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 ID fan sits
- Reciprocating grate or fluidised bed
- Boiler
- Economiser
- Pomace dryer (if fitted)
- Cyclone
- Bag filter
- ID fan
- Chimney
The fan follows the bag filter. Where flue gas dries the pomace, the fan handles the dryer exhaust at about 90 to 130°C, humid and carrying fine fruit particles; without a dryer, it handles boiler gas at 160 to 190°C. Suction at the fan is typically 150 to 250 mmWC, or 250 to 350 mmWC with a dryer in the circuit. The fan stands idle for weeks or months between campaigns, often with deposits still in it.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C without a dryer; 90 to 130°C after a pomace dryer |
| Maximum or upset | Up to 230°C with fouled surfaces or the dryer bypassed |
| Dust at the fan | Fine potassium-rich ash and fruit particles; low after a bag filter |
| Moisture and dew point | Water dew point 55 to 75°C depending on pomace moisture and dryer use |
| Corrosive species | Potassium chloride in the ash; organic acids in condensate from fruit residue |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 fruit pomace fired boilers
| Blade form | Backward curved, with smooth blades that shed deposits |
| Material of construction | Insulated mild steel for boiler-only duty; SS 304 or SS 316 wetted parts after a dryer |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc, and shaft seal |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain, wash access, insulation; provision for slow rotation or dry storage during idle periods |
What this ID fan has to cope with
Corrosion between campaigns
When the campaign ends, ash and fruit residue left in the fan absorb moisture and turn acidic, and the fan stands for weeks. Corrosion pits in the impeller and casing grow each idle period. Washing and drying the fan at the end of each campaign, sealing the inlet and outlet, and rotating the shaft by hand periodically protect both impeller and bearings.
Sticky deposits from sugars
Residual sugars caramelise in the dryer and fine fruit particles carry over to the bag filter and fan, where they bind ash into a hard layer. Smooth stainless blades in dryer duty, access doors for cleaning, and vibration monitoring keep build-up from reaching the point of tripping the fan.
Changing pomace through the year
A plant may fire mango peel in summer, tomato residue in winter and grape pomace after the harvest. Moisture, ash and gas volume differ for each. The fan is selected on the wettest and most ash-rich pomace the plant expects, with a drive to trim it back for drier fuels.
Energy and control
Pomace boilers run in campaigns with heavy steam demand during processing and little between. A variable speed ID fan saves power during the lighter parts of each campaign and when firing drier pomace. Drying the pomace with flue gas cuts the wet gas volume through the boiler, but adds dryer resistance at the fan, so both are assessed 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 Forced Draft (FD) Fan for fruit pomace fired boilers.
Questions buyers ask
Can fruit pomace be used as boiler fuel?
Yes. Grape, apple, mango and tomato pomace are fired once dried to about 15 to 25% moisture, usually blended with wood or husk. Their ash is rich in potassium and fouls heat surfaces, so the boiler and the induced draft fan need margin for fouling and regular cleaning.
How do I protect the ID fan between processing seasons?
Wash out ash and fruit residue at the end of the campaign, dry the casing, seal the inlet and outlet, and turn the shaft periodically. Deposits left in an idle fan absorb moisture and corrode the impeller, which shows as unbalance when the next campaign starts.
Does a fruit pomace fired boiler ID fan need to be stainless steel?
Where the fan follows a pomace dryer and handles cool, humid gas, stainless wetted parts are worthwhile. On a boiler without a dryer, insulated mild steel works if the fan is cleaned and kept dry between campaigns. Drains and inlet boxes are often stainless in either case.
Information needed to quote an ID fan for fruit pomace 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.
- Pomace types. Each fruit residue fired through the year, with moisture and ash, and any blend fuel.
- Campaign pattern. Length of each campaign and of the idle periods between them.
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.