BCR: More About Wet Scrubbers
As the MACT standard now stands, all facilities that are major sources of hazardous air pollutants (HAP), defined as those emitting greater than 10 tons of a single HAP or 25 tons of combined HAPs, will be required to install additional controls. (See sidebar on p. 6: Update on the MACT Standards.) Clearly, the industry will be required to embrace some type of scrubber on many existing and all future large kilns. The question is: What technology will be best able to help brick and structural clay plants meet the new standards once the regulations are in place?
The Wet AlternativeThe Environmental Protection Agency (EPA) is currently basing the rule on dry injection systems, which it says can achieve a high rate of emissions reduction. However, alternative systems are being developed that can also greatly reduce emissions. A properly designed wet scrubber is a highly efficient approach that will collect all acid gases, thereby helping brick and structural clay manufacturers successfully lower kiln emissions.
A wet scrubber should include the following features, listed in priority of plant importance:
Efficiency. The temptation with control devices is to “buy cheap” since the devices themselves do not add to the plant’s bottom line. However, all decision-makers will quickly forget how much money was saved if a system fails to perform on all required emission targets (generally hydrogen fluoride [HF], hydrogen chloride [HCl], sulfur oxides [SOx] and particulate matter [PM]). Efficiency should be the key factor in choosing a control system. A properly designed system includes redundancy on mechanical parts and sufficient mass transfer, and should be relatively easy to maintain.
Low Operating Costs. The system must scrub at the lowest possible cost per unit of pollutant collected. In general, wet scrubbers can be operated at close to stoichiometric ratios (1:1), thereby considerably reducing reagent consumption. The cost analysis should include energy, maintenance, labor hours, reagent usage, disposal costs and expected lifespan of the components. (Acid gases are very corrosive and can limit housing life, for example, without the use of proper construction materials.)
Final system costs, including operating costs, can be determined by studying three key parameters: gas volume (determined by the kiln manufacturer), emissions estimates (determined through experience, stack tests or pyrohydrolysis), and kiln emission moisture content.
Redundancy and Turndown Capability. The system should be rugged, with redundant pumps and sensors, and with easy access to internals for inspection or maintenance. A wet crossflow scrubber gives the operator the advantage a small footprint and horizontal layout. Kilns often have variations in gas flow due to flashing and other techniques. The scrubber should be capable of operating efficiently during these gasflow swings.
What About the Water?A common comment heard from brick plant operators pertains to the fate of the water used in the wet scrubber. “What do you do with the water?” is a common refrain. The answer is, “It depends.” Hydrogen fluoride, for example, can be neutralized to an ionic salt, which is very insoluble. This compound would be separated from the water with a standard lamella-type settler, with the water returning to the scrubber and the fluoride salt returning to the brick.
Similarly, if the sulfur content of the brick body is low, it is possible to return the scrubber solution back into the brick body during extrusion. This has been done with no effect on the fired brick (see Figure 1). Continuing research suggests that the fluoride will re-evolve during firing at a very low rate, allowing the plant to send the fluoride out with the fired brick. In general, it is best to complete a pilot trial to prove that concept in the plant lab prior to a full system being installed.
Another option is to send the wastewater to a sewer if a hook-up is available near the plant site. Pretreatment would still be required to remove solids and low solubility salts from the water prior to disposal.
For moderate to high sulfur brick body materials, two options are viable. First, a closed-loop evaporation system would render the generated salt water (neutralized acid gases) into a powder for landfill disposal. Toxicity Characteristic Leaching Procedure (TCLP) data (an EPA procedure for testing landfill leachability of toxic materials) suggests that this solid is a non-hazardous waste. The waste heat from the kiln is generally sufficient to eliminate free water and produce a landfillable salt cake. This design includes an evaporative loop (spray dryer) with a particulate separator (cyclone or baghouse) connected to the wet scrubber. This design also handles sulfur trioxide (SO3) quite easily, which is a consideration in higher sulfur applications. In essence, the system lets the scrubber scrub and the dryer dry, spreading out the controls to their appropriate devices.
Second, the wet scrubber would complete all the scrubbing, with the pretreated wastewater flowing to the nearby receiving water (NPDES permit) or wastewater treatment plant (POTW). Since scrubber blowdown water is an industrial wastewater, the proper permits would be required for either water disposal situation. A calculated amount of insoluble sludge would exist that could be either landfilled or returned to the brick body. Water disposal permitting will require planning and can be straightforward or complex, depending on the plant location and mood of the state agency.
The issue of SO3 abatement is an important consideration for a wet scrubber in higher sulfur applications. With higher sulfur dioxide (SO2) emissions, the ratio of SO2 to SO3 will be maintained, leading to higher SO3 levels as well. Unfortunately, SO3 converts instantly into sulfuric acid mist in a wet scrubber. This can be seen as a blue or yellow haze, depending on the sun angle. This is a major concern for coal-fired utilities that use high sulfur coal, since mercury controls will likely require wet scrubber controls, thereby creating the paradoxical condition of lower SO2 emissions but higher apparent emissions and particulate emissions. This sulfuric acid mist can be controlled but requires a very carefully designed wet system.
One approach that shows promise is nucleated scavaging,* whereby particulates are grown from 0.2-0.4 micron to 1.0 micron within the scrubber, downstream of the absorption system, allowing the mist to be eliminated more conventionally (through a standard mist eliminator).
The lowest cost approach will be disposal to a sewer or water body, since a pretreatment system typically costs less than a dryer/particle separator system. The liquid flows are generally in the 5-20 gallon per minute range for a large kiln firing high sulfur content bricks. However, in the absence of a sewer hook-up or limitations on total dissolved salts (TDS) in discharges to nearby creeks, rivers or lakes, a permit for this rate may be difficult to obtain. In this case, a wet scrubber with a dry disposal option would be favored.
Meeting MACT StandardsIt is unclear when the new MACT standards will be put in place and what controls the final regulations will require. However, it is fairly certain that additional controls will be required to limit emissions in the brick and structural clay industry. Manufacturers should be prepared to meet the new standards by evaluating the available control technologies, including wet scrubbers.
For More InformationFor more information about wet scrubbers, contact Lucas Process Systems, 10800 Sikes Place, Suite 160, Charlotte, NC 28277; (704) 321-2552; fax (704) 321-2445; e-mail firstname.lastname@example.org; or visit www.lucasprocess.com.
*Available commercially as Lucas NS™, a trademark of Lucas Process Systems
SIDEBAR: Update on the MACT StandardsThe U.S. Environmental Protection Agency (EPA) is required by the Clean Air Act Amendments of 1990 to evaluate numerous industries for control of hazardous air pollutants (HAPs), including the hydrogen fluoride (HF), hydrogen chloride (HCl) and particulate matter (PM) that are emitted by some brick plants. The Maximum Achievable Control Technology (MACT) standards are intended to limit the HAPs that are released into the environment by requiring emission sources to install controls, such as scrubbers.
A task force backed by the Brick Industry Association and other organizations has been successful in getting periodic kilns removed from the preliminary version of the standard. However, existing tunnel kilns that emit more than 10 tons per year (tpy) of a single HAP or 25 tpy of combined HAPs will be required to install an additional emission control. Additionally, all new tunnel kilns (constructed or rebuilt after the proposal date, i.e., the date the Brick National Emission Standards for Hazardous Air Pollutants [NESHAP] are published in the Federal Register), regardless of size, will be required to meet the new standards. Emissions from all processes at a facility must be included in calculating total emissions.
The EPA is currently basing the rule on dry injection systems, which neutralize HF and other acid gases through contact with an alkali reagent, such as limestone, hydrated lime and soda ash. According to the EPA, such systems can achieve 95% reduction of HF or 0.005 lb HF/ton of fired product; 90% reduction of HCl or 0.006 lb HCl/ton of fired product; and 0.12 lb PM/ton of fired product. If the rule is promulgated as it stands, other control technologies can be used, but proof of compliance will need to be presented. The task force is working with the EPA to try to ensure that the requirements are reasonable and attainable. The EPA has said that it is changing the pound per ton limits (the current limits are based on 99% control of HF and 95% control of HCl), but it is currently unclear what the change will be.
For tunnel kilns constructed before the proposal date, compliance will be required three years after promulgation (the date the final rule appears in the Federal Register). For tunnel kilns constructed after the proposal date, compliance will be due at promulgation or at the start-up of the kiln, whichever is later.
After the NESHAP has been proposed, the pre-promulgation phase will begin. A public comment period will begin on the proposal date and last 60 to 90 days. If requested, the EPA will hold a public hearing, typically 30 days prior to the end of the public comment period. The EPA will then address public comments and develop a promulgation package. Promulgation has historically taken 12 to 15 months after proposal. However, due to tight statutory deadlines, the EPA may try to expedite this schedule. After promulgation, the EPA will develop compliance support documents and will issue correction/clarification notices as necessary.
The original proposal date was May 15, 2002. However, at this point, it does not appear that the EPA will meet this deadline. The Clean Air Act [Section 112(j)] mandates that a facility must work with its state/local regulatory authority to establish MACT limits for its operations if the EPA fails to establish a final NESHAP by this date, but this requirement is not expected to be triggered for the brick industry.
For more information about the MACT standards or the industry task force, call task force lead Terry Schimmel, Boral Bricks, at (334) 480-2488; or Susan Miller or Dana Norvell, Clayton Group Services, (919) 851-2160.
Editor’s note: The sidebar information was obtained from a presentation by Terry Schimmel at the 47th International Brick Plant Operator’s Forum in Clemson, S.C., October 1-3, 2001. For additional information about scrubbers, see “Scrubbing Brick Kiln Emissions to Meet Regulations,” CI, Brick & Clay Record, July 1998, pp. 57-62; and “A Wet Solution to Emissions Control,” CI, Brick & Clay Record, January 2001, pp. BCR 12-BCR 15.