While the cement industry differs from the U.S. utility sector in many ways, the technologies currently being used or tested for coal-fired utilities are the same as those being proposed by the U.S. Environmental Protection Agency (EPA) to control various pollutants, including mercury in the cement industry. In this regard, many of the lessons learned by utilities can be applied to the cement industry. This article discusses the implementation of mercury control technologies in the cement industry and some of the potential impacts.
Mercury emissions from cement plants—as they are from coal-fired power plants—are highly variable and depend on a number of factors. Unlike utilities, the greatest contributor to mercury emissions from cement plants is the limestone rather than the fuel. The mercury concentration in the limestone not only varies from location to location but also within an individual quarry.
The kilns used for the manufacture of cement are vastly different than utility boilers. Different temperature profiles and flue gas compositions greatly impact mercury emissions, which means that different types of kilns may result in different mercury emissions both in terms of concentration and type.
EPA has stated that in order to meet the proposed mercury rule, most if not all cement facilities will have to install some form of mercury control. Currently, the best available control technology for mercury is considered to be activated carbon injection (ACI). However, as the Energy & Environmental Research Center (EERC) has learned over the past 15 years in its work with the utility industry, the effectiveness of ACI is site-specific. This will certainly be the case for the cement industry as well. Five primary factors affect mercury collection efficiency using ACI and air pollution control devices, and all are interrelated: mercury speciation, temperature profile, residence time, type of ACI used, and flue gas constituents (particularly sulfur trioxide [SO3]). There are two primary types of mercury species that are commonly formed in a utility of cement system, electrical and oxidized. In general, oxidized is much more easily cultured while elemental can be more challenging. A review of the limited published mercury data from cement plants showed that the form of mercury at the inlet of the particulate control device ranged from almost 100% to near 0% elemental mercury, and elemental mercury is more difficult to collect with ACI.
The temperature of the flue gas at the activated carbon collection point (baghouse for most cement facilities) is critical. Mercury capture by activated carbon is drastically reduced at temperatures greater than 400°F compared to temperatures closer to 250°F. For existing cement plants, the temperature at the primary baghouse ranged from a low of about 200°F to a high of close to 700°F. The effectiveness of ACI at the plants with high temperatures is borderline at best. It may be necessary to increase the ACI to very high levels or use additives. In either case, it is bound to be more expensive.
For the past several decades, the portland cement industry has strived to utilize most if not all baghouse dust by either returning it to the cement kiln or beneficially utilizing it. If ACI is added to the primary particulate collection device, the baghouse dust can no longer be recycled back to an inline kiln/raw mill because it would result in the reemission of the mercury captured by the activated carbon. Also, depending on the amount of activated carbon used, it may limit or prevent beneficial uses as well. This results in a double penalty—increased production costs and increased disposal costs. To ensure the quality of baghouse dust and to maintain the ability to recycle the dust back to the system, a polishing baghouse is required. The activated carbon is added to the polishing baghouse, with most of the return dust collected in the primary particulate control device. Obviously, this will add substantially to the cost of mercury control.
Because standard types of activated carbon may not be effective enough to meet the proposed requirements, some cement plants may need to use more expensive treated carbons or enhancement agents. Typically, this means the addition of halogens. In addition to increased cost, there is a concern regarding increased corrosion and erosion of ductwork. Additional halogen may also have an impact on the overall chemistry of the cement kiln dust.
Another serious concern that may dramatically impact the effectiveness of either standard or treated activated carbons is the presence of even low levels of SO3in the flue gas. As a result, many utilities are considering adding SO3removal technologies. In the utility industry, SO3 removal technologies usually consist of adding an alkali component, but most cement plants are limited by the alkali content of the product and must meet an alkali limit, so this may not be an option.
To effectively control mercury, a reliable mercury emission measurement method is required. Utility experience with continuous mercury monitors has been mixed. Some plants have had good experiences, but others have had tremendous challenges. One plant reported that it had been trying to get its monitor operating consistently since the spring of 2007. To be fair, vendors understand that there have been difficulties, and they are actively trying to reduce or eliminate them. The other method for measuring mercury emissions is the use of sorbent traps (PS-12A). Sorbent traps have been used with good results at a number of utilities, but there are very limited data documenting the problems that may be encountered at cement plants.
So what does all of this mean? Federal regulations are going to force the cement industry to reduce its mercury emissions. There is a very good possibility that several kilns will be permanently shut down because the expense to bring them into compliance is simply too high. Research and demonstration projects conducted by the EERC and others at U.S. utilities have shown that the required mercury reductions can be achieved, but to what level is very site-specific. What is not clear are the long-term ramifications and costs associated with achieving and then consistently measuring the levels required by the portland cement National Emission Standards for Hazardous Air Pollutants ( NESHAPS). For example, EPA has made cost assumptions that may not be accurate, in particular regarding the possible need for a polishing baghouse. For most existing facilities, achieving a standard of 55 lb of mercury per million tons of clinker will most likely be possible, but achieving 21 lb of mercury per million tons of clinker produced for new facilities will be substantially more difficult.
In summary, each facility will need to incorporate new control devices along with sorbent injection systems to meet regulations, and mercury behavior at each facility will be unique and must be treated as such. There may well be long-term detrimental effects that have not yet surfaced from the use of mercury reduction technologies, including lifetime reducing corrosion and erosion of ductwork and other components. In conclusion, much more data are certainly needed to adequately understand the overall implications of mercury regulation on the cement industry.
Dennis Laudal, Senior Research Advisor
John Kay, Research Manager
Energy & Environmental Research Center
Energy & Environmental Research Center