Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2001-09-11
2004-01-06
Lu, Jiping (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S106000, C110S246000
Reexamination Certificate
active
06672865
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to method and apparatus for the improved operation efficiency and reduced emissions from mineral processing kilns and in particular those kilns wherein the processed mineral liberates gas during thermal processing. More particularly the invention is directed to the injection of high velocity/high energy air into the kiln gas stream to mix gas stream components and dissipate the released gases blanketing the mineral bed allowing for more efficient heat transfer to in-process the mineral and concomitantly to reduce pollutants in the kiln gas effluent stream.
BACKGROUND AND SUMMARY OF THE INVENTION
In the widely used commercial process for the manufacture of cement, the steps of drying, calcining, and clinkering cement raw materials are accomplished by passing finely divided raw materials, including calcareous minerals, silica and alumina, through a heated, inclined rotary vessel or kiln. In what is known as conventional long dry or wet process kilns the entire mineral heating process is conducted in a heated rotating kiln cylinder, commonly referred to as a “rotary vessel.” The rotary vessel is typically 10 to 15 feet in diameter and 200-700 feet in length and is inclined so that as the vessel is rotated, raw materials fed into the upper end of the kiln cylinder move under the influence of gravity toward the lower “fired” end where the final clinkering process takes place and where the product cement clinker is discharged for cooling and subsequent processing. Kiln gas temperatures in the fired clinkering zone of the kiln range from about 1300° C. (~2400° F.) to about 2200° C. (~4000° F.). Kiln gas exit temperatures are as low as about 250° C. (~400° F.) to 350° C. (~650° F.) at the upper mineral receiving end of so-called wet process kilns. Up to 1100° C. (~2000° F.) kiln gas temperatures exist in the upper end of dry process rotary kilns.
Generally, skilled practitioners consider the cement making process within the rotary kiln to occur in several stages as the raw material flows from the cooler gas exit mineral feed end to the fired/clinker exit lower end of the rotary kiln vessel. As the mineral material moves down the length of the kiln it is subjected to increasing kiln gas temperatures. Thus in the upper portion of the kiln cylinder where the kiln gas temperatures are the lowest, the in-process mineral materials first undergo a drying/preheating process and thereafter move down the kiln cylinder until the temperature is raised to calcining temperature. The length of the kiln where the mineral is undergoing a calcining process (releasing carbon dioxide) is designated the calcining zone. The in-process mineral finally moves down the kiln into a zone where gas temperatures are the hottest, the clinkering zone at the fired lower end of the kiln cylinder. The kiln gas stream flows counter to the flow of in-process mineral materials from the clinkering zone, through the intermediate calcining zone and the mineral drying/preheating zone and out the upper gas exit end of the kiln into a kiln dust collection system. The flow of kiln gases through the kiln can be controlled to some extent by a draft induction fan positioned in the kiln gas exhaust stream. Over the last 10-20 years preheater/precalciner cement kilns have proven most significantly more energy efficient than the traditional long kilns. In precalciner kilns the raw mineral feed is heated to calcining temperatures in a stationary counterflow precalciner vessel before it drops into a heated rotary vessel for the higher temperature clinkering reactions.
Responsive to environmental concerns and more rigorous regulating of emission standards, the mineral processing industry has invested in a significant research and development effort to reduce emissions from cement and other mineral processing kilns. The present invention provides a method and apparatus for improving thermal efficiency and reducing emission of gaseous pollutants during the manufacture of thermally processed mineral products such as cement and limestone.
The invention finds application to both so-called long mineral processing kilns and, in the case of cement manufacture, precalciner kilns, already recognized for their energy efficient production of cement clinker. The invention provides advantage in the form of reduced emissions and enhanced energy efficiency in supplemental fuels, the thermal processing of gas releasing minerals including, but not limited to, talconite, limestone, cement raw materials, and clays for the production of light weight aggregates.
In one aspect of the invention high energy/velocity air is injected into the kiln gas stream to reduce or eliminate stratification of gases in a kiln during thermal processing of a mineral that liberates a gas as it is processed.
In another aspect of this invention kiln gas mixing energy is delivered to the kiln gas stream by injecting air at high velocity into rotary kilns in a manner designed to impart rotational momentum to the kiln gases in the rotary vessel. It has been found that injection of high velocity air to promote cross-sectional mixing in mineral processing kilns works to improve energy efficiency by facilitating energy transfer to the mineral bed, and concomitantly such air injection alters the stoichiometry and temperature profile of combustion in the primary combustion zone to reduce the formation of byproduct nitrogen oxides.
According to one aspect of the present invention, there is provided a method for reducing NO
x
emissions and improving energy efficiency during mineral processing in a rotary kiln. The kiln comprises an inclined rotary vessel having a primary burner and a combustion air inlet at its lower end and an upper end for introducing raw mineral feed. The method finds particular use wherein the mineral in a mineral bed in the rotary vessel undergoes a gas releasing chemical reaction during thermal processing in the kiln. The method comprises the step of injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second, typically from an air pressurizing source providing a static pressure of greater than about 0.15 atmospheres, and in one aspect of the invention, at a point along the lower one-half length of the rotary vessel, where the temperature difference between the kiln gases and the mineral are the greatest, to mix the gas released from the mineral with combustion gases from the primary burner. Preferably the mass flow rate of the injected air is about 1 to about 15% of the mass rate of use of combustion air by the kiln.
In one embodiment air is injected into the rotary vessel preferably through an air injection tube extending from a port in the rotary vessel wall into the rotary vessel and terminating in a nozzle for directing the injected air along a predetermined path in the rotary vessel. Typically air is injected into the rotary vessel through two or more nozzles positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel wherein “H” is the maximum depth of the mineral bed in the vessel. Preferably the predetermined path of the injected air is directed to impart rotational momentum to the combustion gases flowing through the rotary vessel. In one aspect of the invention the method further comprises the step of burning supplemental fuel delivered into the rotary vessel downstream relative to kiln gas flow in the kiln from where the air is injected into the kiln. In still another embodiment of the invention the method further includes the step of injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second at a point downstream, relative to gas flow in the kiln, from the supplemental fuel delivery port to mix the gas released from both the mineral bed and the burning supplemental fuel with the combustion gases from the primary burner. The rate of injection of air into the kiln is generally about 1% to about 15%, more typically about 1% to about 7% of the mass of the total combustion air required per unit time du
Hansen Eric R.
Supelak Ralph A.
Tutt James R.
Way Peter F.
Barnes & Thornburg
Cadence Enviromental Energy, Inc.
Lu Jiping
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