How fruit pomace fired boilers work
Fruit pomace is the residue of skins, seeds, pulp and stems left from juice, pulp, wine and puree processing: grape pomace from wineries and juice plants, apple pomace, mango peel and stones, and tomato skins and seeds. Fresh pomace carries 60 to 80% moisture and residual sugars, and ferments within days. On a dry basis it has 3 to 8% ash, rich in potassium, and a calorific value of about 4,000 to 4,800 kcal/kg, with grape seeds at the upper end. It is usually dried, often with flue gas, to 15 to 25% moisture before firing on a reciprocating grate or in a fluidised bed, and commonly blended with wood or husk. Processing runs in campaigns that follow each fruit's season. The flue gas passes the boiler, economiser, dryer if fitted, cyclone and bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum or air distributor
- Reciprocating grate or fluidised bed
The FD fan pushes ambient air through the air heater, heated to about 150 to 220°C, into the plenum under the reciprocating grate or the air box of a fluidised bed. A separate fan supplies secondary air. Pressure is typically 150 to 250 mmWC for a grate and 700 to 1,000 mmWC for a bubbling bed. Between campaigns the fan stands idle in a humid boiler house.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 42°C |
| Air after the air heater | 150 to 220°C, downstream of the fan |
| Dust | Dried pomace fines near the fuel store |
| Pressure at the fan outlet | 150 to 250 mmWC for a grate; 700 to 1,000 mmWC for a fluidised bed |
| Ash melting | Potassium-rich ash softens from about 800 to 950°C |
| 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 fruit pomace fired boilers
| Blade form | Backward curved, for steady flow across fuel changes |
| Material of construction | Mild steel with a protective coating |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard bearings, with idle-period lubrication |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet screen and covers for the inlet and outlet during idle periods |
What this FD fan has to cope with
Clinker from potassium-rich ash
Grape and mango pomace ash softens at low temperature. A bed short of primary air overheats and fuses into clinker that blocks the grate or agglomerates a fluidised bed. The fan must hold primary or fluidising air at every load and for every pomace, and blending with wood or husk helps further.
Different pomace, different air
Wetter tomato or apple pomace needs more hot air for drying and more pressure through a denser bed; drier grape pomace with seeds needs less undergrate air. A drive on measured air flow, with the split reset at each fuel change, keeps combustion steady. Selecting the fan on the wettest pomace gives it the pressure for the hardest case.
Bearings and casing after idle months
A fan that stands for months in a humid boiler house can suffer bearing corrosion and false brinelling from vibration of nearby machines. Turning the shaft periodically, keeping bearings greased, and covering the inlet and outlet avoid a difficult start to the next campaign. A short trial run before the fruit arrives confirms vibration and bearing temperatures are normal.
Energy and control
Pomace boilers run hard during processing campaigns and rest between them. A variable speed FD fan with oxygen trim saves power and fuel as the pomace changes, and reduces the wet gas volume at the ID fan. Drier pomace needs less air and less fan power, so the dryer and fans are tuned together.
ID fan and FD fan in fruit pomace fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, often after a pomace dryer | Before the air heater, feeding grate or bed |
| What it handles | Humid flue gas with potassium-rich ash | Ambient air |
| Temperature at the fan | 160 to 190°C, or 90 to 130°C after a dryer | Ambient, 15 to 42°C |
| Main design concern | Fouling and idle-period corrosion | Primary air against clinker |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for fruit pomace fired boilers.
Questions buyers ask
How do I avoid clinker when firing grape pomace?
Keep the bed below the ash softening temperature with steady primary air from the FD fan, avoid too hot an air heater outlet, and blend pomace with wood or husk. Grape pomace ash is rich in potassium and melts at a lower temperature than wood ash.
Can one FD fan serve different fruit pomaces?
Yes, if it is selected for the wettest, densest pomace the plant fires and controlled on measured air flow. The forced draft fan setting and the air split are adjusted at each change of fuel through the year. Seed-rich grape pomace burns more like wood; wet tomato residue needs much more hot air.
What should be done to the FD fan between pomace campaigns?
Cover the inlet and outlet, keep bearings greased and turn the shaft by hand periodically. This avoids bearing corrosion and flat spots from standing still, so the fan starts smoothly with the next campaign. Before restart, check the screen, the coupling alignment and the vibration on a short trial run.
Information needed to quote an FD fan for fruit pomace fired boilers
- 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.
- Pomace range. Wettest and driest pomace fired, and any blend fuel.
- Firing system. Grate or fluidised bed, and the air heater outlet temperature.
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.