WO1995025075A1 - Method and system for controlling pollutant emissions in combustion operations - Google Patents

Abstract

The present invention relates to methods and systems for reducing pollutant emissions associated with combustion operations, e.g. cement manufacturing. The invention further relates to the disposal of sludge, e.g., produced by waste water treatment plants, by introducing it into the combustion process. In particular, the invention provides for introducing particles of aqueous sludge into a kiln at a point in the kiln effective to reduce pollutant emissions in outlet flue gasses produced by the kiln. Optimally, the sludge is introduced at a point in which maximum NOx emission rductions can occur and maximum HC1 emission reductions can occur downstream (at lower temperatures). In a specific embodiment, introduction of about 10 to about 20 tons per hour of wet sludge results in about a 15 % to 30 % reduction in NOx emissions in a cement kiln.

Description

METHOD AND SYSTEM FOR CONTROLLING POLLUTANT EMISSIONS IN COMBUSTION OPERATIONS

FIELD OF THE INVENTION

The present invention relates to methods and systems for reducing pollutant emissions associated with combustion operations, such as cement production. The invention further relates to the disposal of biological sludge, e.g. , produced by waste water treatment plants, by introducing it into the combustion operation.

BACKGROUND OF THE INVENTION

Oxides of nitrogen are one of the principal contaminants emitted by combustion processes, and various combustion processes are known to generate effluent gases having an unacceptably high NO- content. In every combustion process, the high temperatures at the burner thus results in the fixation of some oxides of nitrogen. These compounds are found in stack gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO ) and only traces of other oxides, such as N₂O. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO₂) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO₂) and N₂O in a sample is determined and referred to as "oxides of nitrogen (NO-)".

Production of hydrogen chloride (HC1) is another undesirable side effect of combustion operations. Upon contacting water in the atmosphere, HC1 forms hydrochloric acid, which contributes, as does NO_x, to the acid rain problem.

The production of cement is a relatively complex process that involves rnining and milling the raw materials, which are then fed directly into a kiln, or fed initially into a heat exchanger (typically a preheater or precalciner) which discharges the material into a kiln, and fired to produce "clinkers". The clinkers are subsequently milled and packaged for sale as cement. This prior art process is schematically illustrated in Figure 1.

Cement kiln operations are among the combustion processes known to involve production of substantial quantities of undesirable NO- and HCl. NO. emission can be decreased by introducing a selective reducing agent, such as ammonia, into the combustion mixture.

With the technology currently available, reduction of HCl emissions requires the use of a wet scrubber. Wet scrubbers increase the complexity of a combustion process, and greatly increase the cost of combustion operations.

Various modifications of kiln operations have been attempted to reduce the level of emissions of pollutants, and to take advantage of the combustion process to incinerate undesirable waste as well. In particular, waste water treatment plant sludge (i.e. , biological sludge) has been used as a feed material in cement kiln operations, which in turn eliminates the sludge.

For example, Yamane et al.. (U.S. Patent No. 5,217,624, issued June 8, 1993) describes a process for introducing waste treatment sludge into a cement kiln, and introducing gases generated during processing of the sludge into the kiln as well in order to reduce NO- content of the exhaust gas. Yamane et al. specifically describes a process of mixing sludge with quicklime to produce slaked lime and sewage sludge. The mixture is dehydrated, releasing gases including ammonia. The remaining solids are introduced into a cement kiln, where the lime and inorganic substances become raw materials for cement, and the organic substances become fuel for cement burning. Dust and water vapor present in the gases produced during dehydration are removed by filtration and distillation, respectively, allowing introduction of the gas containing ammonia into the kiln. The gas containing ammonia helps reduce NO. emissions. Lafser, Jr. et al. (U.S. 4,921,538, issued May 1, 1990) concerns a method for recycling and reusing contaminated earth and mineral matter in the manufacture of cement clinkers. The contaminated material is mixed with inorganic constituents in water to form a raw material slurry suitable for manufacture of Portland cement, and is then charged to a cement kiln.

Wuntz (U.S. 4,306,978, issued December 22, 1981) concerns a method for lime stabilization of waste water sludge. The sludge is dewatered to produce a sludge cake having 10 to 60% solids and this is then mixed with calcium oxide to produce stabilized sludge pellets.

Steinbiss et al. (U.S. 4,640,681, issued February 3, 1987) concerns a method and apparatus for removal of hazardous and waste materials of low heat content, such as refuse. Among the wastes considered are "household wastes, agricultural waste and industrial waste. "

Enkegaard (U.S. 4,984,983, issued January 15, 1991) concerns a method and apparatus for co-firing hazardous organic waste in solid pasty, greasy, or sludge form by introducing it directly into the burning zone of an industrial rotary kiln and burning the waste in such zone simultaneously with cement kiln or lightweight aggregate. In particular, the waste is gasified prior to injecting the gas into the kiln. The gasification may be accomplished in a conventional gasifier such as is used for coal gasification.

Mozes et al. (U.S. 5,058,514, issued October 27, 1991) describes a method for the simultaneous control of sulfur dioxide and NO_x emissions from power plant flue gases. In the method described, an aqueous slurry of limestone and a nitrogenous progenitor such as urea is injected into the furnace at temperatures ranging between 900°C and 1350°C. Somewhat similar to the above, Amrhein et al. (U.S. 5,176,088, issued January 5, 1993) discloses, among other things, the injection of ammonia into a furnace region having a temperature of about 1600° to 2000°F, to reduce NO_x.

There is, therefore, a need in the art to reduce pollutant emissions produced during combustion processes, such as cement kiln operations. In particular, there is a need to reduce NO_x emissions.

There is also a need in the art to reduce HCl emissions produced during combustion processes, particularly cement kiln operations.

There is a further need in the art to utilize and eliminate sewage sludge.

Another need in the art is to provide for an efficient and cost effective means for achieving both of these ends.

These and other needs in the art are addressed by the present invention, as described below.

SUMMARY OF THE INVENTION

The present invention is directed to a method for eliminating sludge by introducing aqueous (wet) sludge into the combustion mixture during a combustion operation in order to reduce emissions of undesirable pollutants, in particular NO_x and HCl. The invention provides for introducing particles of aqueous sludge into a combustion operation in an amount and at a point in the combustion operation effective to reduce NO-, and in some instances, HCl, emissions in outlet flue gasses produced by the combustion operation. Preferably, an amount of sludge effective for optimal reduction in NO_x and HCl emissions is introduced into the combustion operation at a point in which maximum pollutant, e.g. , NO. and HCl, emission reduction can occur. Generally, an optimal point for introduction in the combustion operation is in the outlet flue gases where the temperature is between about 1500°F to about 1800°F.

In a specific aspect in which the sludge is introduced in a combustion operation comprising a kiln and a precalciner, such as in a cement kiln operation, the sludge is introduced into the calcining zone (calcining duct or mixing chamber), which is located at a point downstream of the precalciner combustion zone where the kiln outlet flue gases and precalciner flue gases are mixed. This ensures that NH₃ release from the sludge occurs in the correct temperature zone to effect selective reduction of NO_x, and provides for a long residence time at this temperature.

Sludge particles can be formed by introducing aqueous sludge through an appropriately sized aperture or apertures under pressure, using atomizing air, such that a sufficient amount of sludge is introduced to effect a reduction in NO_x emissions. Atomizer design and atomizer pressure, particularly air pressure, can be varied after determining the optimum amount of sludge to be introduced in order to produce an optimum particle size. The number and location of sludge injectors can also be varied. Optimal particle size and momentum, which depend on atomizer design and air pressure, will be reflected by improved kiln performance and greater reductions in pollutant emissions, or other performance parameters desired for the kiln operation. The particles of sludge are typically not larger than approximately 25 mm, but this may vary with sludge from various sources and moisture content, and is not critical.

Particle size directly affects residence time of the particle, which is critical for NH₃ release and subsequent combustion of the particle. Preferably, the particle residence time is greater than 0.5 seconds.

In a further aspect of the invention, the aqueous sludge is introduced at a point in the combustion operation in which the temperature is approximately 1500°F to approximately 1800°F; preferably, the temperature is approximately 1600°F to 1700°F.

According to the invention, the sludge that is introduced into the combustion operation contains a sufficient amount of moisture to be readily transported to and introduced into the combustion process, e.g. , a kiln, in particle form. Preferably, the moisture (water) content is not excessive, as this will excessively increase the amount of steam produced and thus the volume of stack gases as well as increase fan power consumption. The sludge can typically comprise up to about 80-85% water by weight, and cannot comprise less than about 65-70% water. Preferably, the sludge comprises from about 70% to about 80% water by weight. The optimal water concentration of the sludge is readily achieved by mechanical dewatering operations, e.g. , centrifuges and belt presses, as typically produced and disposed of by sanitation districts. These dewatering operations do not significantly affect the ammonia content of the wet sludge.

If the sludge delivered to the site of the combustion operation is outside the appropriate moisture range, the invention contemplates reducing or increasing water content to compensate. For example, if the moisture content of the sludge is too high, partial reduction in water content, e.g. , by mechanical dewatering or evaporation, may be performed, although enough moisture remains even after reducing the water content to allow for pumping of the sludge and direct introduction into a kiln. If the water content is too low, water can be added to the sludge. Since drying is expensive, it is unlikely that sludge that is too dry will be provided.

The energy content of the sludge is also important, since it has been found that injection of wet sludge can reduce overall kiln fuel consumption. The present invention advantageously provides for maximum introduction of ammonia contained in wet sludge. Most of the ammonia which is present in the wet sludge is dissolved in the adherent aqueous phase. Ammoma is released from the particles of sludge that are wet and that enter at the desired point. Thus, the invention advantageously provides greater economy and efficiency in reducing pollutant emissions.

A particular advantage of the invention is that NO_x reduction appears to be due to selective reduction by NH₃ present in the sludge. Accordingly, the NH₃ to NO_x molar ratio achieved by sewage sludge injection is an important parameter for reducing NO_x emissions. Preferably, a ratio of at least about 0.3 is achieved.

Introduction of aqueous sludge in the combustion operation is a significant advantage over prior art methods of utilizing sludge. Sludge typically contains more than 85% water, and according to the prior art, substantially all of the water is required to be removed from the sludge before the solid materials are introduced into the cement kiln. Mechanical dewatering can bring the moisture content to about 70-80% water. However, removal of water below 70% moisture, especially when drying is required, dramatically increases the expense and complexity of sludge disposal.

In another aspect, the invention provides a combustion operation apparatus, such as a kiln, comprising means for introducing aqueous particles of sludge into the combustion operation apparatus. Such means preferably comprises means for generating particles of sludge of the desired size, e.g. , an atomization system. Thus, the means for introducing aqueous particles of sludge into the combustion operation requires apparatus not normally associated with usual combustion operations, such as a prior art cement kiln as depicted in Figure 1. Such combustion operation apparatus further comprises means and machinery typically associated with the combustion operation, e.g. , cement production. Specific Examples, infra, demonstrated that sewage sludge injection (SSI) reduced NO_x emissions in a cement kiln. The results, obtained at an injection rate of 20 tph wet sludge (two sludge pumps on site), indicated that NO_x reductions of 15-30% were realized. Although CO emissions increased in these tests (from 125-250 ppm to 250-500 ppm), these CO emission increases could be moderated by process changes. Further reductions can be achieved by additional optimization. Importantly, the mechanism of NO_x reduction appears to be selective, and does not require substantial CO formation. The effects on kiln operations were to reduce fuel consumption by 2-10% and to increase fan power consumption by 5-10% (on a constant feed rate basis).

The SSI process used dewatered sewage sludge, which contained NH₃, as an SNCR (Selective Non-Catalytic Reduction) reagent. Sewage sludge was injected into the precalciner kiln at the optimal location for SNCR, the calcining zone, where there is a long residence time at approximately 1600°F. The kiln features affecting SSI performance were the baseline NO_x and CO concentrations. The principal sludge features were the moisture and NH₃ contents. The SSI test equipment was designed to provide sludge break-up, penetration, and mixing, so that sludge could react in the correct zone. The design features to achieve these goals were injector location, number, and design.

Thus, it is a principal object of the present invention to provide for reduction of pollutant emissions in kiln outlet flue gasses efficiently and economically, using specialty equipment modified for this purpose.

A particular object of the invention is the reduction of NO_x emissions.

Another particular object of the invention is the reduction of emissions that cause acid rain. Yet another particular object of the invention is the reduction of HCl emissions.

A further object of the invention is to provide for elimination of sludge waste material.

It is a further object of the invention to provide for introduction of aqueous sludge into a cement kiln.

Yet another object of the invention is to reduce NO- emission on the order of 15-30% or more.

These and other objects of the present invention will be further understood by reference to the following drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic diagram of a typical prior art cement manufacturing operation, from quarrying and crushing the raw materials, milling, and feeding into the kiln for clinker production, to clinker transport, milling, and shipping of the finished product.

FIGURE 2 presents a schematic diagram of a precalciner and cement kiln. A preferred site for introduction of aqueous sludge according to the invention is shown by the arrow.

FIGURE 3 presents a schematic diagram of cement kiln location for sewage sludge introduction, which should be in the mixing chamber or precalciner. The diagram indicates schematically events affecting NO. emissions after introduction of the sludge. In particular, the theoretical reactions involving NH₃ and N volatiles that affect NO_x production are noted in the figure. FIGURE 4 presents a schematic diagram of the fate of sewage sludge nitrogen after introduction into the kiln. The steps in Figure 4 relate to the steps in Figure 3.

FIGURE 5 presents a schematic diagram for NH₃ release from a particle of sludge introduced into the kiln.

FIGURE 6 presents a diagram of sludge particle behavior after introduction into the precalciner of a cement kiln. Horizontal movement proceeds along a vector in the y direction: vertical movement proceeds along a vector in the x direction. Movement of the particle in the x direction is a function of particle size, and in the y direction is a function of injection velocity. Drying of the aqueous particle occurs after injection into the kiln.

FIGURE 7 presents a schematic diagram of the parameters involved in decreasing NO_x emissions in the cement kiln outlet gasses.

FIGURE 8 presents a schematic diagram for a sludge handling and feeding system for use in the method and apparatus of the invention.

FIGURE 9 presents graphs showing the effect of 20 tons per hour wet sewage sludge injection on NO_x emissions (in Table 6). Kiln NO_x production is represented by the open bars; stack NO_x emissions are represented by styled bars. (A) Data obtained on days 1 and 2 of the test. (B) Data obtained at three time points on day 4 of the test.

DETAILED DESCRIPTION OF THE INVENTION

A principal object of the invention is to introduce aqueous sludge at a point in a combustion operation processing stream where it will be effective to reduce pollutant levels in the outlet gases of the combustion operation. In particular, the invention is directed to a method and an apparatus for reducing NO- and HCl emissions in a kiln, in particular, a cement kiln.

As used herein, the term "kiln" is intended to refer generally to a combustion operation apparatus used in combustion operations or processing in the manufacture of materials or the production of energy. Thus, the term "kiln" as used throughout this application, unless otherwise indicated, refers to any such combustion operation apparatus.

In a specific embodiment, the invention provides for introduction of aqueous sludge in a cement kiln used in the production of portland cement clinkers. It is readily appreciated that portland cement is a hydraulic cement produced by numerous cement manufacturers. In addition to kilns used for cement production, the invention specifically contemplates introducing sludge in kilns, particularly rotary kilns, used in the production or manufacture of lime, bricks, minerals, paper, etc. The term "kiln" further encompasses long dry kilns, kilns associated with boilers, preheater kilns, precalciner kilns, wet kilns, and the like. The invention further extends but is not limited to combustion processes in metal manufacturing, glass manufacturing, and energy production, e.g. , combustion of coal to produce electricity.

The present invention provides for introduction of aqueous sludge into the kiln processing stream in which the ratio of ammoma (NH₃) to non-ammonia nitrogen is sufficient to provide for NO. reduction. Although not intending to be bound by any particular theory of the mechanism of NO_x emission reduction, it is believed that a decrease in NO_x emissions following introduction of aqueous sludge into a combustion operation involves selective reduction of NO. by NH₃. Accordingly, the process of the present invention does not require the overall O₂/fuel ratio to become fuel rich (sub-stoichiometric air). Some NO_x reduction, and inhibition of N volatile conversion to NO_x, can also result from local reducing zones produced by sludge combustion. The selective reduction mechanism proposed herein explains why fuel-rich conditions, which would lead to high CO emissions, are not required to achieve the indicated level of NO_x emission reduction. Furthermore, according to the invention, ammoma release accompanies drying and precedes nitrogen volatiles release of the sludge introduced into the kiln processing stream.

In a preferred aspect, the amount of sludge introduced is an amount that achieves an NH₃ to NO_x mole ratio of 0.3.

In a further embodiment, the introduction of sludge can result in introduction of NH₃ that may not react with and reduce NO_x. Instead, this unreacted NH₃ "slip" can react with and neutralize HCl to form NH₄C1 salts further downstream in the flue outlet, e.g. , at temperatures of about 700 °F or less. In cement kiln operations, this reaction will generally occur upstream of or in the baghouse. However, according to the invention, most of the NH₃ reacts to reduce NO_x, rather than escapes as slip, which would result in plume formation.

The sludge is introduced into the kiln in particles, in which particle size range is selected to meet the requirements of particle carryover, particle drying time, and particle penetration necessary to accomplish the drying, ammonia release, and ultimately combustion of the sludge. Particle size is preferably an operational parameter determined by the effects on combustion operation. Particle size may be varied by varying the atomizer type and atomization air pressure, while maintaining the amount of sludge to be introduced. Particle penetration and residence time can be varied by the number and location of injection sites. Preferably, particle residence time at the preferred temperature range is greater than about 0.5 seconds.

The following factors are considered in deterrriining the optimum rate of introduction of the sludge, particle size of the sludge, and other factors for designing kilns according to the present invention. The size, type, fuel consumption, and operating temperature of the kiln are very important. The concentration of oxygen, NO_x, CO, and HCl in the kiln gas outlet and precalciner gas outlet in the absence of sludge are important. Also critical are gas temperatures at the site of introduction, e.g. , the mixing chamber or the precalciner. Another critical factor in designing systems of the invention is the content of ammonia, non- ammonia nitrogen, moisture, and BTUs in the sludge. Once these factors are determined, a sludge injection rate can be determined to provide conditions in the kiln gas outlet sufficient to achieve potential NO_x reductions of as much as 30% (i.e. , with a NH₃/NO_x molar ratio of approximately 0.3). The sludge injection rate is also preferably determined with respect to providing a small amount of NH₃ slip to react with and neutralize hydrogen chloride to form ammonium chloride salt, while avoiding plume formation.

Based on the Examples disclosed, infra, the following observations can be made, regarding obtaining the lowest possible stack NO_x and CO emissions in the presence of sewage sludge injection. The stack NO_x level is dependent on the kiln outlet NO_x level and the precalciner NO_x contribution. The stack CO level is dependent on the kiln outlet CO level, the precalciner CO contribution, and the CO reduction across the tower. Kiln NO_x levels are a function of burning zone temperature and kiln fuel/air ratio, which should be maintained constant. The precalciner NO_x contribution can be minimized by decreasing the precalciner fuel rate by reducing (if possible) sludge H₂O content. Kiln outlet CO levels are a function of kiln outlet O₂ concentration, which should be maintained constant in the presence of SSI. CO formation due to sludge can be nήnimized by reducing (if possible) the sludge moisture content and improving the sludge injector design to reduce particle size. CO reduction across the tower can be maximized by further increasing the fan speed and maintaining the precalciner O₂ concentration in the presence of SSI. In a further aspect, the overall effect of tires (used as a kiln fuel) and sludge is to reduce kiln NO_x emissions by 44%, with a 10% increase in CO emissions (after optimization). Higher sludge injection rates may reduce NO_x emissions even further.

In a particular aspect of the invention, an injection rate of about 5000 to about 10,000 lbs of sludge per hour, calculated on a dry weight basis (corresponding to approximately 10-20 tons per hour [tph] wet, in which the water content is about 75%), is expected to achieve NO- reductions of about 15% to about 30% in a cement kiln having the parameters shown in Table 1. Higher injection rates may further lead to higher NO- reduction levels.

TABLE 1

KILN PARAMETERS (without tire burning)

According to the present invention, the potential for NO_x reduction in the kiln outlet gases is preferably greater than or equal to about 30% . The principal assumptions in determining this value are the content of ammonia present in the wet sludge that is released with water upon introduction of the particles of sludge into the kiln, and the baseline NO_x emissions in the absence of sludge. Furthermore, it is believed that the dried sludge volatile nitrogen conversion to NO_x is minimized by appropriate selection of the injection location and the foregoing parameters, particularly residence time. Thus, the invention provides for a residence time sufficient for optimal NH₃ release, with selective NO_x reduction, followed by complete combustion of the sludge.

As used herein, the following terms have the given meanings:

The term "pollutant" refers to a component of combustion exhaust gases that has harmful or noxious properties. The present invention provides for reduction of pollutant emissions. More particularly, the term "pollutant" refers to compounds that can react in the atmosphere to form acids and contribute to ambient ozone formation. In specific aspects, the term pollutants refers to NO_x and HCl.

The term "aqueous sludge" generally refers to biosolids suspended or admixed in water, such as aqueous sewage produced by waste water treatment plants. It is a particular advantage of the present invention that it provides for the efficient and economical disposal of sewage sludge by using it to make cement. However, the invention contemplates use of aqueous organic solids or biosolids other than sewage sludge. For example, in a particular aspect sludge may be mixed with green waste, such as grass clippings, pruning waste, leaves, shrubs, etc. Such sludge is advantageously high in ammonia content with relatively low water content.

According to the invention, the sludge comprises between about 0.4% and about 1.5% ammoma by weight, calculated on a dry basis as nitrogen. However, it may be possible to use sludge with a greater ammonia content, e.g. , green- waste sludge. The ratio of non-ammonia to ammonia nitrogen by weight is approximately 1.5 to approximately 12.0. Additional components of sludge include but are not limited to carbon, hydrogen, oxygen, sulfur, and other elements. Sludge comprises volatile components as well as solids which upon combustion produce ash.

The sludge for use according to the present invention advantageously contains water, thus eliminating the step of dehydration that is necessary according to prior art methods for combustion of sludge. Generally, the water content of the sludge that is introduced into the combustion operation ranges from about 65-70% up to about 80-85%, by weight. Preferably, the moisture content is about 70% to about 80% water, by weight. In a specific embodiment, the moisture content is approximately 75 % . If the water content is too high, the evaporation of water will lead to overall cooling that can adversely affect the combustion operation or limit the potential reduction in pollutant emissions. If the water content is too low, it may not be possible to introduce particles of sludge into the combustion operation. Parameters of a specific example of a sludge sample are shown in Table 2.

TABLE 2

SLUDGE PARAMETERS

Hyp. Data Hyp. Data

Parameter Units Dry Sludge Wet Sludge Minimum Average Maximum

Solids content % solids 22 22 25 30

Water content % moisture 78 78 75 70 lb/lb dry solids — 3.5 3.5 3.0 2.3

Gross BTU per lb 4,590 4,500 BTU content dry solids

C wt%. dry 24.1 24.1

H wt%. dry 3.8 3.8 o wt%. dry 10.4 10.4

S wt . dry 1.2 1.2

Total volatiles wt%. dry 43.2 43.2

Ash wt . dry 56.8 56.8

Wet Sludge Values

NH, wt%. dry 1.02 0.4 0.8 1.5

NH₃ N wt%, dry 0.84 0.33 0.66 1.24 non-NH, N wt%, dry 2.7* 4.16 2.0 4.0 7.5

Total N wt%. dry ... 5.00 2.3 4.7 9.7

* Dry sludge value for Total N based on ultimate analysis.

According to the invention, the water content of the sludge is only important when the sludge is introduced into the combustion operation. In most cases, no additional processing of the sludge prior to introduction into the combustion operation is required. Nevertheless, the present invention contemplates adjusting the water content of sludge that may have too high or too low a water content, although as a practical matter, neither possibility is likely. This is because sludge of a high water content is expensive to transport, since much of the material is water. Sludge of a low water content is more expensive, since drying requires space for drying ponds and time. Thus, there is no incentive to produce sludge that is too dry for use according the invention. Nevertheless, the invention provides for reducing, but not eliminating, the moisture content of the sludge that contains too much water, e.g. , by mechanical dewatering (with a belt press or centrifuge) or evaporation, so as to reduce the amount of steam produced on combustion of the sludge. The invention further provides for adding water to sludge with too low a moisture content.

It will be further recognized that the sludge itself also acts as a fuel. That is, the sludge has energy content, which can be represented in BTUs. For example, the energy content per pound of dry solids in the sludge is on the order of about 4500 to about 6500 BTU; the energy content of volatiles present in the sludge is on the order of approximately 12,000 BTU/lb volatiles. Consequently, introduction of sludge in the combustion operation results in a small net decrease in energy consumption. This net decrease in energy consumption due to dry sludge combustion is partly offset by the moisture content of the sludge, but the net BTU content of the aqueous sludge is positive (with net fuel savings) at the moisture content of the sludge according to the present invention.

As used herein, the term "atomization system" refers to an apparatus or means for generating particles of defined size from the sludge. For example, the atomization system may be a dual-fluid atomization system in which steam or air forces the aqueous sludge through an atomizer in such a way as to generate particles of sludge, e.g. , or a gunnite nozzle under pressure. In specific embodiments, particles ranging in size from approximately 5 millimeters to approximately 25 millimeters can be generated. However, the actual particle size need not be determined, or necessarily fall within this range if it is determined. The critical point is that the atomization system produces particles of sludge that, upon introduction into the combustion operation, result in a decrease in pollutant emissions.

Accordingly, referring now to Figure 2, which depicts a typical cement kiln 10 and precalciner 20, in a preferred aspect of the invention, the sludge is introduced at a point in the precalciner flue gas inlet 22. More preferably, the sludge is introduced in the precalciner, downstream of the precalciner combustion zone and of the entry point of kiln outlet flue gases (Figure 3). In this configuration, the invention advantageously provides for the reduction of pollutant emissions that result from both kiln and precalciner combustion operations. The present invention contemplates that the temperature of the flue gas at the point of introduction of the sludge is in the range of approximately 1500°F to approximately 1800°F; more preferably, the temperature is approximately 1600°F to 1700°F. In a specific

Example, infra, the sludge is introduced into a calciner (mixing chamber), and held for at least about 0.5 seconds at 1600°F.

Upon introduction of the sludge into the flue gases, a series of events are expected to occur resulting in reduction of NO- emissions. Initially, the water, and more importantly ammoma, present in the sludge are released. The ammonia released in this step combines with NO and oxygen to produce molecular nitrogen and water. This is the point of substantial reduction in the NO- levels in the gases. After the initial release of ammonia and water, volatile nitrogens, including ammonia, present in the solid particles of sludge are released. The volatile nitrogen reaction is one that results in a small amount of NO. formation. The overall effect of the process is a substantial reduction in NO_x emissions, in additional to elimination of the sludge. The reaction is summarized in Figure 4.

Ammoma released upon introduction of the sludge can also effect a reduction in HCl emissions. The sludge NH₃ slip from the reaction between sludge NH₃ and NO_x can react with HCl in the baghouse to form NH₄C1, thus neutralizing the HCl. Neutralizing HCl in this fashion avoids the need to use a wet scrubber to reduce HCl emissions.

As pointed out above, reduction of pollutant emissions in the flue gases depends primarily on release of ammoma from the sludge. A particular advantage of the present invention, therefore, is that the ammonia content of the sludge is high, and at the same time the sludge provides energy. Wet sludge contains 100% of the ammonia present in the sludge (Figure 5). Based on a model system, it is expected that after the sludge particle partially dries at the high temperatures of the kiln, approximately 48% of the initial ammoma remains. Thus, it is clearly an advantage of the present invention that all of the ammonia present in the wet sludge is available for reaction in the flue gases to reduce NO- and HCl emissions.

Figure 6 provides a schematic diagram for the behavior of sludge particles introduced into the kiln. The wet sludge is injected into the appropriate site in the combustion operation. As can be readily appreciated, there is a substantial flow of combustion gases in an upward direction. Thus, there are two component vectors to the motion of each sludge particle; vertical motion in the X direction, which depends on the velocity of flue gases, and horizontal velocity in the Y direction, which depends on the velocity at which the sludge is injected. Each particle of sludge will have a drying time that depends on the particle size. The drying time in turn affects the length of time during which the particle will be subject to the forces in the X and Y directions. The longer the drying time, the farther the distance traveled in each direction. The particle size of the sludge is selected to meet the requirements for particle carryover, particle drying time, and particle penetration necessary to most efficiently effect pollutant reduction in the flue gases.

The amount of sludge introduced in the system is determinative of the extent of NO_x emission reduction. The more sludge introduced, the greater the reduction in NO_x emissions expected. However, too much sludge can overload the combustion system. The actual amount of sludge to be introduced will depend on the characteristics of the kiln, e.g. , the type of kiln, size of the kiln, amount and type of fuel used to fire the kiln, etc., as discussed above. The amount of sludge to be introduced can be varied in order to optimize reductions in the levels of pollutant emissions. According to the present invention, a sludge injection rate of greater than approximately 5000 to approximately 10,000 lb/hr, calculated on a dry weight basis, is expected to provide for approximately a 15-30% reduction in NO_x emissions in a cement kiln having the parameters shown in Table 1.

Since the present invention envisions that ammonia release will primarily accompany drying, and precede nitrogen volatile release, the ammonia will be present in the flue gas before the nitrogen volatiles that can be used in the formation of NO_x.

Key parameters involved in the reduction of NO_x emissions from the flue gases, and disposal of the sludge, are summarized in Figure 7. Sludge breakup into particles depends on the water content of the sludge and the characteristics of the particle formation system, such as sludge pressure, atomizer air pressure, and atomizer design. Release of NH₃ from the sludge is affected by particle size and temperature, as well as partitioning of nitrogens in the sludge and sludge water content. Sludge mixing is affected by the number of injectors and their orientation. The size and velocity of particles also affect sludge mixing. Finally, control of particle deposit in the combustion process is determined by air level and ash fusion temperature in addition to panicle size and spray pattern.

The handling and feeding of sludge into the processing stream where it will be effective to produce NO_x and HCl emission reduction can be according to any desired means. In a specific aspect of the invention, the sludge can be introduced into a precalciner as shown in Figure 8, which diagrams a dual feed operation in which two identical systems introduce sludge into the precalciner. In one exemplary system, the sludge can be moved from a 25 ton hopper truck 30 into a feed hopper 34. A screw feeder 36 can transfer the sludge to a sludge pump 38 operated by a hydraulic drive 40 powered with a 75 horsepower motor 42. Preferably, water is introduced through an injector 44 into the sludge pumped out of the sludge pump to provide lubrication. More preferably, a metering pump 46 is installed to control the rate of injection of water. In a specific aspect, the water is injected at a rate of about 24 gallons per hour. In this specific embodiment, the sludge is pumped from the sludge pump through an 8 inch pipe 48 at a rate of approximately 40-80 gallons per minute of wet sludge containing approximately 25% solids into a dual-fluid atomizer 52. The sludge in the dual-fluid atomizer is pressurized with 200 SCFM 100 PSIG air fed from a compressor 54 through a ball valve 56. Pressure can be monitored with a gauge 58. Thus the sludge is introduced in paniculate form into the precalciner mixing chamber 20.

As noted above, the net heating value of sludge is positive, resulting in a decrease in fuel consumption. For a sludge injection rate of 10,000 lb/hr dry (75% moisture content), there is a 3-6% decrease in fuel consumption. The energy contribution of the dried sludge is partly offset by the loss in energy due to evaporation of the water. Also, the additional water volume in the flue gas that accompanies moist sludge injection requires increased fan power consumption where a fan is used to exhaust flue gases. In the particular embodiment described above, where the injection rate of sludge is 10,000 lb/hr dry (75% moisture content), there is a 5- 10% increase in fan power consumption. Nevertheless, any increase in fan power consumption that results from injection of wet sludge is more than offset by the net positive energy value of the sludge.

In a specific aspect of the invention, the net revenue from sludge, after the sludge transport costs have been subtracted, is greater than $10.00 per ton of wet sludge, as calculated in 1993 dollars. For an injection rate of 20 tons per hour of wet sludge, estimating 8,000 operating hours per year, this represents annual revenues from sludge elimination in excess of $1,600,000.00 per year. Of course, higher sludge utilization rates would result in greater revenues. The foregoing estimates are based on the particular kiln parameters shown in Table 1. As one of ordinary skill in the art can appreciate, these conclusions will vary depending on the specific assumptions or characteristics of the kiln in process in a given embodiment of the invention. Thus, the foregoing are provided by way of example and not by way of limitation.

EXAMPLES

The sewage sludge injection (SSI) demonstration test features can be summarized as follows. Dewatered biosolids (25% solids) were transported in hopper trucks. The sludge handling equipment included a sludge pump, piping with annular water injection for lubrication, dual-fluid atomizers, and air compressors. There were two phases of SSI demonstration testing. During the first phase, a single pump (max. cap. 10 tph wet sludge) was available on-site, while, during the second phase, two pumps were operated in parallel, delivering a total rate of 20 tph wet sludge. The parameters monitored included kiln operating conditions and kiln NO_x, O₂, and CO concentrations at four locations (kiln outlet, precalciner 1st and 4th stages, and stack). Between the first and second phases of testing, the test procedure was modified (to collect baseline data between tests) so that baseline and SSI test data could be compared directly.

Kiln Baseline Data

The baseline NO_x concentrations determined the quantity of sludge required to achieve the desired NH₃/NO_x molar ratio. The baseline CO concentrations determined to what extent CO emissions may increase and indirectly affect NO_x reduction performance. Kiln baseline data are shown in Table 3. TABLE 3

KILN BASELINE DATA

Kiln Units Baseline Baseline % Change Property Without Tires With Tires Due to Tires

Kiln NO_x lb/ton clinker 2.3 1.9 -17 mass rate

Kiln NO_x Percent of 72 85 contribution total

Stack NO_x ppmvd at 320 230 concentration 12% O₂

Stack NO_x lb/ton clinker 3.2 2.3 -28 mass rate

Stack CO ppmvd at 210 120 concentration 12% O₂

Stack CO lb/ton clinker 1.2 0.7 -42 mass rate

Table 3 presents baselines both without and with tires. The presence of tires affected kiln performance, particularly NO_x emissions. In particular, with tires as fuel, stack NO_x emissions were reduced 28%. This was due to a reduction in kiln outlet NO_x emissions and in the precalciner NO_x contribution (precalciner contribution = stack emissions - kiln emissions). The kiln outlet NO_x emissions decrease because the kiln fuel/air ratio increases due to back end fuel addition. The precalciner NO_x contribution decreases because the precalciner fuel rate decreases.

Stack CO emissions were also reduced. This was probably because there was sufficient residence time in the flue gas downstream of the tire combustion location to achieve complete combustion of the CO produced. Less CO is produced in the precalciner because of the precalciner fuel rate reduction.

The data in Table 3 indicate that the stack NO_x emissions in the baseline without tires are lower than expected, and a much greater fraction of these emissions are from the kiln (both without and with tires).

Sludge Parameters Affecting Performance

Based on the characteristics of the sludge and the kiln, including the amount of NH₃ required for the selective reduction process to occur, the following parameters for sludge injection rate were determined (Table 4):

TABLE 4

SLUDGE INJECTION RATES AS A FUNCTION OF SLUDGE PARAMETERS

(Kiln Parameters Shown in Table 3, 225 tph Clinker Rate)

Low Average High

Solids content wt% solids 22 25 30

Moisture content wt% H₂O 78 75 70

Gross BTU BTU per 4500 4500 4500 content (before lb dry evap. H₂O

Net BTU content BTU per 1100 1600 2200 (after evap. H₂O) lb dry

NH₃ content wt% wet 0.12 0.18 0.24

Non-NH₃ N wt% wet 0.6 0.9 1.2 content

Sludge rate required for NH₃/NO_x mole ratio = 0.3

3.2 1b NO₂ per tph wet 33 22 17 ton clinker

2.3 lb NO₂ per tph wet 24 16 12 ton clinker Based on the fuel consumption results obtained, the sludge has a small positive net BTU content.

Based on sludge sampling during the testing period, the ranges of sludge NH₃ and non-NH₃ N content are as shown in Table 4.

At a clinker rate of 225 tph, the sludge rate required to achieve an NH₃/NO_x molar ratio of 0.3 is 16 tph wet for the lower kiln baseline NO_x (baseline NO_x with tires).

Design Features and Sludge Injection Location

Sludge injection was designed with the following goals: achieving sludge breakup, penetration, and mixing; releasing NH₃ from the sludge in the correct temperature zone; separating the process of sludge drying and release of NH₃ from the process of dry sludge combustion (accompanied by N release); and complete combustion of the sludge. Design features chosen to achieve these goals included location and number of injectors, and atomizer design and air pressure. These factors determine sludge momentum and particle size, which, in combination with the kiln parameters, affect the ability of the SSI process to achieve desired emissions reductions.

The sludge injection location was based on obtaining the maximum residence time in the optimal temperature window for SNCR (1500-1800°F). In these Examples, the sludge was injected in the precalciner mixing chamber (see Figure 3). The mixing chamber provides approximately 0.8 seconds residence time at 1600°F.

The sludge was injected downstream of the point where the kiln outlet flue gas and precalciner flue gas contribution mix. The velocity at this location was sufficient to make the sludge particles move upward. It is expected that, as the sludge particles move upward, sludge drying and NH₃ release (Step 1 in Figure 3) will precede N volatiles release (Step 2 in Figure 3). Conversion of the CO formed by the sludge combustion to CO₂ will take place as the flue gas travels further downstream (up through the preheater stages).

Summary of Design Strategy

The SSI process used dewatered sewage sludge, which contains NH₃, as an SNCR reagent.

The principal kiln features affecting SSI performance are the baseline NO_x and CO concentrations, both kiln outlet and stack.

The principal sludge features affecting SSI performance are the sludge moisture and BTU content, which determine the effect on kiln fuel consumption and fan power consumption, and the sludge NH₃ and non-NH₃ N content, which determine the potential NO_x reduction per lb of sludge. The sludge rate for a given kiln feed rate is selected based on the NH₃/NO_x mole ratio.

The design goals include sludge breakup, mixing, and reaction in the correct zone. The design features to reach these goals are injector location, number, and design.

The optimal location in a precalciner kiln for sludge injection was determined to be in the calcining zone (calcining duct or mixing chamber), where there is a long residence time at approximately 1600°F.

Handling and Feeding System

A schematic diagram showing the sludge handling and feeding system is presented in Figure 8. A positive-displacement sludge pump with screw feeder was used to drive the sludge through the piping from ground level to the injector level. Annular water injection was used to reduce the pressure drop in the piping. The actual single sludge pump capacity was 7.5-10 tph. This led to difficulties in achieving measurable sludge effects on emissions. A second phase of testing with two sludge pumps on-line was initiated, yielding the results reported in the Example.

Dual-fluid atomizers, which performed much better than finger mill and gravity-fed injectors, were used. Rough measurements of sludge particle size and penetration obtained with the injectors were made outside the mixing chamber. The sludge particle size was '/₂ to 1 " or more. The sludge pressure drop across the injector was small relative to the line pressure drop (15 psig versus 200-400 psig). Sludge atomization required 200 CFM of 85 psig air per injector.

The sludge handling and feeding data are summarized in Table 5.

TABLE 5 SLUDGE HANDLING AND FEEDING PARAMETERS

Pump sludge flowrate (per pump) 7.5 tph ave. (10 tph max.) 30 GPM ave. (40 GPM max.)

Particle breakup step Breakup in atomizers

Sludge injector design Pump-fed under pressure High-pressure air atomization

Particle size achieved Vi - 1 " or more

(test outside mixing chamber)

Penetration achieved 6 - 12 ft

(test outside mixing chamber)

Sludge injection pressure 15 psig or less

Atomizing air pressure 85 psig

(3 compressors on-line)

Atomizing air flowrate 200 CFM per injector

Test Conditions and Durations

The test conditions used to obtain the data are summarized in Table 6. Comments about these conditions, and the data obtained, are reported below.

TABLE 6

TEST CONDITIONS AND DURATIONS

Sludge Tire Steady Baseline Sludge

Test Rate Tire Shutdown/ state data Test Period tph wet Burning Startup Time reached? available? Duration

Example: Testing with two sludge pumps on-line

4 days 20 Yes Tires Yes Yes 2-4 hrs continuous (between tests)

During demonstration testing, at a sludge injection rate of 20 tph (wet basis), in the presence of tires, the following effects were observed:

• Fuel consumption was reduced 2-10%.

• Fan power consumption was increased 5-10% .

• NO_x emissions were reduced 15-30% .

• CO emissions were increased from 125-250 ppmc to 250-500 ppmc.

The NO_x emission reduction characteristics suggested that the mechanism of NO_x reduction was selective (due to NH₃, like SNCR), and therefore did not depend on substantial CO formation. The CO emission increase characteristics indicated that CO emission increases could be reduced by process optimization, without affecting NO_x reduction. Test Conditions and Durations

For this testing (test conditions and durations shown in Table 6), with two sludge pumps on-line (maximum capacity 20 tph wet sludge), in addition to the potential for more measurable sludge effects due to the higher sludge rate, the test procedure was designed to make the results easier to interpret. First, tires were operated continuously, so there was no need to wait for steady-state to be reached. Second, the SSI test durations were 2 to 4 hours, and a baseline data were collected between SSI tests. Third, the precalciner tower sampling for NO_x was added to provide more accurate (less dilute) and more timely (less lag time) data.

As a result of these modifications to the test procedure, direct comparisons could be made between baseline and SSI data, and an accurate determination could be obtained of the percent change due to sludge. Quantitative data were obtained in this experiment, and the data support a conclusion that SSI can achieve significant reductions in NO_x emissions.

Test Results

The results of SSI at a rate of about 20 tons per hour are summarized in Table 7.

TABLE 7

SSI DEMONSTRATION TEST RESULTS

AT 20 TPH WET SLUDGE

(TESTING WITH TIRES)

Day l Day 2 Day 4 Day 4 Day 4 Day 4 Avg. of

Test A Test Bl Test B2 Test B3 6 tests

Total Fuel Consumption (fuel/feed ratio) Percent Change -2.4 -10 -5.4 -6.6 -4.9 -2.7 -5.4

Fan Power Consumption (current/feed ratio) Percent Change 10 6.2 6.5 5.8 5.8 5.8 6.8

Stack CO Without sludge 104 108 119 134 134 134 122

Concentration With sludge 446 490 224 501 497 554 452

(ppmc) 330 350 88 270 270 310 270

Kiln NO_x Without sludge 2.29 1.23 2.94 1.33 1.33 1.33 1.74 Mass Emissions With sludge 2.68 2.59 2.48 2.07 1.60 1.34 2.13 (lb/ton clinker) Percent change 17 110 -16 56 20 0.6 31

Stack NO_x Without sludge 1.91 1.69 3.42 2.1 1 2.11 2.11 2.23 Mass Emissions With sludge 1.57 1.42 2.72 2.02 1.68 1.47 1.81 (lb/ton clinker) Percent change -18 -16 -21 -4.3 -20 -30 -18

Precalciner NO_x Without sludge -0.38 0.46 0.48 0.78 0.78 0.78 0.48 Contribution With sludge -1.11 -1.17 0.24 0.05 0.08 0.13 -0.31 (lb/ton clinker) Net effect -0.73 -1.63 -0.24 -0.83 -0.70 -0.65 -0.80

The fuel and fan power consumption results of this test indicate that fuel and fan power consumption effects are more consistent than observed in the initial test. In this test, fuel consumption was reduced 2-10% (5.4% on average), and fan power consumption increased 5-10% (6.8% on average). The stack CO emissions results indicate that substantial stack CO emissions increases occurred in all cases, except in Day 4 Test A, in which a gradual start-up of the SSI system was performed, with incremental changes in kiln operating conditions. Notwithstanding the observed increase in CO emission, it is evident that CO emissions could be reduced through optimization. This appears to be the case based on the results of Day 4, Test A. Although CO emissions increases were moderate, NO_x emission reductions were measured, indicating that NO_x emission reductions are not dependent on CO emission increases. Thus, the mechanism of NO_x reduction is selective, and would not be affected by introduction of measures designed to reduce CO emission, e.g. , oxygenation of the combustion processes.

The kiln and stack NO_x emissions are shown in Table 7 and Figure 9. The test data show that, even in cases where kiln NO_x increased between the baseline and the SSI tests due to factors unrelated to SSI, reductions in stack NO_x emissions occurred. Stack NO_x reductions of 4-30% (15-30% once the effects of kiln NO_x increases unrelated to SSI are subtracted) were obtained.

The importance of calculating the precalciner NO_x contribution is illustrated in the data from the Day 4 test set B (Bl, B2, B3), in which all three SSI tests were compared to a single baseline after the SSI test. The kiln NO_x emissions decreased over time (from test Bl to test B2 and from test B2 to test B3), due to factors unrelated to SSI, and then remained constant from test B3 to the baseline (after the SSI test). Therefore, only SSI test B3 is directly comparable to the baseline. The Day 4 test set B shows that, once the kiln NO_x emissions were subtracted from the stack NO_x emissions, the change in NO_x emissions (in the precalciner NO_x contribution) due to sludge is very consistent. This implies that, if the kiln NO had been the same in the Bl and B2 tests as in the B3 test, the stack NO_x emissions in the Bl and B2 tests would have been similar to the B3 test (i.e. , 30% reduction from the baseline.)

The relatively constant magmtude of the NO_x reduction due to SSI in the Day 4 test set B suggests that this fixed level of NO_x reduction corresponds to the point at which the sludge NH₃ content is consumed. Calculations indicate that the magnitude of the NO_x reduction is in agreement with the calculated NH₃/NO_x molar ratio. This supports the contention that the NO_x reduction is selective. (Note that since sludge NH₃ content varies more from day to day than within one truck load, the quantity of NO_x reduction will show variation from day to day, but be relatively constant within one truck load.)

The NO_x emission reductions due to SSI had the following characteristics. Consistent NO_x reductions were measured at a variety of baseline conditions and kiln NO_x levels. The NO_x reduction quantity corresponded to the calculated NH₃/NO_x mole ratio, based on measured sludge NH₃ content. Finally, the NO_x reduction did not depend on O₂ or CO levels. These characteristics indicated that the mechanism of NO_x reduction was selective.

In contrast, the CO emission increases due to SSI varied from test to test. Increasing the atomization air pressure drastically reduced CO emissions. Adjusting fan rates to increase O₂ levels tended to reduce CO emissions, without affecting NO_x reduction. These results indicate that CO emissions can be reduced by process optimization, without affecting NO_x reduction.

Conclusions

The data in Table 8 show that optimum performance objectives can be obtained using this SSI technology. The test results shown in this Example indicate that the optimum pollutant reduction (in this limited data set) is obtained at a sludge injection rate of 20 tph wet sludge (kiln and sludge parameters shown in Tables 3 and 4), using tires to fuel in conjunction with SSI.

TABLE 8

KILN DATA WITH SSI

(Data Obtained at 20 tph Wet Sludge, Testing with Tires)

% Change

Baseline % Change Due to

Kiln Property Without Baseline Due to Tires Data % Change Tires and Tires With Tires With SSI Due to SSI SSI

Stack NOχ Concentration

(ppmvd at

12% 0₂) 320 230 180

Stack NO_x mass rate

(lb/ton clinker) 3.2 2.3 -28 1.8 -22 -44

Stack CO Concentration

(ppmvd at

12% 0₂) 210 120 230*

Stack CO mass rate

(lb/ton clinker) 1.2 0.7 -42 1.3* +90* + 10*

* Values shown are after optimization. These values are based on the Day 4 Test A result, which showed that CO emissions increases could be reduced through optimization.

The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for reducing pollutant emissions from a combustion operation comprising introducing particles of aqueous sludge at a point in a combustion operation effective to reduce pollutant emissions in outlet flue gasses produced by the combustion operation.

2. The process according to claim 1 wherein the sludge comprises at least about 65% to about 70% water, and not more than about 80% to about 85% water by weight.

3. The process according to claim 2 wherein the sludge comprises between about 70% and about 80% water by weight.

4. The process according to claim 1 wherein the pollutant is selected from the group consisting of NO_x and HCl.

5. The process according to claim 1 , wherein the pollutant is NO_x, and the sludge is introduced at a rate which results in reduction of greater than 10% of emissions of NO_x.

6. The process according to claim 5 wherein the sludge is introduced at a rate which results in reduction of approximately 30% of emissions of NO_x.

7. The process according to claim 1 wherein the combustion operation is a cement manufacturing operation having a kiln and a precalciner associated with the kiln, and the sludge is introduced in a precalciner flue gas outlet downstream of a precalciner combustion zone and at a point where kiln outlet flue gases and precalciner flue gases are mixed.

8. The process according to claim 1 wherein the particle size of the sludge is optimal for reduction of pollutant emissions.

9. The process according to claim 1 wherein the sludge comprises between about 0.4% and about 1.5% NH₃ by weight, calculated on a dry basis, as nitrogen, and the ratio of non-NH₃ to NH₃ nitrogen by weight is approximately 1.5 to approximately 12.0.

10. The process according to claim 1 wherein the temperature at the point at which the sludge is introduced is approximately 1500°F to approximately 1800°F.

11. A process for reducing pollutant emissions in a cement kiln comprising introducing particles of aqueous sludge comprising at least about 65% to about 70% water, and not more than about 80% to about 85% water by weight, in a precalciner flue gas outlet downstream of a precalciner combustion zone and at a point where kiln outlet flue gases and precalciner flue gases are mixed, wherein the sludge is introduced at a rate which results in reduction of greater than 10% of emissions of NO_x.

12. The process according to claim 11 wherein the temperature in the precalciner flue gas outlet is approximately 1500°F to approximately 1800°F.

13. A combustion operation apparatus adapted for combustion of aqueous sludge resulting in reduction of NO. emissions, comprising: (a) a combustion operation having a flue gas outlet; (b) means for introducing particles of aqueous sludge into the combustion operation at a point effective to reduce pollutant emissions produced by the combustion operation.

14. The combustion operation apparatus of claim 13 which is a modified cement kiln.

15. The combustion operation apparatus of claim 14 wherein the means for introducing particles of aqueous sludge is located in a precalciner flue gas outlet downstream of a precalciner combustion zone and at a point where kiln outlet flue gases and precalciner flue gases are mixed.

16. The combustion operation apparatus of claim 15 wherein the temperature of at the point of introduction of the sludge is approximately 1500°F to approximately 1800°F.

17. The combustion operation apparatus of claim 13 wherein the means for introducing particles of aqueous sludge comprise an atomization system located at the point at which the particles of sludge are introduced.

18. The combustion operation apparatus of claim 17 wherein the means for introducing particles of aqueous sludge further comprise pumping means in fluid communication with the atomization system, which pumping means transfer the sludge to the atomization system.

19. The combustion operation apparatus of claim 18 wherein the means for introducing particles of aqueous sludge further comprise means for introducing water into the sludge, whereby the water lubricates the sludge.

20. The combustion operation apparatus of claim 13 wherein the means for introducing particles of aqueous sludge introduce approximately 40 to approximately 80 gallons per minute of aqueous sludge.