WO2010138783A1

WO2010138783A1 - Method for eliminating spikes of mercury emissions

Info

Publication number: WO2010138783A1
Application number: PCT/US2010/036506
Authority: WO
Inventors: Ove Jepsen; Peter T. Paone Iii; John Salmento
Original assignee: Flsmidth Inc.
Priority date: 2009-05-28
Filing date: 2010-05-28
Publication date: 2010-12-02
Also published as: MX2011012661A; EP2435161A1; US20100300864A1; CN102548638A

Abstract

A method for the reduction of mercury emissions from an industrial plant that utilizes a cement or minerals kiln that has a high level of mercury emissions during specific operating conditions. The invention reduces a large spike in mercury emissions typically seen in cement kiln systems when the in-line raw mill is shut-down and all preheater gases are vented to the stack directly. The invention diverts the preheater gases to a cooling device that functions as an adsorption reactor to provide mercury absorption during periods when the raw mill is not in operation.

Description

METHOD FOR ELIMINATING SPIKES OF MERCURY EMISSIONS

Background of the Invention

There is an increasing level of awareness concerning the emission of mercury and other contaminants during industrial processes. Cement plants, for example, have a wide range of mercury inputs and resulting emissions because of the wide variety of raw materials and fuels used in the process. In studies of operating plants, it has become apparent that a large increase in emissions of several of these compounds occurs within a short period of time following the stoppage of milling operations. This spike in emissions is significantly greater than the emission level seen during steady-state operation when the mill is in operation or when the mill system has been off-line for some time.

Consequently, there is an interest in developing cost effective options for controlling these emissions, and that is an object of the present invention.

Brief Description of the Invention

This invention is a method of reducing the emission levels, including reducing variability in emissions, of mercury and other contaminants from mineral processing systems such as cement or mineral kiln systems during mill stoppages.

The invention comprises a method and system for the reduction of mercury emissions from an industrial plant utilizing a cement or lime kiln that has a high level of mercury emissions during specific operating conditions. The invention eliminates or significantly reduces a large spike in mercury emissions typically seen in cement kiln systems when the in-line raw mill is shut-down and all preheater gases are vented to the stack directly. The invention treats the preheater gases prior to dedusting and venting by cooling the gases to at least near the condensation temperature (it being understood that when a sorbent is employed mercury and other compounds may absorb at temperatures slightly above— that is, up to about 25° above— their condensation temperature) of certain contaminants, particularly mercury compounds, and absorbing such contaminants on a sorbent either added to the cooling step and/or in the form of the dust exiting the preheater system. The sorbent may be recycled to enhance the capture of mercury. All or some of the recirculated sorbent may be removed and disposed of in order to reduce the overall emissions of mercury in addition to eliminating the spike in emissions.

This invention is not limited to cement plants or plants with preheater towers. It can be used on any industrial processing plant where the emission of volatile metals, VOCs, or dioxin/furans are highly dependant on the mode of operation of equipment downstream of the main processing equipment, for example long dry cement kilns and lime kilns.

Description of the Drawings

Figure 1 is a general diagram of a plant for the production of cement clinker adapted to the continuous cleaning of particulate material of mercury and other contaminants during times when the raw mill of the plant is not in operation.

Figure 2 is an enlarged partial diagram of section 200 of Figure 1 showing in detail the raw grinding area of the cement plant as a first embodiment adapted to the cleaning of particulate material of mercury and other contaminants from the gas stream passing around the raw mill and using a separate collection system. Figure 3 is an enlarged partial diagram of a second embodiment of the present invention more adaptable to a retrofit of an existing plant. Figure 4 is an enlarged partial diagram of a third embodiment of the present invention.

Figure 5 is an enlarged partial diagram of a fourth embodiment of the present invention.

Description of the Invention Although the present invention is particularly directed to the removal of mercury it should be understood that it also applies to the removal of other volatile metals such as cadmium and thallium, sulfur compounds, condensable volatile organic compounds (VOCs), acid gases such as HCl, and dioxin/furans from industrial plants such as cement manufacturing facilities that contaminate kiln manufacturing processes. Also, while emphasis is placed on a cement manufacturing process, it is understood that the present invention is applicable to other kiln manufacturing processes, such as a lime manufacturing process and other kiln manufacturing industrial processes where a variation in operating temperatures and/or dusty concentrations brought about by changes in operation modes may affect levels of emissions of these compounds.

Mercury typically enters an industrial kiln manufacturing process, such as a cement kiln process, in raw materials and fuels. In cement processes the mercury enters in very low concentrations. Due to the phase properties of mercury and mercury compounds, very little mercury exits with the cement clinker product. Most of the mercury re-circulates in the process between the raw mill, main dust filter and the preheater tower. The mercury compounds vaporize in the preheater tower and travel in the gas stream to the raw mill and main dust filter. When the raw mill is running a high percentage of the mercury in the gas stream is captured by the raw meal. The captured mercury is disproportionately concentrated in the kiln dust in the dust filter after the raw mill. Since very little mercury leaves with the clinker or exits the stack when the raw mill is running, the concentration of mercury increases in the kiln feed, kiln dust, conditioning tower dust, raw mill cyclone dust, downcomer dust, downcomer gas stream, and gas streams in the mid to upper stages of the preheater tower to many times the levels found in the original raw materials. When the raw mill is shut down, the mercury emissions from the main stack increase dramatically as the built up mercury is purged from the system.

In practice it is observed that there is a very large increase in the emissions level of mercury compounds in a short duration of time directly after the ceasing of operation of the raw milling system. It has been observed that the emission level in a period of time less than one hour after the raw mill is shut down may spike to about 10 to 20 times the baseline emission level of mercury seen when the raw mill is in steady state operation prior to the shut-down of the mill. The very high level of mercury emissions during this brief period of time can be a substantial portion of the emissions levels seen from the plant. Reducing the size and duration of this increase in emissions level seen at the main plant stack may not directly reduce the level of mercury emissions seen at the plant over a long duration of time, but reducing this emissions spike will greatly reduce the short term severity of the emission level and will assist in the efficient operation of other technologies for the capture and removal of mercury and other compounds downstream of the pollution control device that an end user may employ as a complement to the present invention.

In the present invention, gases exiting from the preheater system which contain mercury are directed away from the raw mill when the raw mill is shut down and through a cooling means for the gases such as, most preferably, a gas suspension absorber (GSA) or a gas cooling tower. The gas suspension absorber consists of a tubular reaction vessel through which these gases are directed maintaining a residence time preferentially between about 0.5 and about 3.0 seconds.

When reference is made herein to a GSA as the cooling means it is understood that a gas cooling tower can also satisfy the requirements of the invention.

A supply of water is provided to the cooling means in order to cool the gases to at least near the condensation temperature of elemental mercury and optionally a number of other contaminant chemical species. By "at least near" it is meant to cool to at least 25°C above the condensation temperature and below. Therefore, if a particular contaminant has a condensation temperature of 100 ⁰C, it should be cooled to about 125 ⁰C and below.

In the case of mercury, as it has a high vapor pressure at temperatures even below its condensation temperature a significant amount of mercury will be in the vapor phase even at its condensation temperature. Therefore, it is most preferred to cool mercury to temperatures significantly below its condensation temperature. Therefore, the temperature at the exit of the cooling means is controlled to maintain a high absorption of mercury between about 40⁰C to about 200⁰C, more preferably about 70⁰C to about 160⁰C and most preferably about 90⁰C to about 135°C. The amount of water supplied is used to control the exit temperature from the reaction vessel to maintain the desired exit temperature. The water sprayed into the vessel may use mechanical atomization or air atomization to ensure evaporation of the water. An amount of sorbent, consisting of hydrated lime, recycled cement kiln dust, activated carbon, or proprietary sorbent, may be added with the water as a slurry or may be supplied separately through direct dry injection to the vessel to assist in the absorption of the pollutants. Alternatively, the dust existing in the gas stream from the preheater stream may contain a sufficient quantity of dust escaping the preheater to act as a sorbent for mercury.

Chemical additives such as oxidizing agents may be optionally added, either upstream, downstream, or in the reaction area, to assist in converting the mercury to the oxidized form to aid in the re-adsorption of mercury when the sorbent or chemical reagent is added downstream of the first dust collector. Suitable oxidizing agents include ozone, peroxide, halogenated species such as a chlorine solution such as calcium chloride, potassium permanganate, hydrochloric acid, iodine and other agents suitable to oxidize mercury. The preferred amount of oxidizing agent will typically be expressed as its concentration in the gas stream downstream from where the agent is injected. For example, when the oxidizing agent is chlorine the preferred concentration of chlorine in the gas stream will generally range from about 50 to about 100 ppm. The addition of such oxidizing agents can be reduced or eliminated, particularly when there are naturally occurring oxidizing agents such as chlorine and other halogens in the hot process gas from the preheater, which may be the case depending on the raw materials utilized in the process, the type and form of the oxidizing agent to be used, the amount of volatized mercury in the preheater gas and whether any oxidizing agents occur naturally in the preheater gas. The practitioner of the invention should take into consideration the emission of un-reacted oxidizing agents such as HCl, ozone, or chlorine when determining levels of injection to be utilized, as these compounds may be subject to governmental emissions controls.

Gases exiting the gas suspension absorber are passed through a separation/collection vessel. The separation/collection vessel is preferentially a cyclone or a series of cyclones, but may also be a drop-out chamber. The separation/collection vessel removes a high percentage of the dust and/or sorbent passing through the gas suspension absorber which has absorbed a large fraction of the mercury in the preheater gas. This dust may be recycled back to the gas suspension absorber to capture additional mercury until saturated, and/or may be taken to other areas of the plant for inclusion into the finished product from the plant, addition to grinding mills, addition to the kiln feed bin or blending silo for introduction to the kiln system, use as a fire suppressant in process equipment such as coal mills and disc reactors, treatment of the dust in a mercury removal system, or disposal. It is preferred to recycle a portion of this dust back to the GSA to reduce or eliminate the requirement of adding sorbent for capture of mercury in the reaction vessel.

The gases exiting from the collection/separation vessel following the gas suspension absorber are then passed to the main air pollution control device in the plant, consisting of a baghouse, an electrostatic precipitator, a gravel bed filter, or any method utilized to maintain the operation of the plant in compliance with dust emission limits. Introduction of additional air to this collector for reduction of temperature may be accomplished with an additional "bleed air" damper located in the ductwork prior to the inlet to the gas suspension absorber or in the ductwork between the collection/separation vessel and the main air pollution control device in order to improve the temperature profile seen in the main air pollution control device, to provide control for the temperature in the main air pollution control device during conditions when gas flows do not allow the use of water for gas cooling (such as during start-up of the cement kiln system), or to provide a means of reducing the dewpoint of the gas stream to avoid the condensation of moisture and acids on the interior surfaces of the main air pollution control device and associated ductwork. Gases may be drafted through the main air pollution control device by means of an induced draft fan following the main air pollution control device, by an induced draft fan between the gas suspension absorber and the main air pollution control device, or by a combination of both of these methods.

The material collected in the main air pollution control device may be recycled back to the gas suspension absorber to capture additional mercury until saturated, and/or may be taken to other areas of the plant for inclusion into the finished product from the plant, addition to grinding mills, addition to the kiln feed bin or blending silo for introduction to the kiln system, use as a fire suppressant in process equipment such as coal mills and disc reactors, treatment of the dust in a mercury removal system, or disposal. It is preferred to recycle a portion of this dust back to the GSA to reduce or eliminate the requirement of adding sorbent for capture of mercury to the reaction vessel. In practice, the use of a main air pollution control device which can provide a product with a resultant differential material size gradation between sections of the main air pollution control device (such as the usage of an electrostatic precipitator for dust capture) will also allow for the return of the coarse fraction of the dust product captured to one location (such as the kiln feed bin or main raw material blending or homogenization silo) and the return of the fine fraction of the dust product captured to a separate location (such as to the gas suspension absorber inlet). The dust captured in the main air pollution control device may also be separated between a fine and coarse fraction using another technology (for example a static or dynamic separator) and then returned to separate locations.

In a typical cement plant, during operation all or a portion of all of the preheater gases are passing through the in-line raw mill before being passed through a main air pollution control device or series of devices for the purpose of dust removal. The mercury emission during this time is variable. At some point in time, the raw mill operation ceases, typically for either mechanical (e.g. maintenance related) or production adjustment reasons, and the preheater exit gases are directed to a bypass conduit where they bypass the in-line raw mill and pass directly to the main air pollution control device or series of devices for the purpose of dust removal. In plants in which the present invention can be advantageously employed, these gases are not cooled between the exit of the preheater and the inlet to the main air pollution control device or series of devices. For a short duration of time after the raw mill stops, there is a noticeable increase in the mercury emission from the plant, which is many times the emission level observed prior to the stoppage of the in-line raw mill. This sharp increase is followed by a decrease in the emission of mercury to a level which, while more than the emission level seen with the raw mill in operation, is at a much lower level than seen in the short duration of time immediately following the stoppage of the mill. The mercury emission remains near this intermediate emission level until the raw mill is started again and all or a portion of all of the preheater gases are redirected to the raw mill. Once the raw mill is restarted, the mercury emission level returns to a relatively lower level of emission.

When the present invention is utilized, at the point when the raw mill operation ceases, the preheater exit gases do not pass directly to the main air pollution control device or series of devices for the purpose of dust removal. Rather, the gases are directed to a cooling means to be cooled to a temperature at which mercury and other contaminants condense, and then are directed through the plant according to the invention described herein. Following the time after the raw mill stops, there may be a noticeable increase in the mercury emission from the plant, slightly above the emission level observed prior to the stoppage of the in-line raw mill. However, the mercury level at this point is much lower than the increase in mercury emissions seen during normal operation of the plant, and in fact the mercury emission remains near the level of emission seen during the period when the raw mill is in operation. Once the raw mill is restarted, the mercury emission level may slightly drop before it returns to a relatively stable level of emission.

The invention is explained in greater detail below with the aid of the drawings.

In the general process of the present invention as illustrated in Figure 1, materials are treated in a kiln 100 which heats the material to undergo chemical changes. In cement kiln systems, the feed entering kiln 100 via conduit 101 may be first preheated in a preheater system 102 consisting of a number of countercurrent heat exchangers 103 in the form of cyclones. The product from the kiln may be cooled in a cooling system 104 provided for the recovery of heat to the process and for the removal of excessive waste heat for handling of the product. Gases exiting from kiln 100 are directed to preheater system 102 and thereafter may be cooled in an optional conditioning tower 107 before being used in other processes. Milling system 200 is used for the preparation of raw materials for the plant through grinding. Gases from the preheater system 102 are directed to milling system 200 via conduit 108 for drying and conveying raw materials. The fine material product from milling system 200 is stored and blended in silo 109 before being introduced through conduit 105 as feed to the kiln 100. Materials from milling system 200 may be cleaned of mercury utilizing an optional mercury removal system 110.

Figure 2 shows one embodiment of the application of the method according to the invention using a kiln installation for the production of cement clinker and shows a more detailed view of the raw mill grinding section 200. Gases and exit dust are provided via conduit 1 from the exit of the preheater tower (not shown in the figure), and are drafted to the system through an induced draft fan 24. These gases may be passed directly from the preheater exit, may be previously treated with water or air for temperature control or may be passed through a waste heat recovery system prior to use in the system of the present invention. The gases may be supplemented by gases coming from a kiln gas bypass or coal mill via one or more supplemental conduits, with exemplary conduits 27 and 28 depicted, for the purpose of adding volume or heat to the combined gas stream. When raw mill 3 is in operation the combined gases pass via conduit 2 to the raw mill 3, where raw materials for the production of cement clinker are introduced via material inlet 4 and are ground to a fine powder. The gases entering the mill are used for the drying of moisture in the raw material and the conveying of ground product through the system. The fine product and the exhaust gases of the mill pass via conduit 30 to a collection cyclone or series of cyclones 5, where the coarse faction of the product is collected via material outlet 6 and conveyed to other locations in the plant for storage, blending and/or feed to the kiln system. Draft of this gas flow is maintained through a mill fan 7. The exhaust gases from the mill fan contain a portion of the final product from the mill and any fine dust from the various gas sources that have not been captured in the mill collection cyclone or cyclones. These exhaust gases are pulled through a main air pollution control device 8 by means of a fan 11 prior to venting to the atmosphere via gas outlet 24 or to subsequent air pollution control devices. The dust captured in the main air pollution control device may be passed via conduit 9 to the storage and/or blending silo, a kiln feed bin, a kiln dust bin, wasted to reduce levels of contaminants, utilized in other areas of the plant for fire suppression in equipment such as coal grinding mills and disc reactors, added to the product grinding mill, added to the product silos, used as a contaminant sorbent, or taken to a device for the removal of harmful contaminants. If the main air pollution control device is of a type which provides a segregation of material fineness between a relatively fine and coarse fraction (such as an electrostatic precipitator) the streams comprised of coarse and fine fractins do not have used in the same location; as just one example, the coarse fraction may be used in the kiln feed bin while the fine fraction may be wasted to reduce levels of contaminants.

In the event that the raw mill is stopped, in the preferred embodiment of the present invention all of the exit gases of the preheater, and if present the gases coming from a kiln gas bypass, coal mill, and/orgases from the cooler vent, are prevented from passing to the raw mill such as by the closing of gas shut-off gates at the inlet of the mill system 12 and at the exit of the mill system 13. These gases are combined in conduit 29 and are diverted away from the raw mill to a cooling means 20 to cool the gases such as a gas suspension absorber or a gas conditioning tower to the point where mercury and, if desired, other contaminants, will form a condensate. A set of shut-off gates at the inlet 14 and outlet 15 of the present invention are provided in the preferred embodiment to prevent air from passing through the system of the invention when the raw mill is in operation, and are opened when the raw mill shuts down, although if the system requirements permit, shut-off gates 14 and 15 may remain partially open to let a small amount of gases into the cooling device 20 while mill 3 is in operation. A supply of water 16 or other coolant may be sprayed into the entrance gas and atomized through a mechanical means or by the addition of atomization air through conduit 17. An optional slurry and/or treatment solution storage vessel 18 may be used to supply a slurry or treatment solution through the supply line 25 for the cooling water or through a separate supply line. The slurry can consist of a combination of water and limestone, lime, hydrated lime, cement kiln dust (CKD), activated carbon, or a proprietary sorbent for the collection of mercury. The treatment solution may consist of a liquid or gaseous form of ozone, peroxide, halogenated species such as chlorine including calcium chloride, potassium permanganate, hydrochloric acid, iodine and other agents suitable to oxidize mercury. It is preferred that both slurry solutions and treatment solutions are atomized in order to ensure proper capture of mercury compounds on the sorbent in use. In addition to any materials already in the gas stream that may act as a sorbent (such as preheater exit dust, clinker cooler dust, or any other dust included in the gas and liquid streams to the system) a dry sorbent from storage vessel 19, consisting of hydrated lime, recycled cement kiln dust, kiln gas bypass dust, main air pollution control device capture, activated carbon, or proprietary sorbent, may optionally be added to the gas stream to assist in collecting the mercury in the gas stream.

A sorbent slurry is preferred in most systems because it minimizes the amount of sorbent required, and therefore the amount of waste produced. A dry sorbent injection is utilized in those installations where a source of potable water may not be available for use in a slurry (e.g., a plant in a desert environment). Non-potable water may be utilized in certain installations, but only if it's relatively free of dissolved minerals, which can cause scaling problems. A treatment solution (whether as a gas or in a solution) may be utilized when the production process results in a high ratio of elemental to oxidized mercury compounds, since the elemental mercury is harder to capture, and the solution would drive more of the mercury to an easier form to capture.

The gas stream and sorbent/sorbent slurry/treatment solution enter the gas suspension absorber (or other cooling means) 20 for a residence time of between 0.5 and 3.0 seconds. The temperature at the exit of the GSA is controlled to maintain a high absorption and condensation of mercury onto the sorbent, generally between 40⁰C to about 200⁰C, more preferably 70⁰C to about 160⁰C and most preferably 90⁰C to about 135°C. It should be noted that these temperature ranges will also serve to condensate other contaminates that may be in the gas stream. For example, in a typical cement kiln these contaminates may include hydrochloric acid and organic compounds such as benzene. If there is need to absorb contaminates having a lower condensation temperature, the temperature at the exit of the GSA can be lower accordingly, such as (for benzene, for example) between about 70⁰C and about 100⁰C.

Water may be used to control the desired exit temperature from the cooling means. A flow measuring device 33 located in the proximity of the gas inlet end of the cooling means may also be used to assist in calculating water spray requirements and/or sorbent concentration based on gas volume as a feed-forward control. The gases and sorbent and/or fine dust in the GSA are vented to a collection device 21, preferably a cyclone or set of cyclones, from which the coarser fraction of the entrained dust is collected. A portion of the collected dust may be returned via conduit 22 to the GSA to absorb a greater amount of mercury and to assist in maintaining a steady outlet temperature from the GSA. Any remaining portion of the dust collected in the GSA may be withdrawn via conduit 23 and used separately in the storage and/or blending silo, a kiln feed bin, a kiln dust bin, wasted to reduce levels of contaminants, utilized in other areas of the plant for fire suppression in equipment such as coal grinding mills and disc reactors, added to the product grinding mill, added to the product silos, used as a contaminant sorbent, or taken to a device for the removal of harmful contaminants.

Exhaust gases from the collection cyclone 21 may be further cooled downstream from the cyclone with the addition of ambient air 31 to assist in maintaining the water dewpoint temperature below the temperature of the gas stream in order to avoid the condensation of moisture on the ducting or in the main air pollution control device 8. The device provided for the addition of ambient air may be an ambient air damper that is also used for the protection of the main air pollution control device (such as a baghouse) during start-up conditions of the kiln system when gas flows may be insufficient to provide protection through alternative means of gas cooling (such as water sprays). A fan 32 for drawing gases through the present invention may be provided if the fan 11 provided for the main air pollution control device 8 does not have sufficient capacity for the induced pressure drop resulting from the present invention. Fan 32 may also be required if the main air pollution control device 8 is a positive air pollution control device such as a positive ESP or a baghouse that does not draw air by means of a fan 11 located on the clean side of the main air pollution control device.

Figure 3 shows a second embodiment of the present invention which may be utilized depending on the specifications of the particular plant in which the invention is being applied. The embodiment of Figure 3 is substantially equivalent to the embodiment of Figure 2, with the notable exception that the off gases, which contain fine product and exhaust gases, from both mill 3 and GSA 20 are directed to the same separation cyclone (or series of cyclones) 21 in which the coarse fraction of the product is collected and conveyed via conduit 6 (or a series of conduits) to other locations in the plant for storage, blending and/or feed to the kiln system. The exhaust gases from the raw mill collection cyclones 21 contain a portion of the final product from the mill and any fine dust from the various gas sources that have not been captured in the mill collection cyclone or cyclones. A portion of exhaust gas from the main air pollution device 8 may also be recycled to the inlet 4 of the mill 3 for the purpose of temperature control or supplementing volume of gas through the mill. The dust captured in the main air pollution control device 8 may be passed to the storage and/or blending silo, a kiln feed bin, a kiln dust bin, wasted to reduce levels of contaminants, utilized in other areas of the plant for fire suppression in equipment such as coal grinding mills and disc reactors, added to the product grinding mill, added to the product silos, used as a contaminant sorbent, or taken to a device for the removal of harmful contaminants via conduit 9.

As indicated, the gases and sorbent and/or fine dust in the cooling vessel are vented to the collection device 21, which is also used to collect the product from the raw mill. Product from this collection device may optionally be returned to other locations in the plant for storage, blending and/or feed to the kiln system via conduit 6.

Figure 4 shows a third embodiment of the application of the present invention which may be appropriate depending on the specifications of the particular plant in which the invention is being applied. The embodiment of Figure 4 is substantially equivalent to the embodiment of Figure 3, with the notable exception that the off gases from mill 3 are directed to main air pollution control device 8 without being directed to an intervening collection cyclone or series of cyclones. The fine product and the exhaust gases of the mill 3 are pulled through a main air pollution control device 8 by means of a fan 11 prior to venting to the atmosphere or to subsequent air pollution control devices via exit conduit 24. A portion of exhaust gas from the main air pollution device may also be recycled to the inlet 4 of the mill 3 for the purpose of temperature control or to supplement gas volume through the mill.

Figure 5 shows yet another embodiment of the present invention which is substantially equivalent to the embodiment of Figure 4, with the exception that mill bypass line 34 is depicted. Bypass line 34 has shut off valves 12 and 36 and regulation damper 35, which provide for the possibility that even when the mill is not in operation a small portion of the preheater off gases may still be sent directly to main air pollution control device 8.

Using this invention, the instability of mercury emissions from a cement plant is significantly reduced. The large spikes in mercury emissions associated with the shutdown of the raw mill for maintenance is eliminated by increasing the adsorption of mercury on dust retained in the system. If mercury reduction is necessary, then the following modifications to the cement plant process can be implemented to decrease the mercury emissions in a more controlled manner and with improved recovery efficiency.

• Removal of a portion of dust from the main air pollution control device 8 and wasting it or removing the mercury contained in the dust through another process.

• Removal of a portion of dust from the gas suspension absorber cyclone 21 and wasting it or removing the mercury contained in the dust through another process. • A combination of these two methods.

The invention having been thus described it will be obvious that the same may be varied in many ways without departing from the spirit and scope thereof. All such modifications are intended to be included within the scope of the invention which is defined by the following claims.

Claims

We claim:

1. A method for the reduction of contaminant emissions and contaminant emission spikes from an industrial plant utilizing a kiln to heat treat a raw material, a preheater to preheat the raw material in the kiln's off gases and a raw mill that is in operation intermittently to grind the raw material, with off gases from the preheater being directed to the raw mill and thereafter to an air pollution control device, said off gases from the preheater containing contaminants in vaporized form; said method comprising (i) diverting at least some of the preheater off gases away from the raw mill into a cooling means wherein the preheater off gases are cooled to near the condensation temperature of the contaminants, and (ii) absorbing such contaminants on a sorbent that is entrained in the preheater off gases.

2. The method of claim 1 wherein the cooling means is a gas suspension absorber.

3. The method of claim 1 wherein the cooling means is gas cooling tower.

4. The method of claim 1 wherein the sorbent is inserted into the cooling means.

5. The method of claim 1 wherein the contaminants are comprised of mercury.

6. The method of claim 5 wherein the preheater off gases are cooled to from about 40⁰C to about 200⁰C.

7. The method of claim 6 wherein step (i) takes place when the mill is not in operation.

8. The method of claim 7 wherein all of the preheater off gases are diverted to a cooling means.

9. The method of claim 6 further comprising: directing the off gases in which the sorbent with absorbed mercury is entrained to a separation device wherein the sorbent is separated from the off gases.

10. The method of claim 9 further comprising: redirecting the separated sorbent with absorbed mercury from the separation device back to the cooling means.

11. The method of claim 10 further comprising: directing the off gases from which the sorbent has been separated to the air pollution control device.

12. The method of claim 6 further comprising: utilizing a flow measuring device located in the proximity of the gas inlet of the cooling to assist in calculating water spray requirements and/or sorbent concentration based on gas volume as a feed-forward control.

13. A system for reducing contaminants in preheater off gases containing contaminants including mercury that are condensable comprising, a kiln for heat treating comminuted raw material; a preheater located, in the direction of material flow, upstream from the kiln for preheating the comminuted raw material with off gases from the kiln; a raw mill adaptable for intermittent operation for comminuting the raw material; means to direct the off gases from the preheater to the raw mill and thereafter to an air pollution control device, a cooling device for cooling preheater off gases to a temperature near the condensation temperature of mercury; means to direct the preheater off gases, when the mill is not in operation, from the mill to the cooling device, and means to direct a sorbent for absorbing the mercury into the cooling device.

14. The system of claim 13 wherein the cooling means is a gas suspension absorber.

15. The system of claim 13 wherein the cooling means is gas cooling tower.

16. The system of claim 13 wherein the sorbent is in the form of a slurry.

17. The system of claim 13 wherein the sorbent is in a dry form.

18. The system of claim 13 wherein the preheater off gases contain dust that is suitable as a sorbent.

19. The system of claim 13 further comprising means to direct at least a portion of the preheater off gases in which there is an entrained sorbent containing absorbed contaminants from the cooling device to the air pollution control device.

20. The system of claim 13 further comprising (i) a separation device located intermediate the cooling device and the air pollution control device for separating entrained sorbent containing absorbed contaminants from the cooled preheater off gases and (ii) means to directing at least a portion of the separated sorbent back to the cooling device.