Process Engineering Equipment Handbook 2009 Part 2 pdf

Source: Altair Filters International Limited... Source: Altair Filters International Limited... Source: Altair Filters International Limited.. Source: Altair Filters International Limite

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It is almost universally agreed that stainless steel is the most cost-effective term solution for construction of the air-inlet system The premium betweenstainless steel over painted carbon steel can now be as low as 20%, whereas thereare no further painting costs and the life is infinitely longer.

long-The grade of stainless steel is also important It is recognized that the lowergrades, such as American Iron and Steel Institute AISI 304 and AISI 321, do nothave sufficient corrosion protection, particularly if the material is work hardened.AISI 316 is the most popular choice since it has up to 18.5% chromium, a metalwhose presence helps to build up a passive protective film of oxide and preventscorrosion Together with 10 to 14% nickel content, the steel has an austeniticstructure that is very ductile and easily welded

It also can have a low carbon content (below 0.03%) as well as a molybdenumcontent of between 2.0 and 3%, which increases its resistance against pitting.Indeed, one operator has paid a significant premium in both cost and delivery time

by insisting that the molybdenum content be no lower than 2.5%

Not only are the filter housings now constructed in AISI 316 but also almost all

of the items such as vane separators, door locks, hinges, and instrumentation aresupplied in this same material These inlet systems will give a long life, and theylook good as well A typical system is shown in Figs A-64 and A-65

A-62 Air Filtration; Air Inlet Filtration for Gas Turbines

FIG A-49 Drain valve (Source: Altair Filters International Limited.)

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The attention to detail is now evident Figure A-65 clearly shows the elaboratedrain systems that are now installed In addition, the stainless-steel housings arecarefully segregated in the manufacturing shop to prevent any cross-contaminationfrom any other ferrous materials, which includes tooling.

Figure A-66 shows a Brunei 4 platform where five of the engines had beenretrofitted with this system

In summary the main requirements of a filtration system in a tropicalenvironment are

1 Protection against tropical rainstorms by vane separators

2 The inclusion of an integrated drain system

3 The selection of AISI 316 stainless steel as the material of construction

4 Protection against droplet carryover by a final stage vane separator

Air Filtration; Air Inlet Filtration for Gas Turbines A-63

FIG A-50 Salt penetration through filters (Source: Altair Filters International Limited.)

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FIG A-51 Salt penetration through filters (Source: Altair Filters International Limited.)

A-52 Corrosion at silencer outlet (Source: Altair Filters International Limited.)

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FIG A-53 Water penetration through the inlet silencer (Source: Altair Filters International Limited.)

A-54 Corrosion in plenum (Source: Altair Filters International Limited.)

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5 Protection against insects with an insect screen

6 The use of dust extract systems only where essential

The Offshore Environment*

In Europe in the late 1960s, the only data generally available on the marineenvironment was generated from that found on ships Since at that time there wasconsiderable interest in using gas turbines as warship propulsion systems, severalattempts were made to define the environment at sea, with particular respect towarships

FIG A-55 A detached plenum lining (Source: Altair Filters International Limited.)

* Source: Altair Filters International Limited, UK Adapted with permission.

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FIG A-56 A corroded inertia filter (Source: Altair Filters International Limited.)

A-57 A corroded weather louvre (Source: Altair Filters International Limited.)

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FIG A-58 Corrosion downstream of filters (Source: Altair Filters International Limited.)

FIG A-59 Corrosion downstream of filters (Source: Altair Filters International Limited.)

Not only was it found difficult to produce consistent data, but other factors such

as ship speed, hull design, and height above water level had major effects It becameapparent that predicting salt in air levels was as difficult as predicting weatheritself

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Since the gas turbine manufacturers had defined a total limit of the amount ofthe contaminants that the turbines could tolerate, some definition of theenvironment was essential to design filter systems that could meet these limits.Many papers and conferences were held with little agreement, as can be seen inFig A-67 However since the gas turbine industry is a conservative one, it adoptedthe most pessimistic values as its standard, namely the National Gas TurbineEstablishment (NGTE) 30-knot aerosol (Table A-11) It was treated more as a teststandard rather than what its name implied In the absence of any other data, thiswas used to define the environment on offshore platforms, despite the fact that theywere much higher out of the water, and did not move around at 40 knots!

This then defined the salt in air concentration, but did not address any otherparticulates In hindsight, it now seems naive that the offshore environments wereoriginally considered to be clean with no other significant problems than salt Many

FIG A-60 Corrosion debris in inlet duct (Source: Altair Filters International Limited.)

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equipment specifications were written at that time saying the environment was

“dust free.”

In the early 1970s there was also a lively debate as to whether the salt in the airwas wet or dry One argument was put forward that if the salt was wet it would

FIG A-61 A paint blister (Source: Altair Filters International Limited.)

FIG A-62 A new filter housing awaiting installation (Source: Altair Filters International Limited.)

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A-64 A stainless-steel filter housing (Source: Altair Filters International Limited.)

FIG A-63 Corrosion on the new housing shown in FIG A-62 (Source: Altair Filters International Limited.)

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FIG A-65 A stainless-steel filter housing (Source: Altair Filters International Limited.)

TABLE A-11 NGTE 30-knot Aerosol

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require a further stage of vane separators as the final stage to prevent dropletreentrainment from the filters The opposing argument maintained that vaneseparators were unnecessary and that a lower humidity resulted in evaporation ofthe droplet, giving a smaller salt particle that required a higher degree of filtration.Snow and insect swarms were largely ignored as a problem.

FIG A-66 Brunei shell petroleum Fairley 4 platform, showing five new filter housings (Source: Altair Filters International Limited.)

FIG A-67 Airborne salt comparisons (Source: Altair Filters International Limited.)

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The Initial Filter Designs

The types of systems that were used on the first phase of the developments in theNorth Sea fell into two categories: high-velocity systems where the design facevelocity was normally around 6 m/s, and lower velocity systems that operated at2.5 m/s

The low-velocity system was a similar system used on land-based installations,and usually comprised a weather louvre, followed by a prefilter and a high-efficiencybag or cartridge filter Sometimes a demister stage was added and occasionally,bleed extract inertials were supplied as a first stage

The high-velocity system was attractive to packagers since it was lighter andoccupied a much smaller space It was the system derived from shipboard use andcomprised a vane, coalescer, and vane system

In general, both systems were housed in mild steel housings with a variety ofpaint finishes The weather louvres and vanes were normally constructed from amarine grade aluminum alloy The filter elements often had stainless steel orgalvanized frames

Bypass doors were used to protect the engine against filter blockage

The emphasis by package designers was to include a provision for a “3 or 4 stagesystem” often without regard for what those stages should comprise

The Actual Offshore Environment

The actual offshore environment is in many ways different from that originallyenvisaged

Salt in air is present, although it is only a problem when the filtration systemleaks or is poorly designed Horizontal rain can be a severe problem although seaspray does not generally reach the deck levels even in severe storms

Flare carbon and mud burning can be a significant problem if the flare stack isbadly positioned or if the wind changes direction (See Fig A-68.) Not only do thefilters block more quickly, but greasy deposits can cover the entire filter system,making the washing of cleanable filters more difficult

The relative humidity offshore was found to be almost always high enough toensure that salt was in its wet form Some splendid work by Tatge, Gordon, andConkey concluded that salt would stay as supersaturated droplets unless therelative humidity dropped below 45% Further analysis of offshore humidities inthe North Sea showed that this is unlikely to happen (Table A-12)

Initially it was thought that the platforms were dust free, but this is far from thecase

Drilling cement, barytes, and many other dusts are blown around the rig as theyare used or moved But the main problem has resulted from grit blasting As theplatforms got older, repainting was found to be an accelerating requirement withgrit blasting a necessary prerequisite

TABLE A-12 Monthly Average Relative Humidity, North Sea

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In order to be effective, grit is sharp and abrasive by design and can bedevastating if ingested into a gas turbine (See Fig A-69.) The quantities used canseem enormous On one platform it was found that over a 12-month period, 700tons of grit blast had been used!

The Problems Encountered

In general, problems were slow to appear, typically taking three to five years afterstart-up, but since a lot of equipment had been installed at about the same time,the problems manifested themselves like an epidemic

These problems could be categorized as follows:

1 Erosion of compressor blading

2 Short intervals between compressor cleaning

3 Frequent filter change-out

4 Turbine corrosion

5 Corrosion of the filters and housing

FIG A-69 Typical turbine damage (Source: Altair Filters International Limited.)

FIG A-68 Flare carbon can cause problems (Source: Altair Filters International Limited.)

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FIG A-70 Typical leak caused by a missing cable gland (Source: Altair Filters International Limited.)

By far the most serious of these problems was the erosion of compressor bladingthat was experienced almost simultaneously on many platforms This occurredabout three to five years after start-up, as this was the time that repaintingprograms were initiated Grit blast found its way into the turbine intakes eitherthrough leaking intakes, bypass doors, or through the media itself (See Fig A-70.)Since the airborne levels were high, the air filters quickly blocked up, allowing thebypass doors to open As filter maintenance is not a high priority on productionplatforms, considerable periods were spent with grit passing straight into theturbine through open bypass doors Even where maintenance standards were moreattentive, there were usually enough leaks in the intake housing and ducting toensure delivery of the grit to the turbine

It often seemed contradictory that the system designers would spend a lot of timespecifying the filter system, but would pay little attention to ensuring theairtightness of the ducting downstream

Since the grit was sharp, it sometimes damaged the filter media itself, reducingthe system efficiency dramatically

Bypass doors were a major problem Early designs failed to take account of theenvironment or the movement in the large structures of the filter housings Veryfew of those initial designs were airtight when shut, and it was not uncommon forthem to be blown open by the wind

Turbine corrosion could almost always be traced to leaky ducting or operationwith open bypass doors Very few systems gave turbine corrosion problems if theducting was airtight The few installations that did give problems were usually theresult of low-velocity systems operating with poor aerodynamics, so that local highvelocities reentrained salt water droplets into the airstream and onward to theengine (See Fig A-71.)

Rapid compressor fouling was usually the prelude to more serious problems later,since it was usually caused by the combined problems of filter bypass

Compressor cleaning almost once a week was fairly standard for systems withthose problems

As time progressed the marine environment took its toll on the carbon steel andsevere corrosion was experienced on the intake housing and ducting (See Fig A-72.) In some cases, corrosion debris was ingested into the turbine causing turbinefailure This again was accelerated by poor design, which allowed dissimilar metals

to be put into contact, leading to galvanic corrosion

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FIG A-71 Inertial filters showing severe corrosion (Source: Altair Filters International Limited.)

FIG A-72 Severe duct corrosion (Source: Altair Filters International Limited.)

Specification of a Typical Filter Used in the Offshore Environment*

Gas turbines are an ideal power source for driving compressors, pumps, andgenerators Since they are relatively small compared to their power output, theycan be used easily in remote locations such as jungles, deserts, and offshoreplatforms They are, however, very complex pieces of machinery, comprised of manyhigh-tolerance rotating parts

The engineering is further complicated by the engine manufacturers’ need toincrease the turbine efficiency by increasing operating temperatures In order toovercome the material stresses associated with these higher temperatures, internalcooling passages have been introduced into the engine Typically, turbine blades arenow of hollow construction with cooling air blown through them, exiting throughtiny holes in the blade surface These holes can be very small and are verysusceptible to blockage The requirement for filtration of the gas turbine air is,therefore, even more stringent than in the past The need to filter the air to the gasturbine is fourfold:

* Source: Altair Filters International Limited, UK Adapted with permission.

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To prevent erosion

To protect against fouling

To prevent particle fusion

To protect against corrosion

Fouling

Engine fouling, by comparison, is normally only a temporary problem and is caused

by a buildup of contamination that adheres to the internal surfaces

Again, deposition on blade surfaces can change profiles, with the resultant loss

in engine power and an increase in fuel consumption Particles of 2mm and less aregenerally the major cause of fouling Smoke, oil mist, and sea salt are commonexamples

The particles are attracted to the metal surfaces by a variety of forces, includingimpaction, electrostatic, and capillary action The composition of the particle, again,

is important in determining the rate of fouling In marine environments, dry dustparticles are often coated in a layer of sea salt, which is viscous by nature and adds

to the fouling action While fouling is basically a temporary problem, it can beremoved by various cleaning techniques It is an irritant to the operator, as many

of these cleaning processes have to be conducted at reduced powers or with theengine stopped In the past, engines were cleaned by injecting a mild abrasive intothe engine to clean off the contamination when the engine was running While themost common material was a mixture of ground coconut shells, rice has been used

on some tropical engines The practice of using online cleaning has now mostly beenabandoned since it tended to transfer large particles of debris into other areas ofthe turbine, causing even more problems There was also a view put forward that

it accelerated hot end corrosion Modern cleaning methods use a detergent sprayedinto the engine on a cold cycle, leaving it to soak and then washing it off with cleanwater

Particle fusion

Dry particles, which range in size from 2 to 10mm, could, on old engine designs,pass through the engine causing little or no damage However, on the newgeneration of hotter engines, these particles can cause problems if their fusiontemperature is lower than the turbine operating temperature, since they will meltand stick to the hot-metal surfaces This can cause severe problems since thismolten mass can block cooling passages and cause thermal fatigue The affectedsurface is permanently disfigured and will need replacement

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The high temperatures of the gas turbine can also cause rapid acceleration of thecorrosion process Even though the hot-metal surfaces are made of some of the mostsophisticated materials, corrosion can still be extremely rapid Blade failures in aslittle as 100 operating hours have been known, and failures within 2000 operatinghours are relatively common Corrosion, however, can be completely prevented bymodern techniques, and yet it still occurs

Normally, corrosion is produced by a salt, such as sodium or potassium, but leadand vanadium are also common contributors Since many gas turbines are basedeither offshore or close to the sea, sea salt (sodium chloride) is the main offender

In the cold parts of the engine it is the sodium chloride that does the damage,whereas in the hot parts of the engine it is sodium sulfate (or sulfidization) thatcreates most of the corrosion Sodium sulfate is produced from the combination ofsulfur in the fuel and sodium chloride in the air

It is important to recognize that the corrosion process is self-propagating, and,once started, will continue even though the source problem has been cured.The modern gas turbine therefore is a sensitive machine and needs to be protected

to provide an acceptable life cycle For this reason, there are limits that arerecommended by the manufacturers in order to achieve this There is not oneuniversal limit that is adopted by all manufacturers Each has its own, which isexpressed in many different forms, either as an absolute limit or one that is timedependent However, all seem to work from the same premise

Previously, it was often thought that providing a gas-turbine air-filter system waswell chosen; it could be used in almost any environment with equal effect This hasproved to be a fallacy, as many operators have found at their cost

The Problems Solved

Since engines appeared to be eroding at a fairly rapid rate, irrespective of whichtype of filter system was fitted, an equally rapid response was required

Phillips decided, after removing a GT22 engine from their Ekofisk Bravoplatform, that they would not operate the repaired engine until the filter systemwas changed, even though the source of the problem had not been identified at thattime

An evaluation of all the likely contaminants was quickly undertaken, with largequantities of various suspicious substances shipped back to the laboratory foranalysis and trial against the installed filter system in the wind tunnel Grit blastwas confirmed as the source of erosion, and the installed filter system gave only a28% protection

Trials against other conventional filter medias proved negative, since the grit wassharp and eventually cut its way through the media

A new type of filter was required with a very strong media and a large capacity

to absorb the huge quantities of grit without blocking too quickly The ability towork at high velocity would also be an advantage

This was achieved by using a bonded polyester fiber, which proved almostimpossible to tear The strength of the polyester allowed the media to be operated

at higher velocities without fear of fiber loss, which can be a limitation with thebrittle glass fibers used in conventional filtration

The old filter makers’ methods of packing in more media with the hope that theincreased area would reduce the media velocity has distinct limitations since theextra area is not effectively used by the poor aerodynamics created Instead acareful aerodynamic design ensured a more even distribution throughout the filter,

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giving a relatively wide pocketed bag filter capable of those higher velocities andgiving protection against the problem contaminants By having a relatively highloft to the media, protection against filter blinding was ensured, with a resultinglonger life Testing showed the new HV2 filter to be over 99% efficient against thedamaging grit blast.

A cleanable prefilter bag of similar but less dense media was also quicklydesigned This PB1 prefilter bag was unique in that it was designed to be tuckedinside the final filter bag and so only took a further 25-mm installation depth.The new filters were installed into a new stainless steel housing protected frontand back by vane separators and delivered on Christmas Eve 1983, just three weeksafter the initial problems were investigated; a record for which all those involvedhave a right to be proud of (See Fig A-73.)

Shell was the next platform operator to experience similar problems, firstly onthe Avon gas turbines on its Brent Delta platform In these installations it waspossible to fit the HV2 filters in an access space between the existing filter stages.Shell undertook a bold and very correct decision to weld up the troublesome bypassdoors, having first revised the alarm and trip levels for the intake depressionpressure switches This system was carefully monitored for a period of nine monthsbefore the remaining 23 Avons on the Brent platforms were similarly converted.Comparison of two adjacent engines, one with the original system and one with aretrofitted system showed that over a three-month period the performance of theretrofitted engine was unchanged, while the other engine showed a steady increase

in exhaust temperature for a given power output amounting to 30°C at the end ofthe period Also, the requirement to change out the filters was reduced from 350hours to, in some cases, over two years

On Shell Leman BK platform, a similar comparison of two Avons wasinvestigated, with an air sampling program constantly monitoring the quality ofthe inlet air over a period of one month This showed the modified installation to

be 10–14 times more effective, in terms of particle penetration

It is not surprising that Shell has now retrofitted 78 installations worldwide Intotal this design has been selected for 212 new and retrofitted gas turbine engines.Filter systems that were an operational irritation every 15 days or so are nowforgotten to such a degree that on some platforms the filters have operated, withoutreplacement, for up to three years

FIG A-73 Typical air filter on a platform (Source: Altair Filters International Limited.)

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Compressor cleaning is operated on a planned maintenance approximately every

2000 hours

The Systems of the Future

With so much experience gained with these retrofitted installations, newinstallations are now designed taking account of the lessons learned

Typically a new offshore system will comprise the following features:

1 A housing made entirely of 316 stainless steel

2 A weather hood or high-efficiency weather louvre constructed in 316 stainlesssteel

3 A prefilter and filter stage capable of high efficiency against grit blast and othercontaminants

4 A final vane separator to protect against droplet reentrainment

5 No bypass doors

6 Pressure switches for alarm and shutdown

7 An integrated drain system

8 All materials capable of withstanding a marine environment, with an exclusion

of dissimilar metals, cardboard, and the like

9 A leak-free intake systemThe argument for the high-velocity (6 m/s) system is now proven, with over 200installations worldwide The advantage of smaller size and lower weight willbecome more important in the future, and may push the current designs evenfurther

The key components of the system* are:

High-efficiency filtration (HerculesTM)

Dynamic water eliminator High-efficiency separator ensures that salt carryover

problems are eliminated (HydraTM)Hercules and Hydra combine to form System Aquila (Fig A-74), providing thefollowing features:

High volume flow Leading to a filter house with a 65% smaller face area than

traditional systems This means a customer saves space and weight, which alsosaves cost (See Fig A-75.)

High efficiency High dust arrestance and salt removal efficiencies provide

excellent protection from erosion, corrosion, and fouling of turbine blades (SeeFig A-76.)

Low pressure loss A typical clean system pressure loss of only 45-mm H2O meanslower fuel consumption, higher output, and longer filter life for operators

High-efficiency filtration

This feature has been aerodynamically designed to ensure that maximumparticulate efficiency is achieved with the minimum resistance to airflow The

* Note: Trademarks are specific to the source for this section Each manufacturer will have its own equivalent terms and trademarks.

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FIG A-74 Filter with water eliminator (Source: Altair Filters International Limited.)

FIG A-75 Pressure loss versus volume flow rate filter characteristic (Source: Altair Filters International Limited.)

semirigid construction, together with the fact that each pocket is divided intosmaller segments by means of a semipermeable “shelving” system, ensures the bestpossible profile throughout all operating conditions This produces an extremelyuniform flow distribution, leading to improved dust-holding capacity andeliminating the likelihood of localized dust breakthrough

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Air Pollution Control A-83

Dynamic water eliminator

This feature conducts water and salt removal The vanes, which are constructedfrom corrosion-resistant marine grade aluminum (other materials are available),are produced with a profile that allows the maximum removal of salt and water,yet produces an extremely low pressure loss This optimal profile has been achieved

by the very latest design methods, and in particular by utilizing a ComputationalFluid Dynamics (CFD) flow modeling system Hydra also incorporates a unique andnovel method of separating water droplets from the air stream, and this has led toimprovements in bulk water removal compared with conventional methods

Reference and Additional Reading

1 Tatge, R B., Gordon, C R., and Conkey, R S., “Gas Turbine Inlet Filtration in Marine Environments,” ASME Report 80-GT-174.

Typical Specifications for Range of Air Filters

This range includes panels and bags as well as high-efficiency, high-velocity systemsand air/water separators

Filter holding frames are constructed in mild or stainless steel, designed toprovide quick and easy removal from upstream, downstream, or sides of ducting,without the use of springs or clips of any kind Filter housings, ducting, louvres,dampers, and silencers can also be designed and fabricated, providing a total systemcapability

Air Pollution Control*

The main methods of combating and controlling air pollution include:

Electrostatic precipitators (for particulates)

Fabric filters (for dust and particulates)

Flue gas desulfurization (for SOxremoval)

SCR DeNOx(for NOxremoval)

Absorbers (for environmental toxins)

End-product–handling systems (for solid and liquid wastes)

Combined unit systems (for some or all of the previous items)

FIG A-76 Efficiency versus pressure loss filter characteristic (Source: Altair Filters International Limited.)

* Source: Alstom Adapted with permission.

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Dynamic water eliminator

This feature conducts water and salt removal The vanes, which are constructedfrom corrosion-resistant marine grade aluminum (other materials are available),are produced with a profile that allows the maximum removal of salt and water,yet produces an extremely low pressure loss This optimal profile has been achieved

by the very latest design methods, and in particular by utilizing a ComputationalFluid Dynamics (CFD) flow modeling system Hydra also incorporates a unique andnovel method of separating water droplets from the air stream, and this has led toimprovements in bulk water removal compared with conventional methods

Reference and Additional Reading

1 Tatge, R B., Gordon, C R., and Conkey, R S., “Gas Turbine Inlet Filtration in Marine Environments,” ASME Report 80-GT-174.

Typical Specifications for Range of Air Filters

This range includes panels and bags as well as high-efficiency, high-velocity systemsand air/water separators

Filter holding frames are constructed in mild or stainless steel, designed toprovide quick and easy removal from upstream, downstream, or sides of ducting,without the use of springs or clips of any kind Filter housings, ducting, louvres,dampers, and silencers can also be designed and fabricated, providing a total systemcapability

Air Pollution Control*

The main methods of combating and controlling air pollution include:

Electrostatic precipitators (for particulates)

Fabric filters (for dust and particulates)

Flue gas desulfurization (for SOxremoval)

SCR DeNOx(for NOxremoval)

Absorbers (for environmental toxins)

End-product–handling systems (for solid and liquid wastes)

Combined unit systems (for some or all of the previous items)

FIG A-76 Efficiency versus pressure loss filter characteristic (Source: Altair Filters International Limited.)

* Source: Alstom Adapted with permission.

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A-84 Air Pollution Control

Electrostatic Precipitators

In combustion processes, the largest quantities of heavy metals and dioxins arefound in the fly ash, or can be contained there by technical means It is thereforeessential to increase even further the very high precipitation efficiencies that arealready being achieved There are two types of ESPs, wet and dry, for collecting particles (See Fig A-77.)

New and retrofit systems are used Retrofitting with new spiral electrodes, arapping system, and pulsed energization pay immediate dividends in the form ofimproved abatement efficiency and lower power consumption

Semipulse ® and Multipulse ® for enhanced separation and energy efficiency

The rather uncomplicated process of charging dust by means of a high-voltage DCsystem, which makes dust stick against collector plates, has undergone high-techrefinement Several of the improvements have been implemented to minimizeenergy consumption Originally, it took about 1 MW of power to operate an ESP in

a large coal-fired power station Pulsed energization is a means to cut energyconsumption substantially while simultaneously improving separation efficiency.Two systems for this purpose have been developed: the Semipulse Concept (SPC)with millisecond pulses, and the Multipulse Concept (MPC) with microsecondpulses (See Fig A-78.)

Since their introduction in 1983, more than 3500 SPC and MPC units have beensupplied SPC can be easily installed in existing plants, while MPC involves ahigher investment and is generally considered for retrofits and new plants

The savings for high-resistivity dust can be substantial Energy consumptionafter installation of SPC or MPC is typically between 10 and 20% of the original

At the same time, dust emissions are reduced to 25–50%

Upgrading or retrofitting with pulsed energization is often the solution when autility wants to switch to low-sulfur coals, which often produce dust of higher

A-77 A typical electrostatic precipitator (Source: Alstom.)

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resistivity It also gives a utility a wider choice of coals that can easily be valued inmoney terms.

Semipulse and Multipulse offer an inexpensive route not only by improvingenergy and separation efficiency, but also by requiring a minimum of supervisionand maintenance

Another reason for the recent success of fabric filters is that they operate bypassing the dust-laden gas through a dust cake that is constantly being built upwith the support of the fabric This enables the removal of a large portion of thefinest particles, a feature that is becoming increasingly important as more stringent emission controls are required (See Fig A-81.)

With the fine particles, several heavy metals can be trapped in the dust cake,together with sulfur dioxide, if lime is introduced in the flue gas

This manufacturer/information source supplies two different kinds of fabricfilters: the high-ratio and low-ratio type, denominated by the air-to-cloth ratio

A major difference between the two filter types is the cleaning system The high-ratio fabric filter is cleaned by the Optipulse®

cleaning system (see following text).

In the Semipulse system, pulsing is achieved by controlling the conventional T/R set of the precipitator In the Multipulse system, special T/R equipment produces intensive bursts of short pulses.

FIG A-78 Pulse systems in precipitators (Source: Alstom.)

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In the low-ratio filter, gas enters the filter bags from the inside, then outward.The filter bags are cleaned “off-line” using either the reverse gas flow, reverse gas with high-energy sound horns, or a cleaning system of the deflate or shakemechanism type (See Fig A-82.)

The Optipulse ® cleaning concept

(Optipulse is a trademark for a proprietary design of this information source.)Pulse-jet fabric filters operate with dust-laden gas approaching the filter elementsfrom the outside, depositing the particles on the fibers of a depth-filtering medium.The clean gas leaves the open end of the filter element, which is typically oftubular design with a diameter of 120–150 mm (5–6 in) An internal wire cagesupports the filter element against the pressure caused by the gas flow (See Fig.A-83.)

Periodically, the dust cake is cleaned off by expanding the filter element with arapid pulse of air The removed dust cake is transported by gravity toward the dusthopper

The effectiveness of the cleaning depends on the character of the pressure pulse.Optipulse produces a forceful pulse by an optimized geometry of the pneumaticsystem delivering the pulse (see Fig A-84):

The pulse air is injected in the filter element, without dissipation of energy in alarge volume of flue gas, through injection nozzles optimally selected in relation

to filter element size

The area of all nozzles on the header serving one row of filter elements is matched

to the area of a large, pilot-operated, fast-opening supply valve

Flue Gas Desulfurization (FGD)

Today, flue gas desulfurization is a well-established method to fight global environmental impairment such as acid rain Most industrial countries have setstandards for SO2 emissions and committed themselves to large reductions ofnational emissions in international agreements

There are several different types of FGD technologies for a wide range of applications

FIG A-79 Fabric filter installation in a metallurgical plant, Höganäs, Sweden (left).

(Source: Alstom.)

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FIG A-80 Typical filter product range (Source: Altair Filters International Limited.)

A-87

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FIG A-81 Air filter elements (Source: Alstom.)

FIG A-82 Low-ratio fabric filter installation at Nevada Power, United States (Source: Alstom.)

A-83 Installation of filter elements in an Optipulse fabric filter (Source: Alstom.)

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FIG A-84 Operating principles of the Optipulse pulse cleaning system (Source: Alstom.)

FIG A-85 Wet/dry flue gas FGD plant, including fabric filters, after two coal-fired boilers at TWS Dampfkraftwerk, Stuttgart, Germany (Source: Alstom.)

Three common technologies for FGD

1 The wet/dry lime spray drying process offers low capital costs and an easily disposable/reusable end product for small and medium-sized plants (See Figs.A-85 and A-86.)

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2 The open spray tower lime/limestone wet FGD process offers low operating costsand proven production of commercial grade gypsum (See Figs A-87 and A-88.)

3 The seawater process offers low operating costs and fully eliminates disposalproblems of end products at plants with access to suitable and sufficient amounts

of seawater (See Figs A-89 and A-90.)

Catalysts solve pollution problems

The SCR (selective catalytic reduction) technique was transferred to Europe fromJapan, where it was first developed

The reduction of nitrogen oxides with ammonia, which occurs spontaneously athigh temperature [about 950°C (1750°F)], can be achieved at a manageabletemperature after the boiler with the aid of a catalyst (See Fig A-91.)

For NOxreduction, the catalyst is usually an active phase of vanadium pentoxideand tungsten trioxide on a carrier of titanium Other types of catalysts areavailable, however

The ideal catalytic reactor is made up of catalyst elements that are assembled inmodules usually 1 ¥ 1 ¥ 2 meters in size A reactor normally has three to four layers

of catalyst modules

Three positions of the reactor are possible in the treatment chain (see Fig A-92):

The reactor in the W/D FGD plant utilizes a spinning disk or a two-fluid nozzle for atomization of lime slurry

The reaction between absorbent and acid gas components takes place

mainly in the wet phase The process is regulated in such a way that the reaction product becomes dry and can

be collected in a conventional dust collector.

FIG A-86 Basic principles of wet/dry FGD installation (Source: Alstom.)

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In a wet FGD plant, line or

linestone slurry is sprayed

through nozzles into the gas flow

The mixture of slurry and reaction

products is gathered at the bottom

of the absorber-tower and

recycled through the

spray-nozzles An important element in

the wet FGD process is the mist

eliminator above the spray nozzlebanks.

Secondary oxidation is normally achieved through introduction of oxygen at the bottom of the slurry tank.

With oxidation, the reaction product, after dewatering, will

be gypsum.

FIG A-87 Wet FGD plant (Source: Alstom.)

FIG A-88 Wet FGD plant with one single absorber installed after a 700-MW coal-fired boiler at Asnæsværket utility in Kalundborg, Denmark The plant produces commercial quality gypsum (Joint venture Alstom and Deutsche Babcock Anlagen.) (Source: Alstom.)

A-91

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The wet FGD process can utilize the alkalinity of seawater to absorb SO2 in the flue gas Absorption takes place in a once through packed bed absorber.

The effluent is aerated in a seawater treatment plant and mixed with cooling water from the condensers before disposal

at sea.

FIG A-90 Basic operation of seawater FGD (Source: Alstom.)

FIG A-89 Seawater FGD plant at Tata Industries, India (Source: Alstom.)

A-92

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FIG A-91 The reduction of nitrogen oxides with ammonia is achieved at a manageable temperature

by the use of a catalyst (Source: Alstom.)

FIG A-92 Flow diagrams for different DeNO x process systems (Source: Alstom.)

A-93

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1 High dust system The reactor is placed before the air preheater, and operates

directly in the dust-laden and acidic gas that leaves the boiler This system dominates the fossil fuel boiler market today (See Fig A-93.)

2 Low dust system The reactor is placed after the hot electrostatic precipitator

but before the air preheater, which is possible, for example, in the case of waste incineration or, in the case of a gas turbine, in the heat recovery boiler

3 Tail end system The reactor is placed after particulate control and after sulfur

dioxide and/or hydrochloric acid removal in the flue gas cleaning train Thisallows for the use of a much more compact catalyst reactor The tail end solution

is used when the particulates or gases are harmful to the catalyst (See Fig A-94.)

SCR reactor design

Although design and operation of an SCR reactor is fairly straightforward andsimple, there are a few issues that require special attention One such issue is gasdistribution at the inlet of the reactor In the case of high dust it is of the utmostimportance that the gas is properly distributed to avoid catalyst erosion problems.The reactor is equipped with guide vanes and distributor plates to ensure even gasdistribution under all operating conditions (See Fig A-95.)

Another important issue is ammonia slip, which must be kept to a minimum forseveral reasons

If the gas still contains sulfuric gases, i.e., in the high dust case, ammonia slipwill react with sulfur trioxide to form ammonia bisulfate when cooled in the air

A-93 SCR reactor of the high-dust type (Source: Alstom.)

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FIG A-94 This coal-fired 550-MWe/900-MWth combined heat and power plant is located centrally in the town of Västerås, Sweden Alstom has gradually extended its flue gas treatment system, which today comprises ESP, FGD, and SCR units for full emission control (Source: Alstom.)

FIG A-95 Stadtwerke München Süd, Germany, has installed a CDAS (Conditioned Dry Absorption System), as well as a tail-end-type SCR unit, at its 300,000-tons-a-year waste-to-energy plant (Source: Alstom.)

A-95

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preheater This formation will take place on the dust particles and make the fly ashunsuitable for direct use in concrete manufacturing

The ammonia will also end up in the effluent from a downstream FGD plant, requiring effluent treatment

The SCR plant is therefore equipped with a control system of the “feed trim back” type In this system, ammonia is injected before the catalyst in relation

forward-to both the measured NO content after the reacforward-tor and the amount of NO present

in the gas fed into the reactor

Catalyst activity will inevitably decrease with time The mechanisms that control

the rate of deactivation are mainly: (i) sintering of the microsurface due to elevated temperatures, (ii) poisoning of the active metal atoms or molecules through a permanent bond or reaction with, for example, alkali metals, and (iii) blocking of

pores by, for example, ammonia bisulfite or dust

The reactor is equipped with a spare layer that can be charged when the efficiency

of the catalyst has dropped below a certain level When the activity drops further,the catalyst has to be replaced

The obvious advantages with the tail end system, with favorable conditionsregarding all three deactivation parameters, are offset only by the cost of bringingthe gas temperature back to the elevated operation temperature of the catalyst.This manufacturer is also conducting research to find catalysts with lower operationtemperatures for various applications

Absorbers for Environmental Toxins

The dioxin and heavy metal problem

This refers to the entrapment of dioxins in dry scrubbers The experiments in thisfield were first conducted in gas cleaning systems for waste-to-energy plants, wheredioxin emissions are a major problem The emission control system describedcombines the dual effect of chemically enhanced adsorption/absorption andfiltration

The fabric filter is also very effective for controlling heavy metals, due to itscapacity for filtering submicron particles The combination of dioxin and heavymetal abatement has been especially important for the environmental acceptance

of waste-to-energy plants This manufacturer has developed the TCR (TotalCleaning and Recycling) concept for complete control of flue gases from waste-to-energy installations

More than 50 Filsorption plants have been installed in Europe and the UnitedStates These plants repeatedly measure dioxin emissions below 0.1 ng/Nm3 (SeeFig A-96.)

The latest development is the Filsorption®II system, which introduces a mixture

of lime and coke in a safe blend to enhance abatement of organic emissions,primarily dioxins, heavy metals, and acidic gases

Filtration and chemisorption (Filsorption ® II)

Filsorption is short for filtration and chemisorption, indicating the dual duty of thesystem The Filsorption II system is primarily aimed at the control of organicemissions such as dioxins The system also offers control of mercury, acidic gases,and particulate emissions Filsorption is an “absolute filter” for securing very lowemission levels

The system includes a storage and injection system for the chemically active

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FIG A-96 Hazardous waste incineration plant with Alstom Filsorption system, Cleanaway Ltd., Ellesmere Port, UK (Source: Alstom.)

sorbent The sorbent is normally a mixture of coke and lime, mixed to a safe blend.This is a significant advantage of this system, which reduces risks for operatingpersonnel

The fabric filter collects the particulate matter that escapes the upstream particulate control units along with the injected sorbents and reaction products.The fabric filter also acts as a chemical reactor for lime with SO2, SO3, HCl, andHF

The ash collected in the filter is discharged for final handling or recirculation back

to the combustion unit to destroy its organic contents The contaminated reactionproduct requires a dust-free handling system This information source provides

an underpressure conveying system to prevent potentially hazardous ash fromcontaminating the working area

The Filsorption system can be used for treatment of gases containing dioxins andheavy metals from other types of industrial processes besides waste-to-energyinstallations (See Figs A-97 to A-103.)

End-Products Handling Systems

The often substantial amounts of solid or liquid wastes resulting from emissioncontrol systems require careful and efficient handling systems within the plantitself, as well as environmentally sound methods of treatment, recycling or disposal.Ash-handling technology needs various types of solids handling, including bottomash submerged drag chain conveyors, wet impounded hoppers, economizer andpyrites systems, and pneumatic fly ash handling

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FIG A-98 Stabilized gypsum from a wet FGD plant The end product is suitable for landfill use (Source: Alstom.)

The DEPAC®

system illustrated is a pneumatic conveying system based on densephase technology

Cost-efficient methods are developed to utilize the solid waste products A number

of commercial operations have already been established such as commercial gradegypsum for wallboard manufacturing and high-strength fill materials

FIG A-97 Schematic for filtration and chemisorption unit (Filsorption II) (Source: Alstom.)

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FIG A-99 Group of silos for short-term storage of reagents and reaction products from an FGD plant (Source: Alstom.)

FIG A-100 The heart of the Fläkt DEPAC system is the dust transmitter, in which the product is fluidized by means of compressed air (Source: Alstom.)

Combining Unit Operations

Total turnkey solutions for emission problems combine the operations of variousunits The example illustrated is the TCR system This system is designed to cleanflue gas from waste incineration and produce certain recyclable end products

In a similar manner, units are combined to form complete flue gas treatmenttrains in modern power plants

Định dạng
Số trang	80
Dung lượng	3,76 MB

Tiêu đề	Air Filtration; Air Inlet Filtration for Gas Turbines
Chuyên ngành	Process Engineering
Thể loại	Handbook
Năm xuất bản	2009