High-efficiency filtration in dry/semi-dry FGD plants

By Florin Popovici

Vo lu me 92 – Is sue 7/2012 Page 81 to 84

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Filtration in dry- and semi-dry FGD VGB PowerTech 7 l 2012

Author

Kurzfassung

Hocheffiziente Filtration in trockenen und halbtrockenen Rauchgasentschwefelungsanlagen

Filtermedien aus Polymerfasern sind im Hin-blick auf ihre chemische und thermische Be-ständigkeit sowie der geforderten Emissions-grenzwerte optimiert. Halbtrockene und tro-ckene Rauchgasreinigungen werden gemeinhin in Verbindung mit einem Gewebefilter betrie-ben. Die Eigenschaften des Filterkuchens, der sich auf dem Filtermedium während des Filt-rationsprozesses ausbildet, ist von erheblicher Bedeutung. Neben der Staubabscheidung findet am Filterkuchen eine Sekundärreaktion statt, bei der SO2 neutralisiert wird. Zum Teil wer-den Filtermedien aus verschiedenen Fasertypen verwendet, um kosteneffiziente Materialien mit herausragenden Qualitäten hinsichtlich der Fil-terkuchenausbildung zu erzeugen.In dem vorliegenden Beitrag wird der Einfluss spezifischer Fasereigenschaften wie Querschnitt und Titer in Verbindung mit den filtrationsre-levanten Eigenschaften und der entsprechen-den chemischen und thermischen Stabilität in Filtern nach trockenen und halbtrockenen Entschwefelungsverfahren in kohlegefeuerten Kesseln dargestellt. Die Partikelemissionen sind ein kritischer Parameter für die Auswahl der Filtermedien, speziell wenn Emissionen unter 5 mg/Nm3 kontinuierlich eingehalten werden sollen.Referenzanlagen mit trockener und halbtro-ckener Entschwefelung von kohlegefeuerten Kesseln werden von der Evaluierung der Effi-zienz über Anlagendesign, Prozessparameter, Filtermedien und Partikelabscheideleistung vorgestellt. l

High-efficiency filtration in dry/semi-dry FGD plantsFlorin Popovici

Florin PopoviciGas Cleaning Specialist Master of Science in Mechanical Engineering Evonik Fibres GmbH Monza/Italy

General

Coal will continue to remain the most im-portant power source over the next decade around the world and it is expected that its demand and consumption will increase at a high rate. It is still the cheapest source of energy, being associated with economic growth in countries like China and India. Former Eastern Block countries will also see a stable coal demand during the next years. In parallel with coal burning power plants, there is an increasing trend for using renewable sources of energy, especially bio-mass. Energy from municipal waste is also expected to grow at a high rate in Eastern Europe and Asia. While the demand for coal and renewable sources will continue to rise, the maximum required particulate emission levels are being reduced to values lower than 5 mg/Nm3. In urban agglomerations these limits can be as low as 1 mg/Nm3. Nowa-days, the plants require flexibility in relation to the type of coal or renewable fuel used. A single source is no longer a viable strat-egy and with each type of fuel the resulting combustion gases and fly ashes have dif-ferent properties in terms of chemical con-stituents and particle size distribution. This requires an emission control technology op-erating consistently within the regulatory emission limits for a wide range of process parameters. An electrostatic precipitator (ESP), which is a constant efficiency device, is not suitable anymore, as its performance is directly influenced by inlet dust burden and the value of fly ash resistivity. Different fuel sources produce particles with different resistivity values and this could lead to in-creased particulate emissions. A fabric filter plant (FFP) is a constant emission control device and represents the only viable tech-nology for the consistent achievement of the present and future tough air pollution targets. In the case of dry and semi-dry flue gas desulphurisation (FGD) plants, the FFP becomes integral part of the FGD system and has a major role in the removal of the SOx and other undesired components from the flue gas. For achieving a successful FFP, the filtration material design and the selec-tion of fibres are of critical importance. The fibres mechanical, chemical and filtration properties associated with the process pa-rameters need particular attention.

Spray dry/semi-dry scrubber

A spray dry/semi-dry FGD system is based on a reactor/scrubber located upstream

of the particulate emission control device where calcium or sodium based reagents are introduced for the neutralisation of the SOx components in the flue gas. The dry/semi-dry FGD systems are de-signed to perform the following:

– collection of solid particulates in the FFP, – absorption of acidic flue gas components

such as HCl, SOx and HF (neutralisation by calcium or sodium based reagents), and

– adsorption of dioxins and furanes (by ac-tivated carbon).

Modern FGD systems use the filter as “fixed bed” reactor for the absorption and adsorp-tion processes. Therefore, the performance of the filter media has an impact on the per-formance of the FGD system. We can dif-ferentiate two main types of reactions: the primary reaction inside the reactor/scrub-ber and the secondary reaction in the par-ticulate emission control device. In cases where the boilers are provided with FFPs, the secondary reaction takes place while the gases pass through the bag filter dust cake. With ESPs the secondary reaction has virtually no effect. In order to increase the reaction rate, the dust (with the non-reacted reagent) is re-circulated from the FFP hoppers to the main reactor. Due to the recirculation an increased dust loading has to be collected in the FFP and particular at-tention has to be paid to the flow distribu-tion in the bag filter area (gas inlet mani-fold and chambers). The complex dust handling system is an important feature of this plant as up to 90 % of the solids are re-circulated from the FFP hoppers to the reactor. The re-circulated material could be up to 1 kg/Nm3. In semi-dry systems water is added to the reactor in order to improve the reaction rate. All added water evapo-rates and the system does not require waste sludge handling and processing. There are different types of dry/semi-dry FGD sys-tems, specific to different manufacturers. The main performance criteria are related to the SOx removal efficiency associated with the reagent consumption.

FFPs versus ESPs

The fabric filter is a constant emission de-vice that can tolerate a reasonable increase in gas flow and particulate inlet burden for a short period without affecting the dust emissions. On the other hand, an ESP is a constant efficiency device that cannot tol-erate any increase in gas flow or inlet fly

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VGB PowerTech 7 l 2012 Filtration in dry- and semi-dry FGD

ash burden without negatively affecting particulate emissions. Due to the fact that in a semi-dry scrubber 90 % of the mate-rial could be re-circulated, the gas dust loading inside the emission control device could be up to 1 kg/m3 and this will nega-tively affect ESP emissions. Another im-portant consideration is the secondary SOx neutralisation reaction which takes place while the gasses pass through the dust cake in case of an FFP. This is not the case with ESPs, where the gasses have only a limited contact with the dust particles and the sec-ondary reaction is actually non-existent. The contribution of the secondary reaction is shown in F i g u r e 1 where the SOx neu-tralisation reaction rate is shown for ESPs and FFPs. In this case the neutralisation agent is sodium sesquicarbonate (Trona). It is important to mention that the differ-ence in performance between an ESP and a FFP is even higher in case of a calcium-based neutralisation agent because of its lower reactivity in relation to the sodium-based reagents. In this case the secondary reaction is of even higher importance.

High-efficiency needle felt materials – dust cake filtration

The bag houses associated with semi-dry scrubbers are based on needle felt type bags. A needle felt comprises of a base material or support scrim upon which a web or batt of fibres is bonded by means of a needling process. The batt consists of one or more layers of fibre orientated ran-domly, where the fibres are arranged in both horizontal directions. A batt can be applied to one or both sides of the support material (scrim). The fibres have to be se-lected in relation to the process conditions: chemical environment, temperature, dust

characteristics and particulate emission requirement. For semi-dry scrubber bag house materials it is critical to select fibres with excellent filtration properties. These have to be present at least on the filtration side of the needle felt. Another critical as-pect of the filter material associated with this type of bag house is the fibre surface area. This is related to the fibre finesse and the cross section irregularity. A fine round fibre of 1.7 dtex has a larger surface area than a 2.2 dtex round fibre and a fine mul-tilobal fibre (1.7 dtex) has an even higher surface area than the 1.7 dtex fine round fibre. The higher the surface area, the

smaller the pores between the fibres and the higher pores total surface. The tra-jectory of a dust particle while travelling across the filtration material is also more complex. Fibres with a high surface area, like the P84 multilobal fibres, are critical for the dry/semi-dry FGD bag houses be-cause the fibre structure is influencing the formation of dust cake. Basically, the fibre structure is transferred to the dust cake structure. The latter one becomes porous and also stable during the cleaning event. A porous dust cake with a high surface area ensures a high contact between the gas molecules and the reagent during the passing of gasses through the dust cake. Therefore, the filter materials used for the dry/semi-dry scrubbers must have at least a layer of multilobal fibres on the filtration surface. F i g u r e 2 shows the specific sur-face area of different types of fibres used in dry filtration. Due to its excellent filtration properties the P84 fibres also prevent the penetra-tion of fine dust particles into the needle felt. This has important benefits in terms of low particulate emissions, low pressure drop across the bag house, low pulsing rate and the capacity to deal with a high dust loading, common to the dry/semi-dry scrubbers. A microscopic schematic cross section of a P84 material and the associ-ated structure of the dust cake are shown in F i g u r e 3. Due to the irregularity of the P84 fibre cross section, the filter material charged with dust maintains a high porosity. An-other explanation for the high porosity of the dust-laden material is related to the fact that the dust collected between the fibre lobes is not in the way of the flow

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SO2

rem

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0 0.5 1.0 1.5

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ESP

2.0

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2.5 3.0 3.5 4.0

Fig. 1. Performance of sodium sesquicarbonate in SO2 mitigation.

Specific surface of different fibre materials as function of the fibre fineness

Surface area in m2/kg

4 3 2 1 0

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Fig. 2. Specific surface area of fibres.

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Filtration in dry- and semi-dry FGD VGB PowerTech 7 l 2012

lines. Typical recommended high filtration efficiency materials based on P84 fibres for dry/semi-dry scrubbers are shown in F i g u r e 4.A summary of the filtration mechanism with P84 multilobal fibres is the following:

– The felt constructed from multilobal fine fibres has a high surface area, thus, it is irregular and porous.

– The dust separates in the low velocity zones of the multilobal fibres forming a dust cake.

– The structure of the filter media is transferred to the structure of the dust cake and the dust cake formed by mul-tilobal fibres is irregular and porous. The flow-lines are affected to a lesser extent if compared to the round fibres dust cake.

– The dust cake formed by multilobal fi-bres is stable and part of it remains at-tached to the felt during the cleaning phase.

Filtration test rig description and latest results

The Evonik filtration test rig (FTR) con-cept was developed for evaluating mainly filtration properties of different types of filter media. Two test rigs were built and were installed at Eskom South Africa and ENEL Italy power plant bag houses. The tests are performed in partnership with site assistance form the two major power producers. The FTR evaluations assist with the selection of a superior type of filter bag for a specific coal-fired boiler bag house ap-plication. The FTR comprises of three main components: platform with suction fans, instrumentation and controls, inspection window adaptor and tube plate bag covers. The FTR schematic is shown in F i g u r e 5.

P84 based needle felt versus PPS round fibres needle felt

A test was performed in South Africa at the Eskom Hendrina power plant to evalu-ate the performance of a P84 based nee-dle felt and two other materials based on PPS round fibres. The FTR test was set up at a filter face velocity of 1.4 m/min with ΔP based cleaning control. In this case the evaluation criteria was the pulsing rate required for each test bag to maintain the ΔP within the set range. The test details and results shown in F i g u r e 6 indicate clearly major differences in terms of pulses per day between the P84 based bag and

Fig. 3. P84 fibres cross section, P84 dust cake structure.

P01 M01FIC

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Fig. 5. Schematic of Evonik filtration test rig.

P84® filtration side

P84® scrim

P84® clean gas side

P84® felt

P84® filtration side

PTFE scrim

P84® clean gas side

P84® felt on PTFE scrim

P84® / PPS filtration side

PPS scrim

PPS clean gas side

Blend of P84® into base materials from otherfibres, e. g. PPS

Fig. 4. Materials for semi-dry scrubber FFPs.

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VGB PowerTech 7 l 2012 Filtration in dry- and semi-dry FGD

the other two PPS bags constructed with fine and normal size (titre) round fibres. Lower pulsing rates induce lower mechani-cal stresses onto the filter bags and this is directly proportional to the bag life. This direct comparison confirms the superiority of the P84 based bag, already proven by the actual performance of such bags in coal-fired boiler bag houses around the world. Eskom, the main utility in South Africa, is operating some of the largest coal-fired power plants provided with bag houses in the world. Most of them are equipped with bag houses based on P84 blended needle felt bag filter materials. These results were

confirmed by other FTR tests performed in Italy in collaboration with ENEL.

P84 needle felt bag versus glass/membrane bag

A further FTR test was performed at the Eskom Hendrina power plant for the evalu-ation of a 100 % P84 needle felt bag and a woven glass with PTFE membrane bag. This time the FTR was set on timer con-trol with pulsing at 20 minute intervals for 17 days/12 minute intervals for the next 23 days and compared in relation to the ΔP achieved by each bag. The results shown

in F i g u r e 7 indicate the superiority of the P84 bag after 40 days of operation at 1.2 m/min filtration face velocity. Results with the same level of magnitude were achieved when the same materials were compared in the Italian FTR installed in an ENEL bag house.

Power plant – Czech Republic – dry scrubbing with bag filter

This coal burning power plant is provided with a fluidised bed boiler, a dry scrubber with calcium hydroxide as a reagent and a bag house. A plant photo and a schematic are shown in F i g u r e 8. The initial set of filter bags was based on a 100 % PPS mate-rial. The bags failed on high-pressure drop caused by filter material blinding. It was decided to replace these bags with bags based on a PPS/P84 blended fibres in or-der to improve the filtration efficiency and prevent the dust penetration into the cross section of the material. F i g u r e 9 shows the condition of the needle felt before the replacement was required and the condi-tion of the new PPS/P84 filter media with a well-defined dust cake and no dust pen-etration .This bag house is achieving more than five years of bag life operation. The plant details are listed in Ta b l e 1 .

References[1] Attwell, M., and Wood Solvay, M.: Solvair

Products Presentation, 2009.[2] Popovici, F.: Eskom Gas Cleaning Manual

2006, South Africa.[3] Eskom/Paul Wurth Presentation at Mittal vd

Bijl Park, South Africa 2006. l

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August/September 2010∆P control: start cleaning 13 mbar – stop cleaning 9/8 mbar

Filtration velocity: 1.4 m/min

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Fig. 6. Eskom/Evonik FTR – PPS/P84 bag versus PPS/PPS bags.

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1.2 m/min gas to cloth ratio, one pulse at 20 min intervals, 17 days1.2 m/min gas to cloth ratio, one pulse at 12 min intervals, 22 days (18 to 40 days)

18 to 40 days

P84 G/M P84 G/M

Fig. 7. Eskom/Evonik FTR – P84 bag versus glass/membrane bag.

Tab. 1. Plant details of Czech power plant.

Boiler Fluidised Bed Boiler

Filter material Procon/(Procon)/P84 multi- lobal + Procon trilobal

Type of cleaning Pulse jet – off line

Gas Flow 550,000 Am3/hour

Temperature 130 to 160 °C

Filtration face velocity 0.75 m3/m2 ∙ min

H2O up to 7 vol%

O2 6 % volume

SOx < 500 mg/Nm3

NOx < 300 mg/Nm3

Fuel Lignite (1 % sulphur)

Fig. 8. FFP schematic. Fig. 9. Old PPS bags – dust penetration, P84 new bags – clean filter media.

Performance and Efficiency for Clean Power Generation.P84® and Procon™ fibre blends for superior filtration efficiency.

P84 and Procon blends for filter bags:Procon fibres with excellent chemical stability offer maximum flexibility when changing coal qualities. Combined with the outstanding filtration efficiency of P84 fibres this material offers long service life and low maintenance cost.

Filtration efficiency: The unique multilobal profile of P84 fibres ensures that even fine particles are collected and no dust can penetrate the needle felt even at very high dust loads. In combination with trilobal Procon fibres the filter material will show low pressure drop during the entire bag life.

High temperature resistance: P84 polyimide fibres and Procon PPS fibres can withstand high temperatures and show chemical resistance over a wide range of pH.

Evonik Fibres GmbHGewerbepark 4 4861 SchörflingAustriaphone +43 7672 701-2891 fax +43 7672 96862www.P84.com