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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 Chemistry a t the 11BLh Meeting of the ~ V E R I C A NCEIEXICAL SOCIETY, htlantio 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 a t low speed. The gases are ejected in short vigorous bursts.
March 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY PREPARATION OF FUEL BLOCKS
The charcoal, the ammonium nitrate, and the procedure for manufacturing pressed fuel blocks have been described (4). A typical pressed block composition is given in Table I. Water, linseed oil, dilute acid, or dilute alkali solution were each added to separate mixtures and in addition to their influence on the reaction served as binders for the pressed units.
;-" K
1
56 B
inch orifice drilled in the side of the pipe. This orifice could be closed gradually by a conical plug. Low pressures were measured by a mercury manometer, higher pressures by a pressure gage. A safety valve was provided. Operation was by remote control behind a wood and steel barricade. The block wm ignited in the surge tester, and when burning smoothly the gas pressure was slowly increased by screwing the conical plug into the exhaust orifice. The surge pressure was the pressure in the fuel compartment when the first mild pressure fluctuations occurred. Surging was easily detected by sound and by the fluctuations in the gas pressure. Increasing the gas pressure above the surge pressure increased the violence of the surging. ACIDITY OF FUEL MIXTURE.A relation between the pH of mixtures and the surge pressure was found for two series of pressed blocks.
TIME
Figure 1. Gas Discharge Rate as Function of Time during Burning of Surging Fuel Mixture
A casting procedure, not previously described, was also used for consolidating the mixture. The ingredients, as shown in the
formulas of Tables I1 and 111, were mixed in a Simpson intensive mixer for 20 minutes and then melted with constant stirring in a steam-jacketed kettle. Charges of up to 30 kg. were melted using 50 to 60 pounds per square inch steam pressure in the jacket. The molten mass was poured (cast) into steel cans and allowed to solidify. The melting point for the mixture ranged from 110' to 125" C. The apparent density of the casting was 1.46 =t0.05 grams per ml. A slurry of starting mixture was spread over the surface of the casting in ribbons and allowed to dry.
Figure 2. Apparatus Used to Measure Surge Pressure of Burning Fuel Mixture
In one series aqueous phosphoric acid, pure water, and aqueous solutions of sodium hydroxide were used as binders. The VARIABLES AFFECTING SURGING finished blocks were 6.4 cm. in diameter and 10.4 em. in height. Data are given in Table I and Figure 3. An increase in the alkali Surging is believed to be due to variations in the surface of the content of the mixture lowers the surge pressure-that is, procharcoal which cause one part to burn more rapidly than another motes surging. The second series was of larger blocks having a part. These variations are accentuated by a number of factors diameter of 17.8 cm., a depth of 4.75 cm., and a density of 1.20 including: the method used in consolidating the loose mixture of grams per d.A curve of the same general shape was obtained. charcoal and ammonium nitrate into a block; the acidity of the The data are shown in Figure 3. fuel mixture; the gas pressure under which the unit burns; the The same general relation between pH and the surge pressure presence of linseed oil in the fuel mixture; the nature and amount was observed qualitatively for cast blocks. These were not of charcoal used; the preignition temperature of the fuel mixture; tested systematically in a surge tester, but were burned in the and the age of the completed fuel block. open with 1 atmosphere gas pressure and also in a chamber proGASPRESSURE. The tendency to surge incremed with an invided with a discharge nozzle. The gas pressure in the chamber crease in the gas pressure under which the mixture burned. The ranged from 10 t o 20 cm. fuel blocks were burned of mercury above the presin a special surge-testing sure of the atmosphere. unit shown in Figure 2, The data are summarized The surge tester was a in Tables I1 and 111. In cylindrical vessel made A large number of fuel blocks consisting principally Table I1 the first composifrom a 3-inch nominal diof ammonium nitrate and hardwood charcoal burned tion is a eutectic mixture ameter extra-strong steel smoothly; a small percentage exhibited a regular cyclic of ammonium nitrate, ampipe. One end waa welded burning termed surging. This was attributed to two monium chloride, and soshut with a 0.25-inch reactions in which two reducing agents compete for the dium nitrate containing same gaseous oxidizing agent. The two reducing agents thick steel plate. The suspended charcoal. The other was threaded and in this case were usually different parts of the same batch mixture burned smoothly of charcoal which possessed somewhat different surface was closed by a pipe cap in the open but surged with reactivities. A number of factors which enhance these which permitted insertion explosive violence when differences and so promote surging are described. The of the fuel block. The comburned in a chamber under bustion gases were exgaseous products from the crest and trough of the surge somewhat greater pressure. hausted through a 0.375have been analyzed.
Vol. 42, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
562
Figure 3. Effect of pH of Fuel Mixture on Gas Pressure a t Which Surging Appears Atmospheric pressure is 75.0 cm. of mercury 0 = small units = large units
+
Blocks containing water surged in some cases but not in others, depending on the charcoal used. A mixture of ammonium nitrate, ammonium chloride, starch, and a charcoal known to have a tendency to surge was divided into two lots, 37, of 0.25 A 1 phosphoric acid was added to the first lot and 3y0of 0.75 M phosphoric acid was added to the second before casting. Table I11 illustrates the effect of lower pH in repressing surging. All three of the blocks treated with 0.75 31 acid did not surge whereas all three treated irith 0.25 J1 acid surged violently in the chamber. PREIGNITION TEMPERATURE, AGE, A B D SHAPE O F BLOCKS. Low preignition temperatures promoted surging in pressed mixtures. A definite increase in surging was noted when the preignition temperature of blocks containing 11% charcoal and 37, linseed oil was lowered from 25' to -40" C. Surging tendencies also became more pronounced after fuel units had been stored for some time a t room temperature. Several sizes and shapes of fuel blocks were tested and no relation between surging and block geometry was found. LINSEED OIL. In general, linseed oil reduced the tendency of the mixture to surge. Pressed blocks containing linseed oil as a binder surged much less frequently than cast or presspd blocks of similar composition containing water, alkali, or very dilute acid. Mixtures from certain lots of charcoal surged even though linseed oil was used. CHARCOAL.The type charcoal used is closely associated a i t h surging. Pressed mixtures containing linseed oil and made from a certain few lots of commercial wood charcoal surged violently when burned in chamber units. Pressed blocks of identical composition made from different lots of the same brand of commercial charcoal burned smoothly. The "surging" charcoal and the "nonsurging" charcoal were examined. h spectroscopic analysis and an x-ray diffraction pattern of the ash showed no appreciable difference in their mineral composition. Many investigators ( 1 , S , 5 , 6 )have attributed variations in the combustion of charcoal to oxides on the charcoal surface. Charcoal with surface oxide absorhs water more rapidly than clean
charcoal and will be wet more readily when floated on a water surface. Samples of the surging and nonsurging charcoal were floated on distilled water and the rate of aetting and settling noted. The nonsurging charcoal netted easily and settled rapidly. The surging charcoal was very difficult to wet. After 24 hours only a small amount of the sample had settled through the water surface; this indicated a much larger amount of surface oxide on the smooth burning charcoal than on the surge charcoal. Regardless of the nature of the difference, a significant difference in surface properties is evident. Data on the two charcoal samples are given in Table IV. The difference in surface wetting is the only distinguishing feature. PERCENTAGE OF CHARCOAL. In a series of pressed blocks containing 3% linseed oil the percentage of normally nonsurging charcoal was varied from 5 to 13% and the percentage of ammonium nitrate adjusted accordingly. The blocks containing only 5% of this charcoal surged mildly when burned in the open and surging was very pronounced when burned under an absolute gas pressure of 77 em. of mercury (atmospheric pressure was 75 cm.). Blocks containing 7 , 9, 11, and 13% of this charcoal did not surge under gas pressures up to 275 cm. of mercury. The gaseous products of combustion from a surging 570 charcoal block were analyzed in an effort to elucidate the surge mechanism. GAS ANALYSIS.The principal products produced in burning are water vapor. carbon dioxide, carbon monoxide, hydrogen, and nitrogen. Smaller amounts of ammonia, nitrogen dioxide, nitric oxide, nitrous oxide, and oxygen are also frequently present. The method used for analysis was selected because it minimized, as much as possible, errors due to the interaction of the gaseous products. The first steps in the procedure Irere based on pressure differences a t constant volume a t a temperature of 120" C., as contrasted to the more conventional methods based on volume changes at constant pressure and room temperature. ii gas sample was taken in an evacuated and weighed liter flask equipped with two outlets so that liquid could be introduced and removed while excluding air. The vapor density of the gas niivture Tyas determined in an oil bath a t 120' C. and the apparent molecular weight of the gas mixture calculated. The flask was cooled and water, ammonia, nitrogen dioxide, and nitric oxide absorbed by introducing a solulion of 0.2 1v potassium permanganate in 6 N sulfuric acid. The number of moles of gas removed \?as determined by measuring the pressure of the gas a t 25' C. and correcting for the vapor pressure of the acidic permanganate solution.
TABLE
I.
I K F L C E N C E O F A&CIDITY O F FUEL >fIXTURE OX PRESSURE O F PRESSED BLOCKS
(Composition of block:
Binder 3.5
,M phosphoric acid
SURGE
NHdNOa, 83%; SH4C1, 3 % : charcoal, 11%; binder, 3%) Surge pH, of Pressure h , mixCm. Hg turea ilbs. Remarks Did total not pressure surge up to 210 em. 3.4 ..
1.5 .M phosphoric acid 0 . 7 5 .M phosphoric acid
5.45 6.0
125
Distilled water
6.1
95
2 M sodium hydroxide
6.5
79-80
10 ilf sodium hydroxide
7,l
77-80
Became more violent with slight increase in presmire
20 M sodium hydroxide
7.2
77-80
...... .....
92-08
Mild, indefinite surging Surging continued below this pressure after once started Surging very mild a t this pressure Surging rather indefinite, becoming more pronounced with pressure
p H of fuel sample determined just before mixture was burned; p H deter ined on slurry containing 40 grams of fuel mixture and 10 grams of water. 6nGas pressure a t which surging first appeared; atmospheric pressure = 75.0 om. Hg. a
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
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563
Ammonia was determined in an aliTABLE 11. SURGING I N C.4ST UNITS O F DIFFERENT COMPOSITION quot of the permanganate solution by nesslerization. Kitrogen dioxide and Block Composition Casting Wt Wt. % Other Temp., nitric oxide were determined together NH48 charcoal ingredients, % 0 C. Burned Surging In chamber Very violent. as nitrate in a second aliquot by a volu66.2 4.3 13.0 NaNOs, 16.5 115 metric method of Kolthoff, Sandwell, unit ble; UP and Muskovitz ( 2 ) . Water was obtained 66.2 4.3 13.0 NaNOs, 16.5 115 I n open None 71.6 7.9 12.6 HzO, 1.9 120 I n chamber Moderate by difference. (Approx.) The gases remaining in the flask (car71.6 7.9 12.6 Hz0. 1.9 120 I n open None (Approx.) bon dioxide, carbon monoxide, hydro75.3 9.7 12.3 Hz0, 2 . 7 . ... In chamber None 75 10 11 HeO, 4 110-15 In chamber Very violent gen, oxygen, and nitrogen) were transferred to a standard gas analysis apTABLE 111. EFFECT OF ACID STRENGTH ON SURGING IN CAST paratus. All but nitrogen were deterFCEL MIXTURES mined by absorption and oxidation. The nitrogen remained as (Block composition: NH4NOs 84.3 . NHiC1, 3.0%: Hapoi, 3.0%; residual gas. Nitrous oxide mas not determined separately in this starch, 3.6h%: ckrcoal, 6.05%) procedure since the chromous chloride solution used to absorb Conon. of Acid pH of oxygen reduced the nitrous oxide to free nitrogen. in Mixture Mixture Surging"
Z;&B
0.26 M Hapoi
6.16 6.03 6.11 5.42 5.32 5.35
SURGING IN BLOCKS CONTAINING 5 % CHARCOAL
A block containing 5% charcoal, 92% ammonium nitrate, and 3% boiled linseed oil was burned under pressure in a chamber equipped so that the gaseous products of combustion could be instantaneously sampled at the crest and trough of a surge. Such gas samples were analyzed by the procedure just described. Data are summarized in Table lr. The trough reaction produced appreciable amounts of nitrogen dioxide and a carbon dioxide to carbon monoxide ratio of about 16 to 1. In contrast the crest reaction producedsmalleramounts of nitrogen dioxide and a carbon dioxide to carbon monoxide ratio of about 4 to 1. This suggests a deficiency of reducing agent (charcoal) during the trough combustion. The reactions COS C -+ 2C0 and NO2 2C -+ 2C0 are retarded. The empirical equations for the reactions which fit the analytical data with reasonable precision are given in Table V. Such equations serve only to indicate the quantitative over-all relation between reactants and products. From these equations the composition of the mixture reacting at the trough and crest were deduced:
a Fuel units were burned under a ga3 pressure of 10 to 20 om. Hg above the pressure of the atmosphere-that is, 85 to 95 om. total pressure.
TABLE IV. ANALYSIS OF SURGING AND KONSURGING CHARCOALS Charcoals Nonsurging Surging
Over-all Composition of Fuel 92 5 3
Separate trials have indicated that charcoal and ammonium nitrate burn much more readily than linseed oil and ammonium nitrate. In fact, a stoichiometric mixture of ammonium nitrate and linseed oil without charcoal could not be ignited. S U R G I NMECHANISM. G A surging mechanism for a 5% charcoal block is proposed. As the crest, reaction begins to build up it appears from the above analyses that the reaction mixture is richer in charcoal (6.6%) than the over-all block composition (5%). B s a result, the charcoal is burned out leaving a slower burning layer of ammonium nitrate and linseed oil over the surface. Below this there is a charcoal-ammonium nitrate oil layer which will again burn rapidly when ignited. Heat supplied by the slower burning ammonium nitrate-linseed oil layer on top serves to reignite the layer under it. The retention of the auto-catalytic products of combustion in the reaction zone makes the charcoal-ammonium nitrate reaction
Mixture Reacting at Crest 91.6 6.6 1.8
R 6 .._
Volatile matter. '3 I "
5 1
8.5 7.2 13 12 3.3 3.3 9.0 9.2 Easy Difficult No difference No difference
%%edian particle diameter, microns Burface area, sq. m./g. 20 ml, HzO) pH (10 g. charcoal Ease of wetting Spectrographio analysis of ash X-ray diffraction analysis of ash
+
+
+
Ingredient Ammonium Nitrate, % Charcoal, % Linseed oil, %
0.75 M HsPO,
Very violent Very violent Violent None None None
go faster and faster until finally a surface explosion occurs, lifting the reacting layer from the block surface. This marks the crest of the surge. The slower burning oil-rich layer which is disrupted but not extinguished by the explosion then settles on the block surface and burns slowly during the trough, giving incomplete combustion until the charcoal-rich layer below is again ignited. Reignition of the fresh block surface starts the cycle
Mixture Reacting at Trough 95.5 0.7 3.6
TABLE V. GASCOMPOSITION DATAFOR SURGING BLOCKSCONTAINIXG 5 AND 11%CHARCOAL Trough Calcd., Obsd. Eq. 1 HzO NHI NOz COa
63.6 0.4 5.0 6.4 0.4 0.1
Gas Composition, % by Volume Crest Trough Calcd., Calcd Obsd. Eq. 2 Obsd. Eq. 3'
Ohsd. 11% Charcoal 47.4 47.4 0.0 0.0
5% Charcoal 52.6 0.0 0.05 5.1 1.6 6.7 10.4 0 0 2.5 0.0 5.5
62.1
Crest Calcd. Eq. 4
52.5 47.4 48.2 0.0 0.0 0.0 1.6 0.0 0.0 0.0 0.0 10.0 13.7 13.9 11.5 11.9 co 2.4 2.5 2.5 4.9 5.1 HZ 6.0 8.7 8.6 7.8 7.8 ozn 1.6 1.5 1.4 0.6 1.3 Nz (residual) 24:Q 25: 1 27:3 24.6 26.0 26.2 28.1 25.7 NzOb 1.0 1.6 n m Wt. 2317 23.9 23:6 23.5 2i:5 23:3 23:6 23:2 Analysis of charcoal gave its approximate formula as CsHa0 8 to 10% ash: using this formula, the following stoichiometrical equations fit the data with considerable precmon. The calculated values were obtained from these eauations. 5% charcoal-trough reaction: 17NHdNOa 0.096HsO 0.06CsrHsoOa ----f 36.8Hz0 4-3SOa 4COa f 14.9Nz 0.6Nz0 (1) 6% charcoal-crest reaction: 17NHbNOa CsHa0 0.O3CnHeoOa 33&0 N0a 6.3COz 4- l 3 C O f 3.8Hz 15.5Ni 0 2 Nz0 (2) 11% charcoal-trough reaction: 17.0NH8"a 0.81
