A Kind of Rare Earth Oxide Composited Superfine Dry Powder Extinguishant With Improved Fire Extinguishing Performance

As one of the most extensively used fire-extinguishing agents, dry powder extinguishant has been widely applied in various fire protection fields, such as building fire, industrial fire, forest fire, urban fire, vehicle fire, and so on [1-4]. Therefore, to reduce casualties and property losses, it is important to improve dry powders' fire-extinguishing performance. There are two main strategies to improve dry powders' fire-extinguishing effect. One is to reduce the particle sizes of dry powders. For example, a series of superfine dry chemical extinguishing agents with an average diameter of micrometers (< 20 μm) was prepared and studied [5, 6]. It was found that the extinguishing efficiency of superfine dry powder is 6–9 times higher than that of the ordinary dry powder extinguishing agent. However, there exists a limit on the extent of reduction in the amount of the required dry powders with decreasing particle size [7]. Moreover, the problems of agglomeration and storage are more serious after super-refinement treatment. The other one is the new powder components or formulas composited with metal ions. The well-known metal ions are potassium, sodium, and magnesium, which are widely used as flame inhibitors and fire suppressants [8-11]. However, the strong base corrosion of alkali metal and alkaline-earth metal compounds and their decomposition products limits the scope of their applications. In recent years, benefitting from the catalysis and radical scavenging effects, some transition metal compounds were found more effective on improving the fire extinguishing performance of dry powder [12]. For instance, Fe(CO)5 was found to exhibit a high flame-suppression efficiency [13]. While, the high toxicity restricts its application. Although, Li et al. studied the less toxicity ferrocene in composite superfine dry powder and found that 0.7% ferrocene in composite superfine dry powder displayed the best inhibitory effect [14], the high cost of ferrocene is another drawback for large scale application. In other study, chromium was more effective than iron at accelerating recombination of radicals [15], while, chromium is one of the environmental pollutants [16]. Ni et al. investigated the performance of three kinds of transition metal ions (Mn2+, Cu2+, and Zn2+) in modified zeolite 4A particles (a kind of molecular sieve with chemical formula of Na12[(AlO2)12(SiO2)12]·27H2O and pore size of 0.42 nm) in suppressing methane/air coflowing flames [17]. The results showed that these transition mental ions exhibited good fire suppression improvement with a ranking order of Mn2+ > Cu2+ > Zn2+. But, for their best fire suppression efficiencies, the metal loadings were up to 8%–12%, which hinders the development of these kind of transition metals. Rare-earth (RE) compounds exhibit good flame retardance properties [18-21]. In our previous work, La, Ce, and Y-based metal organic frameworks (MOFs) were synthesized and studied as flame retardants on epoxy [22]. The results revealed that the improved flame retardance performances of RE-MOFs resulted from the catalytic carbonizing abilities and the radical-trapping abilities of La, Ce, and Y. Moreover, La2O3, CeO2, and Y2O3 are cheap industrial chemicals, and their outputs are very large (more than 20 thousand tons per year in China). Therefore, it is highly desirable to explore the influence of these RE compounds on dry powder and the effect of their combination on fire suppression. In this work, three rare earth compounds (La2O3, CeO2, and Y2O3) were firstly studied as additives to enhance the fire extinguishing ability of the dry powder. Their fire suppression effectiveness was studied through cup burner and bench scale fire extinguishing experiments; then, an in-depth analysis was carried out using thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) to reveal the mechanism of the fire suppression improvement. All reagents were commercially available and used as received without further purification. A commercial superfine dry powder (90 wt% ammonium dihydrogen phosphate (NH4H2PO4), Chuangwei Firefighting Material Co. Ltd., Yantai City, China) was used as the base material. The RE additives (La2O3, CeO2, and Y2O3) were obtained from Baotou Research Institute of Rare Earths. The RE composited superfine dry powders with different RE contents (1.64%, 3.23%, and 7.69%.) were prepared through the mixture of RE additives and commercial superfine dry powder by planetary ball mill (F-P4000, FOCUCY). The ball milling time and frequency were 20 min and 10 Hz, respectively. The particle size and size distribution of the powder samples were measured by a laser particle size analyzer (LS-909, OMEC). The bulk density of the powder extinguishing agent was measured by the following method. Generally, a 35 g dry powder sample was weighed and placed in a measuring cylinder with a plug. The measuring cylinder was inverted within 2 s and this action was repeated 20 times. After that, the total volume was recorded. The bulk density of the powder extinguishing agent was calculated by dividing the total mass by the total volume of the unvibrated granular sample in natural accumulation. Thermogravimetric analysis (TGA) experiments were performed by a TGA/DSC3+ thermal analyzer produced by Mettler Toledo. The initial temperature was set at 30°C, the termination temperature was set at 800°C, and the heating rate was controlled at 10°C min−1. All the experiments were carried out in a nitrogen atmosphere with a gas flow rate of 50 mL·min−1 and a sample mass of 3–5 mg. Differential scanning calorimeter (DSC) tests were carried out by a DSC 3 METTLER TOLEDO. A 5–10 mg sample was sealed in an aluminum pan and heated from 30°C to 400°C at a heating rate of 10°C·min−1 in a nitrogen atmosphere with a gas flow rate of 50 mL min−1. A bench scale experimental apparatus of confined space (1 m3) was built according to the previous works after a minor modification [11, 25]. The experimental platform was mainly constructed with a transparent confined space (1 m × 1 m × 1 m), an oil pan, a sprayer, a powder tank, a driving gas system (nitrogen), a temperature measuring system, a smoke analyzer, and high-speed camera (Figure 2a). The dry powder sample was filled into a tank with a conical bottom. One intake tube connected to the driving system was inserted at 1 cm from the bottom. Another discharge tube linked to the sprayer was inserted at 1 cm from the top. The pressure of the compressed nitrogen was controlled by a mass flowmeter and set to 0.11 MPa for every test. 150 mL of n-heptane was used as fuel in a square oil pan (20 cm × 20 cm × 4 cm) for each test. It should be noted that the oil pan was placed near the edge of the box to prevent it from being directly sprayed by the fire extinguishing agent. The experimental steps are as follows: first, the oil pan is ignited and the fire lasts for 35 s. Then, the suppressant is completely discharged within about 10 s. To obtain more information during the fire-extinguishing experiment, the temperature and flue gas composition were measured by four thermocouples (K type, 2 mm diameter) and a smoke analyzer (Testo 350, TESTO), respectively. Four thermocouples were fixed on the iron stand. The location of the fixed thermocouples and the position between the thermocouple and the flame were schematically illustrated in Figure 2b. The full-scale burning test was carried out with a 100 m3 confined space according to GA 578–2005 (the standard of super fine powder fire extinguishing agent). The experimental platform was schematically illustrated in Figure 3. In a 100 m3 confined space (5.2 m × 5.2 m × 3.7 m), the square oil pan (0.25 m2) was filled with 12.5 L n-heptane and placed in an area where it was not directly sprayed with fire extinguishing agents. Four circular oil tanks (diameter 80 mm, height 100 mm) were filled with 250 mL n-heptane, respectively, and placed at the four corners of the room. The K-type thermocouples were placed above every square oil pan and circular oil tank to detect whether the flame was extinguished. The dry powder sample was filled into the powder storage tank. The pressure of the spray outlet was controlled at 0.5 MPa for every test. The experimental steps are as follows: first, the oil pan and four oil tanks are ignited and the fire lasts for 35 s. Then, the suppressant is completely discharged within about 15 s. The fire extinguishing is considered successful if all flames are extinguished within 30 s after the release of the extinguishing agent and there are remaining fuels in the oil pan and oil tanks. The particle size distributions of the commercial superfine dry powder and RE oxide additives were shown in Figure 4. As can be seen that the particle sizes of D90 for the commercial superfine dry powder, La2O3, CeO2, and Y2O3 were 16.82, 6.37, 17.01, and 11.31 μm, respectively. Therefore, the particle sizes of these raw materials are all less than 20 μm. In other words, the composite dry powders combined from the above components all belong to superfine dry powders. The bulk density refers to the average density of the accumulation of loose particles, including internal and external pores and intergranular spaces. The bulk density of the powder extinguishing agent is essential when calculating the extinguishing dosage of the dry powder. The bulk densities of each sample were shown in Figure 5. It can be seen that, compared with the commercial dry powder, the bulk densities of composite dry powders increased a little. The addition of lanthanum oxide made the greatest increase among other RE compounds, which may be due to the smallest particle size of lanthanum oxide, since the smaller particle will be easier to fill the gap space in the commercial dry powder. The main parameters in the cup burner fire extinguishing experiment were listed in Table 1. The comparison of the extinguishing concentrations among various samples was shown in Figure 6. As can be seen that almost all the composite superfine dry powders with different RE additives showed a higher fire extinguishing efficiency than that of the pure commercial superfine dry powder. For the same additive content, the fire suppression improvement by adding RE compounds has a ranking order of La2O3 > CeO2 > Y2O3. Moreover, all the RE additives have the highest fire extinguishing improvement at the mass content of 3.23%, and 3.23% La2O3 exhibited the best performance (the fire extinguishing concentration decreased by 18.39% compared with the commercial dry powder). The typical flame images in the extinction process with the samples of 3.23% La2O3, 3.23% CeO2, and 3.23% Y2O3 were shown in Figure 7. As can be seen that the RE composited dry powders exhibited very similarly with regard to the flame extinguishment. The methane/air cup burner flame was a nearly axisymmetric laminar diffusion flame with a bright yellow conical flame. Due to the influence of gravity, low-speed coaxial air flow could produce buoyancy-induced flame instability and make the flame pulsate at a lower frequency, but the flame height was basically unchanged and the combustion was relatively stable. In the application of powders to suppress the flame, the flame morphology change was more complex. First, the flame height significantly increased and the flame had a combustive effect when a small amount of powder medium was introduced into the combustion zone. Then, with the increase in the amount of powder, the flame flickered and became unstable. This was because the powder began to play the role of blocking combustion in the flame [12]. When the amount of dry powder agent was further increased, the flame root began gradually to separate from the cup brim and eventually extinguished as the powder concentration reached the extinguishment limit. As can be seen that the flame extinguishment occurs as a result of a series of flame destabilization processes, that is, the flame base detachment, oscillation, and blow-off, which is consistent with the reported observation of gaseous suppressants [24]. The previous researches have revealed that such a blow-off type of flame extinguishment was caused by the reaction kernel, which was dependent on the flux of both heat and chain radicals [26, 27]. Thus, the application of RE composited dry powders on fire can effectively reduce the supply of chain radicals (OH•, O• and H•) that are necessary to support the combustion reaction [17]. Moreover, considering the high radical trapping ability of La, Ce, and Y [22, 28, 29], it is reasonable to conclude that the high radical capturing ability of RE additives is one of the main reasons for the improved fire extinguishing efficiency of RE composited dry powders. Due to the highest fire extinguishing improvement of 3.23% La2O3 composited superfine dry powder, its fire extinguishing performance was further studied with a bench-scale experiment in a 1 m3 confined space. To compare with the commercial superfine dry powder, it was also tested under the same conditions. The powder consumption and extinguishing time of the two samples under the same condition of 0.11 MPa are listed in Table 2. For the similar fire extinguishing time, 3.23% La2O3 composited superfine dry powder showed a higher fire extinguishing efficiency than that of the commercial superfine dry powder. In other words, the addition of 3.23% La2O3 can improve the dry powder's fire extinguishing efficiency by 16.7%, which is consistent with the result of the cup burner fire extinguishing experiment. The temperature variation of the different combustion reaction regions during the fire suppression for the two powder samples were shown in Figure 8. And their temperature drop rates in the same period (30 s) from the highest temperatures were shown in Table 3. As can be seen that 3.23% La2O3 composited superfine dry powder exhibited a fast temperature drop rate than that of the commercial superfine dry powder, which further proves the improved fire suppression performance by addition of La2O3 in dry powder. Fire smoke contains many toxic substances, such as CO, NOx, PAHs, soot, and SOx. Among them, CO, as one of the most abundant ingredients, has a greater impact on human health. In addition, dry powders contain NH4H2PO4 that may form NOx in the process of extinguishing fire. Therefore, the variation of CO and NOx for the application of extinguishant was further studied in this work. Figure 9 shows the changes in CO concentration and NOx concentration of commercial superfine dry powder and 3.23% La2O3-composited superfine dry powder during fire suppression. It can be observed that the CO and NOx concentrations increased slowly due to the inadequate combustion of the pool fire before the application of the dry powders. However, after the introduction of the dry powders, the CO and NOx concentrations rose sharply within a short time. The sudden increases in CO and NOx concentrations are caused by the extinguishing effect of the dry powder on flame combustion. However, compared with the commercial superfine dry powder, the highest CO and NOx concentrations of La2O3-composited superfine dry powder showed a rather low level and were only about a half of that, which indicates that the introduction of La2O3 in dry powder can effectively control the toxic gases of dry powder during the firefighting process. Considering the scale effect of thermal radiation, the fire extinguishing performance of 3.23% La2O3 composited superfine dry powder was further studied with a full-scale (100 m3) burning test. The results and their comparison with different test methods were listed in Table 4. As can be seen for the same extinguishant, the fire extinguishing concentration measured by the full-scale test is larger than that of the small-scale test, which indicates that the large-scale fire is more difficult to extinguish than the small-scale fire. However, the fire extinguishing improvements of 3.23% La2O3 composited dry powder for different scale tests are very similar (close to 18%). In other words, for different kinds of fire tests, although their absolute values are different, their relative values are consistent. This result further proved the effectiveness of RE oxides in enhancing the performance of dry powders. And the thermal decomposition products can inhibit the chain reaction of flame via the catalytic combination of active species (such as OH•, O•, and H•). Thus, to further investigate the suppression mechanism of the RE composited dry powders, the pyrolysis properties of the commercial superfine dry powder and 3.23% La2O3 composited superfine dry powder were comparatively studied by TGA and DSC. As shown in Figure 10a, the two samples have similar weight loss curves, which indicates that their pyrolysis processes are in accord with the above thermal decomposition equations of NH4H2PO4. In the initial decomposition stage (50°C–195°C), the commercial superfine dry powder had no obvious decomposition; however, La2O3 composited dry powder presented a 7% weight loss, which indicates that La2O3 can induce the decomposition of dry powder at low temperature. Moreover, in the second peak decomposition stage (195°C–250°C), the decomposition rate of La2O3 composited dry powder (0.58%·°C−1) was faster than that of the commercial dry powder (0.51%·°C−1) according to DTG curves, which demonstrates that the introduction of La2O3 did accelerate the decomposition of the dry powder. Moreover, from the results of DSC (Figure 10b), it can also be concluded that La2O3 composited dry powder is easier to decompose than the commercial dry powder at lower temperatures. For ammonium dihydrogen phosphate type dry powders, the initial decomposition plays a vital role in the entire reaction [25]. In this work, TGA and DSC results clearly revealed that the addition of La2O3 can apparently induce and accelerate dry powder decomposition in the initial stages, which can be inferred that the catalytic degradation of La2O3 plays an important role in the improved fire extinguishing efficiency of the composite dry powders. Furthermore, in the previous works, it has been proved that RE compounds have a strong radical trapping ability by experimental and computational methods [22, 28, 29]. Accordingly, RE additives could capture and terminate a large number of the active radicals (such as OH•, O•, and H•) that maintain the flame burning, which can further improve the fire extinguishing efficiency of the composite dry powders. According to the research above, RE additives (especially for La2O3) can effectively improve the fire suppression performance of dry powders, and the main reasons for the improvement are attributed to RE additives' catalytic degradation effect and radical capturing ability. This work delivers a promising strategy to enhance the performance of dry powders by compositing with RE oxides. However, the mixture method of ball-milling used in this work is not efficient enough in component combination. Therefore, the high effective composite preparation methods that can give full play to the role of RE oxides in dry powder should be developed for future research. This work was supported by the Key Project of the Fundamental Research Funds for the Central Universities (3122025083). The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.