This week’s selected publications cover a wide range of issues from mathematical modeling of CDR to its energy requirements and potential side-effects. Let’s take a closer look at this week’s collection:
A Sensitivity Study of the Mathematical Model for Carbon Dioxide Removal by Physical Absorption in the Production of Biomethane from Palm Empty Fruit Bunch
Abstract
A non-equilibrium rate-based absorption model based on the two-film theory was adapted for the physical solvent in a packed bed column with co-counter current flow in place of chemical reaction. Carbon dioxide (CO2) removal from fresh biomethane in a palm empty fruit bunch thermochemical conversion plant to improve the purity of the dried gas was modeled from the approximation of mathematical equations. This objective was achieved by improvising and reducing the model assumptions with guaranteed accuracy based on the validation using the established measured data. A better mathematical model with the predicted temperature profile at the liquid side and a mean absolute percentage error of less than 25% contributed to the 2 wt.% differences between the assumed dimethyl ether polyethylene glycol purity and the experiment, which is sufficient to be considered acceptable. To understand the performance of the absorption column, the sensitivity of three input variables on the removal of CO2 was analyzed, including the temperature, pressure, and solvent feed flow rate by manipulating the input value for each variable individually. The optimum temperature of 31 °C, pressure of 1.6 kPa, and solvent feed flow rate of 1:1 liquid-to-gas ratio were established as the baseline values for the sensitivity test. The analysis from the mathematical model indicates a significant influence of the operating temperature on CO2 absorption. This study enhances biomethane purity, optimizes CO2 removal, and improves operational efficiency. It aligns with sustainability goals, reduces emissions, and offers economic benefits, making it valuable for the renewable energy industry.
Asrul, M. A. M., Yun, H. A. H., Atan, M. F., Wei, I. T. A., & Hui, J. L. C. (2024) A Sensitivity Study of the Mathematical Model for Carbon Dioxide Removal by Physical Absorption in the Production of Biomethane from Palm Empty Fruit Bunch. International Journal of Integrated Engineering, 16(6), 216-227.
Read the full paper here: A Sensitivity Study of the Mathematical Model for Carbon Dioxide Removal by Physical Absorption in the Production of Biomethane from Palm Empty Fruit Bunch | International Journal of Integrated Engineering
Biohydrogen with Negative CO2 Emissions from Municipal Solid Waste for Decarbonising the Public Bus Fleet. Application to the Municipality of Madrid
Abstract
This study assesses the production potential, environmental impact, and economic viability of generating biohydrogen from biomethane obtained from the organic fraction of municipal solid waste (MSW) using steam methane reforming with carbon capture and storage (CCS). As the emissions are biogenic, CCS results in negative emissions. The methodology is based on a previously developed model, including techno-economic analysis based on the levelised cost of hydrogen (LCOH) and mobility (LCOM), and environmental assessment, focusing on production potential, cost estimates, and emissions impact. A case study is conducted to assess the feasibility of using this biohydrogen with negative emissions to decarbonize Madrid’s public bus fleet. The findings reveal that Madrid’s MSW could meet the entire hydrogen fuel demand if the fleet consisted of fuel-cell buses. However, given the high costs of replacing the entire fleet, a net-zero solution is proposed, combining 60% fuel-cell buses with existing natural gas-powered buses. In this configuration, the negative emissions from biohydrogen offset the fossil emissions from natural gas and 40% of biomethane is saved. The cost of the net-zero fleet ranges between 192.55 and 209.37 €/100 km, comparable with 100% natural gas fleet, which ranges between 176.19 and 217.69 €/100 km.
Lefranc, L., Linares, J., Santos, A., Arenas, E., Martin, C. & Moratilla, Y. (2024). Biohydrogen with Negative CO2 Emissions from Municipal Solid Waste for Decarbonising the Public Bus Fleet. Application to the Municipality of Madrid. Journal of Environmental Management, 371 (123258), 1-13.
Read the full paper here: Biohydrogen with Negative CO2 Emissions from Municipal Solid Waste for Decarbonising the Public Bus Fleet. Application to the Municipality of Madrid I Journal of Environmental Management
A Geographic Analysis and Techno-Economic Assessment of Renewable Heat Sources for Low-Temperature Direct Air Capture
Abstract
Integrated assessment model (IAM) scenarios examining pathways to achieve the goals of the Paris Agreement stress the necessity of deploying carbon dioxide removal (CDR) methods, of which direct air capture (DAC) is viewed as one of the most promising. This study undertakes both a geospatial analysis and techno-economic assessment of potential heat sources for DAC to examine the economic impact of different renewable heat source systems on the capture costs of large-scale LT-DAC plants. It does this by determining the location of these plants through the paradigm of identifying the ideal geographic and economic environment for the selected heat sources. Thus, the research aims to answer the following research questions: What heat sources are optimally suited for low-temperature (LT) DAC and what conditions are feasible for setup? Which geographic locations represent the ideal environment within Europe for each heat source? How do the selected heat sources and geographic locations impact the economic viability of LT-DAC? Drawing on Climeworks’ LT-DAC approach as a focal case, the heat sources of geothermal energy, parabolic trough collector (PTC), industrial waste heat (IWH), and high-temperature heat pump (HTHP) were chosen, to be separately deployed in Iceland, Spain, Germany, and Norway, respectively. Spain emerged as a highly promising location for the PTC, IWH, and HTHP systems while Iceland is most suitable for the geothermal, IWH, and HTHP systems. Norway is a promising country mostly for deploying a HTHP system, whereas Germany faces primarily environmental and legal barriers. The techno-economic assessment identified great variation in the LCOD costs for the different heat source systems, with the geothermal energy system exhibiting the lowest costs at 175.63 €/tCO2 followed by the IWH, PTC, and HTHP systems. Future LCOD costs could potentially see a significant reduction of up to 66 % depending on the heat source system based on projected decreases in DAC CAPEX costs. A cost comparison revealed that current carbon price levels within the European Emission trading scheme are not expected to be sufficiently high enough to drive large investments in the development and scaling of LT-DAC. Cost levels of CCS technologies and LT-DAC could however be comparable, in particular for the geothermal energy system.
Krull, L., Baum, C., & Sovacool, B. (2025). A Geographic Analysis and Techno-economic Assessment of Renewable Heat Sources for Low-Temperature Direct Air Capture in Europe. Energy Conversion and Management, 323 (119186), 1-18.
Read the full paper here: A Geographic Analysis and Techno-Economic Assessment of Renewable Heat Sources for Low-Temperature Direct Air Capture I Energy Conversion and Management
Resilience of Phytoplankton and Microzooplankton Communities under Ocean Alkalinity Enhancement in the Oligotrophic Ocean
Abstract
Ocean alkalinity enhancement (OAE) is currently discussed as a potential negative emission technology to sequester atmospheric carbon dioxide in seawater. Yet, its potential risks or cobenefits for marine ecosystems are still mostly unknown, thus hampering its evaluation for large-scale application. Here, we assessed the impacts OAE may have on plankton communities, focusing on phytoplankton and microzooplankton. In a mesocosm study in the oligotrophic subtropical North Atlantic, we investigated the response of a natural plankton community to CO2-equilibrated OAE across a gradient from ambient alkalinity (2400 μmol kg–1) to double (4800 μmol kg–1). Abundance and biomass of phytoplankton and microzooplankton were insensitive to OAE across all size classes (pico, nano and micro), nutritional modes (autotrophic, mixotrophic and heterotrophic) and taxonomic groups (cyanobacteria, diatoms, haptophytes, dinoflagellates, and ciliates). Consequently, plankton communities under OAE maintained their natural chlorophyll a levels, size structure, taxonomic composition and biodiversity. These findings suggest a high tolerance of phytoplankton and microzooplankton to CO2-equilibrated OAE in the oligotrophic ocean. However, alternative application schemes involving more drastic perturbations in water chemistry and nutrient-rich ecosystems require further investigation. Nevertheless, our study on idealized OAE will help develop an environmentally safe operating space for this climate change mitigation solution.
Xin, X., Goldenberg, S., Taucher, J., Stuhr, A., Aristegui, J. and Riebesell, U. (2024). Resilience of Phytoplankton and Microzooplankton Communities under Ocean Alkalinity Enhancement in the Oligotrophic Ocean. Environmental Science & Technology, XXXX A-M.
Read the full paper here: Resilience of Phytoplankton and Microzooplankton Communities under Ocean Alkanity Enhancement in the Oligotrophic Ocean I Environmental Science & Technology
Mapping the Global Variation in the Efficiency of Ocean Alkalinity Enhancement for Carbon Dioxide Removal
Abstract
To limit global warming to below 2 °C by 2100, CO2 removal from the atmosphere will be necessary. One promising method for achieving CO2 removal at scale is ocean alkalinity enhancement (OAE), but there are challenges with incomplete air–sea CO2 equilibration, which reduces the efficiency of carbon removal. Here, we present global maps of OAE efficiency, and assess the seasonal variation in efficiency. We find that the equilibration kinetics have two characteristic timescales: rapid surface equilibration followed by a slower second phase, which represents the re-emergence of excess alkalinity that was initially subducted. These kinetics vary considerably with latitude and the season of alkalinity release, which are critical factors for determining the placement of potential OAE deployments. Additionally, we quantify the spatial and temporal scales of the induced CO2 uptake, which helps identify the requirements for modelling OAE in regional ocean models.
Zhou, M., Tyka, M., Ho, D., Yankovsky, D., Bachman, S., Nicholas, T., Karspeck, A. and Long, M. (2024). Mapping the Global Variation in the Efficiency of Ocean Alkalinity Enhancement for Carbon Dioxide Removal. Nature Climate Change, 1-7.
Read the full paper here: Mapping the Global Variation in the Efficiency of Ocean Alkalinity Enhancement for Carbon Dioxide Removal | Nature Climate Change