A biomass combined heat and power flue gas waste heat recovery and utilization system and method

Abstract

The application discloses a biomass combined heat and power flue gas waste heat recycling system, wherein the flue gas enters a first heat exchanger (310), a second heat exchanger (320) and a third heat exchanger (330) in sequence through a flue gas exhaust pipe (301), heats first waste heat water and third waste heat water in a first waste heat water circulating pipe (401) and a third waste heat water circulating pipe (403), and is finally discharged into a biogas tank; the first waste heat water enters an absorption heat pump generator (110) to be cooled; the third waste heat water enters an absorption heat pump evaporator (130) to be cooled; heat network water enters the second heat exchanger (320) through a heat network water first return pipe (511) and a second waste heat water circulating pipe (402) to be heated and then is merged into the heat network; the heat network water also enters an absorption heat pump absorber (140) and an absorption heat pump condenser (120) through a heat network water second return pipe (512) to be heated and then is merged into the heat network; external air (200) enters a third heat exchanger second unit (332) through an air inlet pipe (201) to be heated and then enters a drying device (700).

Description

Technical Field

[0001] This invention belongs to the field of heat energy recovery technology, specifically a system and method for recovering and utilizing waste heat from biomass cogeneration flue gas. Background Technology

[0002] In biomass direct combustion power generation systems, the fuel has a high moisture content, the flue gas has a high moisture content, resulting in significant heat loss and low overall utilization rate. Temperature gaps often exist during flue gas waste heat utilization, making it impossible to achieve "cascaded utilization" and "full utilization" of flue gas energy.

[0003] To address the aforementioned issues, and based on the actual operation of biomass direct-fired power plants, this study investigates the cascade recovery of waste heat from flue gas, adhering to the principle of "matching temperature and grade, and utilizing energy in stages."

[0004] In response to the practical operational problems of biomass cogeneration systems, such as low fuel utilization rate, high feedstock moisture content, large flue gas heat loss, and poor load regulation capacity, as well as the lack of graded utilization of waste heat from biomass direct combustion cogeneration flue gas both domestically and internationally, this project follows the principle of "matching temperature and grade, and cascaded energy utilization," and combines the principles of thermodynamics, heat transfer, and physical chemistry to conduct research using the following technical approaches.

[0005] The final flue gas temperature of the biomass direct-fired boiler is 145℃, and the main components of the flue gas are CO2, N2, and water vapor. The moisture content of the biomass fuel is as high as 30%, the moisture content of the flue gas is as high as 16%, and the flue gas flow rate is as high as 115606 Nm³. 3 With a direct emission heat loss of up to 7.61% per hour, the deep cascade utilization of waste heat from flue gas has significant economic value. Summary of the Invention

[0006] This application aims to address the technical challenges of recovering latent heat from low-grade flue gas generated after biomass power generation and heating, as well as heat pump upgrading and low-grade waste heat utilization. It specifically addresses the problems of low fuel utilization rate, high moisture content, and significant flue gas heat loss in biomass cogeneration systems. The application provides a biomass cogeneration flue gas waste heat recovery system that integrates cascaded waste heat recovery with heat pump upgrading. Part of the recovered waste heat is used to dry biomass fuel to improve combustion efficiency, and the upgraded low-temperature flue gas is used for isothermal fermentation of organic waste, significantly improving the recovery efficiency of biomass cogeneration flue gas.

[0007] The first objective of this invention is to provide a waste heat recovery and utilization system for biomass cogeneration flue gas, including an absorption heat pump, a waste water system, a heating network water system, a flue gas system, a drying system, and a biogas digester;

[0008] The flue gas system includes: a flue pipe and a first-stage heat exchanger, a second-stage heat exchanger, a first-stage heat exchanger unit, and a third-stage heat exchanger unit arranged sequentially on the flue pipe along the flue gas conveying direction, and a second-stage heat exchanger unit connected in parallel to both ends of the first-stage heat exchanger unit via a branch pipe of the flue pipe. The outlet of the flue pipe is located below the liquid surface of the biogas digester.

[0009] The waste water system includes: a primary waste water circulation pipe and a secondary waste water circulation pipe; the outlet of the primary waste water circulation pipe is connected to the inlet of the absorption heat pump generator, the outlet of the absorption heat pump generator is connected to the inlet of the primary waste water circulation pipe, and a section of the primary waste water circulation pipe is located inside the primary heat exchanger; a section of the secondary waste water circulation pipe is located inside the secondary heat exchanger; the outlet of the tertiary waste water circulation pipe is connected to the inlet of the absorption heat pump evaporator, the outlet of the absorption evaporator is connected to the inlet of the tertiary waste water circulation pipe, and a section of the tertiary waste water circulation pipe is located inside the first unit of the tertiary heat exchanger;

[0010] The heating network water system includes: a first return water pipe and a first supply water pipe for the heating network water, wherein the outlet of the first return water pipe is connected to the inlet of the secondary waste water circulation pipe, and the outlet of the secondary waste water circulation pipe is connected to the inlet of the first supply water pipe for the heating network water; it also includes a second return water pipe and a second supply water pipe for the heating network water, wherein the second return water pipe is connected to the inlet of the absorption heat pump absorber, the outlet of the absorption heat pump absorber is connected to the inlet of the absorption heat pump condenser through the second return water pipe, and the outlet of the absorption heat pump condenser is connected to the second supply water pipe for the heating network water;

[0011] The drying system includes an air inlet pipe for supplying external air into the drying device, a section of which is located inside the second unit of the three-stage heat exchanger.

[0012] Furthermore, it also includes a steam system, which includes a steam pipe. The steam pipe inlet is connected to a biomass boiler, and the steam pipe outlet is connected to a steam turbine generator. The high-pressure steam outlet of the steam turbine generator is connected to the exhaust outlet of the industrial steam turbine and connected to the exhaust pipe. The exhaust pipe outlet is connected to a steam-water heat exchanger.

[0013] Furthermore, it also includes a third return water pipe and a third supply water pipe for the heating network. The outlet of the third return water pipe is connected to the inlet of the steam-water heat exchanger, and the outlet of the steam-water heat exchanger is connected to the third supply water pipe for the heating network.

[0014] Furthermore, the absorption heat pump absorber, absorption heat pump generator, absorption heat pump evaporator, and absorption heat pump condenser are single-stage or multi-stage series structures.

[0015] Furthermore, the heat exchange media of the primary heat exchanger, the secondary heat exchanger, and the tertiary heat exchanger are flue gas and water.

[0016] Furthermore, the primary and secondary heat exchangers are indirect heat exchangers, and the tertiary heat exchanger is a submerged heat exchanger.

[0017] Furthermore, the outlet of the drying device is connected to a biomass boiler.

[0018] Furthermore, the absorption heat pump is a vapor-type lithium bromide absorption heat pump.

[0019] The second objective of this invention is to provide a method for recovering and utilizing waste heat from biomass cogeneration flue gas, based on the aforementioned biomass cogeneration flue gas waste heat recovery and utilization system, comprising: Flue gas discharged from the biomass boiler sequentially enters a primary heat exchanger, a secondary heat exchanger, and a tertiary heat exchanger through the exhaust pipe to release heat, heating the primary and tertiary waste water in the primary and tertiary waste water circulation pipes, and finally being discharged into a biogas digester; the tertiary heat exchanger comprises a first tertiary heat exchanger unit and a second tertiary heat exchanger unit connected in parallel.

[0020] The primary waste water enters the absorption heat pump generator through the primary waste water circulation pipe to cool down, and then returns to the primary heat exchanger to be reheated; the tertiary waste water enters the absorption heat pump evaporator through the tertiary waste water circulation pipe to cool down, and then returns to the first unit of the tertiary heat exchanger to be reheated.

[0021] The heating network water enters the secondary heat exchanger for heating through the first return water pipe and the secondary waste water circulation pipe. The heated heating network water then enters the first supply water pipe. The heating network water also enters the absorption heat pump absorber and absorption heat pump condenser in sequence through the second return water pipe, and after being heated, it flows into the heating network.

[0022] Outside air enters the second unit of the three-stage heat exchanger through the air inlet pipe for heating, and the heated outside air then enters the drying device through the air inlet pipe to dry the biomass fuel.

[0023] Furthermore, it also includes the low-pressure steam discharged from the steam turbine generator entering the steam-water heat exchanger through the exhaust pipe to release heat; the heating network water also enters the steam-water heat exchanger through the third return water pipe of the heating network and is heated before flowing into the heating network.

[0024] The above-described technical solution of the present invention has the following beneficial technical effects:

[0025] This application presents a biomass cogeneration flue gas waste heat recovery and utilization system. Based on the energy level distribution characteristics of biomass flue gas waste heat, it constructs a multi-stage, multi-section heat pump cycle driven by medium- and low-temperature flue gas waste heat and employs efficient mass transfer and heat exchange technology for tiered energy utilization, thus more fully recovering waste heat. This reduces energy consumption and improves thermal energy utilization efficiency.

[0026] The innovation of this scheme lies in its approach to the latent heat characteristic of biomass flue gas. It employs a staged energy recovery and utilization method, using 145℃ high-temperature flue gas to drive an absorption heat pump. This pump recovers the latent heat from the 60℃ low-temperature flue gas. The absorption heat pump can be configured with multiple units, each operating in series or independently. A portion of the 60℃ low-temperature flue gas waste heat is recovered for drying biomass fuel, improving system combustion efficiency. The 45℃ low-temperature flue gas, after multi-stage cooling and heat pump enhancement, is discharged below the biogas digester's liquid surface to exchange heat with the biogas liquid, maintaining the fermentation temperature and improving the digester's gas production efficiency. The flue gas, after heat exchange in the biogas digester, is discharged into the atmosphere at near-ambient emission temperatures, significantly improving the efficiency of flue gas waste heat recovery and utilization. Attached Figure Description

[0027] Figure 1 A schematic diagram of a flue gas waste heat recovery and utilization system according to the present invention;

[0028] Figure 2 This is a schematic diagram of an absorption heat pump;

[0029] Figure label:

[0030] The components include: 600, biogas digester; 700, drying device; 800, biomass boiler; 110, absorption heat pump generator; 120, absorption heat pump condenser; 130, absorption heat pump evaporator; 140, absorption heat pump absorber; 201, air inlet pipe; 202, outside air; 301, flue pipe; 302, flue pipe branch pipe; 310, primary heat exchanger; 320, secondary heat exchanger. 331. First unit of the three-stage heat exchanger; 332. Second unit of the three-stage heat exchanger; 401. First-stage waste hot water circulation pipe; 402. Second-stage waste hot water circulation pipe; 403. Third-stage waste hot water circulation pipe; 511. First return pipe of the heating network water; 521. First supply pipe of the heating network water; 512. Second return pipe of the heating network water; 522. Second supply pipe of the heating network water; 513. Third return pipe of the heating network water; 523. Third supply pipe of the heating network water. Detailed Implementation

[0031] The accompanying drawings illustrate a layer structure according to an embodiment of the present invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0032] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. In the description of the present invention, it should be noted that the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0033] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0035] See Figure 1 This application provides a biomass cogeneration flue gas waste heat recovery and utilization system for recovering waste heat from flue gas and improving energy efficiency. It is understood that in other embodiments of this application, the above-described composite heat pump system can also be used in other applications, such as industrial or agricultural hot water systems, and is not the only applicable application here.

[0036] In the embodiments of this application,

[0037] The biomass cogeneration flue gas waste heat recovery and utilization system includes an absorption heat pump, a waste water system, a heating network water system, a flue gas system, a drying system, and a 600m biogas digester.

[0038] The flue gas system includes: a flue pipe 301 and a first-stage heat exchanger 310, a second-stage heat exchanger 320, a third-stage heat exchanger first unit 331, which are sequentially arranged on the flue pipe 301 along the flue gas conveying direction, and a third-stage heat exchanger second unit 332 connected in parallel to both ends of the third-stage heat exchanger first unit 331 through a branch pipe 302 of the flue pipe. The outlet of the flue pipe 301 is located below the liquid surface of the biogas digester 600.

[0039] The waste water system includes: a primary waste water circulation pipe 401 and a secondary waste water circulation pipe 402; the outlet of the primary waste water circulation pipe 401 is connected to the inlet of the absorption heat pump generator 110, and the outlet of the absorption heat pump generator 110 is connected to the inlet of the primary waste water circulation pipe 401; a section of the primary waste water circulation pipe 401 is located inside the primary heat exchanger 310; a section of the secondary waste water circulation pipe 402 is located inside the secondary heat exchanger 320; the outlet of the tertiary waste water circulation pipe is connected to the inlet of the absorption heat pump evaporator 130, and the outlet of the absorption evaporator 130 is connected to the inlet of the tertiary waste water circulation pipe 403; a section of the tertiary waste water circulation pipe 403 is located inside the first unit 331 of the tertiary heat exchanger.

[0040] The heating network water system includes: a first return water pipe 511 and a first supply water pipe 521. The outlet of the first return water pipe 511 is connected to the inlet of the secondary waste water circulation pipe 402, and the outlet of the secondary waste water circulation pipe 402 is connected to the inlet of the first supply water pipe 521. It also includes a second return water pipe 512 and a second supply water pipe 522. The second return water pipe 512 is connected to the inlet of the absorption heat pump absorber 140, and the outlet of the absorption heat pump absorber 140 is connected to the inlet of the absorption heat pump condenser (120) through the second return water pipe 512. The outlet of the absorption heat pump condenser 120 is connected to the second supply water pipe 522.

[0041] The drying system includes an air inlet pipe 201 for supplying outside air 202 into the drying device 700, and a section of the air inlet pipe 201 is located inside the second unit 332 of the three-stage heat exchanger.

[0042] Based on the above structure, flue gas undergoes staged heat exchange via a primary heat exchanger 310, a secondary heat exchanger 320, and a tertiary heat exchanger. The resulting primary, secondary, and tertiary waste water then flows through primary waste water circulation pipe 401, secondary waste water circulation pipe 402, and tertiary waste water circulation pipe 403, respectively, into an absorption heat pump and a compression heat pump to release heat. This staged recovery of heat energy from the flue gas according to different temperatures and grades allows for tiered energy utilization. The flue gas after the tertiary heat exchange is discharged into the fermentation liquid in the biogas digester 600 to maintain the fermentation temperature and improve biogas production efficiency. A portion of the waste heat from the flue gas is further heated via the second unit 332 of the tertiary heat exchanger to heat the external air in the intake pipe 201. The heated external air then enters the drying device 700 to dry the biomass fuel and improve combustion efficiency.

[0043] First, the waste heat from the high-temperature flue gas drives the absorption heat pump generator 110. The absorption heat pump unit uses the high-temperature heat source of the flue gas (145°C) as the driving heat source, lithium bromide solution as the absorbent, and water as the refrigerant to recover and utilize the heat energy from the low-temperature heat source to produce the required high-temperature hot water for process or heating, thus achieving the function of transferring heat energy from low to high temperature. The absorption heat pump consists of the absorption heat pump generator 110, the absorption heat pump condenser 120, the absorption heat pump evaporator 130, and the absorption heat pump absorber 140. It is also equipped with auxiliary devices such as a vacuum pump, a solution pump, and a refrigerant pump. The vacuum pump is used to remove non-condensable gases from the heat pump and maintain a high vacuum state inside the heat pump.

[0044] See Figure 2 This is a schematic diagram of an absorption heat pump. Absorption heat pumps utilize a solution composed of high-boiling-point and low-boiling-point substances to recover and utilize heat energy. Lithium bromide solution is commonly used, with lithium bromide as the absorbent. Lithium bromide has a high boiling point of 1265℃ and is a highly hygroscopic salt. Not only can solid lithium bromide absorb water vapor, but highly concentrated lithium bromide solutions also have a strong ability to absorb water vapor. After absorbing water vapor, the concentrated lithium bromide solution decreases in concentration, becoming a dilute solution. After heating to evaporate and concentrate the water, it can be recycled. Lithium bromide absorption heat pumps use lithium bromide solution as the absorbent, utilizing the gas-liquid transformation process of water to achieve heating.

[0045] See Figure 2 The absorption heat pump 1 mainly comprises four parts: an absorption heat pump generator 110, an absorption heat pump condenser 120, an absorption heat pump evaporator 130, and an absorption heat pump absorber 140. Its operation includes:

[0046] First, in the absorption heat pump generator 110, the lithium bromide solution is heated to boiling by the driving heat source medium in the driving heat source supply pipe, causing the water in it to evaporate and form high-temperature water vapor. The concentration of the lithium bromide solution increases after losing water, becoming a concentrated lithium bromide solution, which flows back to the absorption heat pump absorber 14, while the high-temperature water vapor is transported to the absorption heat pump condenser 120.

[0047] In the second step, inside the absorption heat pump condenser 120, high-temperature water vapor condenses on the surface of the heating network water return pipe 510, releasing a large amount of latent heat, which heats the water in the heating network water return pipe 510, raising the water temperature in the second heating network water return pipe 512. The condensed water then enters the absorption heat pump evaporator 13 after being throttled.

[0048] Thirdly, within the absorption heat pump evaporator 130, a negative pressure environment exists. Water within the evaporator 130 has a very low vaporization temperature, typically reaching saturation and evaporation at 5°C. Therefore, the surface temperature of the waste hot water pipe is sufficient for the condensate to evaporate. The condensate vaporizes upon contact with the surface of the waste hot water pipe, absorbing a large amount of heat during the evaporation process, thus lowering the temperature of the waste hot water and achieving the purpose of heat recovery. The water vapor produced after the condensate vaporizes enters the absorber.

[0049] In the fourth step, within the absorption heat pump absorber 140, the concentrated lithium bromide solution returning from the generator is sprayed onto the outer surface of the second return water pipe 512 of the heating network. This solution encounters water vapor from the absorption heat pump evaporator 130 on the outer surface of the return water pipe 510. The concentrated lithium bromide solution absorbs the water vapor, and during the liquefaction process, a large amount of latent heat is released on the outer surface of the second return water pipe 512, heating the water inside and raising the temperature of the return water. After absorbing the water vapor, the concentration of the lithium bromide solution decreases, becoming a dilute lithium bromide solution. This dilute solution flows into the bottom of the absorption heat pump absorber 140 and is pumped to the absorption heat pump generator 110, where it is again heated by the heat source to boil and evaporate water. After evaporation, the water becomes a concentrated solution, regaining its ability to absorb water vapor. The entire system operates in this cyclical manner.

[0050] In this embodiment, the absorption heat pump can be configured as a single-stage or multi-stage structure. A single-stage structure is easier to manufacture, while a multi-stage structure allows for more complete recovery of waste heat. To ensure more complete recovery of flue gas waste heat, this embodiment configures each unit of the absorption heat pump as a multi-stage structure. To further enhance the system's heating capacity, the absorption heat pump generator 110, absorption heat pump condenser 120, absorption heat pump evaporator 130, and absorption heat pump absorber 140 can be configured as a multi-stage structure. Specifically, a two-stage structure is used as an example. It should be noted that in actual operating conditions, three-stage, four-stage, or even more stages can be configured as needed, all of which are within the scope of protection of this invention.

[0051] For example, in one embodiment, the absorption heat pump 1 can be equipped with two or more absorption heat pump generators 110 connected in series. The primary waste water circulation pipe 401 passes through each absorption heat pump generator 110 in sequence to more thoroughly absorb the heat from the driving heat source. Similarly, two or more absorption heat pump condensers 120 can be connected in series, and the second return water pipe 512 of the heating network passes through each absorption heat pump condenser in sequence to ensure more complete heating of the heating network return water. Similarly, two or more absorption heat pump evaporators 130 can be connected in series, and the tertiary waste water circulation pipe 403 passes through each absorption heat pump evaporator 130 in sequence to ensure more complete recovery of waste water heat. Additionally, two or more absorption heat pump absorbers 140 can be connected in series, and the second return water pipe 512 of the heating network passes through each absorption heat pump absorber 140 in sequence to ensure sufficient heating of the heating network water.

[0052] For example, each unit of the absorption heat pump is configured as a two-stage structure. The flue gas emitted from the biomass boiler 800, with a high-temperature section of 90℃~145℃, serves as a high-temperature heat source. After heat exchange in the first-stage heat exchanger 310, the waste hot water from the heat exchange is used as a driving heat source to drive the first and second stages of the absorption heat pump generator 110. After the drive is completed, the waste hot water is pumped back into the first-stage heat exchanger 310 for heat exchange, forming a loop. After the flue gas passes through the first-stage heat exchanger 310, its temperature decreases, and it enters the second-stage heat exchanger 320. The flue gas with a medium temperature section of 60℃~90℃ passes through the second-stage heat exchanger 320 to release heat. The hot water from the heat exchange is supplied to the heating network users. The cooling return water from the heating network on the user side then passes through the second return water pipe 512 and the secondary waste hot water circulation pipe 402, and re-enters the secondary heat exchanger 320 for heat exchange. The hot water from the heating network passes through the first return water pipe 511 and is heated to 70°C by the secondary heat exchanger 320 before being supplied to heat users. The flue gas, cooled to 60°C after heat exchange in the secondary heat exchanger 320, enters the tertiary heat exchanger. The waste water from the tertiary heat exchanger serves as the heat source for the first and second stages of the absorption heat pump generator. After heat exchange, the waste water from the tertiary heat exchanger re-enters the tertiary heat exchanger for heating, forming a loop.

[0053] The hot water from the heating network passes through the second return pipe 512, then through the first and second stages of the absorption heat pump absorber, and then through the first and second stages of the absorption heat pump condenser for heat exchange, heating the hot water to 70°C before supplying it to the heating network users.

[0054] In this application embodiment, reducing the moisture content of biomass fuel is also considered. Unlike traditional fossil fuels, biomass fuel often has a higher moisture content, leading to poor combustion and affecting heat generation and power generation efficiency. Therefore, reducing the moisture content of biomass fuel has become a pressing problem. Using electric drying equipment to reduce the moisture content of biomass fuel requires additional electricity, increasing system costs. Therefore, this application considers using the low-temperature heat of flue gas for drying, which eliminates the need for additional energy consumption and can also improve the combustion efficiency of biomass fuel.

[0055] Therefore, this application also includes a drying system, which includes an air inlet pipe 201 for sending outside air 202 into the drying device 700, and a section of the air inlet pipe 201 is located inside the second unit 332 of the three-stage heat exchanger.

[0056] The flue gas temperature after heat exchange in the primary heat exchanger 310 and the secondary heat exchanger 320 is around 60℃. This portion of the flue gas then passes through the first unit 331 of the tertiary heat exchanger to heat the waste water, which is then used as a heat source for the absorption heat pump 140. Another portion enters the second unit 332 of the tertiary heat exchanger through the exhaust pipe branch 302 to heat the external air 202. The heated external air 202 then enters the drying device 700 as a heat source to reduce the moisture content of the biomass fuel. This scheme effectively utilizes the waste heat of the low-temperature flue gas section, achieving the purpose of drying biomass fuel without the need for additional computer equipment, further improving the combustion efficiency of biomass fuel.

[0057] After heat exchange in the primary, secondary, and tertiary heat exchangers, the flue gas temperature has dropped to around 45°C. This portion of waste heat is difficult to recover, but direct discharge would also result in a waste of thermal energy. This application considers using this portion of low-temperature flue gas after primary, secondary, and tertiary heat exchange to maintain the fermentation temperature of the biogas digester at 600°C.

[0058] Therefore, in this embodiment of the invention, the outlet of the flue pipe 301 is located inside the biogas digester 600, and the outlet of the flue pipe is located below the fermentation liquid surface of the biogas digester 600. The flue gas and the fermentation liquid in the biogas digester are directly contacted for heat exchange, so as to increase the temperature inside the biogas digester and improve the fermentation gas production efficiency.

[0059] Furthermore, this application embodiment considers that the steam generated by the biomass boiler drives a steam turbine generator to generate electricity. After power generation, the low-pressure steam still contains a large amount of waste heat, which can also be recovered and reused. Therefore, this application also includes a steam system, which includes a steam pipe 901. The inlet of the steam pipe 901 is connected to the biomass boiler, and the outlet of the steam pipe 901 is connected to the steam turbine generator. The high-pressure steam outlet of the steam turbine generator 910 is connected to the industrial steam terminal 920, and the exhaust outlet of the steam turbine is connected to the exhaust pipe 902. The outlet of the exhaust pipe is connected to the steam-water heat exchanger 930.

[0060] Furthermore, it also includes a third return water pipe 513 and a third supply water pipe 523 for the heating network. The outlet of the third return water pipe is connected to the inlet of the steam-water heat exchanger 930, and the outlet of the steam-water heat exchanger is connected to the third supply water pipe 523 for the heating network.

[0061] Furthermore, the first return water pipe 511 and the second return water pipe 512 of the heating network are connected to the outlet of the main return water pipe of the heating network, and the first supply water pipe 521 and the second supply water pipe 522 of the heating network converge into the inlet of the main heat network pipe. The purpose of this arrangement is to divide the heating network water into two paths, which are heated separately in the absorption heat pump and the secondary heat exchanger 320, to fully utilize the recovered heat energy. Since the waste heat temperature of biomass flue gas is around 145℃, its temperature rise on the heating network water is relatively limited. If the heating network water is directly heated through the secondary heat exchanger 320 and the absorption heat pump in series, the temperature of the return water after passing through the first heat pump will inevitably increase, reducing the temperature difference between the heated network water and the heating unit of the second heat pump, thus reducing the heating efficiency in the second heat pump. Therefore, in this embodiment, the return water of the heating network is preferably divided into two lines: a first return water pipe 511 and a second return water pipe 512. The two return water lines are heated by a two-stage heat exchanger 320 and an absorption heat pump, respectively. The heated heating network water is then supplied by a first supply water pipe 521 and a second supply water pipe 522, flowing into the main heating network pipe for use by heating network users. By dividing the return water pipe into two branches and connecting them in parallel with the compression heat pump and the absorption heat pump, the utilization rate of recovered heat energy is improved, and the system's thermal efficiency is enhanced.

[0062] Furthermore, the heating network water is divided into a third path, including a third return water pipe 513 and a third supply water pipe 523, for heating the third path of heating network water in a steam-water heat exchanger 930. This recovers the heat energy from the low-pressure steam and fully recovers the waste heat from various parts of the system.

[0063] In this embodiment of the invention, the return water of the waste hot water system can be selected from various sources according to the actual application scenario. For example, the inlet of the waste hot water return pipe can be connected to the waste heat source, or the inlet of the waste hot water return pipe can be connected to the return water system of the upper-level heating network.

[0064] The choice of heat source for an absorption heat pump can be based on the specific application environment. Different heat sources can be selected for industrial, agricultural, and urban residential applications, with commonly used sources including natural gas, steam, or high-temperature hot water. This embodiment of the invention uses a biomass boiler 800.

[0065] Furthermore, the heat exchange medium can be selected according to the actual operating conditions. The heat exchange mediums of the first-stage heat exchanger 310, the second-stage heat exchanger 320 and the third-stage heat exchanger of the present invention are flue gas and water.

[0066] Furthermore, the type of heat exchanger can be selected according to specific operating conditions. There are many types of heat exchangers, but based on the principle and method of heat exchange between cold and hot fluids, they can be basically divided into three categories: indirect-wall type, mixing type, and regenerative type. Among the three types of heat exchangers, indirect-wall type heat exchangers are the most widely used. The characteristic of an indirect-wall type heat exchanger is that the cold and hot fluids are separated by a solid wall (tube or plate) and do not mix, exchanging heat through the wall. The primary heat exchanger 310 and the secondary heat exchanger 320 described in this invention are indirect-wall type heat exchangers.

[0067] Furthermore, since the flue gas temperature inside the tertiary heat exchanger is relatively low, in order to improve the heat exchange effect, the first unit 310 of the tertiary heat exchanger is configured as a submerged heat exchanger. A submerged heat exchanger is constructed by placing a bundle of tubes arranged in a certain manner into a container of different types, such as a tank. Appropriate space must be maintained around each tube to ensure effective heat exchange on the tube surface.

[0068] Furthermore, there are two main types of absorption heat pumps: ammonia absorption heat pumps and lithium bromide absorption heat pumps. Ammonia units can reach evaporation temperatures as low as -60°C and are often used in cold storage for quick freezing. However, ammonia gas is toxic. When mixed with air to a certain concentration, it can easily explode upon contact with an open flame. Due to the danger, fully automated equipment is generally not available, and manual operation is required.

[0069] Lithium bromide heat pumps use water as a refrigerant, with an evaporation temperature above 0 degrees Celsius and an outlet water temperature above 5 degrees Celsius. They are highly effective at recovering and utilizing waste gas, waste heat, solar energy, and low-temperature thermal energy using low-pressure steam or waste hot water. They are particularly suitable for recovering and utilizing the thermal energy of low-temperature heat sources (such as waste hot water) to produce the required high-temperature heat medium (hot water) for processes or heating, thus enabling the transfer of heat energy from low to high temperatures. Furthermore, lithium bromide is non-toxic, environmentally friendly, and pollution-free. Therefore, in this embodiment of the invention, a steam-type lithium bromide absorption heat pump is preferred.

[0070] The second objective of this invention is to provide a method for recovering and utilizing waste heat from biomass cogeneration flue gas, based on the aforementioned biomass cogeneration flue gas waste heat recovery and utilization system. The method includes: flue gas discharged from a biomass boiler 800 enters sequentially through the exhaust pipe 301 into a primary heat exchanger 310, a secondary heat exchanger 320, and a tertiary heat exchanger to release heat, heating the primary and tertiary waste water in the primary waste water circulation pipe 401 and the tertiary waste water circulation pipe 403, and finally discharging it into a biogas digester. The tertiary heat exchanger includes a first tertiary heat exchanger unit 331 and a second tertiary heat exchanger unit 332 connected in parallel.

[0071] The primary waste water enters the absorption heat pump generator 110 through the primary waste water circulation pipe 401, is cooled down, and then returns to the primary heat exchanger 310 to be heated again; the tertiary waste water enters the absorption heat pump evaporator 130 through the tertiary waste water circulation pipe 403, is cooled down, and then returns to the first unit 331 of the tertiary heat exchanger to be heated again.

[0072] The hot water enters the secondary heat exchanger 320 for heating via the first return water pipe 511 and the secondary waste water circulation pipe 402. After heating, the hot water enters the first supply water pipe 521. The hot water also enters the absorption heat pump absorber 140 and the absorption heat pump condenser 120 in sequence via the second return water pipe 512, and then flows into the hot network after being heated.

[0073] Outside air 202 enters the second unit 332 of the three-stage heat exchanger through the air inlet pipe 201 for heating, and the heated outside air 202 enters the drying device 700 through the air inlet pipe 201 to dry the biomass fuel.

[0074] Furthermore, it also includes the low-pressure steam discharged from the steam turbine generator, which enters the steam-water heat exchanger 930 through the exhaust pipe 902 to release heat; the heating network water also enters the steam-water heat exchanger 930 through the third return water pipe 513 of the heating network and is heated before flowing into the heating network.

[0075] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A biomass combined heat and power flue gas waste heat recovery system, characterized by, The system comprises an absorption heat pump, a waste heat water system, a heat network water system, a flue gas system, a drying system and a biogas tank (600); The flue gas system comprises a flue gas pipe (301) and a first-stage heat exchanger (310), a second-stage heat exchanger (320), a third-stage heat exchanger first unit (331) and a third-stage heat exchanger second unit (332) connected in parallel at both ends of the third-stage heat exchanger first unit (331) and arranged in sequence along the flue gas conveying direction on the flue gas pipe (301); and the outlet of the flue gas pipe (301) is arranged below the liquid level of the biogas tank (600). The waste heat water system comprises a first-stage waste heat water circulation pipe (401) and a second-stage waste heat water circulation pipe (402); the outlet of the first-stage waste heat water circulation pipe (401) is connected with the inlet of an absorption heat pump generator (110), the outlet of the absorption heat pump generator (110) is connected with the inlet of the first-stage waste heat water circulation pipe (401), and a section of the first-stage waste heat water circulation pipe (401) is arranged in the first-stage heat exchanger (310); a section of the second-stage waste heat water circulation pipe (402) is arranged in the second-stage heat exchanger (320); the outlet of a third-stage waste heat water circulation pipe is connected with the inlet of an absorption heat pump evaporator (130), the outlet of the absorption heat pump evaporator (130) is connected with the inlet of the third-stage waste heat water circulation pipe, and a section of the third-stage waste heat water circulation pipe (403) is arranged in the third-stage heat exchanger first unit (331). The heat network water system comprises a heat network water first return pipe (511) and a heat network water first supply pipe (521); the outlet of the heat network water first return pipe (511) is connected with the inlet of the second-stage waste heat water circulation pipe (402), and the outlet of the second-stage waste heat water circulation pipe (402) is connected with the inlet of the heat network water first supply pipe (521); the heat network water system further comprises a heat network water second return pipe (512) and a heat network water second supply pipe (522); the heat network water second return pipe (512) is connected with the inlet of an absorption heat pump absorber (140), the outlet of the absorption heat pump absorber (140) is connected with the inlet of an absorption heat pump condenser (120) through the heat network water second return pipe (512), and the outlet of the absorption heat pump condenser (120) is connected with the heat network water second supply pipe (522). The drying system comprises an air inlet pipe (201) for sending external air (202) into a drying device (700); and a section of the air inlet pipe (201) is arranged in the third-stage heat exchanger second unit (332).

2. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 1, characterized in that, The system further comprises a steam system, which comprises a steam pipe (901); the inlet of the steam pipe (901) is connected with a biomass boiler (800), the outlet of the steam pipe (901) is connected with a steam turbine generator (910), the high-pressure steam outlet of the steam turbine generator is connected with an industrial steam end (920), the exhaust outlet of the steam turbine is connected with an exhaust steam pipe (902), and the outlet of the exhaust steam pipe (902) is connected with a steam-water heat exchanger (930).

3. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 2, characterized in that, The heat network water third return pipe (513) is connected with the inlet of the steam-water heat exchanger (930), and the outlet of the steam-water heat exchanger (930) is connected with the heat network water third supply pipe (523).

4. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 1, characterized in that, The absorption heat pump absorber (140), the absorption heat pump generator (110), the absorption heat pump evaporator (130) and the absorption heat pump condenser (120) are single-stage or multi-stage series structures.

5. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 1, characterized in that, The first-stage heat exchanger (310) and the second-stage heat exchanger (320) are partition wall heat exchangers, the third-stage heat exchanger first unit (331) is an immersed heat exchanger, and the third-stage heat exchanger second unit (332) is a partition wall heat exchanger.

6. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 1, characterized in that, The outlet of the drying device (700) is connected with the biomass boiler (800).

7. The system for recovering and utilizing the flue gas waste heat of a biomass combined heat and power plant according to claim 1, characterized in that, The absorption heat pump is a steam type lithium bromide absorption heat pump.

8. A method for recovering and utilizing the waste heat of the flue gas of a biomass combined heat and power plant, which is realized based on the system for recovering and utilizing the waste heat of the flue gas of a biomass combined heat and power plant according to claim 1, characterized in that, The biomass boiler (800) exhaust gas is sequentially introduced into the first-stage heat exchanger (310), the second-stage heat exchanger (320) and the third-stage heat exchanger through the exhaust pipe (301) to release heat, heat the first-stage waste heat water in the first-stage waste heat water circulation pipe (401) and the third-stage waste heat water in the third-stage waste heat water circulation pipe (403), and finally is discharged into a biogas tank. The first-stage waste heat water is introduced into the absorption heat pump generator (110) through the first-stage waste heat water circulation pipe (401) to be cooled, and then is returned to the first-stage heat exchanger (310) to be heated again. The third-stage waste heat water is introduced into the absorption heat pump evaporator (130) through the third-stage waste heat water circulation pipe (403) to be cooled, and then is returned to the third-stage heat exchanger first unit (331) to be heated again. The heat network water is introduced into the second-stage heat exchanger (320) through the heat network water first return pipe (511) and the second-stage waste heat water circulation pipe (402) to be heated, and then is introduced into the heat network water first supply pipe (521); the heat network water is also introduced into the absorption heat pump absorber (140) and the absorption heat pump condenser (120) through the heat network water second return pipe (512) to be heated, and then is introduced into the heat network. The ambient air (202) is introduced into the third-stage heat exchanger second unit (332) through the air inlet pipe (201) to be heated, and then is introduced into the drying device (700) through the air inlet pipe (201) to dry the biomass fuel. The low-pressure steam discharged by the steam turbine generator is introduced into the steam-water heat exchanger (930) through the exhaust pipe (902) to release heat, and then the heat network water is introduced into the steam-water heat exchanger (930) through the heat network water third return pipe (513) to be heated, and then is introduced into the heat network.

9. The method according to claim 8, wherein the biomass combined heat and power flue gas waste heat recovery method is characterized by, ​

Images