Effects of Different Additives on the Ignition and Combustion

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The effects of different additives on the ignition and combustion characteristics of micron-sized aluminum powder in steam Baozhong Zhu, Fan Li, Yunlan Sun, Yuxin Wu, Qichang Wang, Qi Wang, and Weikang Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01045 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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The effects of different additives on the ignition and combustion characteristics of micron-sized aluminum powder in steam Baozhong Zhu, Fan Li, Yunlan Sun*, Yuxin Wu, Qichang Wang, Qi Wang, Weikang Han School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, China

ABSTRACT: To improve the ignition and combustion characteristics of micron-sized aluminum powder in steam at 900 °C, the present study focuses on the effects of adding different contents of Mg, NaF and NaBH4. Experiments were conducted in two high-temperature tubular resistance furnaces to measure ignition temperatures, maximum combustion temperatures and ignition delay times and to understand the combustion features of all samples. The experimental results show that Mg addition results in a reduced ignition temperature and ignition delay time of the micron-sized aluminum powder and causes two burning stages (a double-peak feature) because the Mg powder preferentially ignites before the micron-sized aluminum powder. The maximum combustion temperature increases with increasing Mg content. The ignition temperature and the ignition delay time are also significantly decreased with increased NaF or NaBH4 addition. However, the addition of NaF or NaBH4 lowers the maximum combustion temperature. The morphology, components and residual aluminum contents of the solid combustion products are analyzed by scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis. The combustion efficiency is slightly improved with the addition of NaF. The different

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mechanisms by which the additives influence the ignition and combustion of micron-sized aluminum powder are believed to underlie these experimental results.

KEYWORDS: Combustion characteristics; Micron-sized aluminum; Steam; Additives

1. INTRODUCTION To minimize the propellant weight for achieving a high specific impulse, a hydro-reactive fuel that can react with water has drawn considerable attention.1-4 This fuel will potentially be used in a future novel ultra-high-speed underwater weapon due to its advantages of high energy density, high emission capacity of hydrogen and fast reaction rate. Excess water is charged into heated steam to improve the working gas of the water ramjet.

（Put Fig. 1 here）） Fig. 1. The mass and volumetric energy densities of different hydro-reactive fuels.

Figure 1 compares the mass and volumetric energy densities of five types of hydro-reactive fuels.5,6 Although Be has much higher energy density than Al, its application is restricted due to its strong toxicity, high cost and high melting point. Li and Na are extremely reactive in water, complicating their storage and transportation. Mg has low energy density. According to the above comparison of these metals, the Al-H2O system is recognized as the first choice for a hydro-reactive fuel used in a water ramjet. Researchers have studied the application of Al-H2O in underwater propulsion systems since the early 1960s,7,8 but the full potential of Al is not easily


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exploited due to its slow kinetics, which cause long ignition delays, high ignition temperatures and incomplete combustion,9 especially for micron-sized particles. Effectively destroying the oxide film on the Al powder surface is a key factor to overcome these problems.10,11 The fundamental reaction characteristics of Al-H2O systems have been studied extensively.1214

Some reports have shown that Al exhibits much better performance in the presence of

additive.15,16 Shmelev et al.17 analyzed hydrogen generation by the reaction of molten Al with water. The addition of 10 wt % KOH into Al developed the hydrogen yield up to 100 %. Fan et al.18 found that a Al-Li alloy prepared using the ball milling method showed excellent hydrolysis characteristics in neutral aqueous solutions. When the Li content was increased to 30 wt %, the alloy had uncontrollable hydrolysis rates, and its conversion efficiency reached 92 %. Vasilev et al.19 studied the ignition and combustion of Al/H2O mixtures with addition of fluorides such as KF, NaF, LiF and AlF3. NaF and KF were more effective than the other additives in enhancing the burning rate of the mixtures. In addition, many studies have reported that the initial temperature is a key factor influencing the reaction of Al and H2O. Mahmoodi et al.20 found that the average hydrogen generation rate of the reaction between Al and water increased from 101 mL min-1 g-1 to 210 mL min-1 g-1 when the initial temperature of the water was varied from 55 °C to 70 °C. Huang et al.21 studied the reactions of Al-Mg alloys and steam in a transparent pipe furnace and observed that the ignition delay time noticeably decreased with increasing temperature of the flowing steam. Yang et al.22 studied the combustion characteristics of milled Al-Li alloy in steam heated to 700 °C. The Al-20% Li alloy yielded the fastest hydrogen rate of 310 mL min-1 g-1. These studies show that the reaction between Al and H2O depends on multiple factors such as additive species, concentration of additive, and initial temperature. However, many issues remain


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unresolved: for example, how an additive improves the ignition and combustion characteristics of Al powder in steam and what mechanisms are responsible for the improved properties, especially for micron-sized aluminum powder (µAl). µAl powder is difficult to ignite and burn completely, which leads to low combustion efficiency.23,24 Very few studies of the ignition and combustion characteristics of µAl in heated steam have been reported, including ignition and combustion temperatures, ignition delay time, combustion efficiency, combustion phenomena and combustion mechanism. The light-weight metal Mg is commonly selected as an additive to Al through the ball milling method and can greatly improve the hydrolysis properties of Al.25,26 Some studies have shown that fluorides, such as NaF, can be used to enhance Al reactivity, by facilitating the destruction of the alumina shell surrounding the Al particles.19,27 Additionally, NaBH4 has excellent hydrolytic performance and high gravimetric hydrogen density, and many studies have reported that NaBH4 can react with water directly to enhance the hydrogen yield in the reaction of Al with water at both low and high temperatures.28,29 To gain new insights into these unresolved issues, we conducted an ignition and combustion experiment to systematically investigate the influences of Mg, NaF or NaBH4 addition and their concentrations on the ignition and combustion characteristics of µAl powder in heated steam, especially the influences of these additives on ignition temperature, maximum combustion temperature, ignition delay time, combustion flame, and combustion efficiency. Based on the experimental results, the mechanisms of the influence of different additives on the ignition and combustion behaviors of µAl powder are discussed. 2. EXPERIMENTAL SECTION 2.1 Materials and sample preparation


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Table 1 lists all reactants (Al and additives) considered in this study, including particle sizes, chemical purity, grade, and suppliers. Fig. 2 shows a scanning electron microscopy (SEM) image of Al powder with an average diameter of 1-2 µm. The active aluminum content is defined as the mass percentage of pure Al in the original Al particles and was 94.7 % as measured by the permanganatometric method.30 The remaining 5.3 % was Al2O3 covering on the Al powder surface. Cyclohexane solution (99.5%, analytical grade) purchased from Sinopharm of China was used as the organic solvent. The deionized water was heated to produce heated steam. Table 1 Reactants used in the test Reagents

Average particle size

Purity (%)

Grade

Supplier

Al

1-2 µm

94.7

Common

Jiaozuo Nano Material Company

Mg

48 µm

99.9

Analytical

Sinopharm

NaF

--

99.99

Chemical

Aladdin

NaBH4

--

99.5

Analytical

Aladdin

（Put Fig. 2 here）） Fig. 2. SEM image of 1-2 µm Al powder.

The energy density of fuel will be reduced if a high level of additive is added to the µAl powder, so the proportion of additive was less than 10 %. Three mixtures of µAl powder and different selected additives (Mg, NaF, or NaBH4) with additive-to-mixture ratios (mass basis) of 3 wt %, 7 wt %, and 10 wt % were prepared, and the total mass of each sample was ~ 300 mg.


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The preparation process of sample was as follows. The required amounts of powders were measured using an electronic analytical balance (ML503, Mettler Toledo, Switzerland). Then, these powders were placed in cyclohexane and sonicated using a sonic oscillator (SB-400DTY, Ningbo Scientz, China) in an ice bath to ensure homogeneous mixing of the powders in the mixture.31-34 Finally, the cyclohexane was evaporated using a vacuum drying box (DZF-6050, Shanghai Yiheng, China), and the powders were collected and stored under argon in glass bottles for further analysis. To provide a baseline to evaluate the effects of the additives at different levels of mixing, the same sample preparation procedures were followed for µAl powder without addition of additives. 2.3 Apparatus and measurement methods A specifically designed experimental apparatus was built as shown in Fig. 3 and included a high-temperature reaction system, a steam supply system, a data acquisition system, and a gas treatment component. The high-temperature reaction system mainly comprised two hightemperature tubular resistance furnaces in which an environment of flowing steam could be established. Before the experiments, the two high-temperature tubular resistance furnaces were heated to 900 °C, with control of the temperature by thermoregulators. Because the steam inside the furnace was prone to condensing, the #1 furnace was used to continuously preheat the steam to 900 °C, and the #2 furnace was employed as the reactor. The steam produced by the steam generator was continually flown into the quartz tube to expel air for 5 min and ensure that there were no gases except steam in the furnaces. The steam flow rate was controlled at 15 mL min-1 in all tests. As soon as the steam flow rate reached the prescribed level and was stable, the data acquisition system was turned on, and the alumina crucible with the sample was introduced into the tubular resistance furnace. An S-type thermocouple was inserted in the middle of the sample


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to monitor the temperature variation.35-41 The temperature data and combustion processes were recorded by a data collecting instrument (Agilent, 34972A, USA) and a high-speed camera (Phantom V311, VRI, USA), respectively. To ensure good repeatability of the measurements, all combustion tests were repeated three times.

（Put Fig. 3 here）） Fig. 3. Schematic diagram of the experimental apparatus.

After the tests ended, the solid combustion products were collected. Microstructural and composition analyses of the products were performed by scanning electron microscopy combined with energy dispersive X-ray (SEM-EDS, SU-70, Japan) and X-ray diffraction (XRD, D8Advance, Bruker) using non-monochromatic Cu-Ka radiation. To study the combustion efficiency, the residual fresh Al contents in the products were measured by a NETZSCH STA449C thermogravimetric analysis (TGA) apparatus at 20-1200 °C in air atmosphere with a flow rate of 40 mL min-1 and a heating rate of 15 K min-1. Before the TGA tests, the solid products was ground in an inert atmosphere to ensure that they were smaller sizes and uniform distributions. 3. RESULTS AND DISCUSSION 3.1 Effects of additives on combustion characteristics


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To understand the effects of different additives and additive contents on the ignition and combustion performances of µAl powder in steam, it is important to identify the combustion characteristics of the samples including the ignition temperature (Ti), the maximum combustion temperature (Tm), the calculated adiabatic flame temperature (Tad), and the ignition delay time (td). In addition, as noted above, experiments on µAl ignition in steam without additives are also required to evaluate the effects of additives. Fig. 4 shows one set of test results for the ignition and combustion temperature curves of the samples with different additive contents in steam at 900 °C. As illustrated in Fig. 4, when the sample is placed into the furnace, the sample temperature gradually increases initially by absorbing heat from the flowing steam. As the sample is ignited, its temperature increases rapidly to a peak value and then decreases fairly quickly to 900 °C. In this work, the ignition temperature is defined as the value at which the slope of the temperature vs time curve increases sharply,37,42 and the noticeable peak in the curve represents the maximum combustion temperature. Additionally, the ignition delay time is routinely calculated as the time elapsed from the moment each sample is placed in the tubular resistance furnace to the time when the sample is ignited. For example, in the first plot in Fig. 4 (a), with no Mg addition, the ignition temperature and the maximum combustion temperature are approximately 816 and 1213 °C, respectively. The corresponding ignition delay time is approximately 48 s. The results shown in Fig. 4 clearly indicate that all three additives enhance the ignition of µAl powder, with Mg the most effective. As shown in Fig. 4 (a), the addition of Mg not only significantly shortens the ignition delay time of µAl powder but also leads to an extra ignition at a much lower temperature, which will be further discussed below. To deal with the variations in the measured temperature and ignition delay time, which may be attributed to a variety of factors including sample size distributions, sample homogeneity, and steam flow


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conditions, and to quantify the repeatability of the data, the ignition temperature, the maximum combustion temperature, and the delay ignition time were repeated three times (n=3). The ignition temperature, the maximum combustion temperature, and the calculated adiabatic flame temperature of different samples are shown in Fig. 5. The error bars represent the standard deviation (SD) of the experimental data.

（Put Fig. 4 here）） Fig. 4. The ignition and combustion temperature curves of µAl powder in steam with the addition of different levels of three additives: (a) Mg, (b) NaF, and (c) NaBH4.

（Put Fig. 5 here）） Fig. 5. Variations of the ignition temperature, maximum combustion temperature, and calculated adiabatic flame temperature of µAl powder in steam with the amount of additives: (a) Mg, (b) NaF, and (c) NaBH4; the error bars represent SD (n=3, relative measurement error < 10 %).

For the µAl powder with Mg addition, Fig. 4 shows that the sample temperature-time profile displays a double-peak feature. The double peaks indicate that the whole combustion process occurs in two stages, including the first stage of Mg combustion and the second stage of µAl combustion. Mg preferentially ignites before the µAl powder because the ignition temperature of the former is lower than that of the latter. The results are consistent with those found by


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Schoenitz et al.43 In the primary combustion stage, Mg is continuously heated until ignition by heat diffusion from µAl. As shown in Fig. 5 (a), increasing Mg content from 3 wt % to 7 wt %, the Mg ignition temperature ( Ti ' ) is increased. There is little difference when Mg content increases from 3 wt % to 7 wt %. In addition, this stage mainly involves Mg combustion in steam, and thus a higher primary maximum combustion temperature ( Tm' ) can be achieved with increasing Mg content. In the secondary combustion stage, the µAl ignition temperature ( Ti'' ) is 818 °C. Among all µAl-Mg samples, it is noteworthy that the µAl-3 wt % Mg has a minimum µAl ignition temperature of 696 °C, corresponding to a decrease of 14.9 % compared with that of µAl without addition of additives. This decrease occurs because the added Mg reacts with steam quickly and releases heat to increase the heating rate of µAl. The high heating rate can easily cause the low ignition temperature. There is a balance between the heating rate and the oxidation of Al. When the content of Mg is less than 3 wt %, the heating rate may play a leading role in ignition temperature of Al. Excessive Mg addition (> 3 wt %) increases the µAl ignition temperature, but the ignition temperature changes little compared with µAl without addition of additives. This may result from the oxidation of Al. The combustion of Mg likely induces µAl oxidation in the first stage, which leads to an increase in the oxide layer thickness of µAl powder. The higher is the amount of magnesium, the thicker is oxide layer. Therefore, the initiation of the µAl-steam reaction becomes more difficult, resulting in a higher µAl ignition temperature. In addition, the energy density of Mg is lower than that of Al, and thus the secondary maximum combustion temperature is slightly reduced when a small amount of µAl powder is replaced by Mg. However, when the Mg content increases from 0 wt % to 10 wt %, the secondary maximum combustion temperature increases from 1213 °C to 1349 °C because the oxide of Mg, which has good permeability, does not hinder the µAl-steam reaction, in contrast to Al2O3.44


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Figure 5 (b) shows the effects of NaF contents on the ignition temperature and the maximum combustion temperature. As shown in Fig. 5(b), both the ignition temperature and the maximum combustion temperature decrease with increasing NaF content, although there is no decrease in the ignition temperature when the NaF content is higher than 7 wt %. The hydrolysis of NaF is accelerated in a high-temperature steam environment. The hydrolysis products react with the oxide film so that the ignition temperature of the exposed fresh µAl significantly decreases. However, combustion is a typical thermodynamic process, and the maximum combustion temperature is closely related to the equilibrium thermodynamic properties. The decrease in the maximum combustion temperature is caused by the loss of a portion of the heat released from the µAl-steam reactions to NaF hydrolysis and other processes, such as by convection and radiation. The ignition temperature and the maximum combustion temperature of µAl powder with different contents of NaBH4 addition are presented in Fig. 5 (c). The addition of 3 wt % NaBH4 is optimal for the ignition and combustion performance of µAl powder, and the ignition temperature of µAl with 3 wt % NaBH4 addition is remarkably decreased compared with that of µAl without addition of additives. These results indicate that addition of NaBH4 is helpful to induce the ignition of µAl. In the case of 3 wt % NaBH4 addition to µAl powder, the ignition temperature and the maximum combustion temperature reach approximately 662 °C and 1208 °C, respectively. When the NaBH4 content is higher than 3 wt %, the ignition temperature basically remains unchanged, whereas a sharp decrease in the maximum combustion temperature occurs, for reasons similar to those for NaF addition to µAl powder. Based on the above discussion, the addition of different contents of Mg, NaF or NaBH4 to µAl powder result in significant variations of the maximum combustion temperature and different


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reactions. The interactions between the different samples and the heated steam can be described according to the following reactions: Al + 3/2H2O → 1/2Al2O3 + 3/2H2

(1)

Mg + H2O → MgO + H2

(2)

NaF + H2O → NaOH + HF

(3)

NaBH4 + H2O → NaOH + BH3 + H2

(4)

To evaluate the effects of different additives on the exothermicity of µAl powder in heated steam,

the

calculated

adiabatic

flame

temperatures

are

obtained

on basis of

thermodynamic theory and compared with the measured combustion temperatures, as shown in Fig. 5. The thermodynamic data for the various reactants and products are taken from the NISTJANAF Thermochemical Tables,** as listed in Table 2. The theoretical adiabatic flame temperatures as a function of additive contents are approximately calculated by the following equations:45 ∆H reac = ∆H prod

(5)

∆H reac = ∑ ni hi reac

    =  ∑ ni hi + c p , H 2O (Tst − T0 ) (1 − ω ) +  ∑ ni hi ω + c p , H 2O (Tst − T0 ) ω  reac ( Al )   reac ( additive ) 

(6)

**

Date available online at http://kinetics.nist.gov/janaf/


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∆H prod

Tf   = ∑ ni h f ,i + ∫ c p ,i dT  prod T0  

=

∑ n [h i

prod ( Al )

f ,i

]

+ c p ,i (T f − T0 ) (1 − ω ) +

∑ n [h

i f ,i prod ( additive )

]

+ c p ,i (T f − T0 ) ω

(7)

In these equations, ∆ H reac and ∆H prod are the enthalpies of the reactants and products, respectively; n i is stoichiometric coefficient of components for reactions (1) - (4); ω is the added mass percentage of additives; Tst is the steam temperature; Tf is the adiabatic flame temperature; T0 is 25 °C (298K) and cp,i is the average molar heat capacity of the products at constant pressure. Additionally, the melting and evaporation of Al must absorb heat produced from the actual Al oxidation process, which can weaken the total enthalpy of the reactants, and thus the equation (6) is rewritten as the following equation (8):   ∆H reac = ∑ ni hi =  ∑ ni hi + c p , H 2O (Tst − T0 ) (1 − ω ) reac  reac ( Al )    +  ∑ ni hi ω + c p , H 2O (Tst − T0 ) ω + (Q sl + Qlg ) (1 − ω ) (8)  reac ( additive ) 

In equation (8), Qsl (10676J/mol) is the latent heat of fusion from solid to liquid of Al, and Qlg (294534J/mol) is the latent heat of vaporization from liquid to gas of Al.


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Table 2 Thermodynamic data of the various reactants and products.

Compound

∆Hf (J mol-1, 25°C)

-1 -1 Cp (J mol K )

Al

0

24.35

Mg

0

24.87

NaF

-575384

46.85

NaBH4

-191836

86.48

H2O

-241826

43.77

Al2O3

-1669800

140.96

MgO

-601600

57.80

NaOH

-425931

59.94

BH3

106692

76.92

HF

-273300

35.32

H2

0

36.11

As shown in Fig. 5, the reaction between µAl-Mg and steam has the highest exothermicity and adiabatic flame temperature, and the reaction between µAl-NaBH4 and steam has the lowest. Fig. 5 also reveals that the adiabatic flame temperature increases substantially with increasing Mg addition but significantly decreases with increasing NaF or NaBH4 addition. The same trends can also be observed in the measured maximum combustion temperatures of different samples. These results verify the correctness and reliability of the above thermodynamic calculations. In addition, the adiabatic flame temperatures are much higher than the corresponding maximum combustion temperatures because the calculation of the adiabatic flame temperature assumes that the exothermic enthalpy of the reactants is fully converted to heat without heat loss to the


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surrounding environment. However, in reality, a large amount of reaction-generated heat is lost to the flowing steam through convection and to the environment by radiation in the present experiments. For further qualitative comparisons of the exothermicity of µAl with Mg, NaF, and NaBH4 addition in steam, the flames for the cases of µAl, µAl-7 wt % Mg, µAl-7 wt % NaF, and µAl-7 wt % NaBH4 are studied, and the evolutions of the flame after ignition are shown in Fig. 6. As shown in Figs. 6 (a) and 6 (b), the combustion intensity improves when Mg is added to µAl powder, and the burning region expands unevenly to the surroundings, with the ejection of particles due to local violent reactions. The ejection phenomenon indicates that µAl particles tend to burn far from the burning surface, consistent with the combustion phenomenon of Al/Ice mixtures containing 10 % µAl powder.46 However, when adding NaF or NaBH4, the bright zone is smaller and the flame brightness is lower than those of µAl powder without addition of additives. It can be seen from Fig. 6 that the durations of the luminous flames are also very different. The durations of the luminous flame of µAl powder without addition of additive is 5.04 s. When the 7 % Mg is added, the durations of the luminous flame increases to 6.8 s. This indicates that the time of heat release increases and the heat release of combustion increases. When adding NaF or NaBH4, the durations of the luminous flames decrease, which shows that the heat release of combustion decreases. These results are in accordance with the results of Fig. 5.

（Put Fig. 6 here）） Fig. 6. Combustion images of different samples in steam: (a) µAl, (b) µAl-7 wt % Mg, (c) µAl-7 wt % NaF, (d) µAl-7 wt % NaBH4.


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The ignition delay times of different samples are compared and the obtained experimental results are shown in Fig. 7. The error bars represent the standard deviation of the experimental data. The compact oxidation film prevents the oxidizer from contacting the µAl particles, which results in an ignition delay time of µAl without addition of additives of as high as 47.7 s. Interestingly, the ignition delay time significantly decreases with the addition of various additives. For example, the µAl with 3 wt % Mg addition shows a significantly reduced ignition delay time of approximately 24 s, a nearly 50 % reduction. And the ignition delay time decreases with the increase of Mg content. The results demonstrate that Mg addition is beneficial for accelerating µAl ignition because Mg reacts with steam quickly and releases a significant amount of heat, and thus promoting µAl ignition. In addition, as shown in Fig. 7, all the ignition delay times of Mg are less than 10 s and display a slightly decreasing trend with increasing Mg content. For samples with addition of NaF or NaBH4, when the additive content increases from 0 wt % to 10 wt %, the ignition delay time is shortened from 47.7 s to 30.4 s and 23.9 s, respectively, much lower than that of the µAl powder without addition of additives. The hydrolysis products of NaF and NaBH4 are conducive to the removal of the oxide layer on the µAl surface, thus reducing the ignition delay time. The effectiveness of these additives with respect to the ignition delay time of µAl powder follows the order: Mg > NaBH4 > NaF. Although the hydrolysis of NaF and NaBH4 produces NaOH, they have different influences on the ignition delay time of µAl powder due to the different hydrolysis characteristics of NaF and NaBH4; in addition, the hydrolysis reaction of NaBH4 has a much lower initiation temperature than NaF. Thus, for NaBH4 addition to µAl powder, the faster production of NaOH provides a more conducive atmosphere for µAl ignition, resulting in a more significant decrease in the ignition delay time.

（Put Fig. 7 here）） Fig. 7. Ignition delay times of different samples. Error bars represent SD (n=3, relative measurement error < 8 %).


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Based on the above discussion, the addition of Mg, NaF or NaBH4 has a significant impact on the ignition and combustion characteristics of µAl. To further reveal the effects of different additives on the combustion behaviors of µAl in steam, the cases of 7 wt % addition of Mg, NaF and NaBH4 are discussed in the next sections. 3.2 Analysis of combustion products To understand how these additives reduce the ignition temperature and the ignition delay time of µAl in steam, the combustion products collected after the experiments were analyzed using the XRD method for the crystalline species. Fig. 8 shows the XRD patterns of the products of the different samples. The chemical compositions of the combustion products of µAl with additives are more complex than those of the µAl powder without additives. As shown in Fig. 8, Al peaks are present in the spectrum for all products, which indicates that the µAl powder burns incompletely in steam.

（Put Fig. 8 here）） Fig. 8. XRD patterns of the combustion products of different samples.

For the µAl-7 wt % Mg sample (the second panel), the XRD peaks include Al, Al2O3, MgO, and MgAl2O4. The presence of MgAl2O4 confirms that the oxidation products of Mg may react with Al2O3 through reaction (2) and the following reaction (9):


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MgO + Al2O3 → MgAl2O4

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(9)

The reactions suggest that removing the Al2O3 product covering the unreacted µAl surface by adding a certain amount of Mg will improve the ignition and combustion performance of µAl powder in steam. When adding 7 wt % NaF (the third panel) to µAl powder, peaks of Al, Al2O3, and NaAl11O17 ·H2O are observed. The possible reaction processes of the µAl-7 wt % NaF sample with steam are reaction (3) and the following reaction (10): 2NaOH + 11Al2O3 + H2O → 2NaAl11O17·H2O

(10)

Based on reaction (3), note that the exhaust of F element is in the form of gas products during combustion, and thus no compounds of F are detected in the products. Similarly, only Al, Al2O3, and NaAlO2 · 1.25H2O peaks are detected in the combustion products of µAl-7 wt % NaBH4 (the last panel). The appearance of NaAlO2·1.25H2O can be explained by reaction (4) and the following reaction (11): 2NaOH + Al2O3 + 1.5H2O → 2NaAlO2·1.25H2O

(11)

For the µAl-7 wt % NaF and µAl-7 wt % NaBH4 samples, the generated NaOH can attach to the µAl surface to react with the oxide film. Thus, the addition of NaF and NaBH4 can lower the ignition temperature and the ignition delay time of µAl powder.


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（Put Fig. 9 here）） Fig. 9. SEM micrographs of the combustion products of different samples: (a) µAl, (b) µAl-7 wt % Mg, (c) µAl-7 wt % NaF, and (d) µAl-7 wt % NaBH4.

The SEM images of the combustion products obtained at magnifications of 1,000× and 5,000× are shown in Fig. 9 and illustrate the effects of the three additives on the morphology and size of the product particles. EDS analysis is performed by irradiation of the surfaces of the products (as white squares #1 and #2), and the results are summarized in Table 3. For µAl without addition of additives after combustion, the surface morphology is composed of large amounts of coral-like grains as shown in Fig. 9 (a). Because µAl particles melt easily and that the molten µAl then reacts with steam, the surfaces of the product appear molten and stick together. The atomic ratio of O/Al should be 1.5 if Al powder is oxidized completely to Al2O3 in steam, and thus the products can be estimated according to the EDS results (Table 3). For these coral-like grains, the atomic ratios of O/Al are less than 1.5, which indicates that the products include unreacted Al and Al2O3. Figure 9 (b) shows the product morphology of the µAl-7 wt % Mg sample, which is also composed of large amounts of coral-like grains, but the coral-like grains are obviously different from that of µAl combustion product. The coalescence of combustion products of µAl-7 wt % Mg sample is more evident than that of µAl without addition of additives. The irregularly shaped micrometer aggregates are more widely distributed, and are assembled from larger amounts of


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smooth, small particles and covered with floccules. According to the corresponding XRD results, these large agglomerations are likely MgAl2O4. Figure 9 (c) shows the unique surface morphology of µAl-7 wt % NaF sample after reaction. The surface contains tiny amount of small spherical grains and a number of fine, coral-shaped broken pieces. The small sizes of these products reflect the ready and dispersed oxidation of µAl particles to Al2O3 with NaF addition during combustion. The EDS results show that the spherical particles are Al2O3 and that the fine broken pieces are composed of Al and Al2O3.

Table 3 EDS analyses of the combustion products of different samples in steam. Element

µAl

µAl-7 wt % Mg

µAl-7 wt % NaF

µAl-7 wt % NaBH4

point 1

point 2

point 1

point 2

point 1

point 2

point 1

point 2

C

10.36

20.16

18.10

30.23

40.32

25.39

39.94

39.15

O

40.84

40.37

43.80

40.49

39.57

36.34

21.78

30.57

Na

--

--

--

--

1.83

1.83

--

1.25

Al

45.68

39.47

5.38

5.09

18.27

36.44

38.28

29.03

Mg

--

--

32.72

24.19

--

--

--

--

As shown in Fig. 9 (d), the surface of the products of the µAl-7 wt % NaBH4 sample is characterized by irregular spherical particles of varying sizes. In addition, irregular particles with a diameter of less than 7 µm accompanied by fragments are highly visible. The sizes of the particles are larger than those of µAl-7 wt % NaF. EDS analysis mainly identifies the particles


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and fragments as unreacted Al because the atomic ratios of O/Al are much less than 1.5. It can be estimated that the µAl particles in this sample are incompletely oxidized in steam. Notably, the C peak is detected in the products of all samples because that the products require spray carbon treatment using a carbon fiber evaporator (EMS150R E, USA) before SEM analyses due to their poor electrical conductivity 3.3 Effects of additives on combustion efficiency The products Al2O3 naturally cover the µAl powder and prevent steam from contacting the fresh Al, and thus these samples cannot be reacted completely even when the steam is heated to 900 °C. To study the combustion efficiency of different samples, the residual active Al contents of different combustion products are determined by TGA experiments.47-50 Figure 10 presents the TG curves of the different products. The TG curves can be divided into three stages: weight-loss, weight-gain, and weight-stable. In the first stage, the moisture and adsorbed gases in the products are released with increasing temperature. Specifically, for µAl with NaF or NaBH4 addition, the volatile substances that mainly contain crystal water in NaAl11O17·H2O and NaAlO2·1.25H2O are quickly removed, and thus the weight loss in the TG curves is much more significant than that observed for µAl without additive addition. In addition, the weight reduction increases with increasing additive content. When the temperature exceeds 600 °C, the second stage is characterized by a major weight gain in the TG curves, which indicates that the unreacted µAl in the products begins to be oxidized to Al2O3. Additionally, oxidation speeds up significantly after the melting point of Al. In the third stage, as shown in the TG curves, a slight weight change is consistently observed for all samples in a narrow temperature range from 1100 °C to 1200 °C because the remaining µAl has essentially


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been fully consumed. The measurements above 1200 °C approach the upper limit of the temperature calibration and thus should be treated with caution. The weight gain in the TG curves is attributed to the oxidation of unreacted active Al, as shown by the following reaction: 4Al + 3O2 → 2Al2O3

(12)

For the reaction of the products with oxygen, the percent of active Al that reacts to form Al2O3 is determined by the following equation:

The active Al% =

108 ∆ m × 100 % 96

(13)

where ∆ m (%) is the percent of the weight gain in the TGA curve. The combustion efficiency (η , %) can be calculated using the following equation:

η=

108 ∆m 96 × 100% C Al

C Al −

(14)

where CAl is the initial activity of the µAl powder, which is 94.7 %. Fig. 11 shows the µAl combustion efficiency of different samples in steam.

（Put Fig. 10 here）） Fig. 10. TG curves of different combustion products.


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（Put Fig. 11 here）） Fig. 11. Combustion efficiencies of different samples.

As shown in Fig. 11, different additives exert significantly different effects on the combustion efficiency. The combustion efficiency of the µAl powder without addition of additive can reach 90.9 %. However, the addition of Mg to the µAl powder significantly decreases the combustion efficiency. For example, the combustion efficiency is reduced to only 73.8 % for the µAl-3 wt % Mg sample. The combustion of µAl with Mg addition is accompanied by the ejection of particles from the samples, which results in weakening of the heat feedback to the burning surface and decreased combustion efficiency. Similarly, the combustion efficiency of the µAl powder declines sharply to 64.5 % when 3 wt % NaBH4 is added to µAl, whereas as for the µAl with added NaF, the combustion efficiency is slightly increased from 90.9 % to 94.3 % as the content of NaF increases from 0 wt % to 10 wt %. There is likely a difference in hydrolysis performance between NaBH4 and NaF. The fast hydrolysis reaction of NaBH4 with steam occurs as soon as the sample is placed in the furnace. The produced NaOH completely reacts with the oxide layer Al2O3 and forms complex compounds covering the µAl surface via reaction (11), thus hindering further µAl-steam reaction. By contrast, the slow hydrolysis process of NaF may persist until the µAl is ignited in steam. As a result, the produced NaOH destroys not only the oxide layer but also the oxidation product of µAl. The removal of oxidation products promotes µAl combustion. Therefore, as shown in Fig. 11, the combustion efficiency of the µAl powder remarkably decreases with the addition of NaBH4 but slightly increases with NaF addition. Although increasing the Mg or NaBH4 content contributes to enhance the combustion efficiency, the


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combustion efficiency of these samples is still much lower than that of µAl without addition of additives. 3.4 Effects of additives on the reaction mechanisms According to the discussion above, the possible reaction mechanisms of µAl with the addition of the three types of additives in steam are schematically illustrated in Fig. 12.

（Put Fig. 12 here）） Fig. 12. Schematic of the reaction mechanisms of µAl with the addition of the three additives in steam: (a) Mg, (b) NaF, and (c) NaBH4.

As shown in Fig. 12 (a), the reaction process of µAl with Mg addition can be described as R1, R2, and R9. Considering the high activity of Mg, R2 should occur first and quickly. After the ignition of Mg, MgO is produced, and R1 and R9 occur simultaneously. R1 can be promoted by the heat released by R2 and the products of R9. The generation of MgAl2O4 reduces the concentration of Al2O3, which is helpful to R1. Therefore, the addition of Mg to µAl powder is favorable for the exposure of µAl to steam and decreasing the ignition temperature and the ignition delay time of µAl. The whole reaction processes for µAl with the addition of NaF or NaBH4 display similar features, as shown in Figs. 12 (b) and 12 (c). Because the µAl powder absorbs heat from the flowing steam, R1 is essentially absent initially due to the relatively low temperature, but R3 and R4 can occur in steam due to the hydrolysis properties of NaF and NaBH4. The hydrolysis product NaOH can react with the oxide layer of µAl and correspondingly


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perform R10 and R11, resulting in significant decreases in the ignition temperature and ignition delay time of µAl. Then, as the temperature of µAl increases until ignition, the oxide products Al2O3 naturally cover the surface of the µAl and restrict the heat released in R1. In addition, the hydrolysis products HF and BH3 can flow out with the flowing steam. However, the hydrolysis of NaF is more difficult than that of NaBH4. In other words, a higher temperature is required to initiate R3. This indicates that a certain content of NaOH continues to be produced during the whole combustion. As a result, the product NaOH of R3 can remove not only the oxide layer but also the oxidation product of µAl. These effects also explain why the combustion efficiency of µAl increases with increasing NaF content and the sizes of the combustion products are smaller than those of µAl powder with NaBH4 addition. 4. CONCLUSION In this paper, the ignition and combustion characteristics of µAl powder with the addition of different contents (3 wt %, 7 wt %, and 10 wt %) of three types of additives (Mg, NaF, and NaBH4) are investigated in steam. Based on the experimental data, the following main conclusions can be reached: (1) The addition of Mg reduces the ignition temperature and the ignition delay time of the µAl

powder. In addition, for the µAl-Mg samples, the measured temperature curves show that the combustion process occurs in two combustion stages (a double-peak feature) because the Mg powder preferentially ignites before the µAl powder. The maximum combustion temperature increases with increasing Mg content. The ignition temperature and the ignition delay time are also significantly decreased with increased NaF or NaBH4 addition. And the addition of NaF or NaBH4 lowers the maximum combustion temperature.


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(2) Based on the XRD,

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SEM, and EDS results, MgAl2O4, NaAl11O17·H2O, or

NaAlO2·1.25H2O is generated with Mg, NaF, or NaBH4 addition. The surfaces of the combustion products have different characteristics for µAl with addition of different additives. The sizes of the particles of µAl-7 wt % NaF are smaller than those of µAl-7 wt % NaBH4. (3) The addition of Mg to the µAl powder leads to a decrease in combustion efficiency. As for the µAl with NaBH4 addition, the newly formed complex compounds covering the µAl surface are responsible for the low combustion efficiency. However, it is gratifying that the combustion efficiency slightly increases as the content of NaF increases.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We greatly appreciate the financial support provided by National Natural Science Foundation of China (No. 51376007, 51676001 and 51206001) and Anhui Provincial Natural Science Foundation (No. 1608085ME104).

REFERENCES


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Fig. 1. The mass and volumetric energy densities of different hydro-reactive fuels. 297x209mm (150 x 150 DPI)


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Fig. 2. SEM image of 1-2 µm Al powder. 62x45mm (220 x 220 DPI)


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Fig. 3. Schematic diagram of the experimental apparatus. 364x175mm (96 x 96 DPI)


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Fig. 4. The ignition and combustion temperature curves of µAl powder in steam with the addition of different levels of three additives: (a) Mg, (b) NaF, and (c) NaBH4. 287x201mm (300 x 300 DPI)


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Fig. 5. Variations of the ignition temperature, maximum combustion temperature, and calculated adiabatic flame temperature of µAl powder in steam with the amount of additives: (a) Mg, (b) NaF, and (c) NaBH4; the error bars represent SD (n=3, relative measurement error < 10 %). 284x197mm (150 x 150 DPI)


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Fig. 6. Combustion images of different samples in steam: (a) µAl, (b) µAl-7 wt % Mg, (c) µAl-7 wt % NaF, (d) µAl-7 wt % NaBH4. 139x78mm (96 x 96 DPI)


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Fig. 7. Ignition delay times of different samples. Error bars represent SD (n=3, the relative measurement error < 8 %). 296x209mm (300 x 300 DPI)


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Fig. 8. XRD patterns of the combustion products of different samples. 287x201mm (300 x 300 DPI)


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SEM micrographs of the combustion products of different samples: (a) µAl, (b) µAl-7 wt % Mg, (c) µAl-7 wt % NaF, and (d) µAl-7 wt % NaBH4. 169x63mm (300 x 300 DPI)


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Fig. 9. SEM micrographs of the combustion products of different samples: (a) µAl, (b) µAl-7 wt % Mg, (c) µAl-7 wt % NaF, and (d) µAl-7 wt % NaBH4.

146x69mm (220 x 220 DPI)


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146x69mm (220 x 220 DPI)


Energy & Fuels

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146x55mm (220 x 220 DPI)


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Energy & Fuels

Fig. 10. TG curves of different combustion products. 287x201mm (300 x 300 DPI)


Energy & Fuels

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Fig. 11. Combustion efficiencies of different samples. 288x200mm (300 x 300 DPI)


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Energy & Fuels

Fig. 12. Schematic of the reaction mechanisms of µAl with the addition of the three additives in steam: (a) Mg, (b) NaF, and (c) NaBH4. 256x183mm (150 x 150 DPI)


Energy & Fuels

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Energy & Fuels



Effects of Different Additives on the Ignition and Combustion

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