(647d) 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.