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 FD fan sits
- Inlet screen
- FD fans (primary, secondary, tertiary air)
- Steam coil air heaters
- Windboxes
- Air ports
- Furnace
Each fan draws ambient air, usually from inside the boiler building, and pushes it through a steam coil air heater that raises it to 120 to 200°C, then into windboxes around the furnace. Primary air ports are just above the hearth around the char bed; secondary air ports are above them; tertiary air ports are above the liquor guns. Port pressure increases with height, from about 100 to 200 mmWC for primary air to several hundred for tertiary air, which must penetrate the furnace.
Air at the FD fan
| Air temperature at the fan | Ambient or boiler house air, 30 to 50°C |
| Air after the heater | 120 to 200°C, downstream of the fan |
| Dust | Boiler house air with some salt cake dust |
| Pressure at the fan outlet | About 150 to 250 mmWC for primary air, higher for secondary and tertiary air |
| Air split | Typically about a third primary, the rest secondary and tertiary, depending on the boiler design |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 black liquor chemical recovery boilers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled; one fan per air level |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on each fan, with port dampers |
| Also specified | Inlet screens, air flow measurement, silencers |
What this FD fan has to cope with
Primary air port plugging
Smelt and char freeze around primary air ports and must be rodded, often by automatic port rodders. Between rodding cycles the ports partly close, resistance rises and air flow falls. A fan with a steep curve, controlled on windbox pressure, keeps the air flow steady and the char bed shape stable while the ports are cleared.
Keeping the air split right
The split between air levels controls bed temperature, smelt reduction efficiency and TRS emissions. A fan without enough pressure at higher firing rates lets its level fall short, which distorts the split and can cool the bed. Each fan is selected for its own port pressure with margin for future increases in liquor solids firing.
Salt cake in the intake
Boiler house air carries fine salt cake and liquor mist, which builds up on impellers and on steam coil fins. The fan slowly goes out of balance, and the steam coils lose output, so the air reaches the furnace cooler. Screens, an outside intake where practical, and periodic cleaning maintain balance and air heater performance.
Energy and control
The air fans run continuously at the rate of liquor firing. Variable speed drives on each air level save power over damper control and give operators finer control of the air split. Warming the air in steam coil heaters with low pressure steam improves combustion, and the fans are sized at ambient density so the heater does not add to fan power.
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 Induced Draft (ID) Fan for black liquor chemical recovery boilers.
Questions buyers ask
How many FD fans does a recovery boiler have?
Typically three, one each for primary, secondary and tertiary air. Some large boilers add a fourth level, such as quaternary air, or split one level between two fans for redundancy. Each fan supplies its own windbox and ports, with its own flow measurement, so the operator can set the air split independently.
Why is the combustion air heated with steam on a recovery boiler?
Black liquor needs warm air to dry and burn well, and steam coil air heaters provide it without the fouling risk of a flue gas air heater. The fans push cold air into the heaters, so they run at ambient density.
What happens if the primary air fan trips on a recovery boiler?
The char bed loses air and its temperature falls, and smelt flow is disturbed. The boiler control usually cuts the liquor firing, and in many plants a trip of the primary air fan leads to a liquor cut-off. Reliability of this fan matters a great deal.
Information needed to quote an FD fan for black liquor chemical recovery boilers
- 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.
- Air levels. Design flow and port pressure for each of the primary, secondary and tertiary air levels.
- Steam coil heaters. Air side pressure drop of each steam coil air heater.
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.