Air Pollution Control Innovations

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

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.