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Research & Development Information

PCA R&D Serial No. 2728

Interactions Among Gaseous Pollutantsfrom Cement Manufacture and Their

Control Technologies

by Walter L. Greer

Portland Cement Association 2003All rights reserved

This information is copyright protected. PCA grants permission to electronically share this document with other professionals on thecondition that no part of the file or document is changed.

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KEYWORDS

Cement kilns, pyroprocess, gaseous pollutants, origins, control technologies, sulfur dioxide, SO2,

nitrogen oxides, NOX, organics, hydrocarbons, carbon monoxide, CO, carbon dioxide, CO2,

ammonia, NH3, dioxins, furans

ABSTRACT

This report presents a qualitative examination of the interactions of gaseous pollutants generatedin 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 TheirControl Technologies, R&D Serial No. 2728, Portland Cement Association, Skokie, Illinois,USA, 2003, 59 pages.

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TABLE OF CONTENTS

Page

KEYWORDS.................................................................................................................................. 2ABSTRACT.................................................................................................................................... 2

REFERENCE.................................................................................................................................. 2

TABLE OF CONTENTS................................................................................................................ 3

INTRODUCTION .......................................................................................................................... 4DESCRIPTION OF THE CEMENT-MAKING PROCESS .......................................................... 5

SOURCES OF GASEOUS POLLUTANTS .................................................................................. 6

Raw Materials ............................................................................................................................. 6Fuel ............................................................................................................................................. 7

Process ........................................................................................................................................ 8

RELATIONSHIPS OF EXISTING AND POTENTIAL CONTROL TECHNOLOGIES FORPRIMARY AIR POLLUTANTS EMITTED FROM A PORTLAND CEMENT PLANT ......... 13

Control of Sulfur Dioxide ......................................................................................................... 19

Control of Nitrogen Oxides ...................................................................................................... 27Control of Organics................................................................................................................... 37

Control of Carbon Monoxide.................................................................................................... 42

Control of Carbon Dioxide ....................................................................................................... 45

Control of Ammonia ................................................................................................................. 48Control of Acid Gases............................................................................................................... 49

Control of Dioxins and Furans.................................................................................................. 52

CONCLUSIONS........................................................................................................................... 55ACKNOWLEDGEMENTS.......................................................................................................... 55

REFERENCES ............................................................................................................................. 55

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InteractionsAmong Gaseous Pollutantsfrom Cement Manufacture and Their

Control TechnologiesBy Walter L. Greer*

INTRODUCTION

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 invarious 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 otherpollutants 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 strategiesfor 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 basicprocess 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 (NOX), sulfur dioxide (SO2), and organic emissions, i.e., in the form oftotal hydrocarbons (THCs) and/or volatile organic compounds (VOCs).

1The emissions of

carbon dioxide (CO2) 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 combustionand/or high-temperature processes. The principal gaseous emissions from the pyroprocessing

system in a typical descending order by volume are nitrogen, CO2, water, oxygen, NOX, SO2,

CO, and hydrocarbons. The volumetric composition range of these constituents is from about 73percent to less than 10 ppm (Greer, Dougherty, and Sweeney, 2000). Emissions of acid gases

(AGs), ammonia (NH3), and dioxins and furans (D/Fs) also are of current interest.

This paper provides a brief description of the cement-making process, an identification ofexpected 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, 25055 West Valley Parkway, Suite 101, Olathe, KS, 66061 USA

(913) 390-9700, www.trinityconsultants.com

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.

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discussion of current and potential control technologies with regard to purpose, process

integration, and potential interactions between them. Listed references provide more detaileddiscussions 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 ofthe listed control technologies; however, site-specific events may result in interactions that are

not consistent with the authors 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 fourtypes 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 beused to independently dry raw materials in dedicated equipment or to simultaneously dry themduring 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 transferfrom 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 commonto 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 1480C (2700F) by an approximately 1870C (3400F) flame in the hottestsection 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 forpollution 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 earths crust as ores consisting primarily of carbonates or oxides, or

are derived from secondary (waste) materials also having an origin in the earths crust.Magnesium, sulfur, sodium and potassium are other elements that appear in clinker in minor

concentrations. Many other elements common to the earths crust can be found in clinker intrace 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 ofthe 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.

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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. Becauseof 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 canbe 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, andthe process itself.

Raw Materials

Calcareous component. The predominant constituent of the cement raw material mix iscalcium 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 mixmust 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 asignificant source of CO2 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 notclearly established, this chlorine is available in the flue gas stream for the generation of hydrogenchloride (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 withouttransformation into SO2, but sulfides and elemental sulfur can result in the generation of SO2

through the oxidation of sulfur in kiln systems. If localized reducing conditions exist in the

pyroprocessing system, sulfates can be converted to SO2.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 componentsof 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.

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sulfates, sulfides (metallic and organic), and elemental sulfur that have the potential to generate

SO2. Organic, CO, and CO2 emissions also may result from kerogens, crude petroleum, orrefined 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 NOX emissions

from cement kilns (Gartner, 1983). Because of the extensive research project that would berequired, this potential contribution has not been documented or quantified in the field; however,

the potential NOX emissions from raw materials can be estimated in the laboratory. These

nitrogenous constituents also may contribute to emissions of NH3.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 cementindustry 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 forcombustion 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 CO2 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 SO2during combustion of the coal. Coal also contains nitrogenous compounds that are oxidized toNOX, i.e., fuel NOX, or converted to small quantities of free NH3 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 SO2 or NOX. In large part, the type of pyroprocess will determine if these impurities have amajor impact on the emissions of NOX and SO2 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 usethan 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. In2000, 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 CO2, and the potential

formation of CO and other PICs.

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The sulfur and nitrogen content of natural gas is insufficient to result directly in

appreciable emissions of SO2 or NOX. However, due to the formation of thermal NOX resultingfrom high flame temperature, the emissions of NOX 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 orheating oil (both known as middle distillates), that is used to bring the kiln system to operatingtemperature 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 CO2, and the potentialformation 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 SO2, NOX, or NH3.

Waste. Combustible wastes of industry and consumers can be used in cement kilns tosupplement 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 mutuallybeneficial 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 non-

hazardous solid wastes (PCA, 2002). In 2000, 57% of the cement plants reporting data to thePCA 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 CO2, 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 SO2, NOX, NH3, HCl, or D/Fs.

Process

Pyroprocess description. The formation of gaseous pollutants primarily occurs in anindependent kiln system or in an in-line kiln/raw mill as the consequence of oxidation or otherprocesses at a relatively high temperature, e.g., greater than 315C (600F). 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 solidfuel 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, thealkali 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, orvertical 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

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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, thesite-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 atmid-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 tocool 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 therotary 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. Inthe 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 thepreheater 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 atabout 870C (1600F) and does not require the 1870C (3400F) 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 anair-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 preheatertower 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 fuel-

efficient and stable of the four types. When compared to the other pyroprocesses, this systemprovides 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 sulfurcontained in the fuel during combustion. In addition, sulfide or elemental sulfur contained in

raw materials may be roasted or oxidized to SO2 in areas of the pyroprocessing system where

sufficient oxygen is present and the material temperature is in the range of 300-600C

(570-1110F) (Miller, Young, and von Seebach, 2001). In addition, sulfates in the raw mix canbe converted to SO2 through localized reducing conditions in the kiln system.

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Figure 1. Wet/long-dry kiln systems.

Figure 2. Preheater/precalciner kiln systems.

Nitrogen oxides. It has been shown that nitrogen oxide (NO) makes up 90% or more of theNOX contained in cement kiln flue gas. Nitrogen dioxide (NO2) comprises the balance of the

nitrogen oxides (Penta, 1991).

There are four mechanisms of NOX formation in cement kilns of which thermal and fuelNOX formation are the most important. Thermal NOX 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 1200C (2200F). Fuel NOX resultsfrom 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 NOX often exceeds that of thermal NOX at

these locations. The generation of feed NOX has been demonstrated only in the laboratory by

SecondaryAir

I.D. Fan Raw Material

Mix Silo

Rotary Kiln Clinker Cooler

Clinker Discharge

MainPMCD

Raw Material Mix to Preheater

Fuel

Fuel andPrimary Air

AlkaliBypass

To PMCD

RawMaterial

In-lineRaw Mill

To PreheaterTower

Tertiar Air Duct

Preheater

Calciner

PMCD

I.D. Fan CKD Bin Rotary Kiln Clinker Cooler

Clinker Discharge

SecondaryAir

Fuel andPrimary Air

MixingAir Fan

Mid-kilnFiring

Raw Material Mixto Kiln

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heating nitrogen-containing cement raw materials to the range of 300-800C (570-1470F) in the

presence of oxygen. Slow heating, such as occurs in wet and long-dry kilns, appears to increasethe yield of NOX for a given raw material. The yield of feed NOX is potentially lower when the

raw material is heated quickly in a preheater or precalciner system. Prompt NOX is generated by

the reaction of certain fuel-derived radicals with elemental nitrogen in a hydrocarbon flame and

is a minor contributor to overall NOX generation (Penta, 1991).

Carbon monoxide. CO is a PIC of carbonaceous fuels resulting from insufficient oxygen atthe 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 completeoxidation.

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 NOX

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 sevencarbon 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 formationof 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 pyroprocessingsystem 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 forVOCs 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 inthe 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 thecombustion sites within a pyroprocessing system.

Carbon dioxide. Carbon dioxide results from the combustion of carbonaceous fuel and thecalcination of the calcareous component of the raw material mix, an essentially unavoidable and

fixed consequence of cement manufacture. Of the total amount of CO2 emitted from a cementkiln, about half of the CO2 originates from the raw material while the other half originates from

the combustion process. There is about one ton of CO2 emitted per ton of clinker produced.

More thermally efficient systems emit slightly less than one ton while less thermally efficientsystems emit slightly more than one ton.

Ammonia. Trace quantities of NH3 in the exhaust gas from a cement kiln gas probably resultfrom 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

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regional haze. In addition, atmospheric reactions occur just outside of the stack between NH3

and the oxides of sulfur or HCl that produce ammonium sulfate, ammonium bisulfate, orammonium 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 NH3 were used as a reagent in a NOX control technology, unreacted NH3 could result in

ammonia slip that would contribute to regional haze and/or a detached plume. Technologies

for cement kilns that use NH3 to control NOX emissions are the only technologies that introducea potential gaseous air pollutant to the cement process to control another air pollutant.

Acid gases. All the oxidants necessary to convert SO2 to sulfur trioxide (SO3) are present inthe combustion products of fossil fuel (Miller, 2001). Therefore, emissions of SO3 and/orsulfuric 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 PMCDsserving 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 204oC

(400oF) 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 temperaturewindow. Most often, this process state occurs in the PMCD serving a rotary kiln and/or an in-

line kiln/raw mill but it can occur elsewhere, e.g., in a lengthy duct.

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RELATIONSHIPS OF EXISTING AND POTENTIAL CONTROLTECHNOLOGIES FOR PRIMARY AIR POLLUTANTS EMITTED FROM APORTLAND 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 andanticipatory 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 analoginstrumentation 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 1969. The Clean Air Act of 1970 necessitated the

installation of PMCDs on the wet and long-dry process kilns that comprised the pyroprocessingsystem 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 andcombustible gas analyzers were the first continuous monitoring systems applied to kiln flue gas.

Shortly thereafter, crude CEMs for NOX and SO2 became available and were applied to a fewcement kilns. There were several interesting discoveries resulting from this instrumentation.The emissions of SO2 from the alkaline environment of a cement kiln were greater and more

prevalent than previously anticipated. The relative emissions of NOX from a specific kiln were

found to be an excellent indicator of changes in its burning zone temperature. The significantvariability of SO2 and NOX emissions from a normally operating kiln was established. The

concentration of NOX 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 SO2 and NOX emissions was observed when excess oxygen in the kilnflue 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 wouldsignal 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 thoseemissions. 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 kilnsystems 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 NOX emissions were an incidental and synergetic benefit. The surprisingpredominance of fuel NOX from a calciner caused the precalciner process to be reconsidered andmodified to reduce NOX 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

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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 todecrease 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 kilnsystems 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 thegeneration 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. Thosetechnologies 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-levelconcentration 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.

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Table 1. Existing control technologies for gaseous pollutants from portland cement manufacturing

Potential effectsExisting control technologies

Pollutant forwhich technology

was intended Synergetic Counteractive

Inherent scrubbing SO2 Process specific Process specific

Increase SO2, THC, CO NOX, CO2Oxygen /excess aircontrol Decrease NOX CO2

SO2, CO, productcolor and quality

Fuel substitution (lower sulfur) SO2 Fuel specific Fuel specific

Lower sulfide SO2 Material specific Material specific

Lower organics THC, CO Material specific Material specific

Lower carbonates CO2 Material specific Material specific

Raw materialsubstitutioncontaining

Lower sulfide orchloride

AG Material specific Material specific

Raw material alkali/sulfur balance SO2 Material specific Material specific

In-line raw mill SO2THC, AG, NH3, D/F,

detached plumeTHC, detached

plume

Preheater upper stage hydratedlime injection

SO2 D/F PM

Calcined feed recirculation SO2 NOX, CO2

Cement kiln dust internalscrubber

SO2 AG, D/F

Preheater upper stage tronainjection

SO2 AG, D/F CKD disposal

Calcium-based internal scrubber SO2D/F, detached plume,

waste disposal

Pyroprocessing system design SO2 Process specific Process specific

Tailpipe wet scrubber SO2 NH3, HClAG, PM, solid wastedisposal, wastewater

Decrease SO2 generation AG SO2

Indirect firing NOX CO2 PM

Low-NOX burner NOXBurner/application

specificBurner/application

specific

Mid-kiln firing NOX Application specific Application specific

Process improvements NOX Project specific Project specific

Process control improvements NOX Project specific Project specific

Low-NOX calciner NOX CO

Staged combustion NOX CO

Semi-direct firing NOX PM

Mixing air fan NOX, THC, CO SO2

Cement kiln dust insufflation NOX CO, CO2, SO2

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Table 1. Existing control technologies for gaseous pollutants from portland cement manufacturing(continued)

Potential effectsExisting control technologies

Pollutant forwhich technology

was intended Synergetic Counteractive

Biosolids injection NOX CO, NH3, detachedplume, metals

Inherent process characteristics(time, temperature, andturbulence)

THC CO

Pyroprocessing system design THC, CO Process specific Process specific

Regenerative thermal oxidizer THC, CO Detached plume, D/FNOX, CO2, SO3, AG,

waste disposal

Good combustion practice CO NOX, CO2, SO2, THC

Improved thermal efficiency CO2 Project specific Project specific

Clinker substitution CO2

Reduction in allgaseous pollutantsper ton of cement

produced

Reduction in allgaseous pollutantsper ton of cement

produced

Improved electrical efficiency CO2

Reduction in allgaseous pollutantsper ton of cement

produced

Reduction in allgaseous pollutantsper ton of cement

produced

Mineralizers CO2 NOX AG

Electricity generation from wasteheat

CO2Reduction in all

pollutants related topower generation

Reduction in allpollutants related topower generation

PMCD inlet temperature control D/F

Reduced residence time attemperature D/F

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Table 2. Potential control technologies for gaseous pollutants from portland cement manufacturing

Potential effects

Potential control technologies

Pollutant forwhich technology

might beintended

Synergetic Counteractive

Mixing air fan SO2, NOX, CO,THC

In-line raw mill hydrated limeinjection

SO2THC, AG, D/F,

detached plume

Fabric filter absorption SO2 AG

Sodium-based internal scrubber SO2AG, D/F, detached

plumeCKD disposal

Calcium/sodium based internalscrubber

SO2 AG, D/F CKD disposal

SO2, THC, CO NOXOxygen enrichment

NOX SO2, CO

Dual-alkali process (soda

ash/lime) SO2 AG Waste disposal

Thermal decomposition (roasting) SO2 THC CO, NOX, CO2

Tailpipe dry scrubber SO2, AG AG, THC, D/FNOX, CO, CO2, waste

disposal

Cement kiln dust tailpipe scrubber SO2THC, NH3, AG,detached plume

Low nitrogencontaining fuel

NOX Fuel/process specific Fuel/process specificFuelsubstitution High hydrocarbon

containing fuelCO2 Fuel specific Fuel specific

Lower nitrogen NOX Material specific Material specific

Lower ammonia NH3 Material specific Material specific

Raw material

substitutioncontainingLower D/F D/F Material specific Material specific

Selective noncatalytic reduction NOX NH3, detached plume

Modified direct firing NOX PM

LoTOX scrubber NOXWater discharges,

ozone slip

Flue gas recirculation NOX CO, SO2

Selective catalytic reduction NOXNH3, CO2, detachedplume, solid catalyst

wastes

Tri-NOX Multi-Chem wet

scrubber NOX SO2, AG Water discharges

Water/steam injection NOX CO, CO2

NOXCatalytic filtration

D/F PM

Non-thermal plasma NOX SO2, THC, D/F

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Table 2. Potential control technologies for gaseous pollutants from portland cement manufacturing(continued)

Potential effects

Potential control technologies

Pollutant forwhich technology

might be

intended

Synergetic Counteractive

Thermal desorption (roasting) THC SO2, CO

Thermal oxidation THC, CO D/F CO2, NOX

Recuperative thermal oxidation THC, CO D/F CO2, NOX

Wet electrostatic precipitator THC, AGSO2, NOX, PM, NH3,D/F, detached plume

Waste disposal,water treatment

Ultraviolet light THC, D/F CO

Catalytic oxidization THC, CO CO2, NOX

Granular activated carbonadsorption

THC, D/F NOX, SO2, metalsWaste disposal, highreagent consumption

Powdered activated carbonadsorption

THC, D/F NOX, SO2, metals D/F, waste disposal,high reagentconsumption

Electricity generation from the sunand wind

CO2Reduction in all

pollutants related topower generation

Reduction in allpollutants related topower generation

Tailpipe wet scrubber NH3, AG SO2, THCPM, acid mist,

wastewater

Fabric filter absorption AG SO2

Tailpipe dry bicarbonate injection AGSO2, D/F, detached

plumeWaste disposal

Temperature control AGSO2, NH3, THC, D/F,

detached plumeWater/waste disposal

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Control of Sulfur Dioxide

Table 3. Sulfur Dioxide Control Technologies.

Potential effectsControl technologies

Synergetic Counteractive

Existing control technologies

Inherent scrubbing Process specific

Oxygen control (increase) CO NOX, CO2

Fuel substitution (lower sulfur) Fuel specific

Raw material substitution (lower sulfide) Material specific

Raw material alkali/sulfur balance Material specific

In-line raw millTHC, AG, NH3, D/F,

detached plumeTHC, detached plume

Preheater upper stage hydrated limeinjection D/F PM

Calcined feed recirculation NOX, CO2

Cement kiln dust internal scrubber AG, D/F

Preheater upper stage trona injection AG, D/F CKD disposal

Calcium-based internal scrubberD/F, detached plume,

waste disposal

Pyroprocessing system design Process specific

Tailpipe wet scrubber NH3, HClAG, PM, solid waste disposal,

wastewater

Potential control technologies

Mixing air fan NOX, CO, THC

In-line raw mill hydrated lime injectionTHC, AG, D/F, detached

plume

Fabric filter absorption 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 NOX

Dual-alkali process (soda ash/lime) AG Waste disposal

Thermal decomposition (roasting) HC CO, NOX, CO2

Tailpipe dry scrubber AG, D/FNOX, CO, CO2, waste

disposal

Cement kiln dust tailpipe scrubberTHC, NH3, AG

detached plume

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Existing control technologies.

Inherent scrubbing. All cement pyroprocessing systems have the characteristics required toremove some SO2 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 authors

experience is that the least effective cement kiln system captures as much as 50% of the sulfurinput 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 theinherent scrubbing potential of any cement pyroprocessing system.

Oxygen control (increase). For control of SO2 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 theburning zone increases the oxygen concentration at the combustion site, there is usually a

concurrent increase in the generation of NOX. If there are localized reducing conditions in the

kiln because of flame impingement or other causes, the increase in oxygen may not reduce SO2emissions but NOX 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 CO2 emissions. Because of the

inherently high removal efficiency for fuel-based SO2 in a calciner, oxygen control at thiscombustion site would not be expected to improve SO2 removal in precalciner systems.

Fuel substitution (lower total sulfur). In precalciner kiln systems, the emission of SO2 thatoriginates in the fuel is often nil because of the inherent ability of the calciner and an alkali-bypass 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

SO2. It is intuitive that a reduction in the sulfur content of a solid fuel or the change to a sulfur-

free fuel, e.g., natural gas, has the potential to reduce SO2 emissions. Because of thecomplexities of the cement pyroprocess, a change in the sulfur content of the fuel does not

always result in expected changes in SO2 emissions. Whenever a fuel is changed, there may be

unintended effects on the process and the resulting pollutants. For example, the replacement ofcoal with natural gas in a long-dry kiln system to reduce SO2 emissions will result in an increase

in NOX 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 andprecalciner kilns, the replacement of a raw material that contains sulfide sulfur with one of lower

sulfide sulfur concentration reduces the potential for generation of SO2 in the upper stages of the

preheater tower. Sulfide sulfur in cement raw materials is most often in the form of iron pyritebut other sulfide compounds, including those of organic origin, may contribute to the potential

for SO2 generation. Selective purchasing, selective quarrying or judicious blending of available

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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 rawmaterial 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 NOX and

CO2 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 rawmaterials to reduce the input of sulfide sulfur. SO2 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 ispotentially 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, SO2 can passthrough the system even though there is an apparent abundance of calcium oxide with which the

SO2 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 SO2 emissions. However, alkali metals are

deleterious to the performance of portland cement in some concretes and the concentration ofthese 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 ischanged, 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 thatpotentially would contribute to increased emissions of NOX. 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 balancethrough 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 theraw materials during grinding is a common practice to improve the overall thermal efficiency ofa 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 calciumcarbonate 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 SO2 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 mixinventory control, the flue gas is not treated in the mill and SO2 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, NH3, and D/F. However, observations

have been made of the release of THC emissions from the matrix of the raw material mix duringgrinding. 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 NH3 has been observed during

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grinding of nitrogen-bearing raw materials that caused a detached plume under favorable

atmospheric conditions. Because an in-line raw mill utilizes most of the sensible heat in the fluegas, the application of tailpipe technologies requiring an elevated temperature, e.g., a dry

scrubber, requires additional fossil fuel combustion (and the resulting increase in CO2 and NOX

emissions) to reheat flue gas that has passed through an in-line raw mill.

Preheater upper stage hydrated lime injection. To serve as an SO2 absorbing reagent,powdered hydrated lime (calcium hydroxide) can be introduced into an appropriate location in

the upper stages of the preheater tower that is an integral component of both preheater or

precalciner kiln systems. The hydrated lime and SO2 can react directly or, more likely, thehydrate is converted to calcium oxide at 522

oC (972

oF) to form an effective scrubbing reagent at

the location in the process where sulfide sulfur is being converted to SO 2. Probably due to

variations in the point of reagent injection, mixing efficiency, and the retention time of thereagent in the required process temperature window, a wide range of scrubbing efficiencies has

been observed for this technology. Due to the receipt and handling of powdered hydrated lime,

there will be a small increase in PM emissions from the plant using this technology. It is

possible that the concurrent removal of HCl and Cl2 from the flue gas at this point in the processwill suppress the subsequent formation of D/F (Richards, 2003).

Calcined feed recirculation. F.L.Smidth has developed a proprietary process, De-SOX, inwhich a small quantity of partially calcined kiln feed, e.g., 5%, is removed from the calciner

vessel of a precalciner kiln system and pneumatically conveyed to an appropriate point in the

upper stages of the preheater tower. The calcium oxide in the calcined feed is an effectivescrubbing reagent at the location in the process where sulfide sulfur is being converted to SO 2.

Scrubbing efficiencies for SO2 of 25-30% have been reported anecdotally. This system is

perhaps more convenient and cost effective than the injection of purchased hydrated lime.

Because of the recirculation of material, there is a small increase in the thermal energyrequirements that results in additional NO

Xand CO

2generation due to additional fuel

consumption. Other vendors may offer similar systems.

Cement kiln dust internal scrubber. F.L.Smidth has developed another proprietaryprocess, Gas Suspended Absorption (GSA), in which dry, lime-rich CKD from the alkali

bypass PMCD on a precalciner kiln system is recirculated to the conditioning tower ahead of the

bypass PMCD in the gas flow path. In the presence of the water in the conditioning tower that is

used for temperature moderation, this calcium oxide becomes an effective SO2 scrubbingreagent. Because of the pyroprocess is the source of the scrubbing reagent, the amount of CKD

that is bled from the GSA system and subsequently wasted is not increased beyond the amount

that ordinarily would be wasted from the alkali bypass. Should enhancement of the CKDscrubbing potential be required, dry hydrated lime can be added to the CKD introduced into the

conditioning tower. In this situation, the amount of wasted CKD would be increased by the

amount of hydrated lime added to the process. The GSA process is only applicable to the gasstream that passes through the alkali bypass and only would control SO2 originating from the

fuel used in the burning zone or from sulfates locally reduced to SO 2 in the rotary kiln (Vu,

2003). The emissions of AGs, e.g., HCl and Cl2 also might be reduced in quantity and

concentration. The reduction of these potential precursors to D/F emissions, may serve tosuppress the generation of D/F in the PMCD. Other vendors may offer similar systems.

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Preheater upper stage trona injection. A variant of the hydrated lime injectiontechnology is the use of powdered trona, a naturally occurring crystalline form of sodium

carbonate/sodium bicarbonate, as the absorbing reagent in a preheater tower. At temperaturesfound in the preheater tower, the sodium salt decomposes to sodium oxide with which the SO 2

can react and form a relatively stable solid. Because the sodium remains in the process, caremust be exercised that an increased concentration of sodium in the clinker does not render theresulting cement unmarketable. If necessary to reduce the alkali content of the clinker, alkali-

rich CKD could be removed as a by-product from the pyroprocess through an alkali-bypass

system. This CKD would require subsequent management including the possibility of disposalin a suitable landfill. Trona is more expensive than hydrated lime but is a more efficient reagent.

Manufactured sodium carbonate and/or sodium bicarbonate may be substituted for trona. To the

degree that this technology also absorbs the AG precursors to D/F formation, HCl and Cl2, thatare present in the flue gas stream, it would tend to suppress the formation of D/F.

Calcium-based internal scrubber. The conditioning tower used for flue gas temperature

control at the discharge of the preheater tower of a preheater or precalciner kiln system initiallycan be installed as or subsequently retrofitted to become an internal dry scrubber using a slurry

of calcium hydroxide as the scrubbing reagent. These systems are effective in the reduction of

SO2 emissions and detached plumes (EnviroCare, 2002). Due to inadequate reagent retentiontime in the conditioning tower, retrofit installations may not be as effective as a properly

designed, new installation. The spent reagent that contains oxidized sulfur as sulfite or sulfate

salts conveniently becomes a raw material constituent and there is no waste to dispose. In anyscrubbing system using abrasive slurry, control of the slurry flow and mechanical maintenance of

the system can be problematic, but the relatively high scrubbing efficiency and the lack of waste

residue are decided benefits for this system. To the degree that this technology also absorbs the

AG precursors to D/F formation, HCl and Cl2, that are present in the flue gas stream, it wouldtend to suppress the formation of D/F.

Pyroprocessing system design. Unique kiln systems of atypical design can take advantageof the inherent properties of the cement manufacturing process to achieve environmental goalsunder site-specific circumstances. These systems often must use additional energy or suffer a

lower production rate than traditional designs but the improved environmental performance and

the avoidance of problems associated with pollution abatement equipment occasionally make

such designs acceptable to an owner. At one plant there is a unique combination of a low-NOXcalciner, a single-stage preheater, and a dryer crusher that exceeded all environmental and

production expectations including very low emissions of SO2. In this case, fuel consumption

with the attendant NOX and CO2 emissions were higher than would have been experienced with atraditional precalciner kiln system (Vu, 2001).

Tailpipe wet scrubber. Based on publicly available permits, tailpipe wet scrubbers usinglimestone as the scrubbing reagent have been installed on four cement kilns in the recent past.

There are no published data detailing the experience to date with these scrubbers. However, thespent reagent (calcium sulfite) must be disposed of or oxidized to calcium sulfate for use in the

finish mill of the cement plant as a set regulator in the cementitious product. There may be a

wastewater stream that requires treatment before discharge. It is expected that the wet scrubber

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would remove NH3 and HCl, if any, from the exhaust gas. Carryover of residual sulfuric acid

mist and of the scrubbing limestone as PM may present environmental compliance problems.

Potential control technologies.

Mixing air fan. Despite the motion of the rotary kiln and the high velocity of the gas inside thekiln, the gas flow in the calcining zone of the kiln is stratified, i.e., experiences laminar flow. Ifthe high-temperature gas in the calcining zone contains sufficient excess oxygen and isthoroughly mixed, SO2 would be oxidized to SO3 that could then react with the abundance of

calcium and alkali metal oxides suspended in the gas stream. The result would be the lowering

of SO2 emissions through the capture of sulfur as a solid sulfate in the clinker or in the CKD that

is expelled from the kiln system. High-pressure air in the range of a 2-10% replacement of thesecondary combustion air can be injected through the shell of the rotary kiln to provide sufficient

oxygen for oxidation of SO2 and the kinetic energy necessary for mixing of the flue gas. This

technology was developed initially to allow for more complete combustion of solid fuelintroduced near the midpoint of the rotary kiln and later was adapted to reduce the generation of

NOX. In a limited number of applications, a mixing air fan attached to a rotary kiln also hasshown simultaneous reductions in the emissions of SO2, CO, and THC. Conceptually, thistechnology could be installed on any of the kiln systems for the primary purpose of SO2 emission

reduction.

In-line raw mill hydrated lime injection. A potential variant of the use of a conditioningtower as a calcium-based internal scrubber is the injection of the calcium hydroxide slurry into

an in-line raw mill. Water frequently is injected into a vertical roller mill to improve operation of

the mill. If this water were replaced with calcium hydroxide slurry, the flue gas and an effectivescrubbing reagent would be intimately mixed within the mill near the optimum reaction

temperature to enhance the inherent scrubbing efficiency of the in-line raw mill. To the degree

that this technology also absorbs the emissions of the AG precursors to D/F formation, HCl andCl2, that are present in the flue gas stream, it would tend to suppress the formation of D/F.

Fabric filter absorption. Older literature suggests that SO2 is removed from cement kiln fluegas in a fabric filter PMCD. This removal seldom has been observed in practice because the

bulk of the material on the filter, i.e., calcium carbonate, is not reactive with SO2 at thetemperature and humidity conditions in the fabric filter. If calcium oxide is present on the filter

medium, e.g., in an alkali-bypass PMCD, a small but measurable reduction in SO2 concentration

in the flue gas may be observed. If a scrubbing reagent, e.g., hydrated lime, were added to theflue gas stream ahead of the PMCD, some adsorption of SO2 might be observed. If the fabric

filter is operated at or below the acid dew point, AGs will be removed from the flue gas.

However, this practice is not acceptable because severe corrosion and ultimate destruction of thePMCD is a certain result.

Sodium-based internal scrubber. This system would be similar in concept to the calcium-based internal scrubber for preheater and precalciner kilns except that a solution of sodium

carbonate/sodium bicarbonate derived from naturally occurring trona or manufactured reagentswould be used in lieu of the calcium hydroxide slurry in the conditioning tower. Trona is more

expensive than hydrated lime but is a more effective reagent. Handling and spraying a solution

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is always easier than handling and spraying slurry. Care must be exercised that the introduction

of sodium into the process does not render the product unmarketable. CKD from an alkalibypass could be wasted to reduce the sodium content of the clinker. The limited use of a sodium

carbonate/sodium bicarbonate solution in the conditioning tower might suffice for control of a

detached plume that is observed only when an in-line raw mill is down. To the degree that this

technology also absorbs the AG precursors to D/F formation, HCl and Cl2, that are present in theflue gas stream, it would tend to suppress the formation of D/F.

Calcium/sodium-based internal scrubber. A possible synergetic combination oftechnologies would be to inject calcium hydroxide slurry into the in-line raw mill during itsoperation and a sodium carbonate/bicarbonate solution into the conditioning tower when the raw

mill was down for maintenance or raw mix inventory control. Some problems with slurry

handling would be eliminated, e.g., wear-sensitive nozzles are not required for calciumhydroxide slurry injection into the in-line raw mill, and the addition of sodium to the process and

clinker is minimized. To the degree that this technology also absorbs the AG precursors to D/F

formation, HCl and Cl2, that are present in the flue gas stream, it would tend to suppress the

formation of D/F.

Oxygen enrichment. Oxygen enrichment through the use of liquid oxygen (LOX) has beenshown to be an effective mechanism for increasing the production of a cement kiln. Because ofthe cost of purchased LOX or its onsite production, the technology has never been economically

justified. Cement kilns are usually fan limited, i.e., the maximum production rate of the kiln is

limited by the amount of flue gas that can be moved by the induced draft fan or fans. Theintroduction of oxygen without the burden of nitrogen from air allows the induced draft fan to

handle the combustion products that result from the combustion of additional fuel that, in turn,

allows more production. The presence of adequate oxygen in the kiln system should reduce SO2

emissions by oxidizing sulfur to a solid sulfate that is retained in the clinker or expelled from thekiln system with the CKD. Generation of CO at the combustion site should be minimized and

easily controlled. The use of LOX in the burning zone of the rotary kiln will have an initial

tendency to increase NOX emissions because of a hotter burning zone and increased oxygen atthe combustion site. However, if the clinkering temperature requirement is unchanged, the NOXemissions should return to their previous level in a relatively short time when process

equilibrium is restored.

Dual-alkali process (soda ash/lime). This is a tailpipe scrubber technology that would usea water solution of sodium sulfite and sodium hydroxide in a spray chamber or packed column at

approximately 120oF to absorb SO2 and AGs in the flue gas stream immediately following the

main or bypass PMCD. The flue gas from a cement pyroprocessing system often would requirecooling before this process could be used. Upon exit from the scrubber, the scrubbing liquid

would be treated with calcium oxide, calcium hydroxide, or calcium carbonate to precipitate

sulfite/sulfate ions and to regenerate sodium hydroxide. The calcium sludge would be removedfrom the liquid using a filter press or other filtration system and must be disposed as a waste in

an appropriate landfill that may or may not be available on site. Water and make-up reagent,

sodium hydroxide or sodium carbonate, would be added to the scrubbing liquid that is

recirculated to the scrubbing device.

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Thermal decomposition (roasting). Smelting of pyritic metal ores is accomplished byroasting them at 600800

oC (11101470

oF) to oxidize sulfide sulfur to SO2. This same

phenomenon occurs in the upper stages of a preheater tower to generate SO2 from cement raw

materials containing small amounts of reduced sulfur. At considerable economic and energycosts, cement raw materials could be roasted outside the normal pyroprocess to liberate the SO2.

The liberated SO2 would have to be captured using one of the tailpipe technologies described inthis paper. The independent roasting of the raw materials would have the advantage of reducingthe volume of flue gas requiring treatment to remove SO2. If present, hydrocarbons also would

be liberated and if combustion conditions were inadequate, CO could be generated. Because of

the independent thermal process, emissions of CO2 and NOX would increase beyond thoseexperienced by an integrated cement pyroprocess in which the SO2 was generated.

Tailpipe dry scrubber. In this technology, a traditional, stand-alone, tailpipe dry scrubber(also known as a spray dryer absorber) would be installed after the main or alkali-bypass PMCDthrough which the flue gas is passed. A limestone or calcium hydroxide slurry would be used as

the scrubbing reagent. Typically, the temperature of the flue gas at the exit of the fabric filter is

too low to accomplish drying of the reagent slurry prior to its collection in a fabric filter.Consequently, the flue gas must be reheated with a clean fuel, e.g., natural gas, to accomplish the

necessary drying of the reagent. In addition to the production of increased CO2 emissions from

the added fuel, there is likely to be an increase in other pollutants, i.e., NO X and CO, resultingfrom reheating the gas and a need to dispose of spent reagent removed from the fabric filter. In

some applications of a spray dryer absorber, activated carbon has been injected upstream of the

absorber to control the emissions of D/F (Richards, 2003).

Cement kiln dust tailpipe scrubber. There has been a full-scale pilot plant installation ofthis technology. Flue gas from a wet process kiln was passed through a slurry of CKD, the SO2

absorbing reagent, in a complicated and prohibitively expensive contact device. Reductions in

SO2

emissions on the order of 95% were obtained. Crystalline potassium sulfate, a fertilizerconstituent, was to have been extracted (leached) from the CKD slurry in commercial quantities.

The leached CKD would have been returned to the kiln in slurry form as a reduced-potassium

raw material. Problems associated with the crystallization of potassium sulfate were not solved.The expensive contact device and the crystallization problems contributed to the commercial

failure of the process. (Young, 2003) This scrubber also would have removed THC, AG, and

NH3 from the exit gas, and the potential for formation of a detached plume would have been

reduced. This technology would be best suited for wet process kiln systems.

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Control of Nitrogen Oxides

Table 4. Nitrogen Oxides Control Technologies.

Potential effectsControl technologies

Synergetic Counteractive

Existing control technologies

Oxygen control (decrease) CO2 SO2, CO, product color and quality

Indirect firing CO2 PM

Low-NOX burner Burner/application specific

Mid-kiln firing Application specific

Process improvements Project specific

Process control improvements Project specific

Low-NOX calciner COStaged combustion CO

Semi-direct firing PM

Mixing air fan SO2, CO, THC

Cement kiln dust insufflation CO, CO2, SO2

Biosolids injection CO, NH3, detached plume, metals

Potential control technologies

Fuel substitution Fuel/process specific

Raw material substitution Material specific

Selective noncatalytic reduction NH3, detached plume

Modified direct firing PM

LoTOX scrubber Water discharges, ozone slip

Oxygen enrichment SO2, CO

Flue gas recirculation CO, SO2

Selective catalytic reductionNH3, CO2, detached plume, solid

catalyst wastes

Tri-NOX Multi-Chem wet scrubber SO2, AG Water discharges

Water/steam injection CO, CO2

Catalytic filtration (ceramic filter)

Non-thermal plasma SO2, THC, D/F

Existing control technologies.

Oxygen control (decrease). For control of NOX originating at the high temperaturecombustion site in a rotary kiln, a decrease in oxygen (excess air) in the burning zone tends to

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minimize the generation of thermal and fuel NOX. Both of these mechanisms of NOX formation

are oxygen dependent. The reduction in excess air reduces the strength of the oxidizingconditions in the rotary kiln and usually causes an increase in SO2 generated from the fuel used

in the main flame. If there is flame impingement on the material in the kiln, i.e., localized

reducing conditions, the decrease in oxygen concentration may further exacerbate SO2

generation. The reduction of oxygen concentration at the primary combustion sites in aprecalciner kiln system also will tend to reduce the generation of fuel NOX. The decrease in

oxygen concentration at combustion sites may cause or increase the generation of CO. There is a

small energy benefit for reducing excess air in the system that will result in a slight decrease infuel consumption and CO2 emissions. The production of cement clinker under generalized or

localized reducing conditions usually has deleterious effects on product color and performance.

Indirect firing. There are two basic mill systems for preparing and burning solid fossil fuel incement kilns. The first of these is called a direct-fired system in which hot air from the clinker

cooler is used to dry the coal while it is being ground in the mill and to pneumatically transport

the powdered coal to the combustion site for immediate combustion. About 20-25% of the air

required for combustion is supplied through the coal mill and burner pipe. This air is calledprimary air. In the second or indirect-fired system, the powdered coal is separated from the

drying and transport air by cyclones and/or fabric filters, and is stored in a bin or silo prior to

being metered and pneumatically transported by ambient air to the combustion site. In anindirect-fired system, the primary air can be reduced to about 10-15% of the total combustion air.

Since the generation of both thermal and fuel NOX is oxygen dependent, the reduction inavailable oxygen in the flame envelope in an indirect-fired coal system results in a reduction in

NOX generation. Indirect coal firing systems generally result in improved thermal efficiency for

the kiln system and a reduction in CO2 emissions. Because the solid fossil fuel is separated fromthe transport air in PMCDs that exhaust to the atmosphere, there is an increase in PM emissions

from an indirect-fired system when compared to a direct-fired system with no vent to the

atmosphere. A variant of the indirect-fired system uses oxygen-deficient flue gas to dry andtransport the coal to the PMCD that separates the powdered coal from the flue gas. The oxygen-

deficient flue gas is used to inert the coal mill atmosphere for safer coal grinding and has no

effect on NOX generation because it is not used as primary air.

Low-NOX burner. Several vendors provide adjustable burners with proprietary designs thatare intended to reduce NOX generation through the mixing scheme for fuel and primary air by

reducing flame temperature, altering turbulence in the flame, and establishing oxygen-deficient

recirculation zones in the flame. While these burners have the capacity to alter the flame

velocity and shape for process purposes, they have met with mixed success in the documentedcontrol of NOX generation. Most often, these burners are installed with an indirect-fired coal

system as part of a retrofit project for a direct-fired system or as part of a new pyroprocessing

system. At the conclusion of a successful project, it is impossible to sort out the relativecontributions of the low-NOX burner and the indirect-fired coal system to the reduction of NOX

generation.

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Mid-kiln firing. Although it could be applied to preheater kiln systems3, mid-kiln firingprimarily provides for staged combustion in wet or long-dry kiln systems. Once per revolution

of the kiln, a single charge of fuel is introduced into the calcining zone through the kiln shell

using a gated feed device. As the carbonaceous fuel charge burns in the low oxygenenvironment of the calcining zone, free radicals are generated that in turn chemically reduce the

NOX that was generated in the burning zone to molecular nitrogen (Eddings, 2003). With mid-kiln firing, there is a risk of increased CO emissions unless there is sufficient residual oxygen,temperature, and mixing downstream of the secondary combustion point in the flue gas flow

path. It was for this reason that the mixing-air fan technology was developed. The fuels most

often used for this purpose are worn or rejected whole passenger car tires and small containers ofcombustible hazardous waste. If cost-effective, any solid fuel could be used for mid-kiln firing.

If the solid fuel used at mid-kiln has a lower sulfur concentration than the fuel it replaces, there is

the potential for reduced SO2 emissions. The intimate mixing of a sulfur-containing fuel andcalcining raw mix at the mid-kiln location also may provide an enhanced opportunity for

absorption of SO2.

Process improvements. Because of the reduced consumption of fuel per unit of production,almost any improvement in an existing kiln system that improves the thermal efficiency of theprocess will be accompanied by a reduction in long-term NOX emissions per ton of clinker. To

help justify the associated capital expenditures, many process improvement projects also result inincreased production that tends to negate decreases in overall NOX emissions. When compared

to a steam boiler or power plant, the high variability of NOX emissions from a properly operated

cement kiln is well known. Process improvements that serve to make the kiln operation morestable without necessarily increasing production also accomplish NOX reductions over the short

term, the long term, and per ton of clinker. Any process improvement project should be

evaluated for its effect on the generation and emission of all pollutants.

Process control improvements. Process control improvements are characterized by theinstallation of new or better instrumentation and/or process control systems. In older kiln

systems, the improvement might mean replacing an analog process control system with a digital

computer. In newer kiln systems with an adequate digital computer, the use of one of the expertor fuzzy logic control systems and the necessary process instrumentation could represent a

significant process control improvement. In essence, the expert systems are satellite computers

that guide the process computer in controlling the kiln system. They are able to detect subtle

changes in the process and to take corrective action more rapidly than the central control roomoperator. The common purposes of most process control improvements on kiln systems are to

improve thermal efficiency and the clinker production rate. However, if a process control

improvement project simply results in a more stable pyroprocessing system, lower NOXemissions over the short term, the long term, and per ton of clinker will result. Any process

control improvement project should be evaluated for its overall effect on the process and on the

generation and emission of all pollutants.

3 The initial purpose of mid-kiln firing was to burn fuel at the point in the process with the maximum requirement

for thermal energy input, i.e., calcination of the calcareous component. In the case of many preheater kiln systemsand all precalciner kiln systems, the majority of this energy requirement has been satisfied prior to the rotary kiln in

the material flow path. In addition, the temperature of the flue gas inside the rotary kiln at the mid-kiln firing access

point for precalciner kiln systems is too hot for the fuel injection apparatus to survive.

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Low-NOXcalciner. All vendors of cement pyroprocessing systems offer proprietary calcinerdesigns that carefully control the mixing sequence of fuel, air, and raw materials in the calciner

vessel. The common feature of all these systems is an oxygen-deficient initial combustion zonein which free radicals are generated that subsequently react with NOX to form molecular nitrogen

and other reaction products (Miller and Bowman, 1989). At the operating conditions found inthe calciner, CO also is formed. The initial combustion zone is followed in the gas flow path by asecondary combustion zone in which residual CO in the flue gas is oxidized to CO2. These

designs have worked quite well in new installations. When applied to calciner replacement or

retrofit projects, physical restraints often limit the ability of the vendor to install a fullyfunctional low-NOX calciner. The result may be an unpredictable and disappointing reduction in

NOX generation in the calciner. In all these designs, there is the possibility of increased

emissions of residual CO if the downstream oxidation is not complete.

Staged combustion. The function of staged combustion (sometimes called secondary firing)in preheater or precalciner kiln systems is to develop a reducing zone in the flue gas flow path

after the burning zone in which free radicals produced during staged combustion of hydrocarbonfuels react with NOX from the burning zone to form molecular nitrogen and other reaction

products. The most prevalent location for staged combustion is in the riser duct between the

discharge of the rotary kiln and the calciner vessel. Typically, natural gas is the most convenientfuel but pulverized (powdered) coal, used oil, and waste-derived fuels also are used for this

purpose. Worn or rejected whole passenger car tires also are inserted into the feed end of the

rotary kiln to accomplish NOX reduction through staged combustion. In preheater kiln systems,there is a high probability of increased CO emissions unless there is a source of air (oxygen) to

oxidize residual CO downstream of the staged combustion site in the flue gas flow path. In

precalciner kiln systems, the residual CO from staged combustion has an opportunity to be

burned in the calciner vessel so the probability of increased CO emissions resulting from stagedcombustion is reduced but not necessarily eliminated.

Semi-direct firing. In one way or another, semi-direct firing systems mechanically separatethe powdered coal in the coal mill exhaust from the coal drying and transport air to provide forbetter combustion control through independent metering of the coal be