Auswahl der wissenschaftlichen Literatur zum Thema „Air-Sea CO2 exchanges“

Abstract. We investigate the ways in which marine biologically mediated heating increases the surface atmospheric temperature. While the effects of phytoplankton light absorption on the ocean have gained attention over the past years, the impact of this biogeophysical mechanism on the atmosphere is still unclear. Phytoplankton light absorption warms the surface of the ocean, which in turn affects the air–sea heat and CO2 exchanges. However, the contribution of air–sea heat versus CO2 fluxes in the phytoplankton-induced atmospheric warming has not been yet determined. Different so-called climate pathways are involved. We distinguish heat exchange, CO2 exchange, dissolved CO2, solubility of CO2 and sea-ice-covered area. To shed more light on this subject, we employ the EcoGEnIE Earth system model that includes a new light penetration scheme and isolate the effects of individual fluxes. Our results indicate that phytoplankton-induced changes in air–sea CO2 exchange warm the atmosphere by 0.71 ∘C due to higher greenhouse gas concentrations. The phytoplankton-induced changes in air–sea heat exchange cool the atmosphere by 0.02 ∘C due to a larger amount of outgoing longwave radiation. Overall, the enhanced air–sea CO2 exchange due to phytoplankton light absorption is the main driver in the biologically induced atmospheric heating.

Abstract. At present, although seasonal sea-ice cover mitigates atmosphere-ocean gas exchange, the Arctic Ocean takes up carbon dioxide (CO2) on the order of −66 to −199 Tg C year−1 (1012 g C), contributing 5–14% to the global balance of CO2 sinks and sources. Because of this, the Arctic Ocean has an important influence on the global carbon cycle, with the marine carbon cycle and atmosphere-ocean CO2 exchanges sensitive to Arctic Ocean and global climate change feedbacks. In the near-term, further sea-ice loss and increases in phytoplankton growth rates are expected to increase the uptake of CO2 by Arctic Ocean surface waters, although mitigated somewhat by surface warming in the Arctic. Thus, the capacity of the Arctic Ocean to uptake CO2 is expected to alter in response to environmental changes driven largely by climate. These changes are likely to continue to modify the physics, biogeochemistry, and ecology of the Arctic Ocean in ways that are not yet fully understood. In surface waters, sea-ice melt, river runoff, cooling and uptake of CO2 through air-sea gas exchange combine to decrease the calcium carbonate (CaCO3) mineral saturation states (Ω) of seawater while seasonal phytoplankton primary production (PP) mitigates this effect. Biological amplification of ocean acidification effects in subsurface waters, due to the remineralization of organic matter, is likely to reduce the ability of many species to produce CaCO3 shells or tests with profound implications for Arctic marine ecosystems

Abstract The sea surface microlayer (SML) at the air–sea interface is <1 mm thick, but it is physically, chemically, and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all air–sea exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the air–sea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterized. MILAN (Sea Surface Microlayer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability show, for example, an influence of (i) changing solar radiation on the quantity and quality of organic material and (ii) diel changes in wind intensity primarily forcing air–sea CO2 exchange. Thus, MILAN underlines the value and the need of multidiciplinary campaigns for integrating SML complexity into the context of air–sea interaction.

Abstract. The air–sea exchanges of CO2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO2 (39 ± 56 mol C m−2 yr−1, positive flux indicates that the water is losing CO2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m−2 yr−1) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m−2 yr−1). With respect to latitude, estuaries between 23.5 and 50° N have the largest flux per unit area (63 ± 101 mmol C m−2 d−1); these are followed by lower-latitude estuaries (23.5–0° S: 44 ± 29 mmol C m−2 d−1; 0–23.5° N: 39 ± 55 mmol C m−2 d−1), and then regions north of 50° N (36 ± 91 mmol C m−2 d−1). Estuaries south of 50° S have the smallest flux per unit area (9.5 ± 12 mmol C m−2 d−1). Mixing with low-pCO2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO2 in estuaries, but wind speed, seldom discussed, is critical to controlling the air–water exchanges of CO2. The total annual release of CO2 from the world's estuaries is now estimated to be 0.10 Pg C yr−1, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high pCO2, are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where pCO2 is lower but wind speeds are much higher, such that the CO2 fluxes are higher than in Asia. Newly emerged CO2 flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO2. Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO2 and absorb CO2 from the atmosphere in all seasons. Shelves between 0 and 23.5° S are on average a weak source and have a small flux per unit area of CO2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO2 values are low, the air-to-sea fluxes are high in regions above 50° N and below 50° S. At low latitudes, the winds tend to be weak, so the sea-to-air CO2 flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr−1 from the atmosphere.

Abstract. At present, although seasonal sea-ice cover mitigates atmosphere-ocean gas exchange, the Arctic Ocean takes up carbon dioxide (CO2) on the order of −65 to −175 Tg C year−1, contributing 5–14% to the global balance of CO2 sinks and sources. Because of this, the Arctic Ocean is an important influence on the global carbon cycle, with the marine carbon cycle and atmosphere-ocean CO2 exchanges sensitive to Arctic Ocean and global climate change feedbacks. In the near-term, further sea-ice loss and increases in phytoplankton growth rates are expected to increase the uptake of CO2 by Arctic surface waters, although mitigated somewhat by surface warming in the Arctic. Thus, the capacity of the Arctic Ocean to uptake CO2 is expected to alter in response to environmental changes driven largely by climate. These changes are likely to continue to modify the physics, biogeochemistry, and ecology of the Arctic Ocean in ways that are not yet fully understood. In surface waters, sea-ice melt, river runoff, cooling and uptake of CO2 through air-sea gas exchange combine to decrease the calcium carbonate (CaCO3) mineral saturation states (Ω) of seawater that is counteracted by seasonal phytoplankton primary production (PP). Biological processes drive divergent trajectories for Ω in surface and subsurface waters of Arctic shelves with subsurface water experiencing undersaturation with respect to aragonite and calcite. Thus, in response to increased sea-ice loss, warming and enhanced phytoplankton PP, the benthic ecosystem of the Arctic shelves are expected to be negatively impacted by the biological amplification of ocean acidification. This in turn reduces the ability of many species to produce CaCO3 shells or tests with profound implications for Arctic marine ecosystems.

Abstract. The University of Bern monitors carbon dioxide (CO2) and oxygen (O2) at the High Altitude Research Station Jungfraujoch since the year 2000 by means of flasks sampling and since 2005 using a continuous in situ measurement system. This study investigates the transport of CO2 and O2 towards Jungfraujoch using backward trajectories to classify the air masses with respect to their CO2 and O2 signatures. By investigating trajectories associated with distinct CO2 concentrations it is possible to decipher different source and sink areas over Europe. The highest CO2 concentrations, for example, were observed in winter during pollution episodes when air was transported from Northeastern Europe towards the Alps, or during south Foehn events with rapid uplift of polluted air from Northern Italy, as demonstrated in two case studies. To study the importance of air-sea exchange for variations in O2 concentrations at Jungfraujoch the correlation between CO2 and APO (Atmospheric Potential Oxygen) deviations from a seasonally varying background was analyzed. Anomalously high APO concentrations were clearly associated with air masses originating from the Atlantic Ocean, whereas low APO concentrations were found in air masses advected either from the east from the Eurasian continent in summer, or from the Eastern Mediterranean in winter. Those air masses with low APO in summer were also strongly depleted in CO2 suggesting a combination of CO2 uptake by vegetation and O2 uptake by dry summer soils. Other clusters of points in the APO–CO2 scatter plot investigated with respect to air mass origin included CO2 and APO background values and points with regular APO but anomalous CO2 concentrations. Background values were associated with free tropospheric air masses with little contact with the boundary layer during the last few days, while high or low CO2 concentrations reflect the various levels of influence of anthropogenic emissions and the biosphere. The pronounced cycles of CO2 and O2 exchanges with the biosphere and the ocean cause clusters of points and lead to a seasonal pattern.

Abstract. The University of Bern monitors carbon dioxide (CO2) and oxygen (O2) at the High Altitude Research Station Jungfraujoch since the year 2000 by means of flasks sampling and since 2005 using a continuous in situ measurement system. This study investigates the transport of CO2 and O2 towards Jungfraujoch using backward Lagrangian Particle Dispersion Model (LPDM) simulations and utilizes CO2 and O2 signatures to classify air masses. By investigating the simulated transport patterns associated with distinct CO2 concentrations it is possible to decipher different source and sink areas over Europe. The highest CO2 concentrations, for example, were observed in winter during pollution episodes when air was transported from Northeastern Europe towards the Alps, or during south Foehn events with rapid uplift of polluted air from Northern Italy, as demonstrated in two case studies. To study the importance of air-sea exchange for variations in O2 concentrations at Jungfraujoch the correlation between CO2 and APO (Atmospheric Potential Oxygen) deviations from a seasonally varying background was analyzed. Anomalously high APO concentrations were clearly associated with air masses originating from the Atlantic Ocean, whereas low APO concentrations were found in air masses advected either from the east from the Eurasian continent in summer, or from the Eastern Mediterranean in winter. Those air masses with low APO in summer were also strongly depleted in CO2 suggesting a combination of CO2 uptake by vegetation and O2 uptake by dry summer soils. Other subsets of points in the APO-CO2 scatter plot investigated with respect to air mass origin included CO2 and APO background values and points with regular APO but anomalous CO2 concentrations. Background values were associated with free tropospheric air masses with little contact with the boundary layer during the last few days, while high or low CO2 concentrations reflect the various levels of influence of anthropogenic emissions and the biosphere. The pronounced cycles of CO2 and O2 exchanges with the biosphere and the ocean cause clusters of points and lead to a seasonal pattern.

Abstract. The air-sea exchanges of CO2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO2 (39 ± 56 mol C m−2 yr−1, negative flux indicates that the water is losing CO2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m−2 yr−1) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m−2 yr−1). With respect to latitude, estuaries between 23.5 and 50° N have the largest flux per unit area (63 ± 101 mmol C m−2 d−1); these are followed by mid-latitude estuaries (23.5–0° S: 44 ± 29 mmol C m−2 d−1; 0–23.5° N: 39 ± 55 mmol C m−2 d−1), and then regions north of 50° N (36 ± 91 mmol C m−2 d−1). Estuaries south of 50° S have the smallest flux per unit area (9.5 ± 12 molC m−2 d−1). Mixing with low-pCO2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO2 in estuaries but wind speed, seldom discussed, is critical to controlling the air-water exchanges of CO2. The total annual release of CO2 from the world's estuaries is now estimated to be 0.10 PgC yr−1, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high in pCO2, are low in sea-to-air fluxes because the wind speeds are lower than previously determined values, which rely heavily on data from Europe and North America, where pCO2 is lower but wind speeds are much higher, such that the CO2 fluxes are higher than in Asia. Newly emerged CO2 flux data in the Arctic reveal that estuaries there mostly absorb, rather than release CO2. Most continental shelves, and especially those at high latitude, are under-saturated in terms of CO2 and absorb CO2 from the atmosphere in all seasons. Shelves between 0° and 23.5° S are on average a weak source and have a small flux per unit area of CO2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO2 values are low, the air-to-sea fluxes are high in regions above 50° N and below 50° S. At low latitudes, the winds tend to be weak, so the sea-to-air CO2 flux is small. Overall, the world's continental shelves absorb 0.4 PgC yr−1 from the atmosphere.

Against the background of climate warming, marine heatwaves (MHWs) and terrestrial drought events have become increasingly frequent in recent decades. However, the combined effects of MHWs and terrestrial drought on CO2 uptake in marginal seas are still unclear. The East China Sea (ECS) experienced an intense and long-lasting MHW accompanied by an extreme terrestrial drought in the Changjiang basin in the summer of 2022. In this study, we employed multi-source satellite remote sensing products to reveal the patterns, magnitude, and potential drivers of CO2 flux changes in the ECS resulting from the compounding MHW and terrestrial drought extremes. The CO2 uptake of the ECS reduced by 17.0% (1.06 Tg C) in the latter half of 2022 and the Changjiang River plume region shifted from a CO2 sink to a source (releasing 0.11 Tg C) in July-September. In the majority of the ECS, the positive sea surface temperature (SST) anomaly during the MHW diminished the solubility of CO2 in seawater, thereby reducing CO2 uptake. Moreover, the reduction in nutrient input associated with terrestrial drought, which is unfavorable to phytoplankton growth, further reduced the capacity of CO2 uptake. Meanwhile, the CO2 sink doubled for the offshore waters of the ECS continental shelf in July-September 2022, indicating the complexity and heterogeneity of the impacts of extreme climatic events in marginal seas. This study is of great significance in improving the estimation results of CO2 fluxes in marginal seas and understanding sea–air CO2 exchanges against the background of global climate change.

La mer Méditerranée est souvent considérée comme un océan laboratoire pour comprendre les changements globaux liés à l’augmentation de CO2 atmosphérique. Ce travail, basé sur l’étude de données recueilles dans trois régions méditerranéennes, étudie les variations du CO2 océanique dans ce bassin. À l’échelle de la saison, outre les changements de température, le contenu en alcalinité influe sur le contenu en CO2 en Méditerranée orientale, tandis que les changements en carbone total sont responsables des variations dans le bassin occidental. En zone côtière urbanisée, l’émission de CO2 anthropique conditionne les échanges air-mer de CO2. Cette étude montre que l’augmentation de carbone et l’acidification à l’échelle de plusieurs années ne sont pas seulement dues à l’augmentation du CO2 atmosphérique : le contenu en alcalinité module ces tendances dans le bassin oriental, tandis que, dans le bassin occidental, ces tendances sont vraisemblablement influencées par la dynamique des courants
The Mediterranean Sea is often considered as a laboratory ocean for understanding global changes related to the atmospheric CO2 increase. This work, based on the study of data collected in three Mediterranean regions, investigates the variations of oceanic CO2 in this basin. On a seasonal timescale, in addition to temperature changes, alkalinity content influences the CO2 content in the Eastern Mediterranean, while total carbon changes are responsible for variations in the Western Basin. In urbanised coastal areas, anthropogenic CO2 emission’ influences air-sea CO2 exchanges. This study shows that the carbon increase and the acidification on a multi-year timescale is not only due to the increase in atmospheric CO2: the alkalinity content modulates these trends in the eastern basin, while, in the western basin, these trends are likely influenced by current dynamics

L’impact anthropique lié à l’augmentation du CO2 atmosphérique a été observé à l’échelle globale océanique, avec comme conséquence l’acidification des océans (AO). Comme l’océan ouvert, les écosystèmes côtiers sont soumis à l’AO. Ces écosystèmes ne représentent que 7% de la surface océanique mais ils sont responsables d’un tiers de la production primaire océanique mondiale, jouant ainsi un rôle clé dans le cycle du carbone global. Les environnements côtiers sont très hétérogènes et influencés par des apports continentaux, ce qui complexifie l’étude du cycle du CO2. Cette thèse étudie à différente échelle spatiale et temporelle la variabilité du cycle du carbone dans les milieux méga tidaux côtiers du nord-ouest de l’Europe. Entre 2015 et 2019, nous avons installé un capteur autonome de pCO2 sur une bouée cardinale de la côte de Roscoff, au sud de la Manche. Les observations proximales et plus au large des paramètres du système CO2 ainsi que de l’ensemble des paramètres physico-chimiques, nous ont permis de décrire précisément l’écosystème et de quantifier la variabilité tidale, diurne et interannuelle. Dans un second temps, nous avons suivi la variabilité de ces paramètres à l’échelle décennale, en se basant sur les prélèvements réguliers réalisés entre 2008 et 2018 dans deux milieux côtiers très proches géographiquement (Brest et Roscoff), mais sous influence plus ou moins importante des rivières. Enfin, nous avons quantifié la dynamique de deux gaz climatiquement actifs dissous le long de deux gradients estuariens : le CO2 et le CH4. Ce dernier, bien que peu étudié, apparaît comme un composé central pour la compréhension du fonctionnement des écosystèmes côtiers
The anthropogenic impact of the raise of atmospheric CO2 has been observed on the global oceanic scale, resulting in the Ocean Acidification (OA). Largely present in the coastal ecosystems, a decrease of their population could have significant socio-economic consequences. Coastal ecosystems represent only 7% of the global ocean but host a third of the total primary production of the oceans, playing a key role in the global carbon cycle. They are highly diversified and influenced by continental inputs, which complexifies the study of the CO2 cycle. This PhD thesis investigated at different spatial and temporal scales the variability of the carbon cycle in megatidal environments of the North Western European Shelves. From 2015 to 2019, we installed an autonomous sensor of pCO2 (Sunburst SAMI-CO2) on a cardinal buoy located off Roscoff, in the south of the English Channel. Coupled with additional proximal and offshore observations of the carbon cycle and biogeochemical parameters, we were able to describe precisely this ecosystem and assess the tidal, diurnal and interannual variability. Secondly, we followed the variability of these parameters at the decadal scale, based on regular sampling from 2008 to 2018 in two coastal environments very close geographically (Brest and Roscoff, NWES), but with different freshwater influence. Finally, since methane is increasingly considered as a key player in the understanding of the coastal ecosystem functioning and Climatically-Actives Gas cycles, we quantified the driving processes of CO2 and CH4 air-sea exchanges in two mega-tidal estuaries influencing our study region

Carbon plays a major role in physical and biogeochemical processes in the atmosphere, the biosphere, and the ocean. CO2 and CH4 are two of the most common carbon-containing compounds in the atmosphere, also recognized as major greenhouse gases. The exchange of CO2 and CH4 between the ocean and the atmosphere is an essential part of the global carbon cycle. The exchange is controlled by the air–sea concentration gradient and by the efficiency of the transfer processes. The lack of knowledge about the forcing mechanisms affecting the exchange of these climate-relevant gases is a major source of uncertainty in the estimation of the global oceanic contributions. Quantifying and understanding the air–sea exchange processes is essential to constrain the estimates and to improve our knowledge about the current and future climate. In this thesis, the mechanisms controlling the air–sea gas exchange in the Baltic Sea are investigated. The viability of micrometeorological techniques for CH4 monitoring in a coastal environment is evaluated. One year of semi-continuous measurements of air–sea CH4 fluxes using eddy covariance measurements suggests that the method is useful for CH4 flux estimations in marine environments. The measurements allow long-term monitoring at high frequency rates, thus, capturing the temporal variability of the flux. The region off Gotland is a net source of CH4, with both the air–sea concentration gradient and the wind as controlling mechanisms. A sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms controlling the air–sea CO2 exchange in the Baltic Sea. This analysis shows that the spatio-temporal variability of CO2 fluxes is strongly modulated by water-side convection, precipitation, and surfactants. The effect of these factors is relevant both at regional and global scales, as they are not included in the current budget estimates.

World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained. Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur. During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7  m s-1.  The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2.  The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

The Carbon dioxide (CO2) concentration in the atmosphere has increased dramatically since the start of the industrialisation. The effects that the increase of CO2 has on the future climate are still not fully investigated. CO2 in the atmosphere contributes to the, for all life on earth, necessary greenhouse effect. It is confirmed that higher CO2 concentration in the atmosphere increases the green house effect, which results in higher temperature. The main source to the increase of CO2 is burning of fossil fuels. The change in land use is also a contribution to the increase of the CO2 concentration in the atmosphere. The largest sinks of CO2 are organic consumption and oceanic uptake. The organic consumption of CO2 varies a lot at higher latitudes due to the difference in vegetation between the seasons. During the warmer seasons the consumption of CO2 is large and during the winters the consumptions of CO2 is practically zero. The ocean uptake of CO2 varies also a lot during the year because the CO2 dissolves more easily in cold water. The purpose of this study is to analyse CO2 concentration and air-sea fluxes of CO2 measured at Östergarnsholm, a small flat island east of Gotland in the Baltic Sea, and compare the results to previous studies. The CO2 concentration data was collected between 1997 – 1999 and 2001 – 2003. The CO2 flux data was collected between 2001 and 2003. The analysis of the CO2 concentration showed that for the period 1997 to 1999, the CO2 concentration at Östergarnsholm was lower than for the reference series from a Polish site in the Baltic Sea. A correction was made by adding 27 ppm to the Östergarnsholm series. The annual fluctuations of CO2 concentration at Östergarnsholm are significant (about 40 ppm). During the summer 1998, the expected decrease was not as large as it should be because of the El Niño outbreak 97/98 and the locally cold and rainy summer. The direct measured CO2 fluxes were corrected with the well known Webb correction before they were analysed. The CO2 fluxes are wind dependant – higher wind speed give higher CO2 flux. The CO2 fluxes are also dependant of the difference in partial pressure between the air and the water. Parameterised CO2fluxes were calculated and compared to the direct measured CO2 fluxes. The parameterisations use a quadratic as well as a cubic wind dependency. To calculate the parameterised CO2 fluxes, a fixed value of the difference in partial pressure between the air and the water was used because the CO2 in the water was not measured. The parameterised CO2 fluxes wind dependency agreed with the direct measured CO2 fluxes.
Koldioxid(CO2)-koncentrationen i atmosfären har ökat stadigt sen början av industrialiseringen. Effekten som de ökade CO2-halterna kommer ha på framtidens klimat är ännu inte helt utrett. CO2 bidrar till den livsviktiga växthuseffekten. Det är en ökning av växthusgaser, bland annat CO2, som leder till en ökning av växthuseffekten. Ökad växthuseffekt leder till högre temperatur på jorden. Den största ökningen av CO2 i atmosfären beror på förbränning av fossila bränslen. Även förändringen i markanvändning leder till ökade halter av CO2. De största sänkorna av CO2 är den organiska konsumtionen av CO2 och havens upptag av CO2. Den organiska konsumtionen av CO2 varierar mycket under året och är som störst under de varmare månaderna. Havens upptag av CO2 varierar också mycket under året eftersom havens förmåga att lösa CO2 beror på vattnets temperatur. Syftet med den här studien är att analysera CO2-koncentrationen och CO2-flödena mellan hav och luft på Östergarnsholm, en liten, låg ö öster om Gotland. Resultaten jämförs med tidigare studier. CO2-koncentrationsdata samlades in mellan 1997 – 1999 och 2001 – 2003. CO2-flödesdata samlades in mellan 2001 och 2003. Analysen av CO2-koncentrationen visar att under perioden 1997 till 1999 är CO2- halterna för låga på Östergarnsholm. En korrektion gjordes genom att lägga till 27 ppm till de uppmätta CO2-halterna. Årsvariationerna av CO2-halterna är mycket tydliga men sommaren 1998 sjunker inte CO2-halten till så låga värden som de borde vara. Att CO2-halterna inte sjönk mer beror dels på El Niño-utbrottet 97/98 och dels på den lokalt kalla och regniga sommaren. De direkt mätta CO2-flödena korrigerades med hjälp av den välkända Webbkorrektionen innan de analyserades. CO2-flödena är beroende av vindhastigheten – högre vindhastighet ger högre CO2-flöden. CO2-flödena beror också på skillnaden i CO2-halt mellan luften och havet. Parameteriserade CO2-flöden beräknades och jämfördes med de direkt mätta CO2-flödena. De parameteriserade CO2-flödena beräknas antingen med kvadratiskt eller kubiskt vindberoende. För att beräkna parameteriserade CO2-flöden användes ett fast värde på skillnaden i CO2-halt mellan luften och vattnet eftersom CO2-halten i vattnet inte mäts. De parameteriserade CO2- flödenas vindberoende stämde överrens med de direkt mätta CO2-flödena.

Carbon dioxide (CO2) is an important greenhouse gas, and the atmospheric concentration of CO2 has increased by more than 100 ppm since prior to the industrial revolution.  The global oceans are considered an important sink of atmospheric CO2, since approximately one third of the anthropogenic emissions are absorbed by the oceans. To be able to model the global carbon cycle and the future climate, it is important to have knowledge of the processes controlling the air-sea exchange of CO2. In this thesis, measurements as well as a model is used in order to increase the knowledge of the exchange processes. The air-sea flux of CO2 is estimated from high frequency measurements using three methods; one empirical method, and two methods with a solid theoretical foundation. The methods are modified to be applicable for various atmospheric stratifications, and the agreement between methods is good in average. A new parameterization of the transfer velocity (the rate of transfer across the air-sea interface), is implemented in a Baltic Sea model. The new parameterization includes also the mechanism of water-side convection. The impact of including the new parameterization is relatively small due to feedback processes in the model. The new parameterization is however more representative for flux calculations using in-situ measurement or remote sensing products. When removing the feedback to the model, the monthly average flux increases by up to 20% in some months, compared to when water-side convection is not included. The Baltic Sea carbon budget was estimated using the Baltic Sea model, and the Baltic Sea was found to be a net sink of CO2. This is consistent with some previous studies, while contradictory to others. The dissimilarity between studies indicates the difficulty in estimating the carbon budget mainly due to variations of the CO2 uptake/release in time and space. Local variations not captured by the model, such as coastal upwelling, give uncertainties to the model. Coastal upwelling can alter the uptake/release of CO2 in a region by up to 250%. If upwelling would be included in the model, the Baltic Sea might be considered a smaller sink of CO2.

Since the beginning of the industrial revolution, a large amount of greenhouse gases such as carbon dioxide (CO2) have been emitted into the atmosphere due to human activities. One of the main consequences of these emissions is a rapid increase in atmospheric CO2 concentration and a profound modification of the Earth's climate system. The ocean plays an important role in the Earth radiative balance since it acts as an important CO2 sink for the atmosphere. By currently absorbing about 25 % of the CO2 emitted by humans it considerably slows down climate change. Understanding the present-day spatial and temporal dynamics of the air-sea CO2 exchange and the different processes that govern this exchange is of critical importance to anticipate the evolution of the oceanic CO2 sink in the future.This thesis was realized in this context and focused on an improved quantification of the exchange of CO2 through the air-sea interface (FCO2) of the global ocean, embracing open ocean waters and coastal regions. The main objective was to fill knowledge gaps in our understanding of the processes that govern the spatial and temporal distribution of FCO2. This objective was mainly achieved through observational approaches and addressed three main aspects: a quantification of the different sources of FCO2 uncertainties at the global scale, an analysis of spatial distribution of the oceanic CO2 exchange with a strong focus on the coastal ocean and a first assessment of the coastal seasonal FCO2 dynamics and its underlying drivers. The latter relied on a data-model fusion approach allowing to decompose the FCO2 seasonality into its main physical and biogeochemical drivers. The quantification of the oceanic FCO2 from observations consists in calculating an air-sea partial pressure CO2 gradient (ΔpCO2) between the atmosphere and the sea surface. Global monthly continuous partial pressure of CO2 (pCO2) products can for example be derived from observational pCO2 databases and statistical interpolation methods. This ΔpCO2 is then multiplied by a gas exchange transfer rate coefficient (k), which depends on wind speed. However, the parametrization of k is still entailed with poorly quantified uncertainties. From a literature review of all k parameterizations available in the literature over the past 25 years, I first quantified the FCO2 uncertainties associated with k globally and regionally for the open ocean. I also quantified the uncertainties associated with the choice of a wind product over another. Our results show that the range of global FCO2, calculated with these k relationships, diverge by 12 % when using CCMP, ERA or NCEP1. Regional discrepancies in FCO2 are more pronounced than global. These global and regional differences significantly increase when using NCEP2 or other k formulations. To minimize uncertainties associated with the choice of wind product, it is possible to recalculate the parametrization of k globally for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. In a second step, we improved the quantification and analysis of the dominant patterns and drivers of the FCO2 spatial distribution for the coastal ocean worldwide. This analysis was performed globally (at 0.25° spatial resolution), using a regional segmentation of the coastal ocean, and latitudinally. I found that coastal regions at high latitudes act as a CO2 sink while tropical regions and along the equator tend to act as an atmospheric CO2 source. Globally integrated, I quantified that the coastal seas act currently as a CO2 sink with a value of -0.20 ± 0.02 Pg C yr-1. For the first time, I also compared the spatial patterns of coastal FCO2 to that of the adjacent open ocean, globally. With the exception of some regions such as those dominated by riverine inputs, I demonstrated that they present similar latitudinal distribution of their FCO2 density per unit of surface area, suggesting analogous responses to increasing atmospheric CO2. I also reevaluated the global ocean CO2 budget and estimated a global anthropogenic CO2 uptake ranging between -2.6 ± 0.4 Pg C yr-1 and -2.9 ± 0.5 Pg C yr-1 for the 1998-2015 period. In a third step, I contributed to the first continuous observational pCO2 data product merging the coastal and open ocean in a consistent manner. This study showed that difference between open ocean and coastal ocean estimates along the overlap area increases with latitude but remains close to 0 µatm globally. Stronger discrepancies, however, exist on the regional level resulting in differences that exceed 10 % of the climatological mean pCO2, particularly in regions constrained by fewer observations, paired with biogeochemical complexity, such as the Peruvian upwelling system and ice covered regions.In a fourth step, a temporal analysis of the FCO2 seasonality was performed for the coastal ocean based on an observational approach. I analyzed and quantified the FCO2 seasonal dynamics globally and for different latitudinal bands. Globally, coastal regions act as a CO2 sink with a more intense uptake occurring in summer (-21 Tg C month-1) because of the disproportionate influence of high latitude shelves in the Northern Hemisphere. I also estimated the contribution of different drivers (sea-ice coverage, wind speed, and ΔpCO2 change) to the FCO2 seasonal amplitude. This data-driven approach allowed me to conclude that the ΔpCO2 is the main driver of the FCO2 variability at the seasonal timescale. I then used a global oceanic biogeochemical model to decompose the seasonal coastal pCO2 variability further into its driving physical and biological processes. From a first qualitative assessment, I concluded that the thermal effect associated to sea surface temperature changes is the main effect governing the coastal seasonal pCO2 variability except at high latitudes where the non-thermal effect associated to changes in biology, circulation, fresh water and the air-sea CO2 exchange itself dominate. I also found that, overall, the thermal effect alone should lead to larger seasonal fluctuations, but its influence is partly offset by the non-thermal effect. Throughout this thesis, I also evaluated the extent to which the continuous observational pCO2 products derived from an artificial neuronal network approach and from the global ocean biogeochemical model MOM6-COBALT could reproduce the raw pCO2 fields extracted from global databases. Overall, I showed that at the regional scale, the two products are in relatively good agreement compared to observations. I also identified regions where discrepancies are the largest and where future observational data are needed in the future, as well as regions where agreement is the most satisfactory and, thus, most suitable for further process-based analyses.
Depuis le début de la révolution industrielle, une grande quantité de gaz à effet de serre tels que le dioxyde de carbone (CO2) a été émise dans l'atmosphère en raison des activités humaines. L'une des principales conséquences de ces émissions est une augmentation rapide de la concentration en CO2 atmosphérique et une modification profonde du système climatique de la Terre. L'océan joue un rôle important dans l'équilibre radiatif de la Terre car il agit comme un important puits de CO2 pour l'atmosphère. En absorbant actuellement environ 25 % du CO2 émis par l'homme, il ralentit considérablement le changement climatique. Comprendre la dynamique spatiale et temporelle actuelle de l'échange de CO2 air-mer et les différents processus qui régissent cet échange est d'une importance cruciale pour anticiper l'évolution du puits océanique de CO2 à l'avenir.Cette thèse a été réalisée dans ce contexte et s'est concentrée sur une meilleure quantification de l'échange de CO2 à travers l'interface air-mer (FCO2) de l'océan global, considérant à la fois l’océan ouvert et les régions côtières. L'objectif principal était de combler les lacunes dans notre compréhension des processus qui régissent la distribution spatiale et temporelle du FCO2. Cet objectif a été principalement atteint grâce à des approches observationnelles et a abordé trois aspects principaux: une quantification des différentes sources d'incertitudes du FCO2 à l'échelle globale, une analyse de la distribution spatiale de l'échange de CO2 océanique avec un fort accent sur l'océan côtier et une première évaluation de la dynamique saisonnière du FCO2 côtier et de ses moteurs sous-jacents. Ce dernier s'est appuyé sur une approche de fusion de modèles et d’approches observationnelles permettant de décomposer la saisonnalité du FCO2 en ses principaux moteurs physiques et biogéochimiques.La quantification du FCO2 océanique à partir d’observations consiste à calculer un gradient de pression partielle air-mer de CO2 (ΔpCO2) entre l'atmosphère et la surface de la mer. Des produits globaux continus mensuels de la pression partielle de CO2 (pCO2) peuvent par exemple être dérivés à partir de bases de données observationnelles de pCO2 et de méthodes d'interpolation statistique. ΔpCO2 est ensuite multiplié par un coefficient de vitesse de transfert d'échange gazeux (k), qui dépend de la vitesse du vent. Cependant, la paramétrisation de k est sujette à de larges incertitudes et mal quantifiées. À partir d'une synthèse de la littérature de toutes les paramétrisations de k disponibles dans la littérature au cours des 25 dernières années, j'ai d'abord quantifié les incertitudes sur FCO2 associées à k à l'échelle globale et régionale pour l'océan ouvert. J'ai également quantifié les incertitudes associées au choix d'un produit éolien par rapport à un autre. Nos résultats montrent que la gamme du FCO2 global, calculée avec ces différentes paramétrisations de k, diverge de 12 % lors de l'utilisation de CCMP, ERA ou NCEP1. En raison des différences dans les pattern de vent régionaux, les différences régionales sur le FCO2 sont plus prononcés que globalement. Ces différences globales et régionales augmentent de manière significative lors de l'utilisation de NCEP2 ou d'autres formulations de k. Afin de réduire les incertitudes associées au choix du produit de vent, il est possible de recalculer la paramétrisation de k pour un produit de vent donné et à une résolution spatio temporelle.Dans un deuxième temps, nous avons amélioré la quantification et l'analyse des principaux pattern et des différents processus sur la distribution spatiale du FCO2 pour l’ensemble des régions côtières. Cette analyse a été réalisée à l'échelle globale (à une résolution spatiale de 0.25°), en utilisant une segmentation régionale de l'océan côtier, et latitudinalement. J'ai trouvé que les régions côtières aux hautes latitudes agissent comme un puits de CO2 tandis que les régions côtières tropicales et le long de l'équateur ont tendance à agir comme une source de CO2 atmosphérique. Globalement, j'ai quantifié que les régions côtières agissent actuellement en tant que puits de CO2 avec une valeur de -0.20 ± 0.02 Pg C an-1. Pour la première fois, j'ai également comparé la distribution spatiale du FCO2 côtier à celle de l'océan ouvert adjacent, à l'échelle globale. À l'exception de certaines régions telles que celles dominées par les apports fluviaux, j'ai démontré que les régions côtières et l’océan ouvert adjacent présentaient une distribution latitudinale similaire sur leur densité de FCO2 par unité de surface, suggérant des réponses analogues à l'augmentation du CO2 atmosphérique. J'ai également réévalué le budget mondial de CO2 de l'océan et estimé une absorption mondiale de CO2 anthropique comprise entre -2.6 ± 0.4 Pg C an-1 et -2.9 ± 0.5 Pg C an-1 pour la période 1998-2015. Dans un troisième temps, j'ai contribué à la création du premier produit continu de pCO2 observationnelles fusionnant le domaine côtier et l'océan ouvert de manière cohérente. Cette étude a montré que la différence entre les estimations provenant du produit de pCO2 de l’océan ouvert à celles dérivant du produit de pCO2 de l’océan côtier le long de leur zone de chevauchement augmente avec la latitude mais reste proche de 0 µatm globallement. Des divergences plus fortes existent cependant au niveau régional, entraînant des différences qui dépassent 10 % sur la moyenne climatologique de pCO2, en particulier dans les régions contraintes par moins d'observations, associées à une complexité biogéochimique, comme le système d'upwelling péruvien et les régions couvertes de glace.Dans une quatrième étape, une analyse temporelle de la saisonnalité du FCO2 a été réalisée pour l'océan côtier sur la base d'une approche observationnelle. J'ai analysé et quantifié la dynamique saisonnière du FCO2 à l'échelle globale et pour différentes bandes latitudinales. À l'échelle globale, les régions côtières agissent comme un puits de CO2 avec une absorption plus intense se produisant en été (-21 Tg C mois-1) en raison de l'influence disproportionnée des régions côtières des hautes latitudes dans l'hémisphère Nord. J'ai également estimé la contribution de différents processus (couverture de glace de mer, vitesse du vent et changement de ΔpCO2) à l'amplitude saisonnière du FCO2. Cette approche basée sur les données observationnelles m'a permis de conclure que ΔpCO2 est le principal moteur de la variabilité du FCO2 à l'échelle saisonnière. J'ai ensuite utilisé un modèle biogéochimique océanique global pour décomposer davantage la variabilité saisonnière du pCO2 côtier en ses processus physiques et biologiques. À partir d'une première évaluation qualitative, j'ai conclu que l'effet thermique associé aux changements de température de la surface de la mer est le principal effet régissant la variabilité côtière saisonnière du pCO2 sauf aux hautes latitudes où l'effet non thermique associé aux changements de biologie, de circulation, d'eau douce et de l’échange de CO2 air-mer domine. J'ai également constaté que, globalement, l'effet thermique à lui seul devrait entraîner des fluctuations saisonnières plus importantes, mais son influence est en partie compensée par l'effet non thermique.Tout au long de cette thèse, j'ai également évalué dans quelle mesure les produits continus de pCO2 observationnelles dérivés d'une approche de réseau de neurones artificiels et du modèle biogéochimique océanique global MOM6-COBALT pourraient reproduire les champs de pCO2 bruts extraits des bases de données globale. Dans l'ensemble, j'ai montré qu'à l'échelle régionale, les deux produits sont relativement en bon accord par rapport aux observations. J'ai également identifié les régions où les différences sont les plus importantes et où de futures données observationnelles sont nécessaires à l'avenir, ainsi que les régions où les deux produits présentent un accord le plus satisfaisant et, par conséquent, le plus approprié pour de futures analyses de compréhension des différents processus.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

The Antarctic Seasonal Ice Zone (ASIZ) is potentially a large contemporary sink for anthropogenic CO2 due to the formation of bottom water along the Antarctic coast. However, south of 55°S, the lack of measurements of the fugacity of CO2 in surface seawater (fCO2), or the concentration and ratio of stable carbon isotopes of atmospheric CO2, has meant that it has been difficult to determine whether the ASIZ acts as a net source or sink for atmospheric CO2. This study contributes to, and is largely based on, new measurement programmes of oceanic fCO 2 and the concentration and 13C/12C ratio of atmospheric CO2 over the region of the Southern Ocean between Australia and the Antarctic continent, with particular emphasis on data from regions of pack ice. Using fCO2 data from six voyages of the RSV Aurora Australis, it was estimated that between 1 October 1992 and 31 March 1993 the ocean south of 55°S, between 60°E and 150°E, sequestered 0.025 ± 0.013 Gt C over an area of ocean equivalent to 19% of the maximum area of open water south of 55°S. The CO2 sink was most pronounced west of 105°E (0.026 ± 0.013 Gt C), where it was associated with intense summer phytoplankton blooms following the melting of sea-ice. In conjunction with the sampling of oceanic fCO 2, flasks were regularly filled on the ship with dry air and later analysed for levels of CO2 and its 13C/12C ratio. This provided the opportunity to observe atmospheric variations directly forced by fluctuations in fCO2, temperature, and the 13C112C ratio of dissolved inorganic carbon (DIC) in the surface ocean. Sea surface temperature and 13C/12C-DIC effects are transmitted to the atmosphere by gross air-sea fluxes of CO2 in the absence of net exchange. Over the ice-free region of the Southern Ocean between 44°S and 60°S, from 85°E to 160°E, atmospheric 13CO2/12CO2 values were dominated by a linear dependence on sea surface temperature (0.0041 ± 0.0003 °/00 °C-1 ), due to the "equilibrium" isotopic fractionation of CO2 during air-sea exchange. During late spring and summer, over the region of the ASIZ south of 60°S, between 60°E and 105°E, the effect of sea surface temperature on atmospheric 13CO2/12CO2 values was overwhelmed by the effect of high marine productivity on 13C/12C -DIC. It is demonstrated that the impact of net air-sea flux of 13CO2 on atmospheric ratios of 13CO2/12CO2 can be measured more easily than the impact of net CO2 flux on atmospheric mixing ratios of CO2 . Long-term changes in sea surface temperature and productivity over the ASIZ, and therefore net ocean uptake, can be more accurately determined from isotopic ratios of 13CO2/12CO2 in baseline air samples from a coastal Antarctic station, than from mixing ratios of CO2 in the same samples.

The Arctic marine system is currently undergoing transition as a result of climate change. This study examines the effects of this transition on rates of air-sea CO2 exchange within the eastern Canadian Arctic. Continuous seawater pCO2 measurements revealed this area to be a strong summertime sink of atmospheric CO2. Total alkalinity and stable oxygen isotopes were utilized as freshwater tracers, revealing areas of significant sea ice melt and riverine inputs. Eastern Baffin Bay and Barrow Strait were found to be strongly influenced by sea ice melt, lowering seawater pCO2; whereas Kennedy Channel contained significant river discharge, raising seawater pCO2. Primary production in surface waters was low throughout the region, with the exception of Petermann Fjord where glacial ice melt likely transports nutrients to the surface. This region is anticipated to represent a weaker CO2 sink in the future, due mainly to predicted decreases in sea ice thickness and extent.
February 2016

Cycle characteristics of closed gas turbines using super critical carbon dioxide as a working fluid are investigated. It is found an anomalous behavior of physical properties of CO2 at pseudo-critical point may limit heat exchange rate of a regenerative heat exchanger due to the presence of pinch point inside the regenerative heat exchanger. Taking such pinch problem into consideration, the cycle efficiency of Brayton cycle is assessed. Its value is found limited to 39% degraded by 8% compared with the case without the pinch present inside. As an alternative a part flow cycle is investigated and its operable range has been identified. It is revealed that the part flow cycle is effective to recover heat transfer capability and may achieve the cycle thermal efficiency of 45% under maximum operating conditions of 20MPa and 800K. Optimal combination of turbine expansion ratio and a part flow ratio is 2.5 and 0.68 respectively. Parametric study is carried out. In neither compressor nor turbine, deteriorated adiabatic efficiency may affect cycle efficiency significantly. However, pressure drop characteristics of heat exchangers govern the cycle efficiency.

Brayton cycles are currently being extensively investigated for possible use with nuclear reactors in order to reduce capital cost, shorten construction period and increase nuclear power plant efficiency. The main candidates are the well-known helium Brayton cycle and the less familiar supercritical CO2 cycle, which has been given increased attention in the past several years. The main advantage of the supercritical CO2 cycle is comparable efficiency with the helium Brayton cycle at significantly lower temperature (550°C/823K), but higher pressure (20MPa/200 normal atmospheres). By taking advantage of the abrupt property changes near the critical point of CO2 the compression work can be reduced, which results in a significant efficiency improvement. Among the surveyed compound cycles the recompression cycle offers the highest efficiency, while still retaining simplicity. The turbomachinery is highly compact and achieves efficiencies of more than 90%. Preliminary assessment of the control scheme has been performed as well. It was found that conventional inventory control could not be applied to the supercritical CO2 recompression cycle. The conventional bypass control is applicable. The reference cycle achieves 46% thermal efficiency at the compressor outlet pressure of 20MPa and turbine inlet temperature of 550°C. The sizing of the heat exchangers and turbomachinery has been performed. The recuperator specific volume is 0.39m3/MWe and pre-cooler specific volume 0.08m3/MWe. For the reference 600MWth reactor this translates to ∼ 99m3 heat exchanger core for the recuperator and ∼ 21m3 for the pre-cooler. Overall the cycle offers an attractive alternative to the steam cycle. The supercritical CO2 cycle is well suited to any type of nuclear reactor with core outlet temperature above ∼ 500°C.

The present writer proposes a new concept of power generation system for CO2 recovery named Hydrogen Decomposed from Natural Gas Turbine (HYDET) in this paper. This concept is natural gas reforming and hydrogen separation. The natural gas is reformed with steam simultaneously the hydrogen is separated from the reformed gas through the hydrogen separation membrane. After the residual gas is combusted by the after burner with pure oxygen, CO2 steam mixture is exhausted. An inorganic hydrogen separation membrane will be assumed to use such as ceramic multi-layer porous membranes. The performance of the proposed system will be over 50% HHV at the sending-end. Though the CO2 recovery ratio will depend upon Hydrogen and CO2 separation ratio of the membrane, theoretically it will be over 90%. And using CO2 liquefaction equipment of LNG cold heat exchanger, liquefied CO2 will be recovered with extra high efficiency. In order to apply this system, the development of the membrane reformer is the key technology.

The extent to which the benefits of hydrocarbon fuel cooling technology can be realized depends on our ability to manage coke deposits. The coke deposits, which may form in heat exchangers, reactors and on inside surfaces of fuel system components, degrade heat transfer, catalyst activity, and fuel flow characteristics and can lead to system failure. Therefore, in situ regeneration of fouled surfaces was investigated as a practical approach for reducing the impact of coke formation on aircraft thermal management systems. Thermogravimetric analysis (TGA) was used to evaluate various surface regeneration techniques (such as carbon burnoff in air or oxygen and carbon gasification using CO2 or steam) and to compile a database for kinetics analysis. The most practical technique for in situ surface regeneration of heat exchangers and reactors is the carbon burnoff method. Although the burnoff method is simple and cost-effective, care must be taken to control strong exothermic reactions. For this reason, a kinetics model and a computer simulation have been developed to guide the selection of the key operating variables (i.e., temperature, pressure, and time) for the in situ regeneration method. A surface regeneration simulator was constructed and used to assess the effectiveness of the in situ regeneration techniques and validate the kinetics model. Use of the computer simulation tool in a real application to specify the conditions for removing coke deposits from a fouled heat-exchanger/reactor panel from a scramjet test engine is discussed in the paper.

This article illustrates aspects of heat recovery steam generator (HRSG) design when employing process integration in an integrated reforming combined cycle (IRCC) with pre-combustion CO2 capture. Specifically, the contribution of the paper is to show how heat integration in a pre-combustion CO2 capture plant impacts the selection of HRSG design. The purpose of such a plant is to generate power with very low CO2 emissions, typically below 100 g CO2/net kWh electricity. This should be compared to a state-of-the-art natural gas combined cycle (NGCC) plant with CO2 emissions around 380 g CO2/net kWh electricity. The design of the HRSG for the IRCC process was far from standard because of the significant amount of steam production from the heat generated by the auto-thermal reforming process. This externally generated steam was transferred to the HRSG superheaters and used in a steam turbine. For an NGCC plant, a triple-pressure reheat steam cycle would yield the highest net plant efficiency. However, when generating a significant amount of high-pressure steam external to the HRSG, the picture changed. The complexity of selecting a HRSG design increased when also considering that steam can be superheated, and low-pressure and intermediate-pressure steam can be generated in the process heat exchangers. For the concepts studied it was also of importance to maintain a high net plant efficiency when operating on natural gas. Therefore the selection of HRSG design had to be a compromise between NGCC and IRCC operating modes.

The possibility of using a Molten Carbonate Fuel Cell (MCFC) to reduce the CO2 emission from Gas Turbine Power Plant (GTPP) is shown. The MCFC is placed after a gas turbine. The main advantages of this solution are: higher total electric power generated by hybrid system and reduced CO2 emission with remained system efficiency. A comparison of three systems: standard GTPP, GT-MCFC, and GT-MCFC with additional heat exchangers is shown. The application of MCFC could reduce CO2 emission of 73% (absolutely) and 77% relative to produced power.

The European electric power industry has undergone considerable changes over the past two decades as a result of more stringent laws concerning environmental protection along with the deregulation and liberalization of the electric power market. However, the pressure to deliver solutions in regard to the issue of climate change has increased dramatically in the last few years and given the rise to the possibility that future natural gas-fired combined cycle (NGCC) plants will also be subject to CO2 capture requirements. At the same time, the interest in combined cycles with their high efficiency, low capital costs and complexity has grown as a consequence of addressing new challenges posed by the need to operate according to market demand in order to be economically viable. Considering that these challenges will also be imposed on new natural gas-fired power plants in the foreseeable future, this study presents a new process concept for natural gas combined cycle power plants with CO2 capture. The simulation tool IPSEpro is used to model a 400 MW single-pressure NGCC with post-combustion CO2 capture, using an amine-based absorption process with Monoethanolamine. To improve the costs of capture the gas turbine, GE 109FB, is utilizing exhaust gas recirculation, thereby increasing the CO2 content in the gas turbine working fluid to almost double that of conventional operating gas turbines. In addition, the concept advantageously uses approximately 20% less steam for solvent regeneration by utilizing preheated water extracted from HRSG. The further recovery of heat from exhaust gases for water preheating by use of an increased economizer flow results in an outlet stack temperature comparable to those achieved in combined cycle plants with multiple pressure levels. As a result, overall power plant efficiency as high as that achieved for a triple-pressure reheated NGCC with corresponding CO2 removal facility is attained. The concept thus provides a more cost-efficient option to triple-pressure combined cycles since the number of heat exchangers, boilers, etc. is reduced considerably.

In the last years greenhouse gas emissions, and in particular carbon dioxide emissions, have become a major concern in the power generation industry and a large amount of research work has been dedicated to this subject. Among the possible technologies to reduce CO2 emissions from power plants, the pre-treatment of the fossil fuels to separate carbon from hydrogen before the combustion process is one of the least energy consuming way to facilitate CO2 capture and removal from the power plant. In this paper several power plant schemes with reduced CO2 emissions were simulated. All the configurations were based on the following characteristics: (1) syngas production via natural gas reforming; (2) two reactors for CO-shift; (3) “pre-combustion” decarbonization of the fuel by CO2 absorption with amine solutions; (4) combustion of hydrogen rich fuel in a commercially available gas turbine; (5) combined cycle with three pressure levels, to achieve a net power output in the range of 400 MW. The base reactor employed for syngas generation is the ATR (Auto Thermal Reformer). The attention was focused on the optimization of the main parameters of this reactor and its interaction with the power section. In particular the simulation evaluated the benefits deriving from the post-combustion of exhaust gas and from the introduction of a gas-gas heat exchanger. All the components of the plants were simulated using Aspen Plus software, and fixing a reduction of CO2 emissions of at least 90%. The best configuration showed a thermal efficiency of approximately 48% and CO2 specific emissions of 0.04 kg/kWh.

The introduction of closed cycle gas turbines with their capability of retaining combustion generated CO2 can offer a valuable contribution to the Kyoto goal and to future power generation. Therefore, research and development at Graz University of Technology has lead to the GRAZ CYCLE, a zero emission power cycle of highest efficiency. The GRAZ CYCLE is still on a theoretical level, first tests with the turbo-machinery equipment were performed. In the GRAZ CYCLE fossil fuels are burned with pure oxygen which enables a cost-effective separation of the combustion generated CO2 by condensation. Cycle efficiencies as high as 63% can be reached. Taking the efforts for the oxygen supply into account the efficiency is reduced to 55% [1]. This work presents a further step towards a GRAZ CYCLE prototype plant, with special emphasis on the layout and design of the heat recovery steam generator (HRSG). The hot exhaust gas of the turbine consists mainly of CO2 and H2O. This exhaust gas causes higher demands on the HRSG. A faster corrosion of the heat exchangers and the recirculation of the cycle fluid have to be considered. Based on the design of conventional HRSGs, the necessary adaptations are discussed and economically evaluated.

In this paper two different power plant processes with their current optimized layouts of the AZEP project and the recently started OXYCOAL-AC project are presented. Both processes are designed for CO2-capture combined with oxygen membrane technology. As a consequence of the implementation of a membrane module there will be essential changes in the plant layout involving modifications to the turbomachinery designs due to the different working medium and interactions of different cycle components. Furthermore, there are different loops included for recirculation of the combustion gas constituents CO2 and H2O. Although, the processes presented have different boundary conditions regarding the selection of fuel and gross power output, they have in common the need for new turbomachinery designs. These two processes are thermodynamically analyzed and compared both at design point and at off-design (part-load operation) mode. Main focus are the different operation modes of AZEP and different turbomachinery layouts for OXYCOAL-AC. Special attention is paid to the modeling of the crucial components common for both power plant processes e.g. the oxygen membrane, the turbine and the compressors. The thermodynamic studies aim at analyzing a) the requirements on turbomachinery at the design point, b) how to reduce the level of requirements for the compressor and the turbine and c) the operation and potential of mismatch for the turbomachinery during part-load operation. Simplified turbomachinery maps and a simplified black box 1-D model of the membrane module and the heat exchangers are used within a commercial heat- and mass-balance program for simulation of part-load operation of both processes. The objective of this conceptual study is the investigation of parameter changes caused by the interaction of process components.