A general binary isotherm model for amines interacting with CO2 and H2O.

CO2 capture by primary or secondary amines has been a topic of great research interests for a century because of its industrial importance. Interest has grown even more, because of the need to eliminate CO2 emissions that lead to global warming. Experimental evidence shows that CO2 sorption by primary or secondary amines is accompanied by co-absorption of H2O. A quantitative analysis of such CO2-H2O co-absorption behavior is important for practical process design and theoretical understanding. Even though there is almost an experimental consensus that water enhances CO2 uptake capacity, an analytical model to explain this phenomenon is not well established. Instead, some empirical models such as the Toth model are used to describe the isotherm without accounting for the presence of water. Recently, we have demonstrated that the isotherm equation of CO2 sorption into strong-base anion exchange materials with quaternary ammonium can be derived from that of strong-base aqueous alkaline solutions by correcting for the drastic change in the water activity and by including an appropriate parameterization of the water activity terms. In this paper, we generalize this model from quaternary ammonium to primary, secondary and tertiary amines either in solutions or as functional groups in polymer resins. For primary, secondary and tertiary amines, the isotherm equation can be derived by extending that of a weak-base aqueous alkaline solution such as aqueous ammonia. The model has been validated using experimental data on CO2 sorption for aqueous ammonia from the literature. This general model even includes quaternary ammonium as a special limit. Hence, this general model offers a platform that can treat the isotherms of solid amines, aqueous amines and aqueous alkaline solutions in a unified manner.

Confronting the Carbon-Footprint Challenge of Blockchain.

The distributed consensus mechanism is the backbone of the rapidly developing blockchain network. Blockchain platforms consume vast amounts of electricity based on the current consensus mechanism of Proof-of-Work (PoW). Here, we point out a different consensus mechanism named Proof-of-Stake (PoS) that can eliminate the extensive energy consumption of the current PoW-based blockchain. We comprehensively elucidate the current and projected energy consumption and carbon footprint of the PoW- and PoS-based Bitcoin and Ethereum blockchain platforms. The model of energy consumption of PoS-based Ethereum blockchain can lead the way toward the prediction of other PoS-based blockchain technologies in the future. With the widespread adoption of blockchain technology, if the current PoW mechanism continues to be employed, the carbon footprint of Bitcoin and Ethereum will push the global temperature above 1.5 °C in this century. However, a PoS-based blockchain can reduce the carbon footprint by 99% compared to the PoW mechanism. The small amount of carbon footprint from PoS-based blockchain could make blockchain an attractive technology in a carbon-constrained future. The study sheds light on the urgency of developing the PoS mechanism to solve the current sustainability problem of blockchain.

Kinetic model for moisture-controlled CO2 sorption.

The understanding of the sorption/desorption kinetics is essential for practical applications of moisture-controlled CO2 sorption. We introduce an analytic model of the kinetics of moisture-controlled CO2 sorption and its interpretation in two limiting cases. In one case, chemical reaction kinetics on pore surfaces dominates, in the other case, diffusive transport through the sorbent defines the kinetics. We show that reaction kinetics, which is dominant in the first case, can be expressed as a linear combination of 1st and 2nd order kinetics in agreement with the static isotherm equation derived and validated in a previous paper. The interior transport kinetics can be described by non-linear diffusion equations. By combining all carbon species into a single equation, we can eliminate - in certain limits - the source terms associated with chemical reactions. In this case, the governing equation is . For a sorbent in a form of a flat sheet or a membrane, one can maintain the same functional form of a diffusion equation by introducing a generalized effective diffusivity DM that combines contributions from both surface chemical reaction kinetics and interior diffusive transport kinetics. Experimental data of transient CO2 flux in a preconditioned commercial anion exchange membrane fit well to the 1st order model as long as very dry states are avoided, validating the theory. The observed DM for a preconditioned commercial anion exchange membrane ranges from 6.6 × 10-14 to 7.1 × 10-14 m2 s-1 at 35 °C. These small values compared to typical ionic diffusivities imply a very slow kinetics, which will be the largest issue that needs to be addressed for practical application. The collected transient CO2 flux data are used to predict the magnitude of a continuous CO2 pumping flux in an active membrane that transports CO2 against a CO2 concentration gradient. The pumped CO2 flux is supported by water flux due to a water concentration gradient.

Isotherm model for moisture-controlled CO2 sorption.

Moisture-controlled sorption of CO2, the basis for moisture-swing CO2 capture from air, is a novel phenomenon observed in strong-base anion exchange materials. Prior research has shown that Langmuir isotherms provide an approximate fit to moisture-controlled CO2 sorption isotherm data. However, this fit still lacks a governing equation derived from an analytic model. In this paper, we derive an analytic form for an isotherm equation from a bottom-up approach, starting with a fundamental theory for an alkali liquid. In the range of interest relevant to CO2 capture from air, an isotherm equation for an alkali liquid reduces to a simple analytic form with a single parameter, Keq. In the limit Keq ≫ 1, a 2nd order approximation simplifies to a Langmuir isotherm that, however, deviates from experimental data. The isotherm theory for an alkali liquid has been generalized to a strong-base anion exchange material. In a strong-base anion exchange material, water concentration inside a sorbent, [H2O], is not large enough to be regarded as constant, which allows us to extend Keq to Keq(AEM)eff = Keq(AEM) × [H2O]-n according to the law of mass action. The final isotherm formula has been validated by experimental data from the literature. For a moisture-controlled CO2 sorbent, Keq(AEM)eff varies significantly with moisture content of the sorbent. Depending on moisture level, the observed Keq(AEM)eff in a specific sorbent ranges from a few times to a few thousand times the value of Keq of a 2 mol L-1 alkali liquid.

The Gutenberg health study: a five-year prospective analysis of psychosocial working conditions using COPSOQ (Copenhagen psychosocial Questoinnaire) and ERI (effort-reward imbalance).

BACKGROUND: Psychosocial working conditions were previously analyzed using the first recruitment wave of the Gutenberg Health Study (GHS) cohort (n = 5000). We aimed to confirm the initial analysis using the entire GHS population at baseline (N = 15,010) and at the five-year follow-up. We also aimed to determine the effects of psychosocial working conditions at baseline on self-rated outcomes measured at follow-up. METHODS: At baseline, working GHS participants were assessed with either the Effort-Reward-Imbalance questionnaire (ERI) (n = 4358) or with the Copenhagen Psychosocial Questionnaire (COPSOQ) (n = 4322); participants still working after five years received the same questionnaire again (ERI n = 3142; COPSOQ n = 3091). We analyzed the association between working conditions and the outcomes job satisfaction, general health, burnout, and satisfaction with life at baseline, at follow-up and also prospectively from baseline to follow-up using linear regression models. We examined the outcome variance explained by the models (R2) to estimate the predictive performance of the questionnaires. RESULTS: The models' R2 was comparable to the original baseline analyses at both t0 and t1 (R2 range: ERI 0.10-0.43; COPSOQ 0.10-0.56). However, selected scales of the regression models sometimes changed between assessment times. The prospective analysis showed weaker associations between baseline working conditions and outcomes after five years (R2 range: ERI 0.07-0.19; COPSOQ 0.07-0.24). This was particularly true for job satisfaction. After adjusting for the baseline levels of the outcomes, fewer scales still explained some of the variance in the distribution of the outcome variables at follow-up. The models using only data from t0 or t1 confirmed the previous baseline analysis. We observed a loss of explained variance in the prospective analysis models. This loss was greatest for job satisfaction, suggesting that this outcome is most influenced by short-term working conditions. CONCLUSIONS: Both the COPSOQ and ERI instruments show good criterion validity and adequately predict contemporaneously measured self-reported measurements of health and (occupational) well-being. However, the COPSOQ provides a more detailed picture of working conditions and might be preferable for improvment strategies in workplaces. Additional prospective research with shorter follow-up times would be beneficial for estimating dose-response relationships.

Postcombustion Capture or Direct Air Capture in Decarbonizing US Natural Gas Power?

This analysis investigates the cost of carbon capture from the US natural gas-fired electricity generating fleet comparing two technologies: postcombustion capture and direct air capture (DAC). Many of the existing natural gas combined cycle (NGCC) units are suitable for postcombustion capture. We estimated the cost of postcombustion retrofits and investigated the most important unit characteristics contributing to this cost. Units larger than 400 MW, younger than 14 years, more efficient than 45%, and with a utilization (capacity factor) higher than 0.5 were found to be the most promising for retrofit. Counterintuitively, DAC (which is usually not considered for point-source capture) may be cheaper in addressing emissions from nonretrofittable NGCC units. DAC can also address the residual emissions from retrofitted units. Moreover, the economic challenges of postcombustion capture for small natural gas-fired units with low utilization, such as gas turbines, make DAC look favorable for these units. After considering the cost of postcombustion capture for the entire natural gas-related emissions and incorporating the impact of learning-by-doing for both carbon capture technologies, our results show that DAC is the cheaper capture solution for at least 1/3 of all emissions.

Sorbents for the Direct Capture of CO2 from Ambient Air.

The urgency to address global climate change induced by greenhouse gas emissions is increasing. In particular, the rise in atmospheric CO2 levels is generating alarm. Technologies to remove CO2 from ambient air, or "direct air capture" (DAC), have recently demonstrated that they can contribute to "negative carbon emission." Recent advances in surface chemistry and material synthesis have resulted in new generations of CO2 sorbents, which may drive the future of DAC and its large-scale deployment. This Review describes major types of sorbents designed to capture CO2 from ambient air and they are categorized by the sorption mechanism: physisorption, chemisorption, and moisture-swing sorption.

Humidity effect on ion behaviors of moisture-driven CO2 sorbents.

Ion hydration is a fundamental process in many natural phenomena. This paper presents a quantitative analysis, based on atomistic modeling, of the behavior of ions and the impact of hydration in a novel CO2 sorbent. We explore moisture-driven CO2 sorbents focusing on diffusion of ions and the structure of ion hydration complexes forming inside water-laden resin structures. We show that the stability of the carbonate ion is reduced as the water content of the resin is lowered. As the hydration cloud of the carbonate ion shrinks, it becomes energetically favorable to split a remaining water molecule and form a bicarbonate ion plus a hydroxide ion. These two ions bind less water than a single, doubly charged carbonate ion. As a result, under relatively dry conditions, more OH- ions are available to capture CO2 than in the presence of high humidity. Local concentrations of dissolved inorganic carbon and water determine chemical equilibria. Reaction kinetics is then driven to a large extent by diffusion rates that allow water and anions to move through the resin structure. Understanding the basic mechanics of chemical equilibria and transport may help us to rationally design next-generation efficient moisture-driven CO2 sorbents.

Net-zero emissions energy systems.

Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO2) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO2 to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

The catalytic effect of H2O on the hydrolysis of CO32- in hydrated clusters and its implication in the humidity driven CO2 air capture.

The hydration of ions in nanoscale hydrated clusters is ubiquitous and essential in many physical and chemical processes. Here we show that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO32- with n = 1-8 water molecules is investigated using an ab initio method. For n = 1-5 water molecules, all the reactants follow a stepwise pathway to the transition state. For n = 6-8 water molecules, all the reactants undergo a direct proton transfer to the transition state with overall lower activation free energy. The activation free energy of the reaction is dramatically reduced from 10.4 to 2.4 kcal mol-1 as the number of water molecules increases from 1 to 6. Meanwhile, the degree of hydrolysis of CO32- is significantly increased compared to the bulk water solution scenario. Incomplete hydration shells facilitate the hydrolysis of CO32- with few water molecules to be not only thermodynamically favorable but also kinetically favorable. We showed that the chemical kinetics is not likely to constrain the speed of CO2 air capture driven by the humidity-swing. Instead, the pore-diffusion of ions is expected to be the time-limiting step in the humidity driven CO2 air capture. The effect of humidity on the speed of CO2 air capture was studied by conducting a CO2 absorption experiment using IER with a high ratio of CO32- to H2O molecules. Our result is able to provide valuable insights into designing efficient CO2 air-capture sorbents.

Spontaneous Cooling Absorption of CO2 by a Polymeric Ionic Liquid for Direct Air Capture.

A polymeric ionic liquid (PIL), with quaternary ammonium ions attached to the polymer matrix, displays CO2 affinity controlled by moisture. This finding led to the development of moisture swing absorption (MSA) for direct air capture of CO2. This work aims to elucidate the role of water in MSA. For some humidity range, CO2 absorption is an endothermic process associated with concurrent dehydration of the sorbent. The thermodynamic behavior of water indicates a decreased hydrophilicity of the PIL as the mobile anion transforms from CO32- to HCO3- during CO2 absorption. The decrease in hydrophilicity drives water out of the PIL, carrying heat away. The mechanism is elucidated by molecular modeling based on density functional theory. The finding of spontaneous cooling during absorption and its mechanism in the PIL opens new possibilities for designing an air capture sorbent with a strong CO2 affinity but low absorption heat.

Kinetic analysis of an anion exchange absorbent for CO2 capture from ambient air.

This study reports a preparation method of a new moisture swing sorbent for CO2 capture from air. The new sorbent components include ion exchange resin (IER) and polyvinyl chloride (PVC) as a binder. The IER can absorb CO2 when surrounding is dry and release CO2 when surrounding is wet. The manuscript presents the studies of membrane structure, kinetic model of absorption process, performance of desorption process and the diffusivity of water molecules in the CO2 absorbent. It has been proved that the kinetic performance of CO2 absorption/desorption can be improved by using thin binder and hot water treatment. The fast kinetics of P-100-90C absorbent is due to the thin PVC binder, and high diffusion rate of H2O molecules in the sample. The impressive is this new CO2 absorbent has the fastest CO2 absorption rate among all absorbents which have been reported by other up-to-date literatures.

A Life Cycle Assessment Case Study of Coal-Fired Electricity Generation with Humidity Swing Direct Air Capture of CO2 versus MEA-Based Postcombustion Capture.

Most carbon capture and storage (CCS) envisions capturing CO2 from flue gas. Direct air capture (DAC) of CO2 has hitherto been deemed unviable because of the higher energy associated with capture at low atmospheric concentrations. We present a Life Cycle Assessment of coal-fired electricity generation that compares monoethanolamine (MEA)-based postcombustion capture (PCC) of CO2 with distributed, humidity-swing-based direct air capture (HS-DAC). Given suitable temperature, humidity, wind, and water availability, HS-DAC can be largely passive. Comparing energy requirements of HS-DAC and MEA-PCC, we find that the parasitic load of HS-DAC is less than twice that of MEA-PCC (60-72 kJ/mol versus 33-46 kJ/mol, respectively). We also compare other environmental impacts as a function of net greenhouse gas (GHG) mitigation: To achieve the same 73% mitigation as MEA-PCC, HS-DAC would increase nine other environmental impacts by on average 38%, whereas MEA-PCC would increase them by 31%. Powering distributed HS-DAC with photovoltaics (instead of coal) while including recapture of all background GHG, reduces this increase to 18%, hypothetically enabling coal-based electricity with net-zero life-cycle GHG. We conclude that, in suitable geographies, HS-DAC can complement MEA-PCC to enable CO2 capture independent of time and location of emissions and recapture background GHG from fossil-based electricity beyond flue stack emissions.

The Effect of Moisture on the Hydrolysis of Basic Salts.

A great deal of information exists concerning the hydration of ions in bulk water. Much less noticeable, but equally ubiquitous is the hydration of ions holding on to several water molecules in nanoscopic pores or in natural air at low relative humidity. Such hydration of ions with a high ratio of ions to water molecules (up to 1:1) are essential in determining the energetics of many physical and chemical systems. Herein, we present a quantitative analysis of the energetics of ion hydration in nanopores based on molecular modeling of a series of basic salts associated with different numbers of water molecules. The results show that the degree of hydrolysis of basic salts in the presence of a few water molecules is significantly different from that in bulk water. The reduced availability of water molecules promotes the hydrolysis of divalent and trivalent basic ions (S2- , CO32- , SO32- , HPO42- , SO42- , PO43- ), which produces lower valent ions (HS- , HCO3- , HSO3- , H2 PO4- , HSO4- , HPO42- ) and OH- ions. However, reducing the availability of water inhibits the hydrolysis of monovalent basic ions (CN- , HS- ). This finding sheds some light on a vast number of chemical processes in the atmosphere and on solid porous surfaces. The discovery has wide potential applications including designing efficient absorbents for acidic gases.

Capture CO2 from Ambient Air Using Nanoconfined Ion Hydration.

Water confined in nanoscopic pores is essential in determining the energetics of many physical and chemical systems. Herein, we report a recently discovered unconventional, reversible chemical reaction driven by water quantities in nanopores. The reduction of the number of water molecules present in the pore space promotes the hydrolysis of CO3(2-) to HCO3(-) and OH(-). This phenomenon led to a nano-structured CO2 sorbent that binds CO2 spontaneously in ambient air when the surrounding is dry, while releasing it when exposed to moisture. The underlying mechanism is elucidated theoretically by computational modeling and verified by experiments. The free energy of CO3 (2-) hydrolysis in nanopores reduces with a decrease of water availability. This promotes the formation of OH(-), which has a high affinity to CO2 . The effect is not limited to carbonate/bicarbonate, but is extendable to a series of ions. Humidity-driven sorption opens a new approach to gas separation technology.

Relative permeability experiments of carbon dioxide displacing brine and their implications for carbon sequestration.

To mitigate anthropogenically induced climate change and ocean acidification, net carbon dioxide emissions to the atmosphere must be reduced. One proposed option is underground CO2 disposal. Large-scale injection of CO2 into the Earth's crust requires an understanding of the multiphase flow properties of high-pressure CO2 displacing brine. We present laboratory-scale core flooding experiments designed to measure CO2 endpoint relative permeability for CO2 displacing brine at in situ pressures, salinities, and temperatures. Endpoint drainage CO2 relative permeabilities for liquid and supercritical CO2 were found to be clustered around 0.4 for both the synthetic and natural media studied. These values indicate that relative to CO2, water may not be strongly wetting the solid surface. Based on these results, CO2 injectivity will be reduced and pressure-limited reservoirs will have reduced disposal capacity, though area-limited reservoirs may have increased capacity. Future reservoir-scale modeling efforts should incorporate sensitivity to relative permeability. Assuming applicability of the experimental results to other lithologies and that the majority of reservoirs are pressure limited, geologic carbon sequestration would require approximately twice the number of wells for the same injectivity.

Co-location of air capture, subseafloor CO2 sequestration, and energy production on the Kerguelen plateau.

Reducing atmospheric CO2 using a combination of air capture and offshore geological storage can address technical and policy concerns with climate mitigation. Because CO2 mixes rapidly in the atmosphere, air capture could operate anywhere and in principle reduce CO2 to preindustrial levels. We investigate the Kerguelen plateau in the Indian Ocean, which offers steady wind resources, vast subseafloor storage capacities, and minimal risk of economic damages or human inconvenience and harm. The efficiency of humidity swing driven air capture under humid and windy conditions is tested in the laboratory. Powered by wind, we estimate ∼75 Mt CO2/yr could be collected using air capture and sequestered below seafloor or partially used for synfuel. Our analysis suggests that Kerguelen offers a remote and environmentally secure location for CO2 sequestration using renewable energy. Regional reservoirs could hold over 1500 Gt CO2, sequestering a large fraction of 21st century emissions.

Moisture-swing sorption for carbon dioxide capture from ambient air: a thermodynamic analysis.

An ideal chemical sorbent for carbon dioxide capture from ambient air (air capture) must have a number of favourable properties, such as environmentally benign behaviour, a high affinity for CO(2) at very low concentration (400 ppm), and a low energy cost for regeneration. The last two properties seem contradictory, especially for sorbents employing thermal swing adsorption. On the other hand, thermodynamic analysis shows that the energy cost of an air capture device need only be slightly larger than that of a flue gas scrubber. The moisture swing separation process studied in this paper provides a novel approach to low cost CO(2) capture from air. The anionic exchange resin sorbent binds CO(2) when dry and releases it when wet. A thermodynamic model with coupled phase and chemical equilibria is developed to study the complex H(2)O-CO(2)-resin system. The moisture swing behaviour is compatible with hydration energies changing with the activity of water on the resin surfaces. This activity is in turn set by the humidity. The rearrangement of hydration water on the resin upon the sorption of a CO(2) molecule is predicted as a function of the humidity and temperature. Using water as fuel to drive the moisture swing enables an economical, large-scale implementation of air capture. By generating CO(2) with low partial pressures, the present technology has implications for in situ CO(2) utilizations which require low pressure CO(2) gas rather than liquid CO(2).