Burning of coal waste piles from Douro Coalfield (Portugal): Petrological, geochemical and mineralogical characterization

Abstract In the Douro Coalfield anthracites were exploited for decades (1795–1994). Besides many small mines Douro Coalfield had two principal mining areas (S. Pedro da Cova and Pejao). Coal mining activities cause several impacts on the environment, one of which is the amount of discard or waste which was disposed of all over Douro Coalfield resulting in one of the most significant and severe impacts on the environment. Over 20 waste piles exist in the old mining areas, geographically dispersed, and three of them are presently burning. Their ignition was caused by forest fires during the summer of 2005. Samples from the burning and unburned zones of the waste piles were studied as were the gas from vents and the minerals resulting after combustion. Geochemical processes and mineralogical transformations in the burning coal waste pile were investigated. Microscopic analyses of the samples identified some particular aspects related with combustion: oxidation of pyrite, the presence of iron oxides, organic particles with cracks and rims with lowered (suppressed) Rr, devolatilization vacuoles and some char structures. The occurrence of vitreous (glassy) material as well as Fe–Al spinels in the burning coal waste provide evidences that the combustion temperature could have reached values above 1000 °C. Due to combustion, and as expected, the samples studied reported high ash yields. Samples taken from the burning zones reported an increase of As, Cr, Li, Nb, Ni, Pb, Rb, Sr and LREE concentrations and a decrease in Zr and HREE concentrations. Enrichment in Cs, Li and Rb was noted when comparing with the geochemical composition of black shales and world coals composition that is related with the contribution of granitic rocks in the sediments that originated the main lithologies of the Douro Coalfield (carbonaceous shale and lithic arenites). Cluster analyses (R-type and Q-type) were performed to understand the trend between the unburned and burning samples and it seems that some chemical variations are responsible for this separation. Elemental sulphur and salammoniac (ammonium salt) are the coal fire gas minerals neoformed on the surface of piles, near the burning zones. They were identified by different techniques, mainly SEM-EDX, XRD and FTIR. Relatively high concentrations of several aromatic compounds were detected in the gas collected at the studied areas, as well as aliphatic hydrocarbons. The highest concentrations of aromatic hydrocarbons were measured in gas samples from S. Pedro da Cova waste pile. The exposure to hazardous compounds present in the gas is a serious risk to human health and the environment.

[1]  F. Goodarzi.  Mineralogy, elemental composition and modes of occurrence of elements in Canadian feed-coals ☆ , 2002 .

[2]  R. Eagar.  The non-marine bivalve fauna of the Stephanian C of North Portugal , 1983 .

[3]  S. Valceva,et al.  Geochemistry of beryllium in the Mariza-east lignite deposit (Bulgaria) , 2003 .

[4]  Robert B. Finkelman,et al.  Potential health impacts of burning coal beds and waste banks , 2004 .

[5]  A. Tomita,et al.  Leaching of ashes and chars for examining transformations of trace elements during coal combustion and pyrolysis , 2004 .

[6]  G. Gürdal.  Geochemistry of trace elements in Çan coal (Miocene), Çanakkale, Turkey , 2008 .

[7]  Yang Li,et al.  Trace element emissions from spontaneous combustion of gob piles in coal mines, Shanxi, China , 2008 .

[8]  A. Pinto de Jesús,et al.  Evolução sedimentar e tectónica da Bacia Carbonífera do Douro (Estefaniano C inferior, NW de Portugal) , 2003 .

[9]  M. J. Sousa,et al.  General description of the terrestrial carboniferous basins in Portugal and history of investigations , 1983 .

[10]  R. G. Wiese,et al.  Spontaneous formation of hydrated iron sulfates on laboratory samples of pyrite- and marcasite-bearing coals , 1987 .

[11]  J. Kortenski,et al.  Trace and major element content and distribution in Neogene lignite from the Sofia Basin, Bulgaria , 2002 .

[12]  M. P. Ketris,et al.  Estimations of Clarkes for Carbonaceous biolithes: World averages for trace element contents in black shales and coals , 2009 .

[13]  Thomas Højlund Christensen,et al.  Effects of Thermal Treatment on Mineralogy and Heavy Metal Behavior in Iron Oxide Stabilized Air Pollution Control Residues , 2000 .

[14]  Edward L. Heffern,et al.  Geologic history of natural coal-bed fires, Powder River basin, USA , 2004 .

[15]  M. J. Sousa,et al.  The Carboniferous megafloras of Portugal ― A revision of identifications and discussion of stratigraphic ages , 1983 .

[16]  Edward Salisbury Dana,et al.  Dana's New Mineralogy: The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana , 1997 .

[17]  E. C. Teixeira,et al.  Geochemical distribution of trace elements in coal: modelling and environmental aspects , 1997 .

[18]  F. G. Bell,et al.  Environmental impacts associated with an abandoned mine in the Witbank Coalfield, South Africa , 2001 .

[19]  John C. Crelling,et al.  Applied coal petrology : the role of petrology in coal utilization , 2008 .

[20]  Magdalena Misz-Kennan,et al.  Thermal transformation of organic matter in coal waste from Rymer Cones (Upper Silesian Coal Basin, Poland) , 2010 .

[21]  E. Sholkovitz.  The aquatic chemistry of rare earth elements in rivers and estuaries , 1995 .

[22]  Dalway J. Swaine,et al.  Trace Elements in Coal , 1990 .

[23]  Harold J. Annegarn,et al.  The spontaneous combustion of coal and its by-products in the Witbank and Sasolburg coalfields of South Africa , 2007 .

[24]  X. Querol,et al.  Mineralogy and geochemistry of coal from the Liupanshui mining district, Guizhou, south China , 2000 .

[25]  A. Guedes,et al.  Correlation between optical, chemical and micro-structural parameters of high-rank coals and graphite , 2009 .

[26]  Y. Prasad,et al.  Effect of coalification on spontaneous combustion of coals , 1990 .

[27]  G. Fernández-Martínez,et al.  Distribution of volatile organic compounds during the combustion process in coal-fired power stations , 2001 .

[28]  David Glasser,et al.  Spontaneous combustion of carbonaceous stockpiles. Part I: the relative importance of various intrinsic coal properties and properties of the reaction system , 2005 .

[29]  B. K. Misra,et al.  Susceptibility to spontaneous combustion of Indian coals and lignites: an organic petrographic autopsy , 1994 .

[30]  Basil Beamish,et al.  Effect of mineral matter on coal self-heating rate , 2008 .

[31]  Eliseo Monfort,et al.  Environmental characterization of burnt coal gangue banks at Yangquan, Shanxi Province, China , 2008 .

[32]  W. Rosenberg.  Clean air act amendments. , 1991, Science.

[33]  S. Parr.  The Classification of Coal , 1922, Science.

[34]  R. Yan,et al.  Chemometric studies of distribution of trace elements in seven Chinese coals , 1995 .

[35]  Van Krevelen,et al.  Coal: Typology - Physics - Chemistry - Constitution , 1993 .

[36]  John C. Crelling,et al.  applied coal petrology , 2008 .

[37]  M. Fabiańska,et al.  Organic components in thermally altered coal waste: Preliminary petrographic and geochemical investigations , 2007 .

[38]  Glenn B. Stracher,et al.  Coal fires burning out of control around the world : Thermodynamic recipe for environmental catastrophe , 2004 .

[39]  R. Finkelman,et al.  The types of data needed for assessing the environmental and human health impacts of coal , 1999 .

[40]  M. P. Ketris,et al.  Arsenic in coal: a review , 2005 .