Carbon dioxide flue gas heat exchanger with reflux and reflux control method thereof

Abstract

The present application belongs to the heat exchanger technology of internal combustion engine, and particularly relates to a backflow type carbon dioxide flue gas heat exchanger and a backflow control method thereof. The heat exchanger is provided with a flue gas outlet and a flue gas inlet. The flue gas outlet is branched with a backflow pipe connected with the flue gas inlet. A blower is arranged on the backflow pipe and driven by a motor. A temperature sensor II is arranged at the flue gas outlet to collect the actual temperature T of the flue gas outlet. 出口 According to the PID system feedback regulation of the T 出口 value, the operation of the regulating valve I, the regulating valve II and the blower is adjusted. In this way, the backflow pipe is arranged on the heat exchanger to avoid the corrosion of the heat exchanger outlet and prolong the service life.

Description

Technical Field

[0001] This invention pertains to heat exchanger technology for internal combustion engines, specifically a reflux carbon dioxide flue gas heat exchanger and its reflux control method. Background Technology

[0002] CO2-based power cycle power generation and refrigeration have high energy efficiency, can directly replace refrigerants, and are more environmentally friendly. CO2 transcritical power cycle technology for waste heat recovery from internal combustion engines can effectively improve petroleum utilization efficiency, reduce energy consumption, and lower CO2 emissions, representing an important technological trend for energy conservation and emission reduction. The CO2 flue gas heat exchanger is a crucial component of CO2 power cycle waste heat recovery technology, where the CO2 working fluid absorbs the high-temperature exhaust heat from the internal combustion engine.

[0003] Currently, in conventional heat exchangers, the heat source is directly discharged into the environment after heat release. Under certain operating conditions of internal combustion engines, the wall temperature of the heat exchanger's heating surface may be lower than a certain ambient temperature (e.g., 120°C). CO2, being an acidic gas, condenses, causing severe low-temperature corrosion of the heat exchanger. Common treatments, such as using glass tube heating surfaces, stainless steel heating surfaces, anti-corrosion coatings, and heat pipe heat exchangers, cannot fundamentally solve the problem of corrosion on the heat exchanger's heat source outlet heating surface.

[0004] Invention patent CN111441837B discloses a waste heat recovery device and method for an internal combustion engine. The waste heat recovery device includes an internal combustion engine cylinder block, a flue gas heat exchanger, an expander, a booster air preheater, a high-temperature regenerator, a low-temperature regenerator, a cooling unit, a booster unit, and a working fluid tank. The waste heat recovery device uses supercritical / subcritical CO2 as the circulating working fluid. However, the flue gas heat exchanger outlet of this device still carries the risk of acid dew point. Summary of the Invention

[0005] To address the aforementioned problem of heat exchanger outlet corrosion, this invention provides a technical solution for a reflux-type carbon dioxide flue gas heat exchanger and a reflux method. A reflux pipeline is installed to prevent heat exchanger outlet corrosion and extend service life. The specific technical solution is as follows:

[0006] A reflux carbon dioxide flue gas heat exchanger is provided, wherein the heat exchanger is provided with a flue gas outlet and a flue gas inlet, a reflux pipe is branched at the flue gas outlet and connected to the flue gas inlet, and a blower is provided on the reflux pipe, the blower being driven by a motor.

[0007] In the aforementioned reflux carbon dioxide flue gas heat exchanger, a regulating valve II is provided near the flue gas outlet of the reflux pipe, and a regulating valve I is provided near the flue gas inlet of the reflux pipe.

[0008] The aforementioned reflux carbon dioxide flue gas heat exchanger also includes a PID controller; a temperature sensor II is provided at the flue gas outlet, and a temperature sensor I is provided at the flue gas inlet; both regulating valve I and regulating valve II are electrically operated regulating valves; the PID controller is electrically connected to regulating valve I, temperature sensor II, regulating valve II, blower and its motor respectively.

[0009] The present invention also provides a method for reflux control using the above-mentioned reflux carbon dioxide flue gas heat exchanger: temperature sensor II collects the actual temperature T_outlet of the flue gas outlet, and sets 120℃ < temperature A < temperature B;

[0010] When outlet temperature T is less than 120℃, the PID controller controls both regulating valve I and regulating valve II to open, and the motor drives the blower to run.

[0011] When 120℃ < T_outlet ≤ temperature A, the speed of blower 10 is adjusted according to the formula n = K × L_return pipe × N_bend / [(T_inlet - T_outlet)(T_outlet - 120)] r / min, where n is the speed of blower 10; K is a set constant; L_return pipe is the length of the return pipe; N_bend is the number of bends through which the return pipe flows; (T_inlet - T_outlet) is the temperature difference between the flue gas inlet and outlet; and (T_inlet - T_outlet) refers to the difference between the flue gas outlet temperature and 120℃. When T_outlet ≥ temperature B, the temperature at the flue gas outlet reaches the upper temperature limit, the PID controller controls both regulating valve I and regulating valve II to close, and the blower motor does not work.

[0012] When outlet temperature T is greater than or equal to temperature B, the PID controller closes both regulating valve I and regulating valve II, and the blower motor stops running.

[0013] Furthermore, when 120℃ < T_outlet ≤ temperature A, when the rotational speed increases to n = 3000 r / min, n is a constant value until the flue gas outlet T_outlet rises to temperature A, at which point the rotational speed n begins to decrease and continues to execute according to the formula; if the flue gas outlet temperature A < T_outlet ≤ temperature B, the PID controller controls the blower to stop immediately.

[0014] Furthermore, in the formula, K is 250,000 at standard atmospheric pressure and room temperature.

[0015] The beneficial effects of this invention are as follows: A reflux pipe connects the inlet and outlet of the heat exchanger, allowing the high-temperature flue gas at the inlet to compensate for the temperature of the flue gas at the outlet, ensuring that the outlet temperature is not lower than the condensation point of carbon dioxide, thus preventing acid corrosion at the outlet. Furthermore, the structure is simple and does not occupy excessive space in the internal combustion engine. The flow rate of the reflux pipe is automatically controlled by PID control, making operation very convenient. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of an embodiment of the present invention;

[0017] Figure 2 This is a schematic diagram of the return pipeline connection according to an embodiment of the present invention;

[0018] Figure 3 This is a schematic diagram of PID control according to an embodiment of the present invention;

[0019] Figure 4 This is a logic diagram of PID control according to an embodiment of the present invention.

[0020] In the diagram: 1 is the heat exchanger, 2 is the flue gas outlet, 3 is the flue gas inlet, 4 is the return pipe, 5 is the regulating valve I, 6 is the temperature sensor I, 7 is the temperature sensor II, 8 is the regulating valve II, 9 is the PID controller, 10 is the blower, 11 is the nut I, and 12 is the nut II. Detailed Implementation

[0021] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0022] like Figure 1 As shown, this embodiment is a reflux carbon dioxide flue gas heat exchanger. The heat exchanger 1 is provided with a flue gas outlet 2 and a flue gas inlet 3. A reflux pipe 4 is branched from the flue gas outlet 2 and connects to the flue gas inlet 3. Nuts I 11 and II 12 are provided at the flue gas inlet 3 to connect to the reflux pipe 4. A blower 10 is provided on the reflux pipe 4. The blower 10 is driven by a motor, and the motor is regulated by a PID controller.

[0023] A regulating valve II8 and a temperature sensor II7 are installed near the flue gas outlet 2 of the return pipe 4, and a regulating valve I5 and a temperature sensor I6 are installed near the flue gas inlet 3 of the return pipe 4. Both regulating valves I5 and II8 are electrically operated. The PID controller 9 is electrically connected to regulating valve I5, temperature sensor II7, regulating valve II8, blower 10, and its motor. The function of temperature sensor I6 is to monitor the temperature at the inlet of the return pipe 4; the system can only operate reflux heat exchange when the temperature is higher than the set value. Figure 4 In the PID logic control, there are two temperature setpoints, namely temperature A and temperature B, both of which are greater than 120℃. Temperature A indicates that when the temperature rises to this value, it means that the temperature of the flue gas outlet 2 is high and can already reach the normal gas discharge state. Therefore, the system starts to reduce the rate of temperature rise by linearly reducing the speed of the blower 10. Temperature B indicates that when the temperature rises to this value, it means that the temperature of the flue gas outlet 2 has reached the upper limit, and the system directly shuts down the temperature value of the blower 10.

[0024] The reflux principle of this heat exchanger is as follows: Temperature sensor II7 collects the actual temperature T_outlet of flue gas outlet 2, where 120℃ < temperature A < temperature B;

[0025] When the outlet temperature is less than 120℃, the PID controller 9 controls both the regulating valve I5 and the regulating valve II8 to open, and the motor drives the blower 10 to rotate at a speed of 1000r / min. This condition is generally when the internal combustion engine has just started and the temperature at the flue gas inlet 3 is slowly rising and has not yet stabilized. This causes the temperature at the flue gas outlet 2 to rise slowly as well, without affecting the normal operation of the internal combustion engine.

[0026] When 120℃ < T_outlet ≤ temperature A, adjust the speed of blower 10 according to the formula n = K × L_return pipe × N_bend / [(T_inlet - T_outlet)(T_outlet - 120)] r / min, where: n is the speed of blower 10; K is a set constant, which is generally taken as 250000 under standard air pressure and normal temperature; L_return pipe is the length of the return pipe; N_bend is the number of bends through which the return pipe flows; (T_inlet - T_outlet) refers to the temperature difference between flue gas inlet 3 and flue gas outlet 2; (T_inlet - T_outlet) refers to the difference between the temperature of flue gas outlet 2 and 120℃. Under this operating condition, (T_inlet - T_outlet) decreases and (T_outlet - 120) increases. The amount of change is related to the speed n of the blower 10. Therefore, it is set that if the speed increases to n = 3000 r / min, then n remains constant. The flue gas outlet 2 heats up relatively quickly. When the temperature T_outlet of the flue gas outlet 2 rises to temperature A, the speed n is reduced. After that, the speed n is controlled according to the above formula. If the temperature of the flue gas outlet 2 rises too quickly, causing T_outlet to be greater than or equal to temperature B, then the blower 10 stops immediately.

[0027] When temperature A < T_outlet ≤ temperature B, the temperature of flue gas outlet 2 reaches the upper temperature limit, PID controller 9 controls regulating valve I 5 and regulating valve II 8 to close, and the motor of blower 10 stops working.

Claims

1. A method for controlling the reflux of a reflux-type carbon dioxide flue gas heat exchanger, characterized in that: The system includes a heat exchanger (1) and a PID controller (9). The heat exchanger (1) is provided with a flue gas outlet (2) and a flue gas inlet (3). A return pipe (4) is branched off from the flue gas outlet (2) and connects to the flue gas inlet (3). A regulating valve II (8) is provided near the flue gas outlet (2) and a regulating valve I (5) is provided near the flue gas inlet (3). A temperature sensor II (7) is provided at the flue gas outlet (2) and a temperature sensor I (6) is provided at the flue gas inlet (3). A blower (10) is provided on the return pipe (4) and the blower (10) is driven by a motor. Both the regulating valve I (5) and the regulating valve II (8) are electric regulating valves. The PID controller (9) is electrically connected to the regulating valve I (5), the temperature sensor II (7), the regulating valve II (8), the blower (10), and its motor. The reflux control method is as follows: Temperature sensor II (7) collects the actual temperature T_outlet of flue gas outlet (2), and sets 120℃ < temperature A < temperature B; When the outlet temperature is less than 120℃, the PID controller (9) controls both regulating valve I (5) and regulating valve II (8) to open and the motor drives the blower (10) to run. When 120℃<T_outlet≤T_temperatureA, adjust the speed of blower (10) according to the formula n=K×L_return pipe×N_bend / [(T_inlet-T_outlet)(T_outlet-120)]r / min, where n is the speed of blower (10); K is a set constant and is 250000 under standard air pressure and normal temperature; L_return pipe is the length of return pipe; N_bend is the number of bends through which return pipe flows; (T_inlet-T_outlet) is the temperature difference between flue gas inlet (3) and flue gas outlet (2); (T_inlet-T_outlet) refers to the difference between the temperature of flue gas outlet (2) and 120℃. When outlet temperature T is greater than or equal to temperature B, the PID controller (9) controls both regulating valve I (5) and regulating valve II (8) to close, and the motor of blower (10) to stop running.

2. The method for reflux control of a reflux-type carbon dioxide flue gas heat exchanger according to claim 1, characterized in that: When 120℃ < T_outlet ≤ temperature A, when the rotation speed increases to n = 3000 r / min, n is a constant value until the flue gas outlet (2) T_outlet rises to temperature A, the rotation speed n begins to decrease and still follows the formula; if the temperature A of the flue gas outlet (2) < T_outlet ≤ temperature B, the PID controller (9) controls the blower (10) to stop immediately.

Images