The use of fossil fuels is known to be the primary source of carbon dioxide (CO₂) emissions, accounting for more than half of global greenhouse gas (GHG) emissions into the atmosphere [1]. Global carbon emissions from fossil fuels have been growing rapidly since 1900 [2]. Despite the introduction of clean energy technologies that have helped to reduce some of the potential GHG emissions, global energy-related carbon emissions have shown little sign of remission, reaching a new high in 2022 of more than 36.8 Gt [3]. Meanwhile, unfortunately, present efforts to optimize fossil fuels combustion and replace fossil fuel with renewable energy sources are unlikely to be sufficient for counteracting this continued growth in carbon emissions in the near future [4].

In this context, carbon capture, utilization, and storage (CCUS) technologies have shown promise for directly reducing the amount of CO₂ released to the atmosphere from significant point sources. In addition to decarbonizing power generation, CCUS is considered a cost-effective solution to mitigate the CO₂ released as a by-product of manufacturing processes from iron, steel, cement, and chemical industries [5]. As of July 31, 2023, the number of commercial CCS facilities in development, construction, or operation is reported to be 392 worldwide (among which 41 are operational) with a total carbon capture capacity of 361 Mtpa, reflecting a 50% increase from 2022 [6].

As mentioned above, the CO₂ feedstock for CCUS technology comes from point sources where the technology is deployed, such as power generation plants or industrial facilities. Usually, these facilities are located away from residential areas or outside dense urban spaces where land is scarce. As such, relatively less research exists on the feasibility and GHG reduction efficiency of CCUS deployment in point sources located within urban areas, despite that an estimated 75% of CO₂ emissions from global final energy use arise from these densely populated spaces [7]. One of the significant point sources of CO₂ emissions located within cities is combined heating and power (CHP) plants, which are known to have higher fuel efficiency and lower environmental impact than other combustion-based power generation methods, as they simultaneously produce heat and power in an integrated process using a common energy source [8]. As such, more cities worldwide have been installing CHP plants for their heating and power needs. Since 2010, the total capacity of CHP plants and installations worldwide has risen by more than 25% in terms of their combined heat output [9].

In the case of South Korea, as of 2021, 99 natural gas-fired power units produce 41.3 GW of power, among which 28 are combined heating and power plants accounting for 7.4 GW of electricity production with further plans for expansion according to the government’s future energy plans [10]. To address the need for a CCUS system that could help reduce the GHG emissions from CHP plants, this study presents an analysis of the GHG emissions reduction efficiency of an integrated CCUS system that can be installed within the CHP plant site performing through a life cycle assessment (LCA). The integrated CCUS system was developed in South Korea as a government-funded project and is currently in construction for full-scale operation after being tested on a pilot scale for three years from 2018 to 2021. This system integrates three different CCUS technologies-microalgae photocultivation, CO₂ membrane separation, and carbon mineralization–to capture and utilize CO₂ in the flue gas from the CHP plants fueled by liquefied natural gas (LNG). Using the pilot stage data, this study aims to perform and quantify the GHG emissions and global warming potentials of the individual processes by LCA. The system’s GHG emissions reduction performance in the entire life cycle system is compared to other existing CCUS technologies.

LCA is a technique outlined by the ISO 14044: 2006 and ISO 14040: 2006 standards for quantifying inputs and emissions throughout the entire life cycle of a product or process to evaluate the overall environmental impact. A typical LCA is carried out in four stages: defining the goal and scope, analyzing the life cycle inventory (LCI), assessing the environmental impact throughout the lifecycle, and interpreting the results of the LCA. LCA is widely applied in various areas and contexts, especially for systematically comparing the environmental impact of different options, such as other waste management strategies and systems [11–18].

LCA is also being increasingly applied as an analytical method to understand the environmental benefits of CCUS technologies, especially since 2007, in line with rising concerns about the need to mitigate global climate change and the quantification of GHGs released into the global atmosphere [19]. LCA studies have been conducted on individual technologies, such as direct air carbon capture and storage [20–21] and membrane-based carbon capture [22], as well as systems integrating anthropogenic CO₂ sources and CCUS technologies, including CO₂-enhanced oil recovery [23], combustion-based electricity generation systems integrated with CCS [24], and many more.

In a recent review of LCA studies on CCUS technologies, Chelvam and Hanafiah [25] point out that carbon capture technologies can indeed reduce GHG emissions to a significant degree. However, they also have the potential to increase other environmental burdens (e.g., acidification, eutrophication, and ecotoxicity) depending on their method, energy penalty, and the rate of nitrogen oxides (NOx) emitted from the CCUS facility. According to the LCA case studies by Edge Environment for a report undertaken by Parsons Brinckerhoff and the Global CCS Institute (SI Table 1 in Supporting information), the ton-CO₂ eq Emitted/Reusing Act of 1 ton-CO₂ computed for different CCU technologies were as follows, in the order of highest to lowest value: Polymers (5.52), formic acid production (3.96), urea yield boosting (2.27), CO₂ concrete curing (2.2), renewable methanol (1.71), enhanced geothermal systems (0.58), Bauxite residue carbonation (0.53), enhanced oil recovery (0.51), enhanced coal bed methane (0.44), algae cultivation (0.42), and carbonate mineralization (0.32) [26]. When the value of ton-CO₂ eq Emitted/Reusing Act of 1 ton-CO₂ is one or above, it means that the process emits more than 1 ton of CO₂ to remove 1 ton of CO₂, while when the value is less than 1, the technology is capable of removing more CO₂ than it emits in the process. Among the case studies evaluated in this report, algae cultivation and carbon mineralization exhibited the lowest values, indicating their efficiency in reducing carbon emissions. Thus, the integrated CCUS system incorporating these two technologies will show a positive GHG emissions reduction effect.

Fig. 1 presents the schematic of the integrated CCUS system examined in this study. The system is installed within the site of the CHP plant and has a compact size.

The flue gas from the CHP plant is sent to the integrated CCUS system via a set of infrastructure: a fixture at the side of the stack (chimney) that extracts the flue gas before it is emitted to the atmosphere, pipelines connecting the stack and the CCUS system, a powerful blower that pushes the flue gas through those pipes, and a separate branch for testing the system. The top of the stack is 100 meters high, from which the flue gas is extracted and sent to ground level. A high-pressure ring blower is coupled with a device for maintaining a constant pressure resistance to ensure a continuous flow rate, and the flue gas supply is measured by comparing the flow meter mounted on the ring blower and the flow meter mounted on the integrated CCUS system.

When the flue gas arrives at the integrated CCUS system, a part of the gas is bypassed for utilization as feedstock for growing microalgae to convert CO₂ into biomass. Microalgae grow quickly in light and convert CO₂ into organic carbon in their growth process, fixing carbon 10–50 times more efficiently than terrestrial plants [27]. Due to this ability of microalgae, microalgae-based processes have emerged as one of the most promising long-term platforms for biological CCU [27]. The set-up for microalgae photocultivation using LNG flue gas and natural light consisted of photobioreactors equipped with TAP-C culture medium. The total capacity of the photobioreactors was 5 tons. In 2,000 hours, this set-up can produce 20 kg of biomass from growing Chlorella sorokiniana based on repeated batch cultivation using a one-stage continuous culture strategy, which adds up to 87.6 kg/year of biomass. After sufficient microalgae growth, 95% is harvested using centrifugation, and the remaining 5% of biomass and the culture medium are reused. More than 90% of the medium solution is reused to minimize water use and maximize water reuse.

The remaining elements of the flue gas after the microalgae photocultivation process are sent back into the main flue gas pipeline, where it joins the rest of the flue gas (i.e., three fourth of total incoming flue gas) to reach the CO₂ separation and capture process using membrane technology. Many existing CCS facilities utilize absorbent agents to capture CO₂, which requires considerable land for installation and are usually developed for processing flue gas from coal-fired plants. Moreover, significant amounts of chemicals and energy use are necessary for regenerating the absorbents, making it less than ideal for further industrial installation [28]. CO₂ membrane separation technology, on the other hand, has been emerging as a promising technology due to its high energy efficiency and minor physical and chemical footprint, as well as its being absorbent-free [28]. This particular process is comprised of a multi-stage membrane system that is continuously operated at an operating pressure ranging from 2 to 5 bar at an operating temperature ranging from room temperature to 50°C.

The concentrated CO₂ is then sent to the carbon mineralization process for conversion into calcium carbonate (CaCO₃) through spontaneous reactions between CO₂ and divalent cations (e.g., Ca²⁺, Mg²⁺, or Fe²⁺) [29]. In recent years, active research has been done on utilizing industrial residue materials and byproducts (e.g., fly and bottom ash from furnace operations, cement kiln dust, and waste concrete, mining waste, etc.) as a source of Ca and Mg cations for carbon mineralization due to their ready availability, low cost, and proximity to significant CO₂ emission sources [30]. The carbon mineralization process of the integrated CCUS system also utilizes an alkaline aqueous solution recovered from industrial waste consisting of abundant (about 30%) CaO and MgO to source the divalent cations for direct carbon mineralization in a slurry, which eliminates the need for absorption agents.

In this study, the flow of the integrated CCUS system takes the order of ‘microalgae photocultivation – CO₂ membrane separation – carbon mineralization,’ however, it is also possible to place the microalgae photocultivation process at the very end, that is, in the order of ‘CO₂ membrane separation – carbon mineralization–microalgae photocultivation.’ In this case, rather than bypassing a part of the flue gas to the microalgae photocultivation process at the beginning, the microalgae photocultivation process becomes responsible for reducing the CO₂ in the flue gas that was insufficiently sequestered by the other two processes.

The goal and scope of this study are clearly defined, which is to verify the GHG emissions reduction effect of the integrated CCUS system from CO₂ source to the final form as a helpful product or service. The life cycle inventories (LCIs) for all three processes of the integrated CCUS system were put together in terms of their inputs (such as the usage of fuel and chemicals, the amount and composition of flue gas processed, electricity and water use, etc.) and outputs (including gas and water emissions and resource recovery, etc.). The data for the following components were obtained from the LCIs in the national LCI database provided by the South Korean government: truck transportation, energy production, chemicals, water supply, calcium carbonate, and wastewater treatment. The inputs and outputs of each process and that of the flue gas supply infrastructure were analyzed and computed regarding CO₂ emissions utilization, and the detailed tables are provided in Table 1 and Supplementary Materials. Table 1 shows the flue gas supply device with 125,000 m³/hr input flow gas and 400 Nm³/hr output flue gas sent to the integrated CCUS system.

As shown in Table S2, the first stage of the integrated CCUS system is the microalgae photocultivation process, which receives 50 Nm³/hr of flue gas, wherein CO₂ takes up 2.1 Nm³/hr (4.2%). The significant inputs are electricity, chemicals, and water, while the outputs include discharged wastewater and excess flue gas after use in algae cultivation. The photobioreactors consume a considerable amount of electricity (406 kWh/year), and a variety of chemicals are used for the culture medium, among which NH₄Cl, sodium bicarbonate, K₂HPO₄, and KH₂PO₄ make up the most significant ratios. The water used in this process is 0.13 kg per 1 ton of CO₂ emissions utilized. However, 80% of the water is recycled, reducing the wastewater discharge to 0.03 kg per 1 ton of CO₂ emissions used. Automized monitoring of long-term operations has shown a 39.6–80.05% CO₂ reduction effect, and the amount of CO₂ emissions utilized in this stage was calculated based on a 39.6% reduction efficiency.

Table S3 in SI shows the inputs and outputs of the CO₂ membrane separation process. The CO₂ membrane separation was set up to process 24,500 Nm³/hr of flue gas at total capacity; however, in the pilot stage, the actual amount of flue gas processed in this process was 1,200 Nm³/hr, of which CO₂ accounted for 4.5% (840 tons/year). The membrane-based CO₂ separation process selectively concentrates the CO₂ in the incoming flue gas, reducing the total volume of gas passing through to 57 Nm³/hr, about 80% of which is CO₂. The only input required for this process is electricity, consuming 692.6 kWh per 1 ton of CO₂ emissions utilized when operated at total capacity. As such, there are no air or water emissions. The CO₂-enriched flue gas can also be produced as liquid carbon dioxide if linked to a liquefaction process, which is not applied in this integrated CCUS system.

The amount of CO₂-enriched mixed gas from the CO₂ membrane separation process that reaches the carbon mineralization process is 884 tons/year, which is composed of 20% nitrogen and 80% carbon dioxide (pure CO₂ input: 707 tons/year). The CO₂ enters and becomes processed at a pace of 57 Nm³/hr, and 99% (700 tons/year) is mineralized, in other words, fixed as calcium carbonate (CaCO₃). This process utilizes recycled industrial waste materials as the alkaline residues for mineralizing CO₂, and 4.3 kg 1 ton of CO₂ emissions utilized (3,009.6 tons/year) of aqueous solution recovered from industrial waste materials (25% CaO concentration) is used. The electricity used to run the process is 25.7 kWh per 1 ton of CO₂ emissions utilized. The amount of CaCO₃ produced annually is 1,454.8 tons/year, which is equivalent to 2,078 kg per 1 ton of CO₂ emissions used. Transportation of the mineralized carbon (CaCO₃) by truck was also included in the inputs, assuming a distance of 10 km (20 km round trip) between the testing facility and the product destination. The final mixed gas emitted into the atmosphere after this process comprised 6 other substances (SOx, NOx, dioxin, CO, HCI, and dust) in addition to CO₂, which take up the bulk of the mixed gas (see Table S4).

The LCA characterized the global warming potential of the integrated CCUS system using methods outlined by IPCC (1996) [1]. The detailed results of the LCA and the GHG emissions reduction effect of each stage are described below, followed by an evaluation of the whole system. Accordingly, the functional unit used in the LCA performed in this study was 1 kg CO₂ eq/ton [1] The carbon offsetting effect was evaluated by adding up the amount of CO₂ emissions utilized through conversion into calcium carbonate (CaCO₃), which is the product outcome of the CCUS system. The flue gas supply device was excluded from the scope of the LCA since this stage does not involve any CO₂ removal. The software used for the LCA is TOTAL 5.0, which was developed and distributed by the South Korean Ministry of Environment [31].

This study presented an integrated CCUS system currently being tested in South Korea that incorporates three different CCUS technologies: microalgae photocultivation, CO₂ membrane separation, and carbon mineralization. LCAs were performed to understand the global warming potential of the individual processes and the whole system. The total GHG emissions of the integrated CCUS system, excluding the carbon offsetting effect, were assessed to be 358 kg CO₂ eq/ton, 95.8% (343 kg CO₂ eq/ton) of which was emitted during the CO₂ membrane separation process. The GHG emissions of the microalgae photocultivation and carbon mineralization processes were 14.6 kg CO₂ eq/ton (4.1%) and 0.371 (0.1%), respectively. When the carbon offsetting effect of CaCO₃ production (−103 kg CO₂ eq/ton) and the CO₂ emissions utilized during the entire process (−1000 kg CO₂ eq/ton) were accounted for, the total GHG emissions of the integrated CCUS system came out to be −745kg CO₂ eq/ton.

Although further studies on the economic feasibility are necessary to determine the path to commercialization for this system, this study objectively verified the integrated CCUS system’s high GHG emissions reduction efficiency through an LCA using global warming characterization factors, thereby widening the applicability of CCUS technologies for CHP plants in urban spaces.