Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-03-19
2003-05-13
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S004000, C096S008000, C096S009000, C096S010000
Reexamination Certificate
active
06562104
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and system for combusting fuel that has direct application to heat consuming devices such as boilers and furnaces as well as reactors that utilize separated oxygen. More particularly, the present invention relates to such a combustion method and system in which combustion is enhanced with oxygen produced by the use of a ceramic membrane system. Even more particularly, the present invention relates to such a method and system in which the ceramic membrane system is subjected to a countercurrent reactive purge or flow of sweep gas.
BACKGROUND OF THE INVENTION
Growing concerns about environmental issues, such as global warming and pollutant emissions, are driving industries to explore new ways to increase efficiency and reduce emissions of pollutants. This is particularly true for fossil fuel fired combustion systems, which represent one of the largest sources of carbon dioxide and air pollution emissions. One effective way to reduce emissions and to increase efficiency is to use oxygen, or oxygen enriched air, in the combustion process. The use of oxygen or oxygen enriched air reduces stack heat losses, which increases the system efficiency, while at the same time reducing NOx emissions. Further, the concentration of carbon dioxide in the flue gas is higher since there is little or no nitrogen to act as a diluent. The higher carbon dioxide concentration enhances carbon dioxide recovery options.
Oxygen using the prior art has been limited to those processes with high exhaust temperatures, such as glass furnaces. In such applications, the fuel savings and other benefits achieved are greater than the cost of the oxygen. In low exhaust temperature systems, such as boilers, the reverse is true. In these systems, the cost of oxygen produced with current technologies is more expensive than the available fuel savings. This makes oxygen use in such systems economically unattractive. Moreover, when the energy required to produce the oxygen is taken into consideration, the overall thermal efficiency decreases.
Oxygen transport membranes have been advantageously utilized in the prior art to produce oxygen for heat consuming devices and processes in a manner that results in a savings of energy that would otherwise have to be expended in the separation of oxygen. Oxygen transport membranes are fabricated from oxygen-selective, ion transport ceramics in the form of tubes or plates that are in themselves impervious to the flow of oxygen. Such ceramics, however, exhibit infinite oxygen selectivity at high temperatures by transporting oxygen ions through the membrane. In oxygen transport membranes, the oxygen is ionized on one surface of the membrane to form oxygen ions that are transported through the membrane. The oxygen ions on the opposite side of the membrane recombine to form oxygen with the production of electrons. Depending upon the type of ceramic, oxygen ions either flow through the membrane to ionize the oxygen or along separate electrical pathways within the membrane, or by an applied electric potential. Such solid electrolyte membranes are made from inorganic oxides, typified by calcium-or yttrium-stabilized zirconium and analogous oxides having fluoride or perovskite structures.
In U.S. Pat. No. 5,888,272 oxygen transport membranes are integrated into a combustion process itself, with all the oxygen produced going directly into the combustor. The heated flue gases can then be routed to a heat consuming process. In one embodiment, flue gases are recycled through a bank of oxygen transport membrane tubes and enriched with oxygen. Typically the flue gas enters the bank containing anywhere from 1 to about 3 percent oxygen and leaves the bank containing from about 10 to about 30 percent oxygen by volume. The enriched flue gas is then sent to a combustion space where it is used to burn fuel. In another embodiment, called reactive purge, the oxygen transport membrane tubes are placed directly in the combustion space. A fuel and flue gas mixture, is passed through the tubes and combust with the oxygen as it passes through the tubes. Thus oxygen production and combustion take place simultaneously inside the oxygen transport membrane with the fuel diluted with flue gas.
As will be discussed, the present invention utilizes oxygen transport membranes to produce oxygen to support combustion that inherently reduces the energy expenditures involved in compressing an incoming oxygen containing feed to the membranes. Combustion can take place at the surface of the oxygen transport membranes in the presence of fuel that is not diluted with flue gas.
SUMMARY OF THE INVENTION
The present invention provides methods and systems for combusting fuel that have direct application to such heat consuming devices as boilers and furnaces or to reactors that separate oxygen from an oxygen-containing feed. Such reactors include devices for separating oxygen to produce a nitrogen-enriched product.
In accordance with one method of the present invention, an oxygen-containing stream is introduced into at least one oxygen transport membrane. The membrane projects into a combustion zone to separate oxygen from the oxygen-containing stream and thereby, to introduce an oxygen permeate into the combustion zone. A fuel stream is introduced into the combustion zone and fuel within the fuel stream is combusted in the presence of the oxygen permeate so that the at least one oxygen transport membrane is subjected to a reactive purge and a portion of heat arising from the combustion of the fuel heats the at least one ceramic membrane to an operational temperature. Radiant heat energy emanating from the at least one oxygen transport membrane is absorbed within a heat sink to promote stabilization of the operational temperature of the at least one oxygen transport membrane.
The at least one oxygen transport membrane can comprise at least one row of oxygen transport membranes spaced apart from one another. The fuel stream is introduced in a cross-flow relationship to the at least one row of oxygen transport membranes.
It is to be noted that the term, “cross-flow” as used herein and in the claims means a flow direction with respect to the oxygen transport membranes that is at right angles to the length of the oxygen transport membranes plus or minus about forty-five degrees. For instance, if tubular oxygen transport membranes are used, the “cross-flow” direction would be at or near right angles to the tube as opposed to a direction parallel to the length of the tube as measured between its ends. As such, in “cross-flow” the fuel stream and therefore, the reactive purge, can be directed anywhere from an angle directly in line with the row to a direction at right angles to the row. Furthermore, the term “row” as used herein and the claims means any arrangement of oxygen transport membranes in a single file. The oxygen transport membranes to be in a “row” do not necessarily, however, have to be positioned so that one oxygen transport membrane is directly in front of or behind another oxygen transport membrane. For instance, oxygen transport membranes may be staggered so that each membrane has full benefit of the reactive purge, or as will be discussed, a sweep gas such that each oxygen transport membrane can take full advantage of such a reactive purge or sweep gas acting at least substantially parallel to the line of oxygen transport membranes making up a row.
It should be pointed out that a cross-flow arrangement is advantageous over flow arrangements that act parallel to the length of the oxygen transport membranes. One major advantage is that all adjacent oxygen transport membranes, as viewed in a transverse direction to the reactive purge will see the same combustion conditions. Furthermore, the fuel composition will be substantially the same from the top to the bottom of an oxygen transport membrane. This will promote uniformity in the oxygen flux and therefore, the combustion flux for the reactive purge along the length of an oxygen transport membrane. Since, the
Bool, III Lawrence E.
Kobayashi Hisashi
Praxair Technology Inc.
Rosenblum David M.
Spitzer Robert H.
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