An Integrated Biofuels Strategy: Catalytic Conversion of Lignocellulosic Biomass to Liquid Hydrocarbon Fuels

An integrated biofuels strategy: Catalytic conversion of lignocellulosic biomass to liquid hydrocarbon fuels

Replacement of fossil fuels with new sustainable resources becomes crucial due to depletion of petroleum reserves, increasing global energy demand and arising environmental concerns. Lignocellulosic biomass provides sustainable and environmentally friendly ways of producing chemicals and fuels as an alternative for fossil fuels. One critical step is the conversion of lignocellulosic biomass to versatile intermediates such as levulinic acid (LA), which can be transformed into liquid fuels, fuel additives and even other specialty chemicals. In this respect, we studied a LA-based catalytic process to convert lignocellulosic biomass into liquid hydrocarbon fuels for use in the transportation sector. Using experimental results for all associated reactions, we synthesized an integrated biomass-to-fuels strategy that has a number of advantages over existing strategies.

The first step of this process is the removal of hemicellulose fraction of the biomass by using dilute acid pretreatment. After the hemicellulose fraction (that mainly contains xylose) is removed, the cellulose portion of the biomass is converted to levulinic acid (LA) and formic acid in a cellulose deconstruction reactor using sulfuric acid. The insoluble lignin and C₅-sugar polymers are sent to a boiler/turbogenerator to produce heat and electricity, covering the utility requirements of our process. Excess electricity is sold to the grid. LA is converted to γ-valerolactone (GVL) over a Ru/C or RuRe/C catalyst in the presence of sulfuric acid. GVL is extracted from the sulfuric acid and GVL aqueous solution using butyl acetate solvent and sulfuric acid is recycled back to the cellulose deconstruction reactor. Finally, GVL is separated from butyl acetate by using a distillation column and purified GVL is converted to butene and to butene oligomers.

To determine the economic potential of this strategy, we carried out detailed process simulation (based on experimental results) and capital/operational cost calculations. A comparison with a lignocellulosic ethanol production facility reveals the potential feasibility of the LA-based catalytic approach. Various feedstock alternatives (corn stover, sugarcane, wheat straw, hybrid poplar, switchgrass, loblolly pine and aspen wood) were analyzed and compared for cost-effective processing. Loblolly pine was identified to be the most cost-effective feedstock due to its high C₆ sugar content. Using loblolly pine as the feedstock, the minimum selling price (MSP) of butene oligomers was calculated as $4.31 per gallon of gasoline equivalent. We also performed sensitivity analysis studies for several process parameters (e.g., feedstock cost, equipment cost etc.) as well as economic parameters (e.g. equipment lifespan, income tax rate, return on investment discount rate) to determine the bottleneck of the base case design. Feedstock price appears to be the major cost driver: a ±20% change results in slightly more than ±8% change in the MSP of oligomers . It was also shown that the MSP of alkenes are sensitive to variations in the cost of the turbogenerator and economic parameters (equipment life span and the ROI discount rate). Finally, we present the results of an energy efficiency analysis for the proposed process. The energy efficiency of biofuel production from C₆ sugar contents was equal to 44.5%.

The presence of sulfuric acid in the GVL production step causes a dramatic decrease in the catalytic activity of the Ru/C catalyst which is preferred because it is cheaper than the RuRe/C catalyst. To address this shortcoming, we considered the production of hydrophobic esters (Sec-Butyl Levulinate and Sec-Butyl Formate) from the reaction of levulinic acid and formic acid with butene. The source of reacting butene is mainly provided from the conversion of GVL to butene and CO₂ by catalytic decarboxylation over an acid catalyst in a later step. Hydrophobic esters can be separated from sulfuric acid without need of an energy intensive distillation and they are converted to GVL over a dual-catalyst-bed system. This sulfuric acid management strategy provides downstream catalytic processing in the absence of sulfuric acid and therefore with higher yields. We investigate the economic impact of integration of the proposed reactive extraction strategy with the original process.