283099 Process Design and Optimization for Hydrocarbon Biofuel Production From Corn Stover Via Gasification
Energy resources are limited but the demand is constantly increasing. In the last decade, researchers have put effort into developing renewable energy sources because of their sustainability and environmental friendly. One of the advantages of biofuel production is it emits less over petroleum-based fuel because it yields less environmental impacts throughout the life cycle compared to their petroleum counterpart. The U.S. Government has proposed the Energy Independence and Security Act (EISA) of 2007 to regulate the development of biofuels in the United States. The main goal is to increase the production of clean renewable fuels that leads to energy independence from imports. 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 . Among all types of biofuels, biomass-derived hydrocarbon fuels provide the most similar characteristics with fossil fuels. Hydrocarbon biofuels have the advantage of compatibility with the existing transportation, storage, engines, and processing technology infrastructure . Therefore, large-scale production of biomass-derived hydrocarbon fuels can help the nation to achieve the EISA requirement in a sustainable and cost-effective way.
In this paper, we optimize the production process of gasoline and diesel from corn stover. We propose a superstructure which demonstrates the network configuration of process. The purpose is to produce hydrocarbon biofuel through various technologies with at least two options. Sections with technology alternatives are gasification, syngas cooling, hydrogen production, and Fischer-Tropsch synthesis. For gasification, we consider two types of gasifier, high-temperature and low-temperature. The choice for syngas cooling is direct quench or indirect quench. Hydrogen can be produced through two pathways, internal production or steam methane reforming. The alternative in Fischer-Tropsch synthesis is the selection of catalyst among cobalt, iron, and nickel. The maximum moisture content for biomass gasification is in the range of 20% to 30% (wet basis), and less than 15% (wet basis) for normal operation . Biomass with 25% moisture on wet basis enters the plant. Therefore, a rotary dryer is needed to reduce the biomass moisture content to 10% on wet basis. Pressurization biomass is then entering the gasifier with 95% purity of oxygen produced from the air separation unit. An air separation unit is used to separate oxygen from air. The gasifier operates at 28 bar and at two different temperatures which 1300°C for high temperature (HT), and 870 °C for low temperature (LT). According to Henrich, the mass ratio of oxygen to biomass is fixed at 0.35 using entrained-flow gasifier . Steam addition to gasifier is fixed at 0.48 mass ratio of steam to biomass . The low-temperature gasifier requires a different mass ratio of oxygen to biomass at 0.26 and mass ratio of steam to biomass at 0.17 . The high-temperature gasifier does not produce hydrocarbons because of its near equilibrium conditions . On the other hand, LT gasifier produces a significant amount of methane, ethane, and ethylene. The raw syngas exiting gasifier is cooled down to lower temperature. There are two methods for cooling, direct quench and indirect quench. Water is added directly to the syngas for direct quench. While for indirect quench, heat exchangers are required. The next step is to modify syngas to the optimal condition needed for Fischer-Tropsch synthesis. Steam methane reforming (SMR) and water gas shift are used to adjust the amount of carbon monoxide and hydrogen to meet the optimal ratio of H2 and CO around 2.1 . The purpose of a SMR step is to reduce methane, ethane, and ethylene content. 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. A pressure swing adsorption (PSA) process is used to isolate hydrogen. Excess hydrogen from steam methane reforming and water gas shift is split into two, one entering hydrocracking, and the rest to hydrotreating. The Fischer-Tropsch synthesis reactor operated at 200 °C and 25 bar using different catalyst, such as cobalt, iron, and nickel. Per-pass carbon monoxide conversion in the reactor is set at 40% . According to Song et al , the production distribution of Fischer-Tropsch synthesis follows the Anderson-Schulz-Flory alpha distribution which the chain growth factor, a, depends of the molar fraction of H2and CO and the temperature of the reactor. The value of alpha is dependent on the type of catalyst. The product composition of C1 to C20 is determined following the Anderson-Schulz-Flory alpha distribution, and the rest is all wax. Methane, ethane, and propane produced from FT synthesis are sent to gas turbine section to combust and generate electricity. The remaining Fischer-Tropsch liquids then distillate and hydrocrack to produce gasoline and diesel. Gasoline includes lighter hydrocarbons (C4~C12) and diesel includes heavier hydrocarbons (C13~C20).
The objective is to maximize the net present value (NPV) and minimize global warming potential (GWP) subject to design and operational constraints that include mass balances, energy balances, economic analysis, and environmental constraints. The equipment costs are based on the capacity of the units which are related to the feed flow rates and sizing factor. Other cost includes annual operational cost and utility cost. The process makes profit from selling gasoline and diesel. The net present value accounts for the investment and discounted annual cost and profit. The environmental impact is measured using the global warming potential metric following the recent development of the life cycle assessment (LCA) procedures. GWP is a relative measure of how much heat a greenhouse gas traps in the atmosphere, and 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. The process is optimized to determine the optimal decision of alternative selections for the production of hydrocarbon biofuel. The optimal choice is using high temperature gasification, cobalt catalyst in Fischer-Tropsch synthesis, direct quench and internal hydrogen production. To obtain the optimal trade-off between the two contradicting objective functions, we use the epsilon constraint method. The 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. It has the minimum GWP of 930 ktonne CO2-equivalent when the NPV is 0. When maximize NPV, GPW has the highest possible value of 3892 ktonne CO2-equivalent. The results show that the capital investment is 40% of the overall costs over 20 years. Among the capital investment, 39% is equipment and installation cost, 13% is engineering and supervision, 13% is construction expenses, 9% is legal and contractors fee, 14% is project contingency, and 12% is working capital.
Feedstock cost is the main expenditure which is around 43% of overall costs. Utility cost is around 2 %, maintenance cost is 5%, and catalyst cost is 10% of overall costs during the 20-year lifespan. GWP calculated in this process including steam, natural gas, electricity, heat, and CO2 emissions. Carbon dioxide emission accounts for 78% of overall GWP, and 13% for electricity. While each of the other GWP categories are ranging from 2% to 5%. Therefore, it is important to consider how to reduce CO2 emissions. One effective method is carbon capture and storage. Carbon capture and storage (CCS) is the technology to avoid releasing CO2 into the atmosphere by capturing CO2and store it to prevent releasing to atmosphere. Future work could include the alternatives of CCS in the process.
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