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CHAPTER 8

MINIMUM IGNITION ENERGY MEASUREMENT

In this experiment, flash powder mixtures which are sensitive

nature to electro static discharge is tested using Hartmann apparatus.Here,

wire exploded nano aluminium powder is also mixed with the flash powder

and tested for comparison.

The methodology for this experimentation is described in the

flowchart (Figure 8.1).

Figure 8.1 Methodologyfor MIE measurement

Nano aluminium powder (Ball

milled)

Measurement of MIE with varying electrode gap / material /

dust concentration

Measurement of MIE using Hartmann apparatus

Preparation of micron and nano flash power mixture

Nano aluminium powder (wire

exploded)

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8.1 PREPARATION OF FLASH POWDER COMPOSITION

8.1.1 Sample preparation for micron flash powder

Table 8.1 shows the basic formulation of the pyrotechnic mixtures

with chemicals for MIE measurement. Various compositions were prepared

by using the chemicals of KNO3, S and Al powders of particle size in 75 µm

(samples S51-S60). The sieves used for this mixing met the ASTM standard.

The number of mixtures required for an analysis was calculated with the help

of Design of Experiment software (Make: Stat-Ease, Inc., Minneapolis). The

complete specifications of the samples are refereed as in the Annexure A1.3,

Set III.

Table 8.1Compositions of micron flash powder

Sample No. Potassium nitrate,% Sulphur, % Aluminium, %

S51 50.0 5.0 45.0 S52 65.0 5.0 30.0 S53 50.0 20.0 30.0 S54 50.0 12.5 37.5 S55 57.5 12.5 30.0 S56 57.5 5.0 37.5 S57 60.0 7.5 32.5 S58 52.5 7.5 40.0 S59 52.5 15.0 32.5 S60 55.0 10.0 35.0

8.1.2 Sample preparation for micron flash powder for different

particle sizes

Samples were prepared from micron sized KNO3, S and Al

powders withdifferentparticle sizes of -100+200, -200+225, -225+325,

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-325+400, -400meshes. The composition for all the sample was KNO3: S: Al

on the ratio of 57: 20: 23 (samples S98-S102). The complete specifications of

samples are described in Annexure A1.6.

8.1.3 Sample preparation for nano flash powder

Nano flash powder composition was prepared by using the

chemicals of nKNO3, nSand nAlpowders, which are prepared as per the

section 4.1. The composition had KNO3: S: Al in the ratio of 57:20:23. Then,

10 samples (S21-S30) were prepared from 10 % nano flash powder to 100 %

nano flash powder by retaining the rest as micron size flash powder as

prescribed in section 4.2 (refer Table 4.1).

8.1.4 Sample preparation with wire exploded nano aluminium

powder

Wire exploded aluminium powder was procured as such from M/s.

Neo-Ecosystems and Software (P) Ltd., Uttarkhand, India, with particle size

367 nm and the purity of this powder in argon atmosphere is 99.99 % (refer

Figure 8.2).This powder is mixed and milled with nano potassium nitrate and

nano sulphur to prepare nano flash powder mixture. Here, the preparation was

done in two methods to check the purity. Glove box in inert condition was

used for preparing flash powder mixtures and also in open

atmosphere.Samples were also prepared from this nano flash powder by the

same procedure and represented as samples S103 S112. The complete

details of these samples are described in Annexure A1.7.

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Figure 8.2 Particle size of wire exploded aluminium

Figure 8.3(a) is the SEM image of wire exploded nAl magnified at

10,000X which is clustered and agglomerated and thus larger in size.

Figure 8.3(b) is the SEM image of nano flash powder with the mixture of

nanopotassium nitrate, nanosulphur and wire explodednAl and is magnified at

3,000X.

In order to check the presence of functional elements of potassium

nitrate, sulphur and aluminium in the composition, the FTIR analysis has been

made. This is shown in Figures 8.4(a-c). Figure 8.4(a) is the FTIR graph of

the nano flash powder made by mechanical milling. Figure 8.4(b) is the FTIR

graph of the nano flash powder consists of the wire exploded nAl and the

composition prepared in normal atmosphere. Figure8.4(c) is the FTIR image

of the nano flash powder consists of nAl made by wire explosion, but the

composition prepared in inert atmosphere. All the graphs show the presence

of potassium sulphate and aluminium oxide.

134

Figure 8.3 SEM images for (a) wire explodednAl (b) nano flash powder

consists of nAl by wire explosion

(b)

(a)

135

Figure 8.4 (a-c) FTIR curves for nano flash powder consists of (a) Mechanical milled nAl (b) wire exploded nAl, prepared in open atmosphere (c) wire exploded nAl, prepared in inert atmosphere

(a)

(b)

(c)

136

From Figure 8.4 (a-b), it is clear that the chemicals have two peaks;

one is in the range of 491-451 cm-1 and another in 447.9 - 439.2 cm-1 which

clearly indicates that these chemicals have aluminium oxide (Al2O3) in the

-modification (Range 491-451 cm-1) and SO4 group (K2SO4 and KHSO4,

Range 447.9 - 439.2 cm-1) as asymmetric bond. From Figure8.4(c), it is

evident that the chemical has only Al2O3 and absence of SO4peak

(447.9 - 439.2 cm-1)and this is because of the manufacturing of powder in

inert atmosphere without Oxygen. It is noticed that, whatever the process of

preparing the mixture, formation of Al2O3 cannot be prevented and Al2O3

forms at the end, due to the manual material handling. Al2O3 is the

passivation coating on pure aluminium metal which reduces the sensitivity of

firework flash powder.

8.2 MIE MEASUREMENT

The variables taken in this study are electrode material, electrode

gap, chemical composition, particle size and dust concentration to estimate

the variations of MIE. The MIE measurement can be conducted by using

Hartmann apparatus.

8.2.1 Hartmann apparatus

Figure 8.5 shows the experimental setup of Hartmann apparatus to

measure Minimum Ignition Energy (MIE). The fundamental setup and

procedures of the MIE tests are similar to the ASTM E2019-03 (2007). It

consists of 1.2 litre vertical tube in which dust was dispersed by air blast. Two

electrodes made of Brass or Stainless Steel (SS) of grade AISI 304 was kept

inside the glass tube at some distance apart. Electrodes were connected with

spark ignitor and serves as ignition source. Spark ignition system was

137

supplied by high voltage DC power. Flame propagation was observed as a

function of dust particle size, dust concentration, DC voltage, etc. The

experimental setup consists of high voltage, dust dispersion, spark ignition

and Hartmann glass apparatus. The DC power supply required for this work is

0 to 7 kV. The dust dispersion system was used to form a uniform dust cloud

in the Hartmann glass. The spark generation circuit is shown in Figure 8.5.

Figure 8.5Hartmann apparatus (a) Skeleton (b) Spark generation unit

(a)

(b)

138

The high voltage capacitor with the rating of 0.1 µF was used to

store the energy required for ignition of dust sample and used electrolytic

capacitor with heavy insulation. In order to make an ignition test, the high

voltage electrode was grounded and the required mass of the prepared dust

was placed in the dispersion cup. The dust was dispersed byopening a valve

and emptying 50 cc pressurised air reservoir at 0.7MPa. Sparks were

generated between the electrodes by using a high voltage pulse to charge a

discharge capacitor, which is subsequently discharged when the breakdown

voltage of the electrode gap is reached. When the dust particles passing

through the spark gap with a preset static high voltage; breakdown may be

triggered with a subsequent spark discharge. The flame may propagate after

the ignition of powder by applying the DC voltage.

The electrode separation and the voltage were then adjusted by trial

and error until sparks of the stored energy on the capacitor reaches 0.5CV2 at

the electrodes, where V is the voltage at which the spark occurs; C is the total

capacitance at the high voltage electrode. The first test usually was performed

with a high spark energy typically 500 mJ. The voltage was then reduced in

steps until no ignitions occurred for ten ignition trials, or until it could not be

reduced any further. During the experiment, visual inspection is required to

determine whether or not the ignition spark has successfully induced a dust

cloud explosion. The electrode material used was Stainless Steel / Brass of

2 mm diameter with round tip. The optimum space between the electrodes for

experimentation was 2 and 4 mm. The nominal dust concentration was also

varied, offering the opportunity to find the ideal conditions for spark ignition

of the dust cloud.

In addition, an investigation of MIE for coal and aluminium

powder using the spark generator has been performed; enabling calibration of

the spark generator by comparison of MIE data with literature values(The

139

Aluminium Association Inc., US, Canadian Centre for Occupational Health

and Safety, 2011 Stephan, 2011) and the results are tabulated in Table 8.2. In

most of the tests the dust was placed in a dust reservoir downstream of the air

reservoir, forcing the particles in the tube, thus reducing agglomeration.

However, some of the dusts have to be placed at the bottom of the Hartmann

tube because of clogging.

Table 8.2 Calibration of results

S.No. Material MIE, mJ

Literature Experimental

1. Aluminium 17 µm 28.0 27.6

2. Aluminium 10 µm 10.0 9.7

3. Coal 30.0 30.7

8.2.2 MIE for different micron flash powder compositions

MIE test is conducted for the various samples as prescribed in the

Table 8.1with the concentration of flash powder 0.3 g/l, electrodes gap 6 mm

and electrode material of brass. The results are shown in Table 8.3, for the

particle size of 75 µm.

From Table 8.3, it is found that the addition of sulphur leads to the

decrease of MIE drastically. As sulphur is a low melting point (110°C)

material and has very low MIE compared to other two materials in the flash

powder composition, it quickly initiates the ignition. For example in sample

S52, Sulphur has 5 %, the MIE is 62.7 mJ despite MIE of sample S53 is

26.7 mJ in which sulphur is 20%. But KNO3 and aluminium powder addition

shows little impact in MIE.

140

Table 8.3 MIE of different micron flash powder compositions

Sample No. DC voltage, kV MIE, mJ S51 1.16 67.3 S52 1.12 62.7 S53 0.73 26.7 S54 1.22 74.4 S55 1.14 65.0 S56 1.15 66.1 S57 1.14 65.0 S58 1.16 67.3 S59 0.78 30.4 S60 1.00 50.4

8.2.3 MIE for micron flash powders with different dust

concentration and particle size

MIE of flash powder with the standard composition as per the

Petroleum and Explosive Safety Organisation, Government of India, consists

of Potassium nitrate (57 %), Sulphur (20 %) and Aluminium (23 %) is found

out for the various particle with brass electrode along 2 mm gap. The results

are shown in Table 8.4. Here, the various dust concentrations (0.3 0.5 g/l)

were taken for this study.

Table 8.4 MIE for different dust concentration for various particle sizes

Concentration, g /l 0.3 0.4 0.5

Sample No. DC voltage, kV

MIE, mJ

DC voltage, kV

MIE, mJ

DC voltage, kV

MIE, mJ

S98 1.28 82.0 1.18 69.6 1.12 62.7 S99 1.21 76.3 1.15 66.1 1.08 58.3

S100 1.18 69.6 1.12 60.7 1.01 52.0 S101 1.12 62.7 1.01 53.0 0.95 45.1 S102 1.05 55.1 0.95 45.1 0.88 38.7

141

Figure 8.6 MIE for various dust concentrations

Table 8.4 and Figure 8.6, shows that when the particle size reduces from -150+75 to -37 µm, MIE is also reduced by 35 % for the dust concentration of 0.4 g/l. This shows that ignition hazard increases when the particle size of flash powder reduces. Table 8.4further infers that MIE is reduced by 23 % for the same particlesize (-150+75), by varying the dust concentration from 0.3 to 0.5 g/l. This is because of the fuel rich mixture is readily available in the Hartmann tube. But in this experiment no steps have been taken to find out lower and upper explosive concentration (LEC and UEC) for safety consideration of the test centre. As all the samples are ignited in the range of 0.3 to 0.5 g/l, it is concluded that this range should be lies within LEC and UEC.

8.2.4 MIE for micron flash powders with different electrode material and gap

MIE test is conducted for the composition of various particle sizes with

different electrode material like Brass, Stainless Steel (SS) and gap of

20

40

60

80

100

S98

S99

S100

S101

S102

MIE

, mJ

Samples

0.3 g/l 0.4 g/l 0.5 g/l

Electrode Material: Brass Electrode gap : 2 mm Dust Concentration:

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2 and 4 mm with the dust concentration of 0.4 g/l. The results are shown in

Table 8.5.

Table 8.5 MIE for different electrode gap and material for various

particle sizes

Electrode gap /

material 4 mm gap 2 mm gap

Brass SS Brass SS

Sample No. DC, kV

MIE, mJ

DC, kV

MIE, mJ

DC, kV

MIE, mJ

DC, kV

MIE, mJ

S98 1.71 146.2 1.61 154.6 1.18 69.6 1.34 89.8 S99 1.68 141.0 1.71 149.2 1.15 66.1 1.31 85.8

S100 1.64 134.5 1.77 145.6 1.12 62.7 1.18 69.6 S101 1.61 129.6 1.54 139.6 1.01 53.0 1.15 66.1 S102 1.54 121.6 1.66 135.7 0.95 45.1 1.08 58.3

Figure 8.7 MIE for various electrode gap

0

30

60

90

120

150

S98

S99

S100

S101

S102

MIE

, mJ

Samples

2 mm 4 mm

Electrode material : Brass Dust concentration : 0.4 g/l Electrode gap :

143

Figure 8.8 MIE for various electrode materials

Table 8.5 and Figure 8.7, indicate that the value of MIE increases

when the gap between the electrode increases. The MIE yielding ignition

decreases upto 50 %, if the gap between the electrodes decreases from 4 to 2

mm irrespective of particle sizes. From the safety point of view, this

represents a quite conservative method to find the safe energy limits of

potential electrostatic sparks in an industrial plant and to design the safe

handling of the flash powders. In addition, this information will be useful to

maintain the gap between the projecting conductors in silos / hoppers which

acts as electrodes.

As shown in Figure 8.8, the electrode material also influences the

MIE of the flash powder. There is a significant change in the results between

brass and SS electrode material. Stainless steel material is the safest one

during powder handling

system.

100

120

140

160

S98

S99

S100

S101

S102

MIE

, m

J

Samples

BrassStainless steel

Dust concentration : 0.4 g/l Electrode gap : 4 mm Electrode material :

144

8.2.5 MIE for different nanoflash powders

MIE test is conducted for the nano flash powders (nfp) mixed with

micron flash powders (µfp) as detailed in sections 8.1.3 and 8.1.4. The dust

concentration is 0.4 g/l. with the brass electrode gap of 2 mm. In this study,

mechanical milled and wire exploded chemicals mixed in normal atmosphere

have been considered, because preparing mixtures in inert condition has its

own limitation. All the processes right from beginning to end stage in

manufacturing fire crackers cannot be done in inert atmosphere and most of

the processes have to be carried out in open atmosphere.

Table 8.6 MIE for nanoflash powders

Sample Nos.

Nano flash powder consists of milled nAl Sample

Nos.

Nano flash powder consists of wire exploded nAl

DC, kV MIE, mJ DC, kV MIE, mJ S21 0.98 48.0 S103 1.11 61.6 S22 0.92 42.3 S104 1.05 55.1 S23 0.89 39.6 S105 0.91 47.4 S24 0.85 36.0 S106 0.88 39.7 S25 0.79 31.2 S107 0.85 36.0 S26 0.79 29.0 S108 0.79 31.2 S27 0.72 26.0 S109 0.71 27.0 S28 0.70 24.5 S110 0.68 23.1 S29 0.62 19.2 S111 0.65 21.1 S30 0.56 15.7 S112 0.62 19.2

145

* 1-10 refers the set of samples S21 S30 and S103 S112

Figure 8.9 MIE for nano powders

From Table 8.6 and Figure 8.9, it is clear that if the nano flash

powder is added with micron powders, MIE has been greatly reduced. The

complete replacement of micron with nano powder (mechanically milled

aluminium) leads to decrease of MIE from 48.0 mJ (sample S21) to 15.7 mJ

(sample S30). However, if the nano flash powder consists of wire exploded

aluminium powder, MIE has reduced from 61.6 (sample S103) to 19.2 mJ

(sample 112). This reveals that handling of nano powders will bear risk and so

utmost care is to be taken. Hence, nAl powders of both mechanical milled and

wire exploded methods provide unsafe handling in fireworks industry.

SUMMARY

The summary of this chapter as below:

The Minimum Ignition Energy measurement shows that MIE

reduces by 65 % when decreasing the particles to nano level. This shows that

the sensitivity to static charge increases.

0

15

30

45

60

75

1 2 3 4 5 6 7 8 9 10

MIE

, mJ

Samples *

Mechanical millWire explosion

Dust concentration : 0.4 g/l Electrode gap : 2 mm Elrctrode material : Brass nAl. making process: