CLIAMS:WHAT IS CLAIMED IS:

1. A process for producing iron in an elongated rotary kiln having a feed end, a discharge end, a heating zone between said feed end and said discharge end, and a reducing zone between said heating zone and said discharge end, comprising:
feeding iron ore or iron pellet, a first carbonaceous material stream and dolomite into the feed end, and injecting a second carbonaceous material stream into the reducing zone through the discharge end;
injecting a first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and injecting a second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream;
heating said iron ore or iron pellet in said heating zone by combustion of said first and second carbonaceous material streams; passing said heated iron ore or iron pellet to said reducing zone, and reducing said heated iron ore or iron pellet in said reducing zone to metallic iron, and recovering iron from said kiln at said discharge end;
wherein the amount of oxygen delivered by one or both of said streams is adjusted in a cyclic manner.

2. The process of claim 1 wherein the total amount of oxygen supplied by the first and second oxygen containing streams is maintained within +/- 5.0% of a target value by adjusting the amount of oxygen delivered by said streams.

3. The process of claim 1 wherein adjustment in oxygen delivered comprises increasing the oxygen delivered by one of the first and second oxygen containing streams and decreasing the oxygen delivered by the other of the said streams.
4. The process of claim 1 wherein the oxygen content or injection rate or both of a given oxygen containing stream can be adjusted to deliver increased or decreased amount of oxygen by that stream.

5. The process of claim 1 wherein adjusting oxygen delivered in a cyclic manner includes maintaining the delivery rate at some starting value for a defined period of time, then either increasing or decreasing the flow in a stepwise manner to a final value and then cycling back to the starting value.

6. The process of claim 4 wherein the stepwise manner could be a linear, polynomial or some other non-linear function.

7. The process of claim 1 wherein the second carbonaceous material stream size distribution and/or pneumatic transport air is adjusted to inject a greater portion of said stream to targeted areas within the rotary kiln.

8. The process of claim 1 producing a byproduct solid fuel containing greater than 20 % by weight carbon.

9. A system for producing sponge iron, comprising:
an elongated rotary kiln having a feed end, a discharge end;
means for feeding iron ore, a first carbonaceous material stream and optionally dolomite into the feed end; means for injecting a second carbonaceous material stream into the reducing zone through the discharge end;
a first conduit means for injecting a first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and a second conduit means positioned concentric or eccentric inside the first conduit means for injecting a second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream;
an oxygen source to provide an oxygen stream and means to split the oxygen stream into a first oxygen stream and a second oxygen stream; and
a process controller configured to adjust the split from a starting value to a final value and then back to starting value in a cyclic manner while maintaining the flows of the first and second oxygen streams at each split value for a defined interval.

10. A method of operating a sponge iron production system comprising:
feeding iron ore, a first carbonaceous material stream and optionally dolomite into the feed end of a rotary kiln, and injecting a second carbonaceous material stream through a discharge end of the kiln;
heating said iron ore or iron ore pellet by complete and incomplete combustion of said first and second carbonaceous material streams in the kiln, and reducing said heated iron ore or iron ore pellet to metallic iron and recovering iron from said kiln at said discharge end;
measuring temperature at one or more location within the kiln and injecting oxygen through the discharge end when all temperature measurements are below a predefined maximum temperature;
wherein oxygen injection comprises:
sourcing oxygen from an oxygen source and dividing the sourced oxygen to form a first oxygen containing stream and a second oxygen containing stream;
injecting the first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and injecting the second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream while adjusting the oxygen delivered by one or both of said streams in a cyclic manner; and
when any measured temperature equals or exceeds the predefined maximum temperature then stopping oxygen injection, and resuming oxygen injection when all measured temperatures are below the predefined maximum temperature.

,TagSPECI:OXYGEN INJECTION IN SPONGE IRON PRODUCTION
Field of the Invention
The present invention relates to a method and system for producing sponge iron in which iron-oxide containing materials are directly reduced in the solid state. More particularly, the present invention relates to a method and system employing a plurality of oxygen containing gases and a plurality of carbonaceous material streams for direct reduction of iron-oxide containing materials in a rotary kiln.
Background of the Invention
Sponge iron is produced by reducing oxides of iron in iron containing raw materials in solid state at temperatures in the range of 800oC. to 1100oC. Such elevated temperatures yield favorable chemical reaction kinetics but are also below melting point of solids for economical production in rotary kilns at commercial scale. Solid carbonaceous materials such as coal are utilized both as a reducing agent and as a fuel. Air, oxygen or mixtures of air and oxygen support combustion reactions that supply thermal energy for heating of materials to reaction temperatures, as well as, for converting iron containing raw material into directly reduced iron product.
Rotary kilns employed in commercial sponge iron production are of elongated type, supported on rollers or the like such that the feed end is inclined above the discharge end and the kiln can be rotated about its longitudinal axis. Rotation of the kiln is provided by a motor that is connected to the kiln exterior, for example through a girth gear. Iron containing raw material is charged from the feed end. Solid carbonaceous material such as coal is charged from the feed end a particulate stream of coal is injected from the discharge end. Dolomite charged through the feed end is utilized as a sorbent for capturing sulfur released from coal and iron containing raw materials. A central blower and/or shell air fans introduce air into the kiln, providing oxygen for combustion reactions within the kiln.
To increase rotary kiln output and/or to improve thermal efficiency, sponge iron producers increase iron ore charge rate in conjunction with one or more of increasing rotary kiln rotational speed, increasing discharge end coal injection rate, injecting oxygen or oxygen enriched air to provide oxygen without the added burden of nitrogen flow. U.S. Patent 3,881,916 and Indian Patent Application Number 134/DEL/2011 describe some of these practices. These approaches work but beyond a certain improvement have operational limitations, for example due to poor mixing, inefficient heat transfer, localized hot spots, agglomeration, and deposits buildup.
The injection of oxygen or oxygen enriched air into the kiln results in oxygen rich zones within the vicinity of the injected stream. The concentration of oxygen and thereby the partial pressure of oxygen is at the highest near the vicinity of injection; heat released due to combustion reactions results in elevated temperatures, potentially much higher than when compared to normally practiced air based operations. This promotes formation of sustained localized hot regions wherein the temperature could exceed the softening/melting point of the solid charge material advancing from the feed end to the discharge end. The hot spots have the potential to promote fusion of the solids. The softening/melting of the charge inventory in the kiln could cause solid material to agglomerate and/or adhere to the walls of the kiln. The sustained injection of oxygen or oxygen enriched air at a fixed location could increase the temperature in the vicinity of the injection point beyond the fusion temperature of the solids in the kiln, and potentially accelerate formation of accretion. Resulting premature shutdown of kiln shortens sponge iron production campaigns, affects increased productivity and reduced specific coal consumption achievable by the injection of oxygen.
As will be discussed, among the advantages, the present invention provides a method and system for sponge iron production in rotary kilns that manages oxygen delivery rates and oxygen concentrations in the vicinity of injection locations to prevent hot spots formation, and accretion buildup. This way premature shutdown of sponge iron production operation can be avoided and increased productivity and reduced specific coal consumption can be realized.

Summary of the Invention
In one aspect the present invention may be characterized as a process for producing sponge iron in a rotary kiln of an elongated type having a feed end, a discharge end, a heating zone between said feed end and said discharge end, and a reducing zone between said heating zone and said discharge end, comprising: feeding iron ore, a first carbonaceous material stream and optionally dolomite into the feed end, and injecting a second carbonaceous material stream into the reducing zone through the discharge end; injecting a first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and injecting a second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream; heating said iron ore in said heating zone by combustion of said first and second carbonaceous material streams; passing said heated iron ore to said reducing zone, and reducing said heated iron ore in said reducing zone to metallic iron, and recovering sponge iron product from said kiln at said discharge end, wherein the amount of oxygen delivered by one or both of said streams is adjusted in a cyclic manner. In exemplary preferred embodiments of the process, the total oxygen supplied by the first and second oxygen containing streams is maintained within +/- 5.0% of a target value by the aforementioned cyclic adjustment of the amount of oxygen delivered by one or both of said streams. The heating zone refers to the region in the rotary kiln proximate to the feed end that predominantly serves to heat the iron ore or iron pellet from feed temperature to elevated temperatures at which direct reducing chemical reactions proceed. The reducing zone serves predominantly to facilitate direct reduction of iron ore or iron pellets to sponge iron product. Combustion/oxidizing reactions occur in the heating zone and in the reducing zone.
The process described above can be carried out wherein adjustment in oxygen delivered comprises increasing the oxygen delivered by one of the first and second oxygen containing streams and decreasing the oxygen delivered by the other of the said streams. The oxygen content or injection rate or both of a given oxygen containing stream can be adjusted to deliver increased or decreased amount of oxygen by that stream. Adjusting oxygen delivered in a cyclic manner includes maintaining the delivery rate at some starting value for a defined period of time, then either increasing or decreasing the flow for example in a stepwise manner to a final value and then cycling back to the starting value. The stepwise manner could be linear, polynomial or some other non-linear function.
The second carbonaceous material stream described in the above process is injected pneumatically using air wherein the second carbonaceous stream particle size distribution and/or pneumatic transport air is adjusted to inject a greater portion of said stream to targeted areas within the rotary kiln. The ratio of the feed rates of first carbonaceous material stream to that of second carbonaceous material stream can be adjusted while maintaining the combined feed rates of the said streams constant.
In addition to the sponge iron product, the process of the present invention can be carried out to optionally produce byproduct solid fuel, steam, and/or power. The byproduct solid fuel contains greater than 20 % by weight carbon. The steam is generated in a waste heat recovery boiler by combusting combustibles in rotary kiln exhaust gases and recovering heat. Optionally a solid fuel can also be burned in the waste heat recovery boiler to increase steam output. The steam can be exported, utilized within the facility, and/or expanded to generate power.
In another aspect the present invention is a system to produce sponge iron. The system comprising: an elongated rotary kiln having a feed end, a discharge end; means for feeding iron ore, a first carbonaceous material stream and dolomite into the feed end, and means for injecting a second carbonaceous material stream into the reducing zone through the discharge end; means for injecting a first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and means for injecting a second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream wherein the oxygen delivered by one or both of said streams is adjusted in a cyclic manner. In some preferred embodiments of the system, the total oxygen supplied by the first and second oxygen containing streams is maintained within +/- 5.0% of a target value by the aforementioned cyclic adjustment of the amount of oxygen delivered by one or both of said streams.
The system described above having provisions to supply oxygen from an oxygen source for forming the first oxygen containing stream and the second oxygen containing stream. The first oxygen containing stream is formed by mixing all or a portion of oxygen from the oxygen source with pressurized air from a central blower and the means for injecting the first oxygen containing stream include a conduit in flow communication with an inlet at the discharge end for injecting pressurized air or oxygen enriched air. The means for injecting second oxygen containing stream include a conduit from here on referred to as a lance that is positioned concentrically or eccentrically within the conduit injecting the first oxygen containing stream, or outside and spaced apart from the conduit injecting the first oxygen containing stream. The provisions for supplying oxygen from the oxygen source include flow control means for the oxygen stream from the oxygen source, as well as those to regulate the flow of oxygen stream that is mixed with pressurized air from central blower and that introduced through the lance. The various flow control means can be configured in several ways, for example to enable increasing the oxygen delivered by one of the first and second oxygen containing streams and decreasing the oxygen delivered by the other of the said streams while maintaining the total oxygen constant. The oxygen content or injection rate or both of a given oxygen containing stream can also be adjusted to deliver increased or decreased amount of oxygen by that stream.
The system described above contains temperature sensing means between kiln feed end and discharge end, a process controller, flow control means to regulate oxygen flow from the oxygen source, flow control means to regulate pressurized air from central blower injected into the kiln, and flow control means to regulate shell air fan flow into the kiln. The process controller configured to receive input signal referable to a temperature within the rotary kiln and generate output signal(s) to one or more of flow control means for oxygen streams, pressurized air from central blower, shell air fans supplying air to the rotary kiln. This process controller or another process controller in the system can be configured and programmed to vary the oxygen delivered by the first and/or second oxygen containing stream in a cyclic manner, that is maintaining the delivery rate at some starting value for a defined period of time, then either increasing or decreasing the flow for example in a stepwise manner to a final value and then cycling back to the starting value. The stepwise manner could be linear, polynomial or some other non-linear function.
The system also includes means for injecting second carbonaceous material stream by pneumatic transport using air wherein the second carbonaceous stream particle size distribution and/or pneumatic transport air can be adjusted to inject a greater portion of said stream to targeted areas within the rotary kiln.
In addition to the sponge iron product, the system contains means to optionally produce byproduct solid fuel, steam, and/or power. The byproduct solid fuel contains greater than 20 % by weight carbon. The steam is generated in a waste heat recovery boiler by combusting combustibles in rotary kiln exhaust gases and recovering heat. Optionally means for feeding a solid fuel and combusting in the waste heat recovery boiler are provided to increase steam output. The steam can be exported, utilized within the facility, and/or expanded to generate power.
In another aspect the present invention is a method of operating a sponge iron production system comprising: producing iron in an elongated rotary kiln having a feed end, a discharge end, a heating zone between said feed end and said discharge end, and a reducing zone between said heating zone and said discharge end, comprising: feeding iron ore, a first carbonaceous material stream and dolomite into the feed end, and injecting a second carbonaceous material stream into the reducing zone through the discharge end; heating said iron ore in said heating zone by complete and incomplete combustion of said first and second carbonaceous material streams; passing said heated iron ore to said reducing zone, and reducing said heated iron ore in said reducing zone to metallic iron and recovering iron from said kiln at said discharge end; measuring temperature at one or more location within the kiln and injecting oxygen through the discharge end when all temperature measurements are below a defined maximum temperature (1100oC.), wherein oxygen injection comprises injecting a first oxygen containing stream having an oxygen content less than 24 volume% oxygen into the reducing zone at a velocity less than 20 meters per second, and injecting a second oxygen containing stream into the reducing zone having both an oxygen content and a velocity greater than that of the first oxygen containing stream wherein the oxygen delivered by one or both of said streams is adjusted in a cyclic manner. In some preferred embodiments of this aspect of the method of operating, the total oxygen supplied by the first and second oxygen containing streams is maintained within +/- 5.0% of a target value by the aforementioned cyclic adjustment of the amount of oxygen delivered by one or both of said streams.
The method of operation described above wherein adjustment in oxygen delivered in a cycle manner comprises increasing the oxygen delivered by one of the first and second oxygen containing streams and decreasing the oxygen delivered by the other of the said streams to pre-specified final values then cycling back to starting values. For example, at the start of the oxygen injection, 30% of the oxygen flow from the oxygen source is mixed with pressurized air from central blower to form first oxygen containing stream and the remaining 70% is injected as second oxygen containing stream. After a pre-defined cycle step time for example “t” hours the split is changed from 30:70 to 50:50 and injection continued for the cycle step time, then the split changed to 70:30 and delivery rates held for “t” hours, then the reverse cycle is started i.e. from 70:30 to 50:50 to 30:70, back to the starting point and then the cycle is repeated. The oxygen content or injection rate or both of a given oxygen containing stream can be adjusted to deliver increased or decreased amount of oxygen by that stream.
The method of operation described above wherein the second carbonaceous material stream is injected pneumatically using air and the second carbonaceous stream particle size distribution and/or pneumatic transport air is adjusted to inject a greater portion of said stream to targeted areas within the rotary kiln.
The method of operation described above that optionally produces byproduct solid fuel, steam, and/or power. The byproduct solid fuel contains greater than 20 % by weight carbon. The steam is generated in a waste heat recovery boiler by combusting combustibles in rotary kiln exhaust gases and recovering heat. Optionally a solid fuel is burned in the waste heat recovery boiler to increase steam output. The steam can be exported, utilized within the facility, and/or expanded to generate power.
Brief Description of the Drawings
While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying sole FIGURE that is a fragmentary, schematic process flow diagram of an apparatus used in carrying out a method in accordance with the present invention.

Detailed Description of the Invention
With reference to the FIGURE the system 1 contains a rotary kiln 2 that is generally cylindrical and has a feed end 4 and a discharge end 6. Kiln 2 is supported on supports 8 and 10 so that the interior of kiln at feed end 4 is inclined above the discharge end 6. Supports 8 and 10 are provided with rollers or the like so that kiln can be rotated about its longitudinal axis. Rotation of the kiln is provided by a motor 12 that is connected to the kiln exterior through a girth gear or the like. The kiln is equipped with means (not shown) to enable rotation at different speeds. Typical rotary kilns are designed for sponge iron production rates in the range of 50 to 500 tons per day. A 100 ton per day kiln is typically 42 meters in length and 2.4 meters in diameter.
Iron-oxide containing raw material (iron ore or pellets) 14, a first carbonaceous material stream (coal) 16, and dolomite 18 are fed through a feed hopper 20 into kiln 2 through line 22 at the feed end 4. A second carbonaceous material stream 24 is injected into the kiln through a conduit situated at the discharge end 6 using pneumatic transport means (air) 26. The second carbonaceous material stream 24 is formed from a coarse fraction 32 and a fines fraction 34, preferably of coal. The coarse and fine fractions have different average or mean particle size as well as particle size distribution, and may differ from each other in terms of proximate and/or ultimate analyses. This way the second carbonaceous material stream 24 characteristics can be adjusted by adjusting the relative proportions of the coarse fraction 32 and fine fraction 34, and/or characteristics of one or both.
Both the first and second carbonaceous material streams serve as fuel as well as a reducing agent in the kiln. The solid carbonaceous material combusts, providing heat which heats the iron ore and the dolomite, and producing carbon dioxide, and carbon monoxide.
The iron ore is heated by the hot gases and the hot solids formed in the kiln by combustion of carbonaceous material streams. The hot iron ore is reduced in the solid state (for instance by reaction with carbon monoxide produced by the aforementioned combustion of the fuel) while at elevated temperatures that are below the melting point to form a sponge iron product. The kiln is programmed to rotate at a speed. As the kiln rotates, solids within the kiln travel in a direction from the feed end towards the discharge end, countercurrent to gas flow path. At steady state the feed rate of materials from hopper 20 and the injected material stream 24 minus that converted to gaseous product should be approximately equal to the rate at which hot solids are discharged from discharge end 6.The residence time in kiln 2 is one of the variables that will determine the rate of production of metallic iron. Typical residence time of the charge in the kiln is 8 to 10 hours. A hot solids stream 36 flows out of the kiln at the discharge end as a hot mixture of sponge iron product, ore residue and unburned carbon containing material. The hot mixture is cooled in a cooler 38. The cooled solids stream is processed in a separation system 40 to separate out the magnetic and non-magnetic material, and recover sponge iron product 42 and at least a carbon containing byproduct 44 for use as a fuel.
Oxygen for combustion of first carbonaceous material stream 16 and the second carbonaceous material stream 24 inside the rotary kiln 2 is provided by ambient air, and oxygen from an oxygen source 60. A plurality of air supply means commonly referred to as shell air fans are utilized to introduce air at different locations in the kiln. These fans are situated spaced apart along the perimeter of kiln 2 and introduce air in a distributed manner. A rotary kiln designed for 100 tons per day sponge iron production typically has 7 shell air fans, 6 meters apart. For illustration purposes only two shell air fans 46 are shown in the FIGURE. A central blower 50 supplies a pressurized air stream 54 sourced from ambient air 48 to the rotary kiln. The pressurized air is injected either as is or mixed with an oxygen stream 64 to be described later. In one kiln configuration, the pneumatically transported second carbonaceous material stream, fuel stream 30 and the air or oxygen enriched air stream 58 emerge into kiln 2 in side-by-side streams. The kiln is equipped with known means (not shown) to regulate shell air fans and central blower air flow into the kiln. Examples of such means include dampers in associated suction or discharge lines, variable speed drives.
The oxygen stream 64 is supplied from an oxygen source 60 such as a cryogenic air separation plant or a VPSA plant or a polymeric membrane based air separation unit or a high temperature ceramic membrane based air separation unit or stored liquid oxygen or stored gaseous oxygen. In the supply of oxygen for direct injection into a rotary kiln and/or for enriching combustion air, it will be appreciated that commercial grade oxygen typically containing oxygen at a concentration of about 90 volume% and higher will be supplied.
Oxygen stream 62 from oxygen source 60 is supplied at metered rates for injection into the rotary kiln. The oxygen injection system is configured to divide oxygen stream 62 into a first oxygen stream 64 and a second oxygen stream 66. The first oxygen stream 64 is mixed with pressurized air 54 from central blower 50, for example using an in-line mixer 56. The resulting oxygen enriched stream also referred to as first oxygen containing stream 58 is injected into the kiln through discharge end 6. The second oxygen 66 also referred to as second oxygen containing stream is injected directly into the kiln through the discharge end 6. The means for injecting the first oxygen containing stream 58 into kiln 2 include a conduit in flow communication with an inlet at the discharge end. The means for injecting second oxygen containing stream include a conduit from here on referred to as a lance that is positioned concentrically or eccentrically within the conduit injecting the first oxygen containing stream, or outside and spaced apart from the conduit injecting the first oxygen containing stream.
The provisions for supplying oxygen from the oxygen source include a flow control means 68 for the oxygen stream from the oxygen source, as well as flow control valves 70 and 72 to regulate the flow of oxygen stream that is mixed with pressurized air from central blower and that introduced through the lance. The flow control means 68 is installed in the supply line for oxygen stream 62 from oxygen source 60 and could be an on/off valve or a flow control valve. The flow control valves 70 and 72 are installed in lines for oxygen streams 64 and 66, respectively. The stream 64 flow is regulated such that the oxygen concentration in stream 58 is always below a maximum threshold value, for example 24 volume% to comply with safety considerations. The control valve 70 and control valve 72 control the injection rate of stream 64 and stream 66 to respective target values. The oxygen injection system flow and control arrangement shown in the FIGURE is one of several possible configurations for formation and controlled injection of the first oxygen containing stream and the second oxygen containing stream. The various flow control means can be configured in several ways, for example to enable increasing the oxygen delivered by one of the first and second oxygen containing streams and decreasing the oxygen delivered by the other of the said streams while maintaining the total oxygen constant. The oxygen content or injection rate or both of a given oxygen containing stream can also be adjusted to deliver increased or decreased amount of oxygen by that stream
Hot gases formed in the kiln leave the kiln from feed end side as stream 74 and flow into an optional waste heat recovery boiler 76. The waste heat recovery boiler is equipped with an after burner (not shown) to limit the concentrations of air pollutants such as CO and other combustibles in flue gas 78 flowing to stack. Optionally a solid fuel (not shown) can be fed to the waste heat recovery boiler 76 and combusted to increase steam production rate. Steam 80 is produced in the waste heat recovery boiler at suitable pressure and temperature for process use and/or as a utility. For example the generated steam can be expanded in a steam turbine system (not shown). The steam turbine can be configured to generate electricity and/or to serve as a mechanical drive. Even though only one steam stream is shown, the system 1 can be configured to generate more than one steam stream, some for export and others for use within the facility. In an alternate rotary kiln arrangement the iron ore or pellets 14 can be preheated first in a preheater kiln (not shown) and then introduced separately into rotary kiln 2. The preheating is accomplished by flowing rotary kiln 2 exhaust gases through the preheater kiln in a countercurrent direction to that of the iron ore or iron pellets 14 through the preheater.
The kiln 2 is equipped with temperature measurement means such as thermocouples 82 (only two shown for illustrative purposes) to monitor temperatures in various zones between the feed end 4 and discharge end 6 and provide signals referable to the temperature to a process controller 84. Any particular thermocouple 82 is utilized as a normal response thermocouple (NRT) in a known manner. The thermocouple 82 produces a signal that can be transmitted by means of electrical conductor 86. The process controller 84 can be a known controller using proportional, integral and differential control methodology. Although not illustrated, but as would be known by those skilled in the art, process controller 84 would be provided with a temperature input that could be varied at will for purposes of setting a maximum temperature set point and the feedback signal would be generated to maintain the temperature in a particular zone of the rotary kiln, for example within +/- 20.0oC. of such a set point. The process controller 84 can be configured to output a signal relating to the status of oxygen injection into the kiln such as start, continue or stop. The signal is transmitted via electrical conductor 88 to oxygen injection system process controller 90; a known controller using proportional, integral and differential control methodology that outputs signals via electrical conductor 92, 94 and 96 to valves 68, 70 and 72, respectively.
The system 1 is equipped with means (not shown) to measure kiln pressure at the feed end and the discharge end; shell air fans flow rates; central blower air flow rate; pneumatic transport air flow rate; rotating speeds of induced draft fan (not shown) and kiln. These measurements are transmitted to process controller 84 or to a different process computer/controller (not shown) in data communication with process controller 84.
The oxygen concentration in stream 58 is maintained below a maximum threshold value for example 24 volume% by adjusting the flow of oxygen stream 64. In one approach the adjustment is based on measured flow rate of oxygen stream 64 and calculated flow rate of pressurized air 54 from central blower air 50. In another approach a known oxygen analyzer (not shown) is utilized to measure the oxygen concentration in stream 58. The oxygen analyzer can be positioned at the discharge (outlet line) of the mixer 56 which could be an in-line mixer or a mixing chamber or the like. The analyzer measurement is utilized as a feedback signal to adjust the flow rate of oxygen stream 64. Alternately, central blower 50 can be modulated, for example by speed control or guide vanes or damper means to vary the pressurized air flow rate.
In accordance with the present invention, oxygen stream 62 is divided between oxygen stream 64 and oxygen stream 66, and the split is varied from a starting value to a final value and then back to the starting value in a cyclic manner. In one situation the starting value of the split could be a low value and the split is varied to increase to a final high value while operating at each value for a specified time period, referred to as cycle step time or step time. Once the split is maintained at the final high value for the specified time period the cycle is reversed, back from the final high value to starting low value in a decreasing manner. Alternately the split between stream 64 and stream 66 can be varied from a starting high value to a final low value in a decreasing manner and then back from final low value to starting high value in an increasing manner. The variation in split can be carried out in a stepwise manner at fixed intervals. For example from an initial low value of zero percent to a final high value of 100% can be carried out in fixed increments such as 10% at fixed intervals and once at the final value can be decreased in same 10% or different step size to the initial value of zero percent. The variation from initial to final value and then back to initial value can be accomplished in a number of ways, including simple stepwise approach to those utilizing functions such as quadratic, polynomial, exponential, sinusoidal, and the likes. These calculations and execution logic are programmed in process controller 90 or a computer or a computer/controller in data communication with process controller 90.
In accordance with the present invention process controller 90 sends an output signal to control valve 70 by electrical conductor 94, and an output signal to control valve 72 by electrical conductor 96 to divide oxygen stream 62 in a desired ratio. Those skilled in the art will appreciate that system 1 will also have process control means for operation of kiln 2 that will supersede the process controller 90 in case of abnormal operating conditions and stop oxygen stream 62.
Under normal operating conditions, temperatures within the kiln are not allowed to exceed a predefined maximum temperature, for example in the range of 1180oC. to 1200oC. Beyond this the rates of accretion formation are higher and that may cause production campaign to end prematurely. For the case under discussion the campaign life is 50 days. Thermocouples of both Normal Response Thermocouple (NRT) type and Quick Response Thermocouple (QRT) type are used to monitor temperatures between the feed end and discharge end. Accretion build-up is also physically inspected as well as shell temperature monitoring. Based on the temperature requirements in the reducing zone closer to the discharge end, oxygen split between that for mixing with central blower air and that for lance injection is decided.
In conjunction with varying oxygen split, adjustment of the ratio of feed coal 16 and injection coal 24 quantities may be desirable. Increased injection of coal could cause local cool down of material in the kiln and help in avoiding localized hot spots. Even the amount of coarse coal 32 and fine coal 34 can be beneficial in maintaining desired thermal profile in the kiln.
While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.