Scholarly article on topic 'The Dust Explosion Characteristics of Coal Dust in an Oxygen Enriched Atmosphere'

The Dust Explosion Characteristics of Coal Dust in an Oxygen Enriched Atmosphere Academic research paper on "Chemical engineering"

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Procedia Engineering
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{"Oxy-coal fuel combustion" / "dust explosion characteristics" / "Minimum ignition energy"}

Abstract of research paper on Chemical engineering, author of scientific article — Frederik Norman, Jan Berghmans, Filip Verplaetsen

Abstract The ability to mix pulverised coal with oxygen at concentrations greater than the currently applied 21% may well provide advantages for burner design in oxy/coal fired systems. However the risk of dust explosions increases significantly with increasing oxygen concentration and temperature. In this study the influence of enriched oxygen concentrations is researched on the dust explosion characteristics of Indonesian (Sebuku) high volatile bituminous coal dust and on Pittsburgh Coal n¡8. Both ignition sensitivity characteristics (minimum ignition energy and minimum ignition temperatures) and explosion severity characteristics (maximum explosion pressure, Pmax, and maximum rate of pressure rise, Kst) are investigated.

Academic research paper on topic "The Dust Explosion Characteristics of Coal Dust in an Oxygen Enriched Atmosphere"

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Engineering

Procedia Engineering 45 (2012) 399 - 402 =

www.elsevier.com/locate/procedia

2012 International Symposium on Safety Science and Technology The dust explosion characteristics of coal dust in an oxygen enriched atmosphere

Frederik Normana, Jan Berghmansb, Filip Verplaetsena*

a NVAdinex, Brouwerijstraat 5, 2200 Herentals, Belgium b Katholieke Universiteit Leuven, Department of Mechanical Engineering, Celestijnenlaan 300A, 3001Leuven, Belgium

Abstract

The ability to mix pulverised coal with oxygen at concentrations greater than the currently applied 21% may well provide advantages for burner design in oxy/coal fired systems. However the risk of dust explosions increases significantly with increasing oxygen concentration and temperature. In this study the influence of enriched oxygen concentrations is researched on the dust explosion characteristics of Indonesian (Sebuku) high volatile bituminous coal dust and on Pittsburgh Coal n°8. Both ignition sensitivity characteristics (minimum ignition energy and minimum ignition temperatures) and explosion severity characteristics (maximum explosion pressure, Pmax, and maximum rate of pressure rise, Kst ) are investigated.

© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Beijing Institute of Technology.

Keywords: Oxy-coal fuel combustion; dust explosion characteristics; Minimum ignition energy.

ELSEVIER

1. Introduction

The work reported here is part of the European FP7 project RELCOM (Reliable and Efficient Combustion of Oxygen/Coal/Recycled Flue Gas Mixtures). The overall aim of the project is to undertake a systematic and focused series of applied research, development and demonstration activities involving both experimental studies and combustion modeling work to resolve existing technical uncertainties and barriers which inhibit commercial deployment of the technology. Attention will be paid to the generation of design rules and methods which can be employed for scaling up results from pilot and laboratory studies to the full-scale. The data obtained in the experimental tests and from the developed modeling tools will be integrated to produce detailed designs for both a retrofit oxy/coal/RFG system

2. Experimental methods

Dust explosion tests were performed on pulverized Sebuku coal and Pittsburg coal n°8 according to the European standards EN 13821, VDI 2263 and EN 14034. The maximum explosion pressure, the maximum rate of pressure rise and the lower flammability limit are measured by means of a standardized test sphere with a volume of 20 liters. The minimum ignition energy is determined by means of the MIKE3 apparatus and of a modified Hartmann tube. The thermal behavior of the coal is measured by means of a Grewer oven. The ignition temperature of the dust in contact with a hot wall is determined by means of a BAM oven. A description of the equipment to determine the explosion characteristics of dusts can be found in e.g. reference [1].The particle size distribution of the coals is established by means of a laser scattering

* Corresponding author. Tel.: +0032-14-270-390; fax: +0032-14-270-399. E-mail address: info@adinex.be

1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.177

technique. Because the explosion characteristics depend on the moisture content of the dust sample, the moisture content of the coal samples is determined. A halogen moisture balance is used for this purpose.

3. Experimental results

The results of the particle distribution investigation show that the Sebuku coal has a median diameter of 14,9 |m with a standard deviation of 17,6 |m. The Pittsburg coal is much coarser. It has a median diameter of 188 |m with a standard deviation of 169,4 |m.

The lowest plate temperature at which a 5 mm thick layer of coal deposited on the plate ignites is 260°C for both coals. The results of the Grewer oven test are shown in Fig. 1. It is found that the Indonesian coal (Sebuku) has more volatile components which are released at lower temperatures than the Pittsburg coal. Sebuku is a bituminous coal rich in volatile components.

700 600

¡1 300

200 100 0

2000 4000 6000 8000 10000 12000 14000

Time / s

Fig. 1 Results of the Grewer test.

The minimum cloud ignition temperature (MCIT) of airborne dust in contact with a hot surface is determined by means of the standardised BAM-oven (VDI 2263, Blatt 1, 2.6.). The lowest temperature of the heated impact plate in the oven at which the dust blown into the oven ignites or decomposes producing flames or explosion in less or equal than 5 seconds is stated as the minimum ignition temperature. This test is normally performed on the fraction of the sample with a particle size less than 63 (xm. It is found that the MCIT of the Sebuku coal is 540°C. The sample contained 9 % of humidity. For the Pittsburg coal one obtains a value of 590°C. The humidity level of this sample was 4,6 %.This result is in agreement with the results of the Grewer tests. Volatile coals have a lower MCIT than non-volatile coals.

Fig. 2 shows the results of the tests in the 20 litre sphere for the Pittsburg coal. It is found that, over the whole range of concentrations the maximum pressure is 6,5 bar, the maximum rate of pressure rise is 290 bar/s and the Kmax value is 79 m

The lower explosive limit is 60g/m3

The results obtained for the Sebuku coal are presented in figure 3. It is found that, over the whole range of concentrations the maximum pressure is 6,6 bar, the maximum rate of pressure rise is 418 bar/s and the Kmax value is 114 m bar/s. The lower explosive limit is 63 g/m3.

The results of the 20 liter sphere test are in agreement with the those of the tests mentioned above. Small particle size and high volatility lead to faster combustion and thus to higher dP/dt and Kmax values.

The minimum ignition energy of the Sebuku coal is 60 mJ. The minimum ignition energy of the Pittsburg coal could not be determined with the Mike3 apparatus. The coal can be ignited in the Hartmann tube however. For this reason the minimum ignition energy of this coal is estimated as to be between 1000 and 10000 mJ.

Grewer-test Relcom Coals

60 250 500 750 1000 gAn3 60 250 500 750 1000 g/m3

Fig. 2 Maximum pressure Pm (bar) and maximum rate of pressure rise dP/dt (bar/s) as a function of dust cloud concentration for Pittsburg coal.

Fig. 3 Maximum pressure Pm (bar) and maximum rate of pressure rise dP/dt (bar/s) as a function of dust cloud concentration for Sebuku coal.

4. Influence of oxygen

The amount of experimental data available in the literature about the effect of oxygen on the explosion characteristics on coal is very limited. An overview of this subject up to 1997 can be found in reference [2]. Most of the work in the past was connected to the technique of inertization. This technique aims at making dust clouds non-ignitable by adding an inert gas to the air in order to reduce the oxygen content of the air-dust mixture. From these studies it can be concluded in general that increased oxygen content will:

- Increase the flame temperature

- Increase the speed of combustion

- Lower the lower flammability limit

- Increase the maximum rate of pressure rise

The maximum explosion pressure may experience a limited increase.

More recently experimental and theoretical studies have been made of coal combustion with emphasis on the behavior of the coal particle. An overview of this work can be found in reference [3]. It is found that there exists a critical particle size below which the volatiles and the char burn simultaneously on or near the surface of the particle. The type of combustion (simultaneous or consecutive volatiles combustion followed by char combustion) has a large impact on the speed of

combustion and the speed of the pressure rise. The critical size depends upon the physical and chemical properties of the coal (size, composition...), the ignition temperature and the composition of the gas phase in which the combustion occurs. Higher O2 concentrations and smaller particle sizes tend to promote simultaneous combustion. The studies also point to the role of CO2. If N2 is replaced by CO2 it is found that simultaneous combustion is promoted (the critical size increases). CO2 on the other hand decreases the flame temperature and the stability of the flame.

The impact of CO2 is important because of the fact that in order to reduce CO2 emission in an oxy-combustion process, CO2 has to be re-circulated into the flame. Assessing the impact of O2 in the context of oxy-combustion therefore also will require the study of the impact of CO2 on the process and on its safety parameters. Based upon the impact of O2 and CO2 on the combustion of coal particles described above, the expected impact of increased CO2 and of O2 is listed in table 1.

Table 1. Impact of increased O2 and CO2 on the main explosion characteristics of coal. (+ is an increase, - is a decrease)

O2 CO2

Flame temperature + -

Speed of combustion + -

Lower flammability limit - +

Maximum rate of pressure rise + -

Minimum ignition energy - +

Minimum ignition temperature - +

5. Conclusions

Knowledge of the phenomena occurring during coal combustion in oxygen enriched atmospheres is necessary for the reliable and efficient combustion of Oxygen/Coal/Recycled Flue Gas Mixtures. In order to identify the safe limits for oxygen use, the explosion characteristics (ignition sensitivity and explosion severity) are determined taking into account the specific atmospheres encountered in oxy/coal combustion. It is concluded that the minimum ignition energy, lower explosive limit and minimum ignition temperatures decrease with increasing oxygen concentration while flame temperature and speed of pressure rise increase. The addition of CO2 during combustion is expected to have the opposite effects.

References

[1] Lees, F.P., 1990, Loss prevention in the process industries, vol 2, Butterworth-Heinemann.

[2] Eckhoff, R.K., 1997, Dust explosions in the process industries, Butterworth-Heinemann.

[3] Khatami, R. et al, 2012, Combustion behavior of single particles from three different coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2

atmospheres, Combustion and flame, vol 159.