Chapter 118

OPTIMISATION OF NITROGEN INJECTION

FOR INERTISATION OF LONGWALL FACES GOAF

IN CdF COAL MINES

J.P. Amartin
Safety General Manager Houilleres du Bassin de Lorraine France

ABSTRACT

Charbonnages de France are presently winning in Lorraine coalfield, seams with a high level of spontaneous combustion risk.

The main method is retreating longwall faces. As the roof is mainly hard rock, goaf is highly porous and important air leakage in the goaf acts against natural methane inerting. Due to this reason, HBL are using nitrogen for inertization of the goaf in order to avoid spontaneous combustion of remaining coal in association with classical methods in order to limit air leakage.

In collaboration with INERIS, HBL developed mathematical and physical modelisations to optimise nitrogen injection. These results are of great help for mining engineers to improve safety in the mined areas, as they give the best nitrogen flow required and the best place of injection to get efficient inertization of the goaf, in accordance with the air leakage ratio of every face

KEYWORDS

Air leakage ratio, numeric code, physical model, numerical simulations, optimisation of nitrogen injection

INTRODUCTION

Charbonnages de France are presently winning, in Lorraine Coalfield, coal seams with a high level of susceptibility regarding spontaneous heating.

The main mining method used is retreating longwall faces. At some places natural roof is soft and we have to keep a coal board under the roof, this board is then entirely caved in the goaf. In other places roof is mainly hard rock and it caves in very large blocks. In such conditions goaf is highly porous.

By the same time as we are facing gassy conditions, large air flow (around 40 to 50 m³/s) are required to maintain safe working conditions.

As it is not always possible to use ashes or sand stowing to tighten the abandoned roadways there is an important air leakage in the goaf, acting against natural methane inertisation and oxidizing remaining coal.

For these reasons in order to decrease the oxygen content in the goaf atmosphere and avoid coal oxidation, HBL is using nitrogen inertisation.

In collaboration with the French Institute INERIS, HBL developed mathematical and physical modelisations to optimise nitrogen injection.

AIR LEAKAGE RATIO

Determination of air leakage ratio

The air leakage ratio represents the percentage of the total district air flow diverted directly in the goaf from main gate to tail gate, and used as an unavoidable leakage.

Instinctively we know that air leakage should be more important in the case of hard rock roof which caves into large blocks than in the case of soft roof which caves smoothly.

One method using tracer gas has been developed in order to get a correct estimation of the air leakage ratio.

The tracer gas (SF6) is injected in the air intake with a constant flow during 30 minutes around 300 m from the face.

Atmosphere sampling is regularly done in the air intake roadway near the face and in air return. The tracer gas content is analysed in every sample and the results are recorded in order to examine the shape differences between the curves of SF6 contents related with the time, from each sampling area.

The difference shows that a more or less important quantity of tracer gas bas been delayed due to the fact it was diverted in the goaf with the air leakage.

Figure 1. SF6 Experiments in a longwall face

Superposition of the curves allows to calculate the quantity of diverted air flow, and the air leakage ratio is then the quotient of the curve area which represents the delayed air flow with the whole area of initial curve which is the representation of the district air flow.

Figure 2. Determination of air leakage ratio

So carefully sampling and analyses can be done there are always minors incertitudes about data, but we can consider this method gives a correct estimation of air leakage ratio and it is a reliable method.

Range of air leakage ratio

The first precaution to observe in the estimation of air leakage ratio is to take care that the normal caving status in the face has been reached. So, in HBL conditions we are waiting the face retreating is no less than 150 metre, before implementation of this method.

In HBL conditions 3 series of data regarding the air leakage ratio are commonly observed.

10 - 15% : soft roof - low ratio (category I)

15 - 30% : medium roof - medium ratio

(category II)

More than 30% : hard rock roof - high ratio

(category III)

Studies carried out in HBL show that, the air leakage ratio is:

• Unrelated with the total district air flow

• Unrelated with the retreating length of the face as far as the normal caving status has been achieved (more than 150 m)

• Slightly related with the face dip and with the air flow direction

• Mainly related with goaf porosity which is a direct characteristic of roof stratigraphy

GAS CIRCULATION IN THE GOAF

Determination of mathematical model

Thanks to several series of experiment using tracer gas it is possible to get other information specially regarding gas velocity in the goaf. For example, in a category I face, gas velocity in the goaf is around the following data :

• 10 to 35 cm/s in the proximate zone close to the power supports in the goaf

• 2 to 10 cm/s in the 20 to 50 m goaf zone

• 0,5 to 2 cm/s at 50 to 80 m in the goaf

• less than 0,5 cm/s in the deeper goaf

Nevertheless, in the case of HBL, where abandoned roadways are not filled with ashes stowing or synthetic foam, gas velocity is slightly higher all along the initial face galleries than in the deep goaf.

All information leads INERIS to the determination of a goaf permeability model. This one is described through a mathematic model and a physical miniature (scale 1/70).

Figure 3. Goaf permeability model

Numeric code

Phenomena are of great complexity and it is compulsory, as a first stage, to use only 2D model despite we are facing a 3D problem. Other phenomena such as thermodynamics exchanges are also neglected.

A Computational Fluid Dynamics (CFD) model under PHOENICS Software is used to give reliable information regarding gas circulation.

Physical model

In parallel with this mathematical model, a miniature has been developed. It allows to follow gas circulation in the goaf thanks to an automatic sequential sampler via needles regularly distributed in the miniature.

Figure 4. Longwall miniature (scale 1/70)

Figure 5. Details of goaf permeability

Adjustments between “in situ” observations, information from the physical model and numeric results have been carried out to improve the efficiency of CFD model simulations.

Examples of numerical simulations

Simulations taking into account :

• permeability distribution related to air leakage ratio,

• total district air flow,

• methane releasing and drainage

give us information about the distribution of oxygen, methane and nitrogen contents in the goaf.

These results reported on a drawing of the considered longwall face being mined give important information about the distribution of highly probable coal oxidation areas.

OPTIMISATION OF NITROGEN INJECTION

Nitrogen injection in the goaf

As far as natural inertisation due to methane released in the goaf and goaf confinement are not efficient enough to insure oxygen content less than 3 to 5%, spontaneous heating is feared in the goaf.

In order to decrease the oxygen content it is then compulsory to inject nitrogen in the goaf as we can't avoid air leakage behind the roof supports.

This can be done from pipes abandoned in the goaf from the air intake roadway or from bore holes distributed in the caved area.

Optimisation of nitrogen injection

The next stage is then to know which are the effects of nitrogen injection in the goaf and which are the most efficient values regarding location of the injection point and level of the nitrogen flow.

Figure 6. CH₄, O₂, N₂ content distribution

In the goaf with the following parameters:

Nitrogen injection point at 50 m in the goaf

Air leakage ratio = 20%

Nitrogen flow = 2 000 m³/h

CH₄ drainage = 1 000 m³/h

Total air flow = 40 m³/s

CFD model implemented by INERIS with CdF collaboration is able to solve this problem through a step by step approach.

For a specific longwall face designed thanks to its geometry, air leakage ratio (permeability model) methane releasing and possibly methane drainage condition , simulations based on the transactional data such as, district air flow, location of nitrogen injection point in the goaf and nitrogen flow leads to drawings of the oxygen contents distribution in the goaf.

It is then possible to choose the more efficient solution to face the present conditions.

Few examples

Simulations carried out in HBL conditions give few advice:

• immediate area at the rear of power supports remains uninertisable due to the important air circulation
  For a face using 30-35 m³/s air flow, the width of this zone is about :

40 m for 15% air leakage ratio

55 m for 20% air leakage ratio

70 m for 30% air leakage ratio

• Nitrogen injection point should not be located too close of the face in order to insure that nitrogen flow is fully efficient for inertisation

• As the district air flow increases the injection point has to be removed far away in the goaf to get the same efficiency.

• Several combinations of nitrogen flow and injection point can insure the same efficiency for example in a longwall face with a total air flow of 35 m³/s and an air leakage ratio of 20%:

• efficient inertisation is obtained at 60 m in the goaf, either with a nitrogen flow of 6500 m³/h injected at 40 m in the goaf or with a nitrogen flow of 4700 m³/h injected at 100 m in the goaf.

• for a 80 m width non inertised zone, in the same conditions, 1800 m³/h nitrogen flow injected at 60 m in the goaf or 3400 m³/h nitrogen flow injected at 40 m are convenient.

Figure 7. CH₄, O₂, N₂ content distribution

In the goaf with the following parameters:

Nitrogen injection point at 50 m in the goaf

Air leakage ratio = 20%

Nitrogen flow = 4 000 m³/h

CH₄ drainage = 1 000 m³/h

Total air flow = 40 m³/s

Figure 8. A few Examples of O₂ content distribution
in the goaf (numerical simulation)

CONCLUSIONS

Thanks to tracer gas studies it is possible to get a convenient representation of longwall face characteristics specially regarding goaf permeability. CFD model implemented by INERIS at HBL colliery allows to solve gas circulation in the goaf and to draw distribution of oxygen, methane and nitrogen contents.

Figure 9. How to choose nitrogen flow and

injection point position

Figure 10. Effects of nitrogen injected at 80 m

in the goaf

Following adjustments of the model, it is then possible to test the effects of nitrogen injection in the goaf and to optimise inertisation.

All data are of great help for mining engineers as they must be considered as guidance to choose the best solution for inertisation of the goaf and to improve safety in the mined areas.

2

I SZKOŁA AEROLOGII GÓRNICZEJ 1999

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852

PROCEEDINGS OF THE 7^TH INTERNATIONAL MINE VENTILATION CONGRESS

853

OPTIMISATION OF NITROGEN INJECTION

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