How pickling acid regeneration plants work
Steel strip and wire pickling lines use hydrochloric acid, and the spent liquor, mainly ferrous chloride, is regenerated rather than neutralised. In a spray roaster the liquor is sprayed into a tall reactor fired by tangential burners on natural gas, LPG or oil; the droplets roast at about 400 to 700°C to iron oxide powder and HCl gas. Fluidised bed roasters do the same in a hot bed of oxide granules. The roaster gas, leaving at about 350 to 450°C, passes a cyclone and a venturi preconcentrator, where it evaporates water from incoming liquor and cools to about 90 to 100°C. HCl is then absorbed in water in an absorption column to give regenerated acid, typically around 18%, and the remaining gas is cleaned in scrubbers before the exhaust fan and stack.
Where the FD fan sits
- Inlet filter
- FD fan
- Air main
- Tangential burners or fluidising grid
- Roaster
The fan sits at the base of the roaster and supplies air through a ring main to the tangential burners, which fire into the lower part of the reactor so that the hot gas swirls up against the falling spray. Pressure is typically 300 to 700 mmWC. On fluidised bed roasters the fan also supplies fluidising air through the grid, which raises the pressure. The fan handles ambient air only, but the intake must be kept away from HCl fumes in the plant.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient |
| Dust and fumes | Possible HCl vapour if the intake is near the plant |
| Pressure at the fan outlet | Typically 300 to 700 mmWC; higher for fluid bed roasters |
| Load pattern | Follows liquor feed rate |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for pickling acid regeneration plants
| Blade form | Backward curved |
| Material of construction | Mild steel with acid-resistant coating |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on burner firing rate |
| Also specified | Intake ducted from clean air, inlet filter and low air pressure trip |
What this FD fan has to cope with
Acid fumes at the intake
Regeneration plants and the pickling lines beside them handle HCl, and any leak or open tank makes the surrounding air corrosive. A fan drawing that air corrodes its impeller and casing and sends acid vapour to the burner parts. Ducting the intake from a clean area, and coating the fan, protects it for a long life.
Burner stability with varying feed
Roaster temperature depends on liquor feed rate and its iron and free acid content, which vary with the pickling line. Firing changes to hold the roaster top temperature. The fan must follow these changes smoothly so the burners stay stable, the oxide is fully roasted and the residual chloride in the oxide stays within specification.
Fluidising air on fluid bed roasters
A fluid bed roaster needs a minimum air flow to keep the bed fluidised, whatever the liquor feed. The fan must not turn down below this, and its pressure must cover the grid and the bed, which grows in weight as oxide granules build up. A minimum speed limit and a low flow alarm protect the bed from slumping.
Energy and control
The roaster runs continuously and the combustion air fan follows liquor feed, which in turn follows the pickling line. Speed control saves power when feed is reduced and gives better ratio control than a damper. Correct excess air also matters for roaster efficiency and oxide quality, and it reduces the volume of gas the exhaust fan, absorber and scrubbers must handle.
ID fan and FD fan in pickling acid regeneration plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the absorption column | Feeding the roaster burners |
| What it handles | Saturated gas with HCl traces | Ambient air from a clean intake |
| Temperature at the fan | 70 to 90°C, saturated | Ambient |
| Main design concern | Acid corrosion and lining temperature limits | Burner stability and clean intake |
| Control | Roaster pressure | Burner firing rate |
See also the Induced Draft (ID) Fan for pickling acid regeneration plants.
Questions buyers ask
What is the FD fan on an acid regeneration plant?
It is the forced draft or combustion air fan for the roaster burners. On a spray roaster it supplies the tangential burners at the base of the reactor; on a fluidised bed roaster it also supplies the fluidising air through the grid. It handles ambient air, while the HCl-bearing roaster gas is handled by the exhaust fan after the absorber.
Does the acid regeneration FD fan need acid protection?
The fan itself handles air, not acid gas, so it is normally mild steel. But the atmosphere around a regeneration plant and pickling line can contain HCl vapour. A protective coating on the fan and an intake ducted from a clean area are recommended to avoid corrosion of the impeller and casing.
How much pressure does a roaster combustion air fan need?
Spray roaster burners typically need 300 to 700 mmWC at the fan, covering the burner, the air main and the roaster pressure. Fluidised bed roasters need more, because the air must also pass through the distribution grid and support the bed. The burner and roaster supplier's data sets the final figure.
Information needed to quote an FD fan for pickling acid regeneration plants
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burners. Number of burners, firing rate and air pressure needed.
- Roaster type. Spray or fluidised bed, and any fluidising air requirement.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.