Typical Biogas Research Articles

UNLOCKING THE POTENTIAL OF BIOENERGY: A SUSTAINABLE SOLUTION FOR ENERGY SECURITY, WASTE MANAGEMENT, AND ENVIRONMENTAL CONSERVATION

The study was carried out to generate biogas from fresh cowdung. Proximate analysis on the cow-dung sample revealed moisture content to be 60.53%, Ash content 11.01%, total solids 39.47%, volatile matter 10.78%, protein content 12%, nitrogen content 1.6%, carbohydrate 5.68% and fixed carbon 17.68%. The pH and Temperature were measured daily at 2:00pm, the temperature ranged from 30OC-38OC and the pH range was 6.7 - 7.2. The gas produced was measured daily by calculating the volume with the height increased daily up to 36 days. The results showed a typical biogas production curve, consisting of lag, acceleration, maturation, and decline phases. The optimal retention time for biogas production is identified as 20-30 days, during which biogas yields are highest. The findings indicate that microorganisms require an initial adaptation period (Days 1-3) before biogas production commences, followed by a significant increase in production (Days 11-20) and a stable production rate (Days 21-30). A decline in production is observed after 30 days. This study highlights the importance of retention time in biogas production and demonstrates the need for monitoring production kinetics to optimize retention time for specific substrates and microbial communities. The results have implications for the design and operation of biogas production systems.

Intrinsic kinetics of steam methane reforming over a monolithic nickel catalyst in a fixed bed reactor system

The intrinsic kinetics of steam methane reforming were experimentally investigated using a monolithic nickel-based catalyst in a specially designed fixed bed system. All kinetic tests were performed under various operating conditions: steam to carbon ratio of 2 to 5, methane volumetric concentration in feed gas of 3 % to 9 %, temperature of 500 to 650 °C, and typical biogas compositions with N2 dilution (carbon dioxide 35.7–55.6 %, methane 44.4–64.3 %, with 86–91 % nitrogen dilution). A deconvolution process was applied during the rate calculation and the reaction orders with respect to different gases were determined, coupled with thermodynamic system analysis. The reaction mechanism was then proposed and a Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model was established, followed by kinetic parameter estimations for the model and a further model validation with experimental data. Based on the model, the intrinsic kinetic of SMR reaction using monolithic catalysts was described. The validity and feasibility of applying the faster approach for kinetic parameter estimation were demonstrated, and this work is the first demonstration of a complex reaction system. Also, the reaction mechanism appeared to show a suitably high correlation with alternative and more sustainable methane sources (i.e. biogas).

Degradation of dissolved sulfide in water using multi-hole dielectric barrier discharge

Biogas obtained from livestock manure is used as fuel for solid oxide fuel cells. Although H2S is a typical biogas, it is a fatal disadvantage for fuel-cell power generation and, thus, must be removed. In this study, we proposed an effective method for sulfide removal from water using a multi-hole dielectric barrier discharge (DBD) system. In this system, active species, such as ozone, ultraviolet rays, hydroxyl radicals, and hydrogen peroxide, were simultaneously generated. Under optimal conditions, dissolved sulfide (initial concentration: 120 mg/L) was completely degraded within 10 min in air plasma and 6 min in oxygen plasma. Changes in the physical properties of the sulfide-treated water were confirmed by measuring the pH, oxidation-reduction potential, and dissolved oxygen. Results of the by-product analysis showed that sulfide was converted into sulfate by reacting with a large amount of ozone, and the active species were emitted from the multi-hole DBD system. In summary, multi-hole DBD technology has demonstrated merit as a water-contaminant purification technology and for the removal of dissolved sulfide.

ANALYSIS OF MACHINE LEARNING ALGORITHMS FOR BIOGAS YIELD PREDICTION

Currently, effective management of biogas production remains a difficult task. The purpose of the research is to analyze machine learning algorithms for predicting biogas output depending on the characteristics of biogas production. Currently, there is no necessary set of data, analyzing which indicators can be obtained to optimize biogas production in our installation. At the same time, testing various optimization algorithms and deciding on the best takes a lot of time, as experience shows. The application of machine algorithms for forecasting biogas production by using existing forecasting methods is considered. Provided that the control systems of typical biogas productions are equipped with the necessary sensing elements, there still remains the task of processing and analyzing data to make the best decision to meet the relevant technological requirements. The reason for this is the large volume of data and the complexity of the interaction of processes that are components of production. In this context, machine learning can be a useful tool for optimizing biogas production.

Renewable biomethane production from biogas upgrading via membrane separation: Experimental analysis and multistep configuration design

In this work, we studied the upgrading of a mixed gas stream having a typical biogas composition by membrane gas separation with the aim of producing a purified biomethane stream. A polyetherimide/polyimide (PEI-PI) membrane module with an area of 1500 cm2 was used to evaluate the mass transport properties and the influence of water vapour and H2S on the separation performance. CO2 permeance was depleted by the presence of CH4 contrarily to this latter, whose permeation was enhanced by CO2. The permeances of both CO2 and CH4 were significantly affected by the presence of water vapour, showing a reduction of about 24–27% with respect to dry gas. Long-term analysis lasted about 650 days, showed an overall permeance variation of about 23% for N2 and 14% for CO2.The experimental permeance and selectivity values measured in wet conditions were used to simulate a multistep membrane system able to purify a “clean” biogas stream up to 98% of methane. It was found that the increasing number of steps from 3 to 7 allows a significant saving of membrane area, paired to an increment of CH4 recovery and CO2 purity.

Thermodynamic and economic analyses of a new compressed air energy storage system incorporated with a waste-to-energy plant and a biogas power plant

In this paper, a novel compressed air energy storage (CAES) system integrated with a waste-to-energy plant and a biogas power plant has been developed and evaluated. In the charging process, the feedwater of the waste-to-energy plant recovers the compressed heat of the compressed air in the CAES system. In the discharging process, the compressed air is heated by the energy obtained from the waste-to-energy boiler, and then replaces the compressor of the gas turbine system by burning with the biogas in the combustor directly. Via the system integration, some major equipment of a conventional CAES system can be saved, and the performance of the CAES system can be improved. Based on a 500 t/d waste-to-energy plant and a typical biogas power plant, the energy, exergy, and economic analyses of the proposed system were performed. As a result, a higher round-trip efficiency of 69.55% can be achieved, and the major exergy loss occurs in the throttle valve. Furthermore, the dynamic payback period of the proposed CAES system is 4.35 years, 4.74 years shorter than that of a conventional CAES system. Overall, the integrated system can improve the performance of the CAES system and promote the development of energy storage technology.

Biogas upgrading using Clostridium autoethanogenum for value-added products

Biogas production from anaerobic digestion is a well-established bioprocess for energy generation, nutrient recovery, and valorisation of waste resources. Typical biogas contains 60 % methane and 40 % carbon dioxide (CH4/CO2 of 1.5). Removing or altering carbon dioxide content facilitates biogas use as biomethane due to its impact on the calorific value. Technologies that effectively remove carbon dioxide to convert biogas to biomethane could play a central role in transforming waste resources. This is important as the world transitions into a low carbon and circular economy. Gas fermentation utilising acetogens and renewable H2 offers an under-explored biological alternative for biogas upgrading. To demonstrate the use of gas fermentation for biogas upgrading, we used Clostridium autoethanogenum to ferment CO2 from synthetic biogas supplemented with H2 to produce ethanol and acetate (2.8 ± 0.1 and 3.9 ± 0.1 g/L, respectively). The bioprocess was able to enrich the biogas (CH4/CO2 of 2.20 ± 0.05) and transform the CO2 into valuable fuels and chemicals in a one-step process. Strikingly, ∼50 % of the off-gas was inert, meaning there is potential for optimising acetogenic fermentation to achieve complete valorisation of CO2 from waste streams. Increasing the gas to liquid mass transfer of H2 improved the culture’s growth rate and stability, but biomass and ethanol titres remained unaffected during chemostat operation. This represents a step towards upgrading biogas and other low-grade CO2 waste streams by utilising renewable hydrogen and gas fermentation as a sustainable platform technology.

Online detection of trace volatile organic sulfur compounds in a complex biogas mixture with proton-transfer-reaction mass spectrometry

Online monitoring of trace contaminants, especially sulfur-containing compounds, is critical in biogas plants to protect downstream process steps, e.g., fuel cells or catalysts. The high biogas purity requirement for fuel cell applications and the presence of a myriad of unknown contaminants at different concentration levels require untargeted measurement techniques with high sensitivity and high dynamic range. Here, we present an online, real-time measurement of trace contaminants in the cleaning stream of a biogas plant with a Vocus proton-transfer-reaction mass spectrometer (Vocus-PTR-MS). We identified more than 300 Volatile Organic Compounds (VOCs) in the raw biogas in the 30–400 m/z range, with total concentrations varying in 54.6–548.3 ppmv on different days. Among these, more than 30 were Volatile Organic Sulfur Compounds (VOSCs), with total concentration in the range of 0.3–9.3 ppmv during measurement days and dimethyl sulfide (C2H6S, DMS) was the most dominant VOSC. Typical biogas tracers were also identified in the ambient air around the biogas plant at low ppbv levels. These results confirm that VOCUS-PTR-MS can be a rapid, scientific online tool of choice to monitor a biogas facility and, hence, shows excellent ability to characterize biogas production.

Long-term performance of highly selective carbon hollow fiber membranes for biogas upgrading in the presence of H2S and water vapor

Biogas is an increasingly attractive renewable resource that needs to be upgraded to meet either the natural gas pipeline quality or the biomethane requirement. The use of carbon membranes is a promising alternative to conventional separation technologies, but, up to now, few attempts have been made to systematically investigate their separation performance under real biogas upgrading conditions, for instance, the presence of water vapor and H2S in the feed stream.In this work, for the first time in literature, the separation performances of as-prepared cellulose-based carbon hollow fiber membranes were monitored for 183 days under continuous exposure to a gas stream also containing H2S and/or water vapor. After the evaluation of their CO2 and CH4 sorption (2,98 mmol g−1 and 2,00 mmol g−1, respectively at 298 K and 10 bar), diffusion(2.45 × 10-7 cm2 s−1 for CO2), and permeation properties (120.9 and 2.3 barrer, respectively at 308 K) with single gases, long-term tests were carried out by feeding gas mixtures with typical biogas composition. It was found that the exposure of the membranes to H2S (up to 500 ppm) and water vapor (relative humidity of 90%) provoked a reduction of CO2 and CH4 permeability compared to the “clean and dry” mixed gas, which resulted in an increment of selectivity, which reached a value of more than 200. Overall, after more than 180 days of continuous testing, the membranes exhibited remarkable CO2/CH4 selectivity with endurability of the H2S and water vapor, confirming the fact of being good candidates for biogas upgrading.

RhNi/CeO2 catalytic activation of alumina open cell foams by dip-spin coating for the CO2 methanation of biogas

Production of CH4 by methanation of biogas provides a promising route towards sustainable energy production for the future. Utilization of biogas from biomass as CO2 source combines the advantage of reduction of greenhouse gas emissions producing a fuel with high energy density. In this study we apply a dip-spin coating method for washcoating Ni(7.5 wt%)Rh(0.5 wt%)/CeO2 catalyst on alumina open cell foams with different cell densities (20, 30 and 40 PPI).The catalytic activity of the structured samples was assessed using a gas mixture that simulates a typical biogas composition (CH4 = 60%vol; CO2 = 40%vol). The effects of the reaction temperature (300–600 °C) and H2/CO2 molar ratio in the feed (3–5) on the CO2 conversion and CH4 selectivity were evaluated under a gas hourly space velocity (GHSV) of 5100 h−1.All samples showed appreciable CO2 conversion ranging between 62.5 and 66.7% at reaction temperature of 500–600 °C, and a CH4 productivity of 3.4 molCH4·gMe−1·h−1 was obtained with the catalyst supported on open cell foam with 30 and 20 PPI under a reaction temperature of 400 °C.

Thermodynamic Insight in Design of Methanation Reactor with Water Removal Considering Nexus between CO2 Conversion and Irreversibilities

The inevitable nexus between energy use and CO2 emission necessitates the development of sustainable energy systems. The conversion of CO2 to CH4 using green H2 in power-to-gas applications in such energy systems has attracted much interest. In this context, the present study provides a thermodynamic insight into the effect of water removal on CO2 conversion and irreversibility within a CO2 methanation reactor. A fixed-bed reactor with one intermediate water removal point, representing two reactors in series, was modeled by a one-dimensional pseudo-homogeneous model. Pure CO2 or a mixture of CO2 and methane, representing a typical biogas mixture, were used as feed. For short reactors, both the maximum conversion and the largest irreversibilities were observed when the water removal point was located in the middle of the reactor. However, as the length of the reactor increased, the water removal point with the highest conversion was shifted towards the end of the reactor, accompanied by a smaller thermodynamic penalty. The largest irreversibilities in long reactors were obtained when water removal took place closer to the inlet of the reactor. The study discusses the potential benefit of partial water removal and reactant feeding for energy-efficient reactor design.

Industrially relevant CHA membranes for CO2/CH4 separation

In the present work, single channel tubular zeolite CHA membranes with a length of 50 cm and a membrane area of 100 cm2 were evaluated by SEM and permeation experiments with single components and industrially relevant humid mixtures. The membranes comprised well-intergrown and smooth CHA films with a thickness of <500 nm supported on the inside of a highly porous tube. For single component permeation, a very low SF6 permeance of 4.5 × 10−10 mol/(m2‧s‧Pa) was observed, which indicated nearly defect free membranes. On the contrary, the membranes displayed a very high CO2 permeance of 128 × 10−7 mol/(m2‧s‧Pa), which illustrated the very high permeability of the CHA pores. Finally, the membranes displayed excellent selectivity for separation of industrially relevant CO2/CH4/H2O mixtures, which was attributed to selective interaction between the CO2 molecules and the polar water molecules in the pores. The highest observed CO2/CH4 separation selectivity was as high as 198 in combination with a CO2 permeance of 14 × 10−7 mol/(m2‧s‧Pa) at a feed pressure of 600 kPa (including 2.2 kPa water) and 293K. The corresponding CO2 flux was 0.39 mol/(m2‧s) and the corresponding CO2/CH4 separation factor was 162. The observed membrane performance was reduced by concentration polarisation due to the limited feed flow in the experimental setup. The corresponding selectivity and CO2 permeance corrected for concentration polarisation were as high as 236 and 16 × 10−7 mol/(m2‧s‧Pa), and the corrected CO2 flux was 0.44 mol/(m2‧s) and corrected separation factor was 198. An estimate showed that even at a low feed pressure of 500 kPa, it would be sufficient with as few as 64 membranes for processing of a feed of 100 Nm3/h raw biogas, i.e. the capacity of a typical biogas plant at a large farm, to biomethane with high purity. These results illustrated that the membranes are promising candidates for industrial separation of CO2 from e.g. natural gas and biogas.

Design of biogas upgrading processes based on ionic liquids

Biogas stands out as an alternative to traditional sources of energy since it presents a high methane content, and it is mainly produced by anaerobic digestions of organic wastes. Typical biogas streams do not only consist of biomethane but also carbon dioxide, water, ammonia, hydrogen sulfide or siloxanes, depending on the source of the organic waste. Therefore, it is important to remove all these contaminants to obtain a high-quality stream in a process known as biogas upgrading. Currently, there is not a predominant technology, all of them presenting advantages and drawbacks to be solved. In this work, we test a biogas upgrading process based on CO2 chemical absorption by ionic liquids (ILs). The complete process was evaluated involving absorber and stripping columns in a wide range of operating temperatures and pressures to reach biomethane of 97% purity from an industrial biogas stream. Best specific energy consumptions are found at 50 °C in the absorber and 95 °C in the stripper at atmospheric pressure. Increasing the operating pressure of the absorber to 6 bar reduces the energy consumption from 0.8 kWh/Nm3 to 0.2 kWh/Nm3. This is mainly because of the reduction in IL flow (almost a half) and that the thermal energy needed is provided by the exothermic reaction and no external requirements are needed. IL-based proposal for biogas upgrading was found able to efficiently retain CO2 but also other main impurities (H2S, H2O, siloxanes) producing biomethane with quality standards. Results from IL-based process reveal savings in operating cost and nearly the same equipment investment costs than available technologies (PSA, water and amine scrubbing, and membranes) for biogas upgrading process.

Thermo-economic optimization of a Rankine cycle used for waste-heat recovery in biogas cogeneration plants

In a typical biogas cogeneration plant, a part of the exhaust gas energy is used for heating buildings within the plant. However, a certain amount of this energy remains unused. This study examined the utilization of this waste-heat through the Rankine cycle and organic Rankine cycle. A multiobjective thermo-economic optimization procedure of a waste-heat recovery unit installed at the exit of an engine before the engine cooling fluid–exhaust gas heat exchanger is proposed herein. The optimization procedure includes sizing all heat exchangers in the waste-heat recovery unit based on the measured Colburn and friction factors. The thermo-economic optimization was performed to investigate the maximal power output and the minimal payback period while preserving the function of the heating system. The procedure was applied to a biogas power plant with two engines (mass flow rate of the exhaust gases from each engine was 1.77 kg/s at a temperature of 410 °C). The electrical efficiency of this system was 42.1% and the measured overall yearly energetic efficiency was 66.7%. Optimization shows that the waste-heat recovery unit based on Rankine cycle is more economical than the unit based on the organic Rankine cycle using toluene. The electrical efficiency of the entire power plant increased by 2.97% and the payback period of the investment was 6.8 years, while the Levelized Cost of Electricity was 0.0419 $/kWh. The proposed method could be used to analyze the investment profitability of waste-heat utilization in other cogeneration plants.

Hydrothermal carbonization of agricultural and municipal solid waste digestates – Structure and energetic properties of the solid products

Digestate is a nutrient-rich substance produced by anaerobic digestion, it contains organic and inorganic compounds as well as micro-organisms. The European Nitrates Directive (91/676/EEC) gives the regulations concerning the wider implementation of digestate. Due to a significant amount of organic matter in the digestate, it can be utilized as a solid biofuel, a soil amendment or a substrate for activated carbon production. HTC is a pretreatment process which can change digestate into potentially more hydrophobic product with higher energy potential. In this study, two kinds of digestate were examined as a potential feedstock for preparing hydrochar applied as a porous material and a solid biofuel. First digestate sample was a clean digestate from a typical biogas plant, performing anaerobic digestion of typical agricultural residues (cow dung). The second one was a digestate from wet fraction of municipal solid waste, after separation at the MSW sorting plant. HTC was performed in the autoclave, at 200 °C process temperature, 4.5 h residence time, under the saturation pressure of water. The proximate and ultimate analysis of hydrochars, along with porosimetry and morphology structure were examined. Additionally, the thermal behaviour under pyrolysis and combustion was analysed using TGA. HTC resulted in increased carbon contents for both types of digestate. Results of DSC and TGA analysis suggests improved ignition due to HTC. HTC increased total pore volume in both of the cases. The change was most visible in the macropore range.

Syngas Production From the Reforming of Typical Biogas Compositions in an Inert Porous Media Reactor.

Syngas production by inert porous media combustion of rich biogas–air mixtures was studied experimentally, focusing on carbon dioxide utilization and process efficiency. Different gas mixtures of natural gas and carbon dioxide, which simulated a typical biogas composition of 100:0, 70:30, 55:45, and 40:60 (CH4:CO2), were comparatively analyzed considering combustion waves temperatures and velocities, and chemical concentrations products, at high equivalence ratios of φ = 1.5 and φ = 2.0. Different CO2 concentrations on biogas composition showed higher H2 productions than on pure methane (100:0), mainly due to CO2 reforming reactions. Also, syngas production, hydrogen yields, and process efficiency by means of biogas filtration combustion were higher than under methane filtration combustion. Results of the thermochemical conversion of biogas show an alternative and promising non-catalytic technique to CO2 utilization.

Evaluation of methane emission flux from a typical biogas fermentation ecosystem in China

To reduce greenhouse gas emissions and promote sustainable agricultural practices in China, this study used conventional method to examine methane (CH4) production from hydraulic biogas digesters in Jiuxi Town, Yunnan Province, China. The static chamber technique was utilized to measure CH4 emission fluxes from the inlet and outlet of the biogas digesters. For the 100 m3 biogas digester, CH4 emission fluxes from the inlet, outlet, and the biogas digester were 1.043 g/(m2·h), 2.328 g/(m2·h), and 52.669 g/(m3·d); while those for the 8 m3 biogas digester were 0.156 g/(m2·h), 0.083 g/(m2·h), and 23.580 g/(m3·d), respectively. The diurnal and monthly variation of CH4 emission fluxes were further analyzed from the 100 m3 biogas digester. Among the environmental factors, the fermentation temperature and feeding quantity had significant effects on the CH4 emission fluxes. Especially, CH4 leakage from the digesters was evaluated, which would help identify and adjust the methane conversion factor for the typical biogas fermentation ecosystem used in China. For the hydraulic biogas digesters of 8 m3 and 100 m3, the respective leakage ratios were 3.01% and 14.56%. Therefore, it is very important for rural farmers in China to improve the management of household biogas digesters for reducing greenhouse gas emissions.

Effect of ultramicropores on the mechanisms of H2S retention from biogas

The pore distribution in activated carbon is crucial for its performance on adsorption processes. In desulfurization applications, the literature highlights the importance of pores from 5 to 10 Å. However, little is known and reported about the range of ultramicropores smaller than 4 Å, which has been found in commercial samples used for H2S retention. In order to clarify the influence of this fraction of pores on the H2S retention of typical biogas mixtures, measurements of multicomponent adsorption of hydrogen sulfide in the presence of carbon dioxide for activated carbon samples further impregnated with potassium hydroxide have been performed in column dynamics. Molecular simulation, XRF analysis and textural characterization were applied to aid the study. Results have shown that the H2S retention capacity in the presence of CO2 decreased significantly mainly in the impregnated samples. The degree of reduction in H2S retention indicates that ultramicropores can be of substantial importance through a synergistic immobilizing effect of H2S molecules. The better understanding of such an effect could improve the development of more efficient adsorbents for chemical industries.

Drying Characteristics of Biogas Digestate in a Hybrid Waste-Heat/Solar Dryer

The use of biogas plants has increased sharply in recent years. A typical biogas plant of 500 kWel produces approx. 10,000 t of digestate per year, with a moisture content of more than 90%. For the purpose of reducing the transport mass and increasing the nutrient concentration, the digestate has to be dried. Using renewable energy is a way to treat biogas digestate without any additional fossil energy requirement for drying. In this study a solar greenhouse dryer was modified to use additional waste-heat from the combined heat and power unit (variant S-CHP), as well as the exhaust gas from a micro turbine (variant S-CHP-MT). The hybrid waste-heat/solar dryer achieved a moisture content for the digestate of 10.9%, and 10.5%, after 13 d of drying for variant S-CHP-MT and S-CHP-MT, respectively. Due to the higher energy input by additional use of the micro turbine, the specific energy consumption is higher for the variant S-CHP-MT. In general, the results showed that the combination of solar energy and waste-heat from electricity generation of a biogas plant is a suitable way to reduce the moisture content of the digestate to a safe level for further handling and storage.

Is the Fischer-Tropsch Conversion of Biogas-Derived Syngas to Liquid Fuels Feasible at Atmospheric Pressure?

Biogas resulting from anaerobic digestion can be utilized for the production of liquid fuels via reforming to syngas followed by the Fischer-Tropsch reaction. Renewable liquid fuels are highly desirable due to their potential for use in existing infrastructure, but current Fischer-Tropsch processes, which require operating pressures of 2–4 MPa (20–40 bar), are unsuitable for the relatively small scale of typical biogas production facilities in the EU, which are agriculture-based. This paper investigates the feasibility of producing liquid fuels from biogas-derived syngas at atmospheric pressure, with a focus on the system’s response to various interruption factors, such as total loss of feed gas, variations to feed ratio, and technical problems in the furnace. Results of laboratory testing showed that the liquid fuel selectivity could reach 60% under the studied conditions of 488 K (215 °C), H2/CO = 2 and 0.1 MPa (1 bar) over a commercial Fischer–Tropsch catalyst. Analysis indicated that the catalyst had two active sites for propagation, one site for the generation of methane and another for the production of liquid fuels and wax products. However, although the production of liquid fuels was verified at atmospheric pressure with high liquid fuel selectivity, the control of such a system to maintain activity is crucial. From an economic perspective, the system would require subsidies to achieve financial viability.