THE SELF-IGNITION TEMPERATURE OF COAL DUST IN OXY-FUEL ATMOSPHERES D. Wu1, F. Norman2, F. Verplaetsen2, J. Berghmans1, E. Van den Bulck1 Katholieke Universiteit Leuven, Department of Mechanical Engineering, Celestijnenlaan 300A, B3001 Leuven, Belgium 2 Adinex NV, Brouwerijstraat 5/3, B 2200 Noorderwijk, Belgium Introduction Oxy-fuel combustion, as one of the most promising technology for CO2 emissions, has been advancing in recent years. In the oxy-fuel combustion system, however, dust may be processed or deposited in oxygen-enriched environments where little or nothing is known about the spontaneous ignition risk. Understanding minimum temperature at which dust layer and deposits ignite is critically important in industries where smouldering fires can occur, causing consequent dust explosion. An experimental investigation into the isothermal slef-ignition of coal dust bulk has been undertaken in oxy-fuel combustion atmospheres, with the oxygen mole fractions in the range of 21-50% (21%, 30%, 40% and 50%). South African (SA) coal was determined for self-ignition temperature (SIT or TSI ) using a device known as a hot oven with temperature controlled by an automated control system (EN 15188) [1]. The typical temperature evolution curve with time is shown in Figure 1. 200 T1 (Ignition) TemperatureC 160 T1 (No ignition) TO =119C 120 TO =117C 80 40 T1 Ignition TO=119C T1 0 40 80 120 160 Time, min Figure 1: Thermal behaviour of the 50ml South African coal in the isothermal oven at 50% O2 in CO2. is the radius or characteristic length of the dust sample, 3 (~600 kg  m ),  is the bulk density of the sample Q is the gross calorific value (27369 kJ  kg ), and  1 1 is the heat conductivity 1 (0.12 W  m  K ), A is the pre-exponential factor ( s 1 ), Ea is the apparent activation energy ( J  mol1 ) and is the universal gas constant R (8.314 J  K 1  mol1 ). Equation (1) also gives a linear correlation between the terms ln( crTSI2 / ro2 ) and 1/ TSI with Ea / R as the slope, which offers the possibility to determine the apparent activation energy and pre-exponential factor [1]. The influence of oxygen concentration and inert gas (N2 or CO2) on the self-ignition temperature for the SA coal sample tested in the hot oven is illustrated in Figure 2. All of them show that the oxygen concentration has a pronounced influence on the self-ignition temperature. With increasing oxygen concentration the SIT of the SA coal decreases. This observation is in line with the findings of Bowes, et al [2] and Schmidt, et al [3]. Corresponding author: [email protected] 120 110 20 25 30 35 40 45 50 O2 concentration, Vol. % In order to evaluate the results of the experimental determination of self-ignition temperatures, the F-K (Frank-Kamenetskii) theory is employed.  T2 E Q Ea ln( cr 2 SI )  ln( A)  a ro  R RTSI 400ml 100ml 50ml 25ml 130 100 18 Air Self-ignition mathematical model 1 ro ratio of 1), 140 No ignition TO=117C 0 where  cr depends on the geometry of the dust sample (e.g.  cr =2.76 for a cylinder with a height to diameter Self-ignition temperature, C 1 (1) Figure 2: The SIT of the SA coal samples in the hot oven test. Figure 3 shows the correlation between the characteristic lengths of the coal samples and the self-ignition temperature using the F-K method at various oxygen concentrations. According to equation (1), the kinetic parameters can be calculated and are shown in Table 1. It suggests that the slight deviation in slope for the lines at different oxygen concentration is not regarded as significant, which is in line with the observation of [2]. As for the pre-exponential factors, they are on the same order of magnitude for the different oxygen concentration. Those results suggest that the reaction mechanism of the coal dust during the self-heating period does not change with elevating of oxygen concentration. Air 21% 30% 40% 50% 21 The authors gratefully acknowledge the financial contribution from the European FP7 project RELCOM (Reliable and Efficient Combustion of Oxygen/Coal/Recycled Flue Gas Mixtures). References [1] CEN, Determination of the Spontaneous Ignition Behavior of Dust Accumulations. European Standard EN15188, European Committee for Standardization, Brussels, 2007. [2] P.C. Bowes, P.H. Thomas. Ignition and extinction phenomena accompanying oxygen-depend selfheating of porous bodies. Combustion and Flame, 10 (3) (1966): 221-230. 2 2 ln(crTSI/r ), crTSI/r in k /m 2 22 Acknowledgements 2 y=50.986-11.861x 20 y=49.352-11.441x 2 2 [3] Schmidt. M., Lohrer, C. & Krause, U. (2003). Self-ignition of dust at reduced volume fractions of ambient oxygen. Journal of Loss Prevention in the Process Industries, 16: 141-147. y=50.122-11.622x y=50.656-11.902x y=50.666-12.026x 19 2,4 2,5 2,6 2,7 1000/TSI, TSI in K Figure 3: Frank-Kamenetskii plot of self-ignition temperatures Table 1: Kinetic parameters of the SA coal at various oxygen concentration Mixture gas A Ea ( kJ  mol ) ( s 1 ) 50% Vol. O2 in CO2 98 ±6 8.6×109 40% Vol. O2 in CO2 96±4 3.7×109 30% Vol. O2 in CO2 99±4 6.2×109 21% Vol. O2 in CO2 100±2 6.2×109 95±3 1.7×109 Air 1 Conclusions A series of experiments have been undertaken to determine the ignition characteristics of the SA coal dust and the conclusions have been derived as follows: 1. As the basket volume increases, the critical temperature for ignition decreases significantly. 2. A remarkable increase of the spontaneous ignition risk of the coal dust with increasing the ambient oxygen concentration. 3. The average value of the apparent activation energy calculated from the gradients of the straight lines in Figure 3 is around 98 kJ  mol1 . The slight variation apparent in the values shown for the different concentrations of oxygen is not regarded as significant. This means that the reaction mechanism is more or less the same.