Toward the decarbonization of hard-to-abate sectors: a case study of the soda ash production

Decarbonizing the so-called "hard-to-abate" sectors is considered more technically challenging than others such as energy or transportation because they entail emissions not only from heat and power generation but also from manufacturing and process industries. The opportunities for them are less obvious and the challenges are greater, so their shift or transition to zero emissions is still relatively unexplored. In this case study, we aim to analyze the environmental impact and the technoeconomic viability of the integration of a carbon capture and utilization (CCU) plant that produces CO2-based methanol (CO2-MeOH) by means of electrochemical reduction (ER) in the hard-to-abate sector of synthetic soda ash. With a rigorous emphasis on the goal of net zero CO2 emissions, life cycle assessment (LCA) and technoeconomic assessment (TEA) were used as tools in order to guide further research and development toward its potential final commercialization. LCA and TEA results have demonstrated that it is possible to reduce the carbon footprint (CF) of the synthetic soda ash production at a reasonable cost within proper medium/long-term developments. Several scenarios have been assessed considering the future innovation of the CCU-ER technology and the future evolution of the electricity and CO2 market prices because of the application of instruments such as Power Purchase Agreements (PPAs) and the European Union Emissions Trading System. The scenarios analyzed suggest that the complete electrification of the integrated plants of soda ash through electric heat (EH) is positive from the environmental perspective. This EH represents the direct conversion of renewable electricity to industrial heat. The results displayed a reduction in the CF of soda ash up to 74% as long as the entire integrated plant was run on renewable electricity and considering the commercialization of the ER side products such as H2 and O2. Not considering the selling of these two products leads to more modest reduction around 41%. However, this complete electrification has major implications on the economic profile under the current combination of electricity and CO2 market prices. Low-cost electricity, for example, using surpluses of renewable electricity and/or PPAs, and a higher CO2 price, which can be expected in the short/mid-term, are required to ensure economic feasibility. A 50% reduction of the current average wholesale electricity price that was used as a reference in the present study (43 €·MW h-1) will ensure economic feasibility under the proper ER technology development. The insights gained in this study may be of assistance in the sustainable implementation of CCU in energy-intensive manufacturing processes.Authors thank to Spanish Ministry of Economy and Competitiveness (MINECO) for the financial support through the project CTQ2016-76231-C2-1-R. We would like also to thank MINECO for providing Marta Rumayor with a Juan de la Cierva postdoctoral contract (IJCI-2017-32621)

Innovative alternatives to methanol manufacture: carbon footprint assessment

Finding and implementing more sustainable alternatives to the fossil-dependence routes for methanol (MeOH) manufacturing is undoubtedly one of the challenges of our model of society. Some approaches can be used to convert CO2 into MeOH as direct hydrogenation or electrochemical reduction (ER). These alternatives lead to lower natural resources consumption respect the conventional routes, but they are still found at different technological readiness levels (TRLs). Therefore some remaining challenges need to be overtaken to achieve a carbon neutral cycle respect the conventional route, especially in the case of ER, which is currently found at its infancy. This would indicate their final industrial competitiveness in a sustainable mode. This study uses Life Cycle Assessment as the main tool in order to compare these two CO2-based manufacture alternatives (found at different TRLs) with the fossil-route. The results allow for evaluating the potential challenges inherited to the alternative based on ER. Utilization of renewable energy is one of the most important key issues to achieve a carbon neutral product using these options. However, its benefit could be neglected due to the high requirement of steam in the purification step, particularly in ER. It was demonstrated that a future scenario using ER leads to a lower natural resources consumption (mainly natural gas) compared to the conventional fabrication, which represents an important step towards more green and efficient MeOH synthesis.Authors thank to Spanish Ministry of Economy and Competitiveness (MINECO) for the financial support through the project CTQ2013-48280-C3-1-R. We would like also to thank MINECO for providing Marta Rumayor with a Juan de la Cierva postdoctoral contract (FJCI-2015-23658)

Mercury removal from MSW incineration flue gas by mineral-based sorbents

Three samples of commercially available mineral-based sorbents (zeolite, bentonite and diatomaceous earth) were selected and evaluated for Hg capture under conditions of simulated dry flue gas atmosphere typical in Municipal Solid Waste Incineration (MSWI). The experiments were carried out in a lab-scale fixed-bed device at temperatures between 120 and 200°C. Two samples of activated carbons (AC) (raw-AC and sulphur impregnated AC) were tested under the same conditions. The mineral-based sorbents were chemically promoted by sulphur, FeCl3 and CaBr2, achieving an improvement in the overall reduction percentage of Hg0out (g) up to 85%, which was comparable to that obtained using a commercial activated carbon for Hg capture (sulphur impregnated AC). The study demonstrates that sorbents with a matrix relatively richer in TiO2, Fe2O3 and Al2O3, as bentonite, favour Hg heterogeneous oxidation. The best Hg capture capacity was achieved with a zeolite sorbent sample characterized by high specific surface (132 m2/g) and impregnated with elemental sulphur. The final form of mercury retained in this sorbent was HgS with proved long-term stability in disposal and landfilling. The higher the temperature, the lower the efficiency of Hg capture being the optimum temperature for Hg-capture in the range of 120-150°C. This study provides a basis for the development of new efficient non-carbon sorbents for mercury removal in the air pollution control lines of MSWI facilities considering the non-hazardous final form of mercury and its long-term landfilling/sequestration

Mitigation of gaseous mercury emissions from waste-to-energy facilities: Homogeneous and heterogeneous Hg-oxidation pathways in presence of fly ashes

This study describes the main mechanisms that take part in the mercury homogeneous oxidation pathway in presence of some of the main reactive compounds formed during waste incineration processes (O2, HCl, SO2 and NO). Series of model, synthetic dry flue gases were used to elucidate the effects of HCl, SO2, NO and their proportions in the gas on mercury behaviour. Three samples of fly ash collected from a MSWI facility were characterized and evaluated both for Hg heterogeneous oxidation and Hg removal in a laboratory scale device. The results obtained in this study showed that homogeneous mercury oxidation in the models MSWI and coal combustion flue gas atmospheres was 52 ± 5% and 25%, respectively. SO2, NO and HCl have a synergetic effect in Hg oxidation in presence of oxygen, but the main differences found are mainly caused by the strong influence of HCl and the likely inhibitory oxidation effects of SO2. Surface area together with carbon and chloride content of the fly ashes were correlated with their capacity for Hg-heterogeneous oxidation and adsorption. The sample of fly ash with relatively high content of unburnt carbon and chlorine, and with BET surface (2.42 m2/g) was able to remove up to 100% of Hg0 (g) during 300 min. The results obtained in this study provide a complete overview of the behaviour of mercury during MSWI processes and may help to clarify the fate/behaviour of mercury in a filter (e.g. electrostatic precipitator) providing a deeper knowledge about the impacts of fly ash properties on mercury fate in waste incineration

Deep decarbonization of the cement sector: a prospective environmental assessment of CO2 recycling to methanol

Current decarbonization pressures are prompting efforts to reimagine the future of the hard-to-abate cement sector. To date, fuel switching has arisen as the most readily operational strategy, and its application in the cement sector is expected in the short to midterm. However, around two-thirds of the cement CO2 emissions come from the calcination of limestone. The implementation of CO2 capture utilization and/or storage will be crucial to support a reliable net-zero carbon future by 2050–2070. CCS is considered as the most carbon-neutral technology in the cement decarbonization roadmap, while CO2 recycling (CCU) has arisen as a suitable strategy for those locations where there is an industrial symbiosis between the cement market and CO2-based chemical markets (e.g., methanol, formic acid, etc.). Despite that the CCU strategy cannot be carbon-neutral by itself, it could be a powerful option in combination with CCS. To date, most CO2 recycling technologies are still emerging, and their development has to be boosted in the next decades. In this study, a prospective environmental analysis has been conducted through life cycle thinking to explore the benefits of cement long-term decarbonization by implementing a carbon recycling plant (CRP) based on the emerging electrochemical reduction (ER) of CO2 to produce methanol (MeOH). The study aims to demonstrate the synergic decarbonization and defossilization for both cement and MeOH markets, respectively. Cell energy efficiency and MeOH concentration have been identified as the key performance parameters that should be around 60% and 40% wt, respectively, to ensure a future sustainable implementation of ER to the MeOH technology. A CRP powered by low-carbon renewable electricity (<0.02 kg CO2eq/kW h) and with a low-fossil depletion (FD) impact (<0.01 kg oileq/kW h) could lead to an integrated cement and MeOH production with sharp reductions in the carbon footprint (∼75%) and FD (∼66%) of the integrated cement and MeOH production compared to the conventional fossil-based productions. The proposed CO2 recycling scheme can contribute to accelerating the innovation of carbon capture and recycling technologies and their deployment in these hard-to-abate sectors.Funding has been obtained though the project PID2020-112845RB-I0

CO2 electroreduction: sustainability analysis of the renewable synthetic natural gas

Capture and utilization of industrial CO2 emissions into low-carbon fuels is a promising alternative to store renewable electricity into chemical vectors while decarbonizing the economy. This work evaluates the viability pathways of producing synthetic natural gas (SNG) by direct CO2 electroreduction (ER) in Power-To-Synthetic Natural Gas electrolyzers (PtSNG). We perform an ex-ante techno-economic (TEA) and life cycle analysis (LCA) for a 2030 framework in Europe. ER performance is varied in defined scenarios and assessed using a built-in process model of the PtSNG system, revealing uncharted limitations and benchmarks to achieve. Results show that substitution of fossil natural gas with renewable SNG could avoid more than 1 kg CO2e/kg SNG under moderate ER conditions when using low-carbon electricity (< 60 kg CO2e/MWh). SNG profitability for 2030 would rely on: i) higher CH4 current densities (800–1000 mA/cm2), ii) improvements in energy efficiency (higher than 60%), and iii) valorization of the anodic product or additional carbon incentives. Our study proves that if market and technology evolve appropriately in the coming years, the SNG by CO2 ER may be a mid-term climate change mitigation technology, among others.The authors thank the Spanish Ministry of Economy and Competitiveness for the financial support through the project PID2020–114,787-RB-I00. Javier Fernández-González and Marta Rumayor would also like to thank the financial support of the Spanish Ministry of Science and Innovation for the concession of a FPU grant (FPU19/05483) and a Juan de la Cierva postdoctoral contract (IJCI-2017-32621), respectively

Prospective life cycle assessment of hydrogen production by waste photoreforming

Identifying sustainable energy vectors is perhaps one of the most critical issues that needs addressing to achieve a climate-neutral society by 2050. In this context, the hydrogen economy has been proposed as a solution to mitigate our current fossil-based energy system while the concept of the circular economy aims to boost the efficient use of resources. Photoreforming offers a promising opportunity for recycling and transforming widely available biomass-derived wastes (e.g., crude glycerol from biodiesel) into clean hydrogen fuel. This processing technology may be a versatile method that can be performed not only under UV light but also under visible light. However, this approach is currently at the lab-scale and some inherent challenges must be overcome, not least the relatively modest hydrogen production rates for the lamps? substantial energy consumption. This study aims to assess the main environmental impacts, identifying the hotspots and possible trade-off in which this technology could operate feasibly. We introduce an assessment of the windows of opportunity using seven categories of environmental impact with either artificial light or sunlight as the source of photocatalytic conversion. We compared the environmental indicators from this study with those of the benchmark water electrolysis and steam?methane reforming (SMR) technologies, which are currently operating at a commercial scale. The results obtained in this study situate biowaste photoreforming within the portfolio of sustainable H2 production technologies of interest for future development in terms of target H2 production rates and lifetimes of sustainable operation.Financial support from projects RTI2018-099407-B-I00 and RTI2018-099407-B-I00 funded by MCIN/AEI/10.13039/501100011033/FEDER and by “ERDF A way of making Europe” by the “European Union” is gratefully acknowledged. We would like also to thank MICIN for providing Marta Rumayor with a Juan de la Cierva postdoctoral contract IJCI-2017-32621

Formic acid manufacture: carbon dioxide utilization alternatives

Carbon dioxide (CO2) utilization alternatives for manufacturing formic acid (FA) such as electrochemical reduction (ER) or homogeneous catalysis of CO2 and H2 could be efficient options for developing more environmentally-friendly production alternatives to FA fossil-dependant production. However, these alternatives are currently found at different technological readiness levels (TRLs), and some remaining technical challenges need to be overcome to achieve at least carbon-even FA compared to the commercial process, especially ER of CO2, which is still farther from its industrial application. The main technical limitations inherited by FA production by ER are the low FA concentration achieved and the high overpotentials required, which involve high consumptions of energy (ER cell) and steam (distillation). In this study, a comparison in terms of carbon footprints (CF) using the Life Cycle Assessment (LCA) tool was done to evaluate the potential technological challenges assuring the environmental competitiveness of the FA production by ER of CO2. The CF of the FA conventional production were used as a benchmark, as well as the CF of a simulated plant based on homogeneous catalysts of CO2 and H2 (found closer to be commercial). Renewable energy utilization as PV solar for the reaction is essential to achieve a carbon-even product; however, the CF benefits are still negligible due to the enormous contribution of the steam produced by natural gas (purification stage). Some ER reactor configurations, plus a recirculation mode, could achieve an even CF versus commercial process. It was demonstrated that the ER alternatives could lead to lower natural resources consumption (mainly, natural gas and heavy fuel oil) compared to the commercial process, which is a noticeable advantage in environmental sustainability terms.This research was funded by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project CTQ2016-76231-C2-1-R. Marta Rumayor contract was funded by the Spanish Ministry of Economy and Competitiveness (MINECO) through a Juan de la Cierva postdoctoral contract (FJCI-2015-23658)

Mercury Retention by Fly Ashes from Oxy-fuel Processes

The objective of this study is to determine the mechanism of mercury retention in fly ashes, the main solid waste from coal combustion power plants, and to evaluate the interactions between the type of mercury and fly ashes. The work was based on the results of mercury speciation in the gas and the solid fly ash before and after mercury retention. The identification of the mercury species in the gas was performed using previously validated methods, but the speciation of the mercury retained in the fly ashes was carried out using a mercury temperature-programmed desorption technique (HgTPD) still under development. The fly ashes were sampled from conventional coal combustion in air and oxy-combustion power plants. The main mercury species identified in the raw fly ashes and after they were subjected to an oxy-combustion atmosphere were mercury bound to organic matter and HgS, the ratio of these species depending on the characteristics of the ashes. The results obtained indicate that fly ashes are the route of mercury oxidation in an oxy-combustion atmosphere, although they hardly retain any mercury unless the unburned carbon content is high. HgTPD analysis shows that the main mechanism for mercury retention in the fly ashes is via the carbon matter.The financial support for this work was provided by the National Research Program under project CTM2011–22921. The authors thank CIEMAT (Department of Energy) and CIUDEN for supplying the fly ashes employed in this study, PCTI Asturias for awarding Ms. Nuria Fernandez-Miranda a pre-doctoral fellowship and the Spanish Research Council (CSIC) for awarding Ms. Marta Rumayor a JAE-predoc fellowship.Peer reviewe

Life cycle assessment of salinity gradient energy recovery by reverse electrodialysis in a seawater reverse osmosis desalination plant

Salinity gradient energy capture by reverse electrodialysis (SGE-RED) can play a part in the shift away from fossil fuels towards a carbon-neutral renewable energy supply; however, like other renewable power technologies, SGE-RED environmental soundness hinges on its whole life-cycle environmental loads. This study surveys the Life Cycle Assessment of SGE-RED technology. We quantified (i) the environmental loads per 1.0 kW h generated by a stand-alone RED unit and then, (ii) the environmental burdens related to the energy provision from an up-scaled RED system to a seawater reverse osmosis (SWRO) desalination plant per 1.0 m3 of desalted water. The RED unit's assessment results show that SGE-RED is environmentally competitive with other renewable sources such as photovoltaics or wind. Regarding the component's contribution analysis, the spacer's fabric material drives the RED environmental burden as the number of cell pairs is increased. The scaling-up of the RED unit, however, improves its full environmental profile. Preliminary results of SGE-RED combination with a SWRO plant suggest that the energy harnessed from SWRO's concentrate streams by RED could enhance the environmental performance of the desalination industry. Further research is required to identify SWRO-RED design alternatives that minimize the life cycle burden while still yielding good technical and economic performance.This work was supported by the Community of Cantabria - Regional Plan through the project Gradisal (RM16-XX-046-SODERCAN/FEDER); and the Spanish Ministry of Science, Innovation and Universities (RTI2018-093310-B-I00 and CTM2017-87850-R). Carolina Tristán is supported by the Spanish Ministry of Science, Innovation and Universities (FPI grant PRE2018-086454). The authors also thank Fumatech for providing information on IEMs properties