Poly(3-hydroxybutyrate) (PHB) is a microbially produced biopolymer that is emerging as a propitious alternative to petroleum-based plastics owing to its biodegradable and biocompatible properties. However, to date, the relatively high costs related to the PHB production process are hampering its widespread commercialization. Since feedstock costs add up to half of the total production costs, ample research has been focusing on the use of inexpensive industrial side streams as carbon sources. While various industrial side streams such as second-generation carbohydrates, lignocellulose, lipids, and glycerol have been extensively investigated in liquid fermentation processes, also gaseous sources, including carbon dioxide, carbon monoxide, and methane, are gaining attention as substrates for gas fermentation. In addition, recent studies have investigated two-stage processes to convert waste gases into PHB via organic acids or alcohols. In this review, a variety of different industrial side streams are discussed as more sustainable and economical carbon sources for microbial PHB production. In particular, a comprehensive overview of recent developments and remaining challenges in fermentation strategies using these feedstocks is provided, considering technical, environmental, and economic aspects to shed light on their industrial feasibility. As such, this review aims to contribute to the global shift towards a zero-waste bio-economy and more sustainable materials.
A combined thermochemical-biochemical Biomass-to-Liquid (BtL) pathway for the production of aviation and maritime liquid fuels is presented. The presence of a semi-commercially proven technology like Dual Fluidized Bed Gasification (DFBG) ensures extended fuel flexibility, syngas of high quality, complete fuel conversion and optimal heat integration while avoiding CAPEX intensive equipment like Air Separation Unit. Then, a twostage biochemical route is proposed: initially syngas fermentation (anaerobic) into acetate and subsequently acetate fermentation (aerobic) into targeted triglycerides (TAGs) that will be finally purified and hydrotreated to form the desired drop-in biofuels. The tolerance of the bacteria to syngas contaminants minimizes the gas cleaning requirements. Moreover, the low-pressure requirements (1-10 bar) along with the mild operating temperatures (30-60°C) reduce drastically the capital and operational cost of the process. In terms of efficiency, the biological process of syngas fermentation inherently has limited side products, a fact that reduces the risk of deactivation of hydrotreatment catalysts. The aim of this study is to develop the process model of this novel biorefinery in Aspen Plus TM and to perform the energy and mass balance calculations of the whole value chain, to determine the appropriate key process specifications and to estimate the production cost of the targeted drop-in biofuels.
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