(412a) An Integrated Strategy for Production of Liquid Transportation Fuels Via Simultaneous Catalytic Conversion of Cellulose and Hemicellulose From Lignocellulosic Biomass

To date, several studies have focused on the development of processes for the catalytic conversion of the cellulose fraction of the lignocellulosic biomass. However, there are few studies conducted to develop a process for the simultaneous catalytic conversion of cellulose and hemicellulose fractions to liquid hydrocarbon fuels. Accordingly, our goal in this study is to develop an integrated strategy for the simultaneous catalytic conversion of these fractions. In the proposed strategy, cellulose and hemicellulose are converted to the same product, levulinic acid (LA), using LA-derived γ-valerolactone (GVL) as a solvent.¹ Then, the LA is catalytically reduced to GVL,² and subsequently, GVL is converted to a mixture of butene oligomers.³ To generate the integrated strategy, we develop interconnecting separation systems to achieve experimentally optimized feed concentrations for the catalytic conversion steps. 

A potential drawback of this strategy is its high heating requirements, hence we perform heat integration to maximize heat recovery. After heat integration, the remaining heating requirement is satisfied from combustion of biomass residues, which are also used to produce high-pressure steam for electricity generation in a turbogenerator unit (basic design). However, the cost of the turbogenerator unit accounts for a significant fraction (35-40%) of the total capital cost.⁴ Thus, we develop an alternative design, in which there is no electricity generation, and electricity is externally supplied. In this case, biomass residues are only used to generate steam for heating, and the excess residues are disposed. The basic and alternative designs are evaluated and compared with a lignocellulosic ethanol production process proposed by the National Renewable Energy Laboratory (NREL)⁵. To this end, we select the same feedstock and processing rate (2,000 dry tons of corn stover per day) and the same economic parameters and assumptions.

Based on the capital and operating costs, we perform a discounted cash flow analysis to determine a minimum selling price (MSP) of butene oligomers that makes the net present value of the project equal to zero. We find that the MSP for the basic design is 33¢ per gallon of gasoline equivalent (GGE) higher than the alternative design ($5.09 GGE^-1) since the electricity credits obtained in the basic design is low (1.6% of the total production cost). Although the ethanol process yields a higher fuel production than the catalytic strategy, the MSP for the alternative design is 4¢ per GGE lower than the ethanol process ($5.13 GGE^-1) since the total annualized cost of the alternative design is significantly lower than that of the ethanol process. 

Furthermore, we evaluate different feedstock alternatives based on their compositions and prices, since the feedstock cost is the major contributor of the operating costs. We find that the use of hybrid poplar is the best option (MSP of $4.29 GGE^-1) among all feedstock alternatives due to its low price ($51 dry ton^-1) and high sugar content (~65 wt%), which in turn, requires less amount of feedstock to produce same amount of fuel. We obtain even better economics ($4.01 GGE^-1) in case of selling biomass residues instead of disposing or burning residues for electricity generation, assuming that biomass residues can be sold at a price ($88 ton^-1) equivalent to that of low-purity lignin that is used as a fuel. Finally, we carry out sensitivity analyses on various key economic parameters. We observe that the total project investment and the internal rate of return (IRR) have the largest impact on the MSP; a 20% increase in the total project investment and the IRR increases the MSP by 10% and 9.5%, respectively.

References:

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