Ex-situ biological CO2 methanation using trickle bed reactor: review and recent advances

Abstract

Power-to-methane technology is a promising solution to facilitate the use of excess variable renewable energy for biomethane production. In this approach, hydrogen produced via electrolysis is used to upgrade raw biogas, which can be subsequently used as fuel or stored in the gas grid. Ex-situ biomethanation is an emerging technology that could potentially replace conventional energy-intensive biogas upgrading methods and allow CO2 utilization for biomethane production. This work provides a comprehensive overview on the current status of ex-situ biomethanation with particular attention to trickle bed reactor. The review includes description of ex-situ biomethanation and summarizes previous works on this topic. The key elements related to operational conditions, efficiency, and microbiology of ex-situ biomethanation using trickle bed reactor are described here. Additionally, the review highlights the technical and economic issues that have to be addressed for future development and large-scale implementation of ex-situ biomethanation.

Microbial community development during syngas methanation in a trickle bed reactor with various nutrient sources

“…Analysis of the inoculum from the WWTP plant (reactor c) in a previous study showed it to be dominated by the acetoclastic genus Methanosaeta, a strict acetoclastic methanogen (Liu et al 2017 ). The highest methane production from syngas was seen for inoculum A (from the manure-based reactor), which could have been caused by factors such as (i) the presence of both methylotrophic and hydrogenotrophic methanogenesis, since hydrogenotrophic methanogens mainly utilise hydrogen during syngas methanation but acetate can be produced via acetogenesis from CO, requiring acetoclastic methanogens for further conversion to methane and (ii) high abundance of order Methanobacteriales , as several studies on biomethanation in TBR have shown enrichment of methanogens within this order, such as genus Methanobacterium and Methanothermobacter , indicating importance for biomethanation in such reactors (Aryal et al 2021 ; Sposob et al 2021 ). Previous studies on syngas methanation in TBR have used inoculum from biogas processes operating with manure (Aryal et al 2021 ) and sludge (Grimalt-Alemany et al 2020; Figueras et al 2021 ; Li et al 2021 ), or a mix of both (Asimakopoulos et al 2020a ), as well as syngas- or H 2 -enriched cultures (Asimakopoulos et al 2020b , 2021 ; Sieborg et al 2020 ) and defined cultures comprising just a few organisms (Kimmel et al 1991 ; Klasson et al 1992 ).…”

“…Some thermophilic processes have even been initiated with mesophilic inocula but have still resulted in well-functioning processes (Kimmel et al 1991 ; Grimalt-Alemany et al 2020; Li et al 2020b ). However, for methanation of CO 2 with H 2 , inoculation with enriched culture is reported to shorten the lag phase during start-up (Sposob et al 2021 ). Microbiological studies of continuously operated TBR have observed a complete change in syngas-enriched communities compared with the inoculum and high adaptive capacity, likely due to intrinsic biological diversity (Asimakopoulos et al 2020a ; Grimalt-Alemany et al 2020).…”

“…Previous studies on productivity during biomethanation in TBR have reported different values, influenced by different parameters such as reactor design, carrier material, inflow gas composition and rate, nutrient composition and loading rate, gas injection, operating time and inoculum source (Asimakopoulos et al 2020a ; Grimalt-Alemany et al 2020b ; Aryal et al 2021 ; Sposob et al 2021 ). For methanation of gas composed of only H 2 and CO 2 , values of around 1.17–3.1 L CH4 /L/d during mesophilic (38–40 °C) operation have been reported (Burkhardt and Busch 2013 ; Burkhardt et al 2015 ; Rachbauer et al 2016 ).…”

“…One challenge for the biomethanation process is gas–liquid transfer. Among different reactor systems available, the trickle bed reactor (TBR) can be considered an efficient technology for producing biomethane from syngas (Sposob et al 2021 ). The TBR consists of a column packed with carrier material with high surface area, for immobilisation of microbial biomass.…”

“…A liquid nutrient medium to support microbial growth and activity is sprinkled at the top of the TBR and trickles over the carrier material to the bottom of the reactor, while input gas flows in a counter-current or co-current direction. Rate-limiting mass transfer of gases is circumvented in TBR, since the carrier material provides a large surface area for interactions between gas, liquid and biofilm (Sposob et al 2021 ). However, combining TBR and biomethanation is a relatively new concept and parameters indicating a stable continuous process and high output have not yet been identified (Grimalt-Alemany et al 2017 ).…”