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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
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Flaming pyrolysis model of the fixed bed cross draft long-stick wood gasifier
A. Saravanakumar a,⁎, T.M. Haridasan b, Thomas B. Reed c
a NSP Green Energy Technologies Private Limited, Renewable Energy Research and Development, Plot No. 5A, NH 3,Maraimalai Nagar Industrial Estate, Maraimalai Nagar, Chennai - 603209, Tamil Nadu, Indiab Department of Bio-Energy, School of Energy Sciences, Madurai Kamaraj University, Madurai - 625 021, Tamil Nadu, Indiac The Biomass Energy Foundation, 46 Broad St., Barre, MA 01005, USA
a b s t r a c ta r t i c l e i n f o
Article history:Received 19 March 2009Received in revised form 6 January 2010Accepted 24 January 2010
The future industrial development of biomass energy depends on the application of renewable energytechnology in an efficient manner. Of all the competing technologies under biomass, gasifiers are consideredto be one of most viable applications. The use of biomass fuel, especially biomass wastes, for distributedpower production can be economically viable in many parts of the world through gasification of biomass.Since biomass, is a clean and renewable fuel, gasification gives the opportunity to convert biomass into cleanfuel gas or synthesis gas for industrial uses. The preparation of feedstock for a gasifier requires time, energyand labour and this has been a setback for gasifier technology development. The present work is focused ongasification of long-stick wood as a feed material for gasifiers. This application makes reduction not only inthe cost but also on the power consumption of feed material preparation. A 50 m3/h capacity gasifier wasfabricated in the cross draft mode. The cross draft mode makes it possible to produce low tar content inproducer gas. This cross draft mode operates with 180 W of blower supply for air to produce 10 kW ofthermal output. The initial bed heights of the long-stick wood and charcoal are 58 cm and 48 cm respectively.Results were obtained for various flow conditions with air flow rates ranging from 20 to 30 m3/h. For modelling,the flaming pyrolysis time for long-stickwood in the gasifier is calculated to be 1.6 min. The length of the flamingpyrolysis zone and char gasification zone is found to be 34 cm and 30 cm respectively. The rate of feed wasbetween 9 and 10 kg/h. Continuous operation for 5 h was used for three runs to study the performance. In thisstudywemeasured the temperature andpressure in the different zones as a functionof airflow.Wemeasured thegas flow and efficiency of the gasifier in order to determine its commercial potential for process and powerindustries.
The handling and preparation of fuels for gasifiers in the form ofwood chips is a stumbling block in the utilization of gasificationtechnology, both in industries and in the domestic sector. This situationis more serious if we have to put gasifier systems in remote rural areaswhere they are really needed. These are the regions mostly sufferingfrom power shortages or absence of power. Feed stock preparation isthus an important aspect of designing a gasifier system. Saravanakumar[1] reports that the cutting of wood involves sizable amounts (10% to15%) of wood wasted as dust. Where cost of wood is high, this wastagewill be a major cost addition. For these reasons we have designed, builtand tested gasifiers that can use long-sticks.
Wood was the primary energy source for cooking, heating andmetals production from the openingof recordedhistoryuntil the time ofthe industrial revolution. At the end of the middle ages, wood was used
across India in smelters producing iron, copper, lead, and othermaterials. Industrialization continued and demand growth for theseproducts. The demand for wood exceeded availability, causing theshutdown of many smelters, shortages of wood for cooking anddeforestation. Woody biomass is considered the renewable energysource with the highest potential to contribute to the energy needs ofmodern society for both the developed and developing economiesworld-wide (European Commission [2] and International EnergyAgency [3]). Woodchips is one of the sources of biomass renewableenergy that can produce gas from gasification. It has the greatestpotential of any renewable energy option for base-load electric powerproduction for electricity generator and heating. High penetration ofbiomass technologies shall require abundant supply of biomassresources. Wood shavings in fine form from wood process industriesand agricultural wastes have also been used to augment the supply ofheat both in domestic and industrial situations. Hislop and Hall [4]reported that the third world where the availability of industrialelectricity is seen as a key factor in assisting development in rural areas,gasification can provide a local source of electricity using local woodybiomass.
The combustion of pyrolysis productswith air in sufficient supply (asin a match) is termed “flaming combustion” [5]. We apply the term“flaming pyrolysis” to the combustion of the same volatiles in aninadequate supply of air so that the products are largely CO and H2
rather than CO2 andH2O. A tar burning gasifier consumes the tars by theprocess called flaming pyrolysis. The time required for pyrolysis tocharcoal plus volatiles depends on the heat transfer rate and the amountof charcoal produced. Bergstorm et al. [6] studied the drying, and initialpyrolysis, and char gasification at different experimental conditions byusing a laboratory-scale furnace. Drying takes place from the moment afuel particle enters the furnaceuntil theflamingpyrolysis starts. Flamingpyrolysis is the simultaneous devolatilization and conversion tocharcoal. Char yield is the remaining material after the pyrolysis. Theflaming pyrolysis gasification reaction in solids is analogous to thecombustion of a gas and proceeds at a finite rate. In steady state gasifiersthe fuel is consumed at the same rate as the reaction progresses, so fuelflow velocity is identical to reaction velocity.
The main purpose of the experiments is to study the flamingpyrolysis andefficiency of the experimental gasifier during a gasificationprocess. The quality and quantity of long-stick wood being used and theflaming pyrolysis time will be measured.
2. Experimental
2.1. Feedstock technology reliability
Clean biomass feedstock are becoming scarce and there is hardly anyreliable supply. In our country, all industrialwoodwaste andotherwoodresidues are consumed completely and there is no other clean biomassavailable to increase the contribution of bio energy. Thus, the industryhas been obliged to look into relative difficult fuels and fuels with littlepractical industrial experience in order to create new market opportu-nities.Waste recovered fuels present the advantage that they often havea negative cost associated with their disposal, which can significantlydecrease the operating costs of a plant. In addition, since the last decadethere has been a significant interest in energy crops and especially shortrotation forestry (SRF) as ameans to increase the production of biomassfuels while simultaneously creating new jobs for the farmingcommunity.
2.2. Design of cross draft long-stick wood gasifier
A cross draft gasifier was designed and constructed using mild steelsheetwith4.3 mmthicknesswithavolume is0.714 m3. The totalheightofthegasifier is 1.34 m.Hopper is theprimary storage area for the fuelwood.Feed stock size for long-stick wood of length 68 cm and thickness of 6 cmis placed into the top of the hopper through the inlet. The hopper isdesigned in such away that it is able to holdwood,which canproduce thegas continuously for 5 to 6 h. Initially, charcoal pieces arefirst loadedup toair nozzle height. Then long-stickwood is packed up to the full capacity ofthe hopper. Hearth is made up of castable cement with a mild steelexterior cladding to encounter higher temperature up to 1600 °C. Cudaand Ziak [7] fabricated the cross-draft gasifier, which has an operatingtemperature of 1500 °C though they used it onlywith charcoal feed stock.The air from the blower for partial combustion enters the hearth throughair nozzle. The air nozzle tube ismadeupof SSmaterial 1.5-inchdiameter.There are twoair nozzlesfixedwithadjacent sideof thehearth. It is shownin Fig. 1. The positioning of the grate below the hearth zone in the gasifieris to help the reduction reactions. The grate directly supports thecombustion zone and must be capable of letting ash fall through withoutan excessive loss of fuel. In addition, the grate is used to control reactorpressure drop and hence to maintain the gas production rate. Tenkilogramsof charcoal is placedon agrate and is ignitedby twofire ports. Itappears that the blower is run in suctionmode to draw air in through thefire ports during the charcoal ignition. 60 kg of long-stick wood is placedon top of the charcoal. The fire ports are closed and the supplied flame
through the twofire ports stops. The fan is run in blowermode and the airenters frombelowthebed. Theprimaryair forpyrolytic gasificationentersat the bottom andmoves up, forming gas in the flaming pyrolysis zone asshown in Fig. 2. This airflow is measured by pressure drop across arestriction. Gases produced by the wood and charcoal exit through a gasport. This gas is combusted and the gas flow rate (via pressure difference)and the producer gas flame temperature are recorded. Drying, pyrolysis,and combustion of the packed bed continue until theflame at the gas portgoes out. We have examined the quantity and the nature of the ashcollected after the different performance trials. The quantity of ash 3%collected with the cross draft mode is smaller compared with that of thebottom lit mode at 5%. The quality of the gas also is better due to thecracking of tar in the cross draft mode.
2.3. Performance of cross draft long-stick wood gasifier
The cross draft gasifier is generally considered suitable only forcharcoal. BrydenandRagland [8]have studied thewhole treebyutilizingadeep, fixed bed combustor/gasifier. Wood logs 20 cm in diameter aresmouldered in a 3.7 m deep fuel bed. In this work we have operated thecross draft mode attaining high energy release rate per unit area due tohigh inlet air velocity andactivated reaction in combustion zone. Reed andLarson [9]havedevelopeda top litupdraft gasifierusing the fuelwoodsize1–3 cm softwood chips, 1–2×10 cm hardwood sticks and 5 mm. Theyused the long-stick size is 2×1×½cm. It is placed vertically. Saravana-kumar et al. [10,11] usedanupdraft gasifierwithpackedbed for long-stickwood of length 68 cm and thickness of 6 cm. We have studied here thecross draft gasifierwith the long-stick length 68 cm and thickness of 6 cmand it is placed on horizontally. Kaupp [12] explained that a down draftgasifier could be operated in a cross draft scheme during the start up inorder to minimise the start up time.
In this work we have operated the gasifier in cross draft modeattaining high energy release rate per unit area due to high inlet airvelocity. This present cross draft gasifier systemwas run ten times, eachfor a period of 5 h and 15min in the cross draft configuration. Amongthese runs, three runs have been selected and discussed. For the feedAcacia Species was utilized and the time needed for the gas to supportcombustion from the initial flaring was 10 min. The total woodconsumption in the run was 45 kg. The performance details are givenin the Table 1. The air/fuel ratio increases from 1.3 to 1.7 as more andmore of the charcoal is gasified and the process approaches completecombustion. The results of operation of the cross draft and conventionalupdraft gasifier runs 1, 2 and 3 are given in Table 2. The cross draftgasifier gas is found to have a calorific value of the producer gas of4300 kJ/Nm3. It had an average efficiency of 79% of the energy in the fuelcollected in the gas, as against 67% for the conventional updraft gasifier.The cross draft gasifier produced 1 to 0.3wt.% tars as indicated in Table 2as compared to 10–30% tars produced in the conventional updraftgasifier used in the present study,. Many gasifier designs produce somuch tar that the gas cleanup equipment cost is several times the
Fig. 1. Pilot model for cross draft long-stick wood gasifier.
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gasifier cost. Fluidized beds typically produce 1–5% tars while updraftbiomass gasifiers produce 10–30% tars. From the overall point ofperformance of the gasifier, the cross draftmode is found to have amoresatisfactory performance. The long-stick wood gasifier was tested forairflow rates of 25,35,38,45, 50 and 53m3/h. Temperature profile indifferent zones as a function of airflow rate is shown in Fig. 3. Thethermocouplemeasurementsweremade at 0.25 m, 0.66 m, 0.78 m, and0.92 m above the bottom of the gasifier. These locations have beendescribed as combustion, gasification, pyrolysis, and drying locations,respectively.
2.4. Description of instruments
2.4.1. Pressure measurementThe pressure across the gasifier was measured using a water gauge
manometer. The pressure ismeasured both at the inlet and outlet of thepipeline. The difference between the atmosphere and either the inlet orthe outlet of the gasifier is the difference in water level in manometer.
2.4.2. Gas flow measurementThe rate of flow of the gas from the gasifier was measured by using
obstruction type of orifice meter.
2.4.3. Temperature measurementThe temperature of the gasifier at different zones, gas outlet and also
that of the flame produced by the gas from burner were measure usingK-type thermocouples accuracy of 1 °C and high precision digitalmeter.The measurement of the temperature, helps in determines the gasquality and heating capacity of the gas generated.
2.4.4. Gas calorific value measurement (Junker's gas calorimeter IRI 023)The gas calorimeter works on the Junker's principle of burning of a
known volume of produced gas and imparting the heat with maximumefficiency to steadilyflowingwater andfindingout the rise in temperatureof a measured volume of water. The given formula is then used todetermine the calorific value of the gas
Calorificvalueof gas =Volumeof water × Rise intemperature
Volumeof gas:
2.4.5. Orsat gas analysisThe gas analyses for CO2, O2 and CO are measured using Orsat gas
apparatus.
2.5. Flaming pyrolysis time — performance of cross draft long-stick woodgasifier
Awide range ofmathematicalmodels have beenpresented describingthe pyrolysis process. Bryden et al. [13] developed a kinetic model ofbiomass devolatilization taking into account biomass drying, moisturerecondensation and pyrolysis. The model is based on three parallelprimary equations and two secondary equations describing the crackingof tars into char and gases. The relation between biomass particle size andthedevolatilizationprocess has been studiedbyHagge andBryden [14]. Inmodelling the flaming pyrolysis zone of the gasifier it is essential to knowthe amount of time and energy that is required for pyrolysis. The heattransfer to each long-stickwood particle in the flaming pyrolysis bed is byconduction down from the charcoal layer to the combustion interface tothe surface. The Convection from the gases streaming outside the long-stick wood particle boundary layer and by radiation from other woodparticles, from the reactor walls, and by the luminous flame. In flamingcombustion the heat for pyrolysis is provided by combustion of thepyrolysis products and radiation from adjacent particles or a surroundingcombustion zone. This heat must be transferred to the particle surfacethrough the flaming pyrolysiswind as shown in Fig. 2. This process is self-adjusting in part. Any factor that increases heat transfer also increases therate of streaming and the thickness of the boundary layer. This in turntends to reduce the heat transferred by convection across the boundarylayer. Based on the equation reported by Reed [15], the time required forflaming pyrolysis of a wood particle, tfp is given by
The time for flaming pyrolysis equation is dependent on particle size,shape, density etc. to describe in a fixed bed. This equation is used for themaximumsize of thewood stick (4.5 cm×0.7 cm×0.57 cm) for the smallsize woody biomass material. The times predicted could be highestbecause long-stick wood length or height is important part of the heattransferred and theheat is utilizedduring theflamingpyrolysis process. Inorder to account for this we have tomodify the tfp equation to include thedenominator using the long-stick wood density, sphericity and height.Therefore flaming pyrolysis time for long-stick wood is given below.
where tfp is the time (in min) required for flaming pyrolysis of a long-stick wood of density ρ=0.5g/cm3, average dimension D=12 cm(equal to the cube root of the volume, V1/3), sphericity Fs=1.8 andmoisture content Fm=0.20 (dry basis). R is the gas constant=1.98,Fo2 is the initial fraction of oxygen in the ambient air and T (°C) is thepyrolysis zone temperature (950 °C) for the gasifier. Table 3 repre-sents the large numbers of variables used for this following equation.
Note that in this equation those terms that involves themass of thewood, density, moisture content and increased time for pyrolysis in alinear mode. The dependence of time on length of the wood, L. is also
linear for large wood particles where L≫1. The dependence becomesquadratic for LN1 where the resistance to heat flows depends on heatflow resistance in the wood as well as in the gas boundary layer. Wehave estimated the velocity of motion of fuel in the gasifier as afunction of the amount of wood being gasified and it is found to be0.3 cm/s
Vf = M = AgFd 1−Fvð Þ: ð3Þ
Table 2Results of operation of cross draft and updraft gasifier for three runs.
Measured parameters Cross draft Updraft
Run 1 Run 2 Run 3 Run 1 Run 2 Run 3
Moisture Content of Acaciawood (%)
20 20 20 20 20 20
Size of wood (cm) 68×6 68×6 68×6 68×6 68×6 68×6Average Air Flow Rate (m3/h) 20.66 24.38 28.90 29.14 27 20.33Average Gas Flow Rate (m3/h) 35.14 37.52 41.47 37.47 37 34.91Average Gas FlameTemperature (°C)
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The length of the flaming pyrolysis zone Lfp, is found to be 65 cm.
Lfp = Vf × tfp = 34:2cm: ð4Þ
We take combustion time tc arbitrarily to be 100s.The length of the charcoal gasification zone is
Lc = Vf × tc = 30cm: ð5Þ
3. Results and discussions
3.1. Validation of gasifier conditions for the measured data
In the pyrolysis section steady state, temperature ranged betweenthat of the hot combustion surfaces and that of the cold unreactedcore from 1185 °C in the combustion zone to 400 °C in the dryingzone. Eqs. (2), (3), (4) and (5) were tested by estimating the pyrolysistime, fuel velocity and length of reaction zone has excellent fit for thetheoretical and practical results. The different conditions for gasifieroperation are as shown in Table 4.
3.2. Results on velocity of fuel flow in the gasifier
Saastamoinen [16] has studied the modelling of wood gasificationin an updraft mode. In this, gasifier was operated by lowering the airrate beyond a critical value increasing the bed height without a limit.The cross draft mode gasifier has a high-energy release rate per unitarea due to high inlet air velocity. If long stick wood is added on top ofthe burning charcoal, it will be pyrolysed by the heat, giving a mixtureof gases and volatile condensable compounds. As they are generated,the charcoal is replenished, so that a steady state can be reached by
adjusting the rate of wood feed. Cross draft gasifier is found to besuitable for long-stick woody biomass materials with high inlet airvelocity.
3.3. Effects of temperature at different zones on the gasifier
Large particles ofwood (N1 cm)may contain char at the surface of theparticle andwet wood in the interior of the particle. Up to this point, charcombustion has been within a specific region (approx 0.15 m tall), andonly a negligible amount of oxygen reaches the wood portion of the bed.Most pyrolysis occurs whenwood temperatures reach 300 to 450 °C. Thegas profiles are at t=1h as shown in Fig. 4. At the gasifier entrance(x=0.0m), oxygen and nitrogen enter the gasifier at 35 °C. Charcombustion reactions consume the oxygen in the first 0.1 m of thegasifier, and CO is produced. The mass fraction of nitrogen drops in thisregion because additional gases are being produced from the combustionreaction. These combustion gases (CO andN2) travel through the gasifier.At x=0.48m, the gases enter the wood region, where water vapour isbeing released fromdryingwoodparticles. Themass fraction ofN2 andCOdrop from x=0.48mto thegasifier exit at x=1.08mdue to the additionalmass of water vapour.
The gas profiles are similar at t=2 h as shown in Fig. 5. The x-locationrepresents the original locationwithin the bed. Oxygen is now consumedfrom x=0.038m to x=0.200m, indicating that the first 0.038 m of charhave been consumed. A small amount of tars and light pyrolysis gases arereleased in the top portion of the gasifier.
Table 4Theoretical and practical results for gasifier conditions.
The gas profile at t=3h is shown in Fig. 6. At t=3h, the charcombustions has moved to higher and higher char particles. Very littlewater is being released because most of the wood particles arecompletely dried. A large amount of light pyrolysis gases, and a lesseramount of tars are released from the pyrolysis reactions. The 6 cmwoodparticles are large enough that tars producedwithin the active pyrolysiszone of the particle can be broken down within the wood particle intolighter pyrolysis gases (CO, H2O, CO2, CH4, and H2).
Up to this point, char combustion has been within a specific region(approx 0.15m tall), and only a negligible amount of oxygen reachesthe wood portion of the bed. The model assumes that CO, O2, and H2Oare all necessary for a three body collision that converts CO to CO2. Themodel assumes no H2O in the char region. The lack of H2O in the charregion, and the lack of O2 in the wood region result in no CO2
production from the char combustion reactions. CO2 concentrationwould increase dramatically if moisture were present in the oxygenrich regions of the gasifier. CO2 may be present from the pyrolysisreactions, as part of the light pyrolysis gases.
Fig. 7 Shows the gas profile at t=4 h. Near the end of the gasifieroperation, a small amount of O2 survives into thewood region (which is
now mostly char). A small amount of CO is converted into CO2 at thistime. Thenewly formed char from thewood is not allowed to combust inthe model. Only light pyrolysis gases are produced (little tar) at thistime, because the pyrolysis gases are coming from pyrolyzation of thecentre of the wood particles. The tars produced at the centre of theparticle are converted into light pyrolysis gases before they can escapethe wood particle.
3.4. Effects of air velocity and gas flow rates on the gasifier
Fig. 8 shows gas production rate increasing with increase of inlet airvelocitywith linearity and steadier gas productionup to anair velocity of1.96 m/s. The inlet air velocity ranges from 2.25 to 2.64 m/s increasinggas production largely. It is obvious that the gas production rate wouldshow a similar behaviour with the change in airflow rate. We haveestimated the air velocity 1.64 m/s of motion of inlet air in the gasifier.
3.5. Effect of the gasifier conditions on the model
Weused long-stickwoodof size 68 cm×6 cm(cylindrical) depictingthe relevant features in pyrolysis zone, char gasification zone, and bufferchar zone. The calculations are shown in Tables 3 and 4 graphically toscale with schematic diagram as shown in Fig. 9.
Fig. 7. Gas profiles at t=4 h in the gasifier.
Fig. 8. Air velocity Vs gas production rate.
Fig. 9. Diagram of 10 kW cross draft long-stick wood gasifier for thermal powergeneration showing projected zone dimensions at a flaming pyrolysis time of 1.6min.
Table 5Comparative study of flaming pyrolysis time of the gasifiers.
Types of gasifier Cross draft Top lit updraft Downdraft
References Presentmodel
A. Saravanakumar et al.[17]
TB Reed et al.[15]
Pyrolysis temperature(T)
850 850 873
Pyrolysis zonePyrolysis time (tfp) 96 s 126 s 111.3 sFuel velocity (Vf) 0.3 m/s 0.3 m/s 0.0026 m/sPyrolysis zonelength (Lfp)
34.2 cm 37.8 cm 28.8 cm
Char gasification zoneChar gasification time(tc)
100 s 120 s 100 s
Char gasificationlength (Lc)
30 cm 37 cm 25.83 cm
Total gasifier activelengthFuel transit time(tfp+ tc)
196 s 240 s 231 s
Active zone length(Lfp+Lc)
64.2 cm 76 cm 55 cm
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3.6. Comparative flaming pyrolysis results of cross draft, top lit updraftand downdraft gasifiers
Saravanakumar et al. [17] noted the flaming pyrolysis time for fuel inthe top lit updraft gasifier as 120 s. In this studyof top lit updraft long-stickwood gasifier the air in the flaming pyrolysis zone moves up through thebed towards incoming oxidant at a constant propagation rate. The bed ismoving down, however, so that the position of the zone may be stable. Itmay move up or move down depending on the operating conditions ofmoisture content, ignition temperature of the fuel, heat loss andthroughput of the gasifier. Reed et al. [15] studied the flaming pyrolysistime for wood chip in the downdraft gasifier for 111 s. The comparativestudy illustrates that pyrolysis time in cross draft is lower than top lit anddowndraft modes as shown in Table 5. This difference is because of hightemperature reached in gasification zone to pyrolysis zone. The hightemperature reached requires a low ash fuel to prevent slagging [12].
4. Conclusions
Cross draft long-stickwood gasification technologies have reached thepoint where the first simple applications withminimal technical risks arebecoming commercial. The development of the technology has movedbeyond the element of the “gasifier” to the critical area of the supply of a“clean gas,” free of particulates and tar. If this will be achieved, then thepower market will be slowly but steadily penetrated on condition thatsufficient feed stocks can be secured. The experience of gasifier userswithregard to the effort and energy needed for wood chip preparation in atypical gasifier has led us to the development of a gasifier suitable toworkwith long-stick woody biomass as the feed material. In the spirit of usingan energy plantation for such a gasifier system, it is preferable to obtainsuch sticks from the branches of trees, which can be replenished at theplantation site in a shorter time-span as compared to big trees being cut.
For themodelling, thegasifierwasdivided intoa fuel reserve, aflamingpyrolysis zone, and a char reduction zone. In all the zones, the gasifierconditions show a fairly good fit between the theoretical and experimen-tal values. The flaming pyrolysis time for fuel in the gasifier is 1.6 min. Eq.(2) has the proper functional dependence on particle size, shape, densityetc. to describe the flaming pyrolysis time. The times predicted could betoo short because oxygen is not present during part of the pyrolysis. Thequality of the gas was also better in the cross draft mode due to thecracking of tar The efficiency of the cross draft long-stick wood gasifierwas found to be 79%. The gasifier designed in the present work can beimplementedatpilot level in a small village at awoodconsumption rate of10 kg/h. Thus it isworkedout to be100 kg for 10 h in twobatchesper day.The gasifier described in the present work can furnish the need of a smallvillage. This can operate with 3W of blower power or other air supply toproduce 8–10 kW thermal for cooking. It is simple and inexpensive tobuild. The gasifierwill bring a litre ofwater to boil in 4–10 min and can beturned down to the simmer level for longer cooking and increasedefficiency. With the fairly satisfactory performance and ease of construc-tion alongwith the convenience of feedpreparation, it is feel that the crossdraft gasifier with long-stick wood as feed would find abundant avenuesof applications in a rural setting for thermal, mechanical and electricalenergy delivery.
NomenclatureAg Cross sectional area of the gasifier (cm2)D Typical dimension (cm)ρ Fuel density (g/cm3)Fm Moisture content (%)
FO2 Fraction of oxygen (%)Fs SphericityFv Void fractionL Length of the wood (cm)Lfp Length of the flaming pyrolysis zone (cm)Lc Length of the charcoal gasification zone (cm)M Mass flow rate of the fuel (kg/h)T Pyrolysis zone temperature (°C)tfp Flaming pyrolysis time (s)tc Char gasification time (s)V Volume of the wood (cm3)Vf Velocity of fuel flow (cm/s)
Acknowledgements
Oneof the authorsA. Saravanakumar thanks theMinistry ofNewandRenewable Energy, Government of India, for financial support in theform of fellowship under the National Renewable Energy FellowshipProgramme.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in theonline version, at doi:10.1016/j.fuproc.2010.01.016.
References
[1] Saravanakumar A. Some Combustion Related Investigations on Gasifiers, Ph. D.Thesis, Copyright to Madurai Kamaraj University, 2007.
[2] European Commission: Energy for the future: Renewable Energy Sources-WhitePaper for a Community Strategy and Action Plan, vol. 97, COM, Brussels, 1997, p. 599.
[3] International Energy Agency, World Energy Outlook, IEA, Paris, 2000.[4] Hislop D, Hall D. Biomass Resources for Gasification Plant, 1996. ETSU B/
M3/00388.[5] T.B. Reed, A. Das, Handbook of Biomass Gasifier Engine Systems, The Biomass
Energy Foundation Press, 1988.[6] D. Bergstorm, S. Israelsson, M. Ohman, S. Axel Dahlqvist, R. Gref, C. Boman, I.
Wasterlund, Effects of rawmaterial particle size distribution on the characteristics ofScots pine sawdust fuel pellets, Fuel Processing Technology 89 (2008) 1324–1329.
[7] Cuda R, Ziak R. Gas Cleaning and Conditioning, Part 1. “Theoretical Background,Construction, Properties, Experiments”, Nordvestuysk Folkecenter for Vedvar-fende Energi, 1995, Hurup, Denmark, 32 pp.
[8] K.M. Bryden, K.W. Ragland, Numerical modelling of a deep, fixed bed combustor,Energy Fuels 10 (1996) 269–275.
[9] T.B. Reed, R. Larson, Wood gas stoves for developing countries, Developments inThermochemical Biomass Conversion Conference, 1996. Canada, 1996.
[10] A. Saravanakumar, T.M. Haridasan, T.B. Reed, R. Kasturi Bai, Operation andmodelling of an updraft long stick wood gasifier, Energy for SustainableDevelopment 9 (2005) 25–39.
[11] A. Saravanakumar, T.M. Haridasan, T.B. Reed, R. Kasturi Bai, Experimentalinvestigations of long stick wood gasification in a bottom lit updraft fixed bedgasifier, Fuel Processing Technology 88 (2007) 617–622.
[12] Kaupp A, Goss JR. State of the art for small (2–50 kW) gas producer enginesystems, Final report to USDA, Forest Service, Contract No. 53–39R-0–141, 1981.
[13] Bryden K.M., K. Ragland, C. Rutland, Modeling thermally thick pyrolysis of wood,Biomass and Bioenergy 22 (2002) 41–53.
[14] M. Haggeb:author>K.M. Bryden, Modeling the impact of shrinkage on the pyrolysisof dry biomass, Chemical Engineering Science 57 (2002) 2811–2823.
[15] T.B. Reed, M.S. Graboski, B. Levie, Fundamentals, Development and scale up of theair-oxygenstratifieddowndraft gasifier, TheBiomassEnergyFoundationPress, 1988.
[16] Saastamoinen, J., “Sub-report. Modelling of counter flow biomass gasifiers”, 34 pp.+ Appendix, EU-project No. STEP-CT-91–0129, Gasification of Waste PreservedWood Impregnated with Toxic Inorganic and/or Organic Chemicals, VTT Energy,Jyvaskyla, 1994.
[17] A. Saravanakumar, T.M. Haridasan, T.B. Reed, R. Kasturi Bai, Experimentalinvestigation and modelling study of long stick wood gasification in a top litupdraft fixed bed gasifier, Fuel 86 (2007) 2846–2856.
