High-efficiency biochar production from oil palm fronds using acidic pretreatment and co-torrefaction with waste cooking oil

Oil palm fronds (OPF) are a potential solid fuel source, similar to empty palm fruit bunches (EFB), yet high potassium (K) content can cause fouling and deposition in boiler systems. To mitigate these issues, an acidic torrefaction liquid pretreatment was applied to reduce OPF potassium level, and co-torrefaction with waste cooking oil (WCO) was employed to enhance the resulting biochar’s higher heating value (HHV). X-ray fluorescence (XRF) analysis indicated that acidic pretreatment lowered potassium content from 35.48% to 20.91%, a reduction of approximately 41%, thereby significantly diminishing K-induced fouling risk. Through systematic variation of pretreatment temperature (30–50 °C), duration (45–75 min), WOPF-to-WCO ratio (1:0–3:1, mass/mass), and co-torrefaction temperature (200–300 °C), optimal conditions were identified at 50 °C for 75 min, an WOPF-to-WCO ratio of 1:1, and co-torrefaction at 250 °C. Under these parameters, WCO contributed additional hydrocarbons, increasing fixed carbon content and reducing oxygen level, thereby improving the fuel stability and HHV. The biochar exhibited a mass yield of 60.95%, an energy yield of 87.43%, and a HHV of 31.97 MJ/kg, surpassing the performance of untreated OPF biochar. By simultaneously reducing potassium content and enriching carbon and hydrogen fractions, this optimized biochar exhibits enhanced combustion properties, positioning it as a sustainable alternative to coal with higher energy density and lower operational risk in combustion systems.

Graphical abstract