Boiler stack temperature rising at constant firing rate: fouled tubes or scale?
Over the last few weeks, I've noticed our boiler's stack temperature has been slowly creeping up, even though we're maintaining the same firing rate and excess oxygen. What could be causing this, and what should I check first to prevent further efficiency drops?
An increasing stack gas temperature at a constant firing rate and excess oxygen concentration is a strong indicator of reduced heat transfer efficiency in the boiler. This is commonly caused by scale or deposits on the waterside of the boiler tubes, which act as insulation. Addressing this issue is crucial to prevent significant fuel loss and increased operating costs.
Likely causes and checks
1Waterside Scale Deposits
- Points to it
- Scale or deposits on the boiler tubes act as insulation, reducing the rate of heat transfer from the hot flue gases to the water. This forces more heat to exit through the stack, increasing flue gas temperature. Fuel losses can range from 1.0% for 1/64" 'Normal' scale to 7.0% for 1/32" 'Iron Plus Silica' scale.
- Check
- Inspect boiler tubes for visible signs of scale or deposits. Review feedwater treatment logs and blowdown practices to identify potential causes of scale formation. Consider a waterside inspection during the next scheduled outage.
2Fireside Fouling (Soot/Ash Deposits)
- Points to it
- Similar to waterside scale, soot or ash deposits on the fireside of the tubes can insulate them, hindering heat transfer. This leads to higher stack temperatures as heat cannot be effectively absorbed by the boiler water.
- Check
- Perform a fireside inspection of the boiler tubes and heat transfer surfaces. Look for accumulation of soot, ash, or other combustion byproducts. Review fuel quality and combustion settings for potential causes of excessive soot formation.
3Incorrect Excess Air/Combustion Issues
- Points to it
- While you mentioned constant excess oxygen, a subtle shift in combustion efficiency could contribute. Too much excess air increases flue gas flow, leading to more heat lost up the stack. Inadequate excess air can lead to unburned combustibles and soot, which can also foul tubes.
- Check
- Verify the accuracy of the oxygen analyzer and recalibrate if necessary. Re-check combustion settings and perform a full combustion analysis to ensure optimal air-to-fuel ratio and minimal unburned combustibles. Compare current readings to baseline or commissioning data.
Next check: Before any internal inspection, isolate and lock out the boiler. For waterside scale, review your feedwater treatment program and blowdown schedule. For fireside fouling, consider a fireside cleaning. Continuously monitor stack temperature and combustion efficiency after any corrective actions.
What it typically looks like in the data
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