Crossing point temperature of coal

Mining Science and Technology(China)21(2011)255-260Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcCrossing point temperature of coalQi Xuyao Deming Wang. James A Milke, Xiaoxing Zhong aa State Key Laboratory of Coal Resourres and Safe Mining Faculty of Safety Engineering. China University of Mining Techmology. Xuzhou 221008, ChinaDepartment of Fire Protection Engineering University of Maryland, College Park, MD 20742, USAARTICLE I OA BSTRACTA further understanding of the self-heating of coal was obtained by investigating thetember 2010temperature(CPT)of different ranks of coal. The tests were carried out using a self-designesystem for coal self-heating. 50 g(+ 0.01 g)of coal particles ranging from 0. 18 mm to 0.38ut into a pure copper reaction vessel attached to the center of a temperature programmed8 mm in sizeyccepted 12 November 2010mperature program increased the temperature at a rate of 0.8 C/min Dry air was permitted to flow intothe coal reaction vessel at different rates. The surrounding temperature and the coal temperature wereKeywords:onitored by a temperature logger. The results indicate that CPT is affected by coal rank, moisture, sulfur.and the experimental conditions. Higher ranked coals show higher CPt values. a high moisture contentntaneous combustioncauses a delay phenomenon during the self-heating of the coal Drying at 40C decreases the effects ofossing point temperatureoisture. The reactivity of sulfur components in the coal is low under dry and low-temperature conditions.These components form a film that covers the coal surface and slightly inhibits the self-heating of the coal.The flow rate of dry air, and the heating rate of the surroundings, also affect the self-heating of the coal. TheCoal rankmost appropriate experimental conditions for coal samples of a given weight and particle size weredetermined through contrastive analysis Based on this analysis we propose that CPTs be determined underthe same, or nearly the same conditions, for evaluation of the spontaneous combustion of coalCopyright 2011, China University of Mining Technology. All rights reserved.1. Introductiona 20 g coal sample [15]. Banerjee et al, proposed a method toevaluate the propensity of coal toward spontaneous combustionThe spontaneous combustion of coal is one of the serious based on the value of the CPT[16). However, Banerjee et al, alsohazards that might develop during the mining and storage of coal. It found that the CPT is usually inaccurate for those coals with a highusually results in personal injury, huge economic loss, and envi- moisture content and suggested the evaluation of such coals byronmental pollution. Many investigators have worked on under- both CPTand temperature increase rate 16). Nandyet al, suggestedstanding the mechanism of spontaneous combustion as a way of that the CPt dropped with an increase in volatile matter, oxygenpreventing this hazard. Among these previous investigations, the content, and moisture content [17] Similar characteristics werenethod of crossing point temperature has been used frequently. later identified by Mahidin[18 Barve and Mahadevan determinedThe rate of the rise in coal temperature during coal oxidation a binary quadratic equation for CPt that related it to moisture andbecomes greater under appropriate conditions. The temperature at ash contents [19]. Kucuk et al, tested CPT using a column reactionthat point where the coal temperature begins to exceed the vessel and suggested that cPt dropped with a decrease in the coalurrounding temperature is the so-called crossing point tempera- particle size and the air humidity but rose with a decrease inture(CPT) Nubling and Wanner first implemented an oil-bath moisture content in the coal 14). Kadioglu and Varamaz suggestedmethod that used a constant heating rate to test the CPT of coal [1]. that CPT rose with an increase in moisture content and the particleOther investigators modified this testing method altering the size of the coal [20 Mandal et al, found that CPt rose if somereaction atmosphere, the heating bath, or the coal reaction vessel. inhibitor was added into the coal sample[21]. Chen et al. proposedThey tested many different kinds of coal [2-14 Bagchi studied the a new testing method to determine Cpt that used a cubic or columneffects of experimental conditions and proposed the most appro- basket [11-13]. Subsequently, Chens method was used by otherpriate nlow rate of oxygen and the surrounding heating rate for investigators [22 23).These previous studies were done under different experimentalconditions. Some of the results are still disputed and the mostCorresponding author. TeL: +86 13585490927.appro中国煤化工 e not been determin1674-5264/s-see front matter Copyright o 2011, China University of Mining& Technology. AllCNMHGdoi:101016 J.-201102024Q Xuyuo et aL/ Mining Science and Technolog(China)21(2011)255-260Table 1of the coal samples.Coal sampleMa(写)写FCad(%)2047yangguang gas-rich coal091uxianzhuang 1/3 coking coal20822329Kabuliang anthracitereveal the mechanism of coal self-heating and it is still widely high gas buffer space segmented with a 0. 15 mm stainless steelused today. This paper analyses the relationship between CPT and mesh aperture, as shown in Fig 1b. This buffer space ensuresmoisture, coal rank, sulfur content, and experimental conditions a steady, dry air flow into the coal sample. Thermocouple 1, fixed atusing a self-designed experimental system for measuring the self- the center of the thermally programmed enclosure, was used toheating of coal.monitor the surrounding temperature while thermocouple 2, fixedat the center of the coal reaction vessel was used to monitor the coal2 Experimentaltemperature.2. L. Coal samples2.3. Testing proceduresDifferent ranks of raw coal were collected from different coal- First, 50 g(+0.01 g)of coal sample were packed into the coalfields. They were delivered to the laboratory as rapidly as possible. reaction vessel. a thin bed of asbestos was laid on top of the coalThe surface was removed and the interior core was crushed toobtain the samples. Then the coal particles ranging from0.18 mm to programmed temperature enclosure was set to run at a constant0.38 mm were sieved to provide the experimental coal test temperature of 40 C while dry air with an oxygen concentration ofa rate of 8 ml/min The temperature logger was used to continu-2. Testing systemously monitor the coal and surrounding temperatures. when thecoal temperature reached 40C. the programmed temperatureThe testing system consisted of a facility for simulation of coal enclosure was set to increase the temperature at a programmedoxidation and a temperature logger, as shown in Fig. 1. The simu- rate of 0.8C/ min while the flow rate of dry air was maintained atlation facility is an enclosure capable of programmed temperature 8 my min. When the coal temperature reached 70., the pro-control, a gas tube, and a coal reaction vessel. the temperature grammed temperature enclosure continued heating at a rate oflogger was a set of thermocouples and an analyzing system.0.8C/min but the flow rate of dry air was changed to 96 ml/minThe programmed temperature enclosure provides a surrounding We ended the experiment when the coal temperature was 5Cenvironment at an accurate and uniform temperature by means of higher than the surrounding temperaturea gas bath. The dry air temperature was the same as the surroundingDry air flow into the coal reaction vessel wastemperature because the dry air was preheated before flowing into a different rate after the temperature rose to 70'Ctonate coalhe coal reaction vessel. Preheating was achieved by passing the air oxidation. During the self-heating of coal. the reaction intensitythrough a 50 m copper tube located inside the programmed becomes stronger and stronger as the temperature rises In the earlyenclosure. The coal reaction vessel was made of pure copper, which stages, the reaction intensity is weak and the corresponding oxygenhas avery good thermal conductivity. At its bottom, there is a 20 mm requirement is also minimal. Hence, a small flow of dry air would beManometerwant n中国煤化工Testing systemCNMHGFig 1. Schematicm of the testing system.Q Xuyao et al/ Mining Science and Technology( China) 21 (2011)255-260g140:201:4000246:403:53:20Time(hh: mm: ss)Time(hh: mm: ss)Coal samples with low moisture contentCoal samples with high moisture contenig 2. Two different trends in coal temperature rise.enough. this small flow rate of dry air not only satisfies the oxygen a delay in the rise of temperature, as shown in Fig 2b. The delay isrequirements in the early stage but also avoids heat loss, which is due to the moisture in the coal. Although moisture has very littlekey for the development of the self-heating effect. In contrast, the effect on the chemical structure of coal 14, 20, 24, 25]. it affects coalreaction intensity becomes stronger in the later stages and it is oxidation through sorption and desorption [26, 27]. The moisturenecessary to increase the dry air flow rate to satisfy the oxygen evaporates with a rise in temperature, which consumes some heatand lessens the heat accumulation in the coal. In addition themoisture forms a film that covers the coal surface. Thus oxygen3. Results and discussionmust first dissolve in the moisture film before contacting andreacting with the coal. However, the solubility of O2 in water is poor.3. 1. Efect of moisture in the coalConsequently, moisture may inhibit the heat transfer, and thecontact between coal and O2, if the moisture content is highMoisture exists, more or less, in all kinds of coal[26, 28-30]. Moisture evaporation and the moisture film, togetherinitial coal samples were tested according to the piresult in the delay phenomenon. Because pores and crannies in theSection 2. 3. The results indicate that the rise in coal tecoal become more numerous after the moisture evaporates, thearies for samples with different moisture contents.coal temperature rises rapidly after the delay stagwith a low moisture content have smooth trend curves as shown inThe moisture in coal exists either as surface moisture inherentFig- 2a. In contrast, samples with a high moisture content show moisture, or as constitutional moisture. Coal oxidation isaffected by surface and inherent moisture. Surface moistureremoved by drying at 40-50C but the inherent moisturebe removed below temperatures of 105-110C.Initial coalYima jet coal, with a moisture content of 12.15%, was dried at18040Cor 50 C The drying process continued until the mass changeDried at50°Cof the coal sample was less than 0. 1% per hour. The initial and driedYima jet coal samples were then tested following the procedures inDried at40°C2100Section 23. The results are shown in Fig. 3. Drying is seen todecrease the obvious effect of moisture and the results of drying at40C or 50 C are nearly the same. Consequently the effect ofmoisture can be reduced when studying the other factors by dryingat 40C before the test, as shown in Fig. 4. Drying at 40C avoids the0:33:204000246:40ffect of moisture but has very little effect on the chemical structureTime(hh: mm: ssof the coal. This is good for the analysis of the other factors such asFig. 3. Temperature trend curves for initial and dried Yima jet coalthe coal rank, sulfur content, or experimental conditionsInitial coalOven0:33:20Time(hh: mm: ss)中国煤化工CNMHGFig. 4. Comparison of initial and dried coalQ Xuyoo et al/ Mining Science and Technology (China)21(2011)255-260Table 3CPT of different rank coals dried at 40C.CPTs of blended and un blended Kabuliang anthraciteCPT(C)Proportion of Fes2(‰lizao ligniteCPT(C2075ima jet coal/3 coking coal 173.2gwan non-caking coal 151.5Chaili gas coalliang anthracite2040oxidation enhancement [39]. Munzer proposed that Fes2 does notDongtan gas coalenhance coal oxidation unless it exists as small particles and itproportion exceeds 5-10% 40 He found there was nearly no effectfor coals whose sulfur content was less than 5% [40).3. 2. Effect of coal rankFeSz was added to the Kabuliang anthracite, after drying at40C in the proportions of 3%, 5%, and 7% Then the blended, anThere is an obvious relationship between coal rank and coal un-blended, Kabuliang anthracite were tested following thereactivity. The effect of coal rank was examined by testing different procedures in Section 23. The results are shown in Fig. 5 andrank coals dried at 40C following the procedures described in Table 3. Fesz inhibits the coal oxidation process slightly undertions. A similar phenomenon was alsodifferences in the CPT of the different rank coals. In general, higher found in Pietrzak and Wachowska's investigation. They suggestedthe difference between the maximum and minimum CPts is found that a lignite sample having a sulfur content of 11.4% had very544C This arises from differences in the micro characteristics ofw reactivity [32]. In general, FeS2 can react with O2 and Hthe coal. The higher a coal rank is, the fewer the types, and amounts, However, the reactivity of Fesa is low and it is almost inert underof active groups in that coal (31). Higher ranks of coal are less dry and low-temperature conditions. During coal oxidation, thesewell [32]. As aresult the higher rank coals show rates of temperature surface. It plugs pores and crannies in the coal and inhibits heatrise that are smaller, and CPTs that are higher, than lower ranked transfer and the contact between the coal and O2. As a result coalcoals. However, coal rank is just one factor that affects the sponta-oxidation is inhibited by these sulfur components under dry andeous combustion of coal Moisture and mineral components alsow-temperature conditions. For equal rank coal samples, the coalfactorspontaneous combustion. The CPT is a reflection of all these with the higher sulfur content usually has a smaller rate oftemperature rise and a higher CPt than the coal with a lower sulfuapproximate relationshipcontent. For example, Changguang gasgh sulfurother coal samples with a very similar rank and moisture content.3.3. Efect of sulfur components in coalHowever because most coals have low sulfur content, the effect ofsulfur is not as obvious as thefactorsThe sulfur components in coal include organic and inorganicsulfur. The organic sulfur components exist as R-SH, R-S-R, 3.4. Effect of experimental conditionsS-S-R-S,and