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WASTE-DERIVED FUEL USE IN CEMENT KILNS
by Beena Tanna and Bert Schipholt
For industry to maintain sustainable development it must use resources wisely and operate to minimise adverse environmental impacts. There are many beneficial effects from use of wastes as fuel or raw materials in the manufacture of cement. Such benefits can include conservation of fossil fuels, reduced CO2 emissions, reduced SOX and NOX emissions, reduced methane emissions from coal mining, movement of waste up the hierarchy, and more globally competitive industries.
Despite this, the practice continues to be controversial in many parts of the world. The arguments against the use of waste as fuel or raw material often overlook the unique aspects of cement process chemistry and engineering that significantly impact capture efficiencies and volatility of inorganic materials. Another common mistake is a failure to account for the natural variability of minor constituents in cement plant raw materials and the beneficial impact that a wide variety of minor elements can have in the cement manufacturing process. To make informed decisions about waste recovery projects all the risks and benefits should be considered using the best available information.Waste fuels, often referred to as Secondary Fuel (SF), have been used by cement kilns for at least 20 years. Much research and testing has been done on emissions, affects on cement, leachability of metals and environmental impacts.
This document examines the issues and reviews some of the extensive data that is available as it relates to use of SF in cement kilns. The use of wastes as raw materials is not a notable topic in this discussion.
Cement production is an extremely energy-intensive process. The average energy requirement to produce 1 ton of cement is approximately 4.4 million Btu1. This is roughly the equivalent to 400 pounds of coal. The most significant benefit to the use of SF in cement kilns is the recovery of energy value from wastes. In the case of cement kilns, the question of what constitutes legitimate energy recovery does not have a simple answer.
Any definition of energy recovery must be based on the ability of the waste to contribute heat to the cement manufacturing process during combustion. One definition of energy recovery from SF is the following:
The SF has an autogenic heating value necessary to make a net positive input to the thermal process that supports calcination.
Cement kilns uses counter-current flow for exchange of heat energy from the process gases to the raw material. There are four important stages in the process of manufacturing cement. The first stage is the heating of the raw material to about 900° C. At this temperature the second stage begins which is a chemical reaction called calcination. The third stage is further heating of the material to about 1300° C. The final stage, sintering, is a chemical reaction resulting in the formation of the clinker.
The sintering reactions take place only after the calcination has occurred and the materials have been brought to high enough temperatures. Once this has been done, these reactions are exothermic and sufficient heat is released to bring the materials to the maximum temperature. The sintering process is not the one requiring large amounts of energy; it just requires high temperatures. The majority of the heat energy is used to preheat the feed solids and convert the limestone to lime (calcination). The maintenance of the raw material temperature at about 1400° C in the sintering zone is accomplished by the exothermic reaction.
From an engineering standpoint, energy recovery occurs when useful energy is transferred from an outgoing process flow to an incoming flow so that the energy efficiency of the system is increased. For the cement kiln, the outgoing flow is the exhaust gas and the incoming flow is the raw material feed. In a preheater/ precalciner kiln the outgoing gas stream contacts incoming feed solids and provides counter-current heat exchange until the gas exits the preheater, typically at 370° C. For long-wet kilns the exit gas temperature is even lower.
The use of SF often lowers the temperature of the combustion gases in the hottest part of the kiln. However, energy recovery occurs as long as there is still enough heat released to contribute to the calcination process. Lowering the combustion gas temperature actually has the benefit of reducing NOX emissions. Some kiln operators have done this with the use of "low NOX" burners but it can also be accomplished with SF.
It is ability of the waste, when combusted, to transfer heat energy to the raw materials in the manufacturing process, not the temperature at the hottest end of the kiln, that is important for determining energy recovery. In fact, not all SF is input at the hottest portion of the kiln. Many kiln operators add SF in the form of whole tires or containerised chemical wastes in the middle portion of the kiln where the temperature is closer to 1000° C.
It is also important to note that wastes with low calorific content can be beneficial to cement manufacturing for other reasons. Kilns operate more efficiently when fuel has a narrow calorific content range. Fuel with calorific content significantly higher or lower than the average makes operation of a kiln more difficult. Blending of wastes with low and high calorific content allows for a more consistent fuel to be produced. The stable operation of a cement kiln has certain environmental benefits that include:
Limits on chlorine input into a cement kiln are so process and chemistry dependent that concentration limits in SF can vary from <0,1% to 5,0% or greater depending on fuel feed rates2. One of the primary factors influencing a cement kiln's ability to handle chlorine is the level of potassium and sodium in the plant's raw materials. Generally, these are undesirable components in cement clinker.
Chlorine entering a cement kiln will quickly react with and tightly bond with potassium and sodium forming the respective salts. Because of the design of the cement manufacturing process, these salts exit the system with the kiln dust. This has the beneficial impact of reducing potassium and sodium (alkali) levels in cement clinker. Some cement plants add calcium chloride to produce this effect. Other factors that influence a cement kiln's capacity for chlorine includes design, CKD recycle rate3, and sulfur input rates. The equations for calculating chlorine capacity are fairly complex and depend on site-specific factors.
It should also be noted that there is no validated method for accurately measuring HCl emissions from a cement kiln. Current methods measure volatile chlorides that include ammonium chloride and alkali chloride salts. There is growing evidence that HCl may actually be formed in the sampling train from the interaction of ammonia, water vapour and potassium/sodium chloride salts in the sampling train. Any existing HCl emission data for cement kilns must be considered as likely biased high for these reasons4.
Sulfur is introduced into cement kilns from both the raw materials and fuels. SF typically contains less than 1% sulfur while coal often contains more than 2%. Actual testing has shown that SOx emissions are often reduced by replacing coal with SF5,6. However, sulfur in fuel can behave quite differently than sulfur in cement plant raw materials. Frequently, changes in fuel sulfur concentrations have no impact on SOx emissions because dominant influences can come from sulfur, usually as pyrites, in the kiln raw material feed. This condition has probably occurred at one cement plant in the UK7 while at least one plant in the UK has shown decreased SOX emissions with the use of SF5.
Sulfur is similar to chlorine in that kiln design and chemistry have a major impact on sulfur capacity. Sulfur limits can vary from 0,5% to 5,0% or higher depending on process chemistry and fuel feed rates. Since SF normally has very little sulfur, it is generally not an issue. There are, however, certain waste streams with sulfur levels comparable to high sulfur coals. These can be utilised in some cement kilns without negatively impacting emissions. It is also important to note that some sulfur input can actually benefit cement clinker in some plants8.
Iodine should not be grouped together with other halogens when determining feed rate limits. The chemistry and volatility of iodine is different than the other halogens in a cement kiln. This is ususally not a concern because iodine is rare in SF. In a cement kiln, iodine is generally quite volatile and likely to be emitted as elemental iodine. For this reason, the use of waste containing significant (>0,1%) iodine is not recommended.
Bromine behaves similar to chlorine9 and an aggregate limit should be applied to chlorine and bromine, although bromine levels in SF are rarely significant.
Fluorine has historically been added to cement kilns to act as a fluxing agent. The optimum concentration for this beneficial impact depends on the raw feed chemistry. Fluorine is highly reactive in the cement kiln and leaves the system in the clinker as a calcium fluorosilicate10. Up to 1,0% fluorine in SF is unlikely to have any impact on fluorine emissions. Fluorine limits in waste derived fuel above 1% may require further process evaluation and testing.
Actual testing has shown that NOx emissions are normally reduced by replacing coal with SF5,6. The flame in a cement kiln must remain very hot in order to provide a favourable reactive atmosphere for the complete combustion of coal. Coal is injected into a kiln as a solid and must under go gasification prior to combustion. This results in a longer flame, providing for the formation of more nitrogen oxides than with a shorter flame typically observed when coal and SF are used together6. It is also possible that the higher water content in SF relative to coal suppresses flame temperatures enough to decrease NOx emissions.
The concept of setting a nitrogen limit on fuels may have some applicability for incinerators and boilers but because of the extremely high flame temperatures (2010-2200°C) in a cement kiln, it does not impact NOX emissions8. The large amount of nitrogen entering the cement kiln as combustion air is the source of NOx emissions. Because of this overwhelming impact from nitrogen in air, any additional nitrogen in the fuel is not likely to impact NOx emissions. Therefore nitrogen limits on waste derived fuel for cement kilns are not needed.
A variety of inorganics such as calcium, aluminium, iron, silicon, titanium and chromium can have beneficial impacts on clinker11. Establishing any limits for ash in SF would require the exclusion of these, and perhaps other, inorganics. This would require the detailed analysis of ash to measure the excluded components. Efforts are more efficiently directed at establishing and monitoring feed rate limits for inorganics of concern to health and the environment. There is no need for a regulatory limit on ash in cement kilns because it is redundant with limits on inorganics of concern.
Cement kilns do not generate ash. The ash from the combustion of fuel is incorporated into the clinker or cement kiln dust (CKD). CKD, which is captured in the air pollution control device, consists primarily of partially calcinated raw material. Most cement kiln operators normally return the majority of the CKD to the kiln to complete the calcination and sintering process. When reintroduced, CKD does not contribute any constituents to clinker production that are not already present in the production process. Metals in CKD and cement are discussed in greater detail below.
METALS CEMENT KILNS
Metals behaviour in cement kilns is not simple. To establish SF metals limits for cement kilns it is helps to understand where the metals come from and what happens to them. Any constituent present in the cement product is derived primarily from either the cement ingredients or fuels. To some extent, ingredients come from the attrition of the grinding media and kiln lining. However, the predominate source of metals input is from the raw materials, even in kilns using SF12,13,14. Depending on the metal, the raw material is normally the source of between 60 to 95% of the metal input in cement kilns15.
Raw materials can include limestone, clay, shale, sand, bauxite, iron ore, coal ash, mill scale, and other industrial by-products. These raw materials contain all the metals and halogens found in SF. Some of the elements reported in raw materials include aluminium, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, phosphorus, potassium, rubidium, selenium, silver, sodium, thallium, strontium, tin, titanium, vanadium, zinc, bromine, chlorine, fluorine and iodine. Concentrations of these elements vary considerably in raw materials.
Ingredients at a specific cement plant can differ substantially in the amounts of minor elements present in the feed. For example, one location of the limestone in a quarry may contain 1-ppm lead and another location may contain 10-ppm lead, an increase by a factor of 10. The use of secondary raw materials in cement manufacturing introduces even more variability. For example, coal ash from electrical power plants is sometimes added in varying amounts as an ingredient to produce the proper proportion of calcium and aluminium silicates. Coal ash, contains trace metals such as arsenic, copper, selenium, tin, cadmium, lead, sulfur, zinc, chromium, nickel and thallium16. As with traditional raw materials, the type of metals and their concentrations in coal ash can vary considerably.
Fuels used in the production of cement may include natural gas, petroleum coke, waste lubricating oil, motor oil, refuse derived fuel, tire derived fuel and most commonly coal. All of these fuels contain trace elements that become constituents of kiln feed upon combustion. Many kilns obtain their coal or petroleum coke from more than one source. Metals in these conventional fuels can vary considerably from one source to another.
Metals in conventional fuels are not necessarily at lower concentrations than in typical SF. In some cases, metals in clinker actually decrease with the use of waste derived fuels12,13. In particular, inputs of arsenic, barium, beryllium, mercury, selenium, silver, thallium, vanadium, silver, and cadmium could be reduced as a result of substituting SF for coal or coke. If overall risk is to be evaluated, then any reductions of metals must also be considered.
Distribution of trace elements in the cement manufacturing process is highly variable. Non-volatile trace elements from the raw materials or fuels tend to become incorporated into the clinker. Semi-volatile elements tend to be more equally distributed between the clinker and CKD. While, volatile metals tend to evaporate at high temperatures in the burning zone, condense at cooler temperatures in the kiln system, and then become incorporated into the CKD. Of the metals likely to be in industrial wastes, only mercury is volatile enough for significant percentages to remain in the vapour phase.
The partitioning of metals is further complicated by the development of recirculating loads. For example, lead oxide and other materials volatilise in the burning zone and condense downstream onto small particles of feed material in the cooler zones of the kiln. As the feed material moves into the hotter regions of the kiln, the volatile components once again evaporate and recirculate within the kiln system. The affect is an enrichment of these volatile materials in the feed. If less CKD is removed from the system, lead will tend to be found at higher concentrations in the CKD. Since lead is abundant in earth materials, recirculating loads of lead can be experienced in virtually any cement kiln.
Obviously, the amount of trace elements entering any kiln can be highly variable. All the parameters described above operate in concert to produce observed emissions and partitioning between cement and CKD.
METALS IN CEMENT
The presence of trace amounts of inorganics should not preclude the use of waste as fuel. SF with metals content as high as a few thousand ppm has been used to produce high quality cement in the United States (US) for over 20 years. These cement plants routinely test their products according to American Society for Testing and Materials (ASTM) standards. It is not uncommon for cement kilns to produce clinker of higher quality with SF than is produced with traditional fuels because clinker quality can be positively affected by the addition of many substances.
The bioavailability of metals in cement and its concrete products is likely quite limited. For the metals of greatest health concern, Environmental Science & Engineering calculated concentrations that would be acceptable in cement under the assumption that the bioavailability of metals in cement is limited to 10%16. These "acceptable levels", shown in Table 1, were derived from occupational exposure limits set by regulatory agencies. The magnitude and length of public exposure to cement dust and cement products are not on the same scale as occupational exposures. Since occupational exposures were used as the most stringent levels of concern, these levels are also protective of public health. These levels are 1 to 4 orders of magnitude greater than the concentrations typically found in cement produced with or without SF.
Repeated studies have shown that metals in cement and mortar cubes produced with and without SF will not leach out in amounts of concern to public health13,17,18,19,20,21. Much cement produced in Europe and the US has been tested according to the National Sanitation Foundation International (NSF) ANSI/NSF 61 standard. All of these cements have been shown to be acceptable for use in drinking water systems18. Germaneau et al tested mortar bars made from nine commercial cements. Ten trace metals were determined in the leachates, all were below the then current US, European, and French specifications after the fourth step of leaching, irrespective of cement quality, original concentration of the metal in cement, curing time, or water to cement ratio20.
Acceptable Level (mg/kg)
The US Environmental Protection Agency (USEPA) has reviewed the results of laboratory analyses of 12 metals (arsenic, antimony, barium, beryllium, cadmium, chromium, lead, mercury, nickel, selenium, silver and thallium) in cement samples collected from 97 North American kilns. After comparing the results from kilns using conventional fuels with those using WDF [waste-derived fuels], the USEPA concluded that "(a) the mean "total metal" concentration of only one metal, chromium, was statistically significantly higher in cement from kilns burning WDF than from kilns burning FF [fossil fuels]; (b) for the remaining 11, some of the "total metal" concentrations were higher and some lower, but none by statistically significant amounts; and (c) none of the "leachable metal" concentrations differed significantly in cement for any of the 12 metals"21. The USEPA cautioned against attributing the higher chromium to the use of WDF because for any particular kiln system, the concentration of these elements in cement ad CKD is a function of the manufacturing process and the total metals input from all sources.
Trace metals have always been present in cement. The evidence does not confirm that the use of SF in kilns materially increases concentrations of metals in cement. The evidence does show that in specific circumstances some metals may slightly decrease while others may slightly increase. This is of little consequence since metal concentrations in cement are below levels that would raise concerns about human health.
METALS IN CKD
The USEPA completed an extensive study of CKD in December 1993. At least seven years in the making, this study included reviewing evidence of damage to human health and the environment, performing direct and indirect risk assessments, and reviewing the results of laboratory analysis of CKD samples. The USEPA has concluded that for "many toxic metals, the concentrations detected in kiln dust were not significantly different whether the dust is generated from kilns that burn or do not burn hazardous waste. However, for lead, cadmium, and chromium, the mean concentration found in CKD generated by kilns that burn hazardous waste is measurably higher than CKD from those that do not burn hazardous waste; conversely, thallium and barium concentrations are measurably higher in CKD from kilns that do not burn hazardous waste. While the lead, cadmium, and chromium were observed to be higher in kilns that burn hazardous waste, the difference in mean concentrations by themselves are not enough (i.e., do not differ by more than a factor of about 2) to result in discernible risk estimates between facilities that do and do not burn hazardous waste". The EPA also noted that concentrations of barium, chromium, and nickel in CKD are within the typical range found in US soils22.
Further, the USEPA determined that in terms of potential constituent solubility and release, leach test results show that no significant distinction can be made between CKD generated from kilns that burn hazardous waste and those that do not burn hazardous waste22.
The USEPA predicted only low or negligible risk potential from on-site management of CKD via conventional direct pathways of constituent transport and exposure (i.e., drinking water, incidental direct ingestion, chemical inhalation) via ground water contamination, surface water runoff to streams or lakes, or windblown dust. They did suggest that further attention needs to be given to windblown dust from uncontrolled CKD waste management units to determine if the USEPA's health-based fine particle (10 micron or less) standard is exceeded at or beyond plant boundaries.
The predominate source of metals in both cement and CKD is the raw material. Further, it is the clinker chemistry, countercurrent materials/gas flow, volatility, and kiln operation that affects where they end up, not their original source. As is the case with cement, trace metals have always been present in CKD. Although in some cases certain metals may slightly increase with the use of SF, other metals are likely to decrease. The available evidence overwhelmingly indicates that most metals limits can safely be established at levels ranging from a few hundred to few thousand ppm.
Extensive scientific research, laboratory analysis, and risk evaluation have failed to show that trace metals in cement or CKD manufactured with SF pose any significant increased risk to the environment when managed properly. The primary concern with metals should be emissions.
The design and operation of the cement kiln determines the emissions of elements. Cement kilns typically have removal efficiencies for non-volatile metals of 99.9% or better. Removal efficiencies for the semi-volatile metals are generally better than 99.5% 15,23. Evidence suggests, that increasing the amount of metals input in the high temperature zone of cement kilns is likely to improve these removal efficiencies further 23,24. Mercury is an exception that has potential for escaping from the system in the vapour phase and should be limited in SF.
Most limits for SF are most appropriately established as feed rates. This is particularly true for metals. Although there are many similarities between kilns, each one has unique design and operational characteristics. One broad-based set of standards for SF is not appropriate. The single most important issue is protection of human health and the environment. This is best accomplished by establishing acceptable emissions levels and then designing SF specifications for individual cement kilns based on the best available techniques not entailing excessive cost (BATNEEC) for cement kilns.
Regulatory authorities are cautioned against establishing low level limits for metals without scientific justification. This may severely and unnecessarily restrict waste materials for use as SF. Seriously limiting the waste that can be used for energy recovery without legitimate cause and diverting it to incineration or landfill is counter to the principal of best practical environmental option (BPEO).
BEST PRACTICAL ENVIRONMENTAL OPTION
The objective behind BPEO is to minimise the chance of polluting the environment, taken as a whole, for any wastes or discharges arising from an industrial process. The UK uses a waste management hierarchy of waste reduction, reuse, recovery or, if these options are not viable, disposal25. Recovery takes on many different forms including recycling and energy recovery. The UK's approach to waste management is consistent with the European Union strategy and EC Directives, 75/442/EEC, 91/156/EEC which promotes the prevention and reuse of wastes prior to incineration and landfill.
According to the Government and the House of Commons Environment Committee, "materials recycling (which includes solvent recovery) should not be seen as automatically preferable to energy recovery"26. The advantages and disadvantages of each technology for a particular waste must be evaluated when determining the overall BPEO.
Using proven process technology, the cement industry has made controlled use of large quantities of wastes as alternative raw materials and fuels for decades. In many ways the cement industry has become an integral actor in waste management schemes. The variety of materials that are suitable for use in cement kilns includes such wastes as tires, plastic, oils, solvents, coal ash from power plants, blast furnace slag, and foundry sand, among many others. Many of these wastes, which are usually generated in large quantities, would require disposal through incineration or landfill if not recovered in cement kilns. For good reasons, there is an increasing trend toward recovering energy from wastes. This is true not only in the UK but also world-wide. The benefits from the use of organic wastes as fuel in cement kilns include:
A discussion of the economical benefits is beyond the scope of this document. However, it is clear that the use of industrial by-products for fuels and raw materials allows the UK cement industry to remain more competitive in a global market. It is also clear that cement kilns are a tremendous resource to assist the UK in moving wastes up the hierarchy, both from disposal to energy recovery and from disposal to materials recovery26.
When determining the BPEO on a case by case basis, it is important to consider all of the global environmental impacts. The specific environmental benefits from the use of SF in cement kilns include:
Pressures on the environment arise from the methods used in extracting coal and from the methods used in converting it into other forms of energy. SF can replace nearly 100% of the coal used in cement kilns although 40% to 60% substitutions are more typical in the US. The use of SF for energy recovery in the UK is effectively limited to 40% by EC Directive 94/67. If a single cement plant producing 800000 tons of clinker per year replaces 40% of its coal with SF, approximately 60000 tons/year of coal are saved.
Replacing coal with SF not only reduces the environmental impacts from mining coal but also reduces global emissions of CO2, a "greenhouse gas". The burning of fossil fuels also releases oxides of sulfur and nitrogen into the atmosphere. Both contribute to acid rain. Nitrous oxide is a "greenhouse gas".
The production of coal results in about 12% of the methane, another "greenhouse gas", emissions in the UK25. Methane emissions are reduced for every ton of coal replaced by SF.
Damages associated with the mining and transportation of coal are significant and well known. They include illnesses, accidents with injuries and deaths, ecological damages, despoiled lands, and many other impacts. Two well know illnesses associated with coal mining include fatal lung cancer from radon and black lung disease. The reduction in these risks must be taken into account when evaluating the overall BPEO with respect to SF. Most risk assessments performed for kilns using SF have shown little or no increased risk. In some cases, reduced risks have been calculated. In any case, the changes are normally slight. It is quite likely that the reduced risks from the reduction in coal mining alone will greatly exceed any increased risk from the use of SF, if there is one.
Resource and waste management strategies must aim toward sustainable development, to maximise natural resources conservation within economically and socially acceptable boundaries and to minimise emissions to the air, water and land. Cement kilns are a valuable resource toward this end because many wastes can be used beneficially in the manufacturing process. Specialised incineration and product recycling by thermal treatment both offer sound environmental and economical solutions to different wastes. The regulatory framework should be designed to favour the use of BPEO for these wastes. Regulatory authorities are should guard against establishment of conditions that hinder the implementation of BPEO.
For the reasons stated above, limits on nitrogen and ash are not needed for SF. The remaining parameters, metals, halogens and sulfur, are too dependent on the specific design, raw materials, and operation of cement kilns to establish broad-based limits. It is most appropriate to establish individual feed rate limitations for these parameters based on acceptable levels of emissions. A one-size-fits all approach cannot be used due to the unique characteristics of each kiln.
There is a need to move beyond extreme positions and focus on what must be done to promote economic growth while protecting the environment. Real solutions are found when industrial processes function in co-ordination and harmony with the environment. Reuse, recycling and waste minimisation is central to this theme. Industry and the Government should promote the complete and truthful information on the environmental advantages and impacts of various waste management practices.
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ERAtech Group LLC is dedicated to the development of waste derived fuels (WDF) operations worldwide. The partners in ERAtech include Robert Kohnen, David Gossman and Bert Schipholt .
ERAtech is firmly committed to the use of its technology to assist in the revalorization of waste materials. Types of fuels that they have successfully utilized not only involve secondary liquid fuels, but also high viscosity organic solids and semi-solids, tyres, paper, plastics, sorted wastes including packaging wastes, pumpable waste streams with heat value and one off materials.
They currently are assisting cement manufactures to reduce their fuel costs in the US, UK, Taiwan, Chile and other corners of the world.45W962 Plank Road