Explosion limits for combustible gases

M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 19 (2009) 0182–0184

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Explosion limits for combustible gases TONG Min-ming, WU Guo-qing, HAO Ji-fei, DAI Xin-lian School of Information and Electrical Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China Abstract: Combustible gases in coal mines are composed of methane, hydrogen, some multi-carbon alkane gases and other gases. Based on a numerical calculation, the explosion limits of combustible gases were studied, showing that these limits are related to the concentrations of different components in the mixture. With an increase of C4H10 and C6H14, the Lower ExplosionLimit (LEL) and Upper Explosion-Limit (UEL) of a combustible gas mixture will decrease clearly. For every 0.1% increase in C4H10 and C6H14, the LEL decreases by about 0.19% and the UEL by about 0.3%. The results also prove that, by increasing the amount of H2, the UEL of a combustible gas mixture will increase considerably. If the level of H2 increases by 0.1%, the UEL will increase by about 0.3%. However, H2 has only a small effect on the LEL of the combustible gas mixture. Our study provides a theoretical foundation for judging the explosion risk of an explosive gas mixture in mines. Keywords: coal mine; gas; explosive gases; explosion limits

1

Introduction

Gas explosions are some of the most serious disasters in coal mines and may cause heavy loss of life and property. Therefore, it is very important to take reliable measures to monitor the variation of gas concentrations to prevent the occurrence of accidents due to gas explosions[1]. The reliability of monitoring depends on an accurate judgment of the risk of explosion of gas[2]. At the present time, various kinds of new gas sensors have been developed both at home and abroad[3–4], such as a gas sensor based on the infrared principle, which can realize a single methane detection. But the gas in coal mines is a mixture containing several components, such as methane, hydrogen and some multi-carbon alkane gases[5]. The multi-carbon alkane gases, such as propane, butane and hexane, have low explosive limits and high explosion danger indices. It is therefore inappropriate to rely only on the detection of the amount of methane and to regard the LEL of methane as the control index. Based on this idea, the explosion limits and the danger indices of a mixture of gases containing C4H10, C6H14, CH4 and H2 have been systematically calculated and the quantitative results are presented by us.

2

Calculation of explosion limits

When combustible gas, mixed with air (or oxygen), reaches a certain concentration, an explosion will take place as soon as it meets fire. This so-called “certain

concentration” is the explosion limit of the combustible gas. Explosion limits can be obtained empirically as well as by numerical methods[6–8]. 2.1 Calculation of lower explosion limit When 1 mole of combustible gas flames completely in the air, its stoichiometric concentration can be calculated according to the following formula: 20.9 100 (1) X0 = = n 0 . 209 + n 0 0 1+ 0.209 where X0 is the stoichiometric concentration of flammable gas, %; n0 the mole number of oxygen molecules needed for complete combustion of 1 mole of flammable gas. The Lower Explosion Limit (LEL) of flammable gas can be approximated by the following formula: X 1 = 0.55 X 0

(2)

where X1 is the LEL of the lammable gas, %.

2.2 Calculation of upper explosionlimit of alkane gas The number of the carbon atoms of one alkane gas molecule is linearly related to the oxygen atomicity needed by the Upper Explosion Limit (UEL) of the alkane gas[8]. Its formula is as follows: 2n = 0.5α + 2.0 (α = 1, 2) 2n = 0.5α + 2.5 (α ≥ 3)

(3) (4)

Received 10 May 2008; accepted 12 July 2008 Projects 706029 supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project of Ministry of Education of China and 2007AA04Z332 by the National High Technology Research and Development Program of China Corresponding author. Tel: +86-516-83884517; E-mail address: [email protected]

TONG Min-ming et al

Analysis and calculation of explosion limits for combustible gases

where α is the carbon atomicity of one alkane gasmolecule, 2n the oxygen atomicity required by UEL of the alkane gas. Since the oxygen atomicity (2n) Table 1 Gas name

Molecular formula

α

Methane

CH4

1

183

can be obtained by introducing α into Eqs.(3) or (4), the UEL of alkane gas can be found from Table 1, using the value of the oxygen atomicity.

Calculated and measured values of explosion limits of alkane gas[9] Stoichiometric concentration

Low X1 (%)

UEL X2 (%)

2n0

X0 (%)

Measured

Calculated

Measured

2n

Calculated

4

9.5

5.0

5.2

14.0

2.5

14.3

Ethane

C2H6

2

7

5.6

3.0

3.1

12.5

3.0

12.2

Propane

C3H8

3

10

4.0

2.1

2.2

9.5

4.0

9.5

Butane

C4H10

4

13

3.1

1.8

1.7

8.5

4.5

8.5

Pentane

C5H12

5

16

2.5

–

1.4

7.6

5.0

7.7

Hexane

C6H14

6

19

2.2

–

1.2

7.0

5.5

7.1

Heptane

C7H16

7

22

1.9

1.05

1.0

6.7

6.0

6.5

2.3 Calculation of explosion limits of gas mixture The explosion limits of a combustible mixture of gases can be calculated according to known explosion limits of the gas components by the following formula[8]: 100 (5) L= P1 P2 P + + ⋅⋅⋅ + n N1 N 2 Nn where P1 , P2 , P3 , …, Pn are mole ratios of each flammable gas component in a mixture of gases, %; P1 + P2 + P3 +…+ Pn =1; N1 , N 2 , N 3 , …, N n are explosion limits of each flammable gas component, %; L is the explosion limit of gas mixture, %. This formula can calculate not only the LEL, but also the UEL of a combustible mixture of gases.

2.4 Calculation of explosive danger index The danger index is regarded as the critical risk level of combustible gases[9]. It can be calculated according to the following formula: H = ( X 2 − X1 ) X1 (6) where H is the explosive danger index; X1 the LEL of combustible gases, %; X2 the UEL of combustible gases, %.

(a) C4H10 and C6H14 vs. LEL

Fig. 1

3 Calculation and analysis of explosion limits of mixture gases in coal mine The composition of combustible gases is quite complex because there are a number of multi-carbon alkane gases such as C4H10, C6H14 and hydrogen found in mines. The LEL of this multi-carbon alkane gas is very low while the UEL of H2 is up to 75%. It is therefore necessary to study the effect on the explosion limits of the mixture of gases caused by C4H10, C6H14 and H2. Eq.(5) is used to calculate the explosion limit of the gas mixture and the effect on explosion limits is studied by means of linear regression analysis.

3.1 Effect of multi-carbon alkane gases on explosion limits From Table 1, the explosion limits (LEL and UEL) of C4H10 and C6H14 can be obtained. According to Eq.(5), the explosion limits of this gas mixture which contains C4H10, C6H14, H2 and CH4 can be calculated. To study the effect of the explosion limit caused by C4H10 and C6H14, the volume fraction of H2 (0.1%) is kept constant. At the end of our study, we calculated the explosive danger index (H) of the gas mixture from Eq.(6). The results are shown in Fig. 1.

(b) C4H10 and C6H14 vs. UEL

(c) C4H10 and C6H14 vs. H

Effect of volume fraction on the LEL, UEL and the explosive danger index

Fig. 1a shows the effect on the LEL of the gas mixture caused by C4H10 and C6H14 while Fig. 1b shows its UEL and Fig. 1c the explosive danger index. The following conclusions can be drawn from ana-

lyzing the calculated results shown in Fig. 1. 1) Given the results of the linear regression analysis, it is found that for every 0.1% increase in C4H10 and C6H14 levels, the Lower Explosion Limit of the

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gas mixture is reduced by about 0.19%, the Upper Explosion Limit decreases by about 0.3% and the explosive danger index increases by about 0.12. It is therefore more dangerous in mines if the gas consists of a mixture of multi-carbon alkane gases such as C4H10, and C6H14. As Fig. 1a shows, this mixture of gases will produce an explosionwhen CH4 is only 1%, if the concentrations of C4H10 and C6H14 adds up to 1%. 2) From Fig. 1, we can see that the amount of CH4 also has a very important effect on the LEL/UEL and on the explosive danger index of the gas mixture. As Fig. 1a shows, the LEL of the gas mixture containing

(a) H2 vs. LEL

No.2

1% CH4 is lower than the LEL of the gas containing 4% CH4. The reason is that the mole ratios of C4H10 and C6H14 in the gas mixture containing 1% CH4 is higher than the gas containing 4% CH4. The LEL of C4H10 and C6H14 is lower than that of CH4, which can be seen from Table 1. The explosion limit of H2 is very wide, its LEL is 4% and its UEL is up to 75%. For this part of our study, we kept the volume fraction of C4H10 (0.1%) and C6H14 (0.1%) in the mixture constant. The explosion limits and explosive danger index of the gas mixture were calculated by the same method. The calculated results are shown in Fig. 2.

(b) H2 vs. UEL

Fig. 2

(c) H2 vs. H

Effect of H2 on the LEL, UEL and the explosive danger index

Fig. 2a shows the effect on the LEL of the gas mixture caused by H2, while Fig. 2b shows its effect on the UEL and Fig. 2c on the explosive danger index. We make the following inferences from analyzing the results shown in these three figures. 1) From Fig. 2a, it is seen that the amount of H2 has a small effect on the LEL of the gas mixture, since the LEL of H2 is similar to that of CH4. 2) From Figs. 2b and c we can see that the amount of H2 has a large effect on the UEL of the gas mixture. The results of our linear regression analysis show that for each increase of 0.1% in H2, the UEL increases by about 0.3% and the explosive danger index increases by about 0.1. This means that H2 has an extremely dangerous effect on the gas mixture and will raise the explosive danger index.

4

Vol.19

Conclusions

The explosion limits and explosive danger index of a combustible gas mixture containing C4H10, C6H14, CH4 and H2 have been systematically calculated. We used a numerical method to analyze the relations between explosion limits and different volumetric amounts of a combustible mixture of gases. The linear regression analysis show that for every 0.1% increase in C4H10, and C6H14 the Lower Explosion Limit of the gas mixture is reduced about 0.19%, the Upper Explosion Limit decreases by about 0.3% and the explosive danger index increases by about 0.12. The results of the analysis also illustrate that as the amount of H2 increases by 0.1%, the UEL increases by about 0.3% and the explosive danger index in-

creases by about 0.1. H2 has a small effect on the LEL. The results provide a theoretical foundation for risk management of explosive gas mixtures in coal mines.

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