Reducing Emissions with Sodium Sorbents

August 1, 2006
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Sodium sorbents can help minimize the waste discharge and operating costs often associated with dry injection scrubbers



The Clean Air Act is placing increasing scrutiny on all industries, mandating compliance with both existing regulations and proposed air pollution control standards. Some industries even face multi-million-dollar class-action lawsuits filed with the stated goal of improving the environment.

In the ceramic, brick and glass industries, the discharge of halide pollutants such as hydrochloric acid (HCl) or hydrofluoric acid (HF) is closely regulated nationwide. Regulations for controlling sulfur dioxide (SO2) and sulfur trioxide (SO3) emissions are already in place in some states and are being promulgated in others.

As with many challenges, the best defense is a good offense. Getting ahead of the standards and implementing good, reliable air pollution control systems can help ward off court action. But how is a "good" system defined?

The high removal rates required in the ceramic and related industries, often more than 90%, were traditionally thought to require a wet scrubbing system, many of which created waste products that could not be recycled. Now, however, alternatives exist.

A dry injection system using sodium scrubbing is both effective and straightforward, and the cost of implementation is lower when compared to the more traditional wet scrubbers.

High-Efficiency Scrubbing

Dry sorbent injection systems typically use a powdered or fine granulated material that, when blown into ductwork, reacts rapidly with pollutants present in the exhaust gas stream. Reaction products generally remain dry and are removed using a baghouse or electrostatic precipitator (ESP). From there, the collected waste is either recycled or disposed of otherwise.

While lime- or calcium-based sorbents have traditionally been used in these systems, sodium-based chemicals such as sodium bicarbonate* and trona** have been shown to be more efficient, especially when higher removal rates are required. (Soda ash, also known as sodium carbonate, does not work well as a dry sorbent.)

Both sodium bicarbonate and trona react rapidly in the duct. It is believed that the products first calcine, giving up their excess CO2 and H2O, before reacting with the acid gases. In this manner, the surface area of the particles increases dramatically, and the resultant particle actively captures acids present in the gas stream.

*SOLVAir Select SBC, supplied by Solvay Chemicals, is an example of sodium bicarbonate used in flue gas treatment.

**SOLVAir Select(tm) 200, supplied by Solvay Chemicals, is an example of a trona product designed for dry sorbent injection.

The Sodium Advantage

Sodium-based chemicals are more expensive than lime- or calcium-based sorbents. For example, lime, which is usually available locally, costs about $70-80 per ton delivered to the plant site, while trona can cost in excess of $110 per ton. However, at high removal rates, lime efficiency has been shown to drop off dramatically, while trona continues to perform. In fact, there is no break-even point when the sorbent does not get the job done. Additionally, in a typical application for acid gas scrubbing, the required amount of trona is approximately half that of lime and about one-third that of limestone in total tons.

Solid waste discharge from a calcium-based system is normally twice as much as sodium because of the efficiencies gained using sodium. (The costs of waste disposal vary widely depending on how and where the waste is removed.)

When using an ESP for final particulate removal, calcium degrades the performance of the ESP due to the resistivity of the calcium byproducts. In addition, since the efficiency of the calcium system is often lower, there are more solids to remove. This can overload the ESP and result in particulate emission problems. When using sodium sorbents, the natural byproducts of the reaction actually enhance the operation of the ESP.

One glass manufacturer had recurring problems with plugging and buildup in its ESP unit, requiring frequent and costly shutdowns for cleaning. When the sorbent being used was switched from sodium bicarbonate to trona, the problem disappeared, and the on-line time of the ESP unit increased tremendously. A ceramic manufacturer constantly battling HF discharge injected trona into ductwork downstream of the furnaces, and the problem was quickly solved.

A Clean Air Option

It is a truism these days that regulations never lessen and compliance never gets easier. However, using a dry scrubber and sodium sorbents can help minimize the impact of compliance on a manufacturer's bottom line by reducing the waste and operating costs associated with emissions control.

For more information about sodium sorbents, contact Solvay Chemicals, Inc., 3333 Richmond Ave., Houston, TX 77098; (713) 525-6829; fax (713) 525-7806; e-mail mike.wood@solvay.com ; or visit http://www.solvaychemicals.us/solvair .

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