high-temperature gasification kinetics of biomass pyrolysis

Jourr.=l o f Arw~.-tioal and Applied Pyro/y.~. 3 (19S1) 161--171 161 ]~.sevier Mentif ie Publishing Company, ~ s t e M a m -- M M i n The .~er.he_,qands

HIGH-TE.~,H~ERATITRE G A S I F I C A T I O N KTN'ETICS O F BIO_~L~.gS P Y R O L Y S I S

G E O R G E SLM.MONS * and bEARIO SA.~CHEZ *

Dcp=rtmer.t o f C h e m ~ l Engineer:.-.g. ~.'nh'¢r~f.~- o f ldch.o..%!o~o,--. ID $35-J3 fL'.S_-I.)

(Received February 14th. 19~0, accepted June ~nd. 1951)

SL~. LALARY

The rate of gas formation from wood pyrolysis has been exper imen~ 'y me~r.~red at temperatures from 300=C to 1000©C. The fo.,Tnztion raze of specific product gases has been measured rather than the rate of solid weight loss. Even for Tery FLue l~-~cles, r.he rate becomes heat trznsfer limited a= high tempera:urea. The p.-oduct gases a~o approach thermodynamic equ,'librium rapidly at high tempe.-'a ~uures. The - ' ~ s are c o . ~ ,-'~;~-g the experimental re~dence time distribut;.on.

L~'TRODUCTION

_Mo~ o f t h e pre~ious w o r k o n p.vrolysis k ine t i c s has b e e n based o n mea- s u r e m e n t s o f w e i g h t loss versus t i m e . S ince pyro lys i s o f ce l lu lose mater ia l s is "known to p r o c e e d t h r o u g h a c o m p l e x series o f consecu t ive a n d c o m p e ~ u g reac t ions [ 1 ] , such -kinetic d e t e r m i n a t i o n s based o n we igh t loss m a y o r m a y n o t accu ra t e ly corre]_-te t h e k ine t ics o f gas f o r m a t i o n . I t is t h e -kinetic raze o f gaseous evo lu t ion t h a t b ~ b e e n s t ud i ed in this w o r k .

S ince t h e f o r m a t i o n o f _~mple gaseous fuels (o r gas f o r s y n r . h ~ ) is a sub- j e c t o f i n c r ~ n g in te res t , w e have u n d e r t a k e n a s n l d y t o d e t e r s ; h e a t w h i c h ra te these l ight gaseous c o m p o n e n t s a re f o r m e d . T h e expex imen ta l analys is suggests t h a t h e a t t r ans fe r b e c o m e s r a t e con t ro l l i ng a t h ighe r t e m p e r a t u r e s , a n d t h a t s e c o n d a r y r eac t ions also t a k e p lace a t h igh t e m p e r a t u r e s ~o b r ing t h e reac t ing gaseous m i x t u r e t o t h e ~ , , , o d y ~ r ~ c equ i l ib r ium. T h e ma te r i a l u sed t h r o u g h o u t th is w o r k was w o o d f lou r (Douglas Fir) o b t a i n e d f r o m t h e Menasha C o r p o r a t i o n . W o o d f lou r was se lec ted fo r i ts ve ry ,fine p a m ~ e ~=e w h i c h w o u l d t e n d to m ~ n i m ~ e h e a t t r ans fe r p rob lems .

F _ ' ~ P E R L M E N T A L D E S I G N

A d iagram o f t h e p3Tolysis r e a c t o r is s h o ~ in Fig. 1. In o r d e r t o k e e p air o u t o f t h e sys t em, t h e s ta r t ing mate r i a l is a d d e d t o T.he heaT.L~d z o n e b y a d o u b l e se t o f vah-es. O n c e t h e mate r i a l t o be p y r o l ~ e d is c o n ~ ; n e d b e t w e e n

t Present address: Escuela Politechnica .~acional, ~.~ico, Ec'.~dor.

0165-2370.s81[0000---0000:$02.50 ~ 19S1 ~.se~er Sc~enLific Publi~ing Co--,;x~uy

1 6 2

~ , " sarn,?, !e i , -det

to h e ~ ' n , ~ . . ~ , to vacuum

l o c a t i o n o f ¢ ~ : e h e a t e d f L ' m m e s h s c r e e n - . --=-- ~ - ' - - h e S u m

: : s w e e p g a s heated zone ~- .:

valve . . - ; e x h a u s t - - - - ~ . . . . . . . . - - c o ! d t r a p

t o c h r o m ~ t o ; r a p h

Fig. I . Exper imenta l pyrolysis .~eacto:.

t he ~ l v e s , t he feed c h a m b e r is e~acuated a n d f'dled w i th he l i um to Isiightl.v above the reac to r pressure. T h e feed ~'alve is t h e n ac tua t ed and t h e w o o d f lour fails by gravi ty in to the r eac t ion zone . To ensure t h a t t h e w o o d f lour remains in the chamber , and decomposes at t he same pos i t ion in t h e reac tor , a f'me mesh screen (425 mesh) is p laced nea r t h e t o p o f t h e reac tor . Wi thou t the screen, w o o d par t ic les were observed to fall t h r o u g h t h e r eac to r Lmme- d ia te ly . T h e screen thus become= a suppo r t fo r t h e reac t ion bed and decom- p o r t i o n takes place a t t h a t loca t ion .

A he l ium sweep gas f lows c o n t i n u o u s l y t h r o u g h t h e r eac to r to r emove the gaseous produc ts . This m e t h o d o f charg ing t h e r eac to r a l lows fo r rapid hea t up o f t h e w o o d f lour and min imizes t h e associa ted p rob lems o f d i f ferent !a t - Lug be tween gas f o z ~ a t i o n k ine t ics and t h e h e a t t r ans fe r rote .

Sample sizes f rom 20 to 200 mg were used du r ing t h e co'.u~v o f t h e expe~- men t s , a n d a he l ium sweep ra te o f 770 cm3/min ( room t e m p e r a t u r e a n d pres- sure) was used for all runs . T h e gases ex i t ing t h e r eac to r were periodica~-'y sampled and ana lyzed us ing s t anda rd gas chro_m-tography. Since t a r f o L i a - t ion is s igni f icant a t t h e lower pyro lys i s t empera tu res , a d r y ice t r ap ~ used to condense th is f r ac t ion ou t . C o n s e q u e n t l y , wa te r ~mpor was also removed ; t he gas a.nalyzed thus c o ~ o f a mLxture o f h y d r o g e n , c a rbon m o n o x i d e , carbon d iox ide , and m e t h a n e . The measu remen t s for h y d r o g e n are less dependab le s ince he l ium was also used as t he ~ r r i e r gas and t h e de t ec to r response is l inear on ly a t ve ry small concen t r a t i ons [2 ] .

RF.SL'LTS

Pyrolys i s da ta were ob t a ined over a t e m p e r a t u r e range f r o m a b o u t 300~C to 1000"C. Fig. 2 represents t h e da ta ob t a ined a t 490~C and is t y p i c a / o f t . h e resul ts a t o the r t empera tu res . Fig. 2 shows a p lo t o f gas c o n c e n t r a t i o n as a func t i on o f t ime , w i th zero Hrne co r respond ing t o t h e t i m e t h e w o o d f lour is added to t h e reac tor . T h e ~1~-t observed gaseous p r o d u c t s occu r a t a ~ n e near ly 0.5 rain late_~. This de lay is d u e to t he res idence t i m e o f t h e gases L-~

163

Concentrat ion x104 ~-

co2 4g

~CO --

(g-moles/liter) - / =_

: -..-. - . :

T' -me (mLn)

Fig. 2. Concenxration ~-ersus zime at 490=C.

t h e r eac to r . O.nly f o u r samples w e r e co l l e c t ed fo r each samp!e feed ; Fig. 9 t hus a!s~o i l lustrates t h e r e p e a t a b ~ t T o f t h e d a m , as it is a c o m p o s i t e o f five d i f f e ren t runs .

K i n e t i c tare cons tan t

T h e r a t e coas~ .u t s fo r gas f o r m a t i o n can be dete~n~.necl f r o m t h e c o n c e n - t r a t i on versus t i m e e u r v ~ (e.g., Fig. 2) . This a n ~ . v . ~ ~ c o m p ~ c a t e d s ince -,he r e a c t o r r e s idence t i m e d i s t r i bu t ion is n o t p!ug f low, z n d ~_.u_~ be t a k e n L~.to a c c o u n t . H o w e v e r , t o ver i fy t ha t "kinetic ra t r~ co-~Id be detezmi_ned b y r:q!s m e t h o d , a n d t o lay t h e g r o u n d w o r k fo r a m o r e d e ~ e d z_~.~ys~_s, it was - ~ , m e d t h a t p lug f low d id exi.~ a n d t h e r a t e consw.-~s w e r e ca l cu lz t ed as fo l lows.

A f i _ ~ o n t e r express ion (charac terL~:c fo r s o l d d e c o m p o ~ t i o n ) is ?~ .~nec l o n t h e mass (m) o f t h e w o o d f lour .

d m - - d---t- = k m ( I )

T h e r a t e c o n s t a n t , k , is t o be d e t e r m i n e d b y a.n~lyzing t h e f o r m a t i o n x~te o f t h e gas, a n d ~-ill t hus be r~]nted to t h e ra te a t w h i c h gas is f o r m e d f r o m ~n.e p.x~rolyzed wood . Application o f a stoichiome~_c factor r - l ~ n g the w o o d d e c o m p o s i t i o n t o t h e q u a n t i t y o f gas f o r m e d resul ts in t h e r a t e c o n s t a n t fo r a specif ic gas. S ince each gas c o n c e n t r a t i o n is -used t o i n d e p e n d e n ~ y d e ~ r - m i n e t h e ra te c o n s t a n t , k , t h e r e is n o r e q u i r e m e n t t h a t th is co.ns÷.ant xs-:ll b e t h e s ame fo r each gas.

Con t inu ing , t h e ~ m o u n t o f gas fo~,,~ed a t a n y t i m e is relzc.ed t o t h e w o o d r eac t ed b y de f in ing a s to ich iome~r ic c o n ~ . n t , a.

e

C F d t = a ( m = - - m ) ( 2 )

w h e r e C = c o n c e n t r a t i o n o f gas p r o d u c e d , F = sweep gas f l ow r~r~, .~.,~ = orig- inal w o o d mass , a n d a = s t o i c h i o m e t r i c ra t io .

I f a is ~ a m e d c o n s t a n t a t a n y 1~emperabxre, t h e n ecln. 2 c.z.n b e differe.n-

1 6 4

C x l 0 4 (g-moles)

2-- J

i

sI0pe "- - k o~ / . , 1 ~

o ~'tcdt =' g-moles~ • liter •-rrfi,-t

g a

. ; f o r c a r b o n m o n o x i d e a t 4 9 0 = C . F i g . 3 . G r a p h i c a l d e t e r m i n a ' . i o n o f r a t e c o n s a .n t

t i a ted t o yie ld

a d m = CF ( 3 )

dt

Subs t i t u t ing eqn. 1 in to eqn. 3:

C F = c .~m (4)

and combin ing eqn. 2 w i th eqn. 4

t C - ahmOF h ./ C d t (5)

O

A plo t o f C v e n u s f C d t shou ld resul*, in a l ine ,~ith a s lope equal to - -k . Fig. 3 is such a p lo t made f rom Fig. 2 fo r ca rbon m o n o x i d e . S ince t h e feed w,l_.] requi re a f in i te t i m e to reach t h e r eac to r t empe ra tu r e , t h e s lope to determLne the ra te co.nsta.nt is c o n s u ~ c t e d us ing on ly t h e l a t t e r pa r t o f t h e cu~-e. By th is m e t h o d , apparen t ra te cons tan t s for t h e f o r m a t i o n o f ca rbon m o n o x i d e were de t e rmined over t h e full t e m p e r a t u r e range and are sho~r , on an Ar- rhen ius p lo t in Fig. 4 . T h e resu l t ing ac t iva t ion energy by th i s m e t h o d is 17.5 kca] (73.2 kJ) .

A t a p p r o x i m a t e l y 500=C, t h e Ar rhen ius p lo t bends over ind ica t ing t h a t some o the r mechan i sm is cont ro l l ing . T w o possible exp lana t ions would be hea t t ransfer l imi t a t ion and secondazy gaseous reac t ions . To e x a m i n e t h e / a t -

In k

- i " 2_. __ .

1/T x l 0 3 ( ° K - l )

F i g . 4 . A_ , ' r hen iu s p l o t " ." z o c ~ r b o n m o n o x i d e r a t e c o n s t a n t , u n c o . - T e e t e ~ f o r r e . ~ d e n c e t . ; ~ e

d ; . s ~ r i b u t i o n .

1 6 5

t e r possibility-, t h e r a t e cons t an t s fo r " t o t a l gas" (H: , CO, C O : , CH~) were also ca lcu la ted ; t h e resu l t ing Axrhenius p lo t curt-ed over a t 5 0 0 : C , as i t d id for CO, ind ica t ing t h a t t.he d e p l e t i o n o f CO b y s e c o n d ~ - r e a c t i o n s was n o t t he exp l ana t i on [ 3 ] . F u r t h e r m o r e , an e x a m i n a t i o n o f t h e equ i l ib r ium com- pos i t ion o f t h e gaseous m i x t u r e ind ica tes t h a t CO is favored a¢ h igh t emper - a tures . I t is t he r e fo re conc luded t h a t t h e overall r a t e o f gas f o r r n - t i o n becomes h e a t t r ans fe r l imi ted a t t empe ra tu r e s above 500=C.

Correction for residence t ime distribution

T h e res idence t i m e d i s t r i bu t ion (RTD) ~ s exper imer . t~ l ly d e t e r m i n e d over t h e en t i re t e m p e r a t u r e range . .At such h i ~ t empera tu res , and us ing heli- u m as t h e sweep gas, t h e resu l t ing R e y n o l d s n u m b e r is on t h e o rde r o f one ; some tTpe o f l amina r R T D was the re fo re ind ica ted . T h e p h e n o m e n a rela;r~g t h e c o n c e n t r a t i o n a n d l amina r ve loc i ty prol~Jes were or iginaUy descr ibed b y S h e r w o o d [4] and have been discussed in subsequen t papers [5 ] . A R T D fo r ]-rninax f low such as descr ibed b y Smi th [6] is a :~np l i f ; ed vers ion neglect .r ig b o t h radial and axia l d i f fus ion . We chose t o use o u r e x . ~ m e n t z l l y de.=er- m ined R T D (Fig. 5) t o co r rec t t h e c o n c e n t r a t i o n p ro~!e . S ince t h e measured c o n c e n t r a t i o n prof i le occur red a t t h e r eac to r ex i t , t h e t a sk vrzs t o d e t e r m i n e t h e c o n c e n t r a t i o n versus t i m e prof'fle a t t h e screen (where t h e p.vxolysis t o o k place) which would re~/t in the measured p.~oi'~le a¢ the exit. It would seem to be a -~mple m a t t e r t o descr ibe t h e proce~_~ in FLmte d i f fe rence fo rm and to calc~,l~te t h e Lnitial c o n c e n t r a t i o n proF_de poLnt b y po in t ; howe~-er, tbAs aop roach ~ x s n o t successful . I f a n y pa r t o f t h e ~n;t~:~l p.-oF~le is m e_,cor, t h e n t h e r ema inde r o f t h e proEde m u s t ad jus t Ln an a t t e m p t t o f i t t h e en t i re curve. Since n o n e o f t h e expe r imen ta ! m e a s ~ e m e n t s t a k e n can be con .dde . r~ exac t , and s ince a fi--~te d~ffexence app roach is a lways zn app.-o=i.~.zdon, t h e m e t h o d is Lnherent ly uns tab le .

Rc~| iz ing t h a t t h e in i tml pro~_fie, once ob ta ined , ~ ; g h t . be used t o de ter - mine a first-order rate co.nstant, a s6mple model wzs developed d~bing ~ t - o r d e r d e c o m p o s i t i o n a n d t h e variable par~mer~-s were ad jus ted to gh-e t h e bes t f i t . The paramete rs w e r e d e t e z - , i n e d using a parte.,~ search program (PATS) [ 7 ] . The re were fou r pa ramete r s over wb_;.ch t h e search ~ s m a d e fo r each c o n c e n t r a t i o n prot-fie: t h e ac~hm~on energy-, E; -..he p re -exponent ia !

R T D

- I . ."

sooOc f

9 0 0 ° C "

. " . - : . 7 . ".~. - = . ¢

Tm~e (m.;.-0

Fig. 5. Experimen~J reside,,ce t ime dis:ribu~.ion at se'.ec:ed ce=pe.--z:u.-es.

166

factor , A; t h e s to ich iomet r i c ra t io , a; and a t i m e cons tan t , ~', govern ing t h e ra te o f t e m p e r a t u r e rise o f t h e ~ m p l e u p o n in jec t ion . B o t h t h e p re -exponen- ti~! fac tor and t h e ac t iva t ion energy were a l lowed to f loa t to achieve t h e best f i t ; however , t h e final actl~'ation energy was d e t e r m i n e d by m a k i n g an Ar- rhen ius p lo t .

T h e c o n c e n t r a t i o n in t h e pyrolysis z o n e is f irst ca lcula ted and t h e n t h e p red ic ted ou t l e t c o n c e n t r a t i o n is calc~!~ted by app ly ing t h e experimen+J~l RTD and c o m p a r e d by least squares t o t h e expe r imen ta l measu remen t s .

In tegra t ing eqn . 1 wi th k be ing r ep re sen ted by a ~ m p l e Arrhen ius e x p ~ s - s ion results in

~- drrt J m i A e -E'Rzx:) d t (6) --'=0 0

where t h e t e m p e r a t u r e is a ssumed to approach t h e r eac to r t empe ra tu r e , T., accord ing t o a first o rde r express ion:

7" = To + ( T ~ - T o ) ( 1 - - e - ' ~ - ) ( 7 )

Th e ILrst-order t i m e cons tan t , ~', is o n e o f t h e adjus table parameters . Numer i - ca] in tegra t ion o f t h e r ight s ide o f eqn . 6 results in

~ : = n o exp - - A ~ e -E-Rr~'k) At (S) .~:ffil

T h e mass m~ cor responds t o t h e in teger ~ a t w h i c h t h e t~me is equal t o _N~ - At. T h e concen t r a t i on , C~, is readi ly de t e~ - , i ned f r o m eqns . 4 and 8, and t h e es t imated ou t l e t c o n c e n t r a t i o n protrde is ca lcula ted by app ly ing t he experi- men ta l RTD:

m J

C o . = t ( t ) = i 0

o r

C~(O) ~r(t - - e ) d e

u ¢

Co.-..~ = ~ C, (O~) J'(t~ - - ® , ) A O (9)

whexe J" is t h e tLme deri~-ative o f t h e res idence t i m e d i s t r ibu t ion and Ck is t h e inlet concenu-a t ion at t h e discre te t ime , e k .

Figs. 6 and 7 axe e ~ m p l e s o f t h e fit ob t a ined by t.his m e t h o d . These figu.-~s P..lso show th e init ial c o n c e n t r a t i o n tha t results in t h e p red ic t ed o u t l e t con- cen t ra t ion by appl.~dng t h e expe.-~nental RTD.

Table 1 lists all o f t h e co r rec t ed ra te cons tan t s for each e.cperimentaI con- cen t ra t ion prof i le . S o m e of t h e expe r imen ta l runs were rn~de analyTing on ly fo r carbon m o n o x i d e , and s o m e o f t h e h y d r o g e n concen t r a t i ons were t o o high to mea>mre by our ch roma tog raph ic techr~que .

Fig. 8 is an Ar rhen ins p lot o f t h e ca rbon d i o x i d e and ca rbon m o n o x i d e ra te cons tan t s l is ted in Table 1. T h e curve still bends over a t app rox ima te ly 500~C indica t ing t h a t a t h igher t empera tu re s hea t t ransfer b e c o m e s IirniH,~g.

! 6 7

C o n c e ~ . r a t i o n

x l O 4

( g - m o l e s / l i t e r )

" , s c r e e n c o n c e n t r a t i o n

" o u t p u t c o n c e n t r a t i o n

- .,. . ~-'.--

: : - - "., .- -.. • .J \

• ~

° = " ~ . ,L'."

T'mm (n,~0

Fig . 6 . In i t i a l c a r b o n z--_onoxide e o n c e - t r a t ; o n a n d ¢90:C.

p . , - e d ; . c ~ o u t p u t = n : n : C O L 'e ~I"~0

s c r e e n c o n c e n t r a t i o n

C o n c e n t r a t i o n

x l 0 4 ; : -

( 9 - m o l e s / l i t e r ) ~ -_ : : - .

l- tree (rr~rO

Fig. 7 Init:---.l c ' ~ b o n m o n o x i d e 9 0 0 " C .

o u t p u t c o n c e n t r a t i o n £

¢ O ' ~ L~'n.. 'e 'J. 1On ~'~d p.,-edic:e,=_" ou~pu : co-,ee.. - =:._::~ ,on. a:

T A B L E 1

R a t e co.,'.sta.,-.-.s f o r w o o d decomposi~; .on

Reaczor Sa.-np!e Ra te co.,,.sL~.ts (~.~.,-1) t emper - we igh t , a ture , ° C mg CO C O - H : CH,;

293 2 0 0 0 .30 0 .659 .,,.o ~ . de:ec~ed 344 2 0 0 0 .54 0 .53 ,-o: de:co;e,= 3S5 2 0 0 1 .18 1.35 ----o; de'ec~e~! 4 4 3 * 2 0 0 2 .~6 - - 4 9 0 2 0 0 " . 1 7 4 .0T 2._~9 537 '* 2 0 0 5 .06 - - 5 9 5 2 0 0 4.2S 4 . 1 2 3 .03 696 100 5 .90 4 .91 3 . . 5 6 9 6 '* 2 0 0 4 .60 - -

. 9 3 100 6.21 6 .11 r.oz'-~inezz respor, se 9 0 0 60 5 .59 7 .74 5 .99

1015 2 0 ~'.6S n o t detec=ed 6 .54 1015 4 0 T . !O n o t (!e-ec~ed no',1;.."-e~ res;>o~.~se 1015 * 60 5 .69 - -

n o : de;ec:e~. - ' o : de:e,.'~ed n o : de :ec :ed

2 .03

3 . $ I ; 65

5 .33 "~.31 L".s'- f~, ~ .e " : . '-e~or, se 10 .55

, O~y CO d=,T,a z~-o.--ded.

168

In k CO " • CO 2 ~',,.

1 / T x l O 3 ( ° K - l )

Fig. S. Arrhenius plot for w o o d decompo~t io '~ bar~i o n carbon m o n o x i d e and car!~,~ dioxide.

in k

: - - --~ - ~ C H H 2 ~ . ~ _ . .

" = . . . ' - -'- ". - - * " 5 1/T x lO 3 ( ° K - 1 )

Fig. 9 . Arrhenius plot for w o o d decompor~l ion based o n hydrogen and me*./~ane.

I

Mole Fract ion

- s. C O 2 -

= . t -

" 2 "

Temperature CJC)

c o • ~ I

Fig. I0 . Expe~menta l mole fraction o f gas produced (water-free hasL$).

c E -

Mole Fract ion .: ~.

=.2-

C 0 2 ...

"3_, 2 ~ : ¢.'C" -r3C" __ e,:@ "- :J:~

Tem~era tw e (u~)

Fig. 11. Predicted equi l ibr ium compos~.tion o f gas produced (water- free bar~).

169

T h e ac t iva t ion energy is n o w seen t o be 11.5 kcal (48.1 kJ ) w h i c h is a signif- icant change resul t ing f r o m t h e RTD cor rec t ion .

T h e ra te cons tan t s fo r m e t h a n e and h y d r o g e n are p l o t t e d in Fig. 9. L'n- fo r tuna te ly , t h e gas q u a n t i t y p r o d u c e d a t t empe ra tu r e s b e l o w 500°C was t o o low for an e s t ima te t o be m a d e o f t h e h y d r o g e n and m e t h a n e ra te con - s tants in t h e k ine t i c con t ro l l ed reg ime. I t is also e x p e c t e d t h a t seconda~- reac t ions are t~k-ing place a t h igher t empera tu res , so t h a t as a m e a s u r e m e n t o f t h e ra te o f gas p r o d u c t i o n , these da t a po in t s above 500"C are somewlm~. meaningless .

By in tegra t ing t h e c o n c e n t r a t i o n versus t i m e cum-e (e.g., Fig. 2) and mu~ti- p ly ing by the h e l i u m f low-rate o f 770 cm3/min , ~he total quan t i~ - Of a spe- cific gas e v o h ~ d can be d e t ~ , , , i n e d . Fig. 10 shows t h e resul t ing m o l e frac- t ion (water=free basis) o f t h e gas f o r m e d in ou r expeffanents , and Fig. 11 a h o ~ this same i n f o r m a t i o n as p red ic t ed by t h e equ i l ib r ium calc111~tions us ing cel lulose as t h e s tar t ing mater ia l . The NASA--Lewis progr~,~ CEC 72 [8] ~ s used t o m a k e these ca lc- l~t ions . I t is clearly seen tha t , even at these sm,1] gas concen t r a t i ons , t h e reac t ion gas m i x t u r e approaches equ i l ib r ium a t h igher t empera tu res .

Quanti~. o f gas produced

T h e in tegra t ion o f t he expe r imen ta l c o n c e n t r a t i o n , -e_~s t i m e curves also gave us t h e s to ich iomet r i c ra t ios fo r each e x ~ e n t a l run . ~-aese values, a long wi th tho.:e d e t e r m i n e d by t h e numer ica l curve fi~;_ug t e c i m i q u e are l is ted in Tabie 2. T h e close a g r e e m e n t b e t w e e n t h e expe r imen ta ! , ~ u e s and t he values ob t a ined by t h e cum'e fit indicate_s s o m e w h a + . surprisin~.v, t h a t a s imple f i rs t-order m o d e l can be used t o cor re la te t h e dam.

TABLE 2

Szo',chiomet.~c rat;.os-

Reactor San'-p!e temperature, weight. =C

Ratio ( r a g m o l e - g ) inz~zed . . ' cu r re f i :

C O C O : H: CH~

2 9 3 2 0 0 0 . 6 S ~ 0 . 5 1 0 . . 5 - 0 . T 5 - - - - 3 4 4 2 0 0 0.92~" 0 . 9 5 o 0 S . ' 2 . 0 S - - - - 3 5 5 2 0 0 1.45~' 1 . 5 5 2 . 0 5 . " 2 . 0 0 - - - - 4 4 3 2 0 0 2 _ 4 7 ! 2 . 6 6 - - - - - - 4 9 0 2 0 0 3 . 3 5 ! 3 . 2 2 3 . 5 0 ~ ' 3 . 2 7 0 . 5 1 : : 0 . 5 4 0 . T S . ' 0 . $ 3 5 3 7 2 0 0 4 ~ 7 t 4 . 2 9 - - - - - - 5 9 5 2 0 0 6 - 9 2 i 6 . 2 2 3 . 6 0 f 3 . 4 4 0 - -50 ] 2 . 3 2 ; . 9 0 - ! . . 3 6 9 6 1 0 0 1 3 . 2 9 : 1 0 9 4 3 . 5 S ; ' 3 . $ 6 7 . 3 0 . ' 6 . 9 0 3 . 5 S - 4 . 1 2 6 9 6 2 0 0 1 0 . 8 7 / 1 0 . 9 5 - - - - - - 7 9 3 1 0 0 1 7 . 3 8 . e 1 6 . 0 0 4 . 2 3 - 4 . 2 0 - - 4 . 1 T - 4 . 2 2 9 0 0 6 0 2 5 . 3 8 / 2 3 . 1 6 1 .4921 .4 -¢ 1 6 . 1 0 ~ 1 5 . 3 ~ 4 . ! 0 - 3 . 9 9

101,5 2 0 1 8 . 9 0 / 1 3 . 5 7 - - 19.S0.'18.15 1 0 1 5 4 0 2 4 - 5 2 . r 2 1 - 6 6 - - - - 2 . 9 0 : ' 3 . 2 0 1 0 1 5 6 0 2 3 . 4 3 i 2 0 . 3 2 - - - - - -

170

DISCUSSIO~

T h e a s sumpt ion o f c o n s t a n t s to i ch iomet r i c r a t io a du r ing t h e course o f a n y given e x p e r i m e n t shou ld be f u r t he r examined . Wha t we ac tua l ly have is an average s toichiome~ric ra t io over t h e en t i re course o f t h e r eac t ion . Recen t d a m in o u r laboratory- a n d f rom o t h e r invest igators ind ica te t h a t more thm~ one ra te cons t an t is involved in w o o d pyroly~.s . I t is n o t y e t c lear to wb_at e x t e n t these d i f fe ren t r a t e cons t an t s are due to k ine t i c changes in t he mecha- ni.~rn or are due to changes in t h e ra tes o f t he tm_nsp, o r t processes.

Fo r engineer ing use , t h e s imple fu-st-order d e c o m p o s i t i o n mode l m a y be adequa te . More compl i ca t ed k ine t i c mode l s wi th va ry ing ac t iva t ion e n e . ~ - and mul t ip le Idnet ic pa ths have been suggested b u t m a y n o t be requ i red con- side_ring the accuracy and precis ion o f t h e avai lable da ta . Solid decompos i - t i on expe r imen t s involve m a n y basic t r a n s p o r t p h e n o m e n a in add i t i on to k inet ics ; i t also appears t h a t neglec t ing t h e res idence t i m e d i s t r i bu t ion o f t h e exper imen ta l appara tus can resul t in s igni f icant errors in da ta cor re la t ion .

Bradbury e t al. [9 ] r epor t t h a t char f o r m a t i o n increases w h e n the reac t ion is ope ra ted at r educed pressures. This p h e n o m e n o n is ascr ibed to t h e result ir .g inh ib i t ion o f secondary- reac t ions b y lower ing t h e c o n c e n t r a t i o n a n d res idence t ime for these species. One i n t e rp re t a t i on o f th i s p h e n o m e n o n is t h a t t h e reac t ion ra te becomes h e a t t r ans fe r l imi ted a t r educed pressures s ince t h e p.~imar~- m o d e of hea t t r ans fe r a t lower t e r m p e r a m r e s is c o n d u c t i o n and.."or forced convec t ion . T o ou r op in ion , it wou ld be more l ike ly t h a t a t rue k ine t i c ra te cons t an t cou ld be d e t e r m i n e d at a tmosphe r i c pre~__~lre r a t h e r t h a n a t ~ c u u m or r educed pressure cond i t ions .

T h e ra te cons t an t s ca lcu la ted he re were all based o n f i rs t -order soEd decompos i t i on . T h e ra te c o n s t a n t p red ic ted d id n o t , 'a~" b y large m o u n t s us ing one g'~s curve or t h e n e x t (Table 1) ind ica t ing t h a t t h e p r o d u c t i o n of these gaseous species are re la ted b y a c o m m o n , p r io r s tep , in t h e overall mechan i sm o f solid decompos i t i on . Pre~-ious f lu idized bed w o r k b y b o t h Maa [10] and Barooah and Long [11] ob t a ined ra te cons t an t s cons i s t en t w i t h ou r expe r imen ta l resul ts , whi le invest igators r e ly ing on thermogra~-Jxnet~_c analys is t echn iques [12 ,13 ] have genera l ly d e t e r m i n e d ra t e c o ~ t s some- wha t higher . I t migh t be expec t ed t ha t t he s lower ra te measured in o u r work is o f a subsequen t s tep to t h e weight loss s tep and is more indica t ive o f t h e ra te cont roUing s tep for gas fo rma t ion . S ince t h e f lu id ized bed w o r k also measured weight loss and agrees ~-ith o u r da ta , th i s exp lana t ion is un l ike ly . Invest igators work ing w i t h kerogen (oil shale) d e c o m p o s i t i o n have also sug- gested t h a t t a r f o r m a t i o n m a y exh ib i t an autocatal~-i~c e f fec t on f~ r t he r decompos i t i on , p roduc ing an overall h igher ra te [ 1 4 ] . Barooah a n d L o n g [11] also observed an appa r en t second-order r eac t ion in t h e l a t t e r stages o f decompos i t i on , which is cons i s t en t w i th some secondarj- e f fec t o f t a r forma- t ion . The various expe r imen t s are a p p a r e n t l y n o t c o n d u c t e d u n d e r s imi lar cond i t ions , and these discrepancies need fu r t he r e x a m i n a t i o n .

O n e of t h e p r imary in teres ts in b iomass p~-rolysls is t h e p r o d u c t i o n o f fuel gas or syn thes i s gas. I t wou ld appea r t h a t t h e expe r imen ta l resul t s ob t a ined b y t h e p resen t w o r k can be used to m a k e re~_ _~o_nable p red ic t ions fo r gas for- m a t i o n us ing var ious r eac to r concep t s , especial ly f lu id ized beds. Also a t t em- pera tures h igher t h a n a b o u t 800 ~ C, t he gas c o m p o s i t i o n is well p red ic ted b y

I T 1

thermod)~-mic equilibria criteria. In our apparatus, and ~ o ' ~ wood flour rn~t~rial, it appears that heat franker becomes rate contro!!ing above 500°C.

REFERE~'~CES

I F. S~/ ' szadeh, Advan. Carbohyd. Chem., 0-3 (196S) 419. 2 R . , '-d/alobos and G.R. _~uss. ISA Trans.. 4(3) (1965) 2S1. 3 -~|. ' | . Sanchez, Experimental S t u d y on the Kinetics o f Wood Pro~uc'.s. M_S. Thesis.

L'nH-er~ty o f Idaho, Mor~-'ow, ID, 19";8. 4 T.K. Sherwood, R.L. Pigf.ord and C.R. ~-tlke, ~ Trz_n~rer, McGntw-H~.!, .~e~r ~-'ork,

19~'5 . p . 8 1 . 5 V. A n a n L b ~ a l s h n a n , W_'~. G ~ and A~J. Bz~u.hn , AIC~E J. , 11 ( !965 ) 1063. 6 J.M. SmiTh. Chemical Enginee.'~ng Kinetics, 2~d ed., M c G . ~ w - ~ , Yew York. 19T0.

p. 249. • "; R. Hooke and T--~. Jee~-es, J . Ass. Comp. Mzch, S (1961) 212.

S CEC T2, NASA--Lew/s Resea.-ch Center, C1evel~d, OH. 9 A.G.W. Bradbury, Y. Sakai and F. Sha-r~adeh, J. App!. Po!.~,n. SoL, 23 (1979) 32.'-1.

10 P. ~ I L . Inf luence o f l~r t ic le S~.e ~..nd F...n~-i.,'o,~,,~e.~,'.al Cond.i--;o=~.~ on ~ , Te-";~e~'-~- ture l~-roly~is o f Cellulor, e Ma:eria!s, Fn.D. D':sr, er~:~.on, We~ ~ , - ~ L'-,~-en~;y. ~Iorgan~own, V[~,', 19";1.

11 .1_~. ~ ~ . d V . D . LonE. Fuel, 55 (19"~6) 116. 12 D.Q. Tt'an and R. C 'han~ i t , Pyro ly t ic G-_sifica;;.on o f B ~ k , A IChE Sy"-p. Set., Fores'.

Products Di,';.sion, 1977. 13 K. Aki~a and M. K z ~ , J. Polym. Sei. P~_~ A - l , 5 (196T) $33. 14 A.K. Mi tyurev, C'ne~.~is~" and Technolo~" o f Corn "~ts-.ib]e ~=~'.es z.=d "1~e]: Pro,~.ue~s,

tr~--~-t~on by Of F~e o f Tee~niea] Services, Was~Lng~n, DC, .~o. OTS 61-11434, 1962.