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Sorption enhanced methanation of biomass

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Sorption enhanced methanation of biomass


Disclosed embodiments provide a system and method for producing hydrocarbons from biomass. Certain embodiments of the method are particularly useful for producing substitute natural gas from forestry residues. Certain disclosed embodiments of the method convert a biomass feedstock into a product hydrocarbon by hydropyrolysis. Catalytic conversion of the resulting pyrolysis gas to the product hydrocarbon and carbon dioxide occurs in the presence of hydrogen and steam over a CO2 sorbent with simultaneous generation of the required hydrogen by reaction with steam. A gas separator purifies product methane, while forcing recycle of internally generated hydrogen to obtain high conversion of the biomass feedstock to the desired hydrocarbon product. While methane is a preferred hydrocarbon product, liquid hydrocarbon products also can be delivered.
Related Terms: Carbon Dioxide Hydrocarbon Hydrogen Ethane Biomass

USPTO Applicaton #: #20130017460 - Class: 429419 (USPTO) - 01/17/13 - Class 429 


Inventors: Bowie G. Keefer, Matthew L. Babicki, Brian G. Sellars, Edson Ng

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The Patent Description & Claims data below is from USPTO Patent Application 20130017460, Sorption enhanced methanation of biomass.

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CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application under 35 U.S.C. §120 of International Patent Application No. PCT/CA2010/001859, filed Nov. 18, 2010, which claims the benefit of the earlier filing date of U.S. Provisional Application No. 61/262,485, filed Nov. 18, 2009. Each of these prior applications is incorporated herein by reference.

FIELD

The process of the invention applies to hydropyrolysis of carbonaceous feedstocks, and particularly of forestry residues, to generate higher value synthetic fuels, in particular methane and optionally liquid hydrocarbons.

BACKGROUND

Thermochemical conversion of biomass such as sawmill wood wastes, forestry residues and agricultural wastes into synthetic fuels is an important emerging avenue for advancement of renewable energy sources to supplement or replace fossils fuels. While air blown gasification is used for generation of lower heating value fuel gas, several variants of oxygen or steam gasification can be used for production of syngas containing minimal nitrogen. Syngas is a gas mixture containing mostly hydrogen and carbon monoxide, and is a versatile feedstock for further chemical processing into a wide range of useful fuels and chemical compounds. Syngas can be catalytically converted into methane, Fischer-Tropsch liquid fuels, methanol, dimethyl ether, or hydrogen. The methanation reaction of syngas to generate methane and byproduct water vapour is typically conducted over nickel catalysts at temperatures in the range of about 300° C. to about 400° C., and preferably at elevated pressure.

Methane is readily marketed and delivered through existing natural gas distribution infrastructure as substitute natural gas (SNG) for numerous end uses including space heating and electrical power generation. Methane has considerably higher energy density than hydrogen, and can be converted into syngas or hydrogen by catalytic steam reforming. Modern combined cycle power plants are conveniently fueled by natural gas. Methane is also a particularly advantageous fuel for future high temperature fuel cell power plants using highly endothermic internal steam reforming of natural gas to recover high grade heat generated by the fuel cell stack.

The reaction of steam with biomass to generate syngas is highly endothermic, hence conducted with direct or indirect heating by partial oxidation with air or oxygen; and is typically conducted at much higher temperature than the subsequent exothermic methanation reaction. The thermal mismatch between gasification and methanation reactions is detrimental to process efficiency.

Hydrogasification has previously been investigated for gasification of biomass. The key reaction is hydrogenation of carbon to form methane, whose exothermicity is a great advantage compared to other gasification approaches. As hydrogen is a premium fuel, its consumption in large amounts has presented the appearance of a major economic barrier.

The endothermic nature of the syngas formation reaction from the reaction of biomass pyrolysis gas and steam requires enthalpy heat to be added (typically by partial combustion with added oxygen). Temperatures well in excess of 650° C. are typically required to reduce tars to reasonable levels.

The gas composition produced in biomass gasification approaches a complex equilibrium established between CO, CO2, H2, H2O and CH4 which is a function of temperature, pressure and overall gas composition. Reforming reactions producing syngas increasingly dominate the equilibrium at temperatures above 650° C. at the expense of hydrocarbons, CO2 and water.

The use of catalysts, such as the use of olivine, dolomite or nickel coated media in fluidized beds, to enhance the rate of syngas formation is well known. These catalysts allow a faster reaction towards syngas equilibrium favoured under the process conditions. Catalysts have also been used in a secondary bed in series with the gasifier for the reduction of tars contained in the syngas or producer gas.

An oxygen blown entrained flow gasifier may typically operate at about 1300° C. to 1500° C., at which temperatures methane and higher hydrocarbons are all nearly entirely converted to syngas. This has the important advantage of almost completely eliminating tar constituents, but the disadvantage for SNG production that all of the product methane must be generated by the exothermic methanation of syngas at much lower temperature than the gasification temperature.

Indirect steam gasifiers (such as the US Battelle/FERCO “Silvagas” system, the Austrian fast internally circulating fluidized bed (FICFB) system, and the Dutch ECN “Milena” system) operate at about 850° C. These systems use twin bed configurations, in which fluidized granular heat transfer media is circulated between a gasification zone in which steam reacts with the biomass to produce syngas and char, and an air-blown regeneration zone in which the char is combusted to reheat the media. The product syngas contains a significant admixture of methane generated within the gasifier. While downstream processing is required to convert or remove tar constituents, an important advantage for SNG production is that only about 55% to 60% of the final product methane must be generated by methanation of syngas, since a useful fraction of the methane was already produced with the syngas.

Some recent improvements to the twin bed gasification approach have been based on adsorption enhanced reforming (“AER”) in which a CO2 acceptor such as lime or calcined dolomite is included in the granular media to remove carbon dioxide by carbonation from the gasification zone operating typically at about 600° C., and to release the carbon dioxide by calcining in the regeneration zone operating typically at about 800° C. The AER process has been disclosed by Specht et al. (European patent publications EP 1,218,290 B1 and EP 1,637,574 A1). The principle of the AER process is to generate hydrogen-rich syngas by shifting the reaction equilibria of the steam reforming and water gas shift reactions by CO2 removal. The AER process has been tested in the FICFB twin bed system, and is being developed for SNG production by using a molten salt methanation reactor to convert the syngas into methane.

Twin bed indirect steam biomass gasifiers, and experimental AER systems derived from twin bed gasifiers, have been operated at atmospheric pressure. Air blown combustion regeneration of pressurized fluidized beds would present challenges. ECN have considered operation of the Milena twin bed gasification system pressurized to about 7 bara.

There is a need to provide more efficient internally self-sustaining generation of the hydrogen needed for hydrogasification, which otherwise is an extremely attractive approach for conversion of biomass and other carbonaceous feedstocks into methane and other high value synthetic fuels.

SUMMARY

While the “sorption enhanced reforming” (SER) process [known in Europe as “absorption enhanced reforming” or AER] concerns generating hydrogen-rich syngas, which may be converted downstream in a separate methanation reactor into SNG, disclosed embodiments of the present invention concern the new principle of absorption enhanced methanation (“SEM”). Whereas carbon is nearly entirely removed from the feed syngas by carbonation of the sorbent in AER, only about half of the carbon is similarly removed in SEM.

Methanation as described in this disclosure is hydroconversion of a pyrolysis gas to produce methane, including but not confined to the conversion of syngas to methane.

It has been found unexpectedly that maintenance of a high hydrogen back-pressure in SEM will inhibit decomposition of methane by steam methane reforming, while carbon oxides are preferentially removed. Because only about half of the carbon contained in the initial syngas is removed by carbonation in SEM, the CO2 sorbent has much lighter duty in SEM as compared with SER.

Thermodynamic modeling indicates that slightly more than half of the carbon not rejected as char or coke deposits can be converted to methane under conditions of hydrogen self-sufficiency. Approximately 20% of the carbon originally in the biomass will typically be rejected as char or coke to be combusted or gasified in the regeneration reactor. If a supplemental source of hydrogen is available, the conversion of feed carbon to methane can be increased within the scope of the present invention, while even less of the carbon will be removed by carbonation of the sorbent.

SEM may be advantageously operated at moderately elevated working pressures in a range of just over 1 bara to about 50 bara, or in a preferred range of from about 5 bara to about 30 bara. While SEM can be conducted at atmospheric pressure, the methane concentration will be lower than at higher operating pressures, thus making the gas separation of hydrogen and methane more difficult. Conventional methanation requires much higher working pressures to achieve satisfactory conversion.

A preferred CO2 sorbent for SEM is CaO, which can be used in any suitable form, or combinations thereof, such as calcined limestone or dolomite, or CaO on a suitable support such as alumina. CaO is readily carbonated at working temperature around 600° C. and moderate pressures from atmospheric upward. Such temperature and pressure conditions have been found to be favourable for the hydrogasification of biomass pyrolysis gas to methane, and for steam reforming of methane to generate hydrogen.

Various CO2 sorbents or “acceptors” will work in the temperature range of from about 500° C. to about 650° C. of interest for SEM. These include calcined dolomite, calcium oxide, calcium hydroxide, lithium zirconate, lithium orthosilicate, and other metal oxides or hydroxides that can react with carbon dioxide to form a carbonate phase.

While hydroconversion of biomass pyrolysis gas to methane works favourably at temperatures in the range of from about 500° C. to about 650° C., productive hydroconversion of pyrolysis gas to liquid hydrocarbons requires lower temperatures in the range of from about 300° C. to about 400° C. CO and CO2 are extracted from the oxygenated pyrolysis gas by decarbonylation and decarboxylation respectively, in parallel with extraction of H2O by hydrodeoxygenation. As CO and H2O can be consumed to generate H2 and CO2 by the water gas shift reaction, it may be advantageous to remove CO2 by a carbonation reaction in order to maximize the generation of hydrogen by water gas shift. Suitable CO2 sorbents for the temperature range of from about 300° C. to about 400° include potassium-promoted hydrotalcites, magnesia supported on alumina, or dolomite in combination with alkali (and particularly potassium) promoters.

Certain disclosed embodiments provide a method for converting a biomass feedstock into a product hydrocarbon comprising: a. subjecting the feedstock to fast pyrolysis with rapid pyrolytic heating in the substantial absence of oxygen, or hydropyrolysis as fast pyrolysis in the presence of hydrogen, in order to generate fractions of pyrolysis gas and char; b. catalytically converting at least a portion of the pyrolysis gas to a product hydrocarbon and carbon dioxide in the presence of hydrogen and steam, while removing carbon dioxide by carbonation of a sorbent; c. generating at least a portion of the hydrogen by reaction between steam and a portion of the pyrolysis gas or a product hydrocarbon; d. separating hydrogen from the hydrocarbon product, and recycling the hydrogen so as to force the conversion of biomass into the hydrocarbon product; and e. regenerating the sorbent by heating through combustion of the char to release the carbon dioxide.

The fast pyrolysis step may be performed with externally heated media, e.g. circulating through a pressurized auger reactor, and preferably as hydropyrolysis in a hydrogen atmosphere. The heat transfer media may include circulating magnetite pellets, which are readily separable from char according to density and magnetic properties. Some impurities such as alkalis, other metals, sulphur, and chloride will be partially entrained by the char. While very fast pyrolysis will minimize char production, slower pyrolysis may also be considered for coproduction of charcoal or biochar with lower yield of methane and any other desirable hydrocarbon products.

The catalytic conversion step includes catalytic hydrogasification, such as steam hydrogasification. Hydroconversion, hydrodeoxygenation, and hydrocracking reactions will take place. The net reaction will be exothermic. This step may be conducted alternatively in any suitable reactor architecture, such as the following reactor architectures: a) bubbling or circulating fluidized bed; b) fixed bed with granular packing or monolithic catalyst, and rotary or directional valve logic for cyclically switching beds between reaction and regeneration steps; c) moving bed with granular catalyst.

The hydrogasification process requires a source of hydrogen, either externally supplied or internally generated. According to certain disclosed embodiments of the present invention, steam addition, plus moisture contained in feed biomass, provides sufficient steam for internal, self-sustaining generation of hydrogen required for the hydrogasification reaction to convert biomass feedstock into methane.

Certain disclosed embodiments of the invention may be realized by any of the following operating modes: 1. Self-sustaining recycle of H2 generated within catalytic stage with sufficient H2 excess to overcome incomplete recovery in downstream gas separation of recycle H2. Methane yield is approximately 50% of carbon after char production, balance primarily to CO2 with preferred use of water gas shift reaction to consume most CO. 2. Supplemental hydrogen may provided from any combination of (a) an external source of hydrogen rich gas, or (b) oxygen or steam gasification of char offgas, or (c) steam methane reforming of a portion of the methane product.

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stats Patent Info
Application #
US 20130017460 A1
Publish Date
01/17/2013
Document #
13474494
File Date
05/17/2012
USPTO Class
429419
Other USPTO Classes
585254, 585251, 585242, 422187, 429423, 60781
International Class
/
Drawings
11


Carbon Dioxide
Hydrocarbon
Hydrogen
Ethane
Biomass


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