Three common reasons baghouses get specified over ESPs: high-resistivity dust, strict PM2.5 limits, and dry sorbent injection for acid gas control. The filter cake on the bags acts as a secondary reactor, providing increased contact between the flue gas and the dry sorbent.
Pulse-jet baghouses now dominate new installations in cement, steel, and general manufacturing because they allow continuous operation with "online" cleaning and a compact footprint. The trade-off: pulse-jet systems require 80-100 psig compressed air and have a significantly higher pressure drop than ESPs, adding operating cost. For very large gas volumes (500,000+ ACFM), some facilities still choose reverse-air designs where the lower operating cost justifies the larger footprint.
How it works
Unlike older shaker or reverse-air designs, pulse-jet systems collect dust on the outside of the bags and use high-pressure air to clean them while the unit remains online. Some baghouses isolate individual cells for pulsing and cleaning while the upstream equipment stays in operation; this reduces compressed air consumption, extends bag life, and increases removal efficiency while eliminating the potential for final particulate to pass through the filters during the pulsing process.
The cleaning cycle
Filtration: Dirty gas flows on the outside and then through the filter bags (usually supported by wire cages). Dust builds up on the exterior, forming a "dust cake" that provides secondary filtration.
Pulse injection: When differential pressure hits a setpoint, a solenoid valve fires a short burst of compressed air (100-200 ms) through a venturi nozzle.
Shockwave: The burst creates a shockwave that travels down the bag, violently flexing the fabric (30-60 g's of acceleration) to shatter the dust cake.
Discharge: The dislodged dust falls into the hopper below. Crucially, this happens to only a row of bags at a time, allowing filtration to continue uninterrupted.
Common failure modes
Bag blinding - Particles penetrate the fabric matrix or moisture causes the cake to turn into mud. The pressure drop spikes, and pulses can no longer clean the bags.
Acid condensation - If gas temperature drops below the acid dew point, acids (H₂SO₄, HCl) attack the fibers and the filter cages (if not properly specified stainless steel). Nomex and fiberglass are particularly vulnerable to chemical attack.
Spark damage - In combustible dust applications, a single spark can burn holes in bags or trigger an explosion. Spark detection and grounding are critical.
Hopper plugging - Bridging or inadequate evacuation angles (below 55°) cause dust to build up, re-entraining into the clean air stream and damaging bags.
Design considerations
In biomass boiler applications, pulse-jet baghouses commonly run air-to-cloth ratios of 2:1 to 4:1. But the A:C ratio is driven by dust characteristics as much as the cleaning method - fine or sticky particulates still require ratios at the lower end regardless of baghouse type.
A temperature excursion can destroy a set of bags in minutes - which is why properly designed systems include suitable bag and cage materials, temperature monitoring, bypass dampers, and alarm setpoints as standard. Poor compressed air quality (moisture or oil) can blind bags just as fast.
Sources
EPA / Sargent & Lundy, "DSI Cost Development Methodology" (April 2017) - baghouse filter cake yields greater SO₂ removal than ESPs (70-90% vs 40-50%) and lower sorbent injection rates.
EPA, "Chapter 5: Emission Control Technologies" (April 2024) - retrofit DSI systems on units with existing ESPs are provided in combination with a fabric filter.
ICAC White Paper, "Dry Sorbent Injection for Acid Gas Control" (July 2016) - baghouse optimization reduced sorbent consumption by up to 33%.
Babcock & Wilcox, "Dry Sorbent Injection Systems" (PS-451).