Multi-stage fuel cell system and control method

Abstract

The invention relates to the technical field of new energy, in particular to a multi-stage fuel cell system and a control method. According to the multi-stage fuel cell system, after anode tail gas of a first-stage electric pile and fuel of a first-stage fuel source are introduced into a fuel gas preheater, the anode tail gas exchanges heat with the fuel in the fuel gas preheater so as to heat the fuel, and then the heated fuel and part of cooled anode tail gas respectively enter an ejector; and under the action of the ejector, the gas enters the first-stage stack steam reformer to react and then enters the anode of the first-stage stack to participate in the reaction. The ejector is used for recycling anode tail gas, parasitic power consumption generated by using a circulating pump and other power structures can be avoided, and reutilization of residual fuel in the anode tail gas is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy, and particularly to a multi-stage fuel cell system and a control method. Background Art

[0002] The improvement of the system fuel utilization rate will bring about an increase in the power generation efficiency of the fuel cell. Considering reliability and performance degradation, a relatively safe fuel utilization rate of the fuel cell stack is set during operation, generally between 60% and 80%, which limits the overall fuel utilization rate of the system. Therefore, the key to improving the power generation efficiency is to improve the system fuel utilization rate as much as possible while ensuring the fuel utilization rate of the fuel cell stack.

[0003] There are two effective methods to improve the system fuel utilization rate at present. One is the cascade utilization of fuel: connecting the anodes of the fuel cell stacks in series, and the anode exhaust gas of the upper fuel cell stack enters the lower fuel cell stack for reuse, which can achieve the cascade utilization of fuel. The other is the recycling of fuel, and the unreacted fuel in the exhaust gas of the fuel cell stack is recycled back to the fuel cell stack through an ejector to achieve the reuse of fuel.

[0004] Chinese Patent Document CN114649548A discloses a multi-stage fuel cell system and its energy conversion method. The multi-stage fuel cell system includes a top fuel cell, a bottom fuel cell, a flue gas preheater, an intermediate reheater, a fuel pre-converter, a fuel preheater, an intermediate cooler, a cooler, and a CO 2 capture device; the flue gas preheater, the cathode of the top fuel cell, the intermediate reheater, and the cathode of the bottom fuel cell are sequentially connected by pipelines; the flue gas preheater is respectively connected to the fuel pre-converter and the cathode of the bottom fuel cell by pipelines; the fuel pre-converter, the anode inlet of the top fuel cell, and the anode outlet of the top fuel cell are respectively connected to the fuel preheater; the fuel preheater, the intermediate cooler, the intermediate reheater, the anode of the bottom fuel cell, the cooler, and the CO 2 capture device are sequentially connected by pipelines.

[0005] However, the above multi-stage fuel cell system has the following disadvantages: the anode exhaust gases of the first-stage fuel cell stack and the second-stage fuel cell stack need to be cooled and dewatered respectively, so multiple water pumps are required to recycle the condensed water, which will not only increase the parasitic power consumption of the system but also increase the complexity of the system.

[0006] Therefore, there is an urgent need for a multi-stage fuel cell system to solve the above technical problems. Summary of the Invention

[0007] The purpose of the present invention is to provide a multi-stage fuel cell system, which can avoid increasing the parasitic power consumption of the system and does not require additional water supply.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A multi-stage fuel cell system, comprising:

[0010] A fuel source, at least including a primary fuel source and a secondary fuel source;

[0011] A fuel cell stack, at least including a primary fuel cell stack and a secondary fuel cell stack, wherein the primary fuel source provides fuel for the primary fuel cell stack, and the secondary fuel source provides fuel for the secondary fuel cell stack;

[0012] A fuel cell stack steam reformer, at least including a primary fuel cell stack steam reformer and a secondary fuel cell stack steam reformer, wherein the primary fuel cell stack steam reformer is communicated with the primary fuel source to provide fuel for the primary fuel cell stack, and the secondary fuel cell stack steam reformer is respectively communicated with the secondary fuel source and the anode of the secondary fuel cell stack;

[0013] A gas preheater, the inlet of the tail gas channel of the gas preheater is communicated with the anode outlet of the primary fuel cell stack, the inlet of the fuel channel of the gas preheater is communicated with the outlet of the primary fuel source, the outlet of the tail gas channel of the gas preheater is communicated with the fuel inlet of the secondary fuel cell stack steam reformer, and the gas preheater is configured to use the anode tail gas to preheat the fuel;

[0014] An ejector, the working fluid inlet of the ejector is communicated with the fuel channel outlet of the gas preheater, and the entrained fluid inlet of the ejector is communicated with the tail gas channel outlet of the gas preheater; the outlet of the ejector is connected to the fuel inlet of the primary fuel cell stack steam reformer.

[0015] As a preferred technical solution of the above multi-stage fuel cell system, a gas mixer is provided between the gas preheater and the secondary fuel source, and the outlet of the tail gas channel of the gas preheater is also communicated with the tail gas inlet of the gas mixer for part of the tail gas to flow into the gas mixer, and the fuel outlet of the gas mixer is connected to the fuel inlet of the secondary fuel cell stack steam reformer.

[0016] As a preferred technical solution of the above multi-stage fuel cell system, the multi-stage fuel cell system further includes an air preheater, the air preheater heats the cathode air entering the primary fuel cell stack, and the cathode outlet of the primary fuel cell stack is connected to the cathode inlet of the secondary fuel cell stack.

[0017] As a preferred technical solution of the above multi-stage fuel cell system, the air channel outlet of the air preheater is communicated with the air inlet of the primary fuel cell stack steam reformer, and the air outlet of the primary fuel cell stack steam reformer is communicated with the cathode inlet of the primary fuel cell stack.

[0018] As a preferred technical solution of the above multi-stage fuel cell system, the multi-stage fuel cell system further includes a burner, the burner is respectively communicated with the anode outlet and the cathode outlet of the secondary stack, and the gas outlet of the burner is communicated with the steam reformer of the secondary stack to provide a heat source for the steam reformer of the secondary stack.

[0019] As a preferred technical solution of the above multi-stage fuel cell system, the tail gas outlet of the steam reformer of the secondary stack is communicated with the tail gas inlet of the air preheater, and the tail gas outlet of the air preheater is communicated with the atmosphere.

[0020] As a preferred technical solution of the above multi-stage fuel cell system, it further includes an air mixer, and the air mixer is configured to mix the cathode tail gas generated by the primary stack with air and input it into the cathode inlet of the secondary stack.

[0021] The present invention also provides a multi-stage fuel cell control method, which is applied to the multi-stage fuel cell system described in any of the above solutions, and includes the following steps:

[0022] Obtain the anode tail gas flow rate of the primary stack, the molar components M% of each substance in the primary stack tail gas, and the fuel flow rate entering the primary stack from the primary fuel source. Each substance in the primary stack tail gas includes CO, CO 2 、H 2 O and H 2 ;

[0023] Calculate and obtain the actual oxygen-carbon ratio according to the anode tail gas flow rate, molar components M% and fuel flow rate;

[0024] Compare the actual oxygen-carbon ratio with the preset maximum oxygen-carbon ratio and the preset minimum oxygen-carbon ratio, and adjust the fuel flow rates of each stack according to the comparison result.

[0025] As a preferred technical solution of the above multi-stage fuel cell control method, adjusting the fuel flow rates of each stack according to the comparison result includes:

[0026] If the actual oxygen-carbon ratio is greater than or equal to the preset minimum oxygen-carbon ratio and less than or equal to the preset maximum oxygen-carbon ratio, the fuel flow rates of the primary stack and the secondary stack remain unchanged.

[0027] As a preferred technical solution of the above multi-stage fuel cell control method, adjusting the fuel flow rates of each stack according to the comparison result includes:

[0028] If the actual oxygen-carbon ratio is less than the preset minimum oxygen-carbon ratio, reduce the fuel flow rate of the secondary stack and increase the fuel flow rate of the primary stack;

[0029] If the actual oxygen-carbon ratio is greater than the preset maximum oxygen-carbon ratio, increase the fuel flow rate of the secondary stack and decrease the fuel flow rate of the primary stack.

[0030] Advantages of the present invention:

[0031] After the anode tail gas of the primary stack and the fuel of the primary fuel source are introduced into the gas preheater, the anode tail gas and the fuel exchange heat in the gas preheater to heat the fuel. Then, the heated fuel and a part of the cooled anode tail gas enter the ejector respectively, and under the action of the ejector, enter the steam reformer of the primary stack to react and then enter the anode of the primary stack to participate in the reaction. By using the ejector to recycle the anode tail gas, the parasitic power consumption generated by using power structures such as circulation pumps can be avoided, ensuring the reuse of the remaining fuel in the anode tail gas without other parasitic power consumption. A part of the anode tail gas flowing out of the gas preheater and the fuel of the secondary fuel source enter the steam reformer of the secondary stack, and the fuel reacts in the steam reformer of the secondary stack and then enters the anode of the secondary stack to participate in the reaction. In this way, the water vapor in the anode tail gas can directly participate in the reforming reaction of the steam reformers of each stack without additional water supply. In addition, the gas preheater can achieve the purpose of connecting the anodes of the primary stack and the secondary stack in series, so as to achieve the step-by-step utilization of fuel.

[0032] The gas preheater uses the primary fuel source to cool the anode tail gas of the primary stack, so that the anode tail gas entering the ejector meets the ejector's ejection requirements. The ejector ejects the anode tail gas of the primary stack cooled to medium temperature and recycles it back into the steam reformer of the primary stack, which can not only provide the water vapor required for the steam reforming reaction but also realize the reuse of the fuel.

[0033] The method provided by the present invention adjusts the fuel flow rates of the primary stack and the secondary stack according to the obtained oxygen-carbon ratio, which can avoid the occurrence of carbon deposition caused by too low a carbon-oxygen ratio and can also avoid low fuel utilization caused by too high a carbon-oxygen ratio. Description of the Drawings

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments of the present invention. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the content of the embodiments of the present invention and these drawings.

[0035] Figure 1 It is a schematic block diagram of a multi-stage fuel cell system provided by an embodiment of the present invention;

[0036] Figure 2Schematic diagram of the fuel flow path in the multi-stage fuel cell system provided by the embodiment of the present invention;

[0037] Figure 3 Schematic diagram of the heat flow path generated by the burner in the multi-stage fuel cell system provided by the embodiment of the present invention;

[0038] Figure 4 Flow chart of the multi-stage fuel cell control method provided by the embodiment of the present invention Figure 1 ;

[0039] Figure 5 Flow chart of the multi-stage fuel cell control method provided by the embodiment of the present invention Figure 2 。

[0040] In the figure:

[0041] 1. Primary fuel source; 2. Secondary fuel source; 3. Primary fuel cell stack; 4. Secondary fuel cell stack; 5. Primary fuel cell stack steam reformer; 6. Secondary fuel cell stack steam reformer; 7. Gas preheater; 8. Ejector; 9. Gas mixer; 10. Air preheater; 11. Burner; 12. Air mixer; 13. Fan. Detailed implementation manners

[0042] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the present invention, rather than limiting the present invention. Additionally, it should be noted that for the sake of description, only parts related to the present invention rather than all structures are shown in the drawings.

[0043] In the description of the present invention, unless otherwise clearly defined and limited, the terms "connected", "connected to", and "fixed" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal connection of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0044] In the present invention, unless otherwise clearly defined and limited, the first feature being "on" or "under" the second feature may include direct contact between the first and second features, or may include indirect contact between the first and second features through additional features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes the first feature being directly above and obliquely above the second feature, or merely indicating that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "below" and "beneath" the second feature includes the first feature being directly below and obliquely below the second feature, or merely indicating that the horizontal height of the first feature is lower than that of the second feature.

[0045] In the description of the present embodiment, the orientation or positional relationship terms such as "upper", "lower", "right", etc. are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of description and simplifying the operation, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention. In addition, the terms "first" and "second" are only used for distinction in description and have no special meaning.

[0046] An embodiment of the present application provides a multi-stage fuel cell system, which can avoid parasitic power consumption when fuel is reused.

[0047] As Figure 1 shown, the multi-stage fuel cell system includes a fuel source, a fuel cell stack, a fuel cell stack steam reformer, a gas preheater 7 and an ejector 8. The fuel source at least includes a primary fuel source 1 and a secondary fuel source 2; the fuel cell stack at least includes a primary fuel cell stack 3 and a secondary fuel cell stack 4. The primary fuel source 1 supplies fuel to the primary fuel cell stack 3, and the secondary fuel source 2 supplies fuel to the secondary fuel cell stack 4. The fuel cell stack steam reformer at least includes a primary fuel cell stack steam reformer 5 and a secondary fuel cell stack steam reformer 6. The primary fuel cell stack steam reformer 5 is used to supply fuel to the primary fuel cell stack 3, and the secondary fuel cell stack steam reformer 6 is respectively communicated with the anode of the secondary fuel source 2 and the secondary fuel cell stack 4.

[0048] The gas preheater 7 has a tail gas channel and a fuel channel that are not connected to each other. The tail gas channel and the fuel channel are arranged in a cross manner and can conduct heat exchange. The inlet of the tail gas channel of the gas preheater 7 is connected to the anode outlet of the first-stage fuel cell stack 3, the inlet of the fuel channel of the gas preheater 7 is connected to the outlet of the first-stage fuel source 1, the outlet of the tail gas channel of the gas preheater 7 is connected to the fuel inlet of the second-stage fuel cell stack steam reformer 6, and the fuel channel and the tail gas channel are not connected to each other. The gas preheater 7 is configured to preheat the fuel with the anode tail gas; the working fluid inlet of the ejector 8 is connected to the outlet of the fuel channel of the gas preheater 7, and the entrained fluid inlet of the ejector 8 is connected to the outlet of the tail gas channel of the gas preheater 7; the outlet of the ejector 8 is respectively connected to the fuel inlets of the first-stage fuel cell stack steam reformer 5 and the second-stage fuel cell stack steam reformer 6.

[0049] After the anode tail gas of the first-stage fuel cell stack 3 and the fuel of the first-stage fuel source 1 are introduced into the gas preheater 7, the anode tail gas and the fuel exchange heat in the gas preheater 7 to heat the fuel. Then, the heated fuel and a part of the cooled anode tail gas enter the ejector 8 respectively, and under the action of the ejector 8, they enter the first-stage fuel cell stack steam reformer 5 to react and then enter the anode of the first-stage fuel cell stack 3 to participate in the reaction. By recycling the anode tail gas using the ejector 8, the parasitic power consumption generated by using power structures such as a circulation pump can be avoided, ensuring the reuse of the remaining fuel in the anode tail gas without other parasitic power consumption. A part of the anode tail gas flowing out of the gas preheater 7 and the fuel of the second-stage fuel source 2 enter the second-stage fuel cell stack steam reformer 6, and the fuel reacts in the second-stage fuel cell stack steam reformer 6 and then enters the anode of the second-stage fuel cell stack 4 to participate in the reaction. In this way, the water vapor in the anode tail gas can directly participate in the reforming reaction of the fuel cell stack steam reformers at all levels without the need for additional water supply. In addition, the gas preheater 7 can achieve the purpose of connecting the anodes of the first-stage fuel cell stack 3 and the second-stage fuel cell stack 4 in series, so that the stepped utilization of the fuel can be realized.

[0050] The gas preheater 7 uses the first-stage fuel source 1 to cool the anode tail gas of the first-stage fuel cell stack 3, so that the anode tail gas entering the ejector 8 meets the entrainment requirements of the ejector 8. The ejector 8 entrains the anode tail gas of the first-stage fuel cell stack 3 cooled to medium temperature and recycles it back into the first-stage fuel cell stack steam reformer 5, which can not only provide the water vapor required for the steam reforming reaction but also realize the reuse of the fuel.

[0051] It should be noted that the fuels provided by the first-stage fuel source 1 and the second-stage fuel source 2 are both methane.

[0052] The anode exhaust gas of the first-stage stack 3 can heat the fuel supplied by the first-stage fuel source 1 to 250 °C to improve the entrainment performance during entrainment. After the anode exhaust gas of the first-stage stack 3 is cooled to 150 °C - 180 °C in the gas preheater 7, it is then mixed with the fuel provided by the second-stage fuel source 2. In this way, while improving the fuel utilization rate, it can prevent carbon deposition due to excessive fuel temperature entering the steam reformer 6 of the second-stage stack.

[0053] In some embodiments of the present application, in combination with Figure 1 and Figure 2 , a gas mixer 9 is provided between the gas preheater 7 and the second-stage fuel source 2. The exhaust gas channel outlet of the gas preheater 7 is also connected to the exhaust gas inlet of the gas mixer 9 for part of the exhaust gas to flow into the gas mixer 9. After part of the exhaust gas enters the gas mixer 9, it is mixed with the fuel supplied by the second-stage fuel source 2. The fuel outlet of the gas mixer 9 is connected to the fuel inlet of the steam reformer 6 of the second-stage stack. The setting of the gas mixer 9 can ensure that the exhaust gas generated by the first-stage stack 3 is fully mixed with the fuel provided by the second-stage fuel source 2, so as to ensure the reaction in the steam reformer 6 of the second-stage stack.

[0054] In order to be able to heat the air entering the cathode of the first-stage stack 3 to meet the temperature requirement of the stack for air. The multi-stage fuel cell system further includes an air preheater 10. The air preheater 10 heats the cathode air entering the first-stage stack 3, and the cathode outlet of the first-stage stack 3 is connected to the cathode inlet of the second-stage stack 4. In this embodiment, the cathode inlet of the first-stage stack 3 is connected in series with the cathode inlet of the second-stage stack 4.

[0055] In some embodiments of the present application, the air preheater 10 has an air channel and an exhaust gas channel. The outlet of the air channel is connected to the air inlet of the steam reformer 5 of the first-stage stack, and the air outlet of the steam reformer 5 of the first-stage stack is connected to the anode of the first-stage stack 3. Such a setting can provide a certain amount of heat for the steam reforming reaction of the steam reformer 5 of the first-stage stack.

[0056] In some embodiments of the present application, in combination with Figure 1 and Figure 3 , the multi-stage fuel cell system further includes a burner 11. The burner 11 is respectively connected to the anode outlet and the cathode outlet of the second-stage stack 4. The gas outlet of the burner 11 is connected to the steam reformer 6 of the second-stage stack to provide a heat source for the steam reformer 6 of the second-stage stack. The setting of the burner 11 can recycle the exhaust gas generated by the second-stage stack 4. Higher heat can be generated by secondary combustion of the exhaust gas, and the heat generated by the burner 11 can provide heat energy for the steam reformer 6 of the second-stage stack to provide heat energy for the steam reforming reaction, thereby improving the fuel utilization rate.

[0057] The air preheater 10 has an air passage and an exhaust gas passage. The temperature of the tail gas flowing out of the secondary stack steam reformer 6 is still relatively high compared to the air in the atmosphere. Therefore, in some embodiments, the tail gas outlet of the secondary stack steam reformer 6 is connected to the tail gas inlet of the exhaust gas passage of the air preheater 10, and the tail gas outlet of the exhaust gas passage of the air preheater 10 is connected to the atmosphere. That is, the high-temperature gas generated by the burner 11 provides heat sources for the secondary stack steam reformer 6 and the air preheater 10 in sequence, further improving the fuel utilization rate. The air preheater 10 uses the high-temperature tail gas generated by the burner 11 to heat the fresh air to meet the temperature requirement of the stack for air. The air inlet of the air passage of the air preheater 10 is connected to the outlet of the blower 13, and the air outlet of the air passage of the air preheater 10 is connected to the air inlet of the primary stack steam reformer 5.

[0058] In order to preheat and raise the temperature of the air entering the secondary stack 4, in this embodiment, the multi-stage fuel cell system further includes an air mixer 12, which is configured to mix the cathode tail gas generated by the primary stack 3 with the air and input it to the cathode inlet of the secondary stack 4. In this way, the heat of the cathode tail gas can be reduced after being mixed with the air, so as to achieve the purpose of cooling the cathode tail gas entering the secondary stack 4 and improving the fuel utilization rate.

[0059] In the embodiments of the present application, a multi-stage fuel cell control method is further provided, which is applied to the multi-stage fuel cell system provided in the embodiments of the present invention, as Figure 4 shown. The method includes the following steps:

[0060] S101. Obtain the anode tail gas flow rate of the primary stack 3, the molar composition M% of each substance in the primary stack tail gas, and the fuel flow rate entering the primary stack 3 from the primary fuel source 1. Each substance in the tail gas of the primary stack 3 includes CO, CO 2 , H 2 O and H 2 ;

[0061] A mass flowmeter (such as a high-temperature vortex street flowmeter) is provided between the gas preheater 7 and the ejector 8. As Figure 2 shown, the tail gas passage outlet of the gas preheater 7 is connected to the ejector 8 and the secondary stack steam reformer 6 through a three-way valve, and the high-temperature vortex street flowmeter is specifically arranged on the pipeline connecting the outlet of the three-way valve and the ejector 8, so as to obtain the anode tail gas flow rate Q j of the primary stack 3 that is drawn and circulated. The molar composition M% of each substance in the tail gas of the primary stack 3 is calculated from the current of the primary stack 3 and the fuel utilization rate, etc. The fuel flow rate Q 1Obtained by a mass flow controller provided between the primary fuel source 1 and the gas preheater 7.

[0062] It should be noted that in practical applications, the molar components of various substances in the tail gas of the primary stack 3 can be obtained by a gas analyzer. Among them, the molar component of CO is M%CO, and the molar component of CO 2 is M%CO 2 , H 2 The molar component of O is M%H 2 O.

[0063] S102. Calculate the actual oxygen-carbon ratio based on the anode tail gas flow rate, molar component M%, and fuel flow rate; the specific calculation method is to calculate the molar ratio of all oxygen and all carbon:

[0064] S103. Compare the size of the actual oxygen-carbon ratio with the preset maximum oxygen-carbon ratio and the preset minimum oxygen-carbon ratio, and adjust the fuel flow rates of each stack according to the comparison results.

[0065] Among them, the values of the preset maximum oxygen-carbon ratio and the preset minimum oxygen-carbon ratio are obtained through multiple experiments. In this embodiment, the preset maximum oxygen-carbon ratio is 2.5, and the preset minimum oxygen-carbon ratio is 1.8.

[0066] In some embodiments, adjusting the fuel flow rates of each stack according to the comparison results includes:

[0067] If the actual oxygen-carbon ratio is greater than or equal to the preset minimum oxygen-carbon ratio and less than or equal to the preset maximum oxygen-carbon ratio, the fuel flow rates of the primary stack 3 and the secondary stack 4 remain unchanged.

[0068] At this time, no regulation is required, so the fuel flow rates of the primary stack 3 and the secondary stack 4 remain unchanged.

[0069] In some embodiments, adjusting the fuel flow rates of each stack according to the comparison results includes:

[0070] If the actual oxygen-carbon ratio is less than the preset minimum oxygen-carbon ratio, reduce the fuel flow rate of the secondary stack 4, increase the fuel flow rate of the primary stack 3, and the total fuel amounts of the primary stack 3 and the secondary stack 4 remain unchanged.

[0071] Reduce the fuel flow rate of the secondary stack 4, and correspondingly increase the fuel flow rate of the primary stack 3, and the entrainment ratio increases until the oxygen-carbon ratio is greater than the threshold lower limit. That is, reduce the fuel output flow rate of the secondary fuel source 2, and correspondingly increase the fuel output flow rate of the primary fuel source 1.

[0072] If the actual oxygen-to-carbon ratio is greater than the preset maximum oxygen-to-carbon ratio, increase the fuel flow rate of the secondary stack 4 and decrease the fuel flow rate of the primary stack 3, and keep the total fuel amount of the primary stack 3 and the secondary stack 4 unchanged, that is, reduce the output flow rate of the primary fuel source 1 and increase the output flow rate of the secondary fuel source 2. Ensure that the total fuel amount of the primary stack 3 and the secondary stack 4 remains unchanged, increase the fuel flow rate of the secondary stack 4, and correspondingly reduce the fuel flow rate of the primary stack 3, and the entrainment ratio decreases until the oxygen-to-carbon ratio is less than the upper threshold.

[0073] As Figure 5 shown, the method specifically includes the following steps:

[0074] S201. Obtain the anode exhaust gas flow rate of the primary stack 3, the molar composition M% of each substance in the exhaust gas of the primary stack 3, and the fuel flow rate entering the primary stack 3. Each substance includes CO, CO 2 , H 2 O and H 2 ;

[0075] S202. Calculate the actual oxygen-to-carbon ratio based on the anode exhaust gas flow rate of the primary stack 3, the molar composition M% of each substance in the exhaust gas of the primary stack 3, and the fuel flow rate entering the primary stack 3.

[0076] S203. Determine whether the actual carbon-to-oxygen ratio is greater than the preset maximum oxygen-to-carbon ratio. If so, execute step S204; if not, execute step S205.

[0077] S204. Increase the fuel flow rate of the secondary stack 4 and decrease the fuel flow rate of the primary stack 3, and keep the total fuel amount of the primary stack 3 and the secondary stack 4 unchanged.

[0078] S205. Determine whether the actual carbon-to-oxygen ratio is less than the preset minimum oxygen-to-carbon ratio. If so, execute step S206; if not, execute step S207.

[0079] S206. Decrease the fuel flow rate of the secondary stack 4 and increase the fuel flow rate of the primary stack 3, and keep the total fuel amount of the primary stack 3 and the secondary stack 4 unchanged.

[0080] S207. Keep the fuel flow rates of the primary stack 3 and the secondary stack 4 unchanged.

[0081] The method provided by the present invention adjusts the fuel flow rates of the primary stack 3 and the secondary stack 4 by obtaining the oxygen-to-carbon ratio, that is, adjusts the flow rates of the primary fuel source 1 and the secondary fuel source 2. This can avoid the occurrence of carbon deposition caused by too low a carbon-to-oxygen ratio, and at the same time can also avoid low fuel utilization caused by too high a carbon-to-oxygen ratio.

[0082] To ensure the oxygen-to-carbon ratio of the first-stage fuel cell stack steam reformer 5 and the first-stage fuel cell stack 3, it is necessary to adjust the performance of the ejector 8. Due to the unique structure of the ejector 8, adjusting the working fluid pressure is the most effective method to achieve the performance adjustment of the ejector 8. When the structure of the ejector 8 is fixed, changing the working fluid flow rate is equivalent to changing the working fluid pressure. For a multi-stage fuel cell system, the oxygen-to-carbon ratio of the system is generally required to be between 1.8 and 2.5. For a multi-stage fuel cell system, when it operates stably, the steam entering the first-stage fuel cell stack 3 is completely ejected by the ejector 8. If the performance of the ejector 8 fails to meet the requirements and cannot be adjusted, the oxygen-to-carbon ratio of the first-stage fuel cell stack 3 will be too low, resulting in carbon deposition. On the premise of ensuring that the total external fuel entering the system remains unchanged, the fuel entering the first-stage fuel cell stack 3 and the fuel entering the second-stage fuel cell stack 4 are regulated to achieve the regulation of the performance of the ejector 8, thereby achieving the regulation of the oxygen-to-carbon ratio of the first-stage fuel cell stack 3.

[0083] In addition, the above are only the preferred embodiments of the present invention and the technical principles applied. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein. Various obvious changes, re-adjustments, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the concept of the present invention, it can also include more other equivalent embodiments, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A multi-stage fuel cell system, characterized in that: include: A fuel source, comprising at least a primary fuel source (1) and a secondary fuel source (2); A fuel cell stack, comprising at least a primary fuel cell stack (3) and a secondary fuel cell stack (4), wherein the primary fuel source (1) provides fuel for the primary fuel cell stack (3), and the secondary fuel source (2) provides fuel for the secondary fuel cell stack (4); A stack steam reformer, comprising at least a primary stack steam reformer (5) and a secondary stack steam reformer (6), wherein the primary stack steam reformer (5) is connected to the primary fuel source (1) for providing fuel to the primary stack (3), and the secondary stack steam reformer (6) is respectively connected to the secondary fuel source (2) and an anode of the secondary stack (4); A gas preheater (7) having an exhaust gas channel and a fuel channel which are not connected to each other, wherein the inlet of the exhaust gas channel of the gas preheater (7) is connected to the anode outlet of the first-stage stack (3), the inlet of the fuel channel of the gas preheater (7) is connected to the outlet of the first-stage fuel source (1), the outlet of the exhaust gas channel of the gas preheater (7) is connected to the fuel inlet of the second-stage stack steam reformer (6), and the gas preheater (7) is configured to use the anode exhaust gas to preheat the fuel; An ejector (8), wherein the working flow inlet of the ejector (8) is connected to the fuel channel outlet of the gas preheater (7), and the ejection flow inlet of the ejector (8) is connected to the exhaust gas channel outlet of the gas preheater (7); the outlet of the ejector (8) is connected to the fuel inlet of the first-stage stack steam reformer (5).

2. The multi-stage fuel cell system according to claim 1, characterized in that: A gas mixer (9) is provided between the gas preheater (7) and the secondary fuel source (2); the tail gas channel outlet of the gas preheater (7) is also connected to the tail gas inlet of the gas mixer (9) so that part of the tail gas flows into the gas mixer (9); and the fuel outlet of the gas mixer (9) is connected to the fuel inlet of the secondary stack steam reformer (6).

3. The multi-stage fuel cell system according to claim 1, characterized in that: The multi-stage fuel cell system further comprises an air preheater (10), wherein the air preheater (10) heats cathode air entering the primary fuel cell stack (3), and the cathode outlet of the primary fuel cell stack (3) is connected to the cathode inlet of the secondary fuel cell stack (4).

4. The multi-stage fuel cell system according to claim 3, characterized in that: The air channel outlet of the air preheater (10) is connected to the air inlet of the primary stack steam reformer (5), and the air outlet of the primary stack steam reformer (5) is connected to the cathode inlet of the primary stack (3).

5. The multi-stage fuel cell system according to claim 3, characterized in that: The multi-stage fuel cell system further comprises a burner (11), wherein the burner (11) is respectively connected to the anode outlet of the secondary fuel cell stack (4) and the cathode outlet of the secondary fuel cell stack (4), and an air outlet of the burner (11) is connected to the steam reformer (6) of the secondary fuel cell stack to provide a heat source for the steam reformer (6) of the secondary fuel cell stack.

6. The multi-stage fuel cell system according to claim 5, characterized in that: The tail gas outlet of the two-stage stack steam reformer (6) is connected to the tail gas inlet of the air preheater (10), and the tail gas outlet of the air preheater (10) is connected to the atmosphere.

7. The multi-stage fuel cell system according to claim 1, characterized in that: It also includes an air mixer (12), which is configured to mix the cathode exhaust gas generated by the first-stage fuel cell stack (3) with air and input the mixed air into the cathode inlet of the second-stage fuel cell stack (4).

8. A multi-stage fuel cell control method, characterized in that: The multi-stage fuel cell system according to any one of claims 1 to 7 comprises the following steps: Obtaining the anode tail gas flow rate of the primary stack (3), the molar component M% of each substance in the primary stack tail gas, and the fuel flow rate entering the primary stack (3) from the primary fuel source (1), wherein each substance in the primary stack tail gas includes CO, CO2, H2O and H2; The actual oxygen-carbon ratio is calculated based on the anode tail gas flow rate, the molar component M% and the fuel flow rate; The actual oxygen-carbon ratio is compared with the preset maximum oxygen-carbon ratio and the preset minimum oxygen-carbon ratio, and the fuel flow of each level of the fuel cell stack is adjusted according to the comparison result.

9. The multi-stage fuel cell control method according to claim 8, characterized in that: The fuel flow rate of each level of the stack is adjusted according to the comparison results, including: If the actual oxygen-carbon ratio is greater than or equal to the preset minimum oxygen-carbon ratio and less than or equal to the preset maximum oxygen-carbon ratio, the fuel flow rate of the first-stage fuel cell stack (3) and the fuel flow rate of the second-stage fuel cell stack (4) remain unchanged.

10. The multi-stage fuel cell control method according to claim 8, characterized in that: The fuel flow rate of each level of the stack is adjusted according to the comparison results, including: If the actual oxygen-carbon ratio is less than the preset minimum oxygen-carbon ratio, the fuel flow rate of the secondary fuel cell (4) is reduced and the fuel flow rate of the primary fuel cell (3) is increased; If the actual oxygen-carbon ratio is greater than the preset maximum oxygen-carbon ratio, the fuel flow rate of the secondary fuel cell stack (4) is increased and the fuel flow rate of the primary fuel cell stack (3) is reduced.

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