Gasifier types - Fixed and Fluidized bed gasification

Gasifiers are devices used to convert solid carbonaceous materials (such as biomass, coal, or waste) into a gaseous fuel known as syngas (synthesis gas) through thermochemical processes. There are several types of gasifiers, each with unique operating principles, advantages, and applications.

1. Gasifier –types
By
Er. T. AYISHA NAZIBA, Dr. D. RAMESH, Dr. S. PUGALENDHI

2. GASIFICATION
Gasificationisapartialoxidationprocesswherebyacarbonsourcesuchas
coal,naturalgasorbiomassisbrokendownintocarbonmonoxide(CO)and
Hydrogen(H
2)pluscarbondioxide(CO
2)andpossiblyhydrocarbon
moleculessuchasmethane(CH
4)
Thismixofgasisknownasproducergasandtheprecisecharacteristicsof
thegaswilldependonthegasificationparameterssuchastemperatureand
alsotheoxidizerused
Theoxidizermaybeair,inwhichcasetheproducergaswillalsocontain
Nitrogen(N
2)orsteamoroxygen

3. GASIFIER types
FIXEDBED
oUPDRAFTGASIFIER(COUNTERCURRENTGASIFIER)
oDOWNDRAFTGASIFIER(CO-CURRENTGASIFIER)
oCROSSDRAFTGASIFIER
FLUDIZEDBEDGASIFIER(Bubblingorcirculating)

4. Updraft DowndraftCrossdraft
TYPES OF GASIFIER

5. UP DRAFTGASIFIER
Reaction zones in an up draft gasifier
Anupdraftgasifierischaracterizedbyacountercurrentflowoffuelandair
orgasinthereactor.Theproducergasexitsfromthetopofthegasifier

6. FEATURES OF UPDRAFT GASIFIER
Thebiomassentersthetopofthereactionchamberwhilesteamandairoroxygenenter
frombottomthroughgrate
Thefuelflowsdownwardandup-flowinghotgasespyrolyzeit.someoftheresulting
charcoalresiduefallstothegrate,whereitburns,producingheatandgivingoffcarbon
dioxideandwatervapor
TheCO
2andH
2Oreactwithothercharcoalparticles,producingcarbonmonoxideand
hydrogengases
Thegasesexitfromthetopandashesfallthroughthegrate
Theupdraftdesignisrelativelysimpleandcanhandlebiomassfuelswithhighashand
moisturecontent
Thegascontains10-20percentvolatileoilsmakingthegasunsuitableforuseinenginesor
gasturbines

7. TYPICAL ZONAL CHEMICAL REACTIONS AND
PRODUCTS IN A UP-DRAF GASIFIER

8. THERMO-CHEMICAL REACTIONS OCCURRING
IN GASIFICATION
REDUCTIONZONE
Productsofpartialcombustion–H
2O,CO
2anduncombustedandpartiallycrackedpyrolyticproductsundergothe
followingchemicalreactionsinthered-hotcharcoalbed
C+CO
2=2CO(-164.9MJ/kgmole)[Boudouardreaction]
C+H
2O=CO+H2(-122.6MJ/kgmole)[watergas]
CO+H
2O=CO+H
2(+42MJ/kgmole)[watershiftreaction]
C+2H
2=CH
4(+75MJ/kgmole)[Methanereaction]
CO
2+H
2=CO+H
2O(-42.3MJ/kgmole)
Lowreductionzonetemperatureyieldslowcalorificvalueofgas
Averagetempis850
0
C

9. UPDRAFT GASIFICATION DESIGN
• Simpledesign
• Notsensitivetofuel
selection
•Hasalongstarttime
•Delayinresponse
•Bestusedforlarge,longuse
applications

10. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN
UPDRAFT GASIFIER

11. Downdraftgasifierischaracterizedbyco-currentflowofairorgasandthe
fuel.Fuelandairorgasmoveinthesamedirection
Reaction zones in a down draft gasifier
DOWN DRAFT GASIFIER

12. FEATURES OF DOWN DRAFT GASIFIER
Gasifierrequiresdryingofbiomassfueltoamoisturecontentoflessthan20percent
Fuelandairoroxygenenterthetopofthereactionchamber
Down-flowingfuelparticlesignite,burningintenselyandleavingacharcoalresidue
Thecharcoalwhichisabout5to15percentofthemassoforiginalfuel,thenreactswiththe
combustiongases,producingCOandH
2gases
Thesegasesflowdownandexitfromthechamberbelowagrate
Theproducergasleavingthegasifierisatahightemperature
Combustionashfallsthroughthegrate
Theadvantageofthedowndraftdesignistheverylowtarcontentoftheproducergas

13. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN
DOWNDRAFT GASIFIER

14. DOWNDRAFT GASIFICATION ANALYSIS
Thegeneralideabehindthisdesignisthatthetarryoilsandvapors
givenoffinthedistillationzonearehighlyunstableathightemperatures.In
ordertoreachthegasoutlettheymustpassthroughthepartialcombustion
zonewhereahighamountoftarwillbecrackedandreducedtonon-
condensablegaseousproductsbeforeleavingthegasifier
Althoughthegeneralprinciplebehindthisseemsconvincingeasily,in
practiceitrequiressometestingandhighskilltocomeupwithadowndraft
gasproducercapableofgeneratingatarfreegasunderequilibrium
conditions

15. TYPICAL CHEMICAL REACTIONS AND PRODUCTS IN A
DOWN-DRAF GASIFIER

16. DOWNDRAFT GASIFICATION
Canusehightarfuels(wood,peat)
•Quickerinresponse
Hasshorterstarttime
Complexdesign
Can’tusehighashcontentfuels
Fuelpelletsizemustbeuniform

17. DOWNDRAFT GASIFICATION SYSTEM

18. Downdraftgasifierwithoutthroat
Downdraftgasifierwiththroatisnotsuitableforgasificationofagro-
residueslikepaddyhusk,groundnutshelletc.Adowndraftgasifierwithout
throatcanbeusedforthegasificationofpaddyhusk,groundnutshell,maize
cob,etc.
Downdraftgasifierwiththroat
Gasifierhasnarrowsectionbelowairentrancepointinthereactor,which
iscalledthroat.Duetodecreaseinthecross-sectionalareaatthethroat,air
velocityincreasesresultinginhighandbettertemperaturedistributioninthe
oxidationzone
.
SIGNIFICANCE OF THROAT IN A DOWN DRAFT GASIFIER

19. Thisgasifiercanoperatewithwide
varietyoffuelscomparedtoanupdraftora
downdraftgasifier
Highgasexittemperature,highergas
velocityatthegasexit
PoorCO
2reductionarecertain
characteristicsofthistypeofgasifier
Thistypeofgasifierhasbeenusedfor
gasificationofcoal
CROSS DRAFT GASIFIER

20. CROSS DRAFT GASIFICATION ANALYSIS
Can’tusehightarfuels
Quickerinresponse
Hasshorterstarttime
Complexdesign
Can’tusehighashcontentfuels
Fuelpelletsizemustbeuniform

21. FLUIDIZED BED GASIFICATION
Afluidizationbedisachamberwithaperforatedfloor
havingpressurizedairflowingverticallywhereaparticlemedium
usuallysand,iscontained.Thepressurizedandflowingairhelps
themediumallowingittoactasafluid

22. PRINCIPLE OF FLUIDIZED BED GASIFICATION
Canusemostfuels(wood,peatandcoal)includingagriculture
wastesuchasstraw,cornstoverandmanure
Haspotentialtousemunicipalwastesuchasgarbage
Quickerinresponse
Hasshorterstarttime
Complexdesign
Lendsitselftocompletecombustionapplicationswhichwould
allowittouseliquidwastessuchasusedengineoil,non-
recyclableplastics&oldshoes,garbageforgenerationofheat

23. Fluidizedbedgasifierisahomogeneousreactorbedofsomeinertsandmaterial.Thefuelis
introducedintheinertbedmaterialandairatthebottomofthebedinthereactor.Thisgasifier
ischaracterizedbyhighgasexittemperature,veryhighsolidparticulatematterinthegasand
relativelylowefficiency.Thegasifiercanoperatewithlowbulkdensitymaterialssuchasagro-
residues,leaves,etc.
FLUIDIZED BED GASIFIER

24. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN
FLUIDIZED BED GASIFIER

25. Sr. NoGasifier Type Advantages Disadvantages
1 Updraft •Small pressure drop
•Good thermal efficiency
•Little tendency towards slag
formation
•Great sensitivity to tar
•Moisture and moisture content of fuel
•Relatively long time required for start up of IC engine
•Poor reaction capability with heavy gas load
2 Downdraft •Flexible adaptation of gas
production to load
•Low sensitivity to charcoal
dust and tar content of fuel
•Design tends to be tall
-not feasible for very small
particle size of fuel
3 Cross-draft•Short design height
•Fast response time to load
•Flexible gas production
•Very high sensitivity to slag formation
•High pressure drop
COMPARISON ON THE PERFORMANCE OF GASIFIERS

26. THANK YOU