The CSI and CEMBUREAU, the European Cement Association, commissioned the University of Liège (ULg) in Belgium to provide an overall inventory of mercury emissions in the cement industry worldwide and, when appropriate, identify the best practices available to reduce such emissions.
The final report is based on technical and scientific literature, on an exhaustive worldwide inventory of mercury emissions in the cement industry and on the analysis of case studies provided by cement companies which are members of the CSI.
Worldwide cement kiln mercury emissions data were collected through CEMBUREAU and the CSI member companies. The enquiry was launched in spring 2008 and data collection took place from summer 2008 until mid 2009. The enquiry was very general as the main objective was to collect as much data as possible. The questionnaire covered 2005, 2006 and 2007. 1681 emission values were obtained from 62 different countries from all continents, with 62 companies taking part. The main findings are:
Mercury enters in the cement manufacturing process as a trace element with the raw materials and the fuels. The mercury content of natural raw materials varies between individual raw material deposits and even within the same deposit. In fuels, the amount of mercury can vary in a similar way, depending on the fuel type and the fuel source. Depending on their origin, alternative raw materials and fuels may have a higher or a lower mercury content than the ordinary materials they replace.
Mass balance tests were conducted on several cement kilns and described in the literature. Moreover, CEMBUREAU and CSI members collected and provided several unpublished case studies. The information and data presented in this report and other studies demonstrate that mercury does not simply volatilize from the fuel and raw materials and directly leave the system through the stack. Rather, cement kiln systems have a significant inherent ability to control mercury stack emissions. The report and case studies largely confirm the experience and the key control factors for mercury abatement in cement kiln systems, as referenced in the available literature.
The key control factors and best environmental practices can be summarized as follow:
In most cases, the major contributors to total mercury input into the kiln system are the natural raw materials, and not the fuels.
Mercury input control is the most important measure for the responsible operation of a kiln. Best environmental practice is to conduct a careful selection and control of all substances entering the kiln in order to avoid too high a mercury input. A dedicated quality assurance system is recommended.
Selective mining may be an option in order to control and avoid mercury input peaks into the kiln system.
Mercury emissions are typically higher in kiln operations with the raw mill-off (“direct” operation) due to the missing adsorption capacity of the freshly ground particles in the raw mill.
Periodic purging (bleeding) of cement kiln dust from the system is an efficient way to control and reduce mercury emissions. Adsorption of mercury is favored due to the very high dust loadings present in the raw gas streams from preheater-precalciner kilns. This purging process is more efficient in the mill-off mode than in the mill-on mode due to the higher mercury concentrations in the dust. The efficiency of this measure depends, in part, on the quantity of dust removed from the system, and on the temperature prevailing in the air pollution control device.
Other techniques to reduce mercury air emissions are available in other industries such as waste incinerators and coal-fired power stations. Some, such as carbon adsorption, are well proven, whilst others are at laboratory or pilot stage. However, most of the test programs completed in those industries cannot be extrapolated to the cement industry. Therefore, those techniques cannot be considered as best environmental practice in the cement industry.