...............  Heavy Fuel Oil (HFO) is a category of fuel oils of a tar-like consistency identified as a "worse case substance".[This quote needs a citation]
Also known as bunker fuel, or residual fuel oil, HFO is the result or remnant from the distillation and cracking process of crude oil.
For this reason, HFO is contaminated with several different compounds including aromatics, sulfur and nitrogen, making emission upon combustion more polluting compared to other fuel oils.[1]
HFO is predominantly used as a fuel source for marine vessel propulsion due to its relatively low cost compared to cleaner fuel sources such as distillates.[2][3]
The use and carriage of HFO on-board vessels presents several environmental concerns, namely the risk of oil spill and the emission of toxic compounds and particulates including black carbon.
Presently, the use of HFOs is banned as a fuel source for ships travelling in the Antarctic as part of the International Maritime Organization's (IMO) International Code for Ships Operating in Polar Waters (Polar Code).[4]
For similar reasons, an HFO ban in Arctic waters is currently underway as IMO committed to the prohibition of the use or carriage of HFO within the next several years.      ...................

What is a ' triple-expansion steam engine ' ?
Seems it has square cylinders?

A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger-volume low-pressure (LP) cylinders. Multiple-expansion engines employ additional cylinders, of progressively lower pressure, to extract further energy from the steam.