Impact of structural modificationThis 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 2FGR-exit NOx and NOx removal efficiency.Figure 3Incinerator structure diagram.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 stagingTo 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.Figure 4Temperature and velocity field distributions for incinerator type A and incinerator type B.Effect of flue gas recirculationThe study investigated the impact of different flue gas recirculation ratios on NOx emissions and boiler efficiency (Table 8).Table 8 Different overfire air arrangements.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.
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.
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.
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 wasteThe 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 278.63Â mg/Nm3 but adjusting the SNCR nozzle position reduced the outlet flue gas NOx content to 245.25Â mg/Nm3, indicating the need for additional denitrification technologies to meet emission standards.Table 9 Different sludge co-combustion conditions.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.
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.
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.
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.
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 compositionsThe 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 technologiesThe 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.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 technologyAir 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 technologiesWhen 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.
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.
b.
Air staging: Reduces the formation of NOx by controlling the combustion process and temperature.
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.