Patent Application
20230030209
The present invention refers to an energy conversion system, which converts gaseous fuels into electricity.
The Allam cycle is an oxy-combustion cycle in which a combination of pure oxygen and methane, natural gas or syngas is burnt at high pressure using a large flow rate of recycled CO2 as temperature moderator. The combustor working pressure is “supercritical” and a regenerator is used to preheat the recycled stream. The Allam cycle is reported to be able to achieve a 55-60% net electric efficiency with 100% CO2 capture. The cycle operates with a turbine inlet temperature of 1100-1300° C. (requiring blade cooling and directionally-solidified superalloys) and a regenerator inlet temperature of 650-750° C. (requiring special alloys, like Ni-based alloy materials such as Inconel 617).
Document U.S. Pat. No. 7,709,118 B2 discloses a regenerative atmospheric Solid Oxide Fuel Cell/Gas Turbine (SOFC-GT) hybrid cycle where the turbine exhaust gases are fed to the SOFC cathode. The fuel cell is placed downstream of the turbine, operating at atmospheric pressure. The gas turbine cycle uses air as working fluid without an Air Separation Unit (ASU) and not a CO2-rich mixture. The system does not separate and make available a stream of nearly pure CO2.
Document U.S. Pat. No. 7,306,871 B2 discloses a hybrid cycle integrating a regenerative gas turbine with a SOFC placed upstream of the combustor. This system uses air as working fluid without an air separation unit and an oxy-combustion unit. A CO2-rich recycle streams are not used as temperature moderator. As a result, this system does not make available a stream of nearly pure CO2.
Document U.S. Pat. No. 9,228,494 B2 discloses a hybrid SOFC-inverted Brayton cycle featuring atmospheric fuel cell and combustor. The SOFC anode outlet flow, available at atmospheric pressure, is used in the combustor. The system features a lower degree of integration between the thermodynamic cycle and the SOFC because the cathode outlet stream is not sent to the combustor as oxidizing agent. The cathode working pressure is atmospheric and the Brayton cycle configuration is the “inverted” one (with atmospheric pressure at the combustor and sub atmospheric pressure at the turbine outlet) without CO2-rich gas recycle and regenerator.
It is the object of the present invention to provide an energy conversion system, which converts gaseous fuels (for example, but not limited to, natural gas, syngas, biogas, biomethane, light hydrocarbons, hydrogen) or liquid fuels (for example, but not limited to, light hydrocarbons, alcohol, methanol, DME), into electricity with the possibility to capture the generated CO2 in an efficient way.
This and other objects are achieved by an energy conversion system according to the claim 1. The dependent claims define possible advantageous embodiments of the invention.
The idea underlying the invention consists of providing a system, which integrates a Solid Oxide Fuel Cell (SOFC) unit working in pressurized conditions up to high pressures (5-500 bar) with a semi-closed oxy-combustion cycle, which uses the heat and the unconverted fuel and oxidant streams discharged by the fuel cell. The oxy-combustion cycle is a semi-closed Brayton cycle featuring CO2 as the main working fluid. If the SOFC unit working pressure is above the critical one, the Brayton cycle becomes a supercritical CO2 cycle. Although with lower efficiency, the system can also work with maximum cycle pressures below the CO2 critical one (73.8 bar). Thus, the CO2 stream is the working fluid of both the thermodynamic cycle and the SOFC unit. Another peculiarity of the system is the use of the unconverted O2 discharged by the SOFC unit within the combustor of the semi-closed Brayton cycle.
The system is referred to as “Solid Oxide Semi-closed CO2 cycle—SOSCO2”.
An advantage of the present system is that the power plant has zero emissions of pollutants and greenhouse gases, if the separated CO2 stream is captured and stored.
Moreover, the claimed system can achieve net electric efficiencies (LHV basis) higher than conventional technologies (75% compared to 55-62% of natural gas combined cycles and 55-60% of the Allam cycle) thanks to the optimized integration with the SOFC unit.
The present system has a superior operational flexibility (i.e., more independent operative variables) compared to conventional gas turbine cycles, as it likely allows achieving higher part-load efficiency and lower minimum turndown ratio. Such operational flexibility is a very important feature for todays and future power plants due to the increasing penetration of intermittent renewables in the electric grid.
Furthermore, the present system provides a higher net electric efficiency (75% vs. 55-60%) which cannot be achieved by the Allam cycle and similar oxy-turbine cycles even using the most advanced gas turbine materials, such as advanced single crystal super alloys.
The operating conditions of the turbine are considerably less severe and suitable for uncooled turbines. As a comparison, in the Allam cycle, the turbine has an inlet temperature of 1100-1250° C. The claimed system, can achieve close-to-optimal efficiency with turbine inlet temperatures of 800-900° C., temperatures suitable for uncooled expander units and less expensive materials (more conventional super-alloys, avoiding the necessity of directionally solidified and single crystal blades), and slightly higher efficiency (with up to 1% increase) with turbine inlet temperatures of 1000-1100° C. with limited cooling. Very high efficiencies, above 70%, can be achieved even using an uncooled expander featuring an inlet temperature below 900° C.
Another advantage is the less severe operating temperatures of the regenerator placed at the outlet of the turbine compared to the Allam cycle. In the claimed system the optimal turbine outlet temperature is in the range 400-600° C., which are also here compatible with less costly materials (e.g., medium-high grade steel, ferritic stainless steels or conventional austenitic stainless steels). In the Allam cycle, the turbine discharges at temperatures in the range 650-750° C., making it necessary to use very expensive nickel-based alloys (e.g., Inconel 617).
At the same time the combustor operates with lower thermal duty (the majority of fuel chemical energy is converted into the fuel cell) and lower maximum temperatures, corresponding to the turbine inlet conditions, with respect to the Allam cycle.
In order to better comprehend the invention and appreciate the advantages thereof, some exemplifying non-limiting embodiments will be described in the following with reference to the attached figures, illustrating an energy conversion system according to possible embodiments of the invention.
For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.
The present disclosure, in at least one of the aforementioned aspects, can be implemented according to one or more of the following embodiments, optionally combined together.
For the purpose of the present description and of the appended claims, the words “a” or “an” should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. This is done merely for convenience and to give a general sense of the disclosure.
The energy conversion system according to the invention comprises:
In a first embodiment (
The energy conversion system can use either a gaseous or a liquid fuel 1. The fuel shall be pressurized, optionally preheated, then is fed to the anode of the SOFC unit A while the oxidant mixture containing CO2 and O2 4, preheated in a regenerative heat exchanger D, is fed to the SOFC cathode side.
The SOFC unit A converts a fraction of the chemical power (e.g., in the range 30-90%, tunable depending on the desired performance) of the fuel directly into electric power through the electrochemical reactions occurring on the anode and cathode sides and involving hydrogen oxidation:
H2+½ O2→H2O+2e− (anode) (Eq. 1)
½ O2+2e−→O= (cathode) (Eq. 2)
with the addition, although with a slower kinetics, of carbon monoxide oxidation:
CO+½ O2→CO2+2e− (anode) (Eq. 3)
The SOFC unit A can be designed to run either with or without internal reforming (e.g., adopting specific catalysts, such as Ni-based materials typically used for commercially available SOFCs), depending on the type of fuel to be used; in the case of natural gas feeding, methane is converted into hydrogen within the cell according to the reactions of steam reforming and water gas shift:
CH4+H2O→CO+3 H2 ΔH
CO+H2O→CO2+H2=ΔH
The reforming reaction, which is highly endothermic, occurs exploiting available heat from the cell losses (thus converting heat into chemical energy, with an advantage for the system electric efficiency) and is driven by the consumption of hydrogen allowed by the electrochemical reactions (Eq. 1,2).
Steam required for hydrocarbons reforming can be supplied directly stream 3 and/or through recycling a fraction of the stream exiting the anode 40. In the latter case, either an ejector (O) or a fan X capable of withstanding high gas temperatures can provide the pressure head required to sustain the stream recycle.
The compressed fuel entering the power plant can therefore be mixed with part of the stream recycled from the anode outlet 40 and/or with steam 3 (which can be generated in the heat exchanger D). Then, the stream entering the anode side can be preheated within the SOFC unit A to the final operating temperature (e.g., 700-850° C.) using the thermal power made available by the electrochemical process, through either a dedicated heat exchanger P (e.g., cooling the product streams) or internally in the fuel cell stack.
Alternatively, and following recent R&D tendencies for new types of intermediate and low temperature SOFC units (e.g. running from 600 to 800° C.), the SOFC can be designed to run directly on natural gas (or other high methane fraction hydrocarbon mixture), developing internally direct oxidation reactions:
CH4+4O=→H2O+CO2+8e− (anode) (Eq. 6)
2O2+8e−→4O= (cathode) (Eq. 7)
In this case the SOFC unit can be fed directly with methane without needing a preliminary mixing with steam or recycled anode exhaust.
The unconverted fuel and oxygen leaving the SOFC unit A are sent to the combustor unit B of the semi-closed cycle. Optionally, to moderate the flame temperature, it is possible to recycle a fraction of the combustion products (stream 23).
Vice versa, optionally, to increase the flame temperature and/or the combustor unit B outlet temperature, it is possible to inject fuel and/or oxidant directly into the combustor B additional fuel 7 and oxidant 8. To increase the efficiency of the proposed system, as a further option, it is possible to preheat the oxidant 26, the temperature moderator 9 and the other streams of the system (e.g., the turbine cooling flows 11, if these are required) in a multi-flow regenerator D. Optionally, if the concentration of fuel and/or oxygen is too low, a catalytic combustor unit B can also be used.
The combustion products 10 are mainly CO2 and H2O, and may contain also some amounts of O2, Ar, N2 and other chemical species.
The product gases are expanded in an expander C to a lower pressure, indicatively in the range 1-50 bar. However, such value depends on the other pressures and temperatures of the cycle, and it may not be limited to such range. Depending on the fuel utilization factor (fraction of the inlet fuel oxidized electrochemically within the fuel cell) of the SOFC unit A, its operative temperature, and the mass flow rate of the stream used as temperature moderator in the combustor unit B, the turbine inlet temperature can be higher or lower, requiring to adopt a cooled or uncooled expander C.
If the expander unit C needs to be cooled (because the gas inlet temperature is above the maximum allowed operating temperature of the turbine materials), the cooling flows 11 can be taken from the stream of recycled CO2 and can be preheated in the regenerator D. The product gases 10 leaving the turbine are cooled in the regenerator D and then in a cooler E to a temperature approaching that of the heat sink (e.g., lake, river, sea, air, cold streams of other plants). Most of the H2O of the product gases condenses, and it is separated with a gas-liquid separator F, such as a flash drum. The outlet gas stream leaving the separator 16 is rich in CO2. A fraction can be recycled 18 to be used as temperature moderator and/or to be mixed with the oxygen stream 21 and/or used as turbine cooling flow 11 while the remaining part 17 can be separated and either vented into the atmosphere or sent to the CO2 purification and utilization/storage system. When compressing the recycle CO2-rich stream above the critical pressure, if water condenses, it is possible to use a liquid-liquid separation process to remove further water.
Depending on the specifications of the final destination of CO2 (e.g., an injection well for enhanced oil recovery or any other storage or utilization option), it could be necessary to treat the separated CO2 stream in a CO2 purification unit (a conventional plant capable of producing nearly pure liquid CO2).
It is also possible to recover heat in the regenerator D from the main compressor of the Air Separation Unit (ASU) and/or the intercoolers of the compressors H,K,L and/or from nearby heat sources. This can result in a further improvement of the efficiency of the proposed energy conversion system.
For systems using natural gas and 99.5% purity O2 produced by a commercially available cryogenic air separation unit, the best performance of the system is achieved adopting the operating parameters reported in Table 1.
It is worth noting that the system is capable of working with good efficiencies also if the operating conditions are outside the above-specified ranges, even if the SOFC unit A and combustor unit B pressures are below the critical pressure of CO2.
In another exemplary embodiment, it also possible to use a second oxidation stage after the expansion (reheating configuration), adding a second SOFC unit AF and/or a second combustor unit AG optionally feeding additional fuel 61 and oxidant 62. Then the working fluid can be further expanded in a second expander AH before entering the regenerator AI (see
The cycle shown in
The assumptions at the basis of the process simulation are reported in the following:
As far as the SOFC unit is concerned, the assumptions are the following:
Under the above-listed assumption, the proposed system still can be designed and operated in a large variety of conditions due to the possibility of varying (i) the SOFC A unit and combustor B operative pressure, (ii) the expander C outlet pressure, (iii) the fraction of the unconverted fuel 40 recycled back to the anode inlet, (iv) the mass flow rate of the temperature moderator 9 of the combustor, (v) the regenerator outlet temperature of the oxidant 26, temperature moderator 9 and steam 3, (vi) the regenerator outlet temperature of the expander cooling flows 11, and (vii) the fraction of the recycled 20 stream mixed with the oxygen to produce the oxidant flow. To determine the most efficient design and operating conditions, the above listed independent variables have been optimized using a systematic process optimization approach.
The objective function to be maximized is the net electric efficiency (net electric power output of the integrated system divided the chemical power of the inlet fuel, LHV basis). The optimization constraints considered in this example are summarized in Table 3:
In the optimization it is assumed that the oxidant, the temperature moderator and the steam for the SOFC unit exit the regenerator at the same temperature. Moreover, it is assumed that the oxidant provides 3% excess of oxygen compared to the stoichiometric condition.
The optimization problem has been tackled using an optimization algorithm specifically developed for process and energy system optimization purposes.
The optimization results are reported in Table 4:
The SOFC produces 72.7% of the plant gross power output while the turbine accounts for the remaining 27.3%.
The intercooled compression and the ASU are the two major penalties, consuming 5.5% and 12.0% of the gross power output respectively.
The compression of the captured CO2, sent to the storage, account only for 0.8% percent and the remaining auxiliaries for 0.9% of the gross production.
The resulting net electric efficiency is 76.2% without CO2 capture (i.e., venting the excess CO2 not recycled), and 75.4% with CO2 capture.
The resulting performance indexes are outstanding compared to state-of-the-art as well as advanced energy systems (with and without CO2 capture) which feature efficiencies in the range 60-63% for the systems without capture, and 40-46% for the systems with capture.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102019000024162 | Dec 2019 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/081054 | 11/5/2020 | WO |
