Major Greenhouse Gas Research Articles

Roles of titania and plasmonic gold nanoparticles of different sizes in photocatalytic methane coupling at room temperature

Methane, one of the most abundant molecules on Earth and a major greenhouse gas, comes from renewable and fossil resources. The utilization of methane as a starting material for synthesis of chemicals is attractive but encounters numerous challenges due to high thermodynamic stability and extreme inertness of methane. Most of methane reactions occur at high temperatures and coincide with major emissions of CO2. Semiconductor photocatalysis offers an efficient alternative for methane conversion at room temperature.In this paper, we studied the plasmonic gold nanoparticles (NPs) with size from 6 nm to 60 nm supported on titania semiconductor in methane nonoxidative and oxidative couplings and elucidated their roles in the reactions. Strikingly, an efficient oxidative coupling with ethane production of 819 μmol/g/h and a coupling selectivity up to 86% was achieved in a flow reactor under optimized conditions. The reaction proceeded through methane activation over oxygen vacancies on Ultraviolet-excited titania. Gold NPs played important roles in charge separation and provided sites for soft oxygen activation. The size of gold NPs was found to be not noticeably affecting methane coupling.

Electrochemical Promotion of CO2 Hydrogenation Using a Pt/YSZ Fuel Cell Type Reactor.

The hydrogenation of CO2 is a reaction of key technological and environmental importance, as it contributes to the sustainable production of fuels while assisting in the reduction of a major greenhouse gas. The reaction has received substantial attention over the years within the catalysis and electrocatalysis communities. In this respect, the electrochemical promotion of catalysis (EPOC) has been applied successfully to the CO2 hydrogenation reaction to improve the catalytic activity and selectivity of conductive films supported on solid electrolytes. However, designing an effective electrocatalytic reactor remains a challenge due to the connections required between the electrodes and the external potentiostat/galvanostat. This drawback could be alleviated if the catalytic reaction occurs in a reactor that simultaneously operates as a power generator. In this work, the Electrochemical Promotion of the CO2 hydrogenation reaction in a low-temperature solid oxide electrolyte fuel cell (SOFC) reactor is studied using yttria-stabilized zirconia (YSZ) and a platinum (Pt) electrode catalyst. The system has been studied in two distinct operation modes: (i) when the necessary energy for the electrochemical promotion is produced through the parallel reaction of H2 oxidation (galvanic operation) and (ii) when a galvanostat/potentiostat is used to impose the necessary potential (electrolytic operation). The performance of the fuel cell declines less than 15% in the presence of the reactant mixture (CO2 and H2) while producing enough current to conduct EPOC experiments. During the electrolytic operation of the electrochemical cell, the CO production rate is significantly increased by up to 50%.

Investigation of physical properties and carbon dioxide solubility in tetramethylammonium bromide and tetraethylammonium bromide ionic liquids solutions

Carbon dioxide is a major greenhouse gas that is responsible for global warming and renders harmful effects on the atmosphere. The unconstrained release of CO2 into the atmosphere should be prevented and various techniques have been developed in this regard to capture CO2 using different solvents and other compounds. Ionic liquids are a suitable candidate to capture CO2 due to their better solubility behaviour. In this work, two ionic liquids namely tetramethylammonium bromide (TMAB) and tetraethylammonium bromide (TEAB) are employed experimentally to capture CO2 and investigate their solubility behaviour. The study is performed at the temperature values of 303 K, 313 K, and 323 K and the pressure values of 5, 10, 15, and 20 bar equivalent to 0.5, 1.0, 1.5, and 2.0 MPa respectively. The concentrations of both ionic liquid solutions are 2.5 wt%, 5.0 wt%, and 10.0 wt%. The solubility results are considered in terms of mol fraction which is the ratio of moles of CO2 captured per moles of ionic liquid. The density and viscosity values are also determined for both compounds at respective conditions. COSMO-RS is used to generate the sigma profile, sigma surface, and Henry's constant of the ions involved in the study. CO2 is found to be soluble in both ionic liquids, but TEAB showed better solubility behaviour as compared to TMAB. The solubility of CO2 is found to be increasing with the increase in pressure while it decreases with the increase in temperature.

Estimation of methane emission from beef cattle of Bangladesh

Agricultural sector emits large quantities of greenhouse gas, which is responsible for global warming. Some of the major greenhouse gases are methane, carbon dioxide, nitrous oxide etc. In our study, we have estimated the methane emission from beef cattle in Bangladesh. We followed the intergovernmental panel of climate change (IPCC) guidelines throughout our whole study. We used Tier 1 and Tier 2 methods for the calculation of methane emissions from beef cattle in Bangladesh. In the tier 1 method, we calculated the methane emission by using the emission factor provided by IPCC 2006 and 2019. According to IPCC 2019, we also calculated the methane emission using the tier 2 method which is based on dry matter intake. Finally, we have compared our calculated value. After calculation, in 2018, 2019, 2020 and 2021 total emission of methane from beef cattle based on Tier-1 (IPCC-2006) is 9990.00Gg, 10777.75Gg, 9996.75Gg and 10111.50Gg, respectively. Again based on IPCC-2019 emissions of methane are 17020.00Gg, 17169.50Gg, 17031.50Gg and 17227.00Gg, respectively. Using Tier 2 method (IPCC2019) methane emissions from beef cattle in 2018, 2019, 2020 and 2021 are 16087.60Gg, 16228.91Gg, 16098.47Gg and 16283.26Gg, respectively.

Effects of nitrogen addition on the combined global warming potential of three major soil greenhouse gases: A global meta-analysis

Increased nitrogen (N) deposition has a great impact on soil greenhouse gas (GHG) emissions, and numerous studies have revealed the individual effects of N addition on three major GHGs (CO2, CH4, and N2O). Nevertheless, quantitative evaluation of the effects of N addition on the global warming potential (GWP) of GHGs based on simultaneous measurements is needed not only to better understand the comprehensive effect of N deposition on GHGs but also for precise estimation of ecosystem GHG fluxes in response to N deposition. Here, we conducted a meta-analysis using a dataset with 124 simultaneous measurements of the three major GHGs from 54 studies to assess the effects of N addition on the combined global warming potential (CGWP) of these soil GHGs. The results showed that the relative sensitivity of the CGWP to N addition was 0.43%/kg N ha−1 yr−1, indicating an increase in the CGWP. Among the ecosystems studied, wetlands are considerable GHG sources with the highest relative sensitivity to N addition. Overall, CO2 contributed the most to the N addition-induced CGWP change (72.61%), followed by N2O (27.02%) and CH4 (0.37%), but the contributions of the three GHGs varied across ecosystems. Moreover, the effect size of the CGWP had a positive relationship with N addition rate and mean annual temperature and a negative relationship with mean annual precipitation. Our findings suggest that N deposition may influence global warming from the perspective of the CGWP of CO2, CH4, and N2O. Our results also provide reference values that may reduce uncertainties in future projections of the effects of N deposition on GHGs.

Techno-economic and life cycle assessment of renewable diesel production via methane-assisted catalytic waste cooking oil upgrading

In this work, a techno-economic and life cycle assessment of a novel catalytic methane-assisted process for the production of renewable diesel has been performed. A systematic method for calculating catalyst production cost is engaged via CatCost utilization. The results are compared with commercial processes (hydrotreatment and alkali-catalyzed process) and other recently published research. The total capital investment in hydrotreating of WCO (hydrocracking of long chain fatty acids by hydrogen under high pressure and temperature) and alkali-catalyzed process (transesterification of fatty acids by NaOH to produce fatty acid methyl ester) are 39% and 97% more than that of the novel methane-assisted catalytic process, respectively. The consumed energy per unit mass of produced renewable diesel calculated from this innovative upgrading technique is 0.93 kW/kg of renewable diesel, which is remarkably lower than that for the aforementioned commercial processes. The production costs have also been investigated and it was found that using natural gas is noticeably cheaper than hydrogen employed for the hydrotreating process and NaOH and methanol used in alkali catalyzed process. The production costs for the methane-assisted catalytic process are 0.365 $/kg renewable diesel and for hydrotreating and alkali-catalyzed process, the production costs are 0.574 $/kg renewable diesel and 0.513 $/kg biodiesel, respectively. The economic results have also been compared with other research and it was found the total production cost of the new process is minimum, however total capital investment is a bit high. The results of the life cycle assessment (LCA) have also proved the advantages of the newly developed process because using NaOH and methanol in the alkali-catalyzed process and hydrogen in the hydrotreating process would have higher environmental impacts. The impact assessment showed that the proposed process contributes significantly to reducing global warming since CO2eq results indicate that the proposed process emits remarkably less CO2 as one of the major greenhouse gases compared to other processes. For examining the risk of investment, the Monte Carlo simulation with 1000 iterations has also been conducted and the results manifest the high profitability of this process.

Economic and environmental impacts of biofuels in Indian context

Carbon dioxide (CO2), one of the major greenhouse gases, is released when fossil fuels are burned. Over the past decade, there has been a significant progress in global energy recovery from biomass resources and the use of biofuels as a renewable and sustainable alternative to fossil fuels. Biofuels are considered a carbon–neutral energy source, due to the fact that the CO2 through photosynthesis is captured by the plants during their growing period. The commonly used feedstocks for the production of biofuels are sugar cane, corn and vegetable oils etc. However, in order to minimize the impact on land use, food and feed prices, and other environmental factors; advanced feedstocks such as agriculture residue, algae, aquatic biomass, and waste derived feedstock are strongly suggested. The aim of this work is to review about different available feedstock for biofuel production, generations of biofuels, and its application as automotive fuels. The study concludes with recent advancements in modern biofuel production technology, significant policy implications, challenges and the prospects of biofuel in India.

Analysis of Long-Term Measurements of Tropospheric Ozone at the SPBU Observational Site in Peterhof

Tropospheric ozone (TO) is one of the major greenhouse gases and a toxic air pollutant. It plays a key role in various chemical and photochemical processes in the troposphere. Ozone concentrations both at surface level and in free troposphere are measured by various local and remote-sensing methods. The SPBU observational site in Peterhof (NDACC site St. Petersburg) is equipped with the Bruker IFS 125HR Fourier spectrometer used for TO measurements and the Thermo Scientific Model 49i gas analyzer for monitoring surface ozone concentrations (SOC). The temporal variability of TO in the 0–8 km layer for the period from April 2009 to October 2022 and of SOC for the period from 2013 to 2021 has been analyzed. Seasonal cycle of TO and SOC is similar to that of total ozone columns, but it is shifted in time by about 1 and 1.5 months, respectively. The maximum variation of TO from the average value for the period falls on the first half of April ~+16%; a minimum of about –(12–14)% is observed from mid-October to the end of December. In the middle of summer, there is also a slight increase in the TO variation at the level of +(7–8)%. A statistically significant decrease in the TO content by 0.34 ± 0.22% per year was also obtained. Comparison of hourly averaged data on TO with synchronized data of SOC measurements revealed an increase in the correlation coefficient (up to 0.5 or more) between the two values 3–4 hours after local noon in the warm season, accompanied by an increase in SOC. The increase in correlations is in good agreement with the increase in the height of the planet boundary layer.

Methane Emission and Carbon Sequestration Potential from Municipal Solid Waste Landfill, India

Quantities of waste generation are drastically increasing every day, and most of the waste is disposed of through open dumps and landfilling. Methane, carbon dioxide, and nitrous oxide are major greenhouse gases (GHGs) produced from landfill sites. However, the global-warming potential of methane is 21 times higher than that of carbon dioxide. Hence, there is immense concern for its utilization from landfill sites. In developing countries, the composition of municipal solid waste (MSW) has high amounts of biodegradable waste (50–60%). This leads to higher emissions of GHGs a per ton of MSW compared to the developed world. In this study, the attempt will be made to estimate the amount of carbon stored in MSW burial in landfills. Tests were conducted in two different locations at the Mavallipura landfill. MSW samples were collected for every meter interval (1–2 m, 2–3 m and so on) up to 6 m. The result shows that carbon stored in organic matter increases with depth from approximately 2.2% at 1.0 m depth to 4.8% at 6 m depth. Based on MSW’s carbon storage factor and data on MSW generation, global carbon sequestration from MSW burial in the Mavallipura landfill is estimated to be at least 10 million metric tons per year. In additional, the study aims to quantify methane-gas production from the ward levels and the Mavallipura landfill site in India.

How does the phytoplankton–light feedback affect the marine N2O inventory?

Abstract. The phytoplankton–light feedback (PLF) describes the interaction between phytoplankton biomass and the downwelling shortwave radiation entering the ocean. The PLF allows the simulation of differential heating across the ocean water column as a function of phytoplankton concentration. Only one third of the Earth system models contributing to the 6th phase of the Coupled Model Intercomparison Project (CMIP6) include a complete representation of the PLF. In other models, the PLF is either approximated by a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the modelled biogeochemical state have not yet been fully assessed and remain a source of multi-model uncertainty in future projection. Here, we evaluate within a coherent modelling framework how representations of the PLF of varying complexity impact ocean physics and ultimately marine production of nitrous oxide (N2O), a major greenhouse gas. We exploit global sensitivity simulations at 1∘ horizontal resolution over the last 2 decades (1999–2018), coupling ocean, sea ice and marine biogeochemistry. The representation of the PLF impacts ocean heat uptake and temperature of the first 300 m of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of ocean stratification, dynamics and oxygen concentration. These perturbations translate into different projection pathways for N2O production depending on the choice of the PLF representation. The oxygen concentration in the North Pacific oxygen-minimum zone is overestimated in model runs with an incomplete representation of the PLF, which results in an underestimation of local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the South Pacific and South Atlantic subtropical gyres but reduced by up to 12 % in oxygen-minimum zones of the Northern Hemisphere. Our results, based on a global ocean–biogeochemical model at CMIP6 state-of-the-art level, shed light on current uncertainties in modelled marine nitrous oxide budgets in climate models.

Energy consumption and carbon footprint of 3D printing in pharmaceutical manufacture

Achieving carbon neutrality is seen as an important goal in order to mitigate the effects of climate change, as carbon dioxide is a major greenhouse gas that contributes to global warming. Many countries, cities and organizations have set targets to become carbon neutral. The pharmaceutical sector is no exception, being a major contributor of carbon emissions (emitting approximately 55% more than the automotive sector for instance) and hence is in need of strategies to reduce its environmental impact. Three-dimensional (3D) printing is an advanced pharmaceutical fabrication technology that has the potential to replace traditional manufacturing tools. Being a new technology, the environmental impact of 3D printed medicines has not been investigated, which is a barrier to its uptake by the pharmaceutical industry. Here, the energy consumption (and carbon emission) of 3D printers is considered, focusing on technologies that have successfully been demonstrated to produce solid dosage forms. The energy consumption of 6 benchtop 3D printers was measured during standby mode and printing. On standby, energy consumption ranged from 0.03 to 0.17 kWh. The energy required for producing 10 printlets ranged from 0.06 to 3.08 kWh, with printers using high temperatures consuming more energy. Carbon emissions ranged between 11.60 and 112.16 g CO2 (eq) per 10 printlets, comparable with traditional tableting. Further analyses revealed that decreasing printing temperature was found to reduce the energy demand considerably, suggesting that developing formulations that are printable at lower temperatures can reduce CO2 emissions. The study delivers key initial insights into the environmental impact of a potentially transformative manufacturing technology and provides encouraging results in demonstrating that 3D printing can deliver quality medicines without being environmentally detrimental.

Excess methane emissions from shallow water platforms elevate the carbon intensity of US Gulf of Mexico oil and gas production

The Gulf of Mexico is the largest offshore fossil fuel production basin in the United States. Decisions on expanding production in the region legally depend on assessments of the climate impact of new growth. Here, we collect airborne observations and combine them with previous surveys and inventories to estimate the climate impact of current field operations. We evaluate all major on-site greenhouse gas emissions, carbon dioxide (CO2) from combustion, and methane from losses and venting. Using these findings, we estimate the climate impact per unit of energy of produced oil and gas (the carbon intensity). We find high methane emissions (0.60 Tg/y [0.41 to 0.81, 95% confidence interval]) exceeding inventories. This elevates the average CI of the basin to 5.3 g CO2e/MJ [4.1 to 6.7] (100-y horizon) over twice the inventories. The CI across the Gulf varies, with deep water production exhibiting a low CI dominated by combustion emissions (1.1 g CO2e/MJ), while shallow federal and state waters exhibit an extraordinarily high CI (16 and 43 g CO2e/MJ) primarily driven by methane emissions from central hub facilities (intermediaries for gathering and processing). This shows that production in shallow waters, as currently operated, has outsized climate impact. To mitigate these climate impacts, methane emissions in shallow waters must be addressed through efficient flaring instead of venting and repair, refurbishment, or abandonment of poorly maintained infrastructure. We demonstrate an approach to evaluate the CI of fossil fuel production using observations, considering all direct production emissions while allocating to all fossil products.

Differential Controls of Greenhouse Gas (CO2, CH4, and N2O) Concentrations in Natural and Constructed Agricultural Waterbodies on the Northern Great Plains

AbstractInland waters are hotspots of greenhouse gas (GHG) cycling, with small water bodies particularly active in the production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, wetland ponds are being replaced rapidly by small constructed reservoirs in agricultural regions, yet it is unclear whether these two water body types exhibit similar physical, chemical, and environmental controls of GHG content and fluxes. Here, we compared the content and regulatory mechanisms of all three major GHGs in 20 pairs of natural wetland ponds and constructed reservoirs in Canada's largest agricultural region. Carbon dioxide content was associated primarily with metabolic indicators in both water body types; however, primary production was paramount in reservoirs, and heterotrophic metabolism a stronger correlate in wetland ponds. Methane concentrations were correlated positively with eutrophication of the reservoirs alone, while competitive inhibition by sulfur‐reducing bacteria may have limited CH4 in both waterbody types. Contrary to expectations, N2O was undersaturated in both water body types, with wetlands being a significantly stronger and more widespread N2O sink. Varying regulatory processes are attributed to differences in age, depth, morphology, and water‐column circulation between water body types. These results suggest that natural and constructed water bodies should be modeled separately in regional GHG budgets.

Estimation of CH4emission based on an advanced 4D-LETKF assimilation system

Abstract. Methane (CH4) is the second major greenhouse gas after carbon dioxide (CO2) which has substantially increased during recent decades in the atmosphere, raising serious sustainability and climate change issues. Here, we develop a data assimilation system for in situ and column-averaged concentrations using a local ensemble transform Kalman filter (LETKF) to estimate surface emissions of CH4. The data assimilation performance is tested and optimized based on idealized settings using observation system simulation experiments (OSSEs), where a known surface emission distribution (the truth) is retrieved from synthetic observations. We tested three covariance inflation methods to avoid covariance underestimation in the emission estimates, namely fixed multiplicative (FM), relaxation-to-prior spread (RTPS), and adaptive multiplicative. First, we assimilate the synthetic observations at every grid point at the surface level. In such a case of dense observational data, the normalized root mean square error (RMSE) in the analyses over global land regions is smaller by 10 %–15 % in the case of RTPS covariance inflation method compared to FM. We have shown that integrated estimated flux seasonal cycles over 15 regions using RTPS inflation are in reasonable agreement between true and estimated flux, with 0.04 global normalized annual mean bias. We then assimilated the column-averaged CH4 concentration by sampling the model simulations at Greenhouse Gases Observing Satellite (GOSAT) observation locations and time for another OSSE. Similar to the case of dense observational data, the RTPS covariance inflation method performs better than FM for GOSAT synthetic observation in terms of normalized RMSE (2 %–3 %) and integrated flux estimation comparison with the true flux. The annual mean averaged normalized RMSE (normalized mean bias) in LETKF CH4 flux estimation in the case of RTPS and FM covariance inflation is found to be 0.59 (0.18) and 0.61 (0.23), respectively. The χ2 test performed for GOSAT synthetic observations assimilation suggests high underestimation of background error covariance in both RTPS and FM covariance inflation methods; however, the underestimation is much higher (>100 % always) for FM compared to RTPS covariance inflation method.

Artificial intelligence and response surface methodology to predict CO2 capture using piperazine‐modified activated alumina

AbstractCarbon dioxide (CO2) is a major greenhouse gas that causes global warming. In this study, piperazine‐modified activated alumina was used for CO2 capture process. The number of 626 experimental data were obtained at different operating conditions to develop the artificial neural networks (ANNs) and response surface methodology (RSM), employed to identify the behavior and performance of the CO2 capture process. Three independent factors, including time (t: 0–1800 s), temperature (T: 20–80°C), and pressure (P: 2.067–10.088 bar), were used as the input variables and CO2 adsorption capacity was used as the response. Among 243 different structures, the multilayer perceptron (MLP) model was optimized with Bayesian regularization backpropagation algorithm and two hidden layers with 17 and 16 neurons. The radial basis function (RBF) model was used with 367 different structures. The optimized structure of the RBF model was obtained with 100 neurons and the spread of 0.5. For optimal networks of MLP and RBF, the best mean square error (MSE) was observed at 1.72e‐04 and 8.28e‐05, and the best coefficient of determination (R2) was obtained 0.998 and 0.999, respectively. Moreover, the response surface methodology (RSM) was used to model the process. The best MSE and R2 values were obtained 0.00454 and 0.9462, respectively. The results showed that ANNs produced more accurate predictions than RSM.

Private Vehicles Greenhouse Gas Emission Estimation at Street Level for Berlin Based on Open Data

As one of the major greenhouse gas (GHG) emitters that has not seen significant emission reductions in the previous decades, the transportation sector requires special attention from policymakers. Policy decisions, thereby need to be supported by traffic emission assessments. Estimations of traffic emissions often rely on huge amounts of actual traffic data whose availability is limited, hampering the transferability of the estimation approaches in time and space. Here, we propose a high-resolution estimation of traffic emissions, which is based entirely on open data, such as the road network and points of interest derived from OpenStreetMap (OSM). We estimated the annual average daily GHG emissions from individual motor traffic for the OSM road network in Berlin by combining the estimated Annual Average Daily Traffic Volume (AADTV) with respective emission factors. The AADTV was calculated by simulating car trips with the open routing engine Openrouteservice, weighted by activity functions based on statistics of the German Mobility Panel. Our estimated total annual GHG emissions were 7.3 million t CO2 equivalent. The highest emissions were estimated for the motorways and major roads connecting the city center with the outskirts. The application of the approach to Berlin showed that the method could reflect the traffic pattern. As the input data is freely available, the approach can be applied to other study areas within Germany with little additional effort.

Greenhouse Gas Emission Dynamics of Saudi Arabia: Potential of Hydrogen Fuel for Emission Footprint Reduction

The growth of population, gross domestic product (GDP), and urbanization have led to an increase in greenhouse gas (GHG) emissions in the Kingdom of Saudi Arabia (KSA). The leading GHG-emitting sectors are electricity generation, road transportation, cement, chemicals, refinery, iron, and steel. However, the KSA is working to lead the global energy sustainability campaign to reach net zero GHG emissions by 2060. In addition, the country is working to establish a framework for the circular carbon economy (CCE), in which hydrogen acts as a transversal facilitator. To cut down on greenhouse gas emissions, the Kingdom is also building several facilities, such as the NEOM green hydrogen project. The main objective of the article is to critically review the current GHG emission dynamics of the KSA, including major GHG emission driving forces and prominent emission sectors. Then, the role of hydrogen in GHG emission reduction will be explored. Finally, the researchers and decision makers will find the helpful discussions and recommendations in deciding on appropriate mitigation measures and technologies.

The Legal Framework for Carbon Tax: A Case of Cameroon

This article builds a case in favour of using carbon tax as a cost-effective tool for regulating greenhouse gas (GHG) emissions as part of the climate change mitigation solution in terms of discouraging further GHG emission. Drawing on existing primary and secondary data, the article makes an assessment of the Cameroon’s legal environment for the design and implementation of carbon tax and articulates on the prospects and challenges of designing and implementing it. The article postulates that in spite of the promising opportunities that carbon tax offers for regulating GHG emissions, the potentials of this climate-smart fiscal policy tool has not yet been explored within the framework of climate governance in Cameroon. The article finds that while the spirit of carbon tax is underpinned by relevant environmental law principles, there is a timid legal framework both at the international and national levels for using carbon to regulate GHG emissions in Cameroon. The article concludes with a recommendation of the strong need for Cameroon to introduce a carbon-tax system as one of the cost-effective measures to reduce the country’s GHG emissions by adopting a carbon tax legal instrument designed specifically to target major GHG emitters across dozens of economic sectors. The model of carbon tax proposed for Cameroon could be useful and replicable elsewhere.

Prolonged operation of a methane biofilter from acclimation to the failure stage

ABSTRACT Global warming needs immediate attention to reduce major greenhouse gas emissions such as methane (CH4). Bio-oxidation of dilute CH4 emissions in packed-bed bioreactors such as biofilters has been carried out over recent years at laboratory and large scales. However, a big challenge is to keep CH4 biofilters running for a long period. In this study, a packed-bed lab-scale bioreactor with a specialized inorganic-based filter bed was successfully operated over four years for CH4 elimination. The inoculation of the bioreactor was the active leachate of another CH4 biofilter which resulted in a fast acclimation and removal efficiency (RE) reached 80% after seven weeks of operation for CH4 inlet concentrations ranging from 700 to 800 ppmv and an empty bed residence time (EBRT) of 6 min. During four years of operation, the bioreactor often recorded REs higher than 65% for inlet concentrations in the range of 1900–2200 ppmv and an EBRT of 6 min. The rate and interval of the nutrient supply played an important role in maintaining the bioreactor’s high performance over the long operation. Forced shutdowns were unavoidable during the 4-year operation and the bioreactor fully tolerated them with a partial recovery within one week and a progressive recovery over time. In the end, the bioreactor’s filter bed started to deteriorate due to a long shutdown of twelve weeks and the extended operation of four years when the RE dropped to below 8% with no sign of returning to its earlier performance.

Recent Applications and Strategies to Enhance Performance of Electrochemical Reduction of CO2 Gas into Value-Added Chemicals Catalyzed by Whole-Cell Biocatalysts

Carbon dioxide (CO2) is one of the major greenhouse gases that has been shown to cause global warming. Decreasing CO2 emissions plays an important role to minimize the impact of climate change. The utilization of CO2 gas as a cheap and sustainable source to produce higher value-added chemicals such as formic acid, methanol, methane, and acetic acid has been attracting much attention. The electrochemical reduction of CO2 catalyzed by whole-cell biocatalysts is a promising process for the production of value-added chemicals because it does not require costly enzyme purification steps and the supply of exogenous cofactors such as NADH. This study covered the recent applications of the diversity of microorganisms (pure cultures such as Shewanella oneidensis MR1, Sporomusa species, and Clostridium species and mixed cultures) as whole-cell biocatalysts to produce a wide range of value-added chemicals including methane, carboxylates (e.g., formate, acetate, butyrate, caproate), alcohols (e.g., ethanol, butanol), and bioplastics (e.g., Polyhydroxy butyrate). Remarkably, this study provided insights into the molecular levels of the proteins/enzymes (e.g., formate hydrogenases for CO2 reduction into formate and electron-transporting proteins such as c-type cytochromes) of microorganisms which are involved in the electrochemical reduction of CO2 into value-added chemicals for the suitable application of the microorganism in the chemical reduction of CO2 and enhancing the catalytic efficiency of the microorganisms toward the reaction. Moreover, this study provided some strategies to enhance the performance of the reduction of CO2 to produce value-added chemicals catalyzed by whole-cell biocatalysts.