Pressed fiber and palm kernel shells leave the press house as “waste” and enter the boiler as renewable fuel—cutting costs, shrinking emissions, and often generating saleable power.
In modern palm oil mills, the mechanical pressing of cooked fruit doesn’t just produce crude palm oil. It also yields two high‑energy solid residues—pressed mesocarp fiber and palm kernel shells (PKS)—that are routinely dried and fed to on‑site steam boilers as biomass fuel. The result is cogeneration: waste becomes steam and electricity, displacing fossil fuels and energy purchases.
The scale is vast. Indonesia processed 200.7 million tonnes of fresh fruit bunches (FFB, fresh fruit bunches) in 2024, yielding 48.17 Mt crude palm oil, according to Palm Oil Magazine (palmoilmagazine.com). That throughput generated roughly 12.0–16.1 Mt of PKS and 26.1–30.1 Mt of mesocarp fibre, along with 46.2 Mt of empty fruit bunches (EFB, empty fruit bunches) (palmoilmagazine.com).
On a per‑tonne basis, studies put residue yields around 5–7% shells and 11–14% fiber (researchgate.net). Thailand reports ~5.5% shells and 13.5% fibre (scijournals.onlinelibrary.wiley.com). In short, each tonne of FFB yields on the order of 60–80 kg PKS and 120–150 kg fiber.
Residue streams and energy content
PKS are dense and carbon‑rich. Dried shells have a gross calorific value (CV, energy content) around ~18.8 MJ/kg, roughly 4,500–5,200 kcal/kg—comparable to coconut shell fuel (palmshells.com; researchgate.net). The mesocarp fiber “press cake” is somewhat lower at about 4,000 kcal/kg (~16–17 MJ/kg) on a dry basis (en.zozen.com).
Moisture matters: shells are often air‑ or mechanically dried to below ~10–15% before firing (en.zozen.com). Fibers are fluffy and low‑density, so mills frequently grind and pelletize or briquette them—sometimes mixing fiber/EFB—to raise bulk density and achieve more uniform combustion (en.zozen.com). Pelletizing also avoids feeding problems like bridging and coking on the grate (en.zozen.com; en.zozen.com).
Boiler preparation and steam cycle
In typical layouts, PKS and fiber move via conveyors from the press house to biomass storage hoppers feeding a chain‑grate or fluidized‑bed steam boiler. Shells are crushed, screened, and dried to optimal size/moisture; pellets or briquettes are often dried in a rotary dryer using waste heat (en.zozen.com).
The biomass mix is burned under a grate, generating high‑pressure steam typically ~2–2.5 MPa (megapascal) at 200–230°C. A back‑pressure turbine generator supplies most of the mill’s electricity, with low‑pressure steam bled off for sterilization and process heating.
Cogeneration outputs and case studies
Many mills run 100% biomass, relying fully on fiber and shells. A 30 t/h mill in Sumatra, for example, can cover its base load from fiber (~542 kW electric‑equivalent), with shells lifting total output to ~580 kW at 20 bar steam; fiber meets the base load and shell adds extra output (researchgate.net). Excess power of ~70–130 kW can be sold to the grid, about ≈1.4–2.6 MWh/day at 20 h operation (researchgate.net).
Similar studies report that a 20 t/h mill might export ~113 kW, while a 54 t/h mill could export ~900 kW beyond on‑site use (scijournals.onlinelibrary.wiley.com). A Thai case (45 t/h mill) found 2.83 MW of surplus generation over plant needs (scijournals.onlinelibrary.wiley.com). In practice, most wet‑process mills are self‑sufficient or net exporters: boilers and turbines often have moderate efficiency (~<80% boiler, ~<35% turbine), but the sheer volume of fuel makes net cogeneration large (scijournals.onlinelibrary.wiley.com). Industry models even identify 40–59 TWh/yr (terawatt‑hours per year) of potential if Indonesian mills converted all solid residues (fiber, shells, EFB, etc.) (palmoilmagazine.com).
Usage ratios, steam demand, surplus power
- Shells (PKS): ~6–8% by weight of FFB. Net CV ≈17.5–18.8 GJ/ton (about 4.2–4.7 kWh/kg). Sources: palmshells.com; researchgate.net.
- Press Fiber: ~13–15% of FFB by weight. Net CV ≈15–17 GJ/ton (~4.0 kWh/kg, dry basis) (en.zozen.com).
- Steam demand: roughly 500–700 kg steam per tonne FFB in milling (scijournals.onlinelibrary.wiley.com).
- Electricity output: many mills achieve ~15–20 kWh per tonne FFB of power surplus. For a 30 t/h mill, solid residues alone (fiber+shell) can generate ~600 kW, typically covering all process needs and yielding a small surplus (scijournals.onlinelibrary.wiley.com; researchgate.net).
Fossil displacement and trade flows
No fossil fuel backup is typically needed. Indonesian government data note only about 10% of mills use coal or gas; most rely on biomass fuels. Fiber and shells effectively displace costly fuel—their marginal cost is essentially zero—so mills save on energy expenses and often sell excess power at favorable rates.
There is an export dimension too. Palm‑Oil‑Magazine reports Indonesia exported 1.5 Mt of PKS in 2023 to Japan, equivalent to ~2–3 TWh of generation (palmoilmagazine.com). Keeping that fuel domestic provides direct mill energy and marketable surplus power instead.
Energy yields, costs, and payback
Because fiber and shell are renewable and high‑energy, utilizing them makes mills virtually energy‑independent. In one 30 t/h study, using all fiber and shell produces roughly 580 kW at 20 bar—enough for the mill’s own load (~450 kW) plus a 130 kW surplus. At 7,500 operating hours/year, that surplus is ~1.0–2.0 GWh/yr (researchgate.net). A 45 t/h mill could generate several MW (even >4 MW including waste‑heat recovery) (scijournals.onlinelibrary.wiley.com). On average, industry finds that fiber and shell alone can supply 100% of a mill’s steam and >80% of its power requirements. Turbine efficiencies are moderate (~25–35% electrical conversion), but because the fuel is free waste, even this yields very cheap electricity.
Using press waste as fuel eliminates fuel purchase. Even if shell could be sold, burning it internally adds more value (cheaper fuel and power). For context, imported coal or diesel might cost on the order of US$50–100 per tonne of oil equivalent; burning shell/fiber is essentially costless aside from handling. Many mills operate boilers near full load so as not to vent steam; mills that can export power even use the revenue to offset capital costs. One Indonesian feasibility study estimated capital payback in <5 years when selling surplus biomass‑generated power (researchgate.net).
Environmental performance and controls
Lifecycle analyses show that GHG (greenhouse gas) emissions from palm waste power are low—<0.25 kgCO₂eq/kWh—versus >0.8 kgCO₂/kWh for coal or oil generation (onlinelibrary.wiley.com). Burning fiber/shell on‑site also avoids alternative waste problems: instead of decomposing (where fibers could emit methane) or being piled, the material is “recycled” as energy. Controlled combustion enables pollution controls; mills typically equip boilers with multi‑cyclones or electrostatic precipitators to remove ash/particulates. The ash produced—rich in minerals—can be recycled to plantations as fertilizer.
Key benefits include: renewable energy use and reduced fossil CO₂; no open burning of waste; valorization of debris rather than disposal; and avoidance of methane from decomposition. Palm biomass contains negligible sulfur, reducing SOx concerns. In effect, every tonne of fiber burnt saves on the order of 0.5–0.6 tonnes of CO₂ that coal would emit for the same energy, and up to ~80–90% of its heating value is captured as steam/electricity in modern boilers (palmshells.com; researchgate.net). Reduced particulates and odor versus traditional open‑burning disposal improve local air quality.
Policy context and market signals
Indonesia’s energy policy emphasizes biomass; national planning (RUU EBT, RPP KEN) prioritizes agricultural residues (fwi.or.id). Utilizing press waste directly contributes to renewable targets (23% by 2025) and to certification standards (RSPO/ISPO encourage no‑burn waste handling). There is no explicit mandate to combust residues, but mills do receive incentives or credits for renewable power. Many mills self‑generate surplus to sell to PLN (the state utility) under bioenergy tariffs.
Environmental regulations on boiler emissions are readily met because the fuel is biomass. On the downside, if mills throw away or dump fiber, they risk fines; thus burning it is both economically prudent and compliant.
Bottom line for sustainability
Fiber and shell from the press are excellent “free” fuels. Properly dried and fed into the boiler, they become the mill’s primary energy source, powering sterilizers and engines. The gains are measurable: energy self‑sufficiency, avoided fuel costs that can reach into the millions for a medium‑size mill, and significantly lower GHG and pollutant emissions. That’s why pressing‑residue cogeneration is now a technical standard and business imperative across Indonesia’s palm sector (palmoilmagazine.com; en.zozen.com; scijournals.onlinelibrary.wiley.com; onlinelibrary.wiley.com).
