standards. 

To download a free white paper on wet electrostatic precipitator for a secondary lead smelter, click the link below.

To view a free wet electrostatic precipitator video, click on the link below. 

Geothermal by Louis Falcon

Refinery by Szeke

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.

On September 15th, 2009, proposed revisions to the New Source Performance Standards (NSPS) and Emission guidelines (EGs) for the HMIWI standards became final. These regulations, originally promulgated in 1997, were established under Section 129 of the Clean air Act (CAA), and serve as the maximum achievable control technology (MACT) standards for hospital, medical, and infectious waste incinerators.

The proposed revisions included both a five year review and a response to a court-ordered Remand.  Many industries have been following these new revisions closely, including the American Forest & Paper Association (AF&PA), the Portland Cement Association (PCA), the Council of Industrial Boiler Owners (CIBO), the National Brick Research Center (NBRC), and the National Lime Association (NLA), among others.  The methodology used by the EPA to formulate these revisions may eventually be applied to the industries of these groups. 

There are a number of objections to the new rules which are viewed by some in industry as unachievable.  One of these objections relates to the establishment of floors for the best-performing units for each of the regulated pollutants individually. This has been considered by some as a MACT-on-MACT approach which may result in the need for a combination of control technologies. 

Below is a summary comparison of the previous emission limits (1997) and the new emission limits (2009) in response to the Remand for new and existing HMIWI units for large, medium and small incinerators.  Many hazardous and medical waste incinerator scrubbers meet the previous emission limits (1997) with a Venturi scrubber system.  However, the new standards for both existing and new HMIWI units are significantly more stringent with respect to particulate, Pd, and Cd.  The new standards, would be met with the same basic technology, however, add-on controls would be needed as a polishing step in most circumstances.  In a future blog post, I'll discuss add-on control strategies that can be used to meet these new requirements.

LARGE INCINERATORS, > 500 LB/HR

MEDIUM INCINERATORS > 200 TO < 500 LB/HR

SMALL INCINERATORS, < 200 LB/HR

All emission limits are measured at 7% oxygen.

For more information about this rule, please download our white paper via the link 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

Back in June I mentioned that I was getting ready to head to the 2009 International Fuel Ethanol Workshop and Expo, which was held June 15-18 in Denver, CO.  I had the opportunity to present a new ethanol scrubber design during the emissions abatement optimization session in the energy & environment track.  My presentation dove- tailed well with other topics of the co-presenters, given below.  

Track 2: Energy & Environment
Emissions Abatement Optimization

• Moderator: Monty McCoy, Technical Manager, US Water Services
• New Ethanol Scrubber Reduces Plant Capital and Operating Costs
  Andrew Bartocci, National Sales Director, Envitech Inc.
• Fail-Safe Scrubber Emissions Compliance for Ethanol Biorefineries
  Monty McCoy, Technical Manager, US Water Services; and Bob Elliott, Environmental Field Manager, American Engineering Testing Inc
• VOC, CO, and NOx Abatement and Optimization with RTO's
  Andy Rodger, Engineer, Pro-Environmental Inc.
• Air Emissions Permit Compliance and Pollution Control Device Optimization Using Advanced Measurement Techniques
  Thomas Dunder, GE Energy

I discussed a new ethanol scrubber used to recover ethanol from the fermentation and other vent streams.  These streams contain CO2, ethanol and low concentrations of various volatile organic compounds (VOC's).  Chilled water is commonly used to recover the ethanol, however, many of the VOC's (acetaldehyde, etheyl acetate, acrolein, and acetone) are highly insoluable in water and do not scrub out well.  Post processing is often required to meet emission limits, which adds costs.  I introduced a new, 2-stage scrubber design that uses re-circulated ethanol in the bottom stage and once-through chilled water in the top stage.  Ethanol is an excellent solvent for the residual VOC's and it is readily available at ethanol plants.  This approach eliminates the need for post processing and significantly reduces the plants capital and operating costs. 

The presentations by Monty McCoy of US Water Services, Bob Elliot of American Engineering Testing, Inc., and Thomas Dunder of GE Energy showed test data from various ethanol scrubbers which illustrate how water flow rate, water temperature, and  bisulfate injection rates impacts scrubber performance.  The data also showed how the emission rate changes throughout the fermentation process.

Other issues of ethanol scrubber performance were discussed including channeling and fouled mist eliminators. Channeling is related to non uniform liquid to gas (L/G) ratio. When a limited amount of water is applied to the packing in a tall, narrow tower, uniform distribution of the water is critically important. If there are parts of the packed section where the L/G is lower than the average value, gas passing through those parts of the packing will not be scrubbed as efficiently.  If any water runs down the tower walls, it won't spend as much time in contact with the gas and will absorb less ethanol than water trickling over the packing.  A properly design scrubber should have internals that help maintain uniform distribution.

Fouled mist eliminators can result in excessive pressure drop.  This can be caused by improper selection of the type of mist eliminator or from not having a proper wash system to keep the mist eliminators clean during operation. It's important to not view the fermentation scrubber as a simple can with packing in it.  There are nuances to the scrubber design that enable it to operate at optimum performance with minimum maintenance.

To download the presentation click the icon below.

photo credit: stefanie says

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

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.

I'm making plans to head to Denver for the upcoming 2009 International Fuel Ethanol Workshop and Expo, June 15-18 in Denver, CO.  Envitech has a booth at #1241.  I'll be giving a talk on our new, patent-pending ethanol scrubber which reduces capital and operating costs. 

The new Envitech ethanol scrubber reduces total organic carbon compounds (TOC) in the exhaust gases to acceptable levels without requiring thermal oxidation or chemical transformation.  The scrubber uses ethanol as a light alcohol solvent for the other, non-ethanol organic carbon compounds.  While these compounds have low solubility in water, they have high solubility in ethanol.  Because ethanol is readily available from the plant, it is an ideal solvent for these difficult to remove compounds. 

My talk is at:
1:30 pm to 3:00 pm, Wednesday, June 17th
Track 2: Energy & Environment
Emissions Abatement Optimization
New Ethanol Scrubber Reduces Plant Capital and Operating Costs