Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2001-10-26
2003-04-01
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C095S045000, C095S054000, C096S004000
Reexamination Certificate
active
06539719
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an integrated system that generates steam and may optionally also provide high purity streams of one or more of oxygen and nitrogen. More particularly, the integrated system combines an oxygen selective ion transport membrane with a boiler furnace. Combustion of a fuel with oxygen transported through the membrane generates heat and combustion products that are used to fire the boiler. Flue exhaust from the boiler is essentially free of NO
x
compounds.
BACKGROUND OF THE INVENTION
Boiler systems operate as a pressurized system in which water is vaporized to steam by heat transferred from a source of higher temperature. The steam may then be used directly as a heating medium or as a working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy. For example, expansion of the steam may be used to drive the blades of a turbine. Although other fluids are sometimes used in boilers, water is by far the most common because of its economy and suitable thermodynamic characteristics. Typically, the required heat is generated by the combustion of burning fuels.
When the fuels are burned in the presence of air, NO
x
compounds may be generated. These NO
x
compounds are deleterious from an environmental standpoint and minimizing or avoiding their formation is desired.
U.S. Pat. No. 5,076,779 to Kobayashi, that is incorporated by reference in its entirety herein, discloses a number of methods to reduce the formation of NO
x
in a combustor. These methods include reduction of the peak flame temperature, diluting the fuel and/or the oxygen content with a diluent and injecting separate oxidant and fuel streams into a furnace at elevated temperatures whereby the oxygen content is diluted by the furnace atmosphere to below 10%, by volume, before contacting the injected fuel.
An article by Heap et al. entitled “Application of NO
x
Control Techniques to Industrial Boilers” recites reducing NO
x
formation in a boiler by reducing the peak flame temperature, reducing the residence time of molecular nitrogen in high temperature zones and having the initial stage of heat release occur in a fuel-rich environment causing nitrogen radical intermediates to convert to N
2
rather than NO
x
.
While the use of oxygen, rather than air, as the oxidant in a combustor would eliminate the formation of NO
x
oxygen from a cryogenic source has historically been too expensive for effective utilization in a boiler system.
Another method to generate oxygen is with an oxygen selective ion transport membrane. This membrane is a non-porous ceramic material that is capable, under proper operating temperature and oxygen partial pressure conditions, of the selective diffusion of either oxygen ions alone or a combination of oxygen ions and electrons. Air, or another oxygen-containing gas, is contacted to a first side of the ceramic material and oxygen ions are transported through the ceramic material while the other constituents of the feed gas are not. The ceramic materials are referred to as “oxygen selective” meaning that only oxygen ions are transported across the membrane with the exclusion of other elements and ions.
Suitable ceramics for use as membrane materials include mixed conductive perovskites and dual phase metal-metal oxide combinations, typified by calcium- or yttrium- stabilized zirconium or analogous oxides having a fluorite or perovskite structure. Exemplary ceramic compositions are disclosed in U.S. Pat. No. 5,702,959 (Mazanec, et al.), U.S. Pat. No. 5,712,220 (Carolan, et al.) and U.S. Pat. No. 5,733,435 (Prasad, et al.). All of the preceding patents are incorporated by reference in their entireties herein.
Use of such membranes in gas purification applications is described in European Patent Application No. 778,069 entitled “Reactive Purge for Solid Electrolyte Membrane Gas Separation” by Prasad, et al.
The ceramic membrane has the ability to transport oxygen ions and electrons at the prevailing oxygen partial pressure in a temperature range of from 450° C. to about 1200° C. when a chemical potential difference is maintained across the membrane. This chemical potential difference is established by maintaining a positive ratio of oxygen partial pressures across the ion transport membrane. The oxygen partial pressure (P
o2
) is maintained at a higher value on the cathode side of the membrane, that is exposed to the oxygen-containing gas, than on the anode side, where transported oxygen is recovered. This positive P
o2
ratio may be obtained by reacting transported oxygen with an oxygen-consuming process or fuel gas. The oxygen ion conductivity of a mixed conductor perovskite ceramic membrane is typically in the range of between 0.01 and 100 S/cm where S (“Siemens”) is reciprocal of ohms (1/ohms).
For effective application of a perovskite for oxygen separation, a number of requirements should be met. (1) The perovskite should have a high oxygen flux, where flux is the rate of oxygen transport through the membrane structure. (2) The perovskite must have a cubic crystalline structure over the entire range of operating temperatures. Perovskites with a hexagonal crystalline structure are not effective for oxygen transport. Some perovskites have a hexagonal crystalline structure at room temperature (nominally 20° C.) and undergo a phase transformation at an elevated temperature. In such a material, the phase transformation temperature represents the minimum temperature at which an oxygen separator containing that material as a membrane element may be operated. (3) The perovskite structure must be chemically stable at the operating temperature and (4) have a degree of mechanical stability.
A number of mixed oxide perovskites are disclosed as useful for oxygen separation. These perovskites are typically of the form ABO
3-*
where A is a lanthanide element, B is a transition metal and O is oxygen. A lanthanide, or rare earth element, is an element between atomic number 57 (lanthanum) and atomic number 71 (lutetium) in the Periodic Table of the Elements as specified by IUPAC. Typically, yttrium (atomic number 39) is included within the lanthanide group. The transition metals are those in Period 4, and between Groups II and III, of the Periodic Table of the Elements and include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. The A component and/or the B component may be doped with other materials to enhance stability and performance.
For stoichiometric balance, the material has three oxygen atoms. However, the oxygen transport membranes are non-stoichiometric and include vacancies at certain of the oxygen lattice points. These vacancies are represented in the formula ABO
3-*
by *, where * may be between about 0.05 and 0.5. The vacancies are mobile and move throughout the ceramic material. Oxygen ions are transported through the membrane by moving from lattice vacancy to lattice vacancy.
A paper by Sirman, “A Study of the Mass Transport and Electrochemical Properties of Materials for Ceramic Oxygen Generators” discloses that the rate of oxygen diffusivity is more dependent on the concentration of vacancies than on the vacancy mobility rate.
U.S. Pat. No. 5,648,304 by Mazanec, et al. discloses an oxygen selective perovskite represented by the formula
[A
1−x
A′
x
][Co
1−y−x
B
y
B′
z
]O
3−d
,
where A is selected from the group consisting of calcium, strontium and barium;
A′ is selected from the lanthanide series defined as elements 57-71 on the Periodic Table of Elements as well as yttrium, thorium and uranium;
B is selected from the group consisting of iron, manganese, chromium, vanadium and titanium;
B′ is selected to be copper or nickel;
x is in the range of between about 0.0001 and 0.1;
y is in the range of from about 0.002 and 0.05;
z is in the range of from about 0.0005 and 0.3; and
d is determined by the valence of the metals.
Mazanec et al. disclose that the addition of a relatively low concentrati
Cook Pauline Jane
Kobayashi Hisashi
Prasad Ravi
Nguyen Hoang
Praxair Technology Inc.
Rosenblum David M.
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