Patent Application
20230265794
The present invention relates to tail gas recycle in a combined cycle power plant.
All patents, patent applications and other publications referred to herein are specifically incorporated herein by reference in their entirety.
Among its many known uses, synthesis gas (syngas) can be used as the fuel in gas turbine driven power plants. Synthesis gas (syngas) is a gas mixture comprising primarily hydrogen (H2), carbon monoxide (CO), water (H2O) and carbon dioxide (CO2), with minor amounts of other compounds (e.g., nitrogen, argon, hydrogen sulfide and methane). It can be produced by a number of known methods, including but not limited to coal gasification, steam methane reforming (SMR) and autothermal reforming (ATR).
In a gas turbine syngas is fed as a fuel together with air. There are three main turbine components:
1. An upstream axial rotating gas compressor section;
2. A downstream turbine expansion section on a common shaft with the compressor;
3. A combustion chamber or area, called a combustor, in between 1. and 2. above.
Atmospheric air flows through the compressor that brings it to higher pressure. Fuel is mixed with the air in a combustor wherein it is ignited to produce high temperature working fluid. In the case of syngas as fuel, energy is added by spraying syngas into the air and igniting it. This high-temperature high-pressure working fluid enters a turbine where it expands down to an exhaust pressure, producing shaft work output in the process. The turbine shaft work is used to drive the compressor; the energy that is not used to drive the compressor exits in the exhaust gases to produce thrust (power). The purpose of the gas turbine determines the design so that the most desirable split of energy between the thrust and the shaft work is achieved.
In the steam methane reforming process, a mixture of water and hydrocarbon, typically natural gas, are contacted at a high temperature, for example, in the range of about 850° to about 900° C., and typically in the presence of a catalyst, to form a mixture of hydrogen and carbon monoxide. Using methane as the hydrocarbon, the theoretical stoichiometry for the steam reforming reaction is as follows:
CH4+H2OCO+3H2
The primary purpose of the steam methane reforming process is to produce hydrogen. To increase the production of hydrogen, it is common practice to subject the hydrogen and carbon monoxide mixture to the water gas shift reaction (WGSR) as follows:
CO+H2OCO2+H2
As illustrated in U.S. Pat. Nos. 9,782,718 and 3,965,675, a gas turbine can be combined with a steam turbine. In this “combined” system, hot exhaust from the gas turbine produces steam in a heat recovery steam generator for feeding as a working fluid to a steam turbine. In the combined system, each of the gas turbine and the steam turbine can be utilized to produce electricity.
Gas turbine power plants and combined gas turbine/steam turbine power plants known as combined cycle power plants (CC) can use the burning of fossil fuel to generate required heat. These systems have known drawbacks, for example harmful air emissions. Natural gas combined cycle power plants (NGCC) produce large quantities of pollutants, especially carbon dioxide. Coal fired integrated gasification combined cycle plants (IGCC) add sulfur oxides, mercury and fine particles. These drawbacks are typically addressed by adding expensive, energy-intensive equipment to reduce or clean up emissions after they are produced. However, the required systems degrade performance, reliability and increase the cost of electricity and the total cost of the power plant. They are expensive to build, complex and energy intensive.
For example, regarding IGCC coal-based plants, see “Commercial Power Production based on Gasification | net1 . . . ”, https://www.net1.doe.gov>energy-systems>gasification>gasifipedia>igcc, [Retrieved Sep. 22, 2019], National Energy Technology Laboratory (NETL), wherein it states,
“Some important challenges to the wide-spread adoption of IGCC technology include cost, availability, and complexity. Cost is widely cited as the greatest barrier to IGCC acceptance. Capital costs for IGCC are high compared with alternative power plant designs, particularly NGCC, and financial viability is often dependent upon subsidies or tax credits. As a relatively new technology relative to pulverized coal power plants (PC) and NGCC, development and design costs are much higher for IGCC. Availability also impacts operating costs and must be high enough to compete favorably with the conventional alternatives. The complexity of IGCC relative to older, more established plant designs also increases operating costs and can impact availability and the generation of capital for plant development. These challenges to gasification, with a focus towards IGCC, are discussed in the introductory discussion about gasification.” See also, “The Three Factors That Doomed Kemper County IGCC”, spectrum.ieee.org/energywise/energy/fossil-fuels/ . . . , Jun. 30, 2017, IEEE Spectrum, [Retrieved Sep. 22, 2019.
In light of the noted problems related to IGCC power plants, and with the advent of relatively cheap natural gas, power plant companies are turning to NGCC power plants. Since natural gas (NG) became readily available as fuel for gas turbines, it was fed directly to the combustion chamber. A non-limiting illustrative example of an NGCC power plant is schematically shown in
With reference to
The present invention relates to a novel process for operating NGCC power plants. According to an embodiment, the process comprises:
According to another embodiment, the process comprises:
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.
All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated herein by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, as well as, any range formed within a specified range, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. For example, recitation of 1-5 is intended to include all integers including and between 1 and 5 and all fractions and decimals between 1 and 5, e.g., 1, 1.1, 1.2, 1.3 etc. It is not intended that the scope of the invention be limited to the specific values recited when defining a specific range. Similarly, recitation of at least about or up to about a number is intended to include that number and all integers, fractions and decimals greater than or up to that number as indicated. For example, at least 5 is intended to include 5 and all fractions and decimals above 5, e.g., 5.1, 5.2, 5.3 etc.
It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise. Unless otherwise expressly indicated herein, all amounts are based on volume.
With reference to illustrative
There are two advantages with membrane separation of carbon as carbon monoxide from unshifted syngas. When feeding unshifted syngas 1b from 1a to membrane separator means 2, membrane selectivity for H2/CO is substantially higher than H2/CO2. In simulations conducted on commercial membranes provided by commercial membrane manufacturers Generon and Ube Industries, Ltd., the Generon membrane, showed a H2/CO selectivity of 38.7 and H2/CO2 selectivity of 2.5 while an Ube membrane, showed a H2/CO selectivity of 27.7 and a H2/CO2 selectivity of 3.2.
Second, in simulations the Generon membrane optimized for H2 production captured 90.42% of the CO in the retentate along with 8.20% of the H2 and recovered 91.80% of the H2 in the permeate along with 9.58% of the CO. According to an embodiment, the captured CO in the retentate is directed to the oxyfuel combined cycle wherein it is captured as high purity CO2 in the exhaust stream. According to simulations, permeate which is directed to H2 production contains 9.58% of the CO and needs substantially less steam (energy penalty) and capital equipment (cost penalty) in a WGSR to convert the CO to CO2. Thus, the WGSR energy and capital cost penalties can be reduced by over 90% by separating CO and H2 in unshifted syngas.
Syngas feed compositions are well known in the art and can vary depending on the source. By way of nonlimiting example, it is believed that syngas feed 1b can comprise H2, CO2, CO, CH4 and H2O in the following amounts. The H2 content can be about 20-80. The CO2 content can be about 2-25%. The CO content can be about 20-60%. The H2O content can be up to 20%. The CH4 content can typically be about 0.1%-2.0%. It is understood that the syngas feed 1b may contain minor amounts of contaminants, e.g., H2S, NH3, HCl, COS, and Hg, depending on whether the syngas is from gasified coal or reformed natural gas, and can be removed by known treatments. By way of example, contaminants could comprise less than about 0.5% of syngas feed 1b.
Separator means 2 can be any known separator means suitable for the purpose of separating the syngas feed stream into a first, CO-rich stream 3 and a second, H2-rich stream 27. For example, separator means can be membrane separator means or pressure swing adsorption means. Membrane separation is preferred.
Gas separation membranes and the operation thereof for separating gas mixtures are well known. See for example, U.S. Pat. Nos. 5,482,539. 4,990,168, 4,639,257, 2,966,235, 4,130,403, 4,264,338, and 5,102,432. Any known membrane that is operable under the conditions of operation to meet the noted product compositions can be used. For example, Ube membranes and Generon® membranes advertised for H2 separations would be suitable, as would a polybenzimidazole (PBI) membrane. Reference is made, respectively, to Haruhiko Ohya et al, “Polyimide Membranes: Applications, Fabrications and Properties” by H. Ohya, V. V. Kudryavtsev and S. I, Semenova (Jan. 30, 1997) co-published by Kodansha LYD., 12-21 Otowa 2-Chome Bunkyo-Ku, Tokyo 112, Japan and Gordan and Breach Science Publishers S. A. Emmaplein 5, 1075 AW Amsterdam, The Netherlands, for the Ube membranes and to Jayaweera, Indira S. “Development of Pre-Combustion CO2 Capture Process Using High-Temperature Polybenzimidazole (PBI) Hollow-Fiber Membranes (HFMs)”, 2017 NETL CO2 Capture Technology Project Review Meeting, Aug. 21-25, 2017, [online] [retrieved Jan. 17, 2019], [https://www.net1.doe.gov/sites/default/files/2017-12/2I-S-Jayaweera2-SRI-PBIHollow-Fiber-Membranes.pdf], and “Celazole® PBI”, [online] [retrieved Jan. 17, 2019], [https://pbipolymer.com/markets/membrane/].
As illustrated in
Concepts of mixed-gas separation, gas permeability and selectivity are discussed in a number of publications, including “Materials Science of Membranes for Gas and Vapor Separation”, Edited by Yampolski et al, 2006 John Wiley & Sons; “Pure and mixed gas CH4 and n-C4H10 permeability and diffusivity in poly(l-trimethylsilyl-1-propyne)” Roy D. Raharjo et al, Polymer 48 (2007) 7329-7344, 2006 Elsevier Ltd., “Carbon Dioxide Separation through Polymeric Membrane Systems for Flue Gas Applications”, Colin A. Scholes et al, Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC, 3010, Australia; and “Recent Patents on Chemical Engineering”, 2008, 1 52-66, 2008 Bentham Science Publishers Ltd.
The CO-rich stream 3 comprises primarily CO, with minor amounts of carbon dioxide and hydrogen.
After optional contaminant removal (not shown), stream 3 should comprise primarily CO and hydrogen. Stream 3 can also comprise a small amount of CO2 and traces of remaining contaminants. For example, stream 3 can comprise at least about 35%, or at least about 50%, or at least about 65%, or at least about 80% CO. Having the benefit of the disclosure of the present invention, it is seen that the H2 content of stream 3 depends on operational and plant design objectives. On that basis, it is believed that the stream 3 should comprise less than about 55%, or less than about 40%, or less than about 25%, or less than about 10% H2. Stream 3 can also comprise a small amount of CO2 and traces of remaining contaminants. Stream 3 should comprise less than about 0.01%, or less than about 0.001%, or less than about 0.0001%, or less than about 0.00001% of contaminants; and CO2 should comprise less than about 25%, or less than about 15%, or less than about 10%, or less than about 5% of stream 3. Any upper limit for the CO content of stream 3 is considered to be limited only by the ability of technology to economically enrich stream 3 in CO. It is believed that using present technology, stream 3 can comprise up to 90-95% CO.
Stream 3 is then fed as oxyfuel combustor feed stream 4 to oxyfuel combustor means 5, wherein it is combined and reacted with high purity oxygen stream 8 of at least about 95% purity from air separation unit means 6 for separating oxygen from air and recycled CO2 stream 34. Following compression in gas turbine compressor section 11 the oxygen CO2 gas mixture as shown at 34a is fed to oxyfuel combustor means 5. As shown in
Air separation units are well known, for example, as illustrated in U.S. Pat. Nos. 2,548,377, 4,531,371 and 4,382,366. See also, Rong Jiang, Analysis and Optimization of ASU for Oxyfuel Combustion [online] [retrieved Feb. 19, 2019] [http://ieaghg.org/docs/General_Docs/5oxy%20presentations/Session%207B/7B-05%20-%20R.%20Jiang%20(SASPG%20Ltd.).pdf], and “History and progress in the course of time, [online] [retrieved Feb. 19, 2019] [https://www.linde-engineering.com/en/images/Air_separation_plants_History_and_progress_in_the_course_of time_tcm19-457349.pdf]. Before the use of a separator means to separate hydrogen from the syngas feed stream 1b in accordance with the present invention, a considerable portion of the oxygen produced in prior air separation units was consumed by reaction with H2 contained in the combustor fuel stream 4. Combustion in accordance with an embodiment of the present invention, results in stream 9 comprised primarily of CO2 working fluid with a substantially reduced amount of steam. The CO2 content of the oxyfuel combustion exhaust in stream 9 will, of course, vary, depending on the amount of H2 recovery in the membrane permeate and the amount of CO2 in the membrane feed stream both of which affects the CO2 content in the CO2 oxyfuel combustion exhaust. In any event, it can comprise at least about 50%, at least about 60% at least about 70%, at least about 80% or at least about 90% CO2, with the balance comprising H2O, and contaminants such as N2+Ar.
Sub-critical CO2 stream 9 formed in combustor means 5 is then fed to the expansion section 10 of sub-critical CO2 turbine means wherein power is produced to power compression section 11 and electricity generator 12. Expanded sub-critical CO2 exhaust 13 is then fed to known heat recovery steam generator means (HRSG) 14, wherein exhaust 13 indirectly heats a water stream (not shown) to produce working fluid steam stream 13b. The working fluid steam stream 13b is fed to a first, known steam turbine means 15 that powers electricity generator 15a. Condensed steam stream 13a is recycled back to the HRSG 14.
Sub-critical CO2 exhaust 25 from HRSG 14 is then fed to heat exchanger cooling means 16 for indirect cooling with cooling fluid 24. Cooled sub-critical CO2 stream 26 is sent to condensed water separator means 17 for removing condensed water 18 from cooled sub-critical CO2 stream 26. Since stream 26 comprises less water due to the separation of hydrogen from stream 1b by separator means 2, cooling means 23 energy and equipment size requirements can be significantly reduced. Cooling fluid 24 for heat exchangers 16 and 20 is provided by known cooling fluid cooling means 23. The sub-critical CO2 working fluid leaving the water separator 17, is compressed in CO2 compressor means 19, and then cooled in aftercooler heat exchanger means 20 to remove heat of compression. Compressed and cooled sub-critical CO2 stream 21 is then circulated for at least partial capture in stream 22 and recirculation in stream 34 and then forwarded back to oxyfuel combustor means 5. As shown in
Stream 27 comprises primarily H2 with small quantities of CO2, CO and trace quantities of H2O. Stream 27 can comprise at least about 40%, or at least about 50% H2 or at least about 60%, or at least about 85% H2. Having the benefit of the disclosure of the present invention, it is seen that the CO content of stream 27 depends on operational and plant design objectives. On that basis, it is believed that stream 27 should comprise less than about 10% CO, or less than about 5% CO, or less than about 3% CO, or less than about 1% CO with the balance comprising other components such as CO2 and H2O. Any upper limit for the H2 content of stream 27 is limited only by the ability of technology to economically enrich stream 27 in H2. It is believed that using present technology, stream 27 can comprise up to about 90-95% H2.
H2-rich gas stream 27 is fed to any known process, for example, pressure swing adsorption or palladium proton membrane treatment, for further enrichment to high purity H2 and further use. Any H2 enrichment process that produces at least an enriched H2 product stream, and a byproduct/tail gas stream would be suitable. Pressure swing adsorption (PSA) is preferred. By way of non-limiting example, the high purity H2 can be used for
There are certain instances in which the industry specifications for hydrogen products require very low amounts of CO. For example, the U.S. Department of Energy issued the following Hydrogen Fuel Quality Specifications for Polymer Electrolyte Fuel Cells in Road Vehicles:
Hydrogen Fuel Quality Specifications for Polymer Electrolyte Fuel Cells in Road Vehicles (energy.gov)
a The hydrogen fuel index is determined by subtracting the “total non-hydrogen gases” in this table, expressed in mole percent, from 100 mole percent.
b Total hydrocarbons include oxygenated organic species. Total hydrocarbons are measured on a carbon basis (μmolC/mol). Total hydrocarbons may exceed 2 μmol/mol due only to the presence of methane, in which case the summation of methane, nitrogen, and argon is not to exceed 100 ppm.
c As a minimum, includes H2S, COS, CS2 and mercaptans, which are typically found in natural gas.
d Includes, for example, hydrogen bromide (HBr), hydrogen chloride (HCl), chlorine (Cl2), and organic halides (R-X)
Other examples of high hydrogen purity specifications are
1International Organization for Standardization (ISO)14687: 2019
2Hydrogen Fuel Quality Specifications for Polymer Electrolyte Fuel Cells in Road Vehicles, Department of Energy, Energy Efficiency & Renewable Energy, Fuel Cells Technologies Office, Nov. 2, 2016
3Hydrogen Diverse supply offers for many industries and applications, Air Liquide, 2022
When necessary to meet those specifications, the H2-rich stream from membrane separator means 2 can be subjected to an additional process step to remove or convert the CO therein to CO2. An additional advantage of using this additional process step is that less CO and higher hydrogen concentration is fed to the hydrogen enrichment step, e.g., the pressure swing adsorption unit which allows for a significant size reduction of the pressure swing adsorption unit.
A preferred additional process step is the well-known water gas shift reaction as follows:
CO+H2OCO2+H2
Using this additional process step, it is believed that CO levels in the hydrogen product 261 can be less than about 6 ppm, or less than about 3 ppm, or less than 1 ppm, or less than about 0.210 ppm, or less than about 0.205 ppm, or less than 0.200 ppm, or even 0.153 ppm or less.
As shown in
1P/I (mm3/s/m2/Pa)
2P/I (cm3/s/cm2/cm Hg)
3P (cm3-cm)/s/cm2/cm Hg) when I = 0.0001
In Tables 1 and 2, UBE Industries, Ltd. (UBE) is a Japanese multinational manufacturer of polyimide hydrogen separation membranes and have supplied membranes globally to industry for many years.
H2 and CO permeability values versus temperature are presented in Table 1 and H2 and CO2 permeability values are presented in Table 2. The GPU unit, also known as permeance, is a pressure normalized steady state flux for a given membrane thickness and is given as volumetric flow per unit area per second per unit differential pressure across the membrane. The Barrer unit, also known as permeability, is a steady state flux normalized for both membrane thickness and pressure differential across the membrane and is given as volumetric flow times membrane thickness, per unit area per second per unit differential pressure across the membrane. Selectivity is the ratio of the respective GPU or Barrer units, e.g., Hz/CO selectivity at 97.37° C. of 75.95 is determined by following ratio:
24.1010 cm3/cm2/s/cm Hg divided by 0.3163/cm2/s/cm Hg=75.95
It can be seen from the Tables 1 and 2 that Hz/CO selectivity is more sensitive to temperature change than H2/CO2 selectivity. The maximum operating temperature for the UBE polyimide membrane is 150° C. Operating an UBE polyimide membrane separator means at the maximum temperature of 150° C. increases overall system thermal efficiency. Further, the trendline equation in Table 3 calculates a H2/CO selectivity of 63.33 at 150° C., a selectivity reduction of only 2.6% compared with 127° C. Furthermore, based a trendline algorithm for temperature vs. H2 GPU values in Table 3, H2 GPU is increased by about 30% at 150° C. compared with 127° C. In general, mixed gas selectivity will be lower than pure gas selectivity.
1P/I (cm3/s/cm2/cm Hg)
2P (cm3- cm)/s/cm2/cm Hg) when I = 0.0001 cm membrane thickness
1P/I (cm3/s/cm2/cm Hg)
2P (cm3 x cm)/s/cm2/cm Hg) when I = 0.0001 cm membrane thickness
Non-limiting examples of mixed gas selectivity concentrations of the first separated CO-rich stream and the second separated H2-rich stream achieved by the Ube membrane and the Generon® membrane.
The present inventor initiated a study to compare membrane performance of an Ube commercial membrane (at 40° C.) and a Generon® commercial membrane (at 38° C.) in separating a syngas. The mixed gas selectivity of these membranes was compared for mixed gas H2/CO2 (shifted syngas) vs. mixed gas H2/CO separations (unshifted syngas).
H2/CO mixed gas selectivity in the Ube and Generon® commercial gas separation membranes for separating unshifted syngas is substantially higher than their H2/CO2 mixed gas selectivity for separating shifted syngas. The increase in mixed gas selectivity is greater by more than an order of magnitude, enabling higher recoveries and purities in unshifted syngas for the respective separated gases.
As an example, for a given unshifted syngas feed composition, the Ube membrane recovers 87.3% of the H2 at 93.3% purity in the permeate and 88.5% of the CO at 64.9% purity in the retentate. In contrast, for a given shifted syngas feed composition, the Ube membrane recovers 89.2% of the H2 at 82.8% purity in the permeate and 45.1% of the CO2 at 52.2% purity in the retentate.
As another example, for a given unshifted syngas feed composition, the Generon® membrane recovers 91.8% of the H2 at 93.7% purity in the permeate and 90.4% of the CO at 73.5% purity in the retentate. In contrast, for a given shifted syngas feed composition, the Generon® membrane recovers 91.8% of the H2 at 79.1% purity in the permeate and 28.1% of the CO2 at 45.9% purity in the retentate. The above comparisons are presented in the table below:
The recoveries and purities of the separated CO2 from shifted syngas is substantially lower than the recoveries and purities of the separated CO from unshifted syngas.
The following data is based on use of a Generon® membrane operated at 38.7° C. with a H2/CO selectivity of 34.1.
The numbers on the left refer to the reference numbers in
The following data is based on use of a Generon® membrane operated at 38.7° C. with a H2/CO selectivity of 34.1.
The numbers on the left refer to the reference numbers in
PSA gas is typically combusted with air to generate process heat. A syngas feed of 10,000 mole/hr. gives a tail gas recycle of 3,246.3 mole/hr. Directing the tail gas to a vented combustor emits 76,718 (metric) tonne/yr. of uncaptured CO2 vented to the atmosphere. As illustrated in
