Biomethanation of synthesis gas in trickle bed reactors

Abstract

Syngas biomethanation is a promising technology for the production of biomethane from 2nd generation biomasses. It has recently started gaining attention due to the increasing interest of global stakeholders and policymakers to reduce CO2 emissions and make a transition towards a sustainable circular economy, in which the production of energy will derive from renewable sources and not from fossil fuels. Biomethane can be introduced in the natural gas grid replacing fossil-fuel derived natural gas and it can play an important role in the transportation sector by fueling vehicle engines. Additionally, it can be used in industrial chemical synthesis processes for the production of added-value products. The core of the syngas biomethanation process are the microbes that convert the syngas components (H2, CO and CO2) to CH4. These chemolithoautotrophic microorganisms consist of bacteria and archaea that collaborate in a synergistic environment to extract the necessary energy for their survival at the thermodynamic limit of life. The major bottleneck of the process is the low mass transfer rate of H2 and CO to the water-based media that results in low cell concentrations and low productivities of CH4. The overall objective of the PhD study was, thus, to select and thoroughly assess an appropriate bioreactor configuration that could overcome the mass transfer limitations and achieve high cell density for an efficient conversion of syngas to CH4. The chosen configuration was the tricklebed reactor that allows the microbes to grow in biofilms at a high surface to volume ratio enhancing the cell density, the cell retention time in the reactor and the mass transfer rate. The first part of the study was devoted on investigating the effects of the operational parameters (pH, liquid recirculation rate and hydraulic retention time) and the media composition on syngas biomethanation in mesophilicconditions. The main byproduct of the process was acetic acid, which decreased the pH to values unfavorable for methanogenesis and further enhanced acetogenic activity. In addition, shortage of trace metals was detected in the medium limiting the potential of microbial cell growth. To solve these issues, a medium with strong buffering capacity and appropriate increased concentration of trace metals was designed. Moreover, it was observed that the liquid recirculation rate affected the wetted fraction of the packed bed and its increase improved the liquid distribution in the bed resulting in higher conversion efficiency of the gas substrate and higher CH4 productivity. The next focal point was to examine the impact of the temperature comparing two trickle bed reactors operated at 37 °C and 60 °C, respectively. The major outcome was that thermophilic conditions were significantly superior to mesophilic conditions exhibiting higher CH4 productivities and higher product selectivity. In addition, by performing population sequencing and metagenomic analysis of samples from the inoculum, the biofilm and the liquid phase of the reactor, it was observed that thermophilic conditions fostered the growth of different microbes compared to mesophilic conditions and that the biofilm was richer in archaea compared to the liquid phase. A noteworthy element was the high abundance of cell debris scavengers in thermophilic conditions indicating that carbon recycling was important part of the process. Syngas produced from biomass gasification faces stoichiometric limitations for its conversion to biomethane and cannot satisfy the criteria for injection in the natural gas grid. The next step of this PhD study addressed the effects of exogenous supply of H2 in the trickle bed reactor for the production of natural gas grade biomethane. The conducted research led to the conclusion that the production of biomethane that could satisfy the natural gas standards was possible when the additional H2 inflow rate was regulated to follow the equation %퐻2 + %퐶푂/%퐶푂2 + %퐶푂 = 4 for the molar gas composition at the inlet of the reactor. Values below 4 corresponded to carbon-moles (CO2) excess, while values above 4 corresponded to electron-moles (H2) excess which deteriorated even more the biomethane quality due to thermodynamic limitations on carboxydotrophic hydrogenogenesis. At the final stage of the PhD study a pilot scale trickle bed reactor with a bed volume 28 times higher than that of the lab scale one was designed, constructed and operated at thermophilic conditions. The scaling-up was considered successful with the pilot scale reactor exhibiting even higher conversion efficiency of the substrate compared to the lab scale one at the same operating parameters. The reason was the improved mass-transfer rate induced (a) by the installation of a spraying nozzle that was distributing the liquid phase in the form of microdroplets at the top of the packed bed and (b) by the improved geometry of the bed that had a 3 times higher height per diameter (H/D) ratio compared to the lab scale reactor. The completion of the PhD study involved the operation of the pilot scale reactor in series with a fluidized bed gasifier producing syngas from wood pellets. The obtained results demonstrated no inhibition of the reactor when supplied with wood pellets syngas. In conclusion, this thesis describes the fundamental and up-scaling research activities carried out for syngas biomethanation in a trickle bed reactor paving thus the way towards commercial application of this promising technology.