A Mathematical Model for Stratified Downdraft Gasifiers

7/29/2019 A Mathematical Model for Stratified Downdraft Gasifiers

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A U4"EEHBTICAL KIDEL FOR STRATIFIED WUNDBAFT GASIFIERSThomas B . Reed, Benjamin Levie, and Michael L. t-hrksonSol ar Energy Research In s t i t u t e , Golden, CO 80401

Michael S. GraboskiColorado School of Mines, Golden, CO 80401

INTRODUCTIONThe downdraft ga s if ie r o r i gi na t i ng f rom World War 11 Swedish designs has proven t obe success fu l in g e n e r a t i n g a c l ean p r o d u c t gas when operated in a n a i r blownmode. Such g a s pr od u ce r s a r e u s e f u l f o r g e n e r a t i n g a n e s s e n t i a l l y t a r f r e e b o i l e rgas o r eng ine fu e l f rom renewable r esources such a s wood and agr icul tural waste.Recently, a new g en e r a t i on of s t r a t i f i e d d ow nd ra ft g a s i f i e r s (1, 2) has beens tud ied . A suc ces s fu l model of t h i s type of g as i f ic a t io n p rocess shou ld be ab l et o c l e a r l y s how th e i n t e rd ep en d en cy of o p e r a t i n g v a r i ab l e s i n o r d e r t o op timizebot h cos t of $gasifier and perform ance. Such a process model would be useful indetermin ing th e p roper ga s i f i ca t i on condi t ions when inpu t condi t ions o r des ignpa ramet er s change .Figure 1 shows th e imp o rt an t f ea tu r e s of t h e g a s i f i e r . Biomass f u e l a n d o x i d i z era r e f ed co cu r r e n t ly t o t h e t o p of t h e g as g en e ra to r w here p y r o ly s i s of t h e f u e lt ak es p lace . The p y r o ly s i s r ea c t i o n is dr iv en by hea t t r an s f er f rom th e gas andhot char bed below. As t h e f r e s h s o l i d is h ea t ed it d r i e s a nd d e v o l a t i l i z e s . ' hev o l a t i l e s ev o lved co n t a in co mb u s t ib l e s p ec i e s w h ich r ea c t w i th o x yg en /a i r t oproduce hea t , CO, C02, H2, H 0 and l i g h t hydrocarbons. Dur ing pyro lys i s , t he gasand so l id a r e a t v a s t l y d i f J e r e n t t e mp e ra t ur e s b ec au se p y r o l y s i s c o ol s t h e s o l i dw h i l e o x id a t io n h e a t s t h e g as . ( A t t he end of py ro ly si s t he gas may be more tha n500K h o t t e r t h a n t h e s o l i d . ) In t h i s zon e of t h e r eac t o r , ab o ut 80% t o 90% o f t h esolid weight loss occurs .Once oxygen is consumed and pyrolysis is completed, red uct ion of char by Cog andH20 can occur in t h e g a s i f i ca t i o n zone. The r eac t i o n s o ccu r r in g a r e end othe rmicso that th e &as and so l i d tempera tu res f a l l a s carbon conver s ion p roceeds . Ther e a c t io n s t en d t o q u i t a t ab o u t lOOOK due t o k i n e t i c l i m i t a t i o n s .I n t h e s t ead y - st a t e o p e r a t i o n of t h e d ow nd ra ft g as i f i e r , a s p e c i f i c o xygen t o f u e lr a t i o e x i s t s f o r a g i v e n f e e ds t o ck a n d ca r bo n c o nv e rs i on l ev e l . I n p r a c t i c e i t isfound tha t ga s i f i e r th roughput does not a f f e c t t h e r e q ui r ed 0 2 / f u e l r a t i o a nd t h eproduct gas composi t ion f or a s u f f i c i e n t l y d e e p char bed. Also, i t is found thatth e ua jo r i ty o f hydrocarbons a re des t royed in th e py rol ys is zone. These datasugges t tha t t h e p y r o l y s i s a n d g a s i f i c a t i o n z o ne s a r e t o a good a pp ro x im a ti o ns e p a r a t e a nd t ha t the whole char bed i n do wn dr af t g a s i f i e r s is n o t t r u l y a c t i v e .I n t h e ch ar g a s i f i ca t i o n zon e t h r ee r eac t i o n s d omin at e ( n eg l ec t i n g t h e c r ack in g ofr e s i d u a l t a r s and hydrocarbon gases f r o m th e p y r o ly s i s zo n e ) :

Boudouard ReactionChar + C02 + 2CO A H = +40,778 k c a l / m o lWater Gas ReactionChar + H20 + CO + H2 A H = +32,472 kcallmol

Water Gas Sh i f t Reac t i o nH20 + CO * H2 + CO2 AH = -8306 kcal/mol

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The ki ne t i cs and thermodynamics of these re ac t io ns determine th e conversion ofchar t o gas and t he subsequent g a s composit ion a t any point in t h e charg as i f i c a t i o n zo n e .In th e reac tion scheme, th e water-gas and Boudouard rea ct io ns a re coup led by th es h i f t reac t ion. Therefo re , on ly two of these re ac t ions can be considered t o bet ru ly independent . The w at er -g as s h i f t r e ac t i o n is f a i r l y r ap i d o ve r ca rb ons u r f a c e s a t g a s i f i c a t i o n t e m pe r at ur e s a nd is assumed t o be in equ i l ib r ium in t h i sinves t iga t ion . In our model we have assumed th a t Edric h e t al . ( 3 ) k i n e t i c s f o rthe Boudouard r ea ct i on o ver ponderosa p ine charcoal appr oxi rat es the carbonred u c ti o n r eac t i o n o ccu r r in g in t h e g a s i f i e r . S in c e t h e a c t i v a t i o n e ne rg y f o rcarbon reduc tion by CO is abou t 35 k c a l , k i n e t i c s in er ro r by a f a c t o r of 2 w i l lbe equivalent to a 5 6 O C o f f s e t . T h is o f f s e t is wi t h in the accuracy of da taav a i l a b l e fo r bed t empera ture . A t a tmospheric p ressu re , t h i s rea c t io n sequenceshou ld adequa te ly descr ibe the k in e t i c p rocesses . A t el eva ted press ure, methaneforming ki ne t i cs should probably be considered. For char p a r t i c l e s w i t h mi n o rdimensions up t o 3 / 4 i n ch , E d ri ch e t a l . ( 3 ) a l so show that i n t r a p a r t i c l e m sst r a n s f e r is no t impor tan t . Therefo re , the same r a t e express ion fo r charg a s i f i c a t i o n a p p l i e s r e g a r dl e s s of p a r t i c l e s i z e ( i f less t han 3 / 4 inch) .The reac to r des ign cond i t ions which a f f e c t the char g as i f i c a t i o n zo n e In c l u d ei n i t i a l c o nd i ti o ns , s uc h as char and gas temperatures, f low ra te and composit ionof t h e incoming g a s , an d g a s i f i e r p a rame te r s , such a s ch a r g a s i f i c a t i o n k i n e t i c s ,c ross-sec t iona l area of r eac t o r , h ea t t r an s f e r f rom t h e g a s t o t h e s o l i d s , a nd t h edensi ty and void f r ac t i on of th e char . An adequate model m st accoun t fo r changesin t h e se i n i t i a l co n d i t io n s an d pa rame te rs .To date t he most e x ten s ive model ling of th e char zone of th e s t r a t i f i e d downdraf tg a s i f i e r has been developed by Reed (1) . Reed's model of the char ga si f i ca t i onzone assumes equ al molar feed r at e s of C02 and carb on ( char) . 'his zone isassumed t o be ad ia ba t i c , y i e l d ing a change in temperature of about 24'K per 1% ofreac t ion of ca rbon. Coupl ing the t empera tu re change t o th e k i ne t i cs of th eBoudouard r ea ct i on then yi el ds the convers ion of carbon and temperature of t herea ct ion versus t ime and pos i t i on (depending on feed ra te ) . ?he model g ives agood f i r s t approximat ion of the g a s i f i e r behavior le adi ng the way towards th e useof theory fo r p r ac t i ca l p red ic t ions .

W)DEL FORWIATIONThe over a l l ga s i f i e r model c on si s t s of two pa r t s ; t hes e a r e a pyro lys i s model anda ga si f i ca t i on model. The pyr oly sis model is u sed t o pro vi d e a s t a r t i n g g a scomposi t ion , f low ra te , and t empera ture fo r the char gas i f i c a t io n zone. Toi n i t i a t e t h e m o d e l l i n g , t h e a i r / f u e l o r O ~ / f u e l a t i o , f ee d u l t im a t e a nd pr ox im atean a l ys i s and a methane l eakage from the gas i f i e r a r e spec i f i ed . 'h e modelgenera tes pyroly sis gas composi t ion and temperature a l ong wi th carbon conversiongas composi t ion and tempe rature a lo ng th e char bed length .Pyrolysis ModelBiomass is assumed t o be a r t i f i c i a l l y composed of f ixe d carbon (char ) and vo la t i l em t t e r . Upon pyro lys i s , f o r b io ms s under downdraf t ga s i f i e r cond i t ions , th e chary i e l d is assumed t o be equ al t o th a t from the proximate yie ld. The cha r ist r e a t ed a s p ure carbo n. With t h e sp e c i f i c a t i o n of a n a i r o r 0 2 / f ue l r a t i o a ndfeed composit ion, a n a d i a b a t i c r e a c t i o n c a l c u l a t i o n ar ou nd t h e p y ro l ys i s z one w i l ly ie ld t empera tu re , gas composi t ion and f low rate . I t i s fu r t he r assumed t ha t anymethane escaping pyrolysis is not cracked in the cha r bed. Therefore , the w s sand energy balance around the py rol ys is zone al l ow s f or the methane leakages p e c i f i e d a s a model input.

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In o r d e r t o ca l cu l a t e t h e ad i a b a t i c fl ame t emp era tu r e of t h e p y r o ly s i s g as , wheni t is o x id i zed by t he a i r or o xy gen, f i r s t t h e en er gy r e l eas ed by the combust ionis determined.

AH298= HHv - AHcombustionwhere AHzg8 is t h e en e rg y r e l e as ed from th e p y r o ly s i s an d p a r t i a l combu st io nassuming N2, CO, C02, H2, H20, CH4, and char a r e t h e o nl y p r od u ct s , HHV i s t h eh ig h h ea t i n g v a lue of t h e b i o m s s ( c a l cu l a t ed by t h e ITG method), and AHc mbustiois th e sum of t h e moles of th e produc ts of p yr ol ys is times t h e i r %eats 04combustion. In th e model p re d ic t io ns p res en ted below, a typ ic a l biomsscomposition of 51 perc ent carbon, 6 percent hydrogen and 45 percent oxygen byweight was assumed f o r t he m t e r i a l b h r . c e c a l c u l a t i o n .Tie ad ia ba t i c f lame tempera tu re can th en be de termined by th e f o l l o w in g eq u a t io n

In t h i s model a n i n t e g r a l a v e r a g e v a l u e of Cpi f o r e a ch of t h e g as c o n s t i t u e n t s isu t i l i z e d . The c a l c u l a t e d a d i a b a t i c f l a m e t e m p e r a t u r e is t h en u s ed t o d e t e r minet h e c o r r e c t Kp and gas compos i t ion i n a second i t e r a t io n . S ince the f lametemp er a tu r e v a r i e s l i t t l e w i th ch an g es i n K , o n ly tw o i t e r a t i o n s a r e necessaryf o r accu r a t e g as co mp os i t i on an d ad i ab a e i c f lame t emp era tu r e p r ed i c t i on s .Speci fy ing the amount o f f ix ed carbon y ie ld ed f rom the b io ms s and th e oxygen t of u e l r a t i o g iv es a u n iq ue g as co mp o s i t io n an d t emp er atur e.Char G a s i f i c a t i o n ModelThe char ga si f i ca t i on model desc r ibe d below assumes that t h e char g a s i f i c a t i o nzone is a d i a b a t i c a n d, a s in t h e p y r o l y s i s zon e, t h e water g as s h i f t r e a c ti o n isa t eq u i li b r iu m. Char g a s i f i ca t i o n k i n e t i c s a r e employed t o compute t h ec o n v e r s i o n / l e n g t h p r o f i l e . H e a t balances on t h e gas a nd s o l i d a r e used t od e t e r min e t emp er a tu r e p r o f i l e s . Material b a la n ce s w r i t t e n f o r g a s a nd char i n t h ere ac to r assume plug f low; however, th e g a s an d ch a r move a t d i f f e r en t r a t e s downt h e r e a c t o r . The frac t iona l co n v e r s io n of t h e char, X , is def ined a s fo l lows:

where h ( 0 ) = molar flow ra t e o f carbon a t t o p of g a s i f i c a t i o n zo ne , an dmolar how r a t e of ca r b o n a t p o s i t i o n "z" in t h e g a s i f i c a t i o n zone.mass balance is then :

h ( z ) =T ~ G ? arbon

where X is t h e f r ac t i o n a l co n v er s io n of t h e ch a r , z is the dis tance down ther e a c t o r , re is t h e r a t e of co n v e r s ion o f in moles char/min, S is t h e c r o s ss e c t i o n a l a r e a of t h e r e a c t o r , a nd h (0) is t h e char f eed ra te in moleschar/min.For t h e Boudouard re ac t i on :

l h e rate is computed from ki ne t5 c dat a.

( k l * PC02)/(1 + k2 * PCO)k l = exp(-E1/RT + 12.3091)

k 2 = exp(-Ep/RT - 28.4295)4 1 2


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where r c , i s in un i t s of l /min , E 1 = 43870 - 19811/Tp (pretreatment) cal/mol, E2 =-67,300 cal /mol and PCO and PC02 are p a r t i a l p r e ss u r es o f CO and CO2r e s p ec t i v e ly . The pre t r ea tment tempera tu re is the tempera tu re a t which theb i o m s s is pyrolyzed. In t h i s mo d e l i t is assumed t o be 1000K. For t h e purposeof t h i s model, th e bed voidage and pa r t i c l e s i z e a r e assumed c o n st a n t . S h i f tk i n e t i c s a r e a ssu med t o b e r ap id . Thus, t h e g as co mp o si t i on is b r ou g ht t o a waterg a s s h i f t e q u i li b r iu m a t e a c h p o s i t i o n in t h e r eac to r . To acco u n t f o r k i n e t i c st h e e q u il i b ri u m c o n s t a n t is disp lace d f rom the gas tempera tu re by 50'C.The energy ba lance inc ludes ind iv i dua l equat ions f o r the char, an d o n e f o r t h e g asphase. In th e gas phase:

where m is t h e m ss f low ra t e of th e gas , cp is t h e h e a t c a p a c i t y of t h e g as, Tis the gas tempera tu re , h is t h e h e a t t r a n s f e k c o e f f i c i e n t b et we en t h e gas an d th gch a r , A is t h e s u r f a ce a r ea p e r g ram ch a r , T is t h e char temperature. P is t h ec h ar d e i s i t y a n d E is t h e v o id f r a c t i o n in th e bed.F or t h e s o l i d D h a s e

where X i s the conver s ion o f char, HB is t h e h ea t of r e ac t i o n f o r t h e Boudouard, Yis the conver s ion o f s t e a m t o hydrogen , and HwGs is t h e h e a t of r e a c t i o n f o r t h ewater g as s h i f t r eac t i o n . The m ss and energy balanc es a r e coupled and solvedus ing a Runge Kut ta in te gr a t io n rou t in e in a n i n t e r ac t i v e mode.The model re su l t s a r e very dependent on i n i t i a l c o n d i ti o ns , i n cl u d in g t h e i n p utf rom the p yro lys i s model ca lc u l a t io ns of tempera tu re and compos i tion o f the gas.lh e average tempera tu re of th e so l i d is not known exactly but is assumed t o besomewhere between th e flame temperature and th e py rol ys is f ro nt temperature. Thes o l id t emp er a tu r e is no t c r i t i c a l , s i n c e t h e h e a t t r a n s f e r c o e f f i c i e n t is l a r g ea nd t h e h e a t c a p a c i t y of t h e s o l i d p ha se is small r e l a t i v e t o t h e g a s phase .The h e a t t r a n s f e r c o e f f i c i e n t in the energy ba lance has b een ca l cu l a t ed by a nemp i r i ca l co r r e l a t i o n of Sa t t e r f i e l d ( 4 ) f o r f i x e d b ed r eac t o r s . The model n eed st h e area t o volume r a t i o of t h e f e ed s t oc k t o c a l c u l a t e t h e h e a t t r a n s f e rc o e f f i c i e n t , h, a nd t h e p a r t i c l e a r e a t o w ei gh t r a t i o , AP'

KXPERMENTAL DATATo compare th e p re d ic t io n o f t he model wi th exper imenta l ga s i f i e r r e su l t s , aq u a rt z t u b e g a s i f i e r 54 mm ou te r diameter , shown in Fig. 2, was employed. A typeK 1/16-in. thermocouple was used f o r tempera tu re measurement th rough th e pyro lys i san d g as i f i ca t i o n zo n es . A 1/16-in. 304 SS tube was p l a ce d d i r e c t l y a lo n g si d e t h ethermocouple, throug h which gas samples were pul led . A 10 cc s y r in g e (+ n eed le )was u se d t o e v a c u at e t h e t u b e a n d t o t a k e t h e sample. Gas a n a l y s i s was done witha Carle # l l l H g as chrouatograph , wi th a h yd ro gen t r an s f e r t u b e an d a t e n f tCarbosieve column. In te gr a t io n of an a l ys i s was performed with a Var ian I CDS111i n t e g r a t o r .Two samples were take n fo r each le v e l measured. Once a s t ead y s t a t e condi ton int h e gas i f i e r was ach i ev ed , t h e p ro bes were i n s e r t e d t o t h e s p e c i fi e d l e v e l an d t h egas sampling tem pera ture rec ord ing proc edu re begun. The probes were then moved a t2 c m i n t e r v a l s up t hr ou gh t h e bed u n t i l t h e t emp er a tu r e r ead below g as i f i ca t i o np y r o ly s i s t emp er a tu r e s (100OC). The t i m e i n t e r v a l in between each sample wasapprox imate ly 1 minute.

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Rl3SULTSTable 1 pr e s e n t s r e s u l t s of t h e py r o l y s i s m a t e r i a l a nd ener gy ba l a nc e model. Thea d i a b a t i c fl am e t e m pe r a t u r e s of t he py r o l y s i s p r oduc t s, a ss um ing f r a c t i on s off i xe d carbon from 0.05 t o 0.20 when burned wi th var iou s r a t i o s of 0 / f ue l ha s be enca lcu la ted . The model a l s o y i e l d s t h e gas c om posi ti on a t t h e e nd o t p y r o l y si s f o re a c h f i xe d c a r bon a nd 02 / f ue l r a t i o . Zhe gas tempe rature and composi t ion is theni n p u t i n t o t h e g a s i f i c a t i o n model. F or p r e d i c t i o n s of t h e l a b o r a t o r y d at a , t h ef i xe d c a r bon is assumed t o be 15%and the 0 / f u el r a t i o is s e t a t 0.45. For th eoxygen runs t h e 0 2/ f u el r a t i o is assumed t o be 0.40. The re su l t s pres ented yi e l dvary ing char l o s s t hr ough t he g r a t e . An a l t e r n a t e c a l cu l a t i o n is t o i t e r a t e ont h e a i r / f u e l r a t i o t o c onsume a s pe c i f i e d m o un t of f i x e d ca rbon .

0.45

0.50

%ble 1. P y r o l y s i s & d e l C a l c u l a t i o n s

Feed Gas 02/Fue l F r ac t i on F ixed Carbon Adiabatic Temperature(K)A i r 0.4 0. 05 57 5

0.10 7640.15 9550.20 11460.05 7450.10 9240.15 1104

(02) (888)(1182)(1475)(1765)(1201)(1490)(1775)(2055)(1504)(1785)(2061)(2330)

0.20 12820.05 8970.10 10670.15 12360.20 1403

Figu re 3 shows th e model pr ed ic t io ns of th e char and gas phase temperatur ep r o f i l e s t hr ou gh t h e g a s i f i c t i o n z on e f o r a i r g a s i f i c a t i o n . The s o l i d phase isr e p r e s e n t ed by t he s o l i d l i ne , t he ga se ous phas e is represented by the do t t ed l i n eand the exper imenta l da ta a r e repr esent ed by stars . Exce l l en t agreement wi th thedata is obs er ve d t h r oughou t t h e c ha r g a s i f i c a t i o n zone , w i t h t h e e xc e pt i on of t hedata poin t a t the gra te . Thi s d i sc repancy is caused by heat l o s s (conduc t ion andr a d i a t i o n ) a t t h e g r a t e , r e s u l t i n g in lower te mpe ratu re measurements th anexpected. In a r e a c t o r w i th a l a ye r of ce ramic balls a b o v e t h e g r a t e i t would beexpec ted t ha t hea t l o s s would not be important .Figu re 4 shows pr ed ic t io ns and exper imen tal data of th e CO/CO2 ra t i o down t h ere ac to r . The agreement between model pr ed ic t io ns and experime ntal data is good

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conf irma tion of t he model. Sinc e no a d j u s t a b l e p a r a m e t e r s were i n p u t into t h emodel t o make t he temp erature p ro f i le and CO/C02 r a t i o p r e d i c t i o n s , t h e modela p p e a r s s u c c es s f ul a t s i m ul a t i ng l a b o r a t o r y c o n di t i on s .The e ff e c t of va ryin g throughput on char conver s ion is shown in Fig. 5 . I n t h i sf i g u r e , t h e r e s u l t of i n p u t t i n g into th e model te n t imes normal, normal, and oneten th normal a i r thorughput i s i l l u s t r a t e d . Q ua nt it at iv el y, a f t e r t h e s o l i d a n dgas have come t o t h e same t empera tu re , th e convers ion of char fo r a g iv en d i s t an ceis l i n ea r ly d ep en d en t on the th roughtpu t . A lthough an inc rea se in th roughputin c r eas es t h e h ea t t r an s f e r fro m th e gas t o t h e s o l i d , t h e n e t e f f e c t on charco n v er s ion , a s y i e ld ed by model ca lcu la t ions , i s the same char co n v e r s io n f o requ iva len t r es idence t i m e s , r eg a r d l e s s of t hr ou gh pu t. I n c r ea s in g t h e s u r f ac ea re a/ vo lu me r a t i o f o r t h e f e e d s t oc k a l s o i n c r e a s e s h e a t t r a n s f e r f rom gas t o c h a r ,bu t aga in no s i g n i f i c a n t d i f f e r e n c e in c o nv e rs i on oc c ur s f o r v a ri o u s r a t i o s , a f t e rt h e gas and so l i d tempera tu res approach th e same po in t .F ig u r e 5 a l s o shows the char conver s ion fo r a throughput c on si s t in g of oxygenins tead o f a i r . An in t e r e s t i n g outcome from usi ng oxygen in the modelc a l c u l a t i o n s i s tha t th e tempera tu re of the char only r ises ab o u t 50K above thepeak t em pe ra tu re of t h e a i r g a s i f i c a t i o n c a s e, a s d ep ic t ed in Fig. 6. 'Ihis i s ins p i t e of t h e i n i t i a l gas te mp era tur e of th e oxygen ru n of 1750K, compared t o 1400Kf o r t h e a i r ca se . S imi l a r r e s u l t s have b een o bs e rv ed in t h e l a b o ra t o ry f o r t h eoxygen g a s if i e r . The reas on f o r t h i s phenomenon is t h e b u f fe r in g e f f ec t of t h ee nd ot he rm ic g a s i f i c a t i o n r e a c t i o n s which i n c r e a s e t h e i r r a t e s a t h i g he rtempera tu res , thus conver t ing g re a t er amounts of se ns ib le hea t to chemicalenergy. The end r e s u l t then i s not higher tempera tu res in t h e r e a c t o r but h i g h e rconversio n of th e char in t h e g a s i f i c a t i o n zone.

CONCLUSIONSA pyrol ysis model has been developed which yiel ds gas temperature andcomposi t ion for both a i r and oxygen ga si f ic at io n. The re s ul ts of th i s modela r e t he n in p u t i n t o a s e p a r a t e char g as i f i ca t i o n mo d e l .A ch a r g as i f i ca t i o n mo d e l has been developed which ut i l izes no a d j u s t a b l ep ar ame te rs t o p r ed i c t d es ig n p a rame te rs an d g a s i f i e r p r o ces s co n d i to n s .The two models y i e l d r e a l i s t i c t emp er a tu re p r o f i l e s of t h e s o l i d and g asphases down the reactor.The CO/CO2 r a t i o pr e di c te d a g r e e w e l l with labora to ry data.'Ihe model shows a n e ss e nt ia l ly linear co r r e l a t io n between th roughput and charconver s ion fo r a g iven r ea c t o r length .The model p red ic t s equ iva len t conver s ions of cha r fo r var iou s sur f ace t ovolume ra t io s of t he same feed stoc k.The model demons t ra tes th e buf fe r in g ef fe c t of the endothermic ga s i f i ca t i onr eac t i o n s in keeping down the char temerpature in a n oxygen g a s i f i e r .

P r e d i c t i o n s compare very well with laboratory data.

RECOMMENDATIONS 'A model shou ld be incorpora ted t o g ive the r es i dence t ime and in te gr a laverage tempera tu re of the char a f t e r pyro lys i s . Th is ad d i t i on would a l lowd e te rmin a tio n s of t h e ap p r o p r i a t e r eac t o r l en g th f o r complet e g a s i f i c a t i o n ,a n d remove t h e e s t i m t i o n of i n i t i a l p a r t i c l e te mp er at ur e in t h e charg a s i f i c a t i o n zone.

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( 2 ) Experimental data s ho l d be t a ke n w i th va r i ous g a s i f i e r c ond i t i ons a nd de s i gnsto check mode1 predic t ions and assumptions .A(XNOWLEDGEI&NTS

Support f o r th i s p ro jec t f rom th e Off i ce of Alcohol fue l s of th e United S ta t esDepartment of Energy is grateful ly acknowledged. Rev i ew of t h is paper by J.Diebold and T. M i l n e is s i nc e r e l y a pp r e c i a t e d .

REFERENCESReed , T.B., M. Markson, A P r e d i c t i v e Yssdel f o r S t r a t i f i e d DowndraftGa s i f ic a t i on of Biomass , Proceedings of t he 14th B i o m s s Thermochemicalconvers ion con t rac tor s Mee ting . At l an t a , GA, 1982.Walawender, Wa lt er P., S. M. Chern and L. T. Fan, Wood C h i p G a s i f i c a t i o n ina Commercia1 Downdraft Ga s i f i e r , P resented a t t he In te rn a t io na l Confe renceon Fundamentals of Thermochemical Biomass Conversion. Estes Park, CO , 18-22October 1982.Edrich, R., T. Bradley , and M. S . Graboski , The Gas i f i c a t io n Kine t i c s ofPonderosa P ine Charcoa l , P resented a t th e In te rn a t io na l Conference onFund amen tals of Thermochemical Biomass Convers ion. Es te s Park, CO, 18-22October 1982.S a t t e r f i e l d , C. N. &ss T r a n s f e r in Heterogeneous Cata lys is . MA: MITPress, 1970.

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AIR/OXYGEN BIOMASS

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A Mathematical Model for Stratified Downdraft Gasifiers

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