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Experimental Investigation on Inerting Mechanism of Dust Explosion
Available online at www.sciencedirect.com
Procedia Engineering 43 (2012) 338 – 342
International Symposium on Safety Science and Engineering in China, 2012 (ISSSE-2012)
Experimental Investigation on Inerting Mechanism of Dust Explosion Bing Dua, Weixing Huanga,*, Niansheng Kuaia, Jingjie Yuana, Zongshan Lia, Yuan Gana a
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
Abstract
Systematic experiments were carried out on explosions of coal dust-inertant mixtures. Explosion severity was gained by using Siwek 20 L vessel and factors strongly influencing inerting effectiveness, such as deflagration mechanism and inertant composition were taken into account. Tests of thermal-gravimetric analysis were conducted to identify the reaction mechanism of coal dust explosions. As a result, bituminous coal dust deflagrates via the homogeneous mechanism, while anthracite dust mainly features the mechanism of heterogeneous reaction. Results reveal that the inerting functional mechanisms are quite different for bituminous coal and anthracite dusts, and the homogeneous-combustion-dust is much easier to be inhibited than the heterogeneous-reaction-dust. Trends of higher inerting effectiveness are associated with better decomposability, more excellent performance of oxygen blocking. Moreover, the particular efficacy of free ammonia decomposed from monoammonium phosphate in flame extinguishing is identified. Added water shows the best inerting effectives due to particle aggregation induced by moisture, which gives the idea that humidification for coal dust is therefore a cost-effective approach for explosion prevention and mitigation. © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Capital University of Economics and Business, China Academy of Safety Science and Technology. Keywords: Dust, Explsion, Inerting, Homogeneous mechanism, Heterogeneous mechanism
1. Introduction Inerting, to intimately premix the combustible dust with inertant prior to ignition, is recognized as an inherent safety approach in prevention and mitigation of dust explosions [1-3]. Various parameters influencing the effectiveness of inertants in explosion suppression have been investigated in previous studies. Unfortunately, although discussion on the inerting mechanism has been referred, the critical functional mechanism, which is of great benefit to seek for high-efficiency inertant for explosion suppression, is still not well understood. The inerting effectiveness of inertant is strongly affected by the explosion characteristic of combustible dust itself. Therefore, researches on the deflagration mechanism of combustible dust are urgently demanded as a basis for explosion prevention and mitigation. Even though several reaction mechanisms have been proposed to describe the dust flame propagation in previous studies [4-7], few methods are proposed to distinguish the deflagration mechanisms of different dusts accurately.
* Corresponding author. Tel.: +86-28-8540-5208; fax: +86-28-8540-3397. E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.058
Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
The present work aims to gain a comprehensive understanding of the explosion behavior of coal dust-inerant mixture, above all, to have an insight into the functional mechanism of inertant on explosion suppression. Explosion severity (maximum explosion pressure pmax and maximum rate of pressure rise (dp/dt)max) were systematically tested. Factors strongly influencing inerting effectiveness, such as deflagration mechanism of combustible dust and inertant properties (including specific heat, decomposition, decomposition temperature and other particular efficacies) were taken into account. This study would provide valuable guidance for improvement of inerting technology and help to seek for high-efficient inertants on dust explosion prevention. 2. Experimental 2.1. Materials and apparatus In the present study, bituminous coal and anthracite powders were employed as combustible dusts. Their particle size distributions and calorific values were illustrated in Table 1. All the tested samples were systematically dried at 50 °C under vacuum for 2 hours before handling. Experiments were performed in the well-known Siwek 20 L apparatus [3,8-9], which is an explosion resistance hollow sphere made of stainless steel in accordance with the recommendations of Chinese standard GB/T 16425, European standard EN 14034 and ASTM standard E 1226. Parameters of explosion severity measured in the chamber are maximum pressure pmax and maximum rate of pressure rise (dp/dt)max, representing the thermodynamic and kinetic of explosion energy liberation respectively. Table 1. Physiochemical properties of tested dust samples Particle size distribution (μm)
Calorific value (kJ.g-1)
Bituminous coal (sample A)
75-125
20.06
Anthracite (sample B)
75-125
30.97
Dust explosion is induced by a pyrotechnical ignitor which consists of zirconium, barium nitrate and barium dioxide by the weight ratio of 4:3:3. The energy released from ignitor is calculated based on the amount of pyrotechnical mixture. For example, the energy release of 1.2 g mixture corresponds to 5 kJ. Ignitors of 5 kJ are chosen in the present work, which is suggested by authors’ previous studies advocating the energy region of 5-10 kJ is the most appropriate for inerting testing [3,8]. 2.2. Inerting of dust samples In this inerting work, calcium carbonate (CaCO3), monoammonium phosphate (NH4 H2 PO4) and water (H2O) were chosen as inertants because they are widely used in explosion prevention and fire extinguishment. Moreover, the reported inertant mixing ratio is the percentage of inertant in the total combustible -inertant mixture. The combustible-inert mixtures were well premixed with a mixing ratio ranging from 0 to 70 %. 2.3. Reaction mechanism of dust explosion The homogeneous (vapor-phase) and heterogeneous mechanisms are two alternative deflagration mechanisms of dust explosions [10-11]. As described, homogeneous mechanism occurs through steps by heating, devolatilization, volatile-air mixing and homogeneous combustion [5-6]. On the contrary, heterogeneous combustion proceeds with oxygen dissolution in the surface of molten particles and no yield of volatile matter. That is, the liberation of volatile is a critical process in the homogeneous mechanism, which can be regarded as the criterion to distinguish the two different deflagration mechanisms. 3. Results and discussion 3.1. Deflagration mechanism analysis of coal samples Prior to discussing the inerting functional mechanism of inertant, thermal-gravimetric analysis (TG) was performed to give an evaluation on the deflagration mechanism of tested coal dust samples. For the TG experiments, coal samples were heated from 30 to 800 °C at a rate of 15 °C/min in both nitrogen and air atmospheres.
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Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
Typical TG curves of bituminous coal and anthracite s are shown in Fig. 1(a) and (b). As can be seen, small weight loss appears in all the coal samples at the low temperature region, which can be associated with losses of physically absorbed water and high-volatile organic matter. For bituminous coal, weight loss becomes significant when temperature exceeds 340 °Cin a nitrogen atmosphere, while the weight loss in the entire heating process reaches up to 67 % attributing to the volatilization. It is clear that devolatilization temperature of bituminous coal ranges from 340 to 600 °C. The volatilization may be due to the breaking of chemical bonds [12] and desorption of low molecular weights [13]. Furthermore, both the nitrogen and air results show the same ash content, which gives the idea that there is little fixed carbon in tested bituminous coal and the fuel of combustion is mainly contributed by volatile matter. Above-mentioned discussion means that the homogeneous combustion mechanism of bituminous coal is verified.
(a)
(b) Fig. 1. TG traces of (a) bituminous coal and (b) anthracite dust in both nitrogen and air atmosphere.
For anthracite, it is essential to point out that only a slight weight loss of 8 % is identified throughout the entire heating process in a nitrogen atmosphere. This behavior is attributed to the low volatile content in tested anthracite. Consequently, the char residue obtained is considered consisting of fixed carbon and ash [12]. Moreover, comparison is made between the nitrogen and air atmospheres, 79 % weight loss can be obviously observed in air atmosphere and the ash content turns out to be 21 % equaling to the weight left of char residue. This is because the combustion of anthracite mainly conducts on the oxidation of fixed carbon via surface direct oxidation and does not undergo the process of devolatilization. As a result, the deflagration mechanism of anthracite features the heterogeneous reaction essentially. 3.2. Inerting of coal dusts with different deflagration mechanisms As the deflagration mechanisms of tested coal samples were identified above, inerting effectiveness of CaCO3 on bituminous coal and anthracite dusts were carried out with the mixing ratio ranging from 0 to 70 %, under coal dust concentration of 500 g/m3.
(a)
(b)
Fig. 2. Evolutions of (a) pmax and (b) (dp/dt)max with CaCO3 mixing ratio for bituminous coal and anthracite dust with the concentration of 500 g/m3.
Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
In Fig. 2, evolutions of pmax and (dp/dt)max were plotted as functions of inertant mixing ratio for both two samples. As can be seen, the inerting effectiveness increases with the rise of inertant mixing ratio significantly. Two samples represent the similar explosion severity without added inertant in spite of the difference on calorific values. pmax of coal-inert mixture represents a gentle downtrend and then undergoes a sharply reduction when the inertant mixing ratio is greater than 50 % for anthracite. Nevertheless, values of pmax and (dp/dt)max for bituminous coal with only 20 % inertant drop to 33 % and 40 % corresponding to the pure coal results. This indicates that the bituminous coal dust with homogeneous combustion mechanism is much easier to be inhibited. For the homogeneous combustion process, the controlling step is pyrolysis/devolatilization (where volatiles are given off by particles) [6]. However, the added CaCO3 can effectively reduce the reaction region temperature by absorbing combustion heat released from burning particles, and then prevent further devolatilization of unburned particles. It follows that the volatilization efficiency of particles gets lower. Nevertheless, for anthracite dust with heterogeneous reaction, the explosion process is controlled by the kinetics of oxygen diffusion on the particle surface. As illustrated in Fig. 2, pmax value of anthracite-CaCO3 mixture somewhat shows a stabilization when CaCO3 mixing ratio ranges from 0 to 50 %, because CaCO3 with small addition amount is unable to cause high resistance of oxygen diffusion evidently. When the inertant mixing ratio is greater than 50 %, combustion heat is absorbed by inertant in a large extent so that there is no sufficient heat to sustain the propagation of combustion flame [14]. Hence, pmax value decreases sharply because particle temperature falls below the kindling temperature of anthracite. That is the reason why CaCO3 shows lower inerting effectiveness on anthracite dust than bituminous coal dust. 3.3. Influence of inertant properties The influence of inertant properties on inerting effectiveness was evaluated for anthracite-CaCO3, anthracite-NH4 H2PO4 and anthracite-H2O mixtures at the coal dust concentration of 250 g/m3. Evolutions of explosion severity were plotted as functions of inertant mixing ratio in Fig. 3. As can be seen, all curves have sudden changes along with the rise of inertant mixing ratio. Values of pmax decrease sharply in ranges of 10-30 %, 10-30 % and 50-70 %, with H2O, NH4 H2PO4 and CaCO3 as inertants respectively, while (dp/dt)max shows pronounced decrease in ranges of 10-30 %, 0-30 % and 60-70 % respectively. For the inerting effectiveness, H2O shows the best result, followed by NH4 H2PO4 and CaCO3.
(a)
(b)
Fig. 3. Evolutions of (a) pmax and (b) (dp/dt)max with CaCO3, NH4 H2 PO4 and H2O mixing ratio for anthracite dust with the concentration of 250 g/m3.
As mentioned previously, the sharp decrease of pmax is attributed to the heterogeneous reaction mechanism of anthracite in a large extent. Inerting of heterogeneous combustion mainly reflects in the synergism between increasing oxygen diffusion resistance and absorbing combustion heat. The difference of inerting effectiveness is attributed to the following reasons: z Firstly, heat-absorbing and inertant decomposition: The decomposition temperature of NH4 H2PO4 (190 °C) is much lower than CaCO3 (825 °C), whereas the kindling temperature of anthracite ranges from 500 to 620 °C, which indicates CaCO3 is somewhat nondecomposable in the explosion process [1]. As described by Chatrathi [15] and Abbasi [16], the decomposition of inertant plays a key role in inerting effectiveness; hence the efficient decomposition of NH4 H2PO4 results in a great thermal sink. However, the nondecomposable H2O represents the best excellent thermal sink due to large specific heat and low gasification temperature (only 100 °C).
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Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
z Secondly, oxygen diffusion resistance: Free ammonia released by NH4 H2PO4 plays a role to dilute oxygen and P2O5 decomposed from NH4 H2PO4 is prone to coating the burning particles, which results in higher resistance of oxygen diffusion and lower ignition sensitivity. The water-vapor not only forms a liquid membrane to cover the particles but also dilutes oxygen. The heterogeneous reaction does not self-sustain until both sufficient oxygen diffuses to the particle surface and the particle surface temperature is kept above the kindling temperature. Whereas CaCO3 seems helpless to increase the oxygen diffusion resistance and thereby its function mechanism mainly depends on heat-absorbing, which is the reason why CaCO3 represents bad inerting effectiveness. z Finally, particle aggregation: For H2O, particle aggregation induced by moisture will significantly decrease the specific surface, as well as reduce the reaction zone of heterogeneous combustion. It is essential to point out that (dp/dt)max of anthracite-NH4 H2PO4 mixture is even lower than anthracite-H2O mixture with 10 % mixing ratio. This is related to the particular efficacy of NH4 H2PO4 in flame extinguishing [17]. Moreover, considering that moisture will not cause the contamination of coal dust, humidification for coal dust is therefore regarded as a cost-effective approach for explosion prevention and mitigation. 4. Conclusions Systematic analysis on inerting functional mechanism of inertant has been preformed by evaluating the influences of deflagration mechanism of coal dust and inertant properties on coal dust-inertant mixture explosion behaviors. As a consequence, deflagration mechanism of dust explosion can be identified by using TG analysis. The homogeneous combustion mechanism features the release of volatile matter, whereas the heterogeneous reaction mechanism does not undergo the process of devolatilization. Results reveal that dust with homogeneous combustion mechanism is much easier to be inhibited than dust with heterogeneous reaction mechanism. The inerting functional mechanism for homogeneouscombustion-dust essentially corresponds to the effect of heat-absorbing which helps to prevent further devolatilization. Nevertheless, the inerting mechanism for heterogeneous-reaction-dust is mainly associated with the synergism between oxygen diffusion resistance and thermal sink. CaCO3 can suppress the explosion of homogeneous-combustion-dust effectively, but it fails to represent excellent inerting effectiveness on heterogeneous-reaction-dust. For inerting of dust with heterogeneous reaction mechanism, NH4 H2PO4 represents higher effectiveness than CaCO3 because of better decomposability, excellent performance of oxygen blocking and particular efficacy of free ammonia in flame extinguishing. Moreover, H2O shows the best result because it has large specific heat, low gasification temperature, and above all, moisture can result in particle aggregation. References [1] Amyotte, P. R., 2006. Solid Inertants and Their Use in Dust Explosion Prevention and Mitigation, Journal of Loss Prevention in the process industries 19, p. 161. [2] Amytte, P. R., Pegg, M. J., Khan, F. I., 2009. Application of Inherent Safety Principles to Dust Explosion Prevention and Mitigation, Process Safety & Environment Protection 87, p. 35. [3] Kuai, N. S., Huang, W. X., Yuan, J. J., Du, B., Li, Z. S., Wu, Y., 2011. Experimental Investigation of Coal Dust-inertant Mixture Explosion Behaviors, Procedia Engineering 26, p. 1337. [4] Haghiri, A., Bidabadi, M., 2010. Modeling of Laminar Flame Propagation Through Organic Dust Cloud with Thermal Radiation Effect, International Journal of Thermal Sciences 49, p. 1446. [5] Benedetto, A. D., Russo, P., 2007. Thermo-kinetic modelling of dust explosions, Journal of Loss Prevention in the process industries 20, p. 303. [6] Benedetto, A. D., Russo, P., Amyotte, P., Marchand, N., 2010. Modelling the effect of particle size on dust explosions, Chemical Engineering Science 65, p. 772. [7] Dreizin, E. L., Hoffmann, V. K., 1999. Constant Pressure Combustion of Aerosol of Coarse Magnesium Particles in Microgravity, Combustion & Flame 118, p. 262. [8] Du, B., Huang, W.X., Kuai, N.S., Yuan, J.J., Li, Z.S., 2012. Experiment-based Investigation on Explosion Behaviors of Coal Dust-inertant Mixtures, Journal of Sichuan University (Engineering Science Edition) 44, Supp. 1, p. 229. [9] Yuan, J. J., Huang, W. X., Ji, H., Kuai, N. S., Wu, Y., 2012. Experimental Investigation of Dust MEC Measurement, Powder Technology 217, p. 245. [10] Eckhoff, R. K., 2003. Dust Explosions in the Process Industries (3rd ed.), Gulf Professional Publishing/Elsevier, Amsterdam, The Netherlands. [11] Hertzberg, M., Zlochower, I. A., Cashdollar, K. L., 1988. Volatility model for coal dust flame propagation and exti nguishment. Proceedings of the 21st symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp.325-333. [12] Vijayakumar C. T., Vinayagamoorthi S., Fink J. K., Sivasamy, P., 2006. Characterization of low rank alpine coals: Thermogravimetric studies, Journal of Analytical and Applied Pyrolysis 76, p. 191. [13] Zoller, D. L., Johnston M. V., 1999. Thermogravimetry-photoionization mass spectrometry of different rank coals, Energy & Fuels 13, p. 1097. [14] Dastidar, A.G., Amyotte, P. R., 2004. Using Calculated adiabatic flame temperatures to determine dust explosion inerting requirements, Process safety Environment 82, p. 142. [15] Chatrathi K., Going J., 2000. Dust deflagration extinction, Process safety Progress 19, p. 146. [16] Abbasi T., Abbasi S. A., 2007. Dust explosions - Cases, Causes, Consequences, and Control, Journal of Hazardous Materials 140, p. 7. [17] Zuo Q. M., Cheng W. M., Tang J. X., 2010. Current Status and Prospects of Application and Research of Powder Coal Mine Explosion Suppression Agent, Coal Technology 29, p. 78.
Procedia Engineering 43 (2012) 338 – 342
International Symposium on Safety Science and Engineering in China, 2012 (ISSSE-2012)
Experimental Investigation on Inerting Mechanism of Dust Explosion Bing Dua, Weixing Huanga,*, Niansheng Kuaia, Jingjie Yuana, Zongshan Lia, Yuan Gana a
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
Abstract
Systematic experiments were carried out on explosions of coal dust-inertant mixtures. Explosion severity was gained by using Siwek 20 L vessel and factors strongly influencing inerting effectiveness, such as deflagration mechanism and inertant composition were taken into account. Tests of thermal-gravimetric analysis were conducted to identify the reaction mechanism of coal dust explosions. As a result, bituminous coal dust deflagrates via the homogeneous mechanism, while anthracite dust mainly features the mechanism of heterogeneous reaction. Results reveal that the inerting functional mechanisms are quite different for bituminous coal and anthracite dusts, and the homogeneous-combustion-dust is much easier to be inhibited than the heterogeneous-reaction-dust. Trends of higher inerting effectiveness are associated with better decomposability, more excellent performance of oxygen blocking. Moreover, the particular efficacy of free ammonia decomposed from monoammonium phosphate in flame extinguishing is identified. Added water shows the best inerting effectives due to particle aggregation induced by moisture, which gives the idea that humidification for coal dust is therefore a cost-effective approach for explosion prevention and mitigation. © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Capital University of Economics and Business, China Academy of Safety Science and Technology. Keywords: Dust, Explsion, Inerting, Homogeneous mechanism, Heterogeneous mechanism
1. Introduction Inerting, to intimately premix the combustible dust with inertant prior to ignition, is recognized as an inherent safety approach in prevention and mitigation of dust explosions [1-3]. Various parameters influencing the effectiveness of inertants in explosion suppression have been investigated in previous studies. Unfortunately, although discussion on the inerting mechanism has been referred, the critical functional mechanism, which is of great benefit to seek for high-efficiency inertant for explosion suppression, is still not well understood. The inerting effectiveness of inertant is strongly affected by the explosion characteristic of combustible dust itself. Therefore, researches on the deflagration mechanism of combustible dust are urgently demanded as a basis for explosion prevention and mitigation. Even though several reaction mechanisms have been proposed to describe the dust flame propagation in previous studies [4-7], few methods are proposed to distinguish the deflagration mechanisms of different dusts accurately.
* Corresponding author. Tel.: +86-28-8540-5208; fax: +86-28-8540-3397. E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.058
Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
The present work aims to gain a comprehensive understanding of the explosion behavior of coal dust-inerant mixture, above all, to have an insight into the functional mechanism of inertant on explosion suppression. Explosion severity (maximum explosion pressure pmax and maximum rate of pressure rise (dp/dt)max) were systematically tested. Factors strongly influencing inerting effectiveness, such as deflagration mechanism of combustible dust and inertant properties (including specific heat, decomposition, decomposition temperature and other particular efficacies) were taken into account. This study would provide valuable guidance for improvement of inerting technology and help to seek for high-efficient inertants on dust explosion prevention. 2. Experimental 2.1. Materials and apparatus In the present study, bituminous coal and anthracite powders were employed as combustible dusts. Their particle size distributions and calorific values were illustrated in Table 1. All the tested samples were systematically dried at 50 °C under vacuum for 2 hours before handling. Experiments were performed in the well-known Siwek 20 L apparatus [3,8-9], which is an explosion resistance hollow sphere made of stainless steel in accordance with the recommendations of Chinese standard GB/T 16425, European standard EN 14034 and ASTM standard E 1226. Parameters of explosion severity measured in the chamber are maximum pressure pmax and maximum rate of pressure rise (dp/dt)max, representing the thermodynamic and kinetic of explosion energy liberation respectively. Table 1. Physiochemical properties of tested dust samples Particle size distribution (μm)
Calorific value (kJ.g-1)
Bituminous coal (sample A)
75-125
20.06
Anthracite (sample B)
75-125
30.97
Dust explosion is induced by a pyrotechnical ignitor which consists of zirconium, barium nitrate and barium dioxide by the weight ratio of 4:3:3. The energy released from ignitor is calculated based on the amount of pyrotechnical mixture. For example, the energy release of 1.2 g mixture corresponds to 5 kJ. Ignitors of 5 kJ are chosen in the present work, which is suggested by authors’ previous studies advocating the energy region of 5-10 kJ is the most appropriate for inerting testing [3,8]. 2.2. Inerting of dust samples In this inerting work, calcium carbonate (CaCO3), monoammonium phosphate (NH4 H2 PO4) and water (H2O) were chosen as inertants because they are widely used in explosion prevention and fire extinguishment. Moreover, the reported inertant mixing ratio is the percentage of inertant in the total combustible -inertant mixture. The combustible-inert mixtures were well premixed with a mixing ratio ranging from 0 to 70 %. 2.3. Reaction mechanism of dust explosion The homogeneous (vapor-phase) and heterogeneous mechanisms are two alternative deflagration mechanisms of dust explosions [10-11]. As described, homogeneous mechanism occurs through steps by heating, devolatilization, volatile-air mixing and homogeneous combustion [5-6]. On the contrary, heterogeneous combustion proceeds with oxygen dissolution in the surface of molten particles and no yield of volatile matter. That is, the liberation of volatile is a critical process in the homogeneous mechanism, which can be regarded as the criterion to distinguish the two different deflagration mechanisms. 3. Results and discussion 3.1. Deflagration mechanism analysis of coal samples Prior to discussing the inerting functional mechanism of inertant, thermal-gravimetric analysis (TG) was performed to give an evaluation on the deflagration mechanism of tested coal dust samples. For the TG experiments, coal samples were heated from 30 to 800 °C at a rate of 15 °C/min in both nitrogen and air atmospheres.
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Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
Typical TG curves of bituminous coal and anthracite s are shown in Fig. 1(a) and (b). As can be seen, small weight loss appears in all the coal samples at the low temperature region, which can be associated with losses of physically absorbed water and high-volatile organic matter. For bituminous coal, weight loss becomes significant when temperature exceeds 340 °Cin a nitrogen atmosphere, while the weight loss in the entire heating process reaches up to 67 % attributing to the volatilization. It is clear that devolatilization temperature of bituminous coal ranges from 340 to 600 °C. The volatilization may be due to the breaking of chemical bonds [12] and desorption of low molecular weights [13]. Furthermore, both the nitrogen and air results show the same ash content, which gives the idea that there is little fixed carbon in tested bituminous coal and the fuel of combustion is mainly contributed by volatile matter. Above-mentioned discussion means that the homogeneous combustion mechanism of bituminous coal is verified.
(a)
(b) Fig. 1. TG traces of (a) bituminous coal and (b) anthracite dust in both nitrogen and air atmosphere.
For anthracite, it is essential to point out that only a slight weight loss of 8 % is identified throughout the entire heating process in a nitrogen atmosphere. This behavior is attributed to the low volatile content in tested anthracite. Consequently, the char residue obtained is considered consisting of fixed carbon and ash [12]. Moreover, comparison is made between the nitrogen and air atmospheres, 79 % weight loss can be obviously observed in air atmosphere and the ash content turns out to be 21 % equaling to the weight left of char residue. This is because the combustion of anthracite mainly conducts on the oxidation of fixed carbon via surface direct oxidation and does not undergo the process of devolatilization. As a result, the deflagration mechanism of anthracite features the heterogeneous reaction essentially. 3.2. Inerting of coal dusts with different deflagration mechanisms As the deflagration mechanisms of tested coal samples were identified above, inerting effectiveness of CaCO3 on bituminous coal and anthracite dusts were carried out with the mixing ratio ranging from 0 to 70 %, under coal dust concentration of 500 g/m3.
(a)
(b)
Fig. 2. Evolutions of (a) pmax and (b) (dp/dt)max with CaCO3 mixing ratio for bituminous coal and anthracite dust with the concentration of 500 g/m3.
Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
In Fig. 2, evolutions of pmax and (dp/dt)max were plotted as functions of inertant mixing ratio for both two samples. As can be seen, the inerting effectiveness increases with the rise of inertant mixing ratio significantly. Two samples represent the similar explosion severity without added inertant in spite of the difference on calorific values. pmax of coal-inert mixture represents a gentle downtrend and then undergoes a sharply reduction when the inertant mixing ratio is greater than 50 % for anthracite. Nevertheless, values of pmax and (dp/dt)max for bituminous coal with only 20 % inertant drop to 33 % and 40 % corresponding to the pure coal results. This indicates that the bituminous coal dust with homogeneous combustion mechanism is much easier to be inhibited. For the homogeneous combustion process, the controlling step is pyrolysis/devolatilization (where volatiles are given off by particles) [6]. However, the added CaCO3 can effectively reduce the reaction region temperature by absorbing combustion heat released from burning particles, and then prevent further devolatilization of unburned particles. It follows that the volatilization efficiency of particles gets lower. Nevertheless, for anthracite dust with heterogeneous reaction, the explosion process is controlled by the kinetics of oxygen diffusion on the particle surface. As illustrated in Fig. 2, pmax value of anthracite-CaCO3 mixture somewhat shows a stabilization when CaCO3 mixing ratio ranges from 0 to 50 %, because CaCO3 with small addition amount is unable to cause high resistance of oxygen diffusion evidently. When the inertant mixing ratio is greater than 50 %, combustion heat is absorbed by inertant in a large extent so that there is no sufficient heat to sustain the propagation of combustion flame [14]. Hence, pmax value decreases sharply because particle temperature falls below the kindling temperature of anthracite. That is the reason why CaCO3 shows lower inerting effectiveness on anthracite dust than bituminous coal dust. 3.3. Influence of inertant properties The influence of inertant properties on inerting effectiveness was evaluated for anthracite-CaCO3, anthracite-NH4 H2PO4 and anthracite-H2O mixtures at the coal dust concentration of 250 g/m3. Evolutions of explosion severity were plotted as functions of inertant mixing ratio in Fig. 3. As can be seen, all curves have sudden changes along with the rise of inertant mixing ratio. Values of pmax decrease sharply in ranges of 10-30 %, 10-30 % and 50-70 %, with H2O, NH4 H2PO4 and CaCO3 as inertants respectively, while (dp/dt)max shows pronounced decrease in ranges of 10-30 %, 0-30 % and 60-70 % respectively. For the inerting effectiveness, H2O shows the best result, followed by NH4 H2PO4 and CaCO3.
(a)
(b)
Fig. 3. Evolutions of (a) pmax and (b) (dp/dt)max with CaCO3, NH4 H2 PO4 and H2O mixing ratio for anthracite dust with the concentration of 250 g/m3.
As mentioned previously, the sharp decrease of pmax is attributed to the heterogeneous reaction mechanism of anthracite in a large extent. Inerting of heterogeneous combustion mainly reflects in the synergism between increasing oxygen diffusion resistance and absorbing combustion heat. The difference of inerting effectiveness is attributed to the following reasons: z Firstly, heat-absorbing and inertant decomposition: The decomposition temperature of NH4 H2PO4 (190 °C) is much lower than CaCO3 (825 °C), whereas the kindling temperature of anthracite ranges from 500 to 620 °C, which indicates CaCO3 is somewhat nondecomposable in the explosion process [1]. As described by Chatrathi [15] and Abbasi [16], the decomposition of inertant plays a key role in inerting effectiveness; hence the efficient decomposition of NH4 H2PO4 results in a great thermal sink. However, the nondecomposable H2O represents the best excellent thermal sink due to large specific heat and low gasification temperature (only 100 °C).
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Bing Du et al. / Procedia Engineering 43 (2012) 338 – 342
z Secondly, oxygen diffusion resistance: Free ammonia released by NH4 H2PO4 plays a role to dilute oxygen and P2O5 decomposed from NH4 H2PO4 is prone to coating the burning particles, which results in higher resistance of oxygen diffusion and lower ignition sensitivity. The water-vapor not only forms a liquid membrane to cover the particles but also dilutes oxygen. The heterogeneous reaction does not self-sustain until both sufficient oxygen diffuses to the particle surface and the particle surface temperature is kept above the kindling temperature. Whereas CaCO3 seems helpless to increase the oxygen diffusion resistance and thereby its function mechanism mainly depends on heat-absorbing, which is the reason why CaCO3 represents bad inerting effectiveness. z Finally, particle aggregation: For H2O, particle aggregation induced by moisture will significantly decrease the specific surface, as well as reduce the reaction zone of heterogeneous combustion. It is essential to point out that (dp/dt)max of anthracite-NH4 H2PO4 mixture is even lower than anthracite-H2O mixture with 10 % mixing ratio. This is related to the particular efficacy of NH4 H2PO4 in flame extinguishing [17]. Moreover, considering that moisture will not cause the contamination of coal dust, humidification for coal dust is therefore regarded as a cost-effective approach for explosion prevention and mitigation. 4. Conclusions Systematic analysis on inerting functional mechanism of inertant has been preformed by evaluating the influences of deflagration mechanism of coal dust and inertant properties on coal dust-inertant mixture explosion behaviors. As a consequence, deflagration mechanism of dust explosion can be identified by using TG analysis. The homogeneous combustion mechanism features the release of volatile matter, whereas the heterogeneous reaction mechanism does not undergo the process of devolatilization. Results reveal that dust with homogeneous combustion mechanism is much easier to be inhibited than dust with heterogeneous reaction mechanism. The inerting functional mechanism for homogeneouscombustion-dust essentially corresponds to the effect of heat-absorbing which helps to prevent further devolatilization. Nevertheless, the inerting mechanism for heterogeneous-reaction-dust is mainly associated with the synergism between oxygen diffusion resistance and thermal sink. CaCO3 can suppress the explosion of homogeneous-combustion-dust effectively, but it fails to represent excellent inerting effectiveness on heterogeneous-reaction-dust. For inerting of dust with heterogeneous reaction mechanism, NH4 H2PO4 represents higher effectiveness than CaCO3 because of better decomposability, excellent performance of oxygen blocking and particular efficacy of free ammonia in flame extinguishing. 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