Ind. Eng. Chem. Prod. Res. Dev. 1982, 27, 97-101
07
Role of Inorganic Additives in the Smoldering Combustion of Cotton Cellulose Fred Shafizadeh,' Alian G. W. Bradbury,' William F. DeGroot, and Thomas W. Aanerud Wood Chemistry Laboratoty, Department of Chemistry, University of Montana, Missoula, Montana 598 12
Inorganic additives are classified according to their ability to enhance or inhibit smoldering combustion in cotton fabric. Thermal analysis shows that the rate of oxidation of chars prepared by pyrolysis of fabric treated with metal salts which enhance smoldering is higher than that of chars prepared from fabric treated with additives such as phosphates or boric acid, which inhibit smoldering. On combustion in air, CO/C02ratios are generally higher and heats of Oxidation generally lower for chars in the latter group. These resub are discussed in terms of a mechanism for smoldering combustion based on oxidation of the carbonaceous residue from pyrolysis.
Introduction Smoldering combustion may be defined as a self-sustaining, low-temperature combustion process involving pyrolysis of the substrate ahead of a solid-phase combustion front. The volatile pyrolysis products escape combustion either because of limited air supply or the inability of the low pyrolysis rates to produce a combustible fuel-air mixture. Smoldering is usually observed in fibrous or porous materials where, in the absence of substantial heat loss due to radiation, conduction, or convection, the slow rate of heat release due to combustion of the char provides the heat flux required for further charring and propagation of the smoldering front. Excessive heat flux can lead to ignition of the volatile products (flaming combustion) and insufficient heat flux results in extinguishment. Smoldering combustion can lead to, but is distinct from, glowing combustion in which rapid oxidation of a char results in glow or incandescence. There is no pyrolysis process associated with glowing combustion. It has been claimed that about 70% of household fires result from an initial smoldering combustion phase, often involving cellulosic materials (Clarke and Ottoson, 1976). However, very little is known about the mechanism of this process and its prevention by chemical additives. Flame retardants, which are generally formulated to reduce the rate of production and heat content of volatile pyrolysis products, may not be effective smoldering inhibitors. They could actually enhance smoldering by catalyzing the formation of an active char capable of supporting smoldering combustion for long periods of time. Differences in the effectiveness of flame retardants in inhibiting flaming and smoldering in cellulose can be better understood by consideration of the two competing thermal decomposition pathways shown in Figure 1 (Shafizadeh, 1968). At low temperature or in the presence of inorganic additives (which decrease the decomposition temperature) reaction occurs by dehydration, decarboxylation, slow depolymerization, and recombination of the decomposition products to produce a carbonaceous char. Smoldering combustion results from oxidation of this char at a front in the solid phase. Conversely, reaction at high temperature involves breakdown of the macromolecule by intramolecular transglycosylation to anhydroglucose units and their subsequent degradation to lower molecular weight volatiles which bum in the gas phase by flaming coml General Foods Corporation, Technical Center, 555 South Broadway, Tarrytown, NY 10591.
0196-432118211221-0097$01.25/0
bustion. The rate of pyrolysis is the same in either air or nitrogen at temperatures above 300 "C, although it is accelerated by the presence of oxygen at lower temperatures (Shafizadeh and Bradbury, 1979a). It has been shown that the addition of inorganic salts to cotton fabric leads to a wide range of reactivities from enhancement to total inhibition of the smoldering combustion (McCarter, 1977). The purpose of this paper is to correlate observation of smoldering behavior of treated fabrics with known mechanisms of the combustion and pyrolysis of cellulosic materials and the effects of inorganic additives on these processes. Materials Purified cotton fabric and cotton linters (supplied by USDA Southern Regional Lab, New Orleans, LA) were extracted 2 days with boiling ethanol and then 2 h with water at 80 "C. The fabric, a desized, scoured, and bleached cotton printcloth had an average weight after extraction of 110 g/m2, and the extraded fabric and linters both contained less than 0.1% ash. Additives were applied to the cotton samples from aqueous solutions. Solution strength was adjusted to give one atom of the expected "active" species, e.g., P or B for phosphates or borates and metal ion for metal salts, per 100 moles of glucose residues in the cellulose sample. After immersion in the additive solutions, samples were rolled to remove excess water and dried in a current of warm air to eliminate migration of the additives in the sample. Chars were prepared by pyrolyzing the fabric at 550 "C for 1.5 min in a preheated oven under flowing nitrogen. The temperature was chosen to be representative of that experienced by smoldering cellulosic fabrics (Shafizadeh and Bradbury, 1979a). One and one-half minutes was the time required for the fabric to reach this temperature under these conditions. Experimental Section The tendency of fabrics to smolder was tested by suspending a 4 X 5 cm sample vertically in a glass tube through which a current of air (100 mL/min) was passed. The ignition source was a small electrical coil. A lighted match was used to test the combustibility of char samples. Thermogravimetry (TG) and derivative thermogravimetry (DTG) were carried out using a Cahn R-100 electrobalance equipped with a time derivative computer. Samples (2 mg) were heated at a rate of 15 "C/min under flowing air or nitrogen (50 mL/min). A DuPont Model 990 thermal analyzer was used for temperature control of the TG furnace and for differential scanning calorimetry (DSC) under the same sample conditions. 0 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
Table I. Effect of Additives on the Smoldering and Pyrolysis Behavior of Fabrics and C h a r s smoldering behavior additive”
fabric
char
X
X
CaCl, w 3 0 3
X
t
X
X
Na2B407
X X
X
X X X
X X
H304
NH4H,P04 (NH4)2HP04
Na3P04 Na,SO, (NH4
X X
)ZS04
X
+
X X
X
untreated
X
+
NaCl NaOH Na,CO, FeCl,
t t
t t
FeSO, Fe(NO3h PWAc),
+
sa,
a 1 mol/l00 glucose
t t
residues.
Key:
14.8 15.1 9.4 14.1 14.0 18.0 19.1 11.1 8.5 18.6 18.6 5.8 14.8 14.4 15.5 18.3 17.1 15.6 9.6 12.1
+
ZnC1,
+ + +
residual char, % dry wt
t t
+ t + + +, smolders; x, does not.
CELLUOSE
Figure 1. Competing pathways in the pyrolysis and combustion of cellulosic materials.
Rates of production of CO and COz from smoldering cotton linters were deterwined by infrared spectroscopy of the evolved gasses. Approximately 1g of cotton linters was loosely packed into a glass tube (2 cm id.). Air purified by bubbling through sodium hydroxide solution and passage through a column packed with Drieritewas passed through the tube at a rate of 200 mL/min. Smoldering combustion was initiated by means of a small electrical coil. Smoldering then propagated in the direction of air flow, and gaseous products were carried in the air stream through a 0.01 N HC1 solution and two U-tubes containing Drierite into a 2.5-cm gas cell positioned in the sample beam of a Beckman Acculab 2 infrared spectrometer. Concentrations of CO and COz were calculated from the absorptions at 2160 and 2350 cm-’, respectively, until evolution of gas had ceased.
Results Using the smoldering test, additives were classified according to the method of McCarter (1977) into (1) those showing no effect or inhibiting smoldering and (2) those enhancing the smoldering reaction. The effects of the additives tested are summarized in Table I. Under the experimental conditions used the untreated fabric did not smolder. If the spark induced by the electric coil led to propagation of the smoldering reaction over at least half of the length of the cotton sample, the smoldering reaction was considered “enhanced”. Chars formed by pyrolysis of the treated substrates were tested for their ability to sustain glowing combustion following application of flame from a match. The results are given in Table I along with the percent char remaining after pyrolysis at 550 OC, the temperature at the peak of the combustion exotherm as
DSC in air, heat release: T (“C) a t peak cal/g of cellulose 4 90 43 5 485 530
2340 2140 2770 4 20 1460 1130 23 20
445 465 475 440
2630 1020 3030 2530 1020 1490
34 5 34 5 385
Shafizadeh et al. (1975).
determined by DSC in air,and heat content of the volatile degradation products evolved on pyrolysis to 500 “C (Shafizadeh et al., 1975). It can be seen from the data in Table I that many of the smolder inhibitors also retard glowing combustion of the char, although this is not a criterion for inhibition. Conversely, enhancers such as alkali or transition metal salts also enhance char combustion. This observation agrees with that of McCarter, who suggested that smolder promoters tended to be either monovalent metal cations or metals such as iron, lead, or chromium, which are known to have catalytic effects in the combustion of carbon (McCarter, 1977). Analysis of the data given in Table I leads to the following conclusions. (1)The additives which enhanced the smoldering combustion of the cotton fabrics also promoted combustion of the char. (2) Conversely, the additives which inhibited smoldering combustion of the fabric also retarded char combustion. The few exceptions to this general rule, borax, sodium sulfate, and calcium chloride, were salts in which the cation could be expected to counteract the activity of the anionic component. (3) There is no correlation between the ability of an additive to inhibit smoldering combustion and ita ability to reduce the heat of combustion of volatile pyrolysis products or “heat release”. For example, the heat release for cellulose treated with smoldering enhancers such as sodium carbonate is often considerably less than that obtained from smoldering inhibitors such as boric acid (Shafiideh et al., 1975). This is in contrast with inhibition of flaming combustion, in which reduction of heat release is a critical factor. The data in Table I1 show the expected correlation between the maximum rate of production of CO and C02 (combustion) and the smoldering behavior of the char as determined empirically. Those chars which smoldered when exposed to a lighted match all gave higher maximum rates of combustion than those which did not. Furthermore, it is significant to note that those additives which enhanced smoldering also enhanced C02 production over CO production (low CO/CO2 ratio). In general, chars which gave a CO/COz ratio substantially less than 1.0 smoldered readily and those for which the ratio was substantially greater than 1.0 did not. Thus, it appears that those additives which catalyze COz formation also catalyze smoldering, due to the much greater heat of formation of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 99
Table 11. Effect of Pre-Pyrolysis Additives o n Rate of Char Oxidation and CO/CO, Ratio at 450 "C in Air smoldering behav- d(CO+CO,)/ CO/CO, ior of -' yield a t chara at %?%b 450 "C
additive
5.7 4.6 6.1 2.9 3.9 5.1 5.3 13.9 15.2
1.01 1.69 1.03 1.71 1.43 0.90 0.95 0.70 0.86
--- N 2
-
Air
80 I
I
Key: +, smolders; x, does not. Units: mmol of g of char-' min-I. Ignition occurred initially in these samples in CO, CO, measurements.
I
100 1 .o
\
I ,
i
200
I
\ \
300
,
--400
Temp
-----__ 500
t
(TI
Figure 3. Thermal analysis of cellulose in air and nitrogen.
t
0 2 1
0 4 0:10
2:8
4:6
6.4
0:2
lo:o
0 o r a x : b o r i c a c i d f w m u 1 d t i o n i p r o v i d i n g t o t a l 101 "add-on w e i g h t "
Figure 2. Cumulative yielde of CO and COz produced by smoldering combustion of treated cotton linters.
COzrelative to the heat of formation of CO (-88.5 kcal/mol vs. -22.9 kcal/mol at 500 "C, respectively). In order to gain further information on the role of these additives in the smoldering combustion of cellulose, rates of CO and COz production were determined for a series of boric acid- and borax-treated cotton linters. Borax and boric acid affect the yields of CO and COz from smoldering cellulose quite differently. Increasing borax content led to higher C02 yields while increasing boric acid contents favored CO production. Figure 2 shows that constant levels of additive (w/w), the concentration of borax and boric acid had a linear effect on production of CO and COz with the molar CO/COz ratio varying from 2.4 at 10% (w/w) boric acid to 0.54 at 10% (w/w) borax. This compares to a molar CO/CO2 ratio of 1.1 for untreated cellulose. As the boric acidborax weight ratio was increased, smoldering combustion became more difficult to initiate and propogation was slower, until at 9:l and 100 weight ratios samples would only smolder if the air flow was increased. The thermal reactions were also investigated by TG, DTG, and DSC. Figure 3 shows the thermal analysis of cellulose in air and nitrogen. The TG and DTG curves show greatest weight loss at about 355 OC in air and 375 "C in nitrogen, due to depolymerization by transglycosylation reactions (Shafizadeh and Fu, 1973;Shafizadeh and Bradbury, 1979). In air additional weight loss takes place due to oxidation of the charred residue. The DSC indicates that pyrolysis under nitrogen is an endothermic process. However, in air an exotherm is superimposed on the pyrolysis endotherm, resulting in an overall
100
200
400
300 Temp
500
(TI
Figure 4. Derivative thermogravimetry (DTG) in air of cellulose treated with boric acid or sodium borate. Moles HaBO,, B per 100 mol of glucose units (a) HaBOa, 1; (b) Na2B407,1; (c) Na2B407,10; (d) Na2B,O7,30.
exothermic peak centered at 375 "C. This is followed by a second exotherm at about 490 "C. The first exotherm, which is simultaneous with pyrolysis, is apparently due to chemisorption of oxygen on the nascent char. This reaction is known to be highly exothermic (Bradbury and Shafizadeh, 1980a) and results in the formation of stable surface oxides which stabilize the char toward combustion. The second exotherm results from the combustion of the char at higher temperatures, where the surface oxides are gasified. The heat release due to the chemisorption reaction, and to a lesser extent the combustion of the char at higher temperatures, provides the driving force for propagation of smoldering combustion in the substrate. The thermal stabilities of sodium borate and boric acid-treated cellulose are compared in Figure 4. For both additives the temperature at which pyrolysis takes place is similar to that of untreated cotton, although the resulting char yield is greater, and the chars differ in their rates of oxidation. The char from the boric acid-treated sample was gradually oxidized up to about 570 "C, whereas the char from borax-treated samples decomposed rapidly around 500 "C. The sharp peak in the DTG trace is indicative of ignition in the samples treated with high levels of borax. Significantly, this peak occurred at lower temperature as the borax content increases.
100
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
~
A
IO0
200
300
400 Temp
500
600
PC)
Figure 5. Derivative thermogravimetry (DTG) in air of cellulose treated with (NH,)*HPOI. Moles of additive per 100 mol of glucose units: (a) 0.2; (b) 1.0; (c) 10.
I
/I
I
I
200
4
300 Temp
400
500
J 600
Pcl
Figure 6. Derivative thermogravimetry (DTG) in air of cellulose treated with additive enhancing smoldering combustion. Moles of additive per 100 mol of glucose units: (a) NaCl, 0.2; (b) NaCl, 1.0; (c) Pb(CH2C00)2,1.0; (d) FeS04, 1.0.
DTG analysis of cellulose treated with diammonium hydrogen phosphate is shown in Figure 5. It is evident from this figure that addition of (NH4)2HP04of only 0.2 P atom per 100 glucose units,corresponding to the addition of 0.16% (w/w) of the salt, is sufficient to lower the temperature of gasification reactions and rate of weight loss, and consequently increase the yield of char. Higher levels of phosphate further lowers the temperature of the pyrolysis reactions and rate of weight loss. Figure 6 shows the effect of some smolder enhancers on the thermal reactions of cellulose as measured by DTG. The decomposition temperature is clearly reduced by all of the enhancers with the onset of ignition (indicated by the rapid weight loss) varying widely, depending on the additive and the level of addition. addition of only 0.2 mol of NaCl per 100 mol of glucose units (0.08% NaCl by weight) reduces the pyrolysis temperature by nearly 50 "C, increases the yield of char, and enhances the char combustion process. The addition of Pb(CH3C02)2and particularly FeS04 are even more effective in reducing the temperatures of both the initial degradation and the combustion reactions. Discussion In order to provide a chemical description of smoldering combustion it is essential to determine the source of heat
flux or the driving force of the smoldering process. In these studies it has been shown that the quantity of volatile pyrolysis products is unrelated to the smoldering behavior of cellulose. Furthermore, oxidation of the char controls the propagation of the smoldering front. Other theoretical and experimental work (Shafizadeh and Bradbury, 1979; Moussa et al., 1976) also indicates that the heat flux needed to sustain pyrolysis is provided by oxidation of the cellulose char. Consequently, enhancement or inhibition of smoldering by an additive should be reflected in the ability of the additive to catalyze or retard char combustion. Relatively little is known about the mechanism of oxidation of cellulosic char, although the high-temperature oxidation of pure carbon has been extensively studied. Many aspects of this reaction are still unexplained, but it is understood that gasification to carbon monoxide or carbon dioxide occurs via the formation of two distinct classes of oxides: (1)a stable oxide associated with the chemisorption of oxygen on the carbon surface and (2) fleeting "mobile oxidesn which are highly mobile on the carbon surface and react to form CO and COz instantaneously (Marsh and Foord, 1973). At the temperature of smoldering, combustion via the mobile oxide is considered to play the major role, while the formation of stable surface oxide retards gasification (Laine et al., 1963; Bradbury and Shafizadeh, 1980a). We have studied the kinetics of oxygen chemisorption on cellulose chars and have found that chars prepared by pyrolysis of cellulose can chemisorb up to 8% of their weight of oxygen at temperatures above 100 "C, and that the kinetics of gasification of the chars is highly dependent on the nature of the prepyrolysis additives (Bradbury and Shafizadeh, 1980b). The effect of various smolder enhancers or inhibitors can be considered in terms of these mechanistic aspects of char oxidation. The mechanism of catalysis of oxidation by metal salts is believed to be due to the effects of dissociation of molecular oxygen and transfer of atomic oxygen to the carbon in contact with the metal catalyst (Vastola and Walker, 1961). Transition metals, such as the ferric salts and lead acetate, tend to decompose on pyrolysis, forming high surface area catalysts which are highly active in promoting the oxidation of the char. In contrast, the gasification of phosphate- and boric acid-treated chars is retarded, although surface oxide formation takes place readily. This could be due to the formation of stable bonds between the cellulose and phoshorus (Hendrix et al., 1972) or boron, which could inhibit mobility of surface oxides in the combustion process. This is supported by the high CO/C02 ratios for chars from cellulose treated with these additives, which is in line with evidence that reducing the mobility of surface oxides increases the ratio of CO/C02 yields in the combustion reaction (Marsh and Foord, 1973). It has also been suggested that the retardant activity of borates and phosphates occurs through B203and P205 acting as a protective barrier preventing diffusion of oxygen to the substrate. This would seem unlikely, however, as prepyrolysis addition of minute quantities of phosphates is capable of preventing char ignition. Acknowledgment The authors thank the Center for Fire Research, National Bureau of Standards, for their interest and support through Grant No. G8-9011. Literature Cited Bradbury, A. G. W.; Shaflzadeh, F. Combust. Flame 19808. 37,85. Bradbury, A. G. W.; Shafizadeh, F. C" 1980b, 78, 109. Clarke, F. B.; Ottoson, J. Fire J . 1976, 70, 117. Hendrlx, J. E.: Drake, G. L., Jr.: Barker, R. H. J . Appl. Powm. Sci. 1972, 76. 257.
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Ind. Eng. Chem. Rod. Res. Dev. 1982, 21, 101-106 bine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J . phys. Chem. 1963, 67, 2030. Lyons. 0. W. “Chemlstw and Use of Fke Reterdants”; Wiley-Intersclence: . New York, 1970, Chfipter 5. McCarter, R. J. J . Consum. prod. FIammabMy 1977, 4 , 346. Marsh, H.; F w d , A. D. Carbon 1973, 1 1 , 421. Moussa. N. A,: T m , T. Y.: Garrls, C. A. “Slxteenth Symposium (Intema-1) on Combwikn”; The Combwtbn Institute: Pittsbw&, PA, 1976. Shafizadeh, F. A&. Carbohya. Chem. 1968, 22, 419. Shafkadeh, F.; Bradbury, A. G. W. J . Appl. Polym. Scl. 1979a, 23, 1481.
Shafkadeh, F.; Bradbury, A. 0. W. J . Therm. Insul. 1979b, 2 , 141. Shahdeh, F.; Chin, P. S.;W m t , W. F. J . Fke FlammbMnylFke Retard. Chem. 1976. 2. 195. Shahdeh, F.; Fu,’Y. L. Carbohydr. Res. 1979, 29, 113. Vast&, F. J.; Walter, P. L., Jr. J . Chem. phys. 1981, 58, 70.
Received for review March 23, 1981 Accepted November 16,1981
Ruminant Rations from Mesquite Biomass Pretreated with Water and Ozone Richard W. Tock,“ C. Reed Richardson,2 I r m a Oancarz,‘ Jeng Chang,‘ and Michele R. Owsiey2 Department of Chemlcal Engineering and Department of Agricultural Sciences, Texas Tech University, Lubbock, Texas 79409
Pulverized mesqulte biomass was chemically treated with ozone and water in both laboratory and pilot plant sized reactors. The objective was to modify the iignin-celiulose structure of the wood chips so that the cellulose fraction becomes more effectively converted by ruminant animals. The measure of success was determined by increases in the level of in vltro digestibility and by direct measurement of holloceilulose and lignin contents before and after treatment. The in vitro digestibility was increased from an average value of 30% for the raw-untreated mesquite to approximately 60% following treatment. This compares favorably with good quality hay. The holloceilulose was found to have been reduced by only 15% foilowing treatment, while the lignin content decreased by more than 40%. These data suggest that the ozone pretreatment process does make more of the cellulose content available for ruminant digestion. This was supported with in vivo studies using sheep.
Introduction The use of wood residues in ruminant animal diets is not a new concept although it does experience periodic renewed interest. Historically it has been considered for feed materials during drought cycles which have provided incentives to tap alternate sources of cellulose. Now, however, we can add concerns over expensive hydrocarbon fuels. The popular view is that abundant, renewable cellulose material, represent a prime reaource for fuel grade alcohols. Should large-scale commercial development of such technology occur, it could significantly alter meat production from ruminant animals. It is conceivable that only the low grade cellulosic substances would be available for animal diets, and even these only on a highly competitive basis. Most wood residues are classified as low grade cellulose sources, not because they are deficient in cellulose but because the cellulose is chemically and structurally bound to the extent that it cannot be readily utilized (Allinson and Osbourn, 1970). Except as fuel, wood biomass must be upgraded if its cellulose content is to be useful. The same is true for cellulose conversion by ruminant animals. One of the most extensive studies on the use of wood cellulose in animal rations was published in a series of papers of Millet et al. (1975). The digestibilities of 20 different hardwoods and 10 softwoods were investigated. However, mesquite was not one of the species included in the study, since it is common only to the semi-arid regions of the Southwest. The authors reported that the in vitro digestibility of untreated wood biomass, which included bark, was low. The values averaged only 30% compared to 60% for good quality alfalfa and 90% for cotton linters. Department of Chemical Engineering. Department of Agricultural Sciences. 0196-4321/82/1221-0101$01.25/0
Because of the low digestibility levels, six pretreatments were postulated and investigated for their potential to enhance cellulose digestion in the rumen. These processes included physical milling, irradiation, chemical treatment ~ t ammonia, h sodium hydroxide and sulfur dioxide, and biological degradation with white rot fungi. No pretreatment with ozone was attempted. This may have been because the study predated large commercial ozone generators. All the pretreatments which were examined by Millet et al. (1975) improved the in vitro digestibility of the wood residue to some extent. The sulfur dioxide process gave the best results. The subsequent in vivo tests indicated that the wood biomass, when pretreated, was an effective roughage replacement at concentration levels of 5 1 5 % of the total ration. Pulp fines which had also been treated to remove most of the lignin could be used at levels of 50 to 75% of the diet for steers, ewes, and beef cows. Weight gains of 0.5 kg per day were observed for steers. Consumption was greater, however, than that for standard rations which indicated that even after pretreatment, the wood-based residues have lower nutritive levels than roughages in standard diets. A significant number of studies specific to the use of mesquite biomass as a potential ruminant ration have been conducted on the Texas Tech University campus. This is part of an ongoing research program sponsored by the State of Texas. The goal is to recover the productivity of rangeland lost through mesquite infestation. Typically, the rancher wants the mesquite irradicated at the lowest possible cost. However, since complete irradication is not practical, control programs are used. It has been proposed that the control system based on periodic harvesting, together with chemical treatment to provide a supplement for cattle feed, might provide an economic advantage for the rancher. This would be especially true during periods 0 1982 Amerlcan Chemical Society
