Impact of water-washing pretreatment on key properties of torrefied palm kernel shells: A statistical optimization study

Rapid industrial growth and rising energy demands necessitate sustainable fuels like palm kernel shells (PKS). This study optimizes washing and torrefaction processes to enhance PKS fuel properties, addressing challenges like high moisture, low calorific value, and alkali metal-induced slagging and fouling. Using response surface methodology (RSM), we optimized the conditions for washing (temperature: 27–85 ºC, time: 1–6 hours) and subsequent torrefaction (300°C for 30 minutes). The proximate analysis revealed significant enhancements in calorific value (CV) and fixed carbon (FC), with reductions in volatile matter (VM) and ash content. Optimized samples exhibited the highest CV (25.30 MJ.kg⁻¹) and FC (38.99 %), and the lowest VM (2 %) and ash content (2.5 %), correlating with improved energy content and combustion efficiency. Besides, the optimized conditions also resulted in substantial potassium removal (up to 116 mg. L⁻¹), mitigating the risks of slagging and fouling. Additionally, the BET analysis showed that there were considerable increases in both surface area and pore volume with increased washing severity, and the surface morphology study revealed significant pore formation. TW3 showed the highest BET surface area (61 m².g⁻¹) and pore volume (0.09 cm³.g⁻¹), which uncovers the role of reopened vascular networks that facilitate better gas transport during torrefaction and boost combustion reactivity. Additionally, the cumulative passing particle size distribution confirms that optimized samples generate finer particles than untreated samples, highlighting their higher susceptibility to pulverization. Overall, rigorous washing conditions significantly improve solid fuel quality and grindability properties, making it a more efficient and cleaner solid fuel for industrial applications.