Composition of Soot

Soot deposits formed in an exhaust gas boiler mainly consist of:

• Unburnt carbon particles, which are the primary component.
• Ash originating from fuel impurities.
• Sulphur compounds (sulphur oxides) produced during fuel combustion.
• Heavy hydrocarbons, including unburnt fuel residues and traces of lubricating oil.

These substances combine to form a sticky and highly combustible layer on the heating surfaces, especially on the boiler tubes.

Reasons for Soot Formation

1. Incomplete Combustion: Inefficient combustion in the main engine—caused by faulty fuel injectors, incorrect fuel–air ratio, or poor-quality fuel—leads to the production of unburnt carbon and hydrocarbons.
2. Prolonged Low Load Operation: Operating the main engine at low RPM for extended periods results in reduced exhaust gas velocity, allowing soot particles to settle on boiler surfaces instead of being carried away.
3. Poor Maintenance Practices: Failure to carry out regular soot blowing or water washing of the economizer leads to gradual accumulation of deposits.
4. Cold Corrosion Effects: When tube surface temperature drops below the dew point of sulphuric acid, condensation occurs, forming a damp surface that promotes adhesion and accumulation of soot.

Soot fires generally develop progressively through the following stages:

Stage 1: Smouldering Fire

• Soot deposits begin to heat up and glow, burning slowly without visible flames.
• Usually initiated by sparks from the engine or elevated exhaust temperatures.
• Heat is localized and may go unnoticed initially.

Stage 2: Small Visible Fire

• The smouldering spreads and visible flames appear.
• Temperature rises significantly.
• Heat transfer to boiler tubes increases, potentially affecting their material strength.

Stage 3: High-Temperature (Hydrogen/Iron) Fire

• At very high temperatures, especially if water is applied incorrectly, steam may dissociate into hydrogen and oxygen.
• This leads to an intense hydrogen fire (iron-burning fire).
• Such fires can melt boiler tubes and cause severe structural damage or failure.

For Initial Stages (Stage 1 and Stage 2)

• Inform the bridge immediately and reduce or stop the main engine to eliminate the heat and spark source.
• Stop soot blowing operations, as steam can intensify the fire under certain conditions.
• Carry out boundary cooling by applying water externally on the boiler casing to prevent heat spread.
• Seal the boiler by closing dampers and air inlets to cut off oxygen supply and smother the fire.

For Advanced Stage (Stage 3 – High-Temperature Fire)

• Avoid applying small quantities of water, as this can lead to hydrogen formation and possible explosion.
• If the fire becomes uncontrollable:
  • Consider flooding the gas side with a large volume of water (e.g., using a fire hose through a manhole) to rapidly reduce temperature below ignition level.
• Continuously monitor casing temperatures until they return to normal.

Dry running refers to operating the exhaust gas boiler without water circulation in the tubes, typically during emergencies or when the boiler is bypassed.

Before undertaking dry running, the following precautions are essential:

1. Ensure Complete Cleanliness
   • All gas-side heating surfaces must be thoroughly cleaned and free from soot.
   • Any remaining soot may harden (bake) onto the tubes and can later ignite due to absence of cooling.
2. Depressurize the Boiler
   • Drain all water from the boiler completely.
   • Keep vent valves open to ensure the boiler is at atmospheric pressure and to prevent pressure buildup from residual moisture.
3. Control Exhaust Gas Temperature
   • Verify and maintain exhaust gas temperature within manufacturer’s specified limits.
   • Excessive temperature may cause tube overheating, sagging, or deformation.
4. Inform Responsible Personnel
   • Notify the Chief Engineer and, where required, relevant shore authorities before commencing dry operation.