Self-Heating of Lignocellulosic Materials - ACS Publications

Chapter 26

Self-Heating of Lignocellulosic Materials Hans Kubler

Downloaded by UNIV OF ARIZONA on May 11, 2017 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch026

Department of Forestry, University of Wisconsin—Madison, Madison, WI 53706

P r o c e s s e s which generate heat i n organic materials are reviewed. At ordinary temperatures, r e s p i r a t i o n of living c e l l s and p a r t i c u l a r l y the metabolism o f microorganisms may cause s e l f - h e a t i n g , w h i l e a t e l e v a t e d temperatures p y r o l y s i s , a b i o t i c o x i d a t i o n , and a d s o r p t i o n o f v a r i o u s gases by c h a r r e d m a t e r i a l s d r i v e temperatures up whenever the r e l e a s e d heat i s unable t o d i s s i p a t e out o f the m a t e r i a l . The c r u c i a l rate o f p y r o l y t i c heat r e l e a s e depends on e x o t h e r m i c i t y and rates of the p y r o l y s i s process.

In s e l f - h e a t i n g m a t e r i a l s , t h e t e m p e r a t u r e r i s e s w i t h o u t i n p u t o f e n e r g y from t h e s u r r o u n d i n g s , and w i t h o u t any e v i d e n t h e a t - g e n e r a t i n g p r o c e s s such as v i s i b l e combustion. Some s e l f - h e a t i n g s start a t ambient conditions, while o t h e r s b e g i n a f t e r t e m p e r a t u r e o f t h e m a t e r i a l h a s been r a i s e d by o u t s i d e e n e r g y . The amount o f h e a t and t h e power p e r u n i t mass o f t h e m a t e r i a l a r e g e n e r a l l y s m a l l , b u t i n l a r g e p i l e s o r p a c k s a n d u n d e r o t h e r c o n d i t i o n s where l i t t l e o f t h e generated heat d i s s i p a t e s , temperatures t e n d to r i s e nevertheless. Self-heating i s quite common, yet i t receives a t t e n t i o n o n l y when i t c a u s e s economic l o s s e s o r l e a d s t o fires. F r e s h g r a s s c l i p p i n g s i n garbage cans and p l a s t i c b a g s h e a t w i t h i n an h o u r o r s o . P i l e s o f manure a n d compost a l w a y s seem t o s e l f - h e a t , a s t h e steam r i s i n g from opened p i l e s i n d i c a t e s . F a r m e r s have c a u s e t o be a l a r m e d when t e m p e r a t u r e s r i s e i n hay, s i l a g e , straw, cotton, 0097-6156/90/0425-0429$06.25/0 © 1990 American Chemical Society

Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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g r a i n , and o t h e r p l a n t m a t e r i a l s ; t h e y e x p e r i e n c e selfh e a t i n g i n e x p o s e d p i l e s , b a l e s and o t h e r f r e e s t o r a g e s as w e l l as i n s i l o s . Forest products industries know t h a t temperature i n c r e a s e s i n p i l e s o f sawdust and b a r k . In p u l p and p a p e r m i l l s , s e l f - h e a t i n g d e v e l o p s i n amassed t r e e chips. Paper r o l l s s t a c k e d h o t t e n d t o s e l f - h e a t , as o c c a s i o n a l l y do s t o r e d b a l e s o f waste p a p e r . The wood-base p a n e l p r o d u c t s p a r t i c l e b o a r d , hardboard, and f i b e r b o a r d s e l f - h e a t after b e i n g s t a c k e d too hot i n the f a c t o r y . Where i n s t r u c t u r e s t h e f r a m i n g lumber, wood-base p a n e l s , and l i g n o c e l l u l o s i c insulation i s heated by items such as steam pipes, t e m p e r a t u r e s t e n d t o r i s e above t h a t o f t h e h e a t s o u r c e . In landfills, organic materials such as grass c l i p p i n g s , b r u s h , k i t c h e n waste, and dumped f o r e s t p r o d u c t s r a i s e the temperature. The m a n u f a c t u r e o f c h a r c o a l from l i g n o c e l l u l o s i c s i n p i t s , r e t o r t s , and k i l n s i n v o l v e s s e l f h e a t i n g , when a f t e r an i n i t i a l p e r i o d o f c o m b u s t i o n t h e a i r s u p p l y i s s h u t o f f and t h e c h a r r e d m a t e r i a l r e l e a s e s h e a t without o x i d a t i o n . S i m i l a r r e a c t i o n s may be t h e c a u s e o f u n d e r g r o u n d fires i n c o a l seams and d r y p e a t b o g s , w h i c h burn f o r months and sometimes y e a r s a l t h o u g h a i r has no access. All s e l f - h e a t i n g s consume m a t e r i a l a n d partially c o n v e r t i t i n t o s u b s t a n c e s such as w a t e r v a p o r and c a r b o n dioxide. T h i s consumption i s d e s i r a b l e i n l a n d f i l l s , but i n n e a r l y a l l other cases u s e f u l m a t e r i a l i s devalued or lost. S e l f - h e a t i n g c a u s e s g r e a t e s t damage when i t l e a d s t o s m o l d e r i n g and f l a m i n g c o m b u s t i o n . The l i t e r a t u r e has b e e n r e v i e w e d e a r l i e r i n g r e a t d e t a i l , w i t h e m p h a s i s on f o r e s t p r o d u c t s (1) . Here the r e v i e w i s u p d a t e d and d r a s t i c a l l y c o n d e n s e d , and i n c l u d e s lignocellulosics o t h e r t h a n wood. Of t h e former 194 r e f e r e n c e s , o n l y those used f o r i l l u s t r a t i o n s , i n support o f c r u c i a l s t a t e m e n t s , and f o r new c o n c l u s i o n s a r e c i t e d again. Some c i t e d e a r l y p u b l i c a t i o n s were n o t a v a i l a b l e i n 1987. S e l f - h e a t i n g i s c a u s e d by a s e r i e s o f p r o c e s s e s , e a c h o f w h i c h has i t s p a r t i c u l a r t e m p e r a t u r e r a n g e , and r a i s e s the temperature up t o t h e l e v e l o r i n t o t h e r a n g e s of ensuing processes. The p r o c e s s e s a r e a b s t r a c t e d one by one i n the sequence o f i n c r e a s i n g temperatures, although the p r o c e s s e s o v e r l a p , and most c a s e s o f s e l f - h e a t i n g i n v o l v e several processes. Some e a r l i e r p u b l i c a t i o n s s h a l l be e v a l u a t e d from a new a n g l e .

Respiration Green l e a v e s of p l a n t s p h o t o s y n t h e s i z e , t r a p p i n g l i g h t e n e r g y f o r t h e c o n v e r s i o n o f c a r b o n d i o x i d e and w a t e r i n t o sugars. As a process which does not generate heat photosynthesis has no place in this review, but p h o t o s y n t h e s i z i n g c e l l s a r e a l i v e , and l i v i n g c e l l s must respire to maintain life. The living cells convert



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c h e m i c a l e n e r g y i n t o h e a t , even t h o u g h t h e c e l l s o f g r e e n l e a v e s a t t h e same t i m e a b s o r b l i g h t e n e r g y and a c c u m u l a t e chemical energy. The n e c e s s i t y o f r e s p i r a t i o n i s b e t t e r known i n t h e c e l l s o f a n i m a l s and p e o p l e : when o u r b r a i n i s n o t s u p p l i e d w i t h o x y g e n by t h e b l o o d s t r e a m , b r a i n c e l l s succumb w i t h i n m i n u t e s . C u t t i n g g r a s s , o r more p r e c i s e l y s e p a r a t i n g l e a f t i p s from lower p a r t s o f the g r a s s p l a n t , i s not immediately l e t h a l , and even c h o p p i n g up t h e s e t i p s k i l l s o n l y a s m a l l percentage of the c e l l s . The l e a v e s c o n t i n u e t o r e s p i r e ; t h e y o x i d i z e s t o r e d c a r b o h y d r a t e s and o t h e r f o o d s i n t o c a r b o n d i o x i d e and water, o r i n t o i n t e r m e d i a t e compounds. In p h o t o s y n t h e s i z i n g l e a v e s o f g r o w i n g p l a n t s t h e h e a t o f r e s p i r a t i o n can d i s s i p a t e w h i l e i n p i l e s o f h a r v e s t e d p l a n t s i t may be t r a p p e d and may a c c u m u l a t e , examples b e i n g heaps o f a l f a l f a , c l o v e r , and c o r n s t a l k s . G a r b a g e cans and p l a s t i c bags w i t h g r a s s c l i p p i n g s have l i t t l e space f o r air. The c l i p p i n g s consume n e a r l y a l l a v a i l a b l e oxygen, then e x t e r n a l oxygen d i f f u s e s i n t o the c o n t a i n e r s to sustain respiration somewhat. Seeds, living cells by d e f i n i t i o n , c o n s i s t m a i n l y o f s t a r c h and o t h e r s t o r e d f o o d which oxidizes extremely slowly, so t h a t these food r e s e r v e s l a s t a l o n g t i m e and seeds can s u r v i v e f o r y e a r s , consuming very l i t t l e o x y g e n and r e l e a s i n g v e r y little heat. T h i s seems t o be why g r a i n s e l f - h e a t s l e s s t h a n o t h e r l i f e c r o p s , i n s p i t e o f t h e huge v o l u m e s i n g r a i n elevators. Wood and b a r k o f l i v i n g t r e e s a r e e s s e n t i a l l y dead tissues. Heartwood i n t h e c e n t e r o f mature t r e e s and o l d b a r k a t t h e v e r y t r e e stem s u r f a c e c o n t a i n no l i v i n g c e l l s , w h a t s o e v e r , whereas i n sapwood and i n young b a r k some c e l l s remain a l i v e . In t h e c a m b i a l l a y e r between wood and b a r k , i n w h i c h t r e e s grow new wood and b a r k , p r a c t i c a l l y a l l cells live. R e s p i r a t i o n of the l i v i n g t r e e c e l l s causes s e l f - h e a t i n g i n p i l e s o f sawdust, o f b a r k , and o f chips f o r pulp; the higher the percentage of l i v i n g c e l l s , t h a t i s of c a m b i a l c e l l s and c e l l s n e a r t h e cambium, t h e more t h e piles self-heat. Whole-tree chips, o b t a i n e d by c h o p p i n g up e n t i r e t r e e s , s e l f - h e a t g r e a t l y e s p e c i a l l y when chopped f r o m young t r e e s , b e c a u s e t h e f o l i a g e i n c l u d e d c o n s i s t s m a i n l y o f l i v i n g c e l l s (2) . P l a n t c e l l s c e a s e l i v i n g and g e n e r a t i n g h e a t when t h e y have e x h a u s t e d t h e i r f o o d r e s e r v e s a f t e r a few d a y s , weeks, o r months. Most p l a n t c e l l s d i e e a r l i e r , due t o l a c k o f m o i s t u r e and oxygen, or from heat. Living cells of l i g n o c e l l u l o s i c s p e r i s h when t h e m o i s t u r e c o n t e n t o f t h e embedding l i g n i f i e d c e l l s d r o p s below f i b e r s a t u r a t i o n n e a r 30% m o i s t u r e c o n t e n t , b a s e d on o v e n d r y w e i g h t . Seeds s u r v i v e down t o much l o w e r m o i s t u r e c o n t e n t s . The u p p e r t e m p e r a t u r e s which p l a n t s c a n t o l e r a t e v a r y from p l a n t s p e c i e s t o s p e c i e s . T r e e c e l l s s u r v i v e up t o a b o u t 50*C. Heat g e n e r a t i o n may c o n t i n u e even a f t e r c e l l d e a t h , when enzymes p r o d u c e d by t h e l i v i n g c e l l s c o n t i n u e c a t a l y z i n g t h e o x i d a t i o n p r o c e s s p o s t mortem. At the


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other, low temperature extreme, many freezing i n the state o f dormancy. optimum, a t w h i c h p l a n t s m e t a b o l i z e t h e many s p e c i e s a r o u n d 25*C; i t depends on o f the atmosphere, s i n c e h i g h degrees o f a s s o c i a t e d w i t h d r y a i r and t h e s t r e s s o f


Metabolism

of

plants survive The temperature fastest lies for r e l a t i v e humidity heat are u s u a l l y moisture l o s s .

Microorganisms

Dead as w e l l as l i v e p l a n t s and a n i m a l s s e r v e as f o o d f o r fungi and bacteria. These microorganisms do not p h o t o s y n t h e s i z e , and l i k e a n i m a l s t h e y f e e d on t h e o r g a n i c materials. The t e r m microorganism indicates that the i n d i v i d u a l s can be s e e n o n l y u n d e r t h e m i c r o s c o p e ; many f u n g i however form l a r g e , c o n s p i c u o u s f r u i t i n g b o d i e s known as mushrooms and t o a d s t o o l s , b u t t h e i r f i l a m e n t s i n t h e h o s t s u b s t r a t e r e m a i n by f a r t o o t h i n t o be v i s i b l e t o t h e unaided eye. The m i c r o o r g a n i s m s b r e a k t h e i r s u b s t r a t e down t o c o n v e r t i t i n t o c a r b o n d i o x i d e and w a t e r ; in this respect their metabolism resembles respiration and combustion. I t i s another heat-generating p r o c e s s . B a c t e r i a and s p o r e s o f f u n g i abound i n t h e a i r as w e l l as a t s u r f a c e s o f a l m o s t everything, multiplying and growing r a p i d l y under s u i t a b l e c o n d i t i o n s . Most p l a n t s fend o f f these microorganisms, w h i l e some l i v e w i t h t h e p a r a s i t e s and o t h e r s a r e k i l l e d by them. Separating higher p l a n t s from t h e i r r o o t s and c h o p p i n g them up i n t o f r a g m e n t s d u r i n g h a r v e s t i n g o p e r a t i o n s weakens t h e i r d e f e n s e s and causes e a r l y death; i t i s then o n l y a q u e s t i o n of time u n t i l m i c r o o r g a n i s m s under s u i t a b l e c o n d i t i o n s consume t h e harvested material. How l o n g p l a n t t i s s u e s r e s i s t d e c o m p o s i t i o n and how q u i c k l y t h e d e c o m p o s i t i o n r e l e a s e s h e a t , t h a t depends on the p l a n t c o n s t i t u e n t s . S t a r c h and s u g a r s i n f r u i t s , r o o t s , and sapwood i s r e a d y f o o d , so t h a t f u n g i f e e d i n g on these low-molecular weight carbohydrates may spread t h r o u g h o u t l a r g e p i l e s o f some m a t e r i a l s w i t h i n a few d a y s . C e l l u l o s e , t h e most common p l a n t c o n s t i t u e n t , i s much more r e s i s t a n t t h a n s t a r c h and s u g a r s , b u t s t i l l may l a s t o n l y a few weeks as we know from t h e d e c o m p o s i t i o n o f l e a v e s and entire herbaceous (not lignified) plants. The decomposition of c e l l u l o s e n e v e r t h e l e s s r e l e a s e s heat a t r e l a t i v e l y f a s t r a t e s , b e c a u s e c e l l u l o s e makes up t h e mass o f t h e p a r t i c u l a r p l a n t and o c c u r s i n g r e a t q u a n t i t i e s , as i n hay f o r e x a m p l e . L i g n i n , n e x t t o c e l l u l o s e t h e main c o n s t i t u e n t o f woody l i g n i f i e d t i s s u e , a d d s months and years to the resistance of lignocellulosics, which t h e r e f o r e r e l e a s e h e a t a t r e l a t i v e l y low r a t e s , b u t huge masses may compensate f o r t h e low r a t e . The h e a r t w o o d o f many tree s p e c i e s and other plant tissues contain substances which poison the tissue as food for m i c r o o r g a n i s m s , and a r e t h e r e a s o n why s u c h t i s s u e s r e s i s t d e c o m p o s i t i o n f o r y e a r s and e v e n d e c a d e s , depending on t e m p e r a t u r e as w e l l as t h e oxygen and m o i s t u r e a v a i l a b l e .


26. KUBLER

Self-Heating of Lignocellulosic Materials

In e a r l y stages of storage of h a r v e s t e d p l a n t s , microorganisms quasi supplement r e s p i r a t i o n , but a f t e r heat has k i l l e d a l l substrate c e l l s around 50" or 60"C, the m i c r o o r g a n i s m s make a g r e a t difference by r a i s i n g temperatures considerably higher. Actually some microorganisms t o l e r a t e no more heat than substrate c e l l s , but some heat r e s i s t a n t thermophilic microorganisms survive and metabolize up to about 80 C . Microorganisms depend on moisture l i k e a l l p l a n t s . They secrete enzymes i n aqueous s o l u t i o n to break the food down i n t o water s o l u b l e compounds, then absorb the compounds i n s o l u t i o n . Hence f u n g i and b a c t e r i a can metabolize i n l i g n o c e l l u l o s i c substrates only as long as the substrate contains at l e a s t 20% moisture. Drying and f r e e z i n g f o r c e microorganisms i n t o dormancy, but a f t e r remoistening and thawing they resume m e t a b o l i z i n g . Thus p i l e s of wood chips s t a r t s e l f - h e a t i n g a l s o i n w i n t e r , although slowly at f i r s t (2). Optimal temperatures of the microorganisms vary with the s p e c i e s ; most metabolize f a s t e s t between 25* and 30*C. Waterlogged m a t e r i a l s e l f heats r e l a t i v e l y l i t t l e , because i t lacks oxygen which most microorganisms need: they are a e r o b i c . Only a few belong to the anaerobic category. Some compact, dense masses s e l f - h e a t l i t t l e i n t h i s stage, due to lack of oxygen and a l s o to s u b s t a n t i a l conduction of heat to the surface (3). Microorganisms t h r i v e i n l a n d f i l l s . In the common sanitary type the r e l a t i v e l y s m a l l , f l e x i b l e , v a r i a b l e pieces of household garbage lend themselves to compaction by the heavy bulldozers and trucks d r i v i n g on the l a n d f i l l . The compaction and interspersed capping layers of c l a y - t y p e d i r t create an e s s e n t i a l l y a i r free mess i n which mainly anaerobic b a c t e r i a decompose the o r g a n i c m a t e r i a l s i n t o methane. This gas b u i l d s up pressure to seep, flow, and d i f f u s e out i n t o the a i r and i n t o surrounding ground. Methane as an energy r i c h gas c a r r i e s much chemical energy away, l e a v i n g l i t t l e for exothermic aerobic decomposition and self-heating. The s i t u a t i o n appears to be d i f f e r e n t i n demolition l a n d f i l l s used f o r dumping wrecked b u i l d i n g s and other structures i n metropolitan areas. There b u l l d o z e r s on studded c a t e r p i l l a r tracks crush and compact the pieces of lumber, wood-base panels, and o t h e r bulky items. Nevertheless the dump holds more a i r than s a n i t a r y landfills do, so t h a t mainly a e r o b i c microorganisms decompose the o r g a n i c , l a r g e l y l i g n o c e l l u l o s i c m a t e r i a l s i n t o carbon oxides and water vapor while r e l e a s i n g much heat. Some a i r , p u l l e d i n from the l a n d f i l l h i l l s i d e where the c o v e r i n g l a y e r of d i r t has been washed away, may sustain the aerobic microorganisms a f t e r they have consumed the innate oxygen. One l a n d f i l l with a t o t a l volume between two and three m i l l i o n cubic meters s e l f - h e a t e d to the l e v e l of p y r o l y s i s . The r e s u l t i n g events w i l l be discussed at the end of the f o l l o w i n g , pyrolysis part. e


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FIRE AND POLYMERS of

Pyrolysis

L i g n o c e l l u l o s i c s r e a c h t e m p e r a t u r e s a r o u n d 80 *C i n many ways b e s i d e s . F o r example, p l a n e r s h a v i n g s and peat insulation around hot p i p e s o r i n w a l l s o f dry 'cilns e a s i l y a t t a i n 100'C, and f o r e s t p r o d u c t i n d u s t r i e s s t a c k s t i l l h o t wood-base p a n e l s a t t e m p e r a t u r e s a r o u n d 8 0 C . In many c a s e s t h e t e m p e r a t u r e s o f t h e h o t m a t e r i a l s l a t e r on r o s e above t h e i n i t i a l 8 0 or 100 C, f i r s t t o l e v e l s o f s m o l d e r i n g c o m b u s t i o n , and f i n a l l y t o t h o s e o f open f l a m e s . In a i r - e x p o s e d and v e n t i l a t e d m a t e r i a l s o x i d a t i o n could c a u s e t h e h e a t i n g above 80* o r 100 *C, b u t i n s i d e tight p a c k s o f p a n e l s p y r o l y s i s must have been t h e h e a t s o u r c e . Up t o about 70*C, p l a n t t i s s u e s a r e t h e r m a l l y s t a b l e , as t h e y must be i n n a t u r e t o a v o i d damage f r o m p r o l o n g e d direct exposure t o the sun. Pyrolysis, the chemical d e c o m p o s i t i o n by h e a t , s t a r t s i n dry lignocellulosics a r o u n d 100"C, i n m o i s t ones below 80*C. I t a c c e l e r a t e s as temperature rises, peaking i n many o r g a n i c m a t e r i a l s between 275 and 300*C, at which point cellulose disintegrates. The e x o t h e r m i c i t y o f p y r o l y s i s had b e e n r e p o r t e d i n t h e l i t e r a t u r e as e a r l y as 1910 (4) . Nevertheless later most a u t h o r s saw t h e r m a l d i s i n t e g r a t i o n as an e n d o t h e r m i c reaction. Recent t h e o r e t i c a l (5) and e x p e r i m e n t a l work (6) c o n f i r m e d the e x o t h e r m i c i t y f o r slow p y r o l y s i s , i n which the e v o l v i n g v o l a t i l e s c o n t a i n l a r g e percentages of carbon dioxide and water, that i s to say of energy-free s u b s t a n c e s , so t h a t t h e sum o f h e a t s o f c o m b u s t i o n o f t h e p y r o l y s i s p r o d u c t s r e m a i n s below t h e h e a t o f c o m b u s t i o n o f the o r i g i n a l m a t e r i a l . Consequently a c c o r d i n g t o the f i r s t law o f t h e r m o d y n a m i c s , s l o w p y r o l y s i s must r e l e a s e h e a t . Fast, high-temperature p y r o l y s i s i n contrast, leads to plenty of high-energy volatiles such as hydrogen, i n d i c a t i n g e n d o t h e r m i c i t y ; the v i g o r o u s m o l e c u l a r motion o f h i g h t e m p e r a t u r e s l i t e r a l l y t e a r s t h e compounds a p a r t i n t o high-energy fragments, w i t h l i t t l e o p p o r t u n i t y f o r o r d e r l y arrangement. In t h i s way expended h e a t e n e r g y winds up as chemical energy i n the v o l a t i l e s . In t r i a l s w i t h wood s i n c e 1910, s e v e r a l r e s e a r c h e r s d i d n o t i c e p y r o l y t i c heat r e l e a s e , but o t h e r s found the r e a c t i o n endothermic. The c o n t r a d i c t i o n s can be e x p l a i n e d w i t h d i f f e r e n t s i z e s o f the samples. It i s believed that primary pyrolysis volatiles interact in secondary, e x o t h e r m i c r e a c t i o n s c a t a l y z e d by t h e s o l i d r e s i d u e . Long residence times of the v o l a t i l e s i n the d i s i n t e g r a t i n g m a t e r i a l favor secondary r e a c t i o n s , of course. Residence t i m e s a r e i n d e e d l o n g i n l a r g e and i n s l o w l y d i s i n t e g r a t i n g s a m p l e s , i n w h i c h t h e v o l a t i l e s have a l o n g p a t h t o t h e s u r f a c e and m i g r a t e out s l o w l y . Many a u t h o r s e x p l o r e d p y r o l y s i s in Differential Thermal Analysis (DTA) and i n Differential Scanning Calorimetry (DSC) t r i a l s , i n w h i c h t i n y samples weighing o n l y one m i l l i g r a m up t o a few grams were h e a t e d r a p i d l y a t e


e

e

e


26.

KUBLER

435

Self-Heating of Lignocellulosic Materials


e

rates f r o m 0 . 5 C / m i n up t o n e a r l y 200'C/min. The e x h i b i t e d h e a t b a l a n c e s c h a n g e d as t e m p e r a t u r e s r o s e and were o v e r a l l on t h e e n d o t h e r m i c s i d e , s i n c e i n t h e t i n y , r a p i d l y h e a t e d samples t h e v o l a t i l e s had l i t t l e chance f o r secondary, exothermic r e a c t i o n s . DTA and DSC t r i a l s a r e t h e r e f o r e not r e p r e s e n t a t i v e o f r e a l s i t u a t i o n s . Amounts o f h e a t r e l e a s e d i n p y r o l y s i s t r i a l s were small. Even h e a t s measured a t a r o u n d 130*C, f a r above t h e 80*C m a r g i n , c o u l d a d i a b a t i c a l l y r a i s e t h e t e m p e r a t u r e by l e s s t h a n one c e n t i g r a d e i n an h o u r (6) . W i t h so l i t t l e h e a t , i n a c c u r a c i e s o f measurements c o u l d c a u s e q u a l i t a t i v e mistakes. Additionally, temperatures were usually determined with thermocouples, whose m e t a l l i c wires c o n d u c t e d h e a t up t o one t h o u s a n d t i m e s f a s t e r t h a n t h e p y r o l z e d m a t e r i a l , and c o u l d a g a i n i n v o l v e substantial e r r o r s , e s p e c i a l l y w i t h s m a l l samples and s t e e p t e m p e r a t u r e gradients. P y r o l y t i c s e l f - h e a t i n g has c a u s e d q u i t e a few f i r e s . When new wood-base p a n e l s were s t a c k e d t o o h o t i n p a c k s f o r storage or for transport, heat of pyrolysis drove temperatures deep i n s i d e t h e p a c k s h i g h e r a n d h i g h e r , p y r o l y z i n g t h e c e n t e r i n t o d a r k e n i n g b r i t t l e m a t e r i a l s , and f i n a l l y i n t o b l a c k c h a r c o a l (7) . Due t o c o n t r a c t i o n from the c h a r r i n g and v o l a t i l i z a t i o n , t h e m a t e r i a l c r a c k e d and c r u m b l e d and l e f t c a v i t i e s i n s i d e t h e p a c k s . The c h a r z o n e s and t h e d e v e l o p i n g c a v i t i e s were r a r e l y e x a c t l y i n the p a c k ' s c e n t e r b u t a l w a y s r e l a t i v e l y f a r f r o m t h e pack s u r f a c e , so t h a t o n l y a f r a c t i o n o f t h e g e n e r a t e d p y r o l y t i c h e a t was c o n d u c t e d t o t h e d i s t a n t s u r f a c e s out o f t h e pack. Some o f t h e p y r o l y s i s v o l a t i l e s e f f u s e d o u t , and c o u l d have been p e r c e p t i b l e symptoms o f t h e d e t e r i o r a t i o n and an imminent disaster, but in open yards, unattended w a r e h o u s e s , o r on t r u c k s nobody was p r e s e n t t o s m e l l t h e odors. The c a v i t i e s grew i n a l l d i r e c t i o n s , a p p r o a c h i n g a p a c k s u r f a c e one o r two weeks a f t e r s t a c k i n g . At t h a t p o i n t a i r g a i n e d a c c e s s t o t h e h o t c h a r r e d m a t e r i a l and triggered ignition. The c o m b u s t i o n s p r e a d r a p i d l y a l o n g the p a c k s u r f a c e , p o s s i b l y a l s o i n t o t h e opened c a v i t y as a i r rushed i n . To p r e v e n t t h e s e l f - h e a t i n g , factories cool fresh f i b e r b o a r d t o t e m p e r a t u r e s o f 80* o r lower b e f o r e s t a c k i n g . The U n i t e d S t a t e s G o v e r n m e n t as b u y e r of the panels r e q u i r e s t h e much s a f e r t e m p e r a t u r e l i m i t o f 5 4 . 5 C (130* F) f o r wood f i b e r b o a r d , and 60 *C (140* F) f o r b o a r d made f r o m s u g a r cane (8) . Experiments with u n r e p r e s e n t a t i v e , small samples c a n be very misleading regarding safe s t a c k i n g t e m p e r a t u r e s , s i n c e i n s m a l l samples t h e s e l f h e a t i n g may d e v e l o p o r become n o t i c e a b l e o n l y above 150*C, w h i l e o x i d a t i v e h e a t i n g and r e s u l t i n g i g n i t i o n a p p e a r o n l y a r o u n d 200'C (7) . In the case of the d e m o l i t i o n l a n d f i l l mentioned above, p y r o l y s i s s u p p l i e d heat a f t e r t h e microorganisms reached t h e i r temperature l i m i t . E i g h t e e n months f r o m t h e time o f l a n d f i l l i n g , obnoxious p y r o l y t i c v o l a t i l e s e f f u s e d e



436

FIRE AND POLYMERS

as dense smoke out o f t h e s e e m i n g l y b u r n i n g dump. Landfill o p e r a t o r s f o u g h t t h e fire f o r s i x months by e x c a v a t i n g s m o k i n g z o n e s deep u n d e r t h e s u r f a c e , a n d by injecting water. The d e g r e e o f c h a r r i n g o f e x c a v a t e d wood i n d i c a t e d t h a t t h e t e m p e r a t u r e had r e a c h e d a b o u t 2 0 0 * C . Some zones which smoked o n l y s l i g h t l y and were n o t e x c a v a t e d c o n t i n u e d to steam f o r s e v e r a l a d d i t i o n a l m o n t h s , as 80*C-hot, m o i s t u r e - l a d e n g a s e s ( w i t h t h e t y p i c a l a c i d o d o r o f steamed v e n e e r l o g s ) and h i g h p e r c e n t a g e s o f c a r b o n d i o x i d e and methane s t i l l s t r e a m e d o u t o f c r e v i c e s i n t h e c l a y - d i r t capping. The r a t e o f p y r o l y t i c h e a t r e l e a s e depends on r a t e and exothermicity of p y r o l y s i s . A c c o r d i n g t o the t h e o r e t i c a l e v a l u a t i o n o f t h e s e f a c t o r s i n t h e APPENDIX, t h e most r a p i d p y r o l y t i c s e l f - h e a t i n g can be e x p e c t e d b e t w e e n 1 2 0 * and 170-C. Abiotic Oxidation A l l k i n d s o f o x i d a t i o n r e l e a s e l a r g e amounts o f h e a t , i n t h e o r d e r o f 15 k J p e r gram o f oxygen consumed i n t h e c a s e of l i g n o c e l l u l o s i c s . M i c r o o r g a n i s m s and o t h e r l i v i n g c e l l s a c h i e v e biotic o x i d a t i o n a t ambient t e m p e r a t u r e by means o f c a t a l y z i n g enzymes. Direct chemical o r abiotic oxidation, a l s o known as atmospheric oxidation, generally occurs only at e l e v a t e d temperatures; i t s t h r e s h o l d of d e t e c t a b l e heat g e n e r a t i o n a p p e a r s a r o u n d 8 0 * C , and somewhat below 8 0 " C i n the presence of c a t a l y z i n g moisture. O x i d a t i o n shows itself i n oxygen consumption, in evolution of carbon d i o x i d e and w a t e r , as w e i g h t l o s s , and o f c o u r s e as h e a t release. O x i d a t i o n and o x i d a t i v e h e a t g e n e r a t i o n i n t e n s i f y w i t h i n c r e a s i n g t e m p e r a t u r e i n a v e r y p r o g r e s s i v e manner, so t h a t i n t h e p r e s e n c e o f a i r a b o v e 1 3 0 * C a t least, l i g n o c e l l u l o s i c s r e l e a s e more o x i d a t i v e t h a n pyrolytic heat. In t h e l a n d f i l l fire described i n the preceding p a r a g r a p h , e x c a v a t e d p i e c e s o f c h a r r e d , a t most 2 0 0 C h o t wood, s t a r t e d g l o w i n g when s u d d e n e x p o s u r e t o a i r and o x i d a t i o n b o o s t e d t h e i r t e m p e r a t u r e t o above 4 7 5 * C . Some e x t r a n e o u s c o n s t i t u e n t s o f p l a n t t i s s u e s , s u c h as alkali s a l t s i n many l i g n o c e l l u l o s i c s and particularly r e s i n i n wood, o x i d i z e a t l o w e r t e m p e r a t u r e s and faster than the r e g u l a r c o n s t i t u e n t s . T h e r e f o r e some f i r e s f r o m self-heating have been attributed to extraneous constituents. E x t r i n s i c s u b s t a n c e s added a f t e r h a r v e s t i n g may a c c e l e r a t e s e l f - h e a t i n g even more; t h e a d d i t i v e s and c o n t a m i n a n t s e i t h e r c a t a l y z e o x i d a t i o n and p y r o l y s i s , o r t h e e x t r i n s i c s t h e m s e l v e s o x i d i z e , w i t h f a t s and o i l s b e i n g examples ( 9 ) . M e t a l l i c c o m p o s i t i o n s s e r v i n g as siccatives catalyze t h e w e l l - k n o w n o x i d a t i o n and p o l y m e r i z a t i o n o f o i l i n p a i n t s and o t h e r f i n i s h e s . L i k e w i s e , f e r r o u s and o t h e r metallic objects boost self-heating in piles of lignocellulosics. Among t h e known c a t a l y z i n g s u b s t a n c e s a r e i r o n s u l p h i d e s and i r o n o x i d e s f r o m c o m b u s t i o n g a s e s o f e



26. KUBLER

Self-Heating ofLignocellulosic Materiah

437

d i r e c t - f i r e d d r i e r s , powdered m e t a l s , m e t a l o x i d e s , some a c i d s , z i n c c h l o r i d e , sodium c a r b o n a t e , and i n o r g a n i c s a l t s w h i c h m o d i f y p y r o l y s i s and a r e u s e d a s f i r e r e t a r d a n t s . The i n f l u e n c e o f oxygen on h o t l i g n o c e l l u l o s i c s shows i n many ways. M i t c h e l l (10) o b s e r v e d p r o p e r t y c h a n g e s i n wood samples h e a t e d a t 150'C f o r 1 t o 16 h o u r s . In ovendry samples, t h e p r o p e r t i e s c h a n g e d i n o x y g e n more t h a n i n n i t r o g e n atmospheres. Moisture amplified the e f f e c t of o x y g e n t h r o u g h h y d r o l y s i s , w h i c h c a n be c o n s i d e r e d a k i n d o f p y r o l y s i s and i s s i m i l a r l y an e x o t h e r m i c r e a c t i o n ( 5 ) . The c l a s s i c example, a n d p r o b a b l y t h e most f r e q u e n t c a u s e o f f i r e s from s e l f - h e a t i n g o f any s u b s t a n c e , a r e o i l s r a g s d i s c a r d e d by p a i n t e r s . In t h e c r i n k l e d , crumpled rags oxygen has a c c e s s t o l a r g e s u r f a c e s o f o i l , w h i l e o n l y some of t h e generated o x i d a t i v e heat d i s s i p a t e s out o f t h e i n s u l a t i n g mess o f a i r a n d r a g s , w h i c h t e n d s t o i g n i t e w i t h i n a few days o r even a f t e r s e v e r a l h o u r s . Whether p y r o l y s i s o r o x i d a t i o n g e n e r a t e s h e a t , t h i s i s what d e t e r m i n e s how t h e s e l f - h e a t i n g s h o u l d be c o n t r o l l e d . Air blown i n t o o r sucked out o f p i l e s works against p y r o l y t i c s e l f - h e a t i n g , whereas i t promotes o x i d a t i o n where much o f t h e i n n a t e o x y g e n h a s b e e n consumed. A i r which f l o w s t h r o u g h a n d c o o l s p i l e s i s h e l p f u l up t o a t l e a s t 100*C. Dense, c o m p a c t e d zones may s e l f - h e a t relatively much (11) due t o t h e l a r g e mass o f h e a t - g e n e r a t i n g m a t e r i a l , while l i t t l e o f the generated heat i s c a r r i e d away by a i r c o n v e c t i o n . In hay w i t h i t s h i g h p r o p o r t i o n o f a i r s p a c e a n d t h e large surfaces of combustible pieces, oxidation i s at e l e v a t e d temperatures hard t o stop. Therefore i n s e l f h e a t i n g hay s t a c k s , even t e m p e r a t u r e s around 60 *C a r e a l r e a d y dangerous and c a l l f o r f i r e f i g h t e r s , who g e n e r a l l y b l o w o r suck o u t t h e h e a t r a t h e r t h a n t r y t o e x c l u d e a i r (11) . Some f a c t o r i e s temper or cure f r e s h hardboard t o i n c r e a s e water resistance, dimensional s t a b i l i t y , and strength. "A c u r i n g f o r f i v e h o u r s o r more a t 165*C i s common" (12) . To f i n d ways t o p r e v e n t i g n i t i o n d u r i n g tempering, Back and Johanson (13) m e a s u r e d t h e h e a t e v o l v i n g f r o m a i r - e x p o s e d s t r i p s o f t h e b o a r d s between 150 and 240*C. "In l i g n i n - c o n t a i n i n g boards, t h e heat e v o l u t i o n r a t e p a s s e d r a p i d l y t h r o u g h a maximum a n d t h e n decreased with time, while f o r l i g n i n - f r e e boards t h e r a t e of heat evolution after r e a c h i n g an i n i t i a l plateau i n c r e a s e d s i g n i f i c a n t l y w i t h t i m e above 200*C o r s t a y e d about c o n s t a n t below 200 C." The r a p i d initial heat e v o l u t i o n may be r e l a t e d t o a v a i l a b l e o x i d a t i o n s i t e s i n the b o a r d s , b u t o v e r a l l t h e r e a c t i o n s must have been v e r y complex. Among lignocellulosic panel products, fiberboard ( c a l l e d fiber insulation board i n e a r l i e r decades) seems t o have c a u s e d more f i r e s t h a n t h e d e n s e r p r o d u c t s h a r d b o a r d , p a r t i c l e b o a r d , and plywood. F i b e r b o a r d s e l f - h e a t s more because i t conducts l e s s o f t h e generated heat out o f the e


438

FIRE AND POLYMERS

pack, but o x i d a t i o n of the r e l a t i v e l y porous m a t e r i a l may be a n o t h e r f a c t o r . S e l f - h e a t i n g and r e s u l t i n g i g n i t i o n o f f i b e r b o a r d have been e x t e n s i v e l y a n a l y z e d ( 1 4 ) .


Adsorptive

Heat

Lignocellulosics as hygroscopic substances release adsorptive h e a t when a d s o r b i n g w a t e r , e s p e c i a l l y i n t h e form of water vapor w i t h i t s l a r g e l a t e n t t h e a t . The a d s o r p t i v e h e a t d i m i n i s h e s as t h e m o i s t u r e c o n t e n t o f t h e adsorbent increases. At r i s i n g temperatures, h y g r o s c o p i c materials usually lose moisture and d r y , whereby the e v a p o r a t i o n p r o c e s s consumes h e a t and r e t a r d s s e l f - h e a t i n g (15, 16) . Hot h y g r o s c o p i c s u b s t a n c e s a d s o r b m o i s t u r e o n l y u n d e r r a r e c i r c u m s t a n c e s , f o r example when o l d steam p i p e s c r a c k and steam e s c a p e s i n t o t h e i n s u l a t i n g sawdust. Wood i n a h o t atmosphere w i t h i n c r e a s i n g r e l a t i v e h u m i d i t y , and panels i n an unevenly dry pack adsorb little, and adsorptions of t h i s kind r a r e l y play a s i g n i f i c a n t r o l e . Gas a d s o r p t i o n by c h a r r e d l i g n o c e l l u l o s i c s i s a n o t h e r m a t t e r , and may c o n t r i b u t e c r u c i a l h e a t i n l a t e s t a g e s o f slow s e l f - h e a t i n g . C h a r r e d l i g n o c e l l u l o s i c s , c h a r c o a l , and t h e l i g n o c e l l u l o s i c s from which most c o m m e r c i a l c h a r c o a l i s pyrolyzed are obviously very d i f f e r e n t substances. The p y r o l y s i s p r o c e s s g r a d u a l l y c o n v e r t s t h e raw m a t e r i a l , so that one cannot define a certain point or pyrolysis condition beyond which the solid r e s i d u e has become charcoal. The h i g h e r t h e t e m p e r a t u r e o f p y r o l y s i s and t h e longer i t lasts, t h e more p r o n o u n c e d are the c h a r c o a l characteristics. Charcoal contains percentagewise more c a r b o n t h a n l i g n o c e l l u l o s i c s ; i t i s a l s o more p o r o u s , has a l a r g e r i n t e r n a l s u r f a c e , and i s c h e m i c a l l y more r e a c t i v e . In t h e p y r o l y s i s p r o c e s s l i g n o c e l l u l o s i c s a p p r o a c h a l l o f these charcoal p r o p e r t i e s . C h a r c o a l r e a d i l y a d s o r b s g a s e s , i n c l u d i n g oxygen o u t of the s u r r o u n d i n g atmosphere. U n l i k e o x i d a t i o n , oxygen a d s o r p t i o n does n o t l e a d t o e f f u s i n g c a r b o n d i o x i d e and water vapor, r a t h e r the s o l i d g a i n s weight; moreover, t h e h e a t r e l e a s e d p e r u n i t mass o f a d s o r b e d oxygen d i m i n i s h e s w i t h i n c r e a s i n g amounts o f t h e a d s o r b a t e , whereas i n c a s e of o x i d a t i o n the heat of combustion remains constant. C h a r c o a l a d s o r b s a l s o c a r b o n monoxide and c a r b o n d i o x i d e . S u c h a d s o r p t i o n s a r e by no means r e s t r i c t e d t o a m b i e n t t e m p e r a t u r e s , o c c u r r i n g i n hot c h a r c o a l t o o a l t h o u g h l e s s t h a n i n c o o l c h a r c o a l , and f u r t h e r m o r e not a l l t y p e s o f h o t c h a r c o a l adsorb. T h e s e a d s o r p t i o n s a p p e a r t o be i n c o n s i s t e n t w i t h t h e e v o l u t i o n o f c a r b o n d i o x i d e and o t h e r v o l a t i l e s o u t o f t h e c h a r r i n g s o l i d i n the p y r o l y s i s p r o c e s s . The a d s o r p t i v e p r o p e r t i e s d e v e l o p as p y r o l y s i s f r e e s s i t e s f o r a d s o r p t i o n ; d e b r i s e s c a p i n g from t h e r m a l l y decomposing l i g n o c e l l u l o s i c s leaves the char r e s i d u e with a h i g h l y r e a c t i v e , e a g e r l y adsorbing inner surface.



26.

KUBLER

Self-Heating of Lignocellulosic Materiah

439

A d s o r p t i o n s a r e e x o t h e r m i c , r e l e a s i n g more t h a n t h e l a t e n t h e a t o f t h e a d s o r b a t e gas, w h i c h q u a s i l i q u e f i e s and s o l i d i f i e s besides being attracted. The f i r s t t r a c e s o f a d s o r b e d oxygen r e l e a s e even more h e a t t h a n t h e c o m b u s t i o n of carbon t o carbon d i o x i d e ; but a f t e r the f i r s t a d s o r p t i o n s i t e s have been o c c u p i e d , t h e a d s o r p t i v e h e a t — b a s e d on u n i t mass o f a d s o r b e d o x y g e n — g r a d u a l l y drops t o the level of condensation heat, f a r below the heat of combustion. I t has been c l a i m e d b u t n o t g e n e r a l l y a c c e p t e d t h a t l o n g - l a s t i n g h e a t between 100 and 200"C c o n v e r t s c e l l u l o s e and l i g n o c e l l u l o s i c s i n t o pyrophoric carbon, which f i n a l l y spontaneously i g n i t e s . I t may be t h a t d u r i n g l e n g t h y slow c h a r r i n g t h e r e s i d u a l s o l i d a d s o r b s o x y g e n and carbon o x i d e s , whereby t h e r e l e a s e d a d s o r p t i v e h e a t r a i s e s t h e t e m p e r a t u r e and t r i g g e r s i g n i t i o n . The c h a r c o a l r e a c t i o n s a r e r a t h e r complex (17) b e c a u s e p r o p e r t i e s o f c h a r c o a l v a r y much, d e p e n d i n g on t h e o r i g i n a l m a t e r i a l , c o n d i t i o n s o f p y r o l y s i s , s i z e o f t h e p i e c e s , and a c c e s s f o r oxygen and moisture during storage, not to speak of storage t e m p e r a t u r e and t i m e .

APPENDIX:

Rate

o f P y r o l y t i c Heat

Release

Heat r e l e a s e e x p r e s s e d i n e n e r g y p e r u n i t mass o f s e l f h e a t i n g m a t e r i a l p e r u n i t t i m e , s u c h as J o u l e s (J) p e r gram (g) and hour (h) , d e p e n d s on the rate and on the e x o t h e r m i c i t y of the p y r o l y s i s process. Both v a r y with the s i z e o f t h e m a t e r i a l sample, as e x p l a i n e d above i n t h e pyrolysis part. The f o l l o w i n g e s t i m a t e s and c a l c u l a t i o n s concern principally relatively large samples of one k i l o g r a m or so. Figure 1 applies to pyrolysis i n w h i c h t h e wood t e m p e r a t u r e i s r a i s e d f r o m 100"C i n about 10 h t o v a r i o u s final temperatures. The weight losses, d e p i c t e d as volatiles, and solid residues have b e e n d e t e r m i n e d a f t e r the trials, and originate mainly from Klason, v. H e i d e n s t a m , and N o r l i n (4), under c o n s i d e r a t i o n o f data p u b l i s h e d by Goos (18) and Stamm (19) . M e a s u r e m e n t s on s m a l l samples by B e a l l and E i c k n e r (20), LeVan and S c h a f f e r (21), and E l d e r (22) have been compared. Up t o about 275 or 300 *C -- t h e t e m p e r a t u r e range i n which cellulose rapidly disintegrates — i n c r e a s e d f i n a l t r i a l temperatures c a u s e i n c r e a s e d i n c r e m e n t s o f v o l a t i l e s ; b e y o n d 300"C t h e increments s t e a d i l y d i m i n i s h . The s l o p e o f t h e volatile = f(final temperature) curve i n F i g u r e 1 amounts t o v o l a t i l e i n c r e m e n t s a t p a r t i c u l a r t e m p e r a t u r e s t e p s , and i s a measure o f t h e p y r o l y s i s r a t e . The s l o p e s h a v e b e e n o b t a i n e d by d i f f e r e n t i a t i n g the volatile c u r v e o f F i g u r e 1 f o r d r a f t i n g F i g u r e 2. For example, v o l a t i l e s a t 2 0 0 C e q u a l 4.6%, and 7.7% a t 220'C; t h a t i s a 3.1% i n c r e m e n t i n t h e 20"C s t e p and amounts t o a #



440

FIRE AND POLYMERS

100

200

300

400

500

Final temperature (°C) F i g u r e 1. P y r o l y s i s o f w o o d h e a t e d f i n a l temperature i n t e n hours.

from

100*C t o t h e



26.

KUBLER

100

Self-Heating ofLignocellulosic Materials

200

300

400

Temperature ( ° C )

F i g u r e 2. P y r o l y s i s i n c r e m e n t s i n e a c h c e n t i g r a d e s t e p , e x p r e s s e d i n mass o f v o l a t i l e s p e r mass o f o r i g i n a l wood.


441

500

442

FIRE AND POLYMERS


e

s e c a n t s l o p e o f 0.16%/ C. D i f f e r e n t i a t i o n o f t h e curve i n F i g u r e 1 a t t h e i n t e r m e d i a t e t e m p e r a t u r e 210*C r e n d e r s t h e n e a r l y i d e n t i c a l t a n g e n t s l o p e 0.15%/*C. Figure 2 gives rates of pyrolysis i n t r i a l s with g r a d u a l l y i n c r e a s i n g temperatures. I n t h e c a s e o f wood h e a t e d t o 500*C f o r e x a m p l e , t h e v o l a t i l e increments i n c r e a s e f r o m t e m p e r a t u r e s t e p t o t e m p e r a t u r e s t e p up t o 300*C, d r o p p i n g o f f i n f u r t h e r s t e p s . The sum o f t h e i n c r e m e n t s r e a c h e s 75% a t 500*C, a s t h e y s h o u l d a c c o r d i n g t o F i g u r e 1. The a r e a under t h e c u r v e c o r r e s p o n d s t o t h a t sum. One c a n e s t i m a t e t h e sum f o r e a c h f i n a l temperature from t h e a r e a s . F o r example, i n t h e c a s e o f h e a t i n g t o 250*C, t h e i n c r e m e n t s a v e r a g e 0.1%, a n d t h e sum o f t h e i n c r e m e n t s becomes 0.1 χ (250 - 100) = 0.1 χ 150 = 15%, o r 0.15 g/g (compare F i g u r e 1). Some p u b l i c a t i o n s g i v e amounts o f p y r o l y s i s h e a t (H) per mass of volatiles; others include numbers f o r e s t i m a t i n g H. The h e a t s appear t o vary g r e a t l y from a u t h o r ( s ) t o a u t h o r ( s ) f o r t r i a l s under s i m i l a r c o n d i t i o n s , b u t one t r e n d i s o b v i o u s : heat o f p y r o l y s i s d e c r e a s e s as temperature i n c r e a s e s . Near t h e m a r g i n a l p y r o l y s i s t e m p e r a t u r e 100"C, m a i n l y the e n e r g y - f r e e gases carbon d i o x i d e and water e f f u s e out o f t h e s l o w l y d i s i n t e g r a t i n g wood, a s m e n t i o n e d earlier. T h e r e f o r e t h e heat o f p y r o l y s i s i s h i g h , p o s s i b l y e x c e e d i n g t h e h e a t o f c o m b u s t i o n o f wood (20 k J / g ) . I n K l a s o n ' s (23) trials, some o f w h i c h a r e i n c l u d e d i n T a b l e I , amounts o f e f f u s i n g w a t e r v a p o r a n d c a r b o n d i o x i d e were i n t h e 5 h trial r e l a t i v e l y s m a l l (16.6% a n d 6%, r e s p e c t i v e l y , a s o p p o s e d t o 26.1% a n d 12.6% i n t h e 336 h t r i a l ) . The relatively large, 800 g sample i n t h e 336 h t r i a l a t a t m o s p h e r i c p r e s s u r e , v e r s u s t h e 100 o r 250 g sample i n t h e 5 h t r i a l i n vacuum, c o n t r i b u t e d t o t h e d i f f e r e n t amounts of e f f u s i n g gases. In t h e 8 h t r i a l w i t h an 800 g sample ( T a b l e I ) , t h e amounts o f w a t e r v a p o r a n d c a r b o n d i o x i d e (20.5 and 10.2%, r e s p e c t i v e l y ) were i n between t h o s e o f t h e 336 h a n d o f t h e 5 h t r i a l s . John (24) r a t i o n a l i z e d h i s e x c e s s i v e p y r o l y s i s h e a t s ( T a b l e I) w i t h c a t a l y t i c c o m b u s t i o n o f p r i m a r y p y r o l y s i s hydrogen. The h e a t o f c o m b u s t i o n o f h y d r o g e n (142 kJ/g) i s indeed very high, but the r a t i o n a l i z a t i o n requires release of hydrogen a t low p y r o l y s i s t e m p e r a t u r e , f o r w h i c h no d i r e c t evidence e x i s t s . B e s i d e s t h a t , oxygen f o r t h e catalytic combustion would have t o be t o r n out of d i s i n t e g r a t i n g wood u n d e r c o n s u m p t i o n o f i m p r o b a b l y s m a l l energy. J o h n ' s (24) e x p l i c i t v a l u e s H a r e n o t c o n s i s t e n t with other data o f h i s experiments, and with H-values r e p o r t e d by o t h e r r e s e a r c h e r s . The 60 k J / g c a l c u l a t e d on b a s i s o f K u b l e r , Wang, a n d B a r k a l o w ' s work (6) d e s e r v e n o t much c o n f i d e n c e e i t h e r , s i n c e t h e t r i a l s were n o t d e s i g n e d f o r e x a c t measurements o f v o l a t i l e s . Heats H of pyrolysis must b e v e r y h i g h a t l o w p y r o l y s i s t e m p e r a t u r e s ( t *C), b u t v a l u e s above 10 k J / g may


26.

KUBLER

443

Self-Heating oj Lignocellulosic Materials

T a b l e I . Heat (H) o f p y r o l y s i s p e r gram v o l a t i l e s i n t r i a l s w i t h wood by v a r i o u s a u t h o r s

Temperature CO

Duration (h)

2250

2400

120 140 160

21 21 21

650 650 650

175

~3

50

275 305 325 375 435

22.9 27.8 24 .2 30.1 25.0

80-130


Sample Mass

Self-Heating of Lignocellulosic Materials - ACS Publications

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