How soda ash calciners work
In the Solvay and dual process, crude sodium bicarbonate from the carbonating towers is calcined at about 160 to 200°C to light soda ash, releasing CO₂, water and ammonia. Large plants use steam-heated rotary calciners whose off-gas is recovered by the CO₂ compressors rather than any fan. Fired calciners, heated by coal, oil or gas burned in a furnace around the shell or in a hot gas generator, remain common on smaller lines and on the dense soda ash route, where light ash is hydrated to monohydrate and dried in a fired rotary or fluid bed dryer at 150 to 250°C. In these fired units an FD fan supplies combustion air, and an ID fan draws the flue gas, or the dryer exhaust carrying soda ash dust, through cyclones and a bag filter.
Where the ID fan sits
- Fired calciner or dense soda dryer
- Dropout box
- Cyclones
- Bag filter
- ID fan
- Stack
On direct-fired dryers and calciners the fan sits after the bag filter, handling the combined combustion gas and water vapour driven off the product at 120 to 180°C. On calciners heated through the shell, the furnace flue gas has its own ID fan, while the process gas goes to CO₂ recovery. Suction at the fan covers the dryer or calciner, cyclones and bag filter, typically 250 to 400 mmWC, and the fan holds the feed end at a slight suction.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the bag filter |
| Maximum or upset | Up to 250°C if feed is lost while burners fire |
| Dust at the fan | Fine soda ash, low after the bag filter, high if a bag fails |
| Moisture and dew point | High water vapour from hydration water; dew point about 60 to 75°C |
| Corrosive species | Alkaline dust that forms a hard crust with moisture; traces of ammonia from Solvay crude bicarbonate |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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. |
ID Fan selection for soda ash calciners
| Blade form | Backward inclined flat blades, which shed alkaline dust better than curved profiles |
| Material of construction | Mild steel with wear liners on the blade leading edges; SS 304 for wetted parts where condensation is likely |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Shaft cooling disc for the higher upset temperature; standard bearings |
| Flow control | Variable speed drive on dryer outlet suction |
| Also specified | Insulated casing, inspection doors and wash connections for removing soda ash crusts |
What this ID fan has to cope with
Caking at shutdown
When the system cools with moist gas inside, soda ash absorbs water and sets hard on the blades and casing, like a weak cement. On restart, uneven crust gives severe vibration. Purging with hot dry gas before stopping and insulated ducts and casing avoid it.
Erosion from bag failure
Soda ash is not very hard, but at high load after a bag failure it erodes blade leading edges and the inlet cone. Wear liners at those points and prompt bag leak detection keep the fan in service.
Corrosion under deposits
Wet sodium carbonate under a crust is strongly alkaline and attacks mild steel slowly, while chlorides carried from brine cause pitting. Stainless steel in wetted zones and regular cleaning prevent through-wall corrosion of the casing bottom.
Energy and control
Soda ash plants run continuously with output set by the carbonating section. Calciner and dryer gas volume follows production and fuel firing. A variable speed drive on dryer suction cuts fan power at partial output and limits air drawn in at seals, which would add to the fuel needed to evaporate the product water.
ID fan and FD fan in soda ash calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Before the hot gas generator or burner |
| What it handles | Exhaust gas with water vapour and soda ash dust | Ambient air |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | Caking of hygroscopic alkaline dust | Steady combustion and dilution air |
| Control | Calciner or dryer suction | Firing rate |
See also the Forced Draft (FD) Fan for soda ash calciners.
Questions buyers ask
Does a steam-heated soda ash calciner have an ID fan?
Usually not on the process side. The CO₂-rich off-gas from a steam calciner goes to coolers, scrubbers and CO₂ compressors. ID fans are found on fired calciners, dense soda dryers and the boilers supplying steam.
Why does soda ash build up on an ID fan impeller?
Soda ash is hygroscopic. If the gas cools near its dew point, or the fan stands with moist gas inside, the dust absorbs water and sets hard. Insulation, hot purging before shutdown and blade forms without pockets reduce build-up.
What fan is used for a dense soda ash dryer?
A centrifugal ID fan after the bag filter, usually backward inclined, sized for the combined combustion gas and evaporated water at 120 to 180°C, with an FD fan supplying combustion air to the dryer furnace.
Information needed to quote an ID fan for soda ash calciners
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Calciner or dryer type. Direct-fired, shell-heated or fluid bed, and whether the fan handles flue gas or process gas.
- Water load. Product moisture in and out, so the water vapour in the exhaust can be calculated.
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.