Opacity (stack)

Opacity is the percentage of light blocked by particulate matter in a stack gas stream. Stack opacity monitors use a light source and detector across the stack path to measure how much light is transmitted. Higher opacity usually means more visible particulate, although particle size, colour, moisture and plume conditions also affect appearance.

Opacity is widely used as an operational indicator for ESP and baghouse performance. It is not the same as mass concentration, but excursions often trigger investigation because they may indicate particulate breakthrough, re-entrainment or combustion upset.

Opacity excursions

Common causes include:

• ESP rapping puffs and re-entrainment;
• hopper high level or poor ash evacuation;
• back-corona or poor ESP electrical performance;
• bag leaks, torn bags or failed tube-sheet seals;
• sootblower events that release deposits downstream;
• combustion upsets that increase unburned carbon or fine particulate;
• wet plume effects or monitor contamination.

The time pattern matters. A sharp spike during rapping points to re-entrainment. A sustained rise may indicate collector underperformance, high inlet loading or a failed bag. A noisy signal may indicate monitor alignment or purge-air problems.

Measurement context

Opacity limits and averaging periods vary by jurisdiction and permit. Older permits may use visible-emission rules; newer systems may rely more on particulate CEMS or stack testing. Operators should interpret opacity with load, fuel, collector power, hopper level, baghouse differential pressure and stack moisture.

How sonic horns reduce opacity

Sonic horns can reduce opacity excursions when they replace or supplement aggressive intermittent cleaning. Instead of allowing dust to accumulate and then releasing it in large puffs, acoustic cleaning encourages more frequent, lower-intensity release. In ESPs, horns may also help keep distribution screens, plate lower regions and hoppers cleaner, reducing re-entrainment sources.

Acoustic cleaning does not fix a torn bag, failed ESP power supply, severe combustion upset or undersized collector. It supports stable particulate-control performance where fouling and deposit release are the limiting mechanisms.

Measurement and troubleshooting detail

Opacity is a visibility-based particulate indicator, so it responds to particle concentration, particle size, moisture plume effects, path length and stack lighting conditions. It is not the same as a mass concentration measurement, although plants often use it as an operating alarm or compliance-related indicator. Continuous opacity monitors must be kept aligned, clean and calibrated, and their readings should be interpreted with process context.

Common causes of opacity excursions include ESP field trips, bag leaks, poor combustion, sootblowing puffs, hopper re-entrainment, wet plumes, startup conditions, high ash loading and rapid load changes. Troubleshooting should compare opacity with ESP power, baghouse differential pressure, fan load, oxygen, CO, hopper levels, rapping events and cleaning cycles. A short spike after a cleaning event has a different meaning from a sustained high-opacity condition.

Acoustic cleaning can reduce opacity when it prevents large intermittent dust releases or keeps collecting surfaces and hoppers from becoming secondary dust sources. It can also be mistuned. If a horn fires into a full hopper or during an unstable ESP condition, it may contribute to a visible puff. Commissioning should therefore tune sequence timing against collector operation and ash discharge.

Related terms

• CEMS
• Electrostatic precipitator
• Re-entrainment
• Sonic horn