Air Pollution Control Innovations

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.

Calcium scale occurs everywhere.  The photo on the right shows calcium carbonate scale lining the rock formations in Lake Mead, just above Hoover Dam.  The calcium from this photo deposited as a result of dissolution of calcium in upstream limestone rock formations along the Colorado River, followed by evaporation and reprecipitation in Lake Mead.  In a roundabout way, this is the same method that calcium scale forms in a scrubber.  Calcium enters via the feedwater (Colorado River), concentrates due to evaporation (the hot Nevada sun), and forms a precipitate as the calcium concentration exceeds the solubility limit (lime deposits).

In my previous post, I detailed how to determine if calcium is a problem in your acid gas scrubber.  Here, I will detail several methods for avoiding calcium scale and, when it arises, cleaning calcium scale.

Avoiding Scaling

The best way to tackle scaling is to avoid scaling. There are three basic methods to avoiding scale:

1.  Remove the calcium

The easiest way to avoid calcium is to remove the calcium!  First, try to select a feedwater that does not contain calcium.  Condensates are often an excellent source of calcium-free feed water.  If calcium-free water sources are not available, investigate the cost associated with a softener.  Companies like Culligan offer a wide range of softeners.  Water softening is a competitive market; there are often local suppliers that can offer low cost solutions.

2. Adjust the chemistry

There are several tricks that can be employed to permit higher concentrations of calcium in your scrubber water without leading to scale formation.  For situations where calcium carbonate is the primary scale, you can decrease the pH of the scrubber.  Acid gas removal efficiency decreases when operating at a lower pH, but the drop in efficiency can be overcome by increasing the packing height (and thus the number of transfer units).  Decreasing the pH decreases the LSI index of the water; LSI indices less than 0 are non-scaling.

Companies like Nalco also sell anti-scalants that reduce the activity of the calcium ion in solution, thus eliminating the potential for calcium scale.  Anti-scalants are usually much more expensive than acid treatment, but also permit operation at a higher pH which is more conducive to acid gas scrubbing.

3.  Increase the blow down

In my previous post, I documented how the calcium concentration was tied to the blow down flow rate.  One of the easiest and most practical solutions therefore is to increase the blow down flow rate.  Increasing the blow down flow rate reduces the concentration of both the calcium and associate scaling anion (CO3, SO4, F) in the scrubber recirculation water.  While increasing the blow down flow rate will add cost by increasing the feed water flow rate (to make up water removed in the blow down) as well as increasing sewer and other wastewater charges, it is often preferable as a quick fix solution for reducing scaling.

Coping with Scaling

In many ways, scaling is inevitable.  A process upset, an exhausted bed, or a blocked blow down valve - these things happen.  How do you remove the scale?

Calcium carbonate is the most common scale.  It is also amongst the easiest to clean.  Lowering the pH is the most aggressive way to dissolve the scale, though running at any LSI << 0 will tend to dissolve the calcium carbonate.

Calcium sulfate is another common scale, especially in an SO2 scrubber.  The best method for removing calcium sulfate is using hot, low pH water to remove the scale.  Calcium sulfate, like calcium carbonate, will dissolve.  Chelants such as EDTA are also helpful for this form of scale.

Calcium fluoride is the worst scale to remove, since it has such a low solubility.  Physical methods work best for calcium fluoride, as dissolving calcium fluoride is difficult.  Some chelants such as EDTA are available on the market for removing calcium fluoride, but are not likely to be as effective as with other forms of scale.

To learn more about Envitech's acid gas scrubbers, please download our packed bed cut sheet.

Note: Photo of Lake Mead by Tim Pearce

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.

Acid gas scrubbers are commonly used for the removal of acid gases, such as sulfur dioxide, hydrochloric acid, chlorine, hydrofluoric acid, and nitrogen dioxide.  These scrubbers typically use a basic reagent; often sodium hydroxide, soda ash, or lime to keep the scrubber liquor at neutral to high pH.

Operation at a high pH can lead to unexpected problems with water chemistry.  Calcium carbonate, calcium sulfate, and calcium fluoride all can cause scale within the scrubber.  Scale can reduce the effective pipe diameters, coat packing, block nozzles, foul instrumentation.  In the worst extremes, calcium scale can shut down the scrubber and in turn shut down the upstream process equipment.

How does one avoid such hazards?  In the first of this two part series, I will go through the five steps for determining if calcium scale is a threat to your acid scrubber.

Step 1: Determine the calcium concentration

Calcium primarily infiltrates an acid scrubber through two sources: water makeup and chemical addition.  Water makeup is typically from a municipal or well source.  The calcium concentration can usually be found in municipal water reports.  Well water can be tested cheaply at an outside laboratory for calcium.

Calcium content for sodium hydroxide and soda ash is a bit more difficult to determine, but can be specified in the procurement of the chemicals.  Using the chemical consumption rate and the blow down flow rate, the amount of calcium left in the scrubber water can be calculated.  Lime actually contains calcium, and should be avoided where calcium scale is possible.

Step 2: Determine the scaling anion concentrations

The three most common calcium scaling anions are carbonate, sulfate, and fluoride. Carbonate primarily enters the system through fresh water, though in acid gases with high carbon dioxide content carbonate can also absorb into the water.  Sulfates enter through fresh water, but due to the high solubility of calcium sulfate, rarely cause problems due to makeup water sulfate.  Instead, sulfates are a byproduct of the neutralization of SO2.  SO2 neutralizes to HSO3 and SO3 in water, and can convert to SO4 depending on the residence time, pH, temperature, and oxygen content of the acid gas.

Step 3: Calculate the buildup of calcium and the scaling anions.

Determining the concentration of ions in the scrubber requires two steps.

1.  Sum all of the mass flow rates (mg/min) of the ion into the scrubber water, including makeup water, chemical, and absorption from the gas.

2.  Divide the mass flow rates by the blow down flow rate (L/min) to give a concentration (mg/L).

Step 4: Calculate the LSI index

The most common form of calcium scale is calcium carbonate.  Calcium carbonate is formed by the interaction of calcium with carbonate alkalinity.  Carbonate alkalinity is often present in fresh water; carbon dioxide can also dissolve in water to create carbonates.

The LSI index offers the best method for determining calcium carbonate scaling; Edstrom provides a wonderful explanation at their website.

Step 5: Calculate the sulfate and fluoride solubilities

The final step is to determine the sulfate and fluoride solubilities. 

Calcium sulfate is relatively soluble, but calcium sulfate scale can still occur in SO2 scrubbers. Scale forms for calcium sulfate if the solubility product exceeds 4.93x10^-5.  The solubility product for calcium sulfate is equal to the concentration of calcium (mol/L) times the concentration of sulfate (mol/L).  The calcium concentration can be converted from mg/L to mol/L by dividing by 40,000.  The sulfate concentration can be converted from mg/L to mol/L by dividing by 96,000.

Calcium fluoride is the opposite of calcium sulfate; it is extremely insoluble.  Scale forms for calcium fluoride if the solubility product exceeds 3.45x10^-11.  The solubility product for calcium fluoride is equal to the concentration of calcium (mol/L) times the square of the concentration of fluoride (mol/L).  The calcium concentration can be converted from mg/L to mol/L by dividing by 40,000.  The fluoride concentration can be converted from mg/L to mol/L by dividing by 96,000.

Click on the icon below to download a white paper about other challenges associated with acid gas scrubbers.

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.

In 1824, the German mathematician M. Hohlfeld described the removal of particles from gas streams by electrical forces.  However, it was almost a century later when Dr. Frederick G. Cottrell at the University of California, Berkeley commercialized the technology by building the first wet electrostatic precipitator. 

A wet electrostatic precipitator uses electrical forces to move particles entrained in a gas stream onto collection surfaces.  Electrodes in the wet electrostatic precipitator are held at high voltage which creates a corona discharge.  Particles receive an electrical charge as they pass through the corona.  The charged particles then follow electric field lines from the charging electrodes to collection surfaces, where they are removed from the gas stream.  

Dr. Cottrell applied wet electrostatic precipitator technology to the removal of sulfuric acid mist and lead oxide dust emitted from various acid-making and smelting activities.  At the time, vineyards in Northern California were being adversely affected by the lead emissions.  Dr. Cottrell's innovative wet electrostatic precipitator solved their problem.

Fast forward to the 2000's.  Envitech brought the control of lead and sulfur dioxide to a new level by installing our most advanced wet electrostatic precipitator technology on a secondary lead smelting facility in Southern California.  The resulting wet electrostatic precipitator system which removes both sulfur dioxide and lead particles is said to set a new standard in air emission control at lead smelting facilities worldwide.

With over thirty years in the industry, we wanted to start sharing the knowledge and expertise that we have gained from cleaning gas streams of unwanted contaminants.  Look for future postings that examine various aspects of state-of-the-art air pollution control technologies.

For more information on our wet electrostatic precipitators, please download a brochure on them.

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