Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2001-12-03
2003-05-27
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S651000, C060S671000
Reexamination Certificate
active
06568185
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
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 “staged combustion” 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 compression—just 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 “air separation” 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.
SUMMARY OF THE INVENTION
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 e
Marin Ovidiu
Penfornis Erwin
L'Air Liquide Societe Anonyme a'Directoire et Conseil
Nguyen Hoang
Russell Linda K.
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