The Effects of Residence Time, Temperature, and Pressure on the

16

Downloaded by UNIV OF MISSOURI COLUMBIA on November 25, 2013 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch016

The Effects of Residence Time, Temperature, and Pressure on the Steam Gasification of Biomass MICHAEL J. A N T A L , JR. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08540

The underlying science of thermochemical conversion of biomass materials to useful gaseous fuels is poorly understood. Recent experimental research in the U.S.A. (1) and Sweden (2) has offered new and important insights into the gasification process. The two research teams independently conclude that biomass gasification occurs in three steps: 1) pyrolysis. producing volatile matter and char; 2) secondary reactions of the evolved volatile matter in the gas phase; and 3) char gasification. Detailed understanding of the rates and products of these three steps offers important guidance for the improved design of biomass gasifiers. Pyrolysis of biomass materials occurs under normal conditions at relatively low temperatures (300° to 500°C), producing volatile matter and char. Very rapid heating causes pyrolytic weight loss to occur at somewhat higher temperatures. In general, the volatile matter content of cellulosic materials approximates 90% of the dry weight of the initial feedstock. Woody materials contain between 70% and 80% volatile matter, and manures contain 60% volatile matter. However, it is known (3) that cellulosic materials can be completely volatilized when subject to very rapid heating (>10,000°C/sec). Several relatively complete reviews of the mechanisms and kinetics of cellulose pyrolysis are available in the literature (4-7).

0097-6156/81/0144-0313$05.50/0 © 1981 American Chemical Society

In Biomass as a Nonfossil Fuel Source; Klass, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

314

BIOMASS AS A NONFOSSIL F U E L SOURCE

V o l a t i l e m a t t e r p r o d u c e d by pyrolysis of t h e biomass b e g i n s t o p a r t i c i p a t e in secondary, gas-phase reactions at t e m p e r a t u r e s e x c e e d i n g 6 0 0 ° C . These reactions o c c u r very rapidly a n d yield a h y d r o c a r b o n - r i c h syngas p r o d u c t . A s recognized by Diebold (8), these reactions resemble t h e h y d r o c a r b o n c r a c k i n g reactions e m p l o y e d in t h e m a n u f a c t u r e of e t h y l e n e a n d p r o p y l e n e by t h e p e t r o c h e m i c a l i n d u s t r y (9.10). T h e s e c o n d a r y gas-phase reactions d o m i n a t e t h e g a s i f i c a t i o n c h e m i s t r y of biomass. A t still higher t e m p e r a t u r e s ( > 7 0 0 C ) . p y r o l y t i c c h a r reacts w i t h s t e a m t o p r o d u c e h y d r o g e n , c a r b o n m o n o x i d e a n d c a r b o n d i o x i d e . Rates of g a s i f i c a t i o n of biomass-derived chars are k n o w n t o be h i g h e r t h a n coal-derived chars (2); h o w e v e r , m u c h higher t e m p e r a t u r e s are required t o a c h i e v e c h a r g a s i f i c a t i o n t h a n w e r e initially required for t h e pyrolysis reactions. Catalysis of c h a r g a s i f i c a t i o n has been reported (1_1.12) w i t h l i m i t e d success.


e

Research d e s c r i b e d in t h i s paper focuses o n t h e s e c o n d step of t h e g a s i f i c a t i o n process, a n d details t h e effects of t e m p e r a t u r e a n d residence t i m e o n p r o d u c t gas f o r m a t i o n . Cellulose is used as a f e e d s t o c k for p y r o l y t i c volatiles f o r m a t i o n . Earlier papers (13.14) have d i s c u s s e d t h e effect of s t e a m o n cellulose pyrolysis kinetics. T w o recent papers (15.16) p r e s e n t e d early results on pelletized red alder w o o d p y r o l y s i s / g a s i f i c a t i o n in s t e a m . Future papers w i l l discuss results u s i n g o t h e r w o o d y materials, c r o p residues, a n d m a n u r e s (1_7.18). Research t o date i n d i c a t e s t h a t all biomass materials p r o d u c e q u a l i t a t i v e l y similar results in t h e g a s i f i c a t i o n reactor d e s c r i b e d in t h e f o l l o w i n g s e c t i o n of t h i s paper. Effects of pressure o n t h e heat of pyrolysis of cellulose are also d i s c u s s e d as a p r e l u d e t o f u t u r e papers d e t a i l i n g t h e m o r e general effects of pressure on reaction rates a n d p r o d u c t slates. EFFECTS O F T E M P E R A T U R E A N D

RESIDENCE T I M E O N THE SEC-

ONDARY. GAS-PHASE REACTIONS Experimental Procedure For t h e e x p e r i m e n t s d e s c r i b e d b e l o w , d r y W h a t m a n No.1 filter paper stored in a d e s i c c a n t b o t t l e w a s used as f e e d s t o c k material. T h e use of an o v e n t o o b t a i n " b o n e d r y " material w a s f o u n d t o be f u t i l e d u e t o t h e h y g r o s c o p i c n a t u r e of t h e cellulose. T h e cellulose w a s a s s u m e d t o have t h e c h e m i c a l c o m p o s i t i o n 0 . 4 4 4 C. 0.062 H, 0.494 0 o n a mass f r a c t i o n basis, a n d t h e c h a r c o m p o s i t i o n w a s d e t e r m i n e d t o be 0 . 7 8 3 5 C. 0.04 H. a n d 0 . 1 7 6 5 by an i n d e p e n d e n t laboratory. A specially d e s i g n e d quartz, t u b u l a r , p l u g - f l o w reactor w a s f a b r i c a t e d t o s t u d y t h e gas-phase reactions. Rates of gas f o r m a t i o n by species can be


16.

ANT AL

315

Steam Gasification of Biomass

m e a s u r e d u s i n g t h e reactor either in a differential or an integral m o d e . Results d e s c r i b e d here emphasize t h e integral aspects of t h e t u b u l a r reactor since t h e y are t h e easiest t o interpret. A s c h e m a t i c of t h e e x p e r i m e n t a l a p p a r a t u s is g i v e n in Figure I. A t y p i c a l experimental procedure was:


1) W i t h all three f u r n a c e s c o l d , a small (0.1 t o 0.5 g) s a m p l e of t h e material t o be pyrolyzed is placed in t h e c e n t e r of t h e pyrolysis reactor. 2) A n inert gas is bled t h r o u g h ports D a n d Ε t o cool t h e s a m p l e a n d p u r g e t h e reactor, w h i l e f u r n a c e s 1 a n d 3 b r i n g t h e s t e a m superheater a n d t h e gas-phase reactor t o t h e desired t e m p e r a t u r e . 3) T h e peristaltic p u m p is a c t u a t e d a n d p u m p s w a t e r into t h e s t e a m generator at a m e a s u r e d rate. C o n c u r r e n t l y , a small a m o u n t of inert tracer gas (argon) is c c n t i n u o u s l y i n j e c t e d t h r o u g h port A into t h e rear of t h e reactor. 4) W h e n c o n d e n s e d w a t e r first b e g i n s t o appear in t h e pyrolysis reactor, f u r n a c e 2 ( w h i c h w a s p r e h e a t e d t o t h e desired

pyrolysis

t e m p e r a t u r e ) is m o v e d i n t o place a r o u n d t h e pyrolysis zone o f t h e reactor. 5) W h e n pyrolysis t e m p e r a t u r e s are r e a c h e d , t h e six-port V a l c o valve is s w i t c h e d a n d t h e 3 4 - p o r t V a l c o valve a u t o m a t i c a l l y takes 15 samples of t h e gas s t r e a m f o r later analysis in t h e H e w l e t t - P a c k a r d 5 8 3 4 a Gas C h r o m a t o g r a p h (HPGC). U n s a m p l e d g a s is c o l l e c t e d in a T e f l o n b a g f o r later analysis. 6) W h e n

all 15 samples

have been t a k e n , t h e six-port valve is

s w i t c h e d again a n d t h e samples are a u t o m a t i c a l l y analyzed b y t h e HPGC. Gases c o l l e c t e d in t h e T e f l o n bag are s a m p l e d using a gas t i g h t syringe a n d analyzed b y t h e HPGC. 7) T h e c h a r a n d tars p r o d u c e d d u r i n g t h e e x p e r i m e n t are c o l l e c t e d a n d w e i g h e d . W a t e r c o l l e c t e d in t h e c o n d e n s e r is also w e i g h e d . T e m p e r a t u r e s w i t h i n t h e reactor are c o n t r o l l e d controllers

and monitored

by Type

by various

Κ thermocouples

with

temperature continuous

r e c o r d i n g o n c h a r t recorders. M e a s u r e d t e m p e r a t u r e variations a l o n g t h e l e n g t h of t h e gas-phase reactor have been d e s c r i b e d in a n earlier p u b l i c a t i o n (1).



Chart Recorder

TracerCarrier Gas

"

u

H

.

f

a

o

t

e

_

r

Figure 1.

8 Thermocouple Leeds

ED

Temperature C o n t r ô l e r s

Movable Pyrolysis Furnace

Furnace 3

Condenser

HP 5834a Gas Chromatography

6as Phase Reactor Furnace

i l l

lir Furnace 2

Gas Purge

Gas Purge

HP Terminal

Schematic of the tubular quartz reactor experiment

Steam Superheater

Furnace 1

Steam Generator

Perl stal tic Pump

Liquid Nitrogen


34 Port Valve

Vent

ta

ο

ι

δ

16.

ANTAL

The

evolved

variation

317

Steam Gasification of Biomass gas c o m p o s i t i o n

during

t h e course

w a s observed

to undergo

of the experiment;

considerable

consequently

t e n gas

s t a n d a r d s w e r e a c q u i r e d t o calibrate t h e HPGC for q u a n t i t a t i v e analysis of t h e f o l l o w i n g gases: Ar, N . H , CO, C 0 . C H , C H . C H , C H , C H , C H 2

C H 5

1 2

, and C H

obscured

6

1 4

2

2

4

2

4

2

6

3

6

4

8

4

1 0

,

. I d e n t i f i c a t i o n of t h e h i g h e r h y d r o c a r b o n s (>C3> is

by t h e fact that some other

pyrolysis p r o d u c t s

have

similar

r e t e n t i o n times. A n a l y s e s g i v e n in this paper f o r light h y d r o c a r b o n s ( ^ C ) 3


have been c h e c k e d using a mass s p e c t r o m e t e r . The HPGC uses a Poropak QS column

in series

with

a Porosil

column

operating

between

— 5 0 °C

(cryogenic) and 2 0 0 ° C f o r gas analysis w i t h a t h e r m a l c o n d u c t i v i t y d e t e c t o r (TCD). T h e carrier gas is an 8.5% H

2

-

91.5% He m i x t u r e . A t y p i c a l gas

analysis takes 1 4 m i n u t e s . The c o m p l e t e recovery of m o i s t u r e a n d tars f r o m t h e reactor s o m e t i m e s poses d i f f i c u l t i e s . T h e m o i s t u r e is a b s o r b e d o n d r y paper t o w e l s a n d w e i g h e d ; w h e r e a s t h e tars c o n d e n s e o n a rolled piece of a l u m i n u m foil inserted in t h e condenser. Mass balances are a l w a y s better t h a n 0.8, b u t can be m i s l e a d i n g because m u c h m o r e w a t e r is used d u r i n g t h e course o f an e x p e r i m e n t t h a n solid reactant. T h e c a r b o n balance is a better measure of t h e e x p e r i m e n t ' s q u a l i t y , a n d c u s t o m a r i l y ranges b e t w e e n 0.7 a n d 1.0 for t h e results reported here. Our inability t o close t h e c a r b o n balance in part reflects t h e f o r m a t i o n o f w a t e r - s o l u b l e c a r b o n a c e o u s c o m p o u n d s w h i c h are n o t s u b j e c t t o analysis b y o u r e x i s t i n g i n s t r u m e n t a t i o n . Their presence is m a n i f e s t e d by t h e color a n d o d o r of t h e c o l l e c t e d w a t e r , w h i c h ranges f r o m clear w i t h an o d o r r e s e m b l i n g a u t o m o b i l e exhaust, t o deep a m b e r w i t h a stronger, m o r e n o x i o u s odor. A s d e s i g n e d , t h e reactor bears s o m e r e s e m b l e n c e t o a d i l u t e - p h a s e t r a n s p o r t reactor in t h a t t h e solids a n d volatile pyrolysis p r o d u c t s are present o n l y in l o w c o n c e n t r a t i o n s in t h e s t e a m reactant. D u r i n g pyrolysis.the c o m p o s i t i o n of gas in t h e gas-phase reactor u s i n g t h e l o w e s t s t e a m f l o w a n d a 0.1 g s a m p l e is n o m i n a l l y 6 8 % s t e a m , 2 8 % volatiles, a n d 4 % a r g o n carrier (on a v o l u m e p e r c e n t basis). S o m e w h a t larger samples, leading t o an increase in volatile c o n c e n t r a t i o n s , d o n o t m a r k e d l y affect t h e results reported here. Rates of gas p r o d u c t i o n c a n be m e a s u r e d using t h e reactor in either a differential or a n integral m o d e . T h e d i f f e r e n t i a l m o d e e m p l o y s t h e V a l c o valve s y s t e m t o o b t a i n f i f t e e n 0.6-ml samples o f gas e v o l v e d d u r i n g t h e course of t h e e x p e r i m e n t . W i t h A r tracer gas i n j e c t e d at a measured rate, t h e d i l u t i o n of t h e tracer gas s a m p l e c a n be d i r e c t l y related t o t h e " i n s t a n t a n e o u s " rate of volatile gas p r o d u c t i o n in t h e reactor. For e x a m p l e , w i t h a tracer gas f l o w of 5 m l per m i n . a d i l u t i o n of 5 0 % in t h e gas s a m p l e w o u l d c o r r e s p o n d t o a n " i n s t a n t a n e o u s " volatile gas p r o d u c t i o n rate of 5 m l per


318


min.

Unfortunately,

(primarily

due

departures

to the

effect

from

of t h e

true

plug

condenser

flow on

within

the

gas f l o w )

reactor

make

the

differential mode experimental data more difficult to interpret than indicated a b o v e . Research r e p o r t e d here e m p h a s i z e s t h e i n t e g r a l a s p e c t s of t h e r e a c t o r design. W h e n used in t h e i n t e g r a l m o d e , t o t a l gas p r o d u c t i o n by species is m e a s u r e d u s i n g t e f l o n bags t o c o l l e c t all t h e reactor e f f l u e n t . T h e d e p e n d e n c e of t o t a l


gas p r o d u c t i o n o n g a s - p h a s e r e s i d e n c e t i m e in t h e g a s - p h a s e zone of t h e r e a c t o r is d e t e r m i n e d u s i n g t h e c o m b i n e d d a t a of m a n y e x p e r i m e n t s . T h i s d a t a c a n be used t o infer rates of gas p r o d u c t i o n w i t h i n t h e g a s - p h a s e reactor. K i n e t i c

models

of g a s e o u s

species

formation

can

be

obtained

t h r o u g h a s t u d y of t h e e f f e c t s of b o t h t e m p e r a t u r e a n d r e s i d e n c e t i m e o n species p r o d u c t i o n . Kinetic Interpretation of Reactor Data Consider t h e p y r o l y s i s of a s m a l l s a m p l e of o r g a n i c m a t e r i a l in t h e p y r o l y s i s zone of t h e t u b u l a r reactor s y s t e m . A t a n y t i m e t. t h e rate of e v o l u t i o n of g a s e o u s v o l a t i l e m a t t e r ( r h ) f r o m t h e p y r o l y z i n g s a m p l e is g i v e n b y : v

m (t) v

m ^

-

mk[V -

V(t)] ; k -

A exp(-E/RT)

n

(1)

w h e r e m is t h e t i m e d e p e n d e n t s a m p l e mass, m j is t h e initial s a m p l e mass. m

f

is t h e final s a m p l e mass. V

= (mj —

m ) / m i . V = (mj — f

m ) / m j , A is t h e

p r e - e x p o n e n t i a l c o n s t a n t . Ε is t h e a p p a r e n t a c t i v a t i o n e n e r g y , R t h e Universal Gas C o n s t a n t . Τ is t h e t i m e - d e p e n d e n t a b s o l u t e t e m p e r a t u r e , a n d η is t h e a p p a r e n t o r d e r of t h e r e a c t i o n . T h e c o n c e n t r a t i o n of v o l a t i l e m a t t e r ( C ) in t h e v

f l o w i n g s t r e a m is g i v e n b y : C (t) -

—

v

m

where m

s

,

—

^

S'Ps +

, , . m /p v

(2)

v

is t h e m a s s f l o w of s t e a m in t h e t u b u l a r r e a c t o r a n d ρ

s

and ρ

v

are t h e d e n s i t i e s of s t e a m a n d v o l a t i l e m a t t e r (respectively). A s s u m i n g p l u g f l o w , t h e v o l a t i l e m a t t e r e v o l v e d at t i m e t e n t e r s t h e gas-phase reactor at t i m e t + θ =

Tj a n d leaves t h e g a s - p h a s e reactor at t i m e t +

r . The residence time 0

T . Tj of t h e v o l a t i l e s in t h e gas phase is g i v e n by: Q

0-L /J v(x)dx^crL/V ο 2

L

(3)

w h e r e L is t h e l e n g t h of t h e g a s - p h a s e reactor, ν is t h e s p a t i a l l y - d e p e n d e n t gas v e l o c i t y in t h e reactor. V is t h e v o l u m e t r i c f l o w of volatiles plus s t e a m in t h e reactor at t h e g a s - p h a s e reactor t e m p e r a t u r e T, a n d σ is t h e reactor's e f f e c t i v e cross s e c t i o n a l area.


16.

ANTAL

319


S u p p o s e t h a t t h e rate of d i s a p p e a r a n c e of c o n d e n s i b l e volatiles (due t o c r a c k i n g , r e f o r m i n g , e t c . a n d t r e a t i n g t h e c o n d e n s i b l e volatiles as a single c h e m i c a l species) in a differential v o l u m e e l e m e n t ν δ τ of gas m o v i n g w i t h average v e l o c i t y ν = L / 0 satisfies a first order rate l a w : ^ dr


w h e r e t h e rate c o n s t a n t r

v

-

-C r v

(4)

v

is g i v e n b y t h e A r r h e n i u s e x p r e s s i o n : r

v

- A

exp(-E /RT)

v

(5)

v

The t i m e variable τ c a n be t h o u g h t t o " t r a c k " t h e p o s i t i o n of t h e differential v o l u m e e l e m e n t w i t h initial v o l a t i l e c o n c e n t r a t i o n C ( t ) as it m o v e s t h r o u g h t h e gas-phase reactor. T h e p r o d u c t i o n of p e r m a n e n t gases p r o d u c e d b y c r a c k i n g / r e f o r m i n g reactions is also a s s u m e d t o satisfy a first order rate l a w : v

dCj -jf.

- C , ,

w h e r e η is t h e rate c o n s t a n t associated w i t h t h e j

(6) p e r m a n e n t gas a n d :

t h

rj - A j e x p ( - E j / R T )

(7)

A s a first a p p r o x i m a t i o n , t h e gas-phase zone c a n be treated as an i s o t h e r m a l reactor (however, see t h e f o l l o w i n g section), leading t o t h e expressions: C (t + T Q ) - C (t) e x p [ - r v

C

j(t

+

v

τ ) 0

— Cj(t)

-

J

°

T

C

v

v

(τ -

(t)rj e x p

[-r

(8)

TJ)]

0

(T-TJ)]CIT

v

Tj

-

C

v

(t)(rj/r ){l-exp[-r v

v

(τ 0

TJ)]}

(9)

w h e r e r a n d r j are c o n s t a n t (by a s s u m p t i o n of c o n s t a n t T), a n d it is also a s s u m e d t h a t t h e t e m p e r a t u r e of t h e pyrolysis zone is s u f f i c i e n t l y " c o l d " so t h a t t h e c r a c k i n g / r e f o r m i n g reactions d o n o t c o m m e n c e u n t i l t h e differential v o l u m e e l e m e n t enters t h e gas-phase reactor. v

The mass of species j in t h e differential v o l u m e e l e m e n t e m e r g i n g f r o m t h e reactor at t i m e t + τ js g i v e n b y : 0

a v ô t C j (t + τ ) 0


(10)

320


a n d t h e t o t a l mass m; of species j p r o d u c e d d u r i n g t h e e x p e r i m e n t is g i v e n by: m j - crJv(t)Cj(t + T ) d t 0

=

vo-/Cj(t + r ) d t -

[vo-(rj/r )]{l-

0

exp [ - r ( T v

0

-

v

Tj)]}/C (t)dt v

+ V a J C j (t)dt

(11)

For s h o r t residence t i m e s [ r ( τ — Tj) < < 1 ] Equation (11) b e c o m e s : Downloaded by UNIV OF MISSOURI COLUMBIA on November 25, 2013 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch016

v

0

mj — ν σ η ( τ

0

-

TJ) J C ( t ) d t + v

vo-/Cj(t)dt

(12!

A p l o t of m j vs. residence t i m e ( τ — τ ) s h o u l d y i e l d a s t r a i g h t line w i t h t e m p e r a t u r e d e p e n d e n t slope ν σ η / C (t)dt — k j . A s u b s e q u e n t plot of / n ( k j ) vs. T " s h o u l d yield s t r a i g h t lines w h o s e slopes are t h e a p p a r e n t a c t i v a t i o n e n e r g y Ej associated w i t h t h e rate of p r o d u c t i o n of species j . T h u s , t h e d a t a o b t r a i n e d f r o m t h e t u b u l a r reactor are s u s c e p t i b l e t o kinetic interpretation. 0

(

v

1

It s h o u l d be n o t e d t h a t t h i s analysis o n l y p r o v i d e s an i n s i g h t i n t o t h e initial rates of t h e c r a c k i n g reactions. Tertiary gas-phase r e a c t i o n s t e n d t o o b s c u r e t h e i n t e r p r e t a t i o n of t h e kinetic data d e r i v e d f r o m t h e reactor. M o r e o v e r , t h e initial rate m e a s u r e m e n t s for t h e c r a c k i n g r e a c t i o n s are m e a n i n g f u l o n l y for short residence t i m e s [ r ( T — τ j )< < 1 ] . Because t h e c r a c k i n g reactions o c c u r rapidly, t h i s c o n s t r a i n t is d i f f i c u l t t o satisfy. v

0

Departures From an Ideal Isothermal Reactor T h e t u b u l a r q u a r t z reactor w a s d e s i g n e d a n t i c i p a t i n g t h e need t o p r o v i d e for long (5 s e c o n d s or more) gas-phase residence t i m e s in order t o r e f o r m the oily v o l a t i l e m a t t e r . S u r p r i s i n g l y , t h e o b s e r v e d reaction rates w e r e so h i g h t h a t residence t i m e s o n t h e order of 0.5 sec or less w e r e n e e d e d t o satisfy t h e c o n d i t i o n r ( 1 — τ j ) < < 1 . T o o b t a i n s u c h s h o r t residence t i m e s , large mass f l o w s w e r e r e q u i r e d w h i c h c a u s e d t h e reactor t o d e v i a t e f r o m its i n t e n d e d use as an ideal i s o t h e r m a l s y s t e m . In a d d i t i o n , t h e L i n d b u r g f u r n a c e s w e r e o b s e r v e d t o be less u n i f o r m in t e m p e r a t u r e t h a n e x p e c t e d . C o n s e q u e n t l y , heat transfer plays a c r i t i c a l role in d e t e r m i n i n g t h e residence t i m e of t h e volatiles at t e m p e r a t u r e . T h e f o l l o w i n g p a r a g r a p h s o u t l i n e o u r " f i r s t o r d e r " a p p r o a c h t o w a r d s r e c o g n i z i n g t h e affects of heat transfer o n t h e kinetic i n t e r p r e t a t i o n of t h e e x p e r i m e n t a l d a t a . v

0


16.

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321


A s i m p l e energy balance for laminar f l u i d f l o w in a long t u b e leads t o t h e equation: — dx

+ aT = a T

(13)

w

w h e r e « = π D h / m c . a n d T(x) is t h e bulk gas t e m p e r a t u r e along t h e l e n g t h p

χ of t h e t u b e . T

w

is t h e c o n s t a n t w a l l - t e m p e r a t u r e , D is t h e t u b e ' s d i a m e t e r , h

is t h e heat transfer c o e f f i c i e n t , m is t h e mass f l o w of t h e gas a n d c

p

is t h e


specific heat of t h e gas. Equation (13) c a n be solved t o d e t e r m i n e t h e distance f

required for t h e gas t o reach t e m p e r a t u r e T, w i t h t h e result:

,

(14l \

*-kN

N u D

\ t

J

w

— τ jy

where h = k N / D . k is t h e t h e r m a l c o n d u c t i v i t y of t h e gas. a n d t h e Nusselt number N = 3.7. N u D

N

u

d

In order t o o b t a i n an a p p r o x i m a t e residence t i m e ( τ — τ , ) for t h e volatiles in t h e gas-phase reactor, t h e l e n g t h f w a s c a l c u l a t e d a s s u m i n g T — Τ = 5°C. A n error of 5°C in t h e gas-phase t e m p e r a t u r e m e a s u r e m e n t gives rise t o a b o u t a 1 0 % error in t h e d e t e r m i n a t i o n of E, . T h e residence t i m e of t h e volatiles at t e m p e r a t u r e w a s t h e n c a l c u l a t e d using t h e f o r m u l a : 0

w

(L - Λ )σ T

° -

T

|

"

m /ps + m / p s

v

v

(15)

w h e r e L is t h e t o t a l l e n g t h of t h e gas-phase section of t h e reactor, a n d σ is t h e a p p a r e n t cross s e c t i o n a l area of t h e reactor. Table I lists values o f . / as a f u n c t i o n of T f o r a s t e a m f l o w of 0.34 g / m i n (used in t h e short residence t i m e kinetic e x p e r i m e n t s ) . From this d a t a , it is a p p a r e n t t h a t at t h e higher gas-phase t e m p e r a t u r e s , t h e volatiles s p e n d a b o u t 5 0 % of their t i m e in t h e gas-phase reactor b e i n g heated t o i s o t h e r m a l c o n d i t i o n s . C o n s e q u e n t l y , kinetic m e a s u r e m e n t s at t h e higher t e m p e r a t u r e s represent i n t e g r a t e d values of t h e rates at l o w e r t e m p e r a t u r e s , in a d d i t i o n t o t h e (assumed) c o n s t a n t rate at T . This result c l o u d s t h e kinetic i n t e r p r e t a t i o n of t h e reactor d a t a . A l t h o u g h m o r e effort c o u l d be m a d e t o e x p l i c i t l y a c c o u n t f o r t h e w a r m u p t i m e in t h e kinetic m o d e l for t h e reactor d a t a , w e have c h o s e n t o f o c u s o u r effort o n t h e d e s i g n of a " s e c o n d g e n e r a t i o n r e a c t o r " w h i c h w i l l p r o v i d e s u f f i c i e n t heat transfer rates t o ensure nearly i s o t h e r m a l c o n d i t i o n s f o r t h e shortest residence t i m e e x p e r i m e n t s . w

w



322


T a b l e I. P A R A M E T E R S U S E D T O C A L C U L A T E G A S - P H A S E RESIDENCE T I M E Effective Reactor Volume, f , cm

w« °C 750 700 675 650

13.4 13.9 14.2 14.5 14.8 15.2 15.6 16.1 17.2

625 600 575 550 500 L « 29.2 c m σ - 3.474 c m

cm

3

46.4 48.1 49.2 50.2 51.3 52.7 54.0 55.8 59.6

Effective Insert V o l u m e , cm 3

30.2 31.3 32.0 32.6 33.3 34.2 35.1 36.2 38.7

2

Bulk v o l u m e of insert ^ 7 0 c m L e n g t h of insert 31.1 m

3

C


16.

ANTAL

323



S o m e efforts have also been m a d e t o e x p e r i m e n t a l l y measure t h e t e m p e r a t u r e rise of t h e s t e a m e n t e r i n g t h e gas-phase reactor. Q u a l i t a t i v e a g r e e m e n t w i t h t h e results of t h e heat transfer c a l c u l a t i o n s w a s f o u n d ; h o w e v e r , a brief c a l c u l a t i o n of t h e effect of radiation o n t h e t h e r m o c o u p l e ' s m e a s u r e m e n t of t h e gas t e m p e r a t u r e p o i n t e d t o a s i g n i f i c a n t error in t h e m e a s u r e m e n t . For e x a m p l e , w i t h a s t e a m f l o w of 0.12 g / m i n a n d a gas-phase t e m p e r a t u r e of 6 0 0 ° C . t h e t h e r m o c o u p l e t e m p e r a t u r e w a s c a l c u l a t e d t o exceed t h a t of t h e gas b y 13°C. Carefully c o n s t r u c t e d radiation shields are needed t o e l i m i n a t e t h i s effect. Because o u r research effort in this area w a s d r a w i n g t o a close, it w a s d e c i d e d t o use t h e m e t h o d o l o g y d e s c r i b e d earlier (Equation 15) t o e s t i m a t e t h e residence t i m e of t h e volatiles at t e m p e r a t u r e . Departures From A n Ideal Plug-Flow Reactor Both t u r b u l e n c e a n d m o l e c u l a r d i f f u s i o n cause t u b u l a r reactors t o d e p a r t f r o m ideal p l u g - f l o w behavior. It is s t a n d a r d p r a c t i c e t o a c c o u n t f o r t h e t w o effects

b y a single

dimensionless

parameter

D / v L , called

t h e vessel

dispersion n u m b e r . This n u m b e r is usually d e t e r m i n e d e x p e r i m e n t a l l y , a n d its m a g n i t u d e indicates t h e degree of d e p a r t u r e of t h e reactor f r o m p l u g f l o w (2VvL) < 0 . 0 1 f o r p l u g f l o w ) . A g o o d d i s c u s s i o n of t h e effects of dispersed p l u g f l o w o n t h e kinetic i n t e r p r e t a t i o n of reactor data is g i v e n b y Levenspiel (19). T h e value Z)/vL = 0.122 w a s m e a s u r e d b y T. M a t t o c k s (V7). A l t h o u g h this value suggests s i g n i f i c a n t d e p a r t u r e s f r o m p l u g f l o w , w e believe m o s t of t h e dispersion o c c u r s in t h e c o n d e n s e r w i t h o u t a f f e c t i n g t h e kinetic measurem e n t s presented here. Results and Discussion Figures II. Ill, a n d IV display t h e d e p e n d e n c e of gas p r o d u c t i o n (g gas per g cellulose or % conversion) b y species o n gas-phase residence t i m e f o r various gas-phase reactor t e m p e r a t u r e s . For these e x p e r i m e n t s , t h e s t e a m superheater w a s m a i n t a i n e d at 3 5 0 ° C , a n d t h e pyrolysis f u r n a c e at 5 0 0 ° C . This latter s e t t i n g gave rise t o a m e a s u r e d s a m p l e h e a t i n g rate of 1 0 0 ° C / m i n . Residence t i m e s w e r e altered b y v a r y i n g t h e peristaltic p u m p ' s w a t e r f l o w rate b e t w e e n 0.06 a n d 0.34 g / m i n . a n d b y inserting a closed quartz c y l i n d e r into t h e gas-phase reactor t o reduce its a p p a r e n t v o l u m e . Data p o i n t s reported in Figures ll-V w e r e a c c u m u l a t e d m o n t h s using e x p e r i m e n t a l t e c h n i q u e s w h i c h evolved t h a t t i m e period. Data p o i n t s w i t h residence t i m e s of w e r e o b t a i n e d using a w a t e r f l o w rate of 0.34 g / m i n

over a period of e i g h t and improved during t w o t o three seconds f o r a 0.25 g sample.


324


χ ο 0.08 0.0*

0

1

?

3

4

S

6

7

8

9

10

11

12

U

12


0.014 ρ

0.006 0.004 0.002

• 0

D 1

2

3

4

S

·

7

8

9

10

CO

•

9

°

Figure 2. Nonhydrocarbon gas production vs. residence time for various gasphase temperatures: Ο 500°, (A) 600°, (O) 650°, (·) 700°, (X) 750°C

0.08 0.07 0.06 0.05 0.04

0.03

0.02

Δ 0

1

2

3

4

5

6

Δ 7

8

9

10

U

12

0.014 0.012 0.010 Ι

0.006

-

0.004

-

Ο

0.002 _J

I

I

L_

Residence T 1 M (sec)

Figure 3. Paraffinic hydrocarbon gas production vs. residence time for various gas—phase temperatures: Ο 500°, (A) 600°, (O) 650°, (%) 700°, (X) 750°C


ANTAL


325

o.os • -

0.04

0.03

-

0.0?

-

.

0.01

L É » .


0

1

' D 0

3

2

4

• Πι

1

6

I

5J

ι

6

7

l

3

2

5

4 . 5

7

Residence T I M

Β

9

1 B

3

10

1

11

1

1

10

11

12

_à 12

(sec)

Figure 4. Olefinic hydrocarbon gas production vs. residence time for various gasphase temperatures: Ο 500°, (A) 600°, (0)650°, (·) 700°, (X) 750°C

CARBON

• —I Ο

EFFICIENCY

•

I

1

OXYCEN

ft

I 3

2

I

Ι S

6

7

8

9

10

11

12

i

6

7

8

9

10

11

12

EFFICIENCY

KO

*

Δ

•

SJ

0

1

Γ

HYDROGEN

EFFICIENCY

1

3

2

2

3

4

4

5

6

7

8

9

Residence Tine (sec)

Figure 5. Carbon, hydrocarbon, and oxygen efficiency vs. residence time for vari ous gas—phase temperatures: Ο 500°, (A) 600°, (O) 650°, (·) 700°, (X) 750°C In Biomass as a Nonfossil Fuel Source; Klass, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

326



Shorter residence t i m e s w e r e o b t a i n e d using a quartz insert to reduce t h e gas-phase reactor's a p p a r e n t v o l u m e . Longer residence t i m e s w e r e o b t a i n e d by r e d u c i n g t h e w a t e r f l o w rate a n d cellulose s a m p l e size p r o p o r t i o n a t e l y . T h u s , all t h e d a t a represents t h e same s t e a m f l o w / c e l l u l o s e w e i g h t ratio. Since cellulose pyrolysis o c c u r s in a b o u t one m i n u t e w i t h a h e a t i n g rate of 1 0 0 ° C / m i n . t h e data c o r r e s p o n d t o a s t e a m d i l u t i o n ratio of a b o u t 1.4 g s t e a m per 1 g cellulose feed. A v a i l a b l e e v i d e n c e s u g g e s t s t h a t h i g h e r s t e a m d i l u t i o n ratios have little affect on t h e g a s i f i c a t i o n results. Efforts are presently b e i n g m a d e t o m o r e f u l l y e l u c i d a t e t h e effects of d i l u t i o n ratio o n s t e a m g a s i f i c a t i o n products. Of t h e various gases represented in Figures ll-IV. t h e behavior of c a r b o n d i o x i d e is s i m p l e s t t o interpret, since it s h o w s t h e least d e p e n d e n c e on g a s phase residence t i m e or t e m p e r a t u r e . A p p a r e n t l y , t h e p r i m a r y m e c h a n i s m for C O 2 f o r m a t i o n rests in t h e initial pyrolysis process. S e c o n d a r y gas-phase reactions at t e m p e r a t u r e s of a b o u t 5 0 0 ° C c o n t r i b u t e less t o C O 2 f o r m a t i o n . In order t o increase g a s i f i c a t i o n e f f i c i e n c y by r e d u c i n g C O 2 f o r m a t i o n (each m o l e c u l e of C O 2 f o r m e d represents a net loss of c a r b o n f r o m t h e c o m b u s t i b l e p r o d u c t s of t h e process), t h e c o n d i t i o n s a f f e c t i n g t h e pyrolysis step of g a s i f i c a t i o n m u s t be carefully e x a m i n e d . For e x a m p l e , t h e use of h i g h h e a t i n g rate m a y r e d u c e C 0 f o r m a t i o n . 2

M e t h a n e f o r m a t i o n is also relatively easy t o interpret. Increasing t e m peratures a n d increasing residence t i m e s result in increased m e t h a n e f o r m a t i o n . T h e slope of t h e d a s h e d lines in Figure III gives t h e a p p a r e n t rate of m e t h a n e p r o d u c t i o n at t h e various t e m p e r a t u r e s s t u d i e d . The d e p e n d e n c e of t h i s p r o d u c t i o n rate on t e m p e r a t u r e is used later in t h i s section t o e s t i m a t e t h e a c t i v a t i o n energy for m e t h a n e f o r m a t i o n . Efforts t o e l u c i d a t e t h e m e c h a n i s m of m e t h a n e f o r m a t i o n (most p r o b a b l y t h e p y r o l y s i s / h y d r o g e n a t i o n of h i g h e r h y d r o c a r b o n s ) are p r e s e n t l y u n d e r w a y . Carbon m o n o x i d e a n d h y d r o g e n p r o d u c t i o n data behave similarly, a n d reach a m a x i m u m at a b o u t 5 sec residence t i m e a n d 7 0 0 t o 7 5 0 ° C . Data for C H p r o d u c t i o n s h o w s o m e s i m i l a r i t y t o t h a t of C H ; h o w e v e r . C 2 H p r o d u c t i o n reaches a m a x i m u m at t e m p e r a t u r e s of 6 5 0 ° t o 7 0 0 ° C a n d residence t i m e s of a b o u t 2 sec. C o m p e t i t i v e rates of f o r m a t i o n by pyrolysis a n d c o n s u m p t i o n by pyrolysis or d e h y d r o g e n a t i o n reactions p r o b a b l y e x p l a i n t h i s o b s e r v e d behavior. 0

2

4

6

6

Ethylene p r o d u c t i o n is m a x i m i z e d at t e m p e r a t u r e s of 7 0 0 ° t o 7 5 0 ° C a n d residence t i m e s of a b o u t 6 sec. w h e r e a s p r o p y l e n e f o r m a t i o n is f a v o r e d by l o w e r t e m p e r a t u r e s (650°C) a n d shorter residence t i m e s (2 sec).


16.

ANT AL

327



In general, these results i n d i c a t e t h a t t h e gas-phase reaction t e m p e r a t u r e is t h e m o s t s i g n i f i c a n t parameter. T h e role of p r i m a r y pyrolysis c o n d i t i o n s a n d gas-phase residence t i m e s are m u c h less significant. Moreover, f o r t e m p e r a t u r e s a b o v e 6 5 0 ° C , t h e initial rates of species f o r m a t i o n are very h i g h , so t h a t m u c h of t h e gas f o r m a t i o n is c o m p l e t e in less t h a n 0.5 sec. These very h i g h rates of gas f o r m a t i o n d u e t o s e c o n d a r y reactions are of great s i g n i f i c a n c e f o r reactor d e s i g n . The p r e c e d i n g c o n c l u s i o n s are s u b s t a n t i a t e d b y Figure V, w h i c h s h o w s t h e effect of gas-phase t e m p e r a t u r e a n d residence t i m e o n t h e c a r b o n , h y d r o g e n a n d o x y g e n g a s i f i c a t i o n efficiencies (carbon e f f i c i e n c y = c a r b o n in gas-rf e e d s t o c k carbon). A g a i n , t h e gas-phase reactor t e m p e r a t u r e m o s t s i g n f i c a n t l y affects t h e c a r b o n a n d h y d r o g e n efficiencies of t h e s y s t e m . Under t h e best c o n d i t i o n s e x a m i n e d t o d a t e , 8 3 % of t h e feedstock's energy w a s retained by t h e gaseous p r o d u c t s of t h e process, a n d t a r p r o d u c t i o n w a s r e d u c e d t o 3 % of t h e f e e d s t o c k w e i g h t . T h e gas h a d a h e a t i n g value of 4 9 0 Btu/SCF. Other p e r t i n e n t statistics are g i v e n in Table II. Initial e x p e r i m e n t s in f l o w i n g a r g o n w i t h n o s t e a m present yield essentially t h e same results as t h e c o m p a r a b l e s t e a m runs. From this, it appears t h a t t h e g a s i f i c a t i o n process is d o m i n a t e d

by c r a c k i n g reactions a n d n o t s t e a m

r e f o r m i n g reactions. V a r i a t i o n s in h e a t i n g rate of t h e cellulose f r o m 5 0 ° C / m i n t o 2 0 0 ° C / m i n d o n o t m a r k e d l y affect results. Using gas-phase residence t i m e s of 0.46 t o 0.97 sec at t e m p e r a t u r e s of 7 5 0 ° a n d 5 0 0 ° C , respectively, a p p a r e n t rates of p r o d u c t i o n w e r e m e a s u r e d for seven gaseous species: C 0 . H . CO. C H , C H , C H a n d C H . Figures VI a n d VII are g r a p h s of log ( k j / m j ) vs. Τ " , w h e r e kj is t h e e x p e r i m e n t a l l y d e t e r m i n e d rate of p r o d u c t i o n of gas species j . A s i n d i c a t e d in Figures VI a n d VII, t h e slope of t h e lines c o n n e c t i n g t h e values of log (kj / m j ) gives t h e a p p a r e n t a c t i v a t i o n e n e r g y Ej associated w i t h t h e rate of p r o d u c t i o n of each species j . Values f o r each Ej are g i v e n in Table III. 2

2

4

2

4

2

6

3

6

1

Several c o n c l u s i o n s c a n be d r a w n f r o m Figures VI a n d VII. A t h i g h t e m p e r a t u r e s (and short residence t i m e s ) , t h e rate of c o n v e r s i o n of volatile m a t t e r t o e t h y l e n e is s e c o n d o n l y t o c a r b o n m o n o x i d e a n d m e t h a n e . T h u s , large yields of e t h y l e n e c a n be e x p e c t e d f r o m a w e l l d e s i g n e d biomass gasifier. In c o n t r a s t t o e t h y l e n e , t h e rate of p r o d u c t i o n of p r o p y l e n e peaks at 6 7 5 ° C a n d rapidly declines at h i g h e r t e m p e r a t u r e s . A s e x p e c t e d , t h e rates of p r o d u c t i o n of h y d r o g e n a n d c a r b o n m o n o x i d e are favored b y h i g h temperatures.


328


T a b l e I I . S E L E C T E D G A S I F I C A T I O N R E S U L T S FOR C E L L U L O S E Steam Superheater Temperature Pyrolysis Reactor T e m p e r a t u r e Gas-Phase Reactor T e m p e r a t u r e Gas-Phase Reactor Residence T i m e Sample W e i g h t


Char Residue W e i g h t Char Residue W e i g h t Percent Tar Residue W e i g h t Tar Residue W e i g h t Percent Gas V o l u m e P r o d u c e d Gas H e a t i n g V a l u e Calorific V a l u e of Gases Calorific V a l u e of Char Calorific V a l u e of Tars Mass Balance Carbon Balance Gas A n a l y s i s (Vol %) CO H 2

co

2

CH

4

C H C H C3H6 Other 2

4

2

6

350°C 500°C 700°C 3.5 sec 0.125 g 0.012 g 10% 0.003 g 2% 8 4 ml 4 9 0 Btu/SCF 13.7 M M B t u / t o n 2.8 M M B t u / t o n 0.5 M M B t u / t o n 0.84 0.96 52 18 8 14 6 1 0.1 0.9

T a b l e I I I . A P P A R E N T L O W T E M P E R A T U R E ( 5 0 0 ° C s= Τ s= 6 7 5 ° C ) A C T I V A T I O N E N E R G Y (Ej) FOR V A R I O U S G A S S P E C I E S

Gas Species C0

(kcal/gmol) 21

2

H C H C2H4

35 38 55

C3H6 CO CH

55 60 67

2

2

6

4


329



ANT AL

l.o

Figure 7.

l.i

looo/T ( · κ - ΐ )

,

I

I

I

I

I

I

I

750"C

700"C

675-C

ISOt

SXt

600*C

575*C

z

,

I 5S0'C

J

I 500*C

Arrhenius plot of the gas—phase production rate for various gas species: (X)C H„ (+) C H (O) C H t

t

kt

s

e


330


T h e rate c u r v e s for m e t h a n e a n d e t h y l e n e " t r a c k " each o t h e r closely, s u g g e s t i n g t h a t t h e s a m e m e c h a n i s m m a y be responsible for t h e f o r m a t i o n of t h e t w o gases. A l l t h e rates e x h i b i t a break at a b o u t 6 7 5 ° C , w i t h l o w e r a p p a r e n t Ej a b o v e 6 7 5 ° C . This m a y i n d i c a t e a c h a n g e in t h e c r a c k i n g m e c h a n i s m , or it m a y be an a r t i f a c t of poor heat t r a n s f e r in t h e gas-phase reactor. A m o r e e x p l i c i t m e c h a n i s t i c i n t e r p r e t a t i o n of t h e d a t a is m a d e d i f f i c u l t b y t h e effects of heat transfer a n d non-ideal p l u g f l o w o n t h e kinetic i n t e r p r e t a t i o n of t h e


e x p e r i m e n t a l d a t a . Nevertheless, t h e d a t a are q u i t e useful for e n g i n e e r i n g d e s i g n purposes, a n d s u g g e s t s criteria t o be used for t h e d e s i g n of s e c o n d g e n e r a t i o n reactors i n t e n d e d t o p r o v i d e m o r e a c c u r a t e m e a s u r e m e n t s of gasphase c r a c k i n g rates. Finally, Figures ll-IV also e x h i b i t t h e f i t (dashed lines) of E q u a t i o n 11 t o t h e e x p e r i m e n t a l d a t a for a g a s - p h a s e t e m p e r a t u r e of 6 0 0 ° C . Values for ν σ / c j (t) d t a n d vo- ( r / r ) J c ( t ) d t at 6 0 0 ° C used in Equation 11 are listed in Table IV. v

v

T h e relatively g o o d a g r e e m e n t of t h e m o d e l e m b o d i e d in E q u a t i o n 11 w i t h t h e e x p e r i m e n t a l d a t a g i v e n in Figures ll-IV is n o t e n t i r e l y f o r t u i t o u s , b u t less g o o d at h i g h e r t e m p e r a t u r e s w h e r e t h e c o n s t r a i n t r

v

(τ

0

—

Τ | ) < < 1 is not

satisfied. A m e c h a n i s t i c i n t e r p r e t a t i o n of gas-phase p h e n o m e n a is n e e d e d t o i m p r o v e our a b i l i t y t o m a t h e m a t i c a l l y p r e d i c t t h e p r o d u c t s of g a s i f i c a t i o n u n d e r a v a r i e t y of c o n d i t i o n s . EFFECTS OF PRESSURE O N T H E P Y R O L Y S I S H E A T OF R E A C T I O N A c o m p r e h e n s i v e e x p e r i m e n t a l research p r o g r a m t o i n v e s t i g a t e t h e effects of pressure o n t h e p r o d u c t s of s t e a m g a s i f i c a t i o n of b i o m a s s is c u r r e n t l y u n d e r w a y . A stainless steel, t u b u l a r m i c r o r e a c t o r similar t o t h e q u a r t z reactor d e s c r i b e d earlier has been f a b r i c a t e d for t h e e x p e r i m e n t a l w o r k . T h e pyrolysis f u r n a c e used w i t h t h e q u a r t z reactor s y s t e m has been r e p l a c e d in t h e pressurized s t e a m s y s t e m by a Setaram Differential S c a n n i n g Calorimeter (DSC). The DSC provides for q u a n t i t a t i v e d e t e r m i n a t i o n of t h e effects of pressure on pyrolysis kinetics a n d heats of reaction. Figure VIII presents t h e results of t h r e e m e a s u r e m e n t s of t h e heat of pyrolysis of cellulose at d i f f e r e n t

pressures. A t

elevated

pressures, t h e

pyrolysis

r e a c t i o n b e c o m e s e x o t h e r m i c , a n d char p r o d u c t i o n increases f r o m a b o u t 1 2 % by w e i g h t of t h e cellulose f e e d s t o c k at 1 bar t o 1 6 % at 6 bars. Future research is e x p e c t e d t o refine this initial data a n d e x t e n d it over a broader range of pressures.


16.

ANTAL

331


Table IV. N U M E R I C A L V A L U E S USED IN T H E GAS-PHASE

KINETIC

MODEL

co H

2

CO Downloaded by UNIV OF MISSOURI COLUMBIA on November 25, 2013 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch016

CH

4

C H

6

C H

4

2

2

C

3

H

va(rj/r )/c (t)dt|

vo-/cj(t)dt 0.061 0.001 0.045

2

6

w

T

= eocc

0.0145 0.0039 0.010

0.0035 0.0009 0.0 0.0002

0.0058

•

Figure 8.

w

0.046 0.0057 0.155

Cellulose ΔΗ p i i

yro ys s

I

Pressure (bars)

vs. pressure


332


CONCLUSIONS


Gas-phase, s t e a m c r a c k i n g reactions d o m i n a t e t h e c h e m i s t r y of b i o m a s s g a s i f i c a t i o n . A t t e m p e r a t u r e s a b o v e 6 5 0 ° C , these reactions p r o c e e d very r a p i d l y a n d g e n e r a t e a h y d r o c a r b o n rich syngas c o n t a i n i n g c o m m e r c i a l l y i n t e r e s t i n g a m o u n t s of e t h y l e n e , p r o p y l e n e , a n d m e t h a n e . Increased pressure appears t o i n h i b i t t h e g a s i f i c a t i o n process. These results i n d i c a t e t h a t b i o m a s s gasifiers s h o u l d be d e s i g n e d t o p r o v i d e for h i g h h e a t i n g rates a n d s h o r t residence t i m e w i t h gas-phase t e m p e r a t u r e s e x c e e d i n g 6 5 0 ° C . T r a n s p o r t reactors, characterized b y large t h r o u g h p u t s , h i g h h e a t i n g rates, m o d e s t pressures, a n d s h o r t residence t i m e s appear t o be ideally s u i t e d f o r t h i s purpose. Future b i o m a s s gasifiers s h o u l d rely o n s t e a m c r a c k i n g t o p r o d u c e fuels a n d c h e m i c a l s . ACKNOWLEDGEMENTS T h e assistance of Mr. W . E d w a r d s a n d Mr. T. M a t t o c k s in p e r f o r m i n g t h e t u b u l a r p l u g - f l o w reactor e x p e r i m e n t s is g r a t e f u l l y a c k n o w l e d g e d . The m e a s u r e m e n t of t h e heat of pyrolysis of cellulose at six bars w a s m a d e by Dr. P. Leparlouer w h i l e t h e a u t h o r v i s i t e d S e t a r a m Laboratory in L y o n . France. T h e assistance of Setaram in t h i s research is also g r a t e f u l l y a c k n o w l e d g e d . This project w a s f i n a n c e d by t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n A g e n c y under Grant No. R 8 0 4 8 3 6 0 1 0 .


16. ANTAL Steam Gasification of Biomass

333


REFERENCES

1.

Antal, M.J. "Symposium Papers", Energy From Biomass and Wastes, Symposium sponsored by the Institute of Gas Technology, Washington, D.C., August 1978; Institute of Gas Technology: Chicago, 1978.

2.

Rensfelt, E.; Blomkvist, G.; Ekstrom, C.; Engstrom, S.; Espenas, B.G.; Liinanki, L. "Symposium Papers", Energy From Biomass and Wastes, Symposium sponsored by the Institute of Gas Technology, Washington, D.C., August 1978; Institute of Gas Technology: Chicago, 1978.

3.

Lewellen, P.C.; Peters, W.A.; Howard, J.B. "Cellulose Pyrolysis Kinetics and Char Formation Mechanism", 16th International Symposium on Combustion, Cambridge, Mass., 1976.

4.

Mackay, G.D.M. Canada Department of Forestry and Rural Development, Publ. 1201, Ottawa, Ont., 1967.

5. Roberts, A.F. Combust. Flame 1970, 14, 261. 6.

Welker, J.R. J. Fire Flammability 1970, 1, 12.

7.

Beall, F.C.; Eickner, H.W. U.S. Forest Service, 1970, FPL-130.

8. Diebold, J; Smith, G. Naval Weapons Center, NWC Technical Publication 6022, April 1978. 9.

Hatch, L.F.; Matar, S. Hydrocarbon Process. 1978, March, 129-139, Part 8.

10.

Hatch, L.F.; Matar, S. Hydrocarbon Process. 1978, March, 129-139, Part 9.

11.

Feber, R.C.; Antal, M.J. U.S. Environmental Protection Agency, Report EPA-600/2-77-147, Cincinnati, Ohio, 1977.

12.

Appell, H.R.; Pantages, P. in "Thermal Uses and Properties of Carbohydrates and Lignin", Shafizadeh, F.; Sarkanen, K.; Tillman, D., Eds. Academic Press: New York, 1976.

13.

Antal, M.J.; Friedman, H.L.; Rogers, F.E. "Kinetic Rates of Cellulose Pyrolysis in Nitrogen and Steam", Eastern Section, The Combustion Institute Fall Meeting, Hartford, Conn., 1977.


334 14.

BIOMASS AS A NONFOSSIL FUEL SOURCE

Antal, M.J.; Friedman, H.L.; Rogers, F.E. Combust. Sci. Technol., in press.

15. Reed, T.B.; Antal, M.J. "Preprints", 176th National Meeting of the American Chemical Society, Miami, Fla., 1978. 16.

Antal, M.J.; Reed, T.B. "Preprints", 176th National Meeting of the American Chemical Society, Miami, Fla., 1978.


17. Mattocks, T. M.S.E. Thesis, Princeton University, Princeton, N.J., 1979. 18. Antal, M.J.; Edwards, W.E.; Friedman, H.L.; Rogers, F.E. "A Study of the Steam Gasification of Organic Wastes", Final Progress Report to the U.S. Environmental Protection Agency, Princeton University, 1979. 19.

Levenspiel, O. "Chemical Reaction Engineering"; J. Wiley and Sons: New York, 1972.

RECEIVED JULY 28,

1980.


The Effects of Residence Time, Temperature, and Pressure on the

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