(483f) Superstructure Optimization of Hydrocarbon Biorefinery Via Fast Pyrolysis, Hydrogen Production and Hydroprocessing Pathway

The burgeoning of global economy relies heavily on stable energy supply which, however, suffers from uncertainty due to limited fossil fuel resources and critical environmental issues such as climate change by accumulated greenhouse gases. This situation has motivated great interests and efforts in looking for advanced fuel alternatives, and thereby the Energy Independence and Security Act (EISA) of 2007 [1] was enacted to reduce the U.S.’s dependence on nonrenewable fuels from foreign countries. In response to EISA, many pathways are currently being studied for the production of sustainable and environmentally friendly biofuels from various biomass feedstocks. Among them, hydrocarbon biorefinery via fast pyrolysis followed by hydrotreating and hydrocracking demonstrates great potential in term of low production cost and small environmental impact [2]. Fast pyrolysis is a thermal process that rapidly heats biomass to around 500 °C in the absence of oxygen and cools the gaseous products quickly to simultaneously obtain biogas, crude bio-oil and char. Since the crude bio-oil is a mixture of oxygenated hydrocarbon compounds and unstable for long-term storage and immiscible with any conventional hydrocarbon based fuels [3], it needs to be further stabilized by oxygen removal and long-chain hydrocarbon breakdown through catalytic hydrotreating and hydrocracking.

In this work, we propose a superstructure for a hydrocarbon biorefinery with hybrid poplar feedstock, which mainly encompasses biomass pretreatment, fast pyrolysis for crude bio-oil, liquid collection, hydrogen production, crude bio-oil upgrading and products separation. The superstructure considers a lot of alternatives of technologies and equipment which are equivalent in making up the entire design but have pros and cons in different situations. Hydrogen production is one of the most important and energy consuming sections within the process. We prepare three options. One involves steam reforming from natural gas purchased from the market; the second option takes advantage of the same approach while using bio-oil from upstream as the feedstock; the last one tries to produce hydrogen through gasification of biomass followed by syngas clean up and water gas shift. The third hydrogen production option also consists of different choices including low temperature gasification, high temperature gasification, partial oxidation and steam reforming. The operation conditions of the hydrotreaters are determined by the specific configuration requirements of one of the three catalyst candidates.

We formulate a mixed integer nonlinear programming (MINLP) model with two objectives, namely maximizing the net present value (NPV) and minimizing the greenhouse gas emission. The selection of different technologies and equipment in the MINLP model is realized by introducing integer variables and applying a series of technology selection constraints. Basic information for feedstock composition, split fractions of separation devises and product distributions in every reactor outlet is given for the formulation of mass balance constraints. Energy balance constraints are imposed by considering the enthalpy differences of the inlet and outlet of each unit. Heat and power consumption will be satisfied by utilities and market power supply, respectively. The economic performance of this hydrocarbon biorefinery is measured by the NPV, which consists of capital cost given by the sum of equipment purchase cost, and annual operating cost given by the sum of feedstock cost, natural gas, transportation cost, utility and power cost, as well as revenue from selling all the products. 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 [2, 4-7]. Greenhouse gas emission, the other objective, is analyzed following standard principle of life cycle assessment (LCA). LCA has 4 steps which are goal and scope definition, inventory analysis, impact assessment and interpretation [8]. 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 given by the sum of the GWP from each greenhouse gas emission source. GWP is calculated by the greenhouse emission times corresponding global warming damage factor reported in the IPCC publication and demonstrates the relative environmental influence of this hydrocarbon biorefinery compared with that of the same mass of carbon dioxide. Furthermore, epsilon-constraint method is employed to deal with the two contradictory objectives and finally we can obtain a Pareto curve which contains all the optimal points and reveals the tradeoffs among them under both economic and environmental objectives.

References

[1]          U. Congress, "Energy independence and security act of 2007," Public Law, p. 2, 2007.

[2]          B. H. Gebreslassie, M. Slivinsky, B. L. Wang, and F. Q. You, "Life cycle optimization for sustainable design and operations of hydrocarbon biorefinery via fast pyrolysis, hydrotreating and hydrocracking," Computers & Chemical Engineering, vol. 50, pp. 71-91, Mar 5 2013.

[3]          S. B. Jones, C. Valkenburg, C. W. Walton, D. C. Elliott, J. E. Holladay, D. J. Stevens, et al., Production of gasoline and diesel from biomass via fast pyrolysis, hydrotreating and hydrocracking: A design case: Pacific Northwest National Laboratory Richland, WA, 2009.

[4]          F. Q. You, L. Tao, D. J. Graziano, and S. W. Snyder, "Optimal design of sustainable cellulosic biofuel supply chains: Multiobjective optimization coupled with life cycle assessment and input-output analysis," Aiche Journal, vol. 58, pp. 1157-1180, Apr 2012.

[5]          F. Q. You and B. Wang, "Life Cycle Optimization of Biomass-to-Liquid Supply Chains with Distributed-Centralized Processing Networks," Industrial & Engineering Chemistry Research, vol. 50, pp. 10102-10127, Sep 7 2011.

[6]          B. H. Gebreslassie, R. Waymire, and F. You, "Sustainable design and synthesis of algae-based biorefinery for simultaneous hydrocarbon biofuel production and carbon sequestration," AIChE Journal, vol. 59, pp. 1599-1621, 2013.

[7]          B. Wang, B. H. Gebreslassie, and F. Q. You, "Sustainable design and synthesis of hydrocarbon biorefinery via gasification pathway: Integrated life cycle assessment and technoeconomic analysis with multiobjective superstructure optimization," Computers & Chemical Engineering, vol. 52, pp. 55-76, May 10 2013.

[8]          J. B. Guinée, "Handbook on life cycle assessment operational guide to the ISO standards," The international journal of life cycle assessment, vol. 7, pp. 311-313, 2002.