Engineering Solutions for Limiting the Increase of Atmospheric Carbon Dioxide

The article describes possible engineering solutions that can reduce the carbon dioxide (CO₂) content of the air. To demonstrate the point fossil fuel power plants will be taken as a model for the source of CO₂. The global mass balance shows that the oceans play a major role in storing CO₂. The hypothesis presented states that the real problem is not the absolute CO₂ content, rather its change. Consequently, present emissions should be stored for future release. Under the current worldwide measures to reduce power consumption CO₂ emissions are unlikely to decrease. A number of strategies for the maritime sequestration of CO₂ are reported in the contribution. One proposal for sequestration is the use of shallow waters which form a thermohaline current: the dissolved gas will travel for hundreds of years in deep sea currents. In the latter case, CO₂ injection is easily achieved. Several scenarios are considered for the fate of this CO₂‐enriched current. The environmental impact is briefly reported. The article will describe current research requirements, demonstrating that similar research in the US and Japan is presently more advanced in comparison to that in Europe. The sequestration of carbon dioxide on land will be the subject of a second publication. It is obvious that the sequestration of CO₂ is the method after all rational chances to save energy.     

Carbon capture from stationary power generation sources: A review of the current status of the technologies

The world will need greatly increased energy supply in the future for sustained economic growth, but the related CO₂ emissions and the resulting climate changes are becoming major concerns. CO₂ is one of the most important greenhouse gases that is said to be responsible for approximately 60% of the global warming. Along with improvement of energy efficiency and increased use of renewable energy sources, carbon capture and sequestration (CCS) is expected to play a major role in curbing the greenhouse gas emissions on a global scale. This article reviews the various options and technologies for CO₂ capture, specifically for stationary power generation sources. Many options exist for carbon dioxide capture from such sources, which vary with power plant types, and include post-combustion capture, pre-combustion capture, oxy fuel combustion capture, and chemical looping combustion capture. Various carbon dioxide separation technologies can be utilized with these options, such as chemical absorption, physical absorption, adsorption, and membrane separation. Most of these capture technologies are still at early stages of development. Recent progress and remaining challenges for the various CO₂ capture options and technologies are reviewed in terms of capacity, selectivity, stability, energy requirements, etc. Hybrid and modified systems hold huge future potentials, but significant progress is required in materials synthesis and stability, and implementations of these systems on demonstration plants are needed. Improvements and progress made through applications of process systems engineering concepts and tools are highlighted and current gaps in the knowledge are also mentioned. Finally, some recommendations are made for future research directions.     

Separation of CO2 from flue gas using electrochemical cells

Past research with high temperature molten carbonate electrochemical cells has shown that carbon dioxide can be separated from flue gas streams produced by pulverized coal combustion for power generation. However, the presence of trace contaminants, i.e., sulfur dioxide and nitric oxides, will impact the electrolyte within the cell. If a lower temperature cell could be devised that would utilize the benefits of commercially-available, upstream desulfurization and denitrification in the power plant, then this CO₂ separation technique can approach more viability in the carbon sequestration area. Recent work has led to the assembly and successful operation of a low temperature electrochemical cell. In the proof-of-concept testing with this cell, an anion exchange membrane was sandwiched between gas-diffusion electrodes consisting of nickel-based anode electrocatalysts on carbon paper. When a potential was applied across the cell and a mixture of oxygen and carbon dioxide was flowed over the wetted electrolyte on the cathode side, a stream of CO₂ to O₂ was produced on the anode side, suggesting that carbonate/bicarbonate ions are the CO₂ carrier in the membrane. Since a mixture of CO₂ and O₂ is produced, the possibility exists to use this stream in oxy-firing of additional fuel.From this research, a novel concept for efficiently producing a carbon dioxide rich effluent from combustion of a fossil fuel was proposed. Carbon dioxide and oxygen are captured from the flue gas of a fossil-fuel combustor by one or more electrochemical cells or cell stacks. The separated stream is then transferred to an oxy-fired combustor which uses the gas stream for ancillary combustion, ultimately resulting in an effluent rich in carbon dioxide. A portion of the resulting flow produced by the oxy-fired combustor may be continuously recycled back into the oxy-fired combustor for temperature control and an optimal carbon dioxide rich effluent.     

Tillage-induced CO2 emission from soil

The influence of agricultural production systems on greenhouse gas generation and emission is of interest as it may affect potential global climate change. Agricultural ecosystems can play a significant role in production and consumption of greenhouse gases, specifically, carbon dioxide. Information is needed on the mechanism and magnitude of gas generation and emission from agricultural soils with specific emphasis on tillage mechanisms. This work evaluated four different tillage methods on the short-term CO₂ and water vapor flux from a clay loam soil in the Northern Cornbelt of the USA. The four tillage methods were moldboard plow only, moldboard plow plus disk harrow twice, disk harrow and chisel plow using standard tillage equipment following a wheat (Triticum aestivum L.) crop compared with no tillage. The CO₂ flux was measured with a large portable chamber commonly used to measure crop canopy gas exchange initiated within 5 minutes after tillage and continued intermittently for 19 days. The moldboard plow treatment buried nearly all of the residue and left the soil in a rough, loose, open condition and resulted in maximum CO₂ loss. The carbon released as CO₂ during the 19 days following the moldboard plow, moldboard plow plus disk harrow, disk harrow, chisel plow and not tilled treatments would account for 134%, 70%, 58%, 54% and 27% respectively of the carbon in the current year's crop residue. The short-term carbon dioxide losses 5 hours after four conservation tillage tools was only 31% of that of the moldboard plow. The moldboard plow lost 13.8 times as much CO₂ as the soil area not tilled while different conservation tillage tools lost only 4.3 times. The smaller CO₂ loss following conservation tillage tools is significant and suggests progress in developing conservation tillage tools that can enhance soil carbon management. Conservation tillage reduces the extent, frequency and magnitude of mechanical disturbance caused by the moldboard plow and reduces the air-filled macropores and slows the rate of carbon oxidation. Any effort to decrease tillage intensity and maximize residue return should result in carbon sequestration for enhanced environmental quality. This revised version was published online in August 2006 with corrections to the Cover Date.     

Syngas Stoichiometric Adjustment for Methanol Production and Co-Capture of Carbon Dioxide by Pressure Swing Adsorption

Methanol is an important raw material in industry and is commonly produced from syngas. The stoichiometric ratio (H₂–CO₂)/(CO + CO₂) of the methanol synthesis reactor feed stream must be adjusted to approximately 2.1. In this study, the replacement of the solvent unit within a coal to methanol process by a pressure swing adsorption (PSA) unit is proposed. The PSA produces a hydrogen enriched stream, to adjust the stoichiometric ratio of the methanol feed stream, and simultaneously captures the carbon dioxide for future sequestration. The feed flow rate is sub divided into eight 4-bed PSA units, operated with a defined phase lag between them in order to flatten the products (composition and flow rate) oscillations. The results show that the stoichiometric adjustment is possible and that oscillations on the products flow rate and composition are reduced to less than 3%. A carbon dioxide stream of 95.15% is obtained with a recovery of 94.2% and a productivity of 82.7 mol CO₂/kg/day. The power consumption of the global process is 119.7 MW, which includes the requirements for the rinse stream (64.4 MW) and the compression of the CO₂ product to 110 bar for sequestration (55.3 MW).     

CO2 sequestration from coal fired power plants

The paper takes into consideration a new approach for CO₂ capture and transport, based on the formation of solid CO₂ hydrates.Carbon dioxide sequestration from power plants can take advantage of the properties of gas hydrates. The formation and decomposition of hydrates from various N₂-CO₂ mixtures has been studied experimentally in a 2 l reactor, to determine the CO₂ separation in terms of hydrate composition and residual CO₂ content in the reacted gas.Carbon dioxide acts as a co-former for the production of hydrates containing nitrogen, besides CO₂. The mixed hydrates that are obtained are less stable than simple CO₂ hydrates. When CO₂ content in the flue gas is higher than 30% by volume, the hydrates formed at 5 MPa are sufficiently concentrated (about 70% CO₂) and carbon dioxide reduction in the reacted gas is acceptable.The application of a process based on hydrate formation could be especially interesting (for CO₂ capture and transport) when connected to an oxy-coal combustion process; in this case the CO₂ content in the flue gas is very high and the hydrate formation is greatly facilitated.     

Carbon dioxide sequestration through novel use of ion exchange fibers (IX-fibers)

Electrical power generation and metal removal processes are practiced globally and share two common attributes that make them ideal candidates to be incorporated in a novel carbon dioxide sequestration scheme using ion exchange fibers (IX-fibers). First, the softening of boiler feed water used in power generation and the removal of metals from finishing wastewaters often employs the use of ion exchange for the purpose of selective separation. Second, both processes represent significant point source CO₂ emissions. This investigation demonstrated that using IX-fibers it is possible to sequester a portion of the CO₂ produced in these practices as carbonate alkalinity during the regeneration step of both the water softening and the trace heavy metal removal processes. Weak acid IX-fibers were used for hardness removal while hybrid cation exchange fibers (HCIX-F) loaded with hydrated Zr(IV) oxide (HZO) were used to remove toxic heavy metals such as zinc, cadmium and copper. IX-fibers offer the unique capability to use and consume CO₂ during the efficient regeneration of IX-fibers, whereas commercial ion exchange resins are not amenable to regeneration with CO₂. A much shorter intraparticle diffusion path length in cylindrical IX-fibers as compared to resin beads is the underlying reason for a highly efficient regeneration of the fibers. In addition to sequestering carbon dioxide, no hazardous or aggressive chemicals/brine solutions are present in the regenerant wastes as compared with traditional ion exchange processes.     

Progress in carbon dioxide capture and separation research for gasification-based power generation point sources

The purpose of the present work is to investigate novel approaches, materials, and molecules for the abatement of carbon dioxide (CO₂) at the pre-combustion stage of gasification-based power generation point sources. The capture/separation step for CO₂ from large point sources is a critical one with respect to the technical feasibility and cost of the overall carbon sequestration scenario. For large point sources, such as those found in power generation, the carbon dioxide capture techniques being investigated by the Office of Research and Development of the National Energy Technology Laboratory possess the potential for improved efficiency and reduced costs as compared to more conventional technologies. The investigated techniques can have wide applications, but the present research is focused on the capture/separation of carbon dioxide from fuel gas (pre-combustion gas) from processes such as the Integrated Gasification Combined Cycle (IGCC) process. For such applications, novel concepts are being developed in wet scrubbing with physical sorption, chemical sorption with solid sorbents, and separation by membranes. In one concept, a wet scrubbing technique is being investigated that uses a physical solvent process to remove CO₂ from fuel gas of an IGCC system at elevated temperature and pressure. The need to define an “ideal” solvent has led to the study of the solubility and mass transfer properties of various solvents. Pertaining to another separation technology, fabrication techniques and mechanistic studies for membranes separating CO₂ from the fuel gas produced by coal gasification are also being performed. Membranes that consist of CO₂-philic ionic liquids encapsulated into a polymeric substrate have been investigated for permeability and selectivity. Finally, processes based on dry, regenerable sorbents are additional techniques for CO₂ capture from fuel gas. An overview of these novel techniques is presented along with a research progress status of technologies related to membranes and physical solvents.     

Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

In the current work, molecular dynamics simulation is employed to understand the intrinsic growth of carbon dioxide and methane hydrate starting from a seed crystal of methane and carbon dioxide respectively. This comparison was carried out because it has relevance to the recovery of methane gas from natural gas hydrate reservoirs by simultaneously sequestering a greenhouse gas like CO₂. The seed crystal of carbon dioxide and methane hydrate was allowed to grow from a super-saturated mixture of carbon dioxide or methane molecules in water respectively. Two different concentrations (1:6 and 1:8.5) of CO₂/CH₄ molecules per water molecule were chosen based on gas–water composition in hydrate phase. The molecular level growth as a function of time was investigated by all atomistic molecular dynamics simulation under suitable temperature and pressure range which was well above the hydrate stability zone to ensure significantly faster growth kinetics. The concentration of CO₂ molecules in water played a significant role in growth kinetics, and it was observed that maximizing the CO₂ concentration in the aqueous phase may not result in faster growth of CO₂ hydrate. On the contrary, methane hydrate growth was independent of methane molecule concentration in the aqueous phase. We have validated our results by performing experimental work on carbon dioxide hydrate where it was seen that under conditions appropriate for liquid CO₂, the growth for carbon dioxide hydrate was very slow in the beginning.     

Design and operation of pilot plant for CO2 capture from IGCC flue gases by combined cryogenic and hydrate method

This project is a trial conducted under contract with CO₂CRC, Australia of a new CO₂ capture technology that can be applied to integrated gasification combined cycle power plants and other industrial gasification facilities. The technology is based on combination of two low temperature processes, namely cryogenic condensation and the formation of hydrates, to remove CO₂ from the gas stream. The first stage of this technology is condensation at −55 °C where CO₂ concentration is expected to be reduced by up to 75 mol%. Remaining CO₂ is captured in the form of solid hydrate at about 1 °C reducing CO₂ concentration down to 7 mol% using hydrate promoters. This integrated cryogenic condensation and CO₂ hydrate capture technology hold promise for greater reduction of CO₂ emissions at lower cost and energy demand. Overall, the process produced gas with a hydrogen content better than 90 mol%. The concentrated CO₂ stream was produced with 95-97 mol% purity in liquid form at high pressure and is available for re-use or sequestration. The enhancement of carbon dioxide hydrate formation and separation in the presence of new hydrate promoter is also discussed. A laboratory scale flow system for the continuous production of condensed CO₂ and carbon dioxide hydrates is also described and operational details are identified.     

Cost of energy analysis of integrated gasification combined cycle (IGCC) power plant with respect to CO2 capture ratio under climate change scenarios

This paper presents the results of the cost of energy (COE) analysis of an integrated gasification combined cycle (IGCC) power plant with respect to CO₂ capture ratio under the climate change scenarios. To obtain process data for a COE analysis, simulation models of IGCC power plants and an IGCC with carbon capture and sequestration (CCS) power plant, developed by the United States Department of Energy (DOE) and National Energy Technology Laboratory (NETL), have been adopted and simulated using Aspen Plus. The concept of 20-year levelized cost of energy (LCOE), and the climate change scenarios suggested by International Energy Agency (IEA) are also adopted to compare the COE of IGCC power plants with respect to CO₂ capture ratio more realistically. Since previous studies did not consider fuel price and CO₂ price changes, the reliability of previous results of LCOE is not good enough to be accepted for an economic comparison of IGCC power plants with respect to CO₂ capture ratio. In this study, LCOEs which consider price changes of fuel and CO₂ with respect to the climate change scenarios are proposed in order to increase the reliability of an economic comparison. And the results of proposed LCOEs of an IGCC without CCS power plant and IGCC with CCS (30%, 50%, 70% and 90% capture-mole basis- of CO₂ in syngas stream) power plants are presented.     

Photon-driven reduction reactions on silver

Irradiation of a polycrystalline silver electrode with near-u.v./visible light in solutions containing either dissolved carbon dioxide or nitrate ions produces strong enhancement of the cathodic current. Maximum photocurrent efficiency is observed for the photon energies of about 3.5 eV characteristic of surface plasmons on silver. In the presence of carbon dioxide in solution, the electrode illumination not only increases the rate of CO₂ to CO reduction but also shifts the onset of the CO production by about 0.5 V to less cathodic potentials.This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday and in recognition of his outstanding contribution to electrochemistry.     

Thermodynamic and transport property models for carbon capture and sequestration (CCS) processes with emphasis on CO2 transport

Carbon capture and sequestration (CCS) is one of the most promising technologies for the reduction of carbon dioxide (CO₂) concentration in the atmosphere, so that global warming can be controlled and eventually eliminated. A crucial part in the CCS process design is the model that is used to calculate the physical properties (thermodynamic, transport etc.) of pure CO₂ and CO₂ mixtures with other components.     

Na2CO3-Based Sorbents Coated on Metal Foil: Post Testing Analysis

Worldwide increase in energy demand coupled with a continued reliance on fossil fuel resources have contributed to a significant increase in atmospheric levels of carbon dioxide. According to the International Energy Agency’s (IEA’s) World Energy Outlook 2010 main scenario, the projected growth in energy demand will translate into a 21 % rise in energy related CO₂ emissions between 2008 and 2035, mostly due to robust economic growth in developing countries. This quantity of greenhouse gas emissions would make it next to impossible to meet a 2 ºC goal thought to avoid the worst consequences of global climate change without additional actions. Scenarios for stabilizing climate-forcing emissions suggest atmospheric CO₂ stabilization can only be accomplished through the development and deployment of a robust portfolio of solutions, of which carbon capture and sequestration represent one component. In previous work [1], Na₂CO₃/Al₂O₃ distributed on metal foil was shown to be effective for CO₂ capture. In the current work, Na₂CO₃/Al₂O₃ prepared in the powder-form and foil-form were tested in a fixed-bed reactor and characterized by X-ray diffraction, BET surface area, Raman spectroscopy and scanning electron microscopy, before and after testing, to better understand the performance of the sorbents. The powder sorbents exhibited higher performance, in general, but one of the foil samples showed the highest performance out of all the power and foil sorbents. The same sorbent was also tested for 500 cycles to understand the long-term stability.     

Sequestration of CO2 using microorganisms and evaluation of their potential to synthesize biomolecules

ABSTRACT

Microalgae are the unicellular or multicellular photosynthetic microorganisms that can efficiently fix carbon dioxide (CO₂) from various sources such as the environment, industrial flue gas, and some carbonate salts. In the present study, one green microalgal strain and a cyanobacterial consortium were used separately for the sequestration of CO₂ at different pHs (7–11), at different initial concentrations of CO₂ (5–20%), and at various inoculum sizes (5–12.5%). The maximum sequestration of CO₂ was found to be 74.37 ± 0.49% and 71.12 ± 0.05% at 5% and 15% CO₂ for green algae and cyanobacterial consortium. The biomass generated after sequestration of CO₂ was utilized for the synthesis of biomolecules.     

The effect of anisotropic dispersion on the convective mixing in long‐term CO2 storage in saline aquifers

Carbon dioxide storage in deep saline aquifers is considered a possible option to bring greenhouse gas emissions under control. The understanding of the underlying mechanisms, such as convective mixing and associated mechanisms, affecting this mixing may have an impact on the long‐term sequestration process in deep saline aquifers. One of the significant aspects of the flow of miscible species in porous media is velocity dependent dispersion. The effect of dispersion on dissolution of carbon dioxide (CO₂) into brine is investigated by full nonlinear numerical simulations. This study reveals that dispersion may dramatically change the trend of CO₂ dissolution into brine. It was found that the dissolution of CO₂ increases as dispersion strength increases. The mixing pattern also shows three different mechanisms: diffusion, convection, and a highly nonlinear interaction mechanism. However, the medium dispersivity ratios were found to slightly affect the mixing, while having an impact on the fingering pattern. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Adsorption and Desorption of Carbon Dioxide and Nitrogen on Zeolite 5A

The adsorption equilibrium data of CO₂ and N₂ at (303, 333, 363, 393, 423) K ranging 0-1 bar on zeolite 5A is reported. The pressure and temperature range covers the operating pressure in adsorption units for CO₂ capture from power plants. Experimental data were fitted by the multi-site Langmuir model. The adsorbent is much more selective to CO₂: loading at 303 K and 100 kPa is 3.38 mol/kg while loading of N₂ at the same pressure is 0.22 mol/kg. The Clausius-Clapeyron equation was employed to calculate the isosteric enthalpy of adsorption. The fixed-bed adsorption and desorption of carbon dioxide and nitrogen on zeolite 5A pellets has been studied. A model based on the bi-LDF approximation for the mass transfer, taking into account the energy and momentum balances, had been used to describe the adsorption kinetics of carbon dioxide and nitrogen. The model predicted satisfactorily the breakthrough curves obtained with carbon dioxide–nitrogen mixtures. Desorption process (consisting of depressurization, blowdown, and purge) was also performed. Following the feasibility of concentration and capture of carbon dioxide from flue gases by Pressure Swing Adsorption (PSA) process was simulated. A CO₂ recovery of 91.0% with 53.9% purity was obtained using a five-step Skarstrom-type PSA cycle.     

Effects of effective stress changes on permeability of latrobe valley brown coal

One of the key issues with geological sequestration of carbon dioxide in coal seams is change of permeability caused by carbon dioxide (CO₂) injection, and especially any resulting reduction in injectivity. Injection causes changes in pressure and effective stress, with further changes caused by coal matrix swelling associated with adsorption of CO₂. In this paper we aim to study how the change in effective stress and coal swelling may influence the gas permeability in brown coal using natural coal and reconstituted coal specimens. Tests were conducted at different confining pressures to represent conditions at different depths. Different gas injection pressures were also employed at each confining stress stage. The test results clearly depicted an exponential reduction of coal permeability to CO₂ gas when effective stress increases. Based on the experimental results, an empirical correlation to represent the effect of stress on permeability was developed. The results also showed that increase in pore pressure can induce further swelling of the coal specimens, and this can lead to further decrease in permeability which can have important impact on field injectivity. Test results for natural brown coal specimens were compared with results of tests on reconstituted coal specimens made from compaction of coal particles obtained from crushing of blocks of natural coal. Observed permeability behaviour of gas in reconstituted coal was similar to the natural coal specimen permeability trend, when effective stress increases.     

Making C–C Bonds from Carbon Dioxide via Transition-Metal Catalysis

Developing more green and sustainable strategies for organic synthesis is an important modern challenge. Although carbon dioxide is an attractive reagent, because of its availability, the low reactivity of this small molecule renders it challenging to use widely in organic synthesis. The discovery of new catalysts thus plays a central role in advancing the science and impact of CO₂ utilization. This perspective describes advances in metal-catalyzed carbon dioxide incorporation since Aresta’s report of the first metal-CO₂ complex.

     

Making C–C Bonds from Carbon Dioxide via Transition-Metal Catalysis

Developing more green and sustainable strategies for organic synthesis is an important modern challenge. Although carbon dioxide is an attractive reagent, because of its availability, the low reactivity of this small molecule renders it challenging to use widely in organic synthesis. The discovery of new catalysts thus plays a central role in advancing the science and impact of CO₂ utilization. This perspective describes advances in metal-catalyzed carbon dioxide incorporation since Aresta’s report of the first metal-CO₂ complex.