(424f) Techno-Economic Analysis of a Perspective Bioethylene Production Process from Photosynthetically-Fixed CO2 in Recombinant Cyanobacteria

Ethylene is the largest petrochemical produced, by volume, worldwide. It serves as a building block for a wide variety of plastics, textiles, and chemicals, and a process has been developed for its conversion into liquid transportation fuels. Today, nearly all ethylene is produced from fossil sources, primarily through steam cracking of naphtha, gasoil, and condensates. There is great interest in developing technologies for ethylene from renewable resources, including biologically-derived biomass and CO₂. Although bioethylene is mainly produced today from the dehydration of bio-ethanol or methanol, additional pathways include direct metabolic synthesis of ethylene by microbes. One of the metabolic pathways used by microbes to produce ethylene is via an ethylene-forming enzyme (Efe), which enables potential valuable biotechnological approaches for biological ethylene production.  A team from the National Renewable Energy Laboratory (NREL) has engineered a genetically stable strain of cyanobacteria that is able to produce ethylene from CO₂ and sunlight by utilizing the Efe pathway¹, with more ongoing research to understand the Efe pathway².

There are techno-economic analysis (TEA) papers on the algae to lipids pathways through photosynthesis. However, there are limited studies on understanding main cost drivers and economic impacts of photosynthetic production of bioproducts or chemicals from cyanobacteria or algae using integrated process concepts. Here, we will present the first of its kind conceptual design and TEA for photosynthetic ethylene from cyanobacteria. Ethylene is an interesting product candidate with high energy content  because of its viability as a building block for other materials, as well as its high demonstrated productivity (739 mg/L/day)³ compared to other possible products such as: 2,3-butandiol (236.3 mg/L/day) ⁴, isobutyraldehyde (149.5 mg/L/day)⁵, and ethanol (95.4 mg/L/day)⁶. However, similar to algae production of lipids, R&D is still in an early development stage. Therefore, the goal of this TEA is to predict near-long term cost targets, rather than judging the current state of technology.  The process model utilizes commercial scale production and nth plant assumptions, and more importantly, the integrated process concepts. The conceptual process design for such technology includes (1) ethylene production in a photobioreactor, (2) ethylene separation from photobioreactor off-gas, (3) ethylene oligomerization to hydrocarbon fuels, (4) distillation of hydrocarbons into fuel range cuts, and (5) anaerobic digestion of cyanobacteria spent biomass in a waste-water treatment facility, as well as nutrients recycling strategies. We will present not only the integrated process concept but also results of the economic analysis. The economic analysis will include capital costs, operating costs, and a minimum ethylene and hydrocarbon fuel (after ethylene upgrading) selling price determined from a discounted cash flow analysis. The process challenges will also be examined including ethylene separation from photobioreactor off gas. Additionally, sensitivity analysis will be discussed exploring key parameters that can significantly impact the overall process economics.  The sensitivity analysis will consider several process options such as, photobioreactor type, ethylene separation method, and microbial ethylene productivity. Finally, prospective options for R&D will be discussed.

Reference:

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2.         Ecket C XW, Xiong W, Lynch S, Ungerer J , Tao L, et al. . Ethylene-forming enzyme and bioethylene production. Biotechnol Biofuels. 2014; 7-33.

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