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 FD fan sits
- Inlet filter
- FD fan
- Non-return valve
- Steam saturation
- Blast inlet
- Coal bed
The air blower sits at grade beside the gasifier and delivers air into the blast pipe, where steam from the water jacket or a saturator is mixed in. The mixture enters below the grate, then passes the ash, oxidation and reduction zones of the bed. The blower must overcome the grate, ash and coal bed resistance and, in hot gas systems, the gas main to the burners as well, typically 300 to 800 mmWC depending on bed depth and coal size.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Blast after saturation | 50 to 65°C saturated air and steam, downstream of the fan |
| Dust | Coal yard dust; inlet filtered |
| Pressure at the fan outlet | Typically 300 to 800 mmWC, varying with bed condition |
| Backflow risk | Producer gas can flow back into the air line if the blower stops |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 coal gasifiers for process heat
| Blade form | Backward curved, with a steep pressure curve so flow holds as bed resistance changes |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive on gas demand, with an inlet damper |
| Also specified | Non-return valve, explosion vent on the air line, and an interlock that closes the gas outlet on blower stop |
What this FD fan has to cope with
Changing bed resistance
As coal is charged, fines settle, clinker forms and ash is withdrawn, the bed resistance moves up and down. A blower with a flat curve loses flow sharply when the bed tightens, and gas quality falls. A steep curve and control on gas pressure or flow keep the blast steady.
Gas backflow on blower stop
If the blower trips while the gasifier is hot, producer gas can flow back up the air line to the blower and building. A non-return valve, a vent on the blast pipe, and interlocks that act on blower failure are part of the fan package.
Channelling at low blast
At very low air flow the blast finds channels through the bed, burning holes and raising CO₂ in the gas. The turndown of the blower is limited to what the bed can tolerate, and low demand is met by reducing the number of gasifiers in service.
Energy and control
The blower follows kiln gas demand, and steam-to-air ratio must stay right as it does. A variable speed drive is preferred over damper throttling, because it also gives smoother control of bed temperature and gas quality. Where a plant runs several gasifiers, running fewer units near their design blast is usually more efficient for both the fans and the gasification than running all of them lightly loaded.
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 Induced Draft (ID) Fan for coal gasifiers for process heat.
Questions buyers ask
What is the air blower on a coal gasifier called?
It is usually called the gasification air blower, blast fan or FD fan. It forces air, mixed with steam, up through the coal bed. In a hot gas system it may be the only fan in the gas path. In a cold clean gas system it works alongside a gas booster.
What pressure does a coal gasifier air blower need?
Typically 300 to 800 mmWC, depending on bed depth, coal size, grate design and whether the blower also pushes the gas to the burners. Pressure margin is needed because bed resistance rises as fines and clinker build up. Hot gas systems need more, because the blower also pushes gas to the burners.
What happens if the gasifier blower stops?
Producer gas can flow back into the air line. The forced draft fan system needs a non-return valve, a blast pipe vent and interlocks that isolate the gas outlet, so a blower trip leaves the plant in a safe state. These are tested at every planned shutdown, not only at commissioning.
Information needed to quote an FD fan for coal gasifiers for process heat
- 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.
- Gasifier data. Gasifier diameter, coal size and the bed depth at normal operation.
- System type. Whether the plant is a hot gas or cold clean gas system, and if a booster is installed.
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.