While the subject of new cement industry compliance legislation remains a delicate issue, the clock on manufacturing plants making the necessary system additions and upgrades is ticking.
Recently, the U.S. Environmental Protection Agency (EPA) published National Emission Standards for Hazardous Air Pollutants (NESHAP) from the Portland Cement Manufacturing Industry and to the New Source Performance Standards (NSPS) for Portland Cement Plants in the Federal Register. The EPA first proposed these standards in June 2008. The new compliance regulations apply to facilities that commence construction, modification, or reconstruction after May 6, 2009. The final amendments to NESHAP were completed Sept. 9, 2010, and affected facilities will have three years from submission to national register to comply.
The NESHAP will regulate the THC during normal operations as follows: Kilns 24 PPMvd1 corrected to 7% O2 (1 corrected to propane). Initial estimates have these new compliance guidelines affecting more than 140 portland cement operations in the United States.
Cement plants are dependent on quarry mining operations that supply clay and limestone to the plant processes. Materials are dried, preheated, calcined, and sintered into a cement clinker. To comply with the passing of the recent cement NESHAP, pollution control equipment may now be necessary for the cement process as plants emit not only carbon dioxide but also acid gasses, mercury, particulate, total hydrocarbons (THC), and hazardous air pollutants (HAP). All of these substances originate from the plants unique source of limestone containing kerogen hydrocarbons, a variable mixture of organic chemical compounds, and fuels.
Choosing the right emission controls
Historically, significant public pressure aimed at these facilities due to visible emissions, noise, and/or odor is not uncommon. Visible emissions from plant exhaust, predominantly from sulfurous compounds and particulate, often justified the installation of improved fabric filters and bag houses. The bag houses replaced electrostatic precipitators that had been utilized in previous decades. A dry-injection scrubber was often deployed to further reduce emissions and volatile materials in the exhaust gas.
To further remove volatile organic compounds and sulfurous emissions, plants often install add-on controls that include acid gas scrubbers and, in some instances, regenerative thermal oxidizers (RTOs). The intent of the RTOs is to further remove CO and THC.
RTOs have been in existence for 30+ years. However, only the latest generation rotary valve systems have demonstrated effective and reliable results for reducing emissions in a cement plant. RTOs with single rotary valves have proven that they can offer a 98% reduction of THC emissions which has, as one might expect, been received favorably by both state regulators and the community.
The advantage of an RTO compared to other means of controlling THC is an RTO is substantially forgiving while maintaining destruction efficiency with little regard to the THC’s expected in the cement process. Catalytic systems, chemical scrubbers, adsorption systems and the like are “tuned” to a particular THC or THC family. The THC will vary with the ore and to some extent the fuel or coal. RTOs will likely perform equally as the THC concentrations and species varies over the life of the quarry.
For a plant to make a commitment to a 98% emission reduction, RTOs need to achieve 98%+ destruction under all operating scenarios. Given the operational issues that cement plants continuously face, a high level of technical expertise and experience in the challenging cement plant environment is important when choosing which environmental equipment supplier.
Choosing the right style RTO
Generally, the environmental equipment supplier conducts an initial engineering study based on a joint effort between the supplier and the cement manufacturer. This primary collaboration combines the plant’s operating issues with the manufacturer’s oxidation equipment experience and system materials knowledge. During the collaborative engineering study, a predominant area of focus is a plant's RTO uptime. A major issue in maintaining a plant’s uptime is the success or failure of a valve on each RTO that controls the inlet, outlet, and purge airflow distribution.
Conventional tower-style RTO uses multiple valves to control the air distribution through the RTO beds. For example, a medium-sized, conventional RTO employs as many as 8-15 valves on a single unit, with as many as 8-12 units required for an average-sized, modern cement plant to abate its emissions properly. Based on this calculation, as many as 180 valves may be required for a conventional RTO installation. Reliability of an RTO is dependent on the reliability of the valves. The valve sustainability in the harsh conditions of a cement application is of paramount importance to the plant’s uptime and THC and CO destruction efficiency.
To minimize potential valve-failure issues, the single rotary valve RTO was developed in 1997. This single rotary valve replaces all of the individual valves normally associated with RTOs. In addition, the single-valve RTO design requires only one drive system, compared to the hundreds of actuators needed to operate the multiple valves related to a conventional tower-style RTO. The rotary valve has proven reliable in the cement industry with more than 14 RTOs operating for as long as 10 years, as well as in hundreds of other applications, markets, and industries.
Secondary areas of RTO focus often relate to purge system design and sizing, RTO materials of construction, maintenance costs to keep the systems running, low-THC destruction efficiencies, and unscheduled downtime. The single-rotary valve system specifically addresses all the issues associated with conventional RTO designs to ensure uptime, promote reliability, and reduce maintenance.
To further ensure plant uptime, a modular approach should be considered, since such a system will utilize smaller RTOs grouped together acting in concert with each other as opposed to a single, “large-scale” RTO, handling the full exhaust volume. The advantage of the modular approach over a large scale system is the practicality of installing “spare” capacity at a sensible cost. This additional capacity provides system redundancy and up time guarantees, but more importantly allows the plant to service a RTO without interrupting the process.
This additional capacity provides system redundancy and up time guarantees, but more importantly allows the plant to service a RTO without interrupting the process.
As an example, if the total exhaust flow requiring abatement is 300,000 cfm, then six smaller modular RTOs, each handling 60,000 cfm, could be installed. With five RTOs on line at any one time, the sixth (or spare) RTO would be available for maintenance or washout without compromising or limiting the plant production capacity. In this case, the cost for the modular smaller RTO as the installed spare is a small fraction of the cost of supplying an installed spare large-scale system. With the maintenance completed, the modular spare RTO would be placed back on line adding capacity if necessary or operating in concert with the other RTO to reduce the overall operating costs and paying back the additional investment. This installed spare capacity would not be reasonable with a single, large-scale RTO and a second as the installed spare.
Choosing the right RTO composition
The secondary, but certainly connected issue when engineering the right RTO for cement plant emissions control is the selection of the RTO materials of construction. In certain cement plants, the RTOs will follow an acid gas scrubber and will require exotic alloys to survive the environment of acids, base materials, and abrasive materials. In other facilities, the RTOs will be installed prior to the scrubber and see a completely different set of conditions.
Cement plant RTOs must be constructed of materials following the analysis of pre-stressed metal coupons removed from RTOs in similar applications and detailed metallurgical consultation. The 316L material is sometimes a suitable and economical choice with excellent resistance to the corrosive, high-temperature conditions under which the RTO and valve would operate. However, 316L is not the proper material for every cement plant RTO.
Conclusions & analysis
Once an RTO solution is selected, some final important issues need to be considered:
- Compact Design – Most cement plants were not planned to accommodate an RTO installation. Site real estate is at a premium. A smaller, compact, modular-designed RTO will allow the most flexibility when installing an RTO solution.
- Heat Recovery Ceramic Media – The RTO scope of supply must include optimization of the ceramic heat recovery media, of which there are several types available, but only a few are proven durable in a cement plant RTO. The media is prone to both chemical attack and abrasion from the cement (gypsum) dust as well as the periodic washouts. In some instances, one type of media in combination with another type or design is the best. In all cases, the media should be selected for maximum performance and fouling resistance. However, testing has proven structured ceramic media (block) is superior to randomly packed ceramic media (saddles) for applications in the cement industry. Structured block media is easier to clean and performance restored to near original condition over several washouts. Randomly packed ceramic media, while less costly, does not clean satisfactorily over time. Further benefits of structured block media are tolerance to particulate and chemical stability.
- Valve Washing System – A properly designed integral valve washing system will prove its value in this application. The harsh cement dust and acidic moisture composition of the process gases, especially after scrubbing, may require the RTO ceramic media to receive a water deluge wash down that consists of an operator using a fire hose at predetermined intervals. This ensures continuous trouble free operation. The need to keep ceramic media within the RTO free of large particulate is essential for the RTO system to maintain the plants environmental compliance. While the topic of new environmental compliance legislation within the cement industry remains somewhat of a delicate issue, the clock on cement plants making the necessary system additions and upgrades has begun. Currently, there is no single be-all and end-all solution. However, the use of RTOs—specifically single rotary valve RTOs—has a track record in abating and controlling CO and THC. The best method is to approach the RTO selection carefully. There is no substitute for experience in this tenuous application.