BIOSOLIDS AND OTHER BIOMASS FUELS

This application is related to and claims the benefit of U.S. Provisional Application No. 61/782,454 entitled "USING KILN WASTE HEAT TO REDUCE MOISTURE CONTENT OF CLASS A BIOSOLIDS AND OTHER BIOMASS FUELS" filed on March 14, 2013, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention pertains to using bio-solid materials as a fuel or fuel additive in a cement-making process. In particular, the present invention pertains to drying biosolids containing Class A biosolids and/or other dried biomass fuels in a cement- making process and cement-making apparatus.

Cement-making processes are based upon the processing of limestone (CaC0 ₃ ) by heating to achieve a cement clinker that contains calcium oxide (CaO) chemically bound with other materials such as alumina, silica, and iron. In a Portland Cement manufacturing process, the main raw ingredient, limestone, is prepared with or without smaller amounts of materials containing alumina, silica, and iron and ground to produce what is called a raw meal. The meal is then conducted to a pyro-processing area, which may include preheaters and calciners to condition the raw meal for introduction into a rotary kiln where the intermediate product clinker is produced.

The main kiln and the preheaters and calciners of a clinker production system are conventionally heated with burners using fossil fuels. Coal, coke, oil, or gas are generally mixed with preheated combustion air and ignited to provide the heat necessary to decarbonate and melt the raw meal to produce clinker. At the discharge end of the rotary kiln, the hot clinker is introduced into a clinker cooler, wherein large amounts of air are blown through the hot clinker to cool it. After sufficient cooling the clinker can be ground into a final product. In the grinding operation of the cooled clinker a small amount of gypsum may be added to produce the finished Portland Cement.

Owing to the large heat requirements of cement-making processes, one of the largest expenses in the manufacture of cement is the cost of fuel. With ever-increasing prices for conventional fossil fuels, alternate fuel sources are being sought for use in the process. Among the materials which have been considered for use as alternate fuels are biosolid materials, such as sewage sludge and other biomass fuels. The qualities of raw biosolid materials available for use as alternate fuel sources vary considerably. Biomass materials have significant moisture content and are often processed to reduce moisture content prior to delivery to a cement manufacturing plant. Typical biosolid materials nevertheless require further processing of the material at the cement manufacturing site before they are suitable for use as a fuel in cement- making. In the case of sewage sludge, precautions must be taken in handling, storage, and treatment of the material to control odors and to eliminate the explosion hazards associated with the volatile materials that these materials produce, evolve, or emit.

U.S. Patent Pub. No. US 2005/0274293 Al describes a method and apparatus for drying wet biosolids using waste heat from clinker cooler or kiln exhaust gases in a cement-making process. The wet biosolids are fed by a closed system into a drying operation to reduce their moisture content to about 10% by weight. The high levels of volatile organic materials evolved from the wet biosolids treated according to the system described cannot be vented to the atmosphere, but are recovered in a closed system and fed to the combustion process of the cement-making system.

U.S. Patent Pub. No. US 2010/0032385 Al describes a cement-making apparatus that used calciner exhaust gases to dry high water-content organic waste such as sewage sludge. Organic waste containing 40% or more by weight is contacted with combustion gas in a flash grinder. The combustion gas exhausted from the grinder has a low oxygen content but a high concentration of volatile, combustible, and odoriferous compounds and must be returned to the calciner exhaust for deodorization treatment.

European Patent Applications No. 0 496 290 A2 and No. 0 664 330 Al describe power plants wherein wet sewage sludge is dried to less than 10% water content by indirect heating with exhaust steam from the power plant turbines or by direct heating with hot flue gas from the power plant furnace. The vapors and gases evolved during the drying of the sludge are recovered and fed into the furnace for combustion. The dried sludge may be mixed with coal, crushed, and further dried for use as fuel in the power plant.

Chinese Patent Application No. 101177331 A describes a method and device of treating sludge containing 70 to 95 % water using exhaust gases from a cement rotary kiln to treat and dry the sludge to 2 to 10% water content. The exhaust gas produced in the drying process must be returned to the kiln exhaust gas system for further treatment before it can be vented to the atmosphere.

Class A biosolids are a manufactured product, produced by biological digestion of, for example, municipal sewage sludge. The full requirements for sewage sludge to be classified as Class A biosolids are set forth in the U.S. Environmental Protection Agency regulations at 40 C.F.R. Part 503. Generally, for biosolids to achieve Class A status they need to be processed by, for example, steps such as composting, pasteurization, drying, heat treatment, alkaline treatment, and the like, to a level that does not pose a risk of infectious disease transmission through casual contact or ingestion. Class A materials are approved for application to land. Similar materials are currently available at home improvement centers as organic fertilizers under the names of Milorganite, Granulite, Vital Cycle, etc.

Most cement-making kilns are fired with coal or coke. In order to fire coal or coke within a kiln system, the coal or coke needs to be dried and ground. The kilns have excess heat available (waste heat), which is used to dry the cement raw materials fed to the kiln as well as the coal or coke used to create the heat source.

Class A biosolids are used as an alternate fuel in cement kiln systems to substitute for traditional fossil fuels such as coal, coke, natural gas, and oil. The processing of, for example, sewage sludge to Class A biosolids can occur by various methods, including mechanical, chemical, biological, and thermal treatments. Depending on the method employed to achieve Class A status, Class A biosolids can have varying amounts of moisture content. Similarly, other biomass fuels are processed in various methods for preparation as a fuel, and not all methods include drying. Other treated biomass fuels will pose the same problem as Class A biosolids having varying amounts of moisture.

The moisture content of Class A biosolids and other biomass fuels will lower the potential calorific value (heating value) of these alternate fuels. If the moisture level is too high, Class A biosolids or other biomass fuels will not have a recuperative calorific (heating) value, because all of the heat available from burning these materials will be used to heat and evaporate the moisture contained within them Moreover, the moisture level where a material becomes heat neutral or negative will vary depending on the material composition, particularly its carbon and hydrogen content. Thus a need persists for a method and apparatus that can treat alternate fuel materials containing Class A biosolids or other biomass fuels for use in a cement-making process such that they contribute a positive heating value to the process while minimizing the environmental and safety hazards associated with the use of these biomass materials.

SUMMARY OF THE INVENTION

The present invention pertains to methods and apparatus for making a fuel or fuel additive for use in a cement-making process. The raw materials for making the fuel or fuel additive are coal and/or coke and a dried biosolid material containing Class A biosolids and/or other biomass fuel. The raw materials for the fuel or fuel additive are mixed, ground, and dried together, the drying being effected by contacting the mixed materials with hot exhaust gases exiting the clinker cooler or kiln of the cement-making process. The fuel or fuel additive can either be stored for future use or incorporated directly into the combustion processes of the cement-making operation. Therefore, in a first aspect, the present invention is a method of making a fuel or fuel additive, for use in a cement-making process, from raw coal and/or coke and dried biosolids containing Class A biosolids and/or other biomass fuel, by mixing the raw coal or coke and dried biosolids; grinding the coal or coke and the dried biosolids together to reduce the particle size and to increase the uniformity of the particle size of the mixture; drying the mixture with kiln or clinker cooler exhaust gases from the cement-making process to form the fuel or fuel additive; and venting the kiln or clinker cooler gases from the drying step to the atmosphere.

In another aspect, the present invention is a method for incorporating dried biosolids comprising Class A biosolids and/or other biomass materials for use as a fuel or fuel additive in a cement-making process, by collecting exhaust gases carrying waste heat from a kiln or a clinker cooler used in the cement-making process; transferring heat from the exhaust gases to the dried biosolids to reduce the total contained moisture content of the biosolids to 10% by weight or less; and burning the dried biosolids alone or with a fuel selected from the group consisting of coal, coke, and mixtures thereof in a burner used in the cement-making process.

In a further aspect, the present invention is an apparatus for making a fuel or fuel additive for use in a cement-making process from raw coal and/or coke and dried biosolids containing Class A biosolids and/or other biomass fuel, including a cement-making apparatus comprising a main cement kiln and a clinker cooler, each producing exhaust gases carrying waste heat; a mixing apparatus for forming a mixture of the coal or coke and the dried biosolids; a drying and grinding apparatus for the mixture of coal or coke and dried biosolids, wherein the apparatus effects the transfer of the waste heat from the kiln or clinker cooler exhaust gases to the mixture of coal or coke and dried biosolids, forming the fuel or fuel additive; and a vent to exhaust the drying gases from the drying and grinding apparatus to the atmosphere after the mixture of coal or coke and dried biosolids is dried.

In yet another aspect the present invention is a cement-making apparatus, including a main cement kiln and a clinker cooler, each producing exhaust gases carrying waste heat; a means to form a fuel mixture containing coal and/or coke and dried biosolids comprising Class A biosolids and/or other biomass fuel; a means to grind the fuel mixture and to dry the fuel mixture by contact with the kiln and/or clinker cooler exhaust gases; and a means to vent the exhaust gases after to the atmosphere after contact with and drying of the fuel mixture.

By introducing the Class A biosolids and other biomass fuels into the coal/coke drying and grinding system, waste heat from the kiln system can be used to dry these materials and create a higher calorific (heating) value fuel. Class A biosolids and other biomass fuels can either be introduced with the raw coal/coke or by a separate introduction point to the coal/coke drying and grinding system . In addition to drying these materials, by processing Class A biosolids and other biomass fuels in the coal/coke drying and grinding system, these materials will be reduced in size to further improve their combustion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a conventional cement making process.

FIG. 2 is a flow diagram of a process and apparatus according to an embodiment of the present invention.

FIG. 3 is a flow diagram of a process and apparatus according to another embodiment of the present invention .

FIG. 4 is a partial schematic of a process and apparatus according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention involves drying Class A biosolids and other treated biomass fuels containing moistu re using a coal/coke drying and grinding system, to increase their calorific (heating) value for use in the combustion operations of a cement-making system. Waste heat from a kiln system can be obtained either from the combustion exhaust gases produced by combustion in the kiln system or cooling air used to cool the clinker exiting the kiln . These gases, which are normally cooled prior to being vented to atmosphere, are contacted with the Class A biosolids and other treated biomass fuels containing moisture in the coal/coke drying and grinding operation . In addition to reducing the moisture content of these materials, size reduction and equalization with the co-ground coal and/or coke will also occu r, improving the combustion characteristics of the Class A biosolids and other biomass fuels contained in the dried mixture.

Biosolids are the nutrient-rich organic materials resulting from the treatment of domestic sewage in a treatment facility. Biosolids differ from sewage sludge, in that biosolids are treated sewage sludge. Biosolids are carefu lly treated and monitored and must be used in accordance with regulatory requirements. Class A biosolids contain no detectible levels of pathogens. Class A biosolids that meet strict vector attraction reduction requ irements and low levels metals contents, only have to apply for permits to ensu re that these very tough standards have been met. In general, exceptional quality (Class A) biosolids used in small quantities by general public have no buffer requirements, crop type, crop harvesting or site access restrictions. When used in bulk, Class A biosolids are subject to buffer requirements, but not to crop harvesting restrictions. Representative standards for EPA Class A biosolids are shown in the following Table 1, alongside a selected state standard and a typical commercially-available material :

Table 1

The compositions and heat values of bituminous coal and Class A biosolids are compared In the following Table 2: Table 2

Other biomass fuels for use in the invention include but are not limited to seeds (corn, wheat, soy bean, etc.), paper sludge, saw dust, wood byproducts, or any other biomass material where reduction in moisture content and/or size will increase the materials' calorific value. These other biomass fuels are pretreated to a moisture content of about 10% by weight or less, consistent with the moisture level required for Class A biosolid status.

Paper pulp biosolids generally consist of organic based byproducts from paper manufacturing. Paper manufacturing uses conventional sources of cellulose such as wood and flax. Paper manufacturing also uses secondary sources of cellulose such as waste paper and waste cardboard. The manufacturing process requires the grinding, particle size classification, heating, and blending of conventional and secondary sources of cellulose. These processes generate by-products that have various concentrations of water and organic components.

Industrial sludges for use in the present invention contain organic byproducts with liquid or solid hydro-carbon components. Petroleum tanker sludge, oil soaked clay, oil soaked filter media, sediment from coal washing operations and wet flexi coke are examples of suitable industrial sludges. Food processing sludges are generated from many process operations. These materials generally consist of grain and vegetable based products that are being ground, mixed and dosed to produce various food products. In other processes diatomaceous earth filter media often becomes saturated with the organic residues form mixing, batching, cooking and pasteurizing processes. These saturated filter media materials are suitable bio-solid sludges for the processes of the present invention.

Agricultural waste sludges consist of by-products from harvesting and processing crops. Materials such as stalks, seeds, shells, pits, skins, rinds, twigs, leaves, and bark with various concentrations of moisture are suitable for the process of the present invention.

The caloric heating value of these biomass materials can be increased by going through the inventive drying and grinding process.

A conventional cement manufacturing process uses large volumes of raw materials such as limestone and coal to produce Portland Cement. The process consists of three main areas, raw meal preparation, pyro-processing, and finished grinding. Limestone (CaC0 ₃ ) is generally the main raw ingredient. Smaller amounts of materials containing alumina, silica and iron are proportioned and ground with limestone to produce a raw meal. A precisely controlled mixture of raw meal is then fed to the pyro-processing area. The pyro-processing area burns large amounts of conventional fuel such as coal, oil and gas to generate the temperatures required to calcine the limestone and allow the new cement components to form. An intermediate product called clinker is produced in the pyro-processing area. Clinker is cooled from high pyro-processing temperatures to ambient temperature. A finished grinding process which incorporates a small percentage of gypsum into the clinker results in a Portland Cement product.

Referring to FIG. 1, a conventional clinkering process is embodied in the apparatus shown as 100. The apparatus 100 includes a preheating section 110 which can include a series of preheater stages 111, 112, 113, 114, and 115, which are interconnected with recovery conduits, e.g. 116, wherein the raw meal represented by arrow 117 introduced by a conduit 118 is gradually preheated for introduction into the main kiln 119 via a conduit 120. Downstream of the preheater section the process apparatus includes a calciner 121, which is fired with a burner 122 to begin the conversion of the limestone to clinker. The calciner portion of the process can include a loop system 123 which includes a conduit 124 to introduce additional fuel into the loop system 123 whereby the fines from the calciner are recycled and eventually introduced into the main kiln 119. Gases and fine particles exiting the preheater section 110 are sent to a cooling tower 125 for cooling of the gases and exhausting via a blower 126. Dust is removed in a dust collector 127.

Main kiln 119 includes a main burner 128 which is fired using a fuel such as coal or coke together with an oxygen-containing fluid such as air. The preheated or pre-calcined meal enters the kiln at a first or entry end 129. As the meal progresses from the entrance 129 to the exit 130 of the kiln 119 it is converted into a clinker. The clinker exits the kiln 119 and is deposited into a clinker cooler 131 where the clinker is cooled to a temperature of about 200° F. Thereafter, the clinker represented by arrow 132 is conducted via a conduit for other delivery device 133 to the grinding operation. The clinker cooler 131 includes a dust recovery system 134 so that dust can be recycled to the pyro-processing portion of the cement-making process.

Referring to FIG. 2, a flow diagram 200 of a method and apparatus according to one embodiment of the present invention is shown incorporated into a cement-making process. In this embodiment, raw coal and/or coke 201 are mixed with dried biosolids containing Class A biosolids and/or other biomass fuels 202 upstream of, and are then fed into, a coal/coke drying and grinding system 203. Waste heat 204 in the form of exhaust gases collected from the kiln and/or clinker cooler elements of the kiln system 205 are fed to the drying and grinding system 203, where they contact the mixture of coal and/or coke and dried biosolids to effect further drying thereof. The finished fuel or fuel additive containing the mixed, ground, and dried coal and/or coke and dried biosolids 206 are fed into the kiln system 205 to be used as fuel in the cement-making process. The exhaust gases 207 from the drying and grinding system 203 are vented to the atmosphere.

Referring to FIG. 3, a flow diagram 300 of a method and apparatus according to another embodiment of the present invention is shown incorporated into a cement- making process. In this embodiment, raw coal and/or coke 301 and dried biosolids containing Class A biosolids and/or other biomass fuels 302 are separately fed directly into a coal/coke drying and grinding system 303. Waste heat 304 in the form of exhaust gases collected from the kiln and/or clinker cooler elements of the kiln system 305 are fed to the drying and grinding system 303, where they contact the mixture of coal and/or coke and dried biosolids to effect further drying thereof. The finished fuel or fuel additive containing the mixed, ground, and dried coal and/or coke and dried biosolids 306 are fed into the kiln system 305 to be used as fuel in the cement-making process. The exhaust gases 307 from the drying and grinding system 303 are vented to the atmosphere.

Referrring to FIG. 4, a schematic 400 of a cement-making method and apparatus according to another embodiment of the present invention is shown. Dried biosolids comprising Class A biosolids and/or other biomass fuels are pneumatically conveyed from a storage silo 401 to a cyclone separator to separate the dried biosolids from the conveying air. The separated biosolids are mixed with raw coal from a coal storage system 403 to be fed to a coal mill 404. The conveying air from the biosolids cyclone separator is fed to a baghouse 405 to remove biosolid fines; the conveying air is vented from the baghouse 405 to the atmosphere, and the solids recovered in the baghouse 404 are added to the unground mixed coal and dried biosolids fed to the coal mill 404.

In the coal mill 404, the mixed coal and dried biosolids are ground and dried. The drying of the coal and biosolid mixture is effected by contacting the mixture in the coal mill 404 with hot exhaust gases collected from the preheater section 406 of a kiln and clinker cooler system 407 and blown by a preheater fan 408 into the coal mill 404. The dried and ground coal and biosolid mixture, now suitable as a fuel or fuel additive for a cement-making process, is collected from the coal mill dust collector 411 and conveyed to storage bins 409, from which the fuel or fuel additive is fed as needed to the main burner 410 or the calciner 414 of the kiln and clinker cooler system 407. The cool exhaust gases exit the coal mill 404 and are fed to a coal mill baghouse 411 to separate ground coal and biosolids from the cooled exhaust gases. The ground coal and biosolids are added to the the coal storage bins 409. The exhaust gases are conveyed from the coal mill bag house 411 by a coal mill fan 412 to the main stack 413 for venting directly to the atmosphere.

The method and apparatus of the present invention may incorporate a biosolids unloading, storage, and metering system where the Class A biosolids and/or other biomass materials are unloaded, stored, and distributed to the drying and grinding system. The storage unit is typically an insulated storage silo, loaded via a pneumatic or mechanical system from transport vehicles carrying the biosolids materials. The silo may be provided with one or more of a pressure relief, odor control, C0 ₂ inerting, or dry air system. The stored material may be metered by volumetric or gravimetric means to the drying and grinding system.

The method and apparatus of the present invention will include a system capable of grinding the coal/coke and biomass materials together and at the same time drying the materials being ground by contact with the kiln or clinker cooler exhaust gases. A suitable apparatus for this operation is a vertical roller mill for crushing and grinding coal or coke. Heated gases, which may be as high as 400 °F (approx. 200 °C) from the clinker cooler and as high as 750 °F (approx. 400 °C) from the kiln, are conducted to coal grinder and directly contact the mixed and ground coal/coke and biosolid materials, which are then dried to form the finished fuel or fuel additive. The finished fuel is conveyed pneumatically to storage bins for distribution as needed into the kiln combustion processes. The dry bio-solids can be stored and used as necessary in the overall process, for example, directly in the main burner of the kiln, or in the calciner burner.

Unlike the methods and systems of the prior art, the method and apparatus of the present invention do not require further treatment of the dryer exhaust gases to remove the moisture and volatile organic compounds evolved from the biomass materials. Thus the method and apparatus of the present invention require no further operations, such as a condenser separator, to treat the dryer exhaust gases before it can be vented to the atmosphere.

The coal grinder/dryer will be operated to obtain less than approximately 2%, preferably 1%, and even more preferably 0.5% moisture content by weight in the dried fuel mixture comprising coal or coke and the dried biosolids materials when it leaves the grinding and drying system. Monitoring the material and vapor discharge temperatures and varying the temperature of the incoming heat to the grinder/dryer makes it possible to control the moisture content of the fuel materials leaving the system. As stated above the dried fuel mixture containing biosolids materials is pneumatically conveyed to a storage device or silo. Depending upon the temperature of the dried material, it may be necessary to either cool the dried fuel in a separate heat exchange device or by using cool conveying gas to transport the dried fuel to the storage silo.

Any storage device must be designed in accordance with all applicable local, state and federal codes and regulations to insure safe handling of such material. It is believed that procedures and methods that are used for the storage and handling of pulverized coal will enable a user to comply with such laws and regulations for storage and handling of dried biosolid materials. Although the storage system will have the ability to withdraw and dose the dried fuel material as needed, alternate transport methods may be practical in some cement processes. Some cement plants have existing pneumatic transport systems for conveying a primary fuel such as pulverized coal. With such a system it is possible to deposit a controlled flow rate of dried fuel materials directly into an existing pneumatic transport line. In this aspect, the dosing systems would discharge through a rotary air lock directly into the pneumatic transport system.

The dried fuel materials may be transported to the combustion (burning) zone of a kiln in several ways. If the fuel is conveyed to the main burning zone of kiln by an independent pneumatic conveying line, a separate burner pipe can be used to introduce the dried fuel into the rotary kiln. The nozzle or discharge end can be placed at any location proximate the discharge end or nozzle end of the main burner. The dried fuel can be conveyed to the burning (combustion) zone of the kiln via an independent pneumatic conveying line that terminates in a conduit inside of the burner. The burner can be a multi-port or a concentric tube burner, such burners being well known in the art.

It should be noted that the particle size of fuels burned in the main combustion zone of the cement kiln is of critical importance. If too many large particles of fuel enter this portion of the kiln there is a possibility that some of them may fall into the reaction zone of the kiln and adversely affect the quality of the clinker produced. For this reason the degree of fineness of the dried biosolids must be monitored closely. As set forth above, the particle size of the dried biosolids is to be reduced and harmonized with the coal or coke with which it is mixed to optimize the combustion characteristics of the resulting fuel mixtures. Preferably, the finished fuel mixture obtained according to the process of the invention will have a particle size distribution wherein no particles exceed 1 mm in size, less than 20% are retained at 90 pm and less than 2% are retained at 200 pm.

Contrary to the requirements of the main kiln burner, particle size is not as critical for dried bio-solids introduced into the calciner and the loop duct combustion zone of the cement plant. Larger particle size dried biosolids can be accommodated in these areas because of the longer retention time in the combustion zone and the fluidizing effect of high gas flows. Furthermore, any particles or fuel that become mixed with the raw meal (feed) will have ample time to oxidize before they are added to the more critical reaction zone of the kiln.

The combustion zone of a cement kiln is one of the hottest industrial processes. Gas temperatures in the main combustion zone can exceed 3500 °F (approx. 1930 °C). Additionally, these gases remain above 1800 °F (approx. 980 °C) for as long as 5 seconds as they move away from the combustion zone. The combination of preheated high oxygen content air, high combustion temperatures and long residence time above 1800 °F (approx. 980 °C) insures complete combustion of all organic compounds. The calciner loop duct combustion zone consists of the main calciner combustion chamber and the loop duct or riser duct. Gas temperatures in the calciner combustion chamber can reach 1650 °F (approx. 900 °C). These gases can remain above 1600 °F (approx. 870 °C) for as long as five (5) seconds as they leave the calciner combustion chamber and pass through the loop duct to the pre-heater cyclone chambers.

The main components of the raw materials used to manufacture cement are calcium, silica, alumina and iron. The inorganic ash components of dried bio-solids have high concentrations of calcium and silica that can supplement the conventional minerals. Therefore, the inorganic ash residue of dried bio-solids can be beneficially recovered by incorporation into the cement clinker. The intimate mixing of the combustion gases and the cement raw materials as the gases leave the combustion zone ensure complete integration of inorganic ash residues into the conventional raw materials. In this manner the inorganic ash residues become an integral part of the process chemistry. There are minor constituents in the inorganic ash residue of dried biosolids that must be monitored to insure the quality of the performance of the cement product. Trace components in the ash such as P ₂ 0 ₅ , CI, Na ₂ 0, and K ₂ 0 must be measured on a regular basis to control any potentially deleterious effect of these components on the cement manufacturing process or the performance of the finished product. Specifically, P ₂ 0 ₅ from dried bio-solids will increase the concentration of phosphorus in the finished clinker. Research has indicated that when the level of phosphorus in a clinker approaches 1.5% the strength development of the resulting concrete will be deleteriously impacted. Additionally, high concentrations of CI, Na ₂ 0, K ₂ 0, and S0 ₃ from dried bio-solids can cause build up in kiln or preheat tower, which, in turn, can cause operational interruptions.