The program output gives the spontaneous combustion potential of the coal, its rank, and SHT .The

program then OU\puts each factor that increases the risk of spontaneous combustion in the underground

mining operation, along with the degree of risk, and details of why the factor increases the risk. The program

output can be printed to the terminal or to a hard copy printer .

DETERMINATION OF SPONTANEOUS COMBUSTION POTENTIAL

The method used to detemline the spontaneous combustion potential of the coal is described by (Smith,

1992). The values from the coal's proximate and ultimate analyses and its heating value are used to

detemline the rank ofthe coal, based on the ASTM 0388 classification system (Anon., 1983). If the rank of

the coal is lignite or subbituminous, the coal is assigned a high spontaneous combustion potential. If the rank

is anthracite, the coal is assigned a low spontaneous combustion potential. If the rank is bItuminous, the

coal's predicted SHT is determined by the empirical expression:

SHT, °c = 139.7- [6.6 x Oxygen, %(DAF)J. (1)

It is generally agreed that the oxidation of coal is a temperature-dependent reaction that obeys an

Arrhenius-type rate law, where the reaction rate increases exponentially with increased temperature. To

account for the increased (or decreased) reactivity of a coal due to ambient in-mine temperatures, the value

for the minimum SHT determined from Equation (1) is adjusted based on the ambient in-mine temperature

of the coal. A value of 12.8 oC (55 oF) is used as the baseline in-mine temperature.

SHT .dj' °c = [SHT -(f .mbl8nt' °c- 12.8)], (2)

Coals with adjusted SHrs <70 °C are assigned a high spontaneous combustion potential, while those with

adjusted SHrs > 100 °C are assigned a low spontaneous combustion potential. Coals with adjusted SHT's

between 70 and 100 ° C are assigned a medium spontaneous combustion potential. The program uses this

adjusted value in the evaluation of the effect of the various factors on the spontaneous combustion risk of the

mining operation.

The program also considers the spontaneous combustion history of the coal and mining operation in the

evaluation of the effect of other factors on the spontaneous combustion risk of the mining operation. If a

spontaneous combustion event has occurred in any in-mine area, the program logic uses a high spontaneous

combustion potential in the determination of the impact of other factors on the spontaneous combustion risk.

If a spontaneous heating has occurred during the storage or transport of the coal, it is noted in the program

output as a spontaneous combustion risk.

FACTORS THAT AFFECT THE SPONTANEOUS COMBUSnON RISK

Coal Properties

The coal properties evaluated in SPONCOM include the moisture content of the coal, its friability, previous

oxidation, pyritic sulfur content, and impurities in the coal. These properties primarily influence the rate of heat

generation during the self-heating of coal. These factors, their ranking criteria, the increased degree of selfheating risk, and the program Output for each factor are shown in table 1.

Setf-heating occurs when the rate of heat generation exceeds the rate of heat dissipation. Two mechanisms

contribute to the rate of heat generation, coal oxidation and the adsorption of moisture, known as the heat-ofwetting effect. The reactivity of a coal is a measure of its potential to oxidize when exposed to air. The coal's

minimum SHT is used as a relative indication of its reactivity .In general, a coal's reactivity increases with

decreasing rank. Although the mechanism of coal oxidation is riot completely understood, it is generally

believed that at low temperatures, the oxidation occurs at the oxygen-containing moieties in the coal molecule.
Table 1: Coal Properties

Moisture high Heat generation due to

heat-of-wetting effect.

medium

>5.0 and <7.5 pct

>7.5 and <10.0pct

> 10.0 pct

>7.5 and <10.0pct

> 10.0 pct

> 10.0 pct

--

low

Generation of fresh

surfaces available for

oxidation.

Friability high

medium

low

>40

>50

>75

high,

medium, or

low

Previous

oxidation

Yes Air and water leakage due

to high pressure

differentials. increase in

coal friability .

pyritic sulfur > 2.67 pct

>2.67 pct

>4.0 pct

high

medium

low

Heat generation due to

pyrite oxidation,

degradation of coal

particles.

Impurities high or

medium

Yes

moderate

moderate

moderate

Heat generation due to

oxidation of impurities.

The amount of moisture that a coal contains is also an important parameter in the rate of heat generation

of the coal. The adsorption of moisture on a dry coal surface is a heat-producing reaction. Thus, if a coal

is partially dried during its mining, storage, or processing, that coal has the potential to readsorb moisture,

producing heat. Therefore, the higher the moisture content of the coal, the greater the potential for this to

occur. In general, the moisture content of coals increases with decreasing rank.

The oxidation of pyritic sulfur is also a heat-producing reaction. The heat generated can increase the

temperature of the surrounding coal, Increasing the rate of oxidation. In addition. as the pyritic sulfur oxidizes,

it expands causing coal degradation to occur. Reactions involving impurities, such as resins, can also be a

source of heat generation. In general, the presence of these types of impurities increases the spontaneous

combustion risk.

Friability and previous oxidation of the coal are also important factors in the self-heating process. The

friability of the coal is a measure of the coal's ability to break apart into smaller pieces. This exposes fresh

coal surfaces to air and moisture, where oxidation and moisture adsorption can occur. Previous oxidation

of coal makes coals more friable. Although the oxidized matter is less reactive, the porous nature of the

oxidized coal makes the coal susceptible to air and water leakage when exposed to high pressure differentials.

Geologic Conditions

Geologic conditions that can influence the self-heating process include properties of the coalbed and

surrounding strata, such as the depth of cover and the presence of geothermal sources, and conditions that

affect the transport of air and moisture to the coalbed, such as the presence of faults, deat density , joints,

channel deposits, clay veins, dikes, and natural burn zones. In addition, the presence of rider beds and pyrite

deposits in the floor or roof, in close proximity to the coal bed, can increase the spontaneous combustion risk.

These factors, their ranking criteria, increased self-heating risk, and program output are shown in table 2.

Table 2: Geologic Conditions--Continued

Factor Ranking

Criteria

Other

Factors

Increased

Self-Heating

Risk

Factor Spontaneous Output

Combustion

Potential

high, medium, distance from

or low floor < 2 times

average coaJbed

ttJlckness mined

floor heave

>30

high Exposure to air and

water due to floor

fracturing, leakage

under seals.

Rider beds in

floor

floor heave

>30

high Exposure to air and

water due to floor

fracturing, leakage

under seals.

pyrite deposits

In floor

high, medium, distance from

or low floor < 2 times

average coalbed

thickness mined

..do. do. floor heave

potential

moderate Do.

Exposure to air and

water in gob after

caving, near seals

due to high pressure

differentials.

Do.

high, medium, distance from

or low roof < 5 times

average coalbed

thickness mined

longwall

mining

Rider beds in moderate

roof

room-andpillar

mining

.do. distance from

roof < 2 times

average coalbed

thickness mined

moderate

Exposure to air and

water in gob after

caving, near seals

due to high pressure

differentials.

Do.

high, medium, distance from

or low roof < 5 times

average coalbed

thickness mined

longwall

mining

pyrite deposits high

in roof

distance from

roof < 2 times

average coal bed

thickness mined

room-andpillar

mining

.do. moderate

The depth of cover, or amount of overburden, can affect the self-heating process in many ways. If the

overburden is shallow, communication paths with the surface can develop, allowing air and moisture to

migrate to mined-out areas. If the overburden is deep, the ambient temperature of the coal bed increases,

which increases the rate of oxidation of the coal.

In general, the rank of the coal Increases with increasing depth of cover, so that deeper mines are usually

less susceptible to self-heating. However, higher ranked, deeper coalbeds usually emit greater quantities of

methane requiring higher ventilation pressures. This increases the potential for air leakage into sealed-off

areas. In addition, deeper mines are usually more prone to rib sloughage, which generates more fresh coal

surfaces for oxidation, and to floor heave, which can expose rider beds or pyrite veins to ventilating air .

The presence of a geothermal source in close proximity to the coalbed can significantly increase the risk

of spontaneous combustion by raising the temperature of the coalbed. The degree of risk depends on the

amount of the coal bed affected, and the temperature increase caused by the geothermal source.

Faulted areas, if relatively close to the surface, can also provide access of air and water to worked-out

areas. The grinding action along the fault can also produce coal fines as well as act as a possible heat

source if the fault is still active. An indirect risk factor of faults is the slowing down of the face advance in a

longwall operation, whid1 can lead to a buildup of heat behind the face supports.