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Inhibiting effect of coal fly ash on minimum ignition temperature of coal dust clouds
Journal of Loss Prevention in the Process Industries 61 (2019) 24–29
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Inhibiting effect of coal fly ash on minimum ignition temperature of coal dust clouds
T
Hongkun Yua, Cheng Wanga,∗, Lei Pangb, Yangyang Cuia, Dongping Chena a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, PR China Department of Safety Engineering, Beijing Institute of Petro Chemical Technology, Beijing, 102617, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal fly ash Coal dust combustion Minimum ignition temperature Explosion suppression
The inhibiting effect of coal fly ash on the combustion of a coal dust cloud was studied for three typical types of coal: anthracite, bituminous coal, and lignite. Using a Godbert–Greenwald furnace, the minimum ignition temperature (MIT) of coal dust clouds containing various quantities of coal fly ash was measured. The results show that the MIT of coal dust clouds increases linearly with increasing quantities of coal fly ash. Compared with calcium carbonate as a common inhibitor, coal fly ash was found to have a better inhibiting effect owing to its larger heat capacity. Furthermore, at the same concentration of coal fly ash, the larger the coal dust particle size, the higher the MIT of the coal dust cloud. Moreover, at the same concentration of coal fly ash, the higher the content of volatile matter in the coal dust, the lower the MIT of the coal dust cloud. Coal fly ash has the strongest inhibiting effect on lignite, the second strongest inhibiting effect on bituminous coal, and the weakest inhibiting effect on anthracite. Combining all the factors examined in this work, a simple empirical model is presented that provides a reasonable estimation of MIT.
1. Introduction During the production, processing, storage, or transportation of coal, large quantities of coal dust are often created, and micro-sized coal dust readily disperses in the surroundings. When a coal dust cloud with a certain concentration comes into contact with a high-temperature surface of a piece of equipment (or an open flame), combustion and deflagration can easily occur, which can lead to safety problems. Such an incident often results in serious casualties and property loss. To prevent such incidents and mitigate the associated risks, explosion parameters such as the maximum explosion pressure (Pmax), minimum ignition energy (MIE), and minimum ignition temperature (MIT) need to be determined (Addai et al., 2016). The MIT is an important parameter of dust combustion and explosion, which is an important index for evaluating dust sensitivity and risk. The lower the MIT of a coal dust cloud, the higher the risk of dust combustion. In industrial production, appropriate preventive measures should be taken to ensure that the surface temperature and ambient temperature of equipment are lower than the MIT of a coal dust cloud, which will minimise the risk of coal dust combustion and explosion. The suppression technique of using retardant powders is the most common method of suppressing dust explosions, as it intrinsically inhibits the combustion process. This technique requires the addition of a ∗
retardant powder to the combustible dust (Amyotte, 2006; Amyotte et al., 2009), and the key lies in mixing retardant materials with combustible dust to reduce its ignition sensitivity (Amyotte and Eckhoff, 2010; Agnes and Douglas, 2013). At coal mines, rock powder is scattered along roadways to suppress dust explosions. Furthermore, when a pulverised coal injection process is applied in thermal power plants, inert dust and pulverised coal are often blended to reduce the danger of a coal dust explosion. Many researchers have conducted theoretical and experimental studies on dust explosion suppression. Du et al. (2012) performed systematic experiments on explosions of coal dust and inert dust mixtures and concluded that factors such as the deflagration mechanism and the inert material composition strongly influenced the suppression effectiveness. Their results revealed that the inerting functional mechanisms of bituminous coal dust and anthracite dust were quite different, and that the inhibition of dust combusted via a homogeneous mechanism is much easier than that of dust combusted via a heterogeneous reaction. Binkau et al. (2015) investigated the effect of inert substances on the auto-ignition temperature of coal dust. Their results indicated that the auto-ignition temperature of coal dust increased upon mixing the coal dust with an inert material with a higher decomposition temperature. In comparison, the auto-ignition temperature decreased when the exothermic decomposition of the inert material occurred at a temperature
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.jlp.2019.05.018 Received 17 January 2019; Received in revised form 29 May 2019; Accepted 29 May 2019 Available online 30 May 2019 0950-4230/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Loss Prevention in the Process Industries 61 (2019) 24–29
H. Yu, et al.
lower than the auto-ignition temperature of coal dust. Addai et al. (2016) examined the influence of adding inert dust on the MIE and MIT of combustible dust mixtures, and concluded that the permissible range for the inert mixture to minimise the ignition risk lies between 60% and 80%. Danzi et al. (2015) studied the effects of two types of inert dust, limestone and fire-extinguishing powder, on the MIT of combustible dust. They found that limestone had a limited influence on the MIT. Kuai et al. (2011) compared the suppression effects of CaCO3 and NH4H2PO4 on dust explosions. They observed that the inerting effectiveness of NH4H2PO4 was higher than that of CaCO3 because of its efficient decomposition and particular flame extinguishing mechanism. Similarly, Wu et al. (2016) measured the MIT of coal dust clouds in atmospheric O2–CO2 mixtures, and found that CO2 had a strong inhibiting effect, especially on coal dust with a high content of volatile matter. In previous studies, the technique of dust suppression focussed mainly on common inhibitors such as CaCO3, BaCO3, SiO2, and NH4H2PO4. However, no previous work has focussed on suppressing coal dust explosions by making use of coal fly ash. Coal fly ash is a type of fine powder collected from the exhaust gas after coal combustion, and it is the main solid waste discharged from thermal power plants. Coal fly ash has a honeycomb structure that has a large specific surface area and high adsorption activity. The main components of coal fly ash are SiO2, Al2O3, and CaO, and it has inert characteristics similar to rock powder. The advantages of this material are its low cost, availability, and recyclability. Therefore, it is a candidate as an inert additive for the suppression of coal dust combustion and explosion. In this study, coal fly ash was tested as an inert medium and inhibitor of coal dust combustion, and its influence on the MIT of coal dust clouds was examined in detail.
Table 2 Composition analysis of coal dust samples.
2.2. Experimental apparatus The experiments for measuring the ignition temperatures of dust clouds were performed using the HY 16429 apparatus (Fig. 1). This apparatus includes mainly a control box, a constant-temperature furnace, and a dust cloud generator. The measurement range of this equipment is 273–1273 K with a temperature resolution of 0.25 K. The injection pressure of the dust can be varied from 0 to 0.1 bar, and the volume of the constant-temperature furnace is 220 mL.
case #4 (106–198)
case #5 (198–270)
Anthracite Bituminous coal Lignite
12.70 9.313 11.30
37.76 27.51 38.38
76.93 65.30 75.71
121.7 159.5 126.6
236.3 256.9 211.1
Fixed carbon (%)
Anthracite Bituminous coal Lignite
4.41 0.80 30.36
10.64 33.37 10.32
8.29 20.62 46.43
76.66 45.21 12.89
3. Results and discussion 3.1. Effect of coal fly ash on MIT of coal dust cloud The MIT of the coal dust cloud varied with different concentrations of coal fly ash. The bituminous coal dust sample with a median diameter of 9.313 μm was selected to examine the effect in detail. In coal dust cloud ignition experiments, there is a mass concentration at which the coal dust is the most reactive, meaning the coal dust cloud can react completely with oxygen. Furthermore, there is an optimal spraying pressure where the coal dust sinks at the most suitable settling speed, and makes the dust cloud evenly distributed. Based on previous experimental results, the most reactive mass concentration and optimal dust pressure were selected to be 1.818 g/L and 0.5bar, respectively. Fig. 2 shows the effect of coal fly ash concentration on the MIT of coal dust cloud, which reveals that the MIT of the coal dust cloud
Table 1 Median diameters of coal dust samples (in μm). case #3 (61–106)
Volatiles (%)
The MITs of coal dust clouds were determined according to the measurement procedures defined in EN ISO/IEC 80079-20-2 (British Standards Institution, 2016) and GB/T 16429–1996 (State Bureau of Technical Supervision, 1996). A coal dust sample was weighed and placed in the dust container, and the furnace temperature and dust injection pressure were set to 773 K and 0.5 bar, respectively. The solenoid valve was opened to spray the dust sample into the heating furnace with high-pressure gas. The criterion for dust ignition was the observation of a combustion flame at the bottom opening of the furnace. If no combustion flame was observed after the first injection, the furnace temperature was increased by 50 K and the experiment was repeated until ignition occurred. Once the ignition temperature was reached, the dust would burn, in which case a bright flame could be observed. Subsequently, the furnace temperature was decreased by 20 K and the MIT was continuously measured. If ignition still occurred at 573 K, the furnace temperature was lowered by 10 K for 10 more tests. During the powder injection, the furnace temperature could be disturbed by cold air. However, the powder injection was sufficiently fast that the cooling time was very short, and the influence on the furnace temperature by the powder injection was considered negligible.
In this study, three types of typical flammable coal dust were used: anthracite, bituminous coal, and lignite. Five representative samples were selected for each type of coal dust. Analysis of particle size was performed using a BT-9300LD laser analyser to obtain the median diameter of each sample; these median diameters are listed in Table 1. Parameters for composition of the samples are listed in Table 2. Coal fly ash was obtained from a thermal power plant in Inner Mongolia (China); the median diameter of the coal fly ash particles was 14.50 μm. Calcium carbonate (a commercial reagent) that had a median diameter of 11.23 μm was also used as an inhibitor for comparison purposes.
case #2 (25–61)
Ash (%)
2.3. Experimental process
2.1. Experimental samples
case #1 (< 25)
Moisture (%)
Fig. 1. Schematic illustration of HY 16429 apparatus used for measuring ignition temperature of dust clouds.
2. Experimental section
Coal sample
Coal sample
Note: All values are in μm. 25
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Fig. 2. MIT of coal dust cloud at different concentrations of coal fly ash.
Fig. 3. Increment in MIT of coal dust cloud with increasing concentration of coal fly ash.
increases linearly with increasing concentration of coal fly ash. During the combustion of coal dust, its temperature rises and the reactions begin with volatiles. The addition of coal fly ash reduces the temperature of the heating furnace and limits the radiative heating of the coal dust particles. As a result, the combustion of coal dust is inhibited. Furthermore, coal fly ash can play an important role in shielding, whereby the radiative heat, heat conduction, and flame propagation between the coal dust particles are blocked. These shielding effects also cause an increase in the MIT of coal dust clouds (Liberman et al., 2015a,b). As the addition of coal fly ash has only a physical inhibiting effect, the heat absorbed by the coal fly ash is linearly proportional to its concentration. Fig. 2 indicates that the MIT of the coal dust cloud and the concentration of coal fly ash can be linearly correlated by Equation (1): Tmin = aw + b
(1)
where Tmin is the MIT of the coal dust cloud, in K; w is the mass concentration of coal fly ash, in %; a is a coefficient; and b is the MIT of the coal dust cloud when coal fly ash is not added, in K.
Fig. 4. MIT of coal dust cloud as a function of concentrations of coal fly ash and calcium carbonate.
was fixed at 0.4 g, and the influence of the inert dust concentration on the MIT was studied to evaluate the inhibiting effect of coal fly ash. Fig. 4 shows that the change trend of the MIT of the coal dust cloud with the concentration of coal fly ash is roughly the same as that with the concentration of calcium carbonate, the MIT increases with increasing concentration of the inert dust. However, the inhibiting effect of coal fly ash is stronger than that of calcium carbonate; in quantitative terms, the MIT at specific concentrations of coal fly ash is approximately 10 K higher than that the MIT at the same concentrations of calcium carbonate. The reason for this difference is the composition of coal fly ash, whose main components include SiO2, Al2O3, and CaO. As stated previously, both coal fly ash and calcium carbonate inhibit the MIT of the coal dust cloud by the same mechanism: heat absorption in the reacting medium. Furthermore, the quantity of heat absorbed by an inert material is proportional to its specific heat capacity. The specific heat capacity of coal fly ash (0.92 kJ/(kg·K)) is slightly higher than that of calcium carbonate (0.84 kJ/(kg·K)). Thus, coal fly ash will have a stronger inhibiting effect than calcium carbonate. Moreover, coal fly ash contains Al2O3, which can distribute evenly on the surface of pulverised coal particles during heating and form a dense protective film to hinder heat transfer, oxygen diffusion, and release of volatiles. Hindrance of all these phenomena has the effect of impeding the combustion of coal dust, which explains the superior inhibition performance of coal fly ash.
Linear fitting of the data as shown in Fig. 2 gives a = 0.424, b = 799, and R2 = 0.9918. The R2 value is a goodness-of-fit index, and a R2 value approximating 1 indicates a high degree of correlation. It is noted from Fig. 3 that the MIT of the coal dust cloud increases rapidly when the concentration of coal fly ash reaches a critical value. At concentrations higher than this critical value, the inert dust shows a better suppression effect. This behaviour provides a basis for explosion protection in actual production, namely that the concentration of the inert medium added to coal dust should exceed the critical value to achieve effective suppression at a production site. Fig. 3 indicates that the increment in the MIT increases to a coal fly ash concentration of 60%, which in this case is the critical concentration. However, coal fly ash cannot completely inhibit the burning of coal dust. 3.2. Effects of coal fly ash and calcium carbonate on MIT of coal dust cloud According to a literature review, techniques for dust suppression employ mainly CaCO3, BaCO3, SiO2, and NH4H2PO4. Coal fly ash has a composition similar to that of calcium carbonate and has additional desirable characteristics such as being resistant to burning and chemically inactive. To compare the inhibiting effects of coal fly ash and calcium carbonate, experiments were performed using bituminous coal with a median diameter of 27.51 μm. Again, the mass of coal powder 26
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quantity of coal dust involved in the oxidation reaction is smaller. All these factors together result in incomplete combustion of coal dust, which makes ignition of the coal dust cloud difficult; therefore, the MITs in these cases are higher. Coal dust with a smaller particle size, as in cases #1 and #2, has a larger specific surface area, which results in a lower MIT, as shown in Fig. 5. Therefore, a larger quantity (higher concentration) of coal fly ash is required for coal dust with a smaller particle size to reach a particular MIT than is required for coal dust with a larger particle size. The influence of coal fly ash on the MITs of coal dust clouds with coal dust particles of five different sizes was also studied; for all five particle sizes, the linear correlation in Equation (1) held true. Because the values of the coefficient a for the five cases were extremely close, the average a value of 0.415 was used for all five cases. Then, the fitted Equation (1) could be rewritten as Tmin = 0.415w + b
(2)
The five fitted lines yielded different b values, where b denotes the MIT of pure coal dust without coal fly ash. As stated previously, the b value is related to the particle size of coal dust. Linear regression yields
Fig. 5. MIT of coal dust cloud with different particle size.
b = 818.5 - (21.03 × 0.99d)
3.3. Effect of coal fly ash on MIT of coal dust cloud with coal dust particles of different sizes
(3)
where d is the median diameter of the coal dust particles (in μm) and R2 = 0.959. The particle size itself has a great influence on the combustion of coal dust. Formation of a dust cloud from large dust particles is difficult because of the gravity effect, which causes the particles to settle. Coal dust with particle size smaller than 1000 μm rapidly participates in the combustion reaction, and the smaller the dust particles, the higher the risk of dust explosions. Therefore, the MIT of the coal dust cloud increases with increasing coal dust particle size up to a certain value. This is consistent with the conclusion of Equation (3).
Five types of bituminous coal dust samples with different particle sizes were selected for the experiments. The corresponding median diameters are listed in Table 1. The mass of coal dust was fixed at 0.4 g. The sensitivity of the MITs of the examined coal dust samples to particle sizes is shown in Fig. 5. The inhibiting tendencies of coal fly ash on the MITs of coal dust clouds with coal dust particles of five different sizes are similar; the MITs increase linearly with increasing concentration of coal fly ash. It can also be observed that a larger particle size corresponds to a lower critical concentration of coal fly ash. The critical concentration of coal fly ash for cases #3, #4, and #5 is 40%, and increases to 60% for cases #1 and #2 (Fig. 6). These values can be explained by the fact that the MIT of the coal dust cloud increases with increasing coal dust particle size (Turns, 2009). This is because both the ignition and the combustion of coal dust occur on the surface of its particles, and that larger particles require a longer heating time for devolatilisation to occur rapidly (intraparticle effect) (Zhang and Wall, 1993). In cases #3, #4, and #5, the particle size is larger, the specific surface area is smaller, the contact area between the coal dust particles and oxygen is smaller, and the
3.4. Effect of coal fly ash on MITs of different types of coal dust Dust samples of three types of flammable coal, anthracite, bituminous coal, and lignite, were selected in this study to evaluate the influence of coal fly ash on the MIT. The particle sizes of the three types of coal corresponded to those of case #2 in Table 1. As shown in Fig. 7, the optimal mass concentration that induces the ignition of a coal dust cloud varies with the type of coal (namely, the volatile matter content). The optimal values are 1.818, 1.818, and 1.363 g/L for anthracite, bituminous coal, and lignite, respectively (Fig. 7). The inhibiting effects of coal fly ash on the three different coal dusts were then studied, and
Fig. 6. Increment in MIT of coal dust cloud with increase in particle size of coal dust.
Fig. 7. MITs of coal dust clouds formed from three types of coal dusts with masses in the range of 0.1–0.5 g. 27
Journal of Loss Prevention in the Process Industries 61 (2019) 24–29
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involves mainly solid carbon. Its combustion mechanism involves surface heterogeneous oxidation, and the reaction is sustained primarily by the diffusion of oxygen onto the particle surface. The addition of a small quantity of coal fly ash cannot effectively suppress the surface heterogeneous reaction, whereas the addition of a large quantity of coal fly ash prevents the diffusion of oxygen, which consequently leads to a lower burning rate. Lignite contains a large quantity of volatiles and a limited quantity of solid carbon. The combustion of lignite is a gasphase reaction in which the volatiles rapidly react with air and burn. When coal fly ash is blended with lignite, the temperature of the reaction zone decreases because of the heat absorption by the coal fly ash; thus, it effectively suppresses the combustion of the coal dust. The combustion of bituminous coal is also dominantly a gas-phase reaction, but its volatile matter content is lower than that of lignite, which results in a lower yield of volatiles during dust combustion. Therefore, at the same concentration of coal fly ash, the inhibiting effect on bituminous coal is weaker than on lignite. It can be seen from the fitted lines show that the value of coefficient a is different for the different types of coal dust, and that it appears to be correlated with the volatile matter content of the coal dust (Fig. 8). The correlation between the MIT and the volatile matter content of the coal dust cloud can be expressed as
Fig. 8. MITs of coal dust clouds formed from three types of coal dusts.
the results are shown in Fig. 8. It can be seen that regardless of the type of coal dust (anthracite, bituminous coal, or lignite), the MIT of the coal dust cloud increases linearly with increasing concentration of coal fly ash. However, the MIT of the coal dust cloud even without coal fly ash varies with the type of coal dust (Fig. 8 at zero concentration). This behaviour is attributed to the significant difference in the quantity of volatiles present in each type of coal dust sample. In the furnace, the coal dust is first heated to release volatiles, which ignite the coal dust in a high-temperature medium. The heat generated during the combustion of volatiles is conducted to the coal dust particles and it further ignites the coal dust. The experimental results show that the MIT of the coal dust cloud decreases with increasing volatile matter content of coal dust. When the volatile matter content of coal dust reduces to a critical value, ignition of the dust becomes difficult (Du et al., 2012). A larger quantity of flammable gas is released from the particle surface of coal dust with a higher volatile matter content upon heating, and as a result the dust can be ignited at a lower temperature. From Figs. 8 and 9, it is found that coal fly ash has different effects on the MITs of the three types of coal dusts tested in this study. It has the strongest inhibiting effect on lignite, the second strongest inhibiting effect on bituminous coal, and the weakest inhibiting effect on anthracite. The critical concentrations of coal fly ash for anthracite, bituminous coal, and lignite are 80%, 60%, and 60%, respectively (Fig. 9). Anthracite has low volatility and its combustion reaction
Tmin = kw + b
(4)
where k is a coefficient whose value depends on the volatile matter content. Because only three types of coal dust were considered in this study, it is not possible to establish the exact correlation between k and the volatile matter content. Therefore, for fitting analysis of the data in Fig. 8, three different values of k were used. From the above-described results, it is ascertained that the MIT of a coal dust cloud is related to the particle size of coal dust, the volatile matter content of coal dust, and the concentration of coal fly ash. Through substitution of Equations (3) and (4) into Equation (1), all these three influencing factors can be combined to obtain a single correlation: Tmin = kw + 818.5 - (21.03 × 0.99d)
(5)
Substitution of k = 0.40, d = 27.51, and w = 20 in Equation (5) gives a Tmin value of 810 K. The error between this predicted value and that the value determined experimentally is only 1.0%. Similarly, the predicted Tmin is 751 K when k is taken to be 0.60; in this case, the experimental MIT is 816 K, and the prediction error is 8.6%. Based on these two simple comparisons, the empirical model expressed by Equation (5) appears to predict the MIT of a coal dust cloud with reasonable accuracy.
4. Conclusion This work examines in detail the influence of coal fly ash on the MIT of a coal dust cloud, and the effects of coal fly ash concentration, particle size of coal dust, and type of coal dust on the MIT. The findings show that the MIT of a coal dust cloud increases linearly with increasing concentration of coal fly ash. Both coal fly ash and calcium carbonate have similar inhibiting effects on the MIT of a coal dust cloud. However, the inhibiting effect of coal fly ash is slightly stronger, owing to its higher specific heat capacity. Furthermore, it is found that the MITs of coal dust clouds with coal dust particles of different sizes are linearly proportional to the concentration of coal fly ash, and that the critical concentration of the coal fly ash gradually decreases with increasing coal dust particle size. Experiments on three different types of coal dust—anthracite, bituminous coal, and lignite—reveal that the MIT of a coal dust cloud is also dependent on the volatile matter content of the coal dust. A simple empirical model was established to reflect the effects of the concentration of coal fly ash, particle size of coal dust, and volatile matter content of coal dust on the MIT of a coal dust cloud.
Fig. 9. Increments in MITs of coal dust clouds formed from three types of coal dusts with increasing concentration of coal fly ash. 28
Journal of Loss Prevention in the Process Industries 61 (2019) 24–29
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Acknowledgements
IEC 80079-20-2. Danzi, E., Marmo, L., Riccio, D., 2015. Minimum ignition temperature of layer and cloud dust mixtures. J. Loss Prev. Process. Ind. 36, 326–334. Du, B., Huang, W.X., Kuai, N.S., Yuan, J.J., Li, Z.S., Gan, Y., 2012. Experimental investigation on inerting mechanism of dust explosion. Procedia Eng. 43, 338–342. Kuai, N.S., Huang, W.X., Yuan, J.J., Du, B., Li, Z.S., Wu, Y., 2011. Experimental investigations of coal dust-inertant mixture explosion behaviors. Procedia Eng. 26, 1337–1345. Liberman, M.A., Ivanov, M.F., Kiverin, A.D., 2015a. Radiation heat transfer in particleladen gaseous flame: flame Acceleration and Triggering Detonation. Acta Astronaut. 115, 82–93. Liberman, M.A., Ivanov, M.F., Kiverin, A.D., 2015b. Effects of thermal radiation heat transfer on flame acceleration and transition to detonation in particle-cloud hydrogen flames. J. Loss Prev. Process. Ind. 38, 176–186. State Bureau of Technical Supervision, 1996. GB/T 16429-1996: Determination of the Minimum Ignition Temperature of Dust Cloud. Ministry of Coal Industry in People's Republic of China, Beijing GB/T 16429-1996. Turns, S.R., 2009. An Introduction to Combustion: Concepts and Applications. The McGraw-Hill Education (Asia) Co. and Tsinghua University Press, pp. 429–430. Wu, D.J., Normanb, F., Verplaetsenb, F., Bulck, E.V.D., 2016. Experimental study on the minimum ignition temperature of coal dust clouds in oxy-fuel combustion atmospheres. J. Hazard Mater. 307, 274–280. Zhang, D.K., Wall, T.F., 1993. An analysis of the ignition of coal dust clouds. Combust. Flame. 92, 475–480.
This research was supported by the National Natural Science Foundation of China under Grant No. 11732003. References Addai, E.K., Gabel, D., Krause, U., 2016. Experimental investigations of the minimum ignition energy and the minimum ignition temperature of inert and combustible dust cloud mixtures. J. Hazard Mater. 307, 302–311. Agnes, J., Douglas, C., 2013. Effect of inerts on ignition sensitivity of dusts. In: LP201314th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, vol. 31. pp. 829–834. Amyotte, P.R., 2006. Solid inertants and their use in dust explosion prevention and mitigation. J. Loss Prev. Process. Ind. 19, 161–173. Amyotte, P.R., Pegg, M.J., Khan, F.I., 2009. Application of inherent safety principles to dust explosion prevention and mitigation. Process Saf. Environ. Protect. 87, 35–39. Amyotte, P.R., Eckhoff, R.K., 2010. Dust explosion causation, prevention and mitigation: an overview. J. Chem. Health Saf. 17, 15–28. Binkau, B., Wanke, C., Krause, U., 2015. Influence of inert materials on the self-ignition of flammable dusts. J. Loss Prev. Process. Ind. 36, 335–342. British Standards Institution, 2016. EN ISO/IEC 80079-20-2: Explosive Atmospheres-Part 20-2: Material Characteristics-Combustible Dusts Test Methods. BSI, London EN ISO/
29
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Inhibiting effect of coal fly ash on minimum ignition temperature of coal dust clouds
T
Hongkun Yua, Cheng Wanga,∗, Lei Pangb, Yangyang Cuia, Dongping Chena a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, PR China Department of Safety Engineering, Beijing Institute of Petro Chemical Technology, Beijing, 102617, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal fly ash Coal dust combustion Minimum ignition temperature Explosion suppression
The inhibiting effect of coal fly ash on the combustion of a coal dust cloud was studied for three typical types of coal: anthracite, bituminous coal, and lignite. Using a Godbert–Greenwald furnace, the minimum ignition temperature (MIT) of coal dust clouds containing various quantities of coal fly ash was measured. The results show that the MIT of coal dust clouds increases linearly with increasing quantities of coal fly ash. Compared with calcium carbonate as a common inhibitor, coal fly ash was found to have a better inhibiting effect owing to its larger heat capacity. Furthermore, at the same concentration of coal fly ash, the larger the coal dust particle size, the higher the MIT of the coal dust cloud. Moreover, at the same concentration of coal fly ash, the higher the content of volatile matter in the coal dust, the lower the MIT of the coal dust cloud. Coal fly ash has the strongest inhibiting effect on lignite, the second strongest inhibiting effect on bituminous coal, and the weakest inhibiting effect on anthracite. Combining all the factors examined in this work, a simple empirical model is presented that provides a reasonable estimation of MIT.
1. Introduction During the production, processing, storage, or transportation of coal, large quantities of coal dust are often created, and micro-sized coal dust readily disperses in the surroundings. When a coal dust cloud with a certain concentration comes into contact with a high-temperature surface of a piece of equipment (or an open flame), combustion and deflagration can easily occur, which can lead to safety problems. Such an incident often results in serious casualties and property loss. To prevent such incidents and mitigate the associated risks, explosion parameters such as the maximum explosion pressure (Pmax), minimum ignition energy (MIE), and minimum ignition temperature (MIT) need to be determined (Addai et al., 2016). The MIT is an important parameter of dust combustion and explosion, which is an important index for evaluating dust sensitivity and risk. The lower the MIT of a coal dust cloud, the higher the risk of dust combustion. In industrial production, appropriate preventive measures should be taken to ensure that the surface temperature and ambient temperature of equipment are lower than the MIT of a coal dust cloud, which will minimise the risk of coal dust combustion and explosion. The suppression technique of using retardant powders is the most common method of suppressing dust explosions, as it intrinsically inhibits the combustion process. This technique requires the addition of a ∗
retardant powder to the combustible dust (Amyotte, 2006; Amyotte et al., 2009), and the key lies in mixing retardant materials with combustible dust to reduce its ignition sensitivity (Amyotte and Eckhoff, 2010; Agnes and Douglas, 2013). At coal mines, rock powder is scattered along roadways to suppress dust explosions. Furthermore, when a pulverised coal injection process is applied in thermal power plants, inert dust and pulverised coal are often blended to reduce the danger of a coal dust explosion. Many researchers have conducted theoretical and experimental studies on dust explosion suppression. Du et al. (2012) performed systematic experiments on explosions of coal dust and inert dust mixtures and concluded that factors such as the deflagration mechanism and the inert material composition strongly influenced the suppression effectiveness. Their results revealed that the inerting functional mechanisms of bituminous coal dust and anthracite dust were quite different, and that the inhibition of dust combusted via a homogeneous mechanism is much easier than that of dust combusted via a heterogeneous reaction. Binkau et al. (2015) investigated the effect of inert substances on the auto-ignition temperature of coal dust. Their results indicated that the auto-ignition temperature of coal dust increased upon mixing the coal dust with an inert material with a higher decomposition temperature. In comparison, the auto-ignition temperature decreased when the exothermic decomposition of the inert material occurred at a temperature
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.jlp.2019.05.018 Received 17 January 2019; Received in revised form 29 May 2019; Accepted 29 May 2019 Available online 30 May 2019 0950-4230/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Loss Prevention in the Process Industries 61 (2019) 24–29
H. Yu, et al.
lower than the auto-ignition temperature of coal dust. Addai et al. (2016) examined the influence of adding inert dust on the MIE and MIT of combustible dust mixtures, and concluded that the permissible range for the inert mixture to minimise the ignition risk lies between 60% and 80%. Danzi et al. (2015) studied the effects of two types of inert dust, limestone and fire-extinguishing powder, on the MIT of combustible dust. They found that limestone had a limited influence on the MIT. Kuai et al. (2011) compared the suppression effects of CaCO3 and NH4H2PO4 on dust explosions. They observed that the inerting effectiveness of NH4H2PO4 was higher than that of CaCO3 because of its efficient decomposition and particular flame extinguishing mechanism. Similarly, Wu et al. (2016) measured the MIT of coal dust clouds in atmospheric O2–CO2 mixtures, and found that CO2 had a strong inhibiting effect, especially on coal dust with a high content of volatile matter. In previous studies, the technique of dust suppression focussed mainly on common inhibitors such as CaCO3, BaCO3, SiO2, and NH4H2PO4. However, no previous work has focussed on suppressing coal dust explosions by making use of coal fly ash. Coal fly ash is a type of fine powder collected from the exhaust gas after coal combustion, and it is the main solid waste discharged from thermal power plants. Coal fly ash has a honeycomb structure that has a large specific surface area and high adsorption activity. The main components of coal fly ash are SiO2, Al2O3, and CaO, and it has inert characteristics similar to rock powder. The advantages of this material are its low cost, availability, and recyclability. Therefore, it is a candidate as an inert additive for the suppression of coal dust combustion and explosion. In this study, coal fly ash was tested as an inert medium and inhibitor of coal dust combustion, and its influence on the MIT of coal dust clouds was examined in detail.
Table 2 Composition analysis of coal dust samples.
2.2. Experimental apparatus The experiments for measuring the ignition temperatures of dust clouds were performed using the HY 16429 apparatus (Fig. 1). This apparatus includes mainly a control box, a constant-temperature furnace, and a dust cloud generator. The measurement range of this equipment is 273–1273 K with a temperature resolution of 0.25 K. The injection pressure of the dust can be varied from 0 to 0.1 bar, and the volume of the constant-temperature furnace is 220 mL.
case #4 (106–198)
case #5 (198–270)
Anthracite Bituminous coal Lignite
12.70 9.313 11.30
37.76 27.51 38.38
76.93 65.30 75.71
121.7 159.5 126.6
236.3 256.9 211.1
Fixed carbon (%)
Anthracite Bituminous coal Lignite
4.41 0.80 30.36
10.64 33.37 10.32
8.29 20.62 46.43
76.66 45.21 12.89
3. Results and discussion 3.1. Effect of coal fly ash on MIT of coal dust cloud The MIT of the coal dust cloud varied with different concentrations of coal fly ash. The bituminous coal dust sample with a median diameter of 9.313 μm was selected to examine the effect in detail. In coal dust cloud ignition experiments, there is a mass concentration at which the coal dust is the most reactive, meaning the coal dust cloud can react completely with oxygen. Furthermore, there is an optimal spraying pressure where the coal dust sinks at the most suitable settling speed, and makes the dust cloud evenly distributed. Based on previous experimental results, the most reactive mass concentration and optimal dust pressure were selected to be 1.818 g/L and 0.5bar, respectively. Fig. 2 shows the effect of coal fly ash concentration on the MIT of coal dust cloud, which reveals that the MIT of the coal dust cloud
Table 1 Median diameters of coal dust samples (in μm). case #3 (61–106)
Volatiles (%)
The MITs of coal dust clouds were determined according to the measurement procedures defined in EN ISO/IEC 80079-20-2 (British Standards Institution, 2016) and GB/T 16429–1996 (State Bureau of Technical Supervision, 1996). A coal dust sample was weighed and placed in the dust container, and the furnace temperature and dust injection pressure were set to 773 K and 0.5 bar, respectively. The solenoid valve was opened to spray the dust sample into the heating furnace with high-pressure gas. The criterion for dust ignition was the observation of a combustion flame at the bottom opening of the furnace. If no combustion flame was observed after the first injection, the furnace temperature was increased by 50 K and the experiment was repeated until ignition occurred. Once the ignition temperature was reached, the dust would burn, in which case a bright flame could be observed. Subsequently, the furnace temperature was decreased by 20 K and the MIT was continuously measured. If ignition still occurred at 573 K, the furnace temperature was lowered by 10 K for 10 more tests. During the powder injection, the furnace temperature could be disturbed by cold air. However, the powder injection was sufficiently fast that the cooling time was very short, and the influence on the furnace temperature by the powder injection was considered negligible.
In this study, three types of typical flammable coal dust were used: anthracite, bituminous coal, and lignite. Five representative samples were selected for each type of coal dust. Analysis of particle size was performed using a BT-9300LD laser analyser to obtain the median diameter of each sample; these median diameters are listed in Table 1. Parameters for composition of the samples are listed in Table 2. Coal fly ash was obtained from a thermal power plant in Inner Mongolia (China); the median diameter of the coal fly ash particles was 14.50 μm. Calcium carbonate (a commercial reagent) that had a median diameter of 11.23 μm was also used as an inhibitor for comparison purposes.
case #2 (25–61)
Ash (%)
2.3. Experimental process
2.1. Experimental samples
case #1 (< 25)
Moisture (%)
Fig. 1. Schematic illustration of HY 16429 apparatus used for measuring ignition temperature of dust clouds.
2. Experimental section
Coal sample
Coal sample
Note: All values are in μm. 25
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Fig. 2. MIT of coal dust cloud at different concentrations of coal fly ash.
Fig. 3. Increment in MIT of coal dust cloud with increasing concentration of coal fly ash.
increases linearly with increasing concentration of coal fly ash. During the combustion of coal dust, its temperature rises and the reactions begin with volatiles. The addition of coal fly ash reduces the temperature of the heating furnace and limits the radiative heating of the coal dust particles. As a result, the combustion of coal dust is inhibited. Furthermore, coal fly ash can play an important role in shielding, whereby the radiative heat, heat conduction, and flame propagation between the coal dust particles are blocked. These shielding effects also cause an increase in the MIT of coal dust clouds (Liberman et al., 2015a,b). As the addition of coal fly ash has only a physical inhibiting effect, the heat absorbed by the coal fly ash is linearly proportional to its concentration. Fig. 2 indicates that the MIT of the coal dust cloud and the concentration of coal fly ash can be linearly correlated by Equation (1): Tmin = aw + b
(1)
where Tmin is the MIT of the coal dust cloud, in K; w is the mass concentration of coal fly ash, in %; a is a coefficient; and b is the MIT of the coal dust cloud when coal fly ash is not added, in K.
Fig. 4. MIT of coal dust cloud as a function of concentrations of coal fly ash and calcium carbonate.
was fixed at 0.4 g, and the influence of the inert dust concentration on the MIT was studied to evaluate the inhibiting effect of coal fly ash. Fig. 4 shows that the change trend of the MIT of the coal dust cloud with the concentration of coal fly ash is roughly the same as that with the concentration of calcium carbonate, the MIT increases with increasing concentration of the inert dust. However, the inhibiting effect of coal fly ash is stronger than that of calcium carbonate; in quantitative terms, the MIT at specific concentrations of coal fly ash is approximately 10 K higher than that the MIT at the same concentrations of calcium carbonate. The reason for this difference is the composition of coal fly ash, whose main components include SiO2, Al2O3, and CaO. As stated previously, both coal fly ash and calcium carbonate inhibit the MIT of the coal dust cloud by the same mechanism: heat absorption in the reacting medium. Furthermore, the quantity of heat absorbed by an inert material is proportional to its specific heat capacity. The specific heat capacity of coal fly ash (0.92 kJ/(kg·K)) is slightly higher than that of calcium carbonate (0.84 kJ/(kg·K)). Thus, coal fly ash will have a stronger inhibiting effect than calcium carbonate. Moreover, coal fly ash contains Al2O3, which can distribute evenly on the surface of pulverised coal particles during heating and form a dense protective film to hinder heat transfer, oxygen diffusion, and release of volatiles. Hindrance of all these phenomena has the effect of impeding the combustion of coal dust, which explains the superior inhibition performance of coal fly ash.
Linear fitting of the data as shown in Fig. 2 gives a = 0.424, b = 799, and R2 = 0.9918. The R2 value is a goodness-of-fit index, and a R2 value approximating 1 indicates a high degree of correlation. It is noted from Fig. 3 that the MIT of the coal dust cloud increases rapidly when the concentration of coal fly ash reaches a critical value. At concentrations higher than this critical value, the inert dust shows a better suppression effect. This behaviour provides a basis for explosion protection in actual production, namely that the concentration of the inert medium added to coal dust should exceed the critical value to achieve effective suppression at a production site. Fig. 3 indicates that the increment in the MIT increases to a coal fly ash concentration of 60%, which in this case is the critical concentration. However, coal fly ash cannot completely inhibit the burning of coal dust. 3.2. Effects of coal fly ash and calcium carbonate on MIT of coal dust cloud According to a literature review, techniques for dust suppression employ mainly CaCO3, BaCO3, SiO2, and NH4H2PO4. Coal fly ash has a composition similar to that of calcium carbonate and has additional desirable characteristics such as being resistant to burning and chemically inactive. To compare the inhibiting effects of coal fly ash and calcium carbonate, experiments were performed using bituminous coal with a median diameter of 27.51 μm. Again, the mass of coal powder 26
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quantity of coal dust involved in the oxidation reaction is smaller. All these factors together result in incomplete combustion of coal dust, which makes ignition of the coal dust cloud difficult; therefore, the MITs in these cases are higher. Coal dust with a smaller particle size, as in cases #1 and #2, has a larger specific surface area, which results in a lower MIT, as shown in Fig. 5. Therefore, a larger quantity (higher concentration) of coal fly ash is required for coal dust with a smaller particle size to reach a particular MIT than is required for coal dust with a larger particle size. The influence of coal fly ash on the MITs of coal dust clouds with coal dust particles of five different sizes was also studied; for all five particle sizes, the linear correlation in Equation (1) held true. Because the values of the coefficient a for the five cases were extremely close, the average a value of 0.415 was used for all five cases. Then, the fitted Equation (1) could be rewritten as Tmin = 0.415w + b
(2)
The five fitted lines yielded different b values, where b denotes the MIT of pure coal dust without coal fly ash. As stated previously, the b value is related to the particle size of coal dust. Linear regression yields
Fig. 5. MIT of coal dust cloud with different particle size.
b = 818.5 - (21.03 × 0.99d)
3.3. Effect of coal fly ash on MIT of coal dust cloud with coal dust particles of different sizes
(3)
where d is the median diameter of the coal dust particles (in μm) and R2 = 0.959. The particle size itself has a great influence on the combustion of coal dust. Formation of a dust cloud from large dust particles is difficult because of the gravity effect, which causes the particles to settle. Coal dust with particle size smaller than 1000 μm rapidly participates in the combustion reaction, and the smaller the dust particles, the higher the risk of dust explosions. Therefore, the MIT of the coal dust cloud increases with increasing coal dust particle size up to a certain value. This is consistent with the conclusion of Equation (3).
Five types of bituminous coal dust samples with different particle sizes were selected for the experiments. The corresponding median diameters are listed in Table 1. The mass of coal dust was fixed at 0.4 g. The sensitivity of the MITs of the examined coal dust samples to particle sizes is shown in Fig. 5. The inhibiting tendencies of coal fly ash on the MITs of coal dust clouds with coal dust particles of five different sizes are similar; the MITs increase linearly with increasing concentration of coal fly ash. It can also be observed that a larger particle size corresponds to a lower critical concentration of coal fly ash. The critical concentration of coal fly ash for cases #3, #4, and #5 is 40%, and increases to 60% for cases #1 and #2 (Fig. 6). These values can be explained by the fact that the MIT of the coal dust cloud increases with increasing coal dust particle size (Turns, 2009). This is because both the ignition and the combustion of coal dust occur on the surface of its particles, and that larger particles require a longer heating time for devolatilisation to occur rapidly (intraparticle effect) (Zhang and Wall, 1993). In cases #3, #4, and #5, the particle size is larger, the specific surface area is smaller, the contact area between the coal dust particles and oxygen is smaller, and the
3.4. Effect of coal fly ash on MITs of different types of coal dust Dust samples of three types of flammable coal, anthracite, bituminous coal, and lignite, were selected in this study to evaluate the influence of coal fly ash on the MIT. The particle sizes of the three types of coal corresponded to those of case #2 in Table 1. As shown in Fig. 7, the optimal mass concentration that induces the ignition of a coal dust cloud varies with the type of coal (namely, the volatile matter content). The optimal values are 1.818, 1.818, and 1.363 g/L for anthracite, bituminous coal, and lignite, respectively (Fig. 7). The inhibiting effects of coal fly ash on the three different coal dusts were then studied, and
Fig. 6. Increment in MIT of coal dust cloud with increase in particle size of coal dust.
Fig. 7. MITs of coal dust clouds formed from three types of coal dusts with masses in the range of 0.1–0.5 g. 27
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involves mainly solid carbon. Its combustion mechanism involves surface heterogeneous oxidation, and the reaction is sustained primarily by the diffusion of oxygen onto the particle surface. The addition of a small quantity of coal fly ash cannot effectively suppress the surface heterogeneous reaction, whereas the addition of a large quantity of coal fly ash prevents the diffusion of oxygen, which consequently leads to a lower burning rate. Lignite contains a large quantity of volatiles and a limited quantity of solid carbon. The combustion of lignite is a gasphase reaction in which the volatiles rapidly react with air and burn. When coal fly ash is blended with lignite, the temperature of the reaction zone decreases because of the heat absorption by the coal fly ash; thus, it effectively suppresses the combustion of the coal dust. The combustion of bituminous coal is also dominantly a gas-phase reaction, but its volatile matter content is lower than that of lignite, which results in a lower yield of volatiles during dust combustion. Therefore, at the same concentration of coal fly ash, the inhibiting effect on bituminous coal is weaker than on lignite. It can be seen from the fitted lines show that the value of coefficient a is different for the different types of coal dust, and that it appears to be correlated with the volatile matter content of the coal dust (Fig. 8). The correlation between the MIT and the volatile matter content of the coal dust cloud can be expressed as
Fig. 8. MITs of coal dust clouds formed from three types of coal dusts.
the results are shown in Fig. 8. It can be seen that regardless of the type of coal dust (anthracite, bituminous coal, or lignite), the MIT of the coal dust cloud increases linearly with increasing concentration of coal fly ash. However, the MIT of the coal dust cloud even without coal fly ash varies with the type of coal dust (Fig. 8 at zero concentration). This behaviour is attributed to the significant difference in the quantity of volatiles present in each type of coal dust sample. In the furnace, the coal dust is first heated to release volatiles, which ignite the coal dust in a high-temperature medium. The heat generated during the combustion of volatiles is conducted to the coal dust particles and it further ignites the coal dust. The experimental results show that the MIT of the coal dust cloud decreases with increasing volatile matter content of coal dust. When the volatile matter content of coal dust reduces to a critical value, ignition of the dust becomes difficult (Du et al., 2012). A larger quantity of flammable gas is released from the particle surface of coal dust with a higher volatile matter content upon heating, and as a result the dust can be ignited at a lower temperature. From Figs. 8 and 9, it is found that coal fly ash has different effects on the MITs of the three types of coal dusts tested in this study. It has the strongest inhibiting effect on lignite, the second strongest inhibiting effect on bituminous coal, and the weakest inhibiting effect on anthracite. The critical concentrations of coal fly ash for anthracite, bituminous coal, and lignite are 80%, 60%, and 60%, respectively (Fig. 9). Anthracite has low volatility and its combustion reaction
Tmin = kw + b
(4)
where k is a coefficient whose value depends on the volatile matter content. Because only three types of coal dust were considered in this study, it is not possible to establish the exact correlation between k and the volatile matter content. Therefore, for fitting analysis of the data in Fig. 8, three different values of k were used. From the above-described results, it is ascertained that the MIT of a coal dust cloud is related to the particle size of coal dust, the volatile matter content of coal dust, and the concentration of coal fly ash. Through substitution of Equations (3) and (4) into Equation (1), all these three influencing factors can be combined to obtain a single correlation: Tmin = kw + 818.5 - (21.03 × 0.99d)
(5)
Substitution of k = 0.40, d = 27.51, and w = 20 in Equation (5) gives a Tmin value of 810 K. The error between this predicted value and that the value determined experimentally is only 1.0%. Similarly, the predicted Tmin is 751 K when k is taken to be 0.60; in this case, the experimental MIT is 816 K, and the prediction error is 8.6%. Based on these two simple comparisons, the empirical model expressed by Equation (5) appears to predict the MIT of a coal dust cloud with reasonable accuracy.
4. Conclusion This work examines in detail the influence of coal fly ash on the MIT of a coal dust cloud, and the effects of coal fly ash concentration, particle size of coal dust, and type of coal dust on the MIT. The findings show that the MIT of a coal dust cloud increases linearly with increasing concentration of coal fly ash. Both coal fly ash and calcium carbonate have similar inhibiting effects on the MIT of a coal dust cloud. However, the inhibiting effect of coal fly ash is slightly stronger, owing to its higher specific heat capacity. Furthermore, it is found that the MITs of coal dust clouds with coal dust particles of different sizes are linearly proportional to the concentration of coal fly ash, and that the critical concentration of the coal fly ash gradually decreases with increasing coal dust particle size. Experiments on three different types of coal dust—anthracite, bituminous coal, and lignite—reveal that the MIT of a coal dust cloud is also dependent on the volatile matter content of the coal dust. A simple empirical model was established to reflect the effects of the concentration of coal fly ash, particle size of coal dust, and volatile matter content of coal dust on the MIT of a coal dust cloud.
Fig. 9. Increments in MITs of coal dust clouds formed from three types of coal dusts with increasing concentration of coal fly ash. 28
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Acknowledgements
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