How coal and biomass co-fired CFBC boilers work
CFBC boilers are well suited to burning coal with a large share of biomass, from wood chips and bagasse to husk and agricultural residue, because the circulating hot solids even out the differences in fuel reactivity. Fuels are fed into the lower furnace and burn at 820 to 880°C, with solids carried up, separated in a hot cyclone and returned through a loop seal. Biomass shares of 30 to 70% by heat are common in captive and independent power plants. Alkali and chlorine from biomass raise the risk of agglomeration in the loop seal and of corrosive deposits on the superheaters. After the back pass and air heater the gas passes an ESP or bag filter and the ID fan.
Where the ID fan sits
- CFB furnace
- Hot cyclone
- Back pass
- Air heater
- ESP or bag filter
- ID fan
- Chimney
The fan, or two fans in parallel on larger units, sits after the ESP or bag filter. Suction at the inlet is typically 250 to 400 mmWC. The cyclone captures most of the coarse solids for recirculation, so the dust at the ESP is mainly fine ash and alkali salts. With a high share of wet biomass, the gas volume at the fan can be 20 to 40% higher than on coal alone for the same steam output.
Gas at the ID fan
| Gas temperature, normal | 145 to 170°C after the air heater |
| Maximum or upset | Up to 210°C with back pass fouling |
| Dust at the fan | Fine coal ash, biomass ash and alkali salts; 30 to 100 mg/Nm³ after the collector |
| Moisture and dew point | Water vapour 10 to 25% by volume depending on blend; water dew point 45 to 65°C |
| Corrosive species | HCl from biomass chlorine and SO₂ from coal; acid dew point depends on blend and limestone use |
| Gas volume range | Varies by 20 to 40% between coal and the wettest biomass blend |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 coal and biomass co-fired CFBC boilers
| Blade form | Backward curved aerofoil or plate, efficient over a wide flow range |
| Material of construction | Mild steel; wear-resistant plate at blade inlets where husk ash is in the blend |
| Drive and arrangement | Direct coupled, arrangement 8; double inlet on larger units |
| Bearings and cooling | Pedestal bearings with cooling disc, oil lubricated on larger fans |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Insulated casing, inspection doors, drains and parallel running dampers |
What this ID fan has to cope with
Wide operating range across blends
A fan sized for the wet biomass case at full load operates far back on its curve when firing coal. With damper control, efficiency at that point is poor, and with two fans in parallel there is a risk of stall at low flow. A variable speed drive and a fan curve that is stable over the full range avoid both.
Alkali chloride deposits
Potassium and sodium chlorides vaporise in the furnace and condense in the back pass on cooler surfaces and fine ash. Deposits increase resistance, and some reach the fan. Selecting the fan with pressure margin for fouled surfaces, and inspecting blades at outages, prevent unplanned limits and unbalance.
Chloride corrosion at low gas temperature
HCl in the flue gas, combined with moisture from wet biomass, makes the cold end of the gas path corrosive. If the fan casing runs cold, especially at low load or in shutdown, chloride corrosion attacks mild steel. Casing insulation and a minimum air heater outlet temperature protect it.
Energy and control
Co-fired CFBC plants often run at high load but change fuel blend with availability and price. The ID fan's power can change by a third or more between the driest and wettest blend at the same output. A variable speed drive keeps the fan efficient across that range. Tracking ID fan power against fuel blend shows the real auxiliary cost of wet biomass.
ID fan and FD fan in coal and biomass co-fired CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and grid |
| What it handles | Flue gas whose volume changes with blend | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Wide gas volume range across blends | Stable fluidisation as PA share changes |
| Control | Furnace draft | Bed fluidisation, with SA taking turndown |
See also the Forced Draft (FD) Fan for coal and biomass co-fired CFBC boilers.
Questions buyers ask
How much more flue gas does biomass co-firing produce in a CFBC?
It depends on moisture and share. Wood chips or bagasse with 40 to 50% moisture at a high share can raise flue gas volume by 20 to 40% compared with coal alone for the same steam. The induced draft fan must be sized for that case at full load.
Why do co-fired CFBC boilers have more back pass fouling than coal-only boilers?
Biomass releases potassium, sodium and chlorine that form chloride vapours in the furnace. These condense on the superheater and economiser, binding fly ash into sticky deposits. The draft loss rises and the ID fan works harder, so soot blowing and fan pressure margin are both important.
Should we use one ID fan or two on a co-fired CFBC boiler?
Larger units often use two fans in parallel, each able to carry around 60% of full load, so the boiler can keep running if one trips. Smaller units use one fan. With two fans, stable characteristics and matched controls are essential because the operating point moves widely with fuel blend.
Information needed to quote an ID fan for coal and biomass co-fired CFBC 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.
- Fuel blends. Analysis and moisture of each fuel and the expected range of biomass shares.
- Operating range. Gas flow at full load for coal alone and for the wettest blend.
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.