How petcoke fired CFBC boilers work
Petroleum coke from refinery delayed cokers has a high calorific value of about 7,800 to 8,300 kcal/kg, low volatile matter of 9 to 12%, ash below 1% and sulphur typically 4 to 7%, with vanadium and nickel in the ash. It is fired in CFBC boilers because the long residence time at 850 to 900°C burns this low reactivity fuel and limestone fed to the bed captures most of the SO₂. Limestone feed is large, often at a calcium to sulphur ratio of about 2 to 2.5, so most of the solids leaving the boiler are spent lime and calcium sulphate rather than fuel ash. After the cyclone, 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 sits after the ESP or bag filter and ahead of the chimney or FGD stage where fitted. Upstream are the furnace, cyclone, superheater, economiser and air heater, giving inlet suction typically 250 to 400 mmWC. In refinery and petrochemical plants, layout space is tight and the fan often sits close to the collector outlet. Its inlet temperature must stay above the acid dew point, which governs how cold the air heater can run.
Gas at the ID fan
| Gas temperature, normal | 150 to 170°C, held above the acid dew point |
| Maximum or upset | Up to 210°C with fouled back pass or air heater bypass |
| Dust at the fan | Fine spent lime, calcium sulphate and a little unburnt carbon; 30 to 80 mg/Nm³ after the collector |
| Moisture and dew point | Low fuel moisture; water dew point about 40 to 50°C |
| Corrosive species | Residual SO₂ and SO₃; acid dew point typically 120 to 145°C depending on capture efficiency |
| Trace metals | Vanadium and nickel compounds in the fly ash |
| Density at the design temperature | About 0.82 kg/m³ at 160°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 petcoke fired CFBC boilers
| Blade form | Backward curved, smooth blade surfaces to limit lime build-up |
| Material of construction | Mild steel with casing insulation to keep metal above the dew point; corrosion-resistant steel at the inlet cone and drains on high sulphur duty |
| Drive and arrangement | Direct coupled, arrangement 8; double inlet on larger units |
| Bearings and cooling | Pedestal bearings with cooling disc and oil lubrication on larger fans |
| Flow control | Variable speed drive on furnace draft |
| Also specified | External casing insulation, inspection doors, drains and a warm air purge connection |
What this ID fan has to cope with
Acid condensation at low load and start-up
With 4 to 7% sulphur, even a small slip of SO₃ past the bed gives an acid dew point well above the water dew point. At low load or during start-up the gas at the fan can fall below it, and acid condenses on cold casing walls. Casing insulation, a minimum gas temperature at the air heater outlet and warm air purging during shutdown protect the fan.
Lime and gypsum build-up
Spent lime is hygroscopic and binds into hard deposits where moisture or acid condenses. Build-up on the blades causes unbalance and on the casing reduces performance. Smooth blade surfaces, insulation and regular inspection keep deposits manageable. Where deposits recur, the inspection door should allow the blades to be cleaned without removing the impeller.
Higher dust load than the fuel ash suggests
Because most of the solids come from limestone, the dust load at the collector inlet is many times what the petcoke ash alone would produce. Collector and fan selection must be based on the ash plus limestone products, so that the fan does not see heavy dust when the collector is stressed.
Energy and control
Petcoke CFBC boilers in refineries and captive plants usually run at steady high load, but steam demand changes with process units going on and off line. A variable speed drive on furnace draft gives efficient control across these changes and avoids running at low damper openings. Holding the air heater outlet temperature only as high as the dew point requires keeps both fuel efficiency and gas volume at the fan in check.
ID fan and FD fan in petcoke fired CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and air box |
| What it handles | High sulphur flue gas with lime-rich ash | Clean ambient air |
| Temperature at the fan | 150 to 170°C, above the acid dew point | Ambient, 25 to 45°C |
| Main design concern | Acid dew point and lime deposits | Stable fluidisation of an added-material bed |
| Control | Furnace draft | Bed fluidisation, with SA taking turndown |
See also the Forced Draft (FD) Fan for petcoke fired CFBC boilers.
Questions buyers ask
What is the acid dew point on a petcoke boiler?
It depends on the SO₃ in the flue gas, which is low when limestone captures most of the sulphur but still higher than on low sulphur coal. Typical values are 120 to 145°C. The flue gas at the ID fan should be kept above this with a margin, especially at low load.
Why does lime build up on my petcoke CFBC ID fan?
The fly ash is mostly spent limestone and calcium sulphate. When it meets condensed moisture or acid, during start-up or on cold casing walls, it hardens into deposits. Insulating the casing, avoiding low gas temperatures and purging with warm air before shutdown reduce the problem.
What material should a petcoke boiler induced draft fan be made of?
Mild steel is normally suitable if the gas stays above its acid dew point and the casing is insulated. Where low load operation is frequent, corrosion-resistant steel at the inlet cone, casing bottom and drains adds protection. The impeller material choice depends on dust and temperature margin.
Information needed to quote an ID fan for petcoke 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 and limestone. Petcoke sulphur, vanadium content and the limestone feed rate or Ca/S ratio.
- Low load. Minimum continuous load and the gas temperature at the fan at that load.
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.