Fabric filtration “is becoming the technology of choice for power generation”
Filtration is a critical element in power generation systems. Whether maintaining the performance of gas-fired turbines or preventing the discharge of fly ash from the chimneys of coal or other sources of solid fuels, proper filtration is key to achieving environmental performance, writes David Appleyard
One characteristic of thermal power generation is the requirement for filtration to maintain performance within environmental parameters. With such standards and regulations on a steadily tightening trajectory, the challenge for the manufacturers and designers of filtration equipment is to maintain appropriate air and gas flow rates, whilst improving filtration performance.
For example, for gas turbines, inlet filtration is key to maintaining the efficiency of the machine, which inevitably has an impact on emissions. Steve Hiner, chief engineer for the Gas Turbine Inlet Filtration business at Clarcor Industrial Air, explains: “Obviously a gas turbine takes in a lot of air, and that air is dirty. Depending on where in the world the turbine is operating, there will be different types of contaminant to consider. That contaminant can cause different problems within the machine – for example, causing erosion if particles are big enough. At 2 microns or less [particle size] you’re likely to get fouling of the compressor, so it won’t permanently damage it, but it will over time reduce the efficiency, meaning the turbine will need to be turned off to be washed. Moving on further into the machine, corrosives can get into the hot section, the combustion chambers and rotating parts of the turbine, particularly when contaminants like salt mix with the sulphur in the fuel and cause hot end corrosion. Sodium combined with sulphur is a particular chemical that attacks the materials that are used in the hot section.
“Filtration stops a lot of the contaminant that mixes with the sulphur from getting to the hot end of the gas turbine. For instance, if you are in a coastal environment, salt going in through the inlet is typically one of the biggest areas where salt can combine with the sulphur – causing your hot end corrosion, so if you stop the salt, you stop the corrosion.”
For solid-fuelled thermal plant, fly ash and the products of combustion are more significant, but the trend for tougher regulatory frameworks is having an impact here too.
David Booth, representative for Germany’s BWF Tec GmbH & Co KG, says: “I think within Europe and most of the rest of the developed world, there is a push for tighter emission control, not just on dust control, but things like HCl, NOx and SOx.”
While filtration will not have any direct impact on, for example, SOx and NOx, a typical control method involves injecting lime or pulverized activated carbon into the gas stream to absorb such gases. Says Booth: “Obviously that increases the dust load, so it increases the burden on the needle felt [filter media]”.
Filter units have seen great strides in efficiency
Credit: Clarcor Industrial Air
Corinne Fields, director of Technical Services at Clarcor Industrial Air, echoes Booth in observing the trend for tighter emissions control: “Globally we’re seeing a shift. It seems to be that big power generators largely use electrostatic precipitators as their filtration technology to filter the fly ash. What we’ve seen is a shift where the electrostatic precipitator efficiencies were no longer meeting the new regulations in the way of particulate matter output. Fabric filtration is becoming the technology of choice for power generation, if we’re burning coal or a mixed fuel.
“What we’re seeing worldwide is a tightening up of the allowable particulate matter that can come out of a power generation stack.”
This trend is driving uptake of fabric filtration as a retrofit solution on existing plant. Says Fields: “There are electrostatic precipitators that, when operated with the right kind of control, can capture very fine dust and can capture to those low limits, but in most cases, because the plant is of an older vintage, so is the control equipment. Therefore, when people have to look at upgrading, they tend to choose fabric filtration because it requires less power to run, capital equipment investment is less and the ROI is better.”
Fields also picks up on some regional trends. “Most recently in China,” she says, “we have seen an outlet emissions target of 5 mg per cubic metre of dust output. In the US most electrostatic precipitators are being taken out of service; if the plant is old enough the entire plant is being taken out of service, and the new equipment that is going in is a fabric filter. In India we are seeing a trend where the Indian power plants are looking at converting their existing electrostatic precipitators using the current housing and making it into a fabric filter. The trend towards fabric filtration is fairly consistent, but how people are going about it is a little different. Some utilities are looking at purchasing brand new fabric filters, and in other cases they are choosing to use their existing infrastructure and box footprint to convert from ESP [electrostatic precipitator] to a fabric filtration system.”
BWF’s Booth also notes the trend toward fabric filtration: “The electrostatic is probably going to fall foul of legislation for the future, so some places will have to change to a bag house to meet the new regulations. The running costs of a precipitator are less than the bag house, but obviously if that precipitator is no longer meeting regulations then something has to be done about it.”
Driving filter performance
Given that the cost of filters is a significant operational expenditure, manufacturers of filters are being pushed to make such units more efficient and simultaneously extend the operational lifespan of the filter bags.
Says Hiner: “It’s about the type and amount of material. It doesn’t always mean that the mass of material goes up; it’s about making the media area that’s in the filter effectively used. The amount of area is going up, you’re doing that with pleating technology and spacing on the pleating-type technology, holding the pleats apart, then – depending on the types of filter – increasing the number of filters or the depth of the filter by making the filter larger or deeper in length.”
In addition, newer filter materials are being used. “Traditionally it used to be glass fibre; cellulose fibres that were available. As the non-woven fabric industry has built up, synthetic fibres and fibres made with polymer have improved over time. The advantage with polymer-based fibres is that they don’t swell and take up liquid, they’re pretty robust and don’t break easily or expand,” notes Hiner.
He points to advances in efficiency that have expanded their use: “Over the last five, six, seven years nanofibre technology has really come in on synthetic fibres in a big way, and they are approaching the same levels of efficiency as glass fibre. In addition, some membrane-type technologies are being used, such as PTFE [polytetrafluoroethylene].”
However, there are some technical challenges associated with PTFE membrane when used in gas turbine inlet filtration. Says Hiner: “There have been a number of problems with that particular material in the field. PTFE is particularly sensitive to hydrocarbons in the real world and, when they have some hydrocarbon on them, the filters react negatively to moisture, mist and fog. What we are seeing as a trend is that, as people go to higher efficiency filters, they tend to become more sensitive to mist and fog.” Hiner does note that some manufacturers have managed to deal with this issue by protecting the filter membrane, for example by putting additional filter layers in front them.
Highlighting general technology trends for filter media, Hiner says: “On the filter side we are talking about lower pressure loss filters, filters with longer life and higher efficiencies. A lot of it is developing these synthetic media to be more efficient and cost-effective.”
Choosing appropriate filter media
When considering fabric filtration, an appropriate choice of filter media suitable for the specific application is emphasized as critical to longevity.
“The fibre choice is determined exactly by the application,” says Booth. “If everything was in an ideal situation, people would use polyester for everything, because it’s cheap and everyone can make polyester needle felt. Unfortunately the technical restrictions that some applications have mean you can’t use polyester, whether that be temperature or aggressive chemicals in the gas stream.”
As a result, alternative solutions have been developed. Booth notes: “We have to make needle felts using, for example, meta-aramid for temperature resistance or PTFE fibres which can stand temperatures up to about 250˚C-260˚C and not are attacked by the aggressive chemicals you might find in some applications, such as municipal waste incinerators.”
He concludes: “We can only use certain fibres dependent on the application and the end process conditions. It is the correct choice of the fibre that suits the application.”
As an example, Booth cites hot gas filtration. To use heat sensitive polyester, the volume of exhaust gas must be diluted with ambient air to bring the temperature down. Polyester could be used, but the bag house would be impractically large.
This issue is also picked up by Fields, who notes: “Some of the developments we have seen are based around the operating conditions in the units themselves. Fabric filtration is very heavily hinged on operating temperature, so if the operating temperatures are controlled in the process train, meaning maybe lime scrubbing systems, air heaters, we’ve got ways to cool the gas very consistently and effectively.”
Where temperatures are in excess of 350˚F, fibr eglass-based media is in more common use as a standard medium. However, Fields explains: “The downside of fibreglass media is the fragility. It needs to be coddled quite a bit, and it also has the lowest cycle life, meaning you can only clean that bag a finite number of times before it mechanically breaks down.”
Fields highlights the emergence of PTFE membranes, as developed by WL Gore several decades ago, as a key breakthrough in filter technology.
“What has happened in filtration across the board,” she says, “is that the efficiencies have been improved by laminating each one of those [traditional, standard] media with PTFE membrane. When we figured out how to laminate that very thin membrane on the dust collection surface of a filter bag, we ended up with an efficiency that was better than what a normal filtration fabric would do.”
The importance of operators
Although filter producers are under pressure to deliver longer warranty periods for their products, the influence of operational characteristics cannot be overemphasized in terms of maintaining filter performance. Says Booth: “Once we’ve supplied the filter media to the end user, the end user is in total control of his application.”
Obviously, warranties for filter media come with a string of operating conditions, and should operators fall foul of these conditions, any warranty will likely be rendered null and void. Booth explains: “For instance, if you take the temperature 20 or 30 degrees up above the operating condition, then we are not responsible for cooking the filter bags.”
Conversely, while operating conditions can adversely affect the filter system, operators can also have a positive impact on filter bag lifespan. Booth says: “What they can do is give it a consistent operating condition. One of the worst things you can do with a hot house application is to constantly go up and down in temperature, particularly where you take the temperature through either the acid or water dew point temperature, because then condensation takes place and you get liquid acid forming that will actually attack the filter bag.”
Fields also picks up on improvements to the lifespan of filter products. “There have been incremental improvements made to the durability of that media, and Clarcor has made advances in the adhesion of the membrane to the base media and the robustness for specific industrial applications. The true advances have been made in better understanding how this impacts the operation of the back end filtration system and improvement of filter life.
“On average, we see an increase in the life of the filter that is somewhere between two and three times what the original base media was able to achieve in the same application. There you have an improvement in both capture efficiency the ability to capture very fine particulate. The efficiencies are somewhere around 99.998 per cent in that capture, but by virtue of the easier ‘clean-ability’ of that media, we’re expanding the total filter life as well.”
Fields explains: “A filter bag will actually fail when it reaches the end of its effective cycle life. You can only pulse it or clean it so many times before it fractures. If you have a filter bag that’s easier to clean down, you clean it less often and less violently, thereby extending its life.”
She continues: “The extension of life span for that filter has everything to do with the PTFE membrane preventing the dust from penetrating into the depth of the filter medium, which would eventually allow the dust to migrate through the depth of the media and then cause emissions which would have you scheduled to replace all of the filters.”
Echoing Booth, Fields also picks up on the importance of selecting application-appropriate filter media: “There are applications out there – for example, where we’re trying to control NOx emissions, HCl emissions, mercury emissions. We have come up with a product, the next-generation filtration technology, that has taken the filtration media and basically manufactures it in an accordion pleat. By accordion pleating the filter media, we now create a peak-and-valley structure that allows for retention of the dust on the surface of the filter. What we’ve found is that, in the presence of hydrochloric acid, if mercury is an issue there is a reaction that takes place that naturally allows for those two chemicals to react and be taken out of the gas, meaning that there is a reduction in total mercury emissions.”
“As a rule in our organization, our first approach is to always use a standard filter media, because it is the most cost-effective for the end user. If there are problems, we start applying what we would refer to as fine filtration options – the PTFE membranes, the pleated filter structure – to solve some inherent problems that the user is having to live with, and it precludes them from having to re-invest in yet another capital project.”
But Fields also notes the impact operators can have on the operational lifespan of filters. “We manage the entire process from engineering, designing and manufacturing that filter, all the way through to ensuring how that filter plays into different applications and industries,” she explains. “The reality is that, in an entire production process, the baghouse dust collector with fabric filters in it is at the very end of that process. Any issue that is happening that may not be chemically correct manifests more often than not in the baghouse, and problems start occurring there whether they are temperature- or moisture-related.”
Operational conditions are crucial
Indeed, Fields emphasizes the impact of operational conditions: “Much of the success of any type of filtration technology, regardless of the base fabrics, surface treatment or geometry of the filter, has a lot to do with the way that people operate their equipment, their understanding of how of all of this plays together for a successful outcome. So much of our time is spent talking to people about how to set the filters up to clean, is there such a thing as cleaning them too much or too violently, how do you start these pieces of equipment up without creating these dew-point conditions and, frankly, operating the dust collector with its own little weather system on the inside, which causes multiple issues, from plugging of material handling systems to corrosion on the metal to extremely high pressures in that system, or plugging of that system. So there’s much more of a holistic approach, where the product is only one part of the entire system. The long and short of it is that, although there are certainly development ideas underway, more often than not it’s much more about operating the entire system.”
David Appleyard is a freelance journalist focusing on the energy, technology and process sectors