Comparative Analysis of Fire and Explosion Properties of Lycopodium Powder

Abstract

Lycopodium (L.) clavatum powder, due to its uniform particle size distribution and low equilibrium moisture content, is often used as a reference material and a calibration benchmark for dust combustion and dust explosion studies. The aim of the study was to determine its fire and explosion parameters, compare them to values obtained in the previous literature findings, and assess the appropriateness of using lycopodium powder as a reference material. The research included the determination of minimum ignition temperatures of dust layer and dust clouds, spontaneous ignition behavior, and explosion characteristics of dust clouds including maximum explosion pressure, maximum rate of explosion pressure rise, and the lower explosion limit of the air/dust mixture. The results reveal that the maximum equipment temperature used with lycopodium dust should not exceed 215 °C for dust thickness up to 5 mm. In order to eliminate the risk of lycopodium dust ignition, the temperature of the equipment surfaces that can come into contact with the dust cloud should not exceed 300 °C. In order to prevent explosions, the concentration of lycopodium dust in air should not be greater than 15 g/m³. Based on the obtained results, it can be seen that lycopodium fire and explosion parameters vary slightly, and its usage as a benchmark is considered legitimate.

1. Introduction

Specific biological, chemical, and physical properties of Lycopodium (L.) clavatum, being one species of the genus Lycopodium, enable its usage in the pharmaceutical industry. It may be used in traditional herbal medicine, i.e., improving osteogenesis and cognitive functions [1]. Lycopodium spores also provide the basis of analytical techniques for the quantification of powdered drugs in pharmacology [2].

A dense core of lycopodium particles is surrounded by a porous structure containing lignin, cellulose, and hemicellulose. The oil found in pores accounts for the 46% of its mass [3]. A yellow–tan lycopodium powder consists of dry spores, which may be highly flammable when mixed with air at sufficient concentration. This is explained by lycopodium powder being high in fat and exhibiting high surface-to-volume ratio [4].

Lycopodium dust is used as a separator in casting to prevent the mass from sticking to the model [5]. In the past, the dust was used in the production of medicines to prevent tablets from sticking together. It has also been used for gastrointestinal irritation, as a wound dressing, and as a baby powder. In theatrical technology and pyrotechnics, it was used to make fireworks due to its explosive properties [6].

Lycopodium dust, due to its uniform particle size and low equilibrium moisture content, shows the ability to retain easy ignitability even after long-term storage. It is used as a reference material for dust combustion and explosion studies [7,8], as well as a benchmark during the calibration of certain test devices applied for verifying the determination of explosive properties of dust atmospheres.

When quantifying the explosive properties of dust, the term “parameters” is deliberately used to distinguish them from physical quantities. The ignitability and explosive parameters of dust depend not only on the properties of the tested dust, but also on the applied method. Therefore, these methods must be precisely defined and respected during testing, because only then can results obtained in different laboratories be compared.

Moreover, the values of these parameters, determined in a standardized way, are the basis for recommendations for assessing the risk of dust explosion and ways to reduce it. The correctness of the application of standardized methods for determining ignitability and explosive parameters, as well as good understanding of obtained results, are, therefore, of primary importance. This underscores the importance of dusts used for validation and calibration [9,10,11,12], such as lycopodium. Although the parameters of basic dusts are relatively well understood, new ones continue to emerge, including hybrid mixtures, the presence of which can threaten worker safety, and explosion parameters should be labeled as accurately as possible.

Although the explosion parameters of lycopodium dust, such as minimum explosible concentration, maximum explosion pressure, maximum rate of explosion pressure rise, dust constant, minimum ignition temperature of the dust layer and the dust cloud, and minimum ignition energy are well known, the fire parameters, including self-heating (with possible self-ignition) are not so thoroughly understood. Knowledge on both types of these parameters is necessary in order to assess safety storage and transport conditions.

The aim of the study was to assess the fire and explosion parameters of lycopodium powder, including the minimum ignition temperatures of dust layer and dust cloud, spontaneous ignition behavior, maximum explosion pressure, maximum rate of explosion pressure rise, the lower explosion limit of the air/dust mixture, and compare the obtained results with the previous literature findings. Moreover, the research was conducted to verify the appropriateness of using lycopodium powder as a reference material for dust combustion and explosion studies, and for validation and calibration of some test equipment in dusty environments, as well as to assess safe storage and transport conditions of lycopodium powder by means of isoperibolic hot storage experiment.

2. Materials and Methods

2.1. Materials

The lycopodium powder (CAS 8023-70-9) was obtained from Sigma Aldrich (Saint Louis, MO, USA).

2.2. Methods

2.2.1. Moisture Content Analysis

The moisture content analysis was conducted according to the standard drying procedure by the use of a moisture analyzer (Radwag, Radom, Poland) with a readability of 0.001 g. The measurement was carried out by the simultaneous weighing and drying of a sample of (2 ± 0.01 g) at a temperature of 120 °C. The measurements were conducted in three repetitions. The moisture content [wt. %] was calculated according to Equation (1):

$$M=\frac{m_0-m}{m_0}\times 100\%$$

(1)

where:

• $m_0$—sample mass before drying [g];
• $m$—sample mass after drying [g].

2.2.2. Elemental Analysis

The CHONS, ash, and humidity content determination by a thermal conductivity detector (TCD) was performed with the use of a VarioEL Cube elemental analyzer (Elementar Analysensysteme, Langenselbold, Germany). The combustion of samples (0.02 ± 0.01 g) was performed at 1200 °C. The measurements were conducted in three repetitions.

2.2.3. Particle Size Distribution Analysis

The IPS UA Analyzer (Kamika Instruments, Warsaw, Poland) with a measuring range from 0.5 µm to 2000 µm and highest accuracy was used to assess the particle size distribution of the lycopodium dust. The measurements were taken at range of medium-sized particle from 1 µm to 143 µm at an ultrasonic feeder vibration frequency of 40 kHz. Additional sieve analysis using 36 µm and 14 µm stackable sieves was conducted. The measurements were conducted in three repetitions.

2.2.4. Minimum Ignition Temperature of Dust Layer

Minimum ignition temperature of dust layer (MITDL), defined as the lowest temperature of a hot surface that causes ignition of a layer of dust of certain thickness laying on that surface [13], was measured according to EN 50281-2-1:1998 (Method A) [14]. Figure 1 presents the test stand for the MITDL determination (ANKO, Warsaw, Poland). A dust layer ignition was recognized as visible smoldering, glowing, burning, the point in time when the furnace temperature reaches 450 °C, or when the dust layer temperature exceeded the temperature of the heating panel by 250 °C. If ignition was observed, another measurement was performed with a use of a new sample, while concurrently lowering the temperature of the heating panel by 10 °C. The heating panel was a metal plate of minimum 200 mm diameter and minimum thickness of 20 mm. The plate temperature was measured with a thermocouple placed 1 ± 0.5 mm below the centre of the heating panel. Temperature was set by an electronic temperature controller of the furnace heating panel. The furnace with temperature control provided heating of the panel without dust to 400 °C combined with consistent and uniform temperature within a range of ±5 °C during the measurement. The ring forming the dust layer was of a nominal diameter of 100 mm and its height was proportional to thickness of tested dust layer.

2.2.5. Minimum Ignition Temperature of Dust Cloud

Minimum ignition temperature of dust cloud (MITDC), defined as the lowest temperature of a hot interior furnace wall that causes ignition of a dust cloud in the air inside the furnace [13], was measured according to EN 50281-2-1:1998 (Method B) [14]. Figure 2 presents the test stand for the MITDC determination (ANKO, Warsaw, Poland). The test was performed with use of tubular furnace. The dust was blown in by an air pulse triggered by opening of a solenoid valve. The temperature of the internal furnace wall was controlled by the supply voltage of heating winding and measured by thermocouple installed inside the furnace. The test commenced with setting the furnace temperature to 500 °C. Next, a sample of 0.1 g was placed inside the holder and the pressure of airflow blowing the dust inwards was set at a value of 0.1 bar above atmospheric pressure. If there was no ignition after blowing in the dust into the furnace, the measurement was repeated and the furnace temperature was increased by 50 °C. The action was repeated until ignition or until the furnace reached a temperature of 1000 °C. Once ignition was observed, the pressure of the dispersing airflow and the dust weight was varied to find the most intense ignition of the dust cloud. For a specific weight and pressure of dust for which the most intense ignition was observed, subsequent measurements of dust cloud ignition were conducted with an incremental decrease in the furnace temperature by 20 °C until the time when no ignition was observed during ten subsequent attempts. A dust cloud ignition was recognized as a visible flame discharge beyond the bottom edge of the furnace. When using that method, minimum dust cloud ignition temperature was considered as the lowest furnace temperature at which ignition occurred, decreased by 20 °C at the furnace temperature exceeding 300 °C, or decreased by 10 °C at the furnace temperature equal or lower than 300 °C.

2.2.6. Spontaneous Ignition Behavior

Spontaneous ignition behaviour of dust accumulations was tested in accordance with standard EN 15188:2020 [15]. Figure 3 presents the test stand for the determination of spontaneous ignition behavior. The self-ignition temperatures T_SI were determined for dust samples filled into mesh wire baskets of volumes of 23.00 cm³, 54.80 cm³, 105.15 cm³, and 207.39 cm³. Baskets were placed in the centre of the furnace for self-heating of solid materials (ANKO, Warsaw, Poland). The measurements were taken until the highest furnace temperature at which the dust did not ignite and the lowest furnace temperature at which the dust ignited were determined with an accuracy of 2 °C.

2.2.7. Explosion Characteristics of Dust Clouds

Explosion characteristics of dust clouds including maximum explosion pressure p_max, maximum rate of explosion pressure rise (dp/dt)_max, and the lower explosion limit of the air/dust mixture were determined according to EN 14034-1:2004 [16], EN 14034-2:2006 [17], and 14034-3:2006 [18]. Figure 4 presents the test stand for the determination of explosion characteristics of dust clouds (ANKO, Warsaw, Poland). The measurement was performed inside a sphere-shaped container of 20 dm³ volume, made of stainless steel, resistant to explosion of tested dust. A water shell was used to dissipate the heat generated during the explosion. A sample of tested dust was dispersed inside the container from the holder of volume of 0.6 dm³ through a quick-acting valve and dispersing nozzle. The valve was opened and closed pneumatically with the use of an auxiliary piston. The compressed air valves were actuated electronically. The source of ignition was placed in the geometrical centre of the 20 L sphere, and in the case of measuring the lower explosion limit, it was made up of two pyrotechnic igniters of total energy of 2 kJ, and in the case of measuring explosion pressure—of two pyrotechnic igniters with energy of each head equal to 5 kJ. The pressure measurement system was equipped with two pressure sensors, a controlling device and recorder. Maximum dust explosion pressure was determined according to test procedure. The first measurement was performed for dust concentration of 250 g/m³, then for 500 g/m³, 125 g/m³, 62.5 g/m³, and for 750 g/m³. At a recorded explosion pressure P_ex ≥ 5.5 bar, the value of maximum explosion pressure was corrected according to Formula (2):

$$p_{max}=0.775·p_{ex}^{1.15}=0.775·{6.91}^{1.15 }=7.16$$

(2)

for the value of recorded pressure P_ex ˂ 5.5 bar, it was corrected according to Formula (3):

$$p_{max}=\frac{5.5·(p_{ex}-p_{ci})}{(5.5-p_{ci})}$$

(3)

$$p_{ci}=\frac{1.6·E_i}{\mathrm{10,000}}$$

(4)

where:

• $p_{ci}$—pressure created by the ignition heads [bar];
• $E_i$—ignition energy [J].

3. Results and Discussion

3.1. Moisture Content Analysis

Moisture content is one of the sample properties that influences the explosion parameters. The results of the moisture content analysis are presented in

Table 1. The moisture content analysis results.

Measurement	Drying Time [s]	Sample Mass before Drying [g]	Sample Mass after Drying [g]	Moisture Content [wt. %]
1	250	2.001	1.942	2.878 ± 0.061
2	225	2.006	1.949
3	215	2.004	1.947

. The moisture content of lycopodium dust is found to be 2.878 [wt. %], which is similar to values obtained in the literature [19,20]. According to studies [21,22], the lower the moisture content of dust, the more likely ignition is to occur, and the explosion flame propagation is faster. The determination of explosion parameters when the dust moisture content is high may lead to heightening of the measurement uncertainty.

3.2. Elemental Analysis

The results of elemental analysis of lycopodium dust are presented in

Table 2. The elemental analysis results.

Element	Content [%]
Carbon	60.1
Hydrogen	8.9
Oxygen	20.8
Nitrogen	2.1
Sulphur	0.1

. The hydrogen (8.1%), oxygen (21.9%), and sulfur (0.1%) contents are similar to previous findings, and their values are in the range of results obtained by Mostafavi [23] or Addai [24]. However, lower carbon (60.1%) and the higher nitrogen (2.1%) contents are observed. The carbon content is 9.2% lower, and the nitrogen content is 0.8% higher than results obtained by Addai [24].

3.3. Particle Size Distribution Analysis

The results of particle size distribution of lycopodium dust, in terms of size frequency, are presented in Figure 5.

The particle size distribution of lycopodium dust is uneven. The analysis shows that the most frequent particle size is 31 µm. The standard lycopodium particle size may have a value of 30 µm [25,26], 30.6 µm [27], 33 µm [28], 35 µm [29], or 38.5 µm [30], and minor differences may arise from different test methods. However, the results show the presence of particles of size greater than 38.5 µm, which may be due to the merging of particles with diameter close to standard size, or to the amplitude of the sieving process [28].

The results of sieve analysis conducted on 36 um and 14 um sieves are presented in

Table 3. Distribution of particles by a sieve mesh of 36 µm.

Measurement	Particles Quantity	Particles of Size <14 µm [%]	Particles of Size <36 µm [%]
1	534,056	1	99
2	720,972	1	99
3	795,423	1	99

. Based on the distribution of particles by a sieve mesh of 36 µm, it can be concluded that lycopodium dust is a relatively homogeneous dust, as 98% of the dust particles are in the size range of 14–36 µm.

3.4. Minimum Ignition Temperatures of Dust Layer and Dust Cloud

Table 4. Minimum ignition temperature of lycopodium dust layer (MITDL).

Dust Layer Thickness [mm]	Time to Ignition [min]	Minimum Ignition Temperature of Dust Layer [°C]
5	4	290
10	25	250
12.5	38	240
15	49	230
25	62	220
50	166	210

shows the results of determination of minimum ignition temperatures of dust layer depending on the layer thickness.

The results show that the minimum ignition temperature of the dust layer is the highest for the dust layer thickness of 5 mm (290 °C) and decreases with the increase in the dust layer with a simultaneous prolonged time to ignition. The lowest MITDL of 210 °C is observed for the dust layer of 50 mm.

Minimum ignition temperature of dust cloud (MITDC) was determined as 450 °C. The obtained MITDL and MITDC values vary slightly compared to the literature values (

Table 5. The comparison of the literature data on the minimum ignition temperatures of lycopodium dust layer and lycopodium dust cloud with the experimental data.

Data	MITDL of Lycopodium of Thickness of 5 Mm [°C]	Reference	MITDC of Lycopodium [°C]	Reference
Experimental	290		450
The Literature	260	[31]	426	[31]
	283	[32]	430, 425	[33]
			460, 455
			420	[19]
			410	[26,34]

).

As the thickness of the lycopodium dust layer increases, the MITDL decreases, which significantly affects the selection of equipment operating in areas where lycopodium dust may accumulate. Figure 6 presents the decrease in maximum permissible equipment surface temperature along with the increase in a dust layer. The maximum temperature of the hot surfaces of electric equipment on which lycopodium dust may lie in processing conditions, if its maximum thickness equals 5 mm, is 215 °C. If the layer is 12.5 mm thick, the value of the temperature is lower and should not be higher than 175 °C.

The maximum permissible temperature of the surfaces of electrical equipment operating near a lycopodium dust cloud equals 300 °C and it accounts for 2/3 of the minimum dust cloud ignition temperature.

3.5. Spontaneous Ignition Behavior

Spontaneous ignition of materials may cause fires and explosions, and may occur during processing, transport, or storage of materials. Exothermic reactions occurring without external heat and/or other sources of ignition may pose a significant threat, thus, the knowledge of the material self-heating behavior is crucial to determine safe storage and transport conditions [35].

The results of hot storage experiments are presented in

Table 6. The spontaneous ignition temperatures of lycopodium dust.

Sample Basket Volume [cm3]	Dust Weight [g]	Bulk Density [g/cm3]	Self-Ignition Temperature TSI [°C]
23.00	6.984	0.304	146
54.80	16.121	0.294	136
105.15	32.349	0.308	134
207.39	62.152	0.300	128

. The highest self-ignition temperature T_SI (146 °C) is observed for the smallest basket volume (23 cm³). As the basket volume increases, the T_SI decreases, and the lowest value of 128 °C is observed for the biggest basket (207.39 cm³).

In order to determine safe storage conditions of lycopodium dust, Arrhenius and Frank-Kamentskii plots were investigated. Figure 7 shows the pseudo-Arrhenius graph of the self-ignition temperature of lycopodium dust, where:

$$Y1=\mathrm{l}\mathrm{g}\left(\frac{V/1 {\mathrm{m}}^3}{A/1 {\mathrm{m}}^2}\right)$$

(5)

$$Y2=V$$

(6)

$$X=\frac{1}{T_{SI}/1K}$$

(7)

where:

• $V$—volume of a cylinder [m³];
• $A$—surface [m²].

The coverage factor of k = 2 and corresponding level of confidence of 95.45% were used to express the expanded uncertainty [15]. The line passing through the T_SI values divides the areas representing the conditions of safe and unsafe storage conditions of lycopodium dust volume.

Figure 8 shows the relation between induction time t_i and dust volume-to-surface ratio, where Y1 and Y2 follow the same as Equations (5) and (6), and:

$$X=lg\frac{t_i}{1h}$$

(8)

The critical Frank-Kamentskii parameter of 2.76 was taken, due to the cylindrical shape of baskets [15]. The Frank-Kamentskii plot shows how long it takes for samples to self-ignite when stored in temperatures slightly higher than T_SI.

The line passing through the t_i values divides the areas representing the conditions of safe and unsafe storage conditions of lycopodium dust volume.

The Arrhenius and Frank-Kamenetskii graphs (Figure 7 and Figure 8) provide information of safe storage conditions. Lycopodium dust stored in a 1 m³ silo needs to be stored at a temperature below 67 °C, and cannot be stored for longer than 7 days.

However, the approximated values may be affected by an error due to limitations of the method regarding the geometry of the samples, thermal boundary conditions, or lack of information on changes in temperature over time.

3.6. Explosion Characteristics of Dust Clouds

Table 7 shows the results of the determination of the explosive parameters of lycopodium dust, including the maximum explosion pressure, the maximum rate of explosion pressure rise, the dust constant, and the lower explosion limit.

The maximum dust explosion pressure and the maximum rate of explosion pressure rise were determined at concentration of 250 g/m³, close to stoichiometric. Although the values of maximum dust explosion pressure for lycopodium do not differ considerably, the values of the maximum rate of dust explosion pressure rise are not similar compared to the literature findings. This may be due to the differences in the vessel volume, size distribution of dust particles, volatile content, or oxygen concentration [36].

According to the dust constant K_st value of 90.5 bar m/s, the lycopodium dust is classified as a weak explosive of the St 1 explosion class. Based on the lower explosion limit, a formation of explosive lycopodium dust–air mixture will occur if its concentration in the air exceeds 15 g/m³.

Table 7. The comparison of the literature data on the minimum ignition temperatures of lycopodium dust layer and lycopodium dust cloud with the experimental data.

Data	Maximum Dust Explosion Pressure [bar]	Maximum Rate of Dust Explosion Pressure Rise [bar/s]	Dust Constant Kst [bar m/s]	Lower Explosion Limit [g/m3]	Reference
Experimental	7.16	333.4 (250 g/m3)	90.5	15	-
The Literature	-	225 (750 g/m3)	-	-	[37]
	6.5	283 (1000 g/m3)	76.8	-	[38]
	7.3	470 (500 g/m3)	-	-	[39]
	7.21	182.7 (250 g/m3)	130.6	-	[12]
	6.3	381.2 (250 g/m3)	103.5	-	[40]
	-	-	119	<15	[26]

Table 7. The comparison of the literature data on the minimum ignition temperatures of lycopodium dust layer and lycopodium dust cloud with the experimental data.

Data	Maximum Dust Explosion Pressure [bar]	Maximum Rate of Dust Explosion Pressure Rise [bar/s]	Dust Constant Kst [bar m/s]	Lower Explosion Limit [g/m3]	Reference
Experimental	7.16	333.4 (250 g/m3)	90.5	15	-
The Literature	-	225 (750 g/m3)	-	-	[37]
	6.5	283 (1000 g/m3)	76.8	-	[38]
	7.3	470 (500 g/m3)	-	-	[39]
	7.21	182.7 (250 g/m3)	130.6	-	[12]
	6.3	381.2 (250 g/m3)	103.5	-	[40]
	-	-	119	<15	[26]

4. Conclusions

The determination of fire and explosion characteristics of flammable dusts is of great practical importance. It enables the evaluation of the explosive hazards of processes including production and storage of flammable or explosive dust. The obtained results are helpful for minimizing risk. They indicate the necessity of careful and frequent removal of collected dust, complying with the operating regimes, and the required frequency of inspections and maintenance of the machines and equipment used in technological processes, as well as ongoing monitoring of temperature, if storage of dust accumulations is required.

The equipment used with lycopodium dust should be properly selected to ensure that maximum temperature of surface is below 215 °C for dust accumulations of thickness up to 5 mm. For dust accumulations thicker than 12.5 mm, the temperature should be lowered to 175 °C. In order to eliminate the risk of lycopodium dust ignition, the temperature of the equipment surfaces that can come into contact with the dust cloud should not exceed 300 °C.

The lower explosion limit of lycopodium dust of 15 g/m³ suggests that in order to prevent explosions, that level of concentration of dust in the air must not be exceeded and/or potential sources of ignition that may trigger explosion must be eliminated. To minimize the effects of explosion, the structure of the building with lycopodium dust explosion hazard should be designed taking into account the value of the maximum explosion pressure and the maximum rate of explosion pressure rise, or relevant safety systems should be installed to reduce the values of the maximum explosion pressure and the maximum rate of explosion pressure rise to values that are safe for the structure.

The obtained test results are comparable to the literature data, which confirms that fire and explosion parameters of lycopodium dust remain the same irrespective of the place of origin. Due to its stable parameters, lycopodium dust is used for calibration of laboratory stands.