In recent years, growing attention has been devoted to the use of lignocellulosic biomass as a feedstock to produce renewable carbohydrates as a source of energy products, including liquid alternatives to fossil fuels. Currently, research is driven by the need to reduce the cost of biomass-ethanol production. One of the prominent methods is to thermochemically hydrolyze (pretreat) the biomass and subsequently, enzymatically hydrolyze the pretreated material to fermentable sugars that can then be converted to ethanol using specialized microorganisms. A key goal of pretreatment is to produce fermentable sugars from hemicellulose and enhance enzymatic conversion of the cellulose fraction, and, hopefully, obtain a higher ethanol yield. Most previous studies on biomass pretreatment have involved pure species, but mixtures of biomass are likely to be feedstocks for agricultural and forest-based biorefineries. The primary goal of this research was to obtain kinetic detailed data for dilute acid hydrolysis from mixtures of several timber species from the Upper Midwest region of the United States and switchgrass, and to determine whether there were any synergistic or antagonistic effects due to the simultaneous pretreatment of mixtures.

Dilute acid-catalyzed hydrolysis (pretreatment) of biomass, including aspen, basswood, balsam, red maple, and switchgrass was studied. Initial experiments were conducted to pretreat each pure species. Following was an experimental design that pretreated 50/50 weight percent mixtures of each possible combination of the five species in duplicate. Overall, there were 25 experiments, 5 pure species and 20 mixtures conducted in a 1-L well-mixed batch reactor (Parr Instruments, Model 4570). 25g of biomass and 500mL of 0.5% sulfuric acid were added to a 1-L glass liner, and then put into the reactor and heated from room temperature at a rate of about 3 degrees C/min. During the experiment, 5 ml samples were taken starting at 130 degrees Celsius at 3 min intervals until reaching the temperature setpoint of 175 degrees Celsius, followed by 4 samples after achieving the setpoint temperature. The collected samples were then cooled in an ice bath to stop the reaction. The cooled samples were filtered using a 0.2 micron MILLIPORE membrane filter to remove suspended solids and analyzed using High Performance Liquid Chromatography (HPLC) with a Bio-Rad Aminex HPX-87P column, and refractive index detection to measure monomeric sugars and diode array detection to measure the main degradation byproduct, furfural. An additional acid hydrolysis was completed to convert any oligomeric sugars into monomers which were also detected by HPLC for a total sugar analysis.

A first order reaction model that takes into consideration simultaneous formation and degradation of xylose was used and the kinetic parameters such as activation energy and pre-exponential constant were obtained for the pure species. Excellent agreement between the model and pure species experimental data was obtained. These parameters were then used to predict the mixtures yields, also with excellent agreement between experimental data and the predicted model. No significant synergistic or antagonistic effects were observed from pretreating mixtures of the five species studied compared to single species results. These results confirm that dilute acid hydrolysis of woody biomass species in mixtures is expected to proceed as if the species existed in the reactor alone (Jensen et al., 2008). This outcome could suggest strategies for maximizing sugar yields from mixtures based on knowledge of how pure species behave during dilute acid hydrolysis.

References

Jensen J, Morinelly J, Aglan A, Mix A, Shonnard DR. Kinetic characterization of biomass dilute sulfuric acid hydrolysis: Mixtures of hardwoods, softwood, and switchgrass. AIChE J. 2008; 54:1637-1645.