High performance and low operating costs could make wet scrubbers a brick manufacturer’s best option for meeting the new MACT standards.
The crossflow wet scrubber efficiently captures HF and other constituents.
New air emission regulations are imminent and could go into effect as early as the end of 2002 (see sidebar: “The MACT Standards”). Manufacturers that operate large kilns (producing more than 10 tons of fired brick per hour) should start planning now for the U.S. Environmental Protection Agency’s (EPA) upcoming Maximum Achievable Control Technology (MACT) standards by evaluating their options for emissions control.
Figure 1. A schematic of the crossflow wet scrubber.
Recently, one attractive option has been tested and certified as being capable of efficiently controlling emissions from brick kilns. In the summer of 1999, a wet scrubber designed to efficiently capture hydrogen fluoride (HF) and other constituents* was installed in a pilot project at a major brick production facility. So far, the design has exceeded 99.5% efficiency on HF, and tests during plant operation have indicated that it will meet anticipated particulate matter emission levels of 0.01 grains per dry standard cubic foot (gr/dscf). The scrubber’s crossflow design can also be set to efficiently remove excess SO2
loading, a factor in non-attainment sites or high sulfur content applications. These values are important because they, too, will most likely be among the emission limits set by the EPA. A schematic of the wet scrubber is shown in Figure 1.
What makes this type of system even more attractive is that its initial cost is often half or less of a conventional dry injection/fabric filter (baghouse) system, and operating costs are also lower, representing a significant savings for brick plants. In addition to demonstrating the necessary efficiency, this new scrubber is simple to operate and requires minimal maintenance.
Handling Discharge Liquid
Several questions are often asked about this approach to emissions control. First, many operators immediately want to know how scrubber discharge liquid is best handled. Fortunately, this question was paramount to original system design. In a word, the discharge is a “trickle.” It helps to understand that even though several hundred gallons per minute (gpm) may be recirculated continuously inside the scrubber, only a trace quantity of water is actually needed for blowdown, or discharge.
Further, one manufacturer has discovered that the insoluble fluoride salts that are formed from the collected HF gases during neutralization can be returned to the brick body without scum or efflorescence effects on the fired brick. This either eliminates discharge or greatly reduces the need to a fraction of a gallon per minute for flushing collected soluble salts out of the scrubber loop (primarily chlorides). For comparison, imagine a leaky faucet—that represents the flow of water needed for discharge for many applications, much less than 1 gpm.
Annual Operating Costs
Other questions relate to annual operating costs—a major issue with injection systems. The system will operate with a very low pressure drop, keeping fan horsepower requirements low. Reagent costs are low as well due to the use of safe, affordable and readily available slurry of magnesia, rather than large quantities of dusty, dry lime or other calcium-based reagent.
Table 1. Estimated operating costs for the crossflow wet scrubber.
A side benefit to using a water-based magnesia system is near-stoichiometric (meaning 1:1 chemical reaction) use of reagent, virtually eliminating the need to dramatically overfeed as in a gas-phase reaction, or baghouse, system. In this application, one-half mole of magnesia is used for each mole of HF. Estimated operating costs are provided in Table 1.
The crossflow configuration of the wet scrubber minimizes maintenance due to the vigorous washing action of sprays on durable mass transfer internals. The housing is accessed from the side, a feature that allows for rapid internal inspection and, if necessary, rapid internal maintenance. This inspection and rapid maintenance feature is not even a possibility with baghouse systems, which use dozens of lengthy, heavy and dusty bags. Routine maintenance requirements have also been demonstrated to be quite low on crossflow installations.
Ensuring Regulatory Compliance
Regulatory compliance is assured with this approach. A normally functioning system cannot be operated to let acid gases and particulates pass through. Acid gases are readily absorbed and converted into non-hazardous salts, unlike a baghouse system where corporate compliance officials must concern themselves with the quality of adsorbant or aggregate and the accuracy of broken bag detectors (bags can break during routine operation and then create particulate emission problems).
The new MACT standards are fast approaching and could spell trouble for any facility that generates a certain amount and type of kiln emissions. The only certain way to comply is to install a reliable scrubber capable of meeting all of the regulatory requirements. A crossflow wet scrubber is an economically and technically attractive option that should be carefully evaluated prior to installation of a control system for any brick facility.
For More Information
For more information about emission control technology, contact Lucas Process Systems, 10800 Sikes Place, Suite 160, Charlotte, NC 28277; (704) 321-2552; fax (704) 321-2445; e-mail email@example.com
; or visit http://www.lucasprocess.com
. *Supplied by Lucas Process Systems.
SIDEBAR: The MACT Standards
The Environmental Protection Agency (EPA) is working to implement what it calls Maximum Achievable Control Technology (MACT) standards—federal regulations developed under Title III of the Clean Air Act Amendments of 1990. These regulations are intended to reduce the emissions of 189 hazardous air pollutants (HAPs) by requiring improved air pollution control equipment and techniques on a wide variety of industrial and commercial sources of air pollution, including brick manufacturing plants.
Most MACT standards apply only to major sources of hazardous air pollutants, defined as facilities that emit or have the potential to emit 10 tons per year (TPY) or more of any HAP or 25 TPY or more of any combination of HAPs. On March 23, 2000, the EPA presented a draft approach of the standards to brick industry representatives. Based on this draft approach, about 100 tunnel kilns with capacities equal to or greater than 10 tons per hour in the brick industry will require some form of control, as well as all new tunnel and periodic kilns. The standards are expected to be proposed this spring and promulgated about one year later, and plants would need to be in compliance anywhere from one to three years after the standards are promulgated.
For more information about the MACT standards, contact Mary Johnson of the EPA at (919) 541-5025.