(401c) Optimal Design and Synthesis of Algal Biorefinery for Hydrocarbon Biofuel Production and Carbon Sequestration Under Economic and Environmental Objectives

In recent decades, international fossil fuels supply has been diminishing due to warfare within crude oil exporting countries and thriving of developing entities. Meanwhile, the combustion of fossil fuels has caused significant concern about climate change [1]. In order to relieve this situation, the Energy Independence and Security Act (EISA) of 2007[2] was proposed that mandates the application of 36 billion gallons of renewable fuels by 2022. Many pathways for biofuel production with respect to various biomass resources have been studied to meet the ambitious target set by EISA. Algae, outweighing others in terms of high area productivity, minimized competition with conventional agriculture, utilization of a variety of water sources, close to zero net carbon dioxide emissions and valuable co-products, stand out to be a very prospective biomass feedstock towards advanced biofuel for aviation transport. The advantages of algae have attracted the attention of nationwide researchers since mid-twentieth century and a number of achievements have been made on algae cultivation, harvesting and dewatering, extraction, downstream upgrading technologies. However scarcity of systematic optimization undoubtedly makes the theoretical algae-based biorefinary design unviable. Therefore, the objective of this work is to develop the optimal process design and operating conditions on the basis of the present technologies from a process system engineering point of view.

In this work, a new superstructure of algae-based biorefinery is proposed to simultaneously produce renewable diesel through hydroprocessing and power from combustion of biogas. In combination with the existing algae cultivation process [3], alternatives for solvent extraction, hydrogen production, residue treatment, hydroprocessing reaction conditions are considered [4-7]. For solvent extraction mainly to obtain lipid from algae, we include two types of solvents, hexane and ethanol. In order to deal with the lipid-extracted remnant, anaerobic digestion or hydrothermal gasification is to be applied for biogas production along with recycled nutrients as co-product. One of the major reactants in hydroprocessing reaction is hydrogen, which will be produced from steam reforming of either natural gas, or biogas, or algal lipid. Two candidate catalysts are responsible for the hydroprocessing reaction and provide different product distribution. The rest of biogas will be fed into a gas turbine to produce power and the flue gas serves as the carbon source for algae cultivation. Wastewater treatment and recycles especially a steam cycle is also integrated to enhance the performance of the entire biorefinery.

We propose a multi-objective mixed integer nonlinear programming (MINLP) model for the superstructure optimization. Integer variables are chosen to account for the selection of one particular option within the pool of feasible technologies. The properties of the flue gas feed are given as well as split fractions, reaction conversions which will contribute to the formulation of mass balance constraints of each unit. Energy balance constraints will be conducted by calculating the enthalpy differences between inlet and outlet. Heat and power consumption will be satisfied by utilities and market power supply, respectively. A Life Cycle Optimization frame work is proposed in this work that integrates the multi-objective superstructure optimization scheme with Life Cycle Assessment and techno-economic analysis of algal biorefinery [3, 8-11]. One of the objectives is to maximize the net present value measured by the profit from selling the renewable diesel and power offset by equipment capital cost and operation cost of purchasing feedstocks along with utilities. The other objective is to minimize greenhouse gas emission analyzed according to the typical life cycle assessment (LCA) procedure. LCA consists of four major steps, namely goal and scope definition, inventory analysis, impact assessment and interpretation [12]. There are many metrics in the impact assessment step to quantify the environmental impact. In this work we choose global warming potential (GWP) based on a time horizon of 100 years which is calculated by the sum of greenhouse emissions times corresponding global warming damage factor reported in the IPCC publications. The total GWP demonstrates the relative environmental influence of this algal biorefinery compared with that of the same mass of carbon dioxide. In order to address the two contradictory objectives, epsilon-constraint method is employed and the final result is demonstrated in a Pareto curve which reveals the tradeoffs among different optimal points under both economic and environmental objectives.

References

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