Comparative energy assessment and net energy return analysis of non-wood lignocellulosic bioethanol systems

The production of second-generation bioethanol from non-woody lignocellulosic feedstocks is constrained not only by biomass recalcitrance but also by the substantial energy burden of deconstruction and conversion. In this study, we compile and normalize energy and yield data from recent publications to evaluate process-level performance across pretreatment, saccharification, fermentation, and distillation stages. The overall energy consumption typically ranges between 25 and 130 kWh per kilogram of ethanol. The stage-level contributions are distributed as pretreatment (5–20%), saccharification (10–25%), fermentation (10–20%), and distillation (40–60%). Heating demand accounts for approximately 70–80% of the total, whereas non-heating electricity contributes around 20–30%. This pattern indicates that heat integration and the adoption of low-carbon steam are essential strategies for reducing thermal loads, while improvements in agitation, milling, and mixing efficiency are more effective for reducing electrical demand. Net energy ratio (NER) values remain below unity when heating loads are included, but they increase substantially when only non-heating inputs are considered, which emphasizes the critical role of heat management. The associated CO2-equivalent emissions further confirm that distillation is the dominant hotspot. Overall, low- to moderate-temperature alkaline and hybrid pretreatments approach the efficiency frontier, while severe high-temperature routes do not necessarily deliver system-level gains. Heat integration could reduce steam demand by 20–40% and thereby improve the sustainability of non-woody lignocellulosic ethanol systems without altering relative performance trends.