전기장을 이용한 생물전기화학 기반 갈탄의 메탄 전환

Abstract

Coal is an energy source with an even distribution worldwide. However, when energy is produced from coal, a large amount of greenhouse gases and environmental pollutants are emitted. Therefore, there is a need for an eco-friendly such as Power-to-Gas (P2G) technology that increases the value of coal as energy production. In addition, low-grade coal has a low calorific value, a high water content, and a high ratio of organic carbon, so it is possible to biologically convert methane. However, biological methane conversion using anaerobic digestion by many researchers produces small amounts of methane. Because toxic intermediates are formed during the decomposition of coal, deterioration of microorganisms occurs and methane conversion rate is lowered. The bioelectrochemical methane conversion is an emerging technology by direct interspecies electron transfer (DIET) using the electrode pair that are polarized by an external power source. Bioelectrochemical methane conversion can overcome the intermediates of coal by DIET in electroactive microorganisms. However, the bioelectrochemical methane conversion of coal has not yet been studied at all. In this thesis, therefore, the following topics were studied to improve the methane conversion of Lignite; 1) methane conversion according to changes in substrates such as glucose and acetic acid in bioelectrochemical reactor with an electric field, 2) bioelectrochemical methane conversion of lignite by high strength of the electric field, 3) replace expensive yeast extracts as biostimulants, 4) the temperature conditions changes in bioelectrochemical reactor 5) control the frequency using the AC power supply. 1. The bioelectrochemical methane production from acetate as a non-fermentable substrate, glucose as a fermentable substrate, and their mixture were investigated in an anaerobic sequential batch reactor exposed to an electric field. The electric field enriched the bulk solution with exoelectrogenic bacteria (EEB) and electrotrophic methanogenic archaea, and promoted direct interspecies electron transfer (DIET) for methane production. However, bioelectrochemical methane production was dependent on the substrate characteristics. For acetate as the substrate, the main electron transfer pathway for methane production was DIET, which significantly improved methane yield up to 305.1 mL/g chemical oxygen demand removed (CODr), 77.3% higher than that in control without the electric field. For glucose, substrate competition between EEB and fermenting bacteria reduced the contribution of DIET to methane production, resulting in the methane yield of 288.0 mL/g CODr, slightly lower than that of acetate. In the mixture of acetate and glucose, the contribution of DIET to methane production was less than that of the single substrate, acetate or glucose, due to the increase in the electron equivalent for microbial growth. The findings provide a better understanding of electron transfer pathways, biomass growth, and electron transfer losses depending on the properties of substrates in bioelectrochemical methane production. 2. The bulk solution in bioelectrochemical reactors was exposed to the electrostatic fields ranged from 0.67 V/cm to 3.33 V/cm by polarizing the insulated electrodes with an external voltage source, and the bioelectrochemical reactors were operated at mesophilic condition (35℃). Another anaerobic batch reactor without the insulated electrode pair was also prepared at the same procedure and used as the control. After the start-up, the methane production was started from the bioelectrochemical reactors without the lag phase. The cumulative methane production was gradually saturated to 176.3 mL in the bioelectrochemical reactor with the electrostatic field of 1.67 V/cm, which was higher than the reactor with 0.67 V/cm or 3.33 V/cm as well as the control. The exoelectrogenic and electrotrophic species in the bulk solution were confirmed by the electrochemical analysis (cyclic voltammetry and electrochemical impedance spectroscopy). This suggests that the electrostatic field of 1.67 V/cm significantly enhanced the biological direct interspecies electron transfer for the methane conversion of coal. Interestingly, after the cumulative methane production was saturated, the residual organic matter, the intermediates produced from the hydrolysis and acidogenic fermentation of coal, was still high in the bioelectrochemical reactors. This suggests that the intermediates have a self-inhibitory effect on methane conversion. The residual intermediates were further converted to methane by supplementing the anaerobic medium and anaerobic sludge. The total methane conversion of coal in the bioelectrochemical reactor exposed to the electrostatic field of 1.67 V/cm reached 168.5 mL/g lignite, which is the highest reported so far. An electric field established by polarizing insulated electrode greatly improves methane conversion of coal in the bioelectrochemical reactor. 3. Bioelectrochemical methane conversion of lignite was investigated under mesophilic- (35℃) and thermophilic (55℃) conditions. An anaerobic batch bioelectrochemical reactor with a pair of electrodes was prepared and a medium containing yeast extract (1.0 g/L), anaerobic seed sludge and lignite powder (5.0 g/L) was added to the reactor. The electrodes were polarized using an external voltage source to expose the bulk solution to 1.67 V/cm. The bioelectrochemical reactor was operated under mesophilic- and thermophilic conditions by mixing the bulk solution using a blade connected to a DC motor. Under thermophilic condition, the methane production in the bioelectrochemical reactor was started without any lag time and gradually saturated to 103.3 mL, which was higher than the thermophilic control. Under mesophilic condition, the trend of methane production in the bioelectrochemical reactor was similar to that under the thermophilic condition, but the cumulative methane production less than the thermophilic condition. When the methane production was saturated at 10th days, the yeast extract and seed sludge were supplemented to the reactor. Under thermophilic condition, the bioelectrochemical methane production was abruptly increased and reached to 362.4 mL (155.6 mL/g lignite), which was much higher than 316.3 mL (133.6 mL/g lignite) under mesophilic condition. Based on the cyclic voltammogram, however, the redox peaks under thermophilic condition was not significantly different from those under mesophilic condition. From the electrochemical impedance spectroscopy data, the charge transfer resistance under mesophilic condition was slightly higher than that under thermophilic condition. The results suggest that the methane conversion of lignite was improved by direct interspecies electron transfer between the electroactive microorganisms. However, the redox condition that convert better the lignite to methane bioelectrochemically under thermophilic condition is shifted to slightly better condition, compared to the mesophilic condition. The thermophilic condition for the bioelectrochemical methane conversion of lignite require more heating energy. The thermophilic bioelectrochemical conversion of lignite to methane requires further studies to assess the economic feasibility. 4. The methane conversion of coal could be improved by bio-stimulation with yeast extract after bio-augmentation with anaerobic sludge in a bioelectrochemical system. However, yeast extract is a bio-stimulant that is too expensive for field use. In this study, it was investigated whether glucose and acetate, as well as yeast extract, could be used as the bio-stimulants to improve the methane conversion of coal in the bioelectrochemical system. The cumulative methane productions in the bioelectrochemical reactors with glucose as bio-stimulant were increased without lag time after the start-up, and gradually saturated to 89.7 mL, which was similar to 90.6 mL in the reactor with yeast extract. This indicates that glucose as bio-stimulant in the bioelectrochemical reactor was as effective as yeast extract in converting coal to methane. However, the methane production in the bioelectrochemical reactor with acetate was only 28.2 mL, which was slightly more than 13.4 mL in the control. The methane conversion of coal could be significantly improved in a bioelectrochemical reactor by the simultaneous use of bio-stimulation and bio-augmentation. As a bio-stimulant, glucose, a fermentable substrate, is as effective as yeast extract for the methane conversion of coal in a bioelectrochemical reactor. However, the bioelectrochemical methane conversion of coal cannot be sufficiently improved by acetate, which is a non-fermentable substrate. In a bioelectrochemical reactor, the anaerobic microorganisms that contribute to the improved methane conversion of coal are mainly exoelectrogens. 5. Recently, research results have been reported that the methane conversion rate of coal can be improved by promoting direct interspecies electron transfer under electrostatic field conditions. However, it was not possible to sufficiently control the substrate inhibitory effect of the intermediate even under electrostatic field conditions. In this study, a study was conducted to further improve the methane conversion rate of coal for lignite under various frequency electromagnetic field conditions. In the anaerobic batch reactor for the experiment, a titanium electrode with a surface insulation coating and an electromagnetic field of 1.67 V/cm was formed in the bulk solution between the electrodes. The methane conversion rate of coal was compared in the frequency range of 50-500K Hz. At the low frequency condition of 50 Hz, the peak current of CV, the HOMO-LUMO energy band gap, and the electron transfer resistance were not significantly different from those of the electrostatic field condition. However, as the frequency increased, the peak current of CV increased, and the energy band gap and electron transfer resistance gradually decreased. The cumulative methane production amount was maximum at 500K Hz as 185.2 mL (69.9 mL/g Lignite). Using a high-frequency electromagnetic field, it was possible to improve the methane conversion rate of coal by reducing the substrate inhibitory effect and promoting decomposition.