                                  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".  

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).  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 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.  


                  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. 

      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,  materials placed 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.
      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. 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. 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. Specifically, 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? 

	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 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')
   * 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
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
   * Tires are sourced through tire brokers 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 - North
                                  Whole 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.


