Valorization of recycled paper mill sludge via mass–energy integration for sustainable onsite power generation: A case study

Abstract

This study presents a site-specific, year-long integrated mass–energy assessment of sludge valorisation at a Thai recycled paper mill, evaluating three sludge-to-energy pathways: (1) refuse-derived fuel (RDF) production from dewatered sludge, (2) anaerobic digestion (AD) of primary and secondary sludge, and (3) a hybrid cascade combining AD with digestate-to-RDF conversion. An auditable organic-carbon balance with a very small deviation (ΔMB ≈ +0.0068%) demonstrates high data quality and traceability. A product-substitution framework was applied using functional units of 1 kWh of on-site electricity and 1 t of on-site steam delivered, with system boundaries explicitly incorporating air-pollution control systems (APCS) for RDF and essential gas-cleaning for AD. The analysis indicates substantial potential for fossil-energy displacement, with differences observed among the three pathways once APCS-related parasitic loads and compliance costs are considered. Over a 20-year project horizon, techno-economic analysis shows that the relative economic performance of the pathways is strongly influenced by the treatment of APCS capital and operating expenditures under on-site utilisation conditions. One-way sensitivity analysis identifies electricity tariff and grid emission factor, sludge and digestate dryness and higher heating value, and APCS costs as the dominant parameters affecting economic outcomes. By integrating experimental data with mass–energy balancing and substitution-based assessment, this study provides a transparent analytical basis for comparing sludge-to-energy options in recycled paper mills under realistic operational and compliance constraints.