How black liquor chemical recovery boilers work
A kraft or soda recovery boiler burns black liquor, the spent cooking liquor from pulping, concentrated in evaporators to typically 65 to 80% solids. The liquor is sprayed into the lower furnace, where it dries and forms a char bed on the hearth. The organic part burns, and the inorganic sodium and sulphur compounds leave as molten smelt through spouts to the dissolving tank. The flue gas carries sodium sulphate and carbonate fume, with chlorides and potassium, which deposits on the superheater and boiler bank and is removed by sootblowers. An ESP recovers this salt cake for return to the liquor. Air enters at primary, secondary and tertiary levels from separate FD fans, and an ID fan draws the gas through the ESP to the stack.
Where the ID fan sits
- Furnace and char bed
- Superheater
- Boiler bank
- Economiser
- ESP
- ID fan
- Stack
The fan sits after the ESP, which removes most of the salt cake fume. The fan still sees fine sodium salts, which are hygroscopic and stick to cool surfaces. Suction covers furnace draft, the heavily fouled superheater and boiler bank, the economiser and the ESP, typically 200 to 350 mmWC, rising as deposits build between sootblowing cycles and water washes. Large boilers use two ID fans in parallel so one can be maintained without a full shutdown.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the economiser and ESP |
| Maximum or upset | Up to 250°C with heavy fouling before a water wash |
| Dust at the fan | Fine sodium sulphate and carbonate fume; low after a working ESP, high if a field trips |
| Moisture and dew point | High moisture from liquor water; salt cake absorbs moisture at shutdown |
| Corrosive species | Chlorides, SO₂ and TRS compounds; corrosive in wet salt deposits |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 black liquor chemical recovery boilers
| Blade form | Backward curved with smooth blades that shed salt cake |
| Material of construction | Mild steel casing, insulated; corrosion-resistant steel for the impeller where deposits stay wet at shutdown |
| Drive and arrangement | Direct coupled, arrangement 8; two fans in parallel on large boilers |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft, inlet damper for isolation |
| Also specified | Wash nozzles, inspection door, casing drain and insulation |
What this ID fan has to cope with
Salt cake deposits and unbalance
Sodium salts that pass the ESP stick to the blades, especially where the gas is near its moisture saturation at low load. Deposits grow unevenly and break away, causing vibration. Smooth blade forms, wash nozzles and insulation keep the build-up under control between shutdowns.
Corrosion of deposits at shutdown
When the boiler stops, the salt cake on the fan absorbs moisture from the air and forms a corrosive, chloride-bearing paste. Washing and drying the fan after shutdown and keeping the casing closed prevent pitting of the impeller and casing.
Draft during an emergency drain
If a tube leak lets water into the smelt bed, the boiler goes through an emergency shutdown and rapid drain. The ID fan must keep the furnace under suction to vent steam and combustion gases safely, so its power supply and controls are part of the boiler safety system.
Rising resistance between washes
Fouling of the superheater and boiler bank raises suction steadily between water washes, and the gas temperature at the fan rises with it. The fan is selected with margin for the fouled condition at full liquor firing, so the boiler does not have to cut load as the run progresses and the mill does not lose pulp production.
Energy and control
Recovery boilers run at a load set by liquor solids flow, which is steady but rises as the mill increases production. A variable speed drive on furnace draft gives savings at reduced firing and during start-up on auxiliary fuel. Efficient sootblowing and regular water washing keep the boiler resistance and gas temperature down, which reduces ID fan power more than any change to the fan.
ID fan and FD fan in black liquor chemical recovery boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the stack | Before the steam coil air heaters, one fan per air level |
| What it handles | Flue gas with sodium salt fume | Ambient or boiler house air |
| Temperature at the fan | 160 to 200°C | Ambient |
| Main design concern | Salt cake deposits and draft reliability | Air split and port pressure |
| Control | Furnace draft | Air flow at each level |
See also the Forced Draft (FD) Fan for black liquor chemical recovery boilers.
Questions buyers ask
What dust does a recovery boiler ID fan handle?
Fine sodium sulphate and sodium carbonate fume, with some chlorides and potassium. A working ESP removes most of it, but what reaches the fan is sticky and hygroscopic, so deposits and corrosion are the main concerns rather than erosion. If an ESP field trips, the load rises sharply and the fan needs washing sooner.
Why do recovery boilers use two ID fans?
Two fans in parallel let the boiler keep running at reduced load if one fan is out for washing or repair, and they give redundancy for the furnace draft, which is a safety function on a recovery boiler. The fans are checked for stable parallel running at low load so that neither stalls during start-up on auxiliary fuel.
How is the ID fan cleaned on a black liquor recovery boiler?
Usually with wash nozzles in the inlet box and casing, used during planned shutdowns with the fan isolated and turning slowly. Washing removes the salt cake before it absorbs moisture and becomes corrosive. The casing is drained and the fan is dried with warm air before restart, then checked for vibration.
Information needed to quote an ID fan for black liquor chemical recovery boilers
- 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.
- Liquor and firing. Dry solids firing rate, liquor solids content, and whether the liquor is wood or agro-based.
- ESP and fouling. ESP outlet dust and the draft profile between water washes.
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.