US3466351A - Closed combustion cycle for cement kilns - Google Patents

Description

Sept. 9, 1969 A LA VELl-E 3,466,351

CLOSED COMBUSTION CYCLE FOR CEMENT KILNS Original Filed May 23, 1966 2 Sheets-Sheet 1 :E'I [3"..1A

mvzsmoa Martin J. LoVelle ATTORNEYS Sept. 9, 1969 M. J. LA VELLE CLOSED COMBUSTION CYCLE FOR CEMENT KILNS 2 Sheets-Sheet 2 Original Filed May 23. 1966 INVENTOR.

United States Patent US. Cl. 26353 4 Claims ABSTRACT OF THE DISCLOSURE In heating devices, such as rotary cement kilns, performance can be improved and the spewing, of combustion products and dust into the atmosphere can be eliminated by a completely closed combustion process wherein a synthetic gas mixture is continuously circulated within the device with fuel and oxygen added to the circulating synthetic gaseous mixture under combustion conditions to add thermal energy for heating while removing 1 a quantity of the circulating synthetic gaseous mixture to provide for the additional gas volume generated by the combustion of the fuel and oxygen. The removed quantities of said mixtures can be processed to remove undesirable air contaminants or largely recovered as by-products.

This application is a continuation of Ser. No. 551,995, filed May 23, 1966, which is now abandoned.

This invention relates to the combustion of fuels, and more particularly to an improved combustion process and devices wherein the fuel is burned while mixed with a synthetic gas mixture composed primarily of CO H O (vapor) and 0 instead of the conventional mixture of fuel and air.

This process technique of this invention can be employed in sustained flame systems, such as boilers, or intermittent cyclical flame systems with many advantages over conventional processes.

Conventional combustion processes usually involve the burning of fuel with air in a combustion zone and a subsequent utilization of the heat energy released to ac complish some useful work function. Air is used as source of oxygen which is necessary for combustion since it is readily available. Air, however, contains only about 21% by volume of oxygen, with the balance composed chiefly of nitrogen which is inert in the combustion process. Thus to have a workable combustion process, it has been necessary to circulate a gas volume approximately five times that of the oxygen necessary if it were pure form, instead of contained in the air.

Such a large gas mass, of it inert in the combustion process, lowers the efficiency, since the temperature of the inert gases is raised considerably within the system and then discharged to the atmosphere, with a resultant heat loss. Further, the large gas mass flow requires higher velocities within the system, lowering heat transfer efficiency therefrom to some useful work function.

The large gas volume flow in conventional combustion processes may be illustrated by referring to combustion engineering literature, which shows that 214 cubic feet of air at standard conditions are necessary for the combustion of a pound of methane. Further, dependent on the specific characteristics of the particular system, up to 100% excess air may be required, or 428 cubic feet of air per pound of methane. If oxygen were used in place of air, only 43 cubic feet would be required.

Thus, in conventional combustion processes at least 20% of the thermal energy liberated is retained in the ice inert nitrogen and is lost to the atmosphere through the stack gases along with the products of combustion, CO H O. The magnitude of this heat loss to the atmosphere through stack gases represents one of the chief disadvantages of the conventional combustion processes.

Still another disadvantage of the conventional combustion process is incomplete conversion of carbon and hydrogen components of fuels to their respective oxides and the liberation of the maximum quantity of thermal energy which is virtually impossible to achieve and maintain with any degree of consistency. Incompletely burned combustible material invariably escapes to the atmosphere as partially oxidized substances or to the ash residue as unburned portions of the fuel. The magnitude of this loss is a direct function of the load on the combustion system and the loss of combustible material increases with increased energy demand and increased fuel utilization.

The significance of the discharge of incompletely combusted fuel products to the atmosphere has become of greater concern in recent years due to the increased awareness of the public to the problems of air pollution. Because of air pollutants in stack emission of conventional combustion processes, industrial management has been required to take steps to prevent air pollution due to these materials emanating from their stacks. While control devices such as mechanical and electrostatic dust collectors have been successful in removing particulate matter from stack gases, within the design ranges of the equipment, the cost of handling the material collected has resulted in serious operating problems for plant management. In addition, the conventional devices are ineffective against the pure carbon, smog producing hydrocarbons and monoxides, which are serious forms of air pollution in urban communities.

In addition to the foregoing, the reliance on atmospheric air as a source of oxygen for combination of hydrocarbon fuels has prevented any significant progress toward development of a more compact combustion system. All combustion processes must be designed for a wide latitude of load conditions if they are to be flexible. Changes in load conditions manifest themselves as significant changes in gas flow within a furnace device. For example, an increment of pounds of methane per hour over a previous fuel feed setting in a conventional combustion system there must be a provision in the device for the increased gas volume to accommodate from 21,400 to 42,800 cubic feet of additional air flow to burn the additional fuel. This example represents a very minor change in load ranges for any typical industrial application of the combustion process, but illustrates the problem.

Conventional combustion systems also have another disadvantage which can be attributed to their dependence on atmospheric air which causes large amounts of nitrogen to be introduced into the system. This is the inability to economically recover the -by-products of combustion, namely H 0 and CO which has not previously been feasible due to the dilution of these combustion products by the large volume of inert nitrogen passing through the system. For example, the average stack concentration of the H 0 and CO components is generally only 6 and 10% of the eflluent of a rotary kiln stack, the remainder being principally nitrogen and excess oxygen.

It is therefore apparent that the thermal and material inefficiencies present in the conventional combustion system represent disadvantages of the present method of liberating heat energy in hydrocarbon fuels through combustion processes. It is therefore evident that there is a need for new combustion processes which avoid the deficiencies in conventional systems.

Accordingly, it is the object of this invention to provide a new, more efficient combustion process for the generation of thermal energy and a subsequent, more efiicient application of that energy to a useful work function.

Another object of the present invention is a novel combustion process which substantially reduces air pollution resulting from the combustion of hydrocarbon fuels while significantly increasing the degree of material and thermal efiiciencies of the process.

Another object of the invention is to provide an mproved combustion method which can be incorporated at modest cost to any existing combustion system and will subsequently improve the efficiency of heat generation and its utilization which have been previously established in the particular combustion system so modified.

An important object of the present invention is a novel combustion process which can be varied between its maximum and minimum heat generating ranges with no appreciable change of material or thermal efficiencies.

Still another object is the provision of a novel combustion process which can be contained in much smaller space than can conventional processes, with the same heat generating capacity.

An ancillary object of the invention is to provide a new combustion process where the valuable by-products of combustion, namely CO and H 0, are sufficiently concentrated to make their recovery practical.

An important object of the invention is the provision of a circulating gaseous mass in the combustion system, which has a greater specific heat capacity and better diathermanous properties than the gas masses employed in conventional combustion pocesses.

A more specific object of the invention is the provision of a combustion process wherein the loss of thermal energy through stack gases is almost eliminated.

Another more specific object of the invention is the provision of a novel process where poor grades of fuels such as those having a high sulfur content, can be utilized in place of the premium grade fuels, with no draw-backs.

The foregoing objects, as well as many others, can be accomplished in a novel combustion system wherein the fuel is burned with a synthetic gas mixture composed primarily of carbon dioxide, water vapor and oxygen instead of air as is used in conventional combustion systems. Generally the process is carried out by recirculating a substantial portion of the hot gaseous products of combustion through combustion Zones and mixing a sufficient amount of oxygen with these gases to support the combustion of the fuel. From 1% to 50% by volume of oxygen can be added to hot gaseous products of combustion to form a hot synthetic gas mixture capable of supporting the combustion of fuels. More specifically, the complete novel combustion system in this invention also includes the recovery of excess by-products of combustion, principally carbon dioxide and water.

In the drawings FIGS. 1A and 1B diagrammatically illustrate a rotary kiln for making cement which has been modified for the practice of this invention. Since the invention has particular applicability to such rotary kilns it will be described in this environment, but it is not intended that it be limited thereto.

Referring to the drawings, a refractory-lined kiln is shown, which has a rotating cylindrical shell 11 supported on concrete piers 12 at a slight incline with rollers 13 through shell rims 14. Rotation of the shell is accom plished by a shell-mounted girth gear 15 driven by pinion 16 powered via gear reducer 17 by motor 18 with the latter mounted on a centrally located pier 19.

The ends of the rotating shell 11 are closed by hoods and connected thereto by sealing ring members 20. A refractory-lined firing hood 21 closes the lower end of the shell, and a dust collecting hood 22 closes the upper end. The sealing ring members prevent gas leakage into or out of the stationary hood structures as the shell is rotated about its longitudinal axis by the motor.

Raw feed to the kiln is introduced through a waterjacketed, slurry feed pipe 23 which extends through the dust collecting hood 22 into the upper end of the rotating shell 11. The slurry feed pipe is fed from a ferris-wheel feeder 24 which picks up feed from the slurry supply line 25, dumping it into a receiving tank 26. The ferris-wheel feeder is driven by a belt or chain 27 connecting it with a driving structure on the rotating shell 11.

At the opposite end of the kiln from the dust collecting hood 22 which is closed by firing hood 21, a burner pipe 29 is mounted in the firing hood so that it terminates in the lower end of the rotating shell 11. This burner pipe discharges .a combustible mixture of fuel from fuel hopper 30, and an oxygen containing gas from primary blower fan 31 into the rotating shell where it is burned to provide direct heat in the kiln.

The bottom of the firing hood 21 is open so that the material discharged into the firing hood from the shell 11 can be removed. The treated products drop from the hood into a product collection chamber 32, and thence .across a travelling grate 33 to a screw conveyor 34 via which it is taken to storage.

When the above-described kiln is used to manufacture cement, often a small portion of the hot gases is taken from the dust collecting hood and forced through the travelling grate to cool the clinker, and returned through the rotating shell to the dust collecting hood. This hot gas stream is referred to as secondary air, and usually represents a very small flow through the kiln, since substantial amounts of air must be used to provide the necessary oxygen for burning the fuel.

The above-described kiln is more or less typical of one that would be employed for manufacturing cement. In such manufacture, a slurry is introduced into the upper end of the rotating shell 11 via feed pipe 23 and travels down the kiln as the shell rotates, While the hot gaseous products of combustion along with a small amount of secondary air, move counter-current thereto up the shell toward the dust collecting hood 22. As the slurry progresses down the shell, water and carbon dioxide are driven out of the slurry and the carbonate material is transformed into a fused product which is discharged from the bottom of the firing hood as clinker."

Normally, the hot products of combustion travelling up the shell 11 enter the dust collecting hood 22 and from there pass through .a breaching 35 to a dust precipitator 36 wherein the solid particles are removed. Subsequent to the passage through the precipitator, the hot gas stream passes through a second breaching 37 to induced draft fan 38 from which it is expelled to the atmosphere through a stack (not shown).

In a conventional process the hot gases within the rotating shell, with the exception of the secondary air, are produced by sucking air through the inlet of the primary blower fan 31, mixing it with fuel in fuel hopper 30 and discharging the combustible mixture into the lower end of shell 11 where it is burned in the combustion Zone. Only a small portion of the hot gases passing through the shell, as mentioned above, can be secondary air, and usually this represents less than 10% of the gas mass flow through the shell.

It is actually in this area of heat generation that the instant invention departs radically from the conventional processes since no air is used to support the combustion of the fuel as is the case in conventional processes. Instead according to the practice of this invention, the hot gas products discharged by the induced draft fan 38 are routed into plenum chamber 39 whose lower end 40 communicates with a subterranean concrete conduit 41 which returns to the lower end of the kiln. A large portion of the hot gases in the conduit are passed through secondary blower 42 and returned to the lower end of shell 11 via conduit 43 for recirculation therethrough, usually passing through the travelling grate structure 33 to cool the clinker slightly.

The remainder of the hot gases in conduit 41 is removed through conduit 44 which connects to the inlet'of the primary blower fan 31, which in .a conventional system has a fresh air inlet. These hot gases entering the blower fan are principally composed of CO and H 0 (vapor), but some CO, ash carbon and uncombusted fuel products are also present. Since these hot gases contain no oxygen they will not support the combustion of fuel. In the instant invention, the lack of oxygen in this hot gaseous mixture is overcome by adding controlled amounts of oxygen to the hot gases in conduit 44 via an oxygen supply line 45 and valve 46 prior to the entry of these gases into the inlet of the primary blower fan. A valve 47 in line 44 is used to control the volume of hot gases passing through conduit 44 so that a proper ratio of hot gases .and oxygen can be obtained through automatic operation of these valves with controller 48.

Since the hot gases passing through the supply conduit 44 contain only minor amounts of CO and uncombusted fuel products plus a small volume of carbon particles, the addition of oxygen to this gas stream is not hazardous, .and can be accomplished safely. Further, since the oxygen is blended with the hot gases prior to the combination of fuel therewith, the oxygen mixture in the hot gases may vary from 1% to 20% prior to contacting the fuel. This amount of oxygen is in a proportion similar to the amount of oxygen in air, and represents no greater hazard than burning the fuel with hot air at the same temperature. Of course, this technique is one of the principal ways the instant invention gains outstanding efficiency-combining the hot synthetic mixture composed of oxygen and hot combustion products with the proper amount of fuel for heat generation by subsequent combustion.

In operation, the synthetic gas mixture entering the inlet of the primary blower fan 31 is discharged into the burner pipe where it is mixed with fuel from hopper 30, and discharged into the combustion zone within shell 11 where it is burned. It may be desirable to mix the fuel with the synthetic gas mixture near the inboard end of the burner pipe so that the burner pipe will not become overheated.

Since the majority of the synthetic gas mixture is composed of hot combustion products, its temperature is substantially higher than the air used in conventional processes, and it will substantially improve the combustion efiiciency of the novel process. Conventional processes often try to recoup some efficiency by warming the inlet air with the hot combustion products in heat exchangers, but such a technique ofiers a very limited improvement. Further, a substantial fuel saving is effected by this novel process since it is not necessary to raise the temperature of a high volume of inert nitrogen to combustion temperature, and thereafter suffer .a heat loss when releasing this hot gas to the atmosphere via the stack to make room for more nitrogen. Because of this advantage, much less heat need be added to the kiln system employing this invention than in a conventional process, and further the gas velocities may be substantially reduced for more efiicient heat transfer, thereby improving the efficiency still further.

Ordinarily, this novel system contemplates a constant volume of gas circulating through the kiln and, since additional gas volume is generated by the combustion process and the addition of oxygen, it is necessary to remove the excess gas volume of the system to keep it at a constant volume. This removal is accomplished through an exhaust fan 49, driven by a variable speed motor 50 with its inlet communicating with the plenum chamber 39. The speed of the exhaust fan will normally be controlled by static pressure transducers within the plenum chamber, which, through a control unit, will vary the motor speed to keep a constant pressure within the system, thus insuring a constant volume.

The outlet 51 of the exhaust fan 49 leads directly to a gas recovery system composed of two gas absorbing columns. Both columns, which are designed for countercurrent flows, are filled with a suitable packing material or contain bubble plates to effect intimate contact between the hot exhaust gases and the absorbing liquids. In the first column which the hot gases from the exhaust fan enter, is the water condensing column 52, which uses a coolant such as a cool water to treat the hot exhaust gases. As the gases are cooled by the cool absorbant flowin-g counter-currently through this column, the water vapor in the gases, which comprises about 50% of the exhaust gases, is condensed out leaving only a gaseous stream of carbon dioxide emanating from the top of the column. A small amount of the carbon dioxide may also be absorbed in the water as well as minor amounts of acidic oxides, ash, and other foreign materials from the combustion process. A recycling system 53 provides for the recirculation of the condensing water to improve the efliciency.

The carbon dioxide remainder of the exhaust gases leaves the top of the column 52 via line 54 and enters the bottom of the CO absorbing column 55 which uses an absorbing liquid, such as ammonium hydroxide, to absorb the CO stream entering this column. A small amount of unabsorbed gases emanates from the top of the column through vent pipe 56, but is a very small amount of pure inert gaseous materials. Also, a recycle system 57 for the hydroxide absorbing liquid is incorporated on this column. The product laden streams are removed from the bottom of the respective columns through lines 58 and 59.

Actually, the collection of carbon dioxide and water from the exhaust gases is made practical by the high concentration levels of these materials in the exhaust gas. Normally the compositon of this gas will b composed principally of carbon dioxide and water vapor in an equal amount. In conventional processes, gases leaving the kiln Via the stack contain only '6%-20% of these useful products since they are composed principally of nitrogen entering the system with the air used to support the combustion. Carton dioxide and water vapor represent nearly of the gas volume exhausted and the recovery of these valuable products will improve the economics of practicing the invention. Further, being a substantially closed system, little, if any atmospheric contamination results from using this novel process, which is becoming of increasing concern in metropolitan areas, even when using low equality fuels, such as those with high sulfur content.

The above description centers around the practice of the-invention in a cement kiln, but it should be appreciated that this invention could be employed in steam genated plants, other boiler systems and installations having continuous or cyclic combustion processes, with the same advantages.

The following example will help illustrate the principles of this invention in practice.

In a typical cement kiln, 1,510,000 pounds of clinker a day are produced from 13,735,335 pounds of raw material introduced into the kiln. This would show a material efficiency of 11.4% for product to raw material ratio. The materials not included in the product appearing as waste materials in the efiluent stack gases flowing through the systems between a conventional kiln an done operated according to this invention demonstrates the following differences using the same quantities of fuel, even though in actual practice the greater thermal efiiciencies of this invention would substantially reduce the amount of fuel consumed and proportionally reduce the gas volume.

Conventional process Invention process CO 1,315,000 H O 1,445,380 Clinker 1,510,000

Total 4,270,3 80

By the same arithmetic used previously it is apparent that from the standpoint of clinker total materials standpoint the material efiiency would be 35.5% If th recovery of water and carbon diovide is allowed as product also, the material efficiency approaches 100%.

Some of the more sophisticated advantages of this invention are not readily apparent and etfect an actual reduction of fuel requirements much greater than would be anticipated. The slow regulated movement of the synthetic gaseous mass through the system having larger heat capacity and resistance to radiant energy losses improves the thermal etficiency of the system.

The diathermous character of synthetic gas mixture, being primarily carbon dioxide and water vapor with oxygen added is superior to that of air since nitrogen, the principal component of air, is much less elfective against impeding radiant energy losses. Likewise the specific heat capacity of the synthetic gas mixture is superior to that of air.

I claim:

1. An improved closed combustion process for rotary cement kilns wherein no air is employed to support combustion comprising:

(a) forming a synthetic gaseous mixture composed principally of carbon dioxide and water vapor;

(b) continuously circulating said synthetic gaseous mixture without the addition of air through the cylindrical shell of said cement kiln;

(c) adding substantially pure oxigen to said synthetic gaseous mixture to form a combustible mixture;

((1) adding fuel to said combustible mixture under combusting conditions to add thermal energy to said circulating gaseous mixture whereby direct heat exchange with the cement in said kiln can be effected; and

(e) withdrawing only that portion of said circulating gaseous mixture necessary to provide gas volume for additional synthetic gaseous mixture formed by the combustion of fuel and oxygen and kiln operation whereby the closed system will operate with a constant volume of synthetic gaseous mixture.

2. The process as defined in claim 1, wherein the synthetic gaseous mixture is principally carbon dioxide and water vapor in similar amounts.

3. The process as defined in claim 1 wherein the quantities of oxygen added to the circulating synthetic gaseous mixture is of 1 to 50% of the volume thereof and in excess of that needed to combust the fuel.

4. The combustion process as defined in claim 1 wherein the portion of synthetic gaseous mixture removed therefrom is treated to recover the CO and H 0 contained therein.

References Cited UNITED STATES PATENTS 797,506 8/1905 Eldred. 1,916,980 7/1933 Horvitz. 1,931,817 10/1933 Hogan et al. 2,016,815 10/1935 Gilmore. 2,111,783 3/1938 Hults 26353 XR 2,210,482 8/1940 Derrom 26353 2,740,693 4/1956 Pomykala. 2,865,344 12/1958 Firl. 2,980,082 4/1961 Firl. 3,232,592 2/1966 Lohman. 3,074,707 1/ 1963 Humphries et a1.

FREDERICK L. MATTESON, ]R., Primary Examiner H. B. RAMEY, Assistant Examiner

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