(724a) Simultaneous Production of Liquid Fuels and Hydrogen From Hybrid Feedstock: Switchgrass and Shale Gas

Over the last decades there has been an enormous effort to develop alternative fuels for transportation. Bioethanol and biodiesel are the most promising ones due to their compatibility with the automobiles and the crude supply chain. However, there are a number of challenges that biofuels must overcome mainly due to the large demand of liquid fuels and the limited production capacity of biofuels from biomass resulting in the need for large harvesting areas and for raw materials that do not compete with the food market. In the mean time hybrid first and second generation biofuels, using corn grain and stover for the production of bioethanol (Cucek et al 2011) or hybrid biomass – fossil fuel feedstock (Baliban et al 2012) can be used to reduce the dependency on fossil fuels and to serve as a bridge between technologies. Recently a new and plentiful source has been found, shale gas. Shale gas is natural gas that is trapped within shale formations. Of the natural gas consumed in the United States in 2009, 87% was produced domestically; thus, the supply of natural gas is not as dependent on foreign producers as is the supply of crude oil, and the delivery system is less subject to interruption and can be used as raw material for liquid fuels.

In this paper, we focus on the conceptual optimal design of multiproduct facilities for the production of green diesel from lignocellulosic biomass and shale gas feedstocks in an attempt to increase the yield to hybrid fuels while reducing the dependency on foreign supply in order to reduce its production cost to make it competitive with bioethanol from switchgrass (Martin & Grossmann 2011). We consider swichgrass as lignocellulosic feedstock since it is the raw material of choice by the DOE due to the harvesting possibilities in the US and its high yield to biofuels.  We propose a limited superstructure optimization approach where we first construct a flowsheet embedding alternative routes for the production of liquid fuels. The process is based on FT technology in which the shale gas is reformed with steam to generate raw syngas. In parallel the switchgrass is gasified, reformed (we consider steam reforming or partial oxidation) to produce raw syngas. The total raw syngas is cleaned up from solids and sour gases. Next its composition may need to be adjusted for the optimal operation at the FT- reactor (using either WGSR, or PSA). The syngas is transformed into liquid fuels in an FT - reactor. The heavy liquids are upgraded using hydrocraking to increase the yield towards FT-diesel. The goal is to optimize the flowsheet, the product distribution (hydrogen, FT –diesel and gasoline production) and the operating conditions to maximize FT-diesel and hydrogen production determining the optimal consumption of raw materials (biomass and shale gas) as function of their price while minimizing the energy input for a number of scenarios where we fix the production level of liquid fuels and/or the maximum availability of biomass and shale gas devoted to fuels production to evaluate the profitability and competitiveness of this kind of hybrid facilities.

It turns out that the selected technology for reforming the raw syngas from the gasifier is steam reforming due to the higher yield to hydrogen. The process operates in such a way that the shale gas that is not required to meet the demand established for liquid fuels is used for the production of hydrogen that is sold to help in the economy of the plant as long as its price is low, otherwise it is better not to produce it. Thus, for this process to produce liquid fuels at around $1/gal, the biomass price must be kept below $100/t and the shale gas should not go beyond $11.5/MMBTU.

References:

Baliban, R.C.; Elia, J.A., Floudas, C. A. (2012) Simultaneous process synthesis, heat, power, and water integration of thermochemical hybrid biomass, coal, and natural gas facilities. Comp. Chem. Eng. 37, 10, 297–327

Cucek, L., Martín, M., Kravanja, Z., Grossmann, I.E., (2011) Integration of Process Technologies for the Simultaneous Production of fuel Ethanol and food from Corn grain and stover. Computers and chemical engineering  35,8, 1547-1557

Martín, M., Grossmann, I.E. (2011) Energy optimization of lignocellulosic bioethanol production via gasification. AIChE J. 57, 12, 3408, 3428