Opacity Spike and Air Heater Cold-End Fouling
A 250 MW pulverized-coal unit burning a medium-sulfur bituminous coal shows stack opacity climbing from 8% to 22% over three days. The electrostatic precipitator is at 92% collection efficiency, down from 99%. Flue-gas exit temperature from the regenerative air heater has risen 18 °C, and the air heater draft loss has increased 0.4 kPa. The unit is at 90% load. Ammonia injection for the selective catalytic reduction system is at its normal rate, and the SCR outlet ammonia slip analyzer reads 2 ppm. Coal feed rate per megawatt is unchanged. The operator must decide whether to derate, adjust sootblowing, or investigate the ESP.
1. Assess whether the opacity rise is most likely an ESP problem, a combustion problem, or an air heater problem, and justify with the given numbers.
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The numbers point to the air heater, and the ESP reading is a symptom rather than the root cause. Three indicators move together: air heater exit gas temperature up 18 °C, draft loss up 0.4 kPa, and ESP efficiency down from 99% to 92%. Rising exit gas temperature with rising draft loss is the signature of cold-end fouling or blockage - the gas side is losing heat transfer and gaining restriction. When the air heater fouls, the gas leaving it is hotter and carries more condensable material and carryover, and the ESP inlet conditions degrade.
A genuine ESP electrical failure would not raise air heater exit temperature or draft loss. A combustion problem would show in coal flow per megawatt, excess oxygen, or unburned carbon - the coal feed per megawatt is unchanged and ammonia slip is normal at 2 ppm, arguing against a combustion upset. The unchanged coal rate also argues against a fuel-quality shift, though a coal change could still be checked. The correlated trio is the deciding evidence.
2. Recommend a sequence of actions and state the risk of aggressive sootblowing on a fouled cold end.
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First, confirm the diagnosis before acting: check air heater gas-side and air-side differential pressures across each sector, compare inlet and outlet temperatures on both sides, and look at cold-end metal temperature against the acid dew point. Verify the ESP fields are all energized and rapping is functioning - rule out the simple cause.
If cold-end fouling is confirmed, sootblowing is the tool, but sequence matters. Bring steam sootblowers on in the gas-flow direction, starting at the cold end, and watch draft loss and exit temperature response blower by blower. The risk of aggressive blowing: heavy water/steam impingement on a fouled, possibly acid-wetted cold end can cause corrosion and thermal shock, and dislodged deposits can plug the gas path downstream or overload the ESP with a burst of particulate, tripping it. Blow in short passes with recovery time.
If exit temperature is near the acid dew point, raise cold-end metal temperature by increasing air heater air bypass or reducing air preheat before blowing. If opacity approaches the permit limit, derate until the ESP recovers.