Innovative approach to produce raw, torrefied almond shells and plastic waste blend pellets

Authors: Tumuluru, Jaya Shankar; OGINNI, OLUWATOSIN - Idaho National Laboratory; SMITH, ZACHERY - Idaho National Laboratory; WAHLEN, BRADLEY - Idaho National Laboratory

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2026
Publication Date: 2/26/2026
Citation: Tumuluru, J., Oginni, O., Smith, Z., Wahlen, B.D. 2026. Innovative approach to combine torrefied almond shells and plastic waste blends into pellets for biofuels and biocomposites applications. Energies. 19(5). https://doi.org/10.3390/en19051159.
DOI: https://doi.org/10.3390/en19051159

Interpretive Summary: There’s a need to develop new uses for almond production byproducts to provide new revenue streams for producers. Work was conducted to explore using plastic and almond shell blends for high-value biocomposite applications. A novel approach was developed that combines high-density polyethylene (HDPE) and polypropylene plastic waste with raw and torrefied almond shells to produce composite pellets. The almond shells underwent a thermal pretreatment process called torrefaction, where samples were heated at 300 °C for 30 minutes. This process enhanced the chemical composition and energy content of the almond shells. For the pelleting process, both the raw and torrefied almond shells and high-density polyethylene and polypropylene plastic waste were ground into smaller particles, blended in a 30%:70% weight ratio with the addition of 10% pure cornstarch, which help bind the pellets. Pelleting tests were conducted using both laboratory and pilot-scale systems. The pellets had bulk density and durability measurements that showed they would transport well using trucks with low breakage, which is important for an economical biocomposite feedstock. Additionally the blend pellets have better storage characteristics and also feed uniformly to the downstream conversion reactors. Development of these blend pellets will help eliminate the adverse environmental impacts of plastic waste but also increase the value of almond byproducts and significantly contribute to achieving the goals of a circular bioeconomy.

Technical Abstract: The increasing demand for sustainable materials has driven interest in biocomposites that incorporate low-value agricultural residues to offset the use of virgin plastics. The study investigated the production of blend pellets from raw and torrefied almond shells and post-consumer plastic waste as a potential feedstock for biocomposite and biofuels applications. Almond shells were torrefied in a lab-scale fixed-bed reactor at 300 °C for 30 min prior to the pelleting tests. High-density polyethylene (HDPE) and polypropylene (PP) wastes were size-reduced in a Crumbler (rotary shear grinder) fitted with a 2 mm head and a 2 mm screen to remove the fines. A portion of the crumbled HDPE, and torrefied almond shells were further ground in a Wiley mill fitted with 2 and 1 mm screens for flat die pelleting tests. The flat die pellet mill used for testing had a 6 mm die and a length-to-diameter (L/D) ratio of 2.0. The blend ratio consisted of 30% torrefied almond shells and 70% HDPE, with a 10% starch binder. The measured pellet properties include unit, bulk and tap densities, durability, and expansion ratio. The bulk density of the blend pellets ranged from 360 to 410 kg/m3, and durability ranged from 80% to 88%. The blend pellet unit density ranged from 830 to 880 kg/m3. The blend pellets produced using crumbled HDPE, PP and raw and torrefied almond shells in a ring die pilot-scale pellet mill with an L/D ratio of 6 and steam conditioning exhibit similar densities to those of HDPE pellets produced using a flat die pellet mill, albeit with lower durability. The study indicated that a smaller grind size and preheating the blend before pelleting produce blend pellets with higher density and greater durability.