The cement manufacturing industry in North America is under increasing pressure to reduce emissions. The rotary kilns used for manufacturing cement produce many byproducts including SO2, SO3, carbon monoxide (CO), and particulate. Depending on the limestone quarry from which the base material is taken, there are also can be volatile organic compounds (VOCs), some of which are high-odor organic sulfur or nitrogen compounds. The levels of these emissions vary depending on whether the cement kiln employs the wet or dry process.

The driving force behind the emissions reduction also depends on the location of the facility. SO2 relates more to acid rain, whereas SO3, although only present in small quantities, creates an acid aerosol mist, which is a major contributor to emissions opacity or visible plume. The VOCs usually are the major source of odor and contribute to the plume since many form aerosols when discharged to the air. CO is not a major concern since it is easily eliminated in treatment devices. Considering all of these factors, the exact combination of technologies can change from location to location.

The cement manufacturing process A variety of calcium-containing raw materials that can include aragonite, limestone, and chalk are gathered from a quarry. The materials are then blended so that the desired physical and chemical properties are achieved. In a dry process, the moisture is typically reduced 1% to 10% through the utilization of drum dryers or impact dryers during the grinding phase. In the wet process, water is added so that the mix becomes a pumpable slurry with a moisture content in the range of 30% to 40%.

In either case, the mixture is directed to a rotary cement kiln for a process commonly called the pyroprocessing phase, which transforms the mix into clinker or raw-fired cement. The rotary kiln is a slightly inclined cylindrical furnace that is lined internally with refractories. The mix is added to the kiln at the raised end and exits after passing through the entire length of the furnace; lengths vary from 400 to 700 ft. While being processed in the furnace, the mix passes though four distinct phases:

1. Evaporation of water

2. Dehydration of minerals

3. Calcination

4. Oxide Reaction

(Phase 4 is at a temperatures as high as 2,600degrees to 2,800degreesF.)

Following the pyroprocessing phase is a cooling process that locks in the mineral structure of the clinker while reducing the temperature so that the material can be handled with conventional conveyors. Cooling is typically done by directing ambient air through the clinker so that it cools to 200degreesF. The heat that is transferred to the air from the clinker is then recovered and reused in the furnace as combustion air. After cooling, the final step is grinding and blending where the clinker is transformed into portland cement. During this phase, varying amounts of gypsum can be added to control the time of setting the cement.

Exhaust treatment options In almost all cases, the final treatment solution is a combination of technologies where corrosion avoidance becomes a driving force behind the technology selection. When below the water dewpoint temperature (160degrees to 180degreesF depending on wet or dry process) any condensed acid will be diluted due to the addition of moisture. When above the acid dewpoint (260degrees to 320degreesF depending on SO3 concentration and moisture) acid condensation will not occur. Therefore, the most dangerous temperature zone is in the range of 180degrees to 300degreesF, where the air is above the water dewpoint but below the acid dewpoint. In this case, a concentrated acid can condense causing rapid corrosion of materials if they are not properly selected.

The first step in all treatment configurations is a baghouse utilized for reduction of particulate, consisting primarily of the filter bags, a bag cleaning device, and dust handling equipment. The filter dust from the baghouse can be returned to the kiln to avoid disposal costs. The next steps in treatment depend on several key factors. The first factor is whether the installation is a retrofit to an existing kiln or a new facility. The second factor relates more to the clean air permit requirements of the site.

The following points contain a description of the three primary post-baghouse treatment technologies and the constituents that they treat:

1. Valveless regenerative thermal oxidizer (VRTO) This technology thermally treats the CO and VOCs by heating them to 1,600degreesF. Internal ceramic heat exchangers ensure that only minimal amounts of additional fuel are required. Unfortunately, the heat that is required to treat the CO and VOCs has an unfavorable reaction on the SO2 and SO3. At elevated temperatures, an equilibrium occurs between these two constituents where, depending on oxygen levels and several other parameters, a portion of the SO2 can convert into SO3, which is far more difficult to treat. The equilibrium tendency to create more SO3 can be reduced by optimizing the system combustion temperature and retention time. The following features of a VRTO will have advantages in cement manufacturing applications:

* Constantly moving rotary distributor to reverse the exhaust flow in the heat-exchanger beds. This enables avoidance of deposits on valves that can cause seal leakage;

* Proper and controllable retention time;

* A heat exchange media that is cleanable and provisions for periodic washing of the media; and

* Proper material selection to avoid corrosion.

2. Wet flue gas desulfurization scrubber (FGD) In this process, limestone is used as a reagent to absorb the SO2 and SO3 from the exhaust stream. The SO2 combines with the limestone (CaCO3) to form CaSO3 and is further oxidized to CaSO4 by a forced oxidation through air injection. SO2 levels are reduced by 96% to 98%, and SO3 is reduced by 30% to 90%. This wide range for the SO3 reduction is due to the fact that aerosols are formed that are very difficult to remove. To achieve the higher values, baffles or trays must be used to force a turbulent reaction with the limestone slurry reagent. Depending on the sequence of technologies selected, high SO3 removal efficiencies may or may not be needed in the scrubber. The resultant byproduct of this scrubbing process is gypsum (CaSO4). If properly dewatered and if the CaSO3 level in the gypsum is kept low, 100% of the gypsum can be reused and returned to the manufacturing process.

The scrubber process equipment consists of a limestone slurry preparation system, spray adsorber with quench zone, and mist eliminator for the exhaust stream. Other components include a combined recycling and forced oxidation tank to produce the gypsum, as well as gypsum dewatering, gypsum handling, and water recycling equipment. The entire system is waste-water free, but it is important to look at the chloride content in the make-up water as well as the limestone to know what the equilibrium chloride concentration will be in the scrubber. This chloride concentration is a major factor in scrubber material selection.

3. Wet electrostatic precipitator (ESP) The wet ESP can be mounted directly on top of the scrubber tower in order to minimize the amount of duct and ground space. With this configuration, all liquids will run into the scrubber and will be neutralized by the pH-controlled slurry in the scrubber. The wet ESP exposes the aerosols and particulate to a negative electric field through the use of discharge electrodes. These particles are then attracted by the collector tube and trapped in a water film within the tube. The water film ensures that particles and aerosols are not re-entrained after capture. This water film also has the function of diluting the acid and maintaining a clean collector surface.

The ESP should be used in applications where very low emission values are required. Although expensive, it is the best device for removing the difficult to control SO3, as well as submicron particulate. Note that if a scrubber and ESP are coupled together, the scrubber can be configured with lower SO3 removal and lower pressure drop since the ESP will eliminate virtually all of the SO3.

Technology combinations There are three main options for combining these technologies:

OPTION 1-Kiln/Baghouse/FGD/Wet Stack- This combination will have the least frequent use since it does not treat VOCs or CO. In some locations, VOCs and CO are not heavily regulated and therefore this combination is possible. Depending on the type of scrubber used, the effect of this treatment will be 96% to 98% SO2 removal and 30% to 90% SO3 removal with very little effect on the other emission compounds.

OPTION 2-KIln/Baghouse/FGD/VRTO/Dry Stack-This configuration is more common. Installing the FGD system prior to the VRTO reduces the amount of particulate entering the unit and reduces the likelihood of heat exchanger blockage. Although the amount of particulate reduction primarily depends on the baghouse, it should be noted that even in this configuration, some moisture will pass through the scrubber mist eliminators carrying particulate that will deposit on the VRTO heat exchanger. An advantage of the Option 2 arrangement is that the amount of SO2 is reduced prior to the VRTO. Therefore, the equilibrium effect in the VRTO that drives SO2 to SO3 is minimized since most of the SO2 has already been removed. A disadvantage of this arrangement is that the exhaust enters the VRTO fully saturated with moisture, meaning that as it is heated, it passes through the temperature range with the highest corrosion potential (above water dewpoint and below acid dewpoint). Thus, very costly nickel alloys must be used at certain areas within the VRTO. Another disadvantage of this arrangement is a costly trade-off. If the VRTO heat exchanger is designed to achieve high thermal efficiency (low gas consumption), the outlet temperature will be below the acid dewpoint. Henceforth, all outlet duct and the stack must be resistant to acid condensation and its high levels of corrosion. If the outlet temperatures are raised above the critical acid dewpoint temperature, then the VRTO will consume very large quantities of natural gas. Therefore, the choice is either high energy costs or high material (alloy) costs.

OPTION 3-Kiln/Baghouse/VRTO/FGD (with optional ESP)/Wet Stack-The advantages of this arrangement are said to be that the VRTO can be sized to the highest energy efficiency without concern for corrosion since the exhaust enters the VRTO already above the acid dewpoint. The second advantage is said to be that the exhaust never is in the high corrosion temperature zone, therefore allowing a lower cost material selection in many areas of the system. As well, an ESP can be added in the base system supply or on top of the scrubber tower if deemed necessary.

The disadvantages of this arrangement are that the VRTO will see a potentially larger amount of particulate in the event that a baghouse bag breaks. This, however, can be handled through an automatic exchanger bed washing system or preventative controls that isolate the broken bag. Another disadvantage is that the total amount of SO2 will now enter the VRTO resulting in a higher equilibrium conversion to SO3. If the scrubber is properly designed, this higher SO3 output can be handled usually without the ESP.

Summary There are several other factors that can affect the system's performance, such as the amount of organic sulfur in the exhaust. The VRTO will convert the organic sulfur to SO2, and therefore, depending on the exact quantities in the exhaust, there can be additional trade-offs between the options listed above. If, for example, there is a significant amount of organic sulfur, then Option 3 is preferred since the SO2 will be removed in the scrubber.

It is clear from the many variables in cement manufacturing process that each application must be reviewed to determine the best combination of technologies. When analyzed properly, the solution should optimize corrosion prevention, emissions reductions, and the low cost of ownership so that long-term emissions compliance can be economically realized.