Development of Bioethanol and Methane Production Processes Combined with Pretreatment from Lignocellulosic Biomass

In this PhD thesis, development of a combined ethanol and methane production method using two different pretreatment techniques was completed with the aim of optimising the newly-proposed method's and perform life cycle analysis to evaluate the environmental impacts of methods. This would ensure the contribution to renewable biofuel production with the utilization of adapted switchgrass grown on marginal land. During the PhD research, detailed and comparative characterisation analyses of Kanlow variety switchgrass were completed and the perennial plant's lignocellulosic structure was determined throughout three harvest seasons. One of the methods used was thermal – NaOH pretreatment of the switchgrass to thermally break the lignocellulosic structure of the crop to achieve enriched ethanol production potential. Second pretreatment method selected was NaOH assisted physicochemical hydrodynamic cavitation. There is limited work on the effects of these two pretretments on lignocellulosic materials biofuel production potential. Thus, the investigation and optimisation of the aforementioned pretreatment methods' effect on biofuel production contributes to the novelty of this work. Independent variables, their ranges and responses for the pretreatment methods were determined for the optimisation. These were used in the response surface methodology, central composite design for the experimental design of the pretreatment methods. Desing Expert® software was utilized in the experimental design. In the first pretreatment method (i.e., thermal – NaOH) independent variables were chosen as; dry matter concentration, NaOH concentration, reaction temperature and reaction time. As for the second pretreatment method (i.e., NaOH assisted hydrodynamic cavitation) NaOH concentration and reaction time were selected as independent variables. Modelling of each independent variable within their selected ranges were suggested by the experimental design software. These were determined after performing prediction models based on the results of pretreatment experiments, ethanol fermentation and biochemical methane potential tests. ANOVA test was also used to estimate the power of these prediction models. Furthermore, to maximise the quantity of ethanol and methane production, two different optimisation approaches were followed one of which was targeted to maximising the biofuel production and the other one to maximising biofuel production whilst minimising process costs. Ethanol production from the solid part of the pretreated samples was performed with simultaneous saccharification and fermentation where enzymatic hydrolysis and ethanol fermentation takes place in the same reactor. Components such as; dry matter and yeast amount, nutrients and enzyme quantities were selected from experimental studies and literature. In order to maximise biofuel potential and minimise the process waste of the system, biochemical methane potential test was performed on the mixture of the solid part of the pretreated samples and the wastes of the ethanol fermentation. This ensured a sustainable biofuel production process design. After the application of optimized pretreatments, to determine the effects of pretreatments on the rigid surface and bond structure of switchgrass, samples were observed using scanning electron microscopy and Fourier Transform InfraRed spectroscopy, respectively. Completed characterisation analyses showed that the Kanlow variety switchgrass is rich in carbon and polysaccharides with experimental results revealed 41.95% of carbon, 36.24% of hemicelluloses and 35.94% of cellulose content. The high cellulose content is particularly promising as this material is an essential substrate for ethanol production which further supports the usability of local switchgrass crops as a biofuel production material. After the simultaneous saccharification and fermentation 14.02 mg EtOH/g of VS ethanol was produced. The highest ethanol production from thermal – NaOH pretreatment was observed under 4% dry matter, 100°C reaction temperature, 2% NaOH concentration and 24 hours reaction time conditions with 36.69 mg EtOH/ g VS. The highest ethanol production from NaOH assisted hydrodynamic cavitation was observed under 1% NaOH concentration and 5 hours reaction time conditions with 27.79 mg EtOH/g VS. Based on the results obtained from various pretreatment conditions, thermal – NaOH pretreatment increased ethanol quantity by 162% compared to raw samples. NaOH assisted hydrodynamic cavitation pretreatment increased this quantity by 97%. Methane potential of Kanlow variety switchgrass was measured as 210.43 mL CH4/g VS. Methane potential of the thermal – NaOH pretreatment sample under 5.5% dry matter, 70°C, 1% NaOH and 6 hours reaction time conditions was measured as the highest with 273.1 mL CH4/g VS and was found to be 29.8% higher than raw switchgrass's methane potential. NaOH assisted hydrodynamic cavitation pretreatment sample under 0% NaOH concentration and 3 hours reaction time conditions was measured as the highest with 264.8 mL CH4/g VS methane potential which is also higher than raw materials' methane potential by 25.8%. Statistical models were created based on the results of pretreatment, fermentation and methane potential experiments. R2 values for ethanol and methane potentials of thermo chemical pretreatment were measured as 0.9280 and 0.3404 respectively. For the maximum biofuel production process, the suggested pretreatment conditions were 4.81% dry matter, 100°C reaction temperature, 2% NaOH concentration and 24 hours reaction time. In order to minimise the process costs the suggested conditions were found as 6.5% dry matter, 70°C reaction time, 0% NaOH concentration and 6 hours reaction time. The predicted and experimentally obtained ethanol production results were 38.25 mg EtOH/g VS and 26.52 mg EtOH/g VS, respectively for maximum biofuel optimization of thermal – NaOH pretreatment. For minimum costs optimization of the same pretreatment, the predicted and experimentally obtained ethanol production results were 12.76 mg EtOH/g VS and 7.74 mg EtOH/g VS, respectively. The predicted and experimentally obtained methane potential results were 274.05 mL CH4/g VS and 223.89 mL CH4/g VS, respectively for maximum biofuel optimization of thermal – NaOH pretreatment. For minimum costs optimization of the same pretreatment, the predicted and experimentally obtained methane potential results were 243.75 mL CH4/g VS and 304.65 mL CH4/g VS, respectively. Similar to the thermo chemical pretreatment, R2 values for ethanol and methane potentials were also calculated after NaOH assisted hydrodynamic cavitation pretreatment. These were found as 0.8449 and 0.6624 for ethanol and methane potentials. The suggested conditions of the pretreatment was 2% NaOH concentration and 5 hours reaction time for maximum biofuel production and 0.71% NaOH concentration and 2.7 hours reaction time for minimum process costs optimization. The predicted and experimentally obtained ethanol production results were 25.09 mg EtOH/g VS and 20.74 mg EtOH/g VS, respectively for maximum biofuel optimization of hydrodynamic cavitation pretreatment. For minimum costs optimization of the same pretreatment, the predicted and experimentally obtained ethanol production results were 18.10 mg EtOH/g VS and 17.07 mg EtOH/g VS, respectively. The predicted and experimentally obtained methane potential results were 255.89 mL CH4/g VS and 149.85 mL CH4/g VS, respectively for maximum biofuel optimization of hydrodynamic cavitation pretreatment. For minimum costs optimization of the same pretreatment, the predicted and experimentally obtained methane potential results were 224.69 mL CH4/g VS and 280.46 mL CH4/g VS, respectively. In order to determine the environmental effects of all the processes included in the biofuel production design, life cycle assessments were performed on the newly-proposed pretreatment combined biofuel production design and alternative biofuel production processes. Life cycle assessment was performed in Denmark Technical University with the scholarship of Turkish Scientific Council. For this purpose EASETECH life cycle assessment software was used in modelling the whole process starting from switchgrass harvest to the last digestate product to be implemented on soil. According to modelling results, scenarios with thermal – NaOH pretreatment processes determined as the most environmentally favourable process configurations. In this thesis combined ethanol and methane production potentials of Kanlow variety switchgrass using optimised pretreatment methods to design a more sustainable biofuel production processes were analysed with life cycle assessment. Best to our knowledge, there is no study in the literature that combined and analysed these components of biofuel production. Therefore, this work is novel in not only providing knowledge and guidance to future work in the field but also contributing greatly to Turkey's development and global warming control potentials.