Bentzen, Jens Dall

Henriksen, Ulrik Birk

Hindsgaul, Claus

Baghouse filters and cartridge filters were tested online with wood gas from a two stage down draft gasifier. The gas contained soot and very low levels (10-30 mg/Nm³) of tar. Particle collection efficiencies were above 95%. Continuous operation with cheap self cleaning baghouse filters were tested for a period of three days with good results

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Low Temperature Particle Filtration of Wood Gas with Low Tar ContentHindsgaul, Claus; Henriksen, Ulrik Birk; Bentzen, Jens DallPublished in:Proceedings of the 12. European Biomass ConferencePublication date:2002Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Hindsgaul, C., Henriksen, U. B., & Bentzen, J. D. (2002). Low Temperature Particle Filtration of Wood Gas withLow Tar Content. In Proceedings of the 12. European Biomass Conference (pp. 494-497). ETA-Florence & WIP-Munich.LOW TEMPERATURE PARTICLE FILTRATION OF WOOD GAS WITH LOW TAR CONTENTC. Hindsgaul*, U. Henriksen* and J. D. Bentzen***Danish Technological University/Mechanical EngineeringBuilding 120, DK-2800 Lyngby, Denmark**COWI Engineers and Planners ASParallelvej 15, DK-2800 Lyngby, DenmarkABSTRACT: Baghouse filters and cartridge filters were tested online with wood gas from a two stage down draftgasifier. The gas contained soot and very low levels (10-30 mg/Nm³) of tar. Particle collection efficiencies were above95%. Continuous operation with cheap self cleaning baghouse filters were tested for a period of three days with goodresults.Keywords: Gas cleaning, gasification, tar removal1  INTRODUCTIONGas cleanup from biomass gasifiers generallyhave to deal with high levels of tar in the gas sticking toe.g. pipe walls and movable parts. For low tar gasifiers,this problem is expected to be very small - or evenabsent. Thus very simple, well known gas cleaningdevices may be sufficient to reach a gas quality suitablefor IC engines and gas turbines.On the two stage gasifier at DTU[1] pyrolysisand gasification reactors are separated by a well definedhot tar cracking zone. The resulting gas contain 10-30 mg/Nm³ tar, but approximately 400 mg/Nm³ particles.The vast majority of the particle mass from the two stagegasifier has been identified as soot with sizes about 0.1-0.5 µm[2].Particles of these sizes can not be efficientlyremoved by inertial particle separation methods such ascyclones or scrubber systems. This explains why theexisting venturi scrubber based gas cleaning system neverremoved more than 85% of the particle mass despiteseveral optimisation attempts.Filtration through fibrous materials orelectrostatic precipitators have excellent performancewith submicron particles. Filtration with baghouse filtersand cartridge filters were chosen for evaluation in thiswork.  Attempts to test an electrostatic precipitator failed.1.1 Fibre filtersIn order to identify the important parametersinfluencing the collection efficiency in fibre filters, abrief filtration theory study was conducted.In fibrous filters the particles are collected onthe fibres by interception and diffusion. Interception iswhen a particle hits a fibre due to inertia effects orbecause the particle is large enough to touch the fibre asit passes. Interception is the most important effect forlarger particles (>1 µm). Diffusion is when the Brownianmotions of the particle brings it in contact with the filtermaterial. Diffusion is the major collection effect for sub-micron particles (<1 µm).A theoretical measure for the efficiency of afibre media is its single fibre efficiency, ηs. This is theefficiency of a single, cylindrical fibre. It is assumed, thatit is surrounded by a cylindrical gas filled volume. Thevolume ratio of fibre and air correspond to that of theactual filter material. The following theoretical equationfor ηs was proposed by Lee and Liu[4]. They included anempirical constant, ε ≈ 1.6:       onintercepti111diffusion158.223/13/1RRKKDUKfS+⋅ −+ ⋅⋅ −⋅=αεαεη(Eq. 1)Where R=Dp/Df, α is the fibre packing (ratio offibre volume to void volume), Df and Dp are the fibre andparticle diameters. The diffusion coefficient, a, can beapproximated assuming spherical particles:vt is the mean thermal speed, lmfp is the meanfree path, n is the number of particles per volume.The relation between ηs and the overall filterefficiency is η = 1−e−ηs·S where fffDmSρpi ⋅⋅=4.Figure 1: Theoretical single fibre efficiency plots varyingdifferent parameters.The plots in Figure 1 show the single fibreefficiency as functions of different parameters usingeq. 1. The following base values were used: fibre packing2564.013420.0420.0ppmmfptDnTAnMTRlva⋅=⋅⋅⋅=⋅≅d) Gas velocitya) Fibre diameter b) Fibre packingc) Temperatureα = 0,1; gas velocity U=0.02 m/s; gas temperatureT=50°C=323 K; particle concentration n=0.03 µm-3; andfibre diameter Df=5 µm.Figure 1c shows that the temperature has onlynegligible impact on ηs (if the gas velocity is keptconstant). This indicate that the advantage of fasterBrownian motions in the filter is negligible under theseconditions. In contrast, fig. 1a indicate that the choice ofmaterials with small fibres (small Df) markedly increasesηs.For submicron particles Figure 1d shows thatthe gas velocity are important to ηs. Thus larger area ofthe filter not only means a lower pressure drop. It alsoresults in better removal of submicron particles.2  METHODSThe filters were tested during a one week testprogramme with the two stage gasifier at DTU fueledwith wood chips. The existing gas cleaning system basedon a venturi scrubber was replaced by a setup in whicheither a baghouse filter or a cartridge filter would cleanthe full amount of produced gas (approx. 60 Nm³/h).A system with two gas coolers was built (seefigure 2). First the gas was cooled from 700°C to 90°Cwhich is intentionally just above the dew point of waterin the gas in order to avoid liquid water to enter thefilters. Cooling reduce gas the volume to be cleaned wasreduced and makes wider range of filter materialspossible. After filtration of the gas further heat wasrecovered in a cooling condenser.Figure 2: Schematic of the test setup.2.1  Baghouse filtersDT/DT 501 PE/PE 501Material Dralon-T PolyethyleneMass 500 g/m² 500 g/m²Area 0.92 m² 0.92 m²Gas velocity 18 mm/s 18 mm/sThickness 2.3 mm 1.8 mmMax. temperature 120 °C 150 °CHydrolysis resist. Good BadPacking density, α 19 % 20 %Table 1: Data for bag filters from data sheets. "Gasvelocity" is the gas velocity occurring in this work.Baghouse filters made of polyethylene (PE) andDralon-T were tested. Polyethylene was the cheaper ofthe two and was the more temperature resistant, but couldbe subject to destruction by hydrolysis in the gas, whichcould quickly break the filter. Two different bag typeswere tested (Table 3).For each of the two bag materials, a setup withtwo bags (1 meter, 0.46 m² each) were tested. The totalfilter area was 0.92 m² corresponding to an average gasvelocity of 0.02 m/s. The baghouse filters were testedwith cleaning pulses of 50 ms with N2 from a pressurisedtank with 3.5 bar gauge.2.2  Cartridge filtersFour different cartridges were tested. One ofthese were of the membrane type, the others fibrous(Table 2).EnviEN-13EnviEN-247EnviEN-712MANNC23 440/1Material Tetratexon PECoatedpaperCellulosePE4MFNpaperFiber  Df Membr. - - 37-50 µmMass 200g/m² 190g/m² 130g/m² -Area 2.0 m² 4.5 m² 4.5 m² 1.7 m²Velocity 8 mm/s 4 mm/s 4 mm/s 10 mm/sα - - - >18 %Table 2: Data for cartridge filters from data sheets. Thefilter areas were measured on the actual filters. "Velocity"is the gas velocity occurring in this work.3  RESULTSFigure 3 shows the particle determinationsduring the test. The determinations of collected particlesare averages over the full operation time of each filter.These are shown as vertical lines filling the time span offilter operation.  For reasons unrelated to this filterevaluation the gasifier was operated in three differentoperating conditions explaining the different particleloads in Figure 3:• 21.11.99-23.11.99: No steam, high particle loads.• 23.11.99-25.11.99: With steam, low particle loads.• 25.11.99-26.11.99: Vortex mode1, medium loads.Particles05001000150021.11.99 22.11.99 23.11.99 24.11.99 25.11.99 26.11.99DateParticles [mg/Nm3]Measured before filterCollected in filterMeasured after filterPE bag (97-98,5%)PE bag(95,5-98,5%)Coated papercartridge filter(98,5-99,5%)Coated papercartridge filterFigure 3: Particle load determinations. The approximateparticle collection efficiencies during the tests are shownin parenthesis.The filters apparently collect more particlesthan were measured in the gas before the filter. This isbecause the particle loads increase dramatically when the                                                                1 See poster V2.82.GasifierCycloneHot coolerCartridge/bag filter90ºC700ºCCondensing coolerCondensateOK for biologicalsurridge plant45ºCgasParticlesgrate supporting the fixed bed in the gasifier is moved.This is done as a routine operation to relieve the fixedbed from excess pressure drops. This operation is doneby the plant personnel but was intentionally avoidedduring particle and tar measurements in the gas beforethe filters in order to have consistent measurements. Thusthe filters collected these particle "bursts" while theywere not included in the gas measurements.Particle measurements after the filters were notas sensitive to grate movements. Partly because theyspanned over longer time.Particle collection efficiencies were calculatedfrom the mass of particles collected by the filters and theparticle measurements after the filters. Inspections of apolice cartridge filter placed after the condensing coolerconfirmed that virtually no visible amounts of particlesreached this filter.3.1  Baghouse filtersUnfortunately dirty gas leaked past the Dralon-T bags due to insufficient fastening of the bags. Thiscaused a collection efficiency of around 50%. The policefilter in effect caught considerable amounts of particlesand the condensate was contaminated. Thus theefficiency evaluation of these filters failed, but the factthat the collected particles were dry and easy to handlewas encouraging.The PE bags were correctly fastened andshowed no signs of destruction due to hydrolysis.Backflush cleaning of the baghouse-filters with N2-pulseswere successfully demonstrated for 50 hours cleaningwoodgas. The pressure of the backflush N2 was 2.5 bargauge. Figure 5 shows the pressure development withbackflushes approximately every 60 minutes.Bag filter PE010020030040010 15 20 25 30HoursPressure drop [mmWG]Filtre pressure dropGrate activationInterruption7 mg/Nm3after filter34 mg/Nm3after filter28 mg/Nm3after filter10 mg/Nm3after filterFigure 5: Pressure development for the PE baghousefilter. The times of two grate movements as well as twoshort stops due to external causes are indicated.The particle load before the filters were 400-500 mg/Nm³ while 10-30 mg/Nm³ was left after filtration.Peaks in the particle load after the filter occurred duringbackflushes. According to the filter producer, this shouldbe avoided if lower the pressure of the backflush N2could be reduced.3.2  Cartridge filtersThe cartridge filters appeared to workeffectively until they were saturated with particles. Themeasured efficiencies of the filters exceeded 98 %. Thepressure developments of the tested cartridges are shownin Figure 4. The cartridge filters are very compact andwould be a good choice for filtering gas streams withsmall particle contents or short operation times. E.g. itwould be good as a "police filter" after other gas cleaningcomponents in order to ensure clean gas even if the maingas cleaning systems should fail.01002003004005000 1 2 3 4Hours of filter operationPressure drop [mmWG]0 mg/Nm3after filterEN-131170 mg/Nm3EN-712  (*)5550 mg/Nm3EN-247450 mg/Nm3Coated paper(1,7 m2MANN C23 8 hoursMANN C23 4401296 mg/Nm32 mg/Nm3after filterFigure 4:  Pressure development for the tested cartridges.The legends show the type and average collected particlemasses. The results of two isokinetic particlemeasurements after the filters are shown.(*) The EN-712 filter test was interrupted by very highparticle loads due to sudden massive movement of thegasifier grate.A similar Danish low tar gasifier have later tested asimilar baghouse filter setup with good results[6].3.3  Tar removalThe tar content found in the particle sampleswere determined by two different methods:1. pyrolysing samples at 600°C.2. extracting samples with acetone.The mass loss due to pyrolysis tend to overestimate thetar levels compared to extraction[5].Figure 6 shows a summary of the tardeterminations during the tests. The x axis is the time ofdetermination. The "pyrolysable" and "acetone solubles"are calculated from the determined particle tar mass.These determinations are averages over the full operationtime of each filter shown as vertical lines filling the timespan of filter operation. The sum of tar determinationsdone by the Danish Technological Institute (TI) areincluded in the figure.Tar025507510022.11.99 23.11.99 24.11.99 25.11.99 26.11.99DateTar [mg/Nm3]Pyrolyzable tar on particlesAcetone solubles on particlesTar before filter (TI)Tar after filter (TI)Figure 6: Tar determinations in the gas measured byparticle and gas samples. Some of the pyrolysisdeterminations were repeated on particle samples fromthe same filter.It is seen from Figure 6 that the tar contentgenerally decreased from 25-40 mg/Nm³ to levels notexceeding 5 mg/Nm³. Extractions of the particlescollected by the PE baghouse filter in acetone showedthat the removed amount of tar was found on thecollected particles, indicating that most of the tars hadcondensed on the particles during cooling. So coldparticle filtration seemed to remove a significant fractionof the tar even though the initial tar levels were very low.It also indicate that even though particles in the gas aregenerally depreciated, certain levels of particles mayimprove tar removal; in the absence of particles, tarwould instead condense on the inner surfaces of coolersand piping.4  CONCLUSIONSTwo dry, cold particle filter types weresuccessfully tested with cooled producer gas from thetwo-stage gasifier at DTU: Baghouse filters and fourdifferent cartridge filters.It was demonstrated that the bag and cartridgefilters had very good cleaning efficiencies (95-99 %mass). It is shown theoretically that larger filter areascould further improve the collection efficiencies of suchfilters for submicron particles such as those found in theproducer gas. This was confirmed by the fact that themeasured collection efficiencies of the cartridge filters(with larger filter areas) were the highest. The choice offilter material will also potentially improve collectionefficiencies.Summarised, baghouse and cartridge filters areeffective for particle removal from producer gas with lowtar content.  Cartridge filters are more compact but havelower particle capacity and should be preferred as apolice filter in the clean gas after the primary filter.ACKNOWLEDGMENTSThis work was sponsored by the Danish Energy Agency.REFERENCES[1] B. Gøbel, C. Hindsgaul, U. Henriksen, J. Ahrenfeldt,F. Fock, N. Houbak, B. Qvale, and J. D. Bentzen. Highperformance gasification with the two-stage gasifier. 12thEuropean conference and Technology Exhibition onBiomass for Energy, Industry and Climate Protection.Amsterdam June 2002. Presentation O2.B1.[2] Claus Hindsgaul, Jesper Schramm, Linda Gratz, UlrikHenriksen, and Jens Dall Bentzen. Physical and chemicalcharacterization of particles in producer gas from woodchips. Bioresource Technology, 73(2):147-155, 2000.[3] K. W. Lee and B. Y. H. Liu. Experimental study ofaerosol filtration by fibrous filters. J. Aerosol Science,1982.[4] Benjamin Y. H. Liu and Kenneth L. Rubow. Airfiltration by fibrous media. In Fluid Filtration: Gas.[5] C. Hindsgaul, U. Henriksen and J. D. Bentzen.Evaluation of Gravimetric Tar Determinations on ParticleSamples. Proceedings of 1st World Conference onBiomass for Energy and Industry, Sevilla, Spain, June 5-9 2000,[6]  M. W. Fock, T. Koch, P. Girard, A. Napoli. BiomassGasification for CHP with Dry Gas Cleaning andRegenerative Heat Recovery. 12th European conferenceand Technology Exhibition on Biomass for Energy,Industry and Climate Protection. Amsterdam June 2002.Poster V2.103.

Low Temperature Particle Filtration of Wood Gas with Low Tar Content

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Low Temperature Particle Filtration of Wood Gas with Low Tar Content

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