Bentzen, Jens Dall

Elmegaard, Brian

Henriksen, Ulrik Birk

Hummelshøj, Reto

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Medium and large-scale low tar and high efficient gasificationBentzen, Jens Dall; Hummelshøj, Reto; Henriksen, Ulrik Birk; Elmegaard, BrianPublished in:Biomass for Energy and IndustryPublication date:2000Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Bentzen, J. D., Hummelshøj, R., Henriksen, U. B., & Elmegaard, B. (2000). Medium and large-scale low tar andhigh efficient gasification. In Biomass for Energy and IndustryPublished by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.1MEDIUM AND LARGE SCALE LOW TAR AND HIGH EFFICIENT GASIFICATIONJens Dall Bentzen, Reto M. Hummelshøj, Ulrik Henriksen* and Brian Elmegaard* COWI - Consulting Engineers and Planners AS, Parallelvej 15, 2800 Lyngby, Denmark, jdb@cowi.dk  rmh@cowi.dk*Technical University of Denmark, Department of Energy Engineering, 2800 Lyngby, Denmark, uh@et.dtu.dk be@et.dtu.dkABSTRACT: A new concept for low tar and high efficient gasification of moist and wet fuels such as wood and sludge hasbeen developed. Designs of medium and large scale CHP-plants based on the new concept are suggested. COWI and TechnicalUniversity of Denmark have a patent pending for the new concept.The new gasification concept combines the advantages of: 1. two-stage gasification, 2. drying with superheated steam, 3.pyrolysis with superheated steam  and 4. gasification with steam.The fuel is dried by superheated steam. The steam produced in the dryer is superheated to 600-800°C and let to the pyrolysiszone. In the inert pyrolysis zone the hot steam will pyrolyse the fuel. The volatiles from the pyrolysis zone is led to a partialoxidation zone, where the oxidation agent is added. Hereby the volatiles are partially oxidised and the tar contents isdramatically reduced. The hot gases from the partial oxidation zone and the char from the pyrolysis zone are led to thegasification zone where the char is gasified.1. PURPOSE OF THE WORKThere is a large market for medium and large scalebiomass gasification combined heat-and power plants(CHP). The existing atmospheric pressure gasificationtechnologies for medium and large scale gasification arecomplicated in order to reduce the tar-content in theproduced gas and have low electrical efficiencies.The purpose of this work was to develop medium andlarge scale CHP biomass gasification concepts with highenergy efficiency and low tar content in the produced gas.2. BACKGROUND OF THE NEW GASIFICA-TION CONCEPTExperience with and insight in- fundamental processes of pyrolysis and gasification- industrial drying technics- the demand of medium and large scale low tar andhigh efficient gasifierswas the inspiration to develop a new concept for mediumand large scale low tar and high efficient gasification.The new gasification concept combines the advantages of1. two-stage gasification2. drying with superheated steam3. pyrolysis with superheated steam4. gasification with steam.2.1 Traditional two-stage gasificationThe two-stage gasification process developed on theTechnical University of Denmark throughout the 1980íesand 1990íes, has a very high cold gas efficiency: 92-97%and the tar content in the produced gas is extreamly low:below 25 mg/Nm3 [1]. The tar content in the gas after thegascleaning (particle filtration) is about 5 tar/Nm3 [1].In the two-stage gasification process, the pyrolysis and thegasification process are separated into two different zones.In between the pyrolysis and the gasification zones, thevolatiles from the pyrolysis are partially oxidised. Hereby,most of the tars are decomposed into gas [2].Fuel Exhaust  gasMDrying and pyrolysisPartialoxidat ionGasif i-cat ionEngineCool ing andgasc leaningPart iclesAir D.H. Air  preheat Bot tomashFigure 1: Traditional two-stage gasifier.In Figure 1 a traditional two-stage gasifier is shown. Thepyrolysis reactor is indirect heated, and the gasificationreactor is a fixed bed.To enable high energy efficiency, the thermal energy inthe gasification gas and the exhaust gas is being used fordrying, air preheating and for pyrolysis.For small gasification plants (up to about 4-6 MWthermal), the traditional two-stage gasification technologycan be used. A 3 MW thermal indirect heated drying andpyrolysis unit at Haslev strawfired power plant has beendemonstrated [3].2.2 Drying with superheated steamDrying with superheated steam involves on or moreexternal heatsource(s) (fluegas, hot oil, steam..) that areheat exchanged with water vapour evaporated from thePublished by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.2product. The superheated steam is led back to the dryer,and is then the drying medium in the dryer (see Figure 2)Drying chamberHeatsourceEvaporatedsteamWet fuel Dryed fuelFigure 2 Principle of drying with superheated steam.Drying with superheated steam is a well knowntechnology, and it offers a number of advantages:- Environment friendly drying: In many gasificationprocesses, the fuel is dried directly by the flue gases, buthereby the exhaust gas becomes contaminated withorganic compounds (acids and phenols) and particles.When the fluegas is heat exchanged with steam thefluegas is not contaminated, and can therefore withoutproblems be used for district heat.- Fire hazards: Since the oxygen content is extreamly lowin the dryer, when drying with superheated steam, thefire hazards are very low.- Better product after drying: The product to be driedfaces a low temperature gradient during the dryingprocess, and therefore it does not break and crack asmuch as normally. The remaining moist in the fuel isalso more evenly distributed when the product is driedin superheated steam.- Loss of product: Due to oxidation, certain losses of theproduct is often seen, i.e. when biomass is dried withfluegasses.- Drying rate: In many cases, drying with superheatedsteam is faster than normal drying processes.- Several heat sources: Possible to use several heatsources in series.- Drying of sludges: Superheated steam dryers aredeveloped to dry very wet fuel such as sludge.Superheated steam dryers have been developed to dryfrom 0,5 to 50 tons vapour pr. hour. This corresponds to2,5 –250 MW thermal input if the fuel is woodchips witha moisture content of 50%2.3 Pyrolysis with superheated steamPyrolysis with superheated steam is a well-knowntechnology, used in kilns to make activated charcoal.Since the produced steam from the dryer is used as theheat carrier for the pyrolysis process, the new two-stagegasification process is most applicable for fuels that arerelatively wet. Moist content of about 40-60% ispreferable. This makes woodchips ideal for this process,which however also will be able to use other biomass,sludge and selected solid waste´s.When dry fuels are used, other heat sources than the steamfrom the fuel must be added to the pyrolysis reactor:• Additional water can be added to the dryer e.g.condensate from gas cooling or a condensing unit• Other heat sources, e.g. returned charcoal, bedmaterial or hot gases from the gasification chamberor radiation heat from the partial combustion zone,can be added to the pyrolysis process.Since the heat carrier to the pyrolysing process is a fluid,the reactor can be formed as a moving bed or a fluidisedbed reactor, where the steam is the fluidisation fluid aswell as the heat carrier.Experiments at DTU indicates pyrolysis reaction times ofwhole wood chips between 1-3 minutes [4].2.4 Gasification with steam as gasification agentIt is well known, that gasification with steam asgasification agent result in higher reaction rates thangasification with CO2 [5]. If steam is the primarygasification agent instead of CO2 the temperature in thegasifier can therefore be lowered. There are several otheradvantages related to steam gasification.- Lower temperatures: Experiments with a two-stagegasifier, with fuels with different moisture content,showed that the temperature in the partial oxidationzone decreases about 200°C when gasifying with steamcompared to gasification of dry fuels. These obser-vations are confirmed by mathematical modelling [6].- Lower soot production: The amount of soot particles arereduced when gasifying with steam [1]. This is partlydue to lower maximum temperatures and partly due tohigher concentration of OH radicals which reduce thesoot production.- Lower emissions: The gascomposition is different whensteam is added as a gasification agent. The H2 content isincreased and the CO content is lowered. The results ina faster combustion and lower emissions of CO andNOx.3. PRESENTATION OF THE UPSCALED TWO-STAGE GASIFICATION CONCEPTIn order to be able to build medium and large scalebiomass or waste gasification plants that are sturdy andhave a high electricity and heat production, the two-stagegasification process should be modified. The challengewas to develop a gasification concept in which the dryingand pyrolysis are directly heated instead of indirectlyheated.Dried fuel CharFuelSuperheated steamfor dryingSuperheatingsteamfor dryingSuperheatingsteam forpyrolysisPyrolysisAirGasificationEngine orgasturbineGascooling andcleaningAir forcombustion250°C150°C 700°C130°C475°C475°C1100°C 800°CC H PPartialoxidationof volatilesAshExhaust gasDryingFigure 3 Basic concept with approximate temperatures.The essential news in the upscaled two-stage gasificationPublished by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.3process shown in Figure 3, is that very hot steam,originating from the fuel, is used as the main heat carrierfor the further process steps. In fluid bed applications thehot steam can also function as the fluidisation medium.4. ADVANTAGES OF THE UPSCALED TWO-STAGE GASIFICATION PROCESSThe new gasification concept shown in Figure 3 can bedesigned in different ways.Common advantages for the gasification plants will bethat:1. Low tar content in produced gas.2. High energy efficiency (Cold gas efficiency about90%  even if the moisture content of the fuel is 60-70%)3. The new two-stage gasification process is acombination of well-known technologies.4. The gas cleaning will be relatively simple and sturdy.5. Stable process: Pyrolysis reaction  times of  1-3minutes will result in a compact pyrolysis reactor anda stabile gasification process even if the feeding ratevaries.6. Clean fluegas to be used for district heat incondensing mode without cleaning of condensate.7. No fire hazards in dryer.8. Low maximum temperatures in gasifier.9. Low soot production.10. Low emissions from combuster11. Fuels with moist content of 40-60% preferrable.12. Pyrolysis of biomass with steam and gasification ofbiomass with steam makes excellent activatedcarbon.5. MOVING BED DESIGNIn a medium size (5-20 MW thermal) two-stagegasification plant, the pyrolysis and the gasificationreactor can be of a moving bed type.Figure 4 Schematic example of a two-stage gasificationplant with pyrolysis and the gasification reactors ofmoving bed type.The design shown in Figure 4 is based on the well knownfixed bed two-stage gasifier. The pyrolysis reactor is inthis design direct heated by the hot steam instead ofindirect heated by hot fluegasses. Hereby the retentiontime of the biomass in the reactor can be reduced fromabout ½-1 hour to about 1-3 minutes. The size of thepyrolysis reactor can therefore be reduced.Model calculations of a CHP plant with gas engines assuggested in Figure 4 shows an electrical efficiencybetween 30-35 % and a total efficiency above 90%.6. FLUID BED DESIGNSThe pyrolysis and gasification reactors in the new two-stage gasification concept can be designed as fluid beds.The pyrolysis and the gasification could take place in twofluid bed reactors as suggested in [7] with a hightemperature partial oxidation zone in between as in Figure3.A more compact solution could be designed if thepyrolysis, the partial oxidation and the gasificationprocess is integrated in one large fluidised bed reactor (seeFigure 5).SandDried fuel PyrolysisPreheated Ai rPart ialoxidat ionGasif i -cat ionHotS teamL O W  T A RT W O - S T A G E  C F BCharProduct  gasAshFigure 5 Possible fluid bed design of a medium and largescale low tar two stage gasifier.In the gasifier design shown in Figure 5, the pyrolysiszone is a bobling bed in inert atmosphere, where steam isfluidisation media, and steam and recirkulating char andsand is heatcarrier. The air inlet is above the pyrolysiszone and the tars are partial oxidised. The gasificationzone is a circulation fluid bed (CFB), with partial oxidisedvolatiles and steam as fluidisation media and heatcarrier.The flows in the different zones and seperation of sandand biomass are demonstrated and visualised in a smalltransparent fluid bed.6.1 Low tar fluid bed and IC engines.The gasifier shown in Figure 5 can produce gas to one orseveral gas engines. This type of combined heat andpower plant could be built in the size range of 10-30 MWthermal input (see Figure 6)Published by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.4Supe rhea teds teamfor  dry ingEngine  Fuel Dry ing Dried fuelSupe rhea teds teamfor pyrolys isCoo l ing  andgascleaningPart ic lesAir D.H. Ai r  preheatSand Pyro-lysisP rehea tedAirPart ia lox idat ionGasi f i -ca t ionLOW TARTWO-STAGE CFBCharProduc t  gasA s hFigure 6  Schematic example of a medium size low tartwo-stage gasification plant with gas engine(s)Model calculations of a CHP plant with gas engines assuggested in Figure 6 shows an electrical efficiency of 30-35 % and a total efficiency above 90%, even at 60%moisture in the fuel.6.2 Low tar fluid bed and Combined CycleIn a large scale (30> MW thermal) two-stage gasificationplant, the gasification plant should be designed as a fluidbed e.g. as the design of the gasifier in Figure 5 and theenergy system should be a Biomass IntegratedGasification and Combined Cycle (BIG/CC) system.When the drying is done by superheated steam withexternal heat supply, different heat sources can be used. Itcan be fluegas but also medium pressure steam from thesteam cycle can be used if the fuel is very vet. Herebyfuels with moist contents above 70% can be gasified withhigh energy efficiencies in an atmospheric low tar gasifier.Model calculations of a BIG/CC plant as suggested inFigure 7  shows electrical efficiencies between 40-45 %and total efficiencies between 80-90%.The electrical efficiency remains high even if the watercontent is as high as 70%. It is therefore  possible to builta high efficient plant using sludge as a fuel using thisgasification concept.Sand Pyro-lysisPreheatedAirPartialoxidationGasif i-cationAirFilter andscrubberGas turbineSteamturbineParticles, wateralkal i  metalsAir preheatingfor gasif ierCondenserLOW TARTWO-STAGE CFBCompression of gasSuperheatingsteam for dryingSuper heaterEvaporatorClean exhaustgas for D.H.CharProductgas AshDrying Dried fuelEvaporatorAddit ional drying(If  water content >64%)PreheatfeedwaterFuelSuperheat ing steamfor pyrolysisFigure 7  Schematic example of a large scale low tar two-stage gasification plant with Combined Cycle.CONCLUSIONSA new concept for low tar and high efficient gasificationof moist and wet fuels combining  the advantages ofknown processes has been developed. Designs of mediumand large scale CHP-plants based on the new concept aresuggested.REFERENCES[1] J. D. Bentzen, U. Henriksen, C. Hindsgaul, P. Brandt.Optimised two-stage gasifier. 1st World BiomassConference and Technology Exhibition.[2] P. Brandt and U. Henriksen, Decomposition of tar inpyrolysis gas by partial oxidation and thermal crac-king. Part 2. Proceedings of Biomass for Energy andIndustry 10th European Conference and TechnologyExhibition, 8-11 June 1998, Würzburg, Germany.[3] R. M. Hummelshøj. Full-scale electricity productionusing pyrolysis gas in Denmark. Proceedings ofBiomass for Energy and Industry 10th European Con-ference and Technology Exhibition 8-11 June 1998,Würzburg, Germany.[4] F. Fock ,K. Thomsen. Optimering af koncepter formedstrømsforgasning. Master Thesis, DTU 2000.[5] Walker. P. L, Jr., et al., Gas reactions of carbon,Advances in Catalysis, D. D. Eley, P. W. Selwood andP. B. Weisz (Eds), XI, 133 (1959)[6] B. Gøbel. Dynamisk modellering af forgasning i fixedkoksbed. Phd Thesis DTU 1999.[7] S. Houmøller, M. W. Hansen and U. Henriksen, Two-stage fluid bed pyrolysis and gasification unit.Proceedings of Biomass for Energy and Industry 9thEuropean Conference and Tech-nology Exhibition 24-27 June 1996, Copenhagen, Denmark.

Medium and large-scale low tar and high efficient gasification

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Medium and large-scale low tar and high efficient gasification

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