A novel membrane-based process to concentrate nutrients from sidestreams of an Urban Wastewater Treatment Plant through captured carbon dioxide from biogas

Among the challenges that wastewater treatment plants face in the path towards sustainability, reducing CO2 emissions and decrease the amount of waste highlight. Within these wastes, those that can cause eutrophication, such as nutrients (nitrogen and phosphorous) are of great concern. Herein we study a novel process to concentrate nutrients via membrane technology. In particular, we propose the use of forward osmosis, applying the carbonated solvent which contains the CO2 captured from the biogas stream as draw solution. This carbonated solvent has a high potential osmotic pressure, which can be used in forward osmosis to concentrate the nutrients stream. To this end, we present the results of an experimental plan specifically designed and performed to evaluate two main parameters: (1) nutrients concentration; and (2) water recovery. The process designed involves pH adjustment, membrane filtration to separate solids, pH reduction and forward osmosis concentration of nutrients. With this process, concentrations factor for nutrients in between 2 and 2.5 and water recovery of approximately 50 % with water flux of 7 to 8 L/(m2h) can be achieved.

Hydrochar and synthetic natural gas co-production for a full circular economy implementation via hydrothermal carbonization and methanation: An economic approach

Herein we study the economic performance of hydrochar and synthetic natural gas co-production from olive tree pruning. The process entails a combination of hydrothermal carbonization and methanation. In a previous work, we evidenced that standalone hydrochar production via HTC results unprofitable. Hence, we propose a step forward on the process design by implementing a methanation, adding value to the gas effluent in an attempt to boost the overall process techno-economic aspects. Three different plant capacities were analyzed (312.5, 625 and 1250 kg/hr). The baseline scenarios showed that, under the current circumstances, our circular economy strategy in unprofitable. An analysis of the revenues shows that hydrochar selling price have a high impact on NPV and subsidies for renewable coal production could help to boost the profitability of the process. On the contrary, the analysis for natural gas prices reveals that prices 8 times higher than the current ones in Spain must be achieved to reach profitability. This seems unlikely even under the presence of a strong subsidy scheme. The costs analysis suggests that a remarkable electricity cost reduction or electricity consumption of the HTC stage could be a potential strategy to reach profitability scenarios. Furthermore, significant reduction of green hydrogen production costs is deemed instrumental to improve the economic performance of the process. These results show the formidable techno-economic challenge that our society faces in the path towards circular economy societies.

Aqueous mineral carbonation of three different industrial steel slags: absorption capacities and product characterization

Heavy carbon industries produce solid side stream materials that contain inorganic chemicals like Ca, Na, or Mg, and other metals such as Fe or Al. These inorganic compounds usually react efficiently with CO2 to form stable carbonates. Therefore, using these side streams instead of virgin chemicals to capture CO2 is an appealing approach to reduce CO2 emissions. Herein, we performed an experimental study of the mineral carbonation potential of three industrial steel slags via aqueous, direct carbonation. To this end, we studied the absorption capacities, reaction yields, and physicochemical characteristics of the carbonated samples. The absorption capacities and the reaction yields were analyzed through experiments carried out in a reactor specifically designed to work without external stirring. As for the physicochemical characterization, we used solid-state Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM). Using this reactor, the absorption capacities were between 5.8 and 35.3 g/L and reaction yields were in the range of 81-211 kg CO2/ton of slag. The physicochemical characterization of the solid products with solid FTIR, XRD and SEM indicated the presence of CaCO3. This suggests that there is potential to use the carbonated products in commercial applications.

Renewable Carbonaceous Materials from Biomass in Catalytic Processes: A Review

This review paper delves into the diverse ways in which carbonaceous resources, sourced from renewable and sustainable origins, can be used in catalytic processes. Renewable carbonaceous materials that come from biomass-derived and waste feedstocks are key to developing more sustainable processes by replacing traditional carbon-based materials. By examining the potential of these renewable carbonaceous materials, this review aims to shed light on their significance in fostering environmentally conscious and sustainable practices within the realm of catalysis. The more important applications identified are biofuel production, tar removal, chemical production, photocatalytic systems, microbial fuel cell electrodes, and oxidation applications. Regarding biofuel production, biochar-supported catalysts have proved to be able to achieve biodiesel production with yields exceeding 70%. Furthermore, hydrochars and activated carbons derived from diverse biomass sources have demonstrated significant tar removal efficiency. For instance, rice husk char exhibited an increased BET surface area from 2.2 m2/g to 141 m2/g after pyrolysis at 600 °C, showcasing its effectiveness in adsorbing phenol and light aromatic hydrocarbons. Concerning chemical production and the oxidation of alcohols, the influence of biochar quantity and pre-calcination temperature on catalytic performance has been proven, achieving selectivity toward benzaldehyde exceeding 70%.

Potential of organic carbonates production for efficient carbon dioxide capture, transport and storage: Reaction performance with sodium hydroxide-ethanol mixtures

Carbon dioxide storage is one of the main long-term strategies for reducing carbon dioxide emissions in the atmosphere. A clear example is Norway's Longship project. If these projects should succeed, the transport of huge volumes of carbon dioxide from the emissions source to the injection points may become a complex challenge. In this work, we propose the production of sodium-based organic carbonates that could be transported to storage sites and be reconverted to CO₂. Solid carbonates can be transported in considerably lower volumes than gases or pressurized liquids. Sodium-based carbonates are insoluble in most of the organic solvents and will therefore precipitate in contrast to in aqueous solutions. Particularly, here we focus on sodium hydroxide-ethanol mixtures as solvents for precipitating sodium ethyl carbonate and sodium bicarbonate. Previous works on this approach used limited sodium hydroxide concentrations, which are insufficient to prove the effectiveness of the proposed process. In this paper, we studied higher sodium hydroxide concentrations in sodium hydroxide-ethanol mixtures than previously reported in the literature. To this end, we use the following strategy: (1) In-line monitoring of the formation of carbonates using an in-line FTIR; (2) In-line measurements of the weight increase, which correspond directly to the captured carbon dioxide and reveal the absorption capacity; (3) Characterization of the solids with X-ray diffraction and scanning electron microscope. Our FTIR results confirmed that both sodium ethyl carbonate and sodium bicarbonate were formed, which agrees with X-ray diffraction and scanning electron microscope. With this reactor design, the absorption capacities reached approximately 80-93% of the theoretical values (4.8-13.3 g/L respectively). We hypothesize that full conversion is hampered because the gas might take preferential paths due to gel formation during the experiments.

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.

Promoting bioeconomy routes: From food waste to green biomethane. A profitability analysis based on a real case study in eastern Germany

Profitability studies are needed to establish the potential pathways required for viable biomethane production in the Brandenburg region of Germany. This work study the profitability of a potential biomethane production plant in the eastern German region of Brandenburg, through a specific practical scenario with data collected from a regional biogas plant located in Alteno (Schradenbiogas GmbH & Co. KG). Several parameters with potential economic influence such as distance of the production point to the grid, waste utilization percentage, and investment, were analyzed. The results illustrate a negative overall net present value with the scenario of no governmental investment, even when considering trading the CO₂ obtained throughout the process. Subsidies needed to reach profitability varied with distance from 13.5 €/MWh to 19.3 €/MWh. For a fixed distance of 15 kms, the importance of percentage of waste utilization was examined. Only 100% of waste utilization and 75% of waste utilization would reach profitability under a reasonable subsidies scheme (16.3 and 18.8 €/MWh respectively). Concerning the importance of investment, a subsidized investment of at least 70% is demanded for positive net present values. Besides, the sensitivity analysis remarks the energy consumption of the biogas upgrading stage, the electricity price, and the energy consumption of biogas production as major parameters to be tackled for the successful implementation of biogas upgrading plants. The results here obtained invite to ponder about potential strategies to further improve the economic viability of this kind of renewable projects. In this line, using the CO₂ separated to produce added-value chemicals can be an interesting alternative.

Optimizing hydrothermal carbonization of olive tree pruning: A techno-economic analysis based on experimental results

In this study the optimization of the hydrothermal carbonization process for the conversion of olive tree pruning into biofuel is presented. To this end, a combined experimental-economic assessment is performed. Experimental data obtained at laboratory scale were used to estimate the economic performance of a hypothetical industrial scale plant. To evaluate the viability of the project, three different plant sizes according to their capacity were selected (1250-625-312.5 kg/h). The discounted cash flow method was applied for the profitability analysis. Different scenarios were analyzed considering the reduction of associate costs or the improvement of the revenues compared to the baseline case. Results indicate that with the sizes studied, none of the alternatives are profitable. Despite that, the larger capacity shows the best outcomes. In this case, minimum selling price of 0.39 €/kg for hydrochar is required to reach profitability. Lower plant sizes would require higher selling prices (i.e., 0.46 €/kg for 625 kg/h capacity and 0.59 €/kg for 312.5 kg/h capacity). Similarly, a reduction of 33% in the electrical energy consumption can make the plan be profitable for the larger capacity. Likewise, a reduction until 0.053 €/kWh in the electricity price must be reached for achieving profitability. Thus, importance of government incentives is revealed in this work given that the reduction of costs along with the improvement in the revenues for the selling of the product can make the project economically viable. Other parameters like the number of workers are also interesting to consider as for example the reduction by two units improves the NPV value in almost 600 k€ for all the plant sizes.

Synergizing carbon capture and utilization in a biogas upgrading plant based on calcium chloride: Scaling-up and profitability analysis

Herein we analyze the profitability of a novel regenerative process to synergize biogas upgrading and carbon dioxide utilization. Our proposal is a promising alternative which allows to obtain calcium carbonate as added value product while going beyond traditional biogas upgrading methods with high thermal energy consumption. Recently we have demonstrated the experimental viability of this route. In this work, both the scale-up and the profitability of the process are presented. Furthermore, we analyze three representative scenarios to undertake a techno-economic study of the proposed circular economy process. The scale-up results demonstrate the technical viability of our proposal. The precipitation efficiency and the product quality are still remarkable with the increase of the reactor size. The techno-economic analysis reveals that the implementation of this circular economy strategy is unprofitable without subsidies. Nonetheless, the results are somehow encouraging as the subsides needed to reach profitability are lower than in other biogas upgrading and carbon dioxide utilization proposals. Indeed, for the best-case scenario, a feed-in tariff incentive of 4.3 €/MWh makes the approach profitable. A sensitivity study through tornado analysis is also presented, revealing the importance of reducing bipolar membrane electrodialysis energy consumption. Overall our study envisages the big challenge that the EU faces during the forthcoming years. The evolution towards bio-based and circular economies requires the availability of economic resources and progress on engineering technologies.

Low-Energy Method for Water-Mineral Recovery from Acid Mine Drainage Based on Membrane Technology: Evaluation of Inorganic Salts as Draw Solutions

In this work, a novel study for acid mine drainage remediation and reutilization by means of a forward osmosis technology is addressed. The proposed process is a potential alternative path, which allows to recover high-quality water and to concentrate metals for its possible reutilization as synthetic minerals. This novel process will help in the mining industry evolving toward more sustainable processes and favors circular economy policies. Four inorganic salts (NaCl, KCl, CaCl₂, and MgCl₂) were evaluated as draw solutions from 1 to 5 M concentrations, in terms of water flux, water recovery, and metal rejection, using a thin-film composite (TFC) membrane. Water flux obtained was in the range of 14-53 L/(m² h). The highest water flux was found for MgCl₂, whereas the lowest correspond to KCl. The metal rejection obtained was greater than 99%. After a discussion and comparison of the results, MgCl₂ was chosen for evaluating long-term assay performance. Scanning electron microscope images of the thin-film composite membrane after long-term assays were taken. The tendency of Mg-Ca and Al-Fe fouling was observed over the membrane surface. The energy consumption was estimated from 4.84-22.3 kWh_e/m³, assuming that osmotically assisted reverse osmosis is used to regenerate the draw solution.

Understanding the effect of Ca and Mg ions from wastes in the solvent regeneration stage of a biogas upgrading unit

This paper reveals the effect of calcium and magnesium ions in carbonation experiments carried out to regenerate sodium hydroxide from a biogas upgrading unit. This novel study arises as an alternative to standard physical process whose elevated energy consumption imposes economic restrictions. Previous works employed alkaline waste to turn them into value added product. Nevertheless, no attractive economical results were obtained due to the low regeneration efficiencies. Our hypothesis is that both calcium and magnesium waste composition percentages have an impact in the result, hence this work propose an isolated study aiming to determine the of each one in the global performance. To this end, the operational parameters (reaction time, reaction temperature and molar ratio) were tuned as well as physicochemical properties of the final solid samples were analyzed by several techniques. The results indicate that calcium is much more prone than magnesium to reach high efficiencies in aqueous carbonation experiments. Additionally, higher quality products were achieved with calcium. The results of this study suppose an important step for understanding the aqueous carbonation through waste in the path to achieve a more sustainable city and society.

Synergizing carbon capture storage and utilization in a biogas upgrading lab-scale plant based on calcium chloride: Influence of precipitation parameters

Herein a strategy for biogas upgrading in a continuous flow absorption unit using CaCl₂ as capturing agent is reported. This process is presented as an alternative to the standard physical regeneration processes to capture carbon dioxide (CO₂) from biogas effluents with inherent high energy penalties. This work showcases a systematic study of the main parameters (reaction time, reaction temperature, and molar ratio reactant/precipitator) affecting calcium carbonate (CaCO₃) precipitation efficiency in a reaction between sodium carbonate (Na₂CO₃) and CaCl₂. In addition, the purity and main characteristics of the obtained product were carefully analysed via in a combined characterization study using Raman, XRD, and SEM. Our results indicate that acceptable precipitation efficiencies between 62 and 93% can be reached by fine tuning the studied parameters. The characterization techniques evidence pure CaCO₃ in a calcite structure. These results confirmed the technical feasibility of this alternative biogas upgrading process through CaCO₃ production.