Nick Nagle

Extraction of Bio‐oils from Microalgae

A wide variety of terrestrial biomass feed stocks have been identified as suitable candidates for fractionation and conversion into biofuels. In particular, microalgae have been promoted as a future source of transportation fuels primarily because of their stated potential to produce up to 10 times more oil per acre than traditional biofuel crops. Their ability to grow relatively fast, be harvested on a daily basis, and grown in earthen ponds or closed photobioreactors that occupy marginal or poor crop lands using salt or brackish water, are often referenced. When these attractive traits are coupled with the potential to concomitantly harvest valuable co‐products such as biopolymers, proteins, and animal feeds, one can see why microalgae are often touted as biotechnologys “green gold”. The development of large‐scale microalgae farms, however, has been slowed by limitations in downstream processing technology. For example, traditional methods to dewater, extract, and recover bio‐oil from oil‐seeds possess little utility for microalgae. The extreme requirement of dewatering poses tremendous hurdles for any technology processing biofuels from microalgae. To further complicate matters, identifying the most appropriate paths for appropriate extraction technology depends heavily on the microalgal species and its cultivation status, both of which are highly characterized for higher plants as compared to microalgae. In this review we discuss existing extraction methodologies as they have and can be applied to microalgae. Commentary is provided on their potential as a unit operation that exists within the framework of an industrial‐scale microalgal cultivation process that extends from the production of biomass in photobioreactors to the fractionation of the recovered bio‐oil.

Production of methyl ester fuel from microalgae

Microalgae are unique photosynthetic organisms that are known to accumulate storage lipids in large quantitites and thrive in saline waters. Before these storage lipids can be used, they must be extracted from the microalgae and converted into usable fuel. Transesterification of lipids produces fatty acid methyl esters that can be used as a diesel fuel substitute.Three solvents, 1-butanol, ethanol, and hexane/2-propanol, were evaluated for extraction efficiency of microalgal lipids. Type of catalyst, concentration of catalyst, time of reaction, temperature of reaction, and quality of lipid were examined as variables for transesterification. The most efficient solvent of the three for extraction was 1-butanol (90% efficiency), followed by hexane/2-propanol and ethanol. Optimal yield of fatty acid methyl esters was obtained using 0.6N hydrochloric acid in methanol for 0.1 h at 70°C.

Acid-catalyzed algal biomass pretreatment for integrated lipid and carbohydrate-based biofuels production

One of the major challenges associated with algal biofuels production in a biorefinery-type setting is improving biomass utilization in its entirety, increasing the process energetic yields and providing economically viable and scalable co-product concepts. We demonstrate the effectiveness of a novel, integrated technology based on moderate temperatures and low pH to convert the carbohydrates in wet algal biomass to soluble sugars for fermentation, while making lipids more accessible for downstream extraction and leaving a protein-enriched fraction behind. We studied the effect of harvest timing on the conversion yields, using two algal strains; Chlorella and Scenedesmus, generating biomass with distinctive compositional ratios of protein, carbohydrate, and lipids. We found that the late harvest Scenedesmus biomass had the maximum theoretical biofuel potential at 143 gasoline gallon equivalent (GGE) combined fuel yield per dry ton biomass, followed by late harvest Chlorella at 128 GGE per ton. Our experimental data show a clear difference between the two strains, as Scenedesmus was more successfully converted in this process with a demonstrated 97 GGE per ton. Our measurements indicated a release of >90% of the available glucose in the hydrolysate liquors and an extraction and recovery of up to 97% of the fatty acids from wet biomass. Techno-economic analysis for the combined product yields indicates that this process exhibits the potential to improve per-gallon fuel costs by up to 33% compared to a lipids-only process for one strain, Scenedesmus, grown to the mid-point harvest condition.

Fermentation of "Quick Fiber" Produced from a Modified Corn-Milling Process into Ethanol and Recovery of Corn Fiber Oil

Approximately 9% of the 9.7 billion bushels of corn harvested in the United States was used for fuel ethanol production in 2002, half of which was prepared for fermentation by dry grinding. The University of Illinois has developed a modified dry grind process that allows recovery of the fiber fractions prior to fermentation. We report here on conversion of this fiber (Quick Fiber [QF]) to ethanol. QF was analyzed and found to contain 32%wt glucans and 65%wt total carbohydrates. QF was pretreated with dilute acid and converted into ethanol using either ethanologenic Escherichia coli strain FBR5 or Saccharomyces cerevisiae. For the bacterial fermentation the liquid fraction was fermented, and for the yeast fermentation both liquid and solids were fermented. For the bacterial fermentation, the final ethanol concentration was 30 g/L, a yield of 0.44 g ethanol/g of sugar(s) initially present in the hydrolysate, which is 85% of the theoretical yield. The ethanol yield with yeast was 0.096 gal/bu of processed corn assuming a QF yield of 3.04 lb/bu. The residuals from the fermentations were also evaluated as a source of corn fiber oil, which has value as a nutraceutical. Corn fiber oil yields were 8.28%wt for solids recovered following prtetreatment.

SSF comparison of selected woods from southern sawmills

Simultaneous saccharification and fermentation (SSF) is recognized as an efficient approach to the cost-effective conversion of biomass to fuel ethanol. This methodology takes advantage of the relief in end-product inhibition realized by conducting cellulose hydrolysis and glucose fermentation in the same well-stirred vessel. In this study, 15 species of hardwoods and softwoods were collected from sawmills located in the Appalachian region of the southern United States. These wood samples were air-dried to 8–10% moisture, pretreated using a dilute sulfuric acid cooking scheme at 160‡C, exhaustively washed, and applied to SSF withSaccharomyces cerevisiae D5A. Although the glucan content of each wood was found to be relatively invariant throughout the samples tested, hemicellulosic sugar and lignin contents were unique to each wood. These and other differences in chemical composition were related to resulting ethanol yields from SSF.

Anaerobic digestion of municipal solid waste : utility of process residues as a soil amendment

Tuna processing wastes (sludges high in fat, oil, and grease [FOG]) and municipal solid waste (MSW) generated on Tutuila Island, American Samoa, represent an ongoing disposal challenge. The biological conversion of the organic fraction of these wastes to useful products, including methane and fertilizer-grade residue, through anaerobic high-solids digestion is currently in scale-up development. The suitability of the anaerobic digestion residues as a soil amendment was evaluated through extensive chemical analysis and greenhouse studies using corn as an indicator crop. Additionally, native Samoan soil was used to evaluate the specific application rates for the compost. Experiments established that anaerobic residues increase crop yields in direct proportion to increases in the application rate. Additionally, nutrient saturation was not demonstrated within the range of application rates evaluated for the Samoan soil. Beyond nutrient supplementation, organic residue amendment to Samoan soil imparts enhanced water- and nutrient-binding capacities.

Effect of pelleting on the recalcitrance and bioconversion of dilute-acid pretreated corn stover under low- and high-solids conditions

Background: Knowledge regarding the performance of densified biomass in biochemical processes is limited. The effects of densification on biochemical conversion are explored here. Results: Pelleted corn stover samples were generated from bales that were milled to 6.35 mm. Low-solids acid pretreatment and simultaneous saccharification and fermentation were performed for pelleted and ground stover (6.35 and 2 mm) formats. Monomeric xylose yields were significantly higher for pellets (∼60%) than for ground formats (∼38%). Pellets achieved approximately 84% of theoretical ethanol yield; ground stover formats had similar profiles, reaching approximately 68% theoretical ethanol yield. Pelleted and 6.35-mm ground stover were evaluated using a ZipperClave® reactor under high-solids, process-relevant conditions for multiple pretreatment severities (Ro); feedstock reactivity increased slightly following combined pretreatment and enzymatic hydrolysis for three of five severities tested. Conclusion: Pelleting did not render corn stover more recalcitrant to dilute-acid pretreatment under low- or high-solids conditions, and even enhanced ethanol yields.

A Process Economic Approach to Develop a Dilute-Acid Cellulose Hydrolysis Process to Produce Ethanol from Biomass

Successful deployment of a bioethanol process depends on the integration of technologies that can be economically commercialized. Pretreatment and fermentation operations of the traditional enzymatic bioethanol-production process constitute the largest portion of the capital and operating costs. Cost reduction in these areas, through improved reactions and reduced capital, will improve the economic feasibility of a large-scale plant.A technoeconomic model was developed using the ASPEN PlusTN modeling software package. This model in cluded a two-stage pretreatment operation with a co-current first stage and countercurrent second stage, a lignin adsorption unit, and a cofermentation unit. Data from kinetic modeling of the pretreatment reactions, verified by bench-scale experiments, were used to create the ASPEN Plus base model. Results from the initial pretreatment and fermentation yields of the two-stage system correlated well to the performance targets established by the model. The ASPEN Plus model determined mass and energy-balance information, which was supplied, to an economic module to determine the required selling price of the ethanol. Several pretreatment process variables such as glucose yield, liquid: solid ratio, additional pretreatment stages, and lignin adsorption were varied to determine which parameters had the greatest effect on the process economics. Optimized values for these key variables became target values for the bench-scale research, either to achieve oridentify as potential obstacles in the future commercialization process. Results from this modeling and experimentation sequence have led to the design of an advanced two-stage engineering-scale reactor for a dilute-acid hydrolysis process.

Horsepower requirements for high-solids anaerobic digestion

Improved organic loading rates for anaerobic bioconversion of cellulosic feedstocks are possible through high-solids processing. Additionally, the reduction in process water for such a system further improves the economics by reducing the overall size of the digestion system. However, mixing of high-solids materials is often viewed as an energy-intensive part of the process. Although the energy demand for high-solids mixing may be minimized by improving the agitator configuration and reducing the mixing speed, relatively little information is available for the actual horsepower requirements of a mechanically mixed high-solids digester system.The effect of sludge total solids content and digester fill level on mixing power requirements was evaluated using a novel NREL laboratory-scale high-solids digester. Trends in horsepower requirements are shown that establish the optimum parameters for minimizing mixing energy requirements, while maintaining adequate solids blending for biological activity. The comparative relationship between laboratory-scale mixing energy estimates and those required for scale-up systems is also established.

Efficacy of hydrolytic enzyme augmentation and thermochemical pretreatments for increased secondary anaerobic digestion of treated municipal sewage sludges

Rising costs for landfill disposal of municipal sewage residues have prompted evaluation of alternative methods for reducing the bulk of the final waste. Representative samples of municipal sewage sludge residues were obtained from three major treatment plants in the United States, including Los Angeles (Hyperion), Denver (North Metro), and Chicago (Stickney). The majority of the treated, dewatered sewage sludge solids was found to be volatile (50–60%) and, presumably, biodegradable. Additionally, much of the volatile content was solubilized by both acid detergent fiber and neutral detergent fiber treatments, and was presumed to be proteineous microbial biomass in nature. Both low- and high-solids anaerobic digester systems, as well as the standard biochemical methane potential (BMP) assay, were utilized to evaluate the anaerobic digestibility of these sewage sludge residues. The low methane yields and, thus, the poor organic waste conversion indicated the need for treatment prior to bioconversion. The effectivenesss of various pretreatments based on assessment of increased soluble protein or organics and anaerobic digestibility as determined by the BMP assay was evaluated.