Boiler having an integrated oxygen producing device

A system is described wherein a boiler 10 is integrated with an oxygen producing device 12. Combustion heat generated in the boiler 10 is used to generate steam in the boiler and is also applied to at least one of a sweep gas stream 13 and a feed gas stream 15 to ensure that the sweep gas and feed gas streams 12, 15 are provided at the appropriate temperature to the oxygen producing device 12. Flue gas generated by fuel combustion within the combustion chamber 14 may be used as the sweep gas stream 13, in which case, the flue gas exiting the oxygen producing device 12 includes the oxygen removed from the feed gas stream. The flue gas/oxygen mixture may be used for fuel combustion within the combustion chamber, and may be provided to an oxygen separator 28 for removing oxygen from the flue gas. Sensible heat contained in the oxygen depleted feed gas from the oxygen producing device 12 may be recovered by the feed gas stream 15.

TECHNICAL FIELD

The present invention relates to a boiler being integrated with an oxygen producing device, and more specifically to a system in which combustion heat generated in the boiler is used to generate steam in the boiler and is applied to at least one of a sweep gas stream and a feed gas stream to ensure that the sweep gas and feed gas streams are provided to the oxygen producing device at temperatures required by the oxygen producing device.

BACKGROUND

The combustion of fossil fuels with relatively pure oxygen has been considered for a variety of reasons, primarily related to the desire to eliminate or effectively reduce the dilution effects of the nitrogen in air. In the past, interest was in reducing the formation of nitrogen oxides during combustion. More recently, the desire to produce a concentrated stream of carbon dioxide for capture and sequestration in order to reduce greenhouse gas emissions has provided a driving impetus. Large quantities of oxygen are commercially produced via the cryogenic separation of oxygen from nitrogen in air. However, the energy requirements for this process are quite high relative to the combustion process, ranging from about 20% to about 30% of the fuel energy depending upon the oxygen purity required. This energy consumption greatly reduces the output of steam and electricity in power plants (see Bozzuto, et al., 2001, OCDO/AEP retrofit study). New technology developments seek to generate oxygen with lower energy consumption. Economic studies indicate that these approaches could improve plant economics (see “Greenhouse Gas Emissions Control By Oxygen Firing In Circulating Fluid Beds” Vol. II (Nsakala, et al., 2003). Advanced systems for the separation of oxygen from a feed gas stream include, for example, pressure swing systems, physical and chemical adsorption systems, and membrane systems. In such systems, the feed gas stream passes over a sorbent, membrane, or the like, and at least a portion of the oxygen within the feed gas is removed. In many of these systems, a high temperature sweep gas is required to provide a chemical driving force for oxygen separation or to regenerate the sorbent material. In effect, the sweep gas stream “sweeps” the oxygen away from the oxygen producing device (e.g., sorbent, membrane, or the like). To ensure the proper operation of the oxygen producing device, the sweep gas and feed gas streams must be provided to the oxygen producing device at temperatures within specified temperature ranges (see U.S. Pat. No. 6,562,104).

One example of a membrane system is the oxygen transport membrane as discussed in U.S. Pat. No. 6,406,518. In this system, air is heated before passing through a ceramic membrane. On the other side of the membrane, a gas with very low oxygen content is also preheated and passed over the outside of the membrane. The difference in oxygen partial pressure provides a driving force for the separation of oxygen from the air through the membrane. The temperature range is given as 450° C. to 1200° C. In an absorption system, the air is passed over a medium that captures oxygen at one temperature and liberates oxygen at a higher temperature. The driving force for separation may also be provided by air compression as described in U.S. Pat. No. 6,702,570. Alternatively, a material may absorb oxygen at high pressure and release oxygen at a lower pressure, as done in pressure swing systems. The goal of all of these systems is to separate oxygen from air at a lower energy penalty than cryogenic separation.

As a high temperature environment is often required in these systems, it is desired to provide a system that allows the use of these various oxygen generation systems with a boiler system to generate steam (and electricity) with reasonable efficiency, provide a concentrated carbon dioxide stream, and provide a high temperature environment necessary to allow the oxygen separation system to function optimally.

SUMMARY

In one aspect, there is provided a system comprising an oxygen producing device and a boiler. The oxygen producing device removes oxygen from a feed gas stream and provides the removed oxygen to a sweep gas stream. The boiler includes a combustion chamber in which fuel combustion provides heat for generating steam. The boiler also includes at least one air heater in which at least one of the feed gas stream and the sweep gas stream are heated before entering the oxygen producing device. In various embodiments, at least a portion of the oxygen producing device is disposed in the boiler and flue gas generated by fuel combustion within the combustion chamber is used as the sweep gas stream. In these embodiments, the flue gas exiting the oxygen producing device includes the oxygen removed from the feed gas stream. The flue gas/oxygen mixture may be used for fuel combustion within the combustion chamber, and may be provided to an oxygen separator for removing oxygen from the flue gas. Sensible heat contained in the oxygen depleted feed gas is recovered by the feed gas stream by heat exchange from the oxygen depleted gas to the feed gas stream (air).

In another aspect, there is provided a boiler comprising a combustion chamber in which fuel combustion provides heat for generating steam, and at least one air heater is disposed in the boiler proximate the combustion chamber. The at least one air heater is in fluid communication with an oxygen producing device, and at least one of a feed gas stream and a sweep gas stream to the oxygen producing device are heated in the at least one air heater.

In yet another aspect, there is provided a method for heating a feed gas stream and a sweep gas stream for use in an oxygen producing device. The method comprises: combusting fuel in a combustion chamber of a boiler to generate heat, the combustion resulting in a flue gas; heating a feed gas stream using the heat generated by combusting the fuel; providing the feed gas stream to an oxygen producing device to remove oxygen from the feed gas stream; and providing the flue gas as a sweep gas stream to the oxygen producing device to remove the oxygen from the oxygen producing device.

DETAILED DESCRIPTION

FIGS. 1-5depict various embodiments of a boiler10integrated with an oxygen producing device12. In each of the systems, the combustion heat generated in the boiler10, which is used to generate steam in the boiler10, is also applied to at least one of a sweep gas stream13and a feed gas stream15to ensure that the sweep gas and feed gas streams12,15are provided to the oxygen producing device12at temperatures required by the oxygen producing device12. In each of the examples described inFIGS. 1-4, the feed gas is air, the sweep gas is flue gas, and the oxygen producing device12transfers oxygen from the feed gas stream15to the sweep gas stream13. However, it will be noted that the sweep gas is steam generated in the boiler10inFIG. 5. It is contemplated, however, that the oxygen producing device12may be any device that separates oxygen from a feed gas and employs a sweep gas to provide a chemical driving force for the oxygen separation or to regenerate a sorbent material used in the oxygen separation. For example, the oxygen producing device12may be a pressure swing system, adsorption system, membrane system, or the like.

In a first embodiment, shown inFIG. 1, a boiler10is integrated with an oxygen producing device12. In the boiler10, recycled flue gas enriched with O2replaces combustion air and is premixed with fuel for firing in a combustion chamber14of the boiler10. Heat of combustion is transferred to the waterwalls of the boiler10to generate steam and to preheat a feed gas (air) stream15flowing through a high temperature air heater16.

In the air heater16, the feed gas stream15is heated to a temperature required by operating characteristics of the oxygen producing device12. In the embodiment shown, the oxygen producing device12is an oxygen transport membrane17that selectively passes oxygen ions through the membrane17without allowing the other gaseous components to pass through the membrane17. Thus, the gas exiting the transport membrane17is hot, O2depleted air, which is essentially pure N2. This N2gas is collected from the oxygen producing device12at an exhaust header18located external to the boiler10, and the N2gas is fed to a low temperature air heater20where heat contained in the N2stream is recovered. A heat recovery steam generator (HRSG) or any other cooling system could be used to replace the low temperature air heater20.

The feed gas stream15is preheated in the low temperature air heater20and is piped to an inlet header22of the high temperature air heater16. From the high temperature air heater16, the feed gas stream15flows to the oxygen producing device12. Connections between high temperature air heater16and the oxygen producing device12are made outside the boiler10by a system of connectors24.

In the boiler10, at least a portion of the oxygen producing device12(e.g., the transport membrane17) is positioned in the path of the flue gas, and the flue gas acts as the sweep gas stream13for the oxygen producing device12. The products of combustion consist essentially of a mixture of water vapor (H2O) and carbon dioxide (CO2), and are well suited for sweeping O2from the oxygen producing device12. The flue gas may be further cooled by convective heat transfer sections26to maintain the temperature of the sweep gas stream13to within the temperature requirements of the oxygen producing device12.

In the embodiment shown, the flue gas flows over the membrane17elements sweeping away the O2that is transferred across the membrane17. The flue gas, now rich in oxygen, contains the same weight percent O2as in the ambient air. The O2rich flue gas may be cooled further before exiting the boiler10. Upon exiting the boiler10, the flue gas is split into two streams: one stream is used for combustion, and the other stream is diverted to a water vapor condenser and CO2and O2separation systems28, where the gases contained in the flue gas are separated, cleaned and prepared for commercial use.

Referring toFIG. 2, the high temperature air heater16may be installed in the lower furnace zone of the boiler10, and the oxygen producing device12may be installed in the upper furnace zone of the boiler10. Connection between the oxygen producing device12and the high temperature air heater16is facilitated outside the boiler10. Where the oxygen producing device12employs a transport membrane17, the transport membrane17may be situated close to, or within, the combustion chamber14to increase the temperature of the sweep gas stream13at the transport membrane17.

FIG. 3depicts an embodiment where the boiler10is a package boiler. The embodiment ofFIG. 3is similar to that ofFIG. 2except for the location of the oxygen producing device12, which is shown inFIG. 3to include a transport membrane17and air inlet header22. InFIG. 3, a high temperature air heater (not shown) may be installed inside the boiler10along with the oxygen producing device12. The oxygen producing device12and/or the high temperature air heater may be incorporated into a wall40defining the combustion chamber14of the boiler10in such a way that the oxygen producing device12and/or the high temperature air heater is removable from the rest of the boiler10along with the wall40(e.g., through the use of a flanged wall40).

Referring toFIG. 4, an embodiment is shown wherein the oxygen producing device12(e.g., a transport membrane17) is installed inside a boiler chamber50forming an integral part of the larger boiler10. Within the chamber50, small temperature control burners52are installed around the oxygen producing device12to ensure that the system12is maintained at a desired temperature during start-up and operation, and to protect the system12against potential thermal shock caused by the flow of the sweep gas stream13over the membrane17.

The sweep gas stream13may be provided by internally-induced recirculation via ports54installed in the waterwalls of the boiler10, by external recirculation of flue gas by a gas recirculation fan56, or by both. The sweep gas stream13consists essentially of either CO2or a mixture of CO2plus H2O. O2contained in the sweep gas stream13is consumed in the combustion within the combustion chamber14of the main boiler10, producing flue gas high in CO2and H2O concentration. The flue gas may be cooled in a series of heat exchangers26,58.

Hot O2depleted air, which is essentially pure N2is collected from the oxygen producing device12and piped to an air heater20where sensible heat contained in the N2stream is recovered by the feed gas stream15. An HRSG or any other cooling system could be used to replace the air heater20.

FIG. 5depicts an embodiment wherein the oxygen producing device12is positioned outside the boiler10. Fuel fired with air (or a flue gas/O2mixture) in the boiler10transfers heat to generate steam and to preheat air, which are transported to the oxygen producing device12as the sweep gas stream13and feed gas stream15, respectively.

High temperature feed gas stream15is provided by an air heater system including a convective (low temperature) section60and a radiant (high temperature) section16installed inside the boiler10. Hot O2depleted air, which is essentially pure N2is collected from the oxygen producing device12and is piped to an HRSG or an air heater20where sensible heat contained in the N2stream is recovered.

Steam generated by the boiler10is used as the sweep gas stream13to sweep O2from the oxygen producing device12. The sweep gas stream13is provided at the temperature required to maintain the membrane17at operating conditions and to prevent thermal shock of the membrane17. The sweep steam with O2is transported from the oxygen producing device12to an HRSG/H2O condenser system62where the steam/O2mixture is cooled, water vapor is condensed and O2separated. The separated O2could be used for combustion in boiler10or in another industrial application.

If the CO2/H2O/O2mixture from the oxygen producing device12is used for combustion in the boiler10instead of combustion air, flue gas leaving the boiler10may be treated in a water vapor condenser and CO2separation system28. Alternatively, if combustion air is used, flue gas leaving the boiler10is emitted to the atmosphere through the stack.

Since the invention is susceptible to various modifications and alternative forms, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the scope of the invention extends to all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.