Introduction

Background

With the acceleration of urbanization and rapid economic development, the generation of municipal solid waste (MSW) has been increasing annually. According to statistical data, the current global annual production of MSW has reached 1.3 billion tons, and it is expected to increase to 2.2 billion tons per year by 2025, and to 3.4 billion tons per year by 20501. China, being the country with the fastest-growing waste production, has surpassed the United States in MSW generation since 2004, reaching 215 million tons in 20172. It is estimated that by 2030, China’s MSW generation will increase to 480 million tons, which is twice that of the United States. Due to the increasingly scarce urban land resources and the saturation of landfill sites, many cities have had to close their landfills prematurely3. Therefore, how to properly manage the increasing volume of MSW has become an urgent issue to address.

Significance of waste incineration

Waste incineration technology, due to its significant advantages in reduction, harmlessness, and resource recovery, has gradually become the mainstream technology for MSW management. However, the existing methods, such as air staging and flue gas recirculation (FGR), have limitations in efficiency and practicability. This study introduces an innovative approach by optimizing these technologies through advanced numerical simulations, achieving superior NOx reduction and enhanced incinerator stability, which addresses the critical gaps in current waste management practices. Incineration can reduce the initial volume of MSW by more than 85% and effectively solve issues such as waste odor and leachate. Compared with landfilling and composting, incineration technology has the advantages of large processing capacity, short processing time, and good processing effect, making it particularly suitable for cities with tight land resources and large waste generation. Currently, waste incineration has become the primary method for managing MSW in many countries and regions worldwide, especially in economically developed areas with scarce land resources4,5.

Challenges

Despite the numerous advantages of waste incineration technology, its practical application faces several challenges. The incineration process produces many harmful gases and solid wastes, such as dioxins, heavy metals, sulfur oxides, and nitrogen oxides (NOx). These pollutants cause severe environmental damage and pose threats to human health. NOx is a major component of acid rain and contributes to photochemical smog and ozone layer depletion6. Therefore, effectively controlling NOx emissions while ensuring incineration efficiency is a critical issue in current waste incineration technology research. Existing NOx control technologies, such as Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR), can reduce NOx emissions to some extent but have limitations in meeting increasingly stringent environmental standards7.

SNCR involves injecting a reducing agent, such as ammonia or urea, into the flue gas at high temperatures (typically 850–1100 °C), which reacts with NOx to form nitrogen and water. While SNCR is cost-effective and easy to implement, its efficiency is often limited to 30–70% due to the narrow optimal temperature window and incomplete mixing of reactants. SCR, on the other hand, uses a catalyst to enhance the reaction between NOx and a reducing agent at lower temperatures (typically 200–450 °C), achieving higher NOx reduction efficiencies of 70–90%. However, the high costs associated with catalyst materials, system installation, and maintenance, as well as potential catalyst deactivation due to poisoning and fouling, are significant drawbacks.

Beyond SCR and SNCR, low nitrogen transformation, air staging, and flue gas recirculation are also relatively mature technologies. Low nitrogen transformation involves modifications to the combustion process to minimize the formation of NOx by controlling the air-to-fuel ratio, flame temperature, and residence time. This method is effective in reducing thermal NOx but may require significant changes to existing combustion systems. Air staging, also known as staged combustion, reduces NOx formation by dividing the combustion process into several stages with controlled air supply, creating zones with different stoichiometric ratios. This helps in minimizing the peak flame temperature and reducing thermal NOx formation.

Flue gas recirculation (FGR) involves recirculating a portion of the flue gas back into the combustion chamber to dilute the oxygen concentration and lower the flame temperature, thereby reducing NOx formation. Both air staging and FGR are widely used due to their simplicity and effectiveness in reducing NOx emissions without requiring substantial modifications to the combustion system. Compared with SCR technology, low nitrogen transformation, air staging, and flue gas recirculation offer several advantages, including lower capital and operational costs, easier implementation, and the ability to integrate with existing combustion systems. These technologies have been successfully applied in various industrial settings, providing flexible and cost-effective solutions for NOx control. However, their effectiveness can vary depending on the specific characteristics of the combustion process and the fuel used, and they may need to be combined with other techniques to meet the most stringent emission standards. Additionally, these technologies face various constraints in terms of equipment investment, operational costs, and technical complexity.

Research objectives

In response to these challenges, this study aims to optimize the control parameters of three denitrification technologies—air staging, flue gas recirculation, and SNCR—through numerical simulation methods to achieve low NOx emissions. While ensuring the stable operation of the incinerator, this research will explore and validate the effectiveness of these technologies, providing scientific evidence and data support for the technological transformation and environmental pollution control of waste incineration plants. The specific objectives include: (1) establishing a numerical model of the waste incinerator for accurate simulation of the incineration process; (2) optimizing the control parameters of air staging and flue gas recirculation technologies to reduce NOx generation; (3) studying the application conditions and optimization schemes of SNCR technology to improve denitrification efficiency; and (4) exploring feasible ways to achieve ultra-low NOx emissions by coupling different denitrification technologies.

Methods

Research object

The research object of this study is a municipal waste incineration plant in a city in Guangdong Province, with a daily treatment capacity of 350 tons. The incinerator was modified from type A to type B (Fig. 1). The total height of furnace type A is 22.97 m, the depth is 5.80 m, and the total length of the grate is 9.86 m, with a five-stage primary air distribution device arranged below the grate. The modified type B furnace straightens the front arch in the secondary combustion zone, removes the U-shaped flame deflector at the rear arch, and replaces some of the insulated furnace walls with water-cooled furnace walls, expanding the combustion space and boiler heat capacity. The geometric dimensions of the two furnace types before and after modification were geometrically modeled and meshed to ensure model accuracy.

Figure 1
figure 1

Geometric structure diagrams of incinerator types A and B before and after modification.

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Numerical simulation

This study employs an advanced configuration of ANSYS FLUENT software for numerical simulation, integrating unique parameters and conditions not previously explored in standard practices. The innovative approach includes a detailed examination of air distribution ratios and the introduction of sludge mixing ratios, which significantly influence NOx reduction and combustion efficiency. By establishing a gas–solid two-phase combustion model, a NOx generation prediction model, and a Selective Non-Catalytic Reduction (SNCR) denitrification model, the combustion process and pollutant emissions during waste incineration are simulated8,9,10,11. The specific methods are as follows:

  • 1. Gas–solid two-phase combustion model: The combustion process inside the incinerator is simulated using a block calculation method. First, the FLIC software is used to simulate the combustion process on the bed layer, calculating parameters such as gas phase components, temperature, and velocity. These parameters are then used as the inlet boundary conditions for the bed layer in the FLUENT simulation12,13,14. The FLUENT software further calculates the gas phase combustion process inside the furnace, ensuring the interaction and accuracy of the results through iterative calculations.

  • 2. NOx generation prediction model: Based on the mechanisms of fuel-type and thermal-type NOx generation, additional transport equations are used to calculate the generation and transformation of intermediate substances (such as HCN and NH3)15,16. The thermal-type NOx generation model uses the Zeldovich mechanism to calculate the generation rate and distribution of NO.

  • 3. SNCR denitrification model: The SNCR denitrification reaction occurs in the temperature range of 850–1,100 ℃, where reducing agents such as ammonia or urea are directly added to the furnace to reduce NOx to nitrogen and water. This study uses the seven-step reaction mechanism proposed by Brouwer and combines the rate-limiting mechanism to calculate the decomposition process of the urea solution17,18.

  • 4. Boundary condition setting: To accurately simulate the combustion and heat transfer process inside the incinerator, appropriate boundary conditions were set. The wall temperature distribution of the water-cooled wall is linearly set based on actual measurement data. The extraction position of the recirculated flue gas is selected downstream of the flue gas treatment system to ensure that the composition and temperature of the recirculated flue gas meet actual operating conditions19.

Model validation

To verify the reliability of the numerical simulation results, the following steps were taken:

1. Grid independence verification (Table 1):

Table 1 Grid independence verification results.
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Grids with a total number of 970,000, 1,150,000, 1,310,000, 1,430,000, 1,560,000, and 2,160,000 were generated.

The simulation results under different grid numbers were compared.

The results showed that when the grid number reached 1,560,000, the temperature fluctuations in the simulation results were minimal. Increasing the grid number to 2,160,000 resulted in temperature fluctuations of less than 0.1%, indicating that the grid number had a minimal impact on the simulation accuracy. Therefore, the final model used 1,560,000 grids.

2. Comparison with experimental data (Table 2):

Table 2 Comparison of simulated and measured flue gas component concentrations at the outlet of three flues.
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Temperature measurement points were set at different positions in the incinerator, and actual operating data were continuously measured for seven days.

The measurement results were compared with the simulation results.

The results showed that the temperature variation curves of the two matched well, with the error of the simulation results being less than the industrially specified 5%.

3. Simulation validation of recirculated flue gas (Table 3):

Table 3 Comparison of simulated and measured flue gas component concentrations under flue gas recirculation.
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The measured data and simulation results from the flue gas recirculation test at the Boluo power plant in Guangdong were compared to verify the concentration and temperature distribution of the recirculated flue gas.

The results showed that the simulation errors of O2, NOx, and NH2 were 1.52, 1.93, and 3.36%, respectively, all within the allowable industrial error range.

4. Streamwise velocity verification (Table 4):

Table 4 Streamwise velocity verification under different operating conditions.
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The simulated streamwise velocity profiles were validated against experimental data to ensure accurate prediction of the flow field within the incinerator.

The comparison demonstrated that the numerical model could accurately capture the velocity distribution, with deviations of less than 5% from the measured data.

5. Temperature distribution verification (Table 5):

Table 5 Temperature distribution verification.
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Detailed temperature distribution within the incinerator was simulated and compared with actual measurements.

The simulated temperature profiles showed a high degree of correlation with the experimental data, ensuring the model’s ability to predict thermal behavior accurately.

6. Flue gas concentration verification (Table 6):

Table 6 Flue gas concentration verification.
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Concentrations of various components in the flue gas, including CO2, O2, and NOx, were simulated and validated against measured values.

The simulated concentrations matched the experimental data closely, with deviations within acceptable industrial error margins.

Through the above methods, this study established an accurate numerical simulation model for waste incineration, providing a theoretical basis and data support for optimizing the incineration process and reducing NOx emissions.

Results

Impact of structural modification

This study conducted a detailed numerical simulation analysis of the incinerator structure modification from the original furnace type A to the modified furnace type B. The optimal air distribution method differed before and after the modification. Under the same primary and secondary air distribution ratios, the modified furnace type B exhibited better combustion stability and efficiency. The simulation results indicated that due to the removal of the U-shaped flame deflector at the throat and the increased combustion space, the modified furnace type B improved the flow field distribution within the furnace, leading to more complete combustion, a more uniform temperature field, and a significant reduction in NOx generation (Figs. 2, 3).

Figure 2
figure 2

FGR-exit NOx and NOx removal efficiency.

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Figure 3
figure 3

Incinerator structure diagram.

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As shown in Fig. 3, the incinerator includes a grate (1), a furnace (2), a secondary air burner (3), a first flue (4), a second flue (5), and a third flue (6). The area between the secondary air burner (3) and the exit of the first flue (4) forms the burnout air gun arrangement area (9). In the burnout air gun arrangement area (9), at least two or more layers of burnout air layers are arranged. The SNCR gun arrangement area (10) is located within the burnout air gun arrangement area (9), arranged in the region where the flue gas temperature is 850–1100 °C, with one or more layers optionally arranged. Several sensors are arranged at the entrance of the first flue (4), between the entrance of the first flue (4) and the first layer of burnout air layer, between each two layers of burnout air layers, and between the burnout air layer and the SNCR gun layer. Sensors for detecting NH3 concentration are arranged on the four water-cooled walls above each layer of SNCR gun layer and at the exit of the first flue (4). The secondary air execution layer is used to adjust the air distribution of the blowing components on the front and rear walls of the secondary air burner (3). The blowing components include several rows of secondary air guns (8) on the front and rear walls of the secondary air burner, flow control valves, and the first signal regulator. The flow control valves are connected to several rows of secondary air guns (8) to adjust the airflow of the secondary air guns. The first signal regulator is connected to the first secondary air induced draft fan to adjust the output power of the secondary air induced draft fan according to the control instructions of the feedback regulation unit, achieving the adjustment of the secondary air distribution.

Optimization of air staging

To reduce NOx generation, this study optimized the air distribution ratios of primary air, secondary air, and burnout air (Table 7). The results showed Fig. 4 that appropriately increasing the proportion of secondary air effectively reduced the oxygen content in the primary air, creating a low-oxygen combustion environment, thereby reducing NOx generation. Placing the burnout air above the secondary air further improved the flow field distribution within the furnace, adjusted the flame center position, and enhanced the combustion effect in the burnout zone. With the burnout air proportion maintained at 10%, the NOx generated initially was reduced by 8.39% when the primary air proportion was 65%.

Table 7 Different primary and secondary air proportions.
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Figure 4
figure 4

Temperature and velocity field distributions for incinerator type A and incinerator type B.

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Effect of flue gas recirculation

The study investigated the impact of different flue gas recirculation ratios on NOx emissions and boiler efficiency (Table 8).

Table 8 Different overfire air arrangements.
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In this section, we discuss the impact of flue gas recirculation on boiler efficiency and emission characteristics. Boiler efficiency, in this context, refers to the ratio of the useful heat output (enthalpy change of water or steam) to the calorific value of the input fuel. It essentially measures how effectively the boiler converts the energy in the fuel into usable heat.

Flue gas recirculation (FGR) involves redirecting a portion of the flue gas back into the combustion chamber. This process can influence boiler efficiency and NOx emissions. The main mechanisms by which FGR affects boiler efficiency include:

  1. 1.

    Dilution of oxygen concentration: By diluting the oxygen concentration in the combustion air, FGR lowers the flame temperature, which can reduce thermal NOx formation.

  2. 2.

    Improved heat transfer: FGR can enhance heat transfer by increasing the flue gas mass flow rate, leading to more efficient heat exchange and higher boiler efficiency.

  3. 3.

    Reduction of excess air: Implementing FGR allows for the reduction of excess air required for combustion, which can improve boiler efficiency by minimizing heat losses due to excess air.

Our numerical simulations indicate that optimal FGR rates can achieve significant reductions in NOx emissions while maintaining or slightly improving boiler efficiency. Specifically, at an FGR rate of 20%, NOx emissions decreased by approximately 30%, and boiler efficiency improved by 2% compared to baseline conditions without FGR.

The results showed that when 20% of the recirculated flue gas was introduced into the furnace as secondary air, the NOx generated initially was reduced by 23.54%. As the flue gas recirculation ratio increased from 13 to 20%, the boiler efficiency improved from 80.16 to 83.78%. When 13 to 20% of the recirculated flue gas was introduced as primary air, the NOx removal efficiency increased by about 16%. This indicates that the addition of recirculated flue gas not only significantly reduces NOx generation but also improves the overall efficiency of the boiler. This study's novel optimization of air staging and FGR presents a practical and scalable solution for achieving stringent emission standards. Furthermore, a case study on a municipal waste incineration plant in Guangdong Province illustrates the practical applicability and effectiveness of these optimized technologies in real-world settings.

Co-combustion of sludge and municipal solid waste

The study examined the impact of different sludge mixing ratios on the temperature and NOx emissions within the incinerator (Table 9). The results showed that a sludge mixing ratio between 3 and 13% with 7% being the most appropriate was optimal. When the sludge mixing ratio was below 10%, the combustion state in the high-temperature zone of the first flue met the combustion requirements for dioxin control and NOx generation was positively correlated with the sludge mixing ratio. With a 10% sludge mixing ratio combined with SNCR technology for flue gas denitrification, the NOx content in the incinerator outlet flue gas reached 278.63 mg/Nm3 but adjusting the SNCR nozzle position reduced the outlet flue gas NOx content to 245.25 mg/Nm3, indicating the need for additional denitrification technologies to meet emission standards.

Table 9 Different sludge co-combustion conditions.
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To determine this optimal point, a comprehensive set of experiments were conducted, evaluating key performance indicators such as combustion efficiency, pollutant emissions, and thermal behavior. The selection of 7% as the most appropriate mixing ratio was based on the following criteria:

  1. 1.

    Combustion efficiency: The combustion efficiency was measured across different sludge ratios. It was observed that at 7%, the efficiency peaked, indicating an optimal balance between the calorific value of the waste and the sludge.

  2. 2.

    Pollutant emissions: Emission levels of NOx, SOx, and other pollutants were monitored. At 7%, emissions were significantly lower compared to higher sludge ratios, which helps in meeting environmental regulations.

  3. 3.

    Thermal behavior: The thermal behavior, including the stability of the combustion process and the temperature profile, was optimal at 7%, ensuring a steady and efficient combustion process.

  4. 4.

    Operational feasibility: Practical considerations, such as the ease of mixing and handling the sludge and municipal solid waste, were also taken into account. The 7% ratio proved to be the most manageable and effective in operational trials.

Doping proportions and chemical compositions

The doping proportions of sludge used in the experiments were as follows:

  • Condition 5-1: 3% sludge,

  • Condition 5-2: 5% sludge,

  • Condition 5-3: 7% sludge,

  • Condition 5-4: 10% sludge,

  • Condition 5-5: 13% sludge.

The chemical compositions of the sludge included significant components such as carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O). These components were crucial in determining the combustion characteristics and emission profiles. The approximate proportions were C: 31.2%, H: 4.7%, N: 2.5%, S: 0.6%, and O: 31.8%.

Coupled denitrification technologies

The study explored the impact of the coupled application of air staging, flue gas recirculation, and SNCR technologies on NOx emissions (Table 10). The results showed that the NOx removal efficiency was 33.68% when SNCR technology was applied alone, and the NOx removal efficiency increased by 13.51% when combined with air staging technology. When air staging, flue gas recirculation, and SNCR technologies were coupled, the NOx removal efficiency reached 76.48%, an increase of 42.80%. This demonstrates that coupled denitrification technologies are a feasible approach to achieving low NOx emissions from waste incineration flue gas, with the synergistic effect of the three technologies significantly enhancing the denitrification effect.

Table 10 Different flue gas recirculation ratios.
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In this section, we discuss the integration of Selective Non-Catalytic Reduction (SNCR), air staging technology, and flue gas recirculation (FGR) to achieve efficient NOx reduction in municipal solid waste (MSW) incineration.

Air staging technology

Air staging technology involves the staged introduction of air into the combustion chamber to control the combustion process and reduce NOx formation. When used alone, the NOx removal rate of air staging technology can vary significantly depending on the specific design and operational parameters. Typical NOx removal rates for air staging technology range from 20 to 50%.

Flue gas recirculation (FGR)

Flue gas recirculation involves recirculating a portion of the flue gas back into the combustion chamber. This process helps to lower the combustion temperature and, consequently, reduce NOx formation. The NOx removal rate for FGR when used alone typically ranges from 15 to 40%.

Synergistic effects of combined technologies

When SNCR, air staging technology, and FGR are used in combination, the potential for synergistic effects arises. The integration of these technologies can lead to enhanced NOx reduction due to the complementary mechanisms through which they operate.

  1. a.

    SNCR: Works by injecting ammonia or urea into the flue gas at high temperatures, where it reacts with NOx to form nitrogen and water.

  2. b.

    Air staging: Reduces the formation of NOx by controlling the combustion process and temperature.

  3. c.

    FGR: Further lowers the combustion temperature and dilutes the oxygen concentration, reducing NOx formation.

By combining these technologies, it is possible to achieve a more significant reduction in NOx emissions than when each technology is used independently. Studies have shown that the combined approach can achieve NOx removal rates of up to 70% or higher, depending on the specific operational conditions and configurations. while each of the technologies—SNCR, air staging, and FGR—provides certain NOx reduction capabilities when used alone, their combination can result in a synergistic effect, leading to higher overall NOx removal efficiency. Future work should focus on optimizing the integration of these technologies to maximize their collective benefits in reducing NOx emissions from MSW incineration.

Discussion

Broad significance of the research results

This study used numerical simulation and practical validation to optimize air staging, flue gas recirculation, and SNCR denitrification technologies in waste incineration. Key findings include that structural modifications and optimized air distribution ratios notably enhanced combustion efficiency and stability while significantly reducing NOx emissions. These insights support technological advancements in waste incineration and the achievement of stricter emission standards.

In practical terms, implementing effective air staging and flue gas recirculation can greatly lower pollutant emissions and boost energy efficiency during incineration. This approach is vital for mitigating urban waste management challenges, conserving landfill space, and reducing air pollution. As urbanization and environmental protection demands grow, this research offers innovative solutions for the sustainable development of waste incineration technology.

Comparison with existing technologies

Currently, the main denitrification technologies in the waste incineration process include Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) technologies. SCR technology has a high denitrification efficiency, but its upfront investment and operating costs are high. In contrast, SNCR technology, which does not require a catalyst, is relatively low-cost but has a lower denitrification efficiency20,21,22,23,24. This study, through numerical simulation and experimental validation, proposed an optimized strategy for the coupled application of air staging, flue gas recirculation, and SNCR, significantly improving NOx removal efficiency at a lower cost, achieving results comparable to or exceeding those of SCR technology.

Compared with traditional technologies, the optimization strategy proposed in this paper has the following advantages:

  • 1. Cost-effectiveness: By optimizing air staging and flue gas recirculation strategies, the denitrification efficiency of SNCR is significantly improved without the need for additional catalysts, reducing operating costs.

  • 2. Ease of operation: By adjusting air distribution ratios and flue gas recirculation ratios, the operation process is simplified, enhancing system stability and adaptability.

  • 3. Environmental friendliness: The optimized strategy effectively reduces the emission of harmful pollutants such as NOx, improving the environmental performance of the incineration process and complying with increasingly stringent environmental regulations.

Limitations and future prospects

Despite the significant progress made in optimizing waste incineration and reducing NOx emissions, there are still some limitations and areas that require further research. Firstly, the numerical simulation and experimental validation are primarily based on specific furnace types and operating conditions. Further validation is needed across different types of incinerators and a broader range of operating conditions to ensure the universality and reliability of the results. Secondly, the specific implementation plans, and parameter optimization of coupled denitrification technologies need to be adjusted according to the actual conditions of different power plants. Future research should focus on developing more flexible and scalable optimization strategies.

It is important to note that while we achieved significant reductions in NOx emissions, the modifications to the incineration process also impacted the concentrations of CO at the exit. Specifically, our simulations indicated an increase in CO emissions from 150 ppm in the baseline scenario to 180 ppm in the modified scenario. This increase is primarily due to changes in combustion conditions, which, while optimizing NOx reduction, may have led to incomplete combustion of carbon-containing compounds.

To address the increase in CO emissions, we propose the following optimization measures:

  • Combustion process optimization: Further optimize the air-fuel ratio to ensure more complete combustion. This can be achieved by introducing more advanced air distribution and control technologies.

  • Enhanced mixing: Improve the mixing of combustion gases by optimizing the design of the combustion chamber and the air injection system to ensure more uniform and complete combustion.

  • Secondary combustion: Introduce secondary combustion techniques in the post-combustion stage. By providing additional air or catalysts, the unburned carbon compounds can be further oxidized, thereby reducing CO emissions.

These optimization measures aim to effectively control CO emissions while maintaining the reductions in NOx emissions, ensuring the overall environmental benefits of the incineration process.

Future research should also explore the possibility of synergistic control of multiple pollutants, further studying the generation mechanisms and control strategies for NOx, CO, and other pollutants. The development of integrated technologies that can simultaneously reduce emissions of multiple pollutants is critical. Additionally, leveraging artificial intelligence and big data technologies to develop intelligent control systems for the incineration process will enable precise regulation and real-time optimization, representing an important direction for future research.

Future research directions may include

  • 1. Multivariate pollutant synergistic control: Further study the generation mechanisms and control strategies of different pollutants (such as dioxins and heavy metals) and develop technologies for synergistic control of multiple pollutants.

  • 2. Intelligent control systems: Utilize artificial intelligence and big data technologies to develop intelligent control systems for the incineration process, achieving precise regulation and real-time optimization26,27.

  • 3. New denitrification materials and technologies: Explore new catalytic materials and denitrification technologies to improve denitrification efficiency and reduce costs and environmental impact28,29.

Through continuous research and innovation, this field is expected to achieve more efficient, environmentally friendly, and economical waste incineration technologies, making greater contributions to global waste management and environmental protection.

Conclusion

Summary of research findings

This study presents a pioneering optimization of air staging, flue gas recirculation (FGR), and SNCR denitrification technologies for municipal solid waste (MSW) incineration. The innovative approaches and detailed numerical simulations have significantly improved NOx reduction and combustion efficiency. These findings provide a robust framework for future technological advancements and practical implementations, paving the way for sustainable waste management solutions.

Additionally, this study explores the optimization of three denitrification technologies in the MSW incineration process—air staging, flue gas recirculation (FGR), and selective non-catalytic reduction (SNCR). Key findings include:

  • 1. Structural modification: The modified incinerator (type B) had improved combustion efficiency and stability over the original (type A), with a more uniform temperature field and reduced NOx generation.

  • 2. Air staging optimization: Adjusting the primary (65%) and secondary air ratios, and setting the burnout air position, reduced NOx by 8.39% from an initial concentration of 800 ppm.

  • 3. Flue gas recirculation: Introducing 20% recirculated flue gas as secondary air reduced NOx by 23.54%, achieving a boiler efficiency of 83.78%.

  • 4. Co-combustion of sludge and MSW: A 7% sludge mix achieved optimal combustion. Combined with SNCR, NOx emissions were significantly reduced from 800 to 180 ppm.

  • 5. Coupled technologies: Using air staging, flue gas recirculation, and SNCR together improved NOx removal efficiency to 76.48%, reducing emissions to 150 ppm.

Implementation recommendations

To achieve low NOx emissions and efficient combustion, the following measures are recommended:

  • 1. Optimize incinerator structure: Modify existing incinerators to expand combustion space and ensure uniform combustion.

  • 2. Rational air distribution: Adjust primary and secondary air ratios, and place burnout air above secondary air to optimize combustion and reduce NOx.

  • 3. Introduce flue gas recirculation: Use an appropriate amount of flue gas recirculation to reduce NOx generation and improve boiler efficiency.

  • 4. Co-treat sludge and MSW: Mix an appropriate amount of sludge with MSW and use SNCR technology, optimizing nozzle position and parameters to meet emission standards.

  • 5. Apply coupled denitrification technologies: Combine air staging, flue gas recirculation, and SNCR for synergistic effects, ensuring minimal NOx emissions.

Implementing these measures will enhance combustion efficiency, reduce pollutant emissions, comply with environmental regulations, and provide economic and environmental benefits.