Final Report: Innovative Filters Using Nanomaterials for Removal of Gaseous Pollutants and Particulates from Contaminated Air Streams

EPA Contract Number: EPD16003
Title: Innovative Filters Using Nanomaterials for Removal of Gaseous Pollutants and Particulates from Contaminated Air Streams
Investigators: McKenna, John D
Small Business: ETSVP-JV, LLC
EPA Contact: Richards, April
Phase: II
Project Period: February 1, 2016 through January 31, 2018
Project Amount: $299,985
RFA: Small Business Innovation Research (SBIR) - Phase II (2015) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , Nanotechnology , SBIR - Air Pollution Monitoring and Control

Description:

A uniquely qualified team consisting of ETSVP-JV, LLC, a small business joint venture, and subcontractor RTI International, brought together all of the skills and experience needed for the development and testing of innovative reactive nanofiber filtration media for controlling PM2.5 and volatile organic compounds (VOCs). Materials developed for the Department of Defense to protect troops from chemical and biological agents are the foundation for next generation filter media for pollution control of Hazardous Air Pollutants (HAPs). For the Phase II project effort, our team set out to develop a novel dual stage reactive nanofiber filtration media that significantly enhances combined PM and HAP baghouse control capabilities over current state-of-the-art.

 

The reactive nanofiber filtration media is comprised of two components: a nanofiber membrane and a reactive nanoscale coating. For the Phase II project our team focused on the methods for forming and combining the two components of the reactive nanofiber membrane into a single filtration system. The nanofiber membrane consists of fibers with a fiber size and distribution controlled to below 200nm. The membrane is supported by a nonwoven backing mat to support the nanoscale architecture of the nanofiber membrane. The technique developed to deposit the nanofibers on the nonwoven support arranges the fiber orientation into a unique three-dimensional structure of fibers that controls the pore size and slip flow velocity through the media. The unique nanofiber structure improves the interception and collection of particles, such as fine particulates like PM2.5, because the particle size is much larger than the fibers in the media. The nanofiber membrane when compared to other filtration materials has significantly better particle filtration collection efficiency as a function of pressure drop. The second component of the filtration media consists of a method to coat fibers with highly conformal, reactive nanoscale coatings that are typically less than 25nm. The nanoscale coatings can be deposited from a wide range of chemistries including some with catalytic properties. The commercial target for the nanofiber, reactive media is aimed at pollution control and reduction of emissions from sources such as coal fired power plants. The membrane offers two routes for commercialization and improving air quality. The first approach is targeted at collection of fine particulate matter from hazardous air streams by protecting the nanofiber membrane with a thin nanoscale coating that encapsulates the fiber media with a hard shell and protects it in aggressive high temperature stack conditions. The second approach is targeted at reducing HAPs such as VOCs by applying reactive or catalytic nanoscale coatings on the media. The reactive coatings take advantage of the high surface area of the media by increasing the available surface area for reaction and oxidation without sacrificing any of the high particle filtration capabilities.

Summary/Accomplishments (Outputs/Outcomes):

In the first case fine particulate emission control was demonstrated at a level below the project goal of 0.00004 grains per dry standard cubic foot (gr/dscf). Using the ASTM D6830-02 test method and the EPA/ETV protocol, the filtration performance testing resulted in PM2.5 and total mass outlet particle concentrations below the detection limit of 0.0000073 gr/dscf. Air permeability values in the range of 4-10 ft3/min of gas per ft2 of cloth at 0.5 inch w.g. (fpm) were measured along with average Mullen burst strengths greater than 600 psi. Average MIT flex endurance values of greater than 100,000 flexes in the warp direction and greater than 65,000 flexes in the fill direction were registered. An objective of the Phase II effort was to scale up the nanofiber membrane production and bond the membrane to the polyphenylene sulfide (PPS) felt to form the composite media. This pilot-scale produced media would then be used to make both small and full-sized filter bags for testing. Building off of the Phase I work, we focused on the same polymer chemistry for the nanofibers for most of the project. A pilot run by a commercial vendor provided a 63-inch wide, 22-yard long roll of nanofibers on a nonwoven backing that met the specifications of good filtration for fine particles (>99.5% for 0.3 µm particles) and a permeability in the range of 7-13 fpm. A variety of bonding technologies were explored including: adhesive bonding, ultrasonic welding, and laminating by calendering (heat and pressure bonding). In Phase I we used a patterned heat and pressure technique to bond the membrane to the PPS felt. After exploring the three techniques with various vendors, the laminating by calendering was selected as the preferred method. Ultrasonic welding could work if further refinements were made in the process. Swatches of material that were bonded using a pattern, heat, and pressure yielded the best results for bond strength and air permeability. However, larger scale production, e.g. roll-to-roll processing, by a toll service provider did not duplicate this performance or structure due to equipment limitations (i.e. they did not have the ability to apply a pattern). Another vendor in the nonwovens industry was able to provide roll-to-roll bonding of nanofiber membrane to PPS felt with good permeability (within the target range of 5-10 fpm) and good bond strength. Potentially better results could be obtained if a tool was used that could include patterning.   Media to form filter bags was prepared using the pilot-scale produced nanofiber membrane, PPS felt, and the laminating process. Both mini filter bags and full-length filter bags were fabricated for testing.   Exposure of the composite nanofiber filter media to power plant emissions revealed that the nanofiber membrane could degrade, potentially due to exposure to acid gases. Alternate polymer chemistries were explored late in the project to provide both good heat and acid gas resistance. A small pilot-scale roll (23-inches wide, 15-yards long) was prepared with nanofibers that have improved acid gas resistance and temperature resistance to over 420ºF. Bonding and stack gas testing were not completed by the end of the Phase II project.  

For the Phase II effort the process for applying reactive catalyst nanocoatings on fiber media was improved upon and expanded to include nonwoven media including PPS felt, nylon and glass. During the Phase II project a VOC testing apparatus was constructed for testing catalyst coated fiber media for VOC removal. It was found over 90% conversion of a 200ppm toluene in air gas stream could be achieved at oxidation temperatures around 175°C (347°F). The lower oxidation temperature was a major success, thus enabling use on PPS felt media. Optimal reduction of the targeted VOC was found with less than 6% fractional gain on the fiber mat or with less than 10nm of catalyst coatings on PPS felt media. Catalyst coatings were successfully applied onto glass fiber swatches as large as 12”x12” and third party partners have been identified and discussions have been held to scale and apply the catalyst coatings on larger swatches. Discussions included developing new tool designs for applying coatings using continuous web or roll-to-roll coating methods.

Conclusions:

As particulate emission regulations have become more stringent, the use of baghouses rather than electrostatic precipitators (ESPs) for fly ash control has become prevalent. The application of the baghouse for control of fly ash emissions from Utility and industrial coal fired boilers (CFBs) predominantly employs PPS felt bags with pulse jet cleaning. As the emission codes have tightened, the use of an expanded polytetrafluoroethylene (ePTFE) membrane on the filtration media has been added so as to achieve compliance. Given the PM2.5 filtration performance demonstrated, the use of composite nanofiber filtration media for particulate emission control is technically competitive with ePTFE membranes. An Electric Power Research Institute (EPRI) survey indicated that there were 142 pulse jet baghouses installed by 2012 in U.S. power plants. Our current estimate is approximately 85 domestic CFB pulse jet baghouses. The typical pulse jet bag life is approximately 3 to 6 years, thus the bag replacement market opportunity for an alternative bag type is significant. Likewise, further study should allow application to the reverse air baghouses utilizing woven fiberglass filter bags. While fly ash control is the single largest market, there are other industrial baghouse applications where composite nanofiber filtration media can be applied. As the VOC capture is proven, this will expand the CFB opportunity as well as allow for market inroads in a number of other applications such as many areas within the refinery and chemical process industries.   It is noted that the U.S. CFB baghouse market has been recently shrinking as plants are switching fuels from coal to natural gas. The Utility coal usage market share has shrunk from greater than 50% to approximately 30%. Forecasts now indicate a leveling off for coal at close to 30% of the Utility fuel market. While the U.S. CFB baghouse market has leveled out, the International CFB baghouse installations have grown vigorously. Today, China has approximately four times the U.S. installed base of coal-fired power plants. It is also forecasted that India will double its coal use by 2020. Air pollution is a transboundary, global problem that affects us all.

References:

Journal Articles:

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SBIR Phase I:

Innovative Filters Using Nanomaterials for Removal of Gaseous Pollutants and Particulates from Contaminated Air Streams  | Final Report