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Influence of inert materials on the self-ignition of flammable dusts
Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Influence of inert materials on the self-ignition of flammable dusts Benjamin Binkau*, Christoph Wanke, Ulrich Krause €tsplatz 2, 39106, Magdeburg, Germany Otto-von-Guericke-University Magdeburg, Dept. of Plant Design and Process Safety, Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2014 Received in revised form 18 November 2014 Accepted 26 November 2014 Available online 27 November 2014
Flammable solid bulk materials, including dusts, often undergo spontaneous combustion and the spread of reaction fronts. By addition of inert substances, the ignition and combustion behavior can be influenced. In a series of experiments different types of coal were mixed with inert powders to study the effect of the composition on the self-ignition temperature and on the formal kinetic parameters. Hot storage tests as well as simultaneous-thermal analysis were used as experimental techniques with the latter being coupled to FTIR measurements to analyze the composition of gaseous reaction products. All conducted hot storage experiments led to the conclusion that the self-ignition temperature was increased by admixing inert material if the decomposition temperature of the inert matter was higher than the self-ignition temperature of the combustible component at the sample characteristic length. If (exothermic) decomposition of the inert material occurred before a noticeable growth of reaction rate of the combustible material, even a reduction in the self-ignition temperature could be observed. In addition, significantly higher maximum reaction temperatures were observed for the mixtures than for the combustible material alone. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Combustible dusts Inert substances Self-ignition Simultaneous thermal analysis
1. Introduction Combustible bulk materials are usually stored in quantities up to several thousands of tons over extended periods of time. It is a wellknown fact, that large amounts of combustible material tend to self-ignition, even if stored at ambient temperature. Packages of porous solids at smaller dimensions can undergo self-ignition if exposed to elevated temperatures. Well known examples are filters in gas cleaning systems and cakings in dryers. Self-ignition occurs, if the heat, released from a low rate oxidation at the active surfaces of the individual particles cannot be dissipated via the outer surface of the deposit. This leads to a temperature rise, which itself further accelerates the reaction. This is ending in a positive feedback loop, until the material catches fire (Babrauskas, 2003). Once a fire is ignited, it produces large amounts of smoke and possibly toxic gases. Depending on volume and shape of the deposit, even more than one hot spot may occur and initiate the formation of hidden smoldering nests. Such hidden smoldering nests may break through to the surface of the deposit several days, weeks or even months after the onset of the self-ignition. Because
* Corresponding author. E-mail address: [email protected] (B. Binkau). http://dx.doi.org/10.1016/j.jlp.2014.11.017 0950-4230/© 2014 Elsevier Ltd. All rights reserved.
of the very time consuming and costly firefighting, a prevention of the spontaneous combustion is always preferable (Hoischen and Kayser, 2009). While a vast knowledge has been gained on selfignition of non-mixed combustibles, comparatively little is known about the effect of non-reactive additives on the process of selfignition. Studies discussing this effect were reported by Smith (Smith et al., 1988), Zhan (Zhan et al., 2011) and Zhang (Sujanti and Zhang, 1999; Watanabe and Zhang, 2001). The aim of the present study is to determine the influence of mixing inorganic salts on the self-ignition behavior of different types of coal dust. The salts where chosen due to their application in fire-extinguishing powder, as flame retardants or respectively due to their property to emit inert gases when decomposed, respectively. The samples were investigated by hot storage experiments and simultaneous-thermal analysis (STA) in combination with Fourier transformed infrared spectroscopy (FTIR) measurements to perform a qualitative analysis of the gaseous reaction products. 2. Experimental set-up and material properties In hot storage tests the self-ignition temperature (TSI), the activation temperature E/R and the induction time tind were determined. The experimental setup is shown in Fig. 1. The experimental procedure was in accordance with the European
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B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342 Table 1 Material properties of the pure substances (rates in %).
Nomenclature d E k0 Q r R TSI t T cp m c
diameter (m) activation energy (kJ/mol) pre-exponential factor (1/s) reaction heat (kJ/kg) radius (m) universal gas constant (J/K-mol) self-ignition temperature ( C) time (h) temperature (K) specific heat capacity (J/kg-K) mass (kg) volume concentration (%)
Greek
d l r
Frank-Kamenetskii parameter () thermal conductivity (W/m2-K) bulk density (kg/m3)
Subscripts c critical ind induction max maximum dis dissolved
Particle diameter [mm]
Moisture [%]
0e20 20e40 40e80 80e160 160e240 240e500 500e760 760e1000 1000e1360
LC
AC
BC
Caox
AS
AP
26.1 32.8 41.1 0 0 0 0 0 0 9.2
37.1 43.8 19.1 0 0 0 0 0 0 9.6
18.8 17.2 35.1 25.2 3.4 0.3 0 0 0 7.9
90.9 8.6 0.2 0.3 0 0 0 0 0 0.6
0 0 0 0.2 0.7 19.0 45.2 39.4 0 0.3
0 0 0 2.2 7.2 54.5 27.3 8.8 0 2.0
ignites, the oven temperature has to be reduced in the subsequent experiment; if not, the oven temperature has to be increased. This procedure is repeated, always with new sample material, until an ignition and a non-ignition can be discriminated within an oven temperature range of 2 K. The highest oven temperature for which ignition did not occur, is considered to be the self-ignition temperature TSI with respect to the volume-to-surface ratio of the sample. This procedure has to be performed for at least three, better four, different volume-to-surface ratios of the baskets. Note, that for cylinders with d ¼ h the volume-to-surface ratio is equal to d/6. To extrapolate the laboratory scale experiments to technical scale and to calculate the activation energy, the theory of thermal explosion by Frank Kamenetzkii (Frank-Kamenetzkii, 1959) can be used, from which Eq. (1) can be derived.
ln dc $
2 TSI 2 r
!
E 1 E$Q $r$k0 ¼ $ þ ln R TSI R$l
(1)
By plotting the data in an Arrhenius diagram (Fig. 2 and Fig. 6), the slope of the resultant line equals the activation temperature E/R, from which the activation energy can be calculated. It is also possible to extrapolate the self-ignition temperature to larger volume-to-surface ratios. 3. Results
Fig. 1. Experimental setup.
standard DIN EN 15188. The experiments took place in a laboratory oven with an internal volume of 64 l and a naturally driven air exchange rate of 30 h1. The sample baskets were equidistant cylinders (diameter d ¼ height h), made out of wire mesh, such that the ingress of fresh air was ensured. The temperature was measured in the centre of the sample and in the oven atmosphere. For the temperature measurements type K sheath thermocouples were used (d ¼ 1 mm). Multiple experimental series with different mixtures of combustible dusts (lignite coal (LC), activated carbon (AC), a mixture of activated carbon and bituminous coal (BC)) and the noncombustible additives calcium oxalate (Caox), ammonium sulphate (AS) and ammonium phosphate (AP) were performed. The particle size distribution and moisture content of all materials is given in Table 1. According to DIN EN 15188 (DIN EN 15188, 2007) the following steps must be taken to determine the TSI: The samples have to be placed in a preheated oven at a constant temperature. If the sample
The self-ignition temperatures of the “pure” combustibles (indicated by bold letters) and the mixtures investigated, obtained in the isoperibolic hot storage experiments with different sample volumes, are shown in Table 2. The numbers after the abbreviations correspond to the percentages of mass in the mixture. The right column of Table 2 contains the maximum reaction temperatures observed during the smoldering propagation. 3.1. Mixtures with lignite coal, bituminous coal and activated carbon The following paragraph describes the influence of the different inert materials on the self-ignition of the lignite coal, activated carbon and bituminous coal. The diagrams in Fig. 2 show the Arrhenius plots for different mixtures, where the red (in the web version) lines indicate the self-ignition temperatures of the pure coals. Considering the mixtures with lignite (LC) in Table 2, it can be seen, that the self-ignition temperatures were between 4 and 7 K higher compared to the pure lignite, but the slope of the lines remained almost the same, which means that there was little influence of mixing inert matter on the activation energy. The exact values of the activation temperatures are given in the second right column of Table 2.
B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
337
Fig. 2. Arrhenius diagram of lignite coal (LC), bituminous coal (BC) and activated carbon (AC) with different inert materials.
Fig. 3. Temperature-time curves for the isoperibolic hot storage of 1600 cm3 activated carbon with different inert materials. Fig. 4. Temperature-time curves for the isoperibolic hot storage of 100 cm3 lignite with different admixtures of ammonium phosphate.
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B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
Fig. 5. Lignite particle (left) and lignite with dissolved ammonium phosphate (right) under a scanning electron microscope with 1000-fold enlargement.
Fig. 6. Arrhenius diagram of lignite (LC).
For the bituminous coal (Fig. 2, diagram in the middle), the admixture of inert substances led to a significant increase in the self-ignition temperatures. The addition ammonium sulfate and calcium oxalate increased the self-ignition temperature for about 10 K or 25 K in all basket sizes. Due to this the slope of the lines and thereby the activation energy were not changed. The addition of ammonium phosphate also led to an increase in the self-ignition temperature; however, this revealed to be dependent on the sample volume examined. A 30 K higher selfignition temperature was determined for the 100 cm3 basket, whereas the self-ignition temperature rose for only 12 K in the 3200 cm3 basket. This dependence led ultimately to a change in the slope of the line and thus to a lower activation energy. Different results were obtained from the experiments with the activated carbon (Fig. 2, diagram at the bottom). An admixture of 20% calcium oxalate rose the activation energy up to 15% compared to the “pure” AC. The addition of 20% ammonium phosphate reduced the self-ignition temperatures about 60e70 K. The addition of 20% ammonium sulfate also reduced the self-ignition temperatures, especially for the bigger sample volumes. This led to a decrease in the activation energy of about one-third. This was caused by the already high self-ignition temperatures of the activated carbon. Ammonium sulfate melts at about 235 C and at temperatures above 280 C the decomposition into ammonium
hydrogen sulfate, ammonia, sulfuric oxides, hydrogen sulfide and nitrogen-containing gases takes place. The decomposition is exothermic what leads to additional heating of the activated carbon. Ammonium phosphate melts at 190 C and decomposes at temperatures above 200 C. Only calcium oxalate is thermally stable in the temperature range up to 400 C. Fig. 3 shows temperature time curves for mixtures of AC with three different inert dusts. The oven temperature Toven being constant for the test runs is given in brackets whereby the self-ignition temperature is at maximum 2 K below this temperature. The illustrated temperatures represent the reaction temperature in the sample center. Using a 1600 cm3 basket, the pure activated carbon (red (in the web version) curve) ignited at 298 C after 3 h. Due to the addition of 20% ammonium sulfate and ammonium phosphate the self-ignition temperature was reduced for slightly more than 60 K. Considering these experiments, ignition occurred after relatively long induction times. For example the mixture with the ammonium sulfate entered into a runaway after more than 60 h. This can be explained by the high heat absorption capacity of the salt which delayed the ignition. In contrast to this, only the addition of calcium oxalate led to an increase of the self-ignition temperature of the activated carbon. However, the temperature rose in this mixture up to 900 C, while the embers in the combustion of the pure activated carbon reached
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Table 2 Self-ignition temperatures for lignite coal dust (LC), activated carbon (AC) and activated carbon mixed with bituminous coal (BC) and mixtures of these with inert substances; activation temperatures and maximum reaction temperatures. Samplea
Self-ignition temperature TSI 100 cm3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a b
LC LC80-AP20 LC80-AP20 dis LC80-AS20 LC80-AS20 dis LC80-Caox20 LC60-AP40 LC60-AP40 dis LC60-AS40 LC60-AS40 dis LC60-Caox40 BC BC90-AP10 BC80-AP20 BC80-AS20 BC80-Caox20 BC60-AP40 BC60-AS40 BC60-Caox40 AC AC80-Caox20 AC80-AS20 AC80-AP20
118 125 142 125 126 126 132 158 131 129 144 149 163 179 160 169 199 182 185
C C C C C C C C C C C C C C C C C C C
340 C 286 C
200 cm3 114 119 136 117 120 120 124 148 125 124 136 140 152 172 150 167 192 170 181 341 341 297 272
C C C C C C C C C C C C C C C C C C C C C C C
Tmaxb
E/R [K] 400 cm3
107 C
113 C 114 C
800 cm3
101 C 105 C 123 C
134 C
162 C
96 C 102 C
3200 cm3 91 C 95 C
101 C
95 C
108 C
95 C 101 C
106 C
102 C
108 C 113 C 125 C
120 C 118 C
1600 cm3
111 C 126 131 144 134 154 177 150
102 115 121 127 123
C C C C C C C
175 C 312 C
240 C
C C C C C
141 C 165 C 135 C 160 295 303 233 232
C C C C C
287 292 220 223
C C C C
12,060 11,988 12,449 12,224 12,234 11,053 12,377 7,782 12,364 12,312 8,855 11,407 9,781 8,097 10,951 11,809 14,132 9,520 14,729 12,221 13,881 6,873 10,709
± 308 ± 526 ± 175 ± 493 ± 42 ± 172 ± 432 ± 256 ± 492 ± 589 ± 191 ± 366 ± 881 ± 450 ± 563 ± 1,995 ± 129 ± 296 ± 1,232 ± 669 ± 37 ± 392 ± 553
649 649 607 623 611 774 661 582 547 631 774 599 615 665 637 725 598 657 842 779 938 804 623
C C C C C C C C C C C C C C C C C C C C C C C
The numbers after the abbreviations correspond to the percentages of mass in the mixture. Maximum sample temperature, which occurred in the smoldering combustion in the respective test series.
only about 700 C. Also in the mixtures of lignite and bituminous coal with calcium oxalate significantly higher combustion temperatures were observed. The highest sample temperatures measured in the different series of experiments are shown in the right column of Table 2. Similar observations have previously been reported by Schossig et al. (Schossig et al., 2010), where various mixtures of cellulose with diatomite dust were investigated. Up to now a clear explanation for the observed increase in the reaction temperature for the mixtures compared to the combustibles alone cannot be given.
3.2. Different mixing methods The influence of the method used to produce the mixtures on the self-ignition behavior of coal was examined in a further series of test runs. Smith et al. (Smith et al., 1988) had found that there is a greater inhibition effect by the slurry addition of the additives. Watanabe and Zhang (Watanabe and Zhang, 2001) examined wetmixed and ion-exchanged coal samples with different acetates. In the present study, the ammonium sulfate and ammonium phosphate were mixed to the lignite either as dry solid particles (further referred to as “undissolved”) or as an aqueous solution with subsequent drying to reach the initial moisture content of the lignite (further referred to as “dissolved”). Fig. 4 shows the temperaturetime curves for the isoperibolic hot storage of “pure” lignite as well as the mixtures with 20% and 40% dissolved or undissolved ammonium phosphate. The red (in the web version) solid curve shows the temperaturetime behavior for lignite without an additive, which is used as a reference curve for the mixtures. After about 350 min, a sudden increase of the sample temperature occurred and the lignite started to burn. The temperature in the coal sample rose to 650 C. After 500 min, the coal burned out entirely and the remaining ash took the oven temperature. Comparing the adjusted oven temperatures (in brackets in Fig. 4), it is obvious that higher temperatures were necessary for the
ignition of the mixtures compared to lignite, which is also reflected in the higher TSI. To ignite the lignite mixed with the dissolved ammonium phosphate the oven temperature had to be another 20 K higher, than for the lignite mixed with ammonium phosphate in dry state. Due to the higher oven temperature, the sample temperature in the mixtures increased earlier than in the pure lignite. However, the mixture with 40% dissolved ammonium phosphate is an exception. Here the sample temperature stagnated at 180 C for several hours, before it came to a further temperature rise and consequently to the ignition. The melting point of the ammonium phosphate lies at 190 C. The temperature remained constant during the fusion process. Fig. 5 shows a lignite particle (left) and lignite with dissolved and subsequently solidified ammonium phosphate (right) under a scanning electron microscope with 1000-fold enlargement. In Table 2 (line 7 and 8) it can be seen that the additional influence of the dissolved ammonium phosphate decreased with increasing volume. Whereas in a 40% admixture the TSI at a volume of 100 cm3 rose by further 26 K, at a volume of 800 cm3 the rise was only 12 K. Fig. 6 shows the measured TSI for the mixtures with the dissolved and undissolved ammonium phosphate in an Arrhenius plot. Based on the theory of Frank Kamenetzkii (Frank-Kamenetzkii, 1959) the TSI values for each series of measurements lie on a straight line with the slope E/R, from which the activation energy can be determined. For the mixtures the line shifts due to the higher TSI further to the left. However, the slope, and therefore also the activation energy, do not change. The only exception is the above-mentioned mixture with 40% dissolved ammonium phosphate, which leads to a reduction of the activation energy. The activation temperature determined for each sample is shown in the second column from the right of Table 2. By the adding ammonium sulfate, the TSI of the lignite could be also raised. Adding the undissolved salt nearly led to the same results like the mixtures with the ammonium phosphate. However,
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no significant difference could be ascertained here in the kind of the mixing (Table 2, line 4/5 and 9/10). 3.3. STA experiments To verify the results of the hot storage basket tests, the samples were also examined by simultaneous-thermal analysis (STA), coupled with a Fourier-transformed infrared spectrometer (FTIR) to analyze the composition of gaseous reaction products. STA is a combination of thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The initial sample mass was 20 mg, the heating rate was set to 5 K/min. The measuring system was flushed with synthetic air at a rate of 10 ml/min. In Fig. 7 the TG and DSC curves of different mixtures with lignite and calcium oxalate are shown. The red (in the web version) line in
the mass diagram characterizes the mass loss of the pure lignite. A two-step process is recognizable. As soon as the coal is heated up, the moisture contained in the coal is desorbed and highly volatile matters evaporate. Therefore in the first step a mass loss of 10% occurred. At temperatures above 200 C the combustion of the lignite began, showing a further decrease in the TG signal and a mass loss of 85%. The light blue line expresses the mass loss of the pure calcium oxalate, which undergoes a three-step reaction. At temperatures of about 100e200 C the water vapor was removed from the calcium oxalate. Above 450 C carbon monoxide was released, whereby calcium carbonate was formed. The carbon monoxide was further oxidized to carbon dioxide. Above 700 C carbon dioxide was removed from the calcium carbonate with calcium oxide
Fig. 7. TG-curve, DSC-curve, CO2 and CO concentration in STA experiments coupled with FTIR of lignite and mixtures with calcium oxalate.
B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
remaining. Mass losses of about 13% in the first, 20% in the second and 30% in the third step were measured. The TG curves of the mixtures reflect the typical reaction steps of the pure substances. The more Calcium oxalate is added the more the influence of the three reaction steps becomes dominant. The mass signals are ordered by the percentage of calcium oxalate, due to the higher amount if non-reactive material. The final mass of each experiment consists of the inert decomposition product calcium oxide and the ash from the lignite coal. It is evident that the combustion of the lignite is inhibited at the beginning by the addition of calcium oxalate. The mass loss of the mixtures in the 2nd step occurs at 10e20 K higher temperatures. On the one hand is calcium oxalate a heat sink on the other hand the water vapor formed by the decomposition of calcium oxalate creates an inert atmosphere. This is also a reason for the higher selfignition temperatures, which were determined in the hot storage tests. The second diagram in Fig. 7 shows the corresponding DSC curves. In relation to the increasing amount of inert material, the large exothermic peaks which are characterizing the combustion process are narrower and shallower. The released energy is, following this, smaller. The light blue DSC curve of the pure calcium oxalate clearly shows three peaks, which are attributed to the three characteristic reactions mentioned. The first and the last peak are endothermic. The peak between 400 and 500 C characterizes the separation of carbon monoxide. However, the heat of reaction of CO (þ63 kJ/mol) is compensated by the strongly exothermic oxidation to CO2 (281 kJ/mol). Therefore only an exothermic peak is visible (Schulz, 2003). The last endothermic reaction (elimination of carbon dioxide), is not able to interfere the combustion of lignite, since the lignite is already burned. The three peaks could be found in a weakened form also in the DSC curves of the mixtures. The results from the gas analysis are also shown in Fig. 7. At temperatures below 200 C no carbon dioxide and carbon monoxide could be identified. Above 200 C the CO and CO2 concentrations increased rapidly. Due to the high concentration of CO compared to CO2 it can be concluded that the combustion reaction was incomplete. The evolution of CO for “pure” calcium oxalate (light blue) shows a distinct peak at temperatures around 500 C. at this temperature carbon monoxide was released from the calcium oxalate.
Fig. 8. Logarithmic plot of the TG signal for lignite coal and a mixture with 40% ammonium phosphate.
341
Since the CO was partly oxidized, there is also a peak at this point in the CO2 signal. The other peak at 800 C shows the CO2 separation from the remaining calcium carbonate. These peaks are visible in a weakened form in the concentration profiles of the mixtures. The curves arrange themselves according to the mass fraction of the calcium oxalate. The large peak of the lignite is correspondingly smaller. The activation energy can be calculated also from the TG signal. The basic principle is the mass change per time for a first order reaction. According to Eq. (2), the activation temperature E/R can be calculated (Torrent and Querol, 2009).
1 dm E ¼ þ lnðk0 Þ ln $ m dt R$T
(2)
Eq. (2) can be transformed is a linear equation with the slope E/R. To find the steepest slope in a diagram (Fig. 8) with the abscissa 1/T and the ordinate ln(1/m*dm/dt) a program in MATLAB was used. This tool calculated all slopes within a given length range using the MATLAB curve fitting toolbox. The program flowchart is given in Fig. 9. The slopes where evaluated by the steepness and the sum of squares of differences. Finally the results had to be checked for plausibility. In Table 3 the calculated activation temperatures from the TGsignal of the STA are compared with those from the hot storage basket tests. The activation energy calculated from the TG is slightly lower than that of the hot storage tests. The reason is that the STA
Fig. 9. Program flowchart.
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B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
Table 3 Comparison of the activation temperatures of the hot storage basket tests and the TG. Sample
E/R [K]
Temperature range [ C]
Average E/R [K]
E/R from hot storage test [K]
LC
11,070 11,509 11,358 10,387 9,369 9,348 10,115 9,484 9,026 10,990 9,082 8,549 9,868 9,318 9,588 9,935 9,240 9,601 11,699 9,075 9,638 9,232 13,286 10,338 10,598 15,339 14,790 19,562 22,165 10,141
222e240 225e243 232e250 220e238 236e254 230e246 244e262 236e254 237e255 246e264 247e265 236e263 218e248 219e240 217e238 244e262 220e241 216e237 242e259 261e281 238e259 178e198 171e189 170e188 190e206 173e188 172e190 172e187 173e188 179e192
11,312
12,060
9,701
11,053
9,542
8,855
LC80-Caox20
LC60-Caox40
LC40-Caox60
LC80-AS20
LC60-AS40
LC40-AS60
LC80-AP20
LC60-AP40
LC40-AP60
9,540
e
9,591
12,224
9,592
12,377
10,137
e
10,952
11,988
13,576
12,377
17,289
e
experiments start at ambient temperature and then the sample is heated at 5 K/min. Therefore also the reactions in the coal are detected which occur at low temperatures. The hot storage basket tests are carried out at a constant temperature in the range of 100e350 C (depending on the sample volume) and presumably low-temperature reactions may be overridden. For the mixtures with ammonium phosphate the activation temperature calculated from the TG-experiments is higher than the one determined from the hot storage test due to an overlap of the melting and the burn away. Evaluating the slopes of the mixtures, it is difficult to determine the steepest slope which reflects the self-ignition reaction (Fig. 8). Due to the melting and the decomposition of the additives, different slopes occur in the TG signal (Fig. 8 e black line). In this way uncertainties cannot be excluded. Furthermore, the question arises to what extent the sample mass examined in the STA (approximately 20 mg) is representative. Furthermore, it is difficult to achieve homogeneity of the mixtures. 4. Conclusions The experimental results show, that the addition of inert materials has a significant influence on the self-ignition temperature as well as on the maximum reaction temperature of the coals under
investigation. Also the kind of the admixture has an influence on the TSI. Thus the TSI of lignite could further be increased by using previously dissolved ammonium phosphate instead of dry mixing. A possible explanation for the different influences of the additives is the different self-ignition temperatures of the combustibles. This depends on the one hand on the substance, but on the other hand also on the sample size. If the melting/decomposition point of the additive is in the same range of temperature as the self-ignition temperature of the coal, the influence is highest. Obviously, the comparatively small amount of heat released during thermal decomposition of the inert material promotes self-ignition of the combustible fraction. However, the additives have different effects on the different investigated coals: The TSI of the lignite could be raised by the additives in all experiments. The activation energy was widely unchanged. In the experiments with the activated carbon only the admixture of calcium oxalate led to a rise of the TSI. The TSI of the mixtures with the BC were higher than those of the pure coal. However the influence decreased with increasing volumes, which led to a slight reduction of the activation energy. Furthermore for the mixtures with calcium oxalate substantially higher combustion temperatures were reached. For this no conclusive explanation could be found so far. New results are expected from extended measurements and simulations, which map the different heat and mass transfer mechanisms in the mixtures of the different bulk materials. Furthermore investigations are planned with quenching the self-ignition at different stages of its development. In this way it will hopefully become possible to look directly in the reaction process.
References Babrauskas, V., 2003. Ignition Handbook. Fire Science Publishers, Issaquah, WA. DIN EN 15188, 2007. Bestimmung des Selbstentzündungsverhaltens von Staub€tze. Beuth schüttungen. In: Normenausschuss Sicherheitstechnische Grundsa Verlag, Berlin. €rmeübertragung in der chemischen Frank-Kamenetzkii, D.A., 1959. Stoff- und Wa €ttingen/Heidelberg, pp. 130e134. Kinetik. Springer-Verlag, Berlin/Go Hoischen, U., Kayser, J., 2009. In: Krause, U. (Ed.), Fires in Silos. WILEY-VCH Verlag GmbH, Weinheim (Chapter 7). Schossig, J., Berger, A., Malow, M., Krause, U., 2010. Beurteilung und Verhinderung von Selbstentzündung und Brandgasemmission bei der Lagerung von Massenschüttgütern und Deponiestoffen. BAM Forschungsbericht 291. Schulz, B., 2003. Über die Thermogravimetrie und die Differenzthermoanalyse. URL: http://www.lernmaus.de/cont/ch/dta-tgzvo.pdf. Smith, Alex C., Miron, Yael, Lazzara, Charles P., 1988. Inhibition of Spontaneous Combustion of Coal. Report of Investigations e USA Bureau of Mines, RI 9196. Sujanti, Wiwik, Zhang, Dong-ke, 1999. A laboratory study of spontaneous combustion of coal: the influence of inorganic matter and reactor size. Fuel 78 (5), 549e556. Torrent, J.G., Querol, E., 2009. In: Krause, U. (Ed.), Fires in Silos. WILEY-VCH Verlag GmbH, Weinheim (Chapter 3). Watanabe, Wiwik S., Zhang, Dong-ke, 2001. The effect of inherent and added inorganic matter on low-temperature oxidation reaction of coal. Fuel Process. Technol. 74 (3), 145e160. Zhan, Jing, Wang, Hai-Hui, Song, Sheng-Nan, Hu, Yuan, Li, Jiao, 2011. Role of an additive in retarding coal oxidation at moderate temperatures. Proc. Combust. Inst. 33 (2), 2515e2522.
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Influence of inert materials on the self-ignition of flammable dusts Benjamin Binkau*, Christoph Wanke, Ulrich Krause €tsplatz 2, 39106, Magdeburg, Germany Otto-von-Guericke-University Magdeburg, Dept. of Plant Design and Process Safety, Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2014 Received in revised form 18 November 2014 Accepted 26 November 2014 Available online 27 November 2014
Flammable solid bulk materials, including dusts, often undergo spontaneous combustion and the spread of reaction fronts. By addition of inert substances, the ignition and combustion behavior can be influenced. In a series of experiments different types of coal were mixed with inert powders to study the effect of the composition on the self-ignition temperature and on the formal kinetic parameters. Hot storage tests as well as simultaneous-thermal analysis were used as experimental techniques with the latter being coupled to FTIR measurements to analyze the composition of gaseous reaction products. All conducted hot storage experiments led to the conclusion that the self-ignition temperature was increased by admixing inert material if the decomposition temperature of the inert matter was higher than the self-ignition temperature of the combustible component at the sample characteristic length. If (exothermic) decomposition of the inert material occurred before a noticeable growth of reaction rate of the combustible material, even a reduction in the self-ignition temperature could be observed. In addition, significantly higher maximum reaction temperatures were observed for the mixtures than for the combustible material alone. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Combustible dusts Inert substances Self-ignition Simultaneous thermal analysis
1. Introduction Combustible bulk materials are usually stored in quantities up to several thousands of tons over extended periods of time. It is a wellknown fact, that large amounts of combustible material tend to self-ignition, even if stored at ambient temperature. Packages of porous solids at smaller dimensions can undergo self-ignition if exposed to elevated temperatures. Well known examples are filters in gas cleaning systems and cakings in dryers. Self-ignition occurs, if the heat, released from a low rate oxidation at the active surfaces of the individual particles cannot be dissipated via the outer surface of the deposit. This leads to a temperature rise, which itself further accelerates the reaction. This is ending in a positive feedback loop, until the material catches fire (Babrauskas, 2003). Once a fire is ignited, it produces large amounts of smoke and possibly toxic gases. Depending on volume and shape of the deposit, even more than one hot spot may occur and initiate the formation of hidden smoldering nests. Such hidden smoldering nests may break through to the surface of the deposit several days, weeks or even months after the onset of the self-ignition. Because
* Corresponding author. E-mail address: [email protected] (B. Binkau). http://dx.doi.org/10.1016/j.jlp.2014.11.017 0950-4230/© 2014 Elsevier Ltd. All rights reserved.
of the very time consuming and costly firefighting, a prevention of the spontaneous combustion is always preferable (Hoischen and Kayser, 2009). While a vast knowledge has been gained on selfignition of non-mixed combustibles, comparatively little is known about the effect of non-reactive additives on the process of selfignition. Studies discussing this effect were reported by Smith (Smith et al., 1988), Zhan (Zhan et al., 2011) and Zhang (Sujanti and Zhang, 1999; Watanabe and Zhang, 2001). The aim of the present study is to determine the influence of mixing inorganic salts on the self-ignition behavior of different types of coal dust. The salts where chosen due to their application in fire-extinguishing powder, as flame retardants or respectively due to their property to emit inert gases when decomposed, respectively. The samples were investigated by hot storage experiments and simultaneous-thermal analysis (STA) in combination with Fourier transformed infrared spectroscopy (FTIR) measurements to perform a qualitative analysis of the gaseous reaction products. 2. Experimental set-up and material properties In hot storage tests the self-ignition temperature (TSI), the activation temperature E/R and the induction time tind were determined. The experimental setup is shown in Fig. 1. The experimental procedure was in accordance with the European
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B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342 Table 1 Material properties of the pure substances (rates in %).
Nomenclature d E k0 Q r R TSI t T cp m c
diameter (m) activation energy (kJ/mol) pre-exponential factor (1/s) reaction heat (kJ/kg) radius (m) universal gas constant (J/K-mol) self-ignition temperature ( C) time (h) temperature (K) specific heat capacity (J/kg-K) mass (kg) volume concentration (%)
Greek
d l r
Frank-Kamenetskii parameter () thermal conductivity (W/m2-K) bulk density (kg/m3)
Subscripts c critical ind induction max maximum dis dissolved
Particle diameter [mm]
Moisture [%]
0e20 20e40 40e80 80e160 160e240 240e500 500e760 760e1000 1000e1360
LC
AC
BC
Caox
AS
AP
26.1 32.8 41.1 0 0 0 0 0 0 9.2
37.1 43.8 19.1 0 0 0 0 0 0 9.6
18.8 17.2 35.1 25.2 3.4 0.3 0 0 0 7.9
90.9 8.6 0.2 0.3 0 0 0 0 0 0.6
0 0 0 0.2 0.7 19.0 45.2 39.4 0 0.3
0 0 0 2.2 7.2 54.5 27.3 8.8 0 2.0
ignites, the oven temperature has to be reduced in the subsequent experiment; if not, the oven temperature has to be increased. This procedure is repeated, always with new sample material, until an ignition and a non-ignition can be discriminated within an oven temperature range of 2 K. The highest oven temperature for which ignition did not occur, is considered to be the self-ignition temperature TSI with respect to the volume-to-surface ratio of the sample. This procedure has to be performed for at least three, better four, different volume-to-surface ratios of the baskets. Note, that for cylinders with d ¼ h the volume-to-surface ratio is equal to d/6. To extrapolate the laboratory scale experiments to technical scale and to calculate the activation energy, the theory of thermal explosion by Frank Kamenetzkii (Frank-Kamenetzkii, 1959) can be used, from which Eq. (1) can be derived.
ln dc $
2 TSI 2 r
!
E 1 E$Q $r$k0 ¼ $ þ ln R TSI R$l
(1)
By plotting the data in an Arrhenius diagram (Fig. 2 and Fig. 6), the slope of the resultant line equals the activation temperature E/R, from which the activation energy can be calculated. It is also possible to extrapolate the self-ignition temperature to larger volume-to-surface ratios. 3. Results
Fig. 1. Experimental setup.
standard DIN EN 15188. The experiments took place in a laboratory oven with an internal volume of 64 l and a naturally driven air exchange rate of 30 h1. The sample baskets were equidistant cylinders (diameter d ¼ height h), made out of wire mesh, such that the ingress of fresh air was ensured. The temperature was measured in the centre of the sample and in the oven atmosphere. For the temperature measurements type K sheath thermocouples were used (d ¼ 1 mm). Multiple experimental series with different mixtures of combustible dusts (lignite coal (LC), activated carbon (AC), a mixture of activated carbon and bituminous coal (BC)) and the noncombustible additives calcium oxalate (Caox), ammonium sulphate (AS) and ammonium phosphate (AP) were performed. The particle size distribution and moisture content of all materials is given in Table 1. According to DIN EN 15188 (DIN EN 15188, 2007) the following steps must be taken to determine the TSI: The samples have to be placed in a preheated oven at a constant temperature. If the sample
The self-ignition temperatures of the “pure” combustibles (indicated by bold letters) and the mixtures investigated, obtained in the isoperibolic hot storage experiments with different sample volumes, are shown in Table 2. The numbers after the abbreviations correspond to the percentages of mass in the mixture. The right column of Table 2 contains the maximum reaction temperatures observed during the smoldering propagation. 3.1. Mixtures with lignite coal, bituminous coal and activated carbon The following paragraph describes the influence of the different inert materials on the self-ignition of the lignite coal, activated carbon and bituminous coal. The diagrams in Fig. 2 show the Arrhenius plots for different mixtures, where the red (in the web version) lines indicate the self-ignition temperatures of the pure coals. Considering the mixtures with lignite (LC) in Table 2, it can be seen, that the self-ignition temperatures were between 4 and 7 K higher compared to the pure lignite, but the slope of the lines remained almost the same, which means that there was little influence of mixing inert matter on the activation energy. The exact values of the activation temperatures are given in the second right column of Table 2.
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Fig. 2. Arrhenius diagram of lignite coal (LC), bituminous coal (BC) and activated carbon (AC) with different inert materials.
Fig. 3. Temperature-time curves for the isoperibolic hot storage of 1600 cm3 activated carbon with different inert materials. Fig. 4. Temperature-time curves for the isoperibolic hot storage of 100 cm3 lignite with different admixtures of ammonium phosphate.
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Fig. 5. Lignite particle (left) and lignite with dissolved ammonium phosphate (right) under a scanning electron microscope with 1000-fold enlargement.
Fig. 6. Arrhenius diagram of lignite (LC).
For the bituminous coal (Fig. 2, diagram in the middle), the admixture of inert substances led to a significant increase in the self-ignition temperatures. The addition ammonium sulfate and calcium oxalate increased the self-ignition temperature for about 10 K or 25 K in all basket sizes. Due to this the slope of the lines and thereby the activation energy were not changed. The addition of ammonium phosphate also led to an increase in the self-ignition temperature; however, this revealed to be dependent on the sample volume examined. A 30 K higher selfignition temperature was determined for the 100 cm3 basket, whereas the self-ignition temperature rose for only 12 K in the 3200 cm3 basket. This dependence led ultimately to a change in the slope of the line and thus to a lower activation energy. Different results were obtained from the experiments with the activated carbon (Fig. 2, diagram at the bottom). An admixture of 20% calcium oxalate rose the activation energy up to 15% compared to the “pure” AC. The addition of 20% ammonium phosphate reduced the self-ignition temperatures about 60e70 K. The addition of 20% ammonium sulfate also reduced the self-ignition temperatures, especially for the bigger sample volumes. This led to a decrease in the activation energy of about one-third. This was caused by the already high self-ignition temperatures of the activated carbon. Ammonium sulfate melts at about 235 C and at temperatures above 280 C the decomposition into ammonium
hydrogen sulfate, ammonia, sulfuric oxides, hydrogen sulfide and nitrogen-containing gases takes place. The decomposition is exothermic what leads to additional heating of the activated carbon. Ammonium phosphate melts at 190 C and decomposes at temperatures above 200 C. Only calcium oxalate is thermally stable in the temperature range up to 400 C. Fig. 3 shows temperature time curves for mixtures of AC with three different inert dusts. The oven temperature Toven being constant for the test runs is given in brackets whereby the self-ignition temperature is at maximum 2 K below this temperature. The illustrated temperatures represent the reaction temperature in the sample center. Using a 1600 cm3 basket, the pure activated carbon (red (in the web version) curve) ignited at 298 C after 3 h. Due to the addition of 20% ammonium sulfate and ammonium phosphate the self-ignition temperature was reduced for slightly more than 60 K. Considering these experiments, ignition occurred after relatively long induction times. For example the mixture with the ammonium sulfate entered into a runaway after more than 60 h. This can be explained by the high heat absorption capacity of the salt which delayed the ignition. In contrast to this, only the addition of calcium oxalate led to an increase of the self-ignition temperature of the activated carbon. However, the temperature rose in this mixture up to 900 C, while the embers in the combustion of the pure activated carbon reached
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Table 2 Self-ignition temperatures for lignite coal dust (LC), activated carbon (AC) and activated carbon mixed with bituminous coal (BC) and mixtures of these with inert substances; activation temperatures and maximum reaction temperatures. Samplea
Self-ignition temperature TSI 100 cm3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a b
LC LC80-AP20 LC80-AP20 dis LC80-AS20 LC80-AS20 dis LC80-Caox20 LC60-AP40 LC60-AP40 dis LC60-AS40 LC60-AS40 dis LC60-Caox40 BC BC90-AP10 BC80-AP20 BC80-AS20 BC80-Caox20 BC60-AP40 BC60-AS40 BC60-Caox40 AC AC80-Caox20 AC80-AS20 AC80-AP20
118 125 142 125 126 126 132 158 131 129 144 149 163 179 160 169 199 182 185
C C C C C C C C C C C C C C C C C C C
340 C 286 C
200 cm3 114 119 136 117 120 120 124 148 125 124 136 140 152 172 150 167 192 170 181 341 341 297 272
C C C C C C C C C C C C C C C C C C C C C C C
Tmaxb
E/R [K] 400 cm3
107 C
113 C 114 C
800 cm3
101 C 105 C 123 C
134 C
162 C
96 C 102 C
3200 cm3 91 C 95 C
101 C
95 C
108 C
95 C 101 C
106 C
102 C
108 C 113 C 125 C
120 C 118 C
1600 cm3
111 C 126 131 144 134 154 177 150
102 115 121 127 123
C C C C C C C
175 C 312 C
240 C
C C C C C
141 C 165 C 135 C 160 295 303 233 232
C C C C C
287 292 220 223
C C C C
12,060 11,988 12,449 12,224 12,234 11,053 12,377 7,782 12,364 12,312 8,855 11,407 9,781 8,097 10,951 11,809 14,132 9,520 14,729 12,221 13,881 6,873 10,709
± 308 ± 526 ± 175 ± 493 ± 42 ± 172 ± 432 ± 256 ± 492 ± 589 ± 191 ± 366 ± 881 ± 450 ± 563 ± 1,995 ± 129 ± 296 ± 1,232 ± 669 ± 37 ± 392 ± 553
649 649 607 623 611 774 661 582 547 631 774 599 615 665 637 725 598 657 842 779 938 804 623
C C C C C C C C C C C C C C C C C C C C C C C
The numbers after the abbreviations correspond to the percentages of mass in the mixture. Maximum sample temperature, which occurred in the smoldering combustion in the respective test series.
only about 700 C. Also in the mixtures of lignite and bituminous coal with calcium oxalate significantly higher combustion temperatures were observed. The highest sample temperatures measured in the different series of experiments are shown in the right column of Table 2. Similar observations have previously been reported by Schossig et al. (Schossig et al., 2010), where various mixtures of cellulose with diatomite dust were investigated. Up to now a clear explanation for the observed increase in the reaction temperature for the mixtures compared to the combustibles alone cannot be given.
3.2. Different mixing methods The influence of the method used to produce the mixtures on the self-ignition behavior of coal was examined in a further series of test runs. Smith et al. (Smith et al., 1988) had found that there is a greater inhibition effect by the slurry addition of the additives. Watanabe and Zhang (Watanabe and Zhang, 2001) examined wetmixed and ion-exchanged coal samples with different acetates. In the present study, the ammonium sulfate and ammonium phosphate were mixed to the lignite either as dry solid particles (further referred to as “undissolved”) or as an aqueous solution with subsequent drying to reach the initial moisture content of the lignite (further referred to as “dissolved”). Fig. 4 shows the temperaturetime curves for the isoperibolic hot storage of “pure” lignite as well as the mixtures with 20% and 40% dissolved or undissolved ammonium phosphate. The red (in the web version) solid curve shows the temperaturetime behavior for lignite without an additive, which is used as a reference curve for the mixtures. After about 350 min, a sudden increase of the sample temperature occurred and the lignite started to burn. The temperature in the coal sample rose to 650 C. After 500 min, the coal burned out entirely and the remaining ash took the oven temperature. Comparing the adjusted oven temperatures (in brackets in Fig. 4), it is obvious that higher temperatures were necessary for the
ignition of the mixtures compared to lignite, which is also reflected in the higher TSI. To ignite the lignite mixed with the dissolved ammonium phosphate the oven temperature had to be another 20 K higher, than for the lignite mixed with ammonium phosphate in dry state. Due to the higher oven temperature, the sample temperature in the mixtures increased earlier than in the pure lignite. However, the mixture with 40% dissolved ammonium phosphate is an exception. Here the sample temperature stagnated at 180 C for several hours, before it came to a further temperature rise and consequently to the ignition. The melting point of the ammonium phosphate lies at 190 C. The temperature remained constant during the fusion process. Fig. 5 shows a lignite particle (left) and lignite with dissolved and subsequently solidified ammonium phosphate (right) under a scanning electron microscope with 1000-fold enlargement. In Table 2 (line 7 and 8) it can be seen that the additional influence of the dissolved ammonium phosphate decreased with increasing volume. Whereas in a 40% admixture the TSI at a volume of 100 cm3 rose by further 26 K, at a volume of 800 cm3 the rise was only 12 K. Fig. 6 shows the measured TSI for the mixtures with the dissolved and undissolved ammonium phosphate in an Arrhenius plot. Based on the theory of Frank Kamenetzkii (Frank-Kamenetzkii, 1959) the TSI values for each series of measurements lie on a straight line with the slope E/R, from which the activation energy can be determined. For the mixtures the line shifts due to the higher TSI further to the left. However, the slope, and therefore also the activation energy, do not change. The only exception is the above-mentioned mixture with 40% dissolved ammonium phosphate, which leads to a reduction of the activation energy. The activation temperature determined for each sample is shown in the second column from the right of Table 2. By the adding ammonium sulfate, the TSI of the lignite could be also raised. Adding the undissolved salt nearly led to the same results like the mixtures with the ammonium phosphate. However,
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no significant difference could be ascertained here in the kind of the mixing (Table 2, line 4/5 and 9/10). 3.3. STA experiments To verify the results of the hot storage basket tests, the samples were also examined by simultaneous-thermal analysis (STA), coupled with a Fourier-transformed infrared spectrometer (FTIR) to analyze the composition of gaseous reaction products. STA is a combination of thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The initial sample mass was 20 mg, the heating rate was set to 5 K/min. The measuring system was flushed with synthetic air at a rate of 10 ml/min. In Fig. 7 the TG and DSC curves of different mixtures with lignite and calcium oxalate are shown. The red (in the web version) line in
the mass diagram characterizes the mass loss of the pure lignite. A two-step process is recognizable. As soon as the coal is heated up, the moisture contained in the coal is desorbed and highly volatile matters evaporate. Therefore in the first step a mass loss of 10% occurred. At temperatures above 200 C the combustion of the lignite began, showing a further decrease in the TG signal and a mass loss of 85%. The light blue line expresses the mass loss of the pure calcium oxalate, which undergoes a three-step reaction. At temperatures of about 100e200 C the water vapor was removed from the calcium oxalate. Above 450 C carbon monoxide was released, whereby calcium carbonate was formed. The carbon monoxide was further oxidized to carbon dioxide. Above 700 C carbon dioxide was removed from the calcium carbonate with calcium oxide
Fig. 7. TG-curve, DSC-curve, CO2 and CO concentration in STA experiments coupled with FTIR of lignite and mixtures with calcium oxalate.
B. Binkau et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 335e342
remaining. Mass losses of about 13% in the first, 20% in the second and 30% in the third step were measured. The TG curves of the mixtures reflect the typical reaction steps of the pure substances. The more Calcium oxalate is added the more the influence of the three reaction steps becomes dominant. The mass signals are ordered by the percentage of calcium oxalate, due to the higher amount if non-reactive material. The final mass of each experiment consists of the inert decomposition product calcium oxide and the ash from the lignite coal. It is evident that the combustion of the lignite is inhibited at the beginning by the addition of calcium oxalate. The mass loss of the mixtures in the 2nd step occurs at 10e20 K higher temperatures. On the one hand is calcium oxalate a heat sink on the other hand the water vapor formed by the decomposition of calcium oxalate creates an inert atmosphere. This is also a reason for the higher selfignition temperatures, which were determined in the hot storage tests. The second diagram in Fig. 7 shows the corresponding DSC curves. In relation to the increasing amount of inert material, the large exothermic peaks which are characterizing the combustion process are narrower and shallower. The released energy is, following this, smaller. The light blue DSC curve of the pure calcium oxalate clearly shows three peaks, which are attributed to the three characteristic reactions mentioned. The first and the last peak are endothermic. The peak between 400 and 500 C characterizes the separation of carbon monoxide. However, the heat of reaction of CO (þ63 kJ/mol) is compensated by the strongly exothermic oxidation to CO2 (281 kJ/mol). Therefore only an exothermic peak is visible (Schulz, 2003). The last endothermic reaction (elimination of carbon dioxide), is not able to interfere the combustion of lignite, since the lignite is already burned. The three peaks could be found in a weakened form also in the DSC curves of the mixtures. The results from the gas analysis are also shown in Fig. 7. At temperatures below 200 C no carbon dioxide and carbon monoxide could be identified. Above 200 C the CO and CO2 concentrations increased rapidly. Due to the high concentration of CO compared to CO2 it can be concluded that the combustion reaction was incomplete. The evolution of CO for “pure” calcium oxalate (light blue) shows a distinct peak at temperatures around 500 C. at this temperature carbon monoxide was released from the calcium oxalate.
Fig. 8. Logarithmic plot of the TG signal for lignite coal and a mixture with 40% ammonium phosphate.
341
Since the CO was partly oxidized, there is also a peak at this point in the CO2 signal. The other peak at 800 C shows the CO2 separation from the remaining calcium carbonate. These peaks are visible in a weakened form in the concentration profiles of the mixtures. The curves arrange themselves according to the mass fraction of the calcium oxalate. The large peak of the lignite is correspondingly smaller. The activation energy can be calculated also from the TG signal. The basic principle is the mass change per time for a first order reaction. According to Eq. (2), the activation temperature E/R can be calculated (Torrent and Querol, 2009).
1 dm E ¼ þ lnðk0 Þ ln $ m dt R$T
(2)
Eq. (2) can be transformed is a linear equation with the slope E/R. To find the steepest slope in a diagram (Fig. 8) with the abscissa 1/T and the ordinate ln(1/m*dm/dt) a program in MATLAB was used. This tool calculated all slopes within a given length range using the MATLAB curve fitting toolbox. The program flowchart is given in Fig. 9. The slopes where evaluated by the steepness and the sum of squares of differences. Finally the results had to be checked for plausibility. In Table 3 the calculated activation temperatures from the TGsignal of the STA are compared with those from the hot storage basket tests. The activation energy calculated from the TG is slightly lower than that of the hot storage tests. The reason is that the STA
Fig. 9. Program flowchart.
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Table 3 Comparison of the activation temperatures of the hot storage basket tests and the TG. Sample
E/R [K]
Temperature range [ C]
Average E/R [K]
E/R from hot storage test [K]
LC
11,070 11,509 11,358 10,387 9,369 9,348 10,115 9,484 9,026 10,990 9,082 8,549 9,868 9,318 9,588 9,935 9,240 9,601 11,699 9,075 9,638 9,232 13,286 10,338 10,598 15,339 14,790 19,562 22,165 10,141
222e240 225e243 232e250 220e238 236e254 230e246 244e262 236e254 237e255 246e264 247e265 236e263 218e248 219e240 217e238 244e262 220e241 216e237 242e259 261e281 238e259 178e198 171e189 170e188 190e206 173e188 172e190 172e187 173e188 179e192
11,312
12,060
9,701
11,053
9,542
8,855
LC80-Caox20
LC60-Caox40
LC40-Caox60
LC80-AS20
LC60-AS40
LC40-AS60
LC80-AP20
LC60-AP40
LC40-AP60
9,540
e
9,591
12,224
9,592
12,377
10,137
e
10,952
11,988
13,576
12,377
17,289
e
experiments start at ambient temperature and then the sample is heated at 5 K/min. Therefore also the reactions in the coal are detected which occur at low temperatures. The hot storage basket tests are carried out at a constant temperature in the range of 100e350 C (depending on the sample volume) and presumably low-temperature reactions may be overridden. For the mixtures with ammonium phosphate the activation temperature calculated from the TG-experiments is higher than the one determined from the hot storage test due to an overlap of the melting and the burn away. Evaluating the slopes of the mixtures, it is difficult to determine the steepest slope which reflects the self-ignition reaction (Fig. 8). Due to the melting and the decomposition of the additives, different slopes occur in the TG signal (Fig. 8 e black line). In this way uncertainties cannot be excluded. Furthermore, the question arises to what extent the sample mass examined in the STA (approximately 20 mg) is representative. Furthermore, it is difficult to achieve homogeneity of the mixtures. 4. Conclusions The experimental results show, that the addition of inert materials has a significant influence on the self-ignition temperature as well as on the maximum reaction temperature of the coals under
investigation. Also the kind of the admixture has an influence on the TSI. Thus the TSI of lignite could further be increased by using previously dissolved ammonium phosphate instead of dry mixing. A possible explanation for the different influences of the additives is the different self-ignition temperatures of the combustibles. This depends on the one hand on the substance, but on the other hand also on the sample size. If the melting/decomposition point of the additive is in the same range of temperature as the self-ignition temperature of the coal, the influence is highest. Obviously, the comparatively small amount of heat released during thermal decomposition of the inert material promotes self-ignition of the combustible fraction. However, the additives have different effects on the different investigated coals: The TSI of the lignite could be raised by the additives in all experiments. The activation energy was widely unchanged. In the experiments with the activated carbon only the admixture of calcium oxalate led to a rise of the TSI. The TSI of the mixtures with the BC were higher than those of the pure coal. However the influence decreased with increasing volumes, which led to a slight reduction of the activation energy. Furthermore for the mixtures with calcium oxalate substantially higher combustion temperatures were reached. For this no conclusive explanation could be found so far. New results are expected from extended measurements and simulations, which map the different heat and mass transfer mechanisms in the mixtures of the different bulk materials. Furthermore investigations are planned with quenching the self-ignition at different stages of its development. In this way it will hopefully become possible to look directly in the reaction process.
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