Storage of surplus electricity is a growing demand in renewable energy technology, with the generation of electricity in an inherently fluctuating mode of operation, such as wind and direct solar, gaining a rapidly increasing market share. In a popular strategy, electricity is used to split water and generate H2 and O2. There are no mature technologies available for handling H2 today, and its conversion to CH4 therefore seems preferable. In this scheme, electricity is transformed into CH4, which is then stored and transported easily via the existing natural gas grids. Chemical methods to reduce CO2 with H2 have been known for some time and earned the Nobel prize for Paul Sabatier in 1912 [31]. The process requires high temperature, high pressure, and metal catalysts. In alternative electrochemical means of CO2 mitigation, electrical energy input is the driving force [3, 9, 30]. Biological systems can solve the same task under mild conditions in an environmentally friendly manner. The life of hydrogenotrophic methanogens, an odd group of Archaea, relies on the same reaction, which is catalyzed by enzymes at ambient temperature and pressure. The biological route of the power-to-gas process, which is here named as power-to-biomethane (P2B), has been recognized and tested in laboratory and scale-up works [19, 22, 24, 32]. These studies have established that microbes are exceedingly efficient catalysts for the P2B process. Hydrogenotrophic methanogens are difficult to cultivate in pure culture, but they are readily available in the mixed culture of effluents from the anaerobic degradation of organic matter, i.e., the fermentation effluent of biogas plants. The rate-limiting step in the work of CH4-forming microbial cell factories is the low solubility of H2 in the aqueous environment. In previous studies [19, 22, 24, 32], continuously-operating fermentation systems were employed as a rule, which offer several advantageous features for process control and management, but allow short residence time for the injected H2 gas.
In our approach, the fed-batch fermentation technique was selected to increase the contact interaction between the gaseous substrate and the biocatalyst methanogens. It was established that an optimal mixing rate has to be upheld in any P2B system in order to facilitate the dissolution of H2 into the aqueous phase where the microbes and dissolved CO2 reside.
Although CO2 is readily soluble in the aqueous medium, it may become an overall limiting factor if removed from the system either by vigorous reaction with H2 or by degassing the reactors. Depletion of CO2 was accompanied by the elevation of pH, which might be precarious for the activity of hydrogenotrophic methanogens.
CO2 is supplied by the biogas-generating process itself [19, 22] or can be provided from outside sources, e.g., flue gas from internal combustion engines. Consumption of the greenhouse gas CO2 by the process is an additional benefit of the P2B technology from an environmental point of view. Addition of an organic substrate may revitalize the entire biogas microbial community, which generates additional CO2 and thereby stabilizes the pH, but does not facilitate the conversion of H2 to CH4. A proper feeding routine in the fed-batch system leads to a sustained high rate of CH4 formation and the process may operate efficiently for an extended period of time.
Comparison with previous works
Our approach to improve the P2B principle attempts to counteract the low solubility of H2 in the aqueous environment by increasing the contact time of the gas and aqueous phases in a fed-batch fermentation arrangement. This has not been tested earlier.
There are four previous reports available to measure up against this approach. Lee et al. [24] used a fixed-bed reactor, while Reuter [32] developed several versions of a continuous stirred tank reactor (CSTR) design and scaled up the process to an industrial level. Both studies concluded that hydrogenotrophic methanogens in pure or mixed culture were markedly efficient catalysts and converted H2 and CO2 to CH4 in surprisingly high yields and rates. Unfortunately, the published results from those studies contain limited data on process parameters to compare with the fed-batch system examined in the present study.
Two recent papers from the Angelidaki team [19, 22] also used CSTR reactors and reported promising results. Their thoughtfully designed and thoroughly documented reports provided data allowing the assessment with our study. Table 2 summarizes the results.
Besides the use of distinct reactor arrangements and sizes, i.e., fed-batch versus CSTR, several operational parameters differed in those studies from our set-up, e.g., inoculum composition and quality, substrate used for CH4 generation, stirring mode and speed. Therefore, only the major tendencies and not the exact values are suitable for a rigorous comparison.
It was found that at high shaking speed the H2 conversion process may not be limited by the gas–liquid mass transfer [19] at thermophilic temperature. In our experience, this observation could not be repeated under mesophilic conditions, and above 160 rpm CH4 formation was inhibited (Fig. 1). It was concluded that the process in our system was critically limited by the mass transfer of H2 at the gas–liquid interface. Hydrogenotrophic methanogens utilized the dissolved H2 at a high rate, and therefore a concentration gradient developed between the liquid and gas phases, driving H2 into the liquid compartment from the headspace as time advanced. It is likely that the fed-batch operation optimized the condition where the amount of H2 transferred into the liquid phase was close to the amount consumed by the microbes. The data presented in Table 2 clearly indicate that this was indeed the case.
In the CSTR work, H2 was dosed on the basis of the available CO2 from the coupled biogas production [22]. Although significant upgrading of the biogas was achieved, this stipulation limited the rate and amount of H2 injection into the system. The goal in these investigations was to achieve maximal H2 conversion yield. H2 bubbles are difficult to retain in the aqueous system, and diffusers and very low purging rates therefore had to be applied to facilitate the dissolution of H2 and its conversion to CH4 during the short residence time of the gaseous substrate in the reactor. In the fed-batch configuration, the H2 loading rate could be increased to 4 times that of the CSTR operational mode without the loss of H2 (Table 2).
In the present study, mesophilic conditions were maintained. Bassani et al. [22] carried out their experiments at 35 and 55 °C under otherwise identical conditions. A significant improvement in CH4 formation rate was noted at higher temperature. A similar effect can be expected in the fed-batch system; this will have to be established in future studies. A comparison between our mesophilic data with those obtained at thermophilic temperature indicates a 2.0 [19] and 2.7 [22] times higher CH4 production rate from H2 in the mesophilic fed-batch reactors as compared with the thermophilic CSTR, respectively.
The mesophilic process performance parameters of Bassani et al. [22] can be compared directly with our results reported under the “Effect of CO2 addition” subtitle above. Two sections of stable operation in our experimental period were taken into account, i.e., the initial phase without external CO2 addition between days 2 and 28 and the part when stoichiometric CO2 and H2 were injected daily (days 50–80) (Figs. 5, 6). To make a fair assessment, the residual CH4 production in the control reactors (no H2 added) should be taken into account.
The control samples in our work started at an unusually low CH4/CO2 ratio (Table 2), which could be due to the residual biogas potential of the inoculum and the fact that all H2 was removed during initial degassing of the reactors. Therefore, the activity of the hydrogenotrophic methanogens was severely restricted until some H2 became available from the fermentation of the residual, small amount of biomass. The situation changed dramatically in the reactors receiving H2 injections and the system produced bio CH4 of high purity, i.e., containing only 17.71 % CO2.
There was a 6.5-fold increase in CH4 yield from H2 in the fed-batch system relative to the mesophilic CSTR experiments if a stoichiometric amount of CO2 was added to both systems together with the H2 (Table 2). Moreover, the fed-batch system operated at a 4-times higher H2 load than the CSTR reactor. The H2 consumption was above 90–100 % in both systems, indicating that the reaction was carried out very efficiently in both systems. The CSTR operation mode has its benefits and advantages, but apparently does not help overcome the low H2 solubility problem, which seems to be the major bottleneck in the accomplishment of the P2B principle at mesophilic temperature.
As an added value, it should be noted that in the fed-batch system a considerable accumulation of acetate takes place without any observable sign of acidosis-related process failure (Fig. 9). The accumulation of acetate was probably due to the inhibition of acetoclastic methanogenesis and syntrophic acetate oxidation [33] by the high H2 doses. Acetate is a valuable commodity [30, 34] and, if acetate can be recovered by a suitable technology from the reaction mixture, it would be a useful side-product of the fed-batch fermentation-based P2B technology.