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Poziom potencjalnego samozagrzewania węgla w świetle badań

Poziom potencjalnego samozagrzewania węgla w świetle badań

Autor: S . Trenczek

Jednym z elementów zapobiegania pożarom endogenicznym jest rozpoznanie skłonności węgla do samozapalenia. O sklasyfi kowaniu pokładu do odpowiedniej grupy samozapalności decydują wartości wyznaczanych w czasie badań: wskaźnika samozapalności węgla Sza, °C/min oraz energii aktywacji utleniania węgla A, kJ/mol. Klasyfi kacja 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, sklasyfi kowanych 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 klasyfi kacji pokładów węgla nie oddaje w pełni poziomu potencjalnego zagrożenia samozagrzewaniem. Proces samozagrzewania jest rozłożony w czasie i składa się zasadniczo z trzech okresów (rys. 2).

Pierwszy okres – przygotowawczy, doprowadza w końcu do temperatury granicznej T1, leżącej zazwyczaj w przedziale od 60 do 80°C. Drugi – proces samozagrzewania, kończy się po osiągnięciu temperatury zapłonu T2, 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 Tz = 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-chromatografi czną. 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 TNz. Natomiast wartość wskaźnika Grahama uzyskiwaną w warunkach laboratoryjnych w standardowej temperaturze zapłonu określono jako wartość niestandardową wskaźnika Grahama GNz. 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 Tzrw = 200°C.

Wzbogacenie oceny potencjalnych możliwości rozwoju procesu samozagrzewania węgla polega na zastosowaniu klasyfi kacji podatności węgla na samozapłon. Wykorzystuje się w niej niestandardowość parametrów charakteryzujących samozapłon, poprzez wskaźnik utleniania niestandardowego węgla WUN 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 – WUN > 1,0 – samozapłon utrudniony,

– grupa II – 0,9 < WUN ≤ 1,0 – samozapłon standardowy,
– grupa III – 0,8 < WUN ≤ 0,9 – samozapłon ułatwiony,
– grupa IV – WUN ≤ 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 klasyfi kacją węgli do odpowiednich grup samozapalności. Na pewno też tych właściwości jednoznacznie nie wyraża wprowadzona klasyfi kacja podatności węgla na samozapłon. Można jednak przyjąć, że wykorzystanie obydwu tych klasyfi kacji 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 = WIgsi + WIpsi (wzór 3).

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.

Klasyfi kacja według indeksu potencjalnego samozagrzewania IPS pokazuje, że w grupie pokładów o wysokim poziomie (IPS III) znajdują się dwa pokłady sklasyfi kowane do II grupy samozapalności, natomiast w grupie o bardzo wysokim poziomie (IPS IV) znajdują się dwa pokłady sklasyfi kowane do III grupy samozapalności (tabl. 3).

Weryfi kację 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).

Weryfi kacja pokazała, że sześć pożarów wystąpiło w pokładach sklasyfi kowanych 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 klasyfi kację według grup samozapalności jak i podatności na samozapłon, daje szansę właściwej oceny potencjalnego poziomu zagrożenia pożarami endogenicznymi.

 
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