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Back to Library Gossman Consulting, Inc

Petroleum and Petrochemical Waste Reuse in Cement

Kilns

ByDavid Gossman

Gossman Consulting, Inc.

Portions Copyright© Gossman Consulting, Inc., 1989, 1990

Published in Environmental Progress (vol. 11, No. 1) February,1992

Introduction

The high temperatures and long residence times in the combustion zones of cement kilns

have for over fifteen years been used to burn flammable liquid wastes, such as solvents,as fuels. Increasing government regulation and control of this technology has actually

resulted in expanded use as both waste generators and cement manufacturers have grownmore comfortable and confident of this technology. Liquid petroleum and petrochemical

wastes have been a part of this liquid fuel stream from its inception. Solid and sludgy

petroleum and petrochemical wastes present greater handling difficulties. Nevertheless,the desirability of not landfilling many of these wastes has resulted in the motivation to

develop solid and sludge handling processing technologies to allow their use as fuel.

These processing options extend back to the point of generation. Changes in filter press

media and drying technologies are allowing certain petroleum and petrochemical wastesto be pneumatically introduced into cement kilns. Quality control technologies, including

laboratories at both cement kilns and the generating/processing location, have also been acritical part of these developments. The cost effectiveness of this option coupled with thesignificant, positive environmental impacts provides a significant opportunity in

developing countries for infrastructure improvement in a way that maximizes sustainable

development.

The integration of two seemingly diverse technologies, management of wastes and production of cement, appears to be having a profound effect on both industries:

• Study after study has warned of severe consequences of not providing for a

nation's capability for waste management. Yet today, many types of wastes are in

significant demand for use as supplemental fuel, in those countries that have promoted this technology.

• Cement manufacturing plants can evaluate different sources of new revenue for

their facilities by providing waste management services. Usually, each new source

of revenue also helps lower plant fuel or raw material costs in the never endingeffort to remain competitive in a well-established and mature market.

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Review of the Cement Manufacturing Process and Chemistry

The process of using large rotary kilns for manufacturing cement is generally understood

by the waste management industry because of the large quantity of liquid waste fuelscurrently burned in 25 cement plants in the U.S. as well as others in Europe and

elsewhere. Nevertheless, a review of the cement manufacturing process and chemistry iswarranted, since critical characteristics are significantly different from incinerator

technology.

Figure 1 is a schematic of a basic straight rotary cement kiln. Raw materials such as

limestone, shale, clay, sand, fly ash, etc. are fed, either wet or dry, in specific proportions

into the back end of the kiln. This material travels toward the front end of the kiln as thekiln turns. Initially, these raw materials give off water vapor (dehydration) and then give

off CO2 (calcination). Finally, in the hottest end of the kiln, the final chemical reactions

occur and the material falls out of the kiln into a cooler where it is quenched.

Figure 1

Figure 2 is a schematic of the more complex preheater type of cement kiln. In this

system, the dehydration step occurs in the preheater cyclones which offer much better heat exchange efficiencies than the chain section of a straight kiln. A flash furnace may

also be present. A flash furnace, or precalciner as it is also called, allows a good portion

of the calcination reaction to occur prior to material entering the kiln and also allows upto 70% of the fuel to be used in the flash furnace rather than at the hot end of the kiln.

Although the use of a flash furnace does not save energy, it does allow higher production

rates since more materials can travel through a kiln which has a smaller volume of hot



gases flowing through it. In both designs, the gas and material flow is counter current and

exit gas temperatures from the process are generally quite low, 150C to 300C.

Figure 2

There are four major components that make up the clinker that exits the kiln. They areshown with their short hand notations in Figure 3. The various types of Portland Cementgenerally require different proportions of these four major components. This is largely

done by controlling the proportions of raw materials entering the kiln. Figure 4 further

illustrates the chemical reactions which occur during the process of forming portlandcement clinker. The presence of oxidizing conditions during these clinker forming

reaction steps is critical to the production of portland cement.

Major Components of Portland Cement Clinker

• Tricalcium Silicate: 3CaO SiO2 (Alite, C3S)

• Dicalcium Silicate: 2CaO SiO2 (Belite, C2S)• Tricalcium Aluminate: 3CaO Al2O3 (C3A)

• Tetracalcium Aluminoferrite: 4CaO Al2O 3 Fe2O3 (C4AF)

Figure 3

Major Steps in Clinkering1. Decarbonation of Calcite (Calcination)

CaCO3 ---->CaO + CO2 @900C



(Highly Endothermic)

2. Rapid neutralization of free lime (exothermic)

3CaO + Al2O3 ---->C3A || Melt (>1230oC)

2CaO + Fe2O3 ---->C2F|

2CaO + SiO2 ---->C2S (Belite)3. Formation of alite (slow reaction)

CaO + C2S ---->C3S (Alite)

(>1200C)4. Quenching (Cooling)

Figure 4

The Hazardous Waste Incinerator

Figure 5 provides a very generalized schematic flow diagram of a rotary kiln incinerator.

Features of this process, which are critically different form cement kilns and can impact

emissions, are noted below.

Figure 5

Incinerator



There are no chemical reactions in an incinerator other than those induced directly by

incineration (ie. oxidation). Therefore, gases exiting the incinerator directly reflect what

is happening in the combustion zone of the kiln.

In an incinerator, the gases are moving the same direction as solids and liquids. This

tends to drive any phase equilibrium to the gaseous side of the equation thus increasingemissions such as metals. Vapor pressure becomes the critical factor.

The thermal capacity and therefore thermal stability of an incinerator is relatively low.Process upsets can occur within a matter of minutes or even seconds that can allow

uncombusted organics to escape from the process. The use of an afterburner is largely

prompted by this potential.

Cement Kiln

In a cement kiln, the dehydration and calcination steps produce large quantities of gases

that largely mask gaseous emissions from the combustion zone of the cement kiln. Nearlyhalf the mass of raw materials that enter a wet process cement kiln leave the kiln as gases,

primarily water vapor and carbon dioxide, yet they are not combustion by-products.Hydrocarbons found in the raw materials may also be released which are unrelated to fuel

combustion.

In a cement kiln, the counter current flow design tends to entrain in the cement clinker

via the development of recirculating loads. Only if metals reach the cooler chain section prior to condensation are they likely to be enriched in the kiln emissions and even then

primarily as a particulate because of the relatively low exit temperatures.

In a cement kiln, at any given moment there is typically greater than 1,000 times as muchsolids undergoing chemical reactions at 1200C as there is waste fuel being combusted.This provides enormous thermal stability in the kiln. A cement kiln upset can take hours

and generally reflects a small decrease in production capacity rather than any changes

which might affect combustion. The only critical short term factors in maintainingcomplete combustion in a cement kiln while clinker is produced are excess oxygen and

correct draft fan operation.

Table 1 provides a comparative look at combustion zone conditions in a cement kiln

versus an incinerator. Those factors with the most significant difference, gas temperaturesand retention times, play an important role in insuring the destruction of hazardous

wastes.

Typical Combustion Zone Conditions in Cement Kilns vs. Industrial Waste Incinerators

Parameter Typical CementKiln

Typical Industrial WasteIncinerator

Maximum Gas Temperatures >2200C1 <=1480C



Maximum Solid Temperatures 1420-1480C <=1370C

Gas Retention Times at>=2000oF

6-10 Seconds 0-3 Seconds

Solid Retention Times at

>=2000

o

F

0-30 Minutes 2-20 Minutes

Oxidizing Conditions Yes Yes

Turbulence (Reynolds' number) >100,000 >10,000

1Peray, Kurt E., The Rotary Cement Kiln, 1986.

Table 1

Waste Processing Options

The use of cement kiln technology for using a wide variety of wastes is only beginning to

be tapped. It is theoretically possible to produce quality clinker from 100% raw material

and fuel substitution. Some kilns already substitute 100% of their fuel requirements fromwastes. Figure 6 illustrates that liquid wastes are only a small portion of the wastes thatmight be eventually used in cement kilns.

Wastes suitable for Treatment in a Cement Kiln

High Solids

(more

likely

bulk)

Inorganic Solids

Suitable for

blending into

raw feed.

Organic

Contaminated

Solids and Sludges

such as contaminated

soils or filter cake.

Requires some form

of thermal separation

or direct feed for

preheater.

Grindable Solid

HWF

such as Aluminum

Potliner

Sludges

(more

likely

drums)

Inorganic Liquids

and Sludges

Suitable for blendinginto

Wet Process slurries.

Sludges

Difficult to handle.

Can be blended intoliquids or otherwise

processed.



.

Figure 7.Processing Options

Process Description Comments

1. Mixing Mixing of different wastes or wastetypes is performed generally to

provide a more uniform feed and

This very common form of processing is currently used with

liquid wastes and for getting



meet specifications. solids into liquids. It has broadapplication to almost any waste

category.

2. Neutralization The neutralization of acid or caustic

waste with other wastes or newmaterials is sometimes required

prior to further processing.

This is particularly applicable for

certain aqueous inorganic wastestreams, but may have application

in any category.

3. Drying Drying to remove moisture may be

needed for certain solid waste

This process may have particular

application for dry process cementkilns.

4. ParticleSizing

Particle sizing is frequently neededto handle and use a variety of waste

streams. Both grinders and mills

may be used along with separators

to produce the desired particle size.

Particle sizing has been and willcontinue to be needed for a wide

cross-section of waste streams

prior to used as fuel and may be

needed for certain inorganicwastes.

5. Thermal

Separation

or Pyrolysis

Thermal separation or pyrolysis istypically used to separate the

volatile and semivolatile organics

from an inorganic matrix.

Contaminated soil is a goodexample.

This process may prove to bevaluable for cement kiln resource

recovery projects since it would

allow the organic fraction to be

placed in the hot end of the kilnand the inorganic fraction in the

cold end.

6. Pelletization Pelletization is used to producemore or less uniformly sized pellets

from the sludges and solids.

It may be possible to use pelletization for processing

sludges and solids to provide auniformly sized pellet that could

then be injected into the hot end

of a cement kiln.

Table 2

The Effects of Using Waste Fuels on the Cement Manufacturing Process

The use of waste fuels in cement kilns has the potential for impacting the cementmanufacturing process in four different areas: kiln control and operation, cement clinker

(product) quality, cement kiln dust (byproduct) quality, and stack emissions. Both

positive and negative effects have been noted in these categories while using waste fuels.

Appropriate levels of quality control on waste fuels can prevent significant negativeimpacts and enhance positive effects on the process.

Kiln control and operation is generally enhanced when using even small quantities of

waste fuels because the high level of volatiles stabilizes and aids combustion, particularly



while simultaneously using low grade coals or coke. Some operators have found that they

can use a less expensive, lower grade coal or larger quantities of coke while using waste

fuels. Higher substitution levels for coal, in excess of 20-25%, may require someretraining of kiln operators and adjustments to atomizing air in order to prevent

overheating the front end of the kiln. On the other hand, excessive chlorine levels in

waste fuels and/or a lack of compensating adjustments in kiln operations can result inlarge recirculating loads of alkali chlorides. In a straight kiln, this can result in rings

which reduce production. In a preheater or precalciner, complete plug-ups have occurred.

Excessive fluxing of the mix has also occurred resulting in bad product and ruined clinker cooler grates. Under the most extreme circumstances, very high chlorine levels have

stripped the coating and brick from the hot end of a kiln. Other problems in kiln

operations have occurred with excessive fluorine or phosphorous levels. Fuel that has

been allowed to stand unagitated has occasionally phase separated into layers withdifferent physiochemical properties causing severe kiln upsets when used. For these

reasons, quality control and proper storage of the waste is a critical part of the process.

Cement clinker, which is the product of the kiln, is frequently enhanced when usingwaste fuels. Chemically, cement clinker may exhibit lower alkali caused by the presence

of chlorine in the waste fuel. A number of plants have completely eliminated the need to

purchase and add calcium chloride to produce this low alkali cement. Some operators

have also observed that subtle changes in kiln temperature profiles produced while usingwaste fuels enhance the quenching of cement clinker, producing a product with both

better long term strengths and characteristics which allow for easier grinding. The

greatest potential negative impact on clinker quality comes from the ash in waste fuels.Generally, ash levels are lower in waste fuels than in coal. This may require minor

changes in raw mix ratios, particularly at higher substitution rates of waste fuels. Most of

the time, waste fuel ash is chemically very similar to coal ash and therefore does not have

any negative impact. Excessive levels of lead and/or zinc could, however, reduce cementstrengths if specifications and quality control are inadequate. Excessive levels of

chromium, which can also be present in waste fuel could also negatively impact the

safety characteristics of mortar cement.

Cement kiln dust is a byproduct of the cement manufacturing process generated by most

cement plants. Because kiln dust is primarily generated in the cool end of the kiln or

preheater tower by the interaction of the ground raw materials with the high velocity gasstream exiting the system, there is very little effect from the combustion of waste fuel in

the hot end of the kiln. Minor increases in certain volatile heavy metals such as lead and

cadmium have been observed while using waste fuel. These increases are not generally

beyond typical levels found in CKD, nor will they leach at levels in excess of establishedleachate limits so long as adequate specifications and quality control are provided for

waste fuels.

Extensive research by the U.S. EPA on the use of waste fuels in cement kilns and the

associated impact on stack gas emissions reveals little, if any, negative impact. Generallyspeaking, waste fuel burns cleaner than coal in a cement kiln and can reduce S0x

emissions. Trace level increases in lead emissions have been observed while using waste



fuel, but these increases are well below the levels which might indicate health and safety

concerns. The only other impact that has been observed involves the use of waste fuel in

kilns with older marginal electrostatic precipitators (ESPs). In these situations, chlorinelevels in the waste fuel must be controlled to prevent the formation of very fine

particulate chloride salts which can be more difficult for these older ESPs to capture.

Conclusion

With adequate specifications and quality control, the use of waste fuel in cement kilnscan have significant positive impacts in product quality, operations and the environment.

In contrast, incinerators have greater negative impacts on the environment because they

represent a new source of emissions that need not exist if cement kilns can burn thewaste. Existing cement kilns burning waste fuels have consistently shown compliance

with air quality standards. The development and implementation of material handling and

processing technologies is significantly extending the use of solids and sludges as waste

fuel in cement kilns.

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