Bentzen, Jens Dall

Henriksen, Ulrik B.

Hindsgaul, Claus

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Investigation of a Two-Stage GasifierBentzen, Jens Dall; Hindsgaul, Claus; Henriksen, Ulrik BirkPublished in:Proceedings of the 2nd Olle Lindström Symposium on Renewable Energy - BioenergyPublication date:1999Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Bentzen, J. D., Hindsgaul, C., & Henriksen, U. B. (1999). Investigation of a Two-Stage Gasifier. In Proceedingsof the 2nd Olle Lindström Symposium on Renewable Energy - Bioenergy (pp. 117-120). Stockholm: RoyalInstitute of Technology.INVESTIGATIONS OF A TWO-STAGE GASIFIERJens Dall BentzenCOWI - Consulting Engineers and Planners ASParallelvej 152800 Lyngby, DenmarkTel: +45 45 97 13 38Fax: +45 45 97 22 21e-mail: jdb@COWI.dkUlrik Henriksen and Claus Hindsgaul HansenTechnical University of DenmarkDepartment of Energy Engineering2800 Lyngby, DenmarkTel: +45 45 25 41 72Fax: +45 45 93 57 61e-mail: uh@et.dtu.dk and ClausH@et.dtu.dk117ABSTRACTA 100 kW thermal input Two-Stage Gasifier was built at theUniversity of Denmark (DTU). The gasifier consists of anexternally heated pyrolysis unit, a down-draft char gasifier, agas cleaning system and an internal-combustion engine.Several tests and investigations have been made (Bentzen,1998). Between the summer of 1996 and the summer of 1998,11 tests were made with the gasifier. The gasifier had a totaloperating time of 278 hours (excluding warming up).The fuel was mainly wood chips, but also briquettes hasbeen used (Henriksen, 1997). Four of the tests lasted 50-70hours, and stable conditions were reached. All the tests werecompleted without technical problems.The particle sizes and amounts were investigated (Hansen,1998), (Ravn, 1998). The amount of particles depended on theamount of steam led to the gasifier. More than 99% of theparticles after the cyclone were soot. The size of the sootparticles was mainly between 0.2-0.5 µm. Under normalconditions (biomass:steam = 1:1 (dry mass basis)), the particleload was about 300 mg/Nm3.  With biomass:steam = 3:1 (drymass basis) the particle load was above 1000 mg/Nm3.The pressure drop through the char bed was moderate andseemed to stabilise at about 200 mmH2O fuelled with woodchips and about 50 mmH2O fuelled with briquettes.Char from the gasifier was investigated for its properties asactivated carbon. After 72 hours' operation fuelled with woodchips, the gasifier was cooled while nitrogen was led throughthe bed. Samples were taken from 7 different vertical positionsin the bed, and the BET surface area was determined at eachposition. The surface area was between 775-2158 m2/g. Twosamples were analysed thoroughly for properties as activatedcarbon. The adsorption tests (Molasses number, Iodinenumber etc.) showed that these samples had adsorptiveproperties comparable to a lot of commercial activatedcarbons (NORIT, 1998).BACKGROUNDThe Department of Energy Engineering of the TechnicalUniversity of Denmark (DTU) has previously described theTwo Stage Downdraft Gasifier and reported  its mainadvantages namely low tar content and high energy efficiency(Henriksen, 1994).A 100 kW thermal input Two Stage Gasifier was built at theUniversity of Denmark for further investigations.THE 100 KW TWO-STAGE GASIFIERThe 100 kW two-stage gasifier was built in 1995 and wasequipped with gas cleaning system, automatic control anddatalogging in 1996.Figure 1. The 100 kW two-stage gasifier at DTU The gasifier consists of a feeding system, an externally heatedpyrolysis unit and a fixed bed gasifier (fig 1). Two kinds ofGas cleaningsystem.See figure 2.118feeding systems were constructed: A screw feeder for woodchips and a lock-hopper for briquettes.The gas cleaning system consists of a hot cyclone, a gascooler, a venturi scrubber and demistor, and a simple gas filter(fig 2). A Ford VSG 411 IC engine is connected to thegasifier.NEW RESULTS FROM THE TWO-STAGE GASIFIERPrevious results from tests with a two-stage gasifier,showing low tar content and high energy efficiency, werepublished by (Henriksen, 1994). These results were confirmedon the 100 kW gasifier. This paper presents results madeduring a recent series of tests.Overview of testsBetween the summer of 1996 and the summer of 1998, 11tests were made with the gasifier. The gasifier had a totaloperating time of 278 hours (excluding warming).Table 1.  Major tests with 100 kW gasifierDate,length of testFuel Investigations to bepresented in this paperApril 1997,50 hoursSGF(briquettes)Pressure drop through bed.January 1998,72 hoursWood chips Particle contents and sizedistribution, activatedcarbon tests of charcoal,pressure drop through bed.March 1998,63 hoursWood chips Particle contents and sizedistribution, baghouse filterfor gas cleaning.PARTICLE INVESTIGATIONS IN THE GASFigure 2. Gas cleaning system with points of measurements.One of the main purposes of the work with the 100 kWgasifier was to optimise the gas cleaning system. Therefore,measurements were made to determine the amount and tocharacterise the particles from the gasifier. Additionally, aliterature study regarding the formation of soot in gasifierswas made. (Hansen, 1998), (Ravn, 1998.)To measure the amount of particles in the gas, equipmentwas built in compliance with the german VDI 2066 standardof dust measurement (VDI, 1975). To determine the sizedistribution of the particles, a Pilat Mark III cascade impactorand a Berner low-pressure impactor were used.Particles in the raw gasWhen the gasifier was operating in the normal mode with abiomass:steam ratio of 1:1 on mass basis, the particle load inthe raw gas was about 200 mg/Nm3. When the steam generatorwas turned off, the biomass:steam ratio was 3:1, and theparticle load  raised to above 1000 mg/Nm3.Figure 3.  Particle load in the raw gas at different tests and atdifferent biomass:steam ratios.Size distributions of the particles were measured at differentbiomass:steam ratios.Figure 4. Size distribution of the particles at differentbiomass:steam ratios (Bentzen, 1998).The dominant particle size was between 0.2 and 0.5 µm, butsizes between 0.1 –2  µm were observed.Gasifier andengine.See figure 1119Figure 5. SEM (Scanning Electron Microscope) picture ofparticles collected in the raw gas.In figure 5 it can be seen that the particles wereagglomerates of small (50-100 nm) spherical particles (whichwere identified as soot).Literature study of soot formation in gasifiersSoot formation is described to follow many differentpathways (Ravn, 1998). In reducing atmosphere and hightemperatures, soot is formed from the volatiles from thepyrolysis products. The reaction rate increases withtemperature, but also the breakdown of soot by oxidation byOH radicals increases with temperature and with the amountof OH radicals. The amount of OH radicals depends moststrongly on water addition and moisture. This explains whythe soot formation increases when the steam addition is turnedoff. A temperature dependent maximum of soot production isexpected to exist.Particles after the venturi scrubberThe venturi scrubber is the most efficient scrubber conceptwith efficiencies exceeding 99%, (Eckman, 1951), butmeasured particle load after the plants venturi scrubbershowed that its efficiency was 60-85% depending on thepressure loss across the scrubber. The particle load after theventuri scrubber was typically 60-90 mg/Nm3. The reason forthis poor efficiency may be that the particles were too small.The submicron particles follow the gas flow and have nodirect contact with the water droplets.Particles after the gas filter.The gas filter used was a MANN C 23 440/1, which isusually used for air filtration on IC engines. This is known asa 10 µm filter, but the particle load decreased dramaticallyafter the filter. The particle load was below 5 mg/Nm3, from the beginningof a test. After 2 days, the particle load was about 0 mg/Nm3.This high efficiency of particle removal may be explained byBrownian motions. When submicron particles follow theBrownian motions they will flow through a large volume ontheir way through the gas filter. The risk of particles impactingon the rough material is therefore high. The cleaningefficiency increases over time because the particles collectedeventually build up a layer of soot.Figure 6. Particle load after the gas filter as a function of thetime it has been used.Experiments with baghouse filterA baghouse filter would precent an alternative gas cleaningsystem, thus a small baghouse filter was constructed, andseveral filter materials were selected for testing during theMarch 98 test. A Heimbach 302.600 filter replaced the venturiscrubber for 3 hours. When the pressure drop was too high(100-150 mm H2O) it was backflushed. After 16 backflushes itwas still no problem to regenerate the filter. The particle loadafter the baghouse filter was measured to be 5 mg/Nm3.PRESSURE DROP ACROSS CHAR BEDSeveral tests with the 100 kW gasifier were madecontinuously during several days, and the pressure drop overthe char bed evolved slowly and stabilised after about twodays.Figure 7. Pressure drop over char bed during test withbriquettes and with wood chip.During the tests, the grate was only activated once (markedon figure 7). When the ashes on the grate were removed thepressure drop across the char bed was reduced.It is evident that the pressure drop over the bed was smallwhen fuelled with briquettes, and moderate when fuelled withwood chips. The pressure drop in the December and theMarch tests evolved similar to the pressure drop in the Januarytest.120CHARCOAL FROM THE TWO-STAGE GASFIER ASACTIVATED CARBONA number of tests on the char bed have been made after thetest in January 1998. The bed was allowed to cool as nitrogenwas led through it. Samples from different layers in the bedwere analysed at DTU for the BET surface area. The Dutchcompany NORIT has made more detailed analyses of thecharcoal, and SEM (Scanning Electron Microscope) picturesof the pyrolysed and the partial gasified char were taken.The most important parameters to determine how active thecharcoal is, are the pore structure and the total surface area.(NORIT). Commercial activated carbons have a surface areabetween 500-1500 m2/g.The surface area was measured on samples from the sevenlayers from the charbed after the test in January 1998.Table 2. BET surface area of charcoal from thecharbed in different heights above the gratePlace of sampleHeight above the grate [cm]BET surface area [m2/g]121 775101 126979 99559 124939 98317 21585 796In table 2 it is seen that the surface area is high in all parts ofthe charbed.Samples from the layer of 39 cm above the grate were sentto NORIT for further investigations. The molasses numbers(Europe) were 150-185, and the Iodine numbers were 790-865. The conclusion was “…It is obvious that the results ofthe adsorption tests (molasses number etc.) show that thesesamples have absorptive properties, which certainly arecomparable with a lot of activated carbons in the market(powdered activated carbon)…..”Figure 8. Charcoal from 39 cm above the grate from test inJanuary 1998.In figure 8 is seen how the pores of the char open evenlywhile the main structure is maintained.ConclusionsInvestigations of particlesExcept for a very small fraction of gasborne biomassparticles, the particles from the gasifier consist of 50-100 nmsoot particles which have agglomerated to particles rangingfrom 0.1 - 2 µm. Most of the particles were between 0.2-0.5µm.Since the particles were so small, the venturi scrubber didnot have the expected high efficiency. A simple gas filter afterthe scrubber, was very effective. The particle load after the gasfilter was <0-5 mg/Nm3. A bag house filter was successfully tested for 3 hours. Itsefficiency was 97%.Pressure drop over charbedWhen wood chips or briquettes are gasified the pressuredrop develops slowly. After a few days it stabilises. Activationof the grate reduces the pressure drop, since the ashes on thegrate are removed.Charcoal from the two stage gasifier as activated carbonFrom these investigations was concluded that charcoal fromthe two stage gasifier can be utilised as activated carbon.References    Bentzen, J,D, Brandt, P, Gøbel, B, Hansen, C, H, andHenriksen, U, 1998,, G., “100 kW totrinsforgasningsanlæg påDTU”. Report ET-ES 98-11, Institut for Energiteknik, DTU,Denmark. ISBN 87-7475-206..    Ekman, F, Johnstone H, F, 1951."Collection of Aerosols ina Venturi Scrubber". University of Illinois, Urbana,    Hansen, C. H.1998. "Physical and ChemicalCharacterisation of Particles in Producer Gas." DTU,Denmark. ET-EP 98-12.    Henriksen, U, Christensen, Ole. October 1994.“Gasification of Straw in a Two-stage 50 kW Gasifier” DTU,Denmark. Proceedings from "8th European Conference onBiomass for Energy, Environment, Agriculture and Industry".Vol. 2. pp. 1568-1578. Vienna. Austria. October 1994.    Henriksen, U, Nielsen, C, 1997 "Duration Test, SGF. DTU,Two-Stage Gasifier". Task 2, EU Project: JOR3-CT95-0084.    NORIT, 1998. Letter of 27´th of August 1998, Analysis.    NORIT. "Introduction to Activated Carbon". NORIT.    Ravn, C , Rousing, M, 1998. “Dannelsen af Sod iForgasningsprocesser” Master thesis, Institut for Kemiteknik.DTU, Denmark    VDI 2066. 1975. “Particulate matter measurement”,Germany.

Investigation of a Two-Stage Gasifier

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Investigation of a Two-Stage Gasifier

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