GENERAL CONCLUSIONS AND FUTURE WORK Conclusions

Lignocellulosic biomass is a biorenewable resource with the potential to serve a feedstock for liquid transportation fuels and chemicals. The recalcitrant nature of lignocellulosic biomass makes its utilization challenging. In this study, several pretreatment and fractionation technologies were developed and tested to improve subsequent enzymatic hydrolysis and ethanol fermentation yields, to separate hemicellulose and lignin with high purity for value-added co-products, and to reduce the required inputs of energy, chemicals, and water.

The fractionation processes described in Chapters 2 – 4 aimed to separate hemicellulose and lignin, and to provide high-purity streams of each, thereby producing readily-fermented carbohydrates and high value co-products. The production of co-products can contribute to increasing the overall economic viability of a process. Even though the fractionated hemicellulose was fermented to ethanol in this study, the hemicellulose could also have been fermented to higher value products such as xylitol and astaxanthin in a biorefinery. In Chapter 4, this approach of high-value compounds was examined by taking ZnCl2-fractionated hemicellulose into furfural by thermal reaction. Separated lignin fractions from these processes also have potential as feedstocks to make additional high-value products. In addition, the biological conversion yields of residual cellulose were improved with the fractionation of hemicellulose and lignin. Two stage fractionation using ammonia and hot-water produced high purity (over 80%) hemicellulose, and the ethanol yield of the cellulose fraction reached up to 98%. In Chapter 4, ZnCl2 treatment removed over 90% of

hemicellulose. A furfural yield of 58% and an ethanol yield of 98% were obtained with the fractionated hemicellulose and residual solids respectively.

Pretreatment studies (Chapters 5 and 6) were conducted to efficiently utilize biomass from another viewpoint. Pretreatment is one of the most expensive and energy intensive processing steps in the biological conversion process using lignocellulosic biomass. In particular, chemicals, water, and energy inputs for the production and recovery have been at issue. Gaseous ammonia pretreatment and photocatalyst-assisted ammonia pretreatment aimed to reduce these inputs to save production costs. Minimizing the production costs can make the products more competitive in the marketplace. In Chapter 5, low-moisture anhydrous ammonia (LMAA) pretreatment resulted in high ethanol conversion yield (~90%) with significantly reduced ammonia (~10% of biomass weight) and water (~1:1 of biomass:water) inputs. In Chapter 6, photocatalyst-assisted ammonia pretreatment increased the delignification up to 70% within shorter period and enhanced the enzymatic digestibilities of the treated biomass compared to the other aqueous ammonia pretreatment with the same amounts of ammonium hydroxide loading.

The characteristics of lignocellulosic biomass vary depending on the species, environments and other factors, and the conversions methods can also be different with the characteristics and target products. Each method has its own pros and cons. The two-stage fractionation process with ammonia and hot-water can produce high purity of each component; however, the operating and capital costs of this process are relatively high because of the multiple stages. The hybrid fractionation with zinc chloride is a relatively simple process, but the zinc chloride recovery costs may be high. The LMAA pretreatment significantly reduced water and ammonia use compared to other liquid ammonia

pretreatment methods; however, economical viability of the LMAA still needs to be verified.

Photocatalyst-assisted ammonia pretreatment can improve the enzymatic hydrolysis without increasing ammonia concentration. It also shortens the pretreatment time. However, the overall liquid loading is relatively high because of the photocatalyst distribution for efficient biomass pretreatment. The aforementioned approaches can be used by themselves or by combining with other methods depending on the biomass species and target products.

Therefore, the studies in this thesis are worthwhile and contributable in lignocellulosic biomass utilization. Among these approaches, the LMAA process is the closest biomass conversion technology to a commercial process because of its simple process design and low operating costs for chemical, water, and energy inputs.

Future Work

In the fractionation studies (Chapters 2 - 4), fractionation technologies of the three main components and conversion of ethanol and furfural were discussed. However, only one co-product (furfural) was investigated in this thesis. To maximize the advantage of these fractionation processes, further studies for conversion of various co-products are required. In particular, conversion of the lignin fraction has not yet been developed as much as that of carbohydrates in the biomass because of its structural variability and complexity. Lignin is an abundant component in lignocellulosic biomass and has high energy content. Therefore, the utilization of lignin will be an important key to the lignocellulosic feedstock biorefinery.

Secondly, even though several fractionation and pretreatment technologies are suggested, most of the results are limited to lab scale experiments. In the pilot scale process,

several unexpected problems can be encountered; therefore, studies with scale-up process are recommended to ensure the process is technically and economically viable in real industries.

In this study, some of the cost barriers such as chemical, water and energy inputs for lignocellulosic biomass utilization were discussed; however, the effects of these factors on the costs of final products were not studied. To understand the viability of the LMAA process, a techno-economic analysis on this process is required.

Moreover, many other obstacles such as transportation, storage, catalysts recovery and purification of products still exist to overcome for the commercialization of biorenewable products. Thus, further studies are necessary to commercialize the products from lignocellulosic biomass.

ACKNOWLEDGEMENTS

I would like to express my sincerest gratitude to everyone who helped me accomplish my doctoral study. First, I would like to thank my advisor, Dr. Tae Hyun Kim, for his guidance, patience and support. It has been a great experience for me to work with him. I would like to thank Dr. Raj Raman, my co-advisor, for his interest, help and encouragement.

He has always encouraged me to do my best at everything I do. I also thank Dr. Monlin Kuo, Dr. Hans van Leeuwen and Dr. Robert Anex, my POS committee members for their great instruction and valuable advice.

I want to give thanks to my colleagues, Xuan Li, Chao Wang, Simone Soso, Lam Nguyen, Weitao Zhang, Bo-ra Kim and Katrina Christiansen for their cooperation and help. I would also like to express my gratitude to Dr. Nhuan Nghiem in USDA-ARS-ERRC, for his advice and help.

I especially thank my wife, Jiyeon Hong, for her endless love, patience and support. I want to extend my appreciation to my parents and sister. It would not be possible to finish my graduate study without their help and support. I wish to dedicate this thesis to all of them.