Memorandum

TO: Rulemaking Docket

FROM: Keith Barnett, US EPA, Environmental Engineer

SUBJECT: Combustion in a Cement Kiln and Cement Kilns’ Use of Tires as Fuel

DATE: April 25, 2011

A cement kiln that combusts any non-hazardous solid waste is subject to regulation as a 

Commercial or Industrial Solid Waste Incineration (CISWI) unit pursuant to section 129 (g) (1) 

of the Clean Air Act. In order for a cement kiln to be classified as a CISWI unit, it must have an 

input that is a non-hazardous solid waste, and the cement kiln must “combust” the solid waste. 

EPA has recently promulgated a definition of non-hazardous secondary materials which are solid 

wastes. See 76 FR 15456 (March 21, 2011).

This memorandum describes the Portland cement production process and discusses 

whether certain secondary materials used in that process are combusted in the kiln. This 

memorandum also addresses the question of whether tires burned by certain cement kilns in their 

performance testing would have been defined as solid wastes under the recently-promulgated 

definition of non-hazardous secondary materials that are solid wastes had that definition applied 

at the time of the burning.

Since combustion is not defined in the CAA, we use a common definition of combust 

which is “an act or instance of burning” or “a chemical process (as an oxidation) accompanied by

the evolution of light and heat”. 1

Basic Kiln Process

In a cement kiln, there are two types of inputs, fuels and ingredients. Fuels provide the 

energy necessary to produce the heat required to raise the temperature of the ingredients to the 

level required for clinker formation. Ingredients provide the materials that make up the actual

clinker mass. The ingredients are also called kiln feed or raw meal. 

Figure 1 presents a schematic of a typical long wet or dry cement kiln. Ingredients are 

introduced into the back or cold end of the kiln at 300 to 500  ̊F (cold here being a relative

term).

2

 The materials gradually move down the kiln over a period of 60-90 minutes and increase 

in temperature until they reach the temperature required for clinker formation (about 2600 to 

2700  ̊F). The ingredients undergo several different reactions as the temperature increases. It is 

important that the mix move slowly enough to allow each reaction to be completed at the 

 

1 Webster’s Ninth New Colleg1ate Dictionary. Merriam-Webster Inc. 1990

2

Environmental Progress (vol. 11, No. 1) February,1992, Petroleum and Petrochemical Waste 

Reuse in Cement Kilns, David Gossman
appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is 

difficult to heat the mix up to a higher temperature until a given reaction is complete.3

Figure 1. Schematic of a long cement kiln

In contrast to ingredients, fuels are introduced into the hot sections of the kiln (either the 

front end or mid kiln) where gas temperatures are 1800 to 4000  ̊F. At these high temperatures 

the fuels immediately burn when introduced into the kiln. Fuels are never introduced into the

cold end of the kiln. If they were, they would slowly heat resulting in creation of significant 

amounts of carbon monoxide emissions and a loss of fuel heating value. 

The process in a preheater or preheat precalciner kiln is similar to a long kiln. 

Ingredients (feed) enter the top of the preheater tower, which is the equivalent of the cold end of 

a long kiln (see further discussion below).

3

The raw meal passes down the tower while hot gases 

rise up, gradually heating the raw meal. By the time the time the materials have reached the 

bottom of the preheater tower the ingredients have partially calcined.

3 A preheater tower is 

likely to have 4-6 stages. 

 

3 Northwestern University. The Science of Concrete. http://iti.northwestern.edu/cement/
Fuels enter a preheater kiln at the front end of the kiln and also may be introduced at the 

bottom of the preheater tower. In the case of the precalciner kiln, fuel is introduced into a 

separate vessel typically located between the last two stages of the preheater tower. However, as 

is the case with a long kiln, the fuels are introduced into high temperature zones where 

combustion of the fuels occurs.

Figure 2. Preheater or Preheater/Precalciner kiln

Combustion does not occur in the cold end of the cement kiln. As explained above, 

materials4placed in the cold end of the kiln are heated gradually until they reach the temperature 

where clinker formation takes place (as opposed to the hot zone of a cement kiln where higher 

temperatures are applied as quickly as possible, not gradually). This is not a chemical process 

marked by the evolution of light and heat. Rather, it is analogous to cooking as opposed to 

burning. 

In a letter dated April 8, 2011, the Portland Cement Association (PCA) questioned

whether the cold end of a preheater/precalciner kiln engages in combustion when ingredients are 

added at that point. The top of the preheater tower in a preheater/precalciner kiln functions 

identically to the cold end of a long kiln. Ingredients are gradually heated – cooked rather than 

burned. Again, this is not a chemical process marked by the evolution of light and heat. As 

such, the top of the preheater tower equates to the cold end of a long kiln and the ingredients are 

not combusted in those areas of the kiln.

 

4 We note again that fuels are never placed in the cold end of cement kilns (including preheaters).
PCA’s April 8 letter also raised a related question concerning ingredients added at 

different levels of the preheater tower, rather than the top of the tower. (See April 8 letter p. 3 

asking whether ingredients placed in “preheater towers that use the heat produced by the kiln to 

preheat the ingredients as they move through the various stages of the tower” would be 

combusted.) We believe that until the material reaches a high temperature zone of the kiln, 

which is the zone where fuel is being added, combustion does not occur for the reasons just 

given: ingredients are being heated gradually through a controlled process For example, some 

long kilns feed tire fuel at a mid-kiln location, so combustion occurs at mid-kiln. However, the 

colder parts of the kiln represent areas where materials are still being gradually heated. In 

preheater/precalciner kilns, this would also apply to the various level of the preheater tower until 

the materials reached a point where fuel is being introduced, either into a precalciner or the feed 

shelf of the rotary part of the preheater kiln. 

Ingredients processed in high temperature areas of the kiln

Cement kilns process many secondary materials as ingredients, and almost always do so 

by introducing these materials into the cold end of the cement kiln (where combustion of 

ingredients does not occur, as explained above). In its April 8 letter, PCA notes that ingredients 

are sometimes introduced into the hot (fuel) end of the kiln, and asks whether those materials are 

combusted within the meaning of CAA section 129 (g). 

The high temperature regions of cement kilns can engage in combustion, as when fuels 

are burned. However, the secondary material ingredients used by cement kilns would not be 

combusted, as explained below. 

Two types of materials are removed from the kiln exhaust gases, typically by the air 

pollution control devices for particulates, and returned to the process as ingredients. The first is 

incompletely calcined material that is recycled back into the kiln as a normal part of the clinker 

manufacturing process. The second is other fine-grained, solid material removed from the system 

to allow the clinker to meet specific quality standards or to maintain process stabilization. See 

PCA Letter of April 8, 2011, p. 3. These materials are often referred to as cement kiln dust.

The PCA letter noted that both of these materials may be reintroduced to the kilns in 

various places, depending on the kiln design and process. However, in this letter the PCA noted 

that in the case of preheater, preheater/precalciner, and long dry kilns that CKD is mixed with the 

raw meal feed and enter the cold section of the kiln. Therefore, these materials would not be 

considered to be combusted for the reason stated previously.

However, as noted in the April 8 PCA letter, in the case of wet kilns, cement kiln dust 

may be added to higher temperature zone regions (mid-kiln and the hot end). Cement kiln dust 

in this case mainly consists of a material that has previously been heated and even partially 

calcined. Therefore, this material would be expected to be inert from the standpoint of 

combustion, i.e., it would not oxidize producing heat and light. In fact, cement kiln dust injected 

in hot end of the kiln within or in close proximity to the flame has the effect of cooling the 
flame.5

This demonstrates the fact that these materials do not create heat and thus do not 

combust.

In its April 8 letter, PCA states that the only ingredients presently placed in kilns’ 

combustion zones (occasionally) are the cement kiln dusts. However, PCA raises the possibility

of other hypothetical ingredients being added to a cement kiln’s combustion zone. It is not 

necessary to address hypothetical possibilities here. In addition, we believe the potential for 

placing non-inert ingredients into kiln combustion zones is unlikely for two reasons. First, the 

raw materials must be chemically homogeneous.

6

Since ingredients must be combined and 

ground together in the raw mill and thoroughly mixed, feeding separate ingredients into other 

areas of the kiln would mean less through mixing of the ingredients. In addition, for the reason 

discussed on page 3 (necessity to heat ingredients gradually), introducing raw materials into a 

hot section of the kiln would be expected to reduce kiln thermal efficiency. It is also worth 

noting that non-hazardous secondary materials used as an ingredient in a combustion unit are not 

solid wastes, 40 CFR section 241.3 (b), assuming the legitimacy criteria in section 241.3 (d) are 

satisfied. 

. 

Tires Used as Fuel

As part of the development of the CISWI rule, we requested additional information on 

the tire-derived fuels used by all cement kilns in the proposed CISWI rule data base based on 

their historic practices. This included over 30 cement kilns that previously reported using some 

type of tire derived fuels and included all the larger cement companies.7Specifically, the ICR 

asked these questions regarding tires combusted during the emissions tests for the kilns: a) what 

is the source of the whole tires the kiln combusted; b) did they come from tire piles or landfills, 

or from an established tire program, defined in the ICR as “one which harvests tires from 

vehicles and businesses, and then manages the tires carefully so they are not thrown away

between collection and eventual use as fuel, for example by use of a tracking system”, and c) if 

the kiln received tires from sources other than an established program, did they undergo 

processing to produce a tire derived fuel? 8

In general, virtually all of the respondents indicated that they had obtained all of the tires 

they burned from established tire programs (as defined in the ICR). However, the kilns 

acknowledged that they could not account for the source of every tire provided by these 

programs. PCA, in its April 8 letter likewise indicated that “[s]ome companies obtain tires from 

brokers and do not know the source of all tires” (April 8 Letter, n. 5). Some of the ICR 

 

5

PCA R&D Serial No. 2728a, A Qualitative Examination of the Control

of Major Gaseous Pollutants Generated in Portland Cement Kilns. Walter L. Greer and Garth J. 

Hawkins, Portland Cement Association , 2004

6

See http://en.wikipedia.org/wiki/Portland_cement

7

See CISWI ICR Follow Up Request for Information – (ICR No. 2286.01; OMB Control No. 

2060-0616; EPA Form No. 5900-122). January 28, 2011. 

8

 The ICR also asked if the kiln had been burning on-specification or off-specification used oil, 

and whether tires that had been physically landfilled were too damaged or otherwise 

contaminated to be suitable for burning by a cement kiln.
respondents further indicated that they knew that these established programs occasionally (for 

example, once a year, or several days a year) would obtain tires from sources other than 

commercial sources, tire dealerships and other standard collection points. Examples mentioned 

in the responses are the annual tire ‘amnesty day’ or other cleanup programs whereby established 

programs (or in one instance, the kiln itself) accepts tires from individuals.

It is EPA’s position that ultimate users are not responsible for knowing the source of all 

tires obtained from an established tire collection program. The certification required by 40 CFR 

section 60. 2175 (w) requires a non-waste tire user to certify that tires were obtained from an 

established tire collection program, that the tires are not discarded and are handled as valuable 

commodities from the point of removal through arrival at the burning facility. EPA does not 

interpret this language as requiring knowledge of each individual tire as this is a practical 

impossibility. In certifying, users also should not assume that tires from established programs 

which participate in occasional cleanup day are discarded – both because there is no information 

that the tires from the cleanup efforts were discarded (and these programs are designed to 

prevent discarding) and whether the kiln received tires from the sporadic cleanup days in any 

case. Rather, EPA interprets the certification requirement to be satisfied if the user deals with an 

established tire collection program (as defined in Part 241) which program can provide the user 

with reasonable assurance that it manages tires carefully from point of collection to point of 

burning and which does not receive tires which have been abandoned in landfills or otherwise

abandoned. Virtually all of the respondents to the ICR stated that they dealt with such 

established tire collection programs, and provided information reasonably supporting that 

conclusion (or otherwise provided sufficient information from which EPA determined that the 

tires came from established tire management programs). In those instances where the kilns 

indicated that some of the tires they received from an established tire program had been 

discarded or suspected that that was the case, EPA counted the kiln as a CISWI (had the solid 

waste definition applied at the time of the performance test) in this analysis.

The responses are summarized (by quotation or near literal paraphrase) in the following 

table:

Company Plant 

Location

Type of 

Tire Fuel

Tire Sources and Processing

Ash Grove 

Cement

Inkom Whole 

Tires

 Receives tires from ‘four county

landfills’ serving as staging areas

(areas where tires are collected for 

recycling rather than disposal) which 

landfills are ‘designated to receive and 

store tires’ by State of Idaho as part of 

the used tire management system 

(which system is ‘distinct from the 

municipal solid waste management 

system’)

9

 

9

Staging of tires at landfills –using the landfill as a collection point without disposing of the tires 

in the landfill (or otherwise) – is an acceptable means of avoiding their discard. The definition 
 Sufficient information to determine 

that tires come from an established 

program and that kiln would not have 

been a CISWI

Ash Grove 

Cement

Durkee, 

Oregon

Whole 

Tires

 Receives whole tires from seven 

sources, two are landfills which have 

areas designated as tire staging areas

 Oregon regulations for used tires 

similar to Idaho

 Sufficient information to determine 

that tires come from an established 

program and that kiln would not have 

been a CISWI

Ash Grove 

Cement

Midlothian Whole 

Tires

 Receives tires from one privately

owned and operated waste 

management facility.

 Kiln has anecdotal information that 

supplier occasionally picks up tires 

from illegal dumpsites, though most 

(but not all) of these tires are 

unsuitable for Ash Grove and are 

shredded and sent elsewhere.

 Would probably have been a CISWI

Ash Grove 

Cement

Leamington, 

Utah

Whole 

Tires

 Burns only Tire chips. No information 

on ultimate source of tires. 

 Per other responses (i.e. Holcim

Devil’s Slide), UT has a state licensed 

program which oversees safe 

collection and management of tires

 Based on features of state law, tires 

come from an established tire 

collection program and kiln would not 

have been a CISWI

 

of “established tire management programs” in section 241.2 allows landfills to be used as staging 

areas as part of collection programs. Programs with landfill staging of tires can be “a 

comprehensive collection system that ensures scrap tires are not discarded and are handled as 

valuable commodities ... from the point of removal from the vehicle through arrival at the 

combustion facility.” .” A system which utilizes staging areas to collect tires for re-distribution, 

even if that staging area happens to be located at a landfill, meets this definition if the tires are 

placed in the staging area on arrival and properly handled prior to delivery at the combustion 

facility. For example, in this instance, Ash Grove noted its understanding that “all counties (sic) 

landfills purchased van trailers for the sole purpose of accumulating tires as part of the state’s 

used tire management system” and that Idaho state law provides that landfills can be used as 

storage collection points for used tires under the state’s used tire management system.
Ash Grove 

Cement

Seattle Whole 

Tires

 Receives tires from one privately

owned and operated tire source. State 

of Washington regulates transport and 

storage of tires

 Based on features of state law, tires 

come from an established tire 

collection program and kiln and kiln 

would not have been a CISWI

Buzzi Oglesby Whole 

Tires

 Vast majority of whole tire fuel 

supplied from brokers that obtained 

tires directly from the generators (tire 

stores, etc.) and meets definition of 

established tire program

 Program regulated by State of Illinois

 Would not have been a CISWI

Buzzi Pryor, 

Oklahoma

Whole 

Tires

 Unable to determine , but does not 

presently burn TDF at all

 Not a CISWI based on existence of 

OK rules for tire collection programs 

noted by other cement kilns (which 

assure safe management after 

collection and predominantly remove 

tires from vehicles)

Buzzi Maryneal Whole 

Tires

 Vast majority of whole tire fuel 

supplied from brokers that obtained 

tires directly from the generators (tire 

stores, etc.) ‘To the best of our 

knowledge, this TDF was from an 

established management program 

administered under

Texas Administrative Code Title 30, 

Part 1, Chapter 328, Subchapter F.’

 Would not have been a CISWI

Cal Portland Colton Whole 

Tires

 Established tire management program 

run by county. The county’s main tire 

source was not specified in the 

response but is likely tire shops, etc.

 This county also collects tires from the 

public and from illegal tire piles a few 

times a year. These tires are sorted 

and some eventually are sent to the 

cement kiln.

 Would probably have been a CISWI

due to known use of discarded tires

Cemex Demopolis Whole  Tires are sourced through tire brokers 
Tires through an established tire 

management program

 The ultimate source of the tires is not 

specified, but the tire must meet 

certain CEMEX specifications. The 

company cannot rule out the 

possibility that landfilled or disposed 

tires may be included

 CEXEX does partner with local 

municipalities to provide an outlet for 

amnesty tire collections which are 

designed “to avoid illegal tire dumps” 

and consist of individuals bringing 

tires to a kiln and not being charged a 

disposal fee. 

 All CEMEX kilns use only whole tires  Would not have been CISWIs

Cemex Brooksville - NorthWhole 

Tires

Cemex Miami Whole 

Tires and 

TDF

Cemex Brooksville-

South

TDF

Cemex Clinchfield TDF

Cemex Knoxville Whole 

Tires

Cemex Blacones TDF

Essroc Bessemer Whole 

Tires

 Has always used an established tire 

management program, ‘one which 

harvests tires from vehicles and 

businesses, and then manages the tires 

carefully so they are not thrown away

between collection and eventual use as 

a fuel’

 Would not have been a CISWI

Essroc Frederick Whole 

Tires

Florida 

Rock

Newberry Whole 

Tires

 Receives tires from a private company. 

Tires are collected directly from the 

source that removes the tire from the 

vehicle. 

 Would not have been a CISWI

Holcim Midlothian TDF

 Tire Chips obtained from the Ada OK 

facility with no other processing

 OK has a tire cleanup program, and 

facility also accepts tires from 

businesses and tire stores; never from 

landfills

 Would not have been a CISWI due to 

use of tire fuel (but is CISWI based on 

other secondary fuels).

Holcim Morgan TDF

 Receives tires from a private company 

under a State program. The tires are 

processed into tire chips at Ada facility

 Holcim Ada does not accept landfill 

tires (see previous write-up for 

Midlothian)
 Sufficient information provided to 

determine that obtains tire from an 

established program

 Would not have been a CISWI due to 

use of tire fuel (but is CISWI based on 

other secondary fuels).

Holcim 

(This 

facility was 

not on the 

CISWI List)

Ada Whole 

Tires

 Tires from businesses and tire stores 

and from Oklahoma clean-up program.

 Chips tires on site for other facilities.

 Holcim in general does not accept 

landfill tires

 Would not have been a CISWI

Holcim 

(This 

facility was 

not on the 

CISWI List)

Hagerstown Whole 

Tires

 Tires from businesses and tire stores.

 Holcim in general does not accept 

landfill tires

 Sufficient information provided to 

determine that obtains tire from an 

established program and would not 

have been a CISWI

Lafarge Roberta TDF  Sources from tire manufacturer or a 

local retailer under and established tire 

management program

 Would not have been a CISWI

Lafarge Joppa Whole 

Tires

 Tires from a variety of sources 

including tire manufacturers, tire retail 

outlets, tire collection forms, auto and 

racing organizations, and tire 

processers.

 Well established tire management 

program approved by the State of 

South Carolina

 On rare occasions the plant participates 

in community cleanup and/or state 

funded cleanup efforts.

 Would not have been a CISWI

Lafarge Tulsa Whole 

Tires

 Tire brokers who process tires under 

established tire management programs. 

 A State program allows reimbursement 

if 5 percent of the tires come from tire 

piles, landfills, or community clean-up 

efforts. (No mention if Lafarge or 

their tire brokers participate)

 Would not have been a CISWI, since it 

obtains tires from an established tire 

collection program and there is no 
indication discarded tires are burned 

by the kiln

Lafarge Whitehall Whole 

Tires

 Sources from tire dealers or 

automotive shops under an established 

tire management program

 Would not have been a CISWI

Lafarge Harleyville Whole 

Tires

 Tires from a variety of sources 

including tire manufacturers, tire retail 

outlets, tire collection firms, auto and 

racing organizations, and tire 

processers.

 Well established tire management 

program approved by the State of 

South Carolina

 Would not have been a CISWI

Lafarge Seattle Whole 

Tires

 Tires obtained from retailers in an 

established tire management program 

under State regulation

 Would not have been a CISWI

Lehigh 

Cement Leeds

Whole 

Tires

 Use an established tire management 

program

 All Lehigh plants use whole tires

 Would not have been a CISWI

Lehigh 

Cement Redding

Whole 

Tires

Lehigh 

Cement Evansville

Whole 

Tires

Lehigh 

Cement York TDF

Mitsubishi 

Cement

Lucerne 

Valley

Whole 

Tires

 Established tire management program 

but no absolute assurance that some 

tires could have been discarded

 Would not have been a CISWI

Monarch 

Cement Monarch

Whole 

Tires

 Established tire management program

 Would not have been a CISWI

National 

Cement of 

CA Encino TDF

 Tire chips and tire fluff. Fluff is from 

a facility that produces crumb rubber 

products.

 Ultimate tire source not specified.

 Status not certain, but reasonable to 

assume not a CISWI because of 

California rules establishing tire 

management programs and due to 

producing TDF to specification

Conclusion
Based on this information presented above, we conclude that no cement kiln would have 

been classified as a CISWI unit based on the use of secondary materials as an ingredient had the 

solid waste definition in Part 241 been promulgated at the time of the testing or the time of 

promulgation of the final NESHAP. In addition, all cement kilns surveyed obtained tire fuels 

from an established tire collection program. With the exception of the facilities that either 

acknowledged accepting tires that had been discarded or provided information from which some 

acceptance can reliably be inferred, the use of tire derived fuel by itself would not have resulted 

in a cement kiln being defined as a CISWI unit.