Waste incineration power generation, which couples the waste incineration power generation system with other thermal systems with higher thermal parameters, has also become the research focus of improving the energy efficiency of waste incineration power generation. At present, the coupling power generation technology of waste incineration and large coal-fired units will bring significant energy-saving benefits through the joint operation mode. This technology fully combines the technical characteristics of the two power generation modes (Song et al., 2021). On the steam side, the main steam at the outlet of the garbage incinerator is introduced to the thermal system of the coal-fired unit, and the heat of the low-grade garbage incinerator is partially transferred to the high-grade coal-fired unit for power generation, so as to improve the utilization efficiency of the garbage heat; and on the flue gas side, the tail flue gas generated by the garbage incinerator is introduced to the flue gas treatment system of the coal-fired boiler. It will reduce the equipment investment of the flue gas purification system at the outlet of the waste incineration waste heat boiler. In the conventional waste incineration power generation system, the waste generates heat through incineration, and then the flue gas absorbs heat through the heating surfaces of the waste incineration boiler, which drives the steam turbine to do work and drives the generator to generate electricity. At present, the mechanical grate incinerator and liquidized bed boiler are widely used in waste incineration technology. Kozioi (2020) compares the performance, advantages and disadvantages of these two technologies. Due to the more perfect development of mechanical incinerators and more experimental experience, mechanical grate incinerators are widely used in the market. However, the power generation efficiency of the conventional waste incineration generating units is between 20% and 32%, which is far lower than that of large coal-fired generating units. The main reasons for low efficiency are small unit capacity, low steam parameters, imperfect regenerative cycle, etc. Compared with the traditional technology, the coupling technology can increase the thermal efficiency of waste incineration power generation to about 32%, and the efficiency can be increased by 10%, further realizing the existing waste reduction, harmless and low-cost disposal (Bandarra and Quina, 2021).

The quality of the waste entering the incinerator seriously affects the combustion characteristics of the incinerator. If the storage time of the waste is not enough, the moisture content of the waste will be relatively high, especially in the lower part of the waste storage pit. The moisture of the waste in the middle and lower part of the storage pit will be evaporated and gathered in the upper part of the waste. As a result, the garbage in the upper part is more difficult to burn than the garbage at the lower part. Waste slag, waste plastics, sawdust, etc. are sorted out from the garbage by the fuel manufacturing plant. After crushing, drying, pressing and solidification, they are made into harmless solid fuel, namely, RDF fuel (refuse-derived fuel) (Dadario et al., 2021a). Then the RDF fuel is transported to the power plant for RDF power generation. The RDF fuel formed after treatment has a high calorific value. Stable combustion, boiler efficiency, and power generation efficiency can be improved (Khandelwal et al., 2019).

In 2021, Dadario et al. (2021b) discussed the power generation process of a very small power plant (VSPP)-municipal solid waste (MSW) from organic Rankine cycle (ORC) to incinerator. It was found that the temperature and heating rate of hot water were 105.38°C and 240.32 kW, respectively, in a 1,500 L high-pressure cylinder at a processing rate of about 92 kg/h for hot water production and heat storage. The energy efficiency of the incinerator is 31.66% and the energy efficiency is 4.05%. Zhang et al. (2021) tested the flue gas composition and flue gas temperature at the economizer outlet of a 500t/D waste incineration power plant and studied the impact of flue gas re-circulation on the efficiency and NOx emission of the waste heat boiler. The results show that within the test range, the efficiency of the waste heat boiler of the incinerator increases from 80.26% to 80.42% as the flue gas re-circulation rate increases from 0% to 16.43%; Zhou et al. (2019) developed a liquidized bed reactor as an incinerator. But as the number of input variables that need to be considered increases, it becomes quite difficult to optimize the design only by computational fluid dynamics (CFD) simulation because it requires a lot of computing time. Therefore, an artificial neural network (ANN) was used to build a surrogate model of the CFD output to determine the optimal operating conditions.

Waste incineration power generation can not only realize the recovery of waste energy but also reduce the annual waste storage in China. However, the parameters of waste incineration power generation are low, and the thermal efficiency of the corresponding power generation system is low. Therefore, this paper will integrate the waste incineration power generation system into the case coal-fired power generation system with the help of large and efficient coal-fired units. The coupling system is modeled and the parameters of the coupling system are obtained. Through energy analysis, the reason why the power generation efficiency of the new waste incineration system is greatly improved is revealed. To reduce the irreversible loss in the process of primary and secondary air preheating in waste incineration boiler, the furnace temperature is taken as the input of the controller, and the feed grate speed, primary air and secondary air are taken as the output. The research indicates that the control effect of the advanced control algorithm is better. It realizes the efficient utilization of the heat generated by waste incineration, which solves the problem of low efficiency of waste incineration power generation. Then, it considers the variable load operation requirements of coal-fired units, and optimizes an integrated power generation system that adapts to the variable load operation of coal-fired units and has the high thermal economy. At the same time, for coal-fired power plants, the active introduction of a waste incineration systems can also increase their competitiveness and assume certain social responsibilities.

In the traditional coupling system, the heat generated by waste incineration is injected into the coal-fired power generation system for utilization. Therefore, the energy flow process in the original system is quite different from that in the coupling system, as shown in Figure 2.

In the new system and the coupling system, the heat input by the fuel remains unchanged, which is 100%. In the coupling system scheme, the waste incineration boiler transfers 10.5 MW to the coal side for heat utilization, while the coal-fired unit transfers 36.6 MW of heat to the waste incineration system through feed water and condensate water for heating the primary and secondary air. The system structure is shown in Figure 3.

Therefore, from the energy flow process of the original system and the coupling system scheme, it can be concluded that the improvement of the efficiency of waste incineration power generation after coupling is mainly due to the reduction of the system’s total condensation loss.

Optimize the operation of waste incinerator. Study the influence of changing the operation parameters and incinerator structure on the combustion process in the furnace. Simulate and analyze the temperature distribution, flow field distribution, flue gas component distribution, bed solid phase component distribution and pollutant generation and its influence law in the furnace. Figure 4 is a temperature distribution diagram of a central section of a boil with different front and rear arch angles.

Gas-phase combustion is mainly concentrated in the middle of the furnace, which is due to the volatilization of solid-phase garbage in the bed layer, which converges with the secondary air of the furnace and is fully mixed for intense combustion. With the heat exchange of the water wall, the flue gas temperature decreases with the increase of the furnace height. With the increase of the front arch angle of the furnace, the combustion condition of the furnace becomes worse, and the outlet temperature of the first flue decreases from 1154 K in # 1–1108 K in # 3. This is due to the increase of the furnace space, the increase of the airflow disturbance, the increase in the heat exchange coefficient between the flue gas and the water wall, and the increase in the heat exchange quantity. The gas back-flow in the front arch of # 1 and # 3 is serious, which is easy to cause the phenomenon of high temperature sticking to the wall. The combustion condition is better when the front and rear arches are 21°/35°.

The boiler velocity vector field is shown in Figure 5.

With the decrease of the angle of the front arch, the cyclone expands and the back-flow intensity increases, which increases the convective heat transfer coefficient between the flue gas and the water wall, and has a positive effect on preventing slagging on the convective heating surface and improving the thermal efficiency of the boiler. The back-flow of # 1 is obvious, which also causes the front arch to stick to the wall at high temperatures. The high turbulence intensity of the airflow in the furnace also intensifies the generation of the vortex area in the first flue above. # 2 has a good tangential circle phenomenon of the overfire air, which is conducive to concentrating the combustion area in the center of the furnace and reducing the high-temperature corrosion on the water wall. With the increase of the rear arch angle, the scouring effect of the rear arch secondary air on the grate is reduced. The mixing capacity of the secondary air and the flue gas is improved. The residence time of the flue gas in the furnace is increased and the volatile combustion is more sufficient. Due to the influence of flue gas turbulence at the throat, the tangential effect of over-fire air of # 5 is poor, and it is easy to cause slagging on the convection heating surface (Chen et al., 2020).

The intense combustion zone with complete oxygen consumption appears in the center of the furnace, and the combustion zone is basically in the middle zone. With the increase of the front arch angle, the mass fraction of oxygen at the outlet increases from 7. 1% in # 1 to 7. 8% in # 3. With the supplement of secondary air, the unburned flue gas is completely burned in the first flue, and the oxygen content is increased. With the increase of the back arch angle, the combustion zone is gradually stabilized in the middle of the furnace, and the oxygen content in the furnace is increased. Corresponding to the temperature distribution map, the maximum oxygen mass fraction is 7. 4% when the back arch angle is 21°/35°.

According to the heat balance calculation of the incinerator, the calculation process of the flue gas heat output by the boiler is as follows: Q t = Q w + Q d n − Q a s h (7)

Where, Q t is the heat of flue gas output (kJ/h), Q w is the heat of fuel entering the furnace (kJ/h), Q d n is the total heat brought in by ventilation (kJ/h), and Q a s h is the heat taken away by slag discharge (kJ/h).

The calculation process of the flue gas heat generated per kilogram of fuel is as follows: Q B = Q g / B c a l (8)

Where: Q g represents the heat of flue gas generated per kilogram of fuel (kJ/kg).

Determining the temperature of the furnace is a crucial step, since the heat of the flue gas at the outlet is calculated (Ajay et al., 2021).

Through calculation, the theoretical volume of CO₂ in the furnace is 0.384 Nm3/kg, and the theoretical volume of SO₂ is 0.003 Nm3/kg. The O₂ volume, HC1 volume, HC1 mass and SO₂ mass of the furnace are calculated as shown in Formula 9-Formula 12: V O 2 = α − 1 V 0 × 0.21 (9) V H C l = 0.631 C l a r / 100 (10) G H C l = V H C l B c a l × 36.5 / 22.4 (11) G S O 2 = V S O 2 B C a l × 64 / 22.4 (12)

Where: V O 2 is the O₂ volume of the furnace (Nm3/kg); V H C l is the HCl volume (Nm³/kg); G H C l is the mass of HCl (kg/h); G S O 2 is the mass of SO₂ (kg/h).

Through calculation, the volume of CO₂ and HCl in the furnace is 0.213 Nm³/kg and 0.003 Nm³/kg respectively, and the mass of HCl and SO₂ is 7 kg/h and 25.231 kg/h respectively.

In this paper, the volume and mass of flue gas at the outlet of the furnace are calculated as shown in Formula 13-Formula 14: V B 1 = V B + V E C I (13) G f = 1 − A a 100 + 1.306 α V 0 (14)

Where: V B 1 represents the outlet flue gas volume (m3/kg); G f represents the outlet flue gas mass (kg/kg).

Through calculation, the outlet flue gas volume and mass are respectively 4.13 m³/kg and 4.842 kg/h.

The summary is shown in Table 1.

The air of the oxygen supply system is extracted from the air-cooled wall and sent to the air chamber at the bottom of the incinerator by the oxygen supply machine to enter the incinerator to meet the needs of waste incineration and cool the grate. Each boiler is supplied with oxygen by one fan, and the air is supplied by three stages of the grates (Tai et al., 2022). The fan is efficiently controlled by the frequency converter. Each section of grate can achieve reasonable air distribution by adjusting the air door according to the actual situation of waste combustion (Su et al., 2020).

The calculation of the volumetric and flow rates of the oxygen supply is shown in Eqs 15–16: V a l = 0.6 × 1.0161 α ″ V D B c a l (15) G a 1 = 0.6 B c a l L (16)

Where: V a l is the oxygen supply volume flow (Nm³/h); G a 1 is the oxygen supply flow (kg/h).

According to the calculation, the oxygen volume flow is 4,238 Nm³/H, and the primary air flow is 5,378.58 kg/h. The actual flow rate of the oxygen supply is calculated as shown in Eq. 17: G t = V a 1 273 + t 1 / 273 (17)

Where: G t is the actual flow of the oxygen supply (m³/h);

The actual flow rate of the oxygen supply is 5,278 m³/h. The calculation process of heat brought by the oxygen supply is shown in Formula 18. Q a 1 = V a 1 I a 1 (18)

Where: Q a 1 is the enthalpy of the oxygen-supplied (kJ/Nm³); I a 1 is the heat brought by oxygen-supplied air (kJ/h).

Since the design temperature of the oxygen supply is 60°C, the enthalpy value of the oxygen supply is 78.38 kJ/Nm³ according to the enthalpy temperature table, and the heat brought by the oxygen supply is 31273 kJ/h through calculation.

When the amount of waste entering the furnace is gradually stabilized, the temperature will gradually stabilize. With the increase of sludge blending amount, the nitrogen content, NO, HCN, and NH₃ in the volatile matter of solid phase combustion on the grate increase significantly, in which the flow of HCN and N in the fuel increases significantly, while NO and NH₃ increase slowly. The C and H elements in the volatile matter decrease, the N and O elements increase, and the combustible components decrease. The lower calorific value of the overall waste is reduced, the amount of oxygen required for combustion is reduced, the air distribution is reduced, and the flow of flue gas is reduced. The combustion process is delayed, the combustion interval moves along the direction of the grate, the combustion interval corresponding to the highest temperature is shortened, and the overall combustion temperature is reduced.

According to the waste treatment strategy and the basic operation strategy of multi-generation, the waste incineration operation strategy is formulated and connected with MATLAB to realize the hourly simulation operation of waste incineration. The operation performance of waste power generation under different strategies is compared.

By analyzing the operation results of garbage power generation, it can be seen from the operation economy and thermal economy that the garbage incineration operation strategy in this paper is the best. When the unit price of energy consumed is less than 3,650 yuan/kg, the economy of garbage power generation operation is better, and when it is more than 4,000 yuan/kg, the economy of operation is better. In terms of emission reduction, the overall load of operation under the basic strategy is too large. The capability of emission reduction is weak. Compared with other multi-generation garbage power generation, the data shows that the proposed multi-generation garbage power generation has greater thermal efficiency and energy-saving potential than other multi-generation ones.

Comparing the traditional co-generation, the cooling, heating, and power loads generated by the multi-cogeneration integrated energy garbage power generation are converted into standard coal in proportion. This paper studies the advantages of thermal efficiency and energy saving rate compared with traditional co-generation. The results indicate that the energy-saving rate of garbage power generation under the strategy is 18.96%. The power generation scheme of the waste incineration model based on the heat balance method designed in this paper has greater energy-saving potential and emission reduction capacity than the traditional energy supply mode.

In this paper, the waste incineration power generation unit is modeled by using the heat balance method. The main parameters of the waste incineration system and the parameters of the thermal system are calculated, and the integrated power generation system adapted to the variable operation condition of the waste incineration unit is optimized and calculated. The main research contents are as follows:

(1) It establishes a physical model, a mathematical model and a reaction rate model of the waste incinerator and couples the models into a control equation.

According to the experimental analysis, with the decrease of the grate speed, the position of the beginning and completion of the volatile separation and coke combustion will move forward, and the rate will increase. When the grate speed is 15 m/h, the MSW burns stably on the grate and the gas-phase combustion is more complete. With the increase of the air distribution of the grate, the moisture drying rate is accelerated, and the volatile matter release rate and the fixed carbon burning rate are increased. The garbage burns stably with a low burn-off rate. With the increase in wind ratio, the combustion area moves forward, the oxygen consumption area moves forward, the flue gas flows well, the combustion is sufficient, and the pollutant emission is low. It is proved that the energy coupling optimization of the integrated power generation system of the waste incineration boiler and the coal-fired unit proposed in this paper is suitable for the variable load operation requirements of the waste incineration power plant and has the high thermal economy.

Energy loss such as pipeline loss is not considered in the simulation of the waste incineration power plant system. If the energy loss can be further considered in the follow-up study of the system, the accuracy of the system model can be improved.