# US 7,608,439 B2 · Bio-recycling of carbon dioxide emitted from power plants

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- Number: US 7,608,439 B2
- Document ID: US7608439B2
- Status: Granted
- Kind: B2
- Application number: 11/821,935
- Filed: 2007-06-26
- Published: 2007-12-27
- Granted: 2009-10-27
- Earliest claimed: 2006-06-26
- Inventors: Hugh McTavish; John D. Offerman
- Web page: https://patentgrep.com/patent/US7608439B2

## CPC

- C12P 5/023: {Methane}
- Y02E 50/10: Biofuels, e.g. bio-diesel
- Y02E 50/30: Fuel from waste, e.g. synthetic alcohol or diesel
- C12P: FERMENTATION OR ENZYME-USING PROCESSES TO SYNTHESISE A DESIRED CHEMICAL COMPOUND OR COMPOSITION OR TO SEPARATE OPTICAL ISOMERS FROM A RACEMIC MIXTURE {(brewing of beer C12C; producing vinegar C12J; producing specific peptides or proteins C07K; producing enzymes C12N9/00; DNA or RNA concerning genetic engineering, vectors, e.g. plasmids, or their isolation, preparation or purification C12N15/00; measuring or testing processes involving enzymes or microorganisms C12Q; measuring or testing processes involving nucleic acid amplification reactions C12Q1/6844; fermentation processes to form a food composition, A21 or A23; compounds in general, see the relevant compound class, e.g. C01, C07)}
- Y02E: REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION

## Abstract

The invention provides a method to decrease emission of carbon dioxide from combustion of fossil fuels or other hydrocarbons and to enhance the efficiency of methane production from anaerobic biodigesters. The invention involves feeding carbon dioxide from the exhaust gas of hydrocarbon fuel combustion to an anaerobic biodigester where biomass is anaerobically fermented to produce methane. Carbon dioxide is an electron acceptor for anaerobic fermentation, and thus some of the carbon dioxide is reduced to methane, which can again be used for fuel. In this way, at least a portion of the exhaust gas CO2 is recycled to form fuel methane instead of being released into the atmosphere. Thus, the net CO2 emission from burning a given amount of fossil fuel is decreased. Adding carbon dioxide to an anaerobic fermentation also increases the efficiency and amount of methane production in the fermentation.

## Claims

### Claim 1

A method of producing methane comprising:
collecting combustion exhaust gas from a hydrocarbon fuel combustion process;
separating O2 from CO2 in the exhaust gas to generate a CO2-rich anaerobic exhaust gas fraction;
feeding the CO2-rich anaerobic exhaust gas fraction to a fermentation mixture containing methanogenic microorganisms;
feeding a reduced substrate selected from the group consisting of H2 and organic material and a combination thereof, to the fermentation mixture; and
producing a biogas comprising methane in the fermentation mixture by the anaerobic fermentation of methanogenic microorganisms;
wherein feeding the CO2-rich anaerobic exhaust gas fraction to the fermentation mixture increases methane production in the fermentation mixture.

### Claim 2 (depends on 1)

The method of claim 1 wherein the reduced substrate comprises H2.

### Claim 3 (depends on 1)

The method of claim 1 wherein the reduced substrate comprises organic material.

### Claim 4 (depends on 1)

The method of claim 1 wherein heat from the combustion exhaust gas is used to heat the fermentation mixture.

### Claim 5 (depends on 1)

The method of claim 1 wherein the hydrocarbon fuel is coal.

### Claim 6 (depends on 1)

The method of claim 1 wherein the hydrocarbon fuel comprises biogas methane.

### Claim 7 (depends on 1)

The method of claim 1 further comprising feeding a source of a cation that precipitates carbonate to the fermentation mixture to precipitate at least a portion of the CO2 fed to the fermentation mixture as a carbonate.

### Claim 8 (depends on 7)

The method of claim 7 wherein biogas production occurs in an anaerobic digester, after a period of biogas production in the anaerobic digester the fermentation mixture is transferred to a precipitation tank, and the step of feeding a source of a cation that precipitates carbonate to the fermentation mixture to precipitate at least a portion of the CO2 fed to the fermentation mixture as a carbonate occurs in the precipitation tank.