Bentzen, Jens Dall

Elmegaard, Brian

Henriksen, Ulrik Birk

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Low tar and high efficient gasification conceptBentzen, Jens Dall; Elmegaard, Brian; Henriksen, Ulrik BirkPublished in:Proceedings of ECOS 2000Publication date:2000Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Bentzen, J. D., Elmegaard, B., & Henriksen, U. B. (2000). Low tar and high efficient gasification concept. InProceedings of ECOS 2000 Twente, Nederlands.Paper presented at the ECOS 2000 Conference July 5-7, 2000,University of Twente, Enschede, The Netherlands”International Conference on Efficiency, Cost, Optimisation,Simulation and Environmental Aspect of Energy and Process Systems”Low Tar and High Efficient Gasification ConceptJens Dall Bentzen1 and Reto M. Hummelshøj COWI - Consulting Engineers and Planners ASParallelvej 15DK-2800 LyngbyDenmarkTel: +45 45 97 13 38, Fax: +45 45 97 22 12 jdb@cowi.dk,  rmh@cowi.dkBrian Elmegaard and Ulrik HenriksenTechnical University of DenmarkDepartment of Energy EngineeringNils Koppels Alle, Building 403DK-2800 Kgs. LyngbyDenmarkTel: +45 45 25 41 72, Fax: +45 45 93 57 61uh@et.dtu.dk, be@et.dtu.dkAbstractA new concept for low tar and high efficient gasification of moist and wet fuelssuch as wood and sludge has been developed. Designs of medium and large scaleCHP-plants based on the new concept are suggested. COWI and the TechnicalUniversity of Denmark have a patent pending for the new concept.The new gasification concept combines and integrates the well known prosecesses:1. Two-Stage gasification, 2. drying with superheated steam, 3. pyrolysis withsuperheated steam  and 4. gasification with steam.The fuel is dried by superheated steam. The steam produced in the dryer issuperheated to 600-800°C and let to the pyrolysis zone. In the inert pyrolysis zone                                                     1  Corresponding authorthe hot steam will pyrolyse the fuel. The volatiles from the pyrolysis zone is led toa partial oxidation zone, where the oxidation agent is added. Hereby the volatilesare partially oxidised and the tar contents is dramatically reduced. The hot gasesfrom the partial oxidation zone and the char from the pyrolysis zone are led to thegasification zone where the char is gasified.IntroductionThere is a large market for medium and large scale biomass gasification combinedheat-and power plants (CHP). The existing atmospheric pressure gasificationtechnologies for medium and large scale gasification are complicated in order toreduce the tar-content in the produced gas and have low electrical efficiencies.The purpose of this work was to develop medium and large scale CHP biomassgasification concepts with high energy efficiency and low tar content in theproduced gas.Background of the new gasification conceptExperience with and insight in- fundamental processes of pyrolysis and gasification- industrial drying technics- the demand of medium and large scale low tar and high efficient gasifierswas the inspiration to develop a new concept for medium and large scale low tarand 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.1 Traditional two-stage gasificationThe two-stage gasification process developed on the Technical University ofDenmark throughout the 1980íes and 1990íes, has a very high cold gas efficiency:92-97% and the tar content in the produced gas is extreamly low: below 25mg/Nm3 [1]. The tar content in the gas after the gascleaning (particle filtration) isabout 5 tar/Nm3 [1].In the two-stage gasification process, the pyrolysis and the gasification process areseparated into two different zones. In between the pyrolysis and the gasificationzones, the volatiles from the pyrolysis are partially oxidised. Hereby, most of thetars are decomposed into gas [2].Figure 1: Traditional two-stage gasifier developed at DTU (the TechnicalUniversity of Denmark).In Figure 1 a traditional two-stage gasifier is shown. The pyrolysis reactor isindirect heated, and the gasification reactor is a fixed bed.Table 1 The tar content in the raw gas after the gasifier before filtering.Institution Method Date Tar contentmg/Nm3Danish TechnologicalInstituteGraviometricGraviometricGCMSGCMS23 nov 199924 nov 199923 nov 199924 nov 19992525525The Technical University ofDenmark (DTU)GCMSGCMSGCMSGCMS9 sep 199810 sep 199823 nov 199924 nov 199916336Royal Institute ofTechnology, Sweden/DTUSolid phaseadsorption (SPA)8 measurements23-24 nov. 19995-18Average 10Risø National Laboratory,DenmarkGCMS 9 sep 199810 sep 1998162,5Fuel Exhaust  gasMDrying and pyrolysisPartialoxidat ionGasif i -cat ionEngineCool ing andgascleaningPart iclesAir D.H. Air  preheat Bot tomashIt is seen in Table 3.1 that the tar content has been measured by several institutions.and that it is extremely low in the gas after the two-stage gasifier.To enable high energy efficiency, the thermal energy in the gasification gas and theexhaust gas is being used for drying, air preheating and for pyrolysis.For small gasification plants (up to about 4-6 MW thermal), the traditional two-stage gasification technology can be used. A 3 MW thermal indirect heated dryingand pyrolysis unit at Haslev strawfired power plant has been demonstrated [3].2. Drying with superheated steamDrying with superheated steam involves on or more external heatsource(s)(fluegas, hot oil, steam..) that are heat exchanged with water vapour evaporatedfrom the product. The superheated steam is led back to the dryer, and is then thedrying medium in the dryer (see Figure 2)Drying chamberHeatsourceEvaporateds teamWet fuel Dryed fuelFigure 2 Principle of drying with superheated steam.Drying with superheated steam is a well known technology, and it offers a numberof advantages:- Environment friendly drying: In many gasification processes, the fuel is drieddirectly by the flue gases, but hereby the exhaust gas becomes contaminated withorganic compounds (acids and phenols) and particles.When the fluegas is heat exchanged with steam the fluegas is not contaminated,and can therefore without problems be used for district heat.- Fire hazards: Since the oxygen content is extreamly low in the dryer, whendrying with superheated steam, the fire hazards are very low.- Better product after drying: The product to be dried faces a low temperaturegradient during the drying process, and therefore it does not break and crack asmuch as normally. The remaining moist in the fuel is also more evenly distributedwhen the product is dried in superheated steam.- Loss of product: Due to oxidation, certain losses of the product is often seen, i.e.when biomass is dried with fluegasses.- Drying rate: In many cases, drying with superheated steam is faster than normaldrying processes.- Several heat sources: Possible to use several heat sources in series.- Drying of sludges: Superheated steam dryers are developed to dry very wet fuelsuch as sludge.Superheated steam dryers have been developed to dry from 0,5 to 50 tons vapourpr. hour. This corresponds to 2,5 –250 MW thermal input if the fuel is woodchipswith a moisture content of 50%3. Pyrolysis with superheated steamPyrolysis with superheated steam is a well-known technology, used in kilns tomake activated charcoal.Since the produced steam from the dryer is used as the heat carrier for the pyrolysisprocess, the new two-stage gasification process is most applicable for fuels that arerelatively wet. Moist content of about 40-60% is preferable. This makes woodchipsideal for this process, which however also will be able to use other biomass, sludgeand selected solid waste´s.When dry fuels are used, other heat sources than the steam from the fuel must beadded to the pyrolysis reactor:• Additional water can be added to the dryer e.g. condensate from gas cooling ora condensing unit• Other heat sources, e.g. returned charcoal, bed material or hot gases from thegasification chamber or radiation heat from the partial combustion zone, can beadded to the pyrolysis process.Since the heat carrier to the pyrolysing process is a fluid, the reactor can be formedas a moving bed or a fluidised bed reactor, where the steam is the fluidisation fluidas well as the heat carrier.Experiments at DTU indicates pyrolysis reaction times of whole wood chipsbetween 1-3 minutes [4].4 Gasification with steam as gasification agentIt is well known, that gasification with steam as gasification agent result in higherreaction rates than gasification with CO2 [5]. If steam is the primary gasificationagent instead of CO2 the temperature in the  gasifier can therefore be lowered.There are several other advantages related to steam gasification.- Lower temperatures: Experiments with a two-stage gasifier, with fuels withdifferent moisture content, showed that the temperature in the partial oxidationzone decreases about 200°C when gasifying with steam compared to gasificationof dry fuels. These obser-vations are confirmed by mathematical modelling [6].- Lower soot production: The amount of soot particles are reduced when gasifyingwith steam [1]. This is partly due to lower maximum temperatures and partly dueto higher concentration of OH radicals which reduce the soot production.- Lower emissions: The gascomposition is different when steam is added as agasification agent. The H2 content is increased and the CO content is lowered.The results in a faster combustion and lower emissions of CO and NOx.Presentation of the upscaled two-stage gasification conceptIn order to be able to build medium and large scale biomass or waste gasificationplants that are sturdy and have a high electricity and heat production, the two-stagegasification process should be modified. The challenge was to develop agasification concept in which the drying and pyrolysis are directly heated instead ofindirectly heated.Dried fuel CharFuelSuperheated steamfor dryingSuperheatingsteamfor dryingSuperheatingsteam forpyrolysisPyrolysisAirGasificationEngine orgasturbineGascoolingand cleaningAir forcombustion250°C150°C 700°C130°C475°C475°C1100°C 800°CC H PPartialoxidation ofvolatilesAshExhaustgasDryingFigure 3 Basic concept with approximate temperatures.The essential news in the upscaled two-stage gasification process shown in Figure3, is that very hot steam, originating from the fuel, is used as the main heat carrierfor the further process steps. In fluid bed applications the hot steam can alsofunction as the fluidisation medium.Advantages of the upscaled two-stage gasification processThe new gasification concept shown in Figure 3 can be designed in different ways.Common advantages for the gasification plants will be that:1. Low tar content in produced gas.2. High energy efficiency (Cold gas efficiency about 90%  even if the moisturecontent of the fuel is 60-70%)3. The new two-stage gasification process is a combination of well-knowntechnologies.4. The gas cleaning will be relatively simple and sturdy.5. Stable process: Pyrolysis reaction  times of  1-3 minutes will result in acompact pyrolysis reactor and a stabile gasification process even if the feedingrate varies.6. Clean fluegas to be used for district heat in condensing mode without cleaningof 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 of biomass with steam makesexcellent activated carbon.Moving bed designIn a medium size (5-20 MW thermal) two-stage gasification plant, the pyrolysisand the gasification reactor can be of a moving bed type.Figure 4 Schematic example of a two-stage gasification plant with pyrolysisand the gasification reactors of moving bed type.The design shown in Figure 4 is based on the well known fixed bed two-stagegasifier. The pyrolysis reactor is in this design direct heated by the hot steaminstead of indirect heated by hot fluegasses. Hereby the retention time of thebiomass in the reactor can be reduced from about ½-1 hour to about 1-3 minutes.The size of the pyrolysis reactor can therefore be reduced.Model calculations of a CHP plant with gas engines as suggested in Figure 4 showsan electrical efficiency between 30-35 % and a total efficiency above 90%.Fluid bed designsThe pyrolysis and gasification reactors in the new two-stage gasification conceptcan be designed as fluid beds.The pyrolysis and the gasification could take place in two fluid bed reactors assuggested in [7] with a high temperature partial oxidation zone in between as inFigure 3.A more compact solution could be designed if the pyrolysis, the partial oxidationand the gasification process is integrated in one large fluidised bed reactor (seeFigure 5).Figure 5 Possible fluid bed design of a medium and large scale low tar two stagegasifier.In the gasifier design shown in Figure 5, the pyrolysis zone is a bobling bed in inertatmosphere, where steam is fluidisation media, and steam and recirkulating charSandDried fuel PyrolysisPreheated AirPartialoxidationGasif i-cationHotSteamL O W  T A RTWO-STAGE CFBCharProduct gasAshand sand is heatcarrier. The air inlet is above the pyrolysis zone and the tars arepartial oxidised. The gasification zone is a circulation fluid bed (CFB), with partialoxidised volatiles and steam as fluidisation media and heatcarrier.The flows in the different zones and seperation of sand and biomass aredemonstrated and visualised in a small transparent fluid bed.Low tar fluid bed and IC engines.The gasifier shown in Figure 5 can produce gas to one or several gas engines. Thistype of combined heat and power plant could be built in the size range of 10-30MW thermal input (see Figure 6)Figure 6  Schematic example of a medium size low tar  two-stage gasification plantwith gas engine(s)Model calculations of a CHP plant with gas engines as suggested in Figure 6 showsan electrical efficiency of 30-35 % and a total efficiency above 90%, even at 60%moisture in the fuel.Superheateds teamfor dryingEngine  Fuel Dry ing Dried fuelSuperheateds teamfor pyrolysisCool ing andgasc leaningPart ic lesAir D.H. Air  preheatSand Pyro-lysisPreheatedAirPart ialoxidat ionGasif i -cat ionLOW TARTWO-STAGE CFBCharProduct  gasAshLow tar fluid bed and Combined CycleIn a large scale (30> MW thermal) two-stage gasification plant, the gasificationplant should be designed as a fluid bed e.g. as the design of the gasifier in Figure 5and the energy system should be a Biomass Integrated Gasification and CombinedCycle (BIG/CC) system.Figure 7  Schematic example of a large scale low tar two-stage gasification plantwith Combined Cycle.Sand Pyro-lysisPreheatedAirPart ialoxidat ionGasif i-cat ionAirFilter andscrubberGas turbineSteamturbinePart icles, wateralkal i  metalsAir preheat ingfor gasif ierCondenserLOW TARTWO-STAGECFBCompression of  gasSuperheat ingsteam for dryingSuper heaterEvaporatorClean exhaustgas for  D.H.CharProduct  gasAshDrying Dried fuelEvaporatorAddit ional drying(I f  water content >64%)EconomiserFuelSuperheated steamfor pyrolysisWhen the drying is done by superheated steam with external heat supply, differentheat sources can be used. It can be fluegas but also medium pressure steam fromthe steam cycle can be used if the fuel is very vet. Hereby fuels with moist contentsabove 70% can be gasified with high energy efficiencies in an atmospheric low targasifier.Model calculations of a BIG/CC plant as suggested in Figure 7  shows electricalefficiencies between 40-45 % and total efficiencies between 80-90%.Figure 8 preliminary calculations of electrical efficiency versus water content inthe fuel calculated on the plant shown in figure 7. In these calculations the inlettemperature and pressure of the gasturbine is 1150 °C and 20 bars.In figure 8 it is seen, that the electrical efficiency remains high even if the watercontent is as high as 70%. It is therefore  possible to built a high efficient plantusing sludge as a fuel using this gasification concept.ConclusionsA new concept for low tar and high efficient gasification of moist and wet fuelscombining  the advantages of known processes has been developed. Designs ofmedium and large scale CHP-plants based on the new concept are suggested.Electrical efficiency at different water contents43%44%45%46%47%45% 50% 55% 60% 65% 70% 75%Electrical efficiency (LHV) Water content in fuel (wet basis)Medium pressure steam from steam turbine is additional heatsoarce for drying.Fluegas is heatsoarce for drying.References [1] J. D. Bentzen, U. Henriksen, C. Hindsgaul, P. Brandt. Optimised two-stagegasifier. 1st World Biomass Conference and Technology Exhibition, 5-9 June,Sevilla, Spain.[2] P. Brandt and U. Henriksen, Decomposition of tar in pyrolysis gas by partialoxidation and thermal crac-king. Part 2. Proceedings of Biomass for Energyand Industry 10th European Conference and Technology Exhibition, 8-11June 1998, Würzburg, Germany.[3] R. M. Hummelshøj. Full-scale electricity production using pyrolysis gas inDenmark. Proceedings of Biomass for Energy and Industry 10th EuropeanCon-ference and Technology Exhibition 8-11 June 1998, Würzburg,Germany.[4] F. Fock ,K. Thomsen. Optimering af koncepter for medstrømsforgasning.Master Thesis, DTU 2000.[5] Walker. P. L, Jr., et al., Gas reactions of carbon, Advances in Catalysis, D. D.Eley, P. W. Selwood and P. B. Weisz (Eds), XI, 133 (1959)[6] B. Gøbel. Dynamisk modellering af forgasning i fixed koksbed. Phd ThesisDTU 1999.[7] S. Houmøller, M. W. Hansen and U. Henriksen, Two-stage fluid bed pyrolysisand gasification unit. Proceedings of Biomass for Energy and Industry 9thEuropean Conference and Technology Exhibition 24-27 June 1996,Copenhagen, Denmark.

Low tar and high efficient gasification concept

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Low tar and high efficient gasification concept

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