New coal purification-combustion method shows stable low-load performance and ultra-low NOₓ emissions
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Graphical abstract
view moreCredit: Shaobo Yang, Shaobo Han et al.
A research team from the Institute of Engineering Thermophysics, Chinese Academy of Sciences has reported a novel coal purification-combustion technology that enables stable and efficient ultra-low nitrogen oxide combustion under low-load conditions, as published in Engineering. The work was carried out on a 200 kW integrated test system, with a focus on purification performance and nitrogen migration mechanisms at approximately 55% load.
The technology integrates medium-temperature activation, high-temperature purification, and moderate or intense low-oxygen dilution combustion into a continuous process. Pulverized coal first undergoes activation in a circulating fluidized bed at around 850 °C, during which volatile nitrogen is released and partially reduced to N₂, with coal nitrogen conversion to N₂ ranging from 43.8% to 53.1%. This step significantly increases the specific surface area and pore development of char particles, providing more active sites for subsequent high-temperature reactions.
In the high-temperature purification stage operated at more than 150 °C above the ash flow temperature of each coal type, inorganic components are effectively separated with removal rates between 62% and 85%. Coal is converted into high-temperature gaseous fuel dominated by CO and H₂, while char nitrogen is extensively released and reduced. The conversion of coal nitrogen to N₂ reaches 93.6%–96.6% in this section, creating favorable conditions for low-pollutant combustion.
The high-temperature gas–solid fuel then enters the combustion chamber and realizes MILD combustion through multi-stage staged air distribution. In the reduction zone, NH₃ is completely converted to N₂, and the remaining char nitrogen is gradually released and reduced, lifting coal nitrogen conversion to N₂ above 99.6%. The oxidation zone completes char burnout with controlled temperature and mixing intensity to limit NOₓ formation.
Tests using three coal types show that raw NOₓ emissions at the furnace outlet are 34.6 mg·m⁻³ for Shenmu bituminous coal, 42.1 mg·m⁻³ for Beishan lignite, and 28.4 mg·m⁻³ for Yihua coal. Combustion efficiency remains above 99.6% across all tested fuels under the low-load condition. The system demonstrates stable operation and consistent performance, supporting the feasibility of this approach for clean coal utilization in power systems with high renewable penetration.
The study clarifies the transformation pathways of fuel-nitrogen throughout activation, purification, and MILD combustion, providing a technical route for improving operational flexibility and reducing pollutant emissions in coal-fired units under low-load conditions.
The paper “A Novel Coal Purification-Combustion Technology: Purification Characteristics and Ultra-Low Nitrogen Combustion at Low Load,” is authored by Shaobo Yang, Shaobo Han, Ruifang Cui, Linxuan Li, Chen Liang, Shuai Guo, Neng Fang, Wei Li, Qiangqiang Ren. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.026. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
A Novel Coal Purification-Combustion Technology: Purification Characteristics and Ultra-Low Nitrogen Combustion at Low Load
World’s largest 5 MWth lignite fueled chemical looping combustion unit demonstrates stable auto thermal operation
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S1 and S2: gas sampling ports at the outlets of the AR cyclone and FR cyclone, respectively.
view moreCredit: Zhenshan Li, Yang Wang et al.
A new study published in Engineering has reported the successful demonstration of a 5‑MWth chemical looping combustion (CLC) unit fueled by lignite, developed under the Sino‑European CHEERS project co‑funded by China’s Ministry of Science and Technology and the European Union’s Horizon 2020 program. As the largest CLC pilot unit worldwide, the facility features a Chinese configuration in which the air reactor operates as a transport bed and the fuel reactor functions as a bubbling/turbulent fluidized bed, with solid circulation regulated by an overflow method and oxygen carriers from the air reactor cyclone returning to the fuel reactor riser.
From June to September 2024, the demonstration unit was operated continuously with a thermal input ranging from 3.5 to 5.0 MWth, using Norwegian ilmenite as the oxygen carrier and Indonesian lignite as fuel. The entire system achieved auto‑thermal CLC operation without external electric or other heating, relying entirely on heat released from the oxidation of oxygen carriers inside the air reactor; the unit was heated up within 48 hours, and the switch to CLC mode was completed in less than one hour. During stable operation, temperatures in the air reactor and fuel reactor remained steady at 1000–1040 °C and 940–980 °C, respectively.
Operational data show that the maximum CO₂ capture efficiency of the lignite‑fed CLC unit exceeded 97%, while the minimum oxygen demand for unburnt gases at the fuel reactor outlet reached 2.45%. The flue gas from the fuel reactor was dominated by CO₂, with only small amounts of carbon monoxide, hydrogen, and methane; the carbon dioxide concentration at the air reactor outlet remained mostly below 2% due to limited char carryover. Solid carbon conversion and char conversion both reached levels above 99.48%, reflecting nearly complete conversion of lignite fed into the dense bed of the fuel reactor. Concentrations of sulfur dioxide and nitrogen oxides emitted from the fuel reactor were measured at 332–815 ppm and 0–101 ppm (dry basis), respectively.
The research team observed that the primary air ratio in the air reactor and the bed inventory in the fuel reactor both influenced the oxygen demand of unburnt gases, while higher bed inventory generally improved combustible gas conversion. This long‑term auto‑thermal operation with high carbon capture efficiency and low oxygen demand fills the gap between laboratory‑scale research and industrial applications for solid‑fuel CLC systems, supporting the technical advancement of chemical looping combustion toward integration within industrial carbon capture, utilization, and storage chains.
The paper “Demonstration of a 5-MWth Chemical Looping Combustion Unit Fueled by Lignite,” is authored by Zhenshan Li, Yang Wang, Weicheng Li, Geng Wei, Xinglei Liu, Shanhu Lin, Jiaye Li, Dan Li, Qingsong Meng, Li Nie, Vincent Gouraud, Shuting Wei, Patrice Font, Nils Erland L. Haugen, Øyvind Langørgen, Yngve Larring, Zuoan Li, Ningsheng Cai. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.07.017. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
Demonstration of a 5-MWth Chemical Looping Combustion Unit Fueled by Lignite
MIT researchers develop MFRNet digital twin for efficient industrial combustion system optimization
Higher Education Press
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The general workflow of building digital twin for multi-field reconstruction, scalar prediction and multi-objective optimization based on MFRNet.
view moreCredit: Linzheng Wang, Yaojun Li, Sili Deng
A research team from the Massachusetts Institute of Technology has introduced a data-driven digital twin framework named Multi-Field Reconstruction Net (MFRNet) for industrial-scale combustion systems, aiming to support low-carbon energy transition with improved data efficiency and multi-task integration. The work was published in Engineering, providing a scalable solution for 3D multi-physical field reconstruction, emission prediction, and operational optimization under limited high-fidelity data conditions.
Against the background of growing use of carbon-neutral fuels such as biomass in combustion facilities, researchers note that conventional digital twin approaches often separate reconstruction and optimization processes, and demand extensive high-fidelity 3D simulation data that is computationally costly to generate. The MFRNet framework integrates dimension expansion, variable extension, and dynamic feature fusion to address these limitations, unifying multi-field reconstruction and multi-objective optimization within one machine learning pipeline.
In the validation using an industrial biomass grate furnace, the team constructed a hybrid dataset including 288 low-fidelity 2D cases covering eight physical fields and 48 high-fidelity 3D cases covering eleven physical fields. The model is first pre-trained on low-fidelity 2D data to explore the high-dimensional condition space, then fine-tuned on a small set of 3D data through dimension expansion to capture z-direction heterogeneity and boundary effects. Variable extension modules further infer NOₓ-related species distributions including NO, HCN, and NH₃ by reusing latent features from core combustion variables learned during pre-training.
The study shows that MFRNet supports multi-modal inputs consisting of operational parameters and sparse temperature measurements, aligned via contrastive learning to enable robust reconstruction even with partial input information. By leveraging intermediate features from the reconstruction stage, the framework enhances scalar prediction for key indicators such as CO and NO emissions at the furnace outlet. The fused features improve prediction accuracy compared with direct neural network mapping, supporting reliable response surface construction.
The trained digital twin is applied to multi-objective optimization using the NSGA‑II algorithm, targeting minimized CO and NO emissions under fixed capacity conditions. The optimization generates Pareto fronts that reveal trade-offs between combustion efficiency and pollutant control, supporting the identification of practical operating strategies. The framework maintains high precision in 3D multi-field reconstruction while substantially reducing dependence on computationally expensive 3D simulations, showing adaptability to diverse industrial combustion systems.
This integrated digital twin approach offers a data-efficient pathway for active control and real-time optimization of modern combustion facilities, supporting stable operation and low-emission performance in the shift toward low-carbon energy systems.
The paper “Building Digital Twin for 3D Multi-Field Reconstruction and Optimization of Industrial-Scale Combustion Systems,” is authored by Linzheng Wang, Yaojun Li, Sili Deng. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.08.020. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
Building Digital Twin for 3D Multi-Field Reconstruction and Optimization of Industrial-Scale Combustion Systems
Flue gas-driven molten-salt heat exchanger boosts flexibility of coal-fired power plants
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Concept of the heat storage system integrated with a coal-fired power plant.
view moreCredit: Jinliang Xu, Hongliang Su et al.
A new study published in Engineering introduces a flue gas‑driven molten‑salt‑heat‑exchanger (MSHE) designed to enhance the operating flexibility of coal‑fired power plants, supporting grid stability amid growing renewable energy integration. This work marks the first experimental investigation into using furnace flue gas directly to drive a molten‑salt heat storage system, offering an alternative to conventional steam‑driven configurations.
The research addresses the need for improved load‑following capability in coal‑fired units, which traditionally face constraints in ramping speed and operational stability when balancing intermittent wind and solar generation. Unlike steam‑vapor‑driven molten‑salt systems, the flue gas‑driven approach eliminates pinch temperature limitations (PTL) associated with phase‑change heat transfer and simplifies system layout by requiring only a single heat exchanger. The MSHE employs three key design innovations: finned tubes to balance thermal resistance between flue gas and molten salt, a weak inclination angle to enable gravity‑driven drainage of molten salt during shutdown, and a modular structure to promote uniform temperature distribution at the tube bundle outlet.
Researchers designed, fabricated, and tested a 300 kW MSHE prototype coupled with a 10 MW furnace. Experimental results show measured overall heat transfer coefficients agree with model predictions within a deviation of less than 10%, and the unit achieved a thermal power of 320 kW, exceeding the design target. A new heat transfer correlation for molten salt was developed across a wide range of Reynolds numbers, supporting accurate performance prediction for engineering scale‑up. The modular design limited temperature deviations among different tubes to below 4 K, reducing risks of local overheating and molten salt decomposition.
The system supports both heat storage and heat compensation modes, using flue gas to offset heat losses from the storage system to the surroundings and lowering auxiliary electricity consumption during standby periods. Transient tests show stable transitions between operating states within 180 to 600 seconds, supporting responsive control under variable conditions. Long‑term testing over one year revealed no significant degradation in heat transfer or flow performance, confirming operational robustness.
Based on prototype data, a 10 MW MSHE has been designed and integrated into a 350 MWe coal‑fired power plant, helping the unit achieve a load variation rate of 6% Pe/min, comparable to gas turbine levels. Compared with steam extraction schemes, the flue gas route delivers higher round‑trip efficiency and a simpler system structure, reducing investment complexity while maintaining reliable heat storage and release. The technology provides a practical pathway to upgrade conventional coal‑fired units for more flexible grid support in low‑carbon energy systems.
The paper “Developing Flue Gas-Driven Molten-Salt-Heat-Exchanger for Flexible Operation of Coal-Fired Power Plant,” is authored by Jinliang Xu, Hongliang Su, Xinyu Dong, Xiongjiang Yu, Chao Liu, Yan Wang, Jian Xie, Wei Wang, Yupu Yu, Qinghua Wang, Yuguang Niu, Jizhen Liu, Ying Huang, Zhengshun Zhang, Anyou Dong, Yan Pan, Hao Wu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.001. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
Developing Flue Gas-Driven Molten-Salt-Heat-Exchanger for Flexible Operation of Coal-Fired Power Plant
Novel twisted cloverleaf U-tube boosts heat transfer for molten salt-based nanofluids
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(a) three-dimensional view, (b) top view, (c) TCUT cross-section, (d) smooth tube cross-section. θ: bending angle; g: gravitational acceleration.
view moreCredit: Yifan Gui, Yuanqiang Duan et al.
A new numerical study published in Engineering investigates the thermal–hydraulic performance of molten salt-based nanofluids flowing inside a novel twisted cloverleaf U-tube, aiming to improve heat transfer uniformity and efficiency in thermal energy storage systems. Molten salt is widely used in thermal energy storage owing to its favorable thermophysical properties and low cost, yet its high viscosity and density often lead to non-uniform flow and local overheating in conventional U-tube heat exchangers. To address these issues, researchers from Southeast University and Henan University of Science and Technology combined a twisted cloverleaf U-tube structure with molten salt-based nanofluids and carried out systematic numerical simulations to evaluate the effects of tube geometry, operating parameters, and nanoparticle types on flow and heat transfer characteristics.
The study employed Hitec salt as the base fluid and tested SiO₂, Al₂O₃, and Cu nanoparticles to form molten salt-based nanofluids. Numerical simulations used the realizable k–ε turbulence model and considered incompressible Newtonian flow under uniform heat flux boundary conditions. Grid independence tests and model validations were conducted against published correlations and experimental data to ensure computational reliability. Results show that the twisted structure generates intensified secondary flow in the U-bend region, promoting mixing between core and near-wall fluids and effectively reducing wall temperature peaks and circumferential temperature differences. Compared with smooth U-tubes, the twisted cloverleaf configuration enhances turbulence kinetic energy and improves the synergy between velocity and temperature gradient fields, which favors convective heat transfer.
Among the three nanoparticle types, Cu-based molten salt nanofluids exhibit the best overall performance, providing considerable heat transfer enhancement with relatively low pressure drop penalties. The team found that inlet velocity and inlet temperature exert stronger influences on flow and thermal performance than heat flux. Higher nanoparticle volume fractions within a reasonable range help increase convective heat transfer coefficients while maintaining acceptable pressure loss levels. Local heat transfer analyses reveal that the local convective heat transfer coefficient rises notably when fluid passes through the U-bend section, and this enhancement becomes more significant with increasing Reynolds number (Re) and nanoparticle loading.
To balance heat transfer enhancement and flow resistance, the research team adopted the response surface method and the non-dominated sorting genetic algorithm II for multi-objective optimization. Optimization results yield a set of Pareto optimal solutions, from which a balanced operating condition is selected for the twisted cloverleaf U-tube with 5% Cu nanoparticles. Under this condition, the device achieves a peak performance evaluation criterion value of 1.21, with controlled maximum wall temperature difference and pressure drop while maintaining a high average convective heat transfer coefficient.
This work provides quantitative references for the design and operational optimization of heat exchange tubes using molten salt-based nanofluids. The combined application of twisted cloverleaf U-tubes and suitable nanofluids offers a practical approach to alleviate flow maldistribution and local overheating in molten salt heat exchangers, supporting more stable and efficient operation in thermal energy storage systems for renewable energy integration and conventional power plant flexibility upgrades.
The paper “Numerical Study of Heat Transfer Performance of Molten Salt-Based Nanofluid in the Novel Twisted Cloverleaf U-Tube,” is authored by Yifan Gui, Yuanqiang Duan, Shuo Zhang, Yu Huang, Minmin Zhou, Lunbo Duan. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.10.010. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
Numerical Study of Heat Transfer Performance of Molten Salt-Based Nanofluid in the Novel Twisted Cloverleaf U-Tube
Staged pressurized oxy-combustion shows promise for carbon-negative biomass power and boiler retrofits
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(a) combustor-boiler and (b) DCC
view moreCredit: Duarte Magalhaes, Mao Cheng et al.
A new study published in Engineering has evaluated the staged, pressurized oxy-combustion (SPOC) process for boiler retrofits using woody biomass to generate carbon-negative power, with assessments covering both standalone power generation and cogeneration systems. First introduced in 2012, SPOC integrates fuel staging and pressurized oxy-combustion to deliver a more compact plant layout, improved energy efficiency, and lower costs for pollutant and greenhouse gas removal relative to conventional carbon capture setups. The modular boiler design supports flexible operation, which supports grid stability amid growing shares of intermittent renewable generation. While initially developed for coal, SPOC is well suited to biomass combustion because it can recover latent heat from the high moisture content in biomass flue gas, boosting performance compared with atmospheric-pressure biomass combustion systems.
Aspen Plus modeling confirms that SPOC outperforms baseline plants equipped with post-combustion capture (PCC) in both power and cogeneration configurations. The SPOC process relies on a series-parallel boiler arrangement that limits flue gas recirculation to less than 30 percent, far below rates used in first-generation atmospheric oxy-combustion, reducing auxiliary power demands. The integrated system includes an air separation unit, staged pressurized boilers, high-temperature heat recovery, a baghouse filter, a direct contact cooler (DCC), and a CO₂ processing unit, with heat recovery from flue gas moisture condensation and compression waste heat integrated into the steam cycle. The DCC simultaneously cools flue gas, condenses water vapor, and removes SOₓ, NOₓ, and HCl in a single pressurized column, cutting capital and operating expenses compared with conventional flue gas treatment systems. Carbon capture efficiency reaches 95 percent or higher, with the CO₂ stream compressed for sequestration at lower energy cost because the gas remains pressurized throughout the process.
Researchers at Washington University in St. Louis have successfully demonstrated 100 percent biomass firing in a 100 kWₜₕ pressurized oxy-combustor following minor adjustments to the dry feeding system. Tests showed stable startup, steady-state operation, and turndown performance with low excess oxygen, high particle burnout, and low NOₓ emissions, with no burner or combustor modifications needed between coal and biomass operation. The study indicates SPOC retrofits can convert existing coal-fired boilers to carbon-negative service using sustainable woody biomass, providing a practical pathway for industrial decarbonization and repurposing of thermal power assets.
The paper “The Staged, Pressurized Oxy-Combustion Technology: Status and Application to Boiler Retrofits to Yield Carbon-Negative Power via Biomass,” is authored by Duarte Magalhaes, Mao Cheng, Zachariah Wargel, Richard L. Axelbaum. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.11.016. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.
Journal
Engineering
Article Title
The Staged, Pressurized Oxy-Combustion Technology: Status and Application to Boiler Retrofits to Yield Carbon-Negative Power via Biomass
