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Technology review: the potential of ceramic nanofibres in hot gas filtration
18
Cover story
Technology review: the potential of ceramic nanofibres in hot gas filtration
Filtration+Separation May 2006
ceramic materials. This has led to the trademarking of two brands; SootGrabber and FilterHot; these correspond to the needs of the automotive and energy industries respectively. Nanofibres are fibres with diameters measured on the nanometre scale; a nanometre is one millionth of a millimetre. A way to visualise their size is to imagine about 1,000 nanofibres of 50 nm diameters aligned side-by-side as having the same thickness as a human hair.
Figure 1. SiO2 nanofibres with average diameter of 300 mm
Ceramic nanofibres, made by the electrospinning process, could represent a suitable new material for the filtration of hot gases in the range of 1200°C – 1800°C. These ceramic fibres, with dimensions in the range of 10 to 1000 nanometres (nm), can capture nanoparticles by electrostatic, diffusion, as well as direct interception mechanisms (see figure 1). When combined with ceramic fibres that possess micrometre dimensions for structural support, this kind of filter media provides little resistance to flow, and could be ideal for hot gas filtering in a variety of applications.
Electro- spinning ceramic nanofibres Electro- spinning emerged in the early twentieth century in the textile industry, and polymeric nanofibres have been around since the 1930s. Researchers at the Ohiobased University of Akron’s renowned Goodyear Polymer Center, were the first to make ceramic fibres by this method, and in 2005 MemPro Ceramics Corp entered into an agreement with the University of Akron’s Research Foundation to commercialise silicon carbide nanofibres and other
Potential applications for nanofibres • Filtration •
Catalytic systems
•
Structural applications and reinforcement
•
Non-woven fabrics
•
Wound dressings
•
Surgical and internal devices
•
Therapeutic and drug delivery systems
•
Energy generation and recovery
•
Electronics and coatings
•
Military and defense
Electro- spinning produces nanofibres that are very long, and which ‘grow’ very fast. A single polystyrene packing peanut – those small, white, green or pink cushions we use to protect fragile items in shipment – would have enough material to make a nanofibre that would wrap around the Earth at the equator, some 25,000 miles. And electro- spinning can produce a 25,000 mile long fibre in about two hours – a real ‘Jack and the Beanstalk’ growth rate. Electro- spun nanofibres have potential in an array of applications (see below left).
Filtration with ceramic nanofibres The filter industry has embraced the concept of nanofibres, because fibres in this range improve filter performance, particularly when sub-micron particles are of interest. From a production perspective, nanofibres can be made with more robust micro fibres for structural support either in a matrix or in layers. Making a very thin membrane with nanofibres has also been explored by MemPro’s research partners in Akron. For hot gas applications we are pursuing a matrix-type production that allows nanofibres to bridge the gaps in threedimensions between ceramic microfibres. The benefit of this approach is, we believe, a very low resistance to flow, needed in commercial applications. A curious characteristic of ceramic nanofibres is the fact that when formed into a nonwoven mat of several layers, in some cases, the resulting ceramic ‘fabric’ is flexible. We did not expect this from our prior experience with ceramic materials because of their usual brittleness. No doubt this flexibility supports the hypothesis that materials in the nano range have unusual and unexpected characteristics. As we learn more about producing ceramic nanofibres, we expect to develop data that support this and other material characteristics.
Summary of Regulations 1963 1970 1971 1977 1979 1990 2005 2005
Clean Air Act (CAA) EPA empowerment to enforce air quality New Source Performance Standards (NSPS) CAA amendment set limits in underperforming areas NSPS added SO2 emissions Clean Air Act Amendments added NOx reductions by 1995 & 2000 Energy Policy Act Clean Coal Power Initiative
Energy and hot gases In addition to the engine and automotive industry (see main cover story), there is a history of regulations in the USA that suggest growing opportunities in the energy industry for companies that produce ceramic nanofibre filters. The recent run up in energy prices has indeed helped the cause of ceramic nanofibre technology, since coal-based energy appears to be a growth market. US legislation (see above) over the past 40 years supports the need for better hot gas filtration. According to a study by the Massachusetts Institute of Technology, coal represents 50% of energy sources for the US energy industry. A secure, national supply of coal places this source of hydrocarbons in a priority position for use, while the industry faces challenges in meeting environmental regulations for lower emissions. Unwanted by-products of coal-fired energy plants include carbon dioxide (a cause of global warming), nitrogen oxides (smog, soot and acid rain), sulphur dioxide (smog, soot and acid rain), fine particulate matter – designated by PM 2.5 (respiratory and health problems) – and mercury (toxic to humans). The current technologies to address these by-products are listed on page 19.
Ceramic nanofibre benefits Particle capture with ceramic nanofibres can lead to highly efficient, low backpressure filters. Ceramics are durable by nature, and ceramic nanofibres can operate in ‘aggressive environments’ including high and low pH, high pressures and high temperatures. Cleaning and regenerating filters made with ceramic materials can be achieved with a better
Cover story
Filtration+Separation May 2006
result than, for example, the cleaning of polymeric membranes. Ceramic membranes, when properly maintained, can have long useful lives, and this leads to little disruption of service. Nanofibres are also potentially attractive catalyst carriers. The small diameter of the nanofibres produces a large surface area, and catalyst deposition on, or within, nanofibres can consume tiny amounts of catalyst, which are generally high value metals. Because catalysis is broadly used in
Continued from page 17... oxidation of carbon by the nitrogen dioxide so produced. • Active systems; The second approach is to actively trigger regeneration by raising the temperature of soot trapped in the filter through the use of an outside energy source. There are two obvious energy sources that are available on-vehicle: diesel fuel and electricity. • Combined systems; It is also possible to combine passive and active regeneration in which a catalyst-based filter is equipped with some kind of active regeneration system. Passive-active combinations, depending on the type and loading of the catalyst, may be able to sustain fully passive operation during periods of increased exhaust temperature.
industry, the opportunity for ceramic nanofibres is large. Ceramic nanofibres can also be used for energy feedback. Some preliminary research sponsored by the National Science Foundation has suggested the possibility of P/N junctions and electrical conductivity within ceramic nanofibres. If such characteristics are proven, there will be opportunities to use ceramic nanofibres to capture lost energy from heat-producing sources, such as smoke stacks and engine exhaust.
thermal stress on the filter material. If regeneration is performed periodically, the regeneration event must be triggered often enough to prevent high soot loads, as well as the related high exotherms during soot combustion. Most problems with diesel particulate filter systems are caused by: • Insufficient, slow regeneration, leading to excessive pressure drops; • Rapid, “uncontrolled” regeneration leading to runaway temperatures (exotherms). These two problems are indeed related to each other, as slow regeneration leads to an excessively high soot load in the filter, which in turn is one of the conditions predisposing uncontrolled regeneration.
Filter configurations – fibres vs. monoliths
Diesel filtration: the lowdown
• Monoliths;
Diesel exhaust particulate filters have been under development since about 1980* and have been in use since about 1985. The filter substrate is critical to the performance and durability of the filter.
As table 1 shows, ceramic wall-flow monoliths are by far the most common type of filter used either commercially or in research projects. The materials used are either cordierite (2MgO.2Al2O3.5SiO2), available from Corning, NGK and Denso or, less often, the silicon carbide (SiC) cell filter available from Ibiden, NGK, NoTox and LiqTech. SiC offers a superior high-temperature performance, but also a higher thermal expansion coefficient and higher cost.
Its task is to capture solid particulates and retain them until the filter is regenerated, either intermittently or continuously. In addition, the filter material must not adversely affect the exhaust emissions in any way – for example, by disintegrating to form particles that are emitted in the exhaust. The highest media temperatures are typically encountered during regeneration and may be as high as 1500OC. In an ideal situation, particulates – which deposit on the filter – are oxidised without ever creating an excessive soot buildup in the filter medium. The filter maintains a low-to-moderate soot load, which produces an acceptable pressure loss. Continuous regeneration does not produce high temperature peaks due to the exothermic combustion of soot, thus, there is little
Wall-flow monoliths provide very good filtration efficiencies, often greater than 95%, although often at the expense of a high delta P and a rapid increase in delta P with increasing soot loading. Monolith filters tend to be voluminous and rather heavy when containment hardware is included. Both the cordierite and silicon carbide versions can readily be coated with catalysts for use in passive filtration systems.
Current technologies to solve problems with unwanted by-products of coal power plants. By-product Current technologies to minimise CO CO2 scrubbing; acid gas removal NOx Selective catalytic reduction SO2 Scrubbers, gasification, fluidised-bed combustion PM 2.5 Filter bags Mercury Injection of powdered activated carbon
steel mandrel for support. In the other, thin, usually air- or water-laid ceramic fibre sheets resembling paper or fabric are made and then fabricated into cartridges that are often similar in concept to oil or air filters used in other forms of filtration. Types of fibre used have included silicon carbide and various forms of glass fibre. Ceramic fibres have generally not been used for catalysed passive-regeneration filters because of difficulties in applying the catalyst (and in getting it to adhere reliably) to the ceramic fibre substrate. All fibre systems present the possibility of fibre emissions, due to fragmentation or failure caused by thermal stresses. • Knitted fibre filters; Highly flexible 3M Nextel fibres (62 wt% Al2O3, 24 wt% SiO2, 14 wt% B2O3) have been extensively investigated for diesel particulate. Other designs have supported woven or knitted fibre layers on wire mesh that is formed to provide a spiral flow path.* • Fibre papers and fabrics; The fibres used may also be formed into thin sheets using either dry- or wet-laying methods similar to those used for some papers and for low-temperature pleated-paper filter media.
Other configurations • Ceramic foam filter media; Continuous-pore ceramic foam materials were once thought to have great potential as diesel Table 1: approximate market share by DPF type Material
market share (%)
Ceramic wall-flow monolith (cordierite)
70
Ceramic fibre (wound and knitted)
25
• Ceramic fibre filters;
Sintered metal
4
Two basic types of ceramic fibre filter have been tried. In one, the fibre is wound onto a
All other
1
19
Cover story
Technology review: the potential of ceramic nanofibres in hot gas filtration
Filtration+Separation May 2006
ceramic materials. This has led to the trademarking of two brands; SootGrabber and FilterHot; these correspond to the needs of the automotive and energy industries respectively. Nanofibres are fibres with diameters measured on the nanometre scale; a nanometre is one millionth of a millimetre. A way to visualise their size is to imagine about 1,000 nanofibres of 50 nm diameters aligned side-by-side as having the same thickness as a human hair.
Figure 1. SiO2 nanofibres with average diameter of 300 mm
Ceramic nanofibres, made by the electrospinning process, could represent a suitable new material for the filtration of hot gases in the range of 1200°C – 1800°C. These ceramic fibres, with dimensions in the range of 10 to 1000 nanometres (nm), can capture nanoparticles by electrostatic, diffusion, as well as direct interception mechanisms (see figure 1). When combined with ceramic fibres that possess micrometre dimensions for structural support, this kind of filter media provides little resistance to flow, and could be ideal for hot gas filtering in a variety of applications.
Electro- spinning ceramic nanofibres Electro- spinning emerged in the early twentieth century in the textile industry, and polymeric nanofibres have been around since the 1930s. Researchers at the Ohiobased University of Akron’s renowned Goodyear Polymer Center, were the first to make ceramic fibres by this method, and in 2005 MemPro Ceramics Corp entered into an agreement with the University of Akron’s Research Foundation to commercialise silicon carbide nanofibres and other
Potential applications for nanofibres • Filtration •
Catalytic systems
•
Structural applications and reinforcement
•
Non-woven fabrics
•
Wound dressings
•
Surgical and internal devices
•
Therapeutic and drug delivery systems
•
Energy generation and recovery
•
Electronics and coatings
•
Military and defense
Electro- spinning produces nanofibres that are very long, and which ‘grow’ very fast. A single polystyrene packing peanut – those small, white, green or pink cushions we use to protect fragile items in shipment – would have enough material to make a nanofibre that would wrap around the Earth at the equator, some 25,000 miles. And electro- spinning can produce a 25,000 mile long fibre in about two hours – a real ‘Jack and the Beanstalk’ growth rate. Electro- spun nanofibres have potential in an array of applications (see below left).
Filtration with ceramic nanofibres The filter industry has embraced the concept of nanofibres, because fibres in this range improve filter performance, particularly when sub-micron particles are of interest. From a production perspective, nanofibres can be made with more robust micro fibres for structural support either in a matrix or in layers. Making a very thin membrane with nanofibres has also been explored by MemPro’s research partners in Akron. For hot gas applications we are pursuing a matrix-type production that allows nanofibres to bridge the gaps in threedimensions between ceramic microfibres. The benefit of this approach is, we believe, a very low resistance to flow, needed in commercial applications. A curious characteristic of ceramic nanofibres is the fact that when formed into a nonwoven mat of several layers, in some cases, the resulting ceramic ‘fabric’ is flexible. We did not expect this from our prior experience with ceramic materials because of their usual brittleness. No doubt this flexibility supports the hypothesis that materials in the nano range have unusual and unexpected characteristics. As we learn more about producing ceramic nanofibres, we expect to develop data that support this and other material characteristics.
Summary of Regulations 1963 1970 1971 1977 1979 1990 2005 2005
Clean Air Act (CAA) EPA empowerment to enforce air quality New Source Performance Standards (NSPS) CAA amendment set limits in underperforming areas NSPS added SO2 emissions Clean Air Act Amendments added NOx reductions by 1995 & 2000 Energy Policy Act Clean Coal Power Initiative
Energy and hot gases In addition to the engine and automotive industry (see main cover story), there is a history of regulations in the USA that suggest growing opportunities in the energy industry for companies that produce ceramic nanofibre filters. The recent run up in energy prices has indeed helped the cause of ceramic nanofibre technology, since coal-based energy appears to be a growth market. US legislation (see above) over the past 40 years supports the need for better hot gas filtration. According to a study by the Massachusetts Institute of Technology, coal represents 50% of energy sources for the US energy industry. A secure, national supply of coal places this source of hydrocarbons in a priority position for use, while the industry faces challenges in meeting environmental regulations for lower emissions. Unwanted by-products of coal-fired energy plants include carbon dioxide (a cause of global warming), nitrogen oxides (smog, soot and acid rain), sulphur dioxide (smog, soot and acid rain), fine particulate matter – designated by PM 2.5 (respiratory and health problems) – and mercury (toxic to humans). The current technologies to address these by-products are listed on page 19.
Ceramic nanofibre benefits Particle capture with ceramic nanofibres can lead to highly efficient, low backpressure filters. Ceramics are durable by nature, and ceramic nanofibres can operate in ‘aggressive environments’ including high and low pH, high pressures and high temperatures. Cleaning and regenerating filters made with ceramic materials can be achieved with a better
Cover story
Filtration+Separation May 2006
result than, for example, the cleaning of polymeric membranes. Ceramic membranes, when properly maintained, can have long useful lives, and this leads to little disruption of service. Nanofibres are also potentially attractive catalyst carriers. The small diameter of the nanofibres produces a large surface area, and catalyst deposition on, or within, nanofibres can consume tiny amounts of catalyst, which are generally high value metals. Because catalysis is broadly used in
Continued from page 17... oxidation of carbon by the nitrogen dioxide so produced. • Active systems; The second approach is to actively trigger regeneration by raising the temperature of soot trapped in the filter through the use of an outside energy source. There are two obvious energy sources that are available on-vehicle: diesel fuel and electricity. • Combined systems; It is also possible to combine passive and active regeneration in which a catalyst-based filter is equipped with some kind of active regeneration system. Passive-active combinations, depending on the type and loading of the catalyst, may be able to sustain fully passive operation during periods of increased exhaust temperature.
industry, the opportunity for ceramic nanofibres is large. Ceramic nanofibres can also be used for energy feedback. Some preliminary research sponsored by the National Science Foundation has suggested the possibility of P/N junctions and electrical conductivity within ceramic nanofibres. If such characteristics are proven, there will be opportunities to use ceramic nanofibres to capture lost energy from heat-producing sources, such as smoke stacks and engine exhaust.
thermal stress on the filter material. If regeneration is performed periodically, the regeneration event must be triggered often enough to prevent high soot loads, as well as the related high exotherms during soot combustion. Most problems with diesel particulate filter systems are caused by: • Insufficient, slow regeneration, leading to excessive pressure drops; • Rapid, “uncontrolled” regeneration leading to runaway temperatures (exotherms). These two problems are indeed related to each other, as slow regeneration leads to an excessively high soot load in the filter, which in turn is one of the conditions predisposing uncontrolled regeneration.
Filter configurations – fibres vs. monoliths
Diesel filtration: the lowdown
• Monoliths;
Diesel exhaust particulate filters have been under development since about 1980* and have been in use since about 1985. The filter substrate is critical to the performance and durability of the filter.
As table 1 shows, ceramic wall-flow monoliths are by far the most common type of filter used either commercially or in research projects. The materials used are either cordierite (2MgO.2Al2O3.5SiO2), available from Corning, NGK and Denso or, less often, the silicon carbide (SiC) cell filter available from Ibiden, NGK, NoTox and LiqTech. SiC offers a superior high-temperature performance, but also a higher thermal expansion coefficient and higher cost.
Its task is to capture solid particulates and retain them until the filter is regenerated, either intermittently or continuously. In addition, the filter material must not adversely affect the exhaust emissions in any way – for example, by disintegrating to form particles that are emitted in the exhaust. The highest media temperatures are typically encountered during regeneration and may be as high as 1500OC. In an ideal situation, particulates – which deposit on the filter – are oxidised without ever creating an excessive soot buildup in the filter medium. The filter maintains a low-to-moderate soot load, which produces an acceptable pressure loss. Continuous regeneration does not produce high temperature peaks due to the exothermic combustion of soot, thus, there is little
Wall-flow monoliths provide very good filtration efficiencies, often greater than 95%, although often at the expense of a high delta P and a rapid increase in delta P with increasing soot loading. Monolith filters tend to be voluminous and rather heavy when containment hardware is included. Both the cordierite and silicon carbide versions can readily be coated with catalysts for use in passive filtration systems.
Current technologies to solve problems with unwanted by-products of coal power plants. By-product Current technologies to minimise CO CO2 scrubbing; acid gas removal NOx Selective catalytic reduction SO2 Scrubbers, gasification, fluidised-bed combustion PM 2.5 Filter bags Mercury Injection of powdered activated carbon
steel mandrel for support. In the other, thin, usually air- or water-laid ceramic fibre sheets resembling paper or fabric are made and then fabricated into cartridges that are often similar in concept to oil or air filters used in other forms of filtration. Types of fibre used have included silicon carbide and various forms of glass fibre. Ceramic fibres have generally not been used for catalysed passive-regeneration filters because of difficulties in applying the catalyst (and in getting it to adhere reliably) to the ceramic fibre substrate. All fibre systems present the possibility of fibre emissions, due to fragmentation or failure caused by thermal stresses. • Knitted fibre filters; Highly flexible 3M Nextel fibres (62 wt% Al2O3, 24 wt% SiO2, 14 wt% B2O3) have been extensively investigated for diesel particulate. Other designs have supported woven or knitted fibre layers on wire mesh that is formed to provide a spiral flow path.* • Fibre papers and fabrics; The fibres used may also be formed into thin sheets using either dry- or wet-laying methods similar to those used for some papers and for low-temperature pleated-paper filter media.
Other configurations • Ceramic foam filter media; Continuous-pore ceramic foam materials were once thought to have great potential as diesel Table 1: approximate market share by DPF type Material
market share (%)
Ceramic wall-flow monolith (cordierite)
70
Ceramic fibre (wound and knitted)
25
• Ceramic fibre filters;
Sintered metal
4
Two basic types of ceramic fibre filter have been tried. In one, the fibre is wound onto a
All other
1
19