Abstract: In the daily operation of waste incineration plants, a large amount of flue gas will be generated, and the low-temperature waste heat in the flue gas has the value of power generation. This paper analyzes how to improve the low-temperature waste heat power generation technology of flue gas in waste incineration plants, studies the characteristics of different working fluids, and analyzes how to adjust parameters to improve the overall efficiency of power generation.
Key words: waste incineration plant; low temperature waste heat of flue gas; power generation
Introduction: Waste incineration is the main method of waste treatment at present. The heat generated in the incineration process can be used in power generation to realize the recycling of energy, reduce emissions and meet environmental protection requirements. become an effective way to recover waste heat resources. At present, the temperature of the flue gas at the chimney entrance of the waste incineration plant is about 150 °C, which will cause a relatively large loss of waste heat. The use of organic Rankine cycle for low-boiling organic working fluid power generation can effectively recover low-temperature waste heat and has great advantages in the utilization of low-grade thermal energy. , and can reduce carbon dioxide and nitrogen oxide emissions.
1 Overview of waste incineration waste heat power generation
Incineration is one of the main methods of garbage disposal. The waste in the garbage will be incinerated into carbon dioxide and water, and the waste heat released can be used for power generation. Compared with landfill and composting, incineration requires less space, so incineration has become the mainstream of garbage disposal. Since the incineration method will generate a large amount of waste heat, the use of waste heat to generate electricity has also become one of the development directions of waste incineration. Waste heat belongs to secondary energy, mainly refers to the available heat energy released in the production process. Above 500℃) can utilize waste heat. The waste heat of about 150°C at the entrance of the chimney of the waste incineration plant is low temperature waste heat, which can be used to enter the Rankine cycle for power generation.
2 Waste incineration low temperature waste heat power generation process
2.1 Process flow of waste incineration low temperature waste heat power generation process
The waste incineration low-temperature waste heat power generation system includes a waste receiving system, a waste incineration system, a thermal energy utilization system, a flue gas treatment system, and a furnace ash and slag treatment system. In the waste incineration power plant, after the waste enters the plant, it is weighed by the weighbridge and sent to the unloading hall; in order to increase the calorific value of the waste, the waste in the waste bin will be fermented for 5-7 days, and the waste will be hoisted and dumped to form a waste incineration plant. The leachate is collected in a special leachate pool, and the fermented leachate is caught into the garbage hopper. The waste incineration is carried out in a special incinerator, and the waste will be fully burned through the grate of the drying section, the grate of the combustion section and the grate of the burning section. more than 2 seconds.
When using medium and high temperature waste heat, the flue gas from waste incineration enters the waste heat boiler, transfers heat through the heating surface of the boiler, and then heats the working fluid of the boiler, and the evaporation of the working fluid drives the steam turbine unit to generate electricity. In the flue gas treatment work, the flue gas discharged from the waste heat boiler is introduced into the rotary atomizer of the semi-dry deacidification reaction tower, and the acidic substances in the flue gas are absorbed by the alkaline liquid; the subsequent flue gas passes through the activated carbon injection system, cloth bag After the dust collector enters the discharge chimney, the on-line monitoring system on the chimney monitors the discharge process.
2.2 Flue gas waste heat utilization ORC system
After the flue gas discharged from the waste heat boiler is purified by deacidification, dust removal, etc., the flue gas temperature is about 150 °C, and the low temperature waste heat can still be further utilized. In the ORC system of flue gas low-temperature waste heat utilization, organic working fluid is used to perform Rankine cycle. The system configuration is shown in Figure 1. The organic working fluid absorbs heat at constant pressure in the evaporator, and then performs adiabatic work in the expander. The heat is released at constant pressure in the condenser, and finally adiabatic compression is carried out in the working fluid pump, and then returns to the original power cycle process. The use of organic working fluid can make better use of low-temperature waste heat, improve the energy utilization efficiency of the system, and reduce carbon dioxide emissions. The exhausted steam can be condensed into liquid to achieve the purpose of energy recovery.
3 Selection of working medium for waste incineration low temperature flue gas waste heat power generation
3.1 Basic principles of working medium selection
The selection of working fluid for ORC power generation system is very important, and the economy, safety and technicality of working fluid should be fully considered in the selection process. The working fluid must have lower critical temperature and pressure, lower steam superheating requirements and lower viscosity, and smaller volume ratio. good compatibility.
In addition to the performance requirements, the working fluid must also meet the requirements of environmental protection, and it is necessary to control the toxicity of the working fluid and meet the requirements of chemical stability.
3.2 Flue gas waste heat utilization ORC system
When choosing the working fluid, the most important thing is the thermodynamic properties of the working fluid, which will determine the size, stability, and environmental protection level of the equipment. It is very economical. This article compares the commonly used working fluids R245fa, R600a and R601a. Figure 1 and Figure 2 are the T-s diagrams corresponding to the configuration and cycle of the system, respectively. The organic working fluid absorbs heat at constant pressure in the evaporator (4-1 process), and then expands adiabatically in the expander to do work (1-2 process), thus Drive the generator to generate electricity, the exhausted steam will release heat at constant pressure in the condenser (2-3 process), and finally perform adiabatic compression in the working fluid pump (4-1 process), and then return to the evaporator to complete the organic Rankine cycle .
Figure 1 ORC waste heat power generation
Figure 2 ORC cycle T-s
The 1-2 process is an adiabatic work process, and the calculation formula of the work is:
2-3 Constant pressure exothermic process, exothermic is:
The 3-4 process is an adiabatic compression process:
The 4-1 process is a constant pressure endothermic process, and the endothermic is:
The cycle thermal efficiency of the system is
3.3 Analysis of calculation results
The net output power of the ORC system first increases and then decreases with the increase of the evaporation temperature. As shown in Figure 3, within the evaporation temperature range, the maximum net output power of the three working fluids is 385kW, 365kW, and 350kW, and the three working fluids reach the maximum Temperatures at net output power are 100°C, 95°C and 90°C. According to the parameter data of the working fluid, the lower the critical temperature of the working fluid, the greater the net output power of the system, and the higher the evaporation temperature is required. Therefore, in order to obtain a higher system output power, a working medium with a smaller critical temperature should be selected.
Figure 3 Relationship between power generation and evaporation temperature
According to the results in Figure 4, the thermal efficiency of the system increases with the increase of the evaporation temperature. When the evaporation temperature is the same, the thermal efficiency of the system gradually decreases with the increase of the critical temperature of the working fluid.
Fig. 4 Variation of power generation efficiency with evaporation temperature
The relationship between the exhaust gas temperature of the ORC system and the evaporation temperature is shown in Figure 5. The exhaust gas temperature of the system increases with the increase of the evaporation temperature. Under the same evaporation temperature, the lower the critical temperature of the working fluid, The exhaust gas temperature of the system will be lower.
Figure 5 Variation of flue gas temperature with evaporation temperature
After the above analysis, the evaporation temperature of the ORC system should be controlled at 70-11 ℃, and the net output work of the system has a maximum value. Comprehensive analysis of the potential value of the working fluid on the environment, it is more effective to use R600a working fluid, according to the evaporation temperature of 100 ℃ design, the ORC system can obtain 385kW of power generation, which can save 950 tons of standard coal and 2,250 tons of carbon dioxide throughout the year, as well as reduce the emission of nitrogen oxides, which has a very good effect of energy saving and emission reduction.
Conclusion: In the system design of waste incineration low-temperature waste heat power generation, the designer should understand the properties of different working fluids, and select the working fluid correctly according to the requirements of the system; the evaporation temperature of the working fluid has a significant impact on the power generation power, power generation efficiency and flue gas temperature. Significant influence, should be considered comprehensively when selecting working medium.
