Alternative TitleThermal hydrolysis and steam explosion pretreatment of different lignocellulosic aquatic weeds by anaerobic digestion
Note (General)Overgrowth of aquatic weeds has been occurring worldwide, leading to various social and environmental problems. Therefore, it is necessary to harvest and utilise these aquatic weeds. Anaerobic digestion (AD) is considered suitable for treating aquatic plant weeds as it has a lower impact on the environment and can recover energy in the form of biogas; further, the digestate can be used for various purposes. Previous studies pointed out that lignin in the lignocellulose structure of aquatic weeds limits the biodegradability and methane potential of the substrate. Therefore, before AD, a suitable pretreatment is required to make the lignocellulose structure more accessible to produce higher methane. Therefore, in this study, we conducted thermal hydrolysis (TH) and steam explosion (SE) pretreatment on different lignocellulosic aquatic weeds having different chemical compositions. In study 1, the physical and chemical differences of lignocellulosic aquatic weeds pretreated by TH and SE pretreatment will be clarified. The effect of these pretreated aquatic weeds on anaerobic digestion will be discussed in study 2. Therefore, Study 1 and Study 2 compare the TH and SE mechanisms, which have not been studied before. After that, a proposal of an appropriate pretreatment based on biomass type is discussed.Chapter 2 (Study 1) clarifies the physical and chemical differences between the lignocellulosic aquatic weed pretreated by TH and SE. When results were interpreted using SEM analysis, it was seen that more lignin was condensed on the biomass surface during the SE pretreatment than TH. Also, substrate degradation leads to higher TOC readily available, which increases the amount of inhibitory compounds in the pretreated liquid substrate. For TH, this had an increasing linear trend regardless of the substrate's chemical composition. For SE, as the lignin concentration of the substrate increased, the TOC kept increasing, whereas the phenolic compounds reached an optimum point, leading to a different trend than TH. Also, when the relationship between lignin in the solid fraction and inhibitors in the liquid fraction was observed, it showed an increasingly significant trend. In conclusion, the abovementioned results' effect on anaerobic digestion should be evaluated.Chapter 3 (Study 2) evaluated the effect of TH and SE on the anaerobic digestion of lignocellulosic aquatic weeds. The study revealed that all the three substrates' methane yield was enhanced after the TH and SE pretreatment. In Eichhornia crassipes and Ludwigia grandiflora the methane yield enhancement from the untreated condition was much higher for both TH and SE pretreatment than Hydrilla verticillata. In this study, the T80 value was calculated, which showed that the T80 values for TH pretreated biomass were lower than the SE pretreated biomass. In conclusion, it was observed that TH pretreatment was sufficient to improve the methane yield for a wide range of lignocellulosic biomasses, whereas SE pretreatment helped disrupt the substrate structure with maximum lignin content.Lastly, in chapter 4, an appropriate pretreatment based on the biomass type was suggested as a general discussion. After TH and SE pretreatment, the methane yield improvement was enhanced for all the substrates used. The value of the lignin polymer in the untreated substrate can help to evaluate the methane yield improvement yield. The methane yield improvement after the TH pretreatment for E. crassipes, H. verticillata, and L. grandiflora was 89.91, 24.44, and 140.72%, respectively. The methane yield improvement after the SE pretreatment for E. crassipes, H. verticillata, and L. grandiflora was 56.83, 11.33, and 216.13 %, respectively. This evaluation of methane yield improvement can be helpful for new studies/research as only the lignin concentration is used for the evaluation. This is the first study which proposes a methane yield improvement estimation model for both TH and SE pretreatment. In conclusion, when comparing the results with previous studies, it was observed that TH pretreatment could increase the methane yield improvement of substrates at a vaster/broader range of temperature, whereas SE pretreatment improved the methane yield at a lower range of temperature. Techniques such as co-digestion were also discussed to stabilise the methane production process for long-term studies.
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2023-10-11T15:41:03+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション