Palm kernel shells are turning steam boilers into quiet profit centers, with millions of tonnes available and hardware already in place to burn them cleanly. The engineering is straightforward: move the shells, meter the air, manage the ash.
Across Southeast Asia, palm oil mills have long burned palm kernel shells (PKS)—the hard, fibrous casings left after extracting the palm kernel—as an on‑site energy source, often mixed with fibrous mesocarp in steam boilers to meet their own steam and power needs (www.researchgate.net). One Indonesian mill processing 30 t/h (tonnes per hour) has shown the math: running just on fiber and shells, it produced 18 t/h of steam, banked a shell fuel surplus of 441.5 metric tons per month, and covered roughly 734 kW of electrical demand (www.researchgate.net).
The feedstock is abundant. Indonesia alone can supply on the order of 10–11 million t/yr (tonnes per year) of PKS, nearly 3.5 Mt of which are currently exported—volumes that have spurred government efforts to turn shells into higher‑value pellets for premium feed‑in tariff (FIT) markets such as Japan (www.en.infosawit.com) (www.qualitasertifikasi.com). Boiler vendors are following suit: ZOZEN cites Indonesian chain‑grate units in the 6–35 t/h class at 1–2.5 MPa (megapascals; a measure of pressure) that fire PKS with high thermal output and low emissions (en.zozen.com) (en.zozen.com).
Fuel properties and energy value
PKS is a high‑carbon, high‑volatility biomass. Ultimate analysis (elemental composition) comes in at roughly 50–57% carbon, about 11% hydrogen, low nitrogen (~0.5–3%), with the balance oxygen (www.researchgate.net) (www.researchgate.net). Proximate analysis (moisture, volatile matter, fixed carbon, ash; here on a dry basis) typically shows low moisture (~2–8%), very high volatiles (~40–70%), high fixed carbon (~50–55%), and ash around 2–4% (link.springer.com).
Heating value lands in the mid‑teens: higher heating value (HHV) ~16–18 MJ/kg, with lower heating value (LHV) ~15–17 MJ/kg when dry (www.researchgate.net) (www.researchgate.net). One measured HHV was 17 MJ/kg (dry, ash‑free), while calorimetric tests on Nigerian PKS came in at about 15.2 MJ/kg (reflecting 3% ash) (link.springer.com) (www.researchgate.net).
Moisture is usually low (<5–15%); typical shipment moisture is around 15%, and dried fuel can be below 8% (as reported in the cited data set). Natural bulk density runs roughly 200–350 kg/m³ in loose fill, so shells are sometimes pelletized or tightly stored. Even unprocessed, the material is “small size, light weight, not easy to break, convenient for storage and transportation,” while pellet density can reach ~1.1 g/cm³. For export logistics, pelletization to about ~600 kg/m³ is common in practice, supported by Indonesian incentives to build downstream capacity (www.en.infosawit.com).
With very low sulfur and moderate nitrogen, PKS tends to produce minimal SO₂ and modest NOₓ. Co‑firing studies show large flue‑gas emission reductions versus coal—about 30–80% less flue‑gas volume and a 32% drop in SO₂ at 950 °C when swapping coal for PKS (www.researchgate.net). The ash is silica/alkali‑rich; potassium salts can trigger slagging and fouling if unmanaged (link.springer.com) (link.springer.com).
Solid handling and boiler feed
PKS typically arrives from nut‑cracking as coarse lumps, then moves by chain or belt conveyors into covered silos or day hoppers before the boiler. Sizing helps: screening or light crushing to uniform few‑millimeter particles improves flow and burn. One study identified an optimal shell size around ~5.5 mm for steady combustion (link.springer.com).
Because the fuel is dry, enclosed conveyors and vent filters help with dust control; level sensors maintain continuous feed. From storage, screw/auger feeders or vibrating conveyors meter shells to the furnace, often through automated airlocks. Design teams account for bridging by specifying wide discharge openings or hopper agitators. Where export is planned, pelletization (to ~600 kg/m³) reduces transport cost and stabilizes supply to FIT markets like Japan (www.en.infosawit.com).
Combustion behavior and air staging
Thermally, shells ignite readily. On heating, PKS devolatilizes between roughly 200–500 °C—driving off gases from hemicellulose and other carbohydrates—then the residual charcoal burns out up to ~600–800 °C. Thermogravimetric analysis (TGA; a technique that tracks weight loss on heating) shows most mass is lost by 400 °C (www.researchgate.net).
Given the high volatiles (~44%), staged air is standard: primary air through the grate for the fuel bed and secondary air above the bed for the flame. In one inclined‑grate study, a 40:60 primary:secondary split drove peak flame temperature near ~1058 K with very low CO (~285 ppm), indicating nearly complete burnout (link.springer.com).
Energy conversion depends heavily on mixing and residence time. A pilot grate combustor recovered only about ~25–32% of PKS chemical energy as useful heat—losses attributed to poor fuel–air mixing and incomplete char burnout (www.researchgate.net). In service, well‑designed units routinely reach ~80–90% boiler efficiency. As a scale marker, co‑firing 15 t/h of PKS yields about 34.5 MW (thermal) (www.researchgate.net), corresponding roughly to 75–80% efficiency for steam generation. Electrical output then depends on the steam cycle; turbogenerator setups typically convert at around ~20–30% electrical efficiency.
Emissions and particulate control
With low sulfur and low fixed carbon in ash, PKS typically produces less particulate and SO₂ than coal. Some NOₓ arises from residual fuel nitrogen. Indonesian policy prioritizes low dust and NOₓ; modern PKS boilers therefore pair proper draft fans with cyclones or baghouse filters. In co‑firing trials, burning PKS instead of coal reduced NOₓ by about ~1% at 950 °C and cut SO₂ by ~32% (www.researchgate.net) (www.researchgate.net), alongside the ~30–80% flue‑gas volume reduction cited above.
To keep stack dust in check, mills deploy multicyclones; ReCyclone MH units, for instance, have been installed to catch fine ash and limit emissions from boilers firing mesocarp fibre and shells (www.environmental-expert.com).
Boiler designs that work with PKS
The workhorse is the stoker/grate furnace. Chain‑grate or travelling‑grate designs tolerate small, irregular shells and handle low‑melting ash better than fluidized beds. ZOZEN’s SZL series is explicitly configured for PKS and similar fuels, at 6–35 t/h steam, 1–2.5 MPa, and 184–350 °C (en.zozen.com).
Key hardware choices include a fuel hopper sized to PKS bulk density, with feeds that distribute shells evenly onto the grate—ZOZEN says it tailors hopper and grate widths to PKS grain size (en.zozen.com); long moving grates that carry fuel through drying, devolatilization, and burnout; and staged air ports: primary air through the grate and secondary above, with three‑stage options on newer units. The 40:60 primary:secondary split reported in lab work informs tuning on site (link.springer.com).
Automated conveyors or screw feeders maintain bed depth, with steam‑pressure or flame‑view sensors adjusting feed. Heat‑exchange surfaces—water‑wall tubes, superheaters, and economizers—recover energy; metallurgy must withstand ~600–700 °C flue gas temperatures after complete combustion. Ash removal is straightforward given ~≤3% ash yield by weight: clinker and bottom ash fall to rakes or screws, while fly ash heads to cyclones or fabric filters. Advanced packages automate fuel feed, grate shaking, and slag discharge, and use O₂/CO analyzers to modulate the air–fuel ratio (en.zozen.com).
Large units and co‑firing strategy
At larger scales, circulating fluidized beds (CFB; a high‑solids, fluid‑like combustion bed) and pulverized‑fuel boilers have co‑fired PKS. Indonesia’s PLN has begun co‑firing in 25 MW CFB plants (www.researchgate.net). The caveat: PKS’s high potassium can cause FBC bed agglomeration unless mitigated (e.g., with kaolin or MgO) (link.springer.com). As one review notes, “PKS shows a tendency of bed agglomeration due to its high alkali content” (link.springer.com).
That’s why grate furnaces remain the industry default, with CFB reserved for very large, dedicated biomass lines or for co‑firing. Coal plants can typically co‑fire PKS up to about ~10–30% by energy using existing mills with minor modifications and certified supply inventories, and Indonesian power policy supports modest biomass co‑firing in large units at typical mixing ratios around ~1–5% (www.researchgate.net).
Policy, markets, and design checklist
PKS is classified as biomass under Indonesia’s energy policies, and industry groups (e.g., APCASI, working with ministries) are promoting PKS‑based cogeneration. Exports of pelletized PKS into Japan’s ISCC‑certified FIT program are rising (www.qualitasertifikasi.com) (www.en.infosawit.com), and boiler makers report growing orders for PKS‑capable units in Indonesia (en.zozen.com) (en.zozen.com).
The engineering checklist is simple and proven: ensure even feed of clean, sized shells; select a grate or furnace matched to throughput; stage primary/secondary air for complete burn (targeting adequate excess air with about ~5–8% O₂ in the flue); minimize stack temperature using economizers; and manage ash and deposits proactively, recognizing PKS ash can be sticky when chlorides/potassium run high. These steps, backed by both field reports and lab studies, are why well‑designed PKS‑fired boilers routinely reach high thermal efficiency with low emissions, on par with other biomass systems (en.zozen.com) (link.springer.com).
Do that, and mills can fully utilize their shells and fibers to generate steam and electricity—often exceeding internal demand and leaving a surplus for power sales or pellet export.
