(516e) Optimal Design of Hydrocarbon Biorefinery Via Gasification Pathway: Multiobjective Optimization Coupled with Life Cycle Assessment

The amount of available energy is rapidly deceasing because of the large demand and consumption of fuel. Among all the energy alternatives, biomass-derived hydrocarbon fuels are most similar to fossil fuels. Concerns about global warming, national security and the diminishing supply of fossil fuels have being increasing. Therefore, the U.S. government proposed the Energy Independence and Security Act (EISA) of 2007 to encourage the development of biofuels. The EISA requires the total amount of biofuel production to increase to 36 billion gallons per year by 2022 from 4.7 billion gallons per year in 2007 [1]. Among all types of biofuels, EISA defines advanced biofuel as renewable fuel, other than ethanol derived from corn starch. The advanced biofuels would be more chemically similar to gasoline, jet fuel and diesel. Therefore, it would be more cost-effective because of the compatibility with the existing infrastructure. For example, renewable gasoline, renewable diesel, renewable jet fuel, cellulosic biobutanol, and algae-derived biofuels are infrastructure-compatible advanced biofuels. The production process would be more cost-effective and sustainable that utilizes the existing refining and distribution infrastructure.

In this work, we propose a superstructure for the optimization of hydrocarbon biorefinery via gasification pathways. The superstructure demonstrates the network configuration of the process which considers different process designs for the production of biofuels from corn stover. There are at least two options for selected technology blocks. In order to model the different alternatives, a binary variable is used to limit to only one selection when there are several technical options [2]. For gasification, we consider two types of gasifiers, high-temperature and low-temperature. Each type of gasifier would have different product distribution. Therefore, the sequential processing steps would be different. There are two ways to cool the raw syngas, which are direct quench and indirect quench. A significant amount of hydrogen is needed for hydrocracking and hydrotreating. Therefore, hydrogen can be produced through two pathways, internal production or steam methane reforming. Internal production uses the hydrogen within the syngas. While steam methane reforming uses additional natural gas to produce hydrogen. Then the conditioned syngas enters Fischer-Tropsch synthesis unit that uses cobalt, iron, or nickel based catalysts. Fischer-Tropsch liquids then can be refined to gasoline and diesel biofuel.

The properties of the feedstock are known, such as the moisture content and element compositions. The species in each stream and the reactions in each unit are also known from simulation results [3]. Therefore, component balances are performed over each unit and atomic balances are used in units with chemical reactions. Energy balances are also included as constraints. The heat consumption and generation rates are calculated using the total enthalpy differences between inlet and outlet flows. The capital costs are scaled based on the scaling stream and sizing factor. Other cost includes annual operational cost and feedstock cost. Operational cost consists of maintenance, steam, power, catalyst, and natural gas costs. The process makes profit from selling gasoline and diesel at market price. The net present value accounts for the investment and discounted annual cost and revenue.

We proposed a multi-objective mixed-integer nonlinear programming formation for the superstructure optimization of this hydrocarbon biorefinery under economic and environmental concerns. The bi-criteria optimization problem maximizes the net present value (NPV) and simultaneously minimizes the environmental impact. The environmental impact is measured using the global warming potential (GWP) metric following the recent development of the life cycle assessment (LCA) procedures.  GWP is a relative measure of how much greenhouse gas emit during a process, and is expressed in terms of carbon dioxide equivalent. GWP is calculated over a specific time interval that has to be stated, commonly 20, 100 or 500 years. In this work, 100 year is adopted following Kyoto protocol. To obtain the optimal trade-off between the two contradicting objective functions, we use the epsilon constraint method. The resulting Pareto curve demonstrates the trade-off that exists between the NPV and GWP. The optimal solutions will form a Pareto-optimal curve which represents the set of optimal solutions of the two objectives. The curve separates the region of feasible and infeasible solutions. A case study is presented and the results show that the optimal choice is using high temperature gasification, cobalt catalyst in Fischer-Tropsch synthesis, direct quench and internal hydrogen production under both the economic and environmental criteria.

References

[1]        Energy Independence and Security Act of 2007, U. S. Congress, 2007.

[2]        P. Liu, E. N. Pistikopoulos, and Z. Li, "A Multi-Objective Optimization Approach to Polygeneration Energy Systems Design," Aiche Journal, vol. 56, pp. 1218-1234, May 2010.

[3]        R. M. Swanson, A. Platon, J. A. Satrio, and R. C. Brown, "Techno-economic analysis of biomass-to-liquids production based on gasification," Fuel, vol. 89, pp. S2-S10, Nov 1 2010.