Free Radicals in Charcoal and the Combustion of Compositions

Free Radicals in Charcoal and the Combustion of Compositions
Containing Charcoal
Freie Radikale in Holzkohle und die Verbrennung von Holzkohle enthaltenden Verbindungen
Radicaux libres au charbon de bois et la combustion de compositions qui contiennent du
charbon de bois
T. U r b a ń s k i , S. Benbenek, S. B e d y ń s k i and A . V/asilewski
Summary
:
Thermal decomposition of cellulose furnishes charcoal containing free radicals detected by ESR technique. The maximum
concentration of free electrons was found at the temperature of carbonization of c. 475 ° C. The curve: free radical concen­
tration against temperature of carbonization of cellulose is of the same shape as the curve: rate of burning against temperature
of carbonization of charcoal in the "Ammonpulver" the latter having maximum of the rate of burning near 500°. This supports
the view that burning of mixtures containing charcoal is strong/y influenced by the presence of free radicals in charchoal.
Zusammenfassung:
Die thermische Zersetzung von Zellulose liefert Holzkohle, die freie Radikale enthàlt, welche durch ESR-Verfahren ermittelt
wurden. Die maximale Konzentration freier Elektronen wurde bei der Verkohlungstemperatur von 475 ° С festgestellt. Die
Kurve: freie Radikal-Konzentration gegen Verkohlungstemperatur von Zellulose ist von der gleichen Form wie die Kurve:
Verbrennungsgeschwindigkeit gegen Verkohlungstemperatur von Holzkohle im „Ammonpulver",
wobei das Maximum an
Verbrennungsgeschwindigkeit des letzteren bei ca. 500 С liegt. Dies stiitzt die Ansicht, dafl das Verbrennen von Gemischen, die
Holzkohle enthalten, stark von der Anwcsenheit freier Radikale in Holzkohle beeinfluflt wird.
0
Résumé:
La décomposition
thermique de cellulose fournit un charbon de bois qui contient des radicaux libres découverts
par la
technique ESR. Le maximum de concentration d'électrons libres fut trouvé à la température
de carbonisation d'environ 475 ° C .
La courbe: concentration de radicaux libres contre température
de carbonisation de cellulose est de la même forme comme la
courbe: vitesse de combustion contre température
de carbonisation de charbon de bois dans la „poudre d'ammonium", dont le
maximum de la vitesse de combustion se trouve à environ 500° C. Ceci appuie l'opinion que la combustion de mélanges qui
contiennent du charbon de bois est fort influencée par la présence de radicaux libres dans le charbon de bois.
Introduction
The recent publication of Milsch, Windsch and
Heinzelmann [1] prompts us to report our own
research on formation of free radicals when cellu­
lose was subjected to thermal decomposition at
temperatures from 2 4 5 ° to 6 0 0 ° . This was a con­
tinuation of our former experiments on infra-red
spectra of thermally decomposed cellulose [2] and
also was connected with the problem why charcoal
possesses remarkable property of facilitating b u r n ­
ing mixtures of the type of black powder and
similar pyrotechnic compositions. One of the authors
of the present paper advanced a suggestion [3] that
this is due to the presence of free radicals in char­
coal which he found by means of E S R apparatus [4].
In view of this, our technique differed from that
of the said authors: our samples were charred in a
stream of nitrogen free of oxygen, but after cooling
the samples were exposed to the air. This was due
to our desire to investigate stable radicals, which
are permanently found in charcoal, irrespective of
the condition of their storage.
Experimental
Cotton linters (nitration grade) were purified by
extracting with chloroform (18 hours), ethanol (18
hours), washed at room temperature with aqueous
18 % solution of sodium hydroxide (to free cellulose
from (3- and y-cellulose), followed by washing with
cold water to the neutral reaction to litmus, washed
with acetone, ethanol and ether. The content of ucellulose was practically 100 % .
E X P L O S I V S T O F F E N r . 1/1970
Nitrogen was purified in a standard way over
copper wool heated to 700 ° and by passing through
a number of absorption tubes [5].
Cotton cellulose (4.0 g) was placed in a quartz
tube 26 m m diameter heated in an electric oven at
a constant temperature. Nitrogen was drawn through
the tube at the rate of 0.75 dcnvVmin. and the outlet
was connected with a water pump (working at a
slow stream of water) to achieve a quicker removal
of the gaseous products of the decomposition of
cellulose.
After the decomposition was terminated, the
heating was stopped and the samples were cooled in
the stream of nitrogen. They were withdrawn,
weighted, subjected to elementar analysis and
examined in E S R apparatus.
It has been found that 3Va hours of heating are
sufficient to complete the reaction of the decomposi­
tion: the weight of the samples and the intensity of
the R S R signal remained unchanged if the reaction
was prolonged.
The E S R measurements were made at the room
temperature on an X - b a n d spectrometer having
100 k H z magnetic field modulation. F o r calibration
the stable radical diphenylpicrylhydrazyl (DPPH)
was used.
Results and Discussion
The loss of weight and elementar analysis were in
full areement with our previous findings [2].
9
сл
2-10
5754
amp/* 10
amp/"10
Fig. 3
E S R r.ignals for the range of temperatures of c a r b o n i z a t i o n
575—650 °. T h e signals for 600 a n d 6 5 0 ° are tenfold a m p l i f i e d
0
The
f o r m a t i o n of free r a d i c a l s w a s first o b s e r v e d
at c a r b o n i z a t i o n t e m p e r a t u r e of 245 ° . T h e c o n c e n t r a t i o n of f r e e r a d i c a l s i n c r e a s e d w i t h t h e
of
the c a r b o n i z a t i o n temperatures
increase
a n d reached
its
m a x i m u m at 475 ° . F u r t h e r i n c r e a s e of t h e t e m p e r a 250
300
350
400
450
500
550
"С
t u r e p r o d u c e d t h e f a l l of the f r e e r a d i c a l c o n c e n t r a t i e n s ( T a b l e 1 a n d F i g . 1).
In t h e r a n g e 245—475 ° t h e s h a p e of E S R s i g n a l s
Fig. 1
S p i n c o n c e n t r a t i o n i n d é p e n d a n c e on the t e m p e r a t u r e of c a r bonization of cellulose
was
typical
appears
The
for
increase
here
stable
in many
of
free
radical
coal samples
the
accompanied
state
which
of d i f f e r e n t
origin.
carbonization temperature
by
the
decrease
of
the
is
signal
w i d t h f r o m c. 6 G to 0.8 G (Figs. 2 a n d 3).
This
lines
considerable
indicates
n a r r o w i n g of
a strong spin-spin
a c t i o n . T h e b r o a d e n i n g of
the
absorption
exchange
signals
of
the
inter-
samples
h e a t e d at t h e l o w e r t e m p e r a t u r e s (250—325 ° ) s e e m s
to be c a u s e d b y u n r e s o l v e d h y p e r f m e s t r u c t u r e of
t h e s i g n a l s d u e to t h e p r e s e n c e of h y d r o g e n
in
not
fully
carbonized
molecules
of
atoms
cellulose.
S i m i l a r l y , w i t h a n i n c r e a s e of t h e t e m p e r a t u r e
the
g - f a c t o r c h a n g e d f r o m 2.00G8 to 2.0027, close to the
free s p i n v a l u e s ( F i g . 4).
0
A b o v e 475
temperature.
t h e w i d t h of t h e l i n e s i n c r e a s e d w i t h
The beginning
of t h e
curve did
not
c o i n c i d e w i t h t h e e n d , ( m a g n e t i c field s w e e p A H
~
100 G ) ; this d e v i a t i o n i n c r e a s e d w i t h t h e t e m p e r a t u r e .
Grinding of charcoal
If t h e c h a r c o a l w a s
g r o u n d , t h e n u m b e r of
radicals increased. T h i s was i n agreement
previous
finding
free
with
[4]. A b r i e f g r i n d i n g i n a n
the
agate
m o r t a r p r o d u c e d a n i n c r e a s e of t h e n u m b e r of f r e e
e l e c t r o n s b y c. 1 0 ° / o .
Rate of burning of blackpowder
and similar mixtures
325°C
It is w e l l k n o w n t h a t the i g n i t i o n t e m p e r a t u r e of
b l a c k p o w d e r to g r e a t e x t e n d d e p e n d s o n t h e
tempe-
r a t u r e of c a r b o n i z a t i o n of w o o d [6]. A l s o t h e r a t e of
b u r n i n g d e p e n d s o n this t e m p e r a t u r e [7] — t h e f a c t
w e l l k n o w n to t h e m a n u f a c t u r e s of b l a c k p o w d e r [8].
The
475 'C
ESR
signals
for the
Fig. 2
range of t e m p e r a t u r e s
250—475
0
10
problem
was
investigated
by
Urbański
and
T ę s i o r o w s k i [8], B l a c k w o o d a n d B o w d e n [9].
Urbański
and T ę s i o r o w s k i
examined
the r a t e of
b u r n i n g of " A m m o n p u l v e r " — a m i x t u r e c o m p o s e d
of
carbonization
of 8 5 ° / " a m m o n i u m n i t r a t e a n d 1 5 ° / o c h a r c o a l - u s i n g
E X P L O S I V S T O F F E
N r . 1/1970
charcoal obtained by carbonization of wood at tem­
peratures ranging from 300 ° to 900 °.
The rate of burning was determined in a mano­
metrie bomb of Sarrau-Vieille-Burlot [10] at the
dp
density of 0.1 and expressed a s — : — m a x [11]. The
dt
plot of the rates of burning against the temperature
of carbonization gave the curve (Fig. 5) with a maxi­
mum near 5 0 0 ° . The similarity of both curves —
Fig. 1 and F i g . 5 seems to substanciate the view ex­
pressed by the author of the present paper that the
burning of blackpowder and similar mixtures is
mainly a free radical reaction and the rate of b u r n ­
ing is considerably influenced by the number of
unpaired electrons in charcoal [3].
^2.007
2.0O6
2,005
2,004
V
1
!
•
2,00 J
2.002
lia
F/ç.4
300
3SÛ
400
45Ô
Ï35
So
600
'C
Fig. 4
g - F a c t o r dependence on the temperature of c a r b o n i z a t i o n
Table 1
No Temperature
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
245 ° C
250 c
2G5 ° c
270 ° c
285 ° c
300 ° c
325 ° c
375 ° c
425 ° c
4G0 ° c
475 ° c
510 ° c
530 ° c
575 ° c
c
Number of
spins in the
sample
1,09
2,01
1,41
1,82
2,83
3,00
3,51
5,71
1,031
1,49
1,65
1,42
1,13
6,51
1 7
xlO
xlO'
xlO
xlO'
xl0
xlO'
xlO'
xlO
xl0
xl0'
xlO
xlO
xl0
xlO'
7
1 7
7
! 7
7
7
1 7
I H
N
1 8
1 8
1 K
7
Number of
spins per
1 gram
§max
19
l,076xl0
1,084x10»
1,410x10'"
l,791xlO
2,551x10'"
3,00 x l O "
3,321x10
4,450x10'"
6,151x10'"
9,252x10'"
1,012x10-''
8,751x10'"
7,050x10'"
4,110x10'"
te
1
м
2.0068
2.0055
2.0041
2.0040
2.0037
2.0036
2.0035
2.0033
2.0032
2.0031
2.0030
2.0029
2.0029
2.0027
300
400
500
600
700
«00
900
°C
Fig. 5
Fig. 5
Rate of b u r n i n g of A m m o n p u l v e r in dependence on the
rature of c a r b o n i z a t i o n
tempe­
Authors:
T. U r b a ń s k i , S. Benbenek, S. B e d y ń s k i and A . Wasil­
ewski
Department of Chemistry, Warsaw Institute of Tech­
nology (Politechnika), Warszawa 10, Poland
References
[1] B . M i l s c h , W . W i n d s c h a n d H . H e i n z e l m a n n , C a r b o n , 6,
807 (1968).
[2] T . U r b a ń s k i , W . H o f m a n , T . O s t r o w s k i a n d M . W i l a ­
n o w s k i , B u l l . A c a d . P o l . S c i , s é r i e c h i m . , 7, 851, 861 (1939);
C h e m i s t r y & I n d u s t r y ( L o n d o n ) , 95 (1960).
[3] T . U r b a ń s k i , E x p l o s i v s t o f ï e , 200 (1968).
[4] T . U r b a ń s k i , N a t u r e , 216, 577 (1967).
[5] Л . F a r k a s a n d H . W . M e l v i l l e , E x p e r i m e n t a l M e t h o d s
i n G a s R e a c t i o n s , p. 162, M a c M i l l a n , L o n d o n , 1939.
[6] V i o l e t t e , A n n . c h i m . , [3] 23, 475 (1848).
[7] T . U r b a ń s k i , C h e m i s t r y a n d T e c h n o l o g y of E x p l o s i v e s
V o l . I l l , p. 325—326, P e r g a m o n P r e s s — P W N , O x f o r d —
W a r s z a w a , 1967.
[8] T . U r b a ń s k i a n d E . T ę s i o r o w s k i , u n p u b l i s h e d w o r k (1931).
[9] J . D . B l a c k w o o d a n d F . P . B o w d e n , P r o c . R o y . S o c . ( L o n ­
don), A 213, 285 (1952).
[10] L . V e n i n , E . B u r l o t , H . L e c o r c h é , L e s P o u d r e s et E x ­
p l o s i f s , p. 75, B é r a n g e r , P a r i s — L i è g e , 1932.
[11] H . B r u n s w i g , D a s r a u c h l o s e P u l v e r , p. 291, W a l t e r de
G r u y t e r , B e r l i n , 1926.
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