Combustion – Process of combustion or burner operation – Heating feed
Reexamination Certificate
2002-06-28
2004-03-09
Cocks, Josiah (Department: 3749)
Combustion
Process of combustion or burner operation
Heating feed
C431S215000, C431S115000, C095S054000, C095S288000, C122S00100C
Reexamination Certificate
active
06702570
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of firing a heat consuming device such as a boiler or furnace in which combustion within the heat consuming device is supported by oxygen separated from air by an oxygen transport membrane. More particularly, the present invention relates to such a firing method in which the separated oxygen also supports combustion to heat an incoming air stream to the oxygen transport membrane and flue gases from the heat consuming device are recirculated to dilute the oxygen being fed to the heat consuming device.
BACKGROUND OF THE INVENTION
Carbon dioxide emissions arising from the combustion of fossil fuels have been identified as major contributors to the increase in the level of greenhouse gases in the earth's atmosphere. This is especially true for the combustion of coal because of the greater carbon content of coal as compared with other types of fuels. Additionally, plants employing coal firing, for instance older electrical utilities, can operate at a lower thermal efficiency than plants fired by liquid fuels to thereby inherently generate more carbon dioxide emissions than liquid fired plants.
Separation and subsequent sequestration of carbon dioxide has been identified as one possible solution for reducing global warming. Sequestration after separation is achieved by compressing the gas to a high pressure and injecting it into deep formations in the ground or the oceans. Unfortunately, common means for removing carbon dioxide from flue gases such as amine scrubbing are expensive. Combustion that relies on oxygen, produced by cryogenic or pressure swing adsorption separation plants, reduces the cost of separating carbon dioxide from the flue gases since the primary combustion product is water which can easily be condensed. However, the costs involved in separating oxygen by cryogenic distillation or pressure swing adsorption makes such practice economically unattractive.
Although the prior art does not directly address the problem outlined above, like considerations have been dealt with in other fields. For instance, in U.S. Pat. No. 5,976,223, an oxygen transport membrane reactor is disclosed that employs ceramic materials to separate oxygen from oxygen-containing feeds. Such ceramic materials, generally perovskites, when heated and subjected to an oxygen partial pressure differential, can function to separate the oxygen from an oxygen-containing feed.
In a well known manner, oxygen is ionized at a cathode side of a membrane formed by a ceramic that can function to separate oxygen. The oxygen ions are transported through the membrane to an anode side thereof. At the anode side of the membrane, the oxygen ions recombine by losing the electrons gained upon ionization. The electrons are then used to ionize oxygen at the cathode side. In certain types of ceramics, known as mixed conductors, both oxygen ions and electrons are conducted. In ceramics known as ionic conductors, only the oxygen ions are conducted and thus, separate electrical pathways are provided for conducting the electrons.
In U.S. Pat. No. 5,976,233, permeated oxygen is combusted with a fuel at the permeate or anode side of the membrane. This combustion of the fuel reduces the oxygen partial pressure at the anode side of the membrane by consuming the permeated oxygen. Carbon dioxide can be recovered from the permeate effluent.
In U.S. Pat. No. 5,888,272, the permeate side of an oxygen transport membrane reactor is purged with combustion products from a downstream process into which fuel is injected. Combustion of the fuel consumes some of the oxygen produced to heat the membrane and to increase the driving force of oxygen through the membrane. The combustion effluent is then introduced into a downstream burner and used to support combustion within the burner and thereby produce the combustion effluent to be recirculated.
U.S. Pat. No. 6,149,714 discloses purging the permeate side of an oxygen transport membrane reactor with a purge gas stream having a low oxygen concentration. This produces an oxidant that is used to support combustion of the fuel and thereby create combustion products. Water can be condensed out of the combustion products and carbon dioxide can be recovered therefrom.
In all of the foregoing references, the incoming air stream must be heated. This heating consumes fuel and thereby produces carbon dioxide. As will be discussed, the present invention provides an integration involving the use of an oxygen transport membrane for oxy-fuel combustion in a heat consuming device in which the air containing the oxygen to be separated is also preheated with an oxy-fuel combustion.
SUMMARY OF THE INVENTION
The present invention relates to a method of firing a heat consuming device. It is to be noted, that the term “heat consuming device” as used herein and in the claims means any device that consumes heat such as a boiler or a furnace.
In accordance with the present invention, air is compressed to form a compressed air stream. After compression, the compressed air stream is heated to form a heated compressed air stream. The compressed air stream is heated at least in part by a first oxy-fuel combustion. As used herein and in the claims, the term “oxy-fuel combustion” indicates a combustion that is supported by oxygen contained within a gaseous mixture that does not contain molecular nitrogen such as in air. The oxygen is separated from the heated compressed air stream by an electrochemical separation process involving oxygen ion transport through a ceramic material to produce an oxygen permeate stream and a retentate stream. The heat consuming device is fired by a second oxy-fuel combustion producing a carbon dioxide-containing flue gas.
The first and second oxy-fuel combustion is supported with oxygen contained in the oxygen permeate. The oxygen is introduced into the second oxy-fuel combustion as a diluted oxygen stream formed by diluting the oxygen permeate with a diluent formed at least in part by recycling part of the carbon dioxide-containing flue gas. A product stream is extracted from the heat consuming device that is formed from a remaining part of the carbon dioxide-containing flue gas. This product stream can then be used in the downstream process or, alternatively, water and carbon dioxide can be separated from the stream for sequestration of the carbon dioxide.
Preferably the oxygen content of the diluted oxygen stream is between about 10 volume percent and about 40 volume percent. More preferably, the oxygen content of the diluted oxygen stream is between about 15 volume percent and about 25 volume percent. This is especially important when retrofitting a heat consuming device.
The oxygen can be separated from the heated compressed air stream within at least one oxygen transport membrane having a retentate side and a permeate side. At least part of the flue gas stream is formed from the part of the carbon dioxide-containing flue gas. The flue gas stream is introduced to the permeate side of the at least one oxygen transport membrane as a sweep gas stream, thereby to form an oxygen-containing sweep gas stream. The oxygen-containing sweep gas stream is introduced into a fired heater to support the first oxy-fuel combustion with a portion of the oxygen contained therein. This produces a combustion product stream. The diluted oxygen-containing stream is formed at least in part by the combustion product stream.
In another embodiment employing at least one oxygen transport membrane and a fired heater, a sweep gas stream is introduced to the permeate side of the at least one oxygen transport membrane to form an oxygen-containing sweep gas stream. Part of the oxygen-containing sweep gas stream and at least part of a flue gas stream, formed from the part of the carbon dioxide-containing flue gas, are introduced into the combustion chamber of the fired heater. This supports the first oxy-fuel combustion and forms the sweep gas stream. The diluted oxygen-containing stream is formed at least in part from a remaining part of the oxygen-conta
Bool, III Lawrence E.
Gottzmann Christian Friedrich
Kobayashi Hisashi
Shah Minish Mahendra
Thompson David Richard
Cocks Josiah
Praxair Technology Inc.
Rosenblum David M.
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