Gas combustion unit (GCU)
The forced-draught thermal oxidiser that disposes of surplus boil-off gas on a gas-fuelled ship or gas carrier when engine demand falls below the gas generation rate.
A gas combustion unit is a forced-draught thermal oxidiser that burns surplus boil-off gas and discharges the products through a dedicated uptake. It exists because boil-off is continuous while engine gas demand is not, and because venting methane to atmosphere is not an acceptable disposal route.
The sizing case is the difference between the maximum credible boil-off rate and the minimum engine gas demand, which usually means a port stay, an anchorage, or an extended period on diesel-only operation. A unit sized only for the steady sea-going case will not hold the tank below its relief setting in the condition that actually matters.
The full article will cover the burner and combustion air arrangement, the sizing calculation, the interaction with tank pressure accumulation and reliquefaction as alternative disposal routes, the emissions the unit itself produces, and the class and IGF Code requirements that govern its installation. See LNG fuel system and boil-off gas management .