1、1421Mechanism and Controlling of Outcrop Coal FireZHANG Xinhai1,2, XU Jianzhong1,3, XI Guang1, DENG Jun2 2 Xian University of Science 3 Engineering Thermo-physics Institute, China Academy of Science, Beijing 100080, China)Abstract: A model of air flow and reaction of coal in such environment is put
2、forward. On the surface of high temperature coal, oxidation and gasification reaction takes place and produces CO. The CO produced flows upward and burns in porous rocks to generate flaming fire. Since water and CO2 are reactive with high-temperature coal, injecting water or CO2 sometimes can even e
3、nhance oxidation rate of the coal and may cause disastrous explosion, though drenching large amount of water can lower temperature of coal. It proved that combined measure of excavating fire, covering outcrop of coal and injecting composite gel and slurry is an effective and efficient means of extin
4、guishing the fire.Keywords: coal fire; physical model; reaction mechanism; control1 IntroductionCoal fire is very serious in Xinjiang, Ningxia and Inner Mongolia municipalities. According to Guan Haiyan, Ganderen(1998)1, coal fire in some area has been burning for over one thousand years and deepest
5、 fire can last to 400 m beneath the ground. Coal fire area mainly distribute in Xinjiang, Ningxia, Inner Mongolia municipalities, Shaanxi, Shanxi, Qinghai and Gansu Provinces. There are 35 fire areas in both sides of Mt. TIAN and the total area is over 8.26 km2. Total coal fire area in north China a
6、mount to 719.51 km2, and about 17-20 km2 is in burning fire. About 600.11-998.11 million tons of coal burned off every year, which causes serious economic loss and serious air pollution. There are many studies on coal oxidation mechanism. Clemens, Matheson, Rogers and Donald(1991)2 studied reaction
7、mechanism of coal oxidation at lower temperature. Sevil(1995)3 presented the fundamentals of spontaneous combustion together with a review of the findings and proposals on its causes, mechanism, associated factors and methods of prevention. Wang Haihui, Dlugogorski, Bogdan and Kennedy(2003)4 studied
8、 pathways for production of CO2 and CO in low-temperature oxidation of coal. Vamvuka and Woodburn (1998)5 put forward a model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis. Xu Jingcai(2002), Deng Jun and Xu Jingcai (2005)6 studied mech
9、anism and development of coal fire. The above studies all focused on lower temperature ranges of coal self-ignition. Nevertheless, temperature in the centre of coal fire in many places is very high. It is measured that the highest temperature in Shangyi fire zone in Ruqigou basin is over 3 000 K. Re
10、action mechanism, kinetic parameters and developing regulation of coal on high temperature is quite different from that on low temperature. Seldom is there any study on mechanism and development of high-temperature coal fire.The burning coal is always covered by rock. Air can penetrate to the surfac
11、e of coal through cracks. Temperature in fire centre is very high where coal burns very quickly like in a burning oven.Fig.1 High temperature Zone of fire area like a burning oven2 Mechanism of Coal Fire in Ruqigou Shangyi Fire ZoneOutcrop coal in Ruqigou of Shangyi area has been combusting for year
12、s. The combusting area is over 1 000 m in length and spread from ground to over 100 m below surface. There are many crannies in the rock above the coal seam through which air flows to the fire centre to provide oxygen for burning of the coal. New cracks are formed due to subsidence of rock caused by
13、 burn off of coal and uneven temperature in fire zone. In the semi-closured environment, heat of coal oxidation accumulated and makes temperature of coal rise. High temperature zone of the fire area is like a burning oven. From the excavation insect it can be seen that temperature at about 2 m above
14、 the burning coal seam is quite high where flaming fire can be seen at night. Traditional means of extinguishing fire is to excavate the high-temperature coal, but the more firing coal is excavated, the deeper the fire spread to underground.2.1 Reaction Process of Burning Coal at Different Temperatu
15、reTemperature of coal changes remarkably from over 3 000 K in the centre to environmental temperature in the border of the fire Supported by China national tenth five year science and technology key problem tackle programme (2005BA813B08), Shaanxi provincial natural science fund(2005D02) and natural
16、 science fund of educational department, Shaanxi province (05JK261).1422zone. As temperature rises, volatile generation rate increases. The volatile generated is combustible and can form flaming fire. But volatile generating rate decreases with increasing bake time, which follows the following equat
17、ion.(1)VRTEkdVmax0expWhere Vmax is the maximum volatile production rate of coal; V is volatile generation rate on time ; K0 and E is apparent pre-exponential factor and activation energy. According to equation (1), volatile production rate increases but the rate of increment slowed down as baking ti
18、me goes on. After long baking time, the coal turned into carbon by and large. According to combustion theory (Xu Jinyuan and Xu Tongmo, 1990)7, reaction process of carbon with oxygen under different temperature is as follows.1) On the edge of the fire, where coal temperature is below 1000 K, the che
19、mical reactions are expressed like the following.3C+2O2 C3O4 (2)2C3O4+O2 2CO+4CO2 (3)CO+O2 CO2 (4)The process characterized by air flows as well as oxygen diffuse to the surface of coal and followed by coal oxidation, the produced gas CO and CO2 diffuse and flow away, and there is no combustion of C
20、O.2) As temperature rises to about 11001500 K, reaction between coal and oxygen will be like these.3C+2O2 C3O4C3O4 2CO+CO2 (5)CO+O2 CO2Oxygen diffuses to the surface and reacts with carbon, while CO and CO2 diffuse to beyond. The CO formed by the reaction may combust and form flaming fire away from
21、the carbon surface, and the CO2 generated diffuses away. 3) At above 1600 K, reaction on the surface is mainly gasification reaction and can be described as follows.C+CO2 2CO (6)The CO generated diffuses to a certain distance away from the carbon surface where it is oxidized to CO2 and forms flame f
22、ront. Reaction in the flame front follows equation (4). One half of the CO2 generated in the fire front diffuses back to coal surface and react with the carbon and another half diffuses away to the air.2.2 Physical Model of Coal FireThe firing coal and sand rocks above are excavated to extinguish fi
23、re. From excavation section it can be seen that there is mining laneway buried deep in the burning coal through which fire propagates. A lot of cracks are formed in the coal seam as well as the rock above the fire zone. Temperature in the rock several meters above the firing coal is rather high wher
24、e flames can be seen at night. Cutaway view of the fire zone is shown in Fig.2. abandoned laneway high-temperature coal coal seam sand rock cracks sand rock flaming zone air flow direction: CO flow: CO2 and N2 flow: Fig.2 Schematic cutaway view and gas flow direction of Ruqigou Shangyi fire zone It
25、can be seen that high temperature zone is mainly around the abandoned laneway and develops along the laneway, and flaming zone in the rock is about 2 m above the coal seam. The depth of the flaming zone is about 12 m and of the coal seam is somewhat over 10 m. Temperature in the centre of the coal f
26、ire is over 3 000 K and in flaming zone is between 1 0001 500 K. Thereby, heat transfer is from burning coal to surrounding rocks through conduction and convection. Reaction(6) is an endothermic reaction which means if only reaction (6) takes place on the surface of burning coal, temperature of the
27、coal may decrease and the fire will quench ultimately. Long term monitoring of the fire zone showed that temperature in the centre of the coal fire kept over 3 000 K for years. It proved that except for reaction (6) there must be exothermic oxidation reaction, so the fire zone attains dynamic heat e
28、quilibrium. As a result, model of the coal fire in Shangyi fire zone is concluded as follows.Air penetrate through cracks and the laneway into coal surface from the bottom of coal outcrop slope; oxygen in the air react with coal to produce CO and CO2 follows reaction (2) and (5); the CO2 produced ma
29、y directly react with coal to produce CO, which follows reaction (6) in high temperature coal; buoyancy caused by uneven temperature drives air in the high-temperature coal to flow upward through cracks in coal and rocks; air from two sides of high-temperature zone flows through cracks to the flamin
30、g zone and react with CO produced by the reaction and volatile generated due to baking of coal to produce CO2 and H2O and generate flaming 1423fire; the produced CO2, H2O and inert gas nitrogen flows upward to the air, which is shown in Fig.2.3 Analysis of Fire Controlling TechniquesAccording to the
31、 mechanism of coal combustion and the characters in the fire section, the basic requests of controlling coalfield spontaneous combustion are: (1) lowering the temperature of the combustion area by injecting water and slurry; (2) reducing the oxygen supply by covering the fire zone with loess; (3) re
32、moving the combusting coal.Excavating the firing coal can extinguish the fire, but in most cases velocity of fire development into coal seam is lager than that of coal excavating. Herein the excavation technique is used only in the sections where distance between coal and surface is fairly small. Co
33、vering fire zone with loess can inhibit fire by reducing oxygen concentration around coal, but temperature of burning coal cant be lowered quickly by the technique. Cracks can be formed in the covered loess layer and the fire will resume after long time of baking.3.1 Effect of Injecting Water into F
34、ire ZoneDrenching water is one of the most commonly used techniques to extinguish the coal fire. But since the large amount of water may flow away, and water resource in fire area is always very limited, this method is only for weaking development of the fire. Since water is reactive with high-tempe
35、rature coal to produce explosive gases, injecting water into large area high-temperature coal fire like in Shangyi fire zone is sometimes dangerous and may cause disastrous explosion. In 2001, when outcrop of coal is covered and water is injected through bores into high-temperature zone to extinguis
36、h the Shangyi fire zone, explosion took place and mixture of water and vapor spurt out suddenly through the bore like volcanic eruption, as shown in Fig.3. Fig.3 Injecting water into Shangyi fire zone caused dangerous water eruptionWater is evaporated when injected into high-temperature coal and the
37、 vapor is readily to react with high-temperature carbon to produce explosive gases. Reaction between coal and water vapor on high temperature can be written as follows.H2O+CH2+CO (7)When H2 and CO are mixed with oxygen on high temperature explosion may take place due to the following reaction.H2+O2H
38、2OCO+O2CO2 (8)Reaction (6) and (7) are controlled by diffusion of high temperature. Since diffusivity of H2 is the largest and followed by H2O and CO, CO2 is the smallest among the above gases, rate of reaction (7) is larger than that of (6). As a result, injecting smaller amount of water into large
39、 area of high-temperature coal fire may even enhance combustion rate of coal. Since reaction (6) and (7) are endothermic reactions, the reaction can depress temperature of the coal and eventually reduce the rate of the reaction if there is no heat source. Drenching large amount of water can reduce c
40、oal temperature and eventually extinguish the fire.3.2 Controlling Fire by Injecting Composite GelAll the reaction process of coal combustion can be restrained through blocking air leakage passage and lowering coal temperature. Xu Jingcai, Zhang Xinhai, Wen Hu and Deng Jun (2003)8, Zhang Xinhai, Den
41、g Jun Xu Jingcai, et al(2004)9 proposed a so called composite gel material to extinguish coalfield fire, and considered it to be one of the cheapest and most effective means of extinguishing coal fire. One advantage of gel fire-fighting technique is air-leaking cracks takes about 40% of the total vo
42、lume of the rocks in fire zone, as a result, less amount of composite gel is demanded to fill the cracks than the amount of loess required to cover the surface of fire zone to isolate coal from air. Another advantage is injecting gel can lower temperature of fire much more quickly than covering the
43、fire with loess. Since evaporation rate of water in gel is much slowly than that of water, extinguishing coal fire by injecting gel is less possible to cause explosion. When extinguishing fire by injecting water or slurry, large portion of water may flow away through crannies in coal and rocks, whil
44、e gel can stay in cranny of high temperature zone and is more effective and efficient than water and slurry in lowering the temperature of coal.Composite gel fire-fighting technique is composed of 3 parts. (1) Suspension slurry is prepared with local materials like rock powder, sand or loess with wa
45、ter and a few additives; (2) The slurry is pumped to fire site where smaller amount of gelation material is added into the slurry; (3) The mixture is injected into cracks in the rock above the coal fire and the mixture can gelate in1424cranny, block air leaking passage, lower temperature of the coal
46、 and rocks and eventually extinguish the fire. 4 Extinguishing Coal Fire with Combined TechniqueOutcrop of coal in Ruqigou Shangyi area has been combusting tens of years. The fire area is several km2 and last from coal outcrop to deep in the stratum and threatened safety of the nearby coalmine. Comb
47、ined measures of drenching water slurry, injecting composite gel, covering fire zone and excavating high temperature coal are taken to extinguish coal fire, shown in Fig.4. bore rock loes coverage rock coal seam abandoned laneway Fig.4 Schematic cutaway view of stratum and bore distribution of Ruqig
48、ou fire zoneHigh-temperature outcrop coal is excavated and the coal outcrop is covered with loess to prevent air from penetrate into high-temperature coal. Bores are dug from the platform above firing coal through which to inject composite gel and slurry to lower temperature of fire zone. About 9 00
49、0 m3 of composite gel is injected into stratum of 2 000 m2 fire area. After taking the above measure, temperature on the surface of the fire zone approaches environmental temperature and there is no smoke gives off any more. It proved that the technique is of success.5 ConclusionsCoal is reactive with H2O and CO2 on high temperature fire zone and extinguishing coal fire by drench
