How coal gasification and syngas plants work
Coal gasification converts coal or coke into a fuel gas of carbon monoxide, hydrogen and methane by partial oxidation with air or oxygen and steam. Large IGCC and chemical plants use pressurised entrained flow or fluidised bed gasifiers, where the syngas is moved by the gasifier pressure and by compressors, not by fans. Fans appear at lower pressure: on fixed bed and atmospheric gasifiers that supply fuel gas to kilns and furnaces, where an air blower supplies the gasification air and a gas booster or exhauster draws raw gas through the cyclone, cooler, tar removal and ESP; and on syngas-fired boilers, tail gas incinerators and heat recovery units, which use conventional ID and FD fans.
Where the ID fan sits
- Gasifier
- Cyclone
- Gas cooler
- Tar ESP
- Gas exhauster (ID fan)
- Users or flare
On an atmospheric gasifier the exhauster sits after the gas cooler and tar ESP, where the gas is cool and mostly free of tar, and before the distribution main. It holds the gasifier outlet at a small suction or set pressure and boosts the gas to the burners, typically 300 to 800 mmWC across the fan. Placing it after cleaning protects the impeller from tar and dust. On syngas-fired boilers and tail gas incinerators the ID fan sits in the normal position after the economiser or heat recovery section.
Gas at the ID fan
| Gas temperature, normal | 35 to 60°C after the gas cooler and tar ESP |
| Maximum or upset | Up to 90 to 120°C when the cooler water fails |
| Dust and tar at the fan | Light tar mist and fine carbon if the ESP underperforms; deposits on blades over time |
| Moisture and dew point | Gas leaves the cooler close to saturation; condensate drains from the fan casing |
| Corrosive and hazardous species | CO, H₂ and H₂S; toxic and flammable, so the casing must be gas-tight |
| Density at the design temperature | About 1.10 kg/m³ at 50°C, 92% 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 gasification and syngas plants
| Blade form | Backward curved with open blades that can be cleaned of tar |
| Material of construction | Mild steel with corrosion allowance, or SS 304 or SS 316 where H₂S and condensate are present |
| Drive and arrangement | Direct coupled, arrangement 8, flameproof motor suited to the hazardous area classification |
| Bearings and cooling | Pedestal bearings outside the gas path; no cooling needed at these temperatures |
| Flow control | Variable speed drive on gasifier outlet pressure or main pressure |
| Also specified | Gas-tight welded casing, inert gas purged or mechanical shaft seal, spark-resistant inlet ring, casing drain with water seal, and a steam or nitrogen purge connection |
What this ID fan has to cope with
Gas leakage at the shaft
A conventional shaft seal leaks syngas into the fan house. Carbon ring or mechanical seals with a nitrogen or steam purge, and a casing pressure test before commissioning, keep the fan tight. Leakage at the shaft is checked with a gas detector at each shutdown and the seal renewed before it becomes a hazard.
Tar deposits
Tar that passes the ESP condenses on the blades and inside the casing, causing imbalance and restricting flow. Steam cleaning connections, inspection doors and a fan kept warm above the tar condensing point reduce it. Tar removal performance upstream matters as much as the fan.
Air ingress and explosion risk
When the system runs under suction, any leak upstream draws in air and can create an explosive mixture. The fan is purged with inert gas before start-up, the oxygen in the gas is monitored, and the casing withstands the explosion pressure specified for the plant.
Low gas density
Syngas from air-blown gasifiers is lighter than air and flue gas. The fan is selected on the actual gas composition and density, not on air, or it will underperform on pressure. Hydrogen-rich gas is lighter still, so composition swings change the pressure the fan delivers.
Energy and control
Atmospheric gasifiers follow the demand of the kilns or furnaces they serve, often with large swings between shifts. A variable speed drive on main pressure delivers only the gas needed and avoids recirculation or flaring of surplus. For syngas-fired boilers, the ID fan control follows boiler load as for any gas-fired unit.
ID fan and FD fan in coal gasification and syngas plants
| ID fan | FD fan | |
|---|---|---|
| Position | After gas cooling and tar removal | Ahead of the gasifier air main |
| What it handles | Raw syngas, toxic and flammable | Clean ambient air |
| Temperature at the fan | 35 to 60°C | 25 to 45°C |
| Main design concern | Gas tightness and explosion safety | Stable air flow and no back-flow |
| Control | Gasifier or main pressure | Air flow |
See also the Forced Draft (FD) Fan for coal gasification and syngas plants.
Questions buyers ask
Is a gas booster on a coal gasifier the same as an ID fan?
Functionally it does the same job, drawing gas from the gasifier through the cleaning equipment. But it handles combustible and toxic gas, so it needs a gas-tight casing, special shaft seals, purging and flameproof electrics, which a normal ID fan does not. Its motor is selected for the hazardous area classification of the gas plant.
What type of fan is used for a coal gasification plant?
Low-pressure fixed bed gasifiers use centrifugal exhausters and air blowers. Large pressurised gasifiers use compressors, which are outside the fan range. Syngas boilers and incinerators use conventional ID and FD fans. Air blowers on fixed bed gasifiers are conventional fans, selected for stable flow against a changing bed.
How do you prevent explosion in a syngas fan?
Purge the system with inert gas or steam before start-up, avoid air ingress by maintaining positive pressure where possible, monitor oxygen, use spark-resistant construction at the inlet and flameproof motors. The casing is pressure tested before commissioning, and the fan is never started on gas whose oxygen content has not been checked.
Information needed to quote an ID fan for coal gasification and syngas plants
- 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 composition. CO, H₂, CH₄, H₂S and tar content of the gas at the fan inlet, and the gas density.
- Hazardous area and sealing. Area classification for the motor and the shaft seal and purge medium available.
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.