Impact of low-chemical storage pretreatment of loblolly pine bark on biochar from microwave pyrolysis

Forest product residues such as bark represent a low-cost, abundant feedstock for bioenergy, but their high ash and alkaline earth metal (AAEM) content limit thermochemical conversion efficiency. This study evaluates the use of low-severity chemical pretreatments during anaerobic storage to improve the performance of microwave pyrolysis for loblolly pine bark. Bark was treated with dilute sulfuric acid (0.1 % and 1 %, w/w ) or sodium hydroxide (4 %, w/w ) and incubated anaerobically for one or two weeks to simulate in-pile biorefinery storage. The most effective treatment—1 % H₂SO₄ for two weeks—reduced AAEM content by 35.7 % and increased bio-oil yield by 11 % compared to untreated controls, while also reducing pyrolysis gas production. In contrast, alkali treatment did not reduce AAEM levels and led to decreased bio-oil yields with increased gas formation. Although biochar yields were relatively stable across treatments, their physicochemical characteristics varied significantly. Acid-treated bark yielded biochars with higher carbon content, lower O/C and H/C ratios, greater surface area, and enhanced heating values. These improvements suggest that chemical pretreatment during storage can tailor biochar quality for specific end uses. Biochars produced under optimized conditions exhibited properties suitable for soil amendment, carbon sequestration, and solid fuel applications. This integrated approach combining storage, mild chemical conditioning, and microwave pyrolysis provides a viable pathway to enhance the value and sustainability of bark-derived bioenergy products. • Reduce ash and alkaline earth metals in pine bark through mild pretreatment during storage. • Improve microwave pyrolysis performance with pretreated bark. • Increase bio-oil yield by 11 % using 1 % sulfuric acid for two weeks. • Produce biochars with high carbon content and low oxygen-to-carbon ratio. • Demonstrate biochar suitability for soil amendment, carbon storage, and renewable fuel.