Prevention and Combating Mine Fires

Sudhish Chandra Banerjee

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000446692
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Bauingenieurwesen

There is a growing concern about fires in mines, coal mines in particular. Attempts are being made to improve mine fire combat efforts and their prevention, taking advantage of developments in the fields of science and technology. This book looks at those developments and their applications.

CHAPTER 1

The Science of Fires

“Pinğabhrū Śmaśru Késākṣaḥ

Pīnānğa Jaṭharóruṇaḥ

Chāgasthaḥ Sāksyasūtrógniḥ

Saptārcciḥ Saktidhārakaḥ”

A huge bodied, red bellied persona with three tier tassles of golden yellow—the eyebrows, beards and hairs—being seated over the sacrificial being and garlanded with beads of purity, forms the emblem of power and energy, the Fire-God with its effulgent glare of the seven*—the fierce, the fumes, the niveous, the red, the crimson, the golden and the lectious like pollens of lotus.

That is how the ancient Seers of India, who used to personify everything in nature, portrayed fire as the Fire-God.

Such aesthetic depiction of fire by ancient Indians, of course conforms fairly well with our present day knowledge on the attributes of fire. The fire plume of most of the natural fires do have three distinct regions of golden yellow leaping flames, as shown in Fig. 1. Besides the flame and the smoke that helps in spreading the ferocity of fire, the object so affected by fire may have glares of different shades of colours depending on intensity as given in Table 1.

Table 1 (Drysdale, 1985 John Wiley & Sons) Visual colour of hot objects

Temperature (°C)	Appearance
550	First Visible red glow
700	Dull red
900	Cherry red
1100	Orange
1400	White

In fact since the dawn of civilisation, man’s efforts to understand fire have been directed at taming it in order to harness heat and light for the benefit of mankind and to control its unchecked advancement. Different disciplines of science has contributed on this topic since ages and the subject of fire science has assumed an interdisciplinary character. In the context of mine fires, the focus is on understanding the principle of occurrence of fire, nature of propagation with a view to its control and suppression.

Fig. 1 Schematic diagram of the fire plume showing McCaffrey’s three regimes: (In Drysdale, D. John Wiley & Sons 1985)

PRINCIPLES OF OCCURRENCE OF FIRE

Fuels, whether in solid, liquid or gaseous form, may undergo rapid chemical change interacting with oxygen or oxidants releasing large amount of energy at a fast rate that manifests in the form of heat and light. This manifestation is known as fire and the process is said ignition or combustion or burning of fuel. The initiation of such ignition may be of two types viz.,

• piloted ignition, where inputs of external energy (electric spark, independent flame, etc.) is required for the release of energy from fuel that would be sufficient for the exothermic reaction to be self sustaining and thereby propagate the fire (Exogenous Fires).

• Spontaneous ignition, where the heat flux generated from spontaneous heating is sufficiently high to ignite the volatiles released from the fuel bed (Endogenous fires).

In fact, such type of spontaneous ignition requires a higher heat flux than piloted ignition. There is also a lag phase for spontaneous ignition to burst out in flames.

It may be noted that all types of fires associated with flames—in cases of both piloted and spontaneous ignition—are always a gas phase phenomenon. Flaming combustion involving both solid and liquid fuels must precede their conversion into gaseous form. In case of liquids it may be simple evaporation, whence for solid fuels chemical decomposition or pyrolysis is necessary to yield products of low molecular weights that can volatalise from surface and flare up with flames. As this decomposition process requires more energy than simple evaporation, surface temperature of a burning solid tend to be rather high, 400°C or above - excepting of course for solids that directly sublime on heating. The release of gaseous products from solids may either be from direct sublimation of the solid or, evaporation via melting-cum-decomposition or, from direct decomposition/pyrolysis yielding volatiles of low molecular weight.

Certain polymers have the characteristics of decomposing to yield products that may resist further flaming combustion. A typical example is the thermal decomposition of PVC (polyvinyl chloride), which lose molecular HCl at about 250°C and yield a char like residue e.g.

$$R{(C{H}_2-CHCl)}_n{R}^{\prime }\to R(CH=CH)n{R}^{\prime }+nHCI.$$

The charred product formed is difficult to ignite, other than at high temperature. Usually polymers which undergo cross linking during pyrolysis, tend to char on heating—requiring much hig...