In the case of Yara Birkeland there is no engines or fuel cells that generate power, only batteries that is charged at port calls.
Power source is hydroelectric, thus no emission during passage, or from the power generation.
I know, the ship is made of steel and machinery was used to build the dams, drive the tunnels and/or produce the pipes that carry the water to the turbines. (Back to wooden water wheels to make it totally emission free)
BTW; what is it these ship engineers are going to do on board a ship with no combustion engines, boilers, living quarters or life saving appliances?