Published : International Mine Tech. ’98 Symposium, Chongqing/ China October 1998,
China Coal Industry Publishing House (ISBN 7-5020-1651-1/TD8), p.304-310 (English)
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THE PRESENT STATE AND PROSPERTS FOR NITROGEN
FIRE-PREVENTION AND FIRE-ELIMINATION IN COAL MINES OF CHINA
Gao Guangwei1
ABSTRACT
The underground mine fire remains one of the major calamitous hazards for the safe production of coal mines ,
especially the fires on high-output and high-efficiency comprehensively mechanized and roof coal caving faces
would spell a loss all the more serious. Now, nitrogen has become the major means for prevention and
elimination of coal mine fires in many coal-producing countries of the world since it was first used by a British
coal mine in 1953 for the said purpose. By 1996, there are already 21 coal mine admonitions and 34 coal faces
in China who have used nitrogen for tackling the fire problem.
The nitrogen generators used in coal mines are of the following 3 types: the cryogenic air-separation generator,
the pressure-swing-adsorption nitrogen generator and the membrane-separation nitrogen generator， all which
are to have the oxygen and nitrogen in the air separated from each other. The different methods have a different
requirement for the structure and application condition of the generator.
The nitrogen fire preventing and eliminating technology is a complicated system engineering which requires for
a determination in the light of spontaneous combustion property, the critical value by which the coal in nitrogen
environment (i.e. oxygen deficiency) is incapable of spontaneous combustion, a determination of the coal
spontaneous indicative gas index, requires for a selection of different nitrogen-injection modes such as
pipe-embedding (pipe-pulling),borehole and by-pass etc in accordance with the layout of mining section
roadways, requires for regulation of the balanced-pressure ventilation according to the conditions of
nitrogen-injected areas and requires for a calculation of the nitrogen-injection technological parameters on the
basis of the shortest fire interval and mining conditions.
As the nitrogen-injection fire prevention and elimination technology would have a great development, so it is
necessary to select in accordance with mine conditions a different nitrogen generating equipment and
technology with the best equipment, the best technology and the best result obtained.
1 INTRODUCTION
The underground mine fire remains a major calamitous hazard for the safe production of coal mines, especially
the spontaneous combustion in the gob area of high-output and high-efficiency comprehensively mechanized
and roof-coal caving faces not only influences the production, brings losses to coal resources and mining
equipment, but also causes methane explosions and casualties which have commonly occurred.
Much attention has been paid to using nitrogen for preventing and eliminating fires by the safety personnel of
coal mines. In 1953, cylinder nitrogen had been applied by Roslin Mine of Britain to eliminating the
spontaneous combustion occurred in the coal seam nearby the pit bottom. In 1962, Fenher Mine of Welsh had
the gasified liquid nitrogen injected into the sealed-off area for fighting the fire. Starting from the 70th of this
century, using liquid nitrogen for fire prevention and elimination in coal mines of the former West Germany has
had a swift development and large scale application. Later on, this method has also been introduced in Britain,
French , the former Russia and India with a better result.
From the 80th of this century, China has carried out researches and tests of nitrogen-activation fire-preventing
and fire-fighting technology. In 1983, Tianfu Coal Mine Administration had performed the cylinder liquid
1
Fushun Branch, CCRI , Fushun, LN , China Zip 113001
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nitrogen underground fire-fighting tests; In 1987, the Longfeng Mine of Fushun succeeded in using the nitrogen
produced by the surface oxygen plant conveyed by pipelines to underground for preventing the spontaneous
combustion of the float coal in the gob area of comprehensively mechanized roof-coal caving faces; In 1992,the
Duerping. Mine of Xishan Coal Mine Administration used the gaseous nitrogen generated by the surface mobile
pressure-swing -adsorption nitrogen generator, and conveyed through pipelines into the underground to
effectively prevent the coal spontaneous combustion of the close-by coal seam group and in 1995, the
Xinglongzhung Mine of Yanzhou installed the underground mobile membrane-separation nitrogen generator at
the near-by location of the stop-mining line and had effectively brought under control of the float coal
spontaneous combustion in the gob area of the pillarles mining adjacent face. By 1996,there are in China already
21 coal mine administrations and 34 coal faces which have conducted nitrogen injection fire prevention and fire
elimination.
As nitrogen has played a more and more important role in prevention and elimination of coal mine fires, the
Safety Regulations in Coal Mines of China stipulates that the comprehensive fire preventing and fire eliminating
measures with injection of inert gas as the major one should be practiced for prevention and elimination of the
spontaneous combustion in the gob area of comprehensively mechanized roof-coal caving faces. The former
Ministry of Coal Industry plans to install a batch of nitrogen generators during the “9th Five -Year-Plan” for
ensuring the safe production of comprehensively mechanized and roof-coal caving faces.
2 NITROGEN GENERATING EQUIMENTS
The nitrogen generators presently used in fire prevention and elimination projects of coal mines in China are
three types: the cryogenic air-separation nitrogen generator , the pressure-swing-adsorption nitrogen-separation
nitrogen generator ,the pressure-swing-adsorption nitrogen generator and the membrane separation nitrogen
generator. The latter two generators are designed
according to the varied installing and moving modes
into the surface fixed one, the surface mobile one and
the underground mobile one with the underground
mobile cryogenic air-separation nitrogen generator
having not yet been reported.
2.1 Working Principle And Construction
The cryogenic air-separation: it refers to the separating
process in which the air by way of compression and
expansion cycles gets lowered in temperature and
liquefied and separated in accordance with the varied
air components. The cryogenic air-separation
nitrogen generator is composed of the air-compressor,
the filter, the heat exchanger, the expansion machine and
fractionating column etc , the flow process of which is
shown in Fig.1
The pressure-swing-adsorption: the process in which
oxygen and nitrogen are separated through oxyen pressurizing adsorption and nitrogen-discharging by
molecular sieves .The pressure-swing -adsorption nitrogen generator consists of the air-compressor, the filter,
and adsorption column etc , the technological flow process chart provided in Fing.2
The membrane-separation: the process in which enrichment of O2 and N2 takes place respectively within and
outside the membrane wall as a result of the different solving, diffusing and permeating rates of O2 and N2
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molecule in the membrane wall. The membrane
separation nitrogen generator consists of the air
compressor, filter and membrane module etc. with the
technological flow-process shown in Fig.3
2.2 Characteristics And Applicable Conditions Of The
Nitrogen Generator
The cryogenic air-separation nitrogen generator is
the earliest to come out with the technology being the
most matured, and application the widest. At present,
60%air-separation market has been occupied by
the cryogenic air-separation. However, the
market occupancy of this method is step by step
lowered while the other two methods have
showed a fast development, especially the
membrane-separation technology which though
only has a history of over 10 years, has presented
a very remarkable superiority, and according to
the prediction of experts, early in the next
century, the membrane, separation nitrogen
generator would play a leading role in the air
separation market.
The cryogenic air-separation nitrogen generator is
applicable
to
produce
high
purity
nitrogen,
especially the liquid nitrogen produced by this method has a very remarkable effect on lowering the temperature
of the self-heated or burning coal body , the fire-preventing and fire-fighting role of it is incomparable with that
of the other two method. The nitrogen used by the former West Germany for the fire-fighting all belongs to this
type, The liquid nitrogen made by the irseparation plant is conveyed again by the pipeline to the pit head and
after gasified to a temperature of -15. . is conveyed again by pipelines to underground for fire prevention and
elimination purpose However, as the cryogenic air-separation nitrogen plant needs a large initial investment
with a high nitrogen cost and complicated operation and maintenance, especially after the plant stops operation,
the pre-production of nitrogen has a long running time, so it is suitable to the newly constructed coal mines
where mines are centralized and production concentrated.
The pressure-swing-adsorption and membrane-separation nitrogen generators all use the method in which
oxygen and nitrogen is separated by basing on the property of oxygen and nitrogen molecule without any phase
transition. These generators, in comparison with the cryogenic air-separation nitrogen generator, have such
merits of small volume, light weight, lower energy consumption, movable, no foundation operation and little
investment in the initial period, and are highly applicable to the relatively independent mines and faces.
However, some differences do exist between the two generators, the pressure-swing-adsorption one is much
more complicated than the membrane-separation one and its application is largely limited as a result of the poor
reliability caused by timely switch-over and operation of multiple valves in operations and the molecular sieve
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being liable to lose effectiveness under a pressure-changing state.
The membrane-separation nitrogen generator has a remarkably economic benefit in producing low purity
nitrogen: with increase in nitrogen purity, the nitrogen cost rises sharply and as the purity comes to over 99%,
the cost for nitrogen is close to that of the cryogenic air-separation nitrogen generator.
According to the property of membrane-separation nitrogen generator, the nitrogen production is in revere
proportion to the nitrogen concentration. Take MD-350 nitrogen generator for example, when the nitrogen
generator, for example, when the nitrogen concentration is 95% the produced quantity of nitrogen is 530m 3/h
while the concentration is 97%, the nitrogen produced is 350m3/h, and when compared with application to the
fire-prevention of gob area inactivation, the inactivation capacity of nitrogen in 95% concentration is 12~20%
higher. Thus, so long as the fire-preventing and eliminating condition is suitable, properly lowering the nitrogen
purity could result in a lowest mine fire-preventing and fire-fighting cost.
3. NITROGEN-INJECTING TECHNOLOGY PARAMETERS
3.1 Coal Spontaneous Combustion Critical Oxygen Concentrations
The coal spontaneous combustion critical oxygen concentration value is the highest concentration value by
which coal is incapable of spontaneous combustion as a result of oxygen-deficit; this value is usually
determined in the laboratory. For guarding against coal spontaneous combustion , it is required that the oxygen
concentration in the air of the float coal space must not exceed the critical oxygen concentration which is varied
with coal types and coal petrography compositions. The spontaneous combustion critical oxygen concentration
for coal seams of China usually ranges 5. .10%, which should be measured from coal samples before the
nitrogen fire-preventing and fire-fighting design.
3.2 The Coal Spontaneous Combustion Indicative Gas Index
This index is to indicate the gas products of various given stages in the process of coal spontaneous combustion,
the essence of which is by tests to find out the gas products (inanely the indicative gas) of the self-heated (or
spontaneous combustion) coal under a certain temperature condition, so as to show the hazardous stages of coal
spontaneous combustion in the fire-prevention area, and this index is especially important during nitrogen
fire-prevention's. The nitrogen-injection is not continuously performed during the whole mining process, but is
determined from the monitoring of gob area samples and the result predicted in accordance with the coal
spontaneous indicative gas index, this is called as the intermittent nitrogen-injection.
3.3 Gob Area “ Three-zones ”
According to the air-leakage of the gob area behind the face , the following three zones are classified: the
cooling zone ,the oxidation zone and the suffocation zone (see Fig.4).The air-leaking quantity of the cooling
zone is rather great, and the coal body in this zone is incapable of spontaneous combustion as the heat quantity
produced by the self-heated coal is unable to accumulate, the deep part of the gob area is the suffocation zone
which is incapable of spontaneous combustion as a result of oxygen deficit and between these two zones is the
oxidation zone where the oxygen concentration is moderate, resulting in not only coals being oxidized but also
capable of heat -accumulation. The aim of nitrogen fire-prevention is just to have the nitrogen injected into this
zone, causing it to be inactivated, and to lose oxidation conditions with the coal spontaneous combustion
inhibited.
Different coal and rock (mainly refers to roofs)properties and mining and ventilation conditions Determine that
the “three-zones” width distributions are largely different. Before nitrogen-injection fire-preventing designs,
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°three-zones” observations should be conducted. The classification is made by burying tube-bundle for
sampling and analyzing gas compositions and ancIent-rations. The oxidation zone of 3 coal seam in Fushun
mining area is located within 20 ~60m behind the
gob are a of the roof breaking line.
3.4 Nitrogen-injection Modes
The nitrogen-injection modes vary with the
mining conditions of mining sections. The
nitrogen-injections for the primary coal mass
independent faces are usually as the three one of
pipe-pulling, the pipe-burying and the borehole
drilling, while the by-pass mode should be used
for the complex gob area of pillarless mining.
The pipe-pulling nitrogen-injection is evoluted
from the pipe-pulling grouting, in which during
retreat coal mining activities and with face
advancing, the nitrogen-injection pipeline is in
advance buried in the intake gate gob area with
the steel pipe buried into the oxidation zone for nitrogen-injection and the buried steel pipe is pulled forward
with the advancing of the face, causing the nitrogen-injection mouth always positioned in the oxidation zone.
However, as some of the gob area oxidation zone lies in a deeper location which presents a larger difficulty for
pipe-pulling, so, of the several tens of the mined and nitrogen-injected faces in China, only one is of the pipe
pulling nitrogen-injection.
The pipe-burying nitrogen-injection has more applications, mostly the faces which are firstly worked have used
this mode for nitrogen fire-prevention. The procedure is : lay a nitrogen-injection pipeline in the lower gateway
and when the pipeline comes close to the face, bury two branch pipes and control valves in the gob area in
accordance with the interval of the nitrogen-injection mouth. When the nitrogen-injection mouth of the first
branch pipeline reaches the oxidation zone, open
the valve to begin nitrogen-injection. As the face
advances, the oxidation zone comes into the
nitrogen-injection mouth of the second branch
pipe, then open the valve of the second branch
pipe for injecting nitrogen with the first valve
closed, then bury the third branch pipe and on
the analogy of this .
The borehole-nitrogen-injection mode is
applicable to the cases where the working
face is provided with the mother rock
roadway or the return gateway of the next
(mining) section has been in advance drove,
the procedure of which runs as follow: in the
above-mentioned roadway, boreholes are drilled to the face gob area wich pipes cased, valves installed and the
pipeline laid. When the first borehole comes to the oxidation zone, the borehole nitrogen-injection could be
performed, which is to be continued till the borehole is removed from the oxidation zone.. The borehole
arrangement is flexible, and the borehole could be drilled and used for nitrogen injection at any time in
accordance with the gob area fire conditions, and the fire-prevention and fire elimination of this method could
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be coordinated as well with injection of inert foams and gels etc which all have achieved a good result. The bore
hole nitrogen-injection mode used on the comprehensively mechanized roof-coal caving face of the Longfeng
Mine in Fushun is indicated in Fig.5.
The by-pass nitrogen-injection is a special mode for the fire prevention and elimination by way of nitrogen
injection in the complex gob area of pillarless minings. As there exist in the inactivated area the gob area of the
face which is being worked as well as the gob area of the adjacent face, so it is required to select the point
having a high pressure-energy as the nitrogen-injection mouth or to set up multiple nitrogen-injection mouths
with nitrogen being distributed in areas having spontaneous combustion hazards. The by-pas nitrogen-injection
mode of the Chaili Mine in Zaozhuang is indicated in Fig.6.
3.5 Nitrogen-injection Quantity
The nitrogen-injection quantity for fire-prevention of the
gob area is calculated from the following formular:
QN=60KQD(C1-C2)/(CN+C2-1 )
here, QN----nitrogen-injection quantity,m3/h;
QD----air-leakage quantity in the oxidation
zone of the fire area or gob area,
m3/h
C1----average oxygen concentration in the
oxidation zone of the fire area or gob
area , %;
C2----inactivating index of the fire area or
gob area, %
CN----injected nitrogen concentration, %
K----stand-by coefficient, 1.2~1.5
However, as a result of quite a many factors which have
an influence on nitrogen injection, the nitrogen quantity
injected in the fire-prevention zone of gob areas in coal
mines of China is in the range of 200~400m3/h, the
injected nitrogen quantity in sealed fire area is 600~800m3/h
with the quantity for opening fire areas to combat fires being all the more great which is difficult to be satisfied
by the available nitrogen generators, and the fuel inert gas nitrogen generator could be introduced instead.
Besides, when the nitrogen-injection fire -prevention and fire-fighting is practiced in prllarless mining area and
areas having a complicated boundary roadways and gob area methane drainage, it is required for a reasonable
determination of the nitrogen-injection position and quantity with the balanced-pressure or leakage-blocking
measures taken for improving the nitrogen-inactivated effectiveness.
The tube-bundle sampling and automatic or artificial analysis are usually adopted for monitoring the oxygen
concentration of the oxidation zone.
4 PROSPECTS
The recent over ten years have witnessed a considerable progress in the nitrogen fire prevention and elimination
technology in coal mines of China with quite more research achievements and application experiences obtained,
and this technology has become the indispensable means for mine fire-protections. However, so long as the
present situation is considered, the complement of nitrogen fire-prevention and fire-fighting systems still lags
behind the development of the comprehensively mechanized and roof-coal caving technology especially the
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fire-preventing and fire-fighting methods of grouting etc which are very difficult to suit the comprehensively
mechanized roof-coal caving face gob area characteristic of a large three-dimensional space and air-leakage,
resulting in many occurrences of face-sealing accidents due to fire hazards in coal mines of China. For this
reason, work should be done to enhance the extended application of this technology and money investment,
further improve the stability and reliability of nitrogen generators, develop the linkage system of the gob
nitrogen concentration self-monitoring /control and the nitrogen generator and complete the automatic analysis
and transmission of signals, optimize the nitrogen-injection technology, making the nitrogen-injection fire
protection system all the more perfect.
We believe that during the “9th Five-year-plan” and a certain period afterwards, the nitrogen fire-preventing and
fire-fighting equipment and nitrogen-injection technology in coal mines of China would be developed by leaps
and bounds, and are bound to make greater contributions to the safety career of China’s coal mines.
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