Key point: It's all about Kuznetsov's fuel.
For most sailors who served on the Admiral Kuznetzov, Mazut is the stuff of legends. The ultra thick, tarry black substance that powers the ship is known for being rather toxic, sticky, and not easy to get out of clothes. But why did the Soviet navy keep powering its ships with Mazut? What are the advantages and disadvantages of the fuel? Why exactly is the Kuznetsov so smoky?
Not all Russian ships run on Mazut. Of all the large ships the current Russian Navy operates, only the Sovremenny-class destroyers and the Admiral Kuznetsov aircraft carrier run on Mazut. Given the large profile the Admiral Kuznetsov on the global scene—being Russia's only aircraft carrier—naturally, some curiosity has arisen about what it runs on and why it produces so much smoke.
Mazut is a heavy petrochemical fuel. While most sources refer it to being taken from the very bottom of a distillation stack, this is inaccurate as "mazut" is a blanket term for very-heavy oil products, including those that can be formed from blending heavier oils with some slightly lighter ones.
In the West, Mazut would fall into the Bunker B and Bunker C fuel oil classifications, although the ISO 8217 standard has superseded these categories. Under the ISO 8217 standard, Mazut may be classified as RMG or RMK fuel.
These thick, heavy fuels were, by and large, the standard for both military and commercial vessels up until the 1960s and 1970s. Their thick nature gave them a very high volume to energy ratio compared with lighter distillates. But to be burned, they often had to be preheated and pressurized in a complex series of boilers and pipes.
Burning these fuels could also produce large amounts of sulfur, as such heavy minerals tended to settle to the bottom of a distillation stack. As a result, these fuels can be expensive to procure in nations with higher environmental standards as they must be distilled from crude with a low initial sulfur content or removed via a chemical process.