Prognosis of residual coal gas capacity made by the 'Express' method

International Journal of Minerals, Metallurgy and MaterialsVolume 18, Number 2, April 2011, Page 127Do|:10.1007/s12613-201404118Prognosis of residual coal gas capacity made by the 'Express ' methodPavel Prokop, Pavel Zapletal, and vo PegrimekTechnical University of Ostrava, Ostrava 708 00, Czech Republic(Received: 2 March 2010: revised: 21 April 2010: accepted: 9 May 2010)Abstract: An easy, reliable, and inexpensive method, calledExpress'method, was described to determine the residual gas capacity of deepmines using results from an air and gas balance. Air and gas balances are common elements of mine management and must be performed pe-riodically. Using the process described here to obtain balance results, it is straightforward to obtain the residual gas capacity, which is an im-portant parameter for decision-making in current mine operations. After a mine is closed, the residual gas capacity becomes a dominant fac-tor used to select methods to protect against gas emissions from the closed underground area or perhaps to provide information for the use ofgas reserves. The proposed"'Express method is a much simpler method to obtain the residual gas capacity than other procedures used for thispurpose to dateKeywords: coal mines; gas storage; methane; ventilation; degassing[This work was financially supported by the Czech Science Foundation(GACR)(No 105/09/0275). 11 Introductionregularity of gas emissions and also indicates methods ofprotection and possible gas utilization.There has been a reduction of mining since the secondhalf of the last century, as many mines are closed in differ-The size of free gaps in closed underground mines is notent countries. The closure of a mine entails many issues, inthe subject of this paper; however it has been addressed in acluding uncontrolled gas(methane )emissions from theseparate series of works. For the sake of completeness, itderground areas to the surface. These gas emissions need to should be noted that both factors(residual gas capacity andbe measured to enable protection of surface objects andthe size of the free gaps)are used in a copopulation and also to evaluate the potential of collecting ion to reliably determine the actual gas reserve and, there-the gas from the free gaps of closed mines as an altermative ore, the possibility of gas emissions or gas usage. The termenergy resource. various methods may be used to assess theof 'storage'is used for the underground free gaps. A morereal gas reserves in this environment [I]. The value of thedetailed definition for gas(methane ) storage is a closed arearesidual gas capacity [2-3] and the size of the free gaps [ 3]destabilized by elder mining works with free gaps that areare the most significant factors governing the concentrationenriched with methane and other gasesof residual gas reserves in free gaps in a closed mine.The gas storage concems a bulk deposit whose grossThe term of residual gas capacity is used here to articu-volume can reach several tens to millions of cubic metreslate the gas emissions produced in closed mines, and its deThe useful volume corresponds to the volume of cavitiestermination has been the subject of a series of works. Theand gaps incurred by the shifts and porosity of the remainingmethod presented here for the determination of gas emissioncoal, similar to the residual volumes of roads, adits, and un-amount, based on earlier work [1], uses the gas balances asdetermined from individual mines before their closure. ThisThe volume itself does not represent a reserve of minedetermination is significant, since it substantially affects the gas, because methane is stored both in a free form in poresCorresponding author: Pavel Prokop E-mail: pavel- prokop @vsb.cz中国煤化工e University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 201CNMHG2 Springer128Int J Miner. MetalL Mater, VoL 18, No 2, Apr 2011and cavities of coal and rock matrices and in an absorbedThe total volume of the exhausted gas mixture wasform on the surface of coal, inside its microscopic structure. 13452 mIn the storage, the gas pressure fluctuates with the exThe CH4 concentration during that time decreased fromhausted( drained quantity of gas and the total volume in. 43vol% to 24.5vol%flow, which depends on the desorption of methane and theThus, it can be presumed that within another 38 h, theair inflowmethane content would decrease to near zero. this indicatesItis also necessary to note that the residual gas capacity that in this case, the volume of free gaps could be approxinear the surface varies, and this variation depends on theately 26000development of barometric pressure.With this information, it is possible to derive a relation2. Some actual methods for determining the re-nip between the formation of free gaps and tectonic distur-sidual gas capacitybance and to determine whether isolated formations of fregaps occur in the locality of interest.The residual gas capacity follows difIn the Petikovice locality at the MV40 borehole, an exdepending on the individual localities. It can be presented as haust test was performed from 16 to17October2002.Thefollowsaverage exhausted amount of the gas mixture wasAs a constant value over a wide locality270m3h1.As different values for individual parts of the localityThroughout the exhaustion(24 h), 6480 mof methanemixed with other gases were exhausted. However, the CH4As individual values in a very distinctive part, for concentration did not decrease for the period of the test du-example, around a methane drainration, and it reached a fixed value of 45vol%.arious procedures are applied for its determination (4-61To address the high expense of the exhaust tests, a pro-One method determines the volume of mined-out coal using cedure is described for determining the free gaps under-a recalculation of the resulting free area after mining. If this ground. It is based on the findings obtained from an earliergap is filled with gas, we can determine an approximate method [1]and requires no additional costs while providingvalue of the gas capacity using the laws of flow between the the same accuracy. However, it must be carried out in theunderground area and surface. An example accordingfinal stage of mine operations, just before closing the mine.Couillet et al. [3] from France was introduced In the Pey- This procedure is the so-called'Express'methoderimhoff locality, 50 million tons of coal has been extracted;it is estimated that this resulted in 10-15 million cubic me. 3. ' Express'methodtres of residual free area. The recalculation shows that the3. 1. Descriptioncoefficient of compressibility for this case varies from 0.2 to0.3.The residual gas capacity is defined as the quantityA much more precise method of determining the residualee gas contained in the rock mass that is affected by agas capacity is based on the results of exhaust tests, of which given mine's operations. Part of the rock mass is able to re-several were conducted in the Ostrava-Karvina coalfieldlease the gas under ideal conditions after the end of coalHere, I present the results from the Hrusov Mine, HDl borehole, and from the Petikovice area, Mv40 borehole [2]The residual gas capacity is the portion of the gas reservethat is capable of being released from the same part of theIt is necessary to note that exhaust tests are very costly. rorock mass under actual conditions; thus, the residual gas ca-The cost per day of the test is about CZK 45000(USDpacity will always be lower than the total gas reserve [7-10]2555); thus, the test is only performed if it is urgent andThese values are of primary importance for determiningthe intensity of goaf gas emissions to the surface, after thThe exhaust test was performed in the Hrusov Mine lo- termination of mining and disposal operations or after clos-cality(HDl borehole) from 21 to 23 September 2000 withthe minethe following resultsFor the period of the test duration (38 h), on the average, state)354 mh"of the gas mixture was exhausted.YHE中国煤化工 eserve(the theoreticalCNMH Ghane emissions at theLuuvciscly, as ule residual gas reserveP. Prokop et al, Prognosis of residual coal gas capacity made by the'Express'methodincreases, gas emissions at the surface increase(provided intake side and upcast side of the j mine working,that other conditions for degassing remain without change). tively, vol%; S, and Sz the surface area of the crossAn issue is how to determine the value of the gas reserve of the j mine working in the point of measurement ator gas capacity. To address this, we can analyze the methane and upcast, respectively, m; n and v2 the air velocity on theorigin during the earlier time period when the objective intake side and upcast side of the j mine working, respemine was in operation [11-12]. The main sources producing tively, m-s!of methane is mining of the seams by long wall method orEq.(1)is used only if the gauge is located in the minedriving the gate roads. During crosscutting, the origin of working profile while measuring the air velocity; Eq.(2)ismethane emissions is primarily from tectonic faults and is used if the gauge is placed outside the mine working profilerarely from surrounding rock [13-14]. Besides active mineWith this method. the calculated values of methane exworkings, nonactive workings(where neither digging nor halations for the individual sites are dependent on the quan-sed for transportation, ventilation by through-circulatingtity of degassed methane, thus, the gas capacities can be ob-tainedcurrent, or other special purposes( sump cross-cuts, variousgodowns, storage, and transformer stations), may also reIn some cases(using the actual measurement at the mine),e can adjust Eqs. (1)and(2)by 2-S1v1 S2v2, where 2 ismined-out parts are also a significant source of methane the volume flow rate of winds through the site, m'semissions. Considering that this latter methane source isThe real absolute gas capacity of the j mine working istypically present for a much longer time than the average calculated according to Eq (3)as followslifetime of faces or entry driving, the exhaled methaneamount can be treated as the residual gas capacity [15-16]. Pd,=E+PdTaking this into account, the residualty can bedetermined using the method of complex gas balance ofPd2=Q1t,t=2592×10°8mines, namely, the ExpreThe method of complex gas balance is composed of both where Pd is the actual absolute gas capacity of the minethe gas analysis of individual sites and the gas analysis of working, m s, Ei the actual methane exhalation of the jthe mine as a wholemine working according to the Eq(1)or(2),m s-Pd2thedegassed quantity of pure methane from the area of the jThe gas analysis of individual sites, as part of the com- mine working according to Eg.(4), msl; and g1plex gas balance, is necessarily performed on all working monthly amount of pure methane drained by the degassingplaces(here named as site). That is, the gas analysis occurs station from the area of thej mine working, m2in all faces that are actually mined and in all long workingswhere driving is performed (hereinafter referred to as theIf the area of thej mine working is not degassed, thevalue of the actual gas capacity of this working is equal tothe actual exhalation. Thus, Eq. 5)is valid.m过础减m时6methane exhalation is calculated from these values meas-The portion of the complex gas balance from the mineured at each site, according to eqs. (1)or(2):a whole is acquired from measurements at all upcast shaftsE1=[=2(S2-04y2-1(S1-04)h]×102(1) of the mine and from the deggassing station records.Then, the total absolute gas capacity of the mine is de-E=(c2S2v2-S1)×102(2) termined from methane exhalations drained by all upcastshafts and from the total quantity of pure methane degassedis the actual exhaled amount of methane at the jby themine working, m-s",j means the consecutive number ofitv ie ealenlated from Eq.( 6)working; E, the actual exhaled amount of methane at the j中国煤化工mine working, if the gauge is located outside the workingprofile, m s"; cI and c2 the methane concentration on theCNMHGInt J. Miner. MetalL Mater VoL 18, No. 2, Apr 2011E2=c32×102Pd(10)Pd4=Q4t,t=2.592×10°Substituting Eqs. (6)and(9)into Eq. (10), Eq.(11)canwhere Pd, is the actual absolute gas capacity of the entire be obtained.mine, m-s"; m the number of upcast shafts of the mine; E2the methane exhalations drained by all upcast shafts, ms∑E2+1Pd the quantity of pure methane drained by the degassingThis value of methane exhalations(residual gas capacity)station from the entire mine, m''s; Cy the methane concen- is considered to be the initial value from the moment that thetration in the general upcast flow of the j upcast shaft of the mining disturbance of the rock mass is finished.mine, vol%; g2 the volume flow rate at the j upcast shaft,Ps; and @4 the monthly quantity of pure33. Practical applications of the 'Express'methodhausted by the degassing station from the entire mine, mThe results of gas balances of individual mines in the Os-The individual values of actual exhalations for operated trava-Karvina Coalfield(OKR)in the Czech Republic weremine workings are summed; these are the separate values gathered from the OKR databaseTables 1 and 2 provide data gathered by this method offrom the coal faces 2E, and from the long gate roadsthe gas balance of the mine, which is used to determine theactual exhalation of the operated coal faces and the actualE4, the same applies to the actual gas capacities exhalation of operated gate roadsFrom an analogous database [1], the values of methane∑Pd3and∑Pds·E3 is the actual exhalation of the opexhalations from the faces and long mine workings of thePaskov Mine before closing were chosen. The results areated coal faces, m s"; E4 the actual exhalation of the oper- shown in Table 3ated gate roads, ms"; Pds the actual gas capacity of them-s; Pd, the actual gas capacity of theAfter closing the mine(all shafts have been filled up andperated gate roads, m-sI; h the number of operated coal sealed), in a short period of time (3 months ), the methanefaces at the time of measurement; and k the number of the production in the degassing system was stopped. The Pd4operated gate roads at the time of measurement.value dropped virtually to zeroThen, we applyTherefore, the specific solution of the Eq.(Il) for thefter closing the mine is as follows:∑Pd1=∑Pds+∑PdPd=079280.1712-03625=0.2591m3swhere. mk+h.The value of the residual gas capacity is determined un-der low barometric pressure by measurements performed atThe residual gas capacity is then determined by means of a control column located for this purpose in one of the filledEq (10)or(11):up shaftsTable 1. Methane exhalations from coal faces(9 Kveten Mine in the OKR)Face NoCH concentrations/ vol% Volume flow rate of winds,@/(m s-)CH,exhalations, E,/(ms294135.333300103140435313354000.25.98330.8l2.18330085328906400000016032702/1352140.1中国煤化工636113CNMHGO4P. Prokop et aL, Prognosis of residual coal gas capacity made by the"Express'methodTable 2. Methane exhalations from gate roads (9. Kveten Mine in the OKR)Gate road NoCHa concentrations/vol%Volume flow rate of winds,Q/(m3s2)CH, exhalations, E4/(m"001085.483300035352314416700133033.9833000404.633300185375270.0500030287280.03000708.133300163362260.0014200160∑E40.1257Table 3. Determination of the residual gas capacity of therecognition prediction of coal and gas outburst hazard in theabandoned Paskov Mine in OKRsixth mine of Hebi, J. Coal Sci. Eng, 14(2008), No 2, P. 248[6]x. Chen, Z.F. Wang, H M. Yang, and D H. 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