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.
In the future, oil shale mining conditions will worsen and environmental taxes will be increased. Higher calorific value and more homogeneous material are required for more effective usage of boilers and generator units in power stations and oil plants. The solution for the mining industry is related to the optimal usage of mineral and technological resources, utilizing the best available techniques and modelling and visualization of mining impacts for explaining the changes in technology to the influenced parties. The main challenges for technologies in the future are related to the mining in environmentally or socially sensitive areas. One of the solutions could be utilizing selective mining and backfilling. Related tests have shown good results and show a promising future for sustainable oil shale mining.
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.
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