How coke oven gas fired boilers work
Coke oven gas is the by-product gas of a by-product coke oven battery, cleaned of tar, ammonia, benzol and, where a desulphurisation plant is fitted, most of its H₂S. It is a rich fuel, typically 55 to 60% hydrogen and 23 to 27% methane, with a calorific value of about 4,000 to 4,500 kcal/Nm³. Residual H₂S can range from a few hundred mg/Nm³ after desulphurisation to several g/Nm³ without it, and traces of naphthalene and tar remain. Steel plants fire it in captive boilers, often mixed with blast furnace gas. Hydrogen-rich firing gives a hot, fast flame and wet flue gas. An FD fan pushes air through the air heater to the burners, and an ID fan draws the gas to the stack.
Where the ID fan sits
- COG and mixed gas burners
- Furnace
- Superheater and boiler bank
- Economiser
- Air heater
- ID fan
- Stack
The fan sits after the air heater at the base of the stack, normally without a dust collector because gaseous fuel leaves almost no ash. The suction it works against covers furnace draft, the boiler, economiser and air heater, typically 120 to 220 mmWC. Since water from hydrogen combustion makes up close to a fifth of the flue gas, and SO₂ follows the H₂S left in the gas, the gas temperature at the fan is set above the acid dew point rather than at the lowest temperature the air heater could reach.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with air heater bypass for cold-end protection |
| Dust at the fan | Negligible; traces of soot from naphthalene or tar carry-over at the burners |
| Moisture and dew point | Water vapour about 17 to 22% by volume; water dew point near 60°C |
| Corrosive species | SO₂ and some SO₃ from residual H₂S; acid dew point typically 110 to 140°C depending on desulphurisation |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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 coke oven gas fired boilers
| Blade form | Backward curved or aerofoil on clean gas |
| Material of construction | Mild steel with insulated casing on desulphurised gas; corrosion-resistant steel or SS for the impeller and inlet where H₂S is high |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain at the lowest point, insulation and cladding, inspection door |
What this ID fan has to cope with
Acid attack when desulphurisation is down
If the gas desulphurisation plant is bypassed, H₂S in the fuel can rise several times and the acid dew point climbs with it. Casings designed for clean gas then corrode at the scroll bottom and inlet cone. Insulated casings, corrosion-resistant wetted parts and a control that raises gas temperature during such periods protect the fan.
Condensation during cold starts
On start-up the fan casing is cold and the wet flue gas condenses inside it, forming dilute acid. Warming through on air before firing, a casing drain, and keeping the fan running on a slow purge after shutdown limit the condensate that collects.
Wide swings with mixed gas firing
Coke oven gas is often fired with blast furnace gas in proportions that change with what the plant has spare. The flue gas volume per unit heat changes several-fold between COG and BFG firing. The fan is selected for the largest-volume mix and must stay stable at the low volume of pure COG firing.
Energy and control
Coke oven gas supply follows the coke battery and the needs of the reheating furnaces, so the boiler often takes whatever gas is surplus. A variable speed drive on furnace draft handles these swings with much less power than damper control. Controlling flue gas exit temperature just above the acid dew point, not higher, is where the real fuel saving lies, and it depends on good desulphurisation upstream.
ID fan and FD fan in coke oven gas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the air heater, before the stack | Before the air heater, feeding the burners |
| What it handles | Wet flue gas with SO₂, almost no dust | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Acid dew point and corrosion | Fast air-fuel response |
| Control | Furnace draft | Combustion control and oxygen trim |
See also the Forced Draft (FD) Fan for coke oven gas fired boilers.
Questions buyers ask
Does a coke oven gas boiler ID fan need a dust collector ahead of it?
No. Coke oven gas is cleaned before firing and leaves almost no ash, so the ID fan sees clean flue gas. The design concern is corrosion from the wet, sulphur-bearing gas, not erosion. A dust collector is needed only where coal or another solid fuel is co-fired in the same boiler.
Why does the ID fan casing corrode on a COG fired boiler?
Burning hydrogen-rich gas produces a lot of water vapour, and any H₂S left in the gas burns to SO₂ and SO₃. Where the casing or impeller is below the acid dew point, sulphuric acid condenses. This happens most at start-up, low load, and when the desulphurisation plant is out of service.
Can the same ID fan handle coke oven gas and blast furnace gas firing?
Yes, if it is selected for both. Blast furnace gas produces much more flue gas per unit heat, so the fan is sized for the largest BFG share expected, and checked for stable operation at the lower volume of pure coke oven gas firing. A variable speed drive covers the range.
Information needed to quote an ID fan for coke oven gas fired 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.
- Gas quality. Coke oven gas analysis including H₂S after desulphurisation and the value when the desulphurisation plant is bypassed.
- Firing mix. Range of COG, BFG and other fuels fired together, with boiler load for each case.
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.