What are the various stages of fire in the exhaust gas boiler? Discuss the factors responsible for ignition and development of ignition of soot, small soot fires and high temperature fires.

An exhaust gas boiler (economizer) is subject to soot fires (fires of accumulated soot and other particles in the gas passages) during operation, especially when the boiler is not producing steam/water vaporating normally (low load, soot build-up). The "stages" of an exhaust gas boiler fire refer to the progression of soot build-up and the resulting fire:

Stages:

1. Soot accumulation/normal sooting - during normal operation the exhaust carries soot, carbon and unburnt hydrocarbons; a layer accumulates on the tube surfaces if the boiler is run lean/fast or if the engine is at low load, or if combustion is imperfect (poor atomisation, low load/idling) - soot deposits grow.
2. Drying/ignition of soot - as soot builds up and the exhaust gas temperature rises (or when the boiler/engine resumes high load), the deposit becomes dry and can be raised to ignition temperature. The soot begins to glow/smoulder when gas temp is high enough (approx 400-600 C or higher at the tube/hot spots).
3. Small soot fires (smouldering/ localised ignition) - local spots of the soot catch fire; the fire is confined to the soot on the tube (a slow smoulder) often at the hot end of the boiler or where the deposit is thick; it produces smoke/heat and burnt deposits.
4. High-temperature fires / major soot fire - if not controlled, the fire develops into a runaway concentrated fire: the soot defect ignites over the whole surface, the temperature rises (can exceed 800-1000 C) possibly localising at the tube bank bases, and can cause overheating of the boiler tubes, burn-through, distortion, and collapse; the risk of an uncontrolled high-temperature fire and consequential boiler/engine damage.

Factors responsible:

• Sooting: incomplete combustion (poor fuel injection, wrong air/fuel, low load/slow-speed, poor atomisation, dirty fuel/water in fuel, leaking injectors), soot and carbon unburnt accumulating on the boiler tubes; a thick heavily insulating soot layer.
• Low boiler load / opening the bypass or over-firing causing the gas to be too hot; or running with the boiler unable to take the heat (e.g. no steam demand, boiler on "temp", low water) so deposits overheat.
• Ignition: the soot/gas mixture which, when the boiler receives a surge of hot gas (engine load increase) or is exposed to a high-temperature/hot spot (e.g. if a flame impinges on an economizer when burning a separate burner), is raised above the ignition temperature of the soot (autoignition of the carbon/deposit starts at ~350-600 C). Soot is a fuel - a carbon-rich deposit that ignites when hot.
• Development: once ignited, the fire is self-sustaining as long as there is soot, fuel and sufficient temperature; the large surface area of the deposits on the tubes releases a lot of heat; if the boiler/gas flow continues (the engine continuing) the fire peaks; high-temperature fires happen when the soot layer is thick and the local temperature exceeds the melting/deformation temperature of the metal, causing tube collapse/burn-through.
• Contributing: prolonged operation at low load with incomplete combustion, dirty fuel, excessive boiler fouling, and not having cleared soot (e.g. no regular economizer cleaning / poor soot-blowing), or a malfunctioning bypass/control which defeats the boiler cooling, so the tube face gets dangerously hot.

Control/prevention: regular monitoring of boiler/gas outlet temperature (exhaust gas boiler fire can be detected by a sudden rise), soot blowing/cleaning, maintaining proper fuel atomisation and combustion, ensuring correct water/steam pressure (so the boiler absorbs heat), avoiding low-load long-running, cleaning, and having a fire detection/alarm; immediate action - isolate the boiler/engine, reduce/stop the gas flow, apply water/use the fixed economizer firefighting by injecting water mist/CO2? (the normal method is to isolate and damp to starve the fire, and to use the boiler's water washing/ventilation); uncontrolled soot fires can cause serious damage demanding boiler tube replacement.