The Chemistry of Pyrolysis and Combustion - American Chemical

13 The Chemistry of Pyrolysis and Combustion FRED SHAFIZADEH

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1

Wood Chemistry Laboratory, University of Montana, Missoula, M T 59812

Cellulosic materials decompose on heating or exposure to an ignition source by two alternative pathways. The first pathway, which dominates at temperatures below 300 °C, involves reduction in the degree of polymer ization by bond scission; elimination of water; formation of free radicals, carbonyl, carboxyl, and hydroperoxide groups; evolution of CO and CO ; and, finally, produc tion of a highly reactive carbonaceous char. The second pathway, which takes over at temperatures above 300 °C, involves cleavage of molecules by transglycosylation, fission, and disproportionation reactions to provide a mixture of tarry anhydro sugars and lower molecular weight volatile products. Oxidation of the reactive char gives smoldering or glowing combustion, and oxidation of the combustible volatiles gives flaming combustion. Flaming combustion could be retarded by inorganic ma terials that suppress the formation of the combustible volatiles through dehydration and charring of the sub strate. The smoldering combustion could be suppressed or enhanced by catalysts that affect the rates of oxida tion of the char to CO ( Δ Η = 22.9 kcal/mol) and CO (ΔΗ = -88.5 kcal/mol). The kinetics and mechanisms of the thermal decomposition, the rates of combustion and heat release, the composition of the py rolysis products, and the formation and reactivity of char have been investigated extensively to provide a chemical description for combustion and fire prevention. 2

2

ONE OF THE GREATEST ASSETS OF CELLULOSIC MATERIALS

is t h e i r c o m patibility with nature, including their combustibility and degradability w h i c h allow for constant t u r n o v e r a n d regeneration of these 1

Deceased

0065-2393/84/0207-0489/$11.25/0 © 1984 American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

490

T H E CHEMISTRY OF SOLID WOOD

natural resources. A f u n d a m e n t a l u n d e r s t a n d i n g of these properties a n d p o s s i b l e m e t h o d s f o r c o n t r o l l i n g t h e m is e s s e n t i a l f o r p r o t e c t i o n and better utilization of these materials. C o m b u s t i o n of w o o d involves a c o m p l e x series of p h y s i c a l trans formations a n d c h e m i c a l reactions that are f u r t h e r c o m p l i c a t e d b y the heterogeneity of the substrate. W o o d , and cellulosic materials i n general, do not b u r n directly; u n d e r the i n f l u e n c e of sufficiently s t r o n g heat sources t h e y d e c o m p o s e to a m i x t u r e o f v o l a t i l e s , t a r r y c o m p o s i t i o n s , a n d h i g h l y reactive carbonaceous char. Gas-phase ox idation of the combustible volatiles and tarry products produces f l a m i n g combustion. Solid-phase oxidation of the r e m a i n i n g char pro duces g l o w i n g or s m o l d e r i n g c o m b u s t i o n , d e p e n d i n g o n the rate of o x i d a t i o n (see F i g u r e 1). T h e following discussion shows h o w the chemical composition, rate of f o r m a t i o n , a n d heat of c o m b u s t i o n of the pyrolysis products are affected b y the variations i n the c o m p o s i t i o n of the substrate, the t i m e a n d temperature profile, a n d the presence of inorganic additives o r c a t a l y s t s . T h e l a t t e r a s p e c t , h o w e v e r , is d i s c u s s e d i n m o r e d e t a i l i n C h a p t e r 14. Combustion m a y b e d e f i n e d as c o m p l e x i n t e r a c t i o n s among fuel, energy, and the environment. Consequently, the c o m b u s t i o n p r o c e s s is c o n t r o l l e d n o t o n l y b y t h e a b o v e c h e m i c a l f a c t o r s , b u t also b y t h e p h y s i c a l p r o p e r t i e s of the substrate a n d o t h e r p r e v a i l i n g c o n d i t i o n s affecting the p h e n o m e n a of heat a n d mass trans p o r t . D i s c u s s i o n o f t h i s p h e n o m e n o n is b e y o n d t h e s c o p e o f t h i s chapter. T h e g e n e r a l l i t e r a t u r e o n t h i s s u b j e c t is r a t h e r c o n f u s i n g a n d controversial, m a i n l y because of the variations i n the composition of substrate r a n g i n g f r o m different types of w o o d to different types o f p u l p a n d n a t u r a l p l a n t fibers. S u p e r i m p o s e d o n t h e s e v a r i a t i o n s a r e t h e effects o f i n o r g a n i c s o r a s h c o n t e n t , t h e t i m e a n d t e m p e r a t u r e profile, the a m b i e n t atmosphere, a n d the conditions of the heat a n d mass transport, w h i c h are s e l d o m the same i n different e x p e r i m e n t s . T h e r e f o r e , f o r a f u n d a m e n t a l u n d e r s t a n d i n g o f t h e s u b j e c t , i t is e s sential to e x a m i n e the i n d i v i d u a l c o m p o n e n t s of the substrate r a t h e r than a p o o r l y d e f i n e d a n d variable aggregate. T h e properties of the aggregate, h o w e v e r ( d i s c u s s e d later), are e x p e c t e d to c o r r e s p o n d w i t h t h e c o l l e c t i v e p r o p e r t i e s o f its c o m p o n e n t s . I n t h i s c h a p t e r , t h e p y rolysis or t h e r m a l degradation reactions of cellulose are d e s c r i b e d i n d e t a i l . C e l l u l o s e is t h e m a j o r c o m p o n e n t o f w o o d a n d o t h e r c e l l u l o s i c m a t e r i a l s as w e l l as t h e m a j o r s o u r c e o f c o m b u s t i b l e v o l a t i l e s t h a t fuel the f l a m i n g combustion.

Formation of Volatile Products from Cellulose T h e g e n e r a l p a t h w a y s for p y r o l y s i s of c e l l u l o s e , l e a d i n g to p r o d u c t i o n o f c h a r as w e l l as g a s e o u s a n d v o l a t i l e p r o d u c t s , a r e s h o w n

Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

13.

SHAFIZADEH

Pyrolysis and Combustion

491

HEAT FLUX FLAMING

Figure 1. Graphic presentation of the flaming and smoldering combustion showing the respective roles of combustible volatiles and active char produced by pyrolysis under heatfluxat different conditions.

i n S c h e m e 1. T h e g l o b a l k i n e t i c s f o r i s o t h e r m a l e v o l u t i o n o f v o l a t i l e pyrolysis products from purified cotton linter cellulose, i n the t e m perature range o f 2 5 7 - 3 1 0 °C, have b e e n studied i n air and nitrogen (12). T h e A r r h e n i u s p l o t o f t h e r e s u l t s b a s e d o n f i r s t - o r d e r k i n e t i c s ( s h o w n i n F i g u r e 2) g a v e a n a c t i v a t i o n e n e r g y o f 17 k c a l / m o l f o r t h e


492


Glowing

Ignition

Polymers

Flaming

Pyrolysis

combustion

Combustion

Scheme 1. The pyrolysis and combustion of

cellulose.

w e i g h t loss d u e t o t h e o v e r a l l p y r o l y t i c r e a c t i o n o f c e l l u l o s e i n a i r , a n d 37 k c a l / m o l i n a nitrogen atmosphere. A transition a r o u n d 300 ° C ( F i g u r e 3) r e f l e c t s t h e e x i s t e n c e o f t w o d i f f e r e n t p a t h w a y s . T h e rate of pyrolysis d e t e r m i n e d b y t h e r m o g r a v i m e t r i c analysis ( T G A ) u n d e r i s o t h e r m a l c o n d i t i o n s ( F i g u r e 4) s h o w s a n i n i t i a l p e r i o d o f a c c e l e r a t i o n that p r o c e e d s faster i n air t h a n i n the i n e r t a t m o s p h e r e . A s t h e p y r o l y s i s t e m p e r a t u r e is i n c r e a s e d , t h e i n i t i a t i o n p e r i o d a n d the difference between pyrolysis u n d e r nitrogen and air gradually d i m i n i s h a n d d i s a p p e a r at 3 1 0 °C w h e n p y r o l y s i s b y t h e s e c o n d p a t h w a y takes over. First Pathway. T h e reactions i n the first p a t h w a y — w h i c h d o m i n a t e s at l o w e r t e m p e r a t u r e s — i n v o l v e r e d u c t i o n i n t h e d e g r e e of p o l y m e r i z a t i o n b y b o n d scission; appearance of free radicals; e l i m ination of water; formation of carbonyl, carboxyl, and hydroperoxide groups (especially i n air); e v o l u t i o n of C O a n d C 0 ; a n d finally p r o d u c t i o n o f a c h a r r e d r e s i d u e . T h e s e r e a c t i o n s , w h i c h c o n t r i b u t e to the o v e r a l l rates of p y r o l y s i s of c e l l u l o s i c materials, have b e e n i n v e s t i g a t e d i n d i v i d u a l l y (2). R e d u c t i o n i n t h e d e g r e e o f p o l y m e r i z a t i o n o f c e l l u l o s e o n i s o t h e r m a l h e a t i n g i n a i r o r n i t r o g e n at a t e m p e r a t u r e w i t h i n t h e r a n g e o f 1 5 0 - 1 9 0 ° C has b e e n m e a s u r e d b y t h e v i s c o s i t y m e t h o d ( F i g u r e 5). T h e r e s u l t i n g d a t a h a v e b e e n c o r r e l a t e d ; t h e r a t e s of b o n d scission are g i v e n i n Table I a n d are u s e d for c a l c u l a t i n g the A r r h e n i u s p l o t s h o w n i n F i g u r e 6. T h e s e c a l c u l a t i o n s g i v e a n a c t i v a t i o n e n e r g y of 21 k c a l / m o l for b o n d scission i n air a n d 27 k c a l / m o l i n n i t r o g e n . T h i s i n d i c a t e s t h a t at l o w t e m p e r a t u r e s a l a r g e r n u m b e r of b o n d s are b r o k e n i n air than i n n i t r o g e n . 2

T h e r a t e s o f p r o d u c t i o n o f C O a n d C 0 at 1 7 0 ° C i n a i r a n d n i t r o g e n ( F i g u r e 7) i n d i c a t e t h a t t h e r a t e s for e v o l u t i o n o f t h e s e gases are m u c h faster i n a i r t h a n i n n i t r o g e n a n d that these rates accelerate 2



494


ι

100

ι

τ

1

ι -

ι

200

1

1

ι

ι

300

1

»

ι

ι

400

Γ

1

500

1

1

600

Reference Temperature (°C) Figure 3. Differential thermal analysis of untreated wood, cellulose, and lignin run in an oxygen atmosphere. o n c o n t i n u e d h e a t i n g . C o m p a r i s o n of t h e i n i t i a l l i n e a r rates for the e v o l u t i o n o f t h e s e gases w i t h t h e r a t e s o f b o n d s c i s s i o n o b t a i n e d f o r d e p o l y m e r i z a t i o n at 1 7 0 ° C (Table II) s h o w s t h a t t h e r a t e o f b o n d scission i n air a p p r o x i m a t e l y equals the rate of p r o d u c t i o n of C O plus C O i n m o l e s p e r glucose u n i t . I n n i t r o g e n , h o w e v e r , the rate o f b o n d s c i s s i o n is g r e a t e r t h a n t h e r a t e s o f C O a n d C O e v o l u t i o n combined.

z

£

I t is a s s u m e d t h a t C 0 a n d C O a r e f o r m e d b y d e c a r b o x y l a t i o n and decarbonylation, respectively. T h e significance of the former reactions was d e t e r m i n e d b y m e a s u r i n g the net rate of a c c u m u l a t i o n o f c a r b o x y l a n d c a r b o n y l g r o u p s i n c e l l u l o s e u p o n h e a t i n g i n a i r at 190 °C. T h e r e s u l t s s h o w n i n F i g u r e 8 i n d i c a t e a n a l m o s t l i n e a r rate of f o r m a t i o n o n h e a t i n g for 50 h . O n h e a t i n g for l o n g e r p e r i o d s , t h e r a t e o f a c c u m u l a t i o n o f c a r b o x y l g r o u p s falls off, a n d t h e r a t e o f a c c u m u l a t i o n o f c a r b o n y l g r o u p s is i n c r e a s e d . T h e r e w a s a l s o a v e r y 2



496

T H E CHEMISTRY O F SOLID WOOD

N2

Δ

150eC

Air A

Δ

I

1 10

ι

ι 30

20

ι 40

» 50

ι

T i m e (h) Figure 5. Viscosity average degree of polymerization ( P J of cellulose heated in air or nitrogen at 150, 170, and 190 °C.

T a b l e I. R a t e Constants for the D e p o l y m e r i z a t i o n o f Cellulose in A i r and Nitrogen Temperature

CO

a

ko χ 10 (moll 162 g min) 1

Conditions

150

N Air

160

N Air

170

N Air

180

N Air

190

N Air

2

2

2

2

2

0

1.1 6.0 2.8 8.1 4.4 15.0 9.8 29.8 17.0 48.9

162 g represents 1 m o l o f m o n o m e r unit.


13.

sHAFizADEH


497

-12

-13

Ink

.

] 4

-15

-16

2.15

2.2 Ι/Γ

2.25 χ

10

2.3

2.35

3

Figure 6. Arrhenius pfot for the rate of bond scission in air (Φ) and nitrogen (O). small increase i n the n u m b e r of these functions on heating i n n i trogen, w h i c h could be formed by dehydration and rearrangement, as s h o w n f o r m o d e l c o m p o u n d s (3, 4). T h e e x t e n t o f t h e d e c a r b o x y l a t i o n at t h e l o w e r p y r o l y s i s t e m p e r a t u r e s w a s d e t e r m i n e d w i t h s a m ples of carboxylcellulose h a v i n g a l o w degree of substitution, w i t h c a r b o x y l g r o u p s at C - l , C - 2 , C - 3 , a n d C - 6 . T h e r e s u l t s g e n e r a l l y w e r e n o t c o n c l u s i v e , a l t h o u g h t h e s a m p l e o x i d i z e d at C - 2 a n d C - 3 s h o w e d a definite reduction i n carboxyl-group content. T h e t h e r m a l d e g r a d a t i o n of c e l l u l o s e m a y also i n v o l v e a free r a d i c a l m e c h a n i s m . It w a s d i f f i c u l t to o b s e r v e t h e s e r a d i c a l s , b u t i t was p o s s i b l e to m o n i t o r t h e f o r m a t i o n o f h y d r o p e r o x i d e g r o u p s o n h e a t i n g cellulose i n air. T h e h y d r o p e r o x i d e functions are f o r m e d a n d decomposed simultaneously, and their concentration rapidly climbs u n t i l a s t e a d y s t a t e is r e a c h e d . T h e d e c o m p o s i t i o n o f t h e h y d r o p e r o x i d e f u n c t i o n a p p e a r e d t o f o l l o w first-order k i n e t i c s w i t h a r a t e c o n -


498


mmol/ 162 g m i n

200

400

600

Figure 7. Yields of CO and CO from heating cellulose at 170 °C. Key: O , C0 in N ; • , CO in N ; ·, C0 in air; and M, CO in air. 2

2

2

2

2

stant of 2.5 x 1 0 " m i n at 170 ° C . F r o m t h e s t e a d y - s t a t e c o n c e n tration of 3.0 x 1 0 ~ mol/164 g m i n , the rate of h y d r o p e r o x i d e d e c o m p o s i t i o n i s , t h e r e f o r e , 7.5 x 1 0 " m o l / 1 6 2 g m i n . W h e n c o m p a r e d w i t h t h e i n i t i a l r a t e o f b o n d s c i s s i o n i n a i r o f 1.5 X 10 " m o l / 1 6 2 g m i n at 1 7 0 ° C (Table I), i t is a p p a r e n t t h a t h y d r o p e r o x i d e f o r m a t i o n c o u l d m a k e a s i g n i f i c a n t c o n t r i b u t i o n to b o n d s c i s s i o n . 2

-

1

5

7

6

T h e s e c o n s i d e r a t i o n s r e v e a l t h a t t h r e e stages a r e i n v o l v e d i n t h e low temperature pathway of cellulose: initiation of pyrolysis, propa gation, and product formation. T h e initiation period apparently i n T a b l e II.

Initial Rates o f G l y c o s i d i c B o n d Scission a n d of C O and C 0 Formation 2

Rate x 10 in N (moll 162 g h)

Rate x 10 in Air (moll 162 g h)

2.7 0.6 0.4

9.0 6.4 2.1

s

Reaction B o n d scission" C O evolution^ C0 evolution** 2

2

s

N O T E : Values are for reaction at 1 7 0 ° C . C a l c u l a t e d from the rate constants i n Table I. C a l c u l a t e d from the initial linear p o r t i o n of plots in F i g u r e 8.

a

b



500

THE CHEMISTRY O F SOLID WOOD

volves the formation of free radicals facilitated b y the presence

of

oxygen or inorganic i m p u r i t i e s . S u b s e q u e n t reactions of the free r a d icals c o u l d l e a d to b o n d scission, o x i d a t i o n , a n d d e c o m p o s i t i o n of the molecule,

to p r o d u c e

char, water, C O , a n d C 0 . S c h e m e 2 2

the initiation, propagation, and decomposition

shows

reactions i n v o l v e d i n

the t h e r m a l d e c o m p o s i t i o n of cellulose b y this p a t h w a y i n air. I n an inert atmosphere, a lactone could be formed by rearrangement and d e c o m p o s e d b y d e h y d r a t i o n a n d d e c a r b o x y l a t i o n (3,

Second

Pathway.

A t temperatures

4).

of approximately 300

°C,

the s e c o n d p a t h w a y g r a d u a l l y takes o v e r a n d d o m i n a t e s . T h e p r i m a r y reaction i n this pathway involves depolymerization by

transglycosy-

l a t i o n . T h i s r e a c t i o n t a k e s p l a c e w h e n t h e m o l e c u l e has g a i n e d suf ficient

f l e x i b i l i t y (activation) a n d p r o d u c e s levoglucosan

β-D-glucopyranose),

(1,6-anhydro-

its furanose i s o m e r ( 1 , 6 - a n h y d r o ^ - D - g l u c o f u r a -

nose) a n d r a n d o m l y l i n k e d oligosaccharides

as s h o w n i n S c h e m e

3

(5). T h e i n t e r m o l e c u l a r a n d i n t r a m o l e c u l a r t r a n s g l y c o s y l a t i o n s s h o w n i n this s c h e m e are a c c o m p a n i e d

by dehydration, followed by

fission

a n d d i s p r o p o r t i o n a t i o n r e a c t i o n s i n t h e gas p h a s e , a n d f u r t h e r

de

c o m p o s i t i o n a n d c o n d e n s a t i o n o f t h e s o l i d p h a s e to p r o d u c e a m i x t u r e o f gases a n d v o l a t i l e p r o d u c t s a n d a " s t a b l e " c a r b o n a c e o u s c h a r (3, 6). O n raising the t e m p e r a t u r e , the tar-forming reactions accelerate

Scheme 2. Possible mechanism of formation and decomposition of cellu lose hydroperoxide formed thermally in air.


13.

sHAFizADEH

501


CH OH 2

CH OH 2

t3 a S

< χ ο υ

4

Jj ill V1w h

d

f

A

j

12

8

?4

16

Retention time (min)

Figure 14. Typical gas chromatogram of oxidation product. Key: a b sol vent; c, benzenedicarboxylic acid; d benzenemonohydroxydicarboxylic acid; e benzenetricarboxylic acid; /, unknown; g, benzenetetracarboxylic acid; h, unknown; i, benzenepentacarboxylic acid; j, benzenehexacarboxylic acid. y

y

y

t

T a b l e V . E l e m e n t a l C o m p o s i t i o n o f S t a r t i n g M a t e r i a l a n d Its C h a r P r e p a r e d b y t h e I s o t h e r m a l Pyrolysis for 5 M i n at the T e m p e r a t u r e s N o t e d

Temperature Material Cellulose

Wood Lignin

CO

Char Yield (wt%)

Empirical Formula (reftoC )

Composition C

Η

O"

42.8 47.9 61.3 73.5 78.8 80.4

6.5 6.0 4.8 4.6 4.3 3.6

50.7 46.1 33.9 21.9 16.9 16.1

^6^11^5.3

63.3 33.1 16.7 10.5 8.7

no treatment 400

6.4 4.6

47.2 22.2

C ^ H g 9Ο4.6

24.9

46.4 73.2

no treatment 400

73.3

64.4 72.7

5.6 5.0

24.8 22.3

no treatment 325 350 400 450 500

6

^βΗ 0 CeH 0 5 ^βΗ 0 3 9

4

5

3

6

2

4 5

1

C6H3.2O0.9 C

f c

6 H

4

5

1

4

^6Η 0 ο ^6Η θΟ] 3 6 5

C

0

2

5

N O T E : C h a r p r e p a r e d b y isothermal pyrolysis for 5 m i n at the temperatures noted. B y difference C o n t a i n s 1.2% sulfur C o n t a i n s 0 . 6 % sulfur

a

h

c


13.

SHAFIZADEH


350

400

511

4 50

ΗΤΤ,·Ο Figure 15. Aromatic carbon content of the char from benzenedicarboxylic acid (O), benzenetricarboxylic acid (U), benzenetetracarboxylic acid (Δ), benzenepentacarboxylic acid (Φ), benzenehexacarboxylic acid (A), and the total yield of hydroxybenzenecarboxylic acid (V). accompanied b y a r e d u c t i o n of the hydrogen-to-carbon ratio ( H / C ) ( F i g u r e 17). T h e y i e l d s o f a r o m a t i c c a r b o n c o n t e n t s c a n b e c a l c u l a t e d o n the basis of the c a r b o n c o n t e n t of the o r i g i n a l cellulose r a t h e r t h a n t h e c a r b o n c o n t e n t o f t h e c h a r ( F i g u r e 18), as a f u n c t i o n o f H T T . I n F i g u r e 18 t h e y i e l d o f a r o m a t i c c a r b o n s a p p r o a c h e s 2 . 5 / 1 0 0 c a r b o n s o f t h e o r i g i n a l c e l l u l o s e at 4 0 0 ° C H T T a n d t h e n l e v e l s off. T h e a r o m a t i z a t i o n , as i n d i c a t e d b y d r a s t i c r e d u c t i o n o f t h e H / C r a t i o ( F i g u r e 19) a n d t h e i n c r e a s e d f o r m a t i o n o f B 5 C a n d B 6 C ( F i g u r e 17), c o n t i n u e s at H T T s a b o v e 4 0 0 ° C . A b o v e 4 0 0 ° C , t h e n u m b e r o f a r o m a t i c c l u s t e r s t h a t a r e o x i d i z e d to p o l y c a r b o x y l i c a c i d s r e m a i n s c o n stant, b u t the a r o m a t i z a t i o n c o n t i n u e s t h r o u g h c o n d e n s a t i o n a n d the g r o w t h of the i n d i v i d u a l clusters, w h i c h results i n l o w e r H / C ratios. F u r t h e r m o r e , t h e w e i g h t loss i n t h e " s t a b l e " c h a r at t e m p e r a t u r e s above 400 °C, a l t h o u g h r e l a t i v e l y small, m u s t take place b y e l i m i nation of aliphatic substituents and must be accompanied by some dehydrogenation a n d condensation or fusion of the products. These



1.5

1.0 H/C

0.5

Ο

Atomic ratio

Figure 17. Relation between total aromatic carbon and H/C ratio of char.


13.

SHAFIZADEH

513


ο D

3.0 O -

·-

o

2.0

1.0

0 350

400

450

500

HTT Figure 18. Number of aromatic carbons in 100 carbons of original cel lulose. processes apparently take place t h r o u g h h o m o l y t i c cleavage a n d c o n d e n s a t i o n o f t h e r e s u l t i n g free r a d i c a l s , e v i d e n t from t h e m e a s u r e m e n t o f t h e t r a p p e d f r e e - s p i n c o n c e n t r a t i o n i n c h a r t h a t is a f f e c t e d by H T T and other variables. F u r t h e r information about the structure and functionality of c h a r s p r o d u c e d w i t h i n t h e t e m p e r a t u r e r a n g e o f 3 2 5 - 5 0 0 ° C has b e e n obtained by cross-polarized magic-angle spinning ( C P M A S ) C-NMR spectroscopy, w h i c h allows quantitative investigation of the carbon skeleton i n solid a n d b y F o u r i e r transform I R ( F T I R ) spectroscopy, w h i c h detects the functional groups. T h e data obtained from these s t u d i e s a r e s u m m a r i z e d i n F i g u r e s 19 a n d 2 0 a n d T a b l e V I (13). A s c a n b e s e e n i n F i g u r e 19, t h e C P M A S C - N M R spectrum of u n t r e a t e d c e l l u l o s e h a s a s h a r p p e a k c o r r e s p o n d i n g to t h e c a r b o n a t o m s of the glucose units. O n further heating, n e w peaks appear that are associated w i t h aliphatic ( 0 - 6 0 p p m ) , olefinic ( 1 0 0 - 1 1 0 p p m ) , car boxylic and ester ( 1 6 0 - 1 8 0 ppm), and carbonyl carbons ( 1 9 0 - 2 2 0 p p m ) . W h e n t h e s a m p l e is h e a t e d at 3 2 5 ° C , a b o u t 3 7 % w e i g h t loss o c c u r s (Table V ) a n d t h e I R s p e c t r u m ( F i g u r e 20) s h o w s n e w b a n d s at 1 6 0 0 a n d 1 7 0 0 c m " , i n d i c a t i n g t h e f o r m a t i o n o f o l e f i n i c a n d c a r b o n y l f u n c t i o n a l i t i e s , r e s p e c t i v e l y . T h e f u n c t i o n a l i t i e s are d u e to d e hydration and rearrangement i n the glycosylic structure. A t the same t i m e , s m a l l b r o a d peaks are f o u n d i n b o t h sides of the glycosylic carbon region i n the N M R s p e c t r u m . H o w e v e r , these peaks are not q u a n t i f i e d e a s i l y . A t 3 5 0 ° C , t h e w e i g h t loss i n c r e a s e s t o 6 7 % (Table V ) a n d t h e N M R s p e c t r u m t h e n s h o w s n e w d i s t i n c t r e s o n a n c e s at 14 1 3

1 3

1



514

300

200

100

0

CHEMICAL SHIFT, ppm Figure 19. CPMAS C-NMR spectra of cellulose chars prepared for 5 min heating at different temperatures (Table V). Key: a, no treatment; b 325 °C; c, 350 °C; d, 400 °C; e, 450 °C, and f 500 °C. The small peak located in the 240-ppm region is a spinning sideband. 13

y



3

Group

b

a

Total

1 7 0 - •190 1 9 0 - •220

1 1 0 - 150 1 5 0 - 170

6 0 - 110

0 - •30 3 0 - 60

Shift

B a s e d o n t h e c a r b o n atoms i n char B a s e d o n t h e c a r b o n atoms i n original cellulose

CgHg - Ο Subtotal Oxygen Functional Group - C O O H , - C O O R >C = 0 , - C H O Subtotal

ÇcHc — H, CcHc — C

Others Subtotal Glycosylic Aromatic

CH

Paraffinic

Functional

Chemical Region (ppm)

(2)

3

1 0 0 (69)

8 (15)

(2)

3

4 (3) 8 6 (59)

4" (3)

1 0 0 (46.4)

5(2) 5 (1.4) 10 (3.4)

2 3 (11) 13(6) 3 6 (17)

9(4) 15 (7) 2 4 (11) 3 2 (15)

400

Preparation

1 0 0 (29.6)

1 (0.3) 2 (0.2) 3 (1.3)

5 6 (16) 13(4) 6 9 (20)

14(4) 13(4) 2 7 (8) < 1 (0.3)

350

325

b

(%) at Char

Distribution

1 0 0 (18.4)

1 (0.2) 1 (0.2) 2 (0.4)

6 6 (12) 11(2) 7 7 (14)

10 (2) 11(2) 2 1 (4) =0

450

Temperature

Table V I . Effect of C h a r Preparation Temperature on Distribution of C a r b o n Atoms in Various Functional Groups

101 (15.2)

< 1 (0.2) 1 (0.2)

—

7 9 (12) 9(1) 8 8 (13)

6(1) 6(1) 12 (2)

500

(°C)


4400

WAVE NUMBER . em-1 3200 2000 1400

600

200

\ure 20. FTIR spectra of cellulose chars prepared for 5 min heating at different temperatures. Key is the same as in Figure 23.


13.


SHAFizADEH

517

p p m for m e t h y l c a r b o n , 34 p p m for paraffinic c a r b o n w i t h o u t m e t h y l , 132 p p m f o r o l e f i n i c a n d a r o m a t i c c a r b o n a d j a c e n t t o h y d r o g e n o r a n o t h e r c a r b o n , 154 p p m f o r o l e f i n i c a n d a r o m a t i c c a r b o n a d j a c e n t to o x y g e n , 1 7 3 p p m f o r - C O O H , - C O O R , a n d 2 1 1 p p m f o r C = 0 a n d - C H O c a r b o n s . A r o m a t i c f o r m a t i o n p r o b a b l y is i n i t i a t e d at t h i s t e m p e r a t u r e b e c a u s e o f t h e a p p e a r a n c e o f N M R p e a k s at 132 a n d 154 p p m . T h e s e o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d by permanganate oxidation, w i t h w h i c h aromatic carbon contents of 0 % f o r 3 2 5 ° C a n d 2 . 8 % f o r 3 5 0 ° C a r e o b t a i n e d (12). A t t h e p y r o l y s i s t e m p e r a t u r e o f 4 0 0 ° C , t h e g l y c o s y l i c s t r u c t u r e is n o l o n g e r f o u n d i n e i t h e r t h e I R o r N M R s p e c t r u m , as i n d i c a t e d b y t h e d i s a p p e a r a n c e of the 1 2 2 0 - 9 0 0 - c m and 6 0 - 1 1 0 - p p m bands, respectively. T h e N M R s p e c t r u m t e n d s to g a t h e r i n t o t w o m a i n r e g i o n s , paraffinic a n d a r o m a t i c . R e s o n a n c e s of 1 1 0 - 1 7 0 p p m are assigned to a r o m a t i c car b o n s . H o w e v e r , t h i s r e g i o n c o r r e s p o n d s to o l e f i n i c c a r b o n s that are v e r y l i k e l y t o e x i s t at 3 5 0 ° C b u t u n l i k e l y t o s u r v i v e a b o v e 4 0 0 ° C w h e n p o l y c y c l i c s t r u c t u r e s a r e f o r m e d (12). T h e c h a r f o r m e d at 4 0 0 °C c o r r e s p o n d e d to o n l y 17 w t % of t h e o r i g i n a l m a t e r i a l a n d was r e l a t i v e l y s t a b l e b e c a u s e , o n h e a t i n g to s t i l l h i g h e r t e m p e r a t u r e s , t h e w e i g h t loss w a s r e l a t i v e l y s m a l l (Table V ) . I R s p e c t r a o f t h e c h a r s a b o v e 4 0 0 °C w e r e also s i m i l a r to e a c h other, e x c e p t the t r a n s m i t t a n c e of the 1 6 0 0 - c m b a n d b e c o m e s greater t h a n that of the 1 7 0 0 - c m b a n d at t e m p e r a t u r e s a b o v e 4 5 0 ° C . T h e s e o b s e r v a t i o n s s u g g e s t t h e increase a n d p r e d o m i n a n c e of aromatic structures. T h e extent of the a r o m a t i z a t i o n is d e m o n s t r a t e d b y t h e N M R d a t a , w h i c h s h o w t h a t the intensity of the aromatic signal increases w i t h increased char preparation temperature. 1

- 1

- 1

T h e relative yields of various carbon species i n these chars are s h o w n i n Table V I . T h e s e data indicate that the glycosylic carbon d i s a p p e a r s o n h e a t i n g u p to 4 0 0 °C. A t this t e m p e r a t u r e t h e c h a r c o n t a i n s 6 9 % a r o m a t i c a n d 2 7 % paraffinic c a r b o n s w h i c h c h a n g e to 8 8 % a n d 1 2 % , r e s p e c t i v e l y , at 5 0 0 ° C . T h e s e d a t a , i n c o n j u n c t i o n w i t h p r e v i o u s s t u d i e s (12) s h o w e d that stable c h a r contains m a i n l y c o n d e n s e d aromatic s t r u c t u r e w i t h i n t e r m i t t e n t p a r a f f i n i c g r o u p s . T h i s s t r u c t u r e is f o r m e d b y s u c c e s s i v e d e h y d r a t i o n , r e a r r a n g e m e n t , loss o f c a r b o x y l , c a r b o n y l , a n d paraffinic g r o u p s , f o r m a t i o n of free radicals, a n d c o n d e n s a t i o n of the c a r b o n s k e l e t o n to p o l y c y c l i c a r o m a t i c s t r u c t u r e s . T h i s i n v e s t i g a t i o n was e x t e n d e d to w o o d a n d l i g n i n chars p r e p a r e d at 4 0 0 ° C t o d e t e r m i n e t h e effect o f p r e e x i s t i n g a r o m a t i c n u c l e i of lignin i n the c h a r r i n g reactions. T h e permanganate oxidation anal ysis i n d i c a t e d that these chars, l i k e cellulose chars, have c o n s i d e r a b l y c o n d e n s e d o r c r o s s - l i n k e d a r o m a t i c s t r u c t u r e s , e v e n at 4 0 0 ° C . T h e N M R d a t a also s h o w e d that t h e chars f r o m s i m i l a r c e l l u l o s e , w o o d ,


518

THE CHEMISTRY OF SOLID WOOD

and lignin h a d a similar aromatic carbon content of about 7 0 % . T h e latter chars, h o w e v e r , s h o w e d d i s t i n c t N M R peaks for the C H - 0 C H group, arising from the methoxyl groups i n lignin from w h i c h the c h a r y i e l d f r o m the l i g n i n c o m p o n e n t of w o o d was e s t i m a t e d . T h e s e data i n d i c a t e d that preexisting aromatic n u c l e i i n lignin do not increase the a r o m a t i c c a r b o n c o n t e n t (aromaticity of the char) a l though they w i l l increase the char yield. 3

6

5

C h a r Reactivity. F r e s h l y p r e p a r e d c h a r is h i g h l y r e a c t i v e a n d p y r o p h o r i c . T h i s r e a c t i v i t y is c l o s e l y r e l a t e d t o s m o l d e r i n g c o m b u s tion and involves the formation of reactive char by pyrolysis, c h e m i sorption of oxygen on this product, evolution of C O and C 0 , a n d g e n e r a t i o n o f n e w r e a c t i v e sites (14, 15). T h i s p r o c e s s , w h i c h is a c companied by evolution of incompletely oxidized volatile pyrolysis p r o d u c t s at r e l a t i v e l y l o w t e m p e r a t u r e s , is u s u a l l y d i s t i n g u i s h e d f r o m t h e m o r e r a p i d a n d i n c a n d e s c e n t c o m b u s t i o n o f t h e c h a r at h i g h e r t e m p e r a t u r e s a n d i n t h e p r e s e n c e o f m o r e o x y g e n , w h i c h is k n o w n as glowing combustion. F i g u r e s 2 1 (14) a n d 2 2 s h o w t h e w e i g h t i n c r e a s e a n d h e a t o f r e a c t i o n d u e to c h e m i s o r p t i o n o f o x y g e n o n fresh char d e t e r m i n e d by thermogravimetry (TG) and differential scanning calorimetry ( D S C ) . I n l o w - d e n s i t y f i b r o u s c e l l u l o s i c m a t e r i a l s w h e r e t h e h e a t loss is r e s t r i c t e d b u t o x y g e n c a n p e n e t r a t e b y d i f f u s i o n , t h e h e a t f l u x f r o m c h e m i s o r p t i o n c o u l d play a significant role i n the ignition of the active 2

I

I

I 4

I

0

1

j

8 Time (min)

I

I

12

I

ι

I

16

Figure 21. Differential scanning calorimetry and thermogravimetry of oxygen chemisorption on cellulose char at 118 °C. The analysis was carried out on 2.5-mg samples in aluminum pans using a Cahn R-100 electrobalance and a DuPont calorimeter cell attached to a DuPont model 990 thermal analyzer, and nitrogen and oxygen gas flows (60 mhlmin, dried by passing through H S0 ) were rapidly interchangeable for DSC and TG. 2

4


13.

SHAFiZADEH


519

140 Η

120 h

i0

·

1

0.2

1

« 0.4

r 0.6

Adsorbed Oxygen (mmol/g char)

t

I 0.8

Figure 22. Differential heat of chemisorption, as a function of the amount of oxygen adsorbed on cellulose char at 118 °C. char a n d initiation o f the c o m b u s t i o n . F i g u r e 2 2 shows the differential h e a t o f c h e m i s o r p t i o n at 1 1 8 ° C , w h i c h i s g r a d u a l l y r e d u c e d as t h e m o r e r e a c t i v e sites a r e o c c u p i e d . T h e chemisorption data d e t e r m i n e d gravimetrically were ana l y z e d a c c o r d i n g t o E l o v i c h k i n e t i c s (15). T h e s e k i n e t i c s a r e b a s e d o n t h e a s s u m p t i o n t h a t t h e r a t e o f c h e m i s o r p t i o n (dg/dt) d e c l i n e s as t h e m o r e r e a c t i v e sites a r e q u e n c h e d a c c o r d i n g to t h e e q u a t i o n : dg/dt = a e x p ( - bg) w h i c h is i n t e g r a t e d o v e r t i m e b e t w e e n t h e l i m i t s o f - t a n d t to g i v e : G

g = (1/1?) I n ab + (lib) I n (t 4- t ) 0

w h e r e g i s t h e a m o u n t o f o x y g e n c h e m i s o r b e d at t i m e t a n d a a n d b are constants. T h e constant t allows for a n initial " p r e - E l o v i c h i a n " p e r i o d o f c h e m i s o r p t i o n t h a t is g e n e r a l l y m o r e r a p i d t h a n t h a t p r e dicted b y E l o v i c h kinetics. T h e value of t was determined b yan i n t e r a c t i o n m e t h o d t o g i v e t h e b e s t l i n e a r i t y o f p l o t s o f g v s . it + t ). 0

Q

0

T h e activation energy for oxygen chemisorption o n a typical char v a r i e d l i n e a r l y f r o m 13 to 2 2 k c a l / m o l w i t h surface coverage o f 0 - 2 . 5 m m o l 0 / g char ( F i g u r e 23). T h e s e data show t h e significance o f n a s c e n t o r f r e s h u n r e a c t e d c h a r that is p y r o p h o r i c a n d b e c o m e s h a r d e r t o i g n i t e as t h e l o w a c t i v a t i o n e n e r g y sites a r e o c c u p i e d a n d t h e h e a t o f c h e m i s o r p t i o n ( F i g u r e 21) is r e d u c e d . 2


520

T H E CHEMISTRY OF SOLID W O O D

q , mmo|

Ô2 / g

Figure 23. Variation of activation energy for oxygen chemisorption with surface coverage. T h e r e a c t i v i t y o f t h e c h a r is r e l a t e d to a h i g h c o n c e n t r a t i o n o f t r a p p e d free radicals a n d a large surface area. F i g u r e 24 shows the effect o f t e m p e r a t u r e o n t h e w e i g h t a n d free r a d i c a l c o n c e n t r a t i o n o f t h e c h a r s p r o d u c e d b y 1.5 m i n p y r o l y s i s u n d e r n i t r o g e n . T h e f r e e r a d i c a l c o n c e n t r a t i o n r e a c h e s a m a x i m u m f o r c h a r p r o d u c e d at a b o u t 550 °C. T h e s u r f a c e a r e a s o f c h a r s p r e p a r e d f r o m c e l l u l o s e s a m p l e s at different H T T s w e r e d e t e r m i n e d b y application of the D u b i n i n - P o l a n y e q u a t i o n t o C 0 a d s o r p t i o n at r o o m t e m p e r a t u r e a n d c o m p a r e d w i t h the area o c c u p i e d b y surface oxides calculated f r o m oxygen c h e m i s o r p t i o n at 2 3 0 ° C . T h e r e s u l t s s h o w n i n F i g u r e 2 5 i n d i c a t e t h a t c e l l u l o s i c c h a r s h a v e l a r g e s u r f a c e areas t h a t v a r y a c c o r d i n g to t h e H T T , a n d p e a k at a b o u t 5 5 0 ° C . T h e s u r f a c e o x i d e s f o r m e d b y c h e m i s o r p t i o n o c c u p y o n l y a p o r t i o n of the total surface area, a n d the c h e m i s o r p t i o n a l s o s h o w s a p e a k f o r c h a r s f o r m e d at a b o u t 5 5 0 ° C , c o r r e s p o n d i n g to the t e m p e r a t u r e of s m o l d e r i n g c o m b u s t i o n . 2

Combustion Combustibility. T h e intensity of combustion may be expressed by the following general equation: dw


13.

SHAFIZADEH


521

12 ι

0 » 400

(20

1

500

«

600

1

1

700

800

11 90

The Chemistry of Pyrolysis and Combustion - American Chemical

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