리그닌 열분해 공정을 통한 유용 방향족 화합물의 선택적 생산에 관한 연구

Abstract

Biomass is an interesting source for production of energy and chemicals in a biorefinery system. To establish the sustainable biorefinery platform, development of conversion technologies is required to enhance feedstock utilization efficiency in terms of production of value-added biofuels and chemicals. Compared to whole lignocellulosic biomass, one single feedstock of lignin has attracted more attention as an aromatic compound source. Lignin is the only nature resource of polyphenolic compounds because it is composed of variously linked phenylpropane units. Even though microbial degradation of lignin is severely difficult, thermal degradation of lignin is relatively simple and effective to recover aromatic compounds from biomass feedstocks. Throughout lignin pyrolysis, various aromatic compounds could be recovered in the form of bio-oils. Compared to cellulose and hemicellulose, thermal degradation of lignin occurred over the temperature range from 30 to 900 °C with very lower mass loss rate due to strong thermal stability. Within increasing temperature, the bio-oil yield increased from 15.8% to 24.1%, however, the biochar yield decreased from 59.3% to 43.7% in the fixed-bed pyrolysis system of lignin feedstock. As the reaction temperature increased, the relative content of the main aromatic compound also changed. These results denoted that lignin pyrolysis for less-functionalized simple aromatic compounds (i.e., less side chains) would need suitable temperature conditions to prevent undesirable further radical-induced rearrangement, and radical coupling reactions. On the other hand, in the secondary reaction, lignin was mainly transformed to gas products, especially hydrogen resulting from the inherent lignin structure. From the result of response surface methodology, comparing with heating rate and mass loading factors, the temperature factor were significant on both bio-oil and biochar yields in the linear, and quadratic terms while the interaction term was less effective on the biochar yield. Most commercial lignin products have been supplied from wood biomass, however, lignin residues from bioethanol production processes (especially based on non-woody feedstocks) could be an additional potential source. About 75% of lignin inherent in corn stover and rice straw was recoverable from the residues of bioethanol producing processes. In this study, chemical structures and thermolysis features of the lignin residues from corn stover and rice straw obtained through acid-alkali pretreatments were characterized. Due to inherent structural differences, the corn stover- and rice straw lignin were more reactive and had less thermal stability than the wood-based Kraft lignin. The corn stover lignin showed the lowest maximum degradation temperature with the highest mass loss rate in the primary pyrolysis reaction and it was mainly pyrolyzed into monomers of lignin building blocks with a higher phenol content (10%), unlike the other lignin samples (< 6%). During organic matter volatilization of the primary pyrolysis, the lignin building blocks such as guaiacyl and syrigyl units were rapidly decomposed, and the lignin-based carbon material tended to be defunctionalized, so called biochar. In addition, particles of the lignin biochar shifted to smaller size distribution as the temperatures increased. Although the initial lignin source was distributed in the lower and keener size region, the particle size of the lignin biochar was much broadened. The particle of the lignin biochar at higher temperature (600 and 800 °C) tended back to be smaller size distribution from agglomeration effects that occurred at near 400 °C. From the image analysis, as the temperature increased, the lignin biochar had smaller holes and carbonized characteristics, similar to different carbon materials like activated carbon. Even though biochar is a byproduct of lignin pyrolysis; compared to inorganic catalysts, the biochar carbon-based catalyst was more stable and did not have significant leaching of functional groups or by products. Different pyrolysis temperature influenced the surface chemistry properties (acidic, or basic) of biochar product. Within increasing temperature, the surface chemistry of biochar shifted from an acidic functional group-dominant property to a basic functional group-dominant property. Especially, stronger surface basicity of biochar seemed to inhibit radical-induced reactions of lignin phenolic derivatives after the primary pyrolysis; consequently, the pyrolytic bio-oil composition had more lower-molecular-weight compounds composed of simple phenolics (e.g., phenol, and guaiacol). On the other hand, surface acidity of biochar might result in acceleration of radical-induced reactions of lignin phenolic derivatives to produce complex-structured aromatic compounds. This results demonstrated the possibility of lignin biochar as an internal recycling additive resource for selective aromatic compound production; however, further study for enhancement of surface chemistry characteristics of biochar is also required to obtain technical and economic feasibilities. Environmental impact potential and economic feasibility of the residual lignin utilization in a biorefinery platform for cellulosic bioethanol production were analyzed using life cycle assessment. Through both combined heat and power (CHP) and pyrolysis units, the fixed cost of 1 L cellulosic bioethanol production was reduced from $ 0.73 to $0.723 and $0.718, respectively. Consequently, potential of lignin-derived chemical production from bioethanol residuals was found to produce value-added substitutes of petrochemicals and to make cellulosic bioethanol more cost-competitive. However, unlike the CHP unit, the pyrolytic chemical production from the residual lignin resulted in significant environmental impact potential. Even though heat and electricity energy were the most critical factor to the environmental impact of the pyrolysis unit, the increasing target compound (i.e., guaiacol) content in lignin bio-oil was more effective both to reduce the environmental impact potential and to enhance economic feasibility. Finally, this study suggested that enhancement of selectively molecular-level transformations of lignin to value-added chemicals through using biochar-based additives and/or choosing suitable parent lignin feedstock could result in better ultimate efficacy on biorefinery platforms rather than increasing bio-oil and/or process performance via energy efficiency or material recycling.