Oxygen-rich torrefaction of bamboo: Multivariate process optimization and environmental assessment

Abstract

Torrefaction is a promising pretreatment method to enhance the fuel properties of lignocellulosic biomass; however, conventional studies are typically restricted to inert, air-limited, or air or flue gas atmospheres. In addition to inert and air torrefaction, this study also investigates the oxygen-rich torrefaction of bamboo (Phyllostachys makinoi Hay) under O2 concentrations up to 30 % to analyze both process performance and environmental implications. A three-level full factorial design combined with analysis of variance (ANOVA) is employed to optimize torrefaction operations in terms of energy yield, higher heating value, and solid product quality. Oxygen enrichment accelerates the devolatilization of bamboo and enhances carbonization efficiency, resulting in significantly higher energy densification. However, these benefits are accompanied by increased mass loss as oxygen levels rise, highlighting a clear trade-off between biochar quality and solid yield. Statistical modeling reveals a strong predictive capability (R2> 0.95) for key responses, enabling the accurate determination of optimal conditions. For 1 kg of bamboo-derived biochar at a lab scale, the global warming potential (GWP) was found to range from 12 to 13 kg CO2-eq. Higher O2 levels improve fuel quality but increase indirect CO2 emissions from oxygen supply in the life cycle assessment (LCA). This highlights the critical need to strike a balance between process efficiency and overall sustainability goals. Overall, this work provides novel insights into the mechanisms, optimization, and environmental trade-offs of oxygen-rich bamboo torrefaction, offering guidance for designing low-carbon bioenergy systems and expanding the applicability of torrefaction in sustainable energy transitions.