Methane loss from commercially operating biogas upgrading plants

Exploring the potential of biofiltration for mitigating harmful gaseous emissions from small or old landfills: a review

Landfills are widely employed as the primary means of solid waste disposal. However, this practice generates landfill gas (LFG) which contains methane (CH4), a potent greenhouse gas, as well as various volatile organic compounds and volatile inorganic compounds. These emissions from landfills contribute to approximately 25% of the total atmospheric CH4, indicating the imperative need to valorize or treat LFG prior to its release into the atmosphere. This review first aims to outline landfills, waste disposal and valorization, conventional gas treatment techniques commonly employed for LFG treatment, such as flares and thermal oxidation. Furthermore, it explores biotechnological approaches as more technically and economically feasible alternatives for mitigating LFG emissions, especially in the case of small and aged landfills where CH4 concentrations are often below 3% v/v. Finally, this review highlights biofilters as the most suitable biotechnological solution for LFG treatment and discusses several advantages and challenges associated with their implementation in the landfill environment.

A perspective on methodologies and system boundaries to develop abatement cost for on-farm anaerobic digestion

Marginal Abatement Cost Curves compare and assess greenhouse gas mitigation options available to various sectors of the economy. In the Irish agricultural sector, large anaerobic digestion facilities are currently considered a high-cost abatement solution. In prior studies of anaerobic digestion abatement costs, two options were assessed: the generation of heat and electricity from biogas (115 €/tCO2eq) and the production of renewable heat from biomethane (280 €/tCO2eq). Both scenarios encompass single cost values that may not capture the potentially variable nature of such systems. In contrast, prior techno-economic analyses and lifecycle analyses can provide a comparison of the abatement costs of anaerobic digestion systems at a range of scales. This work compares two case studies (based on prior literature) for small and medium-scale on farm anaerobic digestion systems. The small-scale system is set in Ireland with cattle slurry collected in open tanks during the winter, while the medium-scale system is set in the USA with cattle slurry collected periodically indoors all year-round. It was found that the abatement cost can vary between -117 to +79 € per t CO2eq. The key variables that affected the abatement cost were additional revenue streams such as biofertilizer sales, displaced energy savings, and additional incentives and emissions savings within the system boundary. Including only some of these options in the analysis resulted in higher abatement costs being reported. Based on the variation between system topologies and therefore system boundaries, assigning a single mitigation cost to anaerobic digestion systems may not be representative.

Developing a biogas centralised circular bioeconomy using agricultural residues - Challenges and opportunities

Anaerobic digestion (AD) can be used as a stand-alone process or integrated as part of a larger biorefining process to produce biofuels, biochemicals and fertiliser, and has the potential to play a central role in the emerging circular bioeconomy (CBE). Agricultural residues, such as animal slurry, straw, and grass silage, represent an important resource and have a huge potential to boost biogas and methane yields. Under the CBE concept, there is a need to assess the long-term impact and investigate the potential accumulation of specific unwanted substances. Thus, a comprehensive literature review to summarise the benefits and environmental impacts of using agricultural residues for AD is needed. This review analyses the benefits and potential adverse effects related to developing biogas-centred CBE. The identified potential risks/challenges for developing biogas CBE include GHG emission, nutrient management, pollutants, etc. In general, the environmental risks are highly dependent on the input feedstocks and resulting digestate. Integrated treatment processes should be developed as these could both minimise risks and improve the economic perspective.

Methane losses from different biogas plant technologies

Biogas and biomethane production can play an important role in a fossil-fuel-free energy supply, provided that process-related methane (CH₄) losses are minimized. Addressing the lack of representative emission data, this study aims to provide component specific CH₄ emission factors (EFs) for various biogas plant technologies, enabling more accurate emission estimates for the biogas sector and supporting the identification of low emission technologies. Four measurement teams investigated 33 biogas plants in Austria, Germany, Sweden and Switzerland including mainly agricultural and biowaste treating facilities. For the first time, a harmonized measurement procedure was used to systematically survey individual on-site emission sources and leakages. Measurements revealed a large variability in technology specific emissions, especially for biogas utilization and upgrading. Median loss from combined heat and power (CHP) plants was 1.6 % for gas engines (n = 21), and 3.0 % for pilot injection units (n = 3) of the input CH₄. Biogas upgrading units showed median CH₄ slips of < 0.1 % (chemical scrubbers, n = 4), 0.1 % (after exhaust gas treatment, n = 3) and 2.9 % (water scrubbers, n = 2). Not-gastight digestate storage (n = 8) was identified as a major emission source with maximum 5.6 % of the produced CH₄ emitted. Individual leakages (n = 37) released between 0.0 and 2.1 % (median 0.1 %) relative to the CH₄ production. All measurement and secondary data are provided in a harmonized dataset (294 datapoints). A review of IPCC default EFs indicate an underestimation of emissions from biogas utilization (reported in the energy sector) while the impact of leakages on overall plant emissions (waste sector) may be overestimated for European biogas plants.

The Danish national effort to minimise methane emissions from biogas plants

In total, 69 biogas plants representing 59 % of Danish biogas production participated in a national effort to reduce methane (CH₄) emission. Measurements in terms of total plant CH₄ emissions, quantification of emissions from point sources, leak surveys and conceptual design plans to mitigate emissions were performed. Plant-level CH₄ emission rates varied between 1.3 and 81.2 kg CH₄ h^-1, and CH₄ losses expressed in percentages of production varied between 0.3 and 40.6 %. Agricultural plants generally had lower CH₄ loss rates compared to wastewater treatment plants. Biogas plants with a smaller gas production emitted a larger fraction of their production compared to larger plants, which was partly explained by the absence of gas collection from digestate storage tanks at smaller plants. A very commonly observed source of emission was pressure relief valves, where this source of leakage was observed at 53 % of the plants. A national emission factor (sum of CH₄ emissions/sum of CH₄ productions) was determined at 2.5 % for the Danish biogas production, whereof it was 2.1 % for agricultural biogas production and 6.7 % for biogas production at wastewater treatment plants. Measurements of total CH₄ emissions at six plants performed before and after implementation of mitigating actions showed that emissions were reduced by 46 % by carrying out relatively minor technical fixes and adjustments. An economic evaluation showed that, in some cases, mitigating actions could be economically beneficial for the biogas plant (positive net present value over a 10 year time frame), due to an increase in revenue.

Elucidation of microbial interactions, dynamics, and keystone microbes in high pressure anaerobic digestion

High-pressure anaerobic digestion (HPAD) is a promising technology for producing biogas enriched with high methane content in a single-step process. To enhance HPAD performance, a comprehensive understanding of microbial community dynamics and their interactions is essential. For this, mesophilic batch high-pressurized anaerobic reactors were operated under 3 bars (H3) and 6 bars (H6). The experimental results showed that the effect of high-pressure (up to 6 bar) on acidification was negligible while methanogenesis was significantly delayed. Microbial analysis showed the predominance of Defluviitoga affiliated with the phylum Thermotogae and the reduction of Thiopseudomonas under high-pressure conditions. In addition, the microbial cluster pattern in H3 and H6 was significantly different compared to the CR, indicating a clear shift in microbial community structure. Moreover, Methanobacterium, Methanomicrobiaceae, Alkaliphilus, and Petrimonas were strongly correlated in network analysis, and they could be identified as keystone microbes in the HPAD reactor.

Swine manure treatment technologies as drivers for circular economy in agribusiness: A techno-economic and life cycle assessment approach

Anaerobic digestion has been employed as a technology capable of adding value to waste coupled with environmental impact mitigation. However, many issues need to be elucidated to ensure the systems viability based on this technology. In this sense, the present study evaluated technically, environmentally, and economically, four configurations of swine waste treatment systems focused on the promotion of decarbonization and circularity of the swine chain. For this, a reference plant, based on a compact treatment process named SISTRATES® (Portuguese acronym for swine effluent treatment system) was adopted to serve as a model for comparison and validation. The results showed the importance of prioritization of the energy recuperation routes through anaerobic digestion, providing increased economic benefits and minimizing environmental damage. Thus, the SISTRATES® configuration was the one that presented the best designs in a circular context, maximizing the recovery of energy and nutrients, along with the reduction of greenhouse gas emissions, ensuring the sustainability of the pig production chain.

Fugitive methane emissions from two agricultural biogas plants

This study quantified fugitive methane (CH₄) losses from multiple sources (open digestate storages, digesters and flare) at two biogas facilities over one year, providing a much needed dataset integrating all major loss pathways and changes over time. Losses of CH₄ from Facility A were primarily from digestate storage (5.8% of biogas CH₄), followed by leakage/venting (5.5%) and flaring (0.2%). At Facility B, losses from digestate storage were higher (10.7%) due to shorter hydraulic retention time and lack of a screwpress. Fugitive emissions from leakage were initially 3.8% but were reduced to 0.6% after the dome membrane was repaired at Facility B. For biogas to have a positive impact on greenhouse gas emissions and provide a low-carbon fuel, it is important to minimize fugitive losses from digestate storage and avoid leakage during abnormal operation (leakage, roof failure).

Non-Fossil Methane Emissions Mitigation From Agricultural Sector and Its Impact on Sustainable Development Goals

The agriculture sector contributes to ∼40% of methane emissions globally. Methane is also 28 times (Assessment Report 5) more potent greenhouse gas than CO2. In this study, we assess the impact of measures for mitigating methane emissions from the agricultural sector on the achievement of all the 17 United Nations’ Sustainable Development Goals (SDGs). A keyword literature review was employed that focused on finding the synergies and trade-offs with non-fossil methane emissions from the agricultural sector and respective SDGs’ targets. The results were in broad consensus with the literature aimed at finding the relationship between SDGs and measures targeting climate change. There is a total of 88 synergies against eight trade-offs from the 126 SDGs’ targets that were assessed. It clearly shows that measures to mitigate methane emissions from the agricultural sector will significantly help in achieving the SDGs. Since agriculture is the primary occupation and the source of income in developing countries, it can further be inferred that methane mitigation measures in developing countries will play a larger role in achieving SDGs. Measures to mitigate methane emissions reduce poverty; diversify the source of income; promote health, equality, education, sanitation, and sustainable development while providing energy and resource security to the future generations.

Microbial electrochemical approaches of carbon dioxide utilization for biogas upgrading

Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO₂) reduction and biomethane (or value-added products) production. There is limited literature critically reviewing the latest scientific developments on the bioelectrochemical system (BES) based biogas upgrading technologies, including CO₂ reduction efficiency, methane (CH₄) yields, reactor operating conditions, and electrode materials tested in the BES reactor. This review analyzes the reported performance and identifies crucial parameters considered for future optimization, which is currently missing. Further, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. Three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO₂ to CH₄. Most of the studies on the biogas upgrading process conclude hydrogen (H₂) mediated electron transfer mechanism in BES biogas upgrading.

Economic and environmental assessment of organic waste to biomethane conversion

Biomethane and biofertilizer production by anaerobic co-digestion of organic waste serves a promising method for reducing the environmental footprint of organic waste management. This study evaluated the techno-economic feasibility and environmental impacts of organic waste to biomethane development in Glasgow, UK using net present value (NPV) analysis and life cycle assessment. Four different biogas upgrading technologies (pressurized water scrubbing, chemical scrubbing, membrane separation, and pressure swing adsorption) were compared. The membrane separation technology-based biomethane production meets 0.8% of the gas demand for Glasgow households with a conversion efficiency of 83%. The organic waste to biomethane development saved up to 264 kg CO_2-eq annually per tonne of waste treated, with an NPV ranged between £-9.0 million and £-12.0 million based on the upgrading technology. High costs for waste collection and transportation are primarily responsible for the negative NPV. Carbon taxes between £31.30 and £58.02 per tonne of CO₂ are needed for economically viable biomethane production.

Biogas upgrading to biomethane as a local source of renewable energy to power light marine transport: Profitability analysis for the county of Cornwall

In this work, the use of biomethane produced from local biogas plants is proposed as renewable fuel for light marine transport. A profitability analysis is performed for three real biogas production plants located in Cornwall (United Kingdom), considering a total of 66 different scenarios where critical parameters such as distance from production point to gas grid, subsidies, etcetera, were evaluated. Even though the idea is promising to decarbonize the marine transport sector, under the current conditions, the approach is not profitable. The results show that profitability depends on the size of the biogas plant. The largest biogas plant studied can be profitable if feed-in tariffs subsidies between 36.6 and 45.7 €/MWh are reached, while for the smallest plant, subsidies should range between 65 and 82.7 €/MWh. The tax to be paid per ton of CO₂ emitted by the shipping owner, was also examined given its impact in this green route profitability. Values seven times greater than current taxes are needed to reach profitability, revealing the lack of competitiveness of renewable fuels vs traditional fuels in this application. Subsidies to make up a percentage of the investment are also proposed, revealing that even at 100% of investment subsidized, this green approach is still not profitable. The results highlight the need for further ambitious political actions in the pursuit of sustainable societies.

Global warming potential and economic performance of gasification-based chemical recycling and incineration pathways for residual municipal solid waste treatment in Germany

Chemical recycling could facilitate the transition from a linear to a circular carbon economy, where carbon-containing waste is channeled back into the production cycle as a chemical feedstock instead of being incinerated or landfilled. However, the predominant focus on technological aspects of chemical recycling for plastic waste narrows evaluations of its potential in contributing to such a transition. Moreover, it leads to significant controversy about its role in the waste hierarchy as a possible competitor to mechanical recycling. To address these gaps in the literature, this study assesses ecological and economic impacts associated with chemical recycling of residual municipal solid waste in Germany. Combining approaches of life cycle assessment and techno-economic analysis, chemical recycling and conventional incineration-based treatment pathways are comparatively evaluated in terms of global warming potential and economic performance (i.e. fixed capital investment, net present value, dynamic payback period, and levelized cost of carbon abatement). Results indicate that compared to incineration-based conventional pathways, chemical recycling can contribute to reducing greenhouse gas emissions in low-emission energy systems. However, the economic performance of chemical recycling is highly dependent on its scale of operation. Additionally, a price premium for recycling products as well as economic instruments for penalizing CO₂ emissions are identified to play important roles in the economic performance of chemical recycling.

New Carvone-Based Deep Eutectic Solvents for Siloxanes Capture from Biogas

During biogas combustion, siloxanes form deposits of SiO₂ on engine components, thus shortening the lifespan of the installation. Therefore, the development of new methods for the purification of biogas is receiving increasing attention. One of the most effective methods is physical absorption with the use of appropriate solvents. According to the principles of green engineering, solvents should be biodegradable, non-toxic, and have a high absorption capacity. Deep eutectic solvents (DES) possess such characteristics. In the literature, due to the very large number of DES combinations, conductor-like screening models for real solvents (COSMO-RS), based on the comparison of siloxane activity coefficient of 90 DESs of various types, were studied. DESs, which have the highest affinity to siloxanes, were synthesized. The most important physicochemical properties of DESs were carefully studied. In order to explain of the mechanism of DES formation, and the interaction between DES and siloxanes, the theoretical studies based on σ-profiles, and experimental studies including the ¹H NMR, ¹³C NMR, and FT-IR spectra, were applied. The obtained results indicated that the new DESs, which were composed of carvone and carboxylic acids, were characterized by the highest affinity to siloxanes. It was shown that the hydrogen bonds between the active ketone group (=O) and the carboxyl group (-COOH) determined the formation of stable DESs with a melting point much lower than those of the individual components. On the other hand, non-bonded interactions mainly determined the effective capture of siloxanes with DES.

Quantification of methane emissions from UK biogas plants

The rising number of operational biogas plants in the UK brings a new emissions category to consider for methane monitoring, quantification and reduction. Minimising methane losses from biogas plants to the atmosphere is critical not only because of their contribution of methane to global warming but also with respect to the sustainability of renewable energy production. Mobile greenhouse gas surveys were conducted to detect plumes of methane emissions from the biogas plants in southern England that varied in their size, waste feed input materials and biogas utilization. Gaussian plume modelling was used to estimate total emissions of methane from ten biogas plants based on repeat passes through the plumes. Methane emission rates ranged from 0.1 to 58.7 kg CH₄ hr^-1, and the percentage of losses relative to the calculated production rate varied between 0.02 and 8.1%. The average emission rate was 15.9 kg CH₄ hr^-1, and the average loss was 3.7%. In general, methane emission rates from smaller farm biogas plants were higher than from larger food waste biogas plants. We also suggest that biogas methane emissions may account for between 0.4 and 3.8%, with an average being 1.9% of the total methane emissions in the UK excluding the sewage sludge biogas plants.

The pump-mixed anaerobic digestion of pig slurry: new technology and mathematical modeling

Biogas production is a relatively novel and developing branch of the renewable fuel sector, which allows agricultural waste, and more, to be used as a feedstock. New technologies have been integrated into the process to improve its efficiency. In this study, a pump-mixed anaerobic digestion concept is considered for both experimental and modeling approaches. The experiment included a total of nine configurations with the same geometry (140 dm³ of total reactor volume) but different hydraulic retention times and mixing intervals. The measurements were used to create and optimize a mathematical model. The complete-stirring assumption, which underlies most anaerobic digestion (AD) simulations, is no longer valid in this case. Thus, the novel concept is developed by assuming that the liquid phase is split into three separate sections, which approximates the concentration gradient in a real reactor. This method allows partial differential equations to be avoided, which could potentially affect the calculation efficiency. The final mean accuracy of the model in the tested range was estimated to be 86.60% while, in selected parts of the scope, was close to 90%. The pump-mixed anaerobic digestion technique in the experiment achieved high production performance (above 8 dm³ of product per 1 dm³ of feedstock) while maintaining a high methane content (approximately 65%). The comparison between the reactor stirred by an impeller, and the pump-mixed, indicated that the proposed configuration ensures better production stability. Additionally, it was possible to achieve a higher biogas production rate with the same feedstock concentration.

Key factors in the volatile organic compounds treatment by regenerative thermal oxidizer

The regenerative thermal oxidizer (RTO) is widely used in the treatment of volatile organic compounds (VOCs). The RTO with three packed beds has a backflushing process, which makes its operation quite different from common regenerative combustion devices with two beds. However, the published research regarding this kind of RTO was far from sufficient. Thus, models based on an industrial RTO with three beds were developed. Temperature distributions for the RTO were simulated based on the standard k-ε model, heat balance model, DO model, and finite chemical-rate/eddy-dissipation model after model validation. Four operating parameters were selected, and these parameters have a significant impact on key factors, such as the oxidation chamber temperature (OCT), outlet temperature, and thermal efficiency. Multi-factor analysis was performed by orthogonal experiment and regression analysis of the key factors, revealing that different parameters imposed various impacts on the key factors. A linear relationship between the OCT and RTO outlet temperature was identified, yielding a useful formula for engineering applications.Implications: A regenerative thermal oxidizer (RTO) used in volatile organic compounds (VOCs) treatment was studied between engineering and simulation. The simulation results showed a few error compared to the engineering ones. Single parameter simulations, orthogonal experiments and regression analysis were applied to study key factors such as oxidation chamber temperature, outlet temperature, and thermal efficiency. The influence and regularity of the four main operating parameters on the key factors were quite different. The best condition for this RTO was got. The linear formula of the oxidation chamber temperature and the outlet temperature are obtained, respectively.

A simplified model to simulate bioaugmented anaerobic digestion of lignocellulosic biomass: Biogas production efficiency related to microbiological data

Mathematical model applications for the bioaugmented anaerobic digestion (BAD) process seem to be lacking in the scientific literature, even more so when related to microbiological data. The present study suggests a simplified mathematical model to investigate and simulate the process kinetics of bioaugmented anaerobic digestion (BAD) aimed at improving biogas production from wheat straw (WS). Bioaugmented conditions were obtained through a mixed inoculum of anaerobic ruminal fungi (ARF) and hydrogen-producing fermenting bacteria (F210) added to a methanogenic inoculum. The investigation focused on two process configurations characterized by a mono (I-BAD) and two-stage (II-BAD) process and a conventional anaerobic digestion (AD) control test. Each configuration was used on two operating scales (i.e., 120 ml and 12,000 ml reactor volume) to provide different data sets for the calibration and validation of the mathematical model proposed. The model calibration step was used to determine the optimal values of selected parameters displaying higher significance for experimental result predictability. The model calibration results highlighted a similar behavior for both BAD tests, which was further strengthened by a statistical analysis supporting the observed correlation regardless of the BAD configuration involved. The BAD configuration always enhanced the CH₄ production (>70%) with a faster kinetic in the II-BAD test. The microbiological results support the superior performance of the II-BAD test, displaying higher Archaea fractions (up to 14.5% on day 23) with values more than double compared to I-BAD and triple compared to the AD test. Furthermore, the presence of Methanosarcina inside the Archaea guild (6.4% and 4.5% at days 11 and 61, respectively) ensures a greater diversification of the metabolic pathways and supports the strength of the process performance. Cell density values are strongly in line with these results.

Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H₂S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

Biogas Upgrading: A Review of National Biomethane Strategies and Support Policies in Selected Countries

Bioenergy contributes significantly towards the share of renewable energies, in Europe and worldwide. Besides solid and liquid biofuels, gaseous biofuels, such as biogas or upgraded biogas (biomethane), are an established renewable fuel in Europe. Although many studies consider biomethane technologies, feedstock potentials, or sustainability issues, the literature on the required legislative framework for market introduction is limited. Therefore, this research aims at identifying the market and legislative framework conditions in the three leading biomethane markets in Europe and compare them to the framework conditions of the top six non-European biomethane markets. This study shows the global status and national differences in promoting this renewable energy carrier. For the cross-country comparison, a systematic and iterative literature review is conducted. The results show the top three European biomethane markets (Germany, United Kingdom, Sweden) and the six non-European biomethane markets (Brazil, Canada, China, Japan, South Korea, and the United States of America), pursuing different promotion approaches and framework conditions. Noteworthy cross-national findings are the role of state-level incentives, the tendency to utilise biomethane as vehicular fuel and the focus on residues and waste as feedstock for biomethane production. Presenting a cross-country comparison, this study supports cross-country learning for the promotion of renewable energies like biomethane and gives a pertinent overview of the work.

Total methane emission rates and losses from 23 biogas plants

Methane losses from biogas plants are problematic, since they contribute to global warming and thus reduce the environmental benefits of biogas production. Total losses of methane from 23 biogas plants were measured by applying a tracer gas dispersion method to assess the magnitude of these emissions. The investigated biogas plants varied in terms of size, substrates used and biogas utilisation. Methane emission rates varied between 2.3 and 33.5 kg CH₄ h^-1, and losses expressed in percentages of production varied between 0.4 and 14.9%. The average emission rate was 10.4 kg CH₄ h^-1, and the average loss was 4.6%. Methane losses from the larger biogas plants were generally lower compared to those from the smaller facilities. In general, methane losses were higher from wastewater treatment biogas plants (7.5% in average) in comparison to agricultural biogas plants (2.4% in average). In essence, methane loss may constitute the largest negative environmental impact on the carbon footprint of biogas production.