• Energy Research
• chemical engineering close
• Transport Research close

The blending of hydrogen gas into natural gas pipelines is an effective way of achieving the goal of carbon neutrality. Due to the large differences in the calorific values of natural gas from different sources, the calorific value of natural gas after mixing with hydrogen may not meet the quality requirements of natural gas, and the quality of natural gas entering long-distance natural gas and urban gas pipelines also has different requirements. Therefore, it is necessary to study the effect of multiple gas sources and different pipe network types on the differences in the calorific values of natural gas following hydrogen admixing. In this regard, this study aimed to determine the quality requirements and proportions of hydrogen-mixed gas in natural gas pipelines at home and abroad, and systematically determined the quality requirements for natural gas entering both long-distance natural gas and urban gas pipelines in combination with national standards. Taking the real calorific values of the gas supply cycle of seven atmospheric sources as an example, the calorific and Wobbe Index values for different hydrogen admixture ratios in a one-year cycle were calculated. The results showed that under the requirement of natural gas interchangeability, there were great differences in the proportions of natural gas mixed with hydrogen from different gas sources. When determining the proportion of hydrogen mixed with natural gas, both the factors of different gas sources and the factors of the gas supply cycle should be considered.

Abstract According to many prognostic scenarios by the Intergovernmental Panel on Climate Change (IPCC), a scaling-up of carbon dioxide (CO2) capture and storage (CCS) by several orders-of-magnitude is necessary to meet the target of ≤2 °C global warming by 2100 relative to preindustrial levels. Since a large fraction of the predicted CO2 storage capacity lies offshore, there is a pressing need to develop field-tested methods to detect and quantify potential leaks in the marine environment. Here, we combine field measurements with numerical models to determine the flow rate of a controlled release of CO2 in a shallow marine setting at about 119 m water depth in the North Sea. In this experiment, CO2 was injected into the sediment at 3 m depth at 143 kg d-1. The new leakage monitoring tool predicts that 91 kg d-1 of CO2 escaped across the seafloor, and that 51 kg d-1 of CO2 were retained in the sediment, in agreement with independent field estimates. The new approach relies mostly on field data collected from ship-deployed technology (towed sensors, Acoustic Doppler current profiler—ADCP), which makes it a promising tool to monitor existing and upcoming offshore CO2 storage sites and to detect and quantify potential CO2 leakage.

Temporal variations in CO2 stream composition and mass flow rates may occur in a CO2 transport network, as well as further downstream when CO2 streams of different compositions and temporally variable mass flow rates are fed in. To assess the potential impacts of such variations on CO2 transport, injection, and storage, their characteristics must be known. We investigated variation characteristics in a scenario of a regional CO2 emitter cluster of seven fossil-fired power plants and four industrial plants that feed captured CO2 streams into a pipeline network. Variations of CO2 stream composition and mass flow rates in the pipelines were simulated using a network analysis tool. In addition, the potential effects of changes in the energy mix on resulting mass flow rates and CO2 stream compositions were investigated for two energy mix scenarios that consider higher shares of renewable energy sources or a replacement of lignite by hard coal and natural gas. While resulting maximum mass flow rates in the trunk line were similar in all considered scenarios, minimum flow rates and pipeline capacity utilisation differed substantially between them. Variations in CO2 stream composition followed the power plants’ operational load patterns resulting e.g., in stronger composition variations in case of higher renewable energy production.

Abstract We investigated the effect of an artificial CO2 vent (0.0015−0.037 mol s−1), simulating a leak from a reservoir for carbon capture and storage (CCS), on the sediment geochemistry. CO2 was injected 3 m deep into the seafloor at 120 m depth. With increasing mass flow an increasing number of vents were observed, distributed over an area of approximately 3 m. In situ profiling with microsensors for pH, T, O2 and ORP showed the geochemical effects are localized in a small area around the vents and highly variable. In measurements remote from the vent, the pH reached a value of 7.6 at a depth of 0.06 m. In a CO2 venting channel, pH reduced to below 5. Steep temperature profiles were indicative of a heat source inside the sediment. Elevated total alkalinity and Ca2+ levels showed calcite dissolution. Venting decreased sulfate reduction rates, but not aerobic respiration. A transport-reaction model confirmed that a large fraction of the injected CO2 is transported laterally into the sediment and that the reactions between CO2 and sediment generate enough heat to elevate the temperature significantly. A CO2 leak will have only local consequences for sediment biogeochemistry, and only a small fraction of the escaped CO2 will reach the sediment surface.

The knowledge of the phase behavior of carbon dioxide (CO2)-rich mixtures is a key factor to understand the chemistry and migration of natural volcanic CO2 seeps in the marine environment, as well as to develop engineering processes for CO2 sequestration coupled to methane (CH4) production from gas hydrate deposits. In both cases, it is important to gain insights into the interactions of the CO2-rich phase—liquid or gas—with the aqueous medium (H2O) in the pore space below the seafloor or in the ocean. Thus, the CH4-CO2 binary and CH4-CO2-H2O ternary mixtures were investigated at relevant pressure and temperature conditions. The solubility of CH4 in liquid CO2 (vapor-liquid equilibrium) was determined in laboratory experiments and then modelled with the Soave–Redlich–Kwong equation of state (EoS) consisting of an optimized binary interaction parameter kij(CH4-CO2) = 1.32 × 10−3 × T − 0.251 describing the non-ideality of the mixture. The hydrate-liquid-liquid equilibrium (HLLE) was measured in addition to the composition of the CO2-rich fluid phase in the presence of H2O. In contrast to the behavior in the presence of vapor, gas hydrates become more stable when increasing the CH4 content, and the relative proportion of CH4 to CO2 decreases in the CO2-rich phase after gas hydrate formation.

AbstractDue to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies.

The exchange of CH4 by CO2 in gas hydrates is of interest for the production of natural gas from methane hydrate with net zero climate gas balance, and for managing risks that are related to sediment destabilization and mobilization after gas-hydrate dissociation. Several experimental studies on the dynamics and efficiency of the process exist, but the results seem to be partly inconsistent. We used confocal Raman spectroscopy to map an area of several tens to hundreds µm of a CH4 hydrate sample during its exposure to liquid and gaseous CO2. On this scale, we could identify and follow different processes in the sample that occur in parallel. Next to guest-molecule exchange, gas-hydrate dissociation also contributes to the release of CH4. During our examination period, about 50% of the CO2 was bound by exchange for CH4 molecules, while the other half was bound by new formation of CO2 hydrates. We evaluated single gas-hydrate grains with confirmed gas exchange and applied a diffusion equation to quantify the process. Obtained diffusion coefficients are in the range of 10−13–10−18 m2/s. We propose to use this analytical diffusion equation for a simple and robust modeling of CH4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation.