Cement Americas provides comprehensive coverage of the North and South American cement markets from raw material extraction to delivery and tranportation to end user.
Issue link: https://cement.epubxp.com/i/189537
FEATURE USE OF SECONDARY FUELS Rohrdorf cement works is the leader in the use of secondary fuels. In 2008, it was able to substitute 66 percent of its former fossil fuel consumption with secondary fuels. The works is licensed to increase the proportion to 100 percent – under compliance with emission limits. In order to achieve this, the company has invested some 6.1 million euros since 1997. For example, storage, homogenization and metering facilities for a range of secondary fuels have been installed. At the works, close attention is paid to the chemical composition of these fuels in order to prevent harmful emissions and detrimental effects on the environmental compatibility of its products. This enables the plant to fulfill the legal requirements for co-firing secondary fuels. In Germany, the proportion of wastes used developed from 7.4 percent in 1990 to 58.4 percent in 2009. The choice of materials used depends, among other things, upon their availability and heat value. Before secondary fuels are used, they have to be carefully checked to see if they are of suitable quality, and whether they could have any effects on emissions, the process control or the end product cement. A sample is taken automatically from each consignment from a secondary fuel supplier when it is tipped into the receiving bunker. The truck driver has to identify him/herself by a chip card before dumping the load. This card assigns the sample to the corresponding delivery note data, thus facilitating back-tracking. The calorific value of the material is determined from the sample. This value is then input into the process control system for further processing and calculation. THE PROCESS CONTROL SYSTEM Rohrdorf cement works has been using the Siemens Cemat process control system since 1993. Version 3 was installed initially, and the system was upgraded to the current Version 6 in 2005. As with its predecessor versions, Cemat Version 6 has been designed specifically for the cement industry. The current Cemat versions are based on the Siemens Simatic PCS 7 process control system. The latest Cemat Version 7 has additional performance features to facilitate easy, efficient operation of cement works. For example, Advanced Process Control (APC) function blocks have already been integrated as standard, so that operational efficiency can be improved still www.cementamericas.com • Fall 2013 • CEMENT AMERICAS further. As these functions are already included in the standard version, even model-based, predictive multivariable controls can be realized without incurring additional product costs. The Cemat process control system is based on Simatic PCS 7. It uses data collected by the various sources of information (manual input, process and laboratory) to calculate the optimal fuel mix online, while also taking into account the relevant process and commercial constraints. Among the most important of the many possible constraints for clinker production are: ■ Heat balance. ■ Oxygen content. ■ Emission limits (SO2, NOX, etc.). ■ Maximum / minimum values and rate of change of final controlling elements. ■ Operational restrictions on fuel consumption. The objective of kiln fuel control is to maintain the energy input constantly at the specified setpoint in gigajoules (GJ). This is achieved by enabling the fuel control to access up to 10 pre-selectable ingredients of the primary and secondary fuels. The data obtained from the laboratory analyses of the secondary fuels are sent from an input dialog directly to the process control system, and are then automatically assigned to the respective fuel. The calorific value data of the primary fuel, for example coal, also follows this path into the system. The operator then specifies a desired fuel setpoint. This value has been individually determined and tailored to the present kiln and the available raw materials. A higher-level oxygen setpoint controller monitors the oxygen concentration in the kiln. If the deviation from this setpoint becomes too great, the energy setpoint in GJ is adjusted. This requires a reliable and, as far as possible, maintenance-free gas analysis system. This oxygen control smooths out fluctuations in the calorific value of the individual fuel ingredients, and brings the fuel setpoint back to the value actually required. The total energy requirement is divided between the primary fuels, such as coal and oil, and the secondary fuels. The calorific values of the ingredients can be calculated from the resulting energy values in GJ of the individual ingredients which, in turn, enable the setpoints for the ingredients of solid fuels to be calcu- 9