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Gas Cooling and Conditioning Under the EPA NESHAP
Peter Welander, Chemical Products Group
Lechler, Inc., St. Charles, Illinois
Gas cooling and conditioning through the use of spray atomization is not a new idea, and the concepts are well known throughout the cement industry. To some it might even be considered old technology and hardly worth discussion at this point. With the development of more fuel efficient kilns, preheaters that recover more waste heat and higher temperature fabric filters, the need for gas cooling has declined in the last few years.
Gas conditioning provides many benefits:
Additionally, now as environmentalists have turned their eyes to additional classes of pollutants, gas conditioning with water sprays is gaining stature as an important technology, providing a critical weapon in the arsenal against toxic gasses.
During last summer, the EPA finalized and promulgated two new NESHAP’s (National Emission Standards for Hazardous Air Pollutants) that put an additional mandate on the cement industry to reduce toxic gasses and heavy metals along with particulates. Over the next few short years, cement kiln operators will be required to monitor and reduce specific groups of pollutants that have not previously been a concern in all areas.
The two standards in question are the National Emission Standards for Hazardous Air Pollutants for Source Categories; Portland Cement Manufacturing Industry, and Revised Standards for Hazardous Waste Combustors. The first covers non-hazardous waste kiln operators and the second a broader category of waste incinerators, including cement kilns that burn hazardous wastes. Between the two, the entire portland cement industry is covered. The EPA maintains an extensive web site (http://www.epa.gov) and with a little hunting you can find the relevant documents plus additional information.
For purposes of this discussion, we will consider several of the points contained in these standards and suggest how traditional methods of spray gas conditioning can provide abatement of certain pollutants and support additional abatement methods. This is by no means a substitute for your own study of the standards and a full analysis of how they will affect your specific site and process. Given the vast variety of plant and process configurations, there is no single answer that applies to all. It is also impossible to state with any certainty that the requirements will not change before they come into effect. Industry and environmental groups will both exert pressure on the EPA to bring the specifics into line with their thinking. Now is the time to begin to familiarize yourself with the process.
What do the standards require?
As would be expected, there are tighter requirements for particulate emissions and the addition of heavy metals which can be measured with particulates. Mercury is in the picture and even certain hydrocarbons. (The non-hazardous waste kiln standard does not address mercury in any depth, so at this point it is primarily a concern for the hazardous waste burning operators. However, there is at least one lawsuit demanding that the EPA include mercury restrictions in both standards in keeping with President Clinton’s Mercury Action Plan and the requirements of the 1990 Clean Air Act. It would be foolish to suggest that non-hazardous waste kiln operators will escape this for any length of time.)
Both standards have specific requirements for toxic gasses, especially dioxins and furans (D/F). These include tetra-, penta-, hexa-, hepta-, and octa- chlorinated dibenzo dioxins and furans. There are more than 200 variations of these in total, with some regarded as the most toxic substances known. These compounds have become a specific source of concern to environmental groups with calls for their eventual complete elimination. This is nothing new to hazardous waste incinerators, however to the larger cement industry it will be unexplored territory.
Mercury is also under tighter restriction since it is not normally captured in a particulate control device in its gaseous phase. It can pass through the whole system and go out the stack since all the temperatures involved in the process are high enough to keep it from condensing. The amount present depends entirely on the content in the fuel and feedstock since it does not "form" in the process.
Beyond the specific hazardous air pollutants, there are two critical items that they do not address. These new standards do not deal with SOx or NOx. They are criteria pollutants and there are currently no federal emission standards for SOx and NOx from cement kilns. These are handled by the individual states.
Specifics for kiln operators
For non-hazardous waste (NHW) kilns, the EPA is allowing a dual D/F standard of 0.2 ng TEQ/dscm or 0.4 ng TEQ/dscm with a particulate matter (PM) control device inlet temperature of 400°F or less. The rationale for this is that D/F compounds form in specific temperature windows (450°-650°F) and a rapid cool down, such as is provided by spray gas cooling, prevents their formation once the temperature is below 400°F. This allows an operator the choice to continue running at a higher temperature if D/F is not a problem (below 0.2 ng TEQ/dscm) or remaining between the two figures simply by lowering the inlet temperature at the inlet of the PM control device. In some situations, thanks to good combustion control, feedstocks free from D/F precursors or compounds in the feedstock that inhibit D/F formation, additional abatement may not be required. In other cases it may not be possible to go below the 0.4 ng TEQ/dscm level even when the temperature is below 400°F. In those situations there are additional methods for controlling D/F that we will mention later.
Hazardous waste kilns do not have the dual standard. Operators are required to reach the 0.2 ng TEQ/dscm level. There is a great deal of discussion in the standard as to the temperature at the PM control device. The conclusion is the same, specifically that D/F formation is minimized when the temperature is below 400°F. The standard cites a case study at Ash Grove Cement in Chanute, Kansas. During testing in 1992, the D/F level with the PM device inlet temperature at 435°F was 1.7 ng TEQ/dscm. Two years later during similar testing when the temperature was down to 375°F, the level dropped to 0.5 ng TEQ/dscm. With a 60° change in temperature, the emission level dropped by more than two-thirds.
For kilns that operate with ESP’s, inlet temperatures at the PM control device are normally below 400°F, with 300°F not uncommon. ESP’s are normally paired with a spray gas conditioning tower to cool the gasses and raise the humidity level of the gas to reduce the electrical resistivity of the dust. This allows greater electrical stability in the ESP and a higher power input. This results in more effective particulate collection.
One of the advantages that baghouse builders suggest is that low cooling temperatures are not required with bags that can operate continuously at 500°F. This type of bag combined with an efficient preheater can cut the amount of cooling required dramatically. Newer installations take advantage of this and eliminate the cooling tower completely, doing all the water injection in the ducts. While this works for the PM collection, it won’t help with D/F abatement. Some plants may find they have to go to greater lengths when it becomes necessary to bring the temperature level down below 400°F.
Gas conditioning with activated carbon injection
Injecting powdered activated carbon (PAC) into the gas stream provides another approach to eliminating D/F and mercury. Both are adsorbed by the particles and then collected in the PM device. Such technology has been proven after many years of use in a wide variety of applications. This approach can help a facility where lowering the gas temperature is not enough to reach the required emission level.
However, PAC has limits to its effectiveness based on temperature. Here again, 400°F is a critical point in the range. When operating below this level, the adsorbates are less energetic and attach more easily to the particles. They tend to remain attached to the carbon and are not driven back into the gas stream. PAC, when injected after the gas conditioning stage where the gas temperature is below 400°F, can reduce mercury and D/F emissions to levels greater than 95%. PAC cannot perform adequately when the temperature is too high, so it needs to be combined with an effective gas conditioning system.
PAC technology works best with bag houses since a high amount of removal takes place at the surface of the filter bag as the gas passes through the cake in addition to the airborne adsorption in the ducts. ESP’s have to inject a larger amount of carbon since the only means of collection is during the airborne phase. Here the carbon should be injected as soon as possible in the gas stream to maximize the contact time.
For cement producers, using PAC can require a change from the normal procedure of feeding the dust collected in the PM device back into the feedstock. When the mercury and D/F laden carbon goes back into the kiln, the carbon is incinerated and the pollutants return to the gas stream. Over time, this can result in increased concentrations of toxic materials in the dust that turns it into a hazardous material. Each site will have to determine when the dust needs to be removed to avoid such a problem. If a portion of the dust is already being removed to control alkali salts, this may be sufficient.
Gas conditioning techniques
An effective gas conditioning system uses spray nozzles to atomize water and inject it into the gas stream. The droplets absorb heat as their temperature increases until the evaporate completely. As the vapor superheats, it absorbs additional heat until the vapor temperature equals that of the gas.
Since the amount of heat required to evaporate a specific volume of water is easy to calculate, determining how much water to inject is not difficult. The key of system design is making sure all the liquid evaporates before it has an opportunity to hit a wall or bottom and wet the surface. A wet surface provides an excellent point for dust to collect in the tower and robs the system of cooling capacity.
Achieving this requires two important skills: The ability to create droplets small enough to evaporate before they hit a surface combined with trajectory modeling that determines the directions the droplets will travel. From these calculations, the system designer then chooses the nozzle capable of producing a small enough droplet and determines how they should be positioned in the tower or duct. The liquid will not evaporate effectively if it is not mixed sufficiently in the gas stream to avoid hot and cold areas.
Most gas conditioning systems use hydraulic nozzles featuring an internal mechanism that allows a portion of the flow to bypass the orifice or a twin fluid atomizer that uses compressed air to break up the liquid. Each approach has its own advantages related to the turn-down ratio, droplet size and energy consumption.
Since a cement kiln can run at various production levels, there are frequent changes in the volume of gas and the temperature. An effective system has to be able to respond to these changes in a way that will stabilize the outlet temperature and keep it steady in the face of changing conditions. This depends on a well integrated package of nozzles, pumps and control equipment combined with careful design and tuning to ensure flawless performance across a wide range of operating conditions.
For decades, such systems have allowed countless cement producers to improve their particulate removal and control operating costs. Now they carry the additional benefit of helping minimize toxic gas emissions and achieve the EPA’s new requirements.
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