Experimental Study of Minimum Ignition Energy of MethaneâAir

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Experimental study of minimum ignition energy of methaneair mixtures at low temperatures and elevated pressures Gan Cui, Zili Li, Chao Yang, Zhen Zhou, and Jianle Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00366 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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Experimental study of minimum ignition energy of methane-air mixtures at low temperatures and elevated pressures Gan Cui 1, Zili Li 1*, Chao Yang 1, Zhen Zhou 2, Jianle Li 3, 1 College of Pipeline and Civil Engineering in China University of Petroleum (east China), Qingdao 266580, China. 2 Pipeline Changchun Branch in China National Petroleum Pipe Co., Ltd, Changchun 130000, China. 3 East China Branch in China Petrochemical Marketing Co. Ltd, Shanghai 200050, China. KEYWORDS: MIE; Methane; Elevated pressures; Low temperatures ABSTRACT: The previous research results show that in the top of the distillation column of the liquefied process of oxygen-bearing coal-bed methane (CBM), explosion hazard may happen. The minimum ignition energy (MIE) of combustible gas reflects the sensitivity of the explosion. Although MIE has been experimentally and theoretically studied at normal or elevated temperatures, there are no relevant data aimed at the environment (low temperature and high pressure) which exists in the top of the distillation column. Therefore, in this study, the MIE of methane-air mixture was tested using a low temperature experimental vessel at low temperature range (123-273 K) and pressure range (0.1-0.9 MPa). Our research results show that MIE increases with the decrease in initial pressure P0 or temperature T0. When the initial pressure is relatively low, both the initial pressure and temperature significantly affect 1


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the MIE. However, at higher initial pressure, the initial temperature and pressure have relatively smaller impact on MIE. The MIE linearly correlates to 1/P02, whereas linearly correlates to 1/T0. With the decrease in initial temperature, the degree of the impact of initial pressure on MIE gets larger. Additionally, with the increase in initial pressure, the degree of the impact of initial temperature on MIE gets smaller. The changing rule of our experimental MIE with initial temperature and pressure shows good accordance with published results. The energy generated by human body, collision between the metals, lightening and external fire is higher than the MIE of methane in the top of the distillation column, which indicates that an explosion hazard may happen. Thus, corresponding safety measures should be taken.

1. Introduction The distribution of coal-bed methane (CBM) in China was investigated by Ministry of Land and Resources, and the investigation results showed that China is abundant in CBM 1. However, in China, CBM cannot be extracted before the coal mining because of the relatively low permeability of coal seam. Thus, the CBM is extracted from the coal tunnel 2 and O2 is inevitably mixed into the CBM. The CBM containing oxygen is called oxygen-bearing CBM. Generally speaking, the oxygen-bearing CBM is mainly composed of methane and air, of which the methane volume fraction is in the range of 30-80 vol%. Because of the existence of air, the oxygen-bearing CBM can’t be transported or used directly, and thus, most of them are discharged into the atmosphere, which leads to energy waste and environmental pollution. Therefore, making full use of the oxygen-bearing CBM has great economic 2


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and social benefits 3-5. Oxygen is the key constraint to reasonably use the oxygen-bearing CBM. In recent years, the deoxidation technologies of the oxygen-bearing CBM have been developed greatly, and mainly include adsorption method 6, separation membrane method

7, 8

9

, combustion method

and low temperature processing method

10

. And

among these, in China, the last method is attracted much attention. However, the existing research results show that in the top of the distillation column of the liquefied process of oxygen-bearing CBM, the methane composition of the combustible gas is within the range of the flammability limits

11, 12

, which will cause explosion risks.

Therefore, it is of significance to know the MIE of methane-air mixtures under the environmental condition (typically low temperatures and elevated pressures) in the liquefied process of the oxygen-bearing CBM. MIE is the minimum energy to successfully ignite a combustible mixture, and is the key parameter which can be used to evaluate the explosion risk. MIE is the basic parameter to study the explosion of combustible gases13, 14. In the past decades, the MIEs of combustible materials have been measured and studied using experimental or numerical simulation method

14-24

. Unfortunately, there are no relevant studies on the

MIE of combustible gases at lower temperatures and higher pressures which exist in the liquefied process of the oxygen-bearing CBM. As a consequence, the existing data of MIE cannot provide significant guidance for the safety operations of the CBM liquefied process. Therefore, in this study, the MIE of methane-air mixture was tested using a low temperature experimental vessel at low temperature range (123-273 K) 3


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and pressure range (0.1-0.9 MPa).

2. Experimental 2.1 Experimental set up The details of the experimental set up are almost the same with the previous studies 25. Here, we just give an outline of the experimental set up and the difference details. The composition of the experimental device is shown in Fig. 1. It is made up of the gas source, explosion vessel, cooling system, ignition and energy measuring system, data acquisition system and vacuum-pumping system. The detailed size of the explosion vessel is as following: height-300 mm, inner diameter-100 mm which is large enough to ignore the effect of wall on MIE, wall thickness-25 mm. The inner diameter and wall thickness of the explosion vessel determine the limit pressure for 45 MPa, which can guarantee the safety of the explosion vessel in the process of the experiment. The cooling box is used to refrigerate the combustible gas in the explosion vessel. The initial low temperature in the explosion vessel can be tested by a thermocouple (Nanmac E Graduation T). A precision pressure gauge is used to measure the initial pressure. A piezo-electric pressure transducer (Dytran 2300C5) is used to measure the explosion pressure. The pressure transducer has a pressure range of 0-34.5 MPa and a response time of 2 µs.

4


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1-Cooling box, 2-Inner wall of the cooling box, 3-Aluminium powder, 4-Insulating layer, 5-Ignition electrode, 6-Explosion vessel, 7-Sampling valve, 8-Safety valve, 9-Precision pressure gauge, 10-Data acquisition system, 11-Flame arrester, 12-Methane gas cylinders, 13-Oxygen gas cylinders, 14-Nitrogen gas cylinders, 15-Needle valve, 16-Vacuum vessel, 17-Vacuum pump

Figure 1. Experimental set up

2.2 Ignition energy testing system In the past, some researchers calculated the MIE using equation (1). However, because of the resistance in the circuit and stray capacitance, the actual energy of the spark is always much smaller than that calculated using equation (1). In this paper, the ignition energy testing system is the same with that used in the previous study 25. Two stainless steel electrodes are mounted in the center of the explosion vessel. The electrode is pointed rod with a diameter of 1 mm. The angle of the tip is 60 degree and the electrode gap distance is 1 mm. The fundamental circuit diagram is shown in Fig.2. In the process of electric discharge, the current flowing through the ignition electrode is recorded by the current probe, and the voltage of the two discharge electrodes is measured using a voltage probe. The current and voltage probes are connected to the oscilloscope, and thus, the variation of the current and voltage with 5


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ignition time can be displayed in the screen of the oscilloscope, just as shown in Fig.3. As a result, the ignition energy can be estimated by the integration of multiplication of current and voltage with ignition time. The calculated expression of the MIE is shown in equation (2).

1 E = CU 2 2

(1)

t

E = ∫ u (t )i (t )dt 0

(2)

Here, E-MIE; C-energy storage capacitor; U-charging voltage; u(t)-the voltage-time curve; i(t)-the current-time curve; t-spark duration.

Figure 2. Fundamental circuit diagram of the ignition system. 1, 2-switches

6


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Figure 3. Variation of current and voltage with ignition time

2.3 Experimental procedures The experimental procedure is almost the same with that used in the previous study 25. The combustible gas mixture is made by partial pressure method: methane is added into the vessel firstly until it reaches the required partial pressure, and then nitrogen and oxygen are added subsequently. Therefore, the combustible gas is made up of methane, nitrogen and oxygen. However, in our experiment, the initial temperature can reach the value as low as 123 K. Under such low temperature, the gas is no longer treated as ideal gas, and the equation of state of ideal gas is never applicable. The actual concentration of the combustible gas is much different from that calculated using partial pressure method. Therefore, in order to acquire the accurate concentration, before ignition, a bit of combustible gas is taken out from the sampling valve and tested by a gas chromatograph (GC). Finally, the concentration tested by the GC is treated as the real concentration of the combustible gas. The ignition process can be described as following. Firstly, a large voltage (for 7


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example 5000 V) higher than the breakdown voltage is chosen, and at the same time, a relative large capacitance (for example 500 pF) is also selected. If the combustible gas is ignited successfully, then, the capacitance is decreased gradually. In our ignition system, the minimum capacitance is 4 pF. If the combustible gas is still ignited successfully when the capacitance reaches to 4 pF, the voltage should be decreased until the combustible gas is failed to be ignited in a twenty-five times continuous discharge trials. At this time, the energy calculated by equation (2) is regarded as the MIE. It is worth noting that between any two consecutive discharges, a time interval of 15-20 s should be ensured to dissipate the supplied energy by the previous attempt. For the purpose of studying the repeatability of the experiment, at each experimental condition, three sets of parallel experiments should be done 21. In this experiment, the initial temperatures are chosen from 123 to 273 K and the initial pressures are chosen from 0.1 to 0.9 MPa. However, when the initial temperature T0 is low enough and the initial pressure P0 is higher, for instance, T0 is 143 K and P0 is 0.9 MPa, the methane will be liquefied and liquid will exist in the explosion vessel. In this case, the gas composition will be changed with changing T0 or P0, and the composition of the combustible gas is difficult to measure accurately. In addition, a large amount of liquid evaporation will cause some danger to our experimental set up. Therefore, some initial high pressure and low temperature conditions must be avoided in the process of the experiment.

2.4 Analysis of the experimental uncertainty Through the experimental device and procedures, we can conclude that the 8


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uncertainties of our experimental MIEs are mainly caused by the methane concentration measurements using GC, measurements of the initial pressure and temperature, and measurements of the voltage and current. In our experimental device, the accuracy of the GC is 0.1%. Therefore, the initial mole concentration of methane gas has the maximum uncertainty of 0.1 vol%. A pressure gauge which has pressure range of 0 - 1.6 MPa and accuracy of 0.02% is used to measure the initial pressure. Therefore, the measurements of the initial pressure have the maximum uncertainty of ±0.32 kPa. A fast response thermocouple which has temperature range of 88 - 643 K and accuracy of 0.5% is used to measure the initial temperature. Thus, the measurements of the initial temperature have a maximum uncertainty of ±2.7 K. The uncertainties of the voltage and current measuring probes are 1% and 0.5%, respectively. Thus, combined with equation (2), the uncertainty of the spark energy is estimated at approximately 2%.

3. Experimental results 3.1 Sensitive conditions There are many factors which have influence on MIE, for instance, initial temperature, initial pressure, electrode gap, gas composition, storage capacitor, storage voltage, and so on

26-29

. To make the ignition energy minimum, there is a

certain combination of the factors above, which is called the sensitive condition. This paper has been described that MIE is obtained by testing the voltage and current between the two electrodes during the spark duration, and the electrode gap is fixed with 1 mm. Thus, the effect of the capacitor, voltage and electrode gap can be ignored. 9


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However, the MIE is significantly affected by the equivalence ratio. Therefore, in order to eliminate the effect of external conditions on the MIE, the sensitive equivalence ratio must be determined before the experiment. Fig.4 illustrates the variation of MIE with equivalence ratio. The initial temperature is 303 K, initial pressure is 0.1 MPa and electrode gap is 1 mm. The data points and the error bars denote the averaged value of the three parallel experiments and the standard deviations, respectively. With the increase in equivalence ratio, the MIE decreases firstly and reaches the minimum when the equivalence ratio is 1(the mole fraction of methane is 9.5%, and other contents are oxygen and nitrogen, with the volume ratio of oxygen and nitrogen is 1/3.76). Then, with the further increase in equivalence ratio, the MIE increases greatly. This is because when the equivalence ratio is 1, the chemical reaction has the highest activity. Therefore, in our experiment, the sensitive methane content is when the equivalence ratio is 1, and all the MIE at the experimental conditions are measured when the equivalence ratio is 1.

Figure 4. Effect of equivalence ratio on MIE 10


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3.2 Validation of the explosion vessel The new built explosion vessel should be validated on the basis of the standard BS-EN-1839-2003. Table 1 shows the MIE results obtained by our experiment and other researchers. The initial pressure and temperature are 0.1 MPa and 303 K, respectively. From Table 1, we can conclude that the experimental MIE measured by Lewis & Von Elbe 30 has the minimal value (0.28 mJ). In late work, Eckhoff et al.

16

pointed out that the MIE with 0.28 mJ was much conservative, and the view above is thought to be correct at present. Kondo et al.

33

obtained the MIE of methane using

numerical method, and it was inevitable to make some assumptions, which make the MIEs much smaller. The MIEs measured by Huang et al. 31, Yuasa et al.32, and Yang et al.34, respectively, are much close to our experimental results, and the largest absolute error is 0.01 mJ. Therefore, we can conclude that in our experiment, the explosion vessel is valid. Table 1. Comparison of the MIE of methane between this study and other researchers Researchers Lewis & Von Elbe Huang et al. 31 Yuasa et al. 32 Kondo et al. 33 Yang et al. 34 Our experiment

MIE, mJ 30

0.28 0.48 0.50 0.33 0.47 0.49

3.3 The MIE results Table 2 illustrates the MIEs at different low temperatures and elevated pressures. Two MIE results calculated by equation (1) and equation (2) respectively are given, and the standard deviation is also calculated. Through Table 2, we can conclude that the MIE calculated by equation (2) is much smaller than that calculated by equation 11


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(1). The real spark energy is about ten percent of the total energy stored in the capacitor, which illustrates that most energy has been consumed in the ignition circuit. The standard deviations of all the MIE results are smaller than 0.03. Therefore, our experiment has good repeatability. It is worth noting that the MIEs are not measured when the initial temperature is lower than 153 K and the initial pressure is 0.9 MPa. Because at such condition, the methane will be liquefied. The impact of the pressure and temperature on MIE will be discussed in the subsequent sections. Table 2. The experimental and calculated MIE results 273 K

P/MPa

C (pF) U (V)

E=

1 CU 2 (mJ) 2

E = ∫ uidt (mJ)

s.d.

Cal

Exp-Cal

0.1

537

5000

6.712

0.560

0.028

0.573

-0.013

0.3

192

5000

2.400

0.068

0.016

0.065

0.003

0.5

26

5000

0.325

0.026

0.009

0.024

0.002

0.7

14

5000

0.175

0.013

0.007

0.013

0

0.9

4

5000

0.050

0.0071

0.004

0.008

-0.0009

243 K 0.1

615

5000

7.687

0.650

0.022

0.643

0.007

0.3

231

5000

2.887

0.076

0.011

0.073

0.003

0.5

30

5000

0.375

0.030

0.010

0.027

0.003

0.7

16

5000

0.200

0.015

0.008

0.015

0

0.9

6

5000

0.075

0.0082

0.006

0.010

-0.0018

213 K 0.1

693

5000

8.662

0.720

0.016

0.733

-0.013

0.3

270

5000

3.375

0.080

0.008

0.083

-0.003

0.5

36

5000

0.450

0.034

0.006

0.031

0.003

0.7

18

5000

0.225

0.0167

0.006

0.017

-0.0003

0.9

8

5000

0.100

0.0091

0.004

0.011

-0.0019

183 K 0.1

732

5000

9.150

0.880

0.016

0.853

0.027

0.3

309

5000

3.860

0.102

0.007

0.097

0.005

0.5

40

5000

0.500

0.037

0.0014 0.036

0.001

0.7

20

5000

0.250

0.0182

0.0028 0.019

-0.0008

0.9

12

5000

0.150

0.0120

0.006

-0.001

153 K 12


0.013

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0.1

810

5000

10.125

1.020

0.018

1.020

0

0.3

348

5000

4.350

0.120

0.006

0.117

0.003

0.5

44

5000

0.550

0.042

0.0012 0.043

-0.001

0.7

22

5000

0.275

0.022

0.0018 0.024

-0.002

123 K 0.1

888

5000

11.100

1.260

0.020

1.268

-0.008

0.3

387

5000

4.837

0.143

0.010

0.144

-0.001

0.5

50

5000

0.625

0.051

0.0015 0.054

-0.003

0.7

26

5000

0.325

0.026

0.007

-0.003

0.029

3.4 Effect of initial pressure on MIE The variation of the MIE with initial pressure at different initial temperatures is shown in Fig.5. The data points and the error bars denote the averaged value of the three parallel experiments and the standard deviations, respectively. From Fig.5, we can see that the MIE decreases with the increase in initial pressure. When the initial pressure is smaller than 0.5 MPa, the MIE decreases significantly with initial pressure, while decreases gradually with initial pressure when the initial pressure is larger than 0.5 MPa. Therefore, we can conclude that when the initial pressure is relative low, the MIE is significantly affected by initial pressure, whereas less affected by initial pressure at higher initial pressure condition. On the purpose of further studying the changing rule of the MIE with initial pressure, we give the variations of the MIE with 1/P02 which is shown in Fig.6. From Fig.6, we can see that the MIE linearly correlates to 1/P02. We deal with the data in Fig.6 using linear regression method, and the results are shown in Table 3. At different low temperatures, the values of R2 are all bigger than 0.99, which indicates a high degree of linear relationship between MIE and 1/P02. With the decrease in initial temperature, the linear slope increases which indicates that the pressure significantly

13


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affects the MIE at lower temperatures.

Figure 5. Effect of initial pressure on MIE at different low temperatures

2

Figure 6. Relationship between MIE and 1/P0 at different low temperatures 2

Table 3 Linear regression result of the MIE with 1/P0 at different low temperatures T0/K

Slope

Intercept

273

0.00558

0.00281

243

0.00648

0.00248

213

0.00718

0.00197

183

0.00878

0.00187

153

0.01017

0.00307

R2

>0.99

14


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123

0.01259

0.00131

3.5 Effect of initial temperature on MIE The variation of the MIE with initial temperature at different pressures is shown in Fig.7. The data points and the error bars denote the averaged value of the three parallel experiments and the standard deviations, respectively. As can be seen from Fig.7, with the decrease of temperature, the MIE increases. When the pressure is low, i.e. 0.1 MPa, the MIE increases significantly with the decrease in initial temperature, while increases gradually with the decrease in initial temperature when the initial pressure is higher than 0.5 MPa. Therefore, we can conclude that at low pressure, the MIE is significantly affected by initial temperature. However, the temperature has relatively small impact on the MIE at higher pressure. On the purpose of further studying the changing rule of the MIE with initial temperature, we give the variations of the MIE with 1/T0 which is shown in Fig.8. From Fig.8, the MIE linearly correlates to 1/T0. We deal with the data in Fig.8 using linear regression method, and the results are shown in Table 4. At different pressures, the values of R2 are all larger than 0.95, which indicates a linear relationship between MIE and 1/T0. With the increase in initial pressure, the linear slope decreases, indicating that initial temperature has smaller effect on MIE at higher pressures.

15


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Figure 7. Variation of MIE with initial temperature at different pressures

Figure 8. Relationship between MIE and 1/T0 at different pressures Table 4 Linear regression result of the MIE with 1/T0 at different pressures R2

P0/MPa

Slope

0.1

155.83

0.00155

0.995

0.3

17.34

0.00394

0.984

0.5

5.33

0.00768

0.988

0.7

2.86

0.00295

0.992

0.9

2.65

-0.0028

0.950

Intercept

3.6 Calculated equation fitted by experimental MIEs 16


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As the above analysis, MIE linearly correlates to 1/P02and 1/T0, respectively. Therefore, an equation for calculating the MIE at low temperatures and elevated pressures is able to be fitted by experimental MIEs. The calculated expression is as shown in equation (3).

MIE = (0.01899 + 0.07645 / P0 2 )[0.0002984 + 20.3196 / T0 ]

(3)

Where, P0-initial pressure, MPa; T0-initial low temperature, K; MIE is the minimum ignition energy at P0 and T0. The MIEs are calculated using equation (3) and given in Table 2. From Table 2, we can see that the differences between the experimental and calculated values are less than the standard deviations, indicating that equation (3) is able to accurately calculate the MIEs at low temperatures.

4. Discussion 4.1 Comparison with other published results Some researchers have studied the variation of MIE with initial temperature and pressure at elevated temperatures and pressures. Zhang et al.

35

measured the

methane-air mixture at the initial temperature range of 293-353 K and normal pressure, and the results were shown in Fig.9. The variation of our experimental MIE with initial temperature at normal pressure is also shown in Fig.9. From Fig.9, we can see that although the numerical values are not the same, the trend of MIE with initial temperature shows a good consistency. Coronel et al.

36

measured the spark energy

density of H2-N2O mixture at low pressures (0.01-0.03 MPa) and normal temperature, and the results were shown in Fig.10. In order to be compared conveniently, equation (3) is used to calculate the MIE of methane-air mixture at 0.01-0.03 MPa. The 17


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calculated results are also shown in Fig.10. Because of the different combustible gas composition, the MIEs are quite different from each other. However, the changing rule of MIE with initial pressure shows a good consistency. Overall, the changing rule of our experimental MIE with initial temperature and pressure shows good accordance with other results, which indicates the correctness of our experimental results.

Figure 9. Comparison of our results with others at different initial temperatures and normal pressure

18


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Figure 10. Comparison of our results with others at different initial pressures and normal temperature

4.2 Discussion of the effect of initial temperature and pressure on MIE Through our perception, with the increase in initial temperature, the molecular thermal motion of combustible gas gets strengthened, and the combustible gases are easily ignited. With the increase in initial pressure, the spacing between combustible gas molecules gets smaller which increases the frequency of molecular collision. Thus, with the increase in initial pressure, the combustible gas is easier to be ignited and the MIE decreases. Next, we will analyze the effect of initial temperature and pressure on MIE from the point of theoretical model. Kondo et al.

33

used two theoretical models to calculate the MIE of premixed

gases. One of the theoretical models was based on the enough amount of energy which could heat up a certain amount of combustible gases to the combustion temperature. The mathematical expression of the theory model is shown in equation 19


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(4). For the effect of initial temperature on MIE, with the increase in initial temperature T0, Tb -T0 decreases and the critical flame radius also decreases. The combustion temperature Tb is almost unaffected by the initial temperature, thus, ρb and Cav can be regarded as constant. Therefore, the minimum ignition energy Emin decreases. As for the effect of initial pressure on MIE, Chen et al.

37

also proposed

that the minimum ignition energy Emin was in proportion to d3. Kelly et al.

38

studied

the relationship between critical radius of spark-ignited spherical flames and initial pressure using experimental and theoretical method. They found that with the increase in initial pressure, the critical radius decreased. Therefore, the minimum ignition energy Emin decreases with the increase in initial pressure. 1 E min = πd 3 ρ b C av (Tb − T0 ) 6

(4)

Where, Emin is the minimum ignition energy; d is the quenching distance, i.e. critical flame radius; ρb is the density of the burnt gas; Cav is the average mole heat capacity within the temperature range; Tb and T0 are the flame temperature and initial temperature, respectively. Besides, we have also studied that with a decrease in T0, the P0 dependence on MIE increases. Also, with an increase in P0, the impact of T0 on MIE decreases. According to the study of Kelly et al.

38

, we assume that the critical radius is

proportional to the reciprocal of initial pressure, as shown in equation (5). Combing equation (4) and (5), we can acquire the theoretical relationship between Emin and P0, T0, as shown in equation (6). From equation (6), we can conclude that with the decrease in initial temperature, the value of M(Tb-T0) increases which indicates the 20


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slope of the Emin-P0 curve increases. Thus, the effect of initial pressure on MIE increases. Similarly, with increasing the initial pressure P0, the value of M/P03 decreases which indicates the slope of the Emin-T0 curve decreases. Therefore, with an increase in P0, the impact of T0 on MIE decreases. d = K / P0

(5)

1 K3 M E min = π 3 ρ b C av (Tb − T0 ) = 3 (Tb − T0 ) 6 P0 P0

(6)

Where, K and M are constants. It should be noting that equation (5) and (6) are just used to analyze the impact of P0 and T0 on MIE. They are not the real relationship between MIE, P0 and T0 because of the rather complex effect of P0 and T0 on the critical radius.

4.3 Safety measures based on experimental results Given that there may be explosion hazard in the top of the distillation column, corresponding safety measures should be taken to keep the safety of the whole process. As we all know, the occurrence of explosion must fit three criteria: the methane content is within the explosion limit range, sufficient oxygen and enough energy. For the above three criteria, some safety measures can be proposed. Li et al. 12 recommend that the oxygen is first removed from the feed gas before it flows into the liquefied process. Therefore, the oxygen in the top of the distillation column is insufficient, and thus, explosion will not happen. However, a new deoxidization device is needed which increases the cost. Cui et al.

11

recommend that

inert gas nitrogen is added into the distillation column to dilute the oxygen. However, the required amount of nitrogen is too large to be applied in engineering. To decrease 21


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the yield of the LNG at the bottom of the distillation column, the methane content can be higher than the upper explosion limit in the top of the distillation column and no explosion hazard exists. However, the methane recovery will be very low and the economy is very poor. Another aspect, if the energy in the top of the distillation column is smaller than the MIE, explosion will also not happen. In the typical low temperature liquefied process of oxygen-bearing CBM, the temperature is 103 K and pressure is 0.27 MPa. The corresponding MIE can be calculated using equation (3) and the value is 0.21 mJ. Therefore, if the energy in the top of the column is smaller than 0.21 mJ, there is no explosion hazard. Within the column, the static electricity generated by mutual friction between the gases, heavy hydrocarbon liquid droplets and the column wall is about 0.025 mJ which is smaller than 0.21 mJ. Therefore, no explosion hazard will be induced. In our daily life, the static electricity on human body is about 0.45 mJ which is higher than the minimum ignition energy. And because of the collision between the metals, thunder and external fire, the generated energy is much higher than the MIE. As a consequence, an explosion hazard may happen. Thus, corresponding safety measures should be taken to avoid the above energy. For instance, the workers should wear linen material clothes and can’t wear shoes with nails; using metal devices such as the bronze wrench; installation of lightning protection and grounding device; installation of flame arrester or block valves, and water blocking fire explosion venting device, automatically spray powder explosion suppression device and safety monitoring system. 22


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5. Conclusions According to the above studies, some conclusions can be drawn. By comparison with the MIEs obtained by other researchers, the explosion vessel in this work is valid. The sensitive equivalence ratio is 1. The real spark energy is about ten percent of the energy which is stored in the capacitor. When the pressure is high (0.9 MPa) and temperature is low (lower than 153K), the methane will be liquefied. Therefore, the high pressure and low temperature conditions should be avoided. Both the initial pressure and temperature significantly affect the MIE. The MIE increases with decreasing the initial pressure or temperature. At low pressures, the initial pressure and temperature significantly affect the MIE. However, at higher pressures, the pressure and temperature relatively less affect the MIE. The MIE linearly correlates to 1/P02, whereas linearly correlates to 1/T0. With the decrease in initial temperature, the degree of the impact of initial pressure on MIE gets larger. Additionally, with the increase in initial pressure, the degree of the impact of initial temperature on MIE gets smaller. The fitting equation is able to accurately calculate the MIEs at low temperatures and elevated pressures. The changing rule of our experimental MIE with initial temperature and pressure shows good accordance with the published results. There are several measures to keep the safety of the liquefied process of CBM. From the point of the energy, in the top of the distillation column, the MIE of methane is 0.21 mJ, which is higher than static electricity generated by mutual friction between the gases, heavy hydrocarbon liquid droplets and the column wall, while much 23


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smaller than the energy generated by human body, collision between the metals, lightening and external fire. Thus, corresponding safety measures should be taken to avoid the large energy.

AUTHOR INFORMATION Corresponding Author *Tel: +86 15053293355. Email:[email protected]

Notes The authors declare no competing financial interest.

ACKNOELEDGEMENTS This investigation has been performed with the financial support of the Fundamental Research Funds for the Central Universities (No. 15CX06071A).

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