Citation: Giannopoulos, D.; Katsifis, I.; Katsourinis, D.; Rentizelas, A.; Founti, M. An Assessment of Liquid Biofuel Value Chains from Heavy-Metal Contaminated Feedstock. Fuels 2022, 3, 509–532. https://doi.org/10.3390/ fuels3030031 Academic Editor: Mamoru Yamada Received: 7 June 2022 Accepted: 2 August 2022 Published: 13 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Review An Assessment of Liquid Biofuel Value Chains from Heavy-Metal Contaminated Feedstock Dimitrios Giannopoulos 1, * , Ilias Katsifis 1 , Dimitrios Katsourinis 1 , Athanasios Rentizelas 2 and Maria Founti 1 1 Laboratory of Heterogeneous Mixtures and Combustion Systems, School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece 2 Sector of Industrial Management and Operational Research, School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece * Correspondence: digiann@central.ntua.gr Abstract: The present work aims to identify alternative liquid biofuel value chain scenarios utiliz- ing heavy metal (HM)-contaminated biomass feedstocks. The analysis is based on breaking down existing liquid biofuel value chains, focusing on the required adaptations needed for clean biofuel production. State-of-the-art and emerging liquid biofuel production options are reviewed. The potential implications caused by the HM load in the biomass feedstock are analyzed along the whole biofuel production chain, which includes pre-processing, conversion and post-processing stages. The fate of the most common HM species present in contaminated biomass is identified and graphically represented for advanced (second generation) biofuel conversion processes. This information synthe- sis leads to the description of alternative value chains, capable of producing HM-free biofuel. This work goes a step further than existing reviews of experiments and simulations regarding heavy metal- contaminated biomass (HMCB) valorization to biofuels since feasible value chains are described by synthesizing the findings of the several studies examined. By defining the adapted value chains, the “road is paved” toward establishing realistic process chains and determining system boundaries, which actually are essential methodological steps of various critical evaluation and optimization methodologies, such as Life Cycle Assessment, supply chain optimization and techno-economic assessment of the total value chain. Keywords: liquid biofuels; heavy metals; contaminated biomass feedstock; value chains 1. Introduction The transport sector (including international aviation and shipping) is responsible for 32% of the total CO 2 emissions in the EU, and in 2017, 72% of these were attributed to road transport [1]. Given that the target of achieving carbon neutrality by 2050 [2] requires a reduction target of Greenhouse Gas (GHG) emissions of at least 55% by 2030 [3], a respective reduction of transport-related emissions is a priority issue. Until newer technologies can be available at a large scale for the transport sector decarbonization (e.g., electrification or hydrogen), biofuels represent the most convenient and tangible short-term solution. Toward this direction, the EU, since 2009, has introduced Renewable Energy Directives (RED) [4,5], which mandate all country members to cover a significant part of their transport energy demand from renewable sources. In the latest amendment published in July 2021 [6], it is stated that the EU target of at least 32% of renewable energy sources in the overall energy mix must increase to 38–40% by 2030. In this direction, a 13% reduction in the transport fuels’ greenhouse gas intensity is proposed. According to [7], this target expressed in energy-based terms reaches up to a respective reduction of 28%, with an additional sub-target of 2.2% share for advanced biofuels (single counted), corresponding to a significant ambition increase considering that the previous Fuels 2022, 3, 509–532. https://doi.org/10.3390/fuels3030031 https://www.mdpi.com/journal/fuels