Methane emission from longwalls and iits relationship to advance rate

• Autorzy: Drzewiecki J.

Warianty tytułu

PL

Metanowość ścian a postęp eksploatacji

• Języki publikacji: PL

Abstrakty

PL

W artykule przedstawiono charakterystyką zmian zeszczelinowania górotworu w zależności od po­stępu eksploatacji, która wpływa na wielkość wydzielania metanu do wyrobisk. W opracowanym modelu mechanizmu zeszczelinowania stropu zasadniczego eksploatowanego pokładu w obszarze przed frontem ściany, uwzględniono zmienność tego obszaru wynikającą z postępu eksploatacji. W wyróżnionym ob­szarze, zarówno przed i za frontem ściany, zasiąg i zagęszczenie spąkań mają istotny wpływ na wielkość wydzielania metanu do rejonu eksploatowanej ściany. W pracy oparto się na wynikach badań in situ w zakresie wydzielania metanu w funkcji postępu dobowego frontu ściany.

EN

The mining activity brings about the formation of discontinuities at specific locations of the rock masses. Measurements taken in the roof strata ahead of the coalfaces of different heights and for the mining systems with caving and backfilling, have allowed determining the extent to which rock mass fracturing might occur (Drzewiecki, 1997, 2000a. 2000b, 2001, 2003). The extent and location of discontinuities depend on the geological and technical conditions of mining as well as on the rock strata position. The obtained study material has allowed to formulate the relations defining the distances to the locations of discontinuities parallel to bedding ahead of coalface r, and to the surface z (Fig. 1, formulae 1 and 2). The curves shown in Fig. 1 determine the socalled active volume of a rock mass for given coalface line advance rates. The maximum surface S, equal to […] has been obtained from the curve of the rock mass active volume extent defined by the relation 1 for the coalface daily advance rate, p equal to 2.24 m (Fig. 2). In the case of the rock mass, in which coal seam exploitation can be associated with the methane as explosion hazard, it would be advisable to predict its fracturing location in the active volume ahead of the longwall coalface. At these locations, as a result of a change in the coalface daily advance rate, a short-term increase in methane gas emission can occur. Each change in the coalface daily advance rate may be associated with the change in the rock mass active volume area, the latter one being in a definite proportion to the maximum change obtained for the coalface daily advance rate of 2.24 m. Methane gas emissions from the rock mass depend on the distance to the methane gas bearing strata reached by the impact of mining operations. The rock mass active volume, the extent of which is varying with the coalface daily advance rate, will include the reservoir strata volume depending on its extent both ahead of the coalface and towards the surface. The increase in the rock mass active volume extent (in Fig. 3 denoted by A) leads to fracturing of new rock mass regions, which can cause periodical increases in the quantity of methane emitted. The roof rock fracturing parallel to bedding makes the detached strata fail "easier" laterally in regions B and C (Fig. 3), which is vital to the permeability of rock mass and easier migration of methane. The distance between the rock mass active volume and the methane gas bearing strata can be crucial to the coalface environment. This is particularly true if such strata of that volume or if a change in coalface daily advance rate makes them "pass" from the active zone to the inactive one or vice versa. In Fig. 4 the coal seam locations are diagrammatically shown against a background of three selected curves containing the rock mass active volume for the coalface daily advance rates, p, equal to 0.5, 1 and 4 m. Points A, A', B and B' indicate the parting planes initiation locations, whereas the arrows indicate examples of the extents of the rock mass parting planes. It can be seen from Fig. 4 that a coal seam overlying the coal seam being mined at a distance of about 110m will enter the rock mass active volume only if the coalface daily advance rate is lower than 4 m, whereas a coal seam being mined at a distance of about 60 m will always be contained by the rock mass active volume. It should be noted that each variation in coalface daily advance rate could result in variation in the extent of the rock mass-fracturing zone, which can be conductive to the variation in the amount of methane emitted from the zone. In the case of high coalface daily advance rates, the methane gas emission will monthly originate from the coal seam being mined, whereas the migration of methane from longwall gob to the disturbed space will to some degree be delayed. According to the presented method for prediction of absolute methane emissions (Krause & Luko-wicz 2001), the total volume of methane gas […] can be defined as a sum of methane volumes released from the distinguished rock mass fragments (formula 5). Although the equation 5 contains information on the amounts of methane in various areas of the coalface environment, considering the rock mass active volume formation mechanism and its impact on the rock mass fracturing, we can come to a conclusion that it will affect these areas with varying intensity. The size and shape of the rock mass active volume become an important topic as far as both the longwall panel area and the longwall gob are concerned. However, they appear to offer the most promising prospect for degasification of methane reservoir coal seams overlying the coal seam being mined. As follows from the relations 5 and 6 quoted from the words of Krause & Lukowicz 2001, the coalface daily advance rate appears to be a factor exerting an essential influence on real methane emission from rock masses. The in situ measurements have shown that the amount of methane emitted from the rock mass, where mining activity had come to a close, can be about 20% of the rate of methane emission from the rock mass where the mining operations are still being conducted (Krause & Lukowicz 2001). The author has revised the coalface daily advance rate coefficient c, (Fig. 5. equation 8) using the measurements data from which the relation 7 was defined. By taking into account the revised coalface daily advance rate coefficient c', the total absolute methane content, […] can be defined by equation 7. In order to assess the methane emissions during the time period the longwall coalface line passes a section defined by a difference between maximum extents of the rock mass active volumes located ahead of the coalface (Fig. 6), an additional amount of methane released from the area depicted by a dashed line in Fig. 6 should be taken into account. The above-mentioned amount can be defined by-relations 10 and 11 shown in Fig. 9. The total absolute methane emission […] is the sum of the methane emission from a disturbed rock medium with a given excavation advance rate c […], and the additional amount of methane, […], emitted from the non-degasified strata lying in the area defined by a difference between the rock mass active volume extents prior to and after the change in coalface daily advance rate (relation 12). This additional amount of methane can only be released into the coalface environment during the passage of the coalface along the section defined by a difference between maximum extents of the rock mass active volumes located ahead of the coalface. For example, the total methane emissions can periodically reach a value in excess of 21 […] min as compared with a value of 17.4 […] predicted for the coalface daily advance rate variations in the range from 2 to 4 m and for the predicted absolute methane emission of the mining - affected rock mass area amounting to a value of about […]. Such a value of the absolute methane content can, for some time, persist even in the case of a decrease in coalface daily advance rate from 4 to 2 m. To sum up, the roles of the variation of the coalface daily advance rate and the rock mass active volume were taken into consideration in the presented method for prediction of absolute methane emissions in this piece of work. Naturally, the rock mass active volume is the volume, in which formation of discontinuities, caused by movement of the roof strata, occurs. The variation of the coalface daily advance rate may result in intensified fracturing of the rock mass active volume. This effects in occasional increase of volume of methane emitted to the coalface environment. Correction of the coefficient c (7) of the coalface daily advance rate was made. New equations were introduced- the equation (8) in order to calculate its value c' for constant coalface daily advance rate, and the equation (12) for calculation of […] in the time of changing this rate.

Słowa kluczowe

PL

masyw skalny; emisja metanu; wykopy

EN

excavation advance rate; fracturing of rock masses; prediction of methane emission

• Wydawca: Instytut Mechaniki Górotworu PAN
• Czasopismo: Archives of Mining Sciences
• Rocznik: 2004
• Tom: Vol. 49, nr 2
• Strony: 271--284
• Opis fizyczny: Bibliogr. 8 poz., wykr.

Twórcy

autor

Drzewiecki J.

• Główny Instytut Górnictwa, Pl. Gwarków 1, 40-166 Katowice, Poland

Bibliografia

• [1] Drzewiecki J., 2002: Prędkość eksploatacji a zagrożenie wyrobisk górniczych zjawiskami dynamicznymi. Bezpieczeństwo Pracy i Ochrona Środowiska w Górnictwie, s. 18-22.
• [2] Drzewiecki J., 2001: Dependence of active volume of the rock mass on the advance ratę of a longwall coalface. Archives of Mining Sciences. Vol. 46 Issue 1, s. 3-18.
• [3] Drzewiecki J., 2000a: Movement dynamics of detached roof strata ahead of the longwall coalface. Fifth International Symposium on Rockburst and Seismicity in Mines, „ Dynamie rock mass response to mining” 17-19, septem- ber 2000. Johannesburg, The South African Institute Of Mining And Metallurgy, s. 351-354.
• [4] Drzewiecki J., 2000b: Rozwarstwienie górotworu jako proces inicjacji zjawisk dynamicznych powodowanych eksploatacja ścianową. Seria Sympozja i Konferencje, Vol. 41, Kraków, Wydawnictwo ISGMiE PAN, s. 159- 172.
• [5] Drzewiecki J., 1997: Roof rock failure mechanism in area of mining by the longwall system. Int. Conference „Geomechanics 96”, Balkema, Rotterdam, s. 9-11.
• [6] Kowalski. A., 1985:Zmienność parametru tgb zasięgu wpływów głównych w górotworze. Ochrona Terenów Górniczych, nr 72/2, Katowice, s. 17-23.
• [7] Kozłowski B., Kalisz J., 1976: Instrukcja stosowania metody oceny metanowości środowiska ściany. Katowice GIG.
• [8] Krause E., Łukowicz K., 2001: Dynamie predicton of absolute methane emissions to extraction panels. 29th International Conference of Safety in Mines Researrch Institute. Katowice GIG, Vol. 1 s. 91-101.