Environmental and energy performance of cotton stalk residues for sustainable bioenergy production.

This study evaluates the energy and environmental performance of cotton stalk residues as a process-level solid bioenergy resource using a gate-to-gate analytical framework. Briquettes were produced under varying particle sizes (≤ 0.7–3 mm), compaction pressures (3–16 MPa), and moisture contents (6–15%). The characterization included density, total specific energy consumption (SECT), calorific value, ash content, thermogravimetric behavior, and combustion emissions (CO₂, CO, SO₂, and NOx). To quantify the trade-off between densification energy demand and emission-related outputs, two composite indicators, the Energy Performance Index (EPI) and the Environmental Impact Index for Energy (EIIE) were applied and interpreted strictly within a process-level context. The results showed that high briquette density (up to 1180 kg·m⁻³), a gross calorific value of 18.64 MJ·kg⁻¹, and low ash content (3.4%) can be achieved under suitable compaction conditions, while combustion emissions were dominated by CO₂ with comparatively low levels of other pollutants. Optimal energy and environmental performance at the process level was observed at moderate moisture contents and coarser particle sizes, reflecting a balance between mechanical energy input and fuel quality. Overall, the findings indicate that cotton stalk residues can be converted into operationally efficient solid biofuels with favorable process-level emission characteristics, supporting their valorization in waste-to-energy applications. The study further presents a novel process-level integrated assessment that quantitatively links densification energy demand, fuel energy output, and combustion-related emissions through composite performance indicators (EPI and EIIE).