Interactions Among Gaseous Pollutants from Cement Manufacture and Their Control Technologies

Transcription

1 Research & Development Information PCA R&D Serial No Interactions Among Gaseous Pollutants from Cement Manufacture and Their Control Technologies by Walter L. Greer Portland Cement Association 2003 All rights reserved This information is copyright protected. PCA grants permission to electronically share this document with other professionals on the condition that no part of the file or document is changed.

2 KEYWORDS Cement kilns, pyroprocess, gaseous pollutants, origins, control technologies, sulfur dioxide, SO 2, nitrogen oxides, NO X, organics, hydrocarbons, carbon monoxide, CO, carbon dioxide, CO 2, ammonia, NH 3, dioxins, furans ABSTRACT This report presents a qualitative examination of the interactions of gaseous pollutants generated in portland cement kiln systems, and their existing and potential control technologies. The cement-making process is described and the sources of the pollutants of concern are identified. The synergetic and counteractive relationships of the pollutants and technologies are presented in tabular and textual format. REFERENCE Greer, Walter L., Interactions Among Gaseous Pollutants from Cement Manufacture and Their Control Technologies, R&D Serial No. 2728, Portland Cement Association, Skokie, Illinois, USA, 2003, 59 pages. 2

3 TABLE OF CONTENTS KEYWORDS... 2 ABSTRACT... 2 REFERENCE... 2 TABLE OF CONTENTS... 3 INTRODUCTION... 4 DESCRIPTION OF THE CEMENT-MAKING PROCESS... 5 SOURCES OF GASEOUS POLLUTANTS... 6 Raw Materials... 6 Fuel... 7 Process... 8 RELATIONSHIPS OF EXISTING AND POTENTIAL CONTROL TECHNOLOGIES FOR PRIMARY AIR POLLUTANTS EMITTED FROM A PORTLAND CEMENT PLANT Control of Sulfur Dioxide Control of Nitrogen Oxides Control of Organics Control of Carbon Monoxide Control of Carbon Dioxide Control of Ammonia Control of Acid Gases Control of Dioxins and Furans CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES Page 3

4 Interactions Among Gaseous Pollutants from Cement Manufacture and Their Control Technologies INTRODUCTION By Walter L. Greer * The purpose of this white paper is to qualitatively explore the interactions of gaseous pollutants generated in portland cement kiln systems and the emission-control technologies that have been or could be applied to these systems. The individual technologies discussed in this paper are in various states of application ranging from those that are in use and well understood to those that only have hypothetical possibilities. When these technologies are applied, there may be unexpected consequences from their application, e.g., the increased generation of other pollutants of concern. To meet multiple pollution abatement objectives, there will be a tendency to simultaneously apply more than one technology to individual kiln systems, a situation that also may have unexpected outcomes. The synergetic and counteractive interaction of the selected technologies must be considered to optimize and prioritize emission control strategies for minimum overall emissions, maximum energy efficiency, and acceptable cost. Site-specific research may have to be conducted, and compromises and choices may have to be made prior to the selection of a control scheme for a particular plant. Nevertheless, the trends and basic process principles provided in this paper can be utilized in the initial evaluation of gaseous pollutant controls on cement kiln systems. The historic gaseous pollutants of concern from cement kilns are carbon monoxide (CO), the oxides of nitrogen (NO X ), sulfur dioxide (SO 2 ), and organic emissions, i.e., in the form of total hydrocarbons (THCs) and/or volatile organic compounds (VOCs). 1 The emissions of carbon dioxide (CO 2 ) are of increasing interest because of concerns about global climate change. In whole or in part, these emissions from cement kiln systems are the products of combustion and/or high-temperature processes. The principal gaseous emissions from the pyroprocessing system in a typical descending order by volume are nitrogen, CO 2, water, oxygen, NO X, SO 2, CO, and hydrocarbons. The volumetric composition range of these constituents is from about 73 percent to less than 10 ppm (Greer, Dougherty, and Sweeney, 2000). Emissions of acid gases (AGs), ammonia (NH 3 ), and dioxins and furans (D/Fs) also are of current interest. This paper provides a brief description of the cement-making process, an identification of expected and potential gaseous pollutants that are contained in emissions from cement kilns, a general description of the potential sources of the pollutants of concern, and a presentation and * Senior Technical Associate, Trinity Consultants, West Valley Parkway, Suite 101, Olathe, KS, USA (913) , 1 VOCs are organic pollutants regulated under federal New Source Review procedures and are a subset of THC emissions from cement kilns due to the exclusion of some hydrocarbons from the regulatory definition of VOCs. Generally, VOCs are of greater interest for kilns located in ozone non-attainment areas. THCs are of greater interest for kilns burning hazardous waste as supplemental fuel, and greenfield cement kilns and raw material dryers seeking to comply with 40 CFR 63, subpart LLL. 4

5 discussion of current and potential control technologies with regard to purpose, process integration, and potential interactions between them. Listed references provide more detailed discussions of many of these points. For a variety of factors, no two cement plants are alike in design or operation. Invariably, there will be one or more plants that will differ in some aspect from the generalizations presented herein. This white paper seeks to anticipate the reactions to and the benefits from application of the listed control technologies; however, site-specific events may result in interactions that are not consistent with the author s predictions and should be investigated on a case-by-case basis. DESCRIPTION OF THE CEMENT-MAKING PROCESS The production of portland cement is a four step process: (1) acquisition of raw materials, (2) preparation of the raw materials for pyroprocessing, (3) pyroprocessing of the raw materials to form portland cement clinker, and (4) grinding of the clinker to portland cement (Greer, Dougherty and Sweeney, 2000). Listed in order of least to most thermally efficient, the four types of pyroprocessing systems in current use in the United States are wet, long dry, preheater, and precalciner. Other than in the wet process, hot gas from the pyroprocessing system may be used to independently dry raw materials in dedicated equipment or to simultaneously dry them during grinding in the second step of the cement-making process. The latter system is known as in-line kiln/raw mill. All cement pyroprocessing systems employ countercurrent flow to achieve heat transfer from the hot combustion products to the relatively cold raw materials. The temperature profiles of the four process types are quite different but the raw materials are essentially the same, and the final product is physically and chemically similar, i.e., the hard, gray, spherical nodules of hydraulic minerals called portland cement clinker (clinker). The rotary cement kiln is common to all cement pyroprocessing systems, and it is in this device that the raw materials are converted to clinker. Regardless of the pyroprocessing system, the raw materials are heated to incipient fusion at about 1480ºC (2700ºF) by an approximately 1870ºC (3400ºF) flame in the hottest section of the rotary kiln, i.e., the burning zone. Because of their characteristic physical configurations and temperature profiles, the respective pyroprocesses can present different emission rates of some gaseous pollutants while using the same raw materials and fuels. Likewise, site-specific variability in raw materials and fuels can result in the emission of gaseous pollutants in greater or lesser amounts than normally would be expected from a given pyroprocessing system. Each of the pyroprocesses also offers different opportunities for pollution abatement because of its inherent characteristics. Listed in descending order of typical concentration, the four elements that are required for the manufacture of clinker are calcium, silicon, aluminum, and iron. These elements are extracted directly from the earth s crust as ores consisting primarily of carbonates or oxides, or are derived from secondary (waste) materials also having an origin in the earth s crust. Magnesium, sulfur, sodium and potassium are other elements that appear in clinker in minor concentrations. Many other elements common to the earth s crust can be found in clinker in trace amounts. The predominant fuel for cement kilns in the United States is bituminous coal with natural gas, petroleum products, and selected combustible wastes providing the balance of the thermal energy required for pyroprocessing. All the raw materials and fuels used in cement manufacture contain constituents that may contribute to one or more gaseous emissions from a rotary kiln or an in-line kiln/raw mill. 5

6 Although deceptively simple in concept, cement pyroprocessing systems are the most complex continuous chemical reactors in the world. There are many sequential, concurrent, endothermic, and exothermic chemical reactions occurring in a material production process approaching a state of equilibrium. Due to the large mass of reacting materials contained in the system and the heat capacity of the refractory materials within the system, there is a significant thermal inertia that must be considered in process and environmental control strategies. Because of this complexity, the mechanisms of formation of some minor constituents of concern are not now known or well understood; therefore, they cannot be readily controlled at the present time. Conversely, the process itself fortuitously reduces some undesirable emissions. The process can be modified to enhance its inherent ability to abate, and the product and byproduct to absorb, gaseous pollutants and their precursors. SOURCES OF GASEOUS POLLUTANTS The sources of gaseous pollutants from a cement kiln system are the raw materials, the fuel, and the process itself. Raw Materials Calcareous component. The predominant constituent of the cement raw material mix is calcium carbonate in one form or another. Most often, the calcareous component is limestone but it can be marl, chalk, or marine deposits of shell or aragonite. About 75% of the raw mix must be calcium carbonate so the degree of purity of the calcareous component determines the amount that is contained in the raw material mix. Because 48% of the weight of the calcium carbonate is carbon and oxygen, the calcareous component of the raw material mix is a significant source of CO 2 emissions through calcination (decarbonization). Because all sources of calcium carbonate used in cement manufacture originated in an ocean, chlorine is present as a trace element. Although the mechanisms of formation are not clearly established, this chlorine is available in the flue gas stream for the generation of hydrogen chloride (HCl) and D/Fs. Limestone also can contain sulfur in the form of sulfates, sulfides (metallic and organic), and, rarely, elemental sulfur. Generally, sulfates pass through the kiln system without transformation into SO 2, but sulfides and elemental sulfur can result in the generation of SO 2 through the oxidation of sulfur in kiln systems. If localized reducing conditions exist in the pyroprocessing system, sulfates can be converted to SO 2. Limestone also can contain petroleum and/or kerogens that can be partially volatilized or pyrolyzed at temperatures present at the feed end of the pyroprocesses to result in organic emissions. 2 These organic constituents or their nonvolatile residues can result in CO emissions when burned in an oxygen-deficient section of the pyroprocessing system. Siliceous, argillaceous, and ferriferous components. The non-calcareous components of the raw mix may be natural in origin, e.g., sand and shale, or be derived from the wastes of other industries, e.g., steel mill scale or power plant fly ash. These materials can contain 2 Organic matter in sedimentary rocks primarily consists of kerogens, an insoluble material. Petroleum formation results form the thermal maturation of kerogens at depth. 6

7 sulfates, sulfides (metallic and organic), and elemental sulfur that have the potential to generate SO 2. Organic, CO, and CO 2 emissions also may result from kerogens, crude petroleum, or refined petroleum products in these components of the raw mix. Laboratory work at the Portland Cement Association (PCA) has suggested that nitrogenous constituents in cement raw materials have the potential contribute to NO X emissions from cement kilns (Gartner, 1983). Because of the extensive research project that would be required, this potential contribution has not been documented or quantified in the field; however, the potential NO X emissions from raw materials can be estimated in the laboratory. These nitrogenous constituents also may contribute to emissions of NH 3. These raw materials may contain chlorine in trace amounts that could contribute to the formation of HCl or the precursors of D/Fs. Fuel Coal and petroleum coke. Bituminous coal is the predominant fuel used by the cement industry in the United States. Coal is often supplemented (replaced) in part by petroleum coke when the economics are favorable. Bituminous coal and coal/coke blends are prepared for combustion in direct-fired and indirect-fired coal mill systems. In 2000, coal and coke contributed 84.0% of the thermal energy used in cement pyroprocessing systems (PCA, 2002). The combustion of any carbonaceous fuel results in the formation of CO 2 and the potential formation of CO if oxygen deficiency and/or poor mixing of fuel and air exist at the combustion site. In addition, localized combustion conditions affecting combustion reactions may result in the formation of other organic products of incomplete combustion (PICs). However, the relatively long gas residence time at high temperature found in cement kiln systems greatly reduces the possibility of emissions of organic PICs when compared to other processes that burn carbonaceous fuel. The sulfur contained in bituminous coal is in the form of sulfates, sulfides (metallic and organic), and elemental sulfur. The sulfides and elemental sulfur are oxidized readily to SO 2 during combustion of the coal. Coal also contains nitrogenous compounds that are oxidized to NO X, i.e., fuel NO X, or converted to small quantities of free NH 3 during combustion. Because petroleum coke contains the impurities from its crude oil source (Green and Maloney, 1984), it may contain a significant concentration of sulfur or nitrogen that has the potential to be oxidized to SO 2 or NO X. In large part, the type of pyroprocess will determine if these impurities have a major impact on the emissions of NO X and SO 2 from a cement pyroprocessing system. Solid fossil fuel may contain trace amounts of chlorine that could contribute to the formation of HCl or D/Fs. Natural gas. Although no longer a common practice, a few kilns will produce clinker over extended periods of time using natural gas as the primary fuel if it is more economical to use than coal. In some cases, natural gas may be used as a supplemental fuel, e.g., to enhance the burning of excessively wet coal from a direct-fired coal mill system. Today, most natural gas is used to bring kilns to operating temperature in preparation for the firing of solid fossil fuels. In 2000, natural gas contributed only 5.9% of the thermal energy used in cement pyroprocessing systems (PCA, 2002). The combustion of natural gas results in the formation of CO 2, and the potential formation of CO and other PICs. 7

8 The sulfur and nitrogen content of natural gas is insufficient to result directly in appreciable emissions of SO 2 or NO X. However, due to the formation of thermal NO X resulting from high flame temperature, the emissions of NO X increase when natural gas is used in lieu of solid fossil fuels in a particular rotary kiln. Petroleum. Most of the oil now burned in cement kilns is a refined product, e.g., diesel fuel or heating oil (both known as middle distillates ), that is used to bring the kiln system to operating temperature in preparation for firing solid fossil fuel. In 2000, petroleum products contributed just 1.1% of the thermal energy used in cement pyroprocessing systems (PCA, 2002). The combustion of petroleum products results in the formation of CO 2, and the potential formation of CO and other organic PICs. A refined petroleum product normally contains low concentrations of sulfur and nitrogen but could make a minor contribution to the formation of SO 2, NO X, or NH 3. Waste. Combustible wastes of industry and consumers can be used in cement kilns to supplement traditional carbonaceous fuels. In a few cases, a waste-derived fuel has completely supplanted fossil fuel as the primary fuel for a kiln. The use of waste fuels provides a mutually beneficial outcome for the environment and the cement plant. The environment is not impacted by land disposal of the waste or non-beneficial incineration. Less fossil fuel is burned while the cement plant often is able to enjoy a more favorable fuel cost. The four most common wastes burned in cement kilns are used or rejected automobile and truck tires, blended liquid and solid hazardous wastes, used oil, and combustible nonhazardous solid wastes (PCA, 2002). In 2000, 57% of the cement plants reporting data to the PCA used some form of waste-derived fuel (PCA, 2002). In 2000, wastes contributed 9.0% of the thermal energy used in cement pyroprocessing systems (PCA, 2002). The combustion of waste-derived fuel results in the formation of CO 2, and the potential formation of CO and organic PICs. These wastes may contain sulfur, nitrogen, and/or chlorine that could contribute to the formation of SO 2, NO X, NH 3, HCl, or D/Fs. Process Pyroprocess description. The formation of gaseous pollutants primarily occurs in an independent kiln system or in an in-line kiln/raw mill as the consequence of oxidation or other processes at a relatively high temperature, e.g., greater than 315ºC (600ºF). The exception is the potential formation of certain PICs and D/Fs at a lower temperature. There are a few plants at which these pollutants are generated in small quantities in an independent raw material or solid fuel dryer. A discussion of dryer emissions is beyond the scope of this white paper; however, they are quite similar to those formed in the low-temperature section of a pyroprocessing system. Process gas is vented through as many as three points in a cement pyroprocessing system. Each of these vent points, i.e., the discharge of the rotary kiln or the in-line kiln/raw mill, the alkali bypass, and the coal mill, are equipped with a particulate matter control device (PMCD). All pyroprocessing systems have a kiln or an in-line kiln/raw mill vent but not necessarily alkali bypass or coal mill vents. The PMCDs may have monovents, horizontal discharge ducts, or vertical stacks. If there is a stack for the kiln or the in-line kiln/raw mill, it is usually called the main stack. Either or both of the vents from the alkali bypass and the coal mill may be ducted 8

9 to the main stack, or they may be vented independently. The gaseous pollutants that are emitted from a particular process vent are generally dependent on the type of pyroprocessing system, the site-specific raw materials and fuel, and the events occurring in the process upstream of the vent with respect to flue gas flow. In the traditional wet and long-dry pyroprocessing systems (Figure 1), the primary fuel is burned at the hot end of the rotary kiln. Supplemental fuel, e.g., scrap tires, may be burned at mid-kiln using a system or device to introduce the fuel through the rotating kiln shell. In this situation, the amount of supplemental fuel used in the kiln is limited because combustion air required by the supplemental fuel must pass through the burning zone of the kiln and serves to cool the main flame. If cooled sufficiently, the temperature of the main flame will not support the formation of the clinker minerals and the clinkering process cannot be sustained. In preheater kiln systems (Figure 2), the primary fuel also is burned at the hot end of the rotary kiln and all combustion air passes through the burning zone. Supplemental fuel may be dropped into the feed end of the rotary kiln, introduced into the rotary kiln through the kiln shell, or injected into the riser duct between the feed end of the rotary kiln and the preheater tower. In the application of these practices for supplemental-fuel combustion, the cooling of the main flame by excess air passing through the burning zone also occurs. The most modern pyroprocessing system is the precalciner kiln system (Figure 2). In this process, there is a special vessel called a calciner located between the rotary kiln and the preheater tower into which fuel is introduced. It is in this vessel that the bulk of the calcination of the calcareous component of the raw mix takes place. The calcination reaction requires the most thermal energy of any reaction in the cement-making process. This reaction commences at about 870ºC (1600ºF) and does not require the 1870ºC (3400ºF) flame temperature found in the burning zone to be sustained. Typically, hot tertiary air is taken from the clinker cooler or kiln firing hood and ducted outside the kiln to the precalciner vessel for combustion support. In an air-through calciner design, the combustion air for the calciner must pass through the burning zone of the kiln and also results in cooling of the flame in the burning zone. In a precalciner kiln system in which tertiary air is used, approximately 60% of the fuel can be burned in intimate contact with the raw materials in the calciner to achieve approximately 90% calcination of the raw mix in just a few seconds after its introduction into the preheater tower and before it enters the rotary kiln. In a traditional rotary kiln system, calcination requires much more than an hour to complete depending on the kiln size and rotational speed. The precalciner configuration of a cement pyroprocessing system is the most fuelefficient and stable of the four types. When compared to the other pyroprocesses, this system provides the highest clinker production rate with the shortest rotary kiln and the smallest footprint on the ground. Sulfur dioxide. Sulfur dioxide results from the oxidation of sulfide or elemental sulfur contained in the fuel during combustion. In addition, sulfide or elemental sulfur contained in raw materials may be roasted or oxidized to SO 2 in areas of the pyroprocessing system where sufficient oxygen is present and the material temperature is in the range of C ( F) (Miller, Young, and von Seebach, 2001). In addition, sulfates in the raw mix can be converted to SO 2 through localized reducing conditions in the kiln system. 9

10 Raw Material Mix to Kiln PMCD Mixing Air Fan Mid-kiln Firing Fuel and Primary Air Secondary Air I.D. Fan CKD Bin Rotary Kiln Clinker Cooler Figure 1. Wet/long-dry kiln systems. Clinker Discharge Raw Material Mix to Preheater Raw Material Main PMCD Preheater Fuel Calciner Tertiary Air Duct I.D. Fan Raw Material Mix Silo In-line Alkali Raw Mill Bypass To PMCD To Preheater Tower Figure 2. Preheater/precalciner kiln systems. Rotary Kiln Clinker Cooler Fuel and Primary Air Secondary Air Clinker Discharge Nitrogen oxides. It has been shown that nitrogen oxide (NO) makes up 90% or more of the NO X contained in cement kiln flue gas. Nitrogen dioxide (NO 2 ) comprises the balance of the nitrogen oxides (Penta, 1991). There are four mechanisms of NO X formation in cement kilns of which thermal and fuel NO X formation are the most important. Thermal NO X results from the oxidation of molecular nitrogen in air at high temperature. This phenomenon occurs in and around the flame in the burning zone of a cement kiln at a temperature greater than 1200ºC (2200ºF). Fuel NO X results from the oxidation of nitrogen in the fuel at any combustion temperature found in the cement process. Because of the lower combustion temperature in the calciner and some sites of supplemental fuel combustion, the formation of fuel NO X often exceeds that of thermal NO X at these locations. The generation of feed NO X has been demonstrated only in the laboratory by 10

11 heating nitrogen-containing cement raw materials to the range of ºC ( ºF) in the presence of oxygen. Slow heating, such as occurs in wet and long-dry kilns, appears to increase the yield of NO X for a given raw material. The yield of feed NO X is potentially lower when the raw material is heated quickly in a preheater or precalciner system. Prompt NO X is generated by the reaction of certain fuel-derived radicals with elemental nitrogen in a hydrocarbon flame and is a minor contributor to overall NO X generation (Penta, 1991). Carbon monoxide. CO is a PIC of carbonaceous fuels resulting from insufficient oxygen at the combustion site, insufficient mixing of oxygen and fuel at the combustion site, and/or rapid cooling of the combustion products to below the ignition temperature of CO prior to its complete oxidation. CO can be formed unintentionally at any of the combustion sites in the pyroprocessing system. The emission of CO usually represents partially burned and under utilized fuel. However, as a result of using oxygen-deficient combustion in the riser duct or calciner as a NO X control strategy, CO sometimes is generated in the pyroprocess and may appear in the flue gas discharge if it is not somehow oxidized following its formation. Organic emissions. VOCs are organic compounds that generally contain from one to seven carbon atoms in the respective molecules and are a subset of THC emissions from cement kilns. VOC emissions from cement kilns are of interest because of their involvement in the formation of atmospheric ozone and the designation of some VOCs as hazardous air pollutants (HAPs). There is no available continuous emission monitor (CEM) to quantify VOC emissions in stack gas. However, the concentration of THC emissions in the exhaust from a cement pyroprocessing system can be measured by a CEM. As stated in the United States Environmental Protection Agency (USEPA) Test Method 25A, 1.1, a THC CEM may not measure all potential THCs; however, the measurement of THCs serves as an accepted surrogate for organic emissions from cement kilns (USEPA, 1999). For purposes of this paper, THCs also serve as a surrogate for VOCs because molecules with seven carbon atoms or less are thought to comprise more than half of the THCs in cement kiln emissions (Ash Grove, 1998). THCs are primarily generated as a result of evaporation and/or cracking of the constituents of petroleum and kerogens found in the raw material mix. The potential for organic emissions varies with the selection of raw materials and the variability of the concentration of organic constituents within raw material sources. Organic PICs also can be formed as a result of incomplete combustion at any of the combustion sites within a pyroprocessing system. Carbon dioxide. Carbon dioxide results from the combustion of carbonaceous fuel and the calcination of the calcareous component of the raw material mix, an essentially unavoidable and fixed consequence of cement manufacture. Of the total amount of CO 2 emitted from a cement kiln, about half of the CO 2 originates from the raw material while the other half originates from the combustion process. There is about one ton of CO 2 emitted per ton of clinker produced. More thermally efficient systems emit slightly less than one ton while less thermally efficient systems emit slightly more than one ton. Ammonia. Trace quantities of NH 3 in the exhaust gas from a cement kiln gas probably result from the pyrolysis of nitrogenous compounds in fossil fuels and raw materials. Ammonia emissions from cement kilns are of primary concern with regard to their potential contribution to 11

12 regional haze. In addition, atmospheric reactions occur just outside of the stack between NH 3 and the oxides of sulfur or HCl that produce ammonium sulfate, ammonium bisulfate, or ammonium chloride as very fine particulate matter (PM). These reaction products are observed as the undesirable anomaly known as a detached plume. Depending on the location of the stack observer, the detached plume can give the incorrect appearance of poorly controlled PM emissions from a kiln stack. If NH 3 were used as a reagent in a NO X control technology, unreacted NH 3 could result in ammonia slip that would contribute to regional haze and/or a detached plume. Technologies for cement kilns that use NH 3 to control NO X emissions are the only technologies that introduce a potential gaseous air pollutant to the cement process to control another air pollutant. Acid gases. All the oxidants necessary to convert SO 2 to sulfur trioxide (SO 3 ) are present in the combustion products of fossil fuel (Miller, 2001). Therefore, emissions of SO 3 and/or sulfuric acid mist are a possibility from cement plants. The emissions of sulfuric acid mist also may increase for those plants employing tailpipe wet scrubbers. The mechanism for the formation of HCl in cement kilns is not fully understood. However, emissions of HCl from cement kilns have been reported over a wide range of values. Perhaps because of the affinity of chlorine for calcium and alkali metals, there is limited evidence that HCl emissions may be independent of chlorine input to a kiln system. Should there be fluorine naturally present in the raw materials or added as a mineralizer, the emission of hydrogen fluoride (HF) from a cement kiln system is a possibility. Dioxins and furans. The USEPA has determined that D/Fs are generated in the PMCDs serving the main and alkali bypass stacks of cement kilns and in-line kilns/raw mills as a function of the temperature at the inlet of the PMCD. Although the mechanism of formation has not been fully determined, USEPA has concluded that there is sufficient empirical evidence to establish the maximum inlet temperature to the PMCDs serving the pyroprocess at 204 o C (400 o F) as the maximum available control technology for cement kilns (USEPA, 1999). Based on currently available data, process engineers in the cement industry generally agree that the predominant variable in the formation of D/Fs is residence time in the critical temperature window. Most often, this process state occurs in the PMCD serving a rotary kiln and/or an inline kiln/raw mill but it can occur elsewhere, e.g., in a lengthy duct. 12

13 RELATIONSHIPS OF EXISTING AND POTENTIAL CONTROL TECHNOLOGIES FOR PRIMARY AIR POLLUTANTS EMITTED FROM A PORTLAND CEMENT PLANT The production of portland cement clinker in simple rotary kilns was once an industrial art. Process instrumentation and controls were rudimentary. In every plant, the visual and anticipatory skills of the indispensable employee known as a kiln burner had a significant impact on the quality and quantity of clinker that was produced. The application of analog instrumentation in the middle of the twentieth century quickly introduced elements of science and engineering to the process. The first successful digital computer-controlled cement kiln in the United States was commissioned in The Clean Air Act of 1970 necessitated the installation of PMCDs on the wet and long-dry process kilns that comprised the pyroprocessing system inventory at that time. The new electrostatic precipitators (ESPs) and fabric filters required induced draft fans for proper operation. The fans and their associated adjustable dampers also provided a means to control combustion parameters as never before. Oxygen and combustible gas analyzers were the first continuous monitoring systems applied to kiln flue gas. Shortly thereafter, crude CEMs for NO X and SO 2 became available and were applied to a few cement kilns. There were several interesting discoveries resulting from this instrumentation. The emissions of SO 2 from the alkaline environment of a cement kiln were greater and more prevalent than previously anticipated. The relative emissions of NO X from a specific kiln were found to be an excellent indicator of changes in its burning zone temperature. The significant variability of SO 2 and NO X emissions from a normally operating kiln was established. The concentration of NO X emissions in the flue gas from a particular kiln was found to be highest when burning natural gas, lowest when burning coal, and in between when burning oil. The inverse relationship of SO 2 and NO X emissions was observed when excess oxygen in the kiln flue gas was varied. The last two observations were the forerunners of the interrelationships of pollution control technologies that are of interest in this white paper. Later, these two relationships would prove to be invalid when applied to precalciner kiln systems and would signal the fact that each of the cement pyroprocessing systems behaves differently with regard to emissions of pollutants. Increasing concerns about the environmental effects of smokestack industries and the environmental regulations that were promulgated to protect ambient air caused the cement industry to more carefully consider its emissions and to apply new technologies to limit those emissions. At the same time, energy shortages and the resulting price increases for fuel gave rise to new, more efficient pyroprocessing systems that often resulted in less pollution. There was early and intuitive synergy between efficiency and pollution prevention. The first preheater kiln systems in the United States were installed on long-dry kilns to cool flue gas without water sprays so that fabric filters could be used to meet PM standards. The resulting improved thermal efficiency and lower NO X emissions were an incidental and synergetic benefit. The surprising predominance of fuel NO X from a calciner caused the precalciner process to be reconsidered and modified to reduce NO X emissions. New technologies were developed and technologies from other industries were adapted to deal with other pollutants. Emerging technologies are expected to be used to further control pollutant emissions from cement kiln systems. The following tables and text present a discussion of the interaction of currently available and potential emission control technologies for cement kiln systems, and their effects on the pollutants of concern. In addition to the synergetic and counteractive effects on gaseous 13

14 pollutants, mention is made of the effects on PM and other relevant environmental concerns, e.g., detached plumes and waste disposal. A synergetic effect is one that would be expected to decrease the generation or emission of other pollutants and a counteractive effect is one that would be expected to degrade environmental performance or product quality. Even in the same pyroprocessing class, no two cement kilns operate exactly alike. There may be no apparent explanation for the difference in behavior even for identically appearing kiln systems operating at the same site. The following generalities about the interactions of the pollution control technologies are important to understand but there always will be site-specific exceptions. Tailpipe technologies are seldom precluded by pollution abatement technologies that are applied prior to or in the main and/or bypass PMCDs in the cement pyroprocess. More than one of the pre-pmcd technologies can be applied simultaneously to the pyroprocess to reduce the generation or emission of the same or different pollutants. Although this white paper does not attempt to evaluate the economics of any of the pollution abatement technologies, pre-pmcd technologies generally are more desirable from the standpoint of process compatibility and cost. From a practical standpoint, tailpipe technologies tend to be mutually exclusive and often have associated costs that put the economic viability of a cement pyroprocess in jeopardy. Those technologies that result in the cooling of flue gas will affect the dispersion characteristics of the flue gas plume and, if all other factors were equal, would tend to increase the ground-level concentration of residual pollutants of concern. Table 1 presents a summary of the gaseous pollutant-control technologies that are currently available for cement kilns, and their synergetic and counteractive effects. Similarly, Table 2 presents the potential control technologies for gaseous emissions from cement kilns. The existing and potential control technologies are described and discussed in succeeding sections of the white paper on a pollutant-by-pollutant basis. 14

15 Table 1. Existing control technologies for gaseous pollutants from portland cement manufacturing Existing control technologies Pollutant for Potential effects which technology was intended Synergetic Counteractive Inherent scrubbing SO 2 Process specific Process specific Increase SO 2, THC, CO NO X, CO 2 Oxygen / excess air control Decrease NO X CO 2 SO 2, CO, product color and quality Fuel substitution (lower sulfur) SO 2 Fuel specific Fuel specific Raw material substitution containing Lower sulfide SO 2 Material specific Material specific Lower organics THC, CO Material specific Material specific Lower carbonates CO 2 Material specific Material specific Lower sulfide or chloride AG Material specific Material specific Raw material alkali/sulfur balance SO 2 Material specific Material specific In-line raw mill SO 2 Preheater upper stage hydrated lime injection THC, AG, NH 3, D/F, detached plume THC, detached plume SO 2 D/F PM Calcined feed recirculation SO 2 NO X, CO 2 Cement kiln dust internal scrubber Preheater upper stage trona injection SO 2 Calcium-based internal scrubber SO 2 AG, D/F SO 2 AG, D/F CKD disposal D/F, detached plume, waste disposal Pyroprocessing system design SO 2 Process specific Process specific Tailpipe wet scrubber SO 2 NH 3, HCl Decrease SO 2 generation AG SO 2 AG, PM, solid waste disposal, wastewater Indirect firing NO X CO 2 PM Low-NO X burner NO X Burner/application specific Burner/application specific Mid-kiln firing NO X Application specific Application specific Process improvements NO X Project specific Project specific Process control improvements NO X Project specific Project specific Low-NO X calciner NO X CO Staged combustion NO X CO Semi-direct firing NO X PM Mixing air fan NO X, THC, CO SO 2 Cement kiln dust insufflation NO X CO, CO 2, SO 2 15

16 Table 1. Existing control technologies for gaseous pollutants from portland cement manufacturing (continued) Existing control technologies Biosolids injection Inherent process characteristics (time, temperature, and turbulence) Pollutant for Potential effects which technology was intended Synergetic Counteractive NO X THC CO CO, NH 3, detached plume, metals Pyroprocessing system design THC, CO Process specific Process specific Regenerative thermal oxidizer THC, CO Detached plume, D/F Good combustion practice CO NO X, CO 2, SO 2, THC NO X, CO 2, SO 3, AG, waste disposal Improved thermal efficiency CO 2 Project specific Project specific Clinker substitution CO 2 Improved electrical efficiency CO 2 Reduction in all gaseous pollutants per ton of cement produced Reduction in all gaseous pollutants per ton of cement produced Reduction in all gaseous pollutants per ton of cement produced Reduction in all gaseous pollutants per ton of cement produced Mineralizers CO 2 NO X AG Electricity generation from waste heat PMCD inlet temperature control Reduced residence time at temperature CO 2 D/F D/F Reduction in all pollutants related to power generation Reduction in all pollutants related to power generation 16

17 Table 2. Potential control technologies for gaseous pollutants from portland cement manufacturing Potential control technologies Mixing air fan In-line raw mill hydrated lime injection Pollutant for which technology might be intended SO 2, NO X, CO, THC SO 2 Synergetic THC, AG, D/F, detached plume Fabric filter absorption SO 2 AG Sodium-based internal scrubber SO 2 Calcium/sodium based internal scrubber Oxygen enrichment Dual-alkali process (soda ash/lime) AG, D/F, detached plume Potential effects Counteractive CKD disposal SO 2 AG, D/F CKD disposal SO 2, THC, CO NO X SO 2, CO NO X SO 2 AG Waste disposal Thermal decomposition (roasting) SO 2 THC CO, NO X, CO 2 Tailpipe dry scrubber SO 2, AG AG, THC, D/F NO X, CO, CO 2, waste disposal Cement kiln dust tailpipe scrubber SO 2 THC, NH 3, AG, detached plume Fuel substitution Raw material substitution containing Low nitrogen containing fuel High hydrocarbon containing fuel NO X Fuel/process specific Fuel/process specific CO 2 Fuel specific Fuel specific Lower nitrogen NO X Material specific Material specific Lower ammonia NH 3 Material specific Material specific Lower D/F D/F Material specific Material specific Selective noncatalytic reduction NO X NH 3, detached plume Modified direct firing NO X PM LoTOX scrubber NO X Water discharges, ozone slip Flue gas recirculation NO X CO, SO 2 Selective catalytic reduction Tri-NO X Multi-Chem wet scrubber NO X NH 3, CO 2, detached plume, solid catalyst wastes NO X SO 2, AG Water discharges Water/steam injection NO X CO, CO 2 Catalytic filtration NO X Non-thermal plasma NO X SO 2, THC, D/F D/F PM 17

18 Table 2. Potential control technologies for gaseous pollutants from portland cement manufacturing (continued) Potential control technologies Pollutant for which technology might be intended Synergetic Potential effects Counteractive Thermal desorption (roasting) THC SO 2, CO Thermal oxidation THC, CO D/F CO 2, NO X Recuperative thermal oxidation THC, CO D/F CO 2, NO X Wet electrostatic precipitator THC, AG SO 2, NO X, PM, NH 3, Waste disposal, D/F, detached plume water treatment Ultraviolet light THC, D/F CO Catalytic oxidization THC, CO CO 2, NO X Granular activated carbon Waste disposal, high THC, D/F NO adsorption X, SO 2, metals reagent consumption D/F, waste disposal, Powdered activated carbon THC, D/F NO adsorption X, SO 2, metals high reagent consumption Electricity generation from the sun and wind CO 2 Reduction in all pollutants related to power generation Tailpipe wet scrubber NH 3, AG SO 2, THC Reduction in all pollutants related to power generation PM, acid mist, wastewater Fabric filter absorption AG SO 2 Tailpipe dry bicarbonate injection AG SO 2, D/F, detached plume Temperature control AG SO 2, NH 3, THC, D/F, detached plume Waste disposal Water/waste disposal 18

19 Control of Sulfur Dioxide Table 3. Sulfur Dioxide Control Technologies. Control technologies Synergetic Potential effects Counteractive Existing control technologies Inherent scrubbing Process specific Oxygen control (increase) CO NO X, CO 2 Fuel substitution (lower sulfur) Raw material substitution (lower sulfide) Raw material alkali/sulfur balance In-line raw mill Preheater upper stage hydrated lime injection THC, AG, NH 3, D/F, detached plume D/F Fuel specific Material specific Material specific THC, detached plume PM Calcined feed recirculation NO X, CO 2 Cement kiln dust internal scrubber AG, D/F Preheater upper stage trona injection AG, D/F CKD disposal Calcium-based internal scrubber Pyroprocessing system design Tailpipe wet scrubber D/F, detached plume, waste disposal NH 3, HCl Potential control technologies Process specific AG, PM, solid waste disposal, wastewater Mixing air fan In-line raw mill hydrated lime injection Fabric filter absorption NO X, CO, THC THC, AG, D/F, detached plume AG Sodium-based internal scrubber AG, D/F, detached plume CKD disposal Calcium/sodium based internal scrubber AG, D/F CKD disposal Oxygen enrichment CO, THC NO X Dual-alkali process (soda ash/lime) AG Waste disposal Thermal decomposition (roasting) HC CO, NO X, CO 2 Tailpipe dry scrubber AG, D/F NO X, CO, CO 2, waste disposal Cement kiln dust tailpipe scrubber THC, NH 3, AG detached plume 19

20 Existing control technologies. Inherent scrubbing. All cement pyroprocessing systems have the characteristics required to remove some SO 2 from the flue gas stream. These include oxidizing atmospheres, long residence times, appropriate process temperature windows, intimate mixing of gases and reactive solids, and the ability to remove from the process an intermediate material, i.e., cement kiln dust (CKD), that contains absorbed sulfur. Without application of additional technology, the author s experience is that the least effective cement kiln system captures as much as 50% of the sulfur input to the system. Likewise, capture efficiencies for sulfur in the range of 90-95% without added technologies are not uncommon in any of the cement kiln systems. High capture efficiencies are more prevalent in precalciner kiln systems with an in-line raw mill. It is not expected that any pollution control technology mentioned herein would detrimentally affect the inherent scrubbing potential of any cement pyroprocessing system. Oxygen control (increase). For control of SO 2 originating in fuel, an increase in oxygen (excess air) in the rotary kiln tends to oxidize sulfur to a solid sulfate that is retained in the clinker or expelled from the system with the CKD. Because excess air passing through the burning zone increases the oxygen concentration at the combustion site, there is usually a concurrent increase in the generation of NO X. If there are localized reducing conditions in the kiln because of flame impingement or other causes, the increase in oxygen may not reduce SO 2 emissions but NO X emissions may increase. If the initial combustion conditions have resulted in CO in the flue gas, the increase in oxygen concentration may decrease or eliminate the CO. There is a small increased energy requirement to heat the excess air in the system that must be overcome by additional fuel and a resulting slight increase in CO 2 emissions. Because of the inherently high removal efficiency for fuel-based SO 2 in a calciner, oxygen control at this combustion site would not be expected to improve SO 2 removal in precalciner systems. Fuel substitution (lower total sulfur). In precalciner kiln systems, the emission of SO 2 that originates in the fuel is often nil because of the inherent ability of the calciner and an alkalibypass equipped kiln to absorb and/or remove sulfur. In the other systems and under certain process conditions, e.g., a deficiency of alkali metals, sulfur in the fuel can result in emissions of SO 2. It is intuitive that a reduction in the sulfur content of a solid fuel or the change to a sulfurfree fuel, e.g., natural gas, has the potential to reduce SO 2 emissions. Because of the complexities of the cement pyroprocess, a change in the sulfur content of the fuel does not always result in expected changes in SO 2 emissions. Whenever a fuel is changed, there may be unintended effects on the process and the resulting pollutants. For example, the replacement of coal with natural gas in a long-dry kiln system to reduce SO 2 emissions will result in an increase in NO X emissions. Because energy costs nominally represent about one-third of the cost of cement manufacture, fuel substitution at a particular plant may not be economically viable. Raw material substitution (lower sulfide sulfur). Primarily appropriate for preheater and precalciner kilns, the replacement of a raw material that contains sulfide sulfur with one of lower sulfide sulfur concentration reduces the potential for generation of SO 2 in the upper stages of the preheater tower. Sulfide sulfur in cement raw materials is most often in the form of iron pyrite but other sulfide compounds, including those of organic origin, may contribute to the potential for SO 2 generation. Selective purchasing, selective quarrying or judicious blending of available 20

21 raw materials is used to accomplish the replacement. Whenever a raw material is changed, there may be unintended effects on the process and the resulting pollutants. For example, the new raw material with a lower concentration of sulfide sulfur might result in the need for a higher temperature in the burning zone that would tend to require more fuel and cause higher NO X and CO 2 emissions. Because cement plants normally are located at or near their sources of raw materials, there are often critical economic limitations to the practicability of substituting raw materials to reduce the input of sulfide sulfur. SO 2 has been shown to be an inhibitor to the formation of D/Fs (Richards, 2003) Raw material alkali/sulfur balance. Another raw material substitution method that is potentially applicable to all kiln systems is to stoichiometrically balance the sulfur in the kiln system with the alkali metals, sodium and potassium. Under oxidizing conditions in the kiln, the sulfur preferentially forms alkali sulfates. If there is a deficiency in alkali metals, SO 2 can pass through the system even though there is an apparent abundance of calcium oxide with which the SO 2 could react and be retained in the clinker or CKD. Balancing the input of alkali metals to the input of sulfur has been shown to reduce SO 2 emissions. However, alkali metals are deleterious to the performance of portland cement in some concretes and the concentration of these metals in cement is frequently limited by specification. Because of this concern for product quality, it may not be possible to introduce a sufficient quantity of alkali metals into a kiln system to stoichiometrically balance a high-sulfur input. Whenever a raw material is changed, there may be unintended effects on the process and the resulting pollutants. For example, a new, higher-alkali raw material also might contain nitrogenous compounds that potentially would contribute to increased emissions of NO X. Because cement plants are normally located at or near their sources of raw materials, there are often critical economic limitations to the practicability of substituting raw materials to alter the alkali/sulfur balance through selective purchasing, selective quarrying or judicious blending of available raw materials. In-line raw mill. The use of hot exhaust gases from a dry kiln system to simultaneously dry the raw materials during grinding is a common practice to improve the overall thermal efficiency of a plant. In-line raw mills most often are found as components of new or reconstructed preheater and precalciner kiln systems. In-line raw mills are seldom, if ever, added to an existing dry kiln system solely as a pollution control technology. The presence of finely divided calcium carbonate in the high-moisture atmosphere of an in-line raw mill (particularly vertical roller mills), and the intimate contact of the solids and flue gas, result in an excellent SO 2 scrubbing environment. Reductions in the concentration of SO 2 in the flue gas are commonly in the range of 40-60% during mill operation. When the in-line raw mill is down for maintenance or raw mix inventory control, the flue gas is not treated in the mill and SO 2 emissions measurably increase unless otherwise controlled. Most in-line kiln/raw mill systems are designed for the mill to be down only 8-16 hours per week while the kiln is operating. In limited, unpublished testing familiar to the author, operation of in-line raw mills also have been shown to reduce the emissions of THC, AG, NH 3, and D/F. However, observations have been made of the release of THC emissions from the matrix of the raw material mix during grinding. Detached plumes also may be reduced or eliminated when the in-line raw mill is in operation because the plume constituents have been reduced in concentration or eliminated by the gas scrubbing action within the mill. Conversely, a release of NH 3 has been observed during 21