Air Pollution Control Innovations

NOx refers to a class of pollutants that is any binary compound of nitrogen and oxygen and is a main contributor to what is commonly referred to as smog.  It is most commonly produced in combustion processes, but can also be generated in non-combustion processes like metal refining, picking baths, or nitric acid manufacturing to name a few.  The following link: EPA NOX technical bulletin, provides a good overview of NOx as a pollutant, but focuses mostly on NOx formed by combustion processes.

This blog post focuses on non-combustion NOx abatement using packed bed scrubbers.  In general a NOx Scrubber System can be comprised of a single packed bed absorber to several other system components, including:

• Quencher
• Stage 1 Packed Bed: Conversion of NO to NO2
• Stage 2 Packed Bed: Absorption and Reduction of NO2
• Stage 3 Packed Bed: H2S Odor Control.

The actual configuration will depend on a combination of factors including:

The temperature of the inlet gas

• The ratio of NO/NO2
• The Outlet NO limit
• Preferences of the plant regarding the risk of H2S odors.

Each component is further described below.

Quencher - The quench stage is used to cool the gas to the saturation in the case the inlet gas is hot. This is done with an evaporative quencher constructed of metal.

Stage 1 Packed Bed: Conversion of NO to NO₂ - This stage is used in the case there is a sufficiently high concentration of NO in the inlet gas stream or sufficiently low outlet concentration limit for NO.  NO is essentially insoluble in water, but it can be quickly oxidized to NO₂ by chlorine dioxide (ClO₂) or ozone (O₃).  This can be done using a packed bed, but the reaction of NO actually occurs in the gas phase. The packed bed functions as a ClO₂ generator and static mixer, or as a static mixer for ozone injected in the form of an aqueous solution. ClO₂ can be generated by the reaction of sodium chlorite (NaClO₂) with a strong acid. Sulfuric acid (H₂SO₄) is commonly used, but if the air being scrubbed contains enough nitric acid fumes, it may not be necessary to add much H₂SO₄.  Most NO_x scrubbers don't have an oxidation stage. When NO_x is generated by the reaction of nitric acid with metals, it usually consists mainly of NO₂. NO is invisible, and it is much less toxic than NO₂. (The Threshold Limit Value for 8-hour workplace exposure to NO is typically 25 ppm_v, vs. 3 ppm_v for NO₂.)

Stage 2 Packed Bed: Absorption and Reduction of NO₂ - NO₂ reacts only slowly with caustic solutions, and when it does, a competing reaction with water converts part of the NO₂ back to NO. So scrubbing NO₂ using NaOH alone is very inefficient.  For efficient removal of NO₂, a strong reducing agent that reacts faster (usually sodium hydrosulfide: NaHS) is added as required to maintain an ORP of about -400 mV in the scrubbing solution. NaOH is added as required to maintain pH ≥ 12.5, in order to minimize H₂S emissions.

Stage 3 Packed Bed: H₂S Odor Control - This stage is optional. If the pH and ORP settings in Stage 2 are adjusted properly, there will be little H₂S released from that stage. However, the strongly alkaline hydrosulfide solution in Stage 2 is a severe environment for pH and ORP probes, so the probes will tend to get out of calibration faster than they would in a scrubber operating at lower pH levels. Some customers with NOx scrubbers prefer to install a caustic scrubbing stage as a second line of defense against odor emissions.  This may be more important to plants located near other businesses or a residential community.  The wastewater from Stage 3, containing excess NaHS and excess NaOH, can be recycled to the sump of Stage 2 in order to reduce chemical usage there.

Please click the icons below to view a video of a packed bed scrubber and a horizontal quencher video.

Photo Credit: monovinyl

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.

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

By Mike Simon
Director of Simulation Products, Digital Dimensions

Understanding how simple design changes affect the airflow inside of Envitech's products is critical in designing efficient industrial gas cleaning systems.  Engineers who design this equipment need to analyze and understand the behavior of the components if they want to improve performance.  Computational Fluid Dynamics (CFD) is a good tool for studying the effects of different design changes on these systems.  CFD provides a way to save time and money in obtaining the necessary information, and assists engineers in designing better quality air pollution control systems.    The use of CFD makes it possible to minimize the use of physical prototypes and find serious flaws much earlier in the design process. 

SolidWorks is the 3D CAD system used by Envitech to design their industrial gas cleaning systems.  SolidWorks has a number of complementary features to its mechanical CAD system including CFD capabilities that are fully integrated within the main CAD interface.  SolidWorks Flow Simulation is the name of the CFD program inside of SolidWorks that allows engineers to take their 3D CAD models and perform virtual prototyping on their designs without having to fabricate any parts.  To perform a simulation, the following steps are needed:

• 1. Create solid model in the SolidWorks CAD system
• 2. Specify the working fluid ( air was used in this case)
• 3. Specify the flow rate at the duct inlet
• 4. Specify the outlet opening of the duct
• 5. Specify the pressure drop or resistance properties of the filter material (properties taken from filter manufacturer specifications)
• 6. Run the simulation inside of the SolidWorks interface

Envitech's products were particularly challenging since Envitech's products utilized very thin fins and packing materials within a large ducting area.  Thin fins are used to direct the airflow and also to collect water from entering the system.  SolidWorks Flow Simulation was able to capture the geometry of these thin fins and create a corresponding CFD model for the simulations.  Packing material is used to help distribute airflow and trap particulates from being released into the environment.  The porous media feature inside of SolidWorks Flow Simulation was used to simulate the packing material and create the additional resistance to the airflow.  After performing the simulations, the Envitech engineers had the ability see the effectiveness of the scrubber fins in directing the airflow and to understand the pressure drops caused by the packing material.  The simulations helped the Envitech engineers validate their designs and gave them additional insight into how to improve future product performance.

For additional information on SolidWorks CAD or SolidWorks Flow Simulation software, go to http://www.ddicad.com/ or contact Mike Simon, Director of Simulation Products, at msimon@ddicad.com.

For a case study on the impact of CFD analysis, click on the link below.

I am very excited to announce that we will be publishing our first special guest blog post tomorrow, Tuesday, October 27.  The topic of the blog will be CFD analysis in air pollution control systems.

Our guest blog author is Mike Simon, Director of Simulation Products at Digital Dimensions.  Mike is a former SolidWorks/Cosmos Technical Manager and he has 10 years of FEA/CFD experience at companies such as General Atomics and General Dynamics.  Mike earned a BSME from UCSD, a MSME from Stanford, and a MBA from USD.  I am sure you will find his article very interesting.

A little more background Digital Dimensions...In addition to being an authorized reseller of Solidworks, Digital Dimensions hosts a variety of Solidworks training sessions, including topics such as structural analysis, flow simulation, and heat transfer - all tools available in the Solidworks design package.  I personally have benefited tremendously from the training sessions, and would recommend it to anyone interested in CFD or structural analysis using 3D models.  I use the CFD analysis on many of our Venturi scrubber designs as well as for our wet electrostatic precipitators.

As the EPA continues to tighten the emissions belt, I am seeing new industries with air emissions issues. One such industry is pharmaceuticals, who are now more commonly regulated for acid gases on post-combustion devices.

Pharmaceutical air emissions are typically a result of an organic fume from a solvent. The fume, containing vaporized solvent, is captured either within a fume hood or central ventilation system. When regulated, the most effective way of removing a fume is to combust it in a regenerative thermal oxidizer (RTO) or some other combustion device.

The combustion of a solvent such as methyl chloride in an RTO leaves three compounds: carbon dioxide, water vapor, and hydrochloric acid. The last of the three - hydrochloric acid - is often treated as an emission and if so must be removed from the outlet exhaust.

Pharmaceutical Scrubber 

The most best method for removing hydrochloric acid from a gas is the use of a pharmaceutical scrubber. A scrubber offers extremely high efficiencies (greater than 99%, or as required) at a low pressure drop. Recirculating neutralized water across a packed tower, the capital and operating cost of a scrubber is minimal. Further, the effluent from a HCl scrubber contains only sodium chloride - table salt - and can easily be disposed of through a wastewater sewer with little to no further treatment. Using FRP for the scrubber provides a low cost building material highly resistant to acid attack.

Hydrochloric Acid Corrosion

The removal of hydrochloric acid from a combustion exhaust does offer one particular difficulty over other common acid gases, of which designers and operators in the pharmaceutical industry need to be wary. Hydrochloric acid and neutralized chlorides are very aggressive towards most metals, especially so at elevated temperatures typically seen on the outlet of a combustion process. Since the HCl is contained in the exhaust of a combustion process, the inlet gas temperature to the scrubber is high. In turn, the recirculation water temperature is also high, usually well above 100F. Standard metallic materials such as stainless steel will quickly corrode in this environment.

In the past, I have used both AL6XN and hastelloy for metallic materials in HCl scrubber systems. Common metallic items in a pharmaceutical scrubber include the quencher, instrumentation, and downstream devices.  AL6XN is a duplex material that provides very good corrosion resistance to around 1000F. It also has about an order of magnitude greater chloride pitting resistance than stainless steel at neutral pH, and over two magnitudes resistance at low pH.  AL6XN is ideal for quenchers on the exhaust of an RTO, where the outlet temperature is usually around 500F. Hastelloy is more expensive, but it offers heat resistance to 2500F as well as a further order of magnitude resistance to chlorides over AL6XN.

Hydrochloric Acid Mist

The other issue provided by hydrochloric acid in a gas stream is the formation of hydrochloric acid mist, which I have previously touched upon in my acid gas dewpoint post.

Hydrochloric acid mist usually requires a high efficiency mesh pad for removal of any HCl aerosols that may form in the scrubber.  A mesh pad is more expensive than a standard wave form mist eliminator, and is also much more prone to particulate plugging.  If hydrochloric acid is in your gas stream, make sure you consider a mesh pad and beware of particulate!

If you would like to learn more about corrosive acid gas scrubbers, download the free case study below.

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.

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.

Acid gases can be found in the exhaust of a large number of combustion processes.  As a gas, the acid compounds usually are not particularly corrosive and are relatively easy to remove.  However, when the temperature of the gas drops below the acid gas dewpoint, an acid mist can form.  The acid mist can turn into a fine aerosol or it can condense on a cold surface.  Acid mist poses a number of design problems, due to the small size of the mist particles and the corrosivity of the liquid form of the acid.

Aerosol Formation

Aerosol formation occurs when the bulk temperature of the gas drops below the acid dewpoint of the gas.  Much like the formation of fog, the acid gas condenses into tiny liquid droplets.  The size of these droplets can vary widely depending on the acid, the amount of condensation nuclei present in the gas, and degree of supersaturation.

The most common problem that occurs with acid aerosol formation is the inability to capture the aerosol.  Many acid gas exhaust treatment systems utilize a packed bed scrubber to remove the acid.  Packed bed scrubbers are extremely efficient at removing acid in the gas, but unfortunately are ineffective at removing acid aerosol.

The solution to removing acidic aerosol mist is to use a high efficiency entrainment separator or Venturi scrubber, which can effectively capture particles to 1-micron or 0.5-micron, respectively.  If the acidic aerosol mist is primarily sub-micron in nature, a wet electrostatic precipitator also provides a useful solution.

Wall Condensation

Wall condensation occurs when a cold surface is in contact with a hot gas.  If the wall is cooler than the acid dewpoint, acid can condense onto the surface of the wall.  There are several dangers with wall condensation.

First, the material selection for an acid is dependent on its form.  Many acids are not corrosive as a gas, but are very corrosive as acids.  Engineers selecting materials under the assumption that the acid remains a gas often choose materials that are not compatible with the acid in its liquid form.

Second, when condensed, the acid is much more concentrated than it is in the bulk medium.  Instead of selecting a material for a gas containing 10 ppm of SO3 gas, the engineer now has to worry about a nearly pure sulfuric acid droplet.

Finally, the acid can condense in non-ideal locations, leading to pooling and further corrosion concerns.

The solution to preventing the effects of wall condensation is to insulate walls to prevent cold surfaces and select materials for the concentrated, liquid form of the acid in locations where wall condensation is unavoidable.

Acid Dew Point

All gases have a dew point that is dependent on the temperature, pressure, and concentration of the acid in the gas.  This article provides acid gas dewpoint equations for a number of acids.  Below are the formulae for a few of the more common acids in exhaust gases.

Tdp = Dewpoint Temperature, K

Pw = Partial Pressure of water, mmHg

Pa = Partial Pressure of acid, mmHg

Hydrochloric acid (HCl)

1000/Tdp = 3.7368 - 0.1591 * ln (Pw) - 0.0326 ln (Pa) + 0.00269 * ln (Pa) * ln (Pw)

Sulfur Dioxide (SO2)

1000/Tdp = 3.9526 - 0.1863 * ln (Pw) + 0.000867 ln (Pa) - 0.000913 * ln (Pa) * ln (Pw)

Sulfuric Acid (H2SO4)

1000/Tdp = 2.276 - 0.0294 * ln (Pw) - 0.0858 ln (Pa) + 0.0062 * ln (Pa) * ln (Pw)

Click on the button below to down load an Envitech packed bed absorber cut sheet for acid gas removal.

Photo Credit: tinyfroglet