Production of high purity biogas from pressurized anaerobic digester by conductive carbon media and novel $H_2S$ adsorbent

Abstract

Recently, anaerobic digestion, which converts organic wastes into biogas, has been widely applied as a food waste treatment technology. Anaerobic digestion can simultaneously achieve waste treatment and renewable energy production. However, major problems to be overcome in anaerobic digestion technology are the slow methane production rate and the high cost of biogas purification. Therefore, a long hydraulic retention time (> 30 days) is required for biogas from a high organic concentration of waste. In addition, it has been reported that the refining process that the cost of refining process required to use biogas in the existing LNG grid is more than twice the cost of production processes. In particular, hydrogen sulfide ($H_2S$) in biogas is corrosive and can significantly damage the metallic parts of the biogas facilities. This research was proposed to improve the methane conversion rate of organic waste by adding a conductive carbon media (CCM) into an anaerobic digester to promote electron transfer between microorganisms. In addition, to produce high purity biogas (< 100 ppm$_v$ $H_2S$), iron layered double hydroxide (LDH) that selectively removes hydrogen sulfide in biogas was produced, and an anaerobic reactor was operated under pressure (> 1 bar) to increase the solubility of hydrogen sulfide. In Chapter 1, the effect of CCM as a conductive structure was evaluated under various organic loading conditions. The maximum acetogenesis rate and methanogenesis rate of the reactor with CCM were improved up to 1.4-ford compared to the control under an increasing organic loading rate. In addition, the microbial community distribution was changed and the Sytrophomonas genera (4.3%) and Syntrophaceticus genera (4.4%), which enhance the methane conversion rate of propionic acid and acetic acid through interaction with methane bacteria, became abundant. In Chapter 2, the adsorption capacity of $H_2S$ in biogas was evaluated by adding various metal-iron LDH to the anaerobic digester. As a result of the experiment, NiFe-LDH successfully achieved $H_2S$ by ion exchange between $H_2S^-$ and $OH^-$/$CO_3^{-}$ at 88.3 mg/g, twice that of $Fe_2O_3$ (46.5 mg/g). In Chapter 3, methane production rate and concentration of H2S in biogas was evaluated based on different pressure in an anaerobic digester with the adding of CCM. The methane production rate in the reator with CCM under 2 bar showed a maximum enhancement of 1.2 times compared to the control. Furthermore, the concentration of $H_2S$ in biogas decreased by 40% (771 ppm$_v$) compared reactor operating under atmospheric pressure. In Chapter 4, the Techno-economic technology of the conventional and the developed anaerobic digestion were compared to evaluate the field applicability. The application of the new $H_2S$ removal technology in the new process resulted in an economic advantage of 1.8 times compared to the conventional biogas production process. In additions, it has been evaluated that using heat energy as a substitute for natural gas can reduce carbon dioxide emissions by approximately 40 kg $CO_2$ eq/kg-FW.. This study overcomes the limitations of the anaerobic process by minimizing the cost of the biogas refining process from organic waste and achieving rapid biogas conversion. It is expected to improve the competitiveness of anaerobic digestion technology, which is in the spotlight as a renewable energy