Estonian oil shale has been used for 90 years mainly for electricity and oil generation with the ash being used for cement and light brick production. The oil shale usage has always been related to available mining and processing technology, and vice versa, with external influences of worldwide petroleum prices. The same situation is true today, when new technology is being applied in power generation units, in oil generators and in oil shale extracting processes.
We compared four types of 30-year-old forest stands growing on spoil of opencast oil shale mines in Estonia. The stand types were: (1) natural stands formed by spontaneous succession, and plantations of (2) Pinus sylvestris (Scots pine), (3) Betula pendula (silver birch), and (4) Alnus glutinosa (European black alder). In all stands we measured properties of the tree layer (species richness, stand density, and volume of growing stock), understory (density and species richness of shrubs and tree saplings), and ground vegetation (aboveground biomass, species richness, and species diversity). The tree layer was most diverse though sparse in the natural stands. Understory species richness per 100-m 2 plot was highest in the natural stand, but total stand richness was equal in the natural and alder stands, which were higher than the birch and pine stands. The understory sapling density was lower than 50 saplings/100 m 2 in the plantations, while it varied between 50 and 180 saplings/100 m 2 in the natural stands. Growing stock volume was the least in natural stands and greatest in birch stands. The aboveground biomass of ground vegetation was highest in alder stands and lowest in the pine stands. We can conclude that spontaneous succession promotes establishment of diverse vegetation. In plantations the establishment of diverse ground vegetation depends on planted tree species.
The basic parameters of oil shale quality are heating value and grain-size composition. Heating value can vary considerably within the location in a deposit and depends on concretions and limestone content. Grain-size distribution and heating value depend directly on mining technology: breakage, transporting and processing. Energy distribution when using different technologies was determined. New boilers of oil shale power plants and oil retorts require a relatively constant quality of raw materials and fuel. The possibility of improving oil shale separation was investigated.
The research needed for sustainable mining should be performed in nature, but large-scale tests are complicated. These studies are performed by computer modelling. The main task for modelling is to find suitable criteria and demonstration ways. Modelling is a relatively new approach for planning new mines and analysing abandoned ones. Modelling itself is a convenient way for choosing, selecting and visualising the results, but deciding about optimal modelling methods and software is a complicated task. There are three main tasks of modelling to solve: mining technology, planning and development of mining and impact of mining.
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