Reaction between Ammonium Nitrate and Charcoals ALKALI METAL SALTS AS COMBUSTION CATALYSTS R. W. PARRY' AND E. W. COMINGS University of Illinois, Urbana, I l l .
Small pressed blocks consisting principally of ammoniumnitrate and charcoal, withand without smallamounts of a linseed oil binder, were prepared and burned in the open and in volume flowmeters. Many hundreds of blocks were tested. Several series are described in which the effect on the burning rate produced by potassium nitrate in the mixture or by pretreating the charcoal with phosphoric acid or inorganic salts was determined. The burning rate was increased by alkali metal nitrates, sulfates, carbonates, and monohydrogen phosphates. Chlorides and cyanides were less effective. Phosphoric acid decreased the burning rate.
I
N RECENT years some applications have been found for solid fuels composed chiefly of ammonium nitrate and charcoal. Such ammonium nitrate-charcoal mixtures usually burn much less rapidly than comparable pyrotechnic mixtures containing potassium nitrate as the oxidizing agent. Earlier investigators have noted that certain alkali metal salts such as sodium and potassium carbonates catalyze a number of heterogeneous reactions involving carbon as a solid phase reducing agent. The present investigation extends this work to include the catalytic action of inorganic acids and salts in the reaction between ammonium nitrate and charcoal. Results obtained from laboratory and pilot plant observations are given. PREVIOUS WORK
Taylor and Neville (6) studied potassium carbonate, sodium carbonate, lithium carbonate, barium carbonate, calcium carbonate, sodium chloride, ferric oxide, copper, sodium silicate, and borax (Na2B407 10 H20) as catalysts for the reaction:
C
+ 2HzO = COz + 2Ht
Potassium carbonate and sodium carbonate were the only effective catalysts. Blayden, Riley, and Shaw ( 1 ) extended the work of Taylor and Neville to the reactions between carbon and oxygen. They concluded that: Solid alkali carbonate on the carbon surface catalyzes the reaction of carbon with carbon dioxide, oxygen, water vapor, and hydrogen. The catalytic effect of the carbonate increases mith the basicity of the cation-that is, the effect increases in the order lithium, sodium, potassium with potassium equivalent to rubidium and cesium and the alkali metals showing a greater effect than the alkaline earths. Sodium carbonate, cyanate, acetate, formate, and oxalate have relatively high catalytic activity while sodium sulfate, chloride, and phosphate have lower activity. The maximum catalytic activity is reached with 2% carbonate. 1 Present
The reaction which is catalyzed is that between carbon and oxygen and not between volatile matter in the charcoal and oxygen. Carbonate was assumed to act by cleaning the charcoal surface of fixed oxides, thus presenting a clean carbon surface to the oxidizing gases. The method by which carbonate cleans the surface was not clearly defined. EXPERIM EXTA L
The rate of the reaction between ammonium nitrate and hardwood charcoal was studied by measuring the time required to burn small cylindrical pressed blocks consisting of ammonium nitrate, charcoal, and a suitable binder (usually 2 to 3% boiled linseed oil). The burning rate of the mixture was varied by substituting potassium nitrate for ammonium nitrate or by pretreating the charcoal with solutions of phosphoric acid or inorganic salts. Blocks were prepared by mixing the ingredients for 20 minutes in a 2-foot diameter Simpson intensive mixer of the edge-runner type and then pressing the finished powder into a suitable container. Steel cans, Bakelite tubes, and rolled paper tubes were used as containers. Blocks of 50- and 500-gram size were pressed in two increments under a 3300 pounds per square inch pressure; only one increment was used for the 13-gram units. An ignition mixture consisting of 40% silicon, 54% potassium nitrate, and 6% charcoal was pressed on with the final increment of powder in the 500-gram blocks. Finished 500-gram blocks had a density of 1.41 grams per ml., which corresponds to about 16 to 17% void space in the pressed unit. Blocks were stored over calcium chloride until burned. The ammonium nitrate used in the 500-gram blocks and pilot plant work was a technical grade (80% between 20 and 50 mesh) from Mallinckrodt Chemical Company. This was carefully dried a t 110' C.before use. The hardwood charcoal was a commercial grade, 5CC, from the Flower City Charcoal Company of Rochester, N. Y.; 98% of the sample passed a 100-mesh screen; 94% passed a 200-mesh screen, and 7501, passed a 325-mesh screen. The subscreen fraction had a mass median diameter of 11microns. The potassium nitrate was a U.S.P. Grade with a screen analysis showing 25 to 30% between 40 and 60 mesh, 60 to 70% between 60 and 150 mesh, and not more than 10% through 150 mesh. The linseed oil was a commercial pure, boiled linseed oil. EFFECT OF POTASSIUM NITRATE ON BURNING RATE OF FUEL MIXTURE
Five hundred-gram blocks in steel cans were used for this investigation. Small amounts of potassium nitrate (0 to 12%) were substituted for part of the ammonium nitrate. Two sample blocks of each of the compositions shown in Table I were burned in a special gas flowmeter ( 3 )after being stored at 20' to 25' C. for 4 days. After 171 days' storage, four blocks of each composition were burned in the open, under atmospheric pressure. The
address, University of Michigan, Ann Arbor, Mich.
557
e
INDUSTRIAL AND ENGINEERING CHEMISTRY
55d
Vol, 42, No. 3
added 750 grams of clharcoal flour. The slurries wexe allowed to stand for about 24 Container = steel can 6.40 cm. inside diameter hours with occasional stirring, Weight of block = lO-gram starter + 500 grams of fuel mixture and then the water wa4 evdpoilrea of block surface = 32.2 sq. c m . ; block height = 11.0 o m . iated. The air-dry charcoal 500 grams ___ Burning rate = hurnine time (inin.) X 32.2 sa. om, Oyer-all !vas dried a t about 100" C. Burnine for 45 hours, ground to pas5 " k k GZ&. Over-all a -&mesh screen, and used in Re- x Sa. CIll.d Burning Max. Rlax. Gas fuelmixtures. The 13% phosComposition of Mixture Rate, Gas Flow R a t e v~~~~~,a&. coyered Age In when CharG./hlin. Temp., A. C.=, phoric acid treatment is equivS. C.; Gasesc burned, C. Cu. Ft./Gec. A. Cea K N 0 3 , KH4NOa, coal, Oil, X Sq. Cm. 171 days alent to adding 0.177 mole of Age when burned, 4 days % % % % , 15~5 87 2.86 0.185 51.3 2.50 795 0 86 11 3 phosphoric acid t o 100 grams 15.4 86 3.28 0.208 52.5 850 11 3 2.67 1 85 of charcoal. The 25% potas15.3 86 3.68 0.265 57.9 2.94 905 4 82 11 3 14.8 84 4.18 0.908 56.3 3.44 940 7 79 I1 3 sium carbonate treatment cor12 74 11 3 3.73 980 0,350 59.1 15.4 86 4.54 responds to 0.241 mole o f a . 4 C. = gas volume measured a t temperature a n d pressure in volume tester. potassium carbonate per 100 b S.'C. = gas voiume measured a t 60" F. a n d 1 atmosphere absolute. c Molecular weight of evolved gases = 23.5 on basis of gas density a t 120' C. graiiis of charcoal. Nine 500d Each rate is the average of four tests. gram blocks were made from each of the treated charcoals and from an untreated control, a total of 54 units. The steel containers for the blocks were provided with an insulating burning rate of these blocks increases with age, but this change paper liner, 0.015 inch thick, to reduce uneven burning dmvn the is essentially complete after a period of 20 days. Apparently the side. -4 standard formula consisting of 83 weight % smnionium Linseed oil in the block causes a contraction of the block volume nitrate, 11 weight % charcoal, and 3 weight % linseed oil was on setting; the contraction permits some burning down between used. The weight of charcoal was corrected in each case t o the wall of the can and the block edge, and this increases the allow for the solid introduced in treatment, thus the weight of burning surface and causes an increase in the burning rate. Thus actual charcoal was the same in each block. The blocks were the performance of the blocks after 171 days' storage represents burned in the open after storage periods of 3 and 9 days. a stable burning rate. Data from this series of tests are sumThe pH of each charcoal sample wa3 determined with a glass marized in Table I and the burning rate of the blocks is shown in electrode in a slurry made by suspending 10 grams of charcoal in Figure 1 as a function of the percentage of potassium nitrate. 10 grams of distilled water. Test data, summarized in Table 11, indicate that pretreatment 5.00 of charcoal has a large effect on the rate of comhst'ion. A burn% ing rate range from 1.36 to 3.41 grams per minute per square cm 4.00 has been obtained with blocks of ot'herwise identical composition i simply by pretreating charcoal Jvith phosphoric acid or pot,assium 3 . W carbonate. The burning rate of the mixture is shown It: Figure W 2 as a function of the pH of the charcoal.
TABLEI. EFFECT
NITRATE IX A CHARCOAt-AMMoXILX XITRATELINSEED OIL MIXTURE
O F POTASSIUM
-
Figure 1. Burning Rate of Fuel Mixture Containing 11% Charcoal, 3q0 Linseed Oil, Potassium Nitrate as Shown, and Remainder, Ammonium Nitrate
The substitution of potassium nitrate for part of the ammonium nitrate increases the burning rate of the mixture. The temperature of the effluent gas stream increases with an increase in the burning rate. The measured gas volume reduced to a standard pressure of 1 atmosphere and a temperature of 16 ' C. iFas the same for all compositions within the limits of experimental error. About 87% of the mass of the block was accounted for in the effluent gas stream.
Solution Used 13% HsP04
llloles of .Idditive/ 100 Grams Charcoal 0,177X HsPOa
IT20
0
Untreated
0
57, KzC08
0.048.11
10%
0.096.W ILC03
yH of ChaLcoal 1.70
Age of ~ l When Burned, Days 3 9 9
9.26 9 4
Per block 1.36 1.37 1.37
3
2.51 2 51 2.55
3 9 0 3
2.92 3.03 3.03 3.03 3.08 3.15
9 Y
KzC03
~ Burning ~ k Rate,
c./arin&%:S2?:.
9 9
!iverage 1.37
2 ;18
3.03
3.11
EFFECT ON BURNING RATE PRODUCED BY PRETREATIhG CHARCOAL WITH POTASSIUM CARBONATE OR PHOSPHORIC ACID
a p H of slurry consisting of 10 grams of charcoal suspended in 10 grams of boiled distilled water; addition of a n extra 10 grams of water had no appreciable effect on pH. Only 9-day blocks included in average.
In a second series of tests the charcoal was pretreated n ith phosphoric acid or potassium carbonate solutions. The burning rate of mixtures prepared from these charcoals was measured. Solutions containing 13% phosphoric acid by weight and 5, 10, and 25% potassium carbonate by weight were prepared. Distilled water was used as a control. To 1000 grams of each solution were
The results of the preceding series of tests were checked in a second series carried out with different ingredients and with a smaller size block, Baker and Adamson wood charcoal powder and reagent grade ammonium nitrate from General Chemical
*
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1950
AFTERACID OR ALKALITREATMENT TABLE 111. BURNINGRATEOF CHARCOAL 50 grams time (min.) X block area (sa. om.) Burning Burning Block Rate, Block Rate, Age, G./Min. Age G./Min. Days X Sq. Cm. H o d s X Sq. Cm. -Without oildRemarks -With an oil binderc1 hour 0.850 1 1.20 Burns with little heat; 17 0.912 1 1.20 carbon residue 17 0.906 1 1 hour 0.910 1 1 41 More heat evolved as 17 0.932 1 1.42 acid concentration 17 0.917 diminished 1.222 1 1.82 17 1.222 1 1.80 17
Block diameter = 2.5 cm.; burning rate = Moles of Additive/ 100 Grams Charcoala 0 187 M HaPOd
pH 2.30
0 . 1 2 4 Y Hap01
2.65
0 . 0 6 2 31 Hap01
6.07
Distilled water
7 20
~
1 hour 17 17 1 hour 17 17
Blocks containing no linseed oil showed incipient surging e 0 036 111 KKO, 9 30 1 36 Pronounced surgmg In 1 57 blocks with no oil 1 11 caused slow and er1 43 iatic burning a Charcoal differed from that described in Table I; i t was obtained from Baker and Adamson Compaw. b Acid-treated charcoal had salt layer across the top after drying but this layer decreased in amount as the strength of the acid decreased. deposit was identified as ME++, and C a + + , phosphates with traces of N H & +salts. I t was not foudd on charcoal treated with K9COa. 0 Composition: NHdNOs, 86%: charcoal, 12'7. linseed oil, 2%. d Composition: NHdNOs, 88%; charcoal, 12%; wt. of charcoal corrected for additive e Surging a cyclic, irregular type of burning.
..
1 630 1 510 1.610 1 850 1 910 1 850
1 1 1 1
1 47 1 51
Company were used. Samples of charcoal were treated with phosphoric acid solutions of three strengths, a potassium carbonate solution, and distilled water as indicated in Table 111. The charcoal slurry was allowed to stand 20 hours and then dried a t 100 * 5" C. for 72 hours. The ingredients were mixed by hand and 50 grams of the mixture were pressed into Bakelite tubes in two increments. One gram of an ignition powder of barium peroxide, aluminum metal, and aluminum oxide was pressed on with the last increment. The finished blocks were 2.5 cm. inside diameter, 6.87 cm. high, and had a density of 1.52 grams per ml. Blocks of this size were prepared both with and without linseed oil. The blocks which contained linseed oil confirmed the results obtained in the preceding series. In Figure 2 the burning rate of both sizes of blocks is shown as a function of the pH of the charcoal used. The two sets of data do not lie on the same line. This is not surprising because different charcoals were used as well as blocks of different size.
559
mixture. The charcoal treated with acid burned a t a uniform rate and at a relatively low temperature. In fact, the reaction was incomplete and considerable carbon and ash residue were left in the Bakelite tube. The Bakelite tubes containing acid-treated charcoal blocks were not badly charred. The carbonate activated charcoal burned a t a much higher temperature as evidenced by complete combustion of the carbon and the burning of the Bakelite tube. EFFECT ON BURNING RATE PRODUCED BY PRETREATING CHARCOAL WITH VARIOUS SALTS
The role of other salts in the combustion of these mixtures was investigated. Baker and Adamson hardwood charcoal Was pretreated with a number Of salts by outlined for the 50-gram units. The fuel mixture consisted of 86% ammonium nitrate (reagent grade), 12% pretreated charcoal, and 2% linseed oil as binder. The blocks were made by pressing 13 grams of the fuel mixture and 0.5 gram of starting powder into a paper tube in a brass form. Finished blocks were 1.85 cm. in diameter, 3.2 cm. high, and had a density of 1.5 grams per ml. The blocks were burned in the open about 2 hours after pressing. Data are summarized in Table IV.
f
Figure 2. Burning Rate. of Ammonium NitrateC harco al-Linseed Oil Fuel Mixture as Function of pH of Charcoal
$2.m
t
4'' a '
4o ''*O PH OF AQUEOUS CHARCOAL SUSPENSION
DISCUSSION BURNING OF PRETREATED CHARCOAL IN A MIXTURE CONTAINING NO LINSEED OIL
In the other series with 50-gram blocks, ammonium nitrate and charcoal were pressed together without the linseed oil binder. Data for these units are also shown in Table 111. The burning time decreased at first with an increase in the pH of the charcoal but increased again above a PH of 6.1. The longer and.erratic burning t'ime in the case of the alkali charcoal was due to a cyclic and irregular form of combustion called surging. Surging was also encountered in a small percentage of the blocks which contained linseed oil. This could be traced to a particular lot of charcoal. Charcoal known to have little or no tendency to surge was used for all blocks discussed in this paper. Surging is discussed more fully ( 4 ) . The block would burn rapidly and vioIently for a few seconds and then almost extinguish itself. After several seconds, rapid burning would begin again and the cycle would be repeated. The long and irregular burning time of the blocks containing charcoal treated with alkali may be attributed to the irregular periods when burning almost ceased in the trough of the surge. Linseed oil served only to prevent surging; it was not involved directly in the catalyzed reaction between charcoal and ammonium nitrate. This was demonstrated by the fact that cast blocks ( 4 )containing no linseed oil burned faster as potassium nitrate was substituted for ammonium nitrate in the
The substitution of potassium nitrate for part of the ammonium nitrate increases the burning rate of the mixture. The pretreatment of charcoal with phosphoric acid decreases the rate-of charcoal combustion, whereas pretreatment of charcoal with various
T~~~~IV, DATA FOR BLOCKS BURNED IN OPEN HOURS AFTER PRESSING (Block area = 2.68 sq. om.; block weight = 13 grams of fuel) Burning Rate, G./ Moles of Additive/100 Min. X 8s.. Cm. Grams of Charcoal PH (With Oil) 1. Nothing added 7.35 1.70 2 . Distilled water 7.00 1.76 3 . 0.036 M KzCOs 9.50 2.16 4 . 0.036 M NazCOa 9.70 2.16 5 . 0.036 M LizCOa 10.00 2.16 7.50 2.20 6 . 0.036 M KaSOa 7 . 0.036 M NazSOi 8.05 2.16 8. 0.036 M Llzso4 7.90 2.13 9 . 0 . 0 3 6 M M so4 8.45 1.73 7.25 1.94 10. 0 . 0 7 2 M K 8 l 1 1 . 0 . 0 7 2 M NaCl 7.25 1.94 12. 0 . 0 3 6 M MgClz 9.25 1.76 13. 0.036 M CaClz 6.85 1.76 14. 0.036 M BaClz 6.85 1.98 0.80 1 . 80a 15. 0 . 0 2 4 M FeCls 16. 0 . 0 3 6 M KZHPOI 9.10 2.06 . 17. 0 . 0 7 2 M K C N 9.35 1.94 a Values for FeCls rather uncertain; results somewhat Trariable.
560
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
salts increases the rate of burning. Pretreatment Rith alkali metal sulfates and carbonates i s most effective followed by treatment nith alkali metal phosphates. Chlorides and cyanides had only a small effect. Little difference was noted between comparable salts of different alkali metals-that is, lithium, sodium, potassium. Of the alkali earth metals, barium appeared to be most effective. However all chlorides of the alkaline earths showed very low catalytic activity. The retarding action of phosphoric acid appears to be related to removal of catalytically active metal carbonates and othei salts from the charcoal surface. First, a salt crust (largely calcium and magnesium phosphates) formed over the surface of the dry acid-treated charcoal mass, but no salt crust appeared over the alkali treated charcoal. This suggests that an exchange adsorption occurred during acid treatment in which H+ions replaced the metal cations held on the chaicoal surface. Secondly, thc presence of carbonate or a related compound on the charcoal was suggested by the fact that carbon dioxide was given o f f when the charcoal was treated with phospholic acid. Chloiides of calcium and magnesium showed little catalj tic effect in separate experiments, but it is highly probable that well distributed carbonates of magnesium and calcium on the charcoal would have shown marked catalytic effect, since carbonates in general are more effective than chlorides. The linear relation between pH and burning rate in the phosphoric acid series may be qualitatively interpreted in terms of removal of surface active metal carbonates and their replacement by noncatalytic phosphoric acid. The relation between pH and burning rate in the potassium carbonate serie5 may be ascribed to replacement of surface acids or bicarbonates by the more catalytically reactive potassium carbonate. Quantitative interpretation of the relations will require considerably more data. No direct relation betivcen pII and burning rate as observed when salts other than potassium carbonate mere uJed in pretreatment of charcoal. This would be expected, since the pIl relation would be of most direct importance in the acid-carbonate system. The present data together with data in the literature suggest that carbonates, organic salts which produce carbonates on combustion, and salts of oxidizing acids are most effective in catalyxing the oxidation. A qualitative parallel exists between the oxidizing ability of the anion group and its efficacy as a catalyst. For instance, the order of decreasing effectiveness appears to be nitrate, sulfate, and phosphate. Xonodizing anions such chloride and cyanide show little activity. The mechanism of catalytic action can not be unequivocally elucidated from data of this type. Hon-ever, some observations
Vol. 42, No. 3
relevant to the reaction mechanism seem pertinent. Since ammonium nitrate is thermally unstable a t the ignition temperature of charcoal, the ammonium nitrate-charcoal reaction is undoubtedly a solid phase gas reaction. The gases involved include nitrous OYide, nitric oxide, nitrogen dioxide, nitric acid, oxygen, carbon dioxide, and perhaps hydrogen. The effects of alkali carbonate in the reaction between charcoal and gases such as oxygen, carbon dioxide, and hydrogen have been described in the literature. Constable (6) found that the reaction betmen nitrous oxide and mrbon was almost identical in behavior to that between oxygen and carbon. Previous investigators have attributed the action of the carbonate catalysts to their ability to remove an oxide film from the charcoal surface. Recent experiments (b) using radioactive C1h as a tracer have shown conclusively that the slow step in the reaction COS C .-+ 2CO is the desorption of carbon monoxide from the oxide layer covering the charcoal surface. Since the catalyst would be expected to accelerate the slow or rate-controlling process, it would follow that catalytically active salts would aid in the desorption process. The mechanism of such action has never been satisfactorily explained, though many suggestions have been advanced. The role played by both cations and anions in this process cannot be clarified without further work. An understanding of this problem would do much to resolve the mechanism of carbonate catalysis in the complex ammonium nitrate charcoal reaction.
+
ACIGYOWLEDGMENT
The helpful suggestions of J. C. Bailar are gratefully acknowledged, as well as contributions by R. P. Connor, D. C. Ehrlinger, R. C. Johnson, M. H. h i l a , P. N. Rylander, andC. H. Simonson. II. F. Johnstone was technical director of the laboratory. LITERATURE CITED (1)
(2)
(3) (4)
(5) (6)
Blayden, H. E., Riley, H. L., and Shaw,F., Fuel, 2 2 , 3 2 , 6 4 (1943). Bonner, F., and Turkevich, J., presented before the Division of Physical and Inorganic Chemistry at the 116th Meeting of the AXERICANCHEMICAL SOCIETY, Atlantic City, N. J. Comings, E. W., and Johnson, R. C., Anal. Chem., 21, 290 (lY49). Parry, R. W., Comings, E. W., and Raila, M. H., IND. EXG. CHEM., 42, 560 (1950). Strickland-Constable,R. F., Trans. Faraday SOC.,34, 1074 (1938). Taylor, H. S., and Neville, H. A , J. Am. Chem. Soc., 43, 2055 (1921).
RECEIVED January 3, 1949. Presented before the Division of Gas and Fuel SOCIETY, htChemistry a t the 11BLh Meeting of the ~ V E R I C A NCEIEXICAL lantio City, N. J.
(Reaction between Ammonium Nit rate and Charcoals)
SURGING IN COMBUSTION R . W. PARRY’, E. W. COMINGS, AND M. H. RAILA Gniversity of Illinois, Urbanu, IIE.
T
HE action of potassium carbonate as a catalyst in the combustion of a charcoal-ammonium nitrate-linseed oil fuel mixture has been described (4). Many hundreds of fuel blocks of this type burned smoothly. In a ferv instances mixtures with and without linseed oil burned with a cyclic combustion termed “surging.’, An investigation of surging is summarized in this paper. Surging is characterized by a high burning rate followed by a sudden sharp decrease in burning rate. This cycle repeats itself 1 Present
address, University of Xlichigan, Ann Arbor, Mich.
many times. It can be represented graphically by a curve showing the rate of discharge of the gaseous products of combustion as a function of time as shown in Figure 1. The violence of the surge is indicated by the height of each wave. The length of time between successive crests is the period of the surge. Periods ranging from 2 to 50 seconds have been observed. Generally an increase in the length of the period increases the violcnce of the surge, A surging unit burning under pressure sounds much like a steam engine operating at low speed. The gases are ejected in short vigorous bursts.
