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LCA Analysis Decarbonisation Potential of Aluminium Primary Production by Applying Hydrogen and CCUS Technologies

School of Mining and Metallurgical Engineering, National Technical University of Athens (NTUA), 15780 Athens, Greece
Author to whom correspondence should be addressed.
Hydrogen 2023, 4(2), 338-356;
Submission received: 22 March 2023 / Revised: 25 April 2023 / Accepted: 18 May 2023 / Published: 20 May 2023
(This article belongs to the Special Issue Feature Papers in Hydrogen (Volume 2))


The energy intensity and high emissions of extractive industries bring a major need for decarbonisation actions. In 2021, extraction and primary processing of metals and minerals were responsible for 4.5 Gt of equivalent CO2. The aluminium industry specifically accounted for total emissions of 1.1 Gt CO2 eq. per year. Reaching the European milestone of zero emissions by 2050, requires a 3% annual reduction. To achieve this, the industry has searched for innovative solutions, considering the treatment of emitted CO2 with techniques such as Carbon Capture Utilisation and Storage (CCUS), or the prevention of CO2 formation on the first place by utilising alternative fuels such as hydrogen (H2). This study aims to comprehensively compare the overall environmental performance of different strategies for addressing not only greenhouse gas (GHG) emission reduction potential, but also emissions to air in general, as well as freshwater and terrestrial ecotoxicity, which are commonly overlooked. Specifically, a Life Cycle Assessment (LCA) is conducted, analysing four scenarios for primary Al production, utilising (1) a combination of fossil fuels, specifically Natural Gas (NG), Light Fuel Oil (LFO) and Heavy Fuel Oil (HFO) (conventional approach); (2) carbon capture and geological storage; (3) Carbon Capture and Utilisation (CCU) for methanol (MeOH) production and (4) green H2, replacing NG. The results show that green H2 replacing NG is the most environmentally beneficial option, accounting for a 10.76% reduction in Global Warming Potential (GWP) and 1.26% in Photochemical Ozone Formation (POF), while all other impact categories were lower compared to CCUS. The results offer a comprehensive overview to support decision-makers in comparing the overall environmental impact and the emission reduction potential of the different solutions.

1. Introduction

Decarbonisation of energy intensive industries is key in reaching the European milestone of climate neutrality by 2050. In 2021, extraction and primary processing of metals and minerals accounted for an annual 4.5 Gt of CO2 eq. [1]. These emissions are expected to increase significantly as global demand sees a constant rise. In 2022, extraction of metals and minerals accounted for more than 100 billion tonnes annually, compared to 92 billion tonnes in 2017 and 27 billion tonnes in 1970 [2,3]. Global demand is expected to reach as high as 190 billion tonnes by 2060. Specifically, global demand for steel and aluminium (Al) is expected to increase by as much as 30% and 75%, respectively, compared to 2017 [4]. In 2022, global primary Al production reached approximately 68.4 million tonnes [5]. This rise in production significantly increases the environmental impact of the industry.
The Al industry is one of the most energy intensive and CO2 emissive industries, accounting for 275 Mt of CO2 in 2021; 3% of global direct CO2 emissions [6]. When indirect emissions from electricity are considered, this figure climbs as high as 1.1 Gt. Primary production is responsible for over 90% of these emissions [7]. In order to reach the milestone of zero emissions by 2050, a 3% annual reduction is necessary.
Primary Al production typically incorporates the following four stages: alumina production, anode/paste production, Al electrolysis and Al casting. In the alumina production stage, alumina is extracted from bauxite by thermo-chemical digestion [8]. Afterwards, the produced alumina is fed to a primary Al smelter, where it is reduced to liquid Al and O2, emitted as CO2 due to the reaction with the carbon anodes. The anode/paste production process includes the production of prebake anodes and Søderberg paste, required for the Al electrolysis. The liquid Al is then casted into ingots as the final product. Yet, only 30% of Al production emissions are direct, including (1) the consumption of carbon anodes; and (2) the generation of thermal energy to produce industrial heat and steam. In the scope of this study, decarbonisation technologies to address direct emissions related to the thermal energy production, will be investigated.
Carbon Capture Utilisation and Storage (CCUS) is a technologically mature solution, seeing various applications in recent years. Carbon Capture and Storage (CCS) is when a relatively pure stream of CO2 from industrial processes is separated, treated and transported to a long-term storage location. Post-combustion capture operates at low pressures and is suitable for flue-gases of low CO2 concentrations showing high efficiencies from laboratory to commercial scale. In these systems, the off-gas is introduced to the bottom of an absorption column, where it makes contact with a liquid solvent flowing downwards, capturing the CO2, thus leaving treated gas of low CO2 content to exit from the top. Afterwards, the liquid stream containing the captured CO2 is fed to a desorption column, where CO2 is extracted, while the regenerated solvent is recycled back to the absorption column to repeat the process. Of the different solvents, aqueous amines, such as monoethanolamine (MEA) are commonly used, achieving efficiencies as high as 95% and CO2 purities higher than 99% [9]. The captured CO2 is stored at geological storage facilities by injecting CO2 into rock formations deep underground, commonly in depleted oil and gas reservoirs or saline formations [10]. The rock layer itself is covered by an impermeable layer to prevent leakages. Geological storage is a very efficient solution due to its high storage capacities. Specifically, global geological storage capacity is estimated to be between 8000–55,000 Gt, which is more than enough to achieve IEA’s “Sustainable Development Scenario” of 220 Gt stored between 2020 and 2070 [11].
Carbon Capture and Utilisation (CCU) technologies allow for the utilisation of such CO2 feedstocks to produce valuable products, such as urea, methanol (MeOH), formaldehyde, formic acid, carbamates, polymer-building blocks and fine chemicals. Chemical conversion with CO2 hydrogenation for MeOH production has specifically seen increased interest over the years. In this process, CO2 reacts with hydrogen (H2) in catalytic reactors, commonly utilising Cu/ZnO/Al2O3 catalysts. The reaction typically takes place at temperatures of 210–270 °C, high pressures of 50–100 bar and a H2:CO2 molar ratio of 3:1 [6]. These technologies significantly reduce the CO2 emissions of plants, especially when green H2, produced by Renewable Energy Sources (RES), is used, with studies showing that production from green H2 can achieve efficiencies as high as 50% [12]. MeOH finds application in various sectors, from fuel for the automotive and marine sector, to feedstock for plastics manufacturing, to pharmaceutical applications. In 2021, global MeOH demand reached more than 164 million tonnes and sees a continuous rise [13]. In that year, MeOH-based fuel production accounted for 31% of global MeOH consumption [14]. The rising popularity of renewable MeOH as a fuel is the result of its significant environmental benefits, showing reduction capabilities of 95% for CO2, 80% for NOx and almost 100% for SOx and particulate matter.
Regenerative carbon technology solutions, such as CCUS, to capture CO2 from flue gases have been commercially available for decades, showing a technology readiness level (TRL) of about 8 (System complete and qualified) to 9 (System proven in operational environment). Yet, absorption-based capture systems that could be applied to primary Al production during the smelting process to capture CO2 from carbon anode consumption only reach TRL 3 (Experimental proof of concept) to 4 (Technology validated in lab) [15]. In terms of transporting CO2 via pipeline, ship, rail and truck, the technology is mature (TRL 9) but still not showing even limited application to meet future global needs. For storage, the technology used is essentially the same process as is already used in the oil and gas sector, with Enhanced Oil Recovery and saline storage being widely used across the industry (TRL 9), with storage in depleted reservoirs being piloted (TRL 5–8) [16]. Therefore, CCUS is more viable at sites close to geological storage reservoirs or other industrial sites where transportation and storage infrastructure can be utilised; however, it brings its own technical and economic challenges. The cost of carbon capture depends on the concentration of CO2 at the flue gases and the technology used, starting from $15/tonne CO2 for high-concentration streams to over $100/tonne for lower-concentration sources such as the flue gas from aluminium smelters [15]. Nevertheless, most CCUS applications are currently designed to capture 85–90% of point source emissions to optimise the cost per tonne of CO2 captured. Higher capture rates are technically feasible but may result in additional operating costs. For this purpose, a use case is currently being investigated by the Alvance, Trimet, LRF (Rio Tinto’s research centre) and the Fives Groups to evaluate the most economical way to capture carbon in aluminium smelters. The project is focused on amine-based capture technology to determine the feasibility of capturing flue gases directly versus the need to concentrate the CO2 for better capture. Alvance is framing a pilot to launch by 2024 in the hope of capturing up to 70% of emissions from the smelting process [17].
Alternative fuels of low or zero CO2 emissions are a viable solution for replacing fossil fuels used in primary Al production. H2 has seen considerable rise in popularity in recent years, with most net-zero scenarios foreseeing fast growth to address hard-to-abate emissions of industries. H2 combustion accounts for zero CO2 emissions. When combusted with pure oxygen (O2), it also accounts for zero NOx formation and near-zero emissions overall. The majority of emissions related to H2 are production-related emissions. H2 produced by splitting natural gas (NG) emits CO2 as a by-product, while H2 produced by water electrolysis accounts for significant electricity demands. Green H2 accounts for near-zero emissions in production and use. In this direction, various industries are experimenting with gradually substituting NG with green H2, heading towards the final goal of a full-scale green H2 transition. While these efforts are mostly in the energy industry, recent developments have seen the utilisation of green H2 in the cement industry, in furnaces similar to those used in Al refineries. Even if a full-scale green H2 transition is not currently economically justified, operators could work with energy providers to reduce the amount of NG required per tonne of Al.
Despite this, green H2 is still limited at a commercial scale and represents just 1% of all H2 production annually; electrolysers are considered a mature, established technology with a high TRL 8–9. On the other hand, the use for high-temperature processes for industrial use, such as in alumina refining, is at this time theoretically possible, but little H2 is actually used for this purpose today (TRL 2–3) [18]. Additionally, the costs of grey, blue and green H2 are considered the greatest barrier to scalability. Grey H2 costs around $1.50/kg H2, blue H2 costs around $2/kg H2 and green H2 costs around $3–$5/kg H2. In view of this, the integration of H2 as a fuel in the Al industry will require extensive research and efforts to ensure the quality of the end-products. For this purpose, two use cases are indicatively presented at Rio Tinto and Norsk Hydro. Rio Tinto has partnered with the Australian Renewable Energy Agency (ARENA) to evaluate the technical feasibility of H2 to replace NG during calcination at the Yarwun alumina refinery in Queensland. Further, Norsk Hydro aims to investigate the potential to operate H2 as an alternative to NG for its own operations, while exploring an additional revenue stream as H2 plays an increasing role in the green economy. This transition is estimated to reduce Hydro’s CO2 emissions by as much as 30% by 2030 [17].
The scope of this study is to examine the environmental impact of four emission reduction approaches in the primary Al industries, from technologically mature solutions of post-combustion treatment, to novel solutions of great environmental benefit. This work aims to comprehensively analyse the environmental footprint of primary Al production as well as examine other impact categories, considering emissions to air, as well as freshwater and terrestrial ecotoxicity, which are commonly overlooked. Specifically, cradle-to-gate models for four scenarios were examined, utilising: (1) baseline, (2) CCS, (3) CCU for MeOH production and (4) green H2. A partial Life Cycle Impact Assessment (LCIA) was performed, with the following midpoint environmental impact categories reported: Global Warming Potential (GWP), Acidification Potential (AP), Eutrophication Potential (EP), Photochemical Ozone Formation (POF), Freshwater Aquatic Ecotoxicity Potential (FAETP) and Terrestrial Ecotoxicity Potential (TETP). This study’s results will provide robust data for the benefits of incorporating the different emission reduction solutions, as well as establish LCA as an efficient tool for sustainable development and for decision making in the industrial sector.

2. Materials and Methods

2.1. LCA Methodology

For the assessment and comparison of the environmental impact of primary Al production incorporating different CO2 emissions’ reduction approaches, an LCA was performed, using the standardized procedures described by ISO 14040:2006 [19] and 14044:2006/A1:2018 [20] and the International Life Cycle Data (ILCD) Handbook [21]. The LCA framework consists of (1) the goal and scope definition, (2) the Life Cycle Inventory (LCI) preparation, (3) the LCIA and (4) the interpretation of the results. The LCA for the scenarios was conducted using the commercial software package Sphera LCA for Experts of Blackstone company, Chicago, IL, USA [22].

2.2. Goal, Scope, and Functional Unit

This study aims for a thorough assessment of the environmental impact of different emissions’ reduction solutions, covering both air emissions and freshwater and terrestrial ecotoxicity, to obtain more concrete results. Specifically, this study examines the approach of CO2 mitigation by the aftertreatment of the off-gases, as well as CO2 formation prevention with the use of alternative fuels. The scenarios proposed are designed using literature review information and the Sphera LCA FE database. The life cycle inventory data and environmental metrics for the primary Aluminium industry located in Europe, as collected by the International Aluminium [8] corresponding to primary aluminium production processes from alumina production to ingot manufacture, including: raw material inputs, energy and water consumption, emissions to air and water and solid waste generation. Considering CO2 mitigation from fossil fuels combustion, CCS with geological storage and CCU for MeOH production were examined. Considering CO2 formation prevention, exploitation of H2, replacing specifically NG, was investigated. The analysis’ scope is the examination of the energy, materials and emissions flows to estimate the potential of CCS, CCU and replacing NG with green H2 technologies at a European primary Al production industry. For this purpose, the functional unit (FU), which is the quantity of product for which the environmental impact will be calculated for, is selected as 1 tonne of Al ingot.

2.3. Scenario Descriptions and System Boundaries

Overall, four scenarios were conducted. Scenario 1 is designed to be the base case, simulating the primary Al production processes currently applied, using Natural Gas (NG), Light Fuel Oil (LFO) and Heavy Fuel Oil (HFO). Scenario 2 includes a CCS capturing CO2 from the alumina production process and transporting it to a geological storage. Scenario 3 simulates a CCU capturing CO2 from the alumina production process and producing MeOH. Scenario 4 investigates the replacement of NG used for thermal energy to all Al production processes with green H2. The system boundaries were defined to include all processes in the techno-sphere of the FU. The different scenarios are summarized in Table 1.

2.3.1. Scenario 1

In scenario 1, the required thermal energy is provided by fossil fuels combustion, namely a mixture of NG, LFO and HFO. Electricity for the different production stages is supplied by the grid. The anodes and pastes required for the Al electrolysis are produced in the plant’s premises, as is often the case. Overall, this scenario simulates a conventional primary Al production process, to serve as the base case for the analysis. The system boundaries for scenario 1 are shown in Figure 1.

2.3.2. Scenario 2

In scenario 2, CO2 is captured from the off-gases of the alumina production stage, using a conventional amine-based system. CO2 in the off-gases is absorbed by an aqueous amine solution in an absorption column. Afterwards, the high CO2 containing amine (rich amine) is fed to a desorption column where CO2 is separated to be stored, while the amine is fed back to the absorption column to repeat the process. The captured CO2 is compressed and sent by pipelines to a geological storage site, specifically a storage well, constructed in a saline aquifer. The production of Al ingot is considered identical to scenario 1, including both fossil fuel combustion and electricity supply. The system boundaries for scenario 2 are shown in Figure 2.

2.3.3. Scenario 3

Scenario 3 consists of the carbon capture system also used in scenario 2. After the capture, the CO2 is compressed and fed to a CO2 to MeOH unit, utilising a CO2 hydrogenation reactor, where CO2 reacts with H2 for the production of MeOH. Required H2 is produced by water electrolysis, using electricity produced by photovoltaics. The end-use of produced MeOH is not considered in the system’s boundaries. The system boundaries for scenario 3 are shown in Figure 3.

2.3.4. Scenario 4

Scenario 4 utilises green H2 to replace NG for the production of the necessary thermal energy. Accordingly, an H2 burner is selected, as shown in Section 3, to produce high-grade heat for the processes. For the production of 1 MJ thermal energy, the selected H2 burner requires 0.008 kg of H2. The required H2 is produced by water electrolysis, using electricity from photovoltaics. The system boundaries for scenario 4 are shown in Figure 4.

2.4. Life Cycle Impact Analysis

The LCIA quantifies the environmental impacts using the results of the LCI analysis and the impact factors. Higher values of impact categories indicate the hot spots of a production process with the most environmental burden. In this study, six impact categories at the midpoint level were selected for the LCIA. The impact categories include the GWP reduction potential for each scenario, as well as the overall environmental impact in terms of overall emissions to air, freshwater and terrestrial ecotoxicity, and in compliance with ISO 14040 and ISO 14044 standards, containing a broad set of midpoint categories [19,20,23]. The LCIA impact categories examined are summarised in Table 2.

3. Life Cycle Inventory

The LCI consists of all the inputs and outputs data of the system, in terms of materials, energy, emissions, etc. To ensure the validity of the data, processes found in the Sphera LCA for Experts database were exploited as much as possible. As these data mostly derive from industrial measurements, they are considered technologically representative and up-to-date. Data for processes not found in these databases were drawn from the literature.
For all scenarios, data for the production stages, namely alumina production, anode/paste production, Al electrolysis and casting were drawn from the “2019 Life Cycle Inventory (LCI) Data and Environmental Metrics” of International Aluminium referring to European Al industries [8]. Alumina production was considered to take place in facilities refining metallurgical grade alumina only from bauxite. In the alumina production stage, approximately 4.41 tonnes of bauxite were used for the production of 1.88 tonnes of alumina, which was then fed to the Al electrolysis process for the production of 1 tonne of liquid Al, using approximately 494 kg of anodes and pastes. The liquid Al was then casted into ingots, considering no material losses.
The electrical grid was simulated as the average EU-28 country grid mix, 1–60 kV, drawn from the Sphera LCA for Experts database. The data included electricity own consumption, transmission/distribution losses of electricity supply and electricity imports from neighbouring countries. The national energy carrier mixes used for electricity production, the power plant efficiency data, shares on direct to combined heat and power generation, as well as transmission/distribution losses and own consumption values are calculated considering various information sources. The electricity grid delivered approximately 1030 MJ, 127 MJ, 53,500 MJ and 387 MJ of electricity for the alumina production, anode/paste production, Al electrolysis and Al casting, respectively.
For scenario 1, data for the thermal energy demand deriving from the combustion of NG, LFO and HFO were drawn from the Sphera LCA for Experts database. The inventory was based on primary and secondary industry data, considering all processes in the supply chain. The detailed power plant model used combined measurement, e.g., NOx, as well as emission values calculation, e.g., heavy metals. For the production of the thermal energy, the European (EU-28) energy carrier mix was examined for each fuel. NG combustion provided approximately 18,300 MJ, 248 MJ, and 408 MJ of thermal energy for the alumina production, anode/paste production and the Al casting, respectively, a total of 18,956 MJ. LFO combustion delivered 22.1 MJ, 7.85 MJ and 1.82 MJ, while HFO provided 63.1 MJ, 369 MJ and 112 MJ, respectively.
For scenarios 2 and 3, carbon capture was designed to simulate a commercial amine scrubbing system utilising MEA, which has found application in 23 commercial plants worldwide [24]. The system had a capture ratio of 90%, delivering CO2 of 99.6% purity. The energy duties of the system are mainly linked to the solvent regeneration and electricity consumption for ancillaries. Material inputs include an MEA make-up stream, to account for losses during operation, activated carbon to absorb degradation products from MEA and caustic soda (NaOH) to promote the MEA regeneration. Direct emissions from the capture process are mostly linked to uncaptured CO2 and other elements contained in the treated gas, as well as liquid waste materials deriving from MEA use. After the amine scrubbing process, captured CO2 is compressed to approximately 110 bar. The required electricity for the separation and compression processes was provided by the electrical grid. The required thermal energy for the separation process was provided by NG combustion. The carbon capture unit handled approximately 977 kg of CO2, deriving from the alumina production process, capturing and compressing 880 kg.
For the geological storage in scenario 2, an average EU transportation distance from the carbon capture plant to the storage site, of approximately 250 km was considered. The storage took place in deep saline aquifers. Specifically, the data for the Syderiai site were selected with a storage capacity of 21.5 Mt CO2 [25]. The LCI considered material and energy flows and emissions for all stages of geological storage, from the storage well construction and integrity, to CO2 transportation to injection. The study examined the geological storage of the 880 kg CO2 captured, considering negligible CO2 leakages in the process.
For scenario 3, production of MeOH from CO2 took place in a conventional system through catalytic CO2 hydrogenation, utilising a Cu/ZnO/Al2O3 catalyst [6]. After the reaction, produced MeOH was extracted by the product stream in a distillation column. Required H2 is produced by water electrolysis, with data available in the Sphera LCA for Experts database. The electricity required for the electrolysis is provided by a power plant using Compact Linear Fresnel Reflector (CLFR) technology, transforming solar thermal energy to electricity, with data drawn from the Sphera LCA for Experts database. The CO2 to MeOH process produced approximately 605 kg MeOH from the 880 kg CO2 captured, consuming 144 kg of green H2 in the process. Production of the necessary 144 kg H2 required approximately 27.76 MJ of RES electricity.
Considering scenario 4, H2 combustion technologies are relatively new and thus not well reported for industrial use. Therefore, data for H2 were drawn from the literature. For H2 combustion, a 600-kW burner is used, designed to achieve complete combustion [26]. The burner is also designed to achieve optimal flame temperature to minimize NOx formation, combustion rate, flame shape and pattern and optimal radiant heat flux rates for high heat transfer efficiencies. The H2 production process was identical to that of scenario 3. H2 combustion delivered thermal energy to the processes equal to the energy provided by NG in the previous scenarios, 18,956 MJ. Total H2 consumption was 157.45 kg, accounting for 30,348 MJ of RES electricity for its production.
The models developed using the software “Sphera LCA for Experts” can be found in the Supplementary Material S1 of this study. Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9 and Table 10 present in detail the LCI used for the modelling of the scenarios and the evaluation of the alternative thermal energy sources.

4. Results

This section presents the scenarios aiming to identify Al ingot production with better environmental performance as a necessary action for industries to ensure competitive and environmental advantages. Figure 5 summarises the results of the LCIA for the scenarios. For the base case, considering emissions to air, the GWP, AP, EP and POF were 9275.62 kg CO2 eq., 22.07 kg SO2 eq., 1.59 kg Phosphate eq. and 8.64 kg NOx eq., respectively. It is therefore evident, that the greatest environmental impact of Al ingot production is related to CO2 emissions. The majority of them are indirect, deriving from the production of the required electricity for the processes, especially Al electrolysis. Despite this, the direct emissions from the combustion of fossil fuels accounted for 1321.19 kg CO2, with NG specifically accounting for 1270.03 kg CO2 eq., a significant 13.7% of the total GWP. Considering freshwater ecotoxicity, the FAETP was 18.15 kg DCB eq., with the grid electricity consumed from the overall production chain and the production of the anodes and pastes having the highest impact, 58.07% and 28.87%, respectively. In terms of terrestrial ecotoxicity, the TETP was 7.18 kg DCB eq., with grid electricity consumption being responsible for 58.7%.
The GWP for scenario 2 was 8637.26 kg CO2 eq., 6.88% lower than in the base case. Incorporation of CCS for the off-gases of alumina production significantly reduced the direct CO2 emissions of the process. With a capture ratio of 90%, the total GWP for the Al production, carbon capture and geological storage was 339.43 kg CO2 eq., compared to the 977.8 kg CO2 eq. of Al production in scenario 1, a reduction of 65.29%. Operation of the carbon capture system, the storage well construction and the CO2 injection accounted for approximately 70% of these emissions. Despite this significant decrease in GWP, the deployment of these infrastructure and processes significantly increased the impact in terms of other air emissions. Specifically, the EP reached 1315.98 kg Phosphate eq., mostly due to the nitrogen emissions released with the treated gas from the carbon capture system. AP and POF were also elevated, but not significantly, only 0.91% and 2.08%, respectively. Considering freshwater and terrestrial ecotoxicity, FAETP was not increased significantly, only 0.77%, while TETP increased by 3.62%.
Considering scenario 3, deployment of the CO2 to MeOH unit over CCS approach increased the environmental impact across the board. In terms of air emissions, GWP, AP and POF increased by 2.74%, 4.58% and 6.12%, while EP was nearly the same, increased by less than 0.01%. Considering freshwater and terrestrial ecotoxicity, FAETP and TETP increased by 6.29% and 27.69%, respectively. This overall increase in the environmental impact mostly derived from the elevated electricity requirements of the processes, from the production of MeOH to the production of the required H2 by water electrolysis. Despite this, the environmental benefits of this approach are highlighted if the end use of MeOH is examined, especially as an alternative fuel for the transportation sector. When use of MeOH is considered as a thermal energy credit of lesser CO2 footprint than conventional fuels, the overall impact is significantly decreased.
Scenario 4, replacing of NG with H2, appears to be the most environmentally beneficial of the different options. Considering air emissions, combustion of H2 accounted for a GWP reduction of 10.76%, 8277.16 kg of CO2 eq. compared to 9275.62 kg CO2 eq. for the base case. In the same manner, EP and POF were reduced by 1.26% and 2.31%, respectively. On the other hand, AP showed an increase of 3.17%, due to the increased electricity demand for H2 production, which accounted for 1.05 kg SO2 eq., even when produced by RES such as photovoltaics, compared to 0.54 kg SO2 eq. from NG combustion. In terms of freshwater and terrestrial ecotoxicity, the FAETP and TETP increased by 15.60% and 5.51%, respectively, as a result of the electricity required for H2 production.

5. Discussion

The results of the study highlight the energy intensive nature of primary Al production, both in terms of electricity and thermal energy consumption, which is correlated with high environmental impact. As mentioned, the EU commitment of climate neutrality by 2050 calls for immediate actions towards the mitigation of this impact, combining technological maturity, focusing in the aftertreatment of CO2, and innovative solutions for preventing CO2 formation in the first place. To this end, the main achievement of this study is the assessment of the potential of both such solutions in reducing the emissions of the industry, and the synergy of the two towards the EU long-term goals.
Generally, the technological solution of CCS is widely applicable to hard-to-abate emissions across many sectors in which the integration of RES electricity alone is still financially or technically unfeasible. The environmental benefits in relation to industry decarbonisation can be seen not only by this study but also across the literature. CCS is considered ideal in areas with access to cheap fossil fuels and has the potential to bring down emissions in almost all parts of the global energy systems. However, the main challenge this solution faces to industrial use is related to the high Capital Expenditure (CAPEX) of investment of storage infrastructure and the installation of the pipelines needed for widespread use. For the Al industry, the environmental and economic factors appear concerning due to the composition of flue gases comprised by low CO2 concentration. Beyond the consideration of CO2 concentration, some smelter flue gas streams may have too much O2 or SO2 to achieve a good capture rate. As shown from the results, there is a negative effect in terms of impact categories such as EP which shows significant rise. To tackle this issue, industries incorporating these technologies need to adopt further treatment processes to address the elevated nitrous emissions. To deal with the increased FAETP ant TETP, energy efficiency is key. Specifically, reducing the heat duty of this system, coupled with electricity from RES and environmentally friendlier thermal energy sources can significantly reduce the overall impact of these systems. The decarbonisation potential of CCS is undoubtable. However not many industries deploy these technologies, even with the incentive of lower carbon taxation. This can only be attributed to CAPEX of such ventures. In this direction, the creation of incentives in governmental or other frameworks, such as subsidies for industries to deploy CCS technologies, can facilitate their expansion. In general, dissemination of the geological CO2 storage is key, to both stake-holders, as well as the public, who still meets this approach with distrust.
Exploitation of CO2 for MeOH production is another beneficial approach, albeit less technologically developed and commercialised. One significant issue for this approach is the high H2 consumption, which can be economically and even environmentally unviable, if H2 is not produced with electricity from RES. To this end, the development of green H2 production technologies is key. In the same manner, electricity consumption for the system’s operation, besides H2 production, is responsible for the vast majority of emissions. To this end, overall exploitation of electricity from RES is vital. In addition, the development of highly efficient catalysts for CO2 hydrogenation, able to achieve sufficient CO2 conversions at milder operational conditions is also key towards reducing the energy demand of the process. These aforementioned developments could reduce the emissions of CCU to match CCS across all impact categories. For GWP specifically, while results show that CCU is more CO2 emissive than CCS, due to a percentage not converted and released in the atmosphere, when use of the MeOH product is considered, these emissions are significantly lower. MeOH is already commonly used as feedstock for chemical industries and thus can provide revenues for producers. However, its most beneficial nature is that of a fuel, where it accounts for significantly less emissions than other conventional fuels. MeOH as a fuel sees constant rise in application, mostly in the marine sector, but it still has way to go for a large-scale commercialisation. Therefore, further penetration of MeOH in the transportation sector is crucial. The automotive sector specifically is key, not only due to the size of the industry, but also the direct connection with the public, which can make its benefits more apparent, and facilitate its dissemination.
H2 exploitation specifically is expected to be key in the EU’s green energy transition and the decarbonisation of energy-intensive industries. Replacing fossil fuels with H2 in the different production stages of Al ingot production will significantly reduce the industry’s emissions. As shown by this study’s results, replacement of NG with H2 reduces CO2 emissions while not significantly affecting other impact categories. The study did not consider the replacement of all fossil fuels, nor the exploitation of H2 for electricity production, due to limitations on green H2 production, mostly associated with the high-RES electricity requirements. Beyond production costs, H2 poses an economic burden for transportation and storage, which needs local, national and international collaboration between governments and industry. In addition to that, the limited infrastructure available (i.e., H2 pipelines) for supporting and covering the Al industry’s energy supply demands slows down the industrial use of H2 for thermal energy generation. While this is representative of the current status, such limitations will soon be overcome, with the ongoing development of RES, and the new EU target of 40% of gross final energy consumption covered by RES by 2030.

6. Conclusions

Adopting CO2 emission reduction solutions in the energy intensive primary Al industry is key in achieving the European milestone of zero emissions by 2050. CCS and CCU for MeOH production can lead to significant reductions in CO2 emissions. However, the extensive infrastructure required and the increased energy demand of their operation limits the net CO2 reduction capabilities of these systems, while also significantly increasing the environmental impact in terms of non-CO2 related emissions. On the other hand, exploitation of innovative solutions, such as green H2 combustion, have the potential to further reduce the CO2 emissions of the industry by preventing CO2 formation in the first place, while also accounting for significantly lower environmental impact in terms of non-CO2 emissions.
The LCA conducted in this paper examined the impact of four scenarios in terms of CO2 emissions, but also other emissions to air, as well as freshwater and terrestrial ecotoxicity. Results showed that CCS and CCU accounted for 6.88% and 4.33% reduction in GWP, respectively. These relatively small reductions are due to the CO2 emissions linked with the operation and energy consumption of the carbon capture system, the geological storage process and the MeOH production process. Despite this, when considering global primary Al production and the Gt of CO2 produced annually, this reduction is nonetheless very significant. In addition, while the 6.88% reduction for the case of geological storage, where no products of environmental benefits are produced, is indeed accurate, the 4.33% for the case of CCU does not account for the CO2 emissions averted by the combustion of the environmentally friendlier MeOH. When considering MeOH combustion, then, net GWP reduction is significantly higher. Despite this, the use of carbon capture systems significantly increased the nitrous emissions released in the “purified” stream of the system, thus AP, EP and POF were consequently elevated, with EP specifically climbing as high as 1316.07 kg, in the case of CCU. In addition, the infrastructure construction, the increased energy demand, and material wastes significantly increased FAETP and TETP, especially in the case of CCU, by 7.11% and 32.31%, respectively. Exploitation of green H2 on the other hand appeared to be the most environmentally beneficial option. Replacing NG with green H2 specifically accounted for a 10.76% reduction in GWP. While this reduction may seem relatively modest, one must consider that almost 50% of the CO2 emissions of the conventional production process are indirect due to electricity consumption. NG combustion accounted for 13.69% of the total GWP. EP and POF were also reduced by 1.26% and 2.31%, respectively. In addition, while the impact in all other categories inevitably increased, still it was significantly less than in the cases of CCS and CCU.
It is therefore evident that the utilisation of green H2 technologies is key for the decarbonisation of primary Al production. Promoting green H2 penetration as an energy carrier for energy intensive industries is integral. Further research and development of H2 production and exploitation technologies will also be key in the penetration of H2 in energy-intensive industries. Specifically, optimisation of electrolysis processes using electricity from RES, as well H2 combustion, will allow for the reduction of both the overall energy demand, and thus the indirect emissions of H2 utilisation, as well as the direct emissions of combustion, such as NOx. Energy demand minimisation, specifically, will allow for the reduction of the impact of these technologies in both freshwater and terrestrial ecotoxicity, in addition to air pollution, thus establishing them as one of the most environmentally beneficial options across the board.
The green H2 scheme presented in this study can serve as a basis to further examine the potential of H2 penetration in energy production for the primary Al industry, and energy intensive industries in general, moving from the conservative notion of replacing just NG, to a total discarding of all fossil fuels and the 100% energy coverage from green H2. To this end, thorough techno-economic analyses for the full-on transition to green H2 is the next step, which, combined with robust LCA, will facilitate the development of H2 implementation plans in an EU policy level, heading toward CO2 neutrality goals in 2050.

Supplementary Materials

The following supporting information can be downloaded at:, Figure S1: LCA model for scenario 1, as developed in “Sphera LCA for Experts”, Figure S2: LCA model for scenario 2, as developed in “Sphera LCA for Experts”, Figure S3: LCA model for scenario 3, as developed in “Sphera LCA for Experts”, Figure S4: LCA model for scenario 4, as developed in “Sphera LCA for Experts”.

Author Contributions

Conceptualization, A.P.; methodology, A.P. and C.P.; validation, C.P.; investigation, C.P. and S.K.; writing—original draft preparation, S.K.; writing—review and editing, C.P.; visualization, S.K.; supervision, A.P. and M.T. All authors have read and agreed to the published version of the manuscript.


This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data sources used are cited and all data produced are reported in the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. NG-LGO-HFO (scenario 1) system boundaries.
Figure 1. NG-LGO-HFO (scenario 1) system boundaries.
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Figure 2. Carbon capture and geological storage (scenario 2) system boundaries.
Figure 2. Carbon capture and geological storage (scenario 2) system boundaries.
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Figure 3. CCU for MeOH production (scenario 3) system boundaries.
Figure 3. CCU for MeOH production (scenario 3) system boundaries.
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Figure 4. Green H2 replacing NG (scenario 4) system boundaries.
Figure 4. Green H2 replacing NG (scenario 4) system boundaries.
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Figure 5. LCIA results for all scenarios.
Figure 5. LCIA results for all scenarios.
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Table 1. LCA scenarios.
Table 1. LCA scenarios.
No.Thermal Energy SupplyCO2 Mitigation SolutionCO2 Formation Prevention Solution
1NG, LFO, HFONoneNone
2NG, LFO, HFOCCS geological storageNone
3NG, LFO, HFOCCU for MeOH productionNone
4H2, LFO, HFONoneGreen H2, replacing NG
Table 2. LCIA impact categories.
Table 2. LCIA impact categories.
Impact CategorySelected IndicatorUnit
Climate ChangeGlobal Warming Potential (GWP) (CML 2001)kg CO2 eq.
AcidificationAcidification Potential (AP) (CML 2001)kg SO2 eq.
EutrophicationEutrophication Potential (EP) (CML 2001)kg Phosphate eq.
Photochemical Ozone FormationPhotochemical Oxidant Formation (POF) (ReCiPe)kg NMVOC eq.
Aquatic EcotoxicityFreshwater Aquatic Ecotoxicity Potential (FAETP inf.) (CML 2001)kg DCB eq.
Terrestrial EcotoxicityTerrestrial Ecotoxicity Potential (TETP inf.) (CML 2001)kg DCB eq.
Table 3. LCI for production of 1 tonne of Alumina (for all scenarios, NG replaced with H2 in scenario 4) [8].
Table 3. LCI for production of 1 tonne of Alumina (for all scenarios, NG replaced with H2 in scenario 4) [8].
Material FlowsValueUnit
Caustic soda74.899kg
Calcined lime21.731kg
Fresh water3.518m3
Thermal Energy from HFO33.536MJ
Thermal Energy from LFO11.761MJ
Thermal Energy from NG/H29722.082MJ
Emissions to airValueUnit
Carbon dioxide519kg
Sulfur dioxide0.012kg
Nitrous oxides (as NO2)0.245kg
Emissions to waterValueUnit
Fresh water3.575m3
Suspended solids0.233kg
Emissions to soilValueUnit
Bauxite residues (red mud)846.905kg
Non-hazardous waste58.897kg
Hazardous waste4.982kg
Table 4. LCI for production of 1 tonne of Anode/Paste Production (for all scenarios, NG is replaced with H2 in scenario 4) [8].
Table 4. LCI for production of 1 tonne of Anode/Paste Production (for all scenarios, NG is replaced with H2 in scenario 4) [8].
Material FlowsValueUnit
Fresh water3.255m3
Sea water7.077m3
Calcined Coke950.131kg
Refractory material1.509kg
Thermal Energy from HFO747.607MJ
Thermal Energy from LFO15.884MJ
Thermal Energy from NG/H2501.753MJ
Anode paste1000kg
Emissions to airValueUnit
Sulfur dioxide0.805kg
Nitrous oxides (as NO2)0.324kg
SF65.88 × 10−5kg
Particulate fluoride (as F)0.015kg
Gaseous fluoride (as F)0.014kg
Total polycyclic aromatic hydrocarbons0.003kg
Benzo(a)pyrene1.15 × 10−6kg
Carbon dioxide (non-fuel)74.258kg
Carbon dioxide (fuels)84kg
Emissions to waterValueUnit
Fresh water2.561m3
Sea water7.077m3
Suspended solids0.025kg
Fluoride (as F)0.024kg
Polycyclic aromatic hydrocarbons (6 Borneff components)1.09 × 10−4kg
Emissions to soilValueUnit
Waste carbon or mix11.351kg
Scrubber sludges0.064kg
Refractory (excl. spent pot lining)1.38kg
Non-hazardous waste0.745kg
Hazardous waste3.945kg
Table 5. LCI for production of 1 tonne of liquid Al (electrolysis) (for all scenarios) [8].
Table 5. LCI for production of 1 tonne of liquid Al (electrolysis) (for all scenarios) [8].
Material FlowsValueUnit
Fresh water32.405m3
Sea water76.167m3
Refractory material5.631kg
Anodes (gross)494kg
Cathode carbon12.356kg
Aluminium fluoride17.638kg
Liquid Aluminium1000kg
Emissions to airValueUnit
Sulfur dioxide7.408kg
Nitrous oxides (as NO2)0.678kg
SF61.83 × 10−4kg
Particulate fluoride (as F)0.140kg
Gaseous fluoride (as F)0.288kg
Total polycyclic aromatic hydrocarbons0.017kg
Carbon dioxide (non-fuel)1455.319kg
Emissions to waterValueUnit
Fresh water21.446m3
Sea water78.441m3
Suspended solids0.222kg
Oil and grease/total hydrocarbons0.007kg
Fluoride (as F)0.322kg
Polycyclic aromatic hydrocarbons (6 Borneff components)0.001kg
Emissions to soilValueUnit
Spent pot lining11.986kg
Waste alumina1.976kg
Waste carbon or mix32.732kg
Scrubber sludges1.039kg
Refractory (excl. spent pot lining)0.466kg
Table 6. LCI for production of 1 tonne of Al ingot (casting) (for all scenarios, NG is replaced with H2 in scenario 4) [8].
Table 6. LCI for production of 1 tonne of Al ingot (casting) (for all scenarios, NG is replaced with H2 in scenario 4) [8].
Material FlowsValueUnit
Fresh water7.892m3
Electrolysis metal1000.000kg
Alloy additives18.958kg
Thermal Energy from HFO112.364MJ
Thermal Energy from LFO1.824MJ
Thermal Energy from NG/H2407.657MJ
Aluminium Ingot1000kg
Emissions to airValueUnit
Sulfur dioxide0.034kg
Nitrous oxides (as NO2)0.226kg
Hydrogen chloride0.006kg
Carbon dioxide (non-fuel)30.000kg
Carbon dioxide (fuels)0.001kg
Methane (fuels)1 × 10−4kg
Emissions to waterValueUnit
Fresh water7.608m3
Suspended solids0.008kg
Oil and grease/total hydrocarbons2.10 × 10−5kg
Emissions to soilValueUnit
Refractory (excl. spent pot lining)0.389kg
Filter dust0.017kg
Non-hazardous waste0.022kg
Hazardous waste0.072kg
Table 7. Carbon Capture and Compression of CO2 LCI (for scenarios 2 and 3) [24].
Table 7. Carbon Capture and Compression of CO2 LCI (for scenarios 2 and 3) [24].
Material FlowsValueUnit
Carbon dioxide1000kg/tonne CO2 in
Monoethanolamine (MEA)1.440kg/tonne CO2 in
Caustic soda0.120kg/tonne CO2 in
Activated carbon0.070kg/tonne CO2 in
Water18.100kg/tonne CO2 in
Reboiler duty (Thermal energy)3.2GJ/tonne CO2 in
EMEA (Electricity)33.800kWh/tonne CO2 in
ECP,CO2 (Electricity)64.500kWh/tonne CO2 in
Carbon dioxide captured900kg/tonne CO2 in
Emissions to airValueUnit
Water87.500kg/tonne CO2 in
Carbon dioxide99.900kg/tonne CO2 in
Argon54.800kg/tonne CO2 in
Nitrogen3202.200kg/tonne CO2 in
Oxygen128.300kg/tonne CO2 in
Monoethanolamine (MEA)0.060kg/tonne CO2 in
Ammonia0.030kg/tonne CO2 in
Formaldehyde0.00024kg/tonne CO2 in
Acetaldehyde0.00015kg/tonne CO2 in
Emissions to waterValueUnit
Amine reclaimer waste2.900kg/tonne CO2 in
Table 8. Geological Storage LCI (for scenario 2) [27].
Table 8. Geological Storage LCI (for scenario 2) [27].
Material FlowsValueUnit
Diesel1.405 × 10−5tonne /tonne CO2 stored
Portland cement1.349 × 10−5tonne /tonne CO2 stored
Bentonite1.349 × 10−5tonne /tonne CO2 stored
Chemicals inorganic2.851 × 10−6tonne /tonne CO2 stored
Chemicals organic6.102 × 10−7tonne /tonne CO2 stored
Barite1.821 × 10−5tonne /tonne CO2 stored
Lignite1.349 × 10−8tonne /tonne CO2 stored
Lubricating oil4.047 × 10−6tonne /tonne CO2 stored
Reinforcing steel1.416 × 10−5tonne /tonne CO2 stored
Transport, freight, rail0.033tonne /tonne CO2 stored
Transport, lorry5.47 × 10−3tonne /tonne CO2 stored
Drilling waste disposal to landfarming1.598 × 10−5tonne /tonne CO2 stored
Drilling waste disposal to residual material landfarming1.066 × 10−5tonne /tonne CO2 stored
Hazardous waste (disposal to hazardous waste incineration)3.372 × 10−7tonne /tonne CO2 stored
Electricity6.68kWh/tonne CO2 stored
Table 9. CO2 to MeOH production LCI (for scenario 3) [6].
Table 9. CO2 to MeOH production LCI (for scenario 3) [6].
Material FlowsValueUnit
Aluminium oxide0.012kg
Zinc oxide0.029kg
Copper oxide0.062kg
Carbon dioxide1455kg
Emissions to airValueUnit
Carbon dioxide20.200kg
Emissions to waterValueUnit
Waste water (kg)630kg
Table 10. Hydrogen Combustion LCI (for scenario 4) [26].
Table 10. Hydrogen Combustion LCI (for scenario 4) [26].
Material FlowsValueUnit
Thermal Energy2.70kWh
Emissions to airValueUnit
Nitrous Oxides8.37 × 10−7kg
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Peppas, A.; Politi, C.; Kottaridis, S.; Taxiarchou, M. LCA Analysis Decarbonisation Potential of Aluminium Primary Production by Applying Hydrogen and CCUS Technologies. Hydrogen 2023, 4, 338-356.

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Peppas A, Politi C, Kottaridis S, Taxiarchou M. LCA Analysis Decarbonisation Potential of Aluminium Primary Production by Applying Hydrogen and CCUS Technologies. Hydrogen. 2023; 4(2):338-356.

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Peppas, Antonis, Chrysa Politi, Sotiris Kottaridis, and Maria Taxiarchou. 2023. "LCA Analysis Decarbonisation Potential of Aluminium Primary Production by Applying Hydrogen and CCUS Technologies" Hydrogen 4, no. 2: 338-356.

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