Soot Fire is a Exhaust Gas Boiler fire. Today’s high efficiency engine reduced the specific fuel oil consumption and increased thermal efficiency of engine and thereby lower exhaust gas temperature. Generally, exhaust gas temperature after the turbocharger is about 250 – 280 degree C.
Soot blow: Air is preferable than steam, because steam can cause hydrogen fire.
Diesel Engine is blamed for soot fire, because soot particles originate from the combustion of fuel. Practically, it’s impossible to avoid soot particles from diesel engine which is running on HFO.
Soot Fire is determined by three factors i.e. Soot deposits, Oxygen & Ignition. This is also called as warning triangle for soot fire. Oxygen is sufficient in exhaust gas, the main focus for fire is soot deposits and ignition source.
Generally, there are two types of boiler for exhaust gas recovery system i.e. Water Tube & Smoke Tube, but Water Tube Boiler is mostly used at ship as EGB.
Reasons for Soot Fire
Economizer cleaning is not done for long time or regular soot blowing is not carried out
Fuel quality is bad
Operating Engine on Low Load for prolonged time. (Velocity of gas is low, so it stucks in tubes.)
Water is not properly maintained
Fuel pump timing is not correct
Excess cylinder oil
Fuel injector leaks
Accumulation of soot (wet soot)
Soot Fire is categorized in different stages instead of different types according to the intensity of fire.
Stage I: Ignition of Soot
Stage II: Small Soot Fire
Stage III: High Temperature Fire
There are two types of soot i.e. Dry soot & Wet soot. Wet soot needs less ignition temperature for fire and Dry soot needs higher temperature (300-400) for ignition of fire.
Small soot fire mostly occurs during manoeuvering and low engine load.
High temperature fire is the result of small soot fire by chemical reactions. Hydrogen gas and carbon mono-oxide gas combusts and results into hydrogen fire. At high temperature, iron tubes reacts with oxygen and burns.
Hydrogen fire: When water dissociates into hydrogen and oxygen or, in connection with carbon, into carbon mono-oxide and hydrogen then at certain temperature (1000 degree C), fire starts.
Iron fire: It’s an oxidation reaction of iron tubes at very high temperature (1100 degree C). Use of steam for soot blowing can enhance the reaction with water molecules also.
Soot Fire Minimization
Exhaust gas velocity in the boiler should not be too low, as good velocity reduces the soot deposits.
Exhaust gas temperature on the boiler tube surfaces must not be too low. Because sufficient temperature reduces the risk of soot deposits due to the formation of wet soot.
Engine smoke emission should be of maintained quality, because poor emmision increases the soot deposits.
Circulation of water flow velocity and ratio in the boiler must not be too low as it keeps the gas temperature at the boundary layer of the boiler tubes below the ignition temperature of the soot.
Also, Water circulating pump should be started early and stopped later during Main Engine operation.
Action to prevent fire
First analyze the intensity of the fire. If it’s of stage I or stage II, then:
Stop the Main Engine for stopping the oxygen supply.
Continue the operation of water circulating pump.
Don’t use the soot blower during fire-fighting. Because soot blowing by air can supply the sufficient air for fire and soot blowing by steam can lead high temperature fire.
Shut the air circulation in main engine.
If there is water washing system, then use itfor fire fighting.
If fire is of high temperature type, then follow:
Stop Main Engine and Water circulating pump.
Close all water circulation valves.
Cool the surroundings and source of fire with sufficient amount of water.