spontaneous basics

30
ENDOGENOUS FIRES/FIRES DUE TO SELF HEATING / SPONTANEOUS COMBUSTION C+O2= CO2+ heat --------(65.5-90 0 C) Coal undergoes slow oxidation on exposer to air at ambient temperature, with the evolution of heat, gases and moisture. The heat generated, if not dissipated, give rise to an increase in temp. of coal. Rate of oxidation increases with the rise in temp. & If this is allowed to proceed unchecked, it can eventually result in the ignition of coal. This oxidation process is known as spontaneous combustion, spontaneous heating or self heating. 2C+O2= 2CO+heat --------( 100-130 0 C) Spontaneous combustion occurs when rate of heat generation > heat dissipation

Upload: sjain818574

Post on 09-Mar-2015

38 views

Category:

Documents


2 download

TRANSCRIPT

Page 1: Spontaneous Basics

ENDOGENOUS FIRES/FIRES DUE TO SELF HEATING / SPONTANEOUS COMBUSTION

•C+O2= CO2+ heat --------(65.5-90 0C) Coal undergoes slow oxidation on exposer to air at ambient temperature, with the evolution of heat, gases and moisture. •The heat generated, if not dissipated, give rise to an increase in temp. of coal. •Rate of oxidation increases with the rise in temp. & If this is allowed to proceed unchecked, it can eventually result in the ignition of coal. •This oxidation process is known as spontaneous combustion, spontaneous heating or self heating.

2C+O2= 2CO+heat --------( 100-1300C)

• Spontaneous combustion occurs when rate of heat generation > heat dissipation

Page 2: Spontaneous Basics

This process occurs mainly in coal because of the adsorption property of coal. It can occur with sulphide ores also.

• Sorption process of taking up of a substance by the solid

• Adsorption physical process and accumulation occurs at the surface of solid

• Absorption chemical process and uniform penetration occurs in the bulk of the solid

Above mechanism of Spontaneous combustion is based on Coal-Oxygen interaction theory

Page 3: Spontaneous Basics

O2

Adsorption of O2 in coal

Onset of Oxidation

Heat Evolution

Heat Dissipated

No Further Heating Yes

No

Rise in temp of coal

Increase in Rate of Oxidation As Rate α Temperature

Further increase in temperature

Temp = Ign. temp

Yes

Combustion of Coal

No

Page 4: Spontaneous Basics

Theories put forward to understand mechanism of spontaneous combustion:

1. Pyrite Theory:– Coal contains pyrite, with concentration some times

> 12%.– 2FeS2+7O2+16H2O= 2H2SO4+2Fe SO4.7H2O+ 316

Kcal/Kg– It was earlier suspected that oxidation of pyrite is the

main cause of spontaneous combustion. – But now it is established that pyrite present in coal

might assist in oxidation of coal by:• Breaking down coal into small fragements and exposing

large surface areas to the air• By elevating the temp. due to heat liberated by its own

oxidation

Page 5: Spontaneous Basics

2. Bacterial Theory:– Heating in hay stacks and in wood due to bacterial action– Earlier though sp heating in coal was due to bacterial action– Further research revealed bacterial action could cause slight

heating which may not play significant role

3. Heating due to ground movement:– Crushing of coal under pressure of superincumbent strata may at

times be source of heating, contributing to sp. Comb. Sudden roof fall and resultant adiabatic compression of air may also elevate ambient temp.

– But it has been observed that though the heat from crushing would contribute to sp.comb, the oxidation of coal is more important factor.

– Crushing of coal increases surface area of coal for oxidation, making it more potential to self heating

Now Coal-Oxygen interaction theory is universally accepted

Page 6: Spontaneous Basics

Categorisation of coal for its liability to spontaneous combustion

Page 7: Spontaneous Basics

Study of thermal data, criteria:– Measuring the lowest temperature at which the exothermic

reaction becomes self propellant under particular set of experimental conditions

– From measurement of relative heat production during coal oxidation

Methods:1. Crossing Point Temperature Determination2. Crossing Point Temperature and Ignition Point Temperature

Determination3. DTA Technique4. Adiabatic Calorimetry

Page 8: Spontaneous Basics

1. Crossing Point Temperature Determination Criteria:

This method envisages heating coal samples in an oxidising atmosphere at a definitely programmed rate of temperature rise. The lowest temperature at which the exothermic reaction in the coal bed can be observed to be self-propellant under the experimental conditions, has been termed as critical oxidation temperature, or critical ignition temperature, or simply as the crossing point of the coal concerned.

Coals which are highly susceptible to spontaneous heatingwould have lower values of crossing point temperatures, and thepoorly susceptible coals have comparatively higher values.So, it can also be defined as the temperature at which the coaltemperature crosses the bath temperature

Page 9: Spontaneous Basics

Various experimental techniques and designs of apparatus have been used for the determination of crossing point temperature.

The method widely adopted, prior to 1994 , in India consist of allowing humid air, at the rate of 80 ml/min, to pass through a coalbed of 20 g in a reactor tube and allowing it to get heated at the rate of 0.5 deg C per min. in a glycerine bath.

After 1994, as per guidelines of DGMS the parameters were modified as under:– 20 g of coal sample having a size of – 72 mesh + 200 mesh placed in

a reactor tube– Rate of heating as 1 deg C/min– Rate of Oxygen flow 80 ml/min

Page 10: Spontaneous Basics

Crossing Point Temp Determination

Page 11: Spontaneous Basics

Categorisation of coal by DGMS based on CPT:

S. N. CPT & Moisture content Category

1 CPT > 1600C and Moisture content < 2% Poorly Susceptible to Sp Heating

2 CPT 140-1600C and Moisture content 2-5%

Moderately Susceptible

3 CPT 120-1400C and Moisture content >5%

Highly Susceptible

Page 12: Spontaneous Basics
Page 13: Spontaneous Basics
Page 14: Spontaneous Basics

It has been observed that crossing point temperature values varies with the volatile matter, oxygen percentage and the moisture content of coal.

The crossing point temperature normally decreases with the increase in each of these constituents of coal.

But beyond 35 % V. M., 9 % oxygen or 4 to 6% moisture content-there is not much change in crossing point temperature values (Figs.).

In fact, above 4 to 6 % moisture content in coal (as received basis) the crossing point temperature values show rather a rising trend.

Page 15: Spontaneous Basics
Page 16: Spontaneous Basics

• When two coal has same Crossing point Temperature, then:The sample with higher heat evolution rate at the crossing point temperature (i.e. with larger slope) has obviously much higher susceptibility.

• Hence the above parameter, i.e., the rate rise of temperature at the crossing point has also to be taken into consideration besides the values of crossing point temperatures for making coal categorisation.

Page 17: Spontaneous Basics
Page 18: Spontaneous Basics

2. Crossing Point & Ignition temperature Determinetion:In order to obviate the above difficulties, it is suggested to continue the crossing point temperature determination using oxygen to pass through the coalbed till the ignition (i.e. bursting into flame) of the coal bed is reached. The ignition temperature is the minimum temperature from which the coal bed temperature starts shooting up almost vertically. The highly susceptible coals would obviously have minimum time lag to reach the ignition point after attaining the crossing point temperature. Hence susceptibility to spontaneous heating depends more on this time lag between crossing point and ignition point temperatures than depending on the value of the crossing point alone (Fig. 3.8).

Page 19: Spontaneous Basics
Page 20: Spontaneous Basics

But the limitation of this method is that the poorly susceptible coals, or even of moderately susceptible ones, where transition to ignition point is not sharp-it becomes difficult to pinpoint the minimum temperature from which coalbed temperature starts shooting up-reaching ignition.This affects accuracy of the ignition temperature determination and thereby, in distinguishing spontaneous heating susceptibility of different coals.

Page 21: Spontaneous Basics

D.T.A. METHODSThe rate rise of heat evolution of coal during aerial oxidation may be compared using Differential Thermal Analytical (DTA) technique.

This technique involves heating a small test specimen at a constant rate, and continuously recording the instantaneous termperature difference (DT) between it and an identical heated inert reference material. as a function of the temperature (T) prevailing in the inert medium. The resultant thermogram, a record of (DT) against (T), with its characteristic heat changes and intensities, depicts the physical or chemical changes of the material at the particular temperature, and is a characteristic of the material used.

This tool can be used in studying various properties of coal, i.e. the spontaneous heating susceptibility of coals.

In this technique the use of calcined alumina is used as the reference material taking nearly 0.6 g of coal, of particle size-72 B.S. mesh. The oxidising atmosphere is maintained by keeping the coal sample exposed to air in a horizontal sample-holder. The heating rate of 50 C/min was maintained during the experiment

Page 22: Spontaneous Basics
Page 23: Spontaneous Basics

There are three stages of transitions in the D.T.A. thermograms carried up to around 300°C (Fig. 3.9). In the initial stage of heating, (stage I), it is the endothermic reaction that predominates mainly from the release of moisture from coal. This is found to be followed by exothermic reaction (supposed to cover a number of concurrent reaction), in stage II of the process, which ultimately leads to reaction of very high exothermicity in stage III.

The rate rise in heat evolution in stage II is observed to be much lower for coals with lower susceptibility to spontaneous heating.

In cases of poorly susceptible coals, stage II is dragged to a considerable range of temperature, delaying the initiation of stage III. But once initiated, the exothermicity in stage III for even the poorly susceptible coals may be as high as that of highly susceptible ones.

Page 24: Spontaneous Basics

The criteria thus set for categorising coals from D.T.A. studies are :

(a) the sharpness of the slope of thermograms in stage II, and(b) the temperature of initiation of stage Ill.

The limitation in the D.T.A. method is the difficulty met in obtaining the repeatability of results. The sharpness of the peak, even by maintaining the above procedure, may change with a change of the sample-holder, the sample cell or even if the furnace is replaced.

Thus, all experiments involved in making the comparative studies of coals are to he carried out not only under similar experimental conditions but also using the same apparatus.

Page 25: Spontaneous Basics

Adiabatic Calorimetric:Adiabatic type calorimeters have also been used

for coal categorisation measuring heat evolution during oxidation process, under adiabatic conditions. It has been observed from such studies that lignite and sub-bituminous coals have a much higher liability to spontaneous heating in ambient temperature than bituminous coals.

Page 26: Spontaneous Basics
Page 27: Spontaneous Basics

• The relative effectiveness of various additives in suppressing the self-heating process in coal was evaluated in an adiabatic heating oven. The oven is designed to minimize heat losses from the coal sample during the self-heating stage of the test.

• The sample is contained in a brass wire mesh basket, 7.6-cm diam by 5 cm high, which is enclosed in a stainless steel assembly. Preheated, humidified air enters the bottom of the assembly, passes through the sample, and exits at the top. A stainless steel cylinder surrounded by a nickel-chromium wire resistance heater serves as the sample oven. The cylinder surrounds the sample assembly and is attached to an insulated top cover in which another resistance-type heater wired in parallel to the sample oven is located.

• The oven assembly is contained in a 15-cm-ID Dewar flask that is surrounded by a 5-cm layer of insulation in which another heater is embedded. The apparatus is

• instrumented to record sample and oven temperatures, and O2 and CO concentrations of the exit gas stream.

Page 28: Spontaneous Basics

• Briefly, the coal was first pulverized and sieved. The minus 100 plus 2OO mesh(74-150 micron) fraction of the coal was then dried in an oven at 67o C with a 2OO cm3/min flow of dry N2. A 100 g sample was placed in the adiabatic heating oven and brought to a

• preselected initial temperature under a 2OO cm3/min. flow of dry N2.

• The coal was then exposed to a 2OO cm3/min flow of humidified air.

• A series of tests were made each with a fresh sample, in 5° C increments, until the minimum initial temperature that produced a sustained exothermic reaction, or thermal runaway, was determined.

Page 29: Spontaneous Basics
Page 30: Spontaneous Basics

• Limitations• Though a reasonably large number of methods have been tried for

categorising the spontaneous combustion susceptibility of coals from various perspectives, none of the individual tests can take into account all the parameters responsible for spontaneous combustion (e.g. oxygen avidity, heat accumulation chnracteristics, friability, state of weathering of coal, etc.). Nor is it possible to simulate the underground conditions in the laboratory.

• It is, therefore, evident that the degree of liability of a coal to spontaneous combustion cannot be accurately determined from a single or even a group of tests.

• It may also be noted that the onset of spontaneous combustion depends not only on the properties of coal but also on the external conditions, caused due to mining operations, influencing the accumulation of heat. Thus, even if the indexing of spontaneous combustion susceptibility of coal is made using the tests cited. it is apparent that the results would only compare the behaviour of different coals under identical heat transfer conditions.