Parametric Study Of Fixed Bed Biomass Gasifier: A Review
International Journal of Thermal Technologies, Vol.2, No.1 (March 2012)ISSN 2277 - 4114Review ArticleParametric Study of Fixed Bed Biomass Gasifier: A reviewHina Beohara, Bhupendra Guptaa*, V. K. Sethib, Mukesh PandeybaJabalpur Engineering College, Jabalpur, M.P.bUIT, RGPV, Bhopal, M.P.Accepted 22 March 2012, Available online 28 March 2012AbstractIn this paper, various parametric aspects of biomass gasification have been discussed. The most widely usedconfigurations of biomass gasifiers and the effect of various operating parameters on the quality of producer gas andperformance of gasifier are considered. The performance of the biomass gasifier system is evaluated in terms of zonetemperature, calorific value, equivalence ratio, producer gas composition, gas production rate, and cold gas efficiency.Keywords: Fixed bed gasifier, Types, Thermo-Chemical process, Parameters, Equivalence ratio, Cold gas efficiency.1. Introduction1Biomass has been one of the main energy sources for themankind ever since the dawn of civilization, although itsimportance dwindled after the expansion in use of oil andcoal in the late 19th century. There has been a resurgenceof interest in the recent years in biomass energy in manycountries considering the benefits it offers. It is renewable,widely available, and carbon-neutral and has the potentialto provide significant productive employment in the ruralareas. Biomass is also capable of providing firm energy.Estimates have indicated that 15%–50% of the world’sprimary energy use could come from biomass by the year2050. Currently, about 11% of the world’s primary energyis estimated to be met with biomass. For India, biomasshas always been an important energy source. Although theenergy scenario in India today indicates a growingdependence on the conventional forms of energy, about32% of the total primary energy use in the country is stillderived from biomass and more than 70% of the country’spopulation depends upon it for its energy needs.2. Fixed Bed GasifiersIn the fixed bed gasifier, the fuel is fed to the top of thegasifier. Meanwhile, at the bottom, oxygen and steam areinstituted and the slag is withdrawn. The fuel is gasified ina bed layer. The fuel goes through four different zones;drying, pyrolysis, oxidation and reduction. Drying occurswhen the hot producer gas contacts the feed at the top ofthe gasifier. Next the fuel devolatilizes, forming tars andoils. These compounds exit with the raw producer gas, andare captured in downstream cleanup processes andrecycled to the gasifier. The devolatilized fuel then entersthe higher temperature gasification zone where it reactswith steam and carbon dioxide. Near the bottom of thegasifier the resulting char and ash react with oxygencreating temperatures high enough to melt the ash andform slag.2.1 Types of fixed bed gasifierThe choice of one type of gasifier over other is dictated byfuel, its final available form, size, moisture content andash content. Table 1 lists the operations advantages anddisadvantages for fixed bed gasifiers.2.2 Different gasifying agents used in fixed bedDifferent gasifying agents used in fixed bed are shown inTable 22.3 The process of gasificationGasification is the conversion process of biomass toproducer by heating in an oxidant or gasifying agents suchas oxygen, air or steam. Because, the oxidant or gasifyingagents and reaction conditions are always different, so theprocess of biomass gasification is very complex. On thewhole, this process includes four stages: drying, pyrolysis,oxidation, reduction.A. DryingIn this stage, the moisture content of biomass is typicallyreduced to be 5-35%. Drying occurs at about 100-200 Cwith reduction in moisture content of biomass to 5%.* Corresponding author’s email: bhupendra243@yahoo.com134
Hina Beohar et alInternational Journal of Thermal Technologies, Vol.2, No.1 (March 2012)B. PyrolysisPyrolysis is breakdown material by heat. It is the first stepin the combustion or gasification of biomass. Whenbiomass heated in the absence of air to about 3500C, Itfoam charcoal, gases and tar vapors. The tar vapors aregases at the temperature of pyrolysis but condense to foama smoke composed of fine tar droplets as they cool.Table 1 Comparison of Fixed Bed Gasification omass is introduced fromthetopandmovesdownward. Oxidizer (air) isintroduced at the top andflows downward. Producergas is extracted at the bottomat grate level. Tars and particulate inthe producer gas arelowerBiomass is introduced fromthe top and moves downward.Oxidizer is introduced at thebottom and flows upward.Some drying occurs. Producergas is extracted at the top.Biomass is introduced from thetop and moves downward.Oxidizer is introduced at thebottom and flows across the bed.Producer gas is extractedopposite the air nozzle at thegrate. Disadvantages Feed size limitsScale limitationsLow heating value gasMoisture-sensitiveCross-flow Can handle highermoisture biomass.Higher temperatures candestroy some toxins andslag minerals and metal.Higher tar content addsto heating value.Feed size limitsHigh tar yieldsScale limitationsLow heating value gas Simplest of designs.Stronger circulation in thehot zone.Lower temperatures allowthe use of less expensiveconstruction materials.More complicated tooperate.Reported issues withslagging.High levels of carbon (33%)in the ash.Fig.1 types of fixed bed gasifiers135
Hina Beohar et alBiomass heatInternational Journal of Thermal Technologies, Vol.2, No.1 (March 2012)solid, liquid, gases products (O, CO)Table 2 Advantages of different gasifying agentsGasifying AgentsAdvantages1. Partial combustion for heatsupply of gasification2. Moderate char and tarcontent1. High heating value producergas (10–15 MJ N/m3)2. H2-rich producer gas (e.g.,450% by volume)1. High heating value producergas2. High H2 and CO and lowCO2 in producer gasAirSteamCarbon dioxideC. OxidationThis is reaction between solid carbonized biomass andoxygen in the air, resulting in formation of C . Hydrogenpresent in the biomass is also oxidized to generate water.Large amount of heat is released with the oxidation ofcarbon and hydrogen.C CD. ReductionIn absence of oxygen, several reduction reactions occur inthe temperature range of 800-1000ºC. These reactions aremostly endothermic. The major in this category are asfollows:C OC CO2 C CO 2COCO 2.4 Types of fuel used in gasifierWood: Wood fuel has several environmental advantagesover fossil fuel. The main advantage is that wood is arenewable resource, offering a sustainable, dependablesupply. Other advantages include the fact that the amountof carbon dioxide (CO2) emitted during the burningprocess is typically 90% less than when burning fossilfuel.Sawdust: sawdust leads to the problems of excessive tarproduction, inadmissible pressure drop and lack of bunkerflow. Small sawdust particles can be used in fluidized gasprocures to produce gas. If this gas is used to be in internalcombustion engines, fairly good clean-up system isessential.Peat: Peat is the first stage of coal formation. Freshlymined peat contains 90 % moisture and 10 % of solid. Itcannot be utilized unless air dried to reduce moisturecontent to 30 % or less. As peat contains very high level ofmoisture and ash, it creates problems in the gasificationprocess.Agricultural residues: Agricultural residues are basicallybiomass materials that are by product of agriculture. Itincludes cotton stalks, wheat and rice straw, coconutshells, maize and rice husks etc. Coconut shells and maizecobs have been successfully tested for fixed bed gasifiers.Most cereal straws contain ash content above 10% andpresent slagging problem in downdraft gasifier. Rice huskwith ash contents above 20% is difficult to gasify.3. Process ParametersThe performance of gasifier is directly affected thefollowing parametersZone TemperaturesOTemperature is considered as the main parameter forestimating on biomass gasifier performance. Increase intemperature reduces the tar content as well as decreaseschar inside the gasifier. Gas yield increases due to highertar cracking.Calorific value (CV)The calorific value (CV) of a material is an expression ofthe energy content, or heat value, released when burnt inair. The CV is usually measured in terms of the energycontent per unit mass or volume (MJ/m3).Equivalence ratioFig 2 Schematic view of different process zone in fixedbed biomass gasifierEquivalence ratio (ER) is the most influential parameter inany gasification process and often has significant impacton producer gas composition. Equivalence ratio wascalculated from the amount of oxygen fed into the gasifierdivided by the amount of oxygen required for completefuel combustion. For the effective gasification, ER shouldbe range 0.2-0.4.136
Hina Beohar et alInternational Journal of Thermal Technologies, Vol.2, No.1 (March 2012)Table 3 Composition of Producer Gas from various fuelsFuelGasificationmethodVolume 210-1555-6055-50Wheat straw pelletsDowndraft14-1717-19-11-14-4.50Coconut husksDowndraft16-2017-19.5-10-15-5.80Coconut shellsDowndraft19-2410-15-11-15-7.20Corn cobsRice hulls 6.293.25Cotton stalks quivalence ratio (HHV) of the biomass material .cold gas efficiencydepends upon the calorific value and the amount ofproducer gas released at constant HHV of biomass.Producer gas compositionCold gas efficiency Producer gas composition means quantities volume ofdifferent gases in producer gas.Typical composition ofproducer gas is as follows.(Table 4 Typical composition of producer gas composition4. Worldwide Installations of fixed bed gasifiersGasCompositionCarbon rbon dioxide9%–12%Nitrogen45%–55%Gas production rateGas production rate means the producer gas productionrate per unit weight of biomass (Nm3/kg). When increasein the equivalence ratio, producer gas production ratecontinuously increases. Higher equivalence ratio signifieshigher air flow rate for a specific biomass consumptionrate.Cold gas efficiencyCold gas efficiency is defined as the ratio of energy of theproducer gas per kg of biomass to the higher heating value)()Fixed bed updraft gasifier has been installed at a CHPpower plant in Kokemaki. Biomass residues and energycrops are used as fuels. The feedstock is dried to about20% moisture by using low 15 temperature waste heatfrom the plant and fed at the top of the gasifier. Theproduced gas is cleaned by a tar reformer, cooled andscrubbed in a wet scrubber, boost and injected in the threeturbocharged 0.6 MWe gas engines to produce 1.8 MWepower and 4.3 MWth for district heating (Nilsson, 2008).The gasifier was developed and constructed by theEnergy Department of Finish Ministry of Trade andIndustry in co-operation with VTT. Eight BIONEERgasifiers were commercialized in 1985-1986, five inFinland and three in Sweden. Four plants are operatedwith wood or wood and peat mixture, while the rest areoperated with peat only. BIONEER gasifier is an updraftfixed bed gasifier with the outputs of the range of 4-5MWth.A 500 kW (5 x 100 kW) biomass gasifier has beeninstalled and commissioned in Gosaba Island, WestBengal, India in July 1997 for electrification of fivevillages comprising more than 10, 000 people.The biomass CHP facility in Skive, Denmark is an airblown bubbling fluidized bed gasifier developed byCarbona Company and in operation since late 2005. Thegasifier converts 110 tons/day (20 MW) wood fuel into 6MW electricity and 12 MW district heat. The overallefficiency is 87% and electrical efficiency of 28%.137
Hina Beohar et alA biomass gasifier for bark, wood chips, sawdust, etc.has been installed at the 137 MWe pulverized coal firedpower station of Verbund-Austrian Hydro Power AG inZeltweg, Austria.A 1.2-MW gasification plant has established in a ricemill located in Changxing, Zhejiang Province, China. Thesystem has been able to run safely and continuously for 4years, from October 2004 to June 2008. The gasificationsystem consists of an air-blown fluidized bed gasifier, acombined gas cleaner (including an inertial separator, acyclone separator, two Venturi tubes and two waterscrubbers), and a power generation subsystem (containingfour gas engines of 200 kW, and one gas engine rated at400 kW), in addition to a wastewater treatment system.Many countries have developed commercial biomassgasification technologies. Some of the commerciallyavailable fixed bed gasifiers are listed in Table 55. Literature reviewD.F. Fletcher et al (2000) provided of a detailedComputational Fluid Dynamics (CFD) model developed tosimulate the flow and reaction in an entrained flowbiomass gasifier. The model is based on the CFX packageand represents a powerful tool which can be used ingasifier design and analysis. The model provides detailedinformation on the gas composition and Temperature atthe outlet and allows different operating scenarios to beexamined in an efficient manner. The initial calculationssuggest that simulations to examine the effect of gasifierheight and the steam flux in the upper inlets can bebeneficial in process optimization. The simulation ofsawdust gasification in one case gave an exit compositionon a dry basis of 10% CO, 12% CO2, 20% H2 and 1.2%CH4, compared with 16% CO, 14% CO2, 10% H2, 1% CH4measured in the experiments, the hydrogen generation wastoo high. The model with further validation againstdetailed experimental data, will aid with the designprocess of such gasifiers.M. Miltner et al (2006) presented the joint applicationof process simulation and computational fluid dynamics(CFD) is a helpful tool for the design and optimization ofcomplex and innovative concepts in chemical engineeringpractice. The major goals of this paper to comprise themaximization of the thermal efficiency and the reductionof gaseous and particular matter emissions .The modelingapproaches are presented that especially focus on thetreatment of the heterogeneous combustion and predictionof gaseous emissions such as carbon monoxide andnitrogen oxide. The agreement between measured andcalculated temperatures and volume flows is rather good,especially when the uncertainty of flow measurements istaken into account.The maximum deviation between the measured andcalculated values is 0.1% for gas volume flows, 6.4% forthe flue gas temperature, and 3.6% for the combustionchamber wall temperatures.International Journal of Thermal Technologies, Vol.2, No.1 (March 2012)SU Yi et al (2009) Analyses the technical features andinnovative structure of the two-stage gasifier andexperimentally detected the effect of different parameterson gasification performance. In this work, mainparameters, such as ER’s effect on gasifier temperature,content of product gas, low heat value, gas yield rate,carbon conversion, gasification efficiency, wereinvestigated. The effect of equivalence ratio (ER) ongasification performance is detected under a certaincondition: feeding rate is around 100 kg/h, char bed heightis kept about 100 cm high. Results show that: within theexperimental condition, when ER is between 0.3 to 0.35,the heat value of product gas can reach as high as 7247.7kJ/m3, gas yield rate is 1.84 m3/kg, carbon Conversion rateis 91.3%, the overall gasification efficiency is 84.6%.P.N. Sheth, B.V. Babu (2009) Presented a downdraftbiomass gasifier is used to carry out the gasificationexperiments with the waste generated while makingfurniture in the carpentry section of the institute’sworkshop. Generally known as sesame wood or rose woodis mainly used in the furniture and wastage of the same isused as a biomass material in the present gasificationstudies. The effects of air flow rate and moisture contenton biomass consumption rate and quality of the producergas generated are studied by performing experiments. Theperformance of the biomass gasifier system is evaluated interms of equivalence ratio, producer gas composition,calorific value of the producer gas, gas production rate andzone temperatures. Based on the results of this study, withan increase in the moisture content, biomass consumptionrate decreases and the air flow rate increase the biomassconsumption rate also increases. Molar fraction of N 2 andCO2 decrease with an increase in equivalence ratio (λ) tillλ 0.205. The fraction of CO and H2 shows increasing anddecreasing trend exactly oppo site to that of N2 and CO2.The calorific value, pyrolysis zone temperature and theoxidation zone temperature are maximum at λ 0.205.With an increase in λ, the production rate ofproducer gas continuously increases. The value of cold gasefficiency is 0.25 for λ 0.17. It becomes almost doublewith a small increase of 0.035 in the value of λ.Y. Ueki et al (2010) focused on woody biomassgasification fundamentals, using a bench-scale packed-bedreactor. In this experiment, pellets of black pine weregasified, using air as the oxidizing agent. Gasification testswere carried out under both updraft and downdraftconditions. Temperature distributions and compositions ofproducer gas inside the gasifier were continuouslymonitored during gasification experiments at several portson the wall of the reactor. The results are summarized asthe temperature fluctuations in the reactor under downdraftconditions are larger than those under updraft conditions,causing unstable gas composition at the exit of thegasifier. An updraft setup yields a lower heating value ofproducer gas greater than a downdraft setup as a timeaverage. Lower heating values of the producer gas underupdraft and downdraft conditions were 4.8 and 3.8 MJ/m3N, respectively. Tar compounds in producer gas under138
Hina Beohar et alInternational Journal of Thermal Technologies, Vol.2, No.1 (March 2012)Table 5 A Few commercially available fixed bed gasifiersCountryUSADenmarkType of fixedbed FuelSizeOrganization / projectHogged wood, stumpsWoodchips, corn cobsHazardous, leather wasteStraw, woodchips, barkWood residues1 MW40 kW2-15 MW0.5 MW1-15 MWCLEWStwalley Engg.DTIVOLUND R&D CentreHollesen Engg.NewZealandDowndraftWood blocks, chips,coppice willow chips30 kWFluidyneFranceDowndraft100-600 kWMartezoUKDowndraftWood, agriculturalresiduesWood chips, hazel nut shell,MSWIndustrial agriculturalwastes30 kW300 kWNewcastle Universityof technologyShawton EngineeringWoody and agriculturalbiomassWood, wood- waste50-2500kW0.25-4 MWDASAGHTV EnergyAssociated EngineeringWorksAnkur ScientificEnergy TechnologiesSRC GazelDowndraftSwitzer- landStratifiedDowndraftIndiaDowndraftDowndraftWood chips, rice hullsWood stalks, cobs, shells,rice husk100 kg/hBelgiumSmall scaleWood chips160 kWSouthAfricaDowndraftbriquettesWood blocks, chips,30-500 kWSystBM Johansson gasproducersNetherlandDowndraftRice- husk150 kWKARA Energy SystemsChinaDowndraftDowndraftSawdustCrop residues200 kW300 kWHuairou woodEquipmentHuantai IntegrateGassupplySystemdown-draft conditions are smaller than those under updraftconditions, because they can be decomposed duringpassage through the char grains section at hightemperature.S. Murgia et al (2011) presented a comprehensive CFDmodel of an air-blown coal fired gasifier has been settedup and tested using the CFD code MFiX. Thecomprehensive 2D CFD model is able to simulate andevaluate the dynamics of the gasification process in anupdraft air blown coal gasifiers. it is an important means inunderstanding the sequence of the different steps, fromcombustion to devolatilization, needed for the conversionof coal to producer gas and to characterize thetemperature, species concentrations, velocities andreaction rates for the gas and solid phases in function oftime and space. Gas temperature and gas composition onthe gasifier are simulate six different time step. And Resultshow In fir
areas. Biomass is also capable of providing firm energy. Estimates have indicated that 15%–50% of the world’s primary energy use could come from biomass by the year 2050. Currently, about 11% of the world’s primary energy is estimated to be met with biomass. For India, biomass
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