1. Brief Description of the Invention
This invention is related generally to air separation processes, and in particular to air separation processes combined with steam generation facilities.
2. Related Art
The power generation research and development community faces an important challenge in the years to come: to produce increased amounts of energy under the more and more stringent constraints of increased efficiency and reduced pollution. In order to fulfill both of these goals, it now appears that the power plant of tomorrow will likely have to be modular, and the different modules will likely be combined using advanced system integration techniques.
On the other hand, there are a large number of industrial processes requiring significant electric power consumption. One of these processes is the air separation process, where the power consumption can represent around 50 percent of the overall production costs. The need to reduce emissions cost-effectively in boiler operation, especially generation of nitrogen oxides (NOx), has potential to bring together two very different activities: power generation and gas separation.
This invention is based on the combined use of air separation, staged combustion techniques, and oxygen-enrichment processes. Below is a review of the prior art of these three concepts in the field of boiler operation.
a) Staged Combustion
Coal combustion results in a potentially large amount of unburned coal in the stack, thus wasting a large amount of fuel. This problem can be handled by the following process, allowing more complete fuel combustion, as well as a significant reduction of NOx formation. FIG. 1 describes a combustion chamber 2 in a standard boiler. The combustion chamber can be divided into two major zones: Zone I, as denoted in FIG. 1 at 4, represents the area where the burners are located, together with air inlets. The combustion air can enter the combustion chamber together with the fuel (part of the air is used to transport coal into the combustion chamber, namely the primary air), or in different inlets. The combustion air can be introduced into the boiler partially or totally. More modern schemes use a different air inlet in Zone II, as marked in FIG. 1 at 6, in order to improve the combustion process and to lower the NOx emissions. This scheme is termed xe2x80x9cstaged combustionxe2x80x9d since the combustion process occurs in the two zones. It is noted that the above drawing is very general, showing a horizontal flue gas circulation 8. In general this circulation can be in any direction (upward, side-wards, downward, and the like).
The combustion process is thus divided into two major zones: Zone I represents the ignition zone, where the fuel(s) enter the combustion chamber, are heated and ignite. When coal is of a lesser quality, additional fuel (generally natural gas or fuel oil) is required for a fast ignition. Zone II represents the final region allocated for combustion. Additional oxidant may be introduced, as mentioned above. The modern staged combustion systems allow a significant portion of the oxidant to enter in Zone II (between 10 and 50% of the total oxidant).
However, due to the low pressure of the incoming air in Zone II, the flow patterns of the main flue gas stream are sometimes relatively undisturbed, thus the mixing between the two streams is relatively poor, preventing the full combustion of the unburned fuel.
A solution to this problem, which can be seen in the prior art, is to introduce a high velocity of oxidizer into Zone II through a multitude of streams, for a better mixing. This invention will also address this problem of mixing, through the use of an oxygen-enriched gas introduced into Zone II. The addition of enhanced oxidant (when compared to air) to the fuel rich combustion products will lead to a more effective and complete fuel combustion.
b) Oxygen Enrichment
Oxygen-enriched combustion has become a popular technique employed in a series of industrial applications, such as glass, steel, aluminum and cement manufacturing, to name only a few. The employment of the oxygen-enriched technique has proven to lead to significant process improvements in these industries, such as fuel savings, production increase, and waste processing. Presently, there are applications where the employment of oxygen enriched-combustion has not yet started to be applied on a large scale. One of these applications are the steam generators, where very large amounts of fuel are used for combustion purposes.
The existing boilers have a wide range of steam output, requiring an energy input, from a few hundred kilowatts (kW) to thousands of megawatts (MW). However, the very large investment required for a steam generator, together with the already high thermodynamic efficiency of the existing boilers make the introduction of operational changes relatively hard to implement. The boiler operators are reluctant to introduce modifications in the boiler characteristics, due to possible changes in water vapor properties (temperature, humidity, etc.). Different heat transfer patterns into the various areas of the boiler (combustion space, convective regions) will lead to different local vaporization/superheating rates of the steam, with direct impact on the boiler pipes. Local vapor superheating may lead to lower heat transfer coefficients, therefore to local pipe overheating, eventually causing cracks in pipes. It is therefore crucial to maintain relatively unchanged the heat transfer patterns as originally designed, in order to produce safely the designed vapor throughput.
The use of oxygen-enriched combustion has two consequences to the boiler: it reduces the mass fraction of nitrogen, and it increases the adiabatic temperature of the flame, thus increasing the local heat transfer rates, primarily the radiative heat transfer.
The prior art in the use of oxygen-enriched combustion indicates that it may lead to reduced flue gas mass flow rates flowing through the boiler, with negative implications on the convective heat transfer. This invention will also address this issue, by maintaining the flue gas mass flow rate unchanged from the designed parameters, in conditions of reduced NOx emissions
c) Air Separation
Air separation consumes a large amount of electric power, mostly for air compressionxe2x80x94just like the gas turbine cycle. At the same time, the cryogenic air separation unit cools the gases down in order to separate the different components. Pressurizing cold gases requires by comparison a smaller compressor work, when compared to hotter gases. Also, pressurizing liquid leads to important energy savings when compared to pressurizing gas. Finally, some of the separated gases leave the air separation plant with residual pressure (especially when using membranes), which has potential to be used by an integrated process.
It is noted that the term xe2x80x9cair separationxe2x80x9d includes any technology of air separation, including cryogenics, membranes, adsorption methods, and the like. The outputs nitrogen, oxygen, argon, refer to enriched streams, not necessarilyy pure streams. Thus, the oxygen stream can contain anywhere between 35 and 100% oxygen, and the nitrogen stream can contain anywhere from 35 to 100% nitrogen.
There is thus a need for innovative processes and apparatus to reduce NOx in boiler operation, particularly in integrated air separation/power generation plants.
In accordance with the present invention, integrated air separation/power generation plants are provided, so as to provide the required gases cost-effectively while reducing NOx emissions.
One purpose of this invention is to propose a method to reduce significantly the NOx generation in boiler operation, since issues of environmental concern such as pollutant emissions are now crucial for power generation processes. However, no new technological concept can be implemented without thinking in terms of process efficiency and profitability.
The interest of this invention is thus to present an effective, but also cost-effective technique of NOx reduction through the combination of the steam generation process with an air separation unit in a new design, able to optimize the overall efficiency.
A first aspect of the invention is a combination air separation and steam generation process, the process comprising the steps of:
a) feeding air into an air separation unit, and separating the air into an oxygen-enriched stream and a nitrogen-enriched stream;
b)combining at least a portion of said nitrogen-enriched stream with air to produce an oxygen-depleted first stage air stream, and combining at least a portion of said oxygen-enriched stream with air to produce an oxygen-enriched second stage air stream;
c) mixing a fuel and said oxygen-depleted first stage air stream into a combustion chamber of a steam generation unit and initiating combusting of said fuel with said oxygen-depleted first stage air in said combustion chamber to produce a flue gas;
d) feeding water to said steam generation unit to generate steam by indirect contact of said water with said flue gas; and
e) introducing said oxygen-enriched second stage air downstream of said fuel and said oxygen-depleted first stage air to substantially complete combustion of said fuel, thus creating conditions for combustion with substantially low NOx generation.
As used here in xe2x80x9cfuelxe2x80x9d includes, but is not limited to, gaseous, liquid, and solid fuels, as well as combinations and mixtures thereof. Suitable gaseous fuel include, natural gas, refinery off-gas, sour gas and the like. Suitable liquid fuels include fuel oil (number 2, number 6, bunker C for example). Suitable solid fuel is preferably pulverized coal, but may also include coke, biomass, waste, and the like. As used herein the term xe2x80x9coxygen-depletedxe2x80x9d means a stream that has less oxygen than air, in other words, less than about 21 volume percent. xe2x80x9cOxygen-depletedxe2x80x9d and xe2x80x9cnitrogen-enrichedxe2x80x9d may be used interchangeably for the purposes of this invention.
Preferred are processes wherein the air separation unit is selected from the group consisting of cryogenic air separation units, membrane air separation units, adsorption air separation units, and combinations thereof; processes wherein the nitrogen-enriched first stage air stream has an oxygen concentration ranging from about 10% (vol) to just less than 21%; processes wherein the nitrogen-enriched first stage air stream has an oxygen concentration ranging from about 15% (vol) to about 19%; processes wherein the oxygen-enriched second stage air stream has an oxygen concentration ranging from about 21% to about 35%.
Further preferred processes in accordance with the first aspect of the invention are those wherein the oxygen-enriched second stage air stream has an oxygen concentration ranging from about 27% to about 33%; processes wherein the nitrogen-enriched primary air has an oxygen concentration ranging from about 8% to about 15% and a flow of unmodified secondary air is allowed to enter the combustion chamber(wherein both primary and secondary air are part of the first stage air, but primary air is injected together with fuel).
Particularly preferred are processes in accordance with the first aspect wherein prior to combining the oxygen-enriched stream with air, the oxygen content required in the second stage air to complete combustion of the fuel is determined, and, if the oxygen content required may match the one of the oxygen-enriched stream, directly injecting the oxygen-enriched stream into the combustion chamber in step(e).
Yet other preferred processes in accordance with the first aspect of the invention are those wherein a balance of the nitrogen-enriched stream produced by the ASU is used for a gas offer. Other preferred processes are those wherein a balance of the oxygen-enriched stream produced by the ASU is used for a gas offer.
Particularly preferred are processes in accordance with the first aspect of the invention wherein the air seperation unit comprises one or more membrane units, wherein the nitrogen-enriched stream leaves this air separation unit at a first pressure, and is expanded in a turbine to form a nitrogen-enriched stream at a second pressure, prior to premixing with first stage air. Also preferred are processes in accordance with the first aspect wherein the turbine drives an air compressor, the air compressor performing the feeding of air into the air separation unit. Also preferred are processes wherein the nitrogen-enriched stream at the second pressure is heated at least to ambient temperature prior to being premixed with first stage air. Preferably, the nitrogen-enriched air is substantially pure nitrogen.
Other preferred processes are those wherein the air in step(b) is preheated by exchanging heat with the flue gas; processes wherein prior to being expanded in the turbine, the nitrogen-enriched gas is preheated; processes wherein the preheating is performed by heat exchange with the flue gases in a preheater, preferably wherein the preheating is performed by heat exchange with flue gases directly inside of the steam generation unit.
In accordance with another aspect of the invention, if the ASU is a cryogenic ASU, preferred are processes wherein the nitrogen-enriched stream exiting the ASU is pressurized in a compressor before being preheated by exchanging heat with flue gases, either in a preheater or directly inside the steam generation unit. Also preferred are processes wherein this nitrogen-enriched flow is expanded in a turbine prior to mixing with first stage air, the turbine driving at the same time the compressor used to pressurize the nitrogen-enriched stream and the compressor used to feed air into the ASU. More preferably, the nitrogen-enriched stream is pressurized in the ASU, either as a cold gas or as a liquid, before passing through the final heat exchanger of the ASU.
Also preferred are processes wherein the nitrogen-enriched stream generated by the ASU, after being pressurized through the means described above, first exchanges heat with hot flue gas from the steam generation unit, and is then expanded in a turbine, the turbine being connected to a compressor which compresses the oxygen-enriched second stage air; processes wherein this compressed second stage air is finally injected through a Laval nozzle to increase oxidant velocity, preferably above sonic conditions.
A second aspect of the invention is a plant comprising:
a) means for feeding air into an air separation unit, and separating the air into an oxygen-enriched stream and a nitrogen-enriched stream;
b) means for combining at least a portion of said nitrogen-enriched stream with air to produce an oxygen-depleted first stage air stream, and combining at least a portion of said oxygen-enriched stream with air to produce an oxygen-enriched second stage air stream;
c) means for mixing a fuel and said oxygen-depleted first stage air stream into a combustion chamber of a steam generation unit and initiating combusting of said fuel with said oxygen-depleted first stage air in said combustion chamber to produce a flue gas;
d) means for feeding water to said steam generation unit to generate steam by indirect contact of said water with said flue gas; and
e) means for introducing said oxygen-enriched second stage air downstream of said fuel and said oxygen-depleted first stage air to substantially complete combustion of said fuel.