PCA R&D Serial No. 2556

Management of Detached Plumes in Cement Plants

by F. MacGregor Miller

Presented at 2001 IEEE-IAS/PCA Cement Industry Technical Conference

Portland Cement Association 2001 All rights reserved

This information is copyright protected. PCA grants permission to electronically share this

document with other professionals on the condition that no part of the file or document is changed

MANAGEMENT OF DETACHED PLUMES IN CEMENT PLANTS

By F. MacGregor Miller

Construction Technology Laboratories, Inc. Abstract The paper examines the phenomenon of detached plumes in cement plants. A discussion of reasons why such a plume forms is followed by a presentation of the various types of detached plumes (steam plumes, organic plumes, sulfuric acid mist, ammonium salts, etc.) A discussion of the chemistry occurring to cause these plumes is then offered, followed by a number of remediative measures that may be effective in ameliorating these plumes, offered on a site-specific basis. Emphasis is placed on the fact that to eliminate such plumes, one or more of the contributing chemical species must be controlled. Introduction Detached plumes are a phenomenon in industrial processes, characterized by the observation that no opacity is observed directly above the outlet of the stack, but that further downstream, typically two to four stack diameters, a plume appears. The plume may be of various colors—white, blue, gray, and yellow or brown are the most common—and it may be of highly variable density. The opacity associated with such a plume is not read by an in-stack opacity meter, since within the stack there is normally little or no observable opacity. For this reason, detached plumes are sometimes termed “reactive” or “dynamic” plumes, or “pseudoparticulate” plumes, because most of the particulate matter responsible for their visibility is not present as particulate in the stack, or in the inlet to the kiln dust collector. It is important to stipulate that if the opacity is attributable to condensation of water droplets from steam only, there may be a “detached plume”, but it will not be considered as such in this article. Such a detached plume will completely evaporate further downwind, without any “trail off”; no emissions are actually occurring in such a case. Even detached plumes that show a “trail off” of pseudoparticulate matter are often more of a visual nuisance than an actual pollution event. The actual amount of particulate matter that they represent is usually rather small. Their visibility is usually due Nonetheless, their very visibility makes them a potential problem for the manufacturer, because the neighbors can clearly see the emissions, and are often concerned that they may constitute something harmful. For this reason, although there are not yet any nationwide regulations governing the control and elimination of detached plumes, they are assuming increasing prominence among issues that concern manufacturing plants, particularly cement plants, in their dealings with local regulators. Why does a Plume Form? Theory of Light Scattering 100% transmittance is a term implying the light from a source can be viewed without any attenuation. By contrast, 100% absorbance or scattering would result in no light being detected. Light passing through a plume is scattered by particles in the plume. The degree of light scattering is determined by the path length, concentration of the particles, and physical properties of the particles. These physical properties include the mean particle size, particle refractive index, and particle density. Large particles will have a very small effect on opacity, while very small particles will have a large effect. The largest effect will be for particles with diameters

comparable to the wavelength of visible light (about 0.4 to 0.7 µm), as shown in Fig. 1 (Dittenhoefer, 1984).

Fig. 1—Nominal Opacity Factor (light attenuation) vs. Particle Size Examination of Fig. 1 shows that either large (>5 µm) or small particles (<0.1 µm) do not affect opacity as much as particles with diameters in the range of the wavelength of visible light. Assuming that the concentration of particulate matter remains constant, and that the path length (related to the plume thickness) remain constant, it is reasonable to suppose that the opacity will increase as the fraction of the particles in the size range of interest increases, normally either at the expense of the finer particles, or because further particulate matter forms from gases in the plume. Types of Detached Plumes The state of our knowledge of detached plumes does not permit us to identify all the chemical species that may contribute to detached plumes; at present, however, there seem to be five general categories of plumes that may develop during the process of clinker manufacture in rotary cement kilns: 1. Actual dust particulate 2. Condensation of semivolatile organic compounds 3. Formation of sulfuric acid mist 4. Condensation of ammonium salts (usually sulfates or chlorides) 5. Formation of water droplets with dissolved salts Discussion of material that is actually present as particulate before reaching the stack is not part of the scope of this paper. Let us deal with each of the other four plume types in order. Organic Plumes Combustion in cement rotary kilns occurs at very high temperatures (850-950°C in calciners, 1550-2000°C in kiln burning zones). In addition, residence times of fuels and oxidizers at high

temperature are long. For this reason, cement kiln emissions of organic species arising from fuel are unusual. Most organic emissions arising from cement kilns are derived from the raw materials (Hawks and Rose, 1995, Shkolnik and Miller, 1996). Many limestone deposits, and also deposits of other raw materials, contain organic residues. The thermal treatment that these raw materials undergo in the rotary kiln will have a major impact on whether any emissions occur. If the organic material is very nonvolatile (like carbon, or soot, in many high carbon fly ashes), it will tend to be burned in the calcining zone of the kiln. In such a case, it will probably contribute to the fuel value, without creating emissions. Even in cases where the organic is incompletely combusted, it may not create a plume. For example, it may be oxidized only to carbon monoxide. CO forms no plume. Even if only vaporized, it may not create visible emissions; it is clear that the organic material will have to condense in order to create opacity. Small molecules like methane (CH4) and ethane (C2H6), which are gases at normal temperatures, are often created in the pyrolysis of raw meal organic material. These cannot create a detached plume. Even volatile liquids like benzene (C6H6) or methanol (CH3OH) will ordinarily not contribute to visible plume formation. However, heavier semivolatile organics can and do contribute to such plumes. The more volatile the organic impurity is, the more likely it is to vaporize without combustion (of course, it is probably unnecessary to point out that if the organic were a gas at ambient conditions, it would have already evaporated before reaching the raw mill). Formation of Sulfuric Acid Mist In high temperature combustion processes, including boilers and cement kilns, the vast majority of any fuel sulfur is converted to sulfur dioxide. Sulfur trioxide is unstable with respect to decomposition to sulfur dioxide at these temperatures (Bartlett, 1987; Greer, 1988, Miller and Young, in press). The dependence is illustrated in Fig. 2, taken from the work of Bartlett for smelters.

Fig. 2- Conversion of SO2 to SO3 for Various O2 Levels Furthermore, because cement kilns are operated with limited amounts of excess air, and because they contain large amounts of highly reactive lime and alkali for dry scrubbing of sulfur oxides, the vast majority of sulfur oxides derived from fuels are captured within the kiln system and are not emitted. The principal exceptions to this rule are when kilns are operated under reducing (fuel-rich) conditions, or when there is a high circulating load of sulfate and kiln dust is not wasted to relieve the cycle.

By contrast, the sulfur content of raw materials, if it is present in the form of pyritic or organic sulfur, or elemental sulfur, can be emitted as sulfur dioxide because it is generated at low temperatures (400-600°C) where there is very little free CaO, and where the low temperature and short residence time in the system preclude efficient capture by the kiln system. Even here, however, although the temperature may be conducive to oxidation of the SO2 to SO3 (the reaction is favorable at moderate temperatures), the reaction is slow, and the content of oxidant (free oxygen) is low. Even where there is more oxygen, the dry scrubbing of the oxidized sulfur is more efficient (Greer, 1988). As a result, there is usually not very much sulfur trioxide or sulfuric acid emitted from cement kilns, particularly dry process kilns. On the other hand, where there is water vapor present, oxidation of sulfur dioxide is much more favorable (Dittenhoefer, 1984). Especially where ammonia is also present, or other catalytic species, the oxidation is favored (Chadbourne, 1995, Chadbourne et al., 1980). This is attributed to the reaction of hydrogen peroxide from the gas stream, in solution, with SO2, similar to the oxidation of SO2 occurring in the impingers of EPA Method 6.

SO2 + H2O2 H2SO4 (The sulfuric acid is actually in the form of hydrogen and sulfate ions in solution). When the water evaporates, the sulfuric acid remains. For these reasons, cement kilns processing raw materials containing significant levels of pyrite raw material may observe a detached plume due to sulfuric acid, even if ammonia is not present. The opacity attributable to any given level of sulfuric acid mist is a function of the moisture content of the gases. Mueller and Imhoff (1994) illustrate this situation in Fig. 3, where the line represents equivalent opacity. It is clear that the opacity at 100% relative humidity occurs with only 10% of the sulfuric acid mist needed at 40% relative humidity.

Fig. 3—Equal Opacity Line for Sulfuric Acid Mist Concentration vs. Relative Humidity Another factor important for the visibility of sulfuric acid mist plumes is the particle size distribution, as observed in Fig. 1. Dittenhoefer (1984) provides an interesting insight into the growth of aerosol particles in a plume as a function of time, as shown in Fig. 4. As plumes ripen, they get higher concentrations of the particles whose size is in the range of visible light wavelengths. Conversely, of course, the plume is dispersing. The relative rates of these processes will dictate how visible the plume becomes.

Fig. 4—Plume Particle Diameter Size Distribution vs. Time These consideration help to explain why these sulfuric acid mist plumes, or indeed any detached plumes, are more visibly prevalent in cool mornings, with high relative humidity, as opposed to warm, dry afternoons. Formation of Ammonium Sulfoxide and Ammonium Chloride Plumes Although fuels contain nitrogen, this nitrogen is usually completely oxidized to nitrogen or oxides of nitrogen in the combustion zones of cement kilns, and does not result in emissions of ammonia. However, for the same reason that cement raw materials contain organic impurities, they also contain organic nitrogen compounds. Proteins are the building blocks of life, and are composed of amino acids, which are organic nitrogen compounds. It is reasonable to postulate that when such compounds are heated, ammonia will be driven off, just as heating organic carbon compounds breaks them down into simpler molecules like methane or ethane. Cheney and Knapp (1987) showed that heating the shale from a plant experiencing a detached ammonium chloride plume resulted in the generation of a significant amount of ammonia; Gray and Ankenman (in press) have made similar observations. Ammonia can be burned at high temperatures to nitrogen, or to nitric oxide:

2NH3 + 1 ½ O2 N2 + 3H2O 4NH3 + 5O2 4NO + 6H2O

At lower temperatures, however, ammonia may escape oxidation. If the ammonia is driven from the kiln feed at a low enough temperature, it may be emitted. Also, if the quantity is too high, some may escape oxidation even if generated or introduced in the temperature range where oxidation is favorable. Such a situation may arise, for example, if ammonia is deliberately injected into the calcining zone to reduce NO emissions:

2NH3 + 3NO 2 ½ N2 + 3H2O This is the so-called SNCR (selective non-catalytic reduction) process for control of NOx emissions. If the ammonia has an opportunity to react with sulfur oxides generated from oxidation of kiln feed pyrites, or if it encounters hydrogen chloride from chlorides introduced with fuel or kiln feed, ammonium salts may form. In the case of the interaction with HCl, this will result in the formation of ammonium chloride salt in the plume:

NH3 + HCl NH4Cl The ammonium chloride will normally not form inside the stack, because the temperature is too high; Chadbourne (1995) and Hawks and Rose (1995) have established that the HCl and ammonia are still unreacted until emitted from the stack. This is the reason that the opacity meter does not react to the material. But the water droplets formed by condensation of water vapor from the stack gases provide a medium for the reaction. Guemez-Garcia and Ganatra (1994) give an example of this situation. Ammonia and HCl were derived from the limestone. The ammonia was driven off between the second and third stages of the preheater. Chloride was largely captured in the lower stages of the preheater, but some HCl was driven off, presumably by reaction of chlorides with sulfur oxides and water vapor at high temperature. The preheater exit temperature was 385°C, high enough that the ammonia and HCl were not captured in stage 1 of the preheater. HCl levels in the stack were 55-90 ppmv, while ammonia was about 27 ppmv. The plume was highly visible. The plant solved the problem by increasing the number of preheater stages, such that the preheater exhaust temperature was now down to 260°C. At this temperature, the chloride was effectively captured by the limestone. One of the two reactants responsible for the plume (chloride) was reduced to less than 5 ppmv, and the plume disappeared. This result was despite the increase in ammonia emissions, to 40-50 ppmv. The situation with ammonium sulfoxide compounds is similar. Ammonium sulfates have higher boiling points, so they may show some opacity in the stack. (Ammonium sulfate has a dissociation temperature of 280°C). The actual compounds formed may vary, depending on the relative amounts of ammonia and sulfur oxides, and on which sulfur oxides are present:

NH3 + SO3 + H2O NH4HSO4 (ammonium bisulfate)

2 NH3 + SO3 + H2O (NH4)2SO4 (ammonium sulfate)

NH3 + SO2 + H2O NH4HSO3 (ammonium bisulfite)

Other compounds can be formed, such as ammonium sulfite, or ammonium sulfamate (NH4SO3NH3) (Chadbourne, 1995; Chadbourne et al., 1980). Any of them can result in a detached plume. We referred earlier to ammonium sulfoxides, rather than ammonium sulfates, because of the plurality of compounds possible. In the author’s observation, these ammonium sulfoxide plumes are the most common of all cement plant detached plumes, since so many raw materials contain pyrite impurities and organic nitrogen compounds. Such a plume was the subject of work by Dellinger et al. (1980). The authors determined the actual chemistry of the plume, and found 4.5-ppmv ammonium sulfate, 200-ppmv sulfur dioxide, 170-ppmv ammonia, and water. The plume is only visible when the conditions are cool and moist; when the sun rises high in the sky and the temperature increases, the water cannot condense until the plume is dispersed, and very little opacity results.

The problem with ammonium salts and detached plumes is especially prevalent for plants operating with in-line raw mills. The raw mill often operates at temperatures around 150-200°C. At this temperature, the ammonium salts may condense. They go back into the kiln feed, back into the kiln system, where they vaporize again. They subsequently condense again in the raw mill (unless a small amount of the ammonia is burned or decomposed). When the raw mill is taken off line, the volatile salts are no longer captured in the raw mill, and go to the dust collector. Since the dust collector cannot capture the new, high concentrations efficiently, the plume becomes highly visible. When the raw mill is put back into operation, the plume ceases again. This cycle continues indefinitely, unless something is done to break it. Studying Plumes at Cement Plants One of the problems in evaluating detached plumes at plants is sampling. It is difficult to get samples of the actual species responsible for the plumes by stack sampling. EPA Method 5 is a good method for determining actual particulate matter, but the condensable salts will be found in the probe, on the filter, or in the impingers, depending upon which salts are present and at what temperatures the collecting devices are maintained. Methods that involve impinger collection of stack gases will bring virtually all potential plume reactions to completion, so that concentrations of ammonium salts, for example, will be overestimated (MacIver et al, 1988, Chadbourne et al., 1980). It is really necessary to sample the actual plume to determine concentrations. This was actually done at several cement plants (Hawks and Rose, 1995; Dellinger et al., 1980; Cheney and Knapp, 1987, Mueller and Imhoff, 1994). This is an arduous and expensive process, but unavoidable to obtain realistic data. Plume Remediation Now we finally get around to the title of this presentation—“Management of Detached Plumes in Cement Plants”. We need to determine what measures to adopt to get rid of the detached plume. To do so, we first must determine which of these plume types is applicable. For example, if the plume is attributable to organic compounds, we need to determine the source of the organic compound. Let us examine a few possibilities: 1. The organics are due to poor combustion in the calciner. Testing reveals that the calciner

exhaust has 60 ppmv of total hydrocarbons. Maybe the fuel and air are not properly mixed in the calciner. Perhaps the oxygen or CO analyzers at the calciner exhaust are not working properly. Fixing these problems may solve the plume problem.

2. The organics are due to oil in the mill scale used for an iron source in the raw mix. This is established by running organic testing on each raw material. The mill scale is replaced with an alternate iron source, and the problem goes away.

3. The organics are due to the limestone. Maybe the quarrying plan can be changed. Otherwise, it may be difficult to solve the problem. In Colorado, two plants changed their pyroprocessing systems and installed one-stage calciner kilns. These kilns took the raw materials immediately up to 850°C—at this temperature, organics are immediately combusted and are therefore not emitted.

Let us examine another case—a plant has a persistent detached plume that is attributable to ammonium sulfate. To get rid of the ammonium sulfate, we need to get rid of one of the two reactants that ultimately form ammonium sulfate—either the ammonia or the SO2. The use of hydrated lime to react with SO2 has been used successfully to remediate some plumes (Schwab et al., 1999; Hawks, Schwab and Stuehmer, 1999; Hawks and Rose, 1995; Wolf and Seaba, 1994). The hydrated lime is introduced into the downcomer, or into the kiln feed, or into the dust collector, in the form of slurry, or a dry powder. In the case of Inland Cement (Hawks, Schwab, and Stuehmer), the use of very fine lime, with a high specific surface area, was successful in remediating the plume. Because of the low temperature region in which the lime is introduced, the reaction rate is slow, and needs to be enhanced with reactive lime. When the lime is introduced in the form of slurry, the additional water also helps the reaction.

Perhaps the optimum solution involves getting rid of the source of ammonia. Certain industrial byproducts may contain precursors of ammonia. Aluminum dross, as an example, often contains aluminum nitride that may generate ammonia when contacted with water or steam. If aluminum dross is used as an aluminum source, it may be responsible for the generation of ammonia. It is important to remember that at least one of the gases (either ammonia or sulfur oxides) must be treated if the plume is to be removed. Guemez-Garcia and Ganatra (1994) provide another type of solution to the plume problem. In this case, as earlier noted, the issue was ammonium chloride. A reduction in temperature, achieved by introducing an additional preheater stage, brought about capture of the chlorine in the system—chlorine which had earlier escaped the preheater. Again, one of the two reactants, the HCl, is removed from the exit gases (even though the ammonia concentration actually increased!). Of course, the plant needed to install air cannons to deal with the higher circulating load of chloride that resulted. Another possible plume problem is sulfuric acid mist. Here, the literature provides worthwhile input as to remediation. The excess oxygen concentration will affect the SO2/SO3 balance. The more SO2 and less SO3 in the plume, the less likelihood of a detached mist plume. Of course, the moisture level will also play a role, whether in the plume itself or in the atmosphere. Wolf and Seaba (1994) provide another solution for the acid mist—they recommend hydrated lime injection. As with the sulfur component of the ammonium sulfoxide plume, the hydrated lime will reduce the SO2, SO3, and/or H2SO4 concentration, and thereby remediate the plume. Finally, we can cite certain measures that may be efficacious on a case-by-case basis. It will be noted that dispersion of the plume, before condensation of its components can occur, has been cited as a reason why the plume disappears when the day warms up and the relative humidity decreases. Another way of causing the plume to disperse more rapidly is to increase its velocity. This may or may not be practical on a particular site. The same argument can be applied to heating the flue gases after the dust collector; they will disperse further before the dew point of the gases is reached. On the other hand, if the flue gas temperature entering the dust collector can be reduced, it may capture certain particulates that would otherwise pass through the dust collector in the gaseous state. This solution may be particularly applicable to ammonium sulfoxide plumes.

BIBLIOGRAPHY Bartlett, R.W., (1987), “Sulfate Aerosol Particulates,”, Journal of Metals, 38-41 Baskerville, J., (1981) “Formation of a Detached Plume in the Exhaust Gas of a Portland Cement Kiln, Contract No. 68-01-4146, Task 84, Prepared for the U.S. EPA by Engineering-Science, McLean, VA Cadle, R.D., (1972) “Formation and Chemical Reactions of Atmospheric Particles”, Journal of Colloid and Interface Science, Vol. 39, 25-31 Chadbourne, J. F., Baker, R. A., and Brown, R. L., (1980) “Reactive Plumes—Sampling and Opacity Concerns for Cement Kilns,” presented at the 73rd Annual Meeting of the Air Pollution Control Association, Montreal, Quebec Chadbourne, J. F., (1995) “Dynamic Plumes Hazardous Air Pollutants and the Community Right to Know,” Proceedings, Rock Products 30th International Seminar Cheney, J. L. and Knapp, K. T., (1987) “A Study of Ammonia Source at a Portland Cement Production Plant”, Journal of the Air Pollution Association, 37 1298-1302 Damle, A .S., Ensor, D. S, and Sparks, L.E., (1987) “Options for Controlling Condensation Aerosols to Meet Opacity Standards, Journal of the Air Pollution Control Association 37, 925-933 Dellinger, B., Grotecloss, G., Fortune, C. R., Cheney, J. L., and Homolya, J. B., (1980), “Sulfur Dioxide Oxidation and Plume Formation at Cement Kilns”, Environmental Science and Technology, pp. 1244-1249 Dittenhoefer, A. C., (1984), “Evidence of Aqueous Phase Oxidation in Power Plant Plumes,” Journal of the Air Pollution Control Association, 11 pp. Gray, K. and Ankenman, B., (in press) “The Detached Plume Study: Statistical Analysis of Causative Factors in Portland Cement Manufacturing Plants,” sponsored by the Portland Cement Association Manufacturing Technical Committee Greer, W. L., (1988) “SO2/NOx Control Compliance with Environmental Regulations,” 30th IEEE Cement Industry Conference, Quebec City, Quebec, Canada Guemez-Garcia, R., and Ganatra, C. P., (1994) “Elimination of Emission Problems at Cementos Guadalajara, World Cement, December, pp. 50-57 Hawks, R. L., and Rose, T., (1995) “A Proactive Approach to Minimizing Opacity from Cement Kilns”, IEEE Cement Industry Technical Conference, June 4-9, San Juan, Puerto Rico, 451-463 Hawks, R. L., Schwab, J. J. and Stuehmer, K., “Abatement of Secondary Plume at Inland Cement, Edmonton, Alberta, 41st Cement Industry Technical Conference, Roanoke, VA, 395-418 MacIver, D.Y, Yannone, M. A., Klemm, W. A., and Adams, L.D., (1988) “Reaction Products of Pseudoparticulate Issues in Testing Portland Cement Plant Emissions, presented at the 81st Annual Meeting of the Air Pollution Control Association, Dallas, Texas Miller, F. M., and Young, G. L., (2001) “Formation and Techniques for Control of Sulfur Dioxide and other Sulfur Compounds in Portland Cement Kiln Systems,” PCA R&D Serial No. 2460, in press

Mueller, S. F., and Imhoff, R. E., (1994)“Estimates of Particle Formation and Growth in Coal-Fired Boiler Exhaust—I. Observations,” Atmospheric Environment, 28, 595-602 Mueller, S. F., and Imhoff, R. E., (1994) “Estimates of Particle Formation and Growth in Coal-Fired Boiler Exhaust—II. Theory and Model Simulations,” Atmospheric Environment, 28, pp. 603-610 Shkolnik, E. and Miller, F. M., (1996) “Differential Scanning Calorimetry for Determining the Volatility and Combustibility of Cement Raw Meal Organic Matter,” World Cement, May Schwab, J. Wilber, K, and Riley, J, (1999), “SO2 Reduction Using Micro-fine Lime” N&S American Cement Magazine, p. 22 Weir, A., Jr., Jones, D.G., Papay, L.T., Calvert, S., and Yung, S. C., (1976), “Factors Influencing Plume Opacity, Environmental Science and Technology, Vol. 10, No. 6, 539-545 Wolf, D. E., and Seaba, J. P., (1994) “Opacity Reduction Using Dry Hydrated Lime Injection”, Air and Waste, pp. 908-912