Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
2001-03-28
2004-02-03
Price, Carl D. (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S008000, C431S009000
Reexamination Certificate
active
06685464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to oxygen enrichment in industrial heating applications, in particular in power boilers.
2. Related Art
In a previous disclosure of the same assignee of the present invention, Air Liquide Serie file 5167, filed Nov. 10, 1999, application Ser. No. 09/437,526, now U.S. Pat. No. 6,314,896, issued Nov. 13, 2001, there was a proposed scheme of oxygen-enrichment in boilers using large amounts of oxygen, up to full oxy-fuel combustion. That patent application involved a certain ration between the oxygen enrichment and the flue gas recirculation, such that the design boiler parameters are maintained constant.
While quite inventive, the above-referenced methods in said patent application may not be desirous in particular industrial heating applications, in particular coal fired boilers, primarily pulverized coal, but with the application to fluidized beds also. Coal combustion results in a potentially large amount of unburned coal in the stack, thus losing a large amount of fuel. Also, due to the incomplete combustion of coal, as well as to the sometimes difficult ignition process, a support fuel such as natural gas is frequently used in significant quantities (from about 10% to about 50% of the total amount of fuel). The ease and completeness of combustion is directly related to the volatile content of the coal, and indirectly related to the percentage of moisture in the coal. In other words, more moisture means more difficult and possibly incomplete combustion, while more volatiles in the coal means more complete combustion.
FIG. 1
presents in schematic, a prior art combustion chamber
2
in a boiler
4
, where the combustion chamber
2
is divided into two major zones: Zone I, as denoted in
FIG. 1
at
6
, represents the area where combustion burners are located, together with air inlets. Combustion air can enter combustion chamber
2
together with a fuel (part of the air is used to transport coal into combustion chamber
2
), or in different inlets. Combustion air can be introduced into the boiler partially or totally at this location. More modern schemes use a different air inlet, denoted as Zone II, and noted as
8
in
FIG. 1
, in order to improve the combustion process and to lower the NO
x
emissions. The combustion scheme illustrated schematically in
FIG. 1
is termed “staged combustion” since the combustion process occurs in two zones. It is noted that the schematic in
FIG. 1
is very general, showing a generally horizontal flue gas circulation
10
. In general, flue gas circulation can be in any direction (vertical, horizontal, circular, and the like).
As illustrated in
FIG. 1
, the combustion process is 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 supra. Modern staged combustion schemes allow a significant portion of the oxidant to enter Zone II (between 10% and 50% of the total oxidant). Due to the low pressure of the incoming air in Zone II, the flow patterns of the main flue gas stream are relatively undisturbed, thus the mixing between the two streams is relatively poor, preventing the full combustion of the fuel. This is represented by the shaded area
16
in FIG.
2
.
FIG. 2
illustrates that the mixing zone
16
between the flue gas stream
10
and the balance of the oxidizer in Zone II and exiting into the mixing zone
16
is not total, thus an important part of the fuel will not mix with the oxidant, thus remaining unburned.
It would be an advantage if the fuel from Zone I could be mixed with oxidant from Zone II to provide better mixing between fuel and secondary oxidant.
SUMMARY OF THE INVENTION
In accordance with the present invention, methods and apparatus are provided which overcome problems associated with the prior art methods. The present invention involves introducing a high velocity stream of an oxygen-enriched gas into Zone II through a multitude of streams, preferably uniformly distributed for better mixing. “Oxygen-enriched” as used herein, is considered any gas having concentration of oxygen higher than 21% (the oxygen concentration in air). The results of this inventive process are: providing the fuel and/or fuel-rich combustion products with enhanced oxidant (when compared to air), and also improving the mixing between the fuel and/or fuel-rich combustion products and the oxidant. The combined effect of high oxygen concentration and improved mixing leads to a more effective and complete fuel combustion.
One aspect of the invention is a method of increasing combustion of a hydrocarbon fuel in a combustion chamber of a furnace, the combustion normally using only air as an oxidant, part of the air entering the combustion chamber near (preferably in) one or more fuel burners, and a remaining portion of air entering the combustion chamber at a plurality of locations downstream of said fuel burners, the method comprising the steps of injecting oxygen-enriched gas through a plurality of lances into a flue gas in the combustion chamber at the plurality of downstream locations, the oxygen-enriched gas injected at a velocity ranging from subsonic to supersonic, the oxygen-enriched gas being present in an amount sufficient to provide an oxygen concentration in the flue gas of no more than 2% on a volume basis greater than when air is used alone as oxidant. Preferably, the velocity is subsonic for the oxygen-enriched gas in each of the plurality of lances, or in some embodiments the velocity is supersonic for the oxygen-enriched gas in each of the plurality of lances. In any case the velocity of the oxygen-enriched gas is greater than velocity of air injection.
As used herein the term “combustion chamber” includes any area where combustion of fuel can occur in a furnace or boiler.
In other preferred embodiments, some of the oxygen-enriched gas is injected at subsonic velocity in one or more lances while a balance of the oxygen-enriched gas is injected at supersonic velocity through one or more lances.
The oxygen-enriched gas is preferably injected through the lances at an angle with respect to a wall of the combustion chamber, the angle ranging from about 20° to about 160°, the angle measured in a plane that is perpendicular to the wall. Preferably, the plurality of locations are arranged so that one-half of the lances are on a first wall and one-half of the lances are on a second wall. Also preferred are embodiments where lances on the first wall are separated by distance L
L
, wherein L
L
<L
CH
/2, wherein L
CH
is selected from the group consisting of height, length, and width of the combustion chamber, and wherein the lances on the first wall are positioned a distance
1
from the lances on the second wall, wherein 0<1<L
L
/2, wherein L
L
is the distance between lances on the first wall.
Preferably, the combustion chamber is rectangular, wherein there is one lance on each of four walls of the rectangular combustion chamber, and wherein each lance is a distance L′ from a wall wherein an adjacent lance is positioned. Preferably, L′<L
CH
/2. In this embodiment, each lance is preferably positioned at a first angle ranging from about 20° to about 160°, the first angle measured in a first plane which is substantially vertical and substantially perpendicular to its corresponding wall, and each lance is preferably positioned at a second angle ranging from about 20° to about 160°, the second angle measured in a plane substantially perpendicular to the first plane.
In the embodiments which employ rectangular combustion chambers, the remaining portion of air preferably enters the combustion chamber through one or more rectangular slots, or through one or more substantially circular slots.
Other preferred methods are those wherein the oxyge
Bugeat Benjamin
Di Zanno Pietro
Marin Ovidiu
Streicher Eric
Cronin Christopher J.
L'Air Liquide - Societe Anonyme à Directoire et Conseil de
Price Carl D.
Russell Linda K.
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