Biogas Upgrading Research Articles

Methane and Carbon Dioxide Separation Characteristics of Quaternary Ammonium Salt Hydrates for Biogas Upgrading under Static Conditions of Gas–Solid Contact

This report describes the methane and carbon dioxide separation characteristics of stoichiometric quaternary ammonium salt (QAS) hydrates for gas–solid contact with simulated biogas (CH4, 0.613 mol fraction; CO2, 0.387 mol fraction). Tests for elucidating the separation characteristics were conducted using tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium chloride (TBAC), and tetra-n-butylammonium bromide (TBAB) hydrates as solid phases. To ascertain system temperature and pressure effects on their gas separation performance, the tests were conducted at 253–293 K under the initial fed pressures of 3.0 and 0.9 MPa. The gas–solid contact between the simulated biogas with 0.387 mol fraction of CO2 and the QAS hydrates reduced CO2 in the gas phase to 0.26–0.37 mol fraction and enriched CO2 in the hydrate phases to 0.54–0.89 mol fraction. All QAS hydrates showed selective incorporation of carbon dioxide into the hydrate phase. Under all comparable conditions, TBAC hydrate exhibited the highest separation factor (15.2 ± 3.0 under initial pressure of 3.0 MPa at 253 K and 13.9 ± 2.5 under initial pressure of 0.9 MPa at 253 K), indicating that TBAC hydrate had the highest CO2 separation performance for biogas upgrading among the QAS hydrates we examined. At the same initial pressure, high gas separation performance was shown for low-temperature ranges. The separation factors of the QAS hydrates were maintained or improved by lowering the initial pressure at the same contact temperature, suggesting that the separation performance of the QAS hydrates, even at lower pressures, was nearly equal to or better than that achieved at higher pressures. The results contrasted against those reported for tetrahydrofuran hydrate, for which the separation factor decreases with lower initial pressure. The use of the QAS hydrates is expected to lower a system’s operating pressure without compromising separation performance.

Simultaneous biosuccinic production and biogas upgrading: Exploring the potential of sugar-based confectionery waste within a biorefinery concept

The imminent need for fossil fuel independence in EU countries has led to an increased development of organic waste valorisation technologies for the production of biomethane and chemical building blocks, such as bio-succinic acid (SA). In this work, the potential of two confectionery waste, in the form of wastewater (SCWW) or a side-stream rejection (SSCW), as inexpensive carbon sources for simultaneous SA production and biogas upgrading was evaluated for the first time. Both substrates were tested batchwise with evolved Actinobacillus succinogenes cultures at different nutrient conditions, SSCW at 100 g L−1 resulting in the highest titres/productivities (∼80 g L−1 and 1.3 g L−1h−1, respectively). Then, simultaneous biogas upgrading under continuous gas feeding was studied at bioreactor-scale, higher gas residence times and pressurization leading to desirable biomethane purities (>98%). The research here conducted is crucial for the cost-effectiveness and scale-up of the technology along this new waste-based biorefinery concept.

In-situ biogas enrichment using generated photovoltaic hydrogen

Hydrogen is the richest element in energy per unit of mass that exist. The challenge nowadays is its handling for different usages, as biogas upgrading. This research aims to study the behaviour of biogas upgrading with photovoltaic in-situ hydrogen generation with different type of substrates. It operates into batch reactors with 85 % of humidity of the mixture, with stainless steel electrodes at a controlled direct current voltage of 1.2 V at 38 °C, using pig slurry, chicken dung and rabbit poop (p > 0.05). Results showed a hydrogen generation during the process of 0.13 mol, 0.21 mol and 0.192 mol for pig slurry, chicken dung and rabbit poop, respectively. Leading to energy balance and energy recovery efficiency of 302.24 Wh.kg−1 and 12.45 %, 596.53 Wh.kg−1 and 19.99 %, and 1158.84 Wh.kg−1 and 30.33 % for pig slurry, chicken dung and rabbit poop, respectively. This upgrading technique improved biomethanization, but varied depending on substrate chemical characteristics involved.

A structure-property study for the effect of methyl substituents on the gas separation of spirodifluoranthene-based polymers of intrinsic microporosity

A simple methyl substitution on the structural building blocks of a PIM can alter its chain stacking behavior. Hence, we have introduced differing amounts of methyls on bulky and rigid spirodifluoranthene (EN) configured as two fluorenyl groups perpendicular to anthracene to obtain a series of spirodifluoranthene-based PIMs with increased intrinsic microporous, as demonstrated by nitrogen adsorption and enhanced gas permeability. The methyl substitution increased the free volume of polymer membranes, but the free volume does not increase in proportion to the number of methyl groups, and the increment is relatively slight at high methyl substitution amounts. Also, the effect of methyl substitution was closely associated with the molar contents of EN structures in PIMs. The series of polymers exhibited excellent CO2/N2, CO2/CH4, and O2/N2 gas separation performances, ranking among state-of-the-art polymeric materials for gas separation. Particularly, PIM-OMEN-40 exhibited the highest CO2 permeability, up to 12,431 barrer, which corresponds to 2.94 times higher than that of PIM-1 and along with comparable gas selectivity. Even after more than 100 days of aging, the series of PIMs still showed higher gas permeability and selectivity than fresh PIM-1, demonstrating great potential in separation areas for CO2 capture and storage, natural gas and biogas upgrading of prime importance to energy and the environment.

Assessment of the mass transfer strategy and the role of the active bacterial population on the biological degradation of siloxanes

Upgrading of biogas to remove siloxanes is mandatory to meet the standards required for its use as a substitute of fossil fuels. The biological degradation of these pollutants is a low cost and environmental friendly alternative to conventional techniques, albeit certain limitations, such as the low solubility of siloxanes, still hinder its application. In the present work, two parameters were optimized in aerobic and anoxic two-phase biotrickling filters (TP-BTF): the trickling liquid velocity (TLV) and the internal gas recirculation, with the aim of improving siloxanes biological removal. The results obtained showed that the increase in TLV from 2 to 10 m h−1 resulted in higher removal efficiency (RE) under both anoxic and aerobic conditions, reaching maximum values of 55 and 47%, respectively. This effect was more significant for the linear siloxanes. On the contrary, a further increase in the TLV to 20 m h−1 together with the implementation of internal gas recirculation caused an excessive turbulence in the liquid side, detaching the biofilm and having a negative effect for the RE. The cyclic siloxanes were more effectively eliminated along the process (maximum REs of 75% were recorded for decamethylcyclopentasiloxane (D5)), but the studied system modifications exerted a minor effect on their RE. The active bacterial population involved in siloxanes degradation (studied throughout RNA extraction and sequencing) was dominated by the clade Acidithiobacillacea KCM-B-112 and the genus Parvibaculum in aerobic conditions, while the members of the family Phyllobacteriacea and the genera Nocardia and Baekduia dominated in anoxic conditions.

Fruit waste (pomelo peels)-derived activated carbons for biogas upgradation and capture of green house gases from flue gas and low concentration coalbed methane

Rising emissions of greenhouse gases, such as CO2 and CH4, due to anthropogenic activities cause several adverse environmental impacts and encourage technological development for their effective capture and separation. The present work addresses a facile hydrothermal approach to impregnate K2CO3 and K2C2O4 into pomelo peels (fruit waste), followed by thermochemical activation for preparing activated carbons, i.e., PKC and PKOX, respectively, as adsorbents for biogas upgradation (CO2/CH4) and capture of greenhouse gases from flue gas (CO2/N2) and low concentration coalbed methane (CH4/N2). The high CO2 sorption capacity of PKC (4.67 mmol.g-1) and PKOX (3.96 mmol.g-1) at 298 K and 1 bar is attributed to their high surface area (1224–1371 m2.g-1), excellent microporosity, nitrogen-based functionalities, and turbostratic structural features. The ideal adsorbed solution theory (IAST) model is applied to estimate separation selectivity for binary gas mixtures, i.e., CO2/N2 (15:85), CO2/CH4 (45:55), and CH4/N2 (30:70) using PKC and PKOX adsorbents. The adsorption selectivity of PKC and PKOX for CO2/CH4 is found to be 143% and 296% higher compared to a widely used commercial activated carbon, signifying the potential of PKOX for biogas upgradation via adsorptive separation of CO2. The adsorption selectivity of PKC and PKOX for CO2/N2 is estimated to be 124% and 119% higher than commercial activated carbon, revealing the potential of both PKC and PKOX for capturing CO2 from flue gas to minimize its release in the atmosphere.

Two-phase pressure loss correlation for co-current flow in corrugated plate static mixers and structured packing

Co-current contacting of gas and liquid with a gas-continuous phase is a process with industrial relevance for natural gas processing, biogas upgrading, and carbon capture. Correlations that quantify two-phase, pressure losses under co-current flow conditions in corrugated geometries such as static mixers and structured packing at high gas-phase Reynolds numbers (ReG>105) are currently lacking. To fill this void, two-phase pressure losses in corrugated plate geometries under co-current flow conditions were measured at: 3,000<ReG<320,000 and 9<ReL<590. Our previously developed single-phase pressure loss correlation is extended to predict the results from this multiphase scenario by utilizing the separated flow modeling framework where each phase contributes to the pressure loss in an independent manner. The previously developed single-phase correlation matches the measurements with an overall Mean-Absolute Percentage Error (MAPE) of 20% at high ReG compared to a MAPE of 24 – 109% obtained from previous literature correlations. The new proposed correlation, employing the critical gas Reynolds number for the onset of liquid entrainment, reduced the MAPE to 7% for ReG>110,000 and 23% for ReG<110,000 compared to the dataset. A preliminary Computational Fluid Dynamics analysis shows that a simple, reflecting droplet boundary condition provides an adequate representation of the interfacial pressure loss effects for the range of conditions investigated in this study.

Evaluation of CO2 sources for Power-to-Liquid plants producing Fischer-Tropsch products

In addition to the climate crisis’s looming dangers, Europe was recently affected by profoundly volatile energy markets, entailing soaring inflation and political uncertainty. Power-to-Liquid processes have the potential to curb global warming by valorizing CO2 to produce synthetic fuels and platform chemicals while simultaneously substituting fossil energy imports. The impact of the CO2 source, i.e., cement production, biogas upgrading and solid biomass combustion, on Power-to-Liquid plants was evaluated by implementing the designed configuration, including CO2 capture, solid-oxide electrolyzer, Fischer-Tropsch synthesis and steam reforming, in IPSEpro, a stationary equation-based process simulation tool. Maximum Power-to-Liquid efficiency of 63.8% and maximum carbon efficiency of 88.6% were obtained by exploiting CO2 emitted by a biogas upgrading unit. Solid-oxide electrolyzers ranging from 23 MWel. (biogas) to 504 MWel. (cement) are required to process CO2 streams from 4.5 to 100 t/h. In addition, the mass and energy balances of the three considered configurations were determined and embedded in a process flow diagram. The presented study aims to facilitate future decisions concerning carbon capture and utilization policy by assessing the CO2 source’s influence on Power-to-Liquid plants’ key performance indicators. Furthermore, the underlying work supports a sustainable realization of Power-to-Liquid plants by offering a framework for exploiting CO2 sources.

Boosting membranes for CO2 capture toward industrial decarbonization

Membrane technology for carbon capture is becoming increasingly attractive to combat the excessive greenhouse gas emitted into the atmosphere, which involves the benefits of cost-effectiveness, environmental-friendly, easy scalability, high energy efficiency, simplicity in design, etc. However, most state-of-the-art membrane materials suffer from either low CO2 permeability, low selectivity towards CO2 separation, poor resistance to plasticization, or inadequate long-term stability, rendering it still challenging to be upscaled to an industrial level. Therefore, the development of advanced membrane materials as well as a reasonable design of the membrane separation process is crucial and urgent for its real-life application in the future. This account reviews the details of some recent research progress in our group on carbon capture from different scenarios including post-combustion carbon capture, biogas upgrading and natural gas sweetening and hydrogen purification. Notably, considerable efforts have been invested in the development of some novel membrane materials in our group, such as facilitated transport membranes, carbon molecular sieving membranes, mixed matrix membranes, composite membranes, and poly(ionic liquids)-based membranes. Meanwhile, some studies focusing on the techno-economic feasibility analysis of membrane technology have also demonstrated its promising application on practical carbon capture.

Coupling biogas upgrading and carbon capture and utilization for enhanced environmental performance of water reclamation plants

One of the primary sources of greenhouse gas (GHG) emissions in water reclamation plants is the anaerobic digestion process, which releases a significant amount of biogas. Today, wastewater treatment operators face immense pressure to find solutions that reduce the GHG emissions associated with this process. This paper proposes a conceptual design for a plant that couples biogas upgrading and carbon capture and utilization to mitigate GHG emissions from biogas release at a water reclamation facility in Singapore. A comprehensive model of the plant is first developed using a process simulation tool to assess its technical feasibility. Next, the plant’s economic and environmental implications are evaluated by calculating the net carbon abated, operating cost, and carbon abated cost. The results indicate that the plant has the potential to reduce GHG emissions by 84–90%, depending on the type of hydrogen used (gray, blue, or green). Although the plant exhibits a negative cash flow, its operating costs are lower than Singapore’s projected carbon tax rate in 2030, except in the scenario where green hydrogen is used. Overall, the study demonstrates the technical, economic, and environmental viability of the plant for reducing GHG emissions from water reclamation facilities.

Recovery of methane and acetate during ex-situ biogas upgrading via novel dual-membrane aerated biofilm reactor

Biological biogas upgrading has been well-proven to be a promising approach for renewable bioenergy recovery, but hydrogen (H2)-assisted ex-situ biogas upgrading is hindered by a large solubility discrepancy between H2 and carbon dioxide (CO2). This study established a new dual-membrane aerated biofilm reactor (dMBfR) to improve the upgrading efficiency. Results showed that dMBfR operated at 1.25 atm H2 partial pressure, 1.5 atm biogas partial pressure, and 1.0 d hydraulic retention time could significantly improve the efficiency. The maximum methane purity of 97.6%, acetate production rate of 34.5 mmol L−1d−1, and H2 and CO2 utilization ratios of 96.5% and 96.3% were achieved. Further analysis showed that the improved performances of biogas upgrading and acetate recovery were positively correlated with the total abundances of functional microorganisms. Taken together, these results suggest that the dMBfR, which facilitates the precise CO2 and H2 supply, is an ideal approach for efficient biological biogas upgrading.

Knowledge Mapping of High-Rate Algal Ponds Research

High-rate algal ponds (HRAPs) are a highly promoted wastewater treatment system that uses sunlight as an energy source to provide the oxygen needed in the system through photosynthesis and has a high nutrient and organic matter removal capacity. In addition, the microalgae in the system can use wastewater as a growth substrate to produce valuable bioproducts, biomaterials, and bioenergy, so it is receiving more and more attention. This review uses bibliometric analysis to explore current research hotspots and future research trends in this emerging technology. By analyzing research papers related to HRAPs published in the Web of Science (WOS) from 1987 to 2021 based on the co-occurrence and clustering of keywords, it shows that the research hotspots of HRAPs are mainly focused on wastewater treatment, nutrient removal, microalgal biomass, biofuel, and biogas upgrading. In the future, in-depth research will continue to be added on the contribution of HRAPs to environmental sustainability, including E. coli removal, biogas upgrading and oxygen removal, treatment of aquaculture wastewater, purple phototrophic bacteria, aqueous biorefineries, and biorefineries. The results assist scholars in systematically understanding the current research status, research frontiers, and future trends of HRAPs from a macro perspective.

Strategic Fast Induction Heating to Combat Hysteresis Barriers in a Flexible MOF for Rapid CO2 Desorption in Biogas Upgrading.

A major challenge in Cyclic Swing Separation using flexible adsorbents that have high equilibrium CO2 adsorption capacity is their very low-pressure hysteresis that hinders efficient desorption. Mg-Gallate MOF is such a flexible adsorbent that only begins to release CO2 at its pore closing pressure at 0.08bar and 30°C, showing very slow and inefficient desorption in pressure or temperature swing. Therefore, a novel strategy is presented that combines state of art technique Magnetic Induction Heating with a vacuum swing for fast and efficient CO2 desorption from flexible adsorbents at a moderately elevated temperature (70°C).

Pressurised Anaerobic Digestion for Reducing the Costs of Biogas Upgrading

The overall purpose of this study is to investigate the potential for producing higher energy biogas at elevated fermentation pressures. Upgrading of biogas is often carried out to increase its methane (energy) content by removing carbon dioxide. Upgrading is used, for example, to give methane of sufficient purity that it can be injected directly into the gas supply grid. In this research, freshwater algae are used as the feedstock for anaerobic digestion (AD) to produce biogas as a source of renewable energy. Although this has been the subject of extensive research over the past few decades, the main reason why AD has not been more widely commercialised is because it can have poor economic viability. In this paper, we used two similar bioreactors of capacity 1.5 L to generate biogas at different pressures. The methane concentration of the biogas increases to at least 70.0% for a headspace pressure greater than 4 bara compared to 57.5% or less when the pressure is less than 1.6 bara. The higher pressure operation therefore reduces the amount of upgrading required leading to a reduction in the cost of this step. Another interesting finding of this study is that the solubility of biogas in the digestate is estimated to be only 3.7% (best fit value) of its solubility in pure water, which is much lower than the values previously reported in the literature.

A novel structure of natural gas, electricity, and methanol production using a combined reforming cycle: Integration of biogas upgrading, liquefied natural gas re-gasification, power plant, and methanol synthesis unit

A novel structure of natural gas, electricity, and methanol production using a combined reforming cycle: Integration of biogas upgrading, liquefied natural gas re-gasification, power plant, and methanol synthesis unit

Removal of CO2 from Biogas during Mineral Carbonation with Waste Materials

Biogas represents a source of renewable energy that could provide a replacement for fossil fuels to meet the increasing demand for energy. The upgrading of biogas through the removal of CO2 to a content of 95-97% of CH4 is necessary to increase its calorific value. This review focuses on biogas upgrading technologies using wastes or residues that enable the performing of mineral carbonation. In this research, we analyzed a natural biogas or synthetic one with a content of about (40-50%) of carbon dioxide. The chemical absorption is also briefly described in this study, due to its being the first step in innovative absorption and regeneration processes using mineral carbonization. Wastes with high calcium contents, i.e., ashes, steel-making slags, and stabilized wastewater anaerobic sludge, were considered for direct carbonization, taking into account the leaching of particles from carbonated wastes/residues. Moreover, the different types of reactors used for mineral carbonation have been described. The presented technological solutions are easy to use and economical, and some of them also take into account the regeneration of reagents. However, in the context of their direct use in biogas plants, it is necessary to consider the availability of wastes and residues.

Soft-templated NiO–CeO2 mixed oxides for biogas upgrading by direct CO2 methanation

The catalytic performance in the direct CO2 methanation of a model biogas is investigated on NiO–CeO2 nanostructured mixed oxides synthesized by the soft-template procedure with different Ni/Ce molar ratios. The samples are thoroughly characterized by means of ICP-AES, XRD, TEM and HR-TEM, N2 physisorption at −196 °C, and H2-TPR. They result to be constituted of CeO2 rounded nanocrystals and of polycrystalline needle-like NiO particles. After a H2-treatment at 400 °C for 1 h, the surface basic properties and the metal surface area are also assessed using CO2 adsorption microcalorimetry and H2-pulse chemisorption measurements, respectively. At increasing Ni content the Ni0 surface area increases, while the opposite occurs for the number of basic sites. Using a CO2/CH4/H2 feed, at 11,000 cm3 h−1 gcat−1, CO2 conversions in the 83–89 mol% range and methane selectivities >99.5 mol% are reached at 275 °C and atmospheric pressure, highlighting the very good performances of the investigated catalysts.

H2-rich syngas production from biogas reforming: Overcoming coking and sintering using bimetallic Ni-based catalysts

Dry reforming of methane is a very appealing catalytic route biogas (mainly composed by greenhouse gases: carbon dioxide and methane) conversion into added value syngas, which could be further upgraded to produce liquid fuels and added value chemicals. However, the major culprits of this reaction are coking and active phase sintering that result in catalysts deactivation. Herein we have developed a highly stable bimetallic Ni–Rh catalyst supported on mixed CeO2–Al2O3 oxide using low-noble metal loadings. The addition of small amounts of rhodium to nickel catalysts prevents coke formation and improves sintering resistance, achieving high conversions over extended reaction times hence resulting in promising catalysts for biogas upgrading.

Role of C/N ratio in a pilot scale Microbial Electrolysis Cell (MEC) for biomethane production and biogas upgrading

Microbial electrolysis cells (MECs) permit to couple the oxidation of waste organic streams (e.g., wastewater, fermentate or digestate) with the reduction of carbon dioxide into products with a high market value (e.g., methane or acetic acid). MECs exploit the ability of electroactive microorganisms to use a solid electrode as final electron acceptor or donor. Here, a micro pilot tubular MEC has been set up combining the anodic oxidation of the organic matter with the bioelectromethanogenesis reaction in the cathodic chamber. Seven different synthetic feeding solutions, simulating a domestic wastewater or an acidogenic fermentate, have been used to test different C/N ratio on the performance of the MEC bioanode in the range between 25 and 0.4 (molC/molN). As a main result it was found that, under the same operating conditions (i.e., anode potential controlled at + 0.2 V vs SHE and HRT of 0.5 d), a high C/N ratio (e.g., 19 mol/mol) promotes the bioelectrochemical metabolism of the electroactive biofilm. These findings are relevant for a practical application of the technology considering the variable content of carbon and nitrogen in real feedstocks.

Study of On-Site Upgraded Livestock Biogas Production and Carbon Emission Reduction By Substituting Coals For Thermal Power Generation

The objective of this project is to integrate a farm-scale bio-desulfurization facility with a novel biogas hollow fibre adsorption module for biogas desulfurization and bio-natural gas production. In this study, the desulfurization experimental results showed that the bio-desulfurization system can remove 96.7 ± 6% of H2S from the biogas after an approximately two-month enrichment period. The average CH4, N2, and CO2 concentrations in raw biogas were 63.4, 15.2, and 21.1%, respectively. As for biogas upgrading experiments, the inlet biogas flow rates were applied from 5 to 20 L/min. The removal efficiency of CO2 under all biogas flow rates was 100%. Meanwhile, methane was promoted from 60% to nearly 94% (i.e. 57% increase in methane concentration). The replacement of anthracite and coking coal by upgraded biogas might reduce 44.4% and 42.5% of CO2 equivalent, respectively. The achievement of this project pursues the mitigation of carbon dioxide emissions by using upgraded pig biogas which can be enlarged and extended to all decentralized pig farms worldwide.