Recent Advances in CO₂ Capture, CO₂ Utilization, and Green Chemistry for Sustainability

To know the sustainable performance of calcium-based adsorbents is one of the important aspects to realize efficient and economical carbon capture, and to systematically study the properties of natural adsorbents is conducive to their industrialization. The cyclic calcination and carbonation characteristics of a typical natural limestone were investigated using a thermal gravimetric analyzer. Two kinds of over-sintering conditions were selected to emphatically study the cyclic separation of CO₂ from limestones through prolonging the calcination time and increasing the calcination temperature. The results showed that the untimely end of the chemical reaction control stage caused by excessive sintering is the direct reason for the reduction in cyclic carbonation conversion, and the changes in surface morphology of calcined products due to pore collapse and fusion are the fundamental reasons for the reduction in cyclic carbonation conversion. The excessive sintering caused by extending the calcining time or increasing the calcining temperature has great inhibition on this cycle only; the inhibition decreases rapidly in subsequent cycles. In addition, SEM and BET–BJH tests further confirm the influence of the over-sintering phenomenon. With the further increase in cycle number, the early excessive sintering has certain stimulative effects on the subsequent carbonation reaction. It is expected to provide a reference for the subsequent research and development of natural calcium-based adsorbents. Full article

The catalytic applications of micron Cu powder are limited due to its large particle size and small specific surface area. Modifying micro-Cu powder to achieve a high catalytic performance is a challenge in the application of micron copper. In this work, micro-Cu was used to synthesize a CeO₂–Cu catalyst, and the phase composition and surface pore structure were analyzed using XRD, BET, etc. The CO₂ hydrogenation performance of the CeO₂–Cu catalyst was analyzed in comparison with CeO₂ and Cu, and we found that the CeO₂–Cu catalyst exhibited a synergistic effect between Cu and cerium, resulting in a much higher hydrogenation performance at 500 °C than CeO₂ or Cu alone. H₂-TPR and TEM characterization revealed that the CeO₂–Cu catalyst formed interfacial interactions with a relatively large Ce–Cu interface, where cerium oxide could promote the reduction of CuO and lower the reduction temperature. Additionally, cerium oxide formed a confinement structure for Cu, and the CeO₂–Cu catalyst exhibited a higher oxygen vacancy concentration, thereby promoting the CO₂ hydrogenation performance. Cu–CeO₂ interaction provides valuable insights into the catalytic application of micron Cu powder. Full article

The absorption heat exchanger with a large temperature difference has a higher heat transfer superiority than the other heat exchangers (including plate heat exchanger), which is more suitable for long-distance heating. To improve its system performance, parameter collaborative optimization (including building accurate predictive models) has become an effective method because it does not require too much investment. In this study, a heat exchange station was chosen as a case study, and a model of a long short-term memory (LSTM) neural network was used to predict the temperatures of primary return water and secondary return water. Accordingly, the reliability of the fitting result based on the model was confirmed through a contrastive analysis with the prediction results of a support vector machine (SVM) model, a random forest (RF) model, and an extreme gradient boosting (XGBoost) model. In addition, the algorithm of particle swarm optimization was used to optimize the flow rate of primary supply water. The results showed that the temperature of primary-side return water decreased from 29.6 °C to 28.2 °C, the temperature of secondary-side return water decreased from 39.8 °C to 38.6 °C, and the flow rate of primary-side supply water decreased from 39 t/h to 35.2 t/h after the optimization of the flow rate of primary supply water. The sensibility assessment emerged that the secondary-side flow rate to the secondary-side supply water temperature was about 7 times more sensitive than the primary-side supply water temperature, and concretely, the lower the temperature, the higher the sensibility. In summary, the accuracy of the proposed prediction model was validated and the optimization direction was pointed out, which can be used to provide guidance for designing and planning absorption heat exchange stations with large temperature differences. Full article

The integrated CO₂ capture and utilization coupled with the reverse water-gas shift reaction (ICCU-RWGS) presents an alternative pathway for converting captured CO₂ into CO in situ. This study investigates the effectiveness of three calcium-based materials (natural limestone, sol-gel CaCO₃, and commercial CaCO₃) as dual-functional materials (DFMs) for the ICCU-RWGS process at intermediate temperatures (650–750 °C). Our approach involves a fixed-bed reactor coupled with mass spectrometry and in situ Fourier transform infrared (FTIR) measurements to examine cyclic CO₂ capture behavior, detailed physical and chemical properties, and morphology. The in situ FTIR results revealed the dominance of the RWGS route and exhibited self-catalytic activity across all calcium-based materials. Particularly, the natural limestone demonstrated a CO yield of 12.7 mmol g⁻¹ with 100% CO selectivity and 81% CO₂ conversion. Over the 20th cycle, a decrease in CO₂ capture capacity was observed: sol-gel CaCO₃, natural limestone, and commercial CaCO₃ showed reductions of 44%, 61%, and 59%, respectively. This suggests inevitable deactivation during cyclic reactions in the ICCU-RWGS process, while the skeleton structure effectively prevents agglomeration in Ca-based materials, particularly in sol-gel CaCO₃. These insights, coupled with the cost-effectiveness of CaO-alone DFMs, offer promising avenues for efficient and economically viable ICCU-RWGS processes. Full article

In this study, circulating fluidized bed fly ash (CFBFA) non-sintered ceramsite was innovatively developed. The CFBFA was addressed by adding ternary activator (including cement, hydrated lime, and gypsum) to prepare ceramsite. In the curing process, the use of power plant flue gas for curing not only captured greenhouse gas CO₂, but also enhanced the compressive strength of the ceramsite. The compressive strength of the composite gravels prepared by the CFBFA was modeled using a novel approach that employed the response surface methodology (RSM) and artificial neural network (ANN) coupled with genetic algorithm (GA). Box–Behnken design (BBD)-RSM method was used for the independent variables of cement content, hydrated lime content, and gypsum content. The resulting quadratic polynomial model had an R² value of 0.9820 and RMSE of 0.21. The BP-ANN with a structure of 3-10-1 performed the best and showed better prediction of the response than the BBD-RSM model, with an R² value of 0.9932 and RMSE of 0.19. The process parameters were optimized using RSM optimization tools and GA. Validation experiments showed that the GA-ANN prediction results were more accurate than the BBD-RSM results. Full article

Coal is the primary energy source in China, and coal pyrolysis is considered an essential and efficient method for clean coal utilization. Three high arsenic coals collected from the southwestern Guizhou province of China were chosen in this study. Low-temperature ashing plus X-ray diffraction analysis (XRD) was used to identify the minerals in coals. The three coals were pyrolyzed in a tube furnace in an N₂ atmosphere at 950 °C, 1200 °C, and 1400 °C, respectively. Environment scanning electron microscope (ESEM), XRD, X-ray fluorescence analysis (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) were adopted to determine the morphology, mineral compositions, and element compositions and arsenic contents of the coal pyrolysis ashes, respectively. It can be found that minerals in coal are mainly composed of quartz, pyrite, muscovite, and rutile. The minerals in the ashes generated from coal pyrolysis mainly contain quartz, dehydroxylated muscovite, iron oxide minerals, mullite, and silicon nitride. Oldhamite and gupeite exist at 950 °C and 1400 °C, respectively. The morphologies of oldhamite and gupeite at these temperatures are irregular block-shaped particles and irregular spherical particles, respectively. The mineralogical transformations in the process of coal pyrolysis affect coal utilization. The arsenic release rate is higher than 87% during pyrolysis at 1400 °C. The arsenic in organic matter is more able to be volatilized than mineral components. The retention time can slightly influence the arsenic release rate, and the influence of temperature is much more significant than the influence of retention time. The understanding of mineral evolution and arsenic environmental emission is helpful for the safety of high-arsenic coal pyrolysis. Full article

Carbon sink afforestation (CSA) has become one of the most concerned issues of countries around the world under the background of climate change. The northern forest ecosystem, located in mid- and high latitudes, is a huge terrestrial carbon pool and is very sensitive to climate change. Studying the carbon emission accounting of CSA in northern forests helps clarify the contribution of CSA to forestry carbon sequestration and forecasts the carbon sink effect of forest ecosystems. It is of great significance for the assessment of forest carbon sink and carbon cycling by providing a scientific basis and reference for the development, utilization, and management of carbon sink afforestation, as well as the coordinated development of ecology and social economy. At present, research on the carbon emission accounting of the CSA in northern China is still lacking. According to the relevant models and parameters of estimating live biomasses with the default method from the IPCC’s (Intergovernmental Panel on Climate Change) Technical Guidelines for National Forestry Carbon Sink Accounting and Monitoring, carbon stock, carbon emission, and carbon leakage of the Weihe CSA (carbon sink afforestation) pilot demonstration area in the boreal Longjiang Forest Industry in a baseline scenario and CSA scenario were measured, and the CSA’s net carbon sink was estimated. The conclusions were as follows: (1) By the end of the crediting period of the project’s baseline, carbon fixation reached 101.85 t CO₂, with an average annual CO₂ fixation of 5.09 t. By the end of the CSA term, carbon sequestration was accumulated as 382.13 t CO₂, with an average annual sequestration of 19.11 t CO₂, nearly four times that of the baseline period. (2) By the end of the CSA term, the carbon sequestration of the coniferous standing forest was 46.2% higher than that of the broad-leaved standing forest, accounting for 65% of the total carbon sequestration of the forest. The carbon sequestration of the tree species in the coniferous forest in descending order is Picea koraiensis, Pinus koraiensis, Larix olgensis, Fraxinus mandshurica, and Populus cathayana. The carbon sink density of the coniferous standing forest was 8.7% higher than that of the broad-leaved standing forest. (3) The carbon fixation of the coniferous standing forest nearly doubled that of the broad-leaved standing forest. The highest carbon fixation belongs to Fraxinus mandshurica, closely followed by Picea koraiensis and Pinus koraiensis at a high level, and then Larix olgensis and Populus cathayana. The carbon fixation of Fraxinus mandshurica was 20 times that of Populus cathayana. (4) The accumulated greenhouse gas emissions within the boundary during the CSA period were 2.53 t CO₂-e. The accumulated greenhouse gas leakage outside the boundary was 0.05 t CO₂-e. Carbon emissions occurred in the first, second, and third years of the crediting period, while carbon leakage occurred only in the first year. (5) This result appeared as carbon sources during the first three years of the CSA period but changed to carbon sink from the fourth year and then accumulated to 280.28 t (70.07 t CO₂-e·hm⁻²) as a net carbon sink by the end of the term. The Weihe CSA appeared to have a relatively strong ability of carbon sequestration in temperate forests. The CSA activity is influenced by factors such as policies, environment, management, etc., resulting in uncertainties in carbon sequestration accounting. Therefore, it is suggested that comparison studies and investigations should be strengthened, and multiple methods should be integrated into carbon sequestration estimation and accounting, leading the carbon accounting of forest ecosystems to a high-level and comprehensive development. Full article

Greenhouse gases, the main cause of global warming, are generated largely in the energy sector. As the need for technology that has reduced greenhouse gas emissions while producing energy is on an increase, CCU technology, which uses CO₂ to produce CH₄ (SNG energy, synthetic natural gas), is drawing attention. Thus, the reaction for converting CO₂ to CH₄ at a specific temperature using a catalyst is CO₂ methanation. The field of CO₂ methanation has been actively studied, and many studies have been conducted to enhance the activity of the catalysts. However, there is a lack of research on the variables that may appear when CO₂ methanation is attempted using emissions containing CO₂ generated from industrial fields and bio-plants. According to previous studies, it is reported that realistic feed gases from gasification or biomass plants contain a significant amount of CO. this study is a follow-up study focused on the application of CO₂ methanation in various real processes. In the CO₂ methanation reaction, a study was conducted on the catalyst efficiency and durability of CO gas that can coexist in the inlet gas rather than CO₂ and H₂ gas. The CO₂ methanation activity was observed at 200–350 °C when 0–15% CO coexisted using the Ni-Ce-Zr catalyst, and the operating variables were set for optimal SNG production. As a result of adjusting the ratio of inlet gas to increase the yield of CH₄ in the produced gas, the final CO₂ conversion of 83% and CO conversion of 97% (with 15% CO gas at 280 °C) were obtained. In addition, catalytic efficiency and catalyst surface analysis were performed by exposing CO gas during the CO₂ methanation reaction for 24 h. It showed high activity and excellent stability. The results of this study can be used as the basic data when applying an actual process. Full article

Nitrogen, phosphorus, and metals’ pollutants discharged from industrial sources eventually accumulate in lake sediment, hence increasing the difficulty of sediment treatment and disposal. In this work, the water storage ceramsite is prepared from dredged lake sediment and cyano-bacterial powder. The effects of pyrolysis temperature and cyanobacterial sediment on the porosity of ceramsite were investigated. The results showed that the pyrolysis of organic matter and the de-composition of compounds or salts can produce gas, causing a rich pore structure inside the ceramsite. When the temperature increased to 1150 °C, vitrification would collapse the pore structure inside the material. At the cyanobacterial-to-sediment ratio of 3:7, the porosity and water absorption of the material could reach 81.82% and 92.45% when the pyrolysis temperature was 500 and 1050 °C, respectively. The internal macropore structure of ceramsite improved the water absorption performance, and the mesoporous structure was responsible for its long water release time and stable water release structure. The ceramsite exhibited a superior metals’ retention effect. Under different pH and temperature conditions, the consolidation rates of Fe, Ni, Mn, Cr, and Pb in ceramsite were all more than 99%, suggesting the safety of the material in environmental applications. This study demonstrates the feasibility of the resourceful production of water storage ceramsite from lake sediment and cyanobacterial slurry, which helps to reduce the impact of solid waste on the environment. Thus, this work provides a practical basis for guiding water storage ceramsite in the construction of sponge cities. Full article