ERI Home IRR Home Last updated 8/10/04
The thorniest environmental choices now challenging the cement industry, their regulators, and the public are not inherent in the nature of the product or of the process. To be sure, the industry processes large quantities of material while consuming a great deal of energy, and cement manufacture creates a fine dust that can pose a particulate problem. But these factors can be managed with a reasonably straightforward measures.
The real dilemma facing the sector is tied to an opportunity. Cement is produced in an intensely hot flame, hot enough to convert many combustible hazardous wastes into harmless end products. Properly handled, and applied to appropriate types of hazardous wastes, the combustion can be carried out thoroughly and safely. And the energy extracted from the wastes can save on the fuel which would otherwise be necessary. It would seem to be a classic synergy -- solve a waste disposal problem and save resources at the same time.
The challenge lies not in the theory, but in the practice. Is hazardous waste combustion a Faustian bargain for the industry? Does the public have a right to expect that cement companies, whose expertise lies in high volume production of a low profit margin commodity, should acquire a comparable expertise in hazardous waste management? On the other hand, would alternative ways of dealing with the waste create even worse problems? Or would it be better to force manufacturers throughout industry in general, those who generate the waste in the first place, to have to work harder at source reduction by denying them the safety valve of relatively easy disposal afforded by cement kilns?
Attempts to resolve these questions have brought forth some creative approaches to regulation. The next few years will indicate how well they fare.
Environmental impacts and risks
Quantitative impact data
Effects of existing and future regulations on impacts
The U.S. Economic Census for 1997 indicates that 178 companies were involved in cement manufacturing (NAICS 327310, SIC 324), with a total annual sales of about $6.5 billion.
In 1999, U.S. companies manufactured 86.0 million metric tons of cement, according to data from the Energy Information Administration (EIA) at the Department of Energy.
Modern Portland cement is composed primarily of calcium silicate compounds. The manufacturing process involves:
Since the lime-containing materials are the primary constituents, cement mills are generally located near the quarrying site for those materials, and the remaining constituents are transported in as necessary. Limestone quarries and other suitable lime sources are fairly widespread geographically, and it is advantageous to minimize the transportation distance between the heavy, relatively low-cost product and its end market, so cement kiln operations are also widely distributed. About three-quarters of the fifty states have at least one operating Portland cement plant.
There are two different types of process methods, referred to as "wet" and "dry". The essential difference between the two types is the medium used to mix the powdered raw materials prior to heating, and the consequent degree of moisture in the materials entering the kiln. In the wet method, water is added to the raw materials after milling to promote thorough mixing, and the mixture is added to the kiln as a slurry, containing 30-40% water. In the dry method, the powders are generally blended in a silo using compressed air. To appreciate the consequences of the choice of method, it is necessary to dissect the process in a bit more detail.
Cement kilns are formidable installations. A kiln generally consists of a cylindrical outer steel shell, lined with refractory brick. They can range from 10 to 25 feet wide, and from 150 to 750 feet long (750 feet is over an eighth of a mile). According to the Portland Cement Association (PCA), the cement kiln is the "world's largest piece of moving industrial equipment". (See the "Raw materials" tab in the "Kiln" section of their "Virtual Plant Tour".)
The kiln is laid out with its axis close to horizontal, but with a slight slope. The powdered and mixed raw material is introduced at the higher end, and gradually rolls down to the lower end. The fuel is blown into the lower end, producing an intense flame. As the material passes down through the kiln, it gets continually hotter. First it loses any remaining water, then it loses the carbonate component, which comes off as carbon dioxide (a process known as calcination), and finally it begins to fuse together into clinker nodules. The clinker emerges from the lower end of the kiln and falls into the cooling unit. Material typically spends several hours passing through the kiln.
If the material entering the kiln is wet, it stays cooler relatively longer. This in turn requires a longer kiln, to provide sufficient residence time. In addition, fuel is consumed driving off the water. One reason to choose the wet process is if the starting materials have high moisture content to begin with. The wet process was the dominant older technology, presumably because, in the age of abundant cheap energy, it was cheaper to burn more fuel and add length to the kiln than to add extra devices.
Newer kilns use the dry method. There are substantial energy savings involved, as well as a higher throughput. Several subtypes of the dry method have evolved in recent years. In the earlier versions, the kilns were shortened to take advantage of the shorter residence time required. Since hot air emerged from the upper end of the kiln at a higher temperature than was the case with the wet method, it became advantageous to capture the heat and use it to run utility boilers to generate electricity, resulting in a cost savings for the plant as a whole. Alternatively, the kiln could be kept long, but heat exchangers could be added to the upper end to retain more of the flame's heat inside the process area and increase the heat transfer efficiency. However, the energy advantages of the dry process were more fully realized with the addition of pretreatment equipment to condition the powdered raw materials before their introduction into the kiln. One variation uses suspension preheaters to transfer heat from the kiln exhaust gases to the incoming material in a series of cyclones, which both improves heat transfer and promotes good mixing. A further development, called a precalciner, pumps even more heat into the pretreatment phase, often combining some additional fuel with air from the clinker cooling stage, which has thus been preheated. The precalciner system is the most energy efficient arrangement, and also has the highest throughput, with the shortest kiln. It is slowly replacing the earlier technologies.
As hazardous waste legislation moved forward in the 1970's and early 1980's, and as disposal options became more limited, it was recognized that the intense heat of cement kilns could be used to incinerate certain types of combustible wastes, such as solvents, waste oils, printing inks, old tires, and materials recovered during remediation activities. Such an option was particularly attractive for wet process kilns, since their fuel costs were highest to begin with, and since they could offset their lower efficiencies with revenues derived from accepting the wastes, as well as decreased fuel costs. This situation will be discussed more extensively below.
The cement manufacturing industry takes environmentally benign starting materials -- limestone, clay, and sand -- and creates an environmentally benign product, primarily composed of calcium silicate. Thus the industry's environmental impacts are associated more with the process conditions required by cement manufacturing than with the materials:
The intense heat required by process entails:
air quality concerns associated with high temperature combustion, with nitrogen oxides being of particular concern
high energy consumption
That intense heat has a tangential consequence: cement kilns can be very effective in destroying combustible hazardous wastes. Many cement operations now accept hazardous wastes as a portion of their fuel inputs, leading to public concerns regarding air emissions and non-combustible residues in the product or solid waste stream from those facilities.
The process is a significant source of carbon dioxide, from two different causes:
The calcination of limestone or other carbonate-containing starting materials
The high rate of carbon fuel consumption
The product is a fine powder, and various process steps involving grinding, both of the input materials and of the finished product, have the potential to emit fugitive dust. Particulate control systems on exhaust air from the clinker cooling and grinding processes produce a waste material known as cement kiln dust (CKD). Many facilities are equipped to return CKD to the process, but some ends up in landfills, or is land applied as an agricultural supplement.
Several of these impacts are discussed in more detail below.
Nitrogen in the atmosphere is very stable. It takes a very hot flame to disrupt it. Nitrogen oxides are therefore generally produced only by processes involving high temperature combustion. But cement kilns run very hot.
The cement industry is responsible for about 1.5% of all nitrogen oxides emissions of sources in the AIRS database (see below). Technologies exist to mitigate the impact, including the use of catalytic reduction units. A full analysis of the costs and benefits is provided in a document prepared by EPA-OAR.
The cement industry affects the global warming issue in two major ways:
In terms of greenhouse gas emissions, cement manufacture (NAICS code 327310) is responsible for 71.6 teragrams (million metric tons, Tg) of carbon dioxide equivalent emissions, according to 2000 data. This includes 30.5 Tg from fuel consumption, and 41.1 Tg from non-fuel sources. Cement manufacture is different from most of the major greenhouse gas contributing sectors in that non-fuel sources account for significantly more carbon dioxide emissions than fuel requirements. Cement is a major source, but is well behind several other sectors such as petroleum refining, chemicals, iron and steel, and pulp and paper, in total greenhouse gas emissions. More information on the calculation of greenhouse gas emissions is provided in another document in this series, Greenhouse Gas Estimates for Selected Industry Sectors.
One way to mitigate the carbon dioxide emission, and to make beneficial use of a solid waste at the same time, is to use a readily available source of lime that has already been calcinated, and will not add additional carbonates.
A vigorous endorsement for the use of fly ash from coal and of granulated blast furnace slag from steel production as a substitute for cement and clinker may be found in a document from Holnam, Inc., a U.S. subsidiary of a Swiss corporation that claims to be the largest cement producer in the United States. The document states that, on average, producing a ton of Portland cement clinker results in the release of a ton of carbon dioxide to the atmosphere, with 60% due to calcination, and 40% due to fuel consumption. Replacing a percentage of the cement with the waste materials will result in an acceptable (in fact, in some cases, an improved) product, with a proportional decrease in the amount of carbon dioxide released. The United States is behind other countries in its acceptance of this substitution.
EPA itself has weighed in on the matter of taking advantage of this opportunity to combine pollution prevention and waste recycling. The Office of Solid Waste (OSW) has prepared a web page containing recommendations for procuring agencies on cement and concrete specifications. In the "Guide Specifications" section, for example, EPA explicitly "recommends that procuring agencies ensure that their guide specifications do not inappropriately or unfairly discriminate against the use of coal fly ash or GGBF [ground granulated blast-furnace] slag in cement and concrete."
The environmental issue with the highest public profile is only tangentially related to the cement manufacturing process itself. As a consequence of legislation enacted in 1980 (the "Bevill Exclusion"), cement kilns became the method of choice for burning hazardous wastes. The Bevill exclusion is discussed in greater length in the document in this series on the mining sector.
The case against hazardous waste burning in cement kilns from the public interest group perspective can be accessed on numerous websites. A few of the more cogent presentations include:
An industry response to the NCA, along with other presentations of the industry viewpoint, can be found at the CKRC website.
After much controversy, the EPA issued a rule in 1999 spelling out the Maximum Achievable Control Technology (MACT) standards that would be applicable to all hazardous waste combustion operations, including cement kilns.
Hazardous waste combustion
A search of the TRI database for 1999 under SIC 3241 (hydraulic cement) at the RTK Net website shows the following data:
Number of SIC 3241
According to the explanatory notes provided by RTK Net, their "Total waste" figure includes, among other items, "Amount Destroyed by Energy Recovery On-site".
This can be compared with the summary generated for all industries by EPA's TRI Explorer. The entire SIC category 32, which includes manufacturers of stone and glass, as well as cement, reported 651,242,324 pounds under "energy recovered on site". The bulk of this material is presumably hazardous wastes burned in cement kilns.
To put this number in perspective, the total for all reporting industries reported under "energy recovered on site" was 2,808,598,629 pounds in 1999. The chemical industry (SIC 28) accounted for 49% of that total, and SIC category 32 (primarily cement) accounted for 23%. The petroleum industry was third, with 15%, and paper a distant fourth, with 7%.
The CKRC website provides some additional data. According to a graph showing data from 1989-1997, the "volume" [sic] of hazardous waste used as fuel in cement kilns rose from about 800,000 tons in 1989 to about one million tons in 1992, and has remained at close to that level since. The one million ton per year figure is also quoted in their rebuttal to NCA's "Myths and Facts" document. One million tons corresponds to two billion pounds, compared to TRI numbers of closer to 650 million pounds for the entire SIC 32 category. Apparently not all of this is reported under TRI (assuming the1999 total was comparable to that of 1997.)
A summary of carbon dioxide emissions from the Energy Information Administration indicates that the 86.0 million metric tons of cement manufactured in 1999 "directly released" 10.9 million metric tons of "carbon equivalent" (the weight of the carbon contained in carbon dioxide) into the atmosphere, independent of the energy consumed. This is apparently a stoichiometric calculation based on the carbonate content of the raw material. Although carbon burned for fuel accounts for most of the carbon dioxide released in the U.S. each year, about 1.3% of the total carbon dioxide generated from all U.S. sources comes from non-fuel industrial processes. Such processes, including manufacture of lime and soda ash, and oxidation of carbon anodes during electrolytic aluminum smelting as well as cement production, contributed a total of 19.2 million metric tons of carbon equivalent in 1999. Cement manufacture thus accounted for 57% of all non-fuel carbon from industrial processes.
Comparing this to the industry figure cited above, according to which the manufacture of one ton of cement clinker releases one ton of carbon dioxide (60% directly from calcination, and 40% from the fuel consumed), we have the following calculation:
This is roughly equal to the 11 million metric tons cited by EIA. This is sufficiently consistent to warrant combining the two approaches to arrive at an estimate for the total (direct plus fuel) carbon equivalent contribution of the cement industry to the national total. If we take the 11 million metric ton figure provided by EIA for direct calcination, and assume that it is 60% of the total due to cement manufacture, we can estimate the total at 11/0.6, or 18.3 million metric tons carbon equivalent. The total U.S. emission from all sources in 1999 was 1,527 million metric tons, so that cement manufacture was responsible for 1.2% of total U.S. carbon dioxide emissions in 1999. That would have to be considered a significant contribution from a single sector.
Air emissions data for certain key criteria pollutants (ozone precursors) are available from the National Emission Trends (NET) database (1999), and hazardous air pollutant emissions data are available from the National Toxics Inventory (NTI) database (1996 is the most recent year for which final data are available). For SIC code 3241 (Hydraulic cement), the total emissions listed in these databases are:
The contributions from cement manufacture to VOC and HAP emission do not put cement in the category of major contributors -- it is 37th on the list of sectors broken down to the four digit SIC level, a factor of ten less than sectors such as petroleum refining and organic chemicals, that lead the list. But in nitrogen oxide emissions, it is the fourth largest contributor. The high process temperatures, and the consequent need for intense combustion as an integral part of the manufacturing process, is the root cause.
The following table summarizes the relative contribution of SIC 3200 (stone, clay, glass, and concrete industries) to the total release of priority air pollutants from all sources. Cement manufacture accounts for the bulk of the SIC 3200 total. The figures are from 1994 (based on data from the AIRS Database, as cited in the 1995 OECA sector notebook for the industry).
From the table, it is evident that:
The main risks associated with these impacts would be those associated with air quality, and would be most pronounced in the vicinity of the manufacturing operations. A secondary risk would involve leachate from cement kiln dust that has been landfilled.
Concern has been expressed about the potential for contamination of the product by hazardous waste residues, but this is controversial.
The document on tire burning in cement kilns referred to above includes several assertions regarding common industry practices which, if true, significantly impact the assessment of risks involved in burning all kinds of hazardous wastes, not just tires, in cement kilns. It is beyond the scope of this analysis to attempt to assess the truth of those claims -- that would be more appropriate for a survey, or perhaps for an investigation involving actual sampling, than for an analytical exercise. However, it seems reasonable to assess the plausibility of the assertions, if only to provide a rough indication of the degree of risk that hazardous waste burning presents in practice.
The main operating practice issue described in the document can be summarized as follows: In order to realize the advantage of the extremely high operating temperatures characteristic of cement kilns for thorough destruction of the combustible portion of hazardous wastes (in particular, of the relatively stable multiple ring aromatics), it is essential that the waste have sufficient residence time in the combustion region, and that adequate oxygen be supplied to the kiln.
The article asserts that kiln operators will take steps to run at maximum combustion efficiency during stack tests, but will revert to less desirable practices under normal operation.
The economic incentive suggests that such a scenario must be considered at least potentially plausible for some fraction of operators. As long as stack testing is a sporadic event, there are no real compliance incentives to counterbalance the economic incentives, other than a general commitment to corporate citizenship, between tests.
One way to mitigate this kind of risk would involve more intrusive monitoring, such as continuous emissions sensors or unalterable records of process parameter settings. Such measures would go beyond what is currently considered to be an acceptable level of regulatory intrusion.
Another option would be reliance on an environmental management system (EMS). There are many good arguments to justify the value, both for a company and for the public, of developing an EMS, and this is not the place to enumerate them. The question here is the extent to which the existence of an EMS is likely to affect the risk to the environment posed generally by companies operating industrial processes, and specifically by cement kiln operators burning hazardous wastes.
According to one point of view, an EMS is fundamentally system for
with regard to practices and operations involving potential environmental impact. Industrial processes carried out under an EMS are not immune from problems. But an EMS can definitely serve to reduce risk. First, operating policies can be clearly spelled out, and can reduce the risk of deviations from acceptable practice due to negligence. Second, recordkeeping requirements, while not ensuring compliance, can only make it that much more difficult for individuals to cut corners surreptitiously and escape accountability. In essence, the EMS can help restore the balance by providing some measure of counterweight to the economic incentive. With a clear understanding of its uses as well as of its limitations, establishing an EMS would appear, in this light, to be a worthwhile goal for a public interest group concerned about the effect of a cement kiln's operating practices on its emissions potential.
Portland cement manufacturing is covered under a NESHAP finalized in 1999.
An Effluent Limit Guideline (ELG) for Cement Manufacturing appears in Title 40 of the Code of Federal Regulations, Chapter 1, Part 411.
Cement kiln dust falls under the Bevill exclusion and thus cannot be regulated under RCRA. EPA has approached this regulatory dilemma by developing a set of management standards for managing cement kiln dust waste. The proposed standards, published in 1999, cover such items as ensuring that landfills containing cement dust are properly lined to prevent leaching, keeping landfilled dust compacted and wetted down, transporting dust in closed containers, and limiting toxic metals concentrations for dust used for agricultural applications. The standards would continue to treat the dust as a nonhazardous waste, unless the management standards were violated, in which case the dust could become a regulated hazardous waste. It is not clear whether that would apply to individual facilities or to companies (the proposed rule refers to "persons"). EPA-OSWER has provided an index page with links to the standards themselves, and to a number of supporting documents.
The Federal Register notice containing the proposed rules is interesting to read from the standpoint of gaining perspective on the range of regulatory options potentially applicable to the situation. Among the options considered are:
This suite of approaches is, of course, not limited to the cement industry. The proposed rule, for example, refers to an MOU approach implemented in the pulp and paper industry in 1994 for land application of pulp mill sludges. There is also an enlightening discussion in the justification for the approach that the agency selected, wherein the disadvantages of treating cement kiln dusts under RCRA are enumerated. It is worth listing them. Treating cement kiln dusts as hazardous wastes under RCRA:
It has probably not escaped the reader's notice that these arguments could apply to a wide variety of situations, well beyond the cement industry. In this context, it might be interesting to refer to the observations on the implications that experience with the Bevill exclusions may have for the RCRA framework in general, in the document in this series analyzing the mining industry.
Cement kilns that burn hazardous wastes are covered under new Hazardous Waste Combustion NESHAPs published on September 30, 1999, with a final compliance date of September 30, 2002.
Extensive information, including technical support documents, can be found from the HWC MACT webpage on the EPA site.
The cement industry is covered in one of the invariably useful OECA sector notebooks, specifically in the volume on the stone, clay, glass, and concrete products industries, at http://es.epa.gov/oeca/sector/sectornote/pdf/stclglsn.pdf
In addition, there is a particularly detailed and useful industry and process overview contained in Chapter 3 of an analysis prepared by OAQPS (EPA-OAR) of NOx emissions from cement kilns, "Alternate Control Techniques Document -- NOx emissions from Cement Manufacturing", at http://www.epa.gov/ttn/catc/dir1/cement.pdf
The PCA website provides:
Other industry data:
Other source documents
Hazardous Waste Combustion MACT
Management of cement kiln dust waste
Environmental and public interest groups