AU2012202891A1 - Method for extinguishing coal mine fires and unit for performing said method - Google Patents

Abstract

The invention relates to a method for extinguishing coal mine shaft fires, consisting in: fully or partially swelling at least one superabsorbent (co)polymer in the presence of water, and subsequently injecting the resulting fully or partially swollen superabsorbent (co)polymer(s) directly into the shaft. The invention also relates to the unit used to perform said method.

Description

1 METHOD OF EXTINGUISHING COAL MINE FIRES AND INSTALLATION FOR IMPLEMENTING THE METHOD Coal mines have always constituted a safety problem. Some unexploited mines have 5 been burning for centuries. However, the number of fires increased considerably starting with the exploitation of coal for the production of steel or energy. These fires are often started by accidents or overheating of equipment (tractors, conveyor belts, welding, etc.), and once started, the fire can be spread by the gases 10 present in the galleries, especially methane and hydrogen. In other cases, fires on the surface spread underground, especially in mines that are not very deep (lightning, forest fires, incineration of refuse). 15 In certain cases, however, spontaneous combustion is observed, through gradual oxidation and build-up of heat, for example in the heaps at the pithead, in the case of coal contaminated with fuel oil, or in the presence of organic matter, for example in the form of wood-coal mixtures. This is in particular the case in the presence of pyrites which, on exposure to the atmosphere, tend to accelerate the phenomenon. 20 These coal fires are also frequent on the surface: heaps of fines or silos, especially in the presence of coal with a high sulfur content. China is a major home of such fires in the provinces of Xinjiang Uygur and Ningxia 25 Hui. These fires consume about 200 million tonnes of coal per year. For example, the Liu Huangou fire has been burning for 20 years and will burn for another 40 years with the extinguishing means currently employed, polluting the town of Urumqi with toxic gases (CO, H 2 S, C0 2 , etc.). 30 These fires cause caving-in and fissures which feed the pits with oxygen. Temperatures can exceed 800'C. The action taken consists in particular of injecting mud and earth for sealing these fissures. However, the latter are spread over wide areas and they are constantly changing, on ground that is often very hot and inaccessible. 35 The second country most affected is the United States of America and especially the large mines of Pennsylvania, where fires started around 1772. The largest fire began 2 in 1869 (Avondale, where the mine fire went out spontaneously after a year, after killing 110 people). There are currently 140 large coal mine fires in the USA. The conventional methods of fire extinguishing consist either of injecting mud for 5 blocking the air inlets and fissures, or of closing the galleries by means of walls of brick, cement or clay. However, the technique most used recently consists of injecting dense chemical foams with inert gas (N 2 - C0 2 ) or nitrogen directly. In certain regions, entire towns have been moved, and roads diverted, for example in Centralia. 10 The third country is India, where fires began in the 18th century. The Jharia mine is the world's largest mine fire, which started in 1916. Today, there are 70 fires in this very large coalfield, which is still in operation (coking coal). 15 The problem of fires in coal mine pits (and some surface mines) is far from being solved. In fact, injection of water is very ineffective as it trickles and infiltrates while evaporating to a slight extent. This does not provide sufficient cooling of the burning 20 coal. Water at the bottom of the mine can also evaporate and cause explosions due to the pressure. 25 The filling of fissures is random. In fact, over very large areas, there are numerous fissures, which are sometimes invisible. It is also dangerous to send trucks carrying earth and mechanical shovels above these combustion zones, which sometimes emit flames several meters high above the ground. 30 Dense foam laden with inert gas has a relatively high angle of repose and, on injection, can block the horizontal galleries, decreasing the horizontal transfers of oxygen. However, the foam does not have an extinguishing effect as it does not contain 35 sufficient water to cool the walls. The foam in fact disappears at relatively low temperatures (50 to 80'C).

3 In the case of injection of mud, for example bentonite mud, the mud has a tendency to flow like water and to separate at the bottom of the pits. Moreover, their very small angle of repose does not permit the sealing of horizontal galleries. 5 Tests were carried out in Australia (Controlling underground heating using Innovative Fire suppressant Injection - Proof of concept study - Shenggen Hu, Sheng Xue - SCIRO project (14021, Feb 2009) using a solution of polymer (polyacrylamide, carboxymethyl cellulose, acrylamide-xanthan acrylate copolymer) mixed with an aluminum complex (citrate in particular). The mixture is injected in 10 liquid form in the presence of a crosslinking agent. In other words, the mixture crosslinks after injection leading to the formation of a gel after a certain time. This liquid does not initially have an angle of repose and only coats the bottom of the galleries unless it is stored between two walls, which is difficult to achieve. Moreover, chemical manipulations consisting of extemporaneous mixing of 15 crosslinking agent and polymer near fires at high temperature are always hazardous. Particularly in China, polymers have been used as binders and water retainers for the production of gels or mineral pastes containing silica, kaolin, bentonite, fly ash, various muds, etc., which serve to seal the air inlets in coal mines that are on fire. 20 We may mention: 1 - Gel retarder for fire prevention and extinguishing - its application for spontaneous combustion of coal seams Wen, Hu; Xu, Jingcai; Dai, Aiping 25 Proceedings - Annual International Pittsburgh Coal Conference (1998), 15th, 593 597 CODEN PICNE4; ISSN. 1075-7961 2 - Thickening colloid for treating coal seam fire Zhu, Hongqing, Suo, Hongli; Zhang, Teng; Liu, Min; Ma, Lin; Wang, Yi; Yuan, 30 Huiping; Guo, Aidong; Zhao, Feng; Liu, Defeng; Zhang, Xinya Faming Zhuanli Shenqing, 6pp. CODEN CNXXEV 4 3 - Extinguishing spontaneous combustion in coal seam with heat-resistant, rich water gel Xu, Jingcai; Deng, Jun; Wen, Hu; Guo, Xingming; Zhang, Xinghai Mining Science and Technology '99, Proceedings of the International Symposium on 5 Mining Science and Technology, Beijing, China, Aug. 29-31, 1999 (1999), 137-140. Editor(s): Xie, Heping; Golosinki, Tad S. Publisher: A. A. Balkema, Rotterdam, Neth. CODEN: 69BDV4 4 - An inorganic foaming gel for preventing spontaneous combustion in a top-coal 10 falling region Yu, Shui-jun; Yu, Ming-gao; Feng-cheng; Pan, Rong-kun; Lu, Lai-xiang; Chang, Xu hua Zhongguo Kuangye Daxue Xuebao (2010), 39(2), 173-177 COden: ZKDXER 15 5 - Production of a composite gel material for the fireproofing and fire-extinguishing of coal seams Suo, Hongli; Zhu, Hongqing; Guo Aidong; Zhang, Teng; Yuan, Huiping; Wu, Ziping; Yan, Binjie Faming Zhuanli Shenqing, 5pp. CODEN: CNXXEV 20 6 - Acrylamide-type copolymer as blocking agent for fire control in coal seams Cao, Dacheng; Wen, Hu; Zhang, Xinhai; Li, Bo; Wang, Jingang; Liu, Qingning; Liu, Guicheng Faming Zhuanli Shenqing Gongkai Shuomingshu, 5pp. CODEN: CNXTEV 25 7 - The technique of fire extinguishing with gel for coal spontaneous combustion in fully mechanized caving face Wen, Hu; Xu, Jingcai; Deng, Jun; Zhang, Xinhai Proceedings in Mining Science and Safety Technology, [International Symposium], 30 Jiaozuo, China, 2002 (2002), 448-452. Editor(s): Zhang, Chuanxiang; Jing, Guoxun; Zhou, Ying. Publisher: Science Press, Beijing, Peop. Rep. China. CODEN: 69DDL4; ISBN: 7-03-010255-X In all these cases, it is the mineral portion, more or less dehydrated by the heat, 35 which serves to seal the oxygen passages to extinguish the fire. However, these methods are effective for small fires but are ineffective for large fires with ground subsidence.

5 There has been very limited success in extinguishing coal mine fires and it is often decided to abandon the regions and leave the fire to spread for decades in view of the costs and the slight chance of success. 5 The objective of the invention is therefore to develop a method of extinguishing coal mine fires combining the following characteristics: - Limited cost, - Application of a large amount of water, - Angle of repose of the injection fluid making it possible to obstruct 10 horizontal galleries, - Storage of injection fluids in intermediate horizontal zones for cooling the walls, - Filling of cavities even of large size, - Mobility of the injection fluids as changes of shape occur within the 15 mine or on the surface. The applicant has developed a method for solving all of these problems by using superabsorbent (co)polymers or SAPs with a high degree of swelling in water. By definition, these polymers are crosslinked. The polymers according to the invention 20 are injected in the form of partially or completely swollen superabsorbents, in contrast to the prior art. There are numerous types of natural or synthetic water soluble polymers that are crosslinked, or that can be crosslinked: polyvinyl alcohol, guar, alginates, carboxymethyl cellulose, dextran, xanthan, poly(ethylene oxide), etc. 25 The superabsorbents are polymers that are well known in specialty chemistry. They are generally in the form of powder. Their structure, based on a three-dimensional network that can be likened to a multitude of small cavities, each of them having the capacity to deform and to absorb water, confers on them the ability to absorb very large amounts of water and therefore to swell. Such polymers are described for 30 example in patent FR 2 559 158, which describes crosslinked polymers of acrylic or methacrylic acid, crosslinked graft copolymers of the polysaccharide / acrylic or methacrylic acid type, crosslinked terpolymers of the acrylic or methacrylic acid / acrylamide / sulfonated acrylamide type and their salts of alkaline-earth metals or alkali metals. 35 6 As already mentioned, the main characteristic of these polymers is a strong capacity to swell in aqueous medium. They can absorb and store large amounts of water up to 100 times their mass or more. They are used notably in agriculture for water retention in soils, in hygiene products for babies intended to contain urine and similar 5 applications. The invention therefore relates to a method of extinguishing fires in coal mine pits consisting of: - partially or completely swelling at least one superabsorbent (co)polymer in the 10 presence of water, - then injecting at least the swollen superabsorbent (co)polymer(s) obtained directly into the mine. Advantageously, the polymers are selected from the group comprising: 15 - crosslinked copolymers obtained by polymerization of acrylamide and of partially or completely salified acrylic acid, preferably in the form of a sodium salt, - crosslinked polyacrylic acids, partially or completely salified, preferably in the form of a sodium salt, which is more sensitive to 20 the salinity of the water and in particular to Ca and Mg 2 +. In a preferred embodiment, the polymers are crosslinked copolymers of acrylamide and of partially or completely salified acrylic acid and contain between 40 and 90 mol% of acrylamide and between 10 and 60 mol% of partially or completely salified 25 acrylic acid. In a particular embodiment, the SAP is a crosslinked terpolymer obtained by polymerization of acrylamide and/or of partially or completely salified acrylic acid and/or of partially or completely salified ATBS (acrylamido tert-butylsulfonate) 30 and/or of NVP (N vinylpyrrolidone). Other hydrophilic monomers, but also monomers of hydrophobic character, can be used for producing the polymers. 35 7 The SAP is partially or completely swollen. A completely swollen SAP is characterized in that all the small cavities of the three-dimensional network are filled with water to saturation, i.e. at a level such that on adding more water, the SAP does not absorb any additional water. 5 The polymers are crosslinked with 100 to 6000 ppm of at least one crosslinking agent selected from the group comprising acrylic compounds for example methylene bis-acrylamide, allylic compounds for example tetra-allylammonium chloride, vinylic compounds for example divinyl benzene, diepoxy, metal salts etc. 10 These polymers have a stability of several years in the ground. It can be improved by carrying out a double crosslinking with an acrylic crosslinking agent, preferably at a rate of 100 to 1000 ppm and an allylic crosslinking agent, 15 preferably at a rate of 1000 to 5000 ppm, for example MBA (methylene bis acrylamide) and tetraallylammonium chloride, which extends the stability of the SAP to more than 5 years. The swelling of these SAPs in distilled water generally reaches 150 to 200 times, but in a standard drinking water, about 100 times and less for very saline waters. 20 Therefore 10 kg of polymer and 990 kg of water are required to make a tonne of "solid water" for swelling of 100 times. The polyacrylamides have the capacity to produce swollen SAPs that are more rigid 25 and less sensitive to divalent metals than the polyacrylates. Advantageously, the water used for swelling the polymer has the lowest possible salinity, so as to obtain the maximum swelling of the SAP for a given amount. The water preferably contains less than 200 g/l, very preferably less than 100 g/l of salts, 30 for example NaCl, CaCl 2 , MgCl 2 . For this purpose, it can be treated by reverse osmosis. When the SAPs are swollen, the angle of repose of the SAPs varies as a function of the initial granulometry of the SAP. With a granulometry of less than 1 mm (less 35 than about 5 mm swollen), the angle of repose is of the order of 10'. With a granulometry of less than 4 mm (less than about 1.5 cm swollen), the angle can reach 15-20'. It is therefore possible to block galleries simply by injection.

8 In contact with a wall that is on fire, the stored water evaporates and the polymer is burnt. However, for 1 g of polymer, 100 g of water has evaporated, or an energy absorption of 226 kJ, to which must be added the heat for temperature rise of the steam generated to 400-600'C. For example, with a flow rate of 1000 1/minute, it is 5 possible to remove 4000 MJ/min, to be compared with the heat of combustion of 1 liter of gas oil of 36 MJ. The swollen superabsorbent therefore has both a mechanical effect like a rigid foam, and a thermal effect like water which has a formation of steam of 1.7 m 3 per liter of 10 water evaporated, and the vapor of which does not dissociate into dioxygen and dihydrogen, explosively, not before 1600'C. Moreover, it can deform as a function of the movements of the ground, maintaining their sealing function. It is also possible to add to the water and/or with the swollen SAP, compounds 15 improving the extinguishing power of this water with additive. It can be ammonium phosphates, potassium bicarbonate, urea etc. However, salts dissolved in the water reduce the swelling. Thus, urea is preferred. Still according to the invention, the swollen SAP is prepared in a zone away from the 20 fire, thus avoiding any dangerous manipulation near flames. The swollen SAP is then transported and then injected by tanker truck under pressure or by pipeline. The partially or completely swollen SAP can be formed continuously or discontinuously. 25 In practice, the partially or completely swollen SAP is injected at the pithead, in vertical or vertical horizontal boreholes or in surface fissures. The invention also relates to an installation employing the method described above 30 and comprising: - a means of storing the superabsorbent (co)polymer, - a means of dosing the superabsorbent (co)polymer, - a means of dosing water, - a means of mixing the water and the superabsorbent (co)polymer, 35 - a means of pumping the partially or completely swollen superabsorbent (co)polymer obtained, 9 - a means of injecting the partially or completely swollen superabsorbent (co)polymer into the mine. Advantageously, the means of mixing the water and the superabsorbent (co)polymer 5 is in the form of two successive tanks, permitting continuous and uniform swelling of the superabsorbent. The invention and the advantages resulting therefrom will be clearer from the following example, together with the appended drawings. 10 Fig. 1 is a representation of an installation employing the method of the invention according to a first embodiment. Fig. 2 is a representation of an installation employing the method of the invention according to a second embodiment. 15 The superabsorbent is supplied in big bags of 750 to 1000 kg, in containers of 2 to 5 tonnes (1) (see Fig. 1) or in a tanker truck (1) (see Fig. 2), then stored in a hopper or a silo (2). It is dosed by a dosing screw (3) in a continuous system with 2 tanks (4, 5) equipped with a powerful propeller or spiral stirrer (6, 7). The superabsorbent (1) and 20 water (8) are metered into the first swelling chamber (4), the water being in excess relative to the swelling of the (co)polymer (e.g. 1200 liters per hour for 10 kg swelling 100 times), which permits more efficient stirring. The residence time required depends on the granulometry and is between 1 hour (less than 1 mm) and 3 hours (less than 4 mm). The second tank (5) operates with a high level-low level 25 stopping the metering screw (3) and water inlet (8) at the top. The swollen SAP is pumped either by means of a pump of the Moineau type (9) up to pressures of about 10 bar or by means of a piston pump up to 200 bar. 30 The swollen SAP is then injected by means of a injection tube (10) at the pithead or in vertical or vertical horizontal boreholes or in surface fissures. The amount of water required for extinguishing is difficult to calculate as a proportion of the volume will be vaporized. But it is hundreds or thousands of tonnes 35 of water, bearing in mind that a tonne of water requires about 10 kg of superabsorbent.

10 A person skilled in the art can adapt these basic conditions to the local characteristics of the fires. In particular, it will be possible to supply the fissures by tanker truck with swollen SAP remaining at a distance from the fire zone. The product can be conveyed from the truck by a pump or by pressurization. The method is much easier 5 to apply than trucks bringing soil, which have to move over ground that is both hot and unstable.