Patent Application
20240010580
The field of the invention is a process to produce pipe-line quality e-methane from low carbon electricity and captured CO2 in one step and at temperatures below about 350° C.
The increase in global atmospheric CO2 concentrations has been linked to changes in the earth's climate. The combustion of fossil fuels in various engines and thermal systems produces atmospheric CO2. Concerns about climate change have led to significant societal changes toward renewable or low carbon electricity. This has also led to increasing activity to decarbonize the economy.
Renewable or low-carbon electric power can be used to produce synthetic methane, kerosine, methanol or other chemicals, which can be gases or liquids. This process is often referred to as Power-to-X (also P2X) which refers to several electricity conversion, energy storage, and reconversion pathways that use renewable power from wind, solar, nuclear, and other sources. P2X in some cases may also refer to the use of surplus electric power, typically during periods where fluctuating renewable energy generation exceeds load.
Specifically, low-carbon methane produced using renewable electric power is referred to as “e-methane”. Electrical power produced from solar, wind or other sustainable sources is commonly called “decarbonized electricity”. This renewable energy can be used to produce low-carbon H2 using electrolysis. The H2 when mixed with captured CO2 can be converted to low-carbon e-methane. One of the advantages of e-methane is that it can be introduced into the well-established natural gas pipeline distribution system; compressed and transported to another location where the e-methane is converted to H2 for the production of low-carbon fuels and chemicals; used as a low-carbon energy source for heating and cooling; compressed and used as a low-carbon fuel for engines; and as a low-carbon alternative to many other natural gas uses (Yugo, 2019).
Currently at least two main steps are required to produce e-methane from low-carbon H2 and captured CO2. The first step is the hydrogenation (usually referred to as the reverse water gas shift, or RWGS, reaction) of the CO2 to produce CO and H2 (syngas) when excess H2 is used in the process. The CO2 hydrogenation is typically carried out catalytically. The second step is the catalytic conversion of the syngas to e-methane.
In order to convert the CO2 efficiently, the CO2 hydrogenation reaction requires temperatures of greater than about 700° C. at the inlet. The objective of the e-methane is that is that it allows for a significant reduction in emissions, since it is produced from decarbonized hydrogen and CO2. Due to this objective, it is required that the processes use electrical heaters powered by renewable energy. This represents a large amount of renewable energy input since the CO2 hydrogenation is endothermic.
The conversion of the syngas to CH4 is highly exothermic—giving off a heat that needs to be removed continuously from the reactor system.
Several processes to convert syngas derived from coal or biomass were developed in the 1970's. Technology development ceased following the drop in oil price, and are now primarily used as purification steps. Currently, methanation of CO and CO2 are widely applied in ammonia synthesis plants, as well as refineries and hydrogen plants.
Thus, the state-of-the-art requires the simultaneous provision of input heat in the syngas formation step, and removal of heat in the syngas conversion step (Aryal et al, 2021; Li et al, 2022). Although the latter partially offsets the former, the net requirement is an input of heat to facilitate the integrated process. Furthermore, and of great practical significance, the process management of these two processes present engineering and operational challenges—with resultant deleterious impacts on both process efficiency and capital cost requirements.
There is accordingly a need in the art for novel processes to produce methane.
The invention describes an improved, one-step process for e-methane production from renewable H2 and CO2 mixtures taking advantage of new and improved catalysts and processes that can perform the CO2C reaction efficiently at temperatures less than 350° C.
A “direct” one-step process would have the substantial advantage of reducing the requirements of these two (coupled) thermal effects—into a single, slightly exothermic (net) reaction with a required heat management challenge of significantly reduced magnitude. The benefits derived from this reaction engineering improvement, relative to the state-of-the-art, include increased process efficiency, operational simplicity, and reduced capital equipment costs.
The objective of the present invention is to take advantage of new and improved processes and catalysts that can facilitate the efficient, direct CO2 conversion (CO2C) reaction to e-methane at temperatures less than about 350° C. in one step.
The catalyst described in this document for the conversion of CO2 and H2 at low temperatures typically meets the following specifications:
1. The catalyst contains low-cost and readily available constituents and nominal levels (<0.3%) of precious metals.
2. The catalyst is robust (e.g., Rockwell hardness greater than Mohr 03-04).
3. It is chemically and physical stable up to about 1,100° C. (2,012° F.). This means that the physical property measurements such as BET surface area are within 20%, more preferably 10%, of the ambient temperature.
4. It can be loaded readily into tubular catalytic reactors.
5. The pressure drop in the catalytic reactor is acceptable.
6. The catalyst activation (reduction with H2) can be carried out in-situ at 275-350° C.
7. The conversion of CO2 to CH4 is greater than about 85% at 275-350° C.
8. The selectivity of CH4 production is greater than about 99% at 275-350° C.
9. The catalyst has a long lifetime (2-3 years) and it doesn't require systematic re-activation (reduction).
Since the catalytic conversion of CO2 and H2 mixtures to CH4 is very exothermic, this reaction should typically be carried out in a tubular reactor with an effective method of removing the heat from the tubes. The maximum, practical operating temperature for a tubular reactor is 275-325° C. (Wikipedia, 2022). Although fluidized and slurry bed reactors can operate at higher temperatures up to 325-350° C., such reactors have not been developed for distributed plants (e.g., less than about 25,000 barrels/day of product output) (Steynberg, 2004).
The most common catalysts are nickel-based since this metal is abundant and inexpensive. Table 1 summarizes the performance of several Ni-based catalysts at 275-400° C. However, the Ni-based catalysts listed in Table 1 suffer from sintering, the creation of gas-phase nickel carbonyls, and formation of carbon deposits (Li et al, 2022; Tan et al, 2022). The only catalyst that was tested for more than 40 hours was reported by Onrubia-Calvo (2022) for which the CO2 conversion decreased by about 3% after 72 hours.
Alarcron (2019) described the performance of a 30 wt. % nickel catalyst impregnated on Al2O3 and the 30 wt. % Ni catalyst with a 30 wt. % CeO2 promoter impregnated on Al2O3. The catalysts were activated at 500° C. resulting in a catalyst surface area of about 5 m2/g. The 30% Ni on Al2O3 converted a 4.0/1.0 mixture of H2/CO2 to CH4 at 300° C. with a CO2 conversion efficiency of 34% and a selectivity of 100%. The addition of 30% CeO2 increased the CO2 conversion efficiency to 57% with a selectivity of 100%. However, the CO2 conversion efficiency rapidly dropped to 10% at 250° C. The catalyst activation temperature of 500° C. is not feasible to achieve in a commercial scale tubular reactor, and the CO2 conversion efficiencies of 57% at 300° C. don't meet the required minimum CO2 conversion efficiency of 85% at this temperature. Although, the CO2 conversion efficiency remained constant for the Ni/CeO2 catalyst for 110 hours, this is not a long enough test to assess long-term catalyst performance.
Jia et al (2019) described a zirconia (ZrO2) supported Ni catalyst for CO2 methanation which was synthesized by impregnation followed by plasma calcining at 150° C. or thermal calcining at 500° C. The catalyst was reduced with H2 at 500° C. The efficiency of CO2 conversion was 72% and 35% at 300° C. for the plasma and thermally calcined catalyst, respectively. Since this catalyst was tested for less than 40 hours, the long-term durability of this catalyst is not known. In addition, it would not be possible to reduce the catalyst in-situ at 500° C.
Although Ru, Pd and Rh based catalysts show much better long-term stability than the Ni-based catalysts, these precious metal catalysts are expensive, and they do not meet the requirements of better than 85% CO2 conversion efficiency at 275-350° F.
The improved catalysts for the efficient conversion of CO2 and H2 to CH4 described in this document consist of the solid-solution catalysts Ni2Mg, Cu3Ni or Cu2Mg. These solid solution catalysts are formed at high temperatures (up to 1,100° C.) when the two metals in the catalysts shown above have similar crystal structures, atomic radii, electronegativities and valences.
The Ni2Mg catalyst was manufactured by mixing two moles of nickel acetate powder with one mole of magnesium acetate powder and producing pellets of the appropriate size (typically 3-10 mm) using a high-pressure, pellet press. The pellets were then calcined at 1,000-1,100° C. The Cu3Ni and Cu2Mg catalysts were produced in a similar manner by producing pellets from the metal acetate salts of Cu, Ni and Mg, and then calcining the pellets at 1,000-1,100° C. The nitrate salts of the Cu, Ni and Mg may also be used to produce the catalyst but NO2 and NO emissions are produced during the calcining process which is difficult to control. The calcining of acetates produces primarily CO2 with some acetic acid. The acetic acid emissions are relatively easy to control by water scrubbing.
The Ni2Mg, Cu3Ni and Cu2Mg catalysts are robust, and they do not change chemically and physically up to about 1,000° C. As an example, the conversion efficiency of the Ni2Mg catalyst did not decrease (less than 2%) over a period of 1,750 operating hours in a process development unit (PDU). After the tests were completed, about 500 mg of the catalyst from the PDU was loaded into a laboratory tubular reactor and heated to 650° C. under an air flow of 100 standard cubic centimeters per minute (sccm) to determine if any carbon (coke) was formed. The exhaust from the reactor was continuously monitored with a mass spectrometer to detect the formation of CO2, which would be consistent with carbon burning off. Since no CO2 was detected, it was verified that no coke was formed.
Electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in slightly different ways. Different electrolyzer designs that use different electrolysis technology that are used include alkaline electrolysis, polymer electrolyte membrane (PEM) electrolysis, solid oxide electrolysis, high temperature electrolysis and other emerging types of electrolysis. Different electrolytes that are used including the liquids KOH and NaOH, and with or without activating compounds. Activating compounds are added to the electrolyte to improve the stability of the electrolyte. Most ionic activators for the hydrogen evolution reaction are composed of an ethylenediamine-based metal chloride complexes. Different electrocatalysts are used on the electrodes including many different combinations of metals and oxides like Raney-Nickel-Aluminum, which are enhanced by adding cobalt or molybdenum to the alloy.
The products from the electrolyzer are a stream comprising H2 called stream 3 in
In
Typically, an alkylamine is used to remove the CO2 from the gas stream. Alkylamines used in the process include monoethanolamine, diethanolamine, methyl diethanolamine, diisopropylamine, aminoethoxy ethanol, or combinations thereof. Metal Organic Framework (MOF) materials have also been used as a means of separating CO2 from a dilute stream using chemisorption or physisorption. Other methods to produce concentrated CO2 steams include chemical looping combustion where a circulating metal oxide material captures the CO2 produced during combustion processes. CO2 can also be captured from the atmosphere in what is called direct air capture (DAC).
Captured CO2 can be converted into useful low-carbon products such as fuels (e.g., synthetic natural gas, diesel fuel, gasoline blend stocks, jet fuel, and other fuels) and chemicals (e.g., solvents, olefins, alcohols, aromatics, etc.). This is what is meant by low-carbon, very low-carbon, zero carbon, or negative carbon fuels and chemicals.
The CO2 that comes from industrial or biological process, or is captured from the atmosphere, or that is available from a commercial CO2 pipeline is not generally pure. These CO2 streams typically contain sulfur containing compounds up to 2,000 parts per million and hydrocarbons up to 10 vol. %. Purification of the CO2 including the removal of sulfur containing compounds and hydrocarbons is necessary to avoid issues with downstream processing. After purification, the purified CO2 is suitable for the generation of low-carbon or zero-carbon fuels and chemicals as per this invention.
At least a portion of the stream 3 comprising hydrogen is blended with the stream 5 comprising CO2 to produce a stream 6 or CO2C Reactor feed stream. The ratio of H2/CO2 is from 2 to 6, or preferably from 3 to 4. The CO2 and H2 in stream 6 are reacted to products in a CO2C reactor shown as Unit 2 in
CO2+H2=CO+H2O(ΔH298=+42.1 kJ/mole) Eq. 1
CO+3H2=CH4+H2O(ΔH298=−206.1 kJ/mole) Eq. 2
CO2+4H2=CH4+2H2O(ΔH298=−165 kJ/mole) Eq. 3
The catalytic methanation of CO and CO2 has been widely studied since its discovery by Sabatier and Senderens in 1902. The first reaction, Eq. 1, is the Reverse Water Gas Shift reaction. At standard temperature, the reaction is endothermic and in other processes requires much higher temperatures than the preferred CO2C reactor temperatures. The second reaction, Eq. 2, is the methanation reaction where CO and H2 are reacted to produce CH4 and H2O. Methanation is very exothermic as can be seen with the high negative enthalpy of reaction
The improved catalysts described previously, Ni2Mg, Cu3Ni or Cu2Mg, are used in the CO2C reactor to facilitate the chemical reactions. These catalysts may also be supported on spinels with surface areas greater than about 10 m2/g)), such as magnesium aluminate at concentrations up to about 25 wt. %.
The product from the CO2C reactor is shown as stream 7 in
When the CO2 conversion in the CO2C reactor is less than about 90%, then the unreacted CO2 is removed prior to further processing after the water removal. CO2 removal from the stream can be done by any number of available techniques. The removal methods include the use of physical solvents. Physical solvents include refrigerated methanol that is used in the Rectisol process and the Solexol process that uses dimethyl ethers of polyethylene glycol to capture the CO2 from the gas stream. Other means include the use of membranes selective for CO2 removal. Another means for CO2 removal is to use aqueous solutions of alkylamines (referred to as amines) to chemically capture the CO2. The amine contactor captures the CO2 in the solution. The overhead of the amine contactor is the CO2 free gas and the amine solution with the CO2 is heated in an amine regenerator where the CO2 is released, and the amine aqueous solution is recycled back to the contactor. Various amines can be used such as diethanolamine (DEA), monoethanolamine (MEA), methyl-diethanolamine (MDEA), and Diisopranolamine (DIPA).
The CO2 stream recovered from the CO2 capture unit is shown as Stream 11 in
Since the conversion of CO2 and H2 mixtures to CH4 is highly exothermic, the design of the CO2C reactor is critical. Management of the heat is a key element of the reactor design. In one embodiment of the invention, the CO2C reactor is a multi-tubular fixed bed reactor (
In one embodiment, the CO2C reactor is a multi-tubular reactor. The reactor tubes have an inner diameter of 18 to 50 mm and are filled with catalyst. In this embodiment, heat transfer is done with a hot oil system. Oil as a working fluid is pumped to the reactor shell. Heat is transferred from the tubes to the oil raising the temperature of the oil. The rate of oil circulation is controlled by the pumping rate that sets the heat removal rate. The hot oil leaving the reactor shell is cooled in an external heat exchanger. Cooling water, refrigerated water, or propylene glycol or other suitable materials can be used in the cold side of the external heat exchanger. The cooled hot oil is then pumped back to the reactor shell. Several suitable oils, have working temperatures that are useful for this service including Therminol XP, Therminol 55, Therminol SP, Therminol 59, or any equivalent or similar oil working fluid. These high temperature fluids allow the tubular reactor to be operated up to about 350° C.
In one embodiment the CO2C reactor is a multi-tubular reactor is cooled with molten salt.
In one embodiment, the catalyst inside the CO2C reactor can be diluted. Dilution of the catalyst aids in the heat transfer so that the rate of exothermic heat generation per unit volume is less and as such the local temperature rise is less. Suitable diluents include alumina, or other refractory materials.
In one embodiment, catalyst activity is modified by the amount of active metal loading in formed catalyst. The amount of active metal in the catalyst is defined to optimize the activity and thermal profile across the reactor, and should be between 1-50 weight percent.
In one embodiment of the invention, the feed gas to the CO2C reactor can be diluted with non-reactive gases. Feed gas dilution aids in heat transfer and temperature control in several ways. It reduces the reaction heat generated per unit time per unit volume. It also adds additional mass flow rate that acts as a heat sink. Various dilution gases can be used. The ideal dilution gas is easily separable from the reactor effluent. As the reactor effluent is cooled, the condensed steam can easily be removed from the effluent by a knockout vessel or separator and since steam is a reaction product, the design change involves just increasing the size of the separator.
In one embodiment, the CO2C reactor is a series of adiabatic reactors or reactor beds. Each reactor bed length or residence time is set such that the maximum conversion that can occur keeps the bed temperature rise to a preset temperature rise of no more than 30° C., preferably less than 20° C., and more preferably less than 15° C. Catalyst and feed gas dilution can also be used to keep the temperature rise to the preset limit. At the outlet of the reactor bed, the effluent gas is cooled in a heat exchanger that produces high pressure saturated steam in the range of 1000 to 1200 psig. This cools the reactor effluent to a temperature similar to the previous bed inlet temperature. The cooled effluent is then fed to a subsequent bed. The total number of beds is chosen to get the desired overall CO2 conversion. Optionally, additional hydrogen or hydrogen and CO2 can be added to the subsequent bed feed gas.
In one embodiment, metal partitions can be placed inside the CO2C reactor tubes prior to the addition of the catalyst such that from the center of the tube to the wall of the tube there are metal pathways or connections. This aids in heat transfer since the high thermal conductivity of the metal increases the rate of heat transfer from the hot center of the tube to the cooled tube walls.
In one embodiment, the reaction is performed in a single adiabatic reactor with at 2-6 distinct reactor zones, or preferably 2-3 reactor zones that are separated by a cooling medium. The inter stage zones are externally cooled by a cooling medium.
In one embodiment, the heat transfer and heat management in the reactor are controlled using sequential H2 injection. The initial H2/CO2 ratio is below stoichiometric for the net reaction. This keeps the CO2 conversion low, and the temperature rise in the reactor bed low. H2 at a temperature substantially below the reactor bed temperature is added at the end of the bed. The cold H2 lowers the overall gas temperature to about the previous bed inlet temperature. The additional hydrogen with the previous bed CO2 reacts in the subsequent bed. The number of reactor beds and cold hydrogen additions sets the overall CO2 conversion.
In one embodiment, the CO2C reactor is operated at low CO2 conversion but with high overall gas recycle. This allows the temperature in the reactor bed to be controlled and the temperature rise within the acceptable range. The space velocity or residence time in the reactor bed is high such that conversion is low. Gas dilution and catalyst dilution can be used to aid in the control of the temperature.
A simpler option is to operate the catalyst at about 225° C. which converts a 4.0/1.0 H2/CO2 to CH4 with a CO2 conversion efficiency of about 40% (Table 3). In this manner, about 3 recycle loops would yield a CO2 conversion of about 90%.
The reactor effluent is fed to the Product Processing Unit where the water in the effluent is removed by cooling and separation and the unreacted CO2 and H2 are removed and compressed and recycled and blended with the fresh feed to the CO2C reactor.
The e-methane produced by the invention meets specifications for natural gas pipelines in the United States and Europe and can therefore be transported and co-mingled with natural gas. Typical specifications for interstate natural gas pipelines in the United States are shown in AGA Report No. 4 A as shown below.
Trace Components:
To be clear, each pipeline can and often does set specifications more stringent than the above for pipeline quality gas. Various other nations have different pipeline quality gas practices and standards.
The e-methane product produced as per the invention, meets the representative specification for pipeline quality natural gas and can be distributed in pipelines. In one embodiment, the invention produces no C2+. In one embodiment of the invention, the overall CO2 conversion in the CO2C reactor is greater than about 95% and as such there is no need for CO2 removal systems downstream of the CO2C reactor.
In this example, CO2 and H2 are converted to CH4 using the Ni2Mg, Cu3Ni or Cu2Mg solid-solution catalyst. H2 and CO2 were fed to the CO2C reactor with a molar H2/CO2 ratio of 4.0. Table 3 shows the conversion of CO2 conversion to CH4 and CO as a function of temperature from laboratory data.
As can be seen in Table 3, substantial conversion of CO2 with high selectivity to CH4 occurs at temperature from 250-350° C. In this example, the CO2C reactor effluent was sent to the product separator. The amount of CO formed in the CO2C reactor in this example was low at 2% or less. At a temperature of 300° C., the CO2 conversion was high at 87%.
Further as an example, the catalyst was used in an integrated process to produce low carbon intensity methane at a CO2C reactor temperature of 300° C. The CO2 conversion was 87 mol % with 100% selectivity to CH4.
Stream 105 was sent to an H2 pressure swing adsorption (PSA) unit, shown as Unit 204. By use of pressure cycling, the H2 was separated from the CH4. Stream 107 is the pure H2 product that was recycled back to the front end of the system and blended with the hydrogen in stream 3 of
The overall chemical reaction for the CO2C reactor in this example is CO2+4H2=CH4+2H2O (g) with an enthalpy of reaction of −165 KJ/mol at 25° C. The reaction is very exothermic at the high CO2 conversion. As such, the CO2C reactor in this example was a multi-tubular fixed bed reactor. In this example, the saturated steam pressure produced in the shell of the reactor was 1,100 psig with a temperature of 292° C. A portion of the steam produced by the multi-tubular reactor was used as the steam to heat the amine regenerator reboiler to allow the amine captured CO2 to be released.
In this example, a 12 wt. % Ni2Mg, Cu3Ni or Cu2Mg catalyst impregnated on a magnesium aluminate spinel substrate was used in the CO2C reactor to convert (CO2) with (H2) to a mixture of H2 and CH4 and CO2 at temperatures from 275 to 350° C. Table 2 shows that the CO2 conversion efficiency was about 86% with a greater than 99% CH4 production selectivity.
The H2 and CO2 was removed with a 95% efficiency from the product stream to produce a CH4 stream with greater than 98% purity. The minor components in the CH4 stream include CO2 at about 1.5 vol. %, H2 at about 0.2%, and H2O at about 0.3%.
The resulting CH4 stream met pipeline quality natural gas standards.
This-application claims the benefit of U.S. Provisional Patent Application No. 63/473,936, filed Jul. 8, 2022, the entire content of which is hereby incorporated by reference into this document.
| Number | Date | Country | |
|---|---|---|---|
| 63473936 | Jul 2022 | US |
