Arch. Min. Sci., Vol. 53 (2008), No 2, p

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1 Arch. Min. Sci., Vol. 53 (2008), No 2, p STANISŁAW TRENCZEK* LEVELS OF POSSIBLE SELF-HEATING OF COAL AGAINST CURRENT RESEARCH POZIOM POTENCJALNEGO SAMOZAGRZEWANIA WĘGLA W ŚWIETLE BADAŃ The paper refers to model characteristics that describe the process of coal self-heating in the context of standardized parameters of the ignition temperature and the Graham s coefficient. Based on investigation of coal samples taken from selected coal seams, different in parameters, this study outlines graphics that correspond to non-standard values of mutual correlations between temperatures and Graham s coefficients that are typical for the samples. The stress was put on importance of such differences for correct evaluation of hazards and the research made it possible to define a correcting factor enabling mapping the results of laboratory examination of coal oxidation onto real oxidation in coal seams. Additionally, the coefficient that reflects vulnerability of coal to self-ignition was developed with account to inconsistency of the coal oxidation process. Finally, a new method for assessment of hazard level is suggested, i.e. the method that takes advantage of the index of possible self-heating of coal that is a merge of coal seam breakdown by groups of self-ignition tendency that is currently in use and the index of coal vulnerability to self-ignition. Verification of the new evaluation method is carried out with use of fires that had occurred in examined coal seams. Keywords: mining industry, hard coal, hazard of endogenous fires, coal oxidation, self-ignition vulnerability, index of possible self-heating of coal Jednym z elementów zapobiegania pożarom endogenicznym jest rozpoznanie skłonności węgla do samozapalenia. O sklasyfikowaniu pokładu do odpowiedniej grupy samozapalności decydują wartości wyznaczanych w czasie badań: wskaźnika samozapalności węgla Sz a, C/min oraz energii aktywacji utleniania węgla A, kj/mol. Klasyfikacja ta informuje, jakie jest prawdopodobieństwo uaktywnienie procesu samozagrzewania danego węgla w sprzyjających temu uwarunkowaniach. Nie może ona jednak stanowić kryterium ostatecznego, gdyż w rzeczywistości występowały i nadal występują pożary w pokładach węgla, sklasyfikowanych zarówno do V, jak i do I grupy skłonności do samozapalenia. Świadczą o tym między innymi pożary endogeniczne z ostatniego dziesięciolecia (rys. 1, tabl. 1), z których ponad połowa (51%) zaistniała w pokładach węgla o bardzo małej i małej skłonności do samozapalenia (grupa I i II). Można więc postawić tezę, że taki sposób klasyfikacji pokładów węgla nie oddaje w pełni poziomu potencjalnego zagrożenia samozagrzewaniem. * CENTRE FOR ELECTRIFICATION AND AUTOMATION CONTROL IN THE MINING INDUSTRY EMAG, UL. LEOPOLDA 31, KATOWICE, trenk@emag.pl

2 294 Proces samozagrzewania jest rozłożony w czasie i składa się zasadniczo z trzech okresów (rys. 2). Pierwszy okres przygotowawczy, doprowadza w końcu do temperatury granicznej T 1, leżącej zazwyczaj w przedziale od 60 do 80 C. Drugi proces samozagrzewania, kończy się po osiągnięciu temperatury zapłonu T 2, w której dochodzi do samozapalenia, a po nim, przy nadal sprzyjających warunkach, do pożaru okres trzeci. Nie ma jednoznacznie określonej temperatury zapłonu, która dla węgla kamiennego wynosi od 270 do 350 C, natomiast przy uziarnieniu o średnicy 1 mm i mniejszej temperatury zapłonu są niższe, wynoszą od 190 do 220 C. Za standardową temperaturę zapłonu przyjmuje się T z = 300 C, co przekłada się na standardową wartość graniczną wskaźnika Grahama wynoszącą co najmniej G = 0,03 (rys. 3). W ocenie potencjalnych skłonności węgla na proces samozagrzewania powinny pomóc badania próbki węgla, pozwalające określić jego temperaturę zapłonu oraz odpowiadającą temu wartość wskaźnika Grahama. Jedne z pierwszych badań produktów utleniania węgla, dokonane prawie czterdzieści lat temu, pokazało, że jako pierwszy pojawia się tlenek węgla, a w dalszej kolejności wodór, propylen i etylen (rys. 4). Rozwój techniki pomiarowej spowodował, że obserwacja produktów utleniania węgla i zachodzących zmian ich stężeń jest coraz dokładniejsza już w temperaturze 50 C pojawia się wiele więcej gazów niż wówczas sądzono, chociaż o małych stężeniach. Badania przebiegu utleniania dokonano na próbkach węgla 20 pokładów, obejmujących wszystkie grupy samozapalności, a reprezentujących pokłady grup: libiąskiej, łaziskiej, orzeskiej, rudzkiej, siodłowej i gruszowskiej. Przeprowadzono je w laboratorium Zakładu Aerologii Górniczej Głównego Instytutu Górnictwa w Katowicach metodą kalorymetryczno-chromatograficzną. Różnotemperaturowe utlenianie badanych próbek węgla pokazało, że przyrosty stężeń gazów charakteryzowały się dużym zróżnicowaniem (rys. 5). Wykonane na ich podstawie obliczenie wartości wskaźnika Grahama (wzór 1) wykazały duże rozbieżności, pomiędzy standardową temperaturą zapłonu, a wartości wskaźnika Grahama (rys. 6). Temperaturę, w której w warunkach laboratoryjnych wskaźnik Grahama osiąga wartość standardową, graniczną określono jako temperaturę niestandardową zapłonu T Nz. Natomiast wartość wskaźnika Grahama uzyskiwaną w warunkach laboratoryjnych w standardowej temperaturze zapłonu określono jako wartość niestandardową wskaźnika Grahama G Nz. Rozbieżność ta dotyczyła wszystkich 20 badanych pokładów, a jedynie w przypadku pokładu 414 była ona minimalna (rys. 7). Ponieważ do badań próbka węgla zmielona jest do średnicy 0,5 mm, czyli jest pyłem węglowym, odbiega ona od wielkości rzeczywistych, jakie najczęściej występuje w zrobach (od brył, poprzez kostkę i groszek do miału). Zostało to więc uwzględnione i skorygowane ocenie potencjalnego zagrożenia, poprzez przyjęcie niższej temperatury zapłonu rozdrobnionego węgla T zrw = 200 C. Wzbogacenie oceny potencjalnych możliwości rozwoju procesu samozagrzewania węgla polega na zastosowaniu klasyfikacji podatności węgla na samozapłon. Wykorzystuje się w niej niestandardowość parametrów charakteryzujących samozapłon, poprzez wskaźnik utleniania niestandardowego węgla W UN wyznaczany według wzoru (2). Z obliczeń wartości wskaźnika utleniania niestandardowego dla węgli badanych 20 pokładów wynika, że wartości te mieszczą się w przedziale od 0,680 do 1,00. Jedynie dla pokładu 414 wartość ta wynosi znacznie powyżej jedności, to jest 1,475. Na tej podstawie przyjęto następujące grupy podatności węgla na samozapłon: grupa I W UN > 1,0 samozapłon utrudniony, grupa II 0,9 < W UN 1,0 samozapłon standardowy, grupa III 0,8 < W UN 0,9 samozapłon ułatwiony, grupa IV W UN 0,8 samozapłon bardzo łatwy. Porównanie badanych pokładów według podatności na samozapłon w stosunku do ich klasyfikacji pod względem samozapalności pokazuje inną hierarchię potencjalnego zagrożenia (tabl. 2, rys. 8). Na podstawie powyżej przedstawionych wyników badań można uznać, że te właściwości węgla, które sprzyjają zagrożeniu pożarami endogenicznymi nie są jednoznacznie wyrażane klasyfikacją węgli do odpowiednich grup samozapalności. Na pewno też tych właściwości jednoznacznie nie wyraża wprowadzona klasyfikacja podatności węgla na samozapłon. Można jednak przyjąć, że wykorzystanie obydwu tych klasyfikacji optymalizuje ocenę poziomu potencjalnych właściwości węgla sprzyjających zagrożeniu pożarami endogenicznymi. Będzie to najbliższe rzeczywistości, a przez to najbardziej reprezentatywne. Tak wyrażone wartości indeksacyjne składają się na indeks potencjalnego samozagrzewania IPS, który oblicza się jako IPS = WIgs i + WIps i (wzór 3).

3 295 Dzieląc cały obszar indeksacji na cztery części, o równych zakresach wartości poziomów najwyższych, to jest grup IV i V (po 0,45) oraz nieco mniejszym zakresie wartości poziomu grupy III (0,35) i najmniejszej grupy I (0,30) uzyskuje się następujący podział potencjalnego samozagrzewania: I, poziom niski 0,45 IPS I < 0,75 II, poziom średni 0,80 IPS II < 1,15 III, poziom wysoki 1,15 IPS III < 1,55 IV, poziom bardzo wysoki IPS IV 1,55. Klasyfikacja według indeksu potencjalnego samozagrzewania IPS pokazuje, że w grupie pokładów o wysokim poziomie (IPS III) znajdują się dwa pokłady sklasyfikowane do II grupy samozapalności, natomiast w grupie o bardzo wysokim poziomie (IPS IV) znajdują się dwa pokłady sklasyfikowane do III grupy samozapalności (tabl. 3). Weryfikację indeksu potencjalnego samozagrzewania przeprowadzono in situ. Uwzględniła ona pożary endogeniczne zaistniałe w ostatnim dziesięcioleciu (tabl. 4). W sześciu przypadkach pożar miał miejsce w pokładzie, którego węgiel jest przedmiotem badań i analiz (tabl. 5). W czterech przypadkach pożary wystąpiły w innych kopalniach niż kopalnie analizowane, jednak w takich samych pokładach, sklasyfikowanych do takich samych grup samozapalności węgla, co węgle pokładów badanych (tabl. 6). Weryfikacja pokazała, że sześć pożarów wystąpiło w pokładach sklasyfikowanych do grupy IPS III, to jest o wysokim poziomie potencjalnego samozagrzewania, a cztery w pokładach sklasyfikowanych do grupy IPS IV, to jest o bardzo wysokim poziomie potencjalnego samozagrzewania. W grupach IPS I o niskim poziomie, i IPS II o średnim poziomie potencjalnego samozagrzewania nie wystąpił ani jeden pożar. Ocena według indeksu potencjalnego samozagrzewania, uwzględniająca zarówno klasyfikację według grup samozapalności jak i podatności na samozapłon, daje szansę właściwej oceny potencjalnego poziomu zagrożenia pożarami endogenicznymi. Słowa kluczowe: górnictwo, węgiel kamienny, zagrożenie pożarami endogenicznymi, utlenianie węgla, samozapalność, indeks potencjalnego poziomu samozagrzewania 1. Introduction Each seam of hard coal exhibits either higher or lower vulnerability to self-heating, whilst the mechanism of such a process is extremely sophisticated (Budryk, 1956; Maciejasz, 1956; Bałtajtis et al., 1968; Maciejasz & Kruk, 1977; Strumiński, 1987, 1996; Dziurzyński & Tracz, 1994; Wacławik et al., 1995; Branny et al., 1995; Pawiński et al., 1995; Bystroń, 1997; Wystemp, 2004; Cygankiewicz, 2006). However, these are rooffallen goafs that are extremely exposed to endogenous fires as coal is able to penetrate the goaf area. In addition, there are some other internal and external factors that are conducive to endogenous fires. Internal factors result from coal properties and conditions of its deposition, whereas external ones are conditioned by mining techniques as well as organization and technical systems of coal extraction (Trenczek, 2006a, 2006c). Among other measures that prevent coal mines from endogenous fires the recognition of coal vulnerability to self-ignition is considered as a crucial one (Maciejasz, 1959; Kłosińska et al., 1967; Holek et al., 1991a, 1991b; Cygankiewicz, 1994). It is why the obligation was imposed onto mining enterprises to conduct reconnaissance hazards related to mining operation, pursuant to the act of 9 th February 1994 Mining and geological law. The entire procedure related to that obligation requires to take coal samples and prepare them for tests accordingly (pursuant to the standards PN-93/G-04501

4 296 i PN-93/G-04502) with further determination of moisture content (to the standard PN- 93/G-04511) and ash content (to PN-93/G-04512) in coal to be extracted as well as evaluation of the coal self-ignition index (to PN-93/G-04558). Eventually, coal of the specific seam or a part of it can be classified to the corresponding group of self-ignition hazard. Classification is carried out on the basis of parameters that can be found out during tests. These include: self-ignition index of coal Sz a, C/min, activation energy of coal oxidation A, kj/mol. Pursuant to the obtained values of parameters, specific coal grades can be classified to one of the following groups of self-ignition hazard: 1 st group with very low vulnerability, when Sz a < 80 and A > 67, 2 nd group with low vulnerability, when Sz a < 80 and 46 A 67, 3 rd group with medium vulnerability, when Sz a 100 and 42 < A < 46, 4 th group with high vulnerability, when 80 Sz a 120 and 34 < A < 42, 5 th group with very high vulnerability, when Sz a > 120 (A is not standardized in that case). Practice of mining operations proves that a single seam, even within a single extraction panel, can be classified to more than one group, i.e. to two or even three groups. The above classification undoubtedly presents an important alert on potential hazard due to endogenous fire and must be seriously considered by those who design coal extraction and are responsible for its course in a specific seam, as it conveys information about the probability of possible activation of the self-heating process for the specific coal under conducive conditions. However, the classification cannot be considered as the final criterion, as endogenous fires have actually occurred and may always occur in coal seams that are classified to both the 5 th and the 1 st group of self-ignition hazard. It is evidenced by endogenous fires of the recent decade (Trenczek, 2006b; Konopko at al., 2006) (Fig. 1, Table 1), as more than half of them (51%) took place in coal seams with very low and low coal vulnerability to self-ignition (1 st and 2 nd groups). 19% 19% 3% group I group II 11% 48% group III group IV group V Fig. 1. Fires according to coal bed self-ignitability groups over the years Rys. 1. Pożary według grup samozapalności pokładów w latach

5 297 Endogenous fires over the years by group of coal self-ignition vulnerability TABLE 1 TABLICA 1 Pożary endogeniczne w latach w zależności od grupy samozapalności węgla Coal selfinflammation Number of fires per year class Total % I II III IV V Total Fire-preventing prophylactic measures are put in practice with no regard to the group of self-ignition hazard assigned to a given coal seam. The relevant regulations in force make no distinction in such measures whether the coal is featured by low or very low vulnerability to self-ignition or is classified to the group with high or very high risk of self-ignition of coal. Thus, the inauspicious statistics make it possible to state that classification of coal seams to the groups of self-ignition hazard incompletely reflect the real hazard due to self-heating of coal. Results of research, examinations and analyses, disclosed in this paper, present an alternative method for evaluation and classification of coal in terms of hazards associated with endogenous fires, with use of the index of potential self-heating that takes account for the existing classification of coal self-heating properties and suggests an additional classification of coal vulnerability to self-ignition. 2. Characteristics of the self-heating process Coal oxidation leads to increase of its temperature and may eventually pose a reason for self-ignition and fire. The process of coal self-heating is long in time and comprises three major periods (Fig. 2). The first period is the initial stage of the phenomenon and is also called the period of incubation. It is the period that is attributable to virtually all coal seams (Budryk, 1956; Olpiński, 1958; Maciejasz & Kruk, 1977; Holek et. al., 1991a; Strumiński, 1996; Bystroń, 1997; Cygankiewicz, 2006). If favourable conditions persist for sufficient time period, even slight rates of coal temperature growth eventually lead to the boundary temperature T 1 that usually falls between 60 and 80 C, whilst the entire range of that interval is recognized as 63 to 102 C (Adamus, 2004). Under continuously conducive conditions

6 298 T 2 Self-ignition Temperature, [ C] T 1 Preparation period Self-heating period Fire Fig. 2. Coal self-ignitability model Rys. 2. Model samozapalenia węgla the process of coal self-heating is initiated. It is a long-lasting process, associated with slow, but continuous growth of coal temperature, with significant dynamic features (Karcz at al., 1983; Kowalewicz, 2000). It comes to the end at the ignition temperature T 2 when the self-ignition of coal may occur. Then, if the fire-favourable circumstances are still maintained, the fire blows up during the third period of the process. Duration of the incubation period depends on many factors, chiefly on local circumstances. The ignition temperature of hard coal cannot be unambiguously defined; usually it is as high as 400 C for anthracite grades and varies between 270 and 350 C for hard coal, although some other tests indicated that in can be even higher. For instance, for coal with the 10% content of volatile particles the ignition temperature ranges from 350 to 380 C whereas it decreases to the interval from 320 to 360 C when that content reaches 30%. Similar self-ignition temperature has been confirmed for steam coal by examination of coal seams from the Ostrava and Karvina Coal Basin (OKR) that constitutes the southern part of the Upper Silesian Coal Basin (Adamus, 2007). Examinations of coal self-ignition temperature as a function of its granulation allowed stating that the temperature varies typically from 330 to 360 C when granulation falls within the interval 1.5 2mm, although some coal grades exhibited much lower value, only 210 C. With granulation diameter below 1 mm and less the ignition temperatures may be even lower, from 190 to 220. Typically, the ignition temperature of hard coal in goafs is assumed as T Z = 300 C (Bystroń, 1997).

7 299 Pursuant to the mining regulations in force, disclosed in the Exhibit No 5 to the Regulation of the Minister of Economy dated on 28 th June 2002, a fire in goafs is considered as the state when the Graham coefficient that reflects composition of goaf gases, is not less than G = The Graham coefficient is calculated by the formula CO G (1) 0.265N O where CO, N 2, O 2 percentage content of carbon oxide, nitrogen and oxide as measured at the measurement station. Evaluation of potential vulnerability of coal to self-heating should be supported by testing of coal samples that make it possible to define its ignition temperature and corresponding value of the Graham coefficient Methodic for investigation of self-heating properties Information, how composition of coal-mining gases varies in time makes it possible to evaluate the level of fire hazard as the gases are released as products of the self-heating process. The most recent experience of mining practice gives the reason to state that steam is the first product of the low-temperature coal oxidation and it evokes the effect of coal sweating, where dew appears at locations of steam origin. Depending on rates at which the subsequent process carries on and environmental conditions of locations where it occurs, releasing of subsequent products may vary. At temperatures about 100 C the oxidation process releases aromatic hydrocarbons with characteristic odour. When coal temperature approaches to the value of ignition and amount of oxygen is still insufficient, concentration of aromatic hydrocarbon increases as well as concentrations of carbon oxide and dioxide. In addition, other gases that are typical for coal carbonization are released, such as hydrogen and methane, but also ethane and propane. One of first research studies on products of coal oxidation were carried out nearly forty years ago (Chamberlain at al., 1970) and confirmed that carbon oxide is released at first and then hydrogen, propylene and ethylene (Fig. 3). Continuous progress in the measurement techniques has facilitated monitoring of coal oxidation products and variations in concentration thereof and made the measurements more accurate. It has made it possible to state that even at temperatures as low as 50 C much more gaseous products is released as compared to the previous beliefs however concentration figures for such gases are still extremely low, not higher than several ppm. Nevertheless, that fact could be used to extremely accurate tracing of the Graham coefficient variation in pace with temperature growth during the process of coal oxidation at variable temperature.

8 carbon oxide Gases concentration, [%] hydrogen ethylene prophylene Coal temperature, [ C] Fig. 3. Dependence gases concentration vs. temperature Rys. 3. Zależność stężeń gazów od temperatury Investigations on the coal oxidation process were carried out for samples from 20 coal seams. The samples covered all the groups with various self-ignition vulnerability and were taken from coal seams located in the following regions: Libiąż the coal bed 118 (the Janina colliery), Łaziska the coal beds 207 and 209 (the Sobieski mining operation), Orzesze the coal beds: 315 (the Silesia coal mine) and 364 (the Brzeszcze coal mine), Ruda Śląska the coal beds: 402 (the Staszic coal mine), 404/2 (the Chwałowice coal mine), 405/1 (the Borynia coal mine), 407/1 (the Knurów coal mine), 408/2 (the Zofiówka coal mine), 409 (the Wujek coal mine, Śląsk division), 413/1-2 (the Jankowice coal mine), saddle-type seams the coal beds: 501 (the Wesoła coal mine), 502 (the Wujek coal mine Śląsk division), 504 (the Sośnica-Makoszowy coal mine Sośnica division), 505 (the Marcel coal mine), 510 (the Wujek coal mine Śląsk division), boundary seams the coal beds: 703/1 and 713/1-2 (the Rydułtowy coal mine). The seams rated to the group of Ruda and saddle-type seams were the predominant type as these types are the most frequent coal beds in collieries of the Upper Silesian Coal Basin. Unfortunately, samples of coal from the seams of Poręba could not be included into the investigation program as these are excavated very rarely.

9 301 The tests of coal oxidation products were conducted in the laboratory operated by Department of Mining Aerology, the Central Mining Institute in Katowice, with use of the combined calorimetric and chromatographic method. They were made up of two equivalent segments: oxidation of the investigated coal samples at various temperatures and determination of gas content with regard to coal oxidation products (Cygankiewicz, 1996). Oxidation of coal samples was carried out in a calorimeter, under conditions that were very close to adiabatic ones. The coal samples delivered for tests, with the weight of 10 g each, were prepared of hard coal crumbled afresh and then ground to the fine grains with size of 0.5 mm. Next, the samples were placed in reaction chambers of calorimeters where synthetic air was supplied, with the content of 20.5% of oxygen and 89.5% of nitrogen, at the flow rate of 1 l/min. Oil in the calorimeter was heated up and temperature of coal samples gradually increased to the values from 50 to 350 C. Oxidation of a singe sample took about 6 hours. After a sample under test reached the temperatures of 50, 100, 150, 200, 250, 300 and 350 o C, oxidation product from the calorimeter were sampled for chromatographic tests, where helium was used as the carrying phase. For the purpose of the tests, accurate chemical analyses of gaseous samples, taken from the carbon oxidation process, were carried out with use of five chromatographs, equipped with units to increase density. Each of the three first chromatographs was used exclusively to determine content of a single gas: carbon oxide CO, with the accuracy of ±0,0005% by volume within the range from 0 do 0,0026% of CO, hydrogen H 2, with the accuracy of ±1 ppm by volume, acetylene C 2 H 2, with the accuracy of ±0,002 by volume. The fourth chromatograph was used to determine content of the following gases: - oxygen O 2, with the accuracy of ±0,1% by volume, - carbon dioxide CO 2, with the accuracy of ±0,03% by volume, methane CH 4, with the accuracy of ± 0,05% by volume, ranged from 0 to 5% CH 4, nitrogen N 2, with the accuracy of ±0,5% by volume. Finally, the fifth chromatograph was used to determine content of propylene C 3 H 6, propane C 3 H 8, ethylene C 2 H 4 and ethane C 2 H 6, with the same accuracy of ±0,01 ppm by volume. Oxidation of coal samples at variable temperature has revealed that rates of concentration growth varied for different gases within a wide range. This is illustrated by examples for 6 selected coal seams of the groups of Libąż, Łaziska, Orzesze, Ruda, saddle-type and boundary ones (Fig. 4). Calculations of the Graham coefficient, carried out on the basis of the measurement results have disclosed discrepancies between their standard ignition temperature and results, obtained from calculations for values of the Graham coefficients (Trenczek, 2007) (Fig. 5).

10 302 a) Coal seam 118 b) Coal seam 207 c) Coal seam 364 d) Coal seam 414 e) Coal seam 502 f) Coal seam 703/1 Fig. 4. Variation of gases concentration during the process of annealing sample of selected coal seams Rys. 4. Przebieg zmian stężeń gazów podczas procesu wygrzewania próbek węgla wybranych pokładów

11 303 a) Coal seam 118 b) Coal seam 207 c) Coal seam 364 d) Coal seam 414 e) Coal seam 502 f) Coal seam 703/1 Fig. 5. Characteristic of non-standard Graham s index value and ignition temperature of sample of selected coal seams Rys. 5. Charakterystyka niestandardowych wartości wskaźnika Grahama i temperatury zapłonu próbek węgla wybranych pokładów

12 304 The standardized value of the Graham coefficient, assumed as G = 0.03, is achieved at temperatures significantly lower than the standardized temperature of coal ignition T z = 300 C, whilst at the temperature T z = 300 C values of the Graham coefficient always exceeded the standardized valued of G = 0.03 and in most cases they were even higher than G = 0.1. Hence, the temperature when the Graham coefficient reaches the standardized boundary value can be defined as the non-standard ignition temperature T Nz. Similarly, the value of Graham coefficient that was obtained under laboratory conditions at the standard ignition temperature can be nominated as the non-standard value of the Graham coeffi cient G Nz. The disclosed discrepancies were associated with all 20 coal seams under investigations, it was only one coal seam (coal bed 414) where difference was insignificant (Fig. 6) standard ignition line G = 0.03; T B = 300 C Non-standard ignition temperature T Nz / /1 408/ /2 713/ / / Non-standard Graham s value index G Nz Fig. 6. Laboratory distribution of the non-standard coal ignition temperatures and the non-standard Graham s value indexes Rys. 6. Rozkład laboratoryjnie uzyskiwanych temperatur niestandardowych zapłonu i wartości niestandardowych wskaźnika Grahama 4. Classification of coal in terms of self-ignition vulnerability The laboratory examinations dedicated to standardized heating of coal are aimed at reproduction of the self-heating process that takes place under natural circumstances.

13 305 However, the examinations never use coal with granulation that actually exists in goafs. Real coal has usually granulation with various sizes of pellets: from coal lumps through cobbles and pea coal up to culm. The coal samples delivered for tests were ground to the fine grains with size of 0.5 mm, thus the samples practically represented coal dust. Thus, it is a factor that must be taken into account for evaluation of possible hazards by appropriate corrections. As it was mentioned before, examination of coal self-ignition have proven (Strumiński, 1996) that temperatures of coal ignition usually range from C and T z = 300 C is assumed as the standardized ignition temperature (Bystroń, 1997). But at it was already mentioned, for granulation with grain diameters of 1 mm and less the ignition temperatures are also lower, namely from 190 to 220 C. Correction of the ignition temperature for granulation that is used for laboratory tests consists in adoption of the temperature of pulverized coal ignition T zrw that is accordingly lower than T z. As determination of such temperature for all the coal grades is practically impossible, the approximated value from the mentioned interval was adopted as T zrw = 200 C, similarly to the standardized ignition temperature T z. Enhanced assessment of possible opportunities for development of the coal self-heating process consists in application of right classification in view of coal vulnerability to self-ignition. For that purpose non-standard properties of coal parameters connected with self-ignition are used, by the coeffi cient of non-standard oxidation of coal W UN calculated with the equation (2) W UN = T T Nz zrw T = Nz 200 = T Nz (2) The experiments revealed that values for coeffi cients of non-standard oxidation of coal from 20 coal seams under investigation fall into the interval from to There was only one coal seam, i.e. No 414, where the coefficient value was much above one, namely The above results served as a basis to distinguish the following groups of coal in view of its vulnerability to self-ignition: group I W UN > 1.0 difficult self-ignition, group II 0.9 < W UN 1.0 typical self-ignition, group III 0.8 < W UN 0.9 easy self-ignition, group IV W UN 0.8 very easy self-ignition. Comparison of the coal seams under investigation in view of their vulnerability to self-ignition against the classification of self-ignition characteristics exhibits altered hierarchy of possible hazards (Table 2, Fig. 7).

14 306 TABLE 2 Classification of the examined coal seams by the group of the coal self-ignition vulnerability Group I difficult self-ignition Group II typical self-ignition Group III easy selfignition Group IV very easy self-ignition TABLICA 2 Zestawienie badanych pokładów węgla według grup podatności węgla na samozapłon Coal seam 414 W UN Coal selfinflammation class II Coal seam 404/2 405/1 510 W UN Coal selfinflammation class III, IV I, II II, III Coal seams /1 408/2 413/ /1 713/1-2 W UN Coal selfinflammation class I, II, III II, III II I II, III III, IV I, II II, III II, III II, III Coal seam W UN Coal selfinflammation class IV V V II, III III II, III The above breakdown demonstrates that a large variety of coal beds are classified to the 4 th group of coal grades, i.e. the grades with very easy ignition. Both those that were already classified into the 4 th and 5 th group of self-ignition vulnerability, i.e. with similar level of possible hazard, and the ones that belonged to the 3 rd and 2 nd group, i.e. with medium and low vulnerability to self-ignition. In turn, the 3 rd group, i.e. coal grades with facilitated vulnerability to self-ignition, encompasses coal beds already classified to various groups of self-ignition hazard, i.e. from 4 th to 1 st. Coal beds with very similar former classification are rated to the 2 nd group, i.e. typical vulnerability to self-ignition. Nevertheless, the 1 st group coal with difficult self-ignition includes only one bed with low vulnerability to self-ignition. The characteristic feature of the both classification is the fact, that the 4 th group coal grades with very easy ignition comprises coal beds already classified to the 5 th group of self-ignition. 4. Index of potential self-heating The investigation results mentioned above lead to the conclusion that coal properties that are conducive to hazards of endogenous fires are not clearly defined by rating of coal grades to relevant groups of self-ignition vulnerability. For sure, such properties cannot

15 307 Fig. 7. Diagram of the non-standard coal oxidation according to self-ignition susceptibility groups Rys. 7. Diagram wartości utleniania niestandardowego węgla badanych pokładów według grup podatności na samozapłon be also unambiguously characterized by the proposed classification of coal vulnerability to self-ignition. However, one can assume that joint advantages of the both classification methods lead to optimized assessment of probable coal properties that are conducive to the hazard of endogenous fires. Such an approach shall be the best adherent to reality and consequently the most representative to actual hazards in the mining industry. To the very beginning, each group of self-ignition and each group of vulnerability to self-ignition must be associated with the corresponding index value respectively WIgs and WIps. It is the value that expresses numerical ratio for the specific classification group that results from assignment of a numerical value to each group (group 1 1 score, group 2 2 scores, group 3 3 scores, group 4 scores and group 5 five scores) divided by the maximum number of groups within a specific classification (i.e. the value of 5 for five groups of self-ignition, the value of 4 for four groups of vulnerability to self-ignition). The lowest index value for the groups of self-ignition is associated with the group No 1 as it amounts to WIgs 1 = Subsequent groups assume the following values: Group No 2 WIgs 2 = 0.40; Group No 3 WIgs 3 = ; Group No 4 WIgs 4 = 0.80; Group No 5 WIgs 5 = The lowest index value for the groups of vulnerability to self ignition is also assigned to the 1st group, but it is higher and amounts to WIps 1 = Values for subsequent

16 308 groups are the following: Group No 2 WIps 2 = 0.50; Group No 3 WIps 3 = 0.75; Group No 4 WIps 4 = The componential indices expressed as above make up the index of potential selfheating IPS that is calculated as (Trenczek, 2007) IPS = WIgs i + WIps i (3) Therefore the lowest value for the index of potential self-heating can be IPS min = 0.45, which corresponds to the lowest, first groups of self-ignition and vulnerability to selfignition, whereas the highest one, IPS max = 2.00 is for groups with highest numbers, respectively for the groups no 4 and 5. In case of coal beds where coal from the specific region is classified to various groups of self-ignition, the suggested classification uses the group with the highest number. The full range of classification stretches from 0.45 to 2.00 and the area of indexation is After having the entire range of indexation split into four sections, with equal values for the upper boundaries, i.e. for the groups IV ad V (0.45 for the both groups) and slightly lower value for the level of the group III (0.35) and the lowest one or the group I (0.30) the following breakdown of potential self-heating indices can be obtained: I, low level 0.45 IPS I < 0.75 II, medium level 0.80 IPS II < 1.15 III, high level 1.15 IPS III < 1.55 IV, very high level IPS IV The subsequent groups of possible self-heating reflect: IPS I low level: maximum: the 2 nd group of self-ignition and 1 st group of vulnerability to self ignition (the maximum value amounts to 0.65), or maximum: the 1st group of self-ignition and the 2 nd group of vulnerability to self ignition (the maximum value amounts to 0.70). IPS II medium level: maximum: the 1 st group of self-ignition and the 3 rd group of vulnerability to self ignition (the maximum value amounts to 0.95), or maximum: the 3 rd group of self-ignition and the 2 nd group of vulnerability to self ignition (the maximum value amounts to 1.10), or maximum: the 4 th group of self-ignition and the 1 st group of vulnerability to self ignition (the maximum value amounts to 1.05). IPS III high level: maximum: the 2 nd group of self-ignition and the 4 th group of vulnerability to self ignition (the maximum value amounts to 1.40), or maximum: the 3 rd group of self-ignition and the 3 rd group of vulnerability to self ignition (the maximum value amounts to 1.35), or maximum: the 4 th group of self-ignition and the 2 nd group of vulnerability to self

17 309 ignition (the maximum value amounts to 1.30), or maximum: the 5 th group of self-ignition and the 2 nd group of vulnerability to self ignition (the maximum value amounts to 1.50). IPS IV very high level: minimum: the 3 rd group of self-ignition and the 4 th group of vulnerability to self ignition (the minimum value amounts to 1.60), or minimum: the 4 th group of self-ignition and the 3 rd group of vulnerability to self ignition (the minimum value amounts to 1.55). In total, 20 coal beds that were sampled and tested. Classification of those coal beds to the index of potential self-heating IPS discloses (Table 3) that only three out of those 20 could be classified into the groups with lowest levels of hazards: the bed No 414 low level and beds 403/1 and 408/2 medium level. All the others coal beds were categorized as beds with high and very high level of potential self-heating. Namely, the group of coal beds with high level risk of potential self-heating of coal comprised 11 units coal beds No 315, 364, 404/2, 407/1, 413/1-2, 502, 504, 505, 510, 703/1 and 713/1-2, whereas the group of coal beds with very high level risk of potential self-heating encompassed 6 units with numbers: 118, 207, 209, 402, 409 and 501. The classification by the indices of potential self-heating IPS exhibits that the group of coal seams with high level (IPS III) encompasses two seams that are normally rated to the 2 nd group of self-ignition vulnerability, while two other coal seams, rated to the 3 rd group of self-ignition vulnerability actually represent very high level (IPS IV) (Table 3). 6. Verification of the index of potential self-heating of coal beds The coal samples from the coal beds in question were submitted for tests due to various reasons. During the time period by July 2006, i.e. to the moment when regulations on compulsory tests with use of the calorimetric and chromatographic method became effective (regulation of the Minister of Economy of 9 th June 2006) the test of coal oxidation at variable temperature used to be carried out only in very specific cases, mostly when symptoms of coal self-heating appeared and the comprehensive assessment of fire hazard had to be carried out in conjunction with the methane hazard estimation. The mentioned regulation imposes the obligation to subcontract the tests whenever level of hazard exceeds the defined threshold. Frequently the tests were initiated by coal mines themselves as they wished to obtain the comprehensive image of possible hazards as their past experience and historical records indicated frequent cases of coal self-heating in coal beds with the 2 nd group of the self-ignition hazard. The above deliberations show that it is very difficult to comprehensively verify the classification of coal beds into the group of self-ignition, which is compulsory in the mining sector, in comparison to the suggested classification to the index of potential

18 310 Levels of possible selfheating Low IPS I Medium IPS II High IPS III Very high IPS IV TABLE 3 Classification of the examined coal seams by levels of possible self-heating TABLICA 3 Zestawienie badanych pokładów węgla według poziomów potencjalnego samozagrzewania Value indices Coal seams 414 Coal self-inflammation class WIgs II 0.40 Easy self-ignition group I WIps IPS 0.65 Coal seams 405/1 408/2 Coal self-inflammation class II I WIgs Easy self-ignition group II III WIps IPS Coal seams /2 407/1 Coal self-inflammation class WIgs III III IV 0.80 II / 1-2 III /1 Easy self-ignition group III III II III III III III IV II III III WIps IPS Coal seams Coal self-inflammation class WIgs Easy self-ignition group WIps. IV 0.80 IV 1.00 IPS V 1.00 IV 1.00 V 1.00 IV 1.00 III IV 1.00 III IV 1.00 II 0.40 IV 0.80 III 0.75 III III III III 713/ 1-2 III

19 311 self-heating. However, the verification is possible for several coal beds, by assessment of classification thereof with use of the on site inspection. Such assessment takes advantage of historical records and accounts for endogenous fires that took place over the recent decade (Table 4) (Trenczek, 2006b ; Konopko at al., 2006). Classification of the endogenous fires over the years Zestawienie pożarów endogenicznych zaistniałych w latach TABLE 4 TABLICA 4 Coal mine Date breeding fire Coal seam / thickness [m] Depth [m] coal self-inflammation class Hazard s methane s rock-bump group category degree Morcinek /1 / II IV I Śląsk / II, III IV III Andaluzja / V III Śląsk breccia coal - barren rock 1050 II Wujek / II I III Knurów /1 / II II Anna /1-2 / II II I Wujek / II I III Julian /2 / IV Porąbka-Klimontów / V I Rydułtowy /1-2 / II, III II I Polska-Wirek / IV II III Bielszowice /2 / II IV Piekary / IV I Knurów /3 / III II Bytom III / IV II III Piast (Czeczott) / V I Brzeziny / V I Sośnica / II, III III Bielszowice /2 / III IV III Sośnica /2 / II IV II Centrum (Dymitrow) / IV I III Bytom II / V I III Bielszowice /1 / I II II Halemba /1 / II IV III Śląsk / II, III II I Centrum (Dymitrow) / IV I III Janina / IV Katowice-Kleofas / II I I

20 Wesoła / III, IV IV III Bielszowice / II IV III Mysłowice / III II I Siltech / II I Polska-Wirek / III II I Budryk /1 / II IV Halemba /1 / II IV III Brzeszcze-Silesia / IV IV The verification may also involve four consecutive cases of fires that occurred in other coal mines, different from the ones under investigation, however in similar coal beds, classified to the same groups of coal self-ignition as coal from the beds under tests (Table 5). Comparative characteristics of examined coal seams where endogenous fires occurred TABLE 5 TABLICA 5 Zestawienie porównawcze badanych pokładów węgla, w których zaistniał pożar endogeniczny Coal mine Date breeding fire Śląsk easy self-ignition easy self-ignition easy self-ignition typical selfignition Rydułtowy Sośnica Śląsk Janina Wesoła Coal seam / thickness [m] / Coal self-inflammation class GS group / value rating index II / low 0.40 III / medium III / medium III / medium IV / 0.80 IV / 0.80 high high Easy self-ignition group group / value index III / 0.75 III / 0.75 III / 0.75 II / 0.50 IV / 1.00 III / 0.75 W UN rating very easy selfignition easy self-ignition Index of potential self-heating IPS levels / value index IPS III / 1.15 IPS III / 1.35 IPS III / 1.35 IPS III / 1.10 IPS IV / 1.80 IPS IV / 1.55 rating high level high level high level high level very high level very high level To do so it is necessary to assume that concentration of gases products of coal oxidation at variable temperature would subject to nearly the same alterations as it was in case of the coal bed under tests. In other words, we have to assume that the coefficient of non-standard oxidation, calculated for coal from those beds, falls into the same group of vulnerability to self-ignition as the coal samples from coal beds under tests (Table 6).

21 313 TABLE 6 Comparison of coal seams where fires occurred against examined coal seams with the same characteristics of self-ignition vulnerability TABLICA 6 Zestawienie porównawcze pokładów węgla, w których wystąpił pożar z badanymi pokładami tej samej grupy samozapalności Coal mine Date breeding fire Anna easy self-ignition very easy self-ignition typical selfignition easy self-ignition Piast (Czeczott) Polska- Wirek Brzeszcze- Silesia Coal seam / thickness [m] 703/ Coal self-inflammation class GS group / value rating index II / low 0.40 V / very 1.00 high III / medium IV / high 0.80 Easy self-ignition group group / value index III / 0.75 IV / 1.00 II / 0.50 III / 0.75 W UN rating Index of potential self-heating IPS levels / value index IPS III / 1.15 IPS IV / 2.00 IPS III / 1.10 IPS IV / 1.55 rating high level very high level high level very high level Analysis of the above breakdowns (Tables 5 and 6) reveals that fires occurred in coal beds where the group of self-ignition was defined in unambiguous way. One fire took place in the coal bed (bed 207 Table 6) that was classified to the 5 th group of self-ignition, while three fires blown up in beds of the 4 th group of self-ignition coal beds No 118 and 501 (Table 5) and 315 (Table 6), i.e. with high vulnerability to self-ignition. As the total number of fires was 37 (Table 4), only 14 of them (38%) were rated to the 5 th and 4 th groups (Table 1). These cases can be considered as reflection of really high hazard of potential fire. The reports disclose that fires occurred in coal beds with medium vulnerability to self-ignition, i.e. classified to the 3 rd group of self-ignition 4 fires altogether, in coal beds 713/1-2, 504 (one per each Table 5) and 510 (2 fires Tables 5 and 6). In relation to the total number of fires it is not much only 11% by the breakdown that uses the group of self-ignition hazard that is more frequent during tests for the specific coal bed (Table 1), or 22%, if the criterion is adopted that the highest possible group of selfignition is assigned to the coal bed as whenever occurred during tests (the worst case) in spite of the number of tests (Table 4). All in all, it is a pretty high contribution of that group with not high level of self-ignition hazard in the total balance of fires. With regard to classification of coal beds in terms of their vulnerability to self-ignition one can state that in two cases fires occurred in the coal bed 510 (Tables 5 and 6), classified to the 2 nd group, with medium self-ignition hazard. There were six fires in coal beds of the 3 rd group with easy self-ignition beds 502, 713/1-2, 504, 501 (Table 5) as well as 703/1-2 and 315 (Table 6) - and two fires in coal beds classified into the highest, 4 th group with very easy vulnerability to self-ignition. These facts indicate that values of vulnerability indices for some coal beds are underestimated in terms of fire hazard.

22 314 The last, totalized classification of coal beds with regard to the index of potential self-heating present pretty good image of coal bed breakdown. Six fires took place in beds classified to the group IPS III, with high level of potential self-heating beds 502, 713/1-2, 504, 510 (Table 5) as well as 703/1-2 and 510 (Table 6), whereas four fires blown up in coal beds classified to he group IPS IV, with very high level of potential self-heating. No fires occurred in groups IPS I and IPS II with low level and medium level of potential self-heating, respectively. The above on site verification make it possible to state with high degree of probability that the current classification of coal beds into groups of self-ignition hazard is conducive to underestimation of hazards due to endogenous fires. Use of the self-ignition index of coal Sz a and activation energy of coal oxidation A as sole criteria seems to be far insufficient. It is confirmed by records of the recent decade, when 51% of endogenous fires blown up in coal beds classified into the 1 st and 2 nd groups of vulnerability to selfignition, i.e. very little and little vulnerable (Table 1). The developed supplementary classification of coal beds in terms of their vulnerability of self-ignition transfers results of laboratory tests (carried out for fine-grained samples, with grain diameters of 0.5 mm) onto real conditions of mining excavations (coal with granulation from lumps through cobbles and pea coal up to culm) as it introduces correction of test results by the coeffi cient of non-standard oxidation. However, even such classification may allow for underestimation of potential fire hazards. The final grading of possible hazard due to endogenous fire, i.e. index of potential self-heating, takes account for the both classifications: into the groups of self-ignition hazard and into the groups of vulnerability to self ignition, which makes the final assessment balanced and unbiased. The on-site verification process, involving 10 cases of endogenous fires, has revealed that these fires took place in coal beds with high and very high levels of indices of potential self-heating. Hence, the new classification method provides opportunity to evaluate hazards of endogenous fires in right way. 5. Conclusions The mining practice proves that classification of coal grades into groups of self-ignition hazard on the basis of laboratory-defined self-ignition index of coal Sz a and activation energy of coal oxidation A as sole criteria poorly reflects actual levels of hazards due to endogenous fires. Progress in measurement techniques makes it possible to monitor variations of concentration of gases that are released as products of the coal self-heating process, whilst the measurements are taken at the level of critical temperatures. Disintegration of coal samples that are taken for tests of coal oxidation at variable temperature serves as a reason that substantial discrepancies occur between standardized

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