Reducing Pollutants with Packed Bed Filters
In the brick and structural clay industry, several pollutant gases—including hydrogen fluoride (HF), sulfur oxides (SOx) and, in some cases, hydrogen chloride (HCl)—are released during the firing process. The U.S. Environmental Protection Agency (EPA) has recently promulgated new national emission standards for these hazardous air pollutants (HAPs), and these standards require that all “major sources” implement a maximum achievable control technology (MACT) to reduce their emissions of these pollutants.* (According to the EPA, major sources are “stationary sources or groups of stationary sources that are located within a contiguous area and under common control that emit or have the potential to emit considering controls, in the aggregate, 9.07 Mg/yr [10 tpy] or more of any one HAP or 22.68 Mg/yr [25 tpy] or more of any combination of HAP.”)
Although a number of control technologies are available that can meet these new standards, some systems can be very expensive to operate and maintain. Companies using such systems are faced with additional manufacturing expenses that negatively impact their bottom line.
Operating PrincipleIn the packed bed filter, the absorbent—typically crushed limestone granules—is stored in a silo on the scrubber’s absorber. (The absorbent material is determined by the type and concentrations of contaminants in the flue gas.) The absorbent trickles vertically from the storage silo and passes through a number of horizontal cascades in the reaction chamber. As the contaminated gas flows through the absorbent, it is captured by the crushed limestone granules. The saturated granules are then collected in the system’s funnel and are discharged to a container, bulk bag or silo either continuously or intermittently, depending on the needs of the plant.
A peeling drum, which peels away the outer reacted surface of the saturated granules, can be used to increase the systems’ economic efficiency. The recovered granules are returned by a pneumatic conveyor to the top of the absorber, where they are then reused. The discharged limestone waste can also be reprocessed in a mill for use as a brick manufacturing material.
High SOx concentrations with simultaneously high HCL, HF and/or dust concentrations can be treated using dry sorption with a fabric filter. In this technology, a dosing system is used to introduce the absorbent to the gas flow. The reacted product is separated in a bag filter (needle felt is used as the filter medium) and is then discharged. The majority of the discharged material can be returned to the process to maximize the use of the absorbent material. Up to 80 percent saturation can typically be achieved before new absorbent material is needed. Some suppliers offer an upgrade that allows an even greater saturation.**
If the temperature of the flue gas is higher than 260 degrees C, cooling air will need to be introduced. Alternatively, some suppliers offer a heat exchanger** that captures the excess heat for use in another part of the plant, such as a dryer, enabling manufacturers to significantly reduce their energy consumption. The exchanger works according to the cross- and counter-current principle: Flue gases flow past flat, square tubes installed in the exchanger block and deliver the thermal energy through the tube walls to the cooling air flowing in the opposite direction. The heat exchanger is always operated above the dew point by adding the recovered warm air to the cooling air. As a result, wall condensation is avoided. Any deposits/buildup arising from the contaminated hot gases are removed through a simple mechanical system called “purification chains.” Since the exchanger blocks are located above each other, a compact, space-saving design can be achieved even when a large amount of heat capture and reuse is required.
Reducing EmissionsWith the publication of the final MACT Rule in the Federal Register, companies will need to begin evaluating technologies that will enable them to meet the new emission standards. However, it is important to look at the overall effectiveness of a given technology in relation to its installation, operating and maintenance costs. Packed bed filters feature lower maintenance requirements, energy consumption and operating costs compared to many other flue gas cleaning technologies. With these systems, brick and structural clay manufacturers can cost-effectively meet strict environmental standards.
References:*The final “National Emission Standards for Hazardous Air Pollutants for Brick and Structural Clay Products Manufacturing; and National Emission Standards for Hazardous Air Pollutants for Clay Ceramics Manufacturing,” commonly known as the MACT Rule, was published in the Federal Register on May 13, 2003. For more information about the final rule, contact Mary Johnson, Combustion Group, Emission Standards Division U.S. EPA, (919) 541–5025 or e-mail firstname.lastname@example.org.
**The saturation upgrade option and heat exchanger were developed and are supplied by Hellmich GmbH & Co. KG, Kirchlengern, Germany.