생물전기화학 혐기성소화에 의한 석탄의 고율 메탄 전환

Abstract

Coal is one of the abundant energy resources widely distributed on the earth, and has contributed greatly to human civilization so far. However, coal is a polluting energy source that releases a large amount of greenhouse gases and dust in the combustion process. On the other hand, the pollutant emission of natural gas is known to be 200 to 500% less than the coal. Therefore, the paradigm of global energy consumption is gradually shifting to clean energy sources including natural gas. The conversion of coal to the natural gas is an essential for securing a stable and sustainable energy source. The main component of natural gas is methane. Coal can be converted to methane through two routes: thermochemical conversion and biological conversion. The thermochemical conversion is a high energy consumption process that requires high temperature and pressure conditions. The biological conversion of coal is an environmentally friendly process that uses anaerobic microorganisms to converts coal to methane under mild conditions of low temperature and pressure. Recent interest in methane as a clean energy source has led to the research on the biological conversion of coal to methane. To date, the biological coal conversion has been improved by the bioavailability improvement of coal, biostimulation, and bioaugmentation. However, the methane yield obtainable from 1 g of coal is still only a few tens of µL to a few mL. The bioelectrochemical anaerobic digestion is an emerging technology that converts organic matter to methane by direct interspecies electron transfer (DIET) in an anaerobic reactor with the electrode pair that are polarized by an external power source. In terms of the kinetics and thermodynamics, it has been found that the limitations in anaerobic digestion could be considerably mitigated in bioelectrochemical anaerobic digestion. 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 coal; i) an electrostatic field that enhances DIET, ii) the yeast extract as a biostimulant, iii) oxygen that promotes the opening of the aromatic rings, and iv) activated carbon as a mediator for DIET. The bioelectrochemical methane conversion of coal was greatly improved under the electrostatic field. The methane yield of coal reached 52.5 mL/g lignite under the electrostatic field of 0.33 V/cm, which was 10.5 times higher than the value reported to the best of our knowledge, but it took long lag phase for the substantial production of methane. However, the lag phase can be reduced in the electrostatic field of 0.67 V/cm higher than the 0.33 V/cm. Interestingly, after the methane production from coal, the soluble organic residue composed of the coal degradation intermediates was still high as above 3,600 mg COD/L in the bulk solution. In the anaerobic toxicity test, it has been revealed that the coal degradation intermediates inhibit the conversion of coal to methane. However, the inhibition of the intermediates on the further conversion to methane could be mitigated by the dilution, and the half-maximal inhibitory concentration (IC50) was 17.5%. The additional methane yield of 55.3 mL/g lignite was obtained from the intermediates when it was diluted 10-fold. It has been concluded that the biological conversion of coal to methane can be significantly improved under the electrostatic field over than 0.33 V/cm, and the dilution of the coal degradation intermediates further improves the methane potential of coal. The electroactive microbial species were enriched in coal medium amended with yeast extract by the electrostatic field of 0.67 V/cm. The electrostatic field promoted DIET between the electroactive species to immediately produce methane from coal. However, the methane production was gradually suppressed by accumulating toxic intermediates, and the methane yield was 25.1 mL/g lignite. When yeast extract and anaerobic sludge were supplemented, methane was produced from the intermediates again and the total methane yield reached 109.9 mL/g lignite, which was the maximum known yield. This suggests that the bioelectrochemical methane conversion of coal can be significantly improved under the electrostatic field of 0.67 V/cm by amending and then supplementing the coal medium with both yeast extract and anaerobic sludge. It has been proved that the oxygen that released from the water electrolysis can be used as an electron acceptor to stimulates the ring opening of the heterocyclic aromatic intermediates of coal degradation, thereby promoting methane conversion. In this experiment, the pairs of ordinary electrodes (OE) and surface-insulated electrodes (IE) were placed in a bioelectrochemical anaerobic batch reactors, respectively, and the electrodes were polarized by applying voltages (0.5, 1.0 and 2.0 V for OE, 2.0 V for IE). The cumulative methane production in the OE with the applied voltage of 2.0 V was slowly saturated to 162.9 mL, which is higher than 136.0 mL in the IE reactor. This indicates that the oxygen from the water electrolysis in OE was served as an electron acceptor to stimulate the ring-opening reaction of the heterocyclic aromatic compounds contained in the coal degradation products. When methane production from OE and IE was saturated by accumulation of toxic coal degradation intermediates, anaerobic medium amended with yeast extract and anaerobic sludge were supplemented to the reactor. The cumulative methane production increased abruptly in IE and was significantly higher than in OE. The total methane yield of coal was 121.0 mL/g lignite in IE and much higher than 64.4 mL/g lignite of OE. This suggests that coal can be effectively hydrolyzed and fermented under the electrostatic field in IE, and the conversion of coal degradation intermediates to methane through biological DIET can be promoted by supplementing yeast extract and anaerobic sludge. The influence of activated carbon on the bioelectrochemical methane conversion of coal was investigated under the electrostatic field ranged from 0.17 V/cm to 0.67 V/cm. Uunder the electric field of 0.67 V/cm, the methane yield of coal with activated carbon was 35.0 mL/g lignite, which was slightly higher than the 25.1 mL/g lignite without activated carbon. After supplementing anaerobic medium amended with yeast extract and anaerobic sludge, the total methane yield of coal increased to 98.0 mL/g lignite within 19 days. This suggests that activated carbon improves the methane conversion rate of coal by mediating the DIET for methane production. The final conclusions are that the electroactive species in the coal medium amended with yeast extract as a biostimulant can be enriched from anaerobic sludge under the electrostatic field of 0.67 V/cm, and the DIET between the electroactive species greatly improves methane conversion of coal. The activated carbon further improves the methane conversion of coal by mediating the DIET. Further studies are still needed to replace the yeast extract as a biostimulant with an inexpensive substance, and to design the continuous bioelectrochemical system for coal, but the findings of this thesis would be greatly contributed to the practical use of the biological methane conversion of coal.