Application Of Heat Recovery Heat Exchanger in ORC Power Generation System
1, The core role of heat recovery heat exchanger in ORC power generation system
The core principle of the ORC system is that the heat from low-grade heat sources (such as industrial flue gas, cooling water, and exhaust steam) is transferred to the organic working fluid through a heat recovery heat exchanger. The organic working fluid, due to its low boiling point, can evaporate into high-pressure steam at lower temperatures, driving the turbine to rotate and generate electricity. The working fluid, after performing work, is cooled and liquefied by a condenser, pressurized by a working fluid pump, and then enters the heat recovery heat exchanger again to complete the cycle.
The core functions of a heat recovery heat exchanger can be summarized into three points:
Efficient heat capture: Maximize the recovery of low-grade waste heat, reduce heat loss on the heat source side, and improve waste heat utilization efficiency;
Precise heating of working fluid: Heating organic working fluid to the evaporating state (saturated steam/superheated steam), providing working fluid parameters (temperature, pressure) that meet the requirements for work to the turbine;
System matching regulation: Adapt to the flow and temperature fluctuations on the heat source side (such as intermittent and variable load characteristics of industrial waste heat), stabilize the output parameters on the working fluid side, and ensure the continuous and safe operation of the ORC system.
Simply put, the heat recovery heat exchanger is the heat exchange bridge between the "heat source" and the "working fluid" in the ORC system, and its heat exchange performance directly determines the power generation efficiency of the ORC system (generally, the total efficiency of the ORC system is about 10%~25%, and the heat exchange efficiency of the heat exchanger is the core influencing factor).
2, Special requirements of ORC system for heat recovery heat exchanger
The heat source of ORC system is mostly low-grade (temperature generally 80-350 ℃), variable operating conditions, and waste heat containing impurities (such as industrial flue gas containing dust and sulfur, and cooling water containing scale), and organic working fluids often have low boiling points, easy volatility, and some working fluids are corrosive/flammable. Therefore, the design, material, and structure of heat recovery heat exchangers are different from traditional thermal power heat exchangers. The core requirements are as follows:
1. Adapt to low-grade heat exchange and enhance heat transfer performance
Low grade heat sources have low temperature and pressure (small temperature difference between the heat source and the working fluid), weak heat transfer driving force, and require heat exchangers to have high-efficiency enhanced heat transfer structures to achieve rapid heat transfer within a limited heat transfer area, avoiding heat exchanger volume and high cost caused by low heat transfer coefficient.
2. Tolerate variable operating conditions and adapt to fluctuations in heat sources
The flow rate and temperature of industrial waste heat (such as flue gas/waste heat steam from steel, chemical, and cement industries) are prone to fluctuations with production load (such as a sudden drop in flue gas temperature from 150 ℃ to 100 ℃ and a decrease in flow rate from 50000m ³/h to 30000m ³/h), requiring the heat exchanger to have good adaptability to changing working conditions. By adjusting the heat exchange area and optimizing the flow channel, the stability of the output parameters on the working fluid side can be ensured.
3. Adapt to the characteristics of organic working fluids, balancing safety and compatibility
Material compatibility: Some organic working fluids (such as fluorocarbons, ketones, and alkanes) may cause slight corrosion to metals at high temperatures. The material of the heat exchanger needs to be matched with the working fluid (such as commonly used 304/316 stainless steel, titanium alloy, and special working conditions using Hastelloy);
Sealing performance: Organic working fluid is prone to volatilization, and the heat exchanger needs to have a high sealing level to prevent leakage of the working fluid (which not only causes heat loss, but may also lead to safety accidents due to the flammability/toxicity of the working fluid);
Anti coking/scaling: Organic working fluids are prone to cracking and coking during local overheating. The heat exchanger needs to optimize the flow channel design to avoid local high temperatures on the working fluid side and ensure uniform flow field.
4. Resistant to the characteristics of the medium on the heat source side, enhancing the ability to resist pollution and corrosion
If the heat source is industrial flue gas: containing dust, sulfur, and acidic gases, the flue gas side of the heat exchanger needs to be wear-resistant, resistant to low-temperature corrosion, and easy to clean (such as installing a cleaning device);
If the heat source is low-temperature cooling water/exhaust steam: prone to scaling and condensation, the heat exchanger needs to be resistant to scaling and electrochemical corrosion;
If the heat source is high-temperature molten salt/heat transfer oil (indirect heat exchange ORC system): it needs to withstand the thermal shock of high-temperature media and the material has good high-temperature strength.
5. Compact, low-cost, suitable for engineering applications
ORC systems are mostly distributed power generation (such as being located near industrial waste heat generation points), with limited site space, requiring compact heat exchanger structures, small volumes, and light weights; At the same time, the profitability of the ORC system depends on the economy of waste heat recovery, and the heat exchanger needs to control manufacturing and operation maintenance costs.
6. Meet thermal matching and achieve temperature matching heat transfer
The heating process of organic working fluid in ORC system is divided into preheating section, evaporation section, and superheating section (some systems do not have superheating section). The heat release on the heat source side is also divided into sensible heat section and condensation section. It is required that the flow channel design of the heat exchanger achieve temperature matching heat transfer, avoid ineffective heat transfer with "large temperature difference and small flow rate", improve thermal efficiency (effective energy utilization rate), and reduce thermal losses.
To improve the overall performance of the ORC system, the design of the heat recovery heat exchanger should revolve around four core aspects: heat transfer efficiency, adaptability to varying operating conditions, fouling resistance, and cost control. Key design and optimization points are as follows:
1. Flow Channel and Heat Exchange Structure Optimization
Employ counter-current heat exchange (heat source and working fluid flow in opposite directions) to maximize temperature and pressure utilization and improve heat exchange efficiency (the average temperature and pressure of counter-current heat exchange is 30%~50% higher than that of co-current heat exchange);
Use reinforced heat transfer tubes (such as threaded tubes, corrugated tubes, and microfinned tubes) on the working fluid side and high-efficiency fins (such as corrugated fins and slotted fins) on the heat source side (flue gas) to improve the heat transfer coefficients on both sides;
Optimize flow channel distribution to ensure a uniform flow field of the medium within the heat exchanger, avoiding local dead zones and flow deviations, and preventing local coking, scaling, and overheating.
2. Precise Material Selection
Based on the heat source medium, organic working fluid, and operating temperature/pressure, the core material selection reference is as follows:
Normal operating conditions (working fluid is R245fa or R1233zd, heat source is clean flue gas/cooling water, temperature <200℃):304 stainless steel;
Corrosive media (flue gas contains sulfur, working fluid is corrosive ketones, temperature 200~300℃):** 316L stainless steel;
Highly corrosive operating conditions (high-temperature acidic flue gas, special working fluid):Titanium alloy, Hastelloy C276;
High-temperature heat source (temperature >300℃, such as high-temperature process waste heat): Heat-resistant steel (such as 15CrMoG, P91)
3. Anti-fouling and Dust Removal Design
For heat sources containing dust and scale, heat exchangers must integrate anti-fouling/dust removal devices to prevent scale buildup on the heat exchange surface, which can reduce the heat transfer coefficient (the heat transfer coefficient can decrease by more than 50% after scaling):
Flue gas side: Install sonic soot blowers, pulse soot blowers, and scraper soot removers to optimize the flue gas velocity (generally controlled at 10~15m/s) to ensure heat transfer while reducing dust deposition;
Liquid side: Employ online chemical cleaning devices and electrostatic descaling devices, with flow channels designed for high flow rates (>1.5m/s) to inhibit scale formation.
