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Carbon dioxide and oxygen are separated from a feed gas, preferably derived from flue gas from an oxyfuel combustion process, by diffusion across at least one membrane in a membrane separation system to produce separated carbon dioxide gas comprising oxygen, which is fed to the oxyfuel combustion pr

Carbon dioxide and oxygen are separated from a feed gas, preferably derived from flue gas from an oxyfuel combustion process, by diffusion across at least one membrane in a membrane separation system to produce separated carbon dioxide gas comprising oxygen, which is fed to the oxyfuel combustion process to improve the performance of the process.

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1. A method for oxyfuel combustion of a fuel selected from the group consisting of carbonaceous fuel; hydrocarbonaceous fuel; and mixtures thereof, said method comprising: combusting said fuel in the presence of oxygen in an oxyfuel combustion unit to produce flue gas comprising carbon dioxide and o

1. A method for oxyfuel combustion of a fuel selected from the group consisting of carbonaceous fuel; hydrocarbonaceous fuel; and mixtures thereof, said method comprising: combusting said fuel in the presence of oxygen in an oxyfuel combustion unit to produce flue gas comprising carbon dioxide and oxygen as a contaminant gas;separating carbon dioxide and oxygen from feed gas comprising carbon dioxide and oxygen by diffusion across at least one permeable membrane in a membrane separation system to produce permeate gas comprising carbon dioxide and oxygen, and vent gas; andfeeding at least a portion of said permeate gas in a conduit arrangement from said membrane separation system to said oxyfuel combustion unit to provide oxygen for combustion therein, wherein carbon dioxide and oxygen in said feed gas originate from said flue gas. 2. The method according to claim 1 wherein said feed gas is a carbon dioxide-depleted fraction separated from said flue gas. 3. The method according to claim 1 comprising: washing at least a portion of said flue gas with water to remove water soluble components and produce washed flue gas;drying at least a portion of said washed flue gas to produce contaminated carbon dioxide gas;cooling and partially condensing at least a portion of said contaminated carbon dioxide gas by indirect heat exchange to produce partially condensed crude carbon dioxide fluid;phase separating at least a portion of said partially condensed crude carbon dioxide fluid to produce impure liquid carbon dioxide and waste vapor; andfeeding at least a portion of said waste vapor as said feed gas to said membrane separation system. 4. The method according to claim 3 comprising compressing at least a portion of said washed flue gas to produce compressed flue gas prior to drying to produce said contaminated carbon dioxide gas. 5. The method according to claim 3 comprising warming at least a portion of said waste vapor by indirect heat exchange to produce waste gas prior to feeding to said membrane separation system. 6. The method according to claim 3 comprising: separating at least a portion of said impure liquid carbon dioxide in a mass transfer separation column system to produce oxygen-enriched overhead vapor and carbon dioxide-enriched bottoms liquid; andrecycling at least a portion of said oxygen-enriched overhead vapor, or a gas derived therefrom, to said contaminated carbon dioxide gas prior to said cooling and partially condensing to produce said partially condensed crude carbon dioxide fluid. 7. The method according to claim 6 comprising: warming at least a portion of said impure liquid carbon dioxide by indirect heat exchange to produce warmed impure liquid carbon dioxide;reducing the pressure of at least a portion of said warmed impure liquid carbon dioxide to produce reduced pressure impure liquid carbon dioxide;feeding at least a portion of said reduced pressure impure liquid carbon dioxide to said mass transfer separation column system for separation. 8. The method according to claim 6 comprising: warming at least a portion of said oxygen-enriched overhead vapor by indirect heat exchange to produce oxygen-enriched gas; andcompressing at least a portion of said oxygen-enriched gas to produce compressed oxygen-enriched gas for recycle to said contaminated carbon dioxide gas. 9. The method according to claim 6 comprising reboiling carbon dioxide-enriched bottoms liquid by indirect heat exchange against at least a portion of said contaminated carbon dioxide gas, or said gas derived therefrom, to provide carbon dioxide-enriched vapor for the column system, wherein said reboiling provides a portion of the refrigeration duty required to cool and at least partially condense said contaminated carbon dioxide gas or said gas derived therefrom. 10. The method according to claim 1 comprising: washing at least a portion of said flue gas with water to remove water soluble components and produce washed flue gas;compressing at least a portion of said washed flue gas to produce compressed flue gas;drying at least a portion of said compressed flue gas to produce contaminated carbon dioxide gas;cooling and partially condensing at least a portion of said contaminated carbon dioxide gas by indirect heat exchange to produce partially condensed crude carbon dioxide fluid;phase separating at least a portion of said partially condensed crude carbon dioxide fluid to produce first carbon dioxide-enriched liquid and carbon dioxide-depleted vapor;cooling and partially condensing at least a portion of said carbon dioxide-depleted vapor by indirect heat exchange to produce partially condensed carbon dioxide-depleted fluid;phase separating at least a portion of said partially condensed carbon dioxide-depleted fluid to produce second carbon dioxide-enriched liquid and waste vapor; andwarming at least a portion of said waste vapor by indirect heat exchange to produce said feed gas. 11. The method according to claim 1 wherein carbon dioxide is separated from said feed gas by diffusion across at least one hollow fiber polymeric membrane in said membrane separation system. 12. The method of claim 1, wherein at least 30 mol % of oxygen in said feed gas is separated from said feed gas in said membrane separation system. 13. Apparatus for oxyfuel combustion of a fuel selected from the group consisting of carbonaceous fuel; hydrocarbonaceous fuel; and mixtures thereof, said apparatus comprising: an oxyfuel combustion unit for combusting said fuel in the presence of oxygen to produce flue gas comprising carbon dioxide and oxygen as a contaminant gas;a membrane separation system comprising at least one permeable membrane for separating carbon dioxide and oxygen from feed gas comprising carbon dioxide and oxygen by diffusion across said membrane(s) to produce permeate gas comprising carbon dioxide and oxygen, and vent gas;a conduit arrangement for feeding permeate gas from said membrane separation system to said oxyfuel combustion unit to provide oxygen for combustion therein; anda separation system for producing said feed gas from said flue gas. 14. The apparatus of claim 13, wherein said at least one permeable membrane provides a permeate fraction for oxygen of at least 0.3. 15. Apparatus as claimed in claim 13 wherein said feed gas comprises at least one inert gas selected from the group consisting of nitrogen and argon, said membrane separation system being selected to provide a balance between the permeate fraction for oxygen and the non-permeate fraction for said inert gas(es). 16. Apparatus as claimed in claim 13 wherein at least one permeable membrane is a hollow fiber polymeric membrane. 17. Apparatus as claimed in claim 13 wherein the surface area to unit volume ratio of the membrane(s) in the membrane separation system is at least 1000 m2/m3. 18. Apparatus according to claim 13 comprising: a gas-liquid contact vessel for washing said flue gas with water to remove water soluble components therefrom and to produce washed flue gas;conduit arrangement for feeding flue gas from said oxyfuel combustion unit to said gas-liquid contact vessel;a gas drying system for drying washed flue gas to produce contaminated carbon dioxide gas;a conduit arrangement for feeding washed flue gas from said gas-liquid contact vessel to said gas drying system;at least one heat exchanger for cooling and partially condensing said contaminated carbon dioxide gas by indirect heat exchange to produce partially condensed crude carbon dioxide fluid; anda conduit arrangement for feeding contaminated carbon dioxide gas from said gas drying system to said heat exchanger; wherein said separation system comprises: a phase separator for separating partially condensed crude carbon dioxide fluid to produce impure liquid carbon dioxide and waste vapor;a conduit arrangement for feeding partially condensed crude carbon dioxide fluid from said heat exchanger to said phase separator;a conduit arrangement for feeding waste vapor from said phase separator as said feed gas to said membrane separation system. 19. Apparatus according to claim 18 wherein said conduit arrangement for feeding washed flue gas from said gas-liquid contact vessel to said drying system comprises: a first compressor arrangement for compressing washed flue gas to provide compressed flue gas;a conduit arrangement for feeding washed flue from the gas-liquid contact vessel to the first compressor arrangement; anda conduit arrangement for feeding compressed flue gas from said first compressor arrangement to said gas drying system. 20. Apparatus according to claim 18 wherein the conduit arrangement for feeding waste vapor from said phase separator to said membrane separation system comprises: a conduit arrangement for feeding waste vapor from said phase separator to said heat exchanger for warming to produce said feed gas;at least one fluid passage in said heat exchanger; anda conduit arrangement for feeding feed gas from said heat exchanger to said membrane separation system. 21. Apparatus according to claim 18 comprising: a mass transfer separation column system for separating impure liquid carbon dioxide to produce oxygen-enriched overhead vapor and carbon dioxide-enriched bottoms liquid;a conduit arrangement for feeding impure liquid carbon dioxide from said phase separator to said column system; anda conduit arrangement for recycling oxygen-enriched overhead vapor or a gas derived therefrom from said column system to said conduit arrangement for feeding contaminated carbon dioxide gas from said drying system to said heat exchanger. 22. Apparatus according to claim 21 wherein said conduit arrangement for recycling oxygen-enriched overhead vapor comprises: a conduit arrangement for feeding oxygen-enriched overhead vapor from said column system to said heat exchanger for warming by indirect heat exchange to produce oxygen-enriched gas;at least one fluid passage in said heat exchanger;a second compressor arrangement for compressing oxygen-enriched gas to produce compressed oxygen-enriched gas;a conduit arrangement for feeding oxygen-enriched gas from said heat exchanger to said second compressor arrangement; anda conduit arrangement for feeding compressed oxygen-enriched gas from said second compressor arrangement to said conduit arrangement for feeding contaminated carbon dioxide gas from said gas drying system to said heat exchanger. 23. Apparatus according to claim 21 comprising: a conduit arrangement for feeding impure liquid carbon dioxide from said phase separator to said heat exchanger for warming by indirect heat exchange to produce warmed impure liquid carbon dioxide;a pressure reduction arrangement for reducing the pressure of warmed impure liquid carbon dioxide to produce reduced pressure impure liquid carbon dioxide;a conduit arrangement for feeding warmed impure liquid carbon dioxide from said heat exchanger to said pressure reduction arrangement; anda conduit arrangement for feeding reduced pressure impure liquid carbon dioxide from said pressure reduction arrangement to said column system. 24. Apparatus according to claim 21 comprising: a reboiler for said column system for reboiling carbon dioxide-enriched bottoms liquid by indirect heat exchange against crude carbon dioxide fluid to provide carbon dioxide-enriched vapor for the column system and cooled crude carbon dioxide fluid;a conduit arrangement for feeding crude carbon dioxide fluid from said heat exchanger to said reboiler; anda conduit arrangement for feeding cooled crude carbon dioxide fluid from said reboiler to said heat exchanger. 25. Apparatus according to claim 13 comprising: a gas-liquid contact vessel for washing flue gas with water to remove water soluble components and produce washed flue gas;a conduit arrangement for feeding flue gas from said oxyfuel combustion unit to said gas-liquid contact vessel;a first compressor arrangement for compressing washed flue gas to provide compressed flue gas;a conduit arrangement for feeding washed flue gas from said gas-liquid contact vessel to said first compressor arrangement;a gas drying system for drying compressed flue gas to produce contaminated carbon dioxide gas;a conduit arrangement for feeding compressed flue gas from said first compressor arrangement to said gas drying system;at least one heat exchanger for cooling and partially condensing said contaminated carbon dioxide gas by indirect heat exchange to produce partially condensed crude carbon dioxide fluid; anda conduit arrangement for feeding contaminated carbon dioxide gas from said gas drying system to said heat exchanger; wherein said separator system comprises: a first phase separator for separating partially condensed crude carbon dioxide fluid to produce first impure liquid carbon dioxide and carbon dioxide-depleted vapor;a conduit arrangement for feeding partially condensed crude carbon dioxide fluid from said heat exchanger to said first phase separator;a conduit arrangement for feeding carbon dioxide-depleted vapor from said first phase separator to said heat exchanger for cooling and partial condensing to provide cooled and partially condensed carbon dioxide-depleted fluid;a second phase separator for separating partially condensed carbon dioxide-depleted fluid to produce second impure liquid carbon dioxide and waste vapor;a conduit arrangement for feeding partially condensed carbon dioxide-depleted fluid from said heat exchanger to said second phase separator;a conduit arrangement for feeding waste vapor from said second phase separator to said heat exchanger for warming to produce said feed gas; anda conduit arrangement for feeding feed gas from said heat exchanger to said membrane separation system.