Contents
EVERYTHING YOU NEED TO KNOW ABOUT Cement Kilns firing Hazardous Waste
Summary
Cement klinker production is a energy-intensive industry producing cement klinker as its main product. As energy demand in cement kilns typically accounting for 30–40% of production costs conventional fuels are substituted by various types of alternative fuels and (hazardous) wastes. A clear differentiation between alternative fuels and (hazardous) wastes currently do not exists, both ingredients are dealt with in this guideline.
As outlined above firing waste in cement kilns aims at energy recovery and substitution of fossil fuels, but in some cases hazardous wastes are disposed of in these installations.
Waste management and minimization systems should be implemented prior to disposal in cement kilns. Demonstrated alternatives to firing wastes in cement kilns are included in normal waste disposal pathways. If properly operated, releases of chemicals listed in Annex C from cement kilns firing hazardous waste are of minor importance. Therefore, alternatives should be considered in the light of the overall impact on the environment.
Other benefits accrue from applying best available techniques and best environmental practices in terms of environmentally sound management. Segregation of wastes will reduce the amounts to be treated. Firing of wastes (including hazardous wastes) recovers energy and substitutes fossil fuels as well as mineral raw materials.
The manufacturing process includes the decomposition of calcium carbonate (CaCO3) at about 900 °C to calcium oxide (CaO, lime) (calcination) followed by the clinkering process in a rotary kiln. The clinker is then ground together with gypsum and other additives to produce cement. According to the physical and chemical conditions the main process routes for the manufacture of cement, especially during the pyroprocessing step, are termed dry, wet, semi-dry and semi-wet. The firing of hazardous wastes may result in the formation and subsequent release of chemicals listed in Annex C of the Stockholm Convention. In addition, releases from storage sites may occur.
In general, the process conditions and primary measures in cement kilns are sufficient to minimize the formation and release of chemicals listed in Annex C and to achieve concentrations of PCDD and PCDF in flue gases of < 0.1 ng TEQ/Nm3. If necessary, additional secondary measures are available. PCDD/PCDF releases via cement kiln dust and possibly clinker have been reported and are currently subject to further investigation.
Preamble
The following draft guidelines shall provide guidance on best available techniques and guidance on best environmental practices for cement kilns relevant to Article 5 and Annex C, Part II of the Convention. This section also considers requirements of Article 6 of the Convention addressing destruction of POPs containing waste.
In these guidelines consideration is also given to the Technical Guidelines developed by the Basel Convention Parties. The Basel Technical Guidelines give guidance for best available techniques to be applied to the destruction or irreversible transformation of persistent organic pollutants and have identified cement kiln-co-incineration as a process that can be used for such destruction and irreversible transformation of POPs in waste.
In the scope of this document co-incineration of alternative fuels and hazardous wastes in cement kilns is dealt with as well. It should be kept in mind in reading these guidelines, that stringent definitions of both terms currently do not exist.
1. Introduction
1.1. Cement industry in general.
Global production of cement is obviously rising. According to investigation of the cement industry, the world wide cement production in 2003 was 1.94 billion tons, increasing from an estimated 1.69 billion tons in 2001 and 1,660 million tons in 2002, a large part of which is based on dry processes (de Bas 2002; DFIU/IFARE 2002).
There has been a steady increase of an estimated 3.6% due to the strong demand in developing countries and countries in transition. At present Europe has a share of 14.4%, USA 4.7%, other America 6.6%, Asia 67,5% (China 41.9%), Africa 4.1% and the rest of the world 2.7%. The cement consumption was estimated to be 260 kg per capita in 2004. (CEMBUREAU, 2004).
Cement production in Europe amounted to 190 million tons per year. More than 75% of this output is based on dry processes, due to the availability of dry raw materials; 16% is based on semi-dry or semi-wet processes and 6 % on wet processes. The typical capacity of a new European kiln is 3,000 tons of clinker per day. (Wulf-Schnabel and Lohse 1999).
In China most cement is produced in vertical shaft kilns, which show low energy efficiency and poor environmental performance. China makes cement using modern dry process kilns as well as it has over 500 preheater and precalciner kilns in operation. (H. Klee, World Business Council for Sustainable Development, personal communication 2004
In the United States of America the average kiln produces 468,000 tons per year (in 2002). Currently, about 81% of the cement produced in the United States is manufactured using dry process technology (Portland Cement Association website).
1.2 Recovery of energy from waste in cement kilns
The cement industry is an energy-intensive industry with energy typically accounting for 30–40% of production costs (i.e., excluding capital costs). A cement plant consumes 3,000 to 6,500 MJ of fuel per ton of clinker produced. Traditionally, the primary fuel used is coal. A wide range of other fuels is also used, including petroleum coke, natural gas and oil (European Commission 2001). In Europe the specific energy consumption of cement industry has been reduced by about 30% over the past 20 years (equivalent approximately 11 million tons of coal per year) (CEMBUREAU, 2004).
In addition to these conventional fuels, the cement industry uses various types of waste as fuel. In the European cement industry the share of waste in total fuel consumption amounts to about 12% (in 2001) (European Commission 2001). (Note: may be updated with CEMBUREAU data of 2004)
Current estimates suggest that the cement industry can increase its energy efficiency by 0.5% to 2% per year by replacing old or outdated equipment. If new, dry-process plants replace older, wet-process units, significant energy efficiency gains are possible (CSI, 2005).
1.3 Hazardous waste destruction in cement kilns
In some countries, the cement industry provides a public or industrial service by disposing of wastes even those with little or no useful energy or mineral content. This may be done at the request of national governments or in response to local demand. It can be done because a cement kiln provides high temperatures, long residence time, and a carefully controlled facility capable of high destruction efficiency.
However, this activity is not part of the fuel or raw material substitution process. Cement kilns have been used in this way for many years in countries such as Japan, Norway, and Switzerland, where there is little space for landfill sites. More recently, modern kilns have been used for waste destruction in some developing countries where the lack of existing waste disposal and incineration infrastructure means that kilns are the most economical option. Even where good waste disposal infrastructure exists, it may be useful to increase local capacity through use of cement kilns.
The use of cement kilns for waste destruction may be less desirable than other approaches, such as recycling or reprocessing, but is a useful alternative to landfill or dumping. This has to be evaluated case by case in the context of an overall waste management concept (see below). Waste destruction in cement kilns must meet strict environmental, health, and safety standards, and must not impair the quality of the final product. The process must be precisely controlled when destroying such wastes, and emissions regularly measured. (CSI, 2005)
2. Links to other sections in the guidelines
2.1 General waste management
Society can manage wastes in a number of ways, depending on their physical and chemical nature, and on the economic, social, and environmental context in which they are produced. Some of these are listed below. Specific decisions will always be influenced by local circumstances such as the availability of waste treatment facilities, alternative markets for materials, and the infrastructure available to safely collect, manage and transport waste materials (CSI, 2005). Figure 1 shows a hierarchy of decision making for waste management.
2.2 Other destruction options
The use of cement kilns for destruction of waste should considered only in a general context of a waste management concept. Waste incineration is also an option for waste treatment and the guidelines for BAT and BEP for this source category should be considered.
2.3 Basel technical guidelines
Technical Guidelines developed by the Basel Convention should also be considered as they provide guidance on best available techniques to be applied to the destruction or irreversible transformation of persistent organic pollutants as wastes.
3. Cement production processes
The basic chemistry of the cement manufacturing process begins with the decomposition of calcium carbonate (CaCO3) at about 900 °C to leave calcium oxide (CaO, lime) and liberate gaseous carbon dioxide (CO2); this process is known as calcination. This is followed by the clinkering process in which the calcium oxide reacts at high temperature (typically 1,400°–1,500° C) with silica, alumina and ferrous oxide to form the silicates, aluminates and ferrites of calcium which comprise the Portland clinker. This clinker is then ground together with gypsum and other additives to produce cement.
3.1 In the rotary kiln
The raw feed material, known as raw meal, raw mix, slurry (with a wet process) or kiln feed, is heated in a kiln, typically a large, inclined, rotating cylindrical steel furnace (rotary kiln). Kilns are operated in a countercurrent configuration. Gases and solids flow in opposite directions through the kiln, providing for more efficient heat transfer. The raw meal is fed at the upper or cold end of the rotary kiln, and the slope and rotation cause the meal to move toward the lower or hot end. The kiln is fired at the hot end, usually with coal or petroleum coke as the primary fuel. As the meal moves through the kiln and is heated, it undergoes drying and pyroprocessing reactions to form the clinker, which consists of lumps of fused, incombustible material.
3.2 After the rotary kiln
The clinker leaves the hot end of the kiln at a temperature of about 1,000° C. It falls into a clinker cooler, typically a moving grate through which cooling air is blown. The clinker is ground with gypsum and other additives, usually in a ball mill, to produce the final product – cement.
The cement is conveyed from the finish cement mill to large, vertical storage silos in the packhouse or shipping department. Cement is withdrawn from the cement storage silos by a variety of extracting devices and conveyed to loading stations in the plant or directly to transport vehicles.
3.3 Production processes in general
The main process routes for the manufacture of cement for the pyroprocessing step of cement production accomplishes the required physical and chemical steps. They vary with respect to equipment design, method of operation and fuel consumption (European Commission 2001). Figure 2 identifies the principle processes and system boundaries of cement production.
3.3.1 The dry process
In the dry process, the raw materials are ground and dried to raw meal in the form of a flowable powder. The dry raw meal is fed to the preheater or precalciner kiln or, more rarely, to a long dry kiln.
3.3.1.1 The preheater dry process
In this process preheaters are used to increase the thermal efficiency. A raw meal preheater consists of a vertical tower containing a series of cyclone-type vessels. Raw meal is introduced at the top of the tower. Hot kiln exhaust gases pass countercurrent through the downward moving meal to heat the meal prior to introduction into the kiln. The meal is separated from the kiln flue gases in the cyclone, and then dropped into the next stage. Because the meal enters the kiln at a higher temperature than with conventional long dry kilns, the length of the preheater kiln is shorter.
With preheater systems, it is sometimes necessary to remove undesirable components, such as certain alkali constituents, through an alkali bypass system located between the feed end of the rotary kiln and the preheater tower. Otherwise, these alkali constituents may accumulate in the kiln, and removal of the scale that deposits on vessel walls is difficult and may require kiln shutdown. This problem can be reduced by withdrawing a portion of the gases with a high alkali content. If this alkali bypass has a separate exhaust stack it can be expected to carry and release the same pollutants as the kiln exhaust.
3.3.1.2 The preheater/precalciner dry process
This process is similar to the preheater dry process, with the addition of an auxiliary firing system to increase the raw materials temperature prior to introduction into the kiln (Figure 3). A precalciner combustion vessel is added to the bottom of the preheater tower. The primary advantage of using the precalciner is that it increases the production capacity of the kiln, as only the clinker burning is performed there. Use of the precalciner also increases the kiln refractory lifetime due to reduced thermal load on the burning zone. This configuration may also require a bypass system for alkali control, which, if released from a separate exhaust stack, can be expected to carry and release the same pollutants as the kiln exhaust.
3.3.2 The semi-dry process
In the semi-dry process dry raw meal is pelletized with 12–14% water and fed into a grate preheater before the kiln or to a long kiln equipped with crosses, on which the pellets are dried and partially calcined by hot kiln exhaust gases before being fed to the rotary kiln.
3.3.3 The semi-wet process
In the semi-wet process the slurry is first dewatered in filter presses. The filter cake is extruded into pellets and fed either to a grate preheater or directly to a filter cake dryer for raw meal production.
3.3.4 The wet process
In the wet process, the raw materials (often with high moisture content) are ground in water to form a pumpable slurry. The slurry is either fed directly into the kiln or first to a slurry dryer. The wet process is an older process used in the case of wet grinding of raw materials. It shows a higher energy demand compared to the dry process because of water evaporation from the slurry.
3.4 Emission control
In general, cement kilns are equipped with either electrostatic precipitators or fabric filters, or both, for particulate matter control. In some cases, the flue gases are cooled prior to the dry air pollution control device. Acid gas pollution control devices are not used at cement kilns as the raw materials are highly alkaline and provide acid gas control (Karstensen 2006), although there are a few kilns equipped with wet scrubbers and nitrogen oxides (NOx) controls.
4. Burning
4.1 Operation with conventional fuels
For smooth operation and complete combustion some important criteria have to be considered for conditioning. Production of a homogeneous clinker requires constant and complete combustion of the fuel. The oxidation of the fuel constituents occurs more quickly when the fuels are well mixed and the specific surface is larger. In the case of liquid fuel, injection has to be as smooth as possible. In the case of solid fuels, thorough mixing with other fuels used at the same time is required
Depending on operational conditions – explicitly in case of improper operation of the installation – emissions of PCDD/F can occur. In proper operation the release of PCCD/F should be below the detectable limit.
A cement plant consumes 3,000 to 6,500 MJ of fuel per ton of clinker produced (electricity and transport not included), depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. As well as providing energy, some of these fuels burn to leave fuel ash containing silica and alumina compounds (and other trace elements). These combine with the raw materials in the kiln contributing to the structure of the clinker and form part of the final product. Energy use typically accounts for 30-40% of the production costs. The different types of fuels are listed in decreasing order of importance below:
Pulverized coal and petroleum coke (petcoke);
(Heavy) fuel oil;
Natural gas.
Potential feed points for supplying fuel to the kiln system are:
Via the main burner at the rotary kiln outlet end;
Via a feed chute at the transition chamber at the rotary kiln inlet end (for lump fuel);
Via secondary burners to the riser duct;
Via precalciner burners to the precalciner;
Via a feed chute to the precalciner/preheater (for lump fuel);
Via a mid-kiln valve in the case of long wet and dry kilns (for lump fuel).
4.2 Operation with possible alternative fuels
The selection of alternative fuels and materials is driven by a number of interrelated considerations, including:
Impact on CO2 emissions and on fuel consumption,
Impact on fuel cost,
Impact on other emissions,
Impact on mining and quarry activity.
4.2.1 Examples of alternative fuels
The substitution of fossil fuels and virgin raw materials with alternatives is a well-developed practice in some countries. Some countries have been using it for almost 30 years, and some national governments actively promote this approach. In a number of countries this practice is well understood and highly developed. Alternative fuels include materials such as (CSI, 2005):
Used tires,
Meat and bone meal, animal fat,
Plastics,
Packaging waste,
Waste wood, impregnated saw dust,
Paper, cardboard,
Sludge (paper fiber, sewage)
Agricultural and organic waste,
Shale, oil shales,
Coal slurries, distillation residues,
Fine/ anodes / chemical cokes,
Waste oils, oiled water,
Spent solvents.
4.2.2 Negative waste list
The following waste materials shall not be used in cement kilns used as a fuel or raw material source under any circumstances:
nuclear waste,
asbestos-containing waste,
wastes containing heavy metals including mercury, lead or cadmium
electronic scrap ,
explosives,
wood treated with copper, chrome, arsenic etc.
mineral acids,
infectious medical waste,
chemical or biological weapons destined for destruction,
entire batteries,
unknown or non-specified waste.
Furthermore, individual companies may exclude additional materials depending on local circumstances.
4.2.3 Considerations for selection of possible alternative fuels
The selection of fuels can be a complex process and is influenced by many parameters including the probability of the formation and releases of POPs. The operator should develop a fuels evaluation and acceptance procedure. Based on this procedure inter alia the effect of the fuel on plant emissions and whether new equipment or procedures are needed to ensure that there is no negative impact on the environment should be assessed.
Variables to consider in the selection of fuels and raw materials as a whole should be applied (CSI, 2005)
Kiln operation:
Chlorine, sulfur, and alkali content: these may build up in the kiln system, leading to accumulation, clogging, and unstable operation; excess in chlorine or alkali may produce cement kiln dust or bypass dust (and may require installation of a bypass) which must be removed, recycled or disposed of responsibly.
Water content: high water content may reduce the productivity and efficiency of the kiln system.
Heat value (fuel): the heat value is the key parameter for the energy provided to the process.
Ash content: the ash content affects the chemical composition of the cement and may require an adjustment of the composition of the raw materials mix.
Clinker and cement quality:
Phosphate content: this influences setting time.
Chlorine, sulfur, and alkali content: these affect overall product quality.
Chromium: this may cause allergic reactions in sensitive users.
Emissions:
High sulfide contents in raw materials: these may result in the release of SO2.
Heavy metals in fuel or raw material: volatile heavy metals, which are not completely captured in the clinker, must be monitored and controlled The choice of fuels and substitute materials can also affect greenhouse gas emissions. For example, substituting alternative materials for limestone reduces CO2 emissions and may decrease fuel use, as discussed elsewhere in this report (Comm..: later to reference, in which chapter to find).
The choice of fuels can also affect greenhouse gas emissions. For example, substituting fossil fuels by biomass results in a decrease of net CO2-emission.
Alternative fuels may also reduce NOx emissions depending on their composition and water content.
4.2.4 Analysis of input material
The operator should develop a fuels and raw materials analysis, evaluation and acceptance procedure that includes the following features:
Each material supplier should be required to prepare a sample of fuel or material, which will be used to evaluate the fuel or material before delivering it to the plant. This should include a datasheet detailing the chemical and physical properties of the fuel or material being supplied, information on relevant health, safety, and environmental considerations during transport, handling, and use, and a typical sample of the material. It should also specify the source of the particular shipments being made.
The sample’s physical and chemical characteristics should be tested and checked against specifications.
A clear management of quality control has to be installed.
4.2.5 Pre-treatment and storage for use of waste as alternative fuel
The storage condition for alternative fuels depends on the type of materials. In general, care has to be taken on emissions, and technical and hygienic demands.
Initial storage: Material mix with strong contaminations (substantial biologic content) and high moisture (up to 40%) is mainly stored in specially designed containers due to hygienic rules and regulations. Animal meal has to be conditioned in absolutely closed systems. It is supplied in containers, and the material is conveyed either pneumatically or by mechanical equipment into storage. Liquid and secondary fuels (waste oil, solvent and sewage sludge) are stored in special containers. Special security guidelines have to be elaborated (taking into account, for example, risk of explosion).
Intermediate storage at the conditioning plant aims at checking the quality of alternative fuels after the preparation process. Here, containers are normally used.
Material (product) storage: The alternative fuels have to be protected from variations in natural conditions, such as humidity, moisture and even rain (for example, by storage in a warehouse).
Initial storage and preparation of different types of waste for use as fuel is usually performed outside the cement plant by the supplier or by waste treatment specialist organizations. This means only the product needs to be stored at the cement plant and then proportioned for feeding to the cement kiln. Since supplies of waste suitable for use as fuel tend to be variable whilst waste material markets are rapidly developing, it is advisable to design storage and preparation plants as multi-purpose (Karstensen 2006). Measures have recently been initiated (2003) in the European Union to standardize solid recovered fuels derived from non-hazardous waste.
4.2.6 Destruction efficiency
Cement kilns utilize wastes commercially (i.e., they accept waste from off-site generators) for use as a fuel substitute in the production of Portland cement clinker. Liquid wastes are typically injected into the hot end of the kiln. Solid wastes may be introduced into the calcining zone at some facilities. For long kilns, this means that the solid waste is introduced mid-kiln, and for preheater/precalciner kilns it is introduced onto the feed shelf in the high-temperature section.
In the case of hazardous wastes a complete decomposition of toxic compounds such as halogenated organic substances has to be ensured. Wastes that are fed through the main burner will be decomposed in the primary burning zone at temperatures up to 2,000° C. Waste fed to a secondary burner, preheater or precalciner will be burnt at slightly lower temperatures but it is anticipated that the burning zone temperatures in the precalciner will be in the range of 1,000°–1,200° C.
Volatile components in material that is fed at the upper end of the rotary kiln or as lump fuel can evaporate and be released from the stack without being combusted, since kiln operation is countercurrent. Batch wastes injected at mid or feed-end locations do not experience the same elevated temperatures as liquid wastes introduced at the hot end. In a worst-case scenario, volatile organic compounds may be released from the charge so rapidly that they are not able to mix with oxygen and ignite before they cool below a critical temperature, forming products of incomplete combustion. CO sensors installed for process control can detect incomplete combustion and allow for corrective measures.
The hazardous waste used as a fuel by the cement industry consists mainly of organic material, but may also contain trace amounts of metal components. To determine whether or not a cement kiln can burn hazardous waste fuel effectively, the fate of the organic constituents must be determined.
Testing of cement kiln emissions for the presence of organic chemicals during the burning of hazardous materials has been undertaken since the 1970s, when the practice of combusting wastes in cement kilns was first considered. The destruction and removal efficiency for chemicals such as methylene chloride, carbon tetrachloride, trichlorobenzene, trichloroethane and polychlorinated biphenyls (PCB) has typically been measured at 99.995% and better (Karstensen, 2006).
The potential for using cement kilns to incinerate PCB has been investigated in many countries. The destruction and removal efficiencies determined from several trial burns indicate that cement kilns are effective at destroying PCB. A destruction and removal efficiency of 99.9999% is required by the United States Toxics Substances Control Act for the incineration of these compounds.
5 Process outputs
5.1 General inputs and outputs
The main environmental issues associated with cement production are emissions to air and energy use, and also groundwater contamination from the storage of waste cement kiln dust. Waste-water discharge is usually limited to surface run-off and cooling water only and causes no substantial contribution to water pollution.
Primary process outputs of cement production are:
Product: Clinker, which is ground to produce cement,
Kiln exhaust gas: Typical kiln exhaust gas volumes expressed as m3/Mg (cubic metres per metric ton) of clinker (dry gas, 101.3 kPa, 273 K) are between 1,700 and 2,500 for all types of kilns. Suspension preheater and precalciner kiln systems normally have exhaust gas volumes around 2,000 m3/Mg of clinker (dry gas, 101.3 kPa, 273 K),
Cement kiln dust (collected in the dust collection equipment): In the United States, some 64% of cement kiln dust is recycled back into the kiln and the remainder, which is generated at the rate of about 40 kg/ton of clinker, is primarily buried in landfills (WISE 2002; EPA 2000). Holcim, one of the world’s largest cement producers, sold or landfilled 29 kg of cement kiln dust per ton of clinker in 2001 (Holcim website). Recycling cement kiln dust directly to the kiln generally results in a gradual increase in alkali content of generated dust, which may damage cement kiln linings, produce inferior cement and increase particle emissions (EPA 1998). Also in Europe, cement kiln dust is usually circulated back to the kiln feed material or added directly to the product cement (Lohse and Wulf-Schnabel 1996). The build up of alkalis in wet and dry process kilns is achieved by disposing a portion of the collected cement kiln dust. For preheater and precalciner kilns, this is sometimes accomplished by alkali bypass systems at the preheater tower that removes alkalis from the kiln system.
Alkali bypass exhaust gas: At facilities equipped with an alkali bypass, the alkali bypass gases are released from a separate exhaust stack in some cases and from the main kiln stack in others. According to the United States Environmental Protection Agency, the pollutants in this gas stream are similar to those in the main kiln exhaust gases so that similar pollution abatement equipment and monitoring is required (EPA 1999). An alkali bypass ratio of more than 10% is commonly required for alkali removal (Sutou, Harada and Ueno 2001). However, a bypass ratio of 30% has also been reported (Holsiepe, Shenk and Keefe 2001),
Alkali bypass exhaust gas dust: Depending on the type of air pollution control used for alkali bypass gases, the collected dust can be expected to be similar in content to cement kiln dust.
5.2 Energy use
New kiln systems with five cyclone preheater stages and precalciner will require, on average, 2,900–3,200 MJ/Mg clinker. To optimize the input of energy in existing kiln systems it is possible to change the configuration of the kiln to a short dry process kiln with multistage preheating and precalcination. This is usually not feasible unless it is part of a major upgrade with an increase of production.
Electrical energy use can be minimized through the installation of power management systems and the utilization of energy-efficient equipment such as high-pressure grinding rolls for clinker comminution and variable speed drives for fans.
Energy efficiency will be increased by most types of end-of-pipe abatement. Some of the reduction techniques described below will also have a positive effect on energy use, for example process control optimization.
5.3. Emissions of PCDD/PCDF
5.3.1 Formation of PCDD/F
Any chlorine input in the presence of organic material may potentially cause the formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in heat (combustion) processes. PCDD/PCDF can be formed in or after the preheater and in the air pollution control device if chlorine and hydrocarbon precursors are available in sufficient quantities in the temperature range between 200ºC and 450ºC. A graph of the temperature profile for gases and materials and their typical residence times in each stage of a clinker kiln with cyclonic pre-heater and pre-calciner is shown in Figure 5 (Fabrellas et al, 2004).
In the case of proper operation, cement production is rarely a major source of PCDD/PCDF emissions. Nevertheless, there would still seem to be considerable uncertainty about PCDD emissions (see data reported in Landesumweltamt Nordrhein-Westfalen 1997).
Figure 5. Graph of temperature profile and typical residence times stages of a clinker kiln with cyclonic pre-heater and pre-calciner (Fabrellas et al, 2004)
5.3.2 Sampling and analysis of PCDD/F
Sampling of PCDD/PCDF emissions is, in most cases, undertaken by using one of three methods:
- United States EPA Method 23;
- EN 1948-1;
- VDI Dilution Method 3499 (also an option in EN 1948-1).
PCDD/PCDF analysis is carried out using high-resolution mass spectrometry. Quality control procedures are required at each stage of the analysis and recovery spike concentrations associated with both sampling and extraction. United States EPA Method 23 specifies that all recoveries should be between 70% and 130%.
The lower detection limits measured during the validation test of EN 1948-1, at a municipal solid waste incinerator, varied between 0.0001 and 0.0088 ng/m3 for the 17 individual PCDD/PCDF toxic congeners (see section I.C, paragraph 3 of the present guidelines). In the new draft of EN 1948 (1948-3) of February 2004, Annex B, the uncertainty for the complete procedure is given to be 30–35%, and the external variability is estimated to be ± 0.05 ng I-TEQ/m3 at a mean concentration of 0.035 ng I-TEQ/m3.
A study performed by Environment Canada assessed the variability of sampling and analysis of 53 sets of PCDD/PCDF emission data from 36 combustion facilities. The limit of quantification for PCDD/PCDF emissions was estimated to be 0.032 TEQ ng/m3, although this limit may vary depending on sampling volume, interfering substances and other factors.
5.3.2.1. Detection/quantification limits and interferences
(Karstensen, 2006) The Lower Detection Limit (LOD) measured during the validation test of EN 1948 at a municipal solid waste incinerator varied between 0.0001 – 0.0088 ng/m3 for the 17 individual PCDD/F toxic congeners (EN 1948 -3, 1996).
In the new draft of EN 1948-3 of February 2004, Annex B, the uncertainty for the complete procedure is given to be 30-35 % and the external variability is estimated to be ± 0.05 ng I-TEQ/m3 at a mean concentration 0.035 ng I-TEQ/m3.
Taking into account the toxic equivalence factors for the individual congeners the resulting over all detection limits varies between 0.001 and 0.004 ng I-TEQ/m3. It´s reasonable to assume that concentrations lower than 0.001 ng I-TEQ/m3 should be considered as being below the detection limit.
In a Canadian study performed in 1999 the variability of sampling and analysis of 53 sets of PCDD/F emission data from 36 combustion facilities was investigated. The Limit of Quantification (LOQ) for PCDD/F was estimated to be 0.032 ng TEQ/m3 (Environment Canada, 1999).
Interferences should be expected to occur from compounds that have similar chemical and physical properties to PCDD/Fs (EN 1948 -3, 1996).
5.3.3 Studies on emissions of PCDD/PCDF into air
A comprehensive survey of PCDD/PCDF emissions from cement kilns is given in (Karstensen 2006). The data represents more than 2200 PCDD/PCDF measurements and covers the period from early 1990s until recently. The data represents PCDD/PCDF levels from both wet and dry kilns, performed under normal and worst case operating conditions, and with the co-processing of a wide range of hazardous wastes fed to both the main burner and to the kiln inlet (preheater/precalciner). The data also covers some developing countries in Africa, Asia and South America but no data however, is available from vertical shaft kilns, which is still the dominating process in China (in number of kilns).
Data from several kilns in the United States show PCDD/PCDF emissions as high as 1.76 ng TEQ/m3 when operating their air pollution control devices in the range of 200°–230° C.[1] Tests in the United States also indicated higher emissions for some kilns where hazardous wastes were fired.
In both the United States and German studies, a positive correlation was identified between PCDD emission concentration and electrostatic precipitator/stack temperature. In the United States tests, at one facility the electrostatic precipitator temperature recorded was between 255 C and 400°C. The PCDD emissions were highest at 400°C, and decreased fifty-fold at 255°C. This correlation was generally observed across all facilities tested. At temperatures lower than 250 C in the electrostatic precipitator/stack inlet there was no correlation between temperature and PCDD emissions. This is consistent with known mechanisms of PCDD formation within municipal waste incinerator systems (Karstensen, 2006).
More detailed investigations suggested that – provided combustion is good – the main controlling factor is the temperature of the dust collection device in the gas-cleaning system. The plants equipped with low-temperature electrostatic precipitators appear to have well-controlled emissions with or without alternative fuels (UNEP 2003).
The possible effect of feeding different alternative fuels to the lower temperature preheater/ precalciner was investigated by Lafarge. Wastes injected at mid- or feed-end locations do not experience the same elevated temperatures and long residence times as wastes introduced at the hot end. The observed concentration level of PCDD/PCDF was low in all measurements (Karstensen 2006).
The reported data indicate that cement kilns can comply with an emission level of 0.1 ng TEQ/Nm3, which is the limit value in several Western European countries’ legislation on hazardous waste incineration plants.
In a recent survey performed by Cembureau, PCDD and PCDF measurements from 110 cement kilns in 10 countries were presented. The countries covered by the survey were Czech Republic, Denmark, France, Germany, Hungary, Italy, the Netherlands, Norway, Spain and the United Kingdom. The measurements were performed under standard conditions (dry gas, 273 K, 101.3 kPa and 10% O2) and showed that the average concentration was 0.016 ng I-TEQ/m3 for all measurements. The lowest and highest concentrations measured were < 0.001 and 0.163 ng I-TEQ/m3 respectively (Karstensen, 2006).
The Holcim Cement Company operates cement kilns worldwide. A recent report from Holcim gives average PCDD/PCDF values for 2001 and 2002 as 0.041 ng TEQ/Nm3 (71 kilns) and 0.030 ng TEQ/Nm3 (82 kilns) respectively. Of these measurements, 120 were from countries within the Organisation for Economic Co-operation and Development (OECD), with an average value of 0.0307 ng TEQ/Nm3; the minimum and maximum values measured were 0.0001 and 0.292 ng TEQ/Nm3 respectively, with nine long wet kilns being above 0.1 ng TEQ/Nm3. For the 29 measurements from non-OECD countries, the average value was 0.0146 ng TEQ/Nm3; the minimum and maximum values measured were 0.0002 and 0.074ng TEQ/Nm3 respectively, with no measurements being above 0.1 ng TEQ/Nm3 (Karstensen, 2006).
5.3.4 Study on releases of PCDD/F through solids
Due to the high temperatures involved in the cement production process, PCDD/PCDF concentrations in solid residues are low. The two main solid materials produced in cement production are cement clinker and dust materials trapped in the air pollution control devices.
Within a European research project, samples from a settling chamber and an electrostatic precipitator of a cement kiln were investigated (Stieglitz et al. 2003). The dust sample from the settling chamber showed concentrations of 0.4 ng/g PCDD and 0.98 ng/g PCDF. The material from the electrostatic precipitator contained 2.6 ng/g PCDD and 0.4 ng/g PCDF.
New analyses of solid materials have been gathered from the cement companies participating in the Cement Sustainability Initiative. (Karstensen 2006). 8 CSI companies reported the PCDD/PCDF concentration in cement clinker dust in 2005. 90 samples showed an average value of 6.7 ng I-TEQ/kg, seemingly strongly influenced by a few high level samples. The highest concentration reported was 96 ng I-TEQ/kg.
8 CSI companies reported the PCDD/PCDF concentration in 57 clinker samples in 2005. The average value of all samples was 1.24 ng I-TEQ/kg. The clinker samples came from wet and dry suspension preheater kilns. The highest concentration reported was 13 ng I-TEQ/kg.
Two CSI companies reported the PCDD/PCDF concentration in 11 kiln feed samples in 2005, consisting of raw meal, pellets, and slurry and raw material components. The average value of 1.4 ng I-TEQ/kg. The kiln feed samples came from wet and dry suspension preheater kilns. The highest concentration reported was 7.1 ng I-TEQ/kg.
5.4 Releases of PCB and HCB
Hexachlorobenzene (HCB) and PCB are not subject to regulatory monitoring in cement plants. Those measurements that have taken place have not detected HCB emissions. As regards PCB emissions, 40 measurements carried out in 13 kilns in Germany in 2001 revealed a maximum concentration of 0.4 µg PCB TEQ/Nm3; in nine measurements, no PCB were detected (Karstensen, 2006).
6. Best available techniques and best environmental practices
The following paragraphs summarize best available techniques and best environmental practices for cement kilns firing hazardous waste.
6.1 General measures for management
Legal aspects:
Appropriate legislative and regulatory framework has to be in place to ensure enforcement and to guarantee a high level of environmental protection.
All relevant authorities have to be involved during the permitting process, and in this regard, among other actions, the cement plant operator must:
- establish credibility through open, consistent, and continuous communication with authorities,
- provide necessary information to ensure that authorities are able to evaluate the processing of hazardous waste and
- install community advisory panels early in the planning process,
Environmental aspects:
The use of hazardous wastes as alternative fuel does not significantly change the emissions from a cement kiln stack. However fuels containing pollutants for which the cement process does not have sufficient retention capability (like mercury) shall not be used;
Emission monitoring is obligatory in order to demonstrate compliance with existing laws, regulations, and agreements, with mechanisms for ensuring the reliability of the initial quality control of the process input materials.
Operational aspects:
Only hazardous waste from trustworthy parties throughout the supply chain should be accepted, with the traceability of the waste ensured prior to reception by the facility, with unsuitable waste refused,
Materials transport, handling and storage must be effectively monitored, in full compliance with existing regulatory requirements.
Health and Safety aspects:
Site suitability avoids risks associated with location (proximity to populations of concerns, impact of releases, logistics, transport), infrastructure (technical solutions for vapours, odours, infiltration into environmental media, etc.).
Adequate documentation and information are mandatory, providing an informed basis for openness and transparency about health and safety measures and standards, and ensuring as well that employees and authorities have such information well before starting any use of hazardous waste derived alternative fuel in a cement kiln facility.
Communication issues and social responsibility:
In the interest of openness and transparency, the cement kiln operator must provide all necessary information to allow stakeholders to understand the purpose of the use of hazardous waste in a cement kiln, the context, the function of the parties involved and decision-making procedures. In summary the following general management aspects should be taken into account:
General infrastructure, paving, ventilation;
General control and monitoring of basic performance parameters;
Control and abatement of gross air emissions (NOx, SO2, particles, metals);
Development of environmental monitoring (establishing standard monitoring protocols);
Development of audit and reporting systems;
Implementation of specific permit and audit systems for use of alternative fuels;
Demonstration by emission monitoring that a new facility can achieve a given emission limit value;
Occupational health and safety provisions: Cement kilns feeding alternative fuels need to have appropriate practices to protect workers handling those materials during the feeding process;
Sufficient qualification and training of staff.
6.2. Specific measures
For new plants and major upgrades, best available techniques for the production of cement clinker are considered to be a dry process kiln with multistage preheating and precalcination. For existing installations, partial (and perhaps considerable) reconstruction is needed.[1]
Indirect measures for control of chemicals listed in Annex C of the Stockholm Convention have a minor impact in specific cases, but are an important element of integrated emission control.
6.2.1 Primary measures and process optimizing
6.2.1.1 Process optimization
Quick cooling of kiln exhaust gases lower than 200° C. The critical range of temperature is usually passed through quickly in the clinker process;
Characterize a good operation and use this as a basis to improve other operational performance. Having characterized a good kiln, establish reference data by adding controlled doses of waste, and look at changes and required controls and practice to control emissions;
Management of the kiln process to achieve stable operating conditions, which may be achieved by applying process control optimization (including computer-based automatic control systems) and use of modern, gravimetric solid fuel feed systems;
Minimizing fuel energy use by means of: preheating and precalcination as far as possible, considering the existing kiln system configuration; use of modern clinker coolers, enabling maximum heat recovery; and heat recovery from waste gas;
Minimizing electrical energy use by means of power management systems, and grinding equipment and other electricity-based equipment with high energy efficiency.
Control of chemicals listed in Annex C: Indirect measures for control of chemicals listed in Annex C have a minor impact in specific cases, but are an important element of integrated emission control. Such measures are generally applicable and are of simple technical construction.
6.2.1.2 Hazardous waste preparation
Pretreatment of hazardous waste, with the objective of providing a more homogeneous alternative fuel and more stable combustion conditions, may include drying, shredding, mixing or grinding depending on the type of waste (see also chapter 4.3.5). Ii is important to give attention to:
Well-maintained and appropriate storage and handling of alternative fuels;
Well-maintained and appropriate storage and handling of wastes and site.
These measures are not typical and specific for the elimination and reduction of Annex C chemicals, but are elements of integrated emission control.
6.2.1.3 Input controls
Consistent long-term supply of alternative fuels (supplies of a month or more) is required to maintain stable conditions during operation;
Careful selection and control of substances (sulphur, nitrogen, chlorine, metals and volatile organic compounds) entering the kiln;
Continuous supply of fuel and waste with specification of heavy metals, chlorine (limitation, product/process dependent), sulphur;
Feeding of waste through the main burner or the secondary burner in precalciner/preheater kilns (ensure temperature > 900o C);
No waste feed as part of raw mix, if it includes organics;
No waste feed during start-up and shutdown.
Control of chemicals listed in Annex C: Indirect measures for control of such chemicals have a minor impact in specific cases, but are an important element of integrated emission control. Such measures are generally applicable and are of simple technical construction. Formation of chemicals listed in Annex C is possible within relevant temperature ranges.
6.2.1.4 Stabilization of process parameters
Regularity in fuel characteristics (both alternative and fossil);
Regular dosage;
Excess oxygen;
Monitoring of CO.
Control of chemicals listed in Annex C: Indirect measures for control of such chemicals have a minor impact in specific cases, but are an important element of integrated emission control. Such measures are generally applicable and help ensure stable operating conditions.
6.2.1.5 Process modification
The off-gas dust should be fed back into the kiln to the maximum extent practicable, in order to reduce issues related to disposal and emissions. Dust that cannot be recycled should be managed in a manner demonstrated to be safe.
Control of chemicals listed in Annex C: Indirect measures for control of such chemicals have a minor impact in specific cases, but are an important element of integrated emission control.
In general, the primary measures mentioned above are sufficient to achieve an emission level below 0.1 ng TEQ/Nm3 in flue gases for new and existing installations. If all of these options do not lead to a performance lower than 0.1 ng TEQ/Nm3 secondary measures may be considered, as described below.
6.2.2 Secondary measures
The secondary measures cited below are installed at cement kilns for other pollution control purposes, but they show a simultaneous effect on emissions of chemicals listed in Annex C.
6.2.2.1 Further improvement of dust abatement and recirculation of dust
Control of chemicals listed in Annex C: Efficiency may decrease with decreasing temperature of dust precipitation; general applicability; medium technical construction; capture of chemicals listed in Annex C bound to particles.
6.2.2.2 Activated carbon filter
This measure has high removal efficiency for trace pollutants (> 90%). Pollutants such as sulphur dioxide (SO2), organic compounds, metals, ammonia (NH3), ammonium (NH4+) compounds, hydrogen chloride (HCl), hydrogen fluoride (HF) and residual dust (after an electrostatic precipitator or fabric filter) may be removed from the exhaust gases by adsorption on activated carbon. The only activated carbon filter installed at a cement works in Europe is that at Siggenthal, Switzerland. The Siggenthal kiln is a four-stage cyclone preheater kiln with a capacity of 2,000 tons of clinker per day. Measurements show high removal efficiencies for SO2, metals and PCDD/PCDF (European Commission 2001).
Control of chemicals listed in Annex C: General applicability; demanding technical construction.
6.2.2.3 Selective catalytic reduction
In general, selective catalytic reduction installations are applied for NOx control. The process reduces NO and NO2 to N2 with the help of NH3 and a catalyst at a temperature range of about 300°-400° C, which would imply heating of the exhaust gases. Up to now selective catalytic reduction has only been tested on preheater and semi-dry (Lepol) kiln systems, but it might be applicable to other kiln systems as well (European Commission 2001). Its high cost could make this solution economically unviable. The first full-scale plant (Solnhofer Zementwerke) has been in operation since the end of 1999 (IPTS 2004).
Control of chemicals listed in Annex C: Demanding technical construction; expected improvement in control of chemicals listed in Annex C by efficient catalysts.
7. Performance requirements based on best available techniques
Performance requirements based on best available techniques for control of PCDD/PCDF in flue gases should be < 0.1 ng TEQ/Nm3. Emission levels shall be corrected to 273 K, 101.3 kPa, 10% O2 and dry gas.
8. Monitoring
8.1 Monitoring of pollutant emissions and operation parameters
To control kiln process, continuous measurements are recommended for the following parameters (European Commission 2001):
Pressure;
Temperature;
O2 content;
NOx;
CO, and possibly when the SOx concentration is high;
SO2 (a technique is being developed to optimize CO with NOx and SO2).
To accurately quantify the emissions, continuous measurements are recommended for the following parameters (these may need to be measured again if their levels can change after the point where they are measured to be used for control):
Exhaust volume (can be calculated but the process is regarded by some as complicated);
Humidity;
Temperature at particulate matter control device inlet;
Particulate matter;
O2;
NOx;
SO2;
CO.
Regular periodical monitoring is appropriate for the following substances:
metals and their compounds;
Total organic carbon;
HCl HF;
NH3;
PCDD/PCDF.
Measurements of the following substances may be required occasionally under special operating conditions:
Destruction and removal efficiency, in case of destruction of persistent organic pollutants in cement kilns;
Benzene, toluene, xylene;
Polycyclic aromatic hydrocarbons;
Other organic pollutants (for example, chlorobenzenes, PCB including coplanar congeners, chloronaphthalenes).
It is especially important to measure metals when wastes with higher metal content are used as raw materials or fuels.
References
CEMBUREAU. 2004. Guidelines on Co-Processing of Waste Materials in Cement Production
CSI, 2005. Guidelines for the Selection and Use of Fuels and Raw Materials in the Cement Manufacturing Process- Fuels and Raw Materials. Cement Sustainability Initiative (CSI) Draft December 2005
De Bas P. 2002. The Economics of Measurement of Emissions into the Air. European Measurement Project. Pembroke College, Oxford, UK.
DFIU/IFARE (French-German Institute for Environmental Research). 2002. Cement/Lime Industry. Draft Background Document in preparation for 5th EGTEI Panel Meeting, 29 November 2002. www.citepa.org/forums/egtei/cement_lime_draft.pdf.
EN 1948-1, 1996 “Stationary source emissions – Determination of the mass concentration of PCDDs/PCDFs – Part 1: Sampling”. European Standard, CEN, rue du Stassart 36, 1050 Brussels.
EN 1948-2, 1996 “Stationary source emissions – Determination of the mass concentration of PCDDs/PCDFs – Part 2: Extraction and clean-up”. European Standard, CEN, rue du Stassart 36, 1050 Brussels.
EN 1948-3, 1996 “Stationary source emissions – Determination of the mass concentration of PCDDs/PCDFs – Part 3: Identification and quantification”. European Standard, CEN, rue du Stassart 36, 1050 Brussels.
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Fabrellas et al, 2004. Begoña Fabrellas, David Larrazabal, M. Angeles Martinez, Paloma Sanz, M. Luisa Ruiz1, Esteban Abad, Josep Rivera. Global Assessment of PCDD/F Emissions from the Spanish Cement Sector. Effect of Conventional /Alternative Fuels Organohalogen Compounds – Volume 66 (2004)
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[1] The dry process is only appropriate in the case of limestone as a raw material feed. It is possible to utilize preheater/precalciner technology to process chalk, with the chalk slurry dried in a flash dryer at the front end of the process.