September/October 2012 | Volume 31, No. 5

Figure 1: Filter media at 300 X.

Temperature dependent industrial manufacturing processes and exhaust emission control systems are making higher demands on gas filtration materials. The filter solutions for accommodating these high-temperature applications are thermal oxidizers, sintered and fiber metal filters, ceramic candles, electrostatic precipitators, and scrubbers. The pleated ceramic fiber filter system provides a number of advantages over these existing high-temperature solutions:

  • Operating temperatures to 1,200 C in air
  • Clean filter backpressures of 0.5 inches of water at 30 cfm/ft2
  • pH resistance from 2 to 14
  • Gas flow capacity from 5 cfm to 500,000 cfm
  • In-situ cleaning for 24 hour operation
  • Engineered filter media design to provide a wide range of porosities
  • Ability to accommodate oils, tars and sticky organics without blinding

The filter media and the filter cartridge design were developed over a six-year period for diesel engine exhaust filtration (diesel particulate filters). Significant funding and testing was provided by the U.S. Department of Energy, the U.S. EPA and the California Air Resource Board. The diesel engine market experienced a severe downturn in 2008 - 2011. New DPF product introduction during this period was difficult, forcing the commercialization of the ceramic fiber pleated filter to investigate new market applications. Fortunately, new markets are emerging for high-temperature filtration.

Figure 2: Pleated filter shapes.

The Ceramic Fiber Filter Media
The ceramic fiber filter can be manufactured in a range of 40% up to 90% porosity, with pore size variations from 5µ to 200µ. Figure 1 shows an electron microscope view of the filter media. All components are made of high-temperature ceramics capable of 1,200 C

operation in air. The patented fiber binder and pleating process allows one to incorporate a large filter surface area in a small footprint, as with cellulosic or polymeric fiber filters. The filter media incorporates large openings to allow a free-flow of gas to minimize backpressure, while a large number of layers of fibers provide a tortuous particle path to attract particles for the desired filtration efficiency. Ceramic fiber filter media types range from 0.015 inches to 0.100 inches, with the most common being 0.070 inches thick. This "depth" filtration allows a considerable particle trapping capacity between filter cleanings. The filtration efficiency is controlled by the thickness of the media, the ceramic fiber diameter and the quantity of the ceramic binder. These variables have been investigated through years of experimental testing to establish a firm understanding of the design of a specific media for the required particle type and efficiency performance.

Filter Cartridges and Systems
The pleated ceramic fiber media can be formed into any size and shape that is possible with other pleated filter media. Figure 2 shows examples of both rectangular and round cartridge shapes. The pleated ceramic fiber media is enclosed securely in lightweight ceramic frames. The proprietary ceramic frame material has been specifically developed for this application. The combined weight of the filter media and the filter frames is less than 1/4th the
Figure 3: Biomass syngas
weight of competing ceramic and metal filter products. This weight reduction assists in lowering the capital cost of the installation, as well as, reducing the energy requirement to clean the filter system in-situ. Figure 3 shows a large cartridge that is currently in use in a commercial biomass application. This filter cartridge has been designed to accommodate temperatures that exceed a 1,000 C operation in air. Other filter systems that remove combustible particulate at room temperature have the ability to periodically raise the temperature of the filter cartridges to approximately 600 C to combust the particulate and return the filters to a clean condition (in-situ regeneration).

Pleated Filter Cartridge
In the early period of the development, most of the testing was accomplished on diesel exhaust filtration. Testing by Oak Ridge National Laboratory on a 1.9 liter Volkswagen TDI engine and on a Mercedes vehicle exhaust produced 96% to 98% filtration efficiency on particulate down to 10 nanometers in size. June 2012 testing by an independent filter laboratory verified a 99% efficiency of the ceramic fiber media on oily mist down to a 0.5 micron size. The filter media has been verified to be 95% efficient at removing airborne grease emissions from restaurants that broil meats by the California South Coast Air Quality Management District. With respect to durability, the pleated ceramic filter has withstood over 20,000 miles of testing on a Ford diesel truck, over a thousand loading and regenerations at an independent diesel-testing laboratory, and weeks of filtration and regenerations on a biomass power generation plant filtering soot and tars.

Figure 4: In-situ cleaning

In-Situ Filter System Cleaning
Thermal oxidizers continuously heat an exhaust stream by burning natural gas to activate the catalyst-coated ceramic "bricks". This allows destruction of all particulate and VOC's in the exhaust. Maintenance problems occur with thermal oxidizers when the particulate overcomes the catalyst to blind further oxidation. At that point the catalyst bricks must be removed for cleaning or replacement. Sintered metal filters and ceramic candles can be cleaned by a high reverse flow air pulse as long as the filtered cake is dry. Sticky, oily or wet particles cannot be removed with the reverse pulse and the system must be disassembled for cleaning. Electrostatic precipitators and scrubbers produce contaminated waste streams that are expensive to clean. The ceramic fiber filter system removes the objectionable particulate by in-situ combustion. Its low backpressure properties allow the system to operate for long periods of time, while accumulating the particulate. When the filters are adequately loaded, the filter container is isolated by valves. The filters are heated to approximately 600 C (Figure 4) until all of the particulate is reduced to CO2 and H2O vapor. This process takes 30 minutes to one hour and leaves the filters completely clean and ready to resume filtering. This in-situ cleaning process can take place while the gas stream is off or one can install a dual system to switch during cleaning for continuous filtering. If VOC's are evolved during the cleaning process, the exhaust from the cleaning process can exit through a small heated catalyst-coat chimney filter to destroy the VOC's before releasing to the atmosphere.

Effective Applications
The pleated ceramic fiber filter can be installed as a "stand-alone" solution to a high temperature filtration problem, such as cleaning hot syngas in biomass power generation plants. This application eliminates expensive gas cooling, currently necessary, to remove particulate and tars before entering the power generator. The ceramic fiber filters can replace sintered metal or candle filters in applications where temperatures are too high or oily, and sticky particulate is encountered in chemical and petrochemical plants. A near-term application in industrial processes, currently in operation, is pre-filtering for those thermal oxidizers, electrostatic precipitators, and scrubbers that are experiencing maintenance problems due to particle overload or oily, wet particulate. In all of these applications, the system capital and operating costs for the pleated ceramic.

For more information contact:
Industrial Ceramic Solutions, LLC

Tel: 1-865-482-7552, Ext. 700