Cement manufacturing plants, like wood-panel manufacturing facilities and many other industries, will require reductions in air pollution emissions by 2002. The compliance date for new plants or plants reconstructed after March 28, 1998, is the date of publication of the final rule or the date start-up, whichever is later.[Footnote: Federal register/40CFR Vol. 63, No 56, Paragraph 63.1347.]

Particulate control equipment in the form of bag houses, ESPs, and other devices already has been installed at most cement plants. In fact, PM-10 emissions from cement manufactures were reduced by more than 85% in two decades, from 1.7 million tons in 1970 to 226,000 tons in 1990.[Footnote: HWF Notes, Gossman Consulting, Inc., April 1994.]

Particulate emissions, although still a concern, are addressed as a minor factor. The emphasis is placed on odor, total hydrocarbons (THC), CO, SOx, and opacity.

The emissions from multiple sources (shown in Table 1)[Footnote: Federal Register/40 CFR Vol. 63, No 56, Paragraph 63.1340 & 63.1343.] will be affected at cement plants, which are a major source and/or an area source. The limit of the emissions (shown in Table 2)[Footnote: Federal Register/40 CFR Vol. 63, No 56, Paragraph 63.1343 & 63.1346.] varies from source to source. The issues and problems, however, are the same. What follows is a discussion of a modular RTO and Scrubber (FGD) system that provides the necessary reductions in opacity, odor, THC, CO, and SOx emissions, along with specific features that address other concerns associated with operating and maintaining the system at a cement operation.

Arrangement The use of a modular rotary valve RTO system (Figure 1) allows for a complete operating spare module RTO, which allows maintenance and cleaning without having to disrupt kiln operation. Potential particulate carryover from the scrubber can necessitate periodic cleaning. The modular RTO approach of approximately 60,000 scfm per module provides the ability to shut down a module without disrupting the kiln operation.

Each individual RTO module is completely functional, and can be removed or put on line as needed. Also, the operating spare RTO module provides additional capacity, thereby reducing the overall electrical consumption and minimizing the system load. Each module must be able to operate independently of the rest of the system in order to cool down, heat up, and purge the module off line. A separate purge fan and individual inlet and outlet isolation damper (Figure 2) are required to utilize this spare module concept without disrupting the kiln operation.

The modular rotary-valve RTO system reduces the number of components typically associated with an RTO unit, decreases the size or footprint about 35%, reduces the weight 70%, reduces the electrical horse power consumption 40% to 60%, and provides a design that increases the SO3 to SO2 conversion through the oxidation process.

Fan arrangement-The operation of a modular RTO and scrubber system and its associated performance depends on either a forced (upstream) or an induced (downstream) draft fan system to carry the process stream through the equipment. The location of the fan system will affect initial capital cost and performance, as well as the operating costs of the total system.

Locating the fan down stream from the scrubber (induced) substantially increases the size and horsepower of the fan due to the added volume of water going through the scrubber system. In addition, the increased moisture level dictates a stainless steel alloy construction to minimize corrosion. These two facts will usually justify a forced draft (up stream) fan system.

The induced draft fan system increases the structural stiffeners required for the ductwork and vessels and, thus, the overall capital costs. Special attention also must be paid to insure cold air leakage into the system is minimized or eliminated, thus preventing condensation and associated corrosion problems that in most other VOC pollution control applications is of little concern.

Table 3 (see page 5) provides a list of additional pros and cons of the forced draft vs. induced draft fan arrangements.

Scrubber and RTO arrangement-The arrangement of the RTO and scrubber down stream of the particulate control device is an important factor in controlling opacity at the stack. Removal of SOx prior to the oxidation process substantially minimizes the potential emissions of SO3 and the associated opacity or plume discharge from the stack. The level of SO3 emission from the RTO system will vary depending on the design and total sulfur input to the oxidation process. Therefore, the scrubber design and locating the RTO after the scrubber are essential in order to control SO3.

SO2 scrubbing system The scrubber system must provide a minimum removal of 98% of the inlet SO2 to achieve a minimum SO3 emission up the stack. Using calcium carbonate in the scrubber will produce gypsum byproduct that can be reintroduced into the cement production process. Typically, a counter-current open spray tower with at least four stages of spray nozzles is used to achieve the 98% removal efficiency. Rich calcium carbonate (CaCO3) slurry is sprayed into the process stream to promote calcium dissolution, forced oxidation, and solid precipitation.

The overall reactions that take place in the scrubber are:

SO2 + CaCO3 > CaSO3 + CO2

SO3 + CaCO3 > CaSO4 + CO2

Calcium ion is formed in the aqueous slurry.

CaCO3 (s) > CaCO3 (aq)

CaCO3 (aq) + H2O > Ca[superscript]++ + HCO3[superscript]- + Oh[superscript]-

The SO3[superscript]-2 anion forms at the flue gas/slurry interface in the scrubber.

SO2 (g) > SO2 (aq)

SO2 (aq) + H2O > H2SO3 > HSO3[superscript]- + H[superscript]+

Gypsum, the primary precipitate is formed under the in-situ forced oxidation environment.

SO3[superscript]= + 11/42 O2 > SO4[superscript]=

SO4[superscript]= + Ca[superscript]++ + 2H2O > CaSO4 * 2H2O

The scrubber will have very little, if any, effect on the odor, THC, or CO from the process. However, it is essential that the mist eliminator be designed to control and minimize the carryover of solids from the scrubber to the RTO system.

SO3 control Opacity at the discharge from the stack is attributed to particulate, NOx, and SO3. Therefore, SO3 concentrations are an important factor in controlling opacity. The level of SO3 out of the RTO system will depend on the SO2, SO3, and total sulfur compounds into the oxidation process of the RTO system and the associated design.

Equilibrium considerations under elevated temperature are not sufficient to predict SO2-SO3 chemistry, especially through a RTO system. The rate approach to equilibrium in a homogeneous SO2-SO3 combustion product environment is relatively slow. As a result, SO3 will be less than equilibrium levels.

In addition, chemical kinetics indicate that the reaction rates controlling the reduction of SO3 back to SO2 increases two orders of magnitude faster than the rate of reaction of SO2 to SO3. Hence, insignificant concentration of SO3 are achieved at temperatures in excess of 600degrees to 800degrees F.

As the SO2-SO3 combustion products cool down, thermodynamic equilibrium shifts back toward SO3. Therefore, the quenching rate of the exhaust stream from the RTO's purification chamber and the associated design of its heat recovery chamber will affect the concentration of SO3. The steeper the temperature gradient through the recovery bed and the smaller the bed, the lower the SO3 emissions. Thus, an RTO with monolithic heat exchange media offers the best design for SO3 control.

The expected SO2-SO3 reaction under thermal conditions of a specific RTO design are shown in Figure 3. A chemical kinetics curve, an equilibrium curve, and a specific RTO design curve are plotted against the purification or combustion temperature of the RTO unit.

RTO heat-exchange medium The heat-exchange medium is one of the major components of the RTO system. In the early 1970s, random (interlox, beryl saddles, etc.) stoneware scrubber packing was used in RTO designs. Then, applications for the RTO were limited to straightforward VOC, or solvent-laden, process streams.

Extensive research and development with monolithic heat-exchange media provide a more efficient and acceptable particulate tolerant media. The distinguishing characteristics of the monolithic media, which led to its broad acceptance in the automotive industry,[Footnote: J.S. Howitt, "Thin Wall Ceramics as Monolithic Catalyst Supports", Society of Automotive Engineers, Inc., Paper # 800082.] also are ideal heat-exchange characteristics for the rotary-valve RTO system:

* Increased geometric surface area;

* Lightweight and more compact;

* Improved thermal shock resistance;

* Lower pressure drop and electrical costs;

* Greater particulate tolerance;

* Greater corrosion resistance;

* Lower maintenance requirements; and

* Surface area andpressure drop.

The increased surface area per unit of volume for the monolithic heat-exchange medium allows for a smaller RTO bed depth and, therefore, a steeper temperature gradient, substantially reducing the potential for SO3 emissions.

The old random-packing heat-exchange medium RTO bed can be as much as 60% greater in depth than the monolithic medium in order to achieve the same thermal energy recovery. The extended bed depth for the random packing media lengthens the retention time, or quench time, below 800degrees F, and dramatically increases the potential for SO3 emissions. It also adds resistance to the airflow, increasing the electrical fan operating cost and compounding any required wash down or cleaning efforts.

Weight and size-The monolithic medium substantially reduces the size and weight of the RTO system, which in turn reduces the number of modules. This reduces the foot print, total weight, and associated foundation. More importantly, it reduces the installation time, associated costs, and number of components of the overall system, thereby reducing maintenance and increasing reliability. A typical single-can rotary-valve RTO with monolithic heat exchange is 40% lighter than an old random-packed heat exchange RTO.

Particulate tolerance and maintainability-The monolithic medium promotes laminar flow through the core of each cell (Figure 4). However, all surface boundary layers are still turbulent, which helps prevent any particulate drop out or accumulation within the monolithic media. The only potential for particulate accumulation is at the entry to the monolithic block; this is due to inertial impaction. Because the flow is reversed every one to two minutes, the turbulence on the exit flow will scrub the face and minimize accumulation.

Since the potential accumulation of CaSO4 carryover from the scrubber system is limited to the entry of the monolithic media, a high-pressure water backwash system can be used to clean the face areas. The straight-through passages from top to bottom allow washing from the top down, which simplifies the process.

The old random packing media has much greater impaction surfaces, and, therefore, the particulate accumulation will be substantially increased. Plus, access to the impaction surfaces within the bed is virtually impossible. Since there are no straight-through passages to allow water flow, the media cannot be high-pressure cleaned in-place. Typically, this type of media requires removal from the equipment before cleaning.

RTO flow-control mechanism The flow-control mechanism is the most critical component of any RTO system. Present rotary RTO valve designs have eliminated the multiple butterfly and/or poppet valves, actuators, and speed controls on the old RTO unit. This dramatically reduces the number of components and associated maintenance, and improves the overall reliability and up time of the system.

The rotary valve should be a simple, rugged, and reliable design. Without these elements, the benefits of reduced components over a typical RTO design can be lost. There are two basic rotary valve designs (Table 4) that have been used in RTO systems to date: a fabricated continuous rotary drum design and an indexing heavy-duty machined rotor. On other VOC process applications, both valves have achieved the minimum leakage required to meet the 99% destruction efficiency for MACT compliance.

The cement kiln exhaust application includes other characteristics (high moisture, corrosive atmosphere, and scrubber particulate carryover) that require special attention in the selection of a rotary-valve RTO. The valve components-especially the bearings, alignment, and thermal expansion features-should be keep out of direct contact with process scrubber exhaust and be easily accessible for maintenance. Thread galling, particulate build up, and corrosion will affect the maintenance and reliability of these components.

Corrosion The presence of SO3 and hydrogen chloride, along with high moisture levels in the process stream, requires special attention in the selection of materials of construction. The high moisture content after the scrubber will greatly affect the acid dew point (ADP). The affects of moisture are reflected in the condensation methods of measurement. Gaksoyr, Ross, and Mueller indicated that ADP temperature differences could be as much as 125degrees F at low SO3 concentrations.[Footnote: Land Combustion Inc. (1981) Operating Manual for the Land Model 410 and dew point monitor system.], [Footnote: Evans F.D. & Targett, B.H. (1976) CEGB Research Notes, Marchwood Engineering Laboratories, R/M/N/849.]

In addition to the materials of selection, the design of the system should ensure that cold air leakage into the equipment and stagnate air zones (where condensation can result) is minimized or eliminated. The internal insulation system must contain an impermeable membrane to prevent surface corrosion.

If an RTO system design greater than 90% thermal energy recovery is used, a means to keep the RTO exhaust temperature higher than the ADP will be required. Otherwise, corrosive resistant materials in the exhaust ductwork and stack are necessary.

Conclusions Certainly the multiple air emissions, VOCs, HAPs, SOx , CO, odor, opacity, and high moisture from a cement plant pose a challenge for control equipment. However, a modular rotary-valve RTO and FGD scrubber system can provide the control necessary to meet MACT requirements. The system also provides a means to deal with corrosion, process/scrubber carryover to the RTO heat exchange, and spare module to facilitate maintenance and achieve the up time required of the kiln operation.

Research data and actual performance tests in the field have confirmed the ability of the specific RTO unit to achieve SO2-SO3 ratios necessary for the application. SO2 scrubber systems operating in cement kilns, along with field pilot tests and collaborating data from other process applications, indicate a high level of success in achieving air emission control of kiln exhaust.

A modular Regenerative Thermal Oxidizer (RTO) and Flue Gas Desulfurization (FGD) system provide a possible solution for controlling cement kiln air emissions of hazardous air pollutants (HAPs), volatile organic compounds (VOCs), carbon monoxide (CO), sulfur oxides (SOx), and odor in order to meet existing and future Maximum Achievable Control Technology (MACT) regulations.

Cement kiln exhaust streams pose several issues and concerns with the application of air pollution control equipment. These include:

Emissions * High levels of SO2 and SO3, opacity (plume) typically associated with combustion of sulfur bound fuels and associated particulate;

* Uncombusted VOCs and HAPs released from the feed stock; and

* Odor and CO.

Process operating parameters * High-process exhaust volumes, moisture, and temperature; and

* Continuous operation and extended up time (equipment reliability).

Air pollution control equipment issues * Corrosion due to sulfides and chlorides released from the feed stock and/or scrubber make up water;

* Controlled final exhaust temperature for system corrosion control, including existing ductwork and stack; and

* Process/scrubber carryover to the RTO module.

Research and development data, along with actual field data, substantiates the relationship between temperature and the ratio of SO2/SO3 through an RTO system and its relation to both the chemical kinetic and equilibrium prediction for SO2/SO3.