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Method of mixing high temperature gases in mineral processing kilns

Imported: 24 Feb '17 | Published: 06 Jan '04

Eric R. Hansen, Ralph A. Supelak, James R. Tutt, Peter F. Way

USPTO - Utility Patents

Abstract

A method is described for reducing NO

x emissions and improving energy efficiency during mineral processing in a rotary kiln. The method comprises injection of air with high velocity/high kinetic energy into the kiln to reduce or eliminate stratification of kiln gases. The method can be applied to mix gases in a rotary kiln vessel or in a preheater/precalciner vessel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are similar and illustrate partially broken away diagrams of mineral processing kilns modified in accordance with the present invention for injection of high velocity mixing air into the rotary vessel.

FIGS. 5,

6, and

7 are similar cross-sectional views of rotary kilns modified in accordance with the present invention illustrating alternative embodiments for delivering high velocity mixing air into the rotary vessel. FIG. 7

a is partially broken away plan view of the fan in FIG. 7 across lines AA.

FIGS. 8

a and

8

b illustrate alternate nozzle orifice configurations.

FIGS. 9

a and

9

b illustrate flow patterns in a cement kiln without high velocity injected air (

9

a) and with high velocity injected air in accordance with this invention (

9

b) upstream of a supplemental fuel (tire) delivery apparatus (not shown).

FIGS. 10

a and

10

b are similar illustrating the stoichiometry of primary burner combustion without high velocity injection air (

10

a) and with 10% injected high velocity air (

10

b).

FIG. 11 is similar to FIG.

10 and shows the stoichiometry of combustion in three zones in a kiln operated with 15% supplemental fuel delivered to the kiln upstream of the injection of 10% high velocity air.

FIG. 12 is similar to FIG. 11 illustrating the stoichiometry of kiln fuel combustion wherein the kiln is modified for burning of supplemental fuel and for injection of high velocity air both upstream and downstream of the point of fuel delivery into the rotary vessel.

FIG. 13 illustrates the effects of injected high velocity air on kiln gas flow in the kiln illustrated in FIG.

12.

FIG. 14 is a cross-sectional view of a rotary kiln vessel containing in-process mineral releasing a gas (carbon dioxide).

FIG. 15 is similar to FIG. 14 showing mixing of the kiln gases by injection of high velocity air into the rotary vessel.

FIG. 16 illustrates the radiant energy transfer to in-process material in the absence of a stratified layer of gases released from the mineral bed.

FIGS. 17-20 illustrates diagrammatically various configurations of commercially available stationary precalciner vessels with “arrows” illustrating points for injection of high velocity air to promote mixing in the stationary vessels with high velocity injected air.

FIGS. 21 and 22 are similar to FIGS. 1-4 and illustrate partially broken away diagrams of mineral processing kilns modified for air injection with diagrammatic representation of kiln gas monitoring and controllers for air injection and steam or fluid gas injection.

FIG. 23 is a partially broken away elevation of the upper end portion of the rotary vessel of a precalciner kiln modified for air injection and supplemental fuel delivery for NO

x reduction.

Claims

1. A method of mixing a high temperature kiln gas stream in a rotary vessel of a mineral processing kiln, said vessel having a cylindrical wall, a combustion air inlet/burner end and a kiln gas exit end, said gas kiln stream having multiple gaseous components consisting essentially of the products of combustion of fuel burned in an oxygen-containing gas comprising combustion air, unburned fuel and the oxygen-containing gas, said method effective to reduce the emission of gaseous pollutants from the kiln and comprising the step of injecting air into the gas stream through an air injection tube terminating in an injection port spaced apart from the vessel wall and the axis of rotation, said air being injected at a mass flow rate of about 1 to about 15% of the mass rate of use of combustion air by the kiln and at an energy input level of at about 1 to about 10 Watt-hour per pound of injected gas, and directed into the kiln gas stream to impart rotational momentum to the kiln gas stream in the vessel at a point along the length of the rotary vessel where the kiln gas temperature is greater than 1800° F.

2. The method of claim 1 wherein the cement air is injected from a pressurizing source providing a static pressure of greater than 0.20 atm.

3. The method of claim 2 wherein the kiln contains a mineral bed of height H and the air injection post is spaced apart from the vessel wall at least the distance H.

4. The method of claim 3 wherein the air injection port is positioned to direct the injected air along a path forming an angle of greater than 45 degrees with a line passing through the port and parallel to the axis of rotation of the vessel and extending through the kiln gas exit end of the vessel.

5. The method of claim 1 wherein steam is added to oxygen-containing gas to provide thermal ballast to the kiln gas stream.

6. The method of claim 1 wherein flue gases are added to the oxygen-containing gas to provide thermal ballast to the kiln gas stream.

7. The method of claim 1 further comprising the step of monitoring the composition of the kiln gas stream exiting the rotary vessel.

8. The method of claim 7 further comprising the step of adjusting the composition of the oxygen-containing gas and/or varying the rate of air injection into the kiln gas stream to minimize NO

x content in the kiln gas stream.

9. The method of claim 1, wherein the mineral processing kiln is preheater or precalciner cement kiln and the air is injected into the rotary vessel at a point within two kiln diameters of the kiln gas exit end of the rotary vessel.

10. The method of claim 9, wherein the air is injected at a lineal velocity of about 100 to about 1000 feet per second.

11. The method of claim 9, wherein supplemental fuel is introduced into the kiln gas stream proximal to the kiln gas exit end of the rotary vessel.

12. A method of mixing a high temperature kiln gas stream in a rotary vessel of an operating mineral processing kiln to reduce emissions of noxious pollutants, said kiln having a cylindrical wall and a combustion air inlet end and a kiln gas exit end, said kiln gas stream having multiple gaseous components consisting essentially of the products of combustion of fuel burned in an oxygen-containing gas comprising combustion air, said method comprising the step of injecting air from a pressurized source into the kiln gas stream through ain injection system, comprising a tube terminating in an injection port in the vessel and spaced apart from both the wall of the vessel and the rotational axis of the kiln, the pressure of the air and the size of the port being selected so that the injected air is delivered through the port at a mass flow rate of less than 15% of the mass rate consumption of combustion air and directed to impact the kiln gas stream in the kiln to impart rotational momentum to the kiln gas stream.

13. The method of claim 12 wherein the air is injected from a pressurizing source providing a static differential pressure of greater than 0.15 atm.

14. The method of claim 12 wherein the injected air has an energy level of about 1 to about 10 Watt-hour per pound of injected gas.

15. A method of mixing a high temperature kiln gas stream in a rotary vessel of an operating mineral processing kiln to reduce emissions of gaseous pollutants, said vessel having a cylindrical wall and a combustion air inlet end and a kiln gas exit end, said kiln gas stream having multiple gaseous component comprising products of combustion of fuel in an oxygen-containing gas comprising combustion air, said method comprising the step of injecting air from an air pressurizing source into the kiln gas stream through an air injection system comprising a tube terminating in an injection port located within the vessel at a point spaced apart from both the wall of the vessel, and the rotational axis of the rotary vessel, the air pressurizing source being selected to provide air at a differential pressure of greater than 0.15 atm and the air injection port being sized in cross-sectional area of deliver air into the kiln through the air injection system at a mass flow rate of less than 15% of the mass consumption of combustion air by the kiln and directed to impact the kiln gas stream so that the major directional vector component of the injected air is orthogonal to a line parallel to the rotational axis of the rotary vessel.

16. The method of claim 15 wherein the air is injected from a pressurizing source providing a static differential pressure of greater than 0.15 atm.

17. The method of claim 15 wherein the injected air has an energy level of about 1 to about 10 Watt-hour per pound of injected gas.

18. A method of mixing high temperature kiln gas stream in an operating preheater or precalciner mineral processing kiln to reduce emission of gaseous pollutants, said kiln having a rotary vessel with a combustion air inlet end and a kiln gas exit end in gas flow communication with a stationary preheater/precalciner tower portion and an intermediate transition shelf, said kiln gas stream having multiple gaseous components comprising products of combustion of fuel burned in an oxygen-containing gas comprising combustion air, said kiln being modified for burning supplemental fuel in a secondary burning zone proximal to the kiln gas exit end of the rotary vessel, optionally to create conditions for reducing NO

X emissions from said kiln, said method comprising the step of injecting air from an air pressurizing source into the kiln gas stream through an air injection system comprising a tube terminating in an air injection port located within two kiln diameters of the kiln gas exit end of the rotary vessel, the pressurizing source and the air injection port being sized to deliver air into the kiln through the air injection system at a mass flow rate of about 1% to about 15% of the rate of mass consumption of combustion air by the kiln and directed to impart rational momentum to the kiln gas stream.

19. The method of claim 18 further comprising the step of delivering supplemental fuel into the kiln gas stream at a point proximal to the kiln gas exit end of the rotary vessel.

20. A method for reducing NO

X in the effluent gas stream from a long rotary cement kiln modified for burning supplemental fuel, wherein the kiln comprises an inclined cylindrical vessel rotating about its long axis and having a cylindrical wall, the vessel being heated at its lower end and charged with raw mineral material at the upper end and having a kiln gas stream flowing from the heated lower end having a primary burner and a combustion air inlet through the upper end, the mineral material forming a mineral bed flowing at a maximum depth H under influence of gravity in the vessel counter-current to the kiln gas stream from a drying zone in the uppermost portion of the rotary vessel, through an intermediate calcining zone, and into a high temperature clinkering zone before exiting the lower end as cement clinker, and wherein the supplemental fuel is charged into the vessel through a port in the vessel wall to burn in contact with calcining mineral material in a secondary burning zone, the method comprising the step of:

21. The method of claim 20, wherein the supplemental fuel is combustible waste delivered through a port in the wall of the vessel into the calcining zone.

22. The method of claim 20, wherein the air is injected at a rate of about 1% to about 10% of the mass of the total combustion air used during kiln operation.

23. A precalciner cement kiln for producing cement clinker from a mineral feed, said kiln modified for reduced NO

x emissions and improved combustion efficiency, said precalciner kiln comprising a rotary vessel heated with a primary burner and a stationary precalciner vessel in gas and mineral flow communication with the rotary vessel and having a secondary burner, said modified kiln comprising a air injection nozzle positioned on said stationary vessel and means for delivering compressed air to said nozzle and into said vessel at a linear velocity of about 100 to about 1000 feet per second.

24. The modified precalciner kiln of claim 23 wherein a plurality of nozzles are positioned to deliver compressed air into the precalciner vessel.

25. A mineral processing kiln modified for operation with reduced NO

x emissions and increased energy efficiency, said kiln comprising an inclined rotary vessel having a primary burner and a combustion air inlet at its lower end and wherein during thermal mineral processing mineral in a mineral bed in said vessel undergoes a gas releasing chemical reaction, said kiln being modified to include

26. The modified mineral processing kiln of claim 25 wherein the kiln is modified to include two or more air injection tubes for injecting air into the rotary vessel, each injection tube terminating in a nozzle for directing the injected air along a predetermined path in said vessel.

27. The modified mineral processing kiln of claim 25 wherein the depth of the mineral bed is H, and the nozzle is positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel.

28. The modified mineral processing kiln of claim 25 wherein the predetermined path of the injected air from each nozzle is effective to impart rotational momentum to the combustion gases flowing through the rotary vessel.

29. The modified mineral processing kiln of claim 25 further comprising a supplemental fuel delivery port and drop tube extending from the port into the rotary vessel at a point on the vessel downstream, relative to gas flow in the kiln, from the location of the air injection tube.

30. The modified mineral processing kiln of claim 29 further modified to include an additional air injection tube for injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second, said additional injection tube extending from a port in the wall of the vessel and into the rotary vessel, and terminating in a nozzle for directing the injected air along a predetermined path in said vessel, said additional air injection tube being located at a point on the rotary vessel downstream, relative to gas flow in the kiln, from the supplemental fuel delivery port, to mix gases released from both the mineral bed and the burning supplemental fuel with the combustion gases from the primary burner, a fan or compressor in air flow communication with the downstream air injection tube, and a controller for the fan or compressor to adjust the rate of air injection into the kiln at the downstream air injection point.

31. A method for reducing NO

x emissions and improving energy efficiency during mineral processing in a rotary kiln comprising an inclined rotary vessel having a primary burner and combustion air inlet at its lower end and an upper mineral feed end and wherein the mineral in a mineral bed undergoes a gas releasing chemical reaction during thermal processing in the kiln, said method comprising the step of injecting air into the rotary vessel at a velocity of about 100 to about 1000 ft. per second from an air pressurinzing soruce providing a static pressure of greater than 0.15 atm to reduce stratification of the gas released from the mineral bed with combustion gases from the primary burner.

32. The method of claim 31 wherein the air is injected into the rotary vessel 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.

33. The method of claim 32 wherein the air is injected into the rotary vessel through two or more nozzles.

34. The method of claim 33 wherein the maximum depth of the mineral bed is H, and the nozzles are positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel.

35. The method of claim 32 wherein the maximum depth of the mineral bed is H, and the nozzle is positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel.

36. The method of claim 32 wherein the nozzle has an orifice of rectangular or elliptical cross-section.

37. The method of claim 31 wherein the kiln is a lime kiln, a cement kiln, a talconite kiln or a lightweight aggregate kiln.

38. The method of claim 37 further comprising the step of burning supplemental fuel delivered through a port in the rotary vessel located downstream, relative to gas flow in the kiln, from where the air is injected into the kiln.

39. The method of claim 38 further comprising 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.

40. The method of claim 39 wherein the rate of injection of air into the kiln is about 1% to about 10% of the mass of total combustion air required during kiln operation.

41. The method of claim 39 wherein the predetermined path of the injected air is effective to impart rotational momentum to the combustion gases flowing through the rotary vessel.

42. The method of claim 37 wherein the rate of injection of air into the kiln is about 1% to about 10% of the mass of total combustion air required during kiln operation.

43. The method of claim 31 wherein the predetermined path of the injected air is effective to impart rotational momentum to the combustion gases flowing through the rotary vessel and the air pressurizing source provides a static pressure of greater than 0.20 atmospheres.

44. The method of claim 43 further comprising the step of burning supplemental fuel delivered through a port in the rotary vessel located downstream, relative to gas flow in the kiln, from where the air is injected into the kiln.

45. The method of claim 44 further comprising 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.

46. The method of claim 43 wherein the rate of injection of air into the kiln is about 1% to about 10% of the mass of total combustion air required during kiln operation.

47. A method for reducing NO

x emissions and improving combustion efficacy in a precalciner cement kiln for producing cement clinker from a mineral feed, said precalciner kiln having a rotary vessel portion heated by a primary burner and a stationary precalciner vessel portion heated by a secondary burner, each of said primary burner and said precalciner portion being supplied with controlled amounts of preheated combustion air, and wherein said precalciner kiln combustion gases from the primary burner flow through the rotary vessel, the precalciner vessel portion, and into a series of cyclones in counterflow communication with mineral feed, said method comprising the step of injecting compressed air into the precalciner portion of said kiln at a point before the first cyclone, at a mass rate corresponding to about 1% to about 7% of the total combustion air and at a velocity of about 100 to about 1000 ft. per second.

48. The method of claim 47 wherein the compressed air is injected into the precalciner vessel portion through two or more nozzles.

49. The method of claim 48 wherein the nozzles are directed into the precalciner vessel to optimize cross-sectional mixing of the gases in the precalciner vessel.

50. The method of claim 48 wherein the nozzles are directed into the precalciner vessel to promote turbulent flow in said vessel.

51. The method of claim 48 wherein the nozzles are directed into the precalciner vessel to promote rotational flow in said vessel.

52. The method of claim 47 wherein the ambient air is compressed to a pressure of about 40 to about 150 psi before being injected into the precalciner vessel portion.