Gas turbine filtration- clean air for efficient power generation

Gas turbine power plants are essentially a jet engine used to generate electrical power. Their compact size allows for their installation in remote areas such as on oil rigs or in the desert where conventional power supplies can't reach. 

A Siemens Gas Turbine generator

Using low cost natural gas, Gas Turbine power units have become more and more commonly used for normal power generation as well, replacing traditional coal or oil power plants. They have a distinct advantage that the efficiency of combustion is extremely high, so whilst they are polluting in terms of greenhouse gases, they are not responsible for soot or acid pollutants such as sulfur dioxide. 

Typical gas turbine power plant 

The principle of operation is identical to a jet engine. Huge amounts of air are taken in and mixed with the fuel and ignited. The power from the rotating turbine can be converted into steam or directly into electrical energy. 

Air Filtration in Gas Turbine Powerplants However unlike normal jet engines, operating at very high altitudes above most of the polluting dust, the air intake of a gas turbine plant is close to the ground and subject to all the pollutants that affect us all including water, soot and sand.

Therefore all GT powerplants have a pre-cleaner with a large bank of filters fitted to protect the blades from erosion and damage. Depending upon the nature of the market, the filters used differ significantly in design, material used and operational function. They divide into two classes:

Typical cylinder component of a GT element under filtration test to EN779

• Pulse cleanable filters: Typically these are large cylinder filters that operate to a pre-determined pressure drop then are cleaned by the simple expedient of a reverse flush of air (the pulse) to dislodge most of the dust from the filter, reducing the pressure drop to close to the clean filter. Typically these are corrugated, cellulose based filter media formed into a cylinder element. Each element assembly is often two elements attached together, a cone element and a cylinder element. These elements are designed to operate in highly dusty conditions of the Middle East where Gas Turbine power generation is the de facto process. They have to be resistant to high levels of sand dust and (for units based close to the coast), a level of water resistance from salt water in the atmosphere. 

• Static filters: These are typically higher efficiency glass based media elements, with an efficiency higher than F9 to EN779, often H11 or H12 to the EN1822 test standard. These elements operate in areas of lower dust concentration in the atmosphere and where much higher levels of efficiency are demanded to protect the element housing. They are most commonly found on Gas Turbine units in Europe, with a particular requirement in offshore operations such as oilfield platforms. The need for a higher level of moisture resistance make such elements is key to their applications performance.  These elements, by their very name, do not have an ability to be cleaned in service, limiting their operational lives. 

  

  Typical glass V-bank element assembly

Pressure Drop saves money and increases output...

The recent increase in filtration efficiency has been matched by an increasing demand for lower pressure drop. Vokes Air state in a White Paper that a reduction in pressure drop of 50Pa on an element for the same efficiency delivers a 0.1% improvement in power output so that a 450MW turbine can deliver an additional 450kW of power.  This has to be at a given efficiency so increasing the efficiency is only one part of the equation. However, as in HVAC, the market demand will rapidly move towards a lower pressure drop technical solution. 

Measurement of efficiency in GT filter media elements

GT element testing, until recently, was rarely standardised outside of some industry norms. For Gas Turbines operating in the Middle East, the original standard was the original 30 year old Aramco test standard. This uses a Aramco specified test dust ($10,000 per element) up to a terminal dP of 2500 and/or 6000Pa with a typical run time of up to 84 hours and is still highly specified by GE for gas turbine elements.

Aramco Test Stand for Gas Turbine Elements
(from Northern Technical, UAE- now Donaldson)

Flatsheet evaluation of efficiency though was undertaken using a standard Palas MFP 2000 looking at fractional efficiency. 

More recently though there has been a move to the standard EN779 test protocol for HVAC. This enables the industry to require a specification to be reached. This has seen a demand for F7 to F9 filter elements. The size and number of the elements in a typical assembly means that the face velocity is actually very low in a Gas Turbine element at only 1.5cm/s versus a typical face velocity of around 11cm/s for a air intake filter for a car or 12.7cm/s for a synthetic HVAC element. 

Fo glass V Bank elements, the performance requirement is measured using the EN1822 standard with a target efficiency of H11, representing a 99.8% reduction in dust reaching the turbine blade. 

The additional function of pulse cleaning of elements has led to the development of a non-standard test for pulse cleanability. A typical test stand from Palas is shown below. 

Palas MMTC Pulse cleaning test stand

Known as the MMTC, this test stand feeds SAE ISO fine test dust into the sample until a pre-determined loading is reached. A reverse pulse is applied allowing the filter to be "cleaned". The clean filter is then reloaded with dust and the process repeated until a predetermined number of cycles are reached e.g. 5,000. The principle of operation is shown below. The rise in the base dP after multiple cycles is inevitable as dust remains entrained in the media following each successive cleaning pulse.  

Principle of pulse cleaning

Cleanability measurements using a MMTC Pulse clean test rig. 

The result is a slow, but steady, increase in the base pressure drop until a terminal dP is reached. 

Filtration Performance of GT Elements

Pulse Clean: The typical cellulose media used for Gas Turbine applications, is similar to the performance of a Heavy Duty Air media with a permeability of around 110 to 240l/m2s at 200Pa pressure drop. At this level of permeability and pressure drop, these materials fail to achieve an efficiency better than M6 rating to EN779 at 1.5cm/s in either flatsheet or elements. 

It is possible to increase the efficiency with cellulose to achieve a higher rating but at a cost of a significant increase in initial dP, which in turn limits the lifetime of the element. The best route forward to improve the performance significantly without a negative impact on pressure drop is through the use of a fine fibre layer on the upstream surface. This is applied in one of two formats: 

• electrospinning a ultrafine layer of polymer fibre from water of solvent onto the surface (typically less than 1 gsm) of the media. Such fibre has a diameter of around 50-150nm and significantly impacts the performance of the media. This is the basis of the Donaldson Spiderweb electrospun used in its Powercore technology and H&V's original Nanoweb treatement (though this is now a meltblown technology). The performance improvement in cleanability due to electrospun fibre is significant. as shown below. The electrospun layer has a significantly lower inherent pressure drop than the treated material in spite of the higher efficiency 

 

Cleanability by MMTC of electrospun media compared with standard cellulose GT media

• meltblown lamination. Melt blown polypropylene fibres are much coarser and therefore the mass required to achieve the same effectiveness in terms of efficiency. Typically the meltblown is fine fibre (average diameter close to 1 micron) and 5-10gsm is added as a lamination process with spay adhesive bonding.  

The one key weakness of these solutions is the current EN779:2012 standard with IPA discharge. The soaking of the fibres with IPA both can damage the fibres (depending on polymer technology) and will significantly impact on the 0.4 micron DEHS discharge efficiency, affecting the minimum efficiency requirement and the final rating. Consequently the 2012 standard is still often not followed in the industry. 

Static Filters: The fineness of the glass microfibre (to 0.3 microns) has a significant efficiency advantage over cellulose. 

Summary  

Gas turbine filters are critical to the performance of environmentally cleaner clean gas turbine power units and have historically been different in the functional design, pulse clean or static, based on the location of the power plant and the need for efficiency performance. Increasing demands on efficiency have been pushing the capability limits of traditional cellulose media used in pulse clean applications to the limit necessitating the use of composite technologies to add further efficiency performance. 
Static filter technology is more water resistant, lower pressure drop and higher efficiency than cellulose based pulse clean technology but is more expensive and can't be pulse cleaned. 
The future will demand more cleanliness efficiency in this technology plus, crucially, reduced pressure drop to maximise turbine efficiency. Whilst the bulk of installations still are glued to existing technology the capital cost of changing technology will restrain the market but newer, more modern technology paradigms will challenge the status quo, driven by the need for higher performance. 

If you have any questions or queries, please let me know and feel free to contact me at any time. 

Tony

Gradient density filtration- 3 dimensional formation control!

Optimising the dust holding capacity of a media is a critical requirement these days. The typical scenario for any sales manager visiting an element manufacturer is "your media is under performing the competitor by x%" or we need a longer service interval on our elements. In a market of commoditising products, filters are an area where added value comes from added performance. 

In order to add performance we need to revisit the basics of how filter materials are manufactured and how contamination is captured. Any process for laying down non-woven fibres creates a density gradient from top to bottom meaning that the fibres are more open at the top (upstream) side of the filter material than at the bottom. The result is that the pore size at the bottom is smaller creating in effect a cone into which the contaminant falls. I've illustrated this in a very simple way below.

Simplistic view of the pore structure of a filter

This means that in an ideal world the filter will load with contaminant from the bottom upwards. As this structure is created by random fibre formation this is not a realistic picture. The reality is that the pore structure has a size distribution and is not evenly distributed. This means that for any single layer structure there is a limited theoretical dust holding capacity based on this concept. 

Gradient density structures in filtration are designed to control this process better. They all into two categories: 

• homogenous gradient density structures formed at the same time with the same materials
• composite layered structures of multiple materials

If we take our simple structure from above and create a gradient density illustration we see something like this. The additional volume allows for a higher level of dust holding capacity to be created.  

Simplistic gradient density filtration structure

  
The downside of this is that the media thickness has significantly increased meaning that there is less media that can be squeezed into a standard pleated element. This therefore reduces surface area and increases face velocity with the impact on DHC and efficiency previously mentioned in my blog of yesterday. The key therefore of any gradient density structures lies in adding a more open upstream side without increasing thickness significantly. This requires different fibres than on the downstream side which leads to more complex product developments. 

I will give you some examples of each of the product strategies. 

Homogeneous Gradient Density Structures- Capaceon Technology
Capaceon filter media technology from Hollingsworth and Vose is an excellent example of controlled z-directional structures. The basic material is 100% cellulose and the key to its design lies in a proprietary process that creates a two layer structure in the forming process on the paper machine with defined permeability in each stage. The result, in terms of performance is a significant improvement in media Dust Holding Capacity at any efficiency rating across the permeability range in air filters. 

Capaceon performance enhancement in air filter applications over traditional single layer materials

The advantage of this approach is two fold. It is possible to retain the same dust holding capacity and manufacture a more efficient element with the same capacity or you can take the added 35% capacity increase as a performance enhancement in element design.  

As pointed out above the media suffers from a slight increase in thickness (about 10%) compared to standard media and in order to ensure that the benefits are seen in the final element, the end user has to optimise the filter element design with slightly fewer pleats and slightly higher face velocity. 

This technology is excellent for air filtration but suffers in liquid filtration as the more open top layer is compressed in the higher viscosity and pressure drops of liquid filtration applications.

Composite Technology 
Composite technology creates a gradient density structure through multiple layers of media. The idea has been well known for many years and is used daily in media and element manufacturers through composite lamination or co-pleating different materials. 

One of the most basic example of this is the cellulose/meltblown or cellulose/synthetic (including electrospinning) technology where a standard cellulose filter media has an enhanced performance through the addition of a synthetic top layer. The top layer acts to disrupt the filter cake and adds significant capacity to the media. The advantage of meltblown (and even more with electrospun fibres) is that the fibre diameters are significantly lower than with cellulose so a high level of fibre density can be created with limited increase in thickness and, more critically, pressure drop. 

To illustrate this I have taken multipass data for a typical 180gsm modern diesel grade, 1723 VH198, manufactured by Hollingsworth and Vose and compared it to the same grade with a 50 gsm PBT meltblown, 1723 K697.

Multipass performance of cellulose filter media v cellulose meltblown composite

The data isn't perfect as the two media didn't show the same efficiency level. However the principle of performance enhancement in terms of the dust holding capacity improvement by adding the synthetic meltblown is clear. There is a >70% increase in media lifetime by the simple expedient of adding a top layer of 50gsm. The impact on thickness and pressure drop is also not significant due to the fine fibres. The resultant material now significantly moves the performance dial whilst at the same time remaining, structurally the same material and able to be pleated on standard process equipment. 

This is not just restricted to fuel media. Performance enhancements by adding very fine fibres such as electrospun, nanometre diameter fibres can significantly increase the performance of pulse clean gas turbine elements in air filtration. The recent move in this market to achieve F8 or F9 efficiency ratings can't be achieved using the traditional cellulose+ PES single layer material. The best efficiency that we saw was a significantly lower M6 efficiency rating. Through the addition of a meltblown layer (typically 10gsm of PP), this rating can be significantly improved to F8 or even F9. Some companies, such as Donaldson, use their own proprietary electrospinning technology to achieve this level of performance.    

The Composite Future  
The concept of composite filter media is moving from the specialty markets to the mainstream as end users look to increase performance advantages over their competitors or as performance specifications increase. 

Dual layer composites are no longer the norm with composite materials up to 5 or 7 layers under development for fuel applications with huge lifetimes at exceptional efficiencies (>99.6% initial efficiency at 4 microns).  

The limiting factor of the efficiency of the cellulose layer is going to be replaced by the simple expedient of an open cellulose layer, with the main efficiency layers being the synthetic layers bonded to the cellulose. This turns the cellulose from being the efficiency layer to being a pleatable backer enabling processing on standard pleating lines. 

The synthetic media itself will be extended further with finer and finer fibres being manufactured to a more consistent level at higher and higher throughput. This will require further advances in meltblown technology to achieve but the technology is slowly evolving in that direction. 

Thanks for reading this blog. If you have enjoyed it or have further topics to discuss please feel free to contact me. 

Tony