To address the demand for low-carbon transition in coal-fired power plants, ammonia, as a zero-carbon fuel and efficient hydrogen storage carrier, provides a novel pathway for carbon reduction in the thermal power industry. The key technologies and research advances in green ammonia synthesis, storage, transportation, and ammonia-coal co-firing are systematically reviewed from the perspective of the “production-storage-transportation-utilization” whole industry chain, and the economic feasibility is also evaluated. The study reveals that, the second-generation low-temperature and low-pressure synthesis technology (Fe/Ru catalysts) exhibits the greatest industrial potential for green ammonia production, but requires breakthroughs in enhancing catalytic activity and dynamic matching technologies for renewable energy-based hydrogen-ammonia synthesis systems. It is urgent to develop 100 000-ton-level cryogenic storage tanks and long-distance liquid ammonia pipelines, and establish a “West-to-East Ammonia Transmission” network to support large-scale applications. Ammonia-coal co-firing can achieve NO_x emissions comparable to pure coal combustion by optimizing ammonia injection positions (post-injection in low-oxygen zones), air staging (equivalence ratio of 1.1~1.3 in primary zone), and ammonia blending ratios, alongside designing low-NO_x co-firing burners. However, the weakened radiative heat transfer and enhanced convective heat transfer post-co-firing necessitate compatibility adjustments in boiler steam-water systems. When the cost of renewable electricity decreases to 0.10 yuan/(kW·h) with carbon price exceeding 370 yuan/t, or by utilizing curtailed wind/solar power (with near-zero electricity costs), green ammonia is more competitive than coal. In the future, it is necessary to promote the implementation of technology through green ammonia cost reduction, carbon price mechanism and policy support. This study provides comprehensive technical references and economic optimization strategies for scaling up green ammonia co-firing in coal-fired power plants.

The technology of co-firing ammonia with natural gas has become a global research focus due to its significant potential in reducing carbon emissions. During the combustion process, ammonia faces challenges such as difficulty in ignition, slow flame propagation speed, and susceptibility to blow-off. The addition of natural gas can significantly improve these combustion characteristics, thereby promoting the widespread application of ammonia fuel and opening up new avenues for the development of clean energy. Firstly, the application potential of natural gas-ammonia co-firing technology is evaluated from the perspectives of technical and economic feasibility, and its positive significance in the energy transition process is analyzed. Then, drawing on research findings at the reaction kinetics level, the chemical reaction mechanisms of ammonia and natural gas co-firing are elucidated. On this basis, the latest domestic and international research progress in this field is reviewed, covering experimental studies, numerical simulations, and low-NO_x stable combustion control strategies. It is pointed out that significant discrepancies still exist among different mechanism models in terms of simulation accuracy and experimental prediction universality. Future research needs to combine multi-scale simulations to develop more adaptable ammonia combustion prediction models that can balance accuracy and efficiency. Finally, the challenges encountered in the practical application of natural gas-ammonia co-firing technology are summarized, and future research directions are proposed, aiming to provide a theoretical basis and practical guidance for the in-depth development of this technology.

Co-firing zero-carbon fuels in coal-fired power plants is one of the important paths to realize low-carbon emissions in power industry, and the common zero-carbon fuels blended at present are biomass, ammonia, and hydrogen, etc. The researches on co-firing zero-carbon fuels in coal-fired circulating fluidized bed (CFB) boilers are discussed, the technical principles and advantages of the technology are analyzed, and the future technical challenges and development trends of the technology are discussed, by combining the characteristics of biomass, ammonia and hydrogen fuels with the progress of the research on blending, to provide theoretical and technical support for the realization of low-carbon emissions from coal-fired boilers. Based on the fluidized combustion characteristics of CFB boilers, the basic fuel characteristics, combustion characteristics and pollutants emission characteristics of the three zero-carbon fuels after blending, problems and future development directions are analyzed. Although there are certain limitations and technical challenges in the co-firing of all three zero-carbon fuels, the optimization of the combustion process and the control of pollutant emissions can be realized through the organization of the gas-solid flow field, the deep grading of fuel or air, and the coupling of other technologies. Co-firing zero-carbon fuels in coal-fired CFB boilers is a feasible route for carbon emission reduction, which helps to develop a new generation of flexible low-carbon coal-fired power generation technologies in CFB boilers, and provides technical support for the promotion of low-carbon transformation of the energy structure in achieving “dual-carbon” target.

Green ammonia co-firing is one of the important technical routes for the low-carbon transformation of coal-fired power units. Currently, the main problem restricting the promotion of green ammonia co-firing projects is the poor economic efficiency of the entire process from green ammonia production, storage and transportation to co-firing. Taking a single 600 MW coal-fired unit co-firing 10% green ammonia as an example, the technical and economic efficiency of the entire process of off-grid/on-grid photovoltaic power generation for green ammonia synthesis and co-firing projects is compared and analyzed. Moreover, the effects of subsidy mechanisms (zero-carbon electricity subsidy, green ammonia production subsidy, carbon emission reduction subsidy and low-interest loan) on project benefits are deeply discussed. The results show that, the price of coal and carbon tax is the main factor affecting the economic efficiency of the project. As the price increases, the economic benefits of green ammonia co-firing in coal-fired units are significantly improved. All subsidy mechanisms can improve the economic efficiency of the project, but the effects vary depending on the scenario. Low-interest loans have the best effect on improving the economic feasibility of the project, while zero-carbon electricity subsidies have the highest sensitivity to the change in the project’s net present value (NPV).

In the context of carbon peak and carbon neutrality, renewable power to ammonia (RePtA) technology has garnered widespread attentions due to its ability to scale up the consumption of renewable energy and green hydrogen. However, the hydrogen production from renewable energy in RePtA systems exhibits significant volatility, posing challenges to stable operation of the Haber-Bosch ammonia synthesis process. To address this issue, a discrete multi-steady-state flexible load operation strategy for ammonia synthesis process is proposed. A two-stage optimization model for capacity configuration and coordinated chemical operation scheduling is established using the PSO-MILP algorithm. A case simulation analysis was conducted on a demonstration project under construction in Inner Mongolia, and the technical and economic performance of three different flexible schemes was compared. The result indicates that, compared with the conventional steady state schemes, the discrete multi-state flexible operation strategy’s economic efficiency improved significantly after capacity and operation coordination optimization, with annual revenue increased by 67 150 000 yuan. Compared with the fully flexible operation strategies, the new strategy significantly enhances the stability of the ammonia synthesis process, reducing production load volatility by 78.16%. The proposed optimization model can balance the investment economic efficiency and operational safety of the RePtA system, and its findings are expected to provide some guidance for actual production operations.

Aiming at the difficulties in renewable energy consumption and the demand for low-carbon development in the integrated energy system, an optimal scheduling method considering the joint operation of hydrogen-doped gas-fired units with hydrogen-doped and ammonia-doped coal-fired units is proposed. Firstly, to account for the uncertainty and correlation of wind and solar power outputs, a joint wind-solar output modeling approach based on the Frank Copula function is adopted. Typical wind-solar scenarios are generated through marginal distribution fitting using kernel density estimation, Monte Carlo sampling, and K-means clustering, thereby enhancing the robustness of the scheduling model. Meanwhile, energy conversion models for power-to-hydrogen and hydrogen-to-ammonia processes are developed to enable the efficient transformation of renewable energy into hydrogen and ammonia. Secondly, the refined operation model of hydrogen-doped combustion of gas-fired units and ammonia-doped combustion of coal-fired units is constructed in response to the demand for low-carbon transformation of conventional fossil energy units, so as to optimize the synergistic utilization of hydrogen and ammonia fuels in the power generation process. Then, the optimal dispatching model is constructed by combining with the laddering-type carbon trading mechanism with the goal of minimizing the total operation cost of the system, which is to minimize the total cost of the system. Moreover, the optimal scheduling model is constructed with the objective of minimizing the total operating cost of the system in combination with the stepped carbon trading mechanism and solved by the CPLEX solver. Finally, different scenarios are set up and comparative analysis are carried out. The results indicate that, the introduction of hydrogen-to-ammonia conversion, building upon hydrogen energy utilization, significantly mitigates wind and solar power curtailment within the system. The combined operation of hydrogen-doped gas-fired unit and ammonia-doped coal-fired unit leads to concurrent reductions in both total operational costs and carbon emissions. The study provides a reference for the development of decarbonization of integrated energy systems.

Ammonia, as a low-carbon fuel, has important application prospects in the field of industrial combustion. However, the dynamic characteristics of liquid ammonia gasification process are complex, and conventional mechanism models are difficult to meet high-precision control requirements. To address the problem of unstable control caused by insufficient modeling accuracy in the liquid ammonia gasification process in ammonia combustion systems, a dynamic modeling method that combines mechanism with data fusion is proposed. By establishing a nonlinear mechanism model based on thermodynamic laws, and combining with a data-driven model based on recursive fuzzy C-means (RFCM) clustering and recursive least square (RLS) algorithm, a hybrid dynamic model with adaptive weight optimization is constructed. On this basis, decoupling control strategies are developed to achieve precise control of the gasification systems. Experimental verification shows that, the proposed model significantly improves the prediction accuracy of the gasification process, and the decoupling control scheme based on this dynamic model achieves stable ammonia supply, verifying the effectiveness and engineering practicality of the dynamic model that integrates mechanism and data for the control system. This method provides an effective solution for intelligent control of ammonia fuel combustion systems and has promotional value for the engineering application of clean energy technology.

Ammonia synthesize through hydrogen produced by green electricity offers an effective solution to the widespread abandonment of wind and solar resources and the shortage of green fuel chemicals. A wind-solar-driven proton exchange membrane (PEM) electrolyzer system in dual-mode operation for hydrogen production and hot standby with integrated ammonia synthesis waste heat storage is proposed, addressing issues of frequent start-stop cycles under fluctuating wind-solar outputs and waste heat recovery in ammonia synthesis processes. The results indicate that, the PEM electrolyzer dual-mode operating system, integrated with ammonia synthesis waste heat storage, can significantly shorten the startup time of the electrolyzer. The startup time at 25 ℃ is 512 seconds, while the hot startup from the standby mode at 47.5 ℃ requires only 274 seconds. Under hot standby mode, the system consumes electricity solely from feedwater pumps, achieving a specific hydrogen production power consumption of only 0.49 kW. The dual-tank thermal storage subsystem is configured with 10.8 tons of Dowtherm-G heat transfer oil. In heat storage mode, it absorbs waste heat gas from the ammonia synthesis unit at a flow rate of 3 kg/s, allowing the thermal tank to reach full capacity within 1 hour. In heat release mode, it heats the electrolyzer inlet water at a flow rate of 0.64 kg/s, enabling the electrolyzer to sustain standby operation for 4.68 hours. Furthermore, the new system is expected to generate long-term benefits that consistently exceed costs, ensuring sustained economic viability.

To optimize the system configuration scheme for green hydrogen co-firing in coal-fired power units at large renewable bases in desert, gobi, and wasteland areas to achieve decarbonization, a comprehensive system framework encompassing hydrogen production, hydrogen storage, energy storage, and hydrogen co-firing in coal-fired power units is established. It develops a system configuration optimization model aiming for the lowest hydrogen production cost under a decarbonization target constraint. The model is solved and analyzed using mixed-integer linear programming to explore the optimal configuration solutions for a decarbonization system via green hydrogen co-firing in different operating scenarios. The model is demonstrated through a case study. Under the constraint of 10% decarbonization for a single coal-fired power unit, if only curtailed wind and solar power are used for hydrogen production, the annual utilization hours of the hydrogen production equipment are only about 2 000 hours, and the green hydrogen cost is as high as 3.02 yuan/m³ (33.8 yuan/kg). This leads to an increase of 0.217 9 yuan/(kW·h) in the per-unit electricity cost for a single coal-fired power unit. Configuring electrochemical energy storage can reduce the scale of hydrogen production and storage systems and increase the utilization hours of hydrogen production equipment. However, limited by high energy storage construction costs, the energy storage scale needs to be optimally determined, and the green hydrogen cost can be reduced to approximately 2.35 yuan/m³ (26.3 yuan/kg) at its lowest, which leads to an increase of 0.165 2 yuan/(kW·h) in the per-unit electricity cost for a single coal-fired power unit. Furthermore, if a small amount of grid electricity can be introduced to assist hydrogen production within the scope of green hydrogen certification, system construction costs can be further reduced. The hydrogen production cost is expected to decrease to approximately 2.12 yuan/m³ (23.7 yuan/kg) and the per-unit electricity cost for a single coal-fired power unit would increase by 0.142 6 yuan/(kW·h).

In response to current issues faced by coal-fired power plants, such as high fuel costs, weakly stable combustion performance at low loads, and insufficient peak-shaving capabilities, an integrated operational scheme is proposed based on the natural endowments of renewable energy surrounding the power plant, which utilizes photovoltaic power generation distributed in plants to produce hydrogen and oxygen via electrolysis of water, then to achieve hydrogen and oxygen co-firing in coal-fired boilers. By constructing a full-size numerical model for a tangentially coal-fired boiler, the calculation accuracy of temperature field, species concentration, and carbon content in fly ash is verified under pure pulverized coal combustion conditions, which could provide a benchmark for optimization of hydrogen and oxygen blending. Based on the typical application scenario of a 20 MW photovoltaic power generation to hydrogen and oxygen production in the plant, the effects of three mixing methods of hydrogen and oxygen on combustion efficiency, burnout characteristics and NO_x formation in the furnace are systematically studied. The results show that, a co-combustion mode which utilizes an independent hydrogen nozzle in conjunction with primary air mixing with oxygen can improve the combustion performance significantly. The carbon content in fly ash at the furnace outlet reduces to 0.97%, and the combustion efficiency is notably enhanced compared with that under pure coal combustion condition. At the same time, the reduction effect of reactive species generated by hydrogen combustion on NO_x leads to a decrease in NO_x emission mass concentration in the flue gas to 294.0 mg/m³, which is decreased by about 25% compared with that under baseline condition of pure coal combustion. This model achieves dual-benefit of coal substitution and combustion optimization through hydrogen and oxygen production from renewable energy, which not only reduces coal consumption but also expands the lower limit of stable combustion load for coal-fired boilers. It provides a technically feasible implementation reference path for the decarbonized retrofitting and flexibility improvement of coal-fired units.

Under the “dual-carbon” target, ammonia as a zero carbon fuel is expected to become a substitute for fossil fuels. Focusing on the problems of slow combustion speed, high ignition energy, and significant ignition delay in ammonia combustion, the effects of initial temperature, pressure, and oxygen volume fraction on ammonia combustion characteristics are studied via Chemkin simulation, based on the different ammonia combustion chemical reaction kinetics mechanisms of Shrestha, Mei, Mei-2021, Stagni, CEU-NH₃, Gotama, and Glarborg. The results show that, as the initial temperature increases, the propagation speed of ammonia laminar flame increases, and the ignition delay time decreases, which is beneficial for ammonia ignition and combustion. The increase in pressure reduces the propagation speed of laminar flames, but significantly shortens the ignition delay time. The increase in pressure is beneficial for ignition but not conducive to flame propagation. As the volume fraction of O2 increases, the laminar flame propagation speed increases and the peak shifts towards lean combustion. Sensitivity analysis reveals that the branching ratios of H+O₂=O+OH, H₂+NO=NNH+OH, and NH₂+NO=H₂O+N₂ have a positive promoting effect on flame propagation, while that of NH₂+O=HNO+H inhibits flame propagation. The reactions H+O₂(+M)=HO₂(+M), NH₃=H+NH₂, HNO=H+NO, and NH₂+HO₂=NH₃+O₂ exhibit high sensitivity at high pressures. The sensitivity coefficients of the reactions between HNO and N_iH_i is relatively high during lean burn combustion. H₂NO is an important intermediate component that affects the ignition delay time at high pressures and low temperatures. By optimizing the conditions of ammonia combustion and regulating key reaction pathways and reaction kinetics, the characteristics of ammonia combustion can be improved.

The effect of co-firing hydrogen/ammonia on nitrogen oxides emissions from boilers is investigated. The reaction kinetics file is modified based on coal quality analysis and experimental results. A psr reactor network based on CFD simulation results is constructed according to the fluid dynamics (CFD) simulation results. Combing with the chemical reaction kinetics analysis method, the NO_x emissions after burning hydrogen/ ammonia at four positions of primary air, peripheral air, secondary air and post secondary air in five schemes are analyzed. The results show that, for the researched boiler, when the hydrogen co-firing position is located in the secondary air scheme, and the hydrogen mixing ratio is 20%, the NO emission reduces by 32.4%, and the emission concentration of unburned carbon does not change much compared to the pure coal condition. When the ammonia co-firing position is located behind the secondary air, the NO emission mass concentration is slightly higher than that under the pure coal condition, and the emission mass concentration of unburned carbon reduces significantly. The above two schemes are recommended for co-firing hydrogen/ammonia in the coal-fired boiler, with nitrogen oxide emissions as the evaluation index. This method and conclusion provides a theoretical basis for the engineering implementation of hydrogen/ammonia co-firing technology.

To tackle the challenges associated with the poor combustion performance of ammonia fuels and the high NO_x emissions in exhaust gases, experimental research on enhancing ammonia combustion through the use of a swirling burner combined with a gliding arc plasma generator was carried out. The effects of various combustion enhancement methods, including methane-assisted combustion, plasma-assisted combustion, and plasma-coupled methane-assisted combustion, on the combustion characteristics of NH₃ swirling flames and the generation of NO were investigated. The experimental results indicated that, compared with the methane-assisted combustion, both plasma-coupled methane-assisted combustion and plasma-assisted combustion significantly enhanced the stability of ammonia combustion. This improvement was evidenced by a substantial expansion of the stable combustion limit range of the NH₃ swirling flame, enabling normal combustion within an NH₃/Air equivalence ratio range of 0~5.0. In comparison to single methane-assisted or plasma-assisted combustion, plasma-coupled methane-assisted combustion (with a plasma power of 0.8 kW and a methane flow rate of 1 L/min) significantly enhanced the active species H_α and OH generated by the discharge, thereby strengthening the chemical effects in plasma-assisted combustion. Under these conditions, the NO emission mass concentration in the exhaust gases rapidly decreased from over 7 000 mg/m³ to approximately 100 mg/m³ as the NH₃/Air equivalence ratio was increased from 0.6 to 0.8. Furthermore, the gas temperature under these conditions was only slightly lower than that observed in pure plasma-assisted combustion, where the flame temperature of ammonia combustion could reach up to approximately 2 030 K.

A simulation study on a 300 MW tangentially-fired boiler with 20% ammonia doping at 60% load was carried out to analyze the combustion process of ammonia doping in pulverized coal boiler, and to seek for the best coal-ammonia co-combustion scheme to ensure the optimal combustion efficiency and the lowest pollutant emission. By adjusting the position of ammonia burners and the ratio of the separated over fire air, the temperature field of the flue gas in the furnace, the molar fraction distribution of the combustion components, as well as the combustion characteristics and the NO_x emission level at the furnace outlet were systematically analyzed using numerical simulation. The comprehensive analysis shows that, the best coal-ammonia co-combustion solution is to place the ammonia burner on top layer (tertiary air) and the CD layer (secondary air) when the ratio of the separated over fire air is 33.5%. This scheme ensures the combustion efficiency and stability while controlling the NO_x emission level comparable to that of the pure coal-fired condition, which provides a new way of thinking for large-scale coal-fired power plants to realize clean and efficient combustion.

A 660 MW opposed firing boiler is designed with staggered lower over fire air (OFA) burner arrangement, in response to the problems of high NO_x emission and low burn-out efficiency at the furnace outlet caused by the use of upper burners on the front wall, numerical simulation is performed to study the effects of the coal mill combinations, as well as the lower OFA ratios, injection angles, heights of the burners on front wall on the combustion and NO_x emission characteristics under full load conditions. The results show that, the NO_x emission mass concentration decreased by 29.49 mg/m³ and the carbon content in fly ash decreased by 0.16% after the upper burner of the front wall was deactivated. The combined operation mode of BCDEF burners should be selected in actual operation. When keeping the lower OFA ratio unchanged, the NO_x emission mass concentration increased after the lower OFA ratio on the front wall was increased from 10.2% to 14.2%. When the air rate exceeded 13.2%, insufficient overfire air at the lower part of the back wall led to a decrease in burnout efficiency. The lower OFA rate on the front wall should be controlled within 12.2%~13.2% during actual operation. The NO_x emission mass concentration reduced by 18.13 mg/m³ and the carbon content in fly ash increased by 0.38 percentage point after the lower OFA was changed from 15° injection to horizontal injection. When the lower OFA burners on the front wall were moved up to the height of the lower OFA burners on the rear wall, the NO_x emission mass concentration decreased by 41.78 mg/m³, and the carbon content in fly ash increased by 0.68 percentage point. Compared to the opposed firing boilers with conventional lower OFA burners, the one with staggered layout of lower OFA burners has relatively weak deep air staged combustion effect, but with high burnout rate and better adjustability.

Co-combustion of biomass and coal can significantly reduce the pollutants and carbon emissions. However, the pollutants emission, ash characteristics and their influence factors during co-combustion of coal and biomass are still obscure. A micro-fluidized bed reactor was used to investigate the co-combustion of rice husk (RH) and bituminous coal (SC). The effects of combustion temperature, atmosphere and blending ratio on the emission characteristics of NO, SO₂ and ash were studied. The results indicated that, with the increase of temperature, the mass concentration of NO emissions during co-combustion rose at first and then fell, while the SO₂ emissions mass concentration gradually increased. High temperature would promote the interaction of minerals in the ash, generating Na₂Al₂SiO₆ and lowering the melting point of the co-combustion ash. In oxygen-deficient atmosphere, NO and SO₂ emission mass concentrations increased with the O₂ volume fraction. In oxygen-enriched atmosphere, the NO emission mass concentration gradually decreased as the O₂ volume fraction increased, while the SO₂ emission mass concentration increased at first and then decreased. The influence of reaction atmosphere on the main composition and crystal structure of the co-combustion ash was relatively minor. NO and SO₂ emissions during co-combustion can be effectively reduced due to the addition of RH. As the RH blending ratio was increased, Ca₃Al₂O₆ in ash tended to form low-melting-point compounds with SiO₂, leading to a noticeable melting phenomenon.

The use of large-scale coal-fired power units mixed with refused derived fuel (RDF) can reduce carbon emissions and solve the problem of waste management. To verify the feasibility of co-firing RDF in coal-fired boilers, initial tests were conducted using a one-dimensional furnace to determine the maximum allowable proportion of RDF. Then, pilot-scale tests were carried out on a 4 MW boiler to study the effect of RDF co-firing on coal grinding, combustion, pollutant emissions, and slagging and fouling. The results showed that, when the co-firing ratio of RDF was less than 10% (mass ratio), the mass concentration of dioxins in the flue gas, and dioxins and heavy metals in the ash residue were all below the pollutant control standards. When 10% of RDF was co-grounded with coal in a medium speed mill, the fineness of R₉₀ increased to 27.2%. When the coal and RDF were mixed and co-fired, the flame temperature and NO_x formation concentration decreased, the slagging in the combustion air zone increased, while the horizontal flue fouling changes were relatively minor, and the combustible content in the bottom slag increased to about 15%. The research will provide reference for co-firing RDF in coal-fired boilers.