Lignin for

15 Product Distribution in the Rapid Pyrolysis of Biomass/Lignin for Production of Acetylene M A R T H A GRAEF, G. G R A H A M A L L E N , and BARBARA B. KRIEGER Department of Chemical Engineering, University of Washington, BF-10, Seattle, WA 98195

Chemical feedstocks and fuels supplied by the petrochemical industry are in ever increasing demand. Due to the projected difficulty of traditional petroleum based resources meeting these demands, alternative materials and technologies are being examined. Although coal will be used as a raw material for these processes, it presents a number of environmental problems. In view of these problems, one might focus attention on the potential of pyrolyzing polymeric renewable resources such as agricultural and urban wastes (lignocellulosics) which have a higher hydrogen to carbon ratio than coal into useful monomeric chemicals and fuels. In particular, the relatively large amounts (1,2), of collected, low cost waste materials generated by the forest products industry represent a low ash, low sulfur, environmentally acceptable alternative feedstock to coal. Dry hardwood and softwood lignocellulosics are chemically characterized as 50% C, 6% H, 44% O, less than 0.1% nitrogen, and 0.3% ash (3). Although the high oxygen and water content of woody waste materials has prevented wide usage of lignocellulosics as feedstocks in the past (4), the cellulose fraction (43%) and hemicelluloses (28% to 35%) containing the oxygen are utilized by the forest products industry, leaving the highly aromatic lignin fraction (22% to 29%) to be further processed (see Figure I).

0097-6156/81/0144-0293$05.00/0 © 1981 American Chemical Society

294

BIOMASS AS A NONFOSSIL F U E L SOURCE

15.

GRAEF ET AL.

Acetylene from Biomass/Lignin

295

A d e t e r e n t t o d e r i v i n g c h e m i c a l s a n d fuels f r o m p o l y m e r s s u c h as coal a n d lignin is precisely t h e w i d e variety of p r o d u c t s , each of a relatively l o w yield, w h i c h are f o u n d in c o n v e n t i o n a l pyrolysis processes. In a t t e m p t s t o c o n t r o l t h e p r o d u c t d i s t r i b u t i o n , several w o r k e r s (5-8) have s t u d i e d t h e effects of h e a t i n g rate, u l t i m a t e t e m p e r a t u r e , a n d q u e n c h i n g rate o n pyrolysis p r o d u c t s , especially t h e a m o u n t of char p r o d u c e d (8). It appears t h a t rapid, severe d e g r a d a t i o n favors gas f o r m a t i o n a n d c a n , in s o m e cases, n a r r o w t h e p r o d u c t d i s t r i b u t i o n c o n s i d e r a b l y (9). Extremely r a p i d , h i g h t e m p e r a t u r e h e a t i n g w i t h rapid q u e n c h i n g c a n be p r o v i d e d

b y plasma h e a t i n g

(10-13). Recently,

plasma h e a t i n g has been successfully used in coal pyrolysis a n d a process unit is c u r r e n t l y u n d e r s t u d y (14). Pyrolysis studies have been carried o u t w i t h lignocellulosics (8,9,15-24), b u t f e w have i n v e s t i g a t e d t h e h e a t i n g rate effect o n p r o d u c t d i s t r i b u t i o n a n d u n f o r t u n a t e l y , o f t e n o n l y a single c o m p o n e n t or t h e large classes of c o m p o u n d s s u c h as char, liquids (tars or c o n d e n s i b l e volatiles), a n d gases are reported. T h e d e t a i l e d characterization of all p r o d u c t s is s e l d o m carried o u t (25). It is t h e i n t e n t of t h i s article t o characterize t h e p r o d u c t s f r o m m i c r o w a v e - i n d u c e d p l a s m a pyrolysis (rapid h e a t i n g rate pyrolysis) of lignin as a p r e l u d e t o d e t e r m i n a t i o n of t h e kinetics a n d e c o n o m i c s of c h e m i c a l p r o d u c t i o n f r o m p i n e w o o d Kraft l i g n i n , a w a s t e material o f t h e p u l p a n d paper industry. PREVIOUS STUDIES Extensive surveys of t h e literature o n m i c r o w a v e pyrolysis appear in Che (26), B i t t m a n (27), a n d Fu a n d Blaustein (28-31), w h o s t u d i e d t h e reactions of a c o m p l e x m o l e c u l e , c o a l , in a m i c r o w a v e - i n d u c e d plasma. M o s t o t h e r studies, h o w e v e r , have been c o n d u c t e d w i t h s i m p l e l o w m o l e c u l a r w e i g h t c o m p o u n d s s u c h as CO a n d C 0 (32) a n d s i m p l e h y d r o c a r b o n s (27,33). These studies, c o n d u c t e d w i t h relatively e x p e n s i v e c h e m i c a l s o f h i g h p u r i t y , lend i n s i g h t i n t o t h e n a t u r e of t h e p l a s m a reactions, b u t c o n t r i b u t e little t o t h e role h e a t i n g rate plays in pyrolysis or t o e c o n o m i c e v a l u a t i o n o f plasma p r o c e s s i n g . In a d d i t i o n , m o s t o f these studies have used resonance c a v i t y m i c r o w a v e a p p l i c a t o r s w h i c h are l i m i t e d t o small size b y d e s i g n a n d are e x c l u s i v e l y research tools. A s discussed in Bosisio (34), w a v e g u i d e a p p l i c a t o r s a n d m o r e recent d e s i g n s (35) a l l o w reactor c o n f i g u r a t i o n s t h a t are s u i t e d t o possible industrial scale-up. Discussion of t h e effect m i c r o w a v e a p p l i c a t o r d e s i g n has o n field i n t e n s i t y a n d u l t i m a t e p r o d u c t d i s t r i b u t i o n is b e y o n d t h e scope o f t h i s s t u d y . A t least t w o p a t e n t s exist c o n c e r n i n g m i c r o w a v e d e g r a d a t i o n o f w a s t e (36,37) a n d hazardous materials (38) b u t specific reactions are n o t discussedTThi"degradation of l i g n i n in a d i s c h a r g e is l i m i t e d t o w o r k b y Goheen (9) in an arc a n d Zaitsev (39) w h o used a 2

296


t u n g s t e n e l e c t r o d e d i s c h a r g e s y s t e m . The e m p h a s i s in b o t h articles w a s t o s t u d y o n l y a single class of p r o d u c t s s u c h as t h e gases or char. S i m p l e t h e r m a l pyrolysis of l i g n o c e l l u l o s i c s w i t h or w i t h o u t a d d i t i v e s has been r e v i e w e d by several a u t h o r s (15-17). In c o n t r a s t t o m i c r o w a v e pyrolysis. reasonably e x t e n s i v e c o n v e n t i o n a l pyrolysis p r o d u c t c h a r a c t e r i z a t i o n has been c o n d u c t e d for certain t y p e s of b i o m a s s (15-25) a n d s o m e of these results w i l l be c o m p a r e d t o t h o s e c i t e d here. T o these a u t h o r s ' k n o w l e d g e , no single s t u d y on m i c r o w a v e pyrolysis (plasma or d i - e l e c t r i c loss mode) has i d e n t i f i e d t h e c o m p o n e n t s of all p r o d u c t f r a c t i o n s nor t h e i r relative a m o u n t s (40.41). T h e w o r k r e p o r t e d here has been e x t e n d e d by others (43.44) t o i n c l u d e pyrolysis s t u d i e s of b i o m a s s f r a c t i o n s a n d o t h e r t y p e s of b i o m a s s w i t h t h e e m p h a s i s o n d e t a i l e d p r o d u c t c h a r a c t e r i z a t i o n , f o r m a t i o n kinetics, a n d effect of t r a n s p o r t rates. EXPERIMENTAL M i c r o w a v e Application S y s t e m and Reactor T h e m i c r o w a v e c i r c u i t used in t h i s s t u d y is s h o w n in Figure II. T h e G e r l i n g M o o r e variable 0-2.5 k W p o w e r g e n e r a t o r operates at 2 4 5 0 MHz. T h r e e c r y s t a l d e t e c t o r s in t h e c i r c u i t m e a s u r e f o r w a r d , r e f l e c t e d , a n d t r a n s m i t t e d p o w e r a n d are m o u n t e d in t h e S-band w a v e - g u i d e (3.8 x 7.6 c m ) . T h e latter t w o p o w e r m e a s u r e m e n t s are r e c o r d e d c o n t i n u o u s l y . C o n s t a n t f o r w a r d p o w e r a n d m a g n e t r o n p r o t e c t i o n are e n s u r e d by t h e presence of a 3-port c i r c u l a t o r . The i m p e d a n c e of t h e c i r c u i t is m a n u a l l y m a t c h e d w i t h t h e E-H field tuner. The reactor is placed t h r o u g h t h e w a v e g u i d e as s h o w n . T h e gas h a n d l i n g a n d reaction s y s t e m is s h o w n in Figure III. T h e reactor (I.D. 18 m m ) is m a d e of V y c o r f u s e d t o s t a n d a r d t a p e r e d glass. Carrier gases ( A r g o n . A i r c o , CP g r a d e ; H e l i u m , L i q u i d Air, CP g r a d e ; a n d H y d r o g e n , L i q u i d Air, CP grade) w e r e dried before f l o w i n g t h r o u g h t h e reactor. I n d u l i n A T , a p i n e w o o d . kraft l i g n i n ( W e s t v a c o C o m p a n y ) w a s e x t r a c t e d w i t h e t h e r in a Soxhlet e x t r a c t o r for 4 8 hours, d r i e d u n d e r v a c u u m , a n d stored over C a S 0 . T h e d r y lignin p o w d e r w a s pellitized (average d e n s i t y . 1.1 ± 0 . 0 5 g / c m ) . w e i g h e d , a n d p l a c e d o n a h o l l o w q u a r t z pedestal in t h e r e a c t i o n zone. Resulting m i c r o w a v e field intensities are a b o u t 5 0 t o 1 0 0 W / c m . T h i s v a l u e is c a l c u l a t e d a s s u m i n g t h e initial i n d i c a t e d a b s o r b e d p o w e r i m p i n g e s o n t h e 4

3

2

pellet cross-section. T h e entire s y s t e m w a s e v a c u a t e d a n d c h e c k e d for leaks prior t o b e g i n n i n g t h e r e a c t i o n . L i q u i d n i t o g e n or ice b a t h s w e r e used t o collect condensible products.

GRAEF ET AL.

297

Acetylene from Biomass/Lignin Ζ REACTOR ι QUARTZ INSERTED

E-H FIELD TUNER

U I

n

FLEXIBLE WAVEGLOE

CRYSTAL DETECTOR

CRYSTAL DETECTOR

MATCHNG (water)

w

SCREENED/! > PORT E

5

FORCED AIR COOLNG ENTRANCE

το Ο το©

το φ

Φ

LOAD FORWARD REFLECTED POWER POWER POWER

POWÉRÊCORDER reflected & load)

CONTROL UNIT

Figure 2.

Figure 3.

^ VARIABLE POWER ADJUSTMENT

Microwave circuit

Gas handling and reactor schematic

MAGNETRON 2450 MHz

298


Analytical Scheme T h e so-called p e r m a n e n t gas f r a c t i o n w a s r o u t i n e l y analyzed o n a 6-ft X 1/8i n c h d i a m e t e r S u p e l c o Porapak Q c o l u m n u s i n g a Perkin-Elmer M o d e l 3 9 2 0 gas c h r o m a t o g r a p h (G.C.). T h e f l o w r a t e of t h e G.C. carrier, h e l i u m , w a s 3 0 m l / m i n . The b r i d g e c u r r e n t w a s set for 1 7 5 m A a n d t h e 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 t e m p e r a t u r e w a s m a i n t a i n e d at 2 0 0 ° C . CO a n d C O 2 peaks w e r e q u a n t i t a t i v e l y analyzed at r o o m t e m p e r a t u r e w h i l e t h e a s s y m m e t r y of t h e a c e t y l e n e peak necessitated e l u t i o n at 100°C. T h e presence of h y d r o g e n w a s d e t e r m i n e d o n a m o l e c u l a r sieve 1 3 X c o l u m n at r o o m t e m p e r a t u r e . Since a c e t y l e n e w a s n o t separated f r o m e t h y l e n e , c o n f i r m a t i o n of a c e t y l e n e w a s m a d e o n a S u p e l c o Porapak Τ c o l u m n (10 ft X % inch) at r o o m t e m p e r a t u r e . T h e l i q u i d f r a c t i o n , d i s s o l v e d in ether, w a s separated o n a S u p e l c o DEGS 5 0 ft x 1 6 - i n c h capillary c o l u m n (liquid l o a d i n g 10%). T h e s a m p l e w a s prepared b y d r y i n g o v e r M g S O * a d d i n g p a r a - b r o m o p h e n o l as a n internal s t a n d a r d a n d b r i n g i n g it a l m o s t t o d r y n e s s in a Danish Kaderna evaporator. A s a m p l e c h r o m a t o g r a m is s h o w n in Figure IV. C o n f i r m a t i o n of t h e major c o m p o n e n t s w e r e m a d e by an associated Hitachi Perkin-Elmer R M S - 4 mass s p e c t r o m e t e r . A d d i t i o n a l analyses are d e t a i l e d in Graef (46). A l i m i t e d n u m b e r of c h a r f r a c t i o n analyses w e r e c o n d u c t e d on a B e c k m a n I.R.-4 infrared s p e c t r o m e t e r . A s a m p l e size of 0.5 m g residual t o 8 0 0 m g KBr w a s f o u n d t o g i v e t h e best s p e c t r a (41). RESULTS Helium Plasma Product Characterization The m i c r o w a v e reactor p a r a m e t e r s distribution and thus, the conditions reaction c o n d i t i o n s . The a b s o r b e d m o n i t o r s a n d t h i s particular p o w e r c o n s i s t e n t l y stable d i s c h a r g e over an rates.

are e x p e c t e d t o affect t h e p r o d u c t in Table I w i l l be c o n s i d e r e d baseline e n e r g y is read d i r e c t l y f r o m p o w e r level w a s t h e l o w e s t t h a t p r o v i d e d a a c c e p t a b l e range of pressures a n d f l o w

T h e gross d i s t r i b u t i o n of p r o d u c t s f o r m e d f r o m lignin in a h e l i u m d i s c h a r g e are s h o w n in Table I. T h e values in parenthesis are t h o s e c a l c u l a t e d by e x c l u d i n g t h e residual f r a c t i o n . The u n c e r t a i n t y s u g g e s t e d by t h e u p p e r l i m i t ( < 1 0 % ) g i v e n for t h e volatile f r a c t i o n is a c o n s e q u e n c e of t h e d e p o s i t i o n of fine particle m a t e r i a l in t h e n i t r o g e n t r a p . T h e i n a b i l i t y t o dissolve this s u b s t a n c e in a v a r i e t y of solvents s u g g e s t s t h a t it s h o u l d be categorized w i t h t h e p o l y m e r i z e d f r a c t i o n , t h u s t h e l o w e r limit ( > 3 % ) given for t h e polymerized fraction.

15.

GRAEF ET AL.


Figure 4. Gas chromatograph of condensable volatiles

299

300

BIOMASS AS A N O N F O S S I L F U E L

SOURCE

T h e p l a s m a pyrolysis values presented in Table I can be c o m p a r e d w i t h t h e c o m p i l a t i o n by A l l a n and M a t t i l a (1_7) of t h e t h e r m a l pyrolysis p r o d u c t s of l i g n i n under solvent-free c o n d i t i o n s over a broad range of t e m p e r a t u r e s . The t w o p r o d u c t d i s t r i b u t i o n s are q u i t e dissimilar. T h e r m a l pyrolysis p r o m o t e s l i q u e f a c t i o n ( 7 8 % o n a residual-free basis) w h i l e p l a s m a p r o c e s s i n g is p r i m a r i l y a g a s i f i c a t i o n r e a c t i o n ( 8 1 % o n a residual-free basis). A s e x p e c t e d t h e p l a s m a reactions cause a m o r e severe d e g r a d a t i o n t o l o w e r m o l e c u l a r weight products. T a b l e I. O V E R A L L P R O D U C T D I S T R I B U T I O N Representative Conditions Reaction Product Residual V o l a t i l e Fraction Permanent G a s e s

3 3 % (0%) < 10% ( 3 % (>4%)

4

Baseline Reactor Parameters H e l i u m Initial Pressure: 2 5 Torr M a x i m u m Pressure: — 1 0 0 Torr Forward Power: 5 5 0 w a t t s Batch Reaction T i m e : 10 m i n Total A b s o r b e d Energy: 7 5 w a t t - h o u r Carrier Flow Rate: 8 6 c m / m i n 3

2.

From Ref. 17. A l l a n a n d M a t i l l a

3. 4.

MW = 1 4 g/mole By d i f f e r e n c e a v e

Gas Fraction Characterization Detailed c h a r a c t e r i z a t i o n of t h e gas f r a c t i o n s of t h e respective

pyrolysis

processes s h o w n in Table II s u g g e s t s t h a t t h e d i s s i m i l a r i t y e x t e n d s b e y o n d t h e d i s t r i b u t i o n of t h e p r o d u c t s .

15.

GRAEF ET AL.

301


Table II. C O M P O S I T I O N OF P E R M A N E N T G A S E S

Carbon Monoxide Carbon Dioxide Hydrogen Methane Ethane Acetylene Higher H y d r o c a r b o n s Sum

1

Helium Plasma

Thermal Pyrolysis

44% 2% 43% 2% Trace 14% Trace

50% 10% None 38% 2% None Trace

105%

3

100%

1. V o l u m e p e r c e n t 2. Reference 17 3. Indicates error in m e a s u r e m e n t s , ± 5 % The analytical s c h e m e f o r these studies p r e c l u d e d m e a s u r e m e n t of w a t e r . Both pyrolysis m e t h o d s evolve 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 in c o m p a r a b l e a m o u n t s . H o w e v e r , plasma processing p r o d u c e s 4 3 % h y d r o g e n a n d 1 4 % acetylene o n a v o l u m e basis w h i l e t h e r m a l pyrolysis gases c o n t a i n neither c o m p o n e n t . Instead, t h e m a j o r h y d r o c a r b o n generated in t h e t h e r m a l pyrolysis s y s t e m is m e t h a n e (38%), w h i l e s a t u r a t e d h y d r o c a r b o n s are m i n o r c o m p o n e n t s in t h e plasma process. These differences illustrate t h a t t h e n a t u r e of c o n v e n t i o n a l pyrolysis reactions is radically different f r o m t h e m i c r o w a v e plasma pyrolysis reactions. Condensible Liquid Characterization The gas phase species w e r e s w e p t f r o m t h e plasma zone b y t h e carrier gas a n d a p o r t i o n o f t h e m c o n d e n s e d in a l i q u i d n i t r o g e n c o l d trap. S o m e of t h e major c o n d e n s i b l e volatiles w e r e i d e n t i f i e d w i t h G.C.-Mass s p e c t r o s c o p y as s h o w n in Figure IV. 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 several of t h e larger peaks is g i v e n in Table III f o r t h e h e l i u m plasma reactor baseline c o n d i t i o n s . C o m p a r i s o n of Table III w i t h t y p i c a l lignin pyrolysis p r o d u c t s f r o m A l l a n a n d M a t t i l a (17) s h o w n in Figure V reveals t h a t w h i l e guaiacol a n d t h e cresols are present in b o t h s y s t e m s , a variety of o t h e r p r o d u c t s , specifically t h e c o n d e n s e d a r o m a t i c s , are n o t c o n v e n t i o n a l pyrolysis p r o d u c t s . T h e major c o m p o n e n t s i d e n t i f i e d represent o n l y 4 % b y v o l u m e of t h e volatile f r a c t i o n w h i l e at least 5 0 a d d i t i o n a l c o m p o u n d s a c c o u n t f o r t h e r e m a i n i n g 96%. No a t t e m p t has y e t been m a d e t o o p t i m i z e or n a r r o w t h e p r o d u c t d i s t r i b u t i o n of t h e l i q u i d f r a c t i o n o b t a i n e d in a h e l i u m p l a s m a since t h e liquids represent only a b o u t 10 w e i g h t p e r c e n t of t h e t o t a l p r o d u c t s . A l t h o u g h c u r r e n t studies are addressing t h e issue of w h i c h e x p e r i m e n t a l variables n a r r o w t h e p r o d u c t

302


COMPOUNO

I D E N T I F I E D AS MAJOR COMPONENTS IN CONDENSIBLE V O L A T I L E FRACTION OF

STRUCTURE

NAME

HELIUM MICROWAVE PLASMA PYROLYSIS

OH

phenol

o-cresol

è

y

OH

ér

OH

p-cresol

Φ

guaiacol

CH

j

CHs

y

OH JsÔCH

3

y

3

OH

2 , 4 dimethyl phenol ο ) (xylenol) 2-methoxy,4-methyl phenol

CH

OH ,OCH, 3

OH ,OCH

2-methoxy, 4 - e t h y l phenol 2-methoxy, 4 - p r o p y l phenol

CH, OH

CH -CH 2

OH

Ψ

^ v . O C H v

3

CH

p - v i n y l phenol

napthalene

3

CH=CH

anthracene

2

-CH

3

2

-CH

3

y

β ί ο ] 2

C H » C H

y 2

y

styrene C = CH

phenyl acetylene

y

acenaphthcene

y

FigureS. Contrast between plasma and thermal pyrolysis condensable volatile fraction (tars) produced in solvent-free pyrolysis of lignin

15.

GRAEF ET A L .

303


d i s t r i b u t i o n (43,44), it is d i f f i c u l t t o generalize a b o u t t h e reactions f o r m i n g t h e c o n d e n s i b l e l i q u i d f r a c t i o n in t h i s reactor g e o m e t r y a n d q u e n c h zone c o n f i g u r a t i o n (44). Even if m o r e favorable yields of t h e a b o v e c o m p o u n d s c o u l d be a c h i e v e d , e c o n o m i c c o n s i d e r a t i o n s s u g g e s t t h e decreased diversity of t h e gas f r a c t i o n s h o u l d take p r e c e d e n c e a n d its c o m p o s i t i o n s h o u l d be optimized. Table III. COMPOSITION OF VOLATILES (Baseline Reactor Parameters) Volume Fraction Major Components

of Tars, %

Styrene Phenyl A c e t y l e n e

0.5 0.5 0.9 0.2

Napthalene Guaiacol O-Cresol P-Cresol Acenaphthene Anthracene

1.0 0.4 0.1 0.5 4.1

Other* Methylphenylacetylene 1,2-Dimethoxybenzene Other unidentified components. 5 0

not quantitative 100%

* Unconfirmed Residual Analysis A n e l e m e n t a l analysis w a s p e r f o r m e d (42) o n t h e residual f r a c t i o n . T h e f o l l o w i n g w e i g h t p e r c e n t s w e r e o b t a i n e d o n t h e char: C. 8 3 . 9 2 % ; H. 2.09%; N, 0.45%; O, 8.5%; a n d remainder, 5.06%. T h e c h a r is a black, porous material w i t h a shape similar t o a n e x p a n d e d pellet. Infrared spectra (41) of t h e char f r a c t i o n s h o w e d v i r t u a l l y n o features a n d n o n e o f t h e original lignin f u n c t i o n a l g r o u p a b s o r p t i o n b a n d s , s u b s t a n t i a t i n g t h a t nearly c o m p l e t e reaction o c c u r r e d . A l t h o u g h f e w residual f r a c t i o n samples w e r e analyzed s p e c t r o s c o p i c a l l y , e a c h w a s v i s u a l l y i n s p e c t e d a n d it is believed t h a t t h e lack of f u n c t i o n a l i t y is representative o f all char fractions. Certain runs n o t reported here s h o w e d u n r e a c t e d l i g n i n in a zone c o n t i g u o u s w i t h t h e reactor w a l l . The p a t t e r n o f t h e reacted a n d u n r e a c t e d zones suggests t h a t t h e pellet

304


was misaligned in the field shielding a portion of the lignin from electron bombardment. This phenomenon, together with short duration runs showing a distinct shell of reacted lignin, lends support to the explanation that the lignin is transformed primarily by electron bombardment, a surface phenomenon, the rate of which depends on the electron concentration and energy. Effects of Some S y s t e m Parameters on Acetylene Production

In order to evaluate the importance of the major system parameters on the product distribution and composition, experiments were conducted in which power, carrier gas composition, flowrate, and time were varied (41J. The variation of carrier composition and power are best described simultaneously since the effect of the input power is dependent on the carrier gas under consideration. This interaction exists since a change in either parameter alters the electron concentration and electron energy in the system (10). The power, expressed as energy absorbed over the 10-minute run. was varied in several experiments and its effect on acetylene production is shown in Figure VI for three carrier gases, helium, argon, and hydrogen. The effects on other products are reported elsewhere (46). The higher concentration of H when it serves as the carrier gas promotes an increase of roughly 1.5 times the acetylene produced in either inert carrier.

2

The mechanism for the homogeneous production of acetylene from the CO and H in the plasma has been suggested by Mertz, et al. (32) as follows: 2

H + e - — 2H- + e H- + CO — CH- + 0 · 2CH* C H 2

2

+

2

Although this mechanism can be expected to occur in H carrier gas experiments, a considerable volume fraction, 43%, of the gaseous products is H derived from the lignin itself (Table II). Thus the CH- radical is being generated directly from lignin and complex hydrocarbon fragmentation. Several studies (13.26,31) of coal plasma pyrolysis report acetylene production in somewhat smaller quantities than reported here consistent with the lower H to C ratio of coal. We suggest, however, that the acetylene production rate is a complex phenomenon dependent on the reactor geometry, local plasma temperature (47.48), plasma applicator configuration, quenching rate, and mass transfer limitations. This matter is under continued investigation. 2

2

15.

GRAEF ET AL.

305

Acetylene front Biomass/Lignin

DISCUSSION Devolatilization kinetics e x p e r i m e n t s (43,44) a n d pellet behavior in w h i c h t h e unreacted-reacted

interface is sharply m a r k e d i n d i c a t e t h a t t h e p r i m a r y

reactions w i t h i n t h e solid c a n be d e s c r i b e d b y a shell-progressive m o d e l of t h e t y p e discussed b y Carberry (45) a n d others. H o w e v e r , t h e e x p e c t e d s e c o n d a r y reactions d e s c r i b e d briefly b e l o w are q u i t e different f o r volatiles e s c a p i n g t o t h e gas phase plasma or volatiles r e m a i n i n g in t h e pellet or charresidual. T h e differences b e t w e e n plasma a n d t h e r m a l pyrolysis r e g a r d i n g p r o d u c t d i s t r i b u t i o n s s h o w n in Figure V a n d Tables II a n d III arise f r o m t h e n a t u r e of t h e secondary reactions. In t h e g a s phase, t h e p l a s m a reactor e n v i r o n m e n t c o n t a i n s e l e c t r o n s g e n e r a t e d a n d s u s t a i n e d b y t h e m i c r o w a v e field (10). T h e e l e c t r o n , b y v i r t u e of its c h a r g e a n d m i n u t e mass, is t h e p r e d o m i n a n t " c a r r i e r " by w h i c h this transfer of e l e c t r o m a g n e t i c e n e r g y t o kinetic energy is a c h i e v e d . Specifically, t h e e l e c t r o n , accelerated b y t h e rapidly o s c i l l a t i n g f i e l d , develops s u f f i c i e n t kinetic energy (1-2 KEV) or p s e u d o - t e m p e r a t u r e s (on t h e order of 1 0 °K) t o dissociate, excite, or ionize o t h e r m o l e c u l e s present in t h e gas. The e n e r g e t i c e l e c t r o n also f r a g m e n t s t h e surface lignin a n d o t h e r h y d r o c a r b o n s u p o n collision. Because of t h e relatively h i g h c o n c e n t r a t i o n o f free radicals a n d o t h e r e n e r g e t i c species, t h e p l a s m a gas reactions are characterized as h i g h t e m p e r a t u r e reactions o c c u r r i n g at rapid rates a n d p r o d u c i n g o t h e r e n e r g e t i c species. T h e p r o d u c t i o n of a c e t y l e n e in a m i c r o w a v e p l a s m a , as c o n t r a s t e d t o e t h a n e or m e t h a n e in c o n v e n t i o n a l pyrolysis, s u p p o r t s t h i s c o n c e p t o f t h e p l a s m a g a s as a h i g h e n e r g y zone, since as Figure VII f r o m Baddour a n d T i m m i n s (10) s h o w s , a c e t y l e n e is t h e r m o d y n a m i c a J l y stable at higher 4

t e m p e r a t u r e w h i l e m e t h a n e a n d e t h a n e are n o t . A l t h o u g h t h e r m o d y n a m i c a r g u m e n t s c a n n o t strictly be used in a m i c r o w a v e p l a s m a as a c o n s e q u e n c e of t h e i n e q u a l i t y of t h e e l e c t r o n t e m p e r a t u r e a n d t h e t e m p e r a t u r e o f t h e ions and molecules, the production of acetylene tends t o suggest that the electron c o n c e n t r a t i o n a n d v e l o c i t y d e t e r m i n e t h e e n e r g e t i c s o f t h e gas phase. Further details of t h e plasma gas reactions are discussed in Graef (41,46). A l t h o u g h e l e c t r o n b o m b a r d m e n t is rapid a n d t h e p r e d o m i n a n t f o r m of heat transfer in t h e gas phase, t r a n s p o r t processes w i t h i n t h e pellet are q u i t e d i f f e r e n t a n d m u c h slower. Increased c h a r yield a n d c o n d e n s e d a r o m a t i c s f o u n d in t h i s s t u d y are c o n s i s t e n t w i t h t h e f o l l o w i n g d e s c r i p t i o n of t h e processes o c c u r r i n g w i t h i n t h e pellet. U p o n collision w i t h t h e pellet surface, t h e f l u x of electrons relases large a m o u n t s o f heat w h i c h volatilizes a n d cracks t h e p o l y m e r i c l i g n i n . D e p e n d i n g o n t h e gas c o m p o s i t i o n as in Figure V I , t h e s t o i c h i o m e t r y (or C / O / H ratios) o f t h e biomass, a n d t h e mass t r a n s p o r t s i t u a t i o n , an a m o u n t of residual or char f o r m s i n w a r d f r o m t h e pellet surface, w h i l e t h e volatiles o u t f l o w increases t h e gas pressure near t h e pellet.

306


PRODUCTION OF ACETYLENE vs ENERGY ABSORBED

140

1201

2

< tr ο \ ο

ΙΟΟι

3 80 >

Ο UJ

60fFigure 6. Acetylene evolved (standard ce) as a function of tubegrabed power absorbed over a 10-min experiment: (0) He, (A) Ar, and (+) H carriers; line is trend only 9

..J I 40 0 40 80 120 160 200 ENERGY ABSORBED (watt-hours)

25h /

2

o

15

υ

I 0 h

^

^ / ^ E T H Y L E N E CjH -| 4

^ / S > V - ÂCETYLENE CjHj

ETHANE

5

C H 2

METHANE C H

6

4

GRAPHITE ( S O L I D ) HYDROGEN(GAS)
1-10 m i n " ) t h a n one m i g h t predict f r o m electron p s e u d o t e m p e r a t u r e s in t h e gas p l a s m a . A f t e r a n initial period, these a p p a r e n t devolatilization rates are c o n s i s t e n t w i t h rates f o r c o n d u c t i o n in porous char (44,49). 1

3.

T h e porous interior of t h e pellet provides a resistance t o mass transfer, i.e., c o n f i n e s t h e volatiles, w h i c h increases reactive f r a g m e n t c o n c e n t r a t i o n s . This p r o m o t e s p o l y m e r i z a t i o n a n d c o n d e n s a t i o n reactions w h i c h f o r m t h e greater char f r a c t i o n a n d c o n d e n s e d a r o m a t i c s (Table III a n d Figure V) t h a n r e p o r t e d f o r c o n v e n t i o n a l pyrolysis.

D e p e n d i n g o n t h e carrier gas f l o w rate, v a c u u m p u m p c a p a c i t y , degree of c r a c k i n g a n d local plasma t e m p e r a t u r e , t h e j u s t - v o l a t i l i z e d higher m o l e c u l a r w e i g h t gases c a n be either s w e p t f r o m t h e plasma zone or u n d e r g o secondary plasma r e a c t i o n s . T h e s e g a s - p h a s e s e c o n d a r y r e a c t i o n s (described previously) o c c u r t o v a r y i n g e x t e n t s d u e t o t h e residence t i m e d i s t r i b u t i o n (laminar f l o w ) a n d spatially n o n u n i f o r m plasma p s e u d o t e m p e r a t u r e (48). T h e c o n d e n s i b l e volatile f r a c t i o n c o m p o n e n t s s u c h as guaiacol a n d cresol t h a t reflect t h e original lignin s t r u c t u r e are rapidly q u e n c h e d volatiles s u b j e c t e d t o short residence t i m e s or l o w - p l a s m a t e m p e r a t u r e s , perhaps f r o m t h e o u t e r m o s t pellet layer reacted. The d e s c r i p t i o n of t h e s e c o n d a r y reactions in t h e gas phase is f u r t h e r c o m p l i c a t e d b y t h e fact t h a t e l e c t r o n c o n c e n t r a t i o n a n d average electron v e l o c i t y or energy d o n o t remain c o n s t a n t f o r t h e entire e x p e r i m e n t because

308


of n o n c o n s t a n t s y s t e m pressure a n d c o m p o s i t i o n . The e l e c t r o n c o n c e n t r a t i o n is inversely related t o pressure a n d h i g h l y d e p e n d e n t o n gas c o m p o s i t i o n via t h e ionization p o t e n t i a l of t h e c o m p o n e n t s (47). T h e a b s o r b e d p o w e r also c h a n g e s w i t h gas c o m p o s i t i o n , f u r t h e r c o u p l i n g t h e variables. T h u s , t h e volatiles o u t f l o w reduces t h e p l a s m a h e a t i n g rate as a f u n c t i o n of t h e rate a n d a m o u n t of pressure increase. E x p e r i m e n t s c o n d u c t e d w i t h increasing initial p o w e r as s h o w n in Figure VI s h o w a c o m p l e x d e p e n d e n c e o n p o w e r a n d e x p e r i m e n t s are b e i n g c o n d u c t e d t o u n c o v e r t h e m e c h a n i s m s . A s t r o n g effect of particle size is o b s e r v e d a n d d e s c r i p t i o n of t h e c o u p l e d t r a n s p o r t a n d reaction processes is r e p o r t e d e l s e w h e r e (43,44). SUMMARY Rapid, severe d e g r a d a t i o n has been s h o w n t o n a r r o w t h e reaction p r o d u c t d i s t r i b u t i o n in p l a s m a pyrolysis of t h e a r o m a t i c f r a c t i o n of b i o m a s s . l i g n i n . This paper reports o n l y f i v e c o m p o u n d s w i t h yields > 2 % if t h e tar (3% t o 10%) a n d char (33%) are c o n s i d e r e d t w o of t h e five. Primarily g a s i f i c a t i o n o c c u r s since yields of 51 w e i g h t p e r c e n t gases are f o u n d . These gases, are 17 w e i g h t p e r c e n t H (43 v o l u m e percent) a n d C H is 13 w e i g h t p e r c e n t (14 v o l u m e percent). Char y i e l d is r e d u c e d as s u g g e s t e d by L e w e l l y n because of c o n s u m p t i o n by increased radical c o n c e n t r a t i o n s , here, associated w i t h t h e p l a s m a . C o n d e n s e d a r o m a t i c s in a d d i t i o n t o p h e n o l - t y p e c o m p o u n d s are f o u n d in t h e tar f r a c t i o n , d u e p r e s u m a b l y t o mass t r a n s p o r t l i m i t a t i o n s in t h e pellet. Despite t h e n a r r o w p r o d u c t d i s t r i b u t i o n a n d h i g h h e a t i n g v a l u e of t h e gas p r o d u c e d , e n e r g y c o n s u m p t i o n for t h e process is h i g h d u e t o t h e necessity of m a i n t a i n i n g t h e p l a s m a a n d t o t h e large pellet size s l o w i n g t h e rate. Because of t h e p o t e n t i a l of very fast reactions in t h e p l a s m a , t r a n s p o r t c o n s i d e r a t i o n s , especially heat transfer, b e c o m e increasingly i m p o r t a n t and point the way to further improvements. 2

2

2

ACKNOWLEDGEMENTS This w o r k w a s s u p p o r t e d by t h e National Science F o u n d a t i o n u n d e r t h e Division of A d v a n c e d Energy a n d Resources Research a n d T e c h n o l o g y Grant No. 7 7 0 8 9 7 9 a n d t h e NSF Engineering Initiation Grant Program. M a r t h a Graef w i s h e s t o a c k n o w l e d g e f i n a n c i a l assistance f r o m t h e D e p a r t m e n t of C h e m i c a l E n g i n e e r i n g . U n i v e r s i t y of W a s h i n g t o n . T h e h e l p f u l s u g g e s t i o n s of Κ. V. Sarkanen, D. H a n s o n , D. E d e l m a n , B. H r u t f i o r d , a n d R. Chan are also g r a t e f u l l y acknowledged.

15. GRAEF ET AL. Acetylene from Biomass/Lignin

309

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Lignin for

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