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Properties of alternatively fueled ammonium nitrate explosives

C. Hurley, V. Petr & S. Liu

Department of Mining Engineering, Colorado School of Mines, Golden, CO, USA

J. Banker

Dynamic Materials Corporation, Boulder, CO, USA

ABSTRACT: This paper presents the results of testing the use of alternative fuels and crushed industrial quality Ammonium Nitrate (AN) as explosive material. By changing the fuels in an explosive mixture, the oxygen balance of the explosive may also change affecting the total amount of energy released as well as detonation velocity. Thus it is necessary to characterize the effects of alternative fuels regarding composition and grain size on detonation velocity and critical diameter for detonation. This paper covers the initial testing of sugar and coal dust as alternative fuels in ammonium nitrate based explosives used for explosive welding. and produce fewer NOX gasses than a similarly fuel rich mixture will. The procedure for calculating oxygen balance of a given explosive material is to determine the number of Mole Units of oxygen that are in excess or deficient for the compound. (1) where: X = number of atoms of carbon, Y = number of atoms of hydrogen, Z = number of atoms of oxygen, and M = number of atoms of metal (metallic oxide produced). (Cooper 1996) This methodology will be used to calculate the oxygen balance of the alternative fuels in a later section. 2 EXPERIMENTAL PROGRAM For this experimental program, two alternative fuels were investigated, sugar and coal dust. These were selected as fuels on the basis of cost and accessibility. Oxygen-balanced ammonium nitrate and fuel oil (ANFO) was used as the control mixture to validate the test results. No additional materials were added to the mixtures to control the detonation. The particle size was controlled for both components for proper mixing and a strict 50% passing a 75-mesh sieve standard was implemented. Particle size analysis was performed on each sample before and after mixing to ensure all tests passed these criteria. Test boxes were constructed to maintain consistent repeatability of sample dimensions and allow

1 INTRODUCTION An explosive may consist of either a chemically pure compound, such as Nitroglycerin, or a mixture of an oxidizer and a fuel, such as ANFO (Ammonium Nitrate and Fuel Oil). Oxidizer is a substance that in a chemical reaction contributes atoms of one or more oxidizing elements, in which the fuel component of the explosive burns. Varying the properties of these components will alter the chemical and physical properties of the explosive. 1.1 Calculating Weight Ratios Using Oxygen Balances Oxygen Balance is defined as "the amount of oxygen, expressed in weight percent, liberated as a result of complete conversion of the explosive material to carbon dioxide, water, sulfur dioxide, aluminum oxide, etc." (Akhavan 1998) The compound is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed. Changing the oxygen balance of an explosive has an impact on both the available energy and the production of harmful gasses. Negative oxygen balances will produce greater quantities of CO and positive oxygen balances will produce more NOx gasses. In order to maximize an explosive's energy potential, the oxygen balance of explosive needs to be approximately zero, with slightly negative balances being preferred over slightly positive ones. Slightly fuel lean mixtures have a greater detonation energy

the explosive to reach steady-state detonation. The dimensions of the test boxes and layout are intended to minimize edge effects at the fiber optic probe locations. The forms were built using 8-mm particle board to minimize the potential for the box to produce fragments during testing. A schematic diagram of a typical experimental box is shown in Figure 1. The height of the wood box is 36 cm. Also shown in figure 1 is a photograph of the manufactured box with explosive and fiber optic probes. A metal base plate was placed under the layer of explosive. The orange booster can be seen on the left side of the box. Also visible are the fiber optic cables (black lines) inserted into the booster and a spacer. The remainder of the box will be filled with containment sand (~20 kg).

tance between fiber optics, VOD can then be calculated using the following equation: VOD = L/T (3) where L is the distance between cables and T is elapsed time between cables.

35±0.1 cm

10±0.005cm

10±0.005cm

Figure 3. Schematic of fiber optic spacer

3 RESULTS AND DISCUSSION 3.1 Oxygen Balance Calculation for the Alternative Fuels Oxygen balances for each composition were calculated based upon the oxygen balances for the individual components. The oxygen balances of the various ingredients are given in Tables 1 and 2.

29 cm

36 cm

40 cm

Table 1. Oxygen Balance for explosive mixes.

Mixes Component % AN ANFO ANCHO ANCoal 94 84 92 Fuel Oil 6 Coal Dust 8 Sugar 16 %OB -1.18 -1.12 -0.16

Figure 1. Top: Schematic of AN Testing form. Bottom: form being used in testing.

Table 2. Oxygen Balances for components

Component AN FO Coal Dust Sugar %OB 0.2 3.33 -2.32 1.12

The densities of the manufactured explosive were measured on site during each test, using a 5-gallon bucket to calculate mass with known volume. The respective densities of sugar and AN (ANCHO), coal dust and AN, and crushed ANFO were measured as 0.862 gram/cm3, 0.836 gram/cm3, and 0.803 gram/cm3, respectively. Using these densities and the area of the wood forms, explosives quantities for a given thickness were calculated. . Detonation velocities (VOD) with not less than 18 cm of sand confinement of the alternative fuel mixes were compared with similarly confined industrial quality crushed ANFO. The critical thickness of ANCHO was also tested and was experimentally established to be approximately 2.4 cm, which is the same as the crushed ANFO. A fiber optic system was used to measure VOD. The fiber optic cables were set a known distance apart as shown in Figure 3 and Table 1, and wired to an amplifier connected to an oscilloscope. As the reaction passed by each cable, the light pulse was detected by oscilloscope, which would then record the time of the passage of the shockwave as a voltage spike. A cable on the booster triggered the oscilloscope. Knowing the time between spikes and the dis-

Notice that the oxygen balances of the explosives are all slightly negative, indicating that the explosives are all fuel lean. 3.2 Grain Size of Solid Fuels An important parameter to consider when using solid fuels is the grain size of the fuel. When using fuel oil, the grain size of the fuel is essentially zero, allowing maximum intimacy between the fuel and the oxidizer. Solid fuels must be crushed to maximize the surface area available to react with the oxidizer. Store bought powdered sugar was used for this testing. The AN and coal was ground at CSM. As seen in Table 3 and the following figures, the commercial powdered sugar has a much smaller grain size than we were able to achieve with the coal dust. We found that our grinding equipment was unable to produce large quantities of sufficiently fine coal

dust. Most of it was lost as airborne dust. A power plant has been found as a source of ultra-fine coal dust and full scale tests on coal dust and AN will be conducted in the future. We also see that there is substantial particle size variation in the crushed AN. PSA data is summarized in Figures 4-7.

Table 3. Particle size distribution for crushed AN and ANCHO. Values are percentage of total mass

Crushed AN Particle Size Analysis 1,2 Test # 7 8,9 Ave ANCHO Particle Size Analysis 10 Test # 11, 12 Ave Sieve Size 20 0 0 0 0 40 8 4 2 5 75 32 23 11 22 100 12 32 34 26 200 18 28 39 28 pan 25 2.3 12 13

Sieve Size 20 0 0 0 40 4 7 5.5 75 30 27 28.5 100 8 9 8.5 200 37 41 39 pan 20 14 17

Fig. 5. Sieve size vs % Retained, crushed AN+ Sugar (AN CHO)andaveragevaluelinefromtwosamples.

Values given as % of material retained by % passing the given screen size

Figure 6. Sieve Size vs average % crushed AN retained and % sugar retained

Fig. 4. Sieve size vs % Retained, crushed AN and average valueline

Figure 7. Sieve size vs % AN and Coal Dust retained

3.3 Testing of Detonation Velocity and Critical Thickness Detonation velocity (VOD) and critical thickness are the primary parameters that determine the suitability of an explosive compound for explosive welding. Critical thickness is the minimum thickness at which an explosive will experience steady state detonation. This testing program was designed to determine first the VOD, then critical thickness. A total of three compositions were tested for VOD in this program. Crushed ANFO was used as a baseline. The second and third compositions were crushed AN + coal dust and crushed AN with powdered sugar. The amounts of each of the explosive mixtures are reported in Table 4.

Table 4. Explosive masses for VOD testing

Composition ANFO Coal Dust + AN Sugar + AN AN (g) 5443.1 5443.1 5443.1 Fuel (g) 409.4 473.1 1036.4 Fuel 6% 8% 16% Un-compacted (g/cm3) 0.803 0.836 0.862

Both mixes experienced successful detonation at 2-cm thickness. ANCHO failed to detonate at 1.5-cm, but AN + coal dust detonated successfully. Explosive quantities and VOD results for each test are shown in Tables 5 and 6. VOD results are summarized in Figure 9.

Table 5. Explosive masses for critical thickness testing

Test 2 cm ANCHO 1.5 cm ANCHO 2 cm AN + coal dust 1.5 cm AN + coal dust AN (g) 3613.4 2710.2 3107.2 2787.6 Fuel (g) 688.2 516.2 248.6 242.4 Fuel (%) 16 16 8 8

Table 6. VOD results for VOD and critical thickness tests

Average VOD (m/s) 3.5-cm ANFO ANCHO AN + coal dust 4140 3218 2349 2-cm 3950 3169 2399 1.5-cm 2314

The above quantities were chosen to give a 3.5-cm thick explosive layer. 3.5-cm was decided on a safe minimum thickness for steady state detonation in all compositions. Results of VOD testing are summarized in Figure 8 and Table 6.

Figure 9. Comparison of VODs for various thicknesses for alternatively fueled AN explosives.

3.4 Cost Analysis

Figure 8. Comparison of VOD between Crushed ANFO, ANCHO, and AN + Coal Dust

Critical thickness testing was performed on the ANCHO and AN + coal dust compositions. Previous testing has set the critical thickness of crushed ANFO at 2-cm. This was used as a starting point for testing the alternative fuel mixtures. Tests were planned for 2-cm and 1.5-cm thicknesses of each. In order to maximize the potential for successful detonation during this testing, booster orientation was changed from vertical (as seen in figure 1) to horizontal. This was done to maximize the amount of energy being transferred from the booster to the charge.

The goal of these studies was to demonstrate the feasibility of these alternatively fuel mixtures for industrial scale explosive welding applications. The final part of this was rough cost analysis. Local costs were used for AN, fuel oil, and powdered sugar. Coal dust is typically a waste product that can be obtained for free. Transportation costs were not taken into account. The results of this analysis are shown in Table 7.

Table 7. Cost analysis for 1-kg charge

AN (kg) 0.94 0.84 0.92 AN ($/kg) $1.32 $1.32 $1.32 Fuel (kg) 0.06 0.16 0.08 Fuel ($/kg) $1.21 $6.00 $0.00 Total Cost ($//kg) $1.31 $2.07 $1.21

Cooper, P. 1996. Explosives Enginering. New York: WileyVCH, Inc.

ANFO ANCHO AN + coal dust

4 CONCLUSIONS This testing confirmed the potential of sugar as alternative fuel in Ammonium Nitrate based explosives. Coal dust also showed potential, but further testing needs to be done to classify its properties. When detonation is successfully achieved, these compounds have a lower VOD than ANFO (3150 m/s for ANCHO and 2300 m/s for AN + Coal Dust compared to the 4140 m/s of ANFO). Grain size of both the fuel and oxidizer was found to be a critical parameter. Grinding procedures need to be refined to produce a more consistent AN product. The use of an outside supplier for coal dust will greatly improve its particle size and consistency. We would also like to note that no work was done to find the Deflagration to Detonation Transition (DDT) distance in either alternative fuel mix. Based on preliminary cost analysis, AN + coal dust could find applications in areas that waste coal dust is readily available. Its economic competitiveness with ANFO is dependant on transportation costs and the current price of fuel oil. ANCHO is not economically feasible due to the high cost of sugar and high quantity required. The potential economic advantages of AN + coal dust will lead to future study. Despite the economic advantages of AN + coal dust from the materials perspective, the applications of it will be limited due to the difficulty achieving homogenous mixing using the solid fuel. This will increase the total cost of use for this explosive. Future study of AN + coal dust will focus on the influence of particle size, coal quality, and moisture content on VOD and brisance. As the properties of this material become better quantified, studies on diluents will be preformed to lower the VOD to optimize it for certain processes. 4 ACKNOWLEDGMENT This project was supported under the Colorado School of Mines, AXPRO Group consortium Project (08/2007-05) supported by Dynamic Materials Corporation. The authors acknowledge and appreciate the assistance of Ray Johnson and Doug Aho from the CSM Mining Engineering Department during the sample preparation and testing. We would also like to thank Roy Hardwick for suggesting the use of sugar as a fuel for AN-based explosives. REFERENCES

Akhavan, J. 1998. Chemistry of Explosives. Tyne, UK: Royal Society of Chemistry

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