Biogas is the product of anaerobic digestion process, which is a biologically mediated process. Besides CH₄ (50-65%), biogas contains a major portion of carbon dioxide (CO₂) (35-50%), and some traces of hydrogen sulfide (H₂S), ammonia (NH₃), and water vapor (H₂O). These impurities limit the application of biogas.
Biogas upgrading is mainly achieved by either physical methods or chemical methods, such as water scrubbing, physical scrubbing, membrane separation, pressure swing method, chemical scrubbing. Even though physical and chemical methods are the most used for biogas upgrading it has some setbacks, like pretreatment of biogas, operational pressure (4-20 bar), conditional temperature. This makes the upgrading process 2-10 times more than the cost of biogas production. Additionally, the release of chemical waste is also a big concern, because of its toxicity in nature. However, biological methods like chemoautrophic methods are still in the research stage.
Autogenerative high-pressure digestion has the advantage of producing CH₄-rich biogas directly from the reactor. However, its continuous operation has rarely been reported, and never been attempted for upflow anaerobic sludge blanket reactor (UASB). Here, UASB was continuously operated at 10 g chemical oxygen demand (COD)/L/d with pressure increase from 1 to 8 bar. As the pressure increased, the CH₄ content in the biogas gradually increased, reaching 96.7±0.8% at 8 bar (309 MJ/㎥ biogas). The pH was dropped from 8.2 to 7.2 with pressure increase, but COD removal efficiency was maintained >90%. With pressure increase, there were no changes in the microbial community and ATPase gene expression, but a 41% increase in carbonic anhydrase gene expression was observed.
Biohydrogen is considered as a suitable alternative to fulfill this demand. But it has one big obstacle, which is low H₂ content (50-60%). For the direct use of H₂, it needs to be increased at least over 80% to fit for the biogas upgradation. However, the low pH during the dark fermentation is one of the biggest obstacles to overcome. Here, a novel approach has been executed to enhance the H₂ content in a single step under high pressure by supplementing Ca2+ for pH regulation above 4.5. A batch experiment was conducted to verify the hypothesis and achieved 93.2%. The obtained results showed that it was possible to obtain H₂ content if the pH can be controlled. A theoretical calculation was solved for before continuous operation to predict the possibility H₂ upgradation by considering the charge balance and COD balance of the reaction. After successful results, continuous operation was executed in a UASB reactor by addition of Ca2+ to regulate the reactor pH. The pressure was automatically increased by regulating the gas flow. A maximum H₂ content increased to 90.1% at 9 bar, having a yield of 1.15 mol/mol hexoseadded. Lactic acid was increased as the pressure increased which was justified by the enhanced abundance of Lactobacillus and L-lactate dehydrogenase gene expression.
Biological biogas upgrading by the addition of H₂ in an anaerobic digester is a new way of storing energy in an efficient manner. Power-to-gas is a promising approach to transform the current energy system. Here, two continuous stirred-tank reactors (CSTRs) (a) control, and (b) extra electrical input (EEI) were operated at different H₂ loading rates (HLR) (2-10 g COD/L/d) having the feeding ratio of H₂:CO₂=4:1. EEI reactor was first operated with 0.3 V direct current supply. Both reactors showed similar performance with more than 95% conversion efficiency till the H₂ loading rate of 8 g COD/L/d. Once HLR increased to 10 g COD/L/d. Both the reactors showed a sharp drop, and the CH₄ conversion efficiency reduced to 42%. At this condition, H₂ content in the biogas reached 72%, while CH₄ content was reduced to 23%. Recovery of the reactors at the HLR of 10 g COD/L/d was gained when the control was supplemented with 2 g Fe3+/L magnetite, and EEI was operated at 0.6 V. The highest CH₄ content obtained at 10 g COD/L/d was 94.3% for the control and 94% for EEI (0.6 V), respectively. In EEI, the voltage was again lowered to 0.3 V at 10 g COD/L/d. However, no drop was observed, which showed that high voltage might have improved the microbial consortia and that lowering the voltage didn't affect the performance. Surprisingly, cutting off the voltage showed a continuous drop in the EEI performance, which further confirms that electrical voltage is required to perform the EEI at 10 g COD/L/d.
Supply of external H₂ has been proven to enhance the biogas upgrading significantly. However, considering the cost it would not be feasible to outsource the H₂ supply from a non-biological channel. Therefore, based on our previous attempts, it was possible to upgrade the high -calorific H₂ generation (H₂ content of 90.1%) via high pressure. In this work, a continuous operation was executed in the UASB reactor for high calorific biogas production with CH₄ content of >95%. Organic loading rate (OLR) was initially fed with 2 g COD/L/d and reached to 4.2 g COD/L/d. The pressure was then gradually increased from 1 bar to 5 bar and found that CH₄ content was initially increased from 73.5% at 1 bar to 92.9% at 5 bar. After that H₂ was supplemented into the reactor having a H₂:CO₂ ratio of 9:1 which was obtained from the previous experiments, considering the obtained COD value of 0.47 g COD/L/d. After H₂ addition the final OLR increased to 4.67 g COD/L/d, additional of extra H₂ was further increased the CH₄ content to 94.8% without increasing the set pressure. It showed that it is possible to increase the CH₄ content without increasing the pressure.