Real stacks stack several control devices in series, each removing a fraction (its capture efficiency η) of what reaches it. Because the devices target different pollutants — particulate matter (ESP), SO2 (wet scrubber), NOx (selective catalytic reduction) — the outlet concentration of the combined pollutant load follows:
C_out = C_in · (1 − η_ESP) · (1 − η_scrub) · (1 − η_SCR)
η_ESP ≈ 0.99 η_scrub ≈ 0.90 η_SCR ≈ 0.80 (typical real-world values)
- ESP — electrostatic precipitator: charges soot/ash particles, pulling them onto oppositely-charged plates.
- Scrubber — sprays an alkaline slurry that dissolves sulfur dioxide out of the gas stream.
- SCR — selective catalytic reduction: ammonia + catalyst convert NOx into harmless N2 and water.
- Emission rate — how much raw pollutant the source (combustion) injects into the duct per second.
Toggle any device off to see its particles sail straight through to the stack — this is exactly why regulators mandate continuous emission monitoring (CEMS) rather than trusting installed capacity alone.