1.0. Introduction 4044 2.0. Biomass Chemistry and Growth Rates 4047 2.1. Lignocellulose and Starch-Based Plants 4047 2.2. Triglyceride-Producing Plants 4049 2.3. Algae 4050 2.4. Terpenes and Rubber-Producing Plants 4052 3.0. Biomass Gasification 4052 3.1. Gasification Chemistry 4052 3.2. Gasification Reactors 4054 3.3. Supercritical Gasification 4054 3.4. Solar Gasification 4055 3.5. Gas Conditioning 4055 4.0. Syn-Gas Utilization 4056 4.1. Hydrogen Production by Water−Gas Shift Reaction 4056

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Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering.

https://faculty.washington.edu/renatab/courses/PSE490/downloads/pretreatment.pdf

Features of promising technologies for pretreatment of lignocellulosic biomass.

Biomass represents an abundant carbon-neutral renewable resource for the production of bioenergy and biomaterials, and its enhanced use would address several societal needs. Advances in genetics, biotechnology, process chemistry, and engineering are leading to a new manufacturing concept for converting renewable biomass to valuable fuels and products, generally referred to as the biorefinery. The integration of agroenergy crops and biorefinery manufacturing technologies offers the potential for the development of sustainable biopower and biomaterials that will lead to a new manufacturing paradigm.

http://science.sciencemag.org/content/sci/311/5760/484.full.pdf

The path forward for biofuels and biomaterials

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

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Microbial cellulose utilization: fundamentals and biotechnology.

Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars for fermentation to biofuels and other biomaterials. Several technologies have been developed during the past 80 years that allow this conversion process to occur, and the clear objective now is to make this process cost-competitive in today's markets. Here, we consider the natural resistance of plant cell walls to microbial and enzymatic deconstruction, collectively known as "biomass recalcitrance." It is this property of plants that is largely responsible for the high cost of lignocellulose conversion. To achieve sustainable energy production, it will be necessary to overcome the chemical and structural properties that have evolved in biomass to prevent its disassembly.

http://science.sciencemag.org/content/sci/315/5813/804.full.pdf

Biomass recalcitrance: engineering plants and enzymes for biofuels production.

Soaking in aqueous ammonia at moderate temperatures was investigated as a method of pretreatment for enzymatic hydrolysis as well as simultaneous saccharification and cofermentation (SSCF) of corn stover. The method involves batch treatment of the feedstock with aqueous ammonia (15–30 wt%) at 40–90°C for 6–24 h. The optimum treatment conditions were found to be 15 wt% of NH3, 60°C, 1 : 6 of solid-to-liquid ratio, and 12 h of treatment time. The treated corn stover retained 100% glucan and 85% of xylan, but removed 62% of lignin. The enzymatic digestibility of the glucan content increased from 17 to 85% with 15 FPU /g-glucan enzyme loading, whereas the digestibility of the xylan content increased to 78%. The treated corn stover was also subjected to SSCF test using Spezyme-CP and recombinant Escherichia coli (KO11). The SSCF of the soaking in aqueous ammonia treated corn stover resulted in an ethanol concentration of 19.2 g/L from 3% (w/v) glucan loading, which corresponds to 77% of the maximum theoretical yield based on glucan and xylan.

Pretreatment of corn stover by soaking in aqueous ammonia at moderate temperatures.

Hydrolysis of α-cellulose by H2SO4 is a heterogeneous reaction. As such the reaction is influenced by physical factors. The hydrolysis reaction is therefore controlled not only by the reaction conditions (acid concentration and temperature) but also by the physical state of the cellulose. As evidence of this, the reaction rates measured at the high-temperature region (above 200°C) exhibited a sudden change in apparent activation energy at a certain temperature, deviating from Arrhenius law. Furthermore, α-cellulose, once it was dissolved into concentrated H2SO4 and reprecipitated, showed a reaction rate two orders of magnitude higher than that of untreated cellulose, about the same magnitude as cornstarch. The α-cellulose when treated with a varying level of H2SO4 underwent an abrupt change in physical structure (fibrous form to gelatinous form) at about 65% H2SO4. The sudden shift of physical structure and reaction pattern in response to acid concentration and temperature indicates that the main factor causing the change in cellulose structure is disruption of hydrogen bonding. Finding effective means of disrupting hydrogen bonding before or during the hydrolysis reaction may lead to a novel biomass saccharification process.

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Heterogeneous Aspects of Acid Hydrolysis of α-Cellulose

Lactic acid production from cellulosic biomass by cellulase and Lactobacillus delbrueckii was studied in a fermenter-extractor employing a microporous hollow fiber membrane (MHF). This bioreactor system was operated under a fed-batch mode with continuous removal of lactic acid by an in situ extraction. A tertiary amine (Alamine 336) was used as an extractant for lactic acid. The extraction capacity of Alamine 336 is greatly enhanced by addition of alcohol. Long-chain alcohols serve well for this purpose since they are less toxic to micro-organism. Addition of kerosene, a diluent, was necessary to reduce the solvent viscosity. A solvent mixture of 20% Alamine 336, 40% oleyl alcohol, and 40% kerosene was found to be most effective in the extraction of lactic acid. Progressive change of pH from an initial value of 5.0 down to 4.3 has significantly improved the overall performance of the simultaneous saccharification and extractive fermentation over that of constant pH operation. The change of pH was applied to promote cell growth in the early phase, and extraction in the latter phase.

Membrane-mediated extractive fermentation for lactic acid production from cellulosic biomass.

Previous kinetic modeling and bench-scale demonstration efforts using batch, percolation, or plug-flow reactors for the dilute sulfuric acid hydrolysis of cellulose have concluded that glucose yields above 70% of theoretical were not possible. This has been explained to be a result of reactions involving glucose or the cellulose itself in a destructive manner, as well as hydrolyzed soluble oligomers which have been modified chemically so as not to release glucose. However, recently, we have demonstrated that near-quantitative yields of glucose from cellulose can indeed be obtained using a bench-scale shrinking-bed percolation reactor in which an internal spring compresses the biomass as the reaction progresses. The present study was initiated to gain a fundamental understanding of the kinetic sequences involved in these high yields. Three reactor configurations (batch, percolation, and shrinking-bed percolation) were studied using similar hydrolysis severities to begin addressing chemical, physical, and h...

Fundamental Aspects of Dilute Acid Hydrolysis/Fractionation Kinetics of Hardwood Carbohydrates. 1. Cellulose Hydrolysis

Recent research work in-house both at Auburn University and National Renewable Energy Laboratory has demonstrated that extremely low concentrations of acid (e.g., 0.05–0.2 wt% sulfuric acid) and high temperatures (e.g., 200–230°C) are reaction conditions that can be effectively applied for hydrolysis of the cellulosic component of biomass. These conditions are far from those of the conventional dilute-acid hydrolysis processes, and the kinetic data for glucose decomposition are not currently available. We investigated the kinetics of glucose decomposition covering pH values of 1.5–2.2 and temperatures of 180–230°C using glass ampoule reactors. The primary factors controlling glucose decomposition are the reaction medium, acid concentration, and temperature. Based on the experimental data, a kinetic model was developed and the best-fit kinetic parameters were determined. However, a consistent discrepancy in the rate of glucose disappearance was found between that of the model based on pure glucose data and that observed during the actual process of lignocellulosic biomass hydrolysis. This was taken as an indication that glucose recombines with acid-soluble lignin during the hydrolysis process, and this conclusion was incorporated accordingly into the overall model of glucose decomposition.

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Y. Y. Lee

Papers

Pretreatment of corn stover by soaking in aqueous ammonia at moderate temperatures.

Heterogeneous Aspects of Acid Hydrolysis of α-Cellulose

Membrane-mediated extractive fermentation for lactic acid production from cellulosic biomass.

Fundamental Aspects of Dilute Acid Hydrolysis/Fractionation Kinetics of Hardwood Carbohydrates. 1. Cellulose Hydrolysis

Kinetics of Glucose Decomposition During Dilute-Acid Hydrolysis of Lignocellulosic Biomass