How coal gasifiers for process heat work
Coal gasifiers for process heat convert lump coal into producer gas for ceramic kilns, glass furnaces, forging and heat treatment furnaces. Air, saturated with steam from the gasifier water jacket, is blown up through a fixed coal bed; the coal dries, devolatilises, then reacts to give a gas of typically 25 to 30% carbon monoxide, 10 to 15% hydrogen, some methane and about 50% nitrogen, with a heating value near 1,200 to 1,500 kcal/Nm³. In hot gas systems the gas goes straight to the burners at 400 to 600°C with its tar. In cold clean gas systems it passes a cyclone, cooler, scrubber and tar precipitator, and a gas booster draws it through this train and delivers it to the kiln burners.
Where the ID fan sits
- Gasifier
- Cyclone
- Gas cooler
- Scrubber
- Tar precipitator
- Gas booster (ID) fan
- Kiln burners
In a cold clean gas system the booster sits after the tar precipitator, where the gas has cooled to 35 to 50°C and most tar and dust are gone. Upstream of the fan the train runs under slight suction, and downstream the gas main is under positive pressure to the burners. The fan works against the gasifier bed, cyclone, cooler, scrubber and precipitator on the suction side, and the gas main and burner pressure on the discharge, typically 300 to 700 mmWC in total. Hot gas systems usually have no booster; the gasifier air blower pushes the gas through.
Gas at the ID fan
| Gas temperature, normal | 35 to 50°C after the tar precipitator |
| Maximum or upset | Up to 80 to 100°C if the cooler or scrubber water fails |
| Gas composition | Producer gas: CO, H₂, CH₄, CO₂ and nitrogen; toxic and flammable |
| Tar and dust at the fan | Residual tar mist, naphthalene and fine soot, more if the precipitator trips |
| Moisture and corrosive species | Saturated; traces of H₂S, ammonia and phenolic condensate |
| Density at the design temperature | About 1.12 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. |
ID Fan selection for coal gasifiers for process heat
| Blade form | Backward curved or radial tipped with open, smooth passages that tolerate tar |
| Material of construction | Mild steel casing with a heavy gauge, gas-tight welded construction; stainless impeller where condensate is acidic |
| Drive and arrangement | Direct coupled, arrangement 8, with motor and bearings outside the gas |
| Bearings and cooling | Pedestal bearings outside the casing; no cooling at this temperature |
| Flow control | Variable speed drive on gas main pressure |
| Also specified | Mechanical or gas-purged shaft seal, steam or nitrogen purge connection, casing drain with a water seal, and steam cleaning nozzles |
What this ID fan has to cope with
Tar and naphthalene deposits
Tar mist that passes the precipitator condenses on the impeller and in the scroll, and naphthalene crystallises in cool weather. Deposits unbalance the wheel and choke the casing. Steam cleaning connections, a drained casing and smooth blade passages let the plant clean the fan during short stops.
Gas leakage at the shaft
Producer gas is high in carbon monoxide, so even a small leak at the shaft is a hazard to people near the fan. A mechanical seal or a gas-purged labyrinth, a welded casing with gasketed joints, and a CO detector at the fan are standard.
Air ingress and explosive mixture
On the suction side the train is below atmospheric pressure, so a leak or open door draws air into the gas. The mixture can become explosive. Oxygen monitoring on the gas main, interlocks with the booster, and purging with steam or nitrogen before start-up are needed.
Unstable flow at low demand
When kilns throttle back, the booster can be pushed toward its stall region and the gas main pressure hunts. A stable curve and speed control on main pressure, rather than a bypass, keep delivery steady. Where one booster serves several gasifiers, the fan curve is checked at the flow with only one gasifier on line.
Energy and control
Gas demand follows the kiln firing schedule, which in ceramic and forging plants changes through the day and between products. The booster spends much of its time below design flow. A variable speed drive on gas main pressure matches output to demand and avoids recirculating gas through a bypass valve. It also lets the operator ramp the fan gently during purging and lighting, which a fixed-speed fan cannot do.
ID fan and FD fan in coal gasifiers for process heat
| ID fan | FD fan | |
|---|---|---|
| Position | After the tar precipitator, before the gas main | At grade, feeding the blast pipe |
| What it handles | Cooled producer gas with tar traces | Clean ambient air |
| Temperature at the fan | 35 to 50°C | Ambient |
| Main design concern | Gas tightness and tar deposits | Bed resistance and backflow |
| Control | Gas main pressure | Gas demand and steam ratio |
See also the Forced Draft (FD) Fan for coal gasifiers for process heat.
Questions buyers ask
What is a gas booster on a coal gasifier?
It is the fan that draws producer gas through the cyclone, cooler, scrubber and tar precipitator and delivers it under pressure to the burners. On a coal gasifier it does the job an ID fan does on a boiler, but it handles fuel gas, so it must be gas tight.
Why does tar build up in my producer gas fan?
Tar mist and naphthalene that pass the cleaning train condense on the cool impeller and casing. Build-up rises if the tar precipitator is off or the gas is cooled too far. Steam cleaning, a drained casing and a working precipitator keep it under control. Keep the precipitator energised.
Does a hot gas coal gasifier need an ID fan?
Usually not. In a hot gas system the gasifier air blower pushes gas directly to the burners, and the gas is not cooled or cleaned. A booster is used in cold clean gas systems, where the cleaning train adds too much resistance for the air blower alone.
Information needed to quote an ID fan for coal gasifiers for process heat
- 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.
- Gas cleaning train. Equipment between gasifier and fan, and their pressure drops.
- Gas main. Pressure required at the burner header and the number of kilns supplied.
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.