Oxygen enrichment of cement kiln system combustion

Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing

Reexamination Certificate

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C106S761000

Reexamination Certificate

active

06488765

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally pertains to the field of Portland cement manufacturing and, more particularly, to the combustion process in Portland cement manufacturing.
2. Description of Related Art
The Portland cement manufacturing process has, in its most general form, long been established. As shown in
FIG. 1
, a raw feed material
12
is fed into a cement kiln system
10
wherein it is heated until it calcines and transforms into a material
13
known as “cement clinker” or “clinker.” More particularly, the raw feed material
12
is fed into the cyclone preheater
14
. The preheater
14
heats the raw feed
12
to a temperature ready for calcination and then passes the preheated feed
12
to the precalciner
16
. The precalcined feed
12
then enters the rotary kiln
18
wherein it is transformed into the clinker
13
that is deposited in is clinker cooler
20
.
The calcination reaction in the precalciner
16
takes place at a relatively narrow temperature range of about 1500 to 1600° F. The heat for calcination is provided by the flameless combustion of fuel injected at point
17
of the precalciner
16
and the preheated feed
12
can reach over 90% calcination before entering the rotary kiln
18
. The residual carbonate is calcined in the rotary kiln
18
, where the temperature of the calcined feed
12
is raised to a clinkering temperature of about 2650° F. by firing a mixture of fuel and air by kiln burner
22
. At the clinkering temperature, approximately 25% of the hot meal is liquefied and, to reach the clinkering temperature, a flame temperature of over 3500° F. is required.
The tumbling action of the rotary kiln forms the partially liquefied feed into clinker nodules
13
that drop into the clinker cooler
20
, where they cool and are taken for pulverization. The air in the clinker cooler
20
cools the clinker
13
by absorbing heat therefrom and the air is thereby heated. This heated air is recycled into the rotary kiln
18
as “secondary air” to support the combustion therein and into the precalciner
16
via a tertiary air duct
21
as “tertiary air.”
In the late 1950's and 1960's, prior to the development of the precalciner, the industry began experimenting with oxygen enrichment of kiln combustion as a potential refinement.
Martin J. La Velle, in 1959, suggested such in “Oxygen Enrichment of Primary Air Can Improve Kiln Production,” published in the journal
Rock Products
, but no practical application is known. Oxygen enrichment of secondary air has been practiced on several occasions. A report by Robert A. Gaydos, entitled “Oxygen Enrichment of Combustion Air in Rotary Kiln,” published by the Portland Cement Association, indicated that oxygen enrichment could improve kiln production.
One common method of oxygen enrichment for a rotary kiln is to place an oxygen lance in between kiln burner
22
and the feed
12
in the rotary kiln
18
. Oxygen is injected through the lance of the burner
22
at a certain tip velocity. Since the underside of the flame is in contact with pure oxygen will exhibit high flame temperature, the oxygen lance is placed so that excessive flame temperature will not impact refractory and kiln coating. The drawbacks of this practice are many: (1) oxygen lance is subject to high temperature, (2) proper direction and velocity of oxygen jet is critical, (3) high flame temperature promotes increased NOx formation, and (4) high flame temperature may adversely impact refractory life.
The precalciner
16
also contains a firing point, which firing point can be equipped with multiple burners. The preheated feed
12
entering the precalciner
16
is suspended and calcined in the vessel thereof. The fuel injected at point
17
supplies the heat necessary to dissociate carbon dioxide from limestone in the feed
12
. The combustion air is primarily supplied as tertiary air from the clinker cooler
20
through the tertiary air duct
21
. Due to the large amount of limestone powder present, the feed
12
undergoes an endothermic reaction in the precalciner vessel and the combustion is flameless. Oxygen enrichment in the precalciner has a reduced risk to refractory life or to increased NO
x
formation, mainly due to the low temperature combustion.
However, a major deterrent to oxygen enrichment in cement kiln systems has always been the cost of oxygen. Some reports have also indicated technical concerns arising from oxygen enrichment in the kiln, such as refractory life, burning zone shift and coating stability. Thus, despite the optimistic tenor of some experiment reports and even though it is commonly used in lime kilns, oxygen enrichment in cement kiln systems never became a common practice because of a variety of technological and economic considerations.
By contrast, so far as is known, nobody has attempted oxygen enrichment of combustion in the precalciner. One authority in the field, Kurt E. Peray in
The Rotary Kiln
(2nd ed. 1985), has suggested this specifically for precalciners without tertiary air ducts or for “air through” systems in order to reduce the excess air drawn through the kiln, but admits not knowing of any previous attempts and makes no mention of oxygen enrichment in systems using a tertiary air duct. Further, Peray admits that “[t]his idea would require research before deciding whether or not it could be feasibly implemented.” Id., at p. 75. However, after approximately 12 years, nobody in the industry has reported such an experiment and certainly there are no known successful attempts.
Thus, it is desirable to develop a cement manufacturing process with higher productivity than is otherwise available from existing pyroprocessing equipment. It would also be desirable to avoid the disadvantages of traditional introduction of oxygen to the kiln burner.
SUMMARY OF THE INVENTION
The invention in its various embodiments includes a method and an apparatus for improving combustion in a cement kiln system. The method in one embodiment comprises enriching the tertiary air stream to the precalciner of a cement kiln system with oxygen. The apparatus in one embodiment comprises a precalciner and an oxygen source coupled to the precalciner.


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