Techno-economic and emissions comparison of waste-to-fuel via hydrothermal liquefaction, transesterification, and incineration

Abstract

The global shift toward sustainable waste management and renewable energy has sparked interest in biofuel production from sewage sludge (SS). This study evaluated four waste-to-biofuel processes like Hydrothermal Liquefaction (HTL) with upgrading, Transesterification, and Incineration with and without energy recovery using ASPEN Plus V12 to assess their techno-economic, energy, and environmental performance. HTL with upgrading emerged as the most efficient, generating ∼4,000,000 MJ/year and emitting ∼700 tonnes/year of CO₂. Transesterification yielded ∼2,850,000 MJ/year, emitting ∼1200 tonnes/year due to post-lipid extraction incineration. Incineration without energy recovery was least efficient, consuming ∼5,000,000 MJ/year and emitting ∼3000 tonnes/year of CO₂, with energy recovery yielding only ∼1,250,000 MJ/year. Financially, HTL with upgrading demonstrated strong profitability with a potential Net Present Value (NPV) of 112.9 million US dollars (MUS$), while Transesterification achieved an NPV of 23.4 MUS$. Both processes were sensitive to operating costs: a 50 % increase could reduce HTL’s NPV to 62.7 MUS$, while pushing Transesterification into a loss. Capital cost reductions could further boost HTL’s profitability, highlighting its economic resilience, unlike incineration, which remained financially unviable. In summary, HTL with upgrading offered 30 % higher energy output and 70 % lower emissions than incineration, making it a scalable, sustainable approach for SS management and biofuel production. However, a complete life cycle assessment could further enhance its potential by identifying additional environmental and economic benefits.