Advantages and disadvantages of an inert-gas smothering system from flue gases compared to an inert-gas generator, why the IG generator is more advantageous, and why it is unsuitable in machinery spaces.

(1) Inert gas from flue gas (exhaust gas inert gas system EGIS):

The exhaust gas from the main engine or boilers is scrubbed (cooled, cleaned of soot and SO2 by a seawater scrubber) to produce inert gas (approx 3-4% O2, remainder N2 and CO2) which is supplied to the tanks/cargo spaces.

Advantages:

• Low initial/running cost: uses the otherwise wasted exhaust gas; cheap operating gas.
• Simple and uses existing engine/auxiliary boiler exhaust.
• No extra fuel burnt for gas production; lower running cost during normal operation.

Disadvantages:

• Only available when the boiler/engine is running at sufficiently high load; limited when the engine is shut down or at low load (e.g. in port) - hence auxiliary supply needed.
• Requires a dedicated scrubber/cooler and the correct combustion and supply; if the exhaust contains excessive oxygen or unburnt gases, the gas may not meet the inert quality.
• Quick response in emergencies (e.g. power loss) is limited.
• The flue gas is only "inert" if thoroughly scrubbed to remove SO2/SOx and particulates; the system is more complex with the scrubber and water treatment.
• On ships where the boiler load fluctuates, quality/temperature control is harder.

(2) Inert gas generator (IGG):

Produces inert gas by burning diesel (a dedicated combustion chamber) with controlled air so that the flue product (mainly N2 and CO2, low O2) is cooled and scrubbed.

Advantages:

• Available on demand independent of the main engine/boiler; can be started at any time including in port for tank cleaning, gas freeing support and COW.
• Better quality control of the inert gas (constant low O2 around 2-5%, can be controlled) with the capability to produce gas to the required oxygen content.
• Quick response and a reserve; can be used flexibly.
• Produces gas to a more reliable, consistent composition for safety of tank operations.

Disadvantages:

• Higher capital cost and running cost (burns its own fuel).
• More equipment (combustion unit, blower, scrubber, control) requiring maintenance and spares.
• Need a reliable fuel supply and cooling.

Why the IG generator is more advantageous:

• Independence and availability (particularly when the main engine or boiler cannot supply or is shut down, e.g. in port during discharge/COW/tank cleaning, or during emergencies), consistent gas quality, responsiveness, and suitability for critical tank inerting/safety operations. On a crude tanker, the inert gas system is essential for COW and safe cargo operations and must be available when needed, hence the dedicated generator is the more dependable choice.

Why unsuitable for use in machinery spaces:

• Inert gas (especially flue-gas inert or generator inert) contains CO2 and low oxygen; introducing it into an occupied/flame-spread control machinery space would asphyxiate personnel. Inert gas does not come "gentle"; it displaces oxygen, so anyone working in an engine room would quickly suffocate. It is therefore not a suitable fire-extinguishing medium for normally occupied spaces such as machinery spaces where people may be present (unlike CO2 total-flood or foam which, though also dangerous, are controlled release with alarms).
• Also, the machinery space is an enclosed occupied volume that needs normal venting/combustion; an inert-gas supply is not a designed extinguishing system there, and the flue gas would contain corrosive SO2 and be hot; and it does not comply with the fixed fire-extinguishing standards prescribed by SOLAS for machinery spaces (which require a gas/foam/water-mist system designed for that space). Inert gas is primarily intended for inerting cargo tanks and void/AFR spaces, not machinery spaces.