MAZUT is a heavy, low quality fuel oil used in power plants and similar applications.  Mazut is blended or broken down with the end product being diesel.

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Mazut is a fuel oil that is manufactured to GOST specifications.  (GOST is the Russian system of standards much like the ASTM)  This product is typically used for large boilers since the energy value is high.

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The most important factor when grading this fuel is the sulfur content, which can mostly be affected by the source feedstock.  For shipment purposes, this fuel is considered a "dirty Oil" product, and because viscosity drastically affects whether it is able to be pumped.  Mazut is like Number 6 Oil (Bunker C), and is part of the products left over after gasoline and lighter components are evaporated from the crude oil.