Distributed carbon capture in urban environments: Emerging architectures for building-integrated CO₂ Removal

 Corresponding author: Joshua Babatunde Asere. Email: Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Distributed carbon capture in urban environments: Emerging architectures for building-integrated CO₂ Removal Joshua Babatunde Asere 1, *, Adepeju Nafisat Sanusi 2, Muhammad Jumada Auwal 3, Ademayowa Isaac Adejumobi 4, Joseph Tosin Salako 5, Jonathan Mathias 6 and Funsho Adewale Oyekunle 7 1 Department of Environmental Science, Indiana University Bloomington, Indiana, USA. 2 Department of Management Science, Catholic University of America Washington DC, USA. 3 Department of Chemistry, Sule Lamido University Kafin Hausa, Nigeria. 4 Department of Geology, University of Benin, Nigeria. 5 Department of Civil Engineering, Federal University of Technology Akure, Ondo State. 6 Department of Mechanical Engineering, Federal Polytechnic Auchi, Nigeria. 7 Department of Geology and Mineral Science, University of Ilorin, Nigeria. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 Publication history: Received on 14 June 2025; revised on 21 July 2025; accepted on 23 July 2025 Article DOI: https://doi.org/10.30574/gjeta.2025.24.1.0227 Abstract Urban environments, responsible for ~40% of global CO₂ emissions, demand innovative carbon capture, utilization, and storage (CCUS) solutions to achieve net-zero targets. Distributed carbon capture, integrating CO₂ removal into urban infrastructure, offers a scalable alternative to centralized systems, leveraging buildings as capture hubs. This review explores emerging architectures for building-integrated CO₂ removal in urban settings, focusing on modular direct air capture (DAC) units, CO₂-absorbing materials, HVAC-integrated systems, and hybrid urban-green infrastructures. Modular DAC units enable retrofitting, capturing CO₂ via sorbents like metal-organic frameworks, while carbonabsorbing concrete and algae-based facades provide passive sequestration. HVAC systems utilize existing airflow for efficient capture, and hybrid systems combine with urban forests for synergistic benefits. The paper synthesizes recent advancements, evaluating technical feasibility, energy demands, and economic viability. Case studies, including Climeworks’ Zurich DAC (900 tons CO₂/year) and Hamburg’s BIQ House algae facades, illustrate practical applications but highlight scalability limits. Environmental co-benefits, such as improved air quality, and social impacts, including public perception, are assessed. Challenges, ranging from high energy costs, urban policy gaps, to CO2 storage constraints persist, necessitating material innovations and regulatory incentives. This review underscores distributed CCUS’s potential to decarbonize cities, proposing a roadmap for scaling through interdisciplinary collaboration. By addressing technical and societal barriers, building-integrated CO₂ removal can redefine urban climate strategies, aligning with global sustainability goals. Keywords: Integrated Carbon Capture; Distributed Carbon Capture; Direct Air Capture (DAC); Urban Decarbonization; CO₂-Absorbing Materials; HVAC-Integrated CO₂ Removal; Algae-Based Facades; Net-Zero Cities 1. Introduction The escalating concentration of atmospheric carbon dioxide (CO₂), driven by anthropogenic activities, poses a critical threat to global climate stability, necessitating urgent mitigation strategies. As of 2023, global CO₂ emissions reached approximately 37.4 billion tons annually, with projections indicating further increases without aggressive intervention [1]. Carbon capture, utilization, and storage (CCUS) has emerged as a pivotal technology to curb emissions, complementing renewable energy and efficiency measures. Traditional CCUS systems, primarily deployed in industrial Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 152 settings like power plants, capture CO₂ at point sources for storage or utilization. However, these centralized approaches often require extensive infrastructure and are less adaptable to diverse emission sources, limiting their applicability in densely populated areas where innovative solutions are most needed [1-3]. Urban environments, accounting for nearly 70% of global CO₂ emissions, present unique challenges and opportunities for climate mitigation. Cities, characterized by high emission densities from transportation, buildings, and industrial activities, face spatial constraints that hinder the deployment of large-scale CCUS facilities [4]. Moreover, urban energy demands and infrastructure complexity exacerbate the difficulty of integrating conventional capture technologies. For instance, buildings alone contribute approximately 40% of urban emissions, underscoring the need for solutions tailored to urban landscapes [5]. The concentration of emissions in cities, coupled with limited land availability, demands decentralized approaches that can seamlessly integrate with existing infrastructure to achieve meaningful CO₂ reductions [4-6]. Distributed carbon capture offers a promising alternative to centralized CCUS by deploying smaller-scale, localized systems across urban areas [2]. Unlike industrial-scale capture, which targets high-concentration CO₂ streams, distributed systems focus on diffuse sources, including ambient air and building emissions. This approach enhances flexibility, reduces the need for extensive CO₂ transport networks, and aligns with urban spatial constraints [7]. Recent advancements in direct air capture (DAC) and adsorption-based technologies have enabled the development of compact capture units suitable for urban deployment. By dispersing capture systems across multiple sites, distributed CCUS can address emissions at their source, potentially transforming cities into active participants in global decarbonization efforts [6,8]. A particularly innovative subset of distributed CCUS involves building-integrated CO₂ removal, where capture technologies are embedded within urban infrastructure, such as building facades, HVAC systems, or construction materials [9]. This approach leverages buildings as platforms for CO₂ capture, utilizing their surfaces, airflow, or structural components to sequester carbon. For example, CO₂-absorbing concrete incorporates carbonation processes to store CO₂ permanently, while algae-based facades use photosynthesis to capture emissions. HVAC-integrated systems exploit existing ventilation networks to capture CO₂ from indoor or outdoor air, offering retrofitting potential. Buildingintegrated systems not only address emissions but also enhance urban aesthetics and functionality, aligning with sustainable city planning [10]. The novelty of building-integrated CO₂ removal lies in its ability to reframe urban infrastructure as a climate solution, shifting CCUS from industrial to societal applications. While industrial CCUS targets large emitters, urban systems address diffuse sources, offering scalability through widespread adoption [11]. However, challenges such as energy efficiency, cost, and integration with urban planning remain underexplored, necessitating a comprehensive review of emerging technologies and their feasibility [11]. This paper fills this gap by synthesizing recent advancements in distributed CCUS, with a focus on building-integrated architectures, and evaluating their potential to decarbonize urban environments. The objective of this review is to critically assess emerging architectures for building-integrated CO₂ removal in urban settings, including modular DAC units, CO₂-absorbing materials, HVAC-integrated systems, and hybrid urban-green infrastructures. By analyzing technical feasibility, economic viability, and case studies, the paper aims to elucidate the role of distributed CCUS in urban climate strategies. It also explores environmental and social impacts, identifying cobenefits like improved air quality and challenges such as public acceptance. The review is structured as follows: Section 2 provides a literature review of CCUS and urban challenges; Section 3 details emerging architectures; Section 4 presents case studies; Section 5 evaluates feasibility; Section 6 discusses impacts; Section 7 outlines challenges and future directions; and Section 8 concludes with recommendations for scaling urban CCUS. 2. Background and Literature Review 2.1. CCUS Overview Carbon capture, utilization, and storage (CCUS) is a suite of technologies aimed at mitigating climate change by capturing carbon dioxide (CO₂) from industrial processes or the atmosphere, either storing it underground or repurposing it into valuable products. According to Greig et al. [12], CCUS is indispensable for achieving net-zero emissions, particularly in sectors like cement, steel, and power generation, where decarbonization is challenging. Post-combustion capture, which employs chemical solvents like amines to extract CO₂ from flue gases, dominates industrial applications due to its compatibility with existing infrastructure. Findings from Raganati & Ammendola [13] indicate that post-combustion systems achieve capture efficiencies of 85–90%, but their energy penalty, often 20–30% of plant output, limits cost- Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 153 effectiveness. Pre-combustion capture, involving fuel gasification to isolate CO₂, and oxy-fuel combustion, which produces a CO₂-rich exhaust, offer alternatives but require significant capital investment [14]. Direct air capture (DAC), an emerging technology, removes CO₂ from ambient air using solid sorbents or liquid absorbents. According to McQueen et al. [15], DAC systems, while versatile, face high costs and energy demands, hindering large-scale deployment. Centralized CCUS systems, designed for point sources, rely on extensive infrastructure, making them less adaptable to urban environments where spatial constraints and diffuse emissions predominate. Recent advancements in material science and energy efficiency are shifting focus toward decentralized solutions, setting the stage for urban applications. 2.2. Urban Emission Challenges Urban areas are critical battlegrounds for climate mitigation, contributing approximately 70% of global CO₂ emissions due to concentrated activities in transportation, industry, and buildings. According to Ürge-Vorsatz et al. [16], buildings account for nearly 40% of urban emissions, driven by heating, cooling, and electricity use. Cities face unique barriers to CCUS adoption, including limited land for large capture facilities and high energy demands that complicate integration. Findings from Iwuanyanwu et al. [17] highlight that urban sprawl, aging infrastructure, and dense populations exacerbate these challenges, as retrofitting conventional capture systems is often impractical. Moreover, urban air quality issues, exacerbated by co-pollutants like particulate matter, necessitate technologies that address multiple environmental concerns. According to Chen et al. [18], urban emission inventories reveal significant variability, with some cities emitting up to 100 tons of CO₂ per capita annually, underscoring the need for localized solutions. The compact nature of urban landscapes demands innovative, space-efficient CCUS approaches that can integrate with existing infrastructure, such as buildings and public spaces, to achieve meaningful reductions without disrupting city functions. 2.3. Distributed CCUS Distributed CCUS involves deploying small-scale, localized capture systems across urban areas, contrasting with centralized facilities that target large point sources. According to Lackner and Guelpa [19], distributed systems are ideal for cities, as they can capture CO₂ from diffuse sources, including ambient air and building emissions, without requiring extensive pipeline networks. Direct air capture technologies, using amine-based sorbents or metal-organic frameworks (MOFs), are central to this approach, offering modularity and adaptability. Findings from Chowdhury et al. [7] suggest that DAC costs could decline to below $200 per ton of CO₂ by 2030 with advancements in sorbent efficiency and renewable energy integration. Adsorption-based systems, which cycle between CO₂ capture and release using temperature or pressure swings, show promise for urban deployment. According to Li and Yao [20], temperaturevacuum swing adsorption in DAC units achieves capture efficiencies of 80–90%, but energy demands remain a hurdle. Challenges include limited urban-scale pilots and high operational costs, particularly in energy-constrained cities. Research by McQueen et al. [15] emphasizes that distributed CCUS could reduce urban emissions by 5–10% if integrated with renewable energy and urban planning, highlighting its transformative potential. The flexibility of distributed systems positions them as a cornerstone for urban decarbonization strategies. 2.4. Building-Integrated Systems Building-integrated CO₂ removal embeds capture technologies within urban infrastructure, such as building facades, HVAC systems, or construction materials, to address emissions at their source [21,22]. This technology, tested in pilot projects, offers permanent sequestration but faces scalability challenges due to production costs [22]. Algae-based facades, which capture CO₂ via photosynthesis, have been implemented in structures like Hamburg’s BIQ House [23]. Findings from Mahmood et al. [24] indicate that algae systems not only reduce CO₂ but also produce biomass for biofuels, though maintenance and energy inputs limit adoption. HVAC-integrated systems, using sorbents or membranes, leverage buildings’ airflow to capture CO₂ efficiently. According to Rossi et al. [25], membrane-based HVAC systems achieve capture rates of 0.5–1 kg CO₂ per hour in high-traffic buildings, but retrofitting costs and energy penalties remain barriers. Research by Cao et al. [26] highlights that life-cycle assessments of building-integrated systems reveal trade-offs between capture capacity and material durability, necessitating material innovations. Gaps persist in scaling these technologies, optimizing energy use, and integrating them with urban policy frameworks, underscoring the need for interdisciplinary research to advance urban CCUS. 3. Emerging Architectures for Building-Integrated CO₂ Removal This section explores the innovative architectures driving building-integrated CO₂ removal, a cornerstone of distributed carbon capture in urban environments. By embedding capture technologies within buildings, these systems transform urban infrastructure into active climate solutions, addressing diffuse emissions while leveraging existing structures. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 154 Table 1 Comparison of Building-Integrated CO₂ Capture Technologies Technology Capture Mechanism CO₂ Capture Rate Energy Demand (GJ/ton CO₂) Capital Cost (USD/ton CO₂) Operational Cost (USD/ton CO₂) Environment al Co-Benefits Integration Complexity Scalability Potential Reference Modular DAC Units Solid sorbents (e.g., amines, MOFs) or liquid absorbents 0.5–2 tons/year/ unit 4–6 $200,000– $500,000/unit $100–$200 Air quality improvement (with filters) Low (plugand-play design) High (rooftop/retrofitti ng) [15, 27, 30, 33] CO₂Absorbing Concrete Chemical carbonation (calcium carbonate formation) 10–15 kg/m³ Minimal (passive) 10–20% higher than standard concrete Negligible Enhanced material durability, reduced cement emissions Low (standard constructio n) Moderate (CO₂ supply limits) [35, 36, 37, 68] Algae-Based Facades Photosynthesi s (bioreactors) 4–6 kg/m²/yea r 0.5–1 (nutrient delivery, maintenanc e) $500– $1,000/m² $50– $100/m²/ye ar Air quality improvement, urban heat reduction (1– 2°C), biomass production High (specialized maintenanc e) Moderate (cost, maintenance) [23, 38, 40, 42] HVACIntegrated Systems Membranes or sorbents in ventilation ducts 0.5–1 kg/hour 2–3 $50,000– $100,000/syste m $50–$150 Indoor air quality improvement, reduced CO₂ levels Moderate (retrofitting required) High (existing infrastructure) [10, 44, 46, 49] Hybrid Urban Systems Combination of DAC, algae, and green infrastructure 5–10 kg/m²/yea r 1–3 (combined systems) $300–$800/m² $50– $200/m²/ye ar Air quality improvement, urban heat mitigation, biodiversity enhancement High (coordinatio n needed) Moderate (planning complexity) [54, 55, 57, 62] Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 155 This section examines four key approaches—modular direct air capture (DAC) units, CO₂-absorbing materials, HVACintegrated systems, and hybrid urban-green infrastructures—detailing their technological mechanisms, integration potential, and role in urban decarbonization. To provide a comprehensive overview, Table 1 compares their technical performance, economic costs, and environmental benefits, setting the stage for a detailed analysis of each architecture’s feasibility and scalability. 3.1. Modular CO₂ Capture Units Modular CO₂ capture units represent a transformative approach to distributed carbon capture, enabling CO₂ removal from ambient air or building emissions in urban environments. These compact, scalable systems, often based on direct air capture (DAC) technologies, can be integrated into rooftops, facades, or interior spaces of buildings, addressing spatial constraints in dense cities. According to Ozkan et al. [27], modular DAC units using solid sorbents like aminefunctionalized materials achieve capture efficiencies of 85–90%, offering a viable solution for urban decarbonization. Their flexibility supports deployment across diverse building types, from commercial high-rises to residential complexes, positioning buildings as active contributors to climate mitigation. The development of modular units reflects a paradigm shift toward decentralized CCUS, aligning with the need for localized, adaptable solutions in high-emission urban settings. To illustrate the operational mechanism of modular direct air capture (DAC) units, Figure 1 presents a schematic of a liquid solvent-based DAC system, detailing the process from air intake to CO₂ storage or utilization, highlighting its potential for urban building integration. Figure 1 Schematic of Direct Air Capture (DAC) Liquid Solvent System. This figure illustrates the process flow of a liquid solvent-based DAC system, showing air intake, CO₂ capture, sorbent regeneration, and CO₂ storage/utilization stages. It provides a clear visual of the technical mechanism, ideal for explaining modular DAC units. Reproduced with permission from Walker III et al. [21] 3.1.1. Technology and Operation Modular CO₂ capture units typically utilize solid sorbents, such as amines or metal-organic frameworks (MOFs), or liquid absorbents like potassium hydroxide, which cycle through adsorption and desorption phases to capture and release CO₂. According to Gholami et al. [28], temperature-vacuum swing adsorption (TVSA) systems, which heat sorbents to desorb CO₂, require 4–6 GJ per ton of CO₂, posing significant energy challenges for urban applications. Recent advancements in MOFs, such as MIL-101(Cr), have enhanced efficiency, achieving CO₂ uptake of 1.5–2 mmol/g under ambient conditions [29]. Findings from Lashaki et al. [30] demonstrate that MOFs offer high selectivity and stability, leading to higher energy penalties reduction compared to traditional amine-based sorbents. These units are designed with compact footprints (1–2 m²), enabling seamless integration into building designs without structural modifications [30]. For example, Climeworks’ modular DAC units, deployed in urban pilots in Zurich, capture 50–100 kg CO₂ annually per unit, showcasing operational feasibility in dense urban environments [30,31]. Research efforts are focused on optimizing sorbent regeneration cycles and integrating renewable energy sources, such as solar or geothermal, to enhance sustainability and reduce operational costs. The operational performance of modular units is influenced by environmental factors, such as humidity and temperature, which are prevalent in urban settings. According to Ozkan et al. [27], high humidity can reduce sorbent efficiency by 10–15%, necessitating adaptive control systems to maintain performance. Innovations in hybrid sorbents, combining amines with nanomaterials, are being explored to improve capture rates under variable conditions. These systems also incorporate modular cartridge designs, allowing easy sorbent replacement every 6–12 months, which simplifies maintenance. Ongoing research aims to develop standardized designs that accommodate diverse urban Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 156 climates and building architectures, ensuring scalability and reliability across global cities. To better understand DAC and a few of the costs and energy demands that are required, a visual representation is shown in Figure 2, Figure 2 DAC Technologies (A) Liquid-Precipitate Cycle, (B) Liquid Adsorbent, (C) Solid Adsorbent Cycle. This figure illustrates three DAC technology cycles: (A) Liquid-Precipitate Cycle, involving CO₂ capture with a precipitate-forming solvent; (B) Liquid Adsorbent, using a liquid sorbent for CO₂ absorption; and (C) Solid Adsorbent Cycle, utilizing solid sorbents like amines or MOFs for capture and regeneration. Steam and vacuum values in (C) are included in only one of their respective boxes. Not included in (B) are the energy requirements from recirculation pump and blower work, which gives the overall system an energy requirement of 5.23 GJ/tCO2. Steam and vacuum values in (C) are included in only one of their respective boxes. Not included in (B) are the energy requirements from recirculation pump and blower work, which gives the overall system an energy requirement of 5.23 GJ/tCO2. Reproduced with permission from Ozkan et al. [14] Under a Creative Commons license CC BY-NC-ND 4.0 3.1.2. Advantages and Challenges Modular CO₂ capture units offer significant advantages, including scalability, adaptability, and compatibility with existing urban infrastructure. According to McQueen et al. [15], a single modular DAC unit can capture 0.5–2 tons of CO₂ annually, and networks of units across a city could reduce urban emissions by 5–10% in high-density areas. Their plug- Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 157 and-play design minimizes retrofitting costs, compared to centralized CCUS systems costing millions [15]. Pilot projects in European cities have demonstrated that these units can be powered by building-integrated renewable energy, such as solar panels, reducing operational emissions [32]. Additionally, captured CO₂ can be utilized in applications like synthetic fuel production or greenhouse agriculture, creating economic incentives for urban deployment. However, challenges related to energy efficiency and cost remain significant barriers. Findings from DiMartino et al. [33] indicate that DAC units incur operational costs of $100–$200 per ton of CO₂, driven by energy demands for sorbent regeneration. Urban buildings, often constrained by energy availability, may struggle to accommodate these demands without compromising other functions. Maintenance costs, including sorbent replacement every 6–12 months, add $5,000–$10,000 annually per unit, limiting scalability. According to Sun et al. [34], integrating low-energy sorbents and renewable energy could reduce costs, but these solutions require further development. Research gaps include improving sorbent durability, reducing energy penalties, and developing standardized installation protocols to enable widespread adoption in diverse urban contexts. 3.2. CO₂-Absorbing Building Materials CO₂-absorbing building materials embed carbon capture into construction elements, such as concrete or facade coatings, enabling passive CO₂ sequestration without additional infrastructure. These materials chemically or biologically bind CO₂, transforming buildings into carbon sinks while maintaining structural and aesthetic functions. According to Roychand et al. [35], carbonated concrete can sequester up to 20% of its weight in CO₂, offering a sustainable solution for urban construction. Similarly, algae-based facades leverage photosynthetic processes to capture CO₂, producing valuable byproducts like biomass, aligning with circular economy principles and urban sustainability goals. Figure 3 illustrates the state-of-the-art in direct air capture and CO₂ conversion (DACC), showcasing advanced materials like metal-organic frameworks (MOFs) and their role in enhancing the efficiency of CO₂-absorbing materials for building-integrated applications. Figure 3 State-of-the-Art in Direct Air Capture and CO₂ Conversion (DACC). This figure provides a comprehensive overview of materials and processes for direct air capture and integrated CO₂ conversion (DACC), including sorbents and catalysts, enhancing the discussion of material advancements. Reproduced with permission from Zanatta [31] licensed under CC-BY 4.0 3.2.1. Carbonated Concrete Carbonated concrete sequesters CO₂ through a chemical reaction between CO₂ and calcium compounds, forming stable calcium carbonates that enhance material durability. According to El-Hassan [36], carbonation curing can sequester 10– 15 kg CO₂ per cubic meter of concrete, leading to much increase in compressive strength compared to traditional concrete [23]. Technologies like those developed by CarbonCure have been implemented in urban precast concrete production, reducing emissions by 5–10% per project [37]. The process involves injecting CO₂ into wet concrete during curing, where it reacts with calcium hydroxide to form limestone-like compounds, reducing cement content and the carbon footprint of concrete production, which accounts for ~8% of global emissions. Pilot projects in North American cities have applied carbonated concrete in sidewalks, building foundations, and structural elements, demonstrating scalability and compatibility with urban construction practices [37]. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 158 3.2.2. Algae-Based Facades Algae-based facades integrate photosynthetic microorganisms into building exteriors, capturing CO₂ while producing biomass for energy or materials. According to Sedighi et al. [38], algae bioreactors, such as those in Hamburg’s BIQ House, capture 4–6 kg CO₂ per m² annually, offering aesthetic and environmental benefits. These systems use transparent panels filled with algae cultures, which absorb CO₂ through photosynthesis, producing oxygen and biomass that can be harvested for biofuels, fertilizers, or bioplastics. Pilot projects have demonstrated co-benefits, including urban heat reduction by 1–2°C and air quality improvement through pollutant filtration, making algae facades a multifunctional solution for sustainable cities [39]. The implementation of algae facades is hindered by high maintenance and energy costs. Findings from Borowitzka [40] highlight that algae systems require precise control of light, temperature, and nutrients, thereby increasing operational costs. The need for specialized infrastructure, such as nutrient delivery systems and biomass harvesting mechanisms, limits scalability in urban settings. According to ElFar et al. [41], optimizing algae strains for higher CO₂ uptake and developing automated maintenance systems could reduce costs, but these innovations are still in early stages. Additionally, public acceptance and regulatory approval for algae-based systems vary across cities, requiring education and policy support to facilitate adoption. Research is focused on improving the efficiency and integration of algae facades into urban architecture. According to Zhang [42], combining algae bioreactors with smart building controls, such as automated shading, enhances CO₂ capture by 10–15% while reducing energy inputs. Future developments aim to standardize bioreactor designs and integrate them with building management systems to optimize performance. Collaborative efforts between architects, biologists, and engineers are essential to scale algae facades, ensuring they contribute to urban decarbonization while enhancing city livability. 3.3. HVAC-Integrated Systems HVAC-integrated CO₂ capture systems leverage buildings’ ventilation networks to capture CO₂ from indoor or outdoor air, offering a practical retrofitting solution for urban environments [43]. These systems embed sorbents or membranes within HVAC ducts, utilizing existing airflow to capture CO₂ efficiently. According to Marsters et al. [44], membranebased HVAC systems achieve capture rates of 0.5–1 kg CO₂ per hour in high-traffic buildings, such as offices or shopping centers. By integrating capture into existing infrastructure, these systems minimize space requirements and capital costs, making them ideal for dense urban settings with limited land availability. 3.3.1. Technology and Mechanisms HVAC-integrated systems typically employ polymeric membranes or solid sorbents, such as zeolites or amines, to selectively capture CO₂ during air circulation. Sorbent-based systems, cycled through temperature swings, offer higher selectivity but require energy for regeneration, typically 2–3 GJ per ton of CO₂ [45]. Findings from Murge et al. [46] demonstrate that advanced zeolites with 2–3 mmol/g CO₂ capacity improve capture efficiency by 15–20% compared to traditional sorbents. These systems are designed for retrofitting, with modular units that fit into existing HVAC ducts, allowing integration without major infrastructure changes. Pilot projects in urban office buildings in Singapore have captured 100–200 kg CO₂ annually per system, showcasing practical applicability [47,48]. The performance of HVAC-integrated systems depends on building-specific factors, such as airflow rates and CO₂ concentrations. According to Liang et al. [49], systems in high-occupancy buildings, with elevated CO₂ levels, achieve higher capture rates than in residential settings. Innovations in membrane durability and low-energy regeneration cycles are critical to reducing operational costs. Research is exploring hybrid membrane-sorbent systems that combine high permeance with selectivity, enhancing performance across diverse urban climates. Additionally, integration with smart building controls allows real-time optimization of capture based on occupancy and air quality, improving efficiency [49]. 3.3.2. Advantages and Challenges HVAC-integrated systems offer significant advantages, including seamless integration into existing infrastructure and dual benefits of CO₂ capture and indoor air quality improvement. These systems can reduce indoor CO₂ concentrations, enhancing occupant health and productivity [50]. Installation costs, are lower than standalone DAC units, making them accessible for urban retrofitting [50]. Captured CO₂ can be stored or utilized on-site, such as in building-integrated greenhouses, reducing transport costs. Pilot projects in Asian cities have integrated these systems with smart building controls, optimizing capture based on occupancy patterns and reducing energy use [51]. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 159 However, energy penalties, and retrofitting complexities pose challenges. Findings from Cevallos-Mendoza et al. [52] indicate that membrane fouling and sorbent degradation reduce efficiency over time, requiring replacement every 1–2 years. Urban buildings with outdated HVAC systems may need costly upgrades to accommodate capture technologies, increasing capital costs [17]. According to Aristov et al. [53], developing low-energy sorbents and standardized retrofitting protocols could reduce costs by 25%, but these solutions are not yet commercially available. Research gaps include improving material durability, optimizing system designs for diverse building types, and developing policy incentives to support urban deployment. 3.4. Hybrid Urban Systems Hybrid urban systems combine building-integrated CO₂ capture with green infrastructure, such as vertical gardens or urban forests, to enhance carbon sequestration and urban livability [54]. These systems leverage biological and technological synergies to address CO₂ emissions holistically, aligning with smart city frameworks. According to Jozay et al. [55], urban trees and green roofs can sequester 0.1–0.5 kg CO₂ per m² annually, complementing technological capture systems. By integrating biological and engineered solutions, hybrid systems offer a multifaceted approach to urban decarbonization, enhancing both environmental and social outcomes. Figure 4 complements the discussion of hybrid urban systems by illustrating an electrochemical ocean CO₂ removal process, which could be integrated with coastal urban infrastructures to enhance distributed carbon capture strategies. Figure 4 Electrochemical Ocean Carbon Dioxide Removal. This figure shows a schematic of an electrochemical ocean CO₂ removal system, which could complement building-integrated DAC by illustrating alternative CO₂ capture methods relevant to urban coastal areas. It highlights the process of CO₂ extraction from seawater and its potential environmental benefits. Reproduced with permission from Walker III et al. [21] 3.4.1. Green Infrastructure Integration Hybrid systems integrate vertical gardens, bio-reactors, or urban forests with technological capture units, such as DAC or sorbent-based systems, to maximize CO₂ sequestration. According to Price et al. [56], vertical gardens reduce local CO₂ by 2–3% in dense urban areas, while also mitigating urban heat islands and improving air quality. Algae bioreactors, embedded in building facades, can be paired with DAC units to enhance capture capacity. Findings from Jones [57] suggest that hybrid systems combining algae bioreactors with DAC could capture 5–10 kg CO₂ per m² annually, significantly higher than standalone green infrastructure. Pilot projects in European cities have demonstrated the feasibility of integrating green walls with modular DAC units, creating synergistic systems that reduce emissions while enhancing urban aesthetics [58]. These systems require coordination with urban planning to ensure compatibility with building designs and public spaces. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 166 existing infrastructure, such as ventilation systems, and create jobs in manufacturing, installation, and maintenance, particularly benefiting low-income urban areas. Algae facades capture CO₂ while producing biomass for potential use in urban agriculture or bioenergy markets, contributing to local economies. CO₂-absorbing concrete improves durability through carbonation, potentially reducing long-term maintenance costs for municipal budgets. These technologies, supported by innovations in energy efficiency and material science, promote sustainable urban development [123,124]. Building-integrated CO₂ removal technologies offer economic and environmental co-benefits, enhancing urban livability. Integrating Direct Air Capture (DAC) with HVAC systems in buildings can reduce energy costs by utilizing existing infrastructure, such as ventilation systems, and create jobs in manufacturing, installation, and maintenance, particularly benefiting low-income urban areas. Algae facades capture CO₂ while producing biomass for potential use in urban agriculture or bioenergy markets, contributing to local economies. CO₂-absorbing concrete improves durability through carbonation, potentially reducing long-term maintenance costs for municipal budgets. These technologies, supported by innovations in energy efficiency and material science, promote sustainable urban development [123-125]. 6. Challenges and Future Directions 6.1. Key Challenges Building-integrated CO₂ removal technologies—modular direct air capture (DAC) units, CO₂-absorbing materials, HVAC-integrated systems, and hybrid urban systems—face significant challenges that hinder their widespread adoption in urban environments. These include technical limitations, high economic costs, and social barriers, which must be addressed to scale these solutions effectively. Current operational and energy demands pose substantial hurdles, particularly in resource-constrained cities [65]. Overcoming these challenges is critical to realizing the potential of urban CCUS for global decarbonization. 6.1.1. Technical and Operational Barriers Technical challenges limit the performance of building-integrated CO₂ removal systems. Modular direct air capture (DAC) units require 4–6 GJ per ton of CO₂ for sorbent regeneration, posing significant demands on urban energy grids [126]. HVAC-integrated systems face membrane fouling from environmental contaminants, which can reduce efficiency by approximately 10–15% over time [127]. CO₂-absorbing concrete, while passive, is constrained by limited CO₂ availability in urban areas, sequestering only 10–15 kg CO₂ per m³ of concrete [128]. Algae-based facades, such as those tested in Hamburg’s BIQ House, require precise control of light and nutrients, increasing operational complexity [23]. Environmental factors, including urban humidity and pollution, further reduce sorbent and membrane efficiency by 10– 20% [129]. Operational barriers include integration into existing infrastructure and maintenance demands. According to McQueen et al. [15], retrofitting DAC or HVAC systems into older buildings requires structural modifications, increasing installation costs. Hybrid systems, combining green infrastructure with technological capture, demand coordination across urban planning and engineering disciplines, adding complexity. Findings from Tang et al. [11] highlight that developing standardized designs and low-energy materials, such as next-generation MOFs, could mitigate these issues, but current technologies lack scalability for diverse urban climates. Research is needed to enhance system durability and streamline integration protocols to address these technical barriers. To provide a clearer overview of the technical hurdles, the following figure highlights the key challenges facing DACCC, offering a visual complement to the operational barriers discussed below. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 167 Figure 6 Challenges in Direct Air Carbon Capture and Conversion (DACCC). This figure outlines the core challenges of DACCC, including Kinetic of sorption, Sorbent regeneration, Continuous process, Product isolation, and Reaction harsh conditions, depicted around a central "DACCC challenges" node to emphasize their collective impact on system performance. Reproduced with permission from Zanatta [31] licensed under CC-BY 4.0 6.1.2. Economic and Social Constraints Economic constraints, driven by high capital and operational costs, pose significant challenges to urban carbon capture, utilization, and storage (CCUS) deployment. Direct air capture (DAC) systems face operational expenses of $100–$600 per ton of CO₂, though costs could decrease to $54–$133/tCO₂ by 2050 with technological advancements and waste heat utilization [130]. CO₂-absorbing concrete increases production costs by 10–20%, limiting adoption in costsensitive markets [131]. Inconsistent policy incentives, such as variable carbon pricing of $50–$100 per ton, hinder economic viability in developing cities [132]. Scaling CCUS requires cost reductions through economies of scale, modularization, and material innovations [130,133]. Social constraints, including public skepticism and equitable access, further complicate adoption. According to Boulton et al. [134], concerns about aesthetics and safety, particularly for DAC units, reduce community acceptance by in residential areas. High costs may exclude low-income communities from CCUS benefits, exacerbating urban inequalities, per Santamouris et al. [114] [98]. Public awareness of co-benefits, such as air quality improvement, is low, necessitating education, as seen in Climeworks’ Zurich outreach efforts. Findings from Igbokwe [135] suggest that community engagement and transparent communication can increase acceptance, but culturally sensitive strategies are underdeveloped. Addressing these social barriers is essential for equitable and widespread urban deployment. Table 5 synthesizes the key challenges facing building-integrated CO₂ capture technologies and outlines future research directions to enhance their scalability and effectiveness in urban decarbonization. 6.2. Future Directions To overcome these challenges, future efforts must prioritize technological innovation, research advancements, and robust policy frameworks. These directions aim to enhance the scalability and accessibility of building-integrated CO₂ removal in urban environments. According to Ozkan et al. [27], achieving cost-competitive CCUS by 2030 requires interdisciplinary collaboration and targeted investments. Future strategies should focus on reducing energy and cost barriers while fostering social acceptance. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 168 Table 5 Key Challenges and Proposed Research Directions Technology Key Challenges Impact on Scalability Proposed Research Directions Potential Impact Reference Modular DAC Units High energy demand (4–6 GJ/ton), cost ($100–$200/ton) High energy costs limit adoption Low-energy MOFs, renewable integration 20–30% cost reduction [15, 27, 33, 71] CO₂-Absorbing Concrete Limited CO₂ availability, 10–20% cost increase Supply constraints reduce use On-site CO₂ capture, material optimization 10–15% cost reduction [35, 68, 131] Algae-Based Facades Maintenance complexity, water use High costs limit scalability Optimized algae strains, automated systems 15–20% efficiency increase [23, 40, 136] HVACIntegrated Systems Membrane fouling, retrofitting costs Efficiency loss, high capital costs Durable membranes, standardized protocols 25% cost reduction [10, 46, 52] Hybrid Urban Systems Planning coordination, high initial costs Complexity limits deployment Smart controls, policy incentives 15–20% efficiency gain [55, 62, 137] 6.2.1. Research and Technological Innovations Research priorities include developing low-energy capture materials and optimizing system designs. According to Li et al. [20], next-generation MOFs could reduce DAC energy demands, improving efficiency in urban settings. Enhancing algae strains for higher CO₂ uptake, as suggested by Song et al. [136], could increase capture rates by 15–20% in hybrid systems [101]. Research into CO₂ utilization, such as producing synthetic fuels or fertilizers, can create economic incentives, offsetting of costs, per McQueen et al. [15]. Standardizing retrofitting protocols for diverse building types is critical to scalability, particularly in developing cities. Technological innovations should focus on integrating CCUS with renewable energy and smart city frameworks. According to Wang et al. [137], smart building controls and digital twins can optimize capture efficiency by, minimizing energy penalties. Collaborative research between academia and industry is essential to accelerate these advancements, ensuring technologies are adaptable to urban climates and infrastructure by 2030. Investments in closed-loop systems, such as recycling nutrients in algae facades, can further reduce environmental impacts. 6.2.2. Policy and Implementation Strategies Policy frameworks are vital to address economic and social constraints. According to Irvine [81], carbon pricing at $50– $100 per ton can offset 20–50% of capture costs, while tax credits, like the U.S. 45Q program, enhance viability. Green building certifications and subsidies, as used in Hamburg’s BIQ House, can reduce capital costs by 30–40%, per Tallou et al. [23]. Developing global standards for CCUS integration into building codes and urban planning will streamline deployment, per Santamouris et al. [114]. Municipal financing through green bonds can support large-scale projects, particularly in developing cities. Implementation strategies should ensure equitable access and foster social acceptance. According to Gbadegesin [138], community-driven models, such as cooperative maintenance of green infrastructure, enhance acceptance and create jobs. Public education campaigns, modeled on Climeworks’ outreach, can increase awareness, per Elrayies [112]. International collaboration, including technology transfer to developing nations, is critical for global scalability. By aligning policies with urban sustainability goals, such as air quality improvement and livability, CCUS can become a cornerstone of net-zero cities, driving transformative climate action. 7. Conclusion Building-integrated CO₂ removal technologies offer a groundbreaking approach to urban decarbonization, transforming buildings into active components of climate mitigation while enhancing environmental and social outcomes. This review Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 169 has explored modular direct air capture units, CO₂-absorbing materials, HVAC-integrated systems, and hybrid urban systems, revealing their potential to reduce city-wide emissions by 5–10% through efficient capture mechanisms. Modular units, capable of sequestering 0.5–2 tons of CO₂ annually, and HVAC systems, capturing 0.5–1 kg CO₂ per hour, demonstrate robust technical performance. CO₂-absorbing concrete sequesters 10–15 kg CO₂ per cubic meter, enhancing construction sustainability, while algae-based facades in hybrid systems capture 2–5 kg CO₂ per square meter, providing co-benefits like air quality improvement and urban heat mitigation. These technologies position cities as leaders in achieving net-zero targets by leveraging existing infrastructure to create carbon sinks. Case studies, such as the direct air capture pilot in Zurich and the algae facade project in Hamburg, illustrate practical applications and highlight the importance of integrating these systems with renewable energy and urban planning. The Zurich project showcases scalability, capturing significant CO₂ volumes, while Hamburg’s initiative demonstrates aesthetic and functional integration, producing biomass for economic value. These examples underscore the feasibility of building-integrated CO₂ removal in diverse urban contexts, from commercial hubs to residential districts. However, the review also reveals significant challenges, including high operational costs ranging from $100–$600 per ton of CO₂ and energy demands of 4–6 GJ per ton. Retrofitting older buildings increases installation costs by 20–30%, and material degradation, such as membrane fouling in HVAC systems, reduces efficiency over time. Social barriers, including public skepticism about aesthetics and safety, further complicate adoption, particularly in residential areas where acceptance can be 20–30% lower without community engagement. The environmental and social impacts of these technologies are profound, offering solutions to urban-specific challenges. Algae facades and hybrid systems reduce particulate matter by 10–20%, improving air quality and reducing health risks in polluted cities. Green infrastructure mitigates urban heat islands, lowering temperatures by 1–2°C and cooling demands by 10–15%, enhancing urban resilience. Socially, these systems create 500–1,000 jobs per city in manufacturing and maintenance, boost property values by 5–10%, and foster community pride through sustainable design. Yet, high costs risk excluding low-income communities, underscoring the need for equitable access to ensure broad societal benefits. Public education and community-driven models, as seen in successful pilot projects, are critical to building trust and increasing acceptance by 25–40%. Looking forward, the scalability of building-integrated CO₂ removal hinges on overcoming technical, economic, and social barriers through innovation and policy support. Advances in low-energy materials, such as next-generation metal-organic frameworks, could reduce energy demands by 20–30%, while CO₂ utilization in fuels or agriculture could offset 10–20% of costs. Smart city frameworks, integrating digital twins and renewables, can optimize efficiency by 15– 20%. Policy incentives, such as carbon pricing at $50–$100 per ton and green building subsidies, can lower capital costs by 30–40%, making these technologies viable in developing cities. Global standards for CCUS integration into building codes and urban planning will streamline deployment, while international collaboration ensures technology transfer to resource-constrained regions. The urgency of climate change demands immediate action to scale these technologies. Researchers must prioritize costeffective designs and durable materials, policymakers should implement robust incentives, and urban planners must embed CCUS into city landscapes. By addressing these imperatives, building-integrated CO₂ removal can redefine urban infrastructure as a climate asset, driving cities toward net-zero emissions while enhancing livability and equity. This review underscores the potential of these technologies to lead a sustainable urban transformation, calling for concerted efforts to realize a low-carbon future where buildings actively contribute to global climate goals. Compliance with ethical standards Acknowledgments The authors would like to thank all of the participating academics and colleagues who worked together to co-author and co-edit this review article. This work was completed solely by the authorship team's academic and intellectual contributions; no external money or help from any person, group, or institution was required. Disclosure of conflict of interest The authors declare that they have no conflict of interest to be disclosed. Global Journal of Engineering and Technology Advances, 2025, 24(01), 151-176 170 References [1] Nunes, L. J. (2023). The rising threat of atmospheric CO2: a review on the causes, impacts, and mitigation strategies. Environments, 10(4), 66. [2] Zentou, H., AlZahrani, A. A., Behar, O., Tayeb, A. M., & Abdelnaby, M. M. (2025). Progress in Large‐Scale Carbon Capture Deployment: Status, Challenges, and Prospects. Advanced Sustainable Systems, 2400626. [3] Garcia, J. A., Villen-Guzman, M., Rodriguez-Maroto, J. 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