H2O Mixture with

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Combustion Characteristics of an Al/H2O Mixture with Polyoxyethylene Yunlan Sun* and Baozhong Zhu School of Metallurgy & Resources, Anhui University of Technology, Maanshan, Anhui 243002, China ABSTRACT: An experimental investigation of the combustion characteristics of an aluminum/water (Al/H2O) mixture with the addition of polyoxyethylene (PEO) was conducted in argon at 0.1 MPa. The burning rates were obtained using a constant-pressure strand burner. Ignition and combustion processes of an Al/H2O mixture were observed with a high-speed camera. The results show that PEO addition can improve the ignition and combustion performance of an Al/H2O mixture. The mixture of nanosized Al powder and H2O cannot be ignited in argon at 0.1 MPa, but the mixture of nanosized Al powder and H2O with the mass fraction 1% PEO or 3% PEO can be ignited, and the mixture can be self-sustaining combustion. The burning rates are 2.57 mm/s and 3.94 mm/s, respectively. In the mixture of nanosized Al powder and H2O with 3% PEO, the ignition can be sustained when 20% nanosized Al powder is replaced by micrometer-sized Al powder, and the burning rate is 3.36 mm/s. However, the mixture would not be ignited when 25% nanosized Al powder is replaced by micrometer-sized Al powder. In addition, the combustion process and flame image characteristics were obtained by using a high-speed photography technique, and the condensed combustion products were characterized by scanning electron microscopy combined with energy dispersive X-ray.

1. INTRODUCTION Aluminum (Al) powders are widely used as fuels in various energetic materials due to its high energy density of 29 MJ/kg,1,2 especially for solid samples.3 The combustion of Al particle has been strongly studied for the last 40 years.4,5 In recent years, Al/ H2O combustion attracts many researchers. Al is used as high energetic fuel and H2O is the main oxidizing component for metal particles in a rocket motor. Al is characterized by large amounts of heat released than magnesium (Mg) during combustion; therefore, it is Al that is used in most of rocket motors. Also, Al/H2O combustion can be used for space propulsion systems since its products are nontoxic and therefore considered as “green”.6 Nanosized particles exhibit interesting traits. The surfaces of nanosized particles have a large number of atoms which are comparable with the number of atoms in the bulk of the particle. Moreover, nanosized particles contain lattices with high concentrations of dislocations and large surface areas. Thus, nanosized particles exhibit high chemical reactivity, which allows a number of nanosized metals to undergo reactions that are previously considered impossible.7 Therefore, the uses of nanosized particles in the Al/H2O reaction can exhibit significant advantages over larger size particles. Polymers, thickening agents play an important role in Al/H2O combustion. For example, the addition of polyacrylamide (PAM) to the Al/H2O mixture can increase combustion stability and make the mixture ignite easily.8,9 In addition, the addition of PAM to the Al/H2O mixture can inhibit water evaporation during combustion.10 13 It can be found that many studies concentrate on the PAM thickening agent. Few fundamental combustion studies have been conducted on the effect of polyoxyethylene (PEO) thickening agent on combustion characteristics of the Al/H2O mixture. In the present research, the effect of PEO thickening agent on combustion characteristics of the Al/H2O mixture is investigated. The results show PEO has a great influence on the combustion characteristics of the Al/H2O r 2011 American Chemical Society

mixture. The studies are specifically extended to investigate the concentration of PEO how to affect combustion characteristics of the Al/H2O mixture by a high-speed camera. The flash pyrolysis of PEO aqueous solution at 700 °C has been studied. The flash pyrolysis products have been detected and identified by gas chromatography combined with mass spectrometry (GC/MS). The residues of combustion have also been studied by scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDS). Such studies may offer useful clues for the application of PEO polymer in the Al/H2O mixture in order to obtain a better combustion performance.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The preparation of mixtures included addition of 1 wt % or 3 wt % PEO in distilled water and mixing the resulting gel with metal powder. In order to improve the combustion efficiency of Al powders, the mass ratio of H2O with 1 wt % PEO or 3 wt % PEO and Al powder was 1.1:1. Average particle size of nanosized Al particle and micrometer-sized Al particle are 40 nm and 1 2 μm, respectively. 2.2. Flash Pyrolysis. The flash pyrolysis of 3% PEO aqueous solution in Ar atmosphere was carried out in a tube-type pyrolyzer at 700 °C for 5 min. The volatile products were detected and identified by GC/MS. The techniques and equipment used were described in ref 14. 2.3. The Flame Image and Burning Rate Measurement. The experimental setup used for flame image consists of a combustion chamber, fitted with two optical viewing windows (shown in Figure 1). Prior to ignition, the desired initial pressure Received: June 11, 2011 Accepted: November 7, 2011 Revised: September 26, 2011 Published: November 07, 2011 14136

dx.doi.org/10.1021/ie201837a | Ind. Eng. Chem. Res. 2011, 50, 14136–14141

Industrial & Engineering Chemistry Research

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Figure 1. Schematic of the strand burner experimental setup.

Table 1. Samples Compositions Studied samples

compositions

A-1

nAl/H2O

A-2

nAl/(1%PEO+H2O)

A-3

nAl/(3%PEO+H2O)

A-4

(nAl:umAl=4:1)/(3%PEO+H2O)

A-5

(nAl:umAl=3:1)/(3%PEO+H2O)

of the combustion chamber was regulated by regulating the flow of purge gas through inlet and exhaust valves. The optical viewports of the combustion chamber were used for realtime recording of the combustion process by a digital video camera. The experimental setup was also used to measure the burning rates of the samples. The luminous combustion wave was recorded using the digital video camera, allowing burning rates to be determined by tracking the position of the reaction front over time. The obtained mixture was placed in a quartz cylinder (height 40 mm, inner diameter 8 mm) and ignited by a hot Nichrome coil embedded in the top layer of loaded sample. The ends of the Nichrome wire were attached to wire leads, allowing a variable voltage power supply to be connected via electrode feedthroughs after the strand assembly was loaded into the pressure chamber. Once the strand assembly was loaded, and wire was made, pressurization of the combustion chamber was initiated. In this investigation, argon was used as the purge gas. The sample was loaded into a quartz tube with a 10 mm OD (8 mm ID) 40 mm length. The experiments were conducted at 0.1 MPa. The samples compositions are shown in Table 1. 2.4. SEM/SED Analysis. In order to study the condensed combustion products of different samples in argon at 0.1 MPa, the condensed combustion products were analyzed by using SEM/EDS.

Figure 2. The burning rates of samples at 0.1 MPa.

3. RESULTS AND DISCUSSIONS 3.1. Burning Rate. The burning rates of mixtures determine the rate of gas generation, which determines the pressure inside the motor and the overall thrust. Burning rates herein are obtained experimentally by burning small mixture strands and measuring the surface regression versus time.15 The burning rate depends on the compositions of sample and combustion conditions. The focus of this paper is on nanosized Al powder or bimodal aluminum distribution in argon atmosphere and how they interact with the water and different concentrations of PEO aqueous solution, thus affecting the burning rates. Literature on combustion of Al particles conjectured that Al particle is covered by an impervious oxide shell, and it does not ignite until the oxide shell melts or breaks up near its melting temperature under the effect of Al thermal expansion.16 Various factors such as additive concentration and particle size can affect the burning rate of the mixture. The burning rates of the Al/H2O mixtures with different concentrations of PEO aqueous solution obtained in argon at 0.1 MPa are shown in Figure 2. 14137

dx.doi.org/10.1021/ie201837a |Ind. Eng. Chem. Res. 2011, 50, 14136–14141


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Table 2. Flash Pyrolysis Products of 3% PEO Aqueous Solution Detected and Identified by GC/MS compounds

formula

relative peak intensity (%)

1 acetic acid

C2H4O2

3.79

2 2-hydroxyisobutyric acid 3 1-ethenoxy-2-methoxyethane

C4H8O3 C5H10O2

3.63 1.55

4 4-hydroxy-2-butanone

C4H8O2

5 4-hydroxy-4-methyl-2-pentanone C6H12O2

25.20 23.09

6 1-methylethyl-benzene

C9H12

3.84

7 1, 3-bis(1-methylethyl)benzene

C12H18

14.20

The PEO thickening agent can not only affect the ignition of the Al/H2O mixture but also affect the burning rate. In argon atmosphere, the A-1 sample would not be ignited without the addition of the thickening agent to the mixture, so we investigated the combustion behavior of the Al/H2O mixture with the PEO thickening agent. It can be seen from Figure 2 that the burning rate of the A-3 sample (3.94 mm/s) is higher than that of the A-2 sample (2.57 mm/s). This suggests that PEO concentration has a dramatic effect on the burning rate of the Al/H2O mixture. Table 2 shows the flash pyrolysis products of 3% PEO aqueous solution. It can be seen from Table 2 that the flash pyrolysis 3% PEO aqueous solution can generate oxidizing species such as 4-hydroxy-2-butanone, acetic acid, and 2-hydroxyisobutyric acid. These oxidizing species can further react with Al. The exothermic reactions of these oxidizing species with Al can increase the heat flux to the burning surface of the Al/H2O mixture and thus increase the burning rate. Also, some flash pyrolysis products such as 4-hydroxy-4-methyl-2-pentanone and most of aromatic hydrocarbons presented, for example, 1-methylethyl-benzene and 1,3-bis(1-methylethyl)benzene, would react with Al and form some metal organic chelate compounds, which may provide an effect to diminish the formation of aluminum oxides. As the concentration of PEO increases, the concentration of pyrolysis products increases. This leads to an increase in the heat flux and a decrease in formation of aluminum oxide, so the burning rate increases. Replacing a portion of the nanosized Al particles with micrometer-sized Al particles in the Al/H2O mixture affects the combustion performance. It can be seen from Figure 2 that the burning rate of the A-4 sample is lower than that of the A-3 sample at 0.1 MPa. Burning is fundamentally a thermal process, and consequently the burning rate of a mixture is determined by the competition between the heat production and heat absorption. When the content of micrometer-sized Al particle increases, the ignition energy increases because the ignition temperature of the micrometer-sized Al particle is difficult to be obtained. Also, the reaction rate of micrometer-sized Al particle is limited by the presence of an oxide ‘‘skin’’ that does not break down until the particle nears the melting point of the oxide (2072 °C).17 These energies come from the combustion of nanosized Al particles and H2O. The combustion flame of nanosized Al particles and H2O is important to the combustion problem of micrometer-sized Al because it is the nearest site to the surface that produces temperatures high enough to melt the oxide skin on the micrometer-sized Al, permitting micrometer-sized Al combustion. Thus, the heat released can be absorbed by a part of micrometer-sized Al and the heat feedback to the burning surface decreases accordingly, rendering a lower burning rate. Another factor contributing to this phenomenon might be the blowing

Figure 3. Captured images of the burning process of the A-2 sample.


effect of the reactive metals which tends to push the primary flame away from the surface. Nanosized Al particles have much shorter ignition delay and combustion time than the micrometersized particles. Upon ignition, they can contribute to enhanced heat feedback and increase the burning rate of the mixture. In addition, the average distance between nanosized particle and oxidizer particle in the mixture is dramatically reduced as compared to the mixture containing micrometer-sized Al powder. So nanosized Al powder has been shown to be a very effective burning rate accelerator for the mixture. Increasing replacement of nanosized Al particles with micrometer-sized Al particles, the results show that with mere 25% micrometer-sized Al content, the mixture would not be ignited, such as the A-5 sample. As above-mentioned, the combustion flame of nanosized Al particles and H2O is important to the micrometer-sized Al combustion. It is a significant heat source to satisfy the conditions of micrometer-sized Al ignition. As the nanosized Al content decreases, the heat released by the combustion of nanosized Al particles and H2O deceases, therefore removing a part of the heat source. The flame cannot raise the temperature high enough to melt the oxide skin on the micrometer-sized Al and does not support micrometer-sized Al combustion, so the A-5 sample cannot be ignited. 3.2. Flame Characteristics. Figures 3 5 present a series of video images showing the burning processes of the samples A-2 A-4 at 0.1 MPa. The onset of ignition is represented by t = 0 s. It can be seen from Figures 3 and 4 that the burning processes of the samples A-2 and A-3 can be divided into three stages: (1) Ignition; in this stage, the sample was ignited and normal deflagration was observed. (2) Diffusion combustion; the flame quickly propagates downward. Combustion flame is bright and accompanies with bright streaks. The bright streaks represent the trajectories of burning Al particles. It can be noted that the bright streaks are few and short but thick. (3) Steady-state combustion; in this stage, the flame continues to propagate downward and the burning surface recedes. The extremely luminous region above 14138



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the wavefront is molten alumina (Al2O3) radiating at a high temperature. Flame becomes dense and intense jet, and the flame is ‘‘slender-shaped’’. The bright streaks of the flame dramatically decrease. This shows that some nanosized Al particles can directly ignite on the burning surface and burn relatively close to the burning surface. This attributes to the large surface area and the short ignition delay time of nanosized Al particles. Compared with the flames of the samples A-2 and A-3, it can be found that the burning of the sample A-3 is more severe than that of the sample A-2. Figure 5 shows the flame images of the burning process of the sample A-4, it can be found that the flame images of the sample A-4 are obviously different from the samples A-2 and A-3. The flame of the sample A-4 has a large number of bright streaks, and the bright streaks are long and thin. The whole burning process of the sample A-4 produces the bright streaks. It is evident that micrometer-sized Al particles burn far from the burning surface, reducing heat feedback from the metal-oxidizer combustion to the unburnt sample zone at 0.1 MPa. This leads to lower combustion efficiency and higher two-phase flow losses. So the burning rate of the sample A-4 is lower than that of the sample A-3. 3.3. Analysis of Condensed Combustion Products. Different techniques are proposed in the literature to collect and analyze the condensed combustion products. Condensed combustion products in the burning of aluminized samples of various classes can be classified into two basic fractions: agglomerates and smoke oxide particles.18 Agglomerates are systems made of aluminum and aluminum oxide, formed during the combustion process. Agglomerates of both known types are formed: agglomerates with ‘‘cap’’ oxide, and ‘‘hollow’’ agglomerates.18 It can be found from Figure 6 that the combustion surfaces contain a number of agglomerates such as spherical particles. As shown in Figure 6(a), the condensed combustion products of the A-2 sample contain a number of spherical particles. The diameters of the particles are about 0.5 4.0 μm. Table 3 shows that dots 1 and 2 (Figure 6(a)) correspond to the absolute concentrations of elements and atoms according to EDS analysis. Assuming that Al particles are completely oxidized to Al2O3, so the atomicity ratio of O/Al should be 1.5. It can be seen from Table 3 that the atomicity ratio of O/Al is greater than 1.5. This indicates that the content of the O element is higher than the Al element, so it can be estimated that the Al particles are completely oxidized to Al2O3. This illustrates that the spherical particles are composed of oxide of aluminum (Al2O3) as the main products. The condensed combustion products of the sample A-3 (shown in Figure 6(b)) are evidently different from that of the sample A-2. It can be seen that there are spherical particles and floccules on the surface. The spherical particles are few on the surface. The

Figure 6. Morphological properties of agglomerates of (a) the A-2 sample, (b) the A-3 sample, and (c) the A-4 sample.

diameter of the biggest particle is ∼4.0 μm. Most of spherical particles’ diameters are a little less than 1.0 μm. EDS analyses show the spherical particles are mainly the oxide of aluminum. For the floccules, the atomicity ratio of O/Al is 1.47 (less than 1.5). This indicates that the content of the O element is less than the Al element, so it can be estimated that the Al particles are incompletely 14139



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Table 3. EDS Analyses of the Representative Agglomerates Displayed A-2 (%) weight element

A-3 (%) atomic

dot 1

dot 2

dot 1

dot 2

C

-

-

-

-

O

50.02

53.35

62.65

65.64

Al

47.15

42.48

35.02

30.99

Mg

2.83

4.17

2.33

3.37

weight dot 1

A-4 (%) atomic

weight

atomic

dot 2

area3

dot 1

dot 2

area3

area1

dot 2

area1

dot 2

-

-

-

-

-

-

-

10.37

-

15.86

55.10

53.71

44.85

67.30

66.03

57.66

45.92

49.47

58.89

56.80

42.58

43.39

51.52

30.84

31.63

39.27

54.08

40.16

41.11

27.34

2.33

2.90

3.62

1.87

2.35

3.06

-

-

-

-

oxidized to Al2O3. Strangely, the impurity of Mg peak is present in the spectrum of the samples A-2 and A-3. This may be caused by the impurities, which are mixed in the process of collecting condensed combustion products. The structure of the burned the sample A-4 is shown in Figure 6(c). The large single particles and the aggregated small particles are observed on their surfaces or located among the floccules. The number of agglomerates of the sample A-4 is more than that of the sample A-3. As mentioned above, floccules are composed of aluminum and aluminum oxide. The EDS analyses of the floccules show that the atomicity ratio of O/Al is 1.43. The atomicity ratio of O/Al of the sample A-4 is less than that of the sample A-3. This explains the condensed combustion products of the sample A-4 contain aluminum more than that of the sample A-3. This shows that the burning of the sample A-3 is more complete than that of the sample A-4. In addition, the carbon peak is detected on the condensed combustion products of the sample A-4. The EDS measurements may be shown incorrect absolute carbon content, and the errors are due to the mathematical interpretations of the measured spectra rather than due to some physical contamination of the sample before or during SEM analyses.

4. CONCLUSIONS The combustion characteristics of Al/H2O-based samples with PEO additive have been characterized for bimodal aluminum distribution and concentrations of PEO aqueous solution. This work has extended previous studies, which considers the effect of PEO on the Al/H2O system and explores a range of bimodal aluminum distribution. The major conclusions of this work are as follows: (1) Nanosized Al powder/H2O mixture cannot be ignited without addition of PEO in argon at 0.1 MPa, but the mixture of nanosized Al powder with 1% PEO or 3% PEO aqueous solution can be ignited and be self-sustaining combustion. Moreover, the burning rate of the mixture of nanosized Al powder with 3% PEO aqueous solution is higher than that of the mixture of nanosized Al powder with 1% PEO aqueous solution. The flash pyrolysis products promote the igniting and combustion performance of Al/H2O-based fuels. Among these flash pyrolysis products, higher amounts of 4-hydroxy-2-butanone, acetic acid, 2-hydroxyisobutyric acid, 4-hydroxy-4-methyl-2-pentanone and most of the aromatic hydrocarbons are produced, which play important roles in enhancing the igniting and combustion performance of Al/H2Obased fuels. (2) When using the micrometer-sized Al powder progressively replaces nanosized Al powder in the mixture of nanosized Al powder with 3% PEO aqueous solution, the

mixture can be still ignited and self-sustaining combustion with nanosized Al powder and micrometer-sized Al powder in the ratio of 4:1. However, the mixture would not be ignited in the ratio of 3:1 because the flame cannot raise the temperature high enough to melt the oxide skin on the micrometer-sized Al. (3) Al trajectories are very few in combustion flame of samples containing pure nanosized Al particles. But using the bimodal aluminum distribution, because a micrometer-sized Al particle tends to burn far from the burning surface, the flame has a large number of bright streaks, and the bright streaks are long and thin. (4) Condensed combustion products contain agglomerates: spherical particles and floccules. Spherical particles are mainly the oxide of aluminum. The floccules are composed of aluminum and aluminum oxide. For the mixture of nanosized Al powder with 3% PEO aqueous solution, the aluminum content of condensed combustion products is less than that of the mixture of nanosized Al powder and micrometer-sized Al powder with 3% PEO aqueous solution.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We greatly appreciate the financial support provided by the National Natural Science Foundation of China (No. 50806001). ’ REFERENCES (1) Price, E. W.; Sigman, R. K. Combustion of Aluminized Solid Samples. In Progress in Astronautics and Aeronautics 185, Solid Sample Chemistry, Combustion, and Motor Interior Ballistics; Yang, V., Brill, T., Ren, W., Eds.; American Institute of Aeronautics and Astronautics: 2000; p 663. (2) Dong, H.; Zhumei, S. Study of the Fast Reaction Characteristics of Aluminized PETN Explosive Powder. Combust. Flame 1996, 105, 428. (3) Sun, Y. L.; Li, S. F. Combustion Characteristics of Coated NanoAluminum in Composite Samples. Defence Sci. J. 2006, 56, 543. (4) Friedman, R.; Macek, A. Ignition and Combustion of Aluminum Particles in Hot Ambient Gases. Combust. Flame 1962, 6, 9. (5) Sarou-Kanian, V.; Rifflet, J. C.; Millot, F.; Matzen, G.; G€ okalp, I. Influence of Nitrogen in Aluminum Droplet Combustion. Proc. Combust. Inst. 2004, 30, 2060. (6) Ingenito, A; Bruno, C. Using aluminium for space propulsion. J. Propuls. Power 2004, 20, 1056. 14140



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