Method and apparatus for improving the energy efficiency of a biomass power plant
By preheating condensed steam with exhaust fumes before degassing, the method reduces steam consumption for degassing, enhancing biomass power plant efficiency and maximizing turbine steam usage.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- CIRQLAR TECH SRL
- Filing Date
- 2025-12-04
- Publication Date
- 2026-06-18
AI Technical Summary
Biomass power plants suffer from reduced efficiency due to the use of steam for degassing condensed steam, which is not transformed into electricity, leading to a lower overall system efficiency.
Introduce a condensed steam preheater using exhaust fumes from the steam generator to raise the temperature of condensed steam before degassing, reducing the amount of steam required for degassing and maximizing the steam available for the turbine.
Enhances energy efficiency by minimizing steam usage for degassing, thereby increasing the steam processed by the turbine and optimizing the energy conversion process.
Smart Images
Figure IB2025062413_18062026_PF_FP_ABST
Abstract
Description
[0001] METHOD AND APPARATUS FOR IMPROVING THE ENERGY EFFICIENCY OF A BIOMASS POWER PLANT
[0002] The present invention relates to a method and a plant for improving the energy efficiency of a biomass power plant. More specifically, the invention relates to a method and a plant for heat recovery between the exhaust fumes of the biomass steam generator and the condensate downstream of the steam turbine.
[0003] Background of the invention
[0004] Biomass power plants are a key element in the production of “green” electricity and represent an important resource for protecting the environment. In fact, biomass is a renewable source and as such allows to reduce dependence on fossil fuels. In general, the term “biomass” refers to any organic material derived from plants, animals, municipal organic waste and the like, which can be burned in suitable combustors to produce steam and convert thermal energy into mechanical energy in a turbine, with the consequent driving of an electric power generator.
[0005] Steam-based electricity generation is based on a cycle in which water is heated to become steam, which drives a turbine coupled to an electric generator. At the outlet of the turbine, the steam is introduced into a condenser, where it is cooled until it returns to the liquid state.
[0006] For the sustainability of the cycle and the correct operation of the process and the plant, the water must possess certain qualitative characteristics, both in terms of mineral salt content and volatile and gaseous substances. Therefore, the water fed to the steam generator must be appropriately conditioned and degassed before being introduced into the heat exchanger that converts it into steam.
[0007] State of the art
[0008] As is known, and as mentioned above, a biomass power plant comprises a steam generator in which water is heated by the combustion of biomass and the steam thus produced is introduced, at an appropriate pressure, into a turbine that drives an electric power generator. Downstream of the turbine, the condensed steam is degassed, preheated in one or more heat exchangers and sent back as feed water to the steam generator to restart the cycle.
[0009] The biomass combustion fumes are appropriately treated to reduce dust and pollutants and, before being released into the atmosphere, are sent to the heat exchanger which preheats the feed water to the steam generator, thus recovering the residual heat from the combustion fumes. The degassing of the condensed steam, i.e., the water, downstream of the turbine is achieved by means of stripping, i.e., by removing the gases contained therein by means of a steam stream produced by the steam generator and taken from an intermediate stage of the steam turbine. The energy associated with this degassing steam, removed from the machine, is therefore not transformed into electricity, resulting in a lower efficiency of the entire system.
[0010] It would therefore be desirable to improve the operating efficiency of a biomass power plant, for example by minimizing the quantity of steam used for degassing the condensed steam. This would consequently allow maximizing the quantity of steam processed entirely in the turbine. Summary of the invention
[0011] An aspect of the present invention therefore relates to a method for improving the energy efficiency of a biomass power plant, comprising the following steps: a) generating steam in a biomass-fired steam generator configured to feed steam into a steam turbine and discharging combustion fumes, which are treated to remove dust and pollutants; b) generating electricity in a generator coupled to said steam turbine; c) condensing the steam discharged from said steam turbine in a condenser configured to produce condensed steam at a temperature Ti; d) degassing said condensed steam in a degasser configured to produce degassed water by stripping it with a portion of the steam produced by said steam generator and extracted from an intermediate stage of the turbine; e) preheating said degassed water from said degasser to a temperature T3 by means of the combustion fumes from said steam generator; f) further preheating said water from said step e) to a temperature T4 by means of steam; g) feeding said steam generator with said water preheated to said temperature T4; characterized in that it comprises a step h) in which said condensed steam at temperature Ti produced by said condenser in said step c) is preheated to a temperature T2 by means of the exhaust fumes of the steam generator and sent to said degasser to carry out said step d), wherein T4 > T3 > T2 > Ti; whereby the quantity of steam required for stripping in said step d) in which said condensed steam is fed at temperature T2 is less than the quantity of steam that would be necessary to degas said condensed steam at temperature Ti in the absence of said preheating step h) to the temperature T2.
[0012] Another aspect of the present invention relates to a plant for a biomass power plant, comprising: - a biomass-fired steam generator, configured to feed steam into a steam turbine and discharge biomass combustion fumes, which are treated in dust and pollutant abatement units located downstream of the steam generator; an electric power generator coupled to said steam turbine; a condenser of the steam discharged from said steam turbine, configured to produce condensed steam; a degasser of said condensed steam, configured to produce degassed water by stripping with a portion of the steam produced by said steam turbine and taken from an intermediate stage of said steam turbine; a first preheater of degassed water, comprising a heat exchanger connected to said degasser, configured to perform a first preheating of said degassed water by means of the combustion fumes of the steam generator; a second preheater of degassed water, comprising a heat exchanger in which a second preheating of said degassed water coming from said first preheater is performed, said second preheater being configured to use steam, with production of further preheated water; means for feeding said further preheated water to said steam generator, characterized by comprising: a condensed steam preheater, consisting of a heat exchanger configured to receive said condensed steam from said steam condenser, preheat said condensed steam by means of the combustion fumes of the steam generator and send said preheated condensed steam to said degasser.
[0013] In the present description, the term “biomass” refers to any organic material derived from plants, animals, municipal organic waste and the like, which can be burned in suitable combustors to produce steam. The preferred biomass is firewood reduced to chips ranging in size from a few millimetres to a few centimetres, produced from logs and twigs, called “chippings”.
[0014] In the present description the terms “condensed steam” and “water” referring to steps c) - h) are used interchangeably, both with reference to the method and to the plant.
[0015] In the present description, the terms “preheater” and “heat exchanger” are used interchangeably with reference to the preheating of the steam generator feed water.
[0016] In the present description, the terms “comprising” and “containing” are used interchangeably, the meaning of which does not exclude the presence of other elements, in addition to those defined after such terms. The term “consisting of’ is therefore also included within this meaning. The terms “comprising” and “containing” have a broader meaning than “consisting of’, but do not exclude it.
[0017] Brief description of the figures
[0018] The invention is described below with reference to the attached figures, in which: - Fig. 1 is a simplified block diagram of an operating method of a biomass power plant according to the prior art;
[0019] - Figure 2 is a simplified block diagram of an operating method of a biomass power plant according to an embodiment of the invention; and
[0020] - Fig. 3 is a simplified diagram of a biomass power plant according to an embodiment of the invention.
[0021] Detailed description of the invention
[0022] With reference to the simplified diagram shown in Figure 1, an operating method of a biomass power plant according to the prior art involves the following steps:
[0023] Step a)
[0024] A biomass 10, consisting for example of chippings, is introduced into a steam generator 14 together with air 12 and burned. Step a) therefore consists of the generation of steam produced from the combustion heat of the biomass. The combustion fumes are treated to abate dust and pollutants in a series of equipment designated overall with the numerical reference 16.
[0025] Step b)
[0026] The steam produced in step a) is fed into a steam turbine 18 by means of a line 11. The turbine 18 is coupled to an electric power generator 20 to produce electricity, according to known methods.
[0027] Step c)
[0028] The steam discharged from the steam turbine 18 in step b) is sent by means of the line 21 into an air condenser 22, where it condenses by cooling, forming water at a temperature Ti.
[0029] Step d)
[0030] In order to be used as feed water for the steam generator 14, the steam condensed at temperature Ti is sent by means of the line 13 to a degasser 24, where it is subjected to degassing, in particular de-aeration. The degassing occurs by means of stripping with a portion of the steam produced by the steam generator 14, which is introduced into the steam turbine 18 from the line 11 and of which the portion intended for stripping is extracted from the steam turbine by means of the line 15, at an intermediate stage upstream of the turbine exhaust.
[0031] Step e)
[0032] The degassed water coming from the degasser 24 is preheated to a temperature T3 in a heat exchanger 26 by means of the combustion fumes coming from the purification treatments carried out in the equipment 16, introduced into the exchanger 26 by means of the duct 17.
[0033] Step f) The water preheated to temperature T3 in step e) is further preheated to a temperature T4 in an exchanger 28, in which the heating medium is steam.
[0034] Step g)
[0035] The water preheated to temperature T4 is fed back to the steam generation step a) by means of the line 19, thus closing the cycle. The use of new water is only required to compensate for any losses and replenish the flow required for the operation of the plant.
[0036] The combustion fumes downstream of the exchanger 26 are sucked into the duct 29 by a fan 32 and introduced into the chimney 34 to be discharged into the atmosphere.
[0037] With reference to Figures 2 and 3, an embodiment of the essential components of a biomass power plant according to the invention and the related operation method will now be described. Fig. 2 shares the same reference numbers as Fig. 1 for the same components present in a biomass power plant according to the known art. For such components, please refer to the description provided in relation to Fig. 1. The components according to the invention are also shown.
[0038] In an aspect of the invention, the quantity of steam used for degassing the condensed steam in the degasser 24 has been reduced or minimized, with the important advantage that the quantity of steam processed entirely by the turbine 18 is maximized.
[0039] This result is achieved by introducing a further combustion exhaust heat exchanger 30 in the plant, placed downstream - with respect to the exhaust fume flow - of the heat exchanger 26. Unlike what is shown in Fig. 1, the steam condensed in the condenser 22 is not sent directly to the degasser 24 but is sent to the heat exchanger 30 by means of the line 23. The condensed steam enters the heat exchanger 30 at the temperature Ti, exits heated at the temperature T2 and is sent by means of the line 25 to the degasser 24. Thanks to the higher temperature of the water (condensed steam) with respect to the known solution shown in Fig. 1, a smaller quantity of steam taken from the line 15 is used to perform the stripping in the degasser 24, thus making a greater quantity of steam available to drive the turbine 18.
[0040] The degassed condensed steam, which satisfies the compositional characteristics required for the steam generator feed water, is sent, by means of the line 27, to the heat exchanger 26, which heats the feed water to the temperature T3. The feed water is further sent to the exchanger 28, in which the heating medium is steam, which further raises the temperature of the water to the value T4, to then be re-fed to the steam generation step a) by means of the line 19, thus closing the cycle. Therefore, the relationship T4 > T3 > T2 > Ti exists between the temperatures of the condensed steam, or feed water to the steam generator.
[0041] As regards the operating method of the biomass power plant according to the invention, it therefore comprises, in addition to steps a) - g), a step h) which consists in preheating to a temperature T2 the condensed steam produced in step c), in which such preheating is achieved by exploiting the residual heat of the exhaust fumes of the steam generator produced in step a), and in sending such condensed steam at a temperature T2 to the degasser 24 for carrying out said step d). Thanks to the higher temperature of the water (condensed steam) in the degasser 24, it is therefore possible to reduce the quantity of steam required for stripping, thus making a greater quantity of steam available for the turbine 18.
[0042] According to an aspect of the invention, the temperature of the combustion fumes in the heat exchanger 30, which is of the tube bundle type, must not be too low in order to avoid the condensation of any acid compounds such as SOx, chlorides, fluorides and the like, which may still be present as residues in the combustion fumes after the abatement treatments carried out in the equipment 16. In fact, the condensed acid compounds would exert a localised corrosive action on the tubes of the tube bundle within which the condensed steam coming from condenser 22 flows.
[0043] To avoid such condensation, the temperature T5 of the combustion gases in the heat exchanger 30 must be kept above the dew point, defined here as Tdp, i.e., T5 > Tdp. Preferably, the temperature T5 is at least 10°C above the dew point, i.e., T5 > Tdp + 10°C. This operating condition is achieved by regulating the water coming from the condenser 22 both in terms of flow rate and temperature, so that the extent of cooling the combustion fumes respects the condition T5 > Tdp, preferably T5 > Tdp + 10°C. A system is therefore present to monitor the chemical composition of the combustion fumes, on the basis of which the Tdp is calculated, and the temperature of the condensed steam. The quantity and temperature of the water introduced into exchanger 30 are then regulated based on the Tdp and the temperature of the condensed steam. This monitoring is carried out with known devices, which are therefore not described herein.
[0044] This regulation is performed in two modes: i. diverting a portion of the water at temperature Ti coming from the condenser 22 from the line 23 directly to the line 25, according to the arrow Ain Fig. 2 and 3, thus bypassing the preheater 30; ii. diverting a portion of the water at temperature T2 downstream of the preheater 30 onto the line 23, according to the arrow B in Fig. 2 and 3.
[0045] These flow regulations are achieved with known valve systems, which are therefore not described herein.
[0046] The mode i) allows to reduce the cooling of the combustion fumes in the exchanger 30 by introducing less water at the temperature Ti , i.e. less “cold” water, while mode ii) allows to reduce the localised cooling of the combustion fumes in the exchanger 30 in contact with the cold water in input by mixing water at the temperature T2 , i.e. “hot” water, in the line 23 upstream of the exchanger 30, thus raising the temperature of the water introduced into the heat exchanger 30 at the temperature Tf high enough to avoid localised acid condensation, where Tf > Ti.
[0047] The modes i) and ii) can also be implemented simultaneously, if necessary to satisfy the condition T5 > Tdp, or T5 > Tdp + 10°C, coupled with T input to the exchanger 30 = Tf.
[0048] According to an aspect of the invention, the temperature T2 of the condensed steam introduced into the degasser 24 must not be too high; more precisely, it must not be too close to the saturation temperature Ts of the liquid inside the degasser, otherwise the degassing would be compromised.
[0049] Preferably, the temperature T2 of the condensed steam introduced into the degasser 24 is at least 5°C lower than the saturation temperature Ts of the liquid, i.e. T2 + 5°C < Ts. More preferably, the temperature T2 of the condensed steam introduced into the degasser 24 is at least 10°C lower than the saturation temperature Ts of the liquid, i.e. T2 + 10°C < Ts.
[0050] The heat exchanger 30 must therefore be managed in such a way as to not only prevent the temperature of the combustion fumes from getting too close to the dew point Tdp of the combustion fumes, but also in such a way as prevent the temperature T2 of the condensed steam from getting too close to the saturation temperature Ts in the degasser 24. This regulation is also made according to the method i) described above.
[0051] As in the diagram of Fig. 1, the combustion fumes downstream of the exchanger 26 are sucked into the duct 29 by a fan 32 and introduced into the chimney 34 to be discharged into the atmosphere, but at a lower temperature, as they have been further used in the heat exchanger 30 to preheat the condensed steam, without however reaching the dew point temperature Tdp in the exchanger 30.
[0052] In an embodiment, the method of the invention is carried out with a temperature Ti of the condensed steam of approximately 40 - 50°C, which is mixed with the hot water in output from the exchanger 30 to reach a value T of approximately 70 degrees in input and then preheated in the heat exchanger 30 to a temperature T2 of approximately 90-110°C. Typically, the temperature T2 is more than 10°C lower than the saturation temperature Ts in the degasser 24. Downstream of the heat exchanger 26, the water temperature T3 is approximately 140-150°C, and downstream of the heat exchanger 28, the temperature T4 of the steam generator feed water is, for example, approximately 160-170°C. The temperature T5 of the combustion fumes in output from the heat exchanger 30, and consequently in the chimney 34, is, for example, 120°C, while in a biomass power plant according to the prior art, i.e. without a condensed steam preheater 30 and a system for calculating the dew point temperature and consequently regulating the condensate flow rate, it is approximately 140°C. It was thus possible to use part of the residual heat from the combustion fumes to preheat the condensed steam and reduce the quantity of steam required in the stripping step d), since it was possible to release less hot combustion fumes into the atmosphere without risking acid corrosion.
[0053] Another aspect of the invention is a plant for a biomass power plant, the main components of which are also shown in Figure 3.
[0054] With reference to Fig. 3, the plant comprises a conveyor belt 40 and a hopper 42 for loading a biomass 10 in the form of chippings into the steam generator, collectively indicated with the numerical reference 14. The chippings are combusted by blowing air 12.
[0055] The steam generator 14 contains a tube bundle 44 into which the feed water is introduced by means of a line 19. A line 11 withdraws the steam produced in the generator 14.
[0056] A series of equipment designated overall with the numerical reference 16, placed downstream of the steam generator 14, treats the combustion fumes 41 for the abatement of dust and pollutants.
[0057] The purified fumes 43 are then fed into the exchanger 26 by means of the duct 17.
[0058] The plant comprises the turbine 18 coupled to an electric power generator 20, as is known in the art. The turbine 18 is driven by the steam fed by means of the line 11.
[0059] A line 21 conveys the steam discharged from the turbine 18 to the air condenser 22, configured to produce condensed steam.
[0060] A line 23 conveys the condensed steam to a condensed steam preheater, consisting of a heat exchanger 30, comprising a tube bundle inserted in the combustion fume exhaust duct.
[0061] A line 25 carries the preheated condensed steam in the heat exchanger 30 to the degasser indicated overall with the numerical reference 24, configured to produce degassed water by means of stripping with a portion of the steam produced by the steam generator. The degasser 24 is therefore connected to a line 15 which withdraws steam from an intermediate stage of the turbine 18 to strip the air and gases dissolved in the condensed steam.
[0062] A line 27 withdraws the condensed and degassed steam from the degasser 24 and introduces it into a degassed water preheater, consisting of a heat exchanger 26 comprising a tube bundle inserted in the combustion fume exhaust duct.
[0063] A line 45 withdraws the preheated water in the heat exchanger 26 and transports it to a second degassed water preheater, consisting of a heat exchanger 28, in which the heating medium is steam. A line 19 feeds further preheated water, or feed water, into the steam generator 14. The plant further comprises: a system for monitoring the chemical composition of the fumes downstream of the purification equipment 16, configured to calculate the dew point temperature Tdp of the fumes fed into the condensed steam preheater 30; hydraulic connection means between the line 23 which conveys the steam to the heat exchanger 30 and the line 25 which carries the preheated condensed steam in the heat exchanger 30 to the degasser 24; means for controlling the flow of condensed steam in the lines 23 and 25 and for diverting the flow of such condensed steam from the line 23 to the line 25 upstream of the heat exchanger 30 (arrow A in Fig. 3). In an embodiment, the means for controlling and diverting the flow of condensed steam between the lines 23 and 25 consist of 3 -way valves. means for controlling the diversion of part of the condensed steam flow from the line 25 to the line 23 upstream of the heat exchanger 30 (arrow B in Fig. 3). In an embodiment, the means for controlling and diverting the flow of condensed steam between lines 25 and 23 consist of recirculation pumps and 2-way control valves.
[0064] The system for monitoring the chemical composition of the fumes and the means for controlling and diverting the flow of condensed steam between the lines 23 and 25 allow for the advantageous implementation of the operating method of the plant described above, both in order to avoid the condensation of any acid compounds in the combustion fumes passing through the heat exchanger 30, and to make maximum use of the residual heat of the fumes to preheat the condensed steam which is sent to the degasser 24, with less use of steam for stripping. This allows the quantity of steam used by the turbine 18 to be maximized.
Claims
CLAIMS1. Method for improving the energy efficiency of a biomass power plant, comprising the following steps: a) generating steam in a biomass-fired steam generator configured to feed steam into a steam turbine and discharging combustion fumes, which are treated to remove dust and pollutants; b) generating electricity in a generator coupled to said steam turbine; c) condensing the steam discharged from said steam turbine in a condenser configured to produce condensed steam at a temperature Ti; d) degassing said condensed steam in a degasser configured to produce degassed water by stripping it with a portion of the steam produced by said steam generator and extracted from an intermediate stage of the turbine; e) preheating said degassed water from said degasser to a temperature T3 by means of the combustion fumes from said steam generator; f) further preheating said water from said stage e) to a temperature T4 by means of steam; g) feeding said steam generator with said water preheated to said temperature T4; characterized in that it comprises a step h) in which said condensed steam at temperature Ti produced by said condenser in said step c) is preheated to a temperature T2 by means of the exhaust fumes of the steam generator and sent to said degasser to carry out said step d), wherein T4 > T3 > T2 > Ti; whereby the quantity of steam required for stripping in said step d) in which said condensed steam is fed at temperature T2 is less than the quantity of steam that would be necessary to degas said condensed steam at temperature Ti in the absence of said preheating step h) to said temperature T2.
2. Method according to claim 1, characterized in that said exhaust fumes in said step h) are maintained at a temperature T5 above the dew point (Tdp) according to the relationship T5 > Tdp, preferably T5 > Tdp + 10°C, by regulating the quantity of said condensed steam at temperature Ti to be preheated to temperature T2 in said step h).
3. Method according to claim 1 or 2, characterized in that said exhaust fumes in said step h) are maintained at a temperature T5 above the dew point (Tdp) according to the relationship T5 > Tdp, preferably T5 > Tdp + 10°C, by raising the temperature of said condensed steam from temperature Ti to temperature Tf, where T > Ti, before carrying out the preheating to temperature T2 in said step h).
4. Method according to claim 2, characterized in that said regulation of the quantity of steam condensed at temperature Ti to be preheated to temperature T2 comprises the exclusionof a part of said condensed steam at temperature Ti from the preheating step to temperature T2 and the introduction of said part of steam excluded from preheating directly to said degassing step d).
5. Method according to claim 3, characterized in that said raising of the temperature of the water of said condensed steam from temperature Ti to temperature T comprises the mixing of the condensed steam at temperature T 1 with a part of the steam preheated at temperature T2, taken downstream of said preheating step.
6. Plant for a biomass power plant, comprising:- a biomass-fired steam generator (14), configured to feed steam into a steam turbine (18) and discharge biomass combustion fumes, which are treated in dust and pollutant abatement units (16) located downstream of the steam generator (14);- an electric power generator (20) coupled to said steam turbine (18);- a condenser (22) of the steam discharged from said steam turbine (18), configured to produce condensed steam;- a degasser (24) of said condensed steam, configured to produce degassed water by stripping with a portion of the steam produced by said steam turbine (18) and taken from an intermediate stage of said steam turbine;- a first preheater (26) of degassed water, comprising a heat exchanger connected to said degasser (24), configured to perform a first preheating of said degassed water by means of the combustion fumes of the steam generator (14);- a second preheater (28) of degassed water, comprising a heat exchanger in which a second preheating of said degassed water coming from said first preheater (26) is performed, said second preheater being configured to use steam, with production of further preheated water;- means for feeding said further preheated water to said steam generator (14), characterized by comprising:- a condensed steam preheater (30), consisting of a heat exchanger configured to receive said condensed steam in said steam condenser (24), preheat said condensed steam by means of biomass combustion fumes and send said preheated condensed steam to said degasser (24).
7. Plant according to claim 6, characterized by comprising a system for monitoring the chemical composition of the fumes downstream of the purification equipment (16), configured to calculate the dew point temperature (Tdp) of the fumes fed into the condensed steam preheater (30).
8. Plant according to claim 6 or 7, characterised by comprising hydraulic connection means between the line (23) which conveys the condensed steam to said condensed steam preheater (30) and the line (25) which conveys the preheated condensed steam from the preheater (30) to the degasser (24).
9. Plant according to one or more of claims 6 to 8, characterised in that it comprises means for controlling the flow of condensed steam in said line (23) which conveys the steam to said condensed steam preheater (30) and said line (25) which conveys the preheated condensed steam from the preheater (30) to the degasser (24).
10. Plant according to claim 9, characterized in that said means for controlling and diverting the flow of condensed steam comprises 3-way control valves, recirculation pumps and two-way control valves.