Materials explained: what are oil shale and oil shale ash?

Oil shale is a black or brown fine-grained sedimentary rock containing kerogen. Oil shale consists of organic matter that has not fully degraded (up to 70%) and various minerals. Organic matter usually consists of kerogen, which is formed from the degradation of algae or bacteria. 

The quantity of oil shale ash generated currently in Estonia per year is 15 times bigger than the quantity of municipal waste.

The use of oil shale generates large quantities of waste products – ash and semi-coke. For example, approximately 5 to 7 million tonnes of ash and a million tonnes of semi-coke is generated in Estonia every year at the current pace and only a very small amount of it is recovered. (Riho Mõtlep: Composition and diagenesis of oil shale industrial solid wastes (dissertation)). 
Oil shale ash, i.e. burnt oil shale, is a mineral material created by the incineration of oil shale. Estonian oil shale contains up to 50% of organic matter. Approximately half of the initial oil shale is left as ash after incineration, which is deposited in ash fields, mostly near the Baltic and Estonian thermal power plants in Ida-Viru County. 

Qualities of oil shale ash

Oil shale ash consists of several ordinary natural materials such as quartz, feldspars, carbonate materials as well as the new materials created during incineration, incl. clinker minerals, which give the ash useful self-cementing qualities. Oil shale ash in itself is not harmful to the environment – it can be used as a fertiliser and for reducing the acidity of soil. Birches and other plants grew on the ash field of the Baltic thermal power plant, which was not used for a long time. The old ash hill, which has been turned into a wind farm, is still covered in greenery. What makes oil shale ash hazardous to the environment is the water that transports it, which is very alkaline. Although the ions found in ash water are mainly the same as those found in nature, the high pH level makes the water hazardous to the environment. However, the water used to transport ash usually circulates and doesn’t escape from the system, but the possibility of a leak always exists. In the case of smaller quantities, however, the damage can be quickly neutralised by natural waters.

Oil shale is widespread as a natural resource, but not as widely used as oil and coal because it’s calorific value and other qualities are not as good. Countries that have large oil shale resources include Estonia, Jordan, the US, Australia, Canada, Brazil and Russia. NB! Very important note is not to confuse oil shale with shale oil, as those are two completely different materials. 

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