EAF Fume Extraction and Baghouse Design: Handling 800,000 Nm3/h of Dirty Gas
Pull up to an operating EAF shop and the first thing you notice after the noise is the ductwork. Massive steel pipes, water-cooled where they pass close to the furnace, climbing up the side of the building, tying into a main header that runs to the baghouse. Inside those ducts, the gas is dirty. Really dirty. Temperatures over 200 degrees C, dust loadings of 5 to 15 grams per cubic meter, particle sizes from sub-micron fume to coarse iron oxide particles. The baghouse has to clean that gas to under 5 mg/Nm3 before discharging to the stack. That is a separation efficiency north of 99.99 percent. Get the design wrong and you are either out of compliance, or you are paying a fortune in fan power and bag replacement.
Here is how EAF fume systems actually get designed and operated.
Start with the two-stream concept.
EAF fume has two distinct sources. Primary fume comes directly from the furnace - the gas drawn through the fourth hole, the slag door, or the elbow gap above the taphole. This gas is hot (200 to 800 degrees C depending on the operation and capture point), heavily loaded with metal oxide dust and CO, and intermittent - it spikes during charging, melting, and oxygen blowing, and drops during flat bath. Secondary fume is everything else: the canopy hood above the furnace, the area around the slag pit, the tapping aisle, the ladle transfer point. Secondary fume is cooler (50 to 100 degrees C typically) and more dilute, but it is continuous throughout the heat.
Most EAF shops capture both streams into a single baghouse. The ductwork combines primary and secondary, and the baghouse handles the combined gas. The alternative - separate primary and secondary baghouses - is more common in integrated mills and gives better control of the primary stream, but it costs more capital.
Primary fume capture is a design decision with real trade-offs.
The fourth-hole evacuation system - a water-cooled duct connected to the delta of the EAF roof, drawing gas directly from the furnace - gives the best capture of primary fume. The furnace runs under negative pressure, fume stays inside the system, and the canopy hood does not have to handle as much. The downside: a fourth-hole evacuation system is intrusive. It ties into the roof, the elbow, and the water cooling network, and any leak pulls in air that cools the gas below the acid dew point. Cold gas plus moisture plus zinc and lead from galvanized scrap means white rust inside the ductwork and rapid corrosion.
Direct shell evacuation (DSE) is the alternative for newer EAFs. The shell itself has water-cooled ducts built into the sidewall, drawing gas from the slag line and the upper shell through dedicated takeoffs. DSE gives even better capture than the fourth hole and is more controllable. The capital cost is higher.
Canopy hoods are the fallback for shops that cannot or do not want to install primary capture. A canopy hood above the furnace, typically 6 to 10 meters above the charging floor, draws fume from the whole area. It is less efficient at capturing fume at the source, but it is simple. The trade-off is large airflow - 600,000 to 1,000,000 Nm3/h for a 60-ton furnace with canopy-only capture - and the corresponding fan power.
A modern EAF with primary capture typically runs total airflow of 400,000 to 600,000 Nm3/h. The difference is huge in operating cost. A 0.5 m3/s difference in fan flow is roughly 50 kW of fan power, and an EAF baghouse fan can be 1000 to 2500 kW. Over a year, the difference between a well-designed primary system and a canopy-only system is millions of kilowatt-hours.
The gas cleaning train has several stages.
First, gas cooling. Hot gas from the primary system enters a water-cooled duct or a forced-draft heat exchanger that drops the temperature to 200 to 250 degrees C. Above 260 degrees C, most filter bags degrade. Below 100 degrees C, you risk condensation and acid corrosion. The cooling system must handle the spikes - the gas temperature during charging can be 600 degrees C or more.
Second, coarse dust removal. A spark arrestor or pre-separator drops out the large particles - typically 100 microns and above - before they hit the filter bags. The pre-separator is a simple device: a cyclone, a settling chamber, or a spark arrestor screen. Without it, the bags load up with coarse dust and the pressure drop spikes.
Third, fabric filtration. The baghouse itself. Filter bags are typically 130 to 160 mm in diameter, 6 to 8 meters long, made of polyester, acrylic, aramid (Nomex), PPS (polyphenylene sulfide), or PTFE, depending on temperature and chemistry. For EAF gas, aramid or PPS is common. Bags are arranged in rows of 10 to 20, with each row on a separate compartment with its own damper. The compartments are taken offline one at a time for bag cleaning or replacement, while the others continue to filter.
Bag cleaning uses pulses of compressed air - typically 5 to 7 bar - injected into the top of the bag. The pulse creates a shock wave that dislodges the dust cake on the outside of the bag, sending it to the hopper below. The dust falls to a screw conveyor or an airlock and is conveyed to a dust silo for disposal or recycling.
The pressure drop across a clean baghouse is typically 1.0 to 1.5 kPa. As the dust cake builds, the pressure drop rises. When it hits 2.0 to 2.5 kPa, the compartment cycles to cleaning mode. With proper cleaning, the pressure drop oscillates between clean and dirty values indefinitely.
Bag life depends on the gas chemistry and temperature. EAF fume contains HCl, SO2, NOx, and various metal oxides. Polyester bags handle these chemicals poorly above 130 degrees C. Aramid handles 200 degrees C but degrades in acid conditions. PPS handles 190 degrees C and resists most EAF fume chemistry. PTFE handles 260 degrees C and is the most chemical-resistant, but it is the most expensive. A typical EAF baghouse uses PPS or aramid bags, with 2 to 4 years of service life. Aggressive fume chemistry can cut that in half.
The induced draft (ID) fan is the heart of the system.
The ID fan pulls the gas through the entire train - hood, ductwork, cooler, pre-separator, baghouse, stack. The fan motor is typically 800 to 2500 kW, depending on the airflow. Variable frequency drives (VFDs) on the ID fan are now standard. They let the operator modulate the airflow based on the heat cycle - higher flow during charging and oxygen blowing, lower flow during flat bath and tapping.
A 60-ton EAF with VFD-controlled airflow typically runs 200,000 to 300,000 Nm3/h during flat bath and 500,000 to 700,000 Nm3/h during charging. The fan power scales with the cube of flow, so a 50 percent reduction in flow during flat bath cuts fan power by 87 percent. The savings are substantial - 500 to 1500 kW of fan power for several hours a day.
Emission limits are tightening worldwide.
The European Union BREF for ferrous metals processing sets a 5 mg/Nm3 dust limit for new EAF baghouses. China, North America, and most other jurisdictions have similar or stricter limits. Older baghouses that were designed for 20 to 50 mg/Nm3 are being upgraded. The upgrade options are: replace bags with finer-micron rated media, add a polishing filter stage after the main baghouse, or use electrostatic precipitators (ESPs) as a pre-cleaner ahead of the baghouse.
Author: MONTE INTELLIGENCE environmental systems team. For EAF fume system audits and baghouse upgrades, contact helenxu@cnlymonte.com.
