In situ methane enrichment in anaerobic digestion

Abstract

A major cost consideration in the use of anaerobic digestion to convert biomass and waste to utility-grade gas is the expense of separating CO(2) from the product gas. Anaerobic digestion has a number of inherent properties that can be exploited to increase the methane content of the gas directly produced by the digester, the most important of which is the high solubility of CO(2)(40-60 times that of methane) in water under digestion conditions. The methane enrichment concept examined in this study involved the recirculation of a liquid stream from the digester through a CO(2) desorption process and the return of the liquid stream back to the digester for absorption of additional CO(2) produced by the conversion of organic materials. A steady-state equilibrium model predicted that a digester gas methane content exceeding 94% could be achieved with this scheme using modest recirculation rates provided a desorption process could be designed to achieve a 60+% CO(2) removal efficiency in the degassing of the liquid recycle stream. Using fixed-film laboratory digesters operated on synthetic feedstocks, the technique of methane enrichment was tested under pressurized and unpressurized conditions. A 93 + 2% methane gas stream was produced from a volatile-acid-fed bench-scale digester simulating the methanogenic stage of two-phase digestion under conditions of (1) a pH swing achieved without caustic addition that allowed digestion at pH 7. 5 and air stripping at pH 6. 5-7. 0, (2) digester pressurization to 30 psig, and (3) a recycle rate of 0. 33 L/L reactor/day. Significant but lower levels of methane enrichment were achieved with the single-stage digester at the low experimental recycle rate. However, the narrow range among all experiments of CO(2) desorption efficiencies achieved in air stripping the recycle stream (35-60% CO(2) removal) suggests that comparable methane enrichment-may be achieved with unpressurized single-stage digestion using greater recycle rates. A materials balance analysis of data from an unpressurized, single-stage digester employing no chemical addition and using laboratory degassing efficiencies indicated that 94% methane could be produced at recycle rates of less than 1. 4 L/L reactor/day with a methane loss of less than 2%.

Integration of a Water Scrubbing Technique and Two-Stage Pressurized Anaerobic Digestion in One Process

“…Methane, in contrast, is less soluble and is thereby more dominant in the gas phase. This phenomenon was observed by the authors [4] and also reported in several other studies [12][13][14].…”

Effects of Organic Loading Rate on the Performance of a Pressurized Anaerobic Filter in Two-Phase Anaerobic Digestion

“…At a higher pressure, more CO 2 is dissolved in the liquid and then CH 4 becomes dominant in the gas phase. Methane enrichment in anaerobic digestion under pressure was also documented elsewhere [5][6][7]19]. However, in terms of the absolute value, the methane content was not as high as reported in other studies.…”

“…The reported few trials showed that pressurized anaerobic digestion had the potential to produce biogas of natural gas quality [6,7]. However, all the experiments were carried out under low organic loading rate (OLR) and a fixed pH value.…”

A new upgraded biogas production process: Coupling microbial electrolysis cell and anaerobic digestion in single-chamber, barrel-shape stainless steel reactor

“…natural gas, used in vehicles and other machinery. Compared with in situ gas purifications which take advantages of differing solubility of CH 4 and CO 2 by controlling pH or pressure [13,14], the new process has no negative effect and increase CH 4 production significantly.…”