Multiparameter optimization of torrefaction for achieving carbon-negative biocoal: integrating quality parameters, thermodynamics, and environmental performance

Abstract

This study advances the torrefaction field by proposing a framework that integrates energy, exergy, and environmental analyses with the quality assessment of torrefaction products, while evaluating process carbon neutrality and carbon-negative outcomes. While biocoal properties are often emphasized, multi-objective analyses addressing critical aspects such as exergy efficiency and life cycle assessment are frequently overlooked. This research critically addresses inconsistencies in life cycle assessment related to functional units, system boundaries, and impact allocation of products, fostering a consistent and robust environmental diagnostic. Experimental data from urban forest waste torrefaction, combined with a two-step kinetic modeling, enabled the simulation of a scaled-up system using Aspen Plus. This integrative approach assessed the properties of biocoal, bio-oil, and torgas, as well as mass and energy flows, irreversibilities, and process emissions. Life cycle assessment quantified and allocated environmental impacts. The framework accounted for CO2 uptake by biomass, revealing trade-offs arising from the severity of torrefaction and the definition of the functional unit. Response surface methodology served as a unifying optimization tool, allowing the simultaneous integration and evaluation of all indicators. Results identified bottlenecks, formulated an equation to evaluate carbon neutrality and determined optimal conditions, offering a scalable and replicable pathway for sustainable torrefaction. Optimal conditions at 256 °C for 41 min yielded biocoal with 87.82 % mass retention, a heating value of 20.98 MJ kg−1, a fuel ratio of 0.34, and an ash content of 4.98 %. The system required 20.99 kWh for drying and 4.04 kWh for torrefaction, with the irreversibility of 81.5 MJ h−1 and a global warming potential of –0.504 kg CO2 eq. per GJ of biocoal.