Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2001-08-29
2004-07-27
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C095S045000, C095S054000
Reexamination Certificate
active
06767527
ABSTRACT:
The present invention relates to a method for recovering substantially all CO
2
generated in a combustion process which includes an application of a mixed conducting membrane. Furthermore, the present invention relates to use of this method.
Due to the environmental aspects of CO
2
, the possibilities for reducing the emissions of this compound to the atmosphere from combustion processes has been widely discussed.
Conventional combustion processes, used for carbon containing fuels and where the oxygen source is air, having carbon dioxide concentrations of 3-15% in the combustion products, hereinafter called exhaust gas, dependent on the fuel and the applied combustion- and heat recovery process. The reason the concentration is this low is because air is made up of about 78% by volume of nitrogen.
Thus, a reduction in the emission of carbon dioxide to the atmosphare makes it necessary to either separate the carbon dioxide from the exhaust gas, or raise the concentration to levels suitable for use in different chemical processes or for injection and deposition in e.g. a geological formation for long term deposition or for enhanced recovery of oil.
CO
2
can be removed from cooled exhaust gas, normally discharged off at near atmospheric pressure, by means of several separation processes e.g. chemical active separation processes, physical absorption processes, adsorption by molecular sieves, membrane separation and cryogenic techniques. Chemical absorption for instance by means of alkanole amines is e.g. considered as the most practical and economical method to separate CO
2
from exhaust gas. The separation processes, however, require heavy and voluminous equipment and will consume a substantial amount of heat or power. Applied in a power generation process, this process will reduce the power output with around 10% or more.
An increase of the concentration of CO
2
in the exhaust gas to levels suitable for use in different chemical processes or for injection and deposition e.g. in a geological formation for long term deposition or for enhanced recovery of oil from an oil reservoir is possible by burning fuel in pure oxygen instead of air.
Commercial air separation methods (e.g. cryogenic separation or pressure swing absorption (PSA)) used to produce pure oxygen require 250 to 300 KWh/ton oxygen produced. Supplying oxygen e.g. to a gas turbine by this methods will decrease the net power output of the gas turbine cycle by at least 20%. The cost of producing oxygen in a cryogenic unit will increase the cost of electric power substantially and may constitute as much as 50% of the cost of the electric power.
However, a less energy demanding methode than the separation methods mentioned above is known from European patent application 0658 367-A2 which desribes an application of a mixed conducting membrane which is integrated with a gas turbine system by heating air in a gas turbine combuster and further by selective permeation of oxygen through the membrane. Pure oxygen near atmospheric pressure or below and at high temperature is recovered from the permeate side of the conducting membrane. An oxygen partial pressure difference causes oxygen to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (rententate side) and oxidation of oxygen ions to oxygen gas on the low oxygen partial pressure side (the permeate side). In the bulk of the membrane oxygen ions are transported by a diffusion process. Simultaneously the electrons flow from the permeate side back to the feed side of the membrane.
Application of a sweep gas in combination with a mixed conducting membrane to lower the oxygen partial pressure to increase the degree of oxygen removal or oxygen recovery is known from the U.S. Pat. No. 5,562,754. In this patent a method for combined production of oxygen and power is disclosed by heating air in a gas turbine combuster and by selective permeation of oxygen through the membrane. In order to improve the efficiency of gas separation by the membrane, the permeate side of the membrane is swept by e.g. steam supplied for instance from the heat recovery section of the power plant. The sweep gas is heated in a separate high temperature heat exchanger. The application of sweep gas will reduce the partial pressure of oxygen on the permeate side of the membrane and thereby increase the flux of oxygen through the membrane. However, this require a certain amount of sweepgas and therefore a certain energy amount to generate this sweep gas. This will therefore decrease the net power output of the power generating process.
Application of a sweep gas in combination with a mixed conducting membrane is also known from Norwegian patent application NO-A-972632 (published Jul. 12, 1998). This patent describes a power and heat generating process where a fuel is combusted with an oxidant, which is an O
2
/CO
2
/H
2
O-containing gaseous mixture, which is supplied from a mixed conducting membrane. The oxygen is picked up from the permeate side of the mixed conducting membrane by means of a sweep gas. The sweep gas is the product or part of the product from at least one combustion process upstream the membrane. In this patent application the sweep gas, or part of the sweep gas, containing a mixture of mainly CO
2
and H
2
O also act as the working fluid in a gas turbine cycle. The amount of sweep gas is related to the amount of working fluid required in the gas turbine cycle i.e. to control the temperature in the gas turbine combuster. Working fluid is the gas (oxidant and fuel) transported through the gas turbine system. Air fed to the retentate side of this membrane is heated by combusting a fuel in the air stream in a burner.
To obtain a sufficient high flux of oxygen through the membrane a rather high temperature is required (600-1500° C.). On the air side of the membrane this may be accomplished by combusting a fuel in the air stream in a burner to increase the temperature of the air fed to the membrane, for instance as disclosed in European patent application 0658 367-A2 or as described in Norwegian patent application NO-A-972632 (published Dec. 7, 1998). The most convenient and least expencive method is to use a carbon containing fuel, e.g. a fossil fuel. However, by means of this method the heated air stream will contain CO
2
generated in the bumer. The CO
2
concentration in the oxygen depleteded air stream discharged from the retentate side of the membrane will be less than about 10% and in most cases less than 3%. If recovery of all generated CO
2
in a combustion process is desirable, due to environmental aspects of CO
2
, an oxygen depleted air stream containing low CO
2
-concentrations is not desirable.
Application of a staged mixed conducting membrane process is known from U.S. Pat. No. 5,447,555 which describes a method for producing pure oxygen. In this process high purity oxygen is recovered from air by a high-temperature ion transport membrane system comprising two or more stages in which each stage operates at a different feed side to permeate side pressure ratio. Operation of the system in multiple stages at controlled pressure ratios produces oxygen at a lower specific power consumption compared with single-stage operation. Sweep gas is not used in this US patent.
The main object of this invention was to arrive at an energy efficient method to recover substantially all CO
2
generated in a combustion process.
The described object can be fulfilled by application of a method which include an application of a mixed conducting membrane.
Hot steam or a mixture of steam and CO
2
(e.g. recycled exhaust gas) is used as sweep gas to pick up oxygen on the permeate side of a mixed conducting membrane (MCM) in a first stage. The membrane is capable of separating oxygen from a hot air stream fed to the retentate side of the membrane. Sweep gas now containing oxygen is applied as oxidant in a catalytic or non catalytic combustion process where a carbon containing fuel is combusted. Heat generated in the combustion process is used to heat air fed to the
Åsen Knut Ingvar
Bruun Tor
Wilhelmsen Kjersti
Hendrickson Stuart L.
Lish Peter J
Norsk Hydro ASA
Wenderoth , Lind & Ponack, L.L.P.
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