Search Results (89)

The novel engines nowadays with high efficiency are operated under the superpressure, supercritical, and supersonic extreme conditions that are situated in the broken reaction zone regime. In this article, the propagation and heat/radical diffusion physics of a high-pressure dimethyl ether (DME)/air turbulent lean-premixed flame are investigated numerically by direct numerical simulation (DNS). A wide range of statistical and diagnostic methods, including Lagrangian fluid tracking, Joint Probability Density Distribution (JPDF), and chemical explosive mode analysis (CEMA), are applied to reveal the local combustion modes and dynamics evolution, as well as the roles of heat/mass transport and cool/hot flame interaction in the turbulent combustion, which would be beneficial to the design of novel engines with high performances. It is found that the three-staged combustion, including cool-flame, warm-flame, and hot-flame fronts, is a unique behavior of DME flame under the elevated-pressure, lean-premixed condition. In the broken reaction zone regime, the reaction zone thickness increases remarkably, and the heat release rate (HRR) and fuel consumption rate in the cool-flame zone are increased by 16% and 19%, respectively. The diffusion effect not only enhances flame propagation, but also suppresses the local HRR or fuel consumption. The strong turbulence interplaying with diffusive transports is the underlying physics for the enhancements in cool- and hot-flame fronts. The dominating diffusive sub-processes are revealed by the aid of the diffusion index. Full article

Ash deposition on economizer heating surfaces degrades convective heat transfer efficiency and compromises boiler operational stability in coal-fired power plants. Conventional time-scheduled soot blowing strategies partially mitigate this issue but often cause excessive steam/energy consumption, conflicting with enterprise cost-saving and efficiency-enhancement goals. This study introduces an integrated framework combining real-time ash monitoring, dynamic process modeling, and predictive optimization to address these challenges. A modified soot blowing protocol was developed using combustion process parameters to quantify heating surface cleanliness via a cleanliness factor (CF) dataset. A comprehensive model of the attenuation of heat transfer efficiency was constructed by analyzing the full-cycle interaction between ash accumulation, blowing operations, and post-blowing refouling, incorporating steam consumption during blowing phases. An optimized subtraction-based mean value algorithm was applied to minimize the cumulative attenuation of heat transfer efficiency by determining optimal blowing initiation/cessation thresholds. Furthermore, a bidirectional gated recurrent unit network with quantile regression (BiGRU-QR) was implemented for probabilistic blowing time prediction, capturing data distribution characteristics and prediction uncertainties. Validation on a 300 MW supercritical boiler in Guizhou demonstrated a 3.96% energy efficiency improvement, providing a practical solution for sustainable coal-fired power generation operations. Full article

This study explores innovative strategies for decarbonizing sludge thermal treatments used in electrical power generation, with a focus on integrating sensor technologies and artificial intelligence. Sludge, a carbon-intensive byproduct of wastewater treatment, presents both environmental challenges and opportunities for energy recovery. The paper provides a comprehensive analysis of thermal processes such as pyrolysis, gasification, co-combustion, and emerging methods, including hydrothermal carbonization and supercritical water gasification. It evaluates their carbon mitigation potential, energy efficiency, and economic feasibility, emphasizing the importance of catalyst selection, carbon dioxide capture techniques, and reactor optimization. The role of real-time monitoring via sensors and predictive modeling through artificial intelligence (AI) is highlighted as critical for enhancing process control and sustainability. Case studies and recent advances are discussed to outline future pathways for integrating thermal treatment with circular economy principles. This work contributes to sustainable waste-to-energy practices, supporting global decarbonization efforts and advancing the energy transition. Full article

Supercritical combustion is a promising technique for improving the efficiency and reducing the emissions of next-generation gas turbines. However, accurately modeling combustion under these conditions remains a challenge, particularly due to the complexity of chemical kinetics. This study aims to evaluate the applicability of a reduced global reaction mechanism compared to the detailed Foundational Fuel Chemistry Model 1.0 (FFCM-1) when performing hydrogen combustion with supercritical carbon dioxide and argon as diluents. Computational fluid dynamics simulations were conducted in two geometries: a simplified tube for isolating chemical effects and a combustor with cooling channels for practical evaluation. The analysis focuses on the evaluation of velocity, temperature, and the water vapor mass fraction distributions inside the combustion chamber. The results indicate good agreement between the global and detailed mechanisms, with average relative errors below 2% for supercritical argon and 4% for supercritical carbon dioxide. Both models captured key combustion behaviors, including buoyancy-driven flame asymmetry caused by the high density of supercritical fluids. The findings suggest that global chemistry models can serve as efficient tools for simulating supercritical combustion processes, making them valuable for the design and optimization of future supercritical gas turbine systems. Full article

As fossil fuel depletion and environmental pollution become increasingly severe, biodiesel has emerged as a promising renewable alternative to conventional diesel due to its biodegradability, low sulfur emissions, and high combustion efficiency. This paper provides a comprehensive review of the evolution of biodiesel feedstocks, major production technologies, and key factors influencing production efficiency and fuel quality. It traces the development of feedstocks from first-generation edible oils, second-generation non-edible oils and waste fats, to third-generation microalgal oils and fourth-generation biofuels based on synthetic biology, with a comparative analysis of their respective advantages and limitations. Various production technologies such as transesterification, direct esterification, supercritical alcohol methods, and enzyme-catalyzed transesterification are examined in terms of reaction mechanisms, process conditions, and applicability. The effects of critical process parameters including the alcohol-to-oil molar ratio, reaction time, and temperature on biodiesel yield and quality are discussed in detail. Particular attention is given to the role of catalysts, including both homogeneous and heterogeneous types, in enhancing conversion efficiency. In addition, life cycle assessment (LCA) is briefly considered to evaluate the environmental impact and sustainability of biodiesel production. This review serves as a valuable reference for improving biodiesel production technologies, advancing sustainable feedstock development, and promoting the commercial application of biodiesel. Full article

Supercritical oxy-fuel combustion, which allows for the high efficiency of power generation with near-zero CO₂ emissions, is considered a promising method to reduce the carbon footprint in the power energy sector. One of the problems in the widespread use of oxy-fuel combustion is a lack of comparative studies on the existing oxy-fuel combustion kinetic mechanisms depending on mixture composition, which complicates the choice of a kinetic mechanism for modeling oxy-fuel combustion. In this paper, a comparative verification of the kinetic mechanisms of GRI-Mech 3.0, UoS sCO₂ 2.0, OXY-NG, and Skeletal was performed using published experimental data on the ignition delay time of methane under conditions of oxy-fuel combustion. A comparative numerical study of the kinetic mechanisms in the wide range of pressures, CO₂ mass fractions in oxidizer (γ), and excess oxidizer ratios (α) by the ignition delay time is also carried out. It was found that the limits of applicability of all of the mechanisms studied are absent when modeling the ignition delay time, the most accurate mechanism to model the IDT of methane in oxy-fuel conditions being UoS sCO₂ 2.0, while the other three mechanisms are overall much inferior to it in terms of accuracy. However, Skeletal and GRI-Mech 3.0 mechanisms can be used to model the IDT during the oxy-fuel combustion of methane under both atmospheric and supercritical conditions, although only in a narrow range of γ. Full article

Aquilaria spp. are a highly valuable plant species found in the Chinese herbal medicine and agarwood fragrance supplement industries for fumigation, combustion and perfume. The phytochemical composition of agarwood oils (extracts) was derived from Aquilaria sinensis and its subspecies ‘Qi-Nan’ using supercritical CO₂ extraction technology. Gas chromatography connected with a mass spectrometry apparatus was employed for qualitative and quantitative analyses. Comparing the agarwood oils from six planting areas, 12 common components were obtained, among which sesquiterpenes and chromones had the highest relative content. Genetic and environmental factors had the greatest impact on the three chromones, especially on 2-phenyl-4H-chromen-4-one. According to the PCA and PLS-DA models, the ‘Qi-Nan’ was derived from a variety selected from the native A. sinensis, and the difference in the volatile components was able to indirectly prove that it was genetically heterogeneous with the native A. sinensis. Using the 73 components obtained from GC–MS analysis, the VIP values and S-plots were generated using the OPLS-DA model. Seven components with VIP values > 1.0 were selected from two groups of agarwood oils of the native A. sinensis and ‘Qi-Nan’ subspecies. In addition, by analyzing 12 common components, the differential components with VIP values > 1 were 2-phenyl-4H-chromen-4-one and 2-(4-methoxyphenethyl)-4H-chromen-4-one. Chromones were the main component of agarwood oils extracted by supercritical CO₂, and 2-phenyl-4H-chromen-4-one could be used as a volatile marker, especially in the ‘Qi-Nan’ subspecies, where this marker exhibited more prominent characteristics. Full article

This study investigates the combustion of hydrogen in supercritical gas turbines, emphasizing the optimization of combustor design through computational fluid dynamics (CFD) simulations. Key parameters analysed include the number of oxygen inlets, operating pressure, excess working fluid in oxygen inlets, power output, and the use of different working fluids: supercritical argon (sAr) and supercritical xenon (sXe). The results highlight how these parameters influence temperature distribution, flame stability, and overall combustion efficiency. Findings suggest that increasing the number of oxygen inlets can significantly affect temperature profiles, while higher operating pressures lead to shorter flames. The dilution of oxygen by argon reduces the peak temperatures, and the choice of working fluid impacts cooling efficiency and flame dynamics. This study provides valuable information on optimizing the design of supercritical combustion chambers for hydrogen combustion in novel supercritical gas turbine systems. Full article

To investigate the safe, stable, and economically viable operation of a boiler under ultra-low-load conditions during the deep peaking process of coal-fired units, a numerical simulation study was conducted on a 660 MW front- and rear-wall hedge cyclone burner boiler. The current research on low load conditions is limited to achieving stable combustion by adjusting the operating parameters, and few effective boiler operating parameter predictions are given for very low-load conditions, i.e., below 20%. Various burner operation modes under ultra-low load conditions were analyzed using computational fluid dynamics (CFDs) methods; this operation was successfully tested with six types of pulverized coal combustion in this paper, and fitting models for outlet flue gas temperature and NO_x emissions were derived based on the combustion characteristics of different types of pulverized coal. The results indicate that under 20% ultra-low-load conditions, the use of lower burners leads to a uniform temperature distribution within the furnace, achieving a minimum NO_x emission of 112 ppm and a flue gas temperature of 743 K. Coal type 3, with the highest carbon content and a calorific value of 22,440 kJ/kg, has the highest average section temperature of 1435.76 K. In contrast, coal type 1 has a higher nitrogen content, with a maximum cross-sectional average NO_x concentration of 865.90 ppm and an exit NO_x emission concentration of 800 ppm. The overall lower NO_x emissions of coal type 3 are primarily attributed to its reduced nitrogen content and increased oxygen content, which enhance pulverized coal combustion and suppress NO_x formation. The fitting models accurately capture the influence of pulverized coal composition on outlet flue gas temperature and NO_x emissions. This control strategy can be extended to the stable combustion of many kinds of coal. For validation, the fitting error bar for the predicted outlet flue gas temperature based on the elemental composition of coal type 6 was 8.09%, whereas the fitting error bar for the outlet NO_x emissions was only 1.45%. Full article

Under the background of achieving carbon dioxide peaking and carbon neutrality, the rapid development of renewable energy power generation poses new challenges to the flexible adjustment capabilities of traditional power plants. To explore the furnace combustion stability and optimal operation modes during deep peak shaving, a simulation of the combustion process under low-load conditions for a 600 MW wall-fired boiler is performed utilizing computational fluid dynamics (CFD) analysis. The impact of burner combination modes on the combustion process within the furnace is explored at 25% and 35% boiler maximum continuous ratings (BMCRs). This study investigates two configurations of burner combinations. One mode operates burners in layers A, B, and C, which include the lower layers of burners on the front and rear walls of the boiler, as well as the middle-layer burners on the rear wall, referred to as OM1. The other mode operates burners in layers A and C, which include the lower layers of burners on the front and rear walls of the boiler, referred to as OM2. The results indicate that OM2 exhibits superior capabilities in orchestrating the distribution of the airflow velocity field and temperature field under the premise of ensuring no more than a 1% decrease in the pulverized coal burnout rate. When OM1 is employed, the airflow ejected from the middle-level burners hinders the upward movement of pulverized coal sprayed from the lower-level burners, causing a larger proportion of pulverized coal to enter the ash hopper for combustion. Consequently, the ash hopper attains a peak mole fraction of CO₂ at 0.163. OM2 delays the blending of pulverized coal with air by enhancing the injection quantity of pulverized coal per burner. As a result, the generation of CO in the ash hopper reaches a notable mole fraction of up to 0.108. The decreased furnace temperature promotes the formation of fuel-based NO_x during low-load operation. Taking the 25% BMCR as an example, the NO_x emissions measured at the furnace outlet are 743 and 1083 ppm for OM1 and OM2, respectively. This study focuses on the impact of combustion combinations on the combustion stability when the boiler is operating at low loads. The findings could enrich previous research on combustion stability and contribute to the optimization of combustion schemes for power plant boilers operating at low loads. Full article

To accommodate the integration of renewable energy, coal-fired power plants must take on the task of peak regulation, making the low-load operation of boilers increasingly routine. Under low-load conditions, the phase transition point (PTP) of the working fluid fluctuates, leading to potential flow instability, which can compromise boiler safety. In this paper, a one-dimensional coupled dynamic model of the combustion and hydrodynamics of a supercritical boiler is developed on the Modelica/Dymola 2022 platform. The spatial distribution of key thermal parameters in the furnace and the PTP position in the water-cooled wall (WCW) are analyzed in a 660 MW supercritical boiler when parameters on the combustion side change under full-load and low-load conditions. The dynamic response characteristics of the temperature, mass flow rate, and the PTP position are investigated. The results show that the over-fire air (OFA) ratio significantly influences the flue gas temperature distribution. A lower OFA ratio increases the flue gas temperature in the burner zone but reduces it at the furnace exit. The lower OFA ratio leads to a higher fluid temperature and shortens the length of the evaporation section. The temperature difference in the WCW outlet fluid between the 20% and 60% OFA ratios is 11.7 °C under BMCR conditions and 7.4 °C under 50% THA conditions. Under the BMCR and 50% THA conditions, a 5% increase in the coal caloric value raises the flue gas outlet temperature by 32.7 °C and 35.4 °C and the fluid outlet temperature by 6.5 °C and 9.9 °C, respectively. An increase in the coal calorific value reduces the length of the evaporation section. The changes in the length of the evaporation section are −2.95 m, 2.95 m, −2.62 m, and 0.54 m when the coal feeding rate, feedwater flow rate, feedwater temperature, and air supply rate are increased by 5%, respectively. Full article

This paper presents an analysis and optimization of a polygeneration power-production system that integrates a concentrating solar tower, a supercritical CO₂ Brayton cycle, a double-flash geothermal Rankine cycle, and an internal combustion engine. The concentrating solar tower is analyzed under the weather conditions of the Mexicali Valley, Mexico, optimizing the incident radiation on the receiver and its size, the tower height, and the number of heliostats and their distribution. The integrated polygeneration system is studied by first and second law analyses, and its optimization is also developed. Results show that the optimal parameters for the solar field are a solar flux of 549.2 kW/m², a height tower of 73.71 m, an external receiver of 1.86 m height with a 6.91 m diameter, and a total of 1116 heliostats of 6 m × 6 m. For the integrated polygeneration system, the optimal values of the variables considered are 1437 kPa and 351.2 kPa for the separation pressures of both flash chambers, 753 °C for the gasification temperature, 741.1 °C for the inlet temperature to the turbine, 2.5 and 1.503 for the turbine pressure ratios, 0.5964 for the air–biomass equivalence ratio, and 0.5881 for the CO₂ mass flow splitting fraction. Finally, for the optimal system, the thermal efficiency is 38.8%, and the exergetic efficiency is 30.9%. Full article

This study focuses on the great challenges for combined cooling and power supply on hypersonic aircrafts. To address the issues of low thermal efficiency and high fuel consumption of heat sink by the existing CO₂ supercritical Brayton cycle, a novel fuel-based CO₂ transcritical cooling and power (FCTCP) system is constructed. A steady-state simulation model is built to investigate the impacts of combustion chamber wall temperatures and fuel mass flow rates on the FCTCP system. Thermal efficiency of the CO₂ transcritical cycle reaches 25.2~32.8% under various combustion chamber wall outlet temperatures and endothermic pressures. Compared with the supercritical Brayton cycle, the thermal efficiency of novel system increases by 54.5~80.9%. It is found from deep insights into the thermodynamic results that the average heat transfer temperature difference between CO₂ and fuel is effectively reduced from 153.4 K to 16 K by split cooling of the fuel in the FCTCP system, which greatly enhances the matching of CO₂–fuel heat exchange temperatures and reduces the heat exchange loss of the system. Thermodynamic results also show that, in comparison to the supercritical Brayton cycle, the cooling capacity and power generation per unit mass flow rate of working fluid in the FCTCP system increased by 75.4~80.8% and 12.9~51.6%, respectively. The FCTCP system exhibits a substantial performance improvement, significantly enhancing the key characteristic index of the combined cooling and power supply system. This study presents a novel approach to solving the challenges of cooling and power supply in hypersonic aircrafts under limited fuel heat sink conditions, laying the groundwork for further exploration of thermal management technologies of hypersonic aircrafts. Full article

In this study, the Density Functional Theory (DFT) Calculations for Molecules and Clusters—ADF module is employed to model carbon-supported nickel catalysts and lignin monomers, integrating the ReaxFF module to simulate molecular dynamics under supercritical water conditions, with a focus on lignin decomposition reactions. Molecular dynamics models for supercritical water gasification are established under various conditions such as catalyst presence, water molecule quantities, and reaction temperature. By comparing simulation systems under different conditions, the yields of and variations in combustible gases (hydrogen and carbon monoxide) are summarized. Introducing heteroatoms into the lattice of the carbon support can alter the electronic structure within graphene, thereby influencing its electrical and electrochemical properties, increasing the number of active sites, and significantly enhancing electrocatalytic activity. Simulation results indicate that carbon-supported nickel metal catalysts can promote the cleavage of C–C bonds in lignin monomers, thereby increasing the rate of water–gas shift reactions and boosting hydrogen production in the system by 105%. Increasing water molecule quantities favored water–gas shift reactions and hydrogen generation while lowering carbon monoxide formation. Moreover, elevating reaction temperatures led to increased hydrogen and carbon monoxide production rates, which were particularly pronounced between 2500 K and 3500 K. These findings offer crucial theoretical insights for advancing efficient hydrogen production through biomass supercritical water gasification. Full article

The self-diffusion coefficients of carbonaceous fuels in a supercritical CO₂ environment provide transport information that can help us understand the Allam Cycle mechanism at a high pressure of 300 atm. The diffusion coefficients of pure CO₂ and binary CO₂/CH₄ and CO₂/C₂H₆ at high temperatures (500 K~2000 K) and high pressures (100 atm~1000 atm) are determined by molecular dynamics simulations in this study. Increasing the temperature leads to an increase in the diffusion coefficient, and increasing the pressure leads to a decrease in the diffusion coefficients for both methane and ethane. The diffusion coefficient of methane at 300 atm is approximately 0.012 cm²/s at 1000 K and 0.032 cm²/s at 1500 K. The diffusion coefficient of ethane at 300 atm is approximately 0.016 cm²/s at 1000 K and 0.045 cm²/s at 1500 K. The understanding of diffusion coefficients potentially leads to the reduction in fuel consumption and minimization of greenhouse gas emissions in the Allam Cycle. Full article