By Dr. Stefan Lindner and Thomas Walther
Rising Energy Costs, Increasing Pressure To Produce Returns, And The Environment Are Creating Tremendous Interest In Energetically Optimized Plant Concepts.
Energy supplies and changes in the climate are two of the major concerns of our day and age. In the opinion of the majority of scientists, the increasing demand for energy is accelerating the rate of climatic change, and that is something that the energy-intensive cement industry wants to work against.
Many cement plants are putting their money on attaining the highest possible efficiency in their production plants and on replacing primary raw materials and fuels by secondary materials in order to meet their obligations to reduce their CO2 emissions.
Energy consumption, the associated pollutant emissions and the consumption of natural resources are undoubtedly the factors with the greatest ecological impacts in cement production, particularly as temperatures of more than 1,450 C have to be maintained in the kilns.
From the point of view of production and environmental compatibility, the use of secondary fuels is a practicable alternative to regular fuels. Research and monitoring of the emissions into the atmosphere produced when secondary fuels are burnt in a rotary kiln have clearly shown that they are no different to those produced by burning primary fuels.
There are a number of advantages to the use of secondary fuels, for example:
■ Conservation of non-renewable fuels.
■ Energy is obtained from materials in line with
the principles of recycling management strategies.
■ Minimization of CO2 emissions by partial use of fuels with biogenic percentage.
Südbayerische Portland-Zementwerk GmbH is part of the Rohrdorfer Baustoffgruppe. The company has been manufacturing cement, transit-mixed concrete, concrete goods and prefabricated concrete parts since 1930. It employs some 1,000 people at 40 production works in Germany and Austria.
The Rohrdorf cement works has an annual capacity of approximately 1 million tons. Therefore every opportunity offered by modern technology is taken to minimize its environmental impact, and there are constant endeavors to prevent harmful effects occurring.
The works facilities have changed over the course of time, and are now equipped with state-of-the-art technology. What has not changed is the way we strive to achieve the highest quality, be a reliable business partner, and do our best to protect the environment around the Rohrdorf cement works.
Rohrdorf cement works faced the challenges of using secondary fuels efficiently, ensuring the trouble-free operation of the plant and the stability of the process. A modern process control system had to be installed in order to achieve these objectives.
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 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 calculated in tons per hour and those for liquid fuels in liters per hour.
Some setpoints have specified limits, breaching them always leads to an increase in the proportion of the primary fuel used. If a malfunction results in the loss of one ingredient from the selected fuel mix, the missing calorific value is automatically made up from the remaining active fuel ingredients.
The individual fuel feed controllers are then driven by the setpoints of the individual ingredients. There is an upstream ramp function to prevent large jumps in the setpoints for the individual controllers (scales).
This means that any deviation from the existing setpoint, whether caused by the loss of a fuel, the recipe specification or by the operator, is continuously readjusted up or down back toward the input value.
As previously mentioned, reliable measurement of the oxygen in the kiln is an important condition for achieving stable kiln operation. Only when this has been achieved can an appropriate process measuring technique succeed in achieving any further optimization.
The process control system delivers a clear fuel mix, and a flexible assignment of the primary and secondary firing controllers.
The control concept integrated in Cemat, which is based on Simatic PCS7, enabled a continuous fuel feed to be achieved. This leads to uniform kiln operation and thus highly efficient energy usage. If one fuel is lost, the heat value is automatically corrected.
That is, the control system responds to malfunctions immediately to maintain uniform kiln operation, even without operator intervention.
This method of kiln operation not only saves energy but also minimizes emissions and keeps within limit values. Uniform kiln operation also guarantees better clinker quality.
The use of an average 75 percent proportion of secondary fuels has enabled the consumption of fossil fuels to be substantially reduced. This high proportion puts Rohrdorf cement plant in a leading position.
Integrated Energy Management
Alongside fossil energy, electrical energy also plays a major role in the cement industry, especially in raw material preparation and cement milling. The German cement industry consumes approximately 3.3 million megawatt hours of electrical energy per annum.
About 100 kWh of electrical energy are used per ton of cement, which also benefits the environment. Electricity prices make up over 50 percent of the industry’s total energy costs, and they are tending to increase. For this reason, it is essential to keep these costs under control and optimize them even further.
An examination of the distribution of the total electrical energy consumption across all the stages of production in a cement works reveals that the grinding mills consume almost two-thirds. The majority are raw meal and cement mills, designed as either vertical or ball mills.
Cement production runs continuously with very limited reserve capacities and little redundant plant or equipment. The greater part of the equipment therefore has to run around-the-clock or – if there are specific restrictions – throughout the day.
Electricity costs can be reduced by shifting production – especially in the cement mill section – to cheaper off-peak periods and by ensuring that the contractually agreed energy consumption limits are not exceeded.
The load-management function integrated into the control system enables increased consumption by hidden electricity guzzlers to be counteracted by, for example, automatically shutting down auxiliary processes if there is a danger of exceeding the limit.
This not only reduces energy costs but also stabilizes the power distribution system by avoiding the need for peak load power from the public grid or the company’s own power generating plants.
The best tools for finding optimization strategies are the “PowerControl” and “power rate” add-ons for the Simatic PCS 7 process control system. The energy management system collects detailed information about energy consumption and costs, and prepares it for energy efficiency analyses.
First and foremost, the transparency of the current flows in the medium and low voltage energy distributions needs to be improved – right down to the individual loads.
This, coupled with the consumption data of other media such as fuels and other process energies, facilitate energy-efficient production processes. If major differences in consumption occur, energy peaks can be identified and eliminated by comparing different parts of the plant or manning of the shifts.
The old adage, “You can’t control what you can’t measure,” can truly be applied to energy management.
In the Rohrdorf cement works, energy consumption is measured by the Simatic S7 automation system, and stored in an SQL database. This can then be evaluated, and displayed in Excel in either graphic or tabular form.
This information can also be accessed by a Web client. The system displays the status of the low-voltage switches, and the current actual values of the electric current, voltage, etc. The link is provided by Profibus devices. However, the current stage of expansion does not permit operation of the low-voltage controls.
The total consumption is accumulated, and the adjustable power setpoint is monitored. These setpoints are input each week in a 7x24 matrix. This enables the Rohrdorf cement works to keep within the supply limits agreed with the power supplier.
Parts of the plant have to be switched on and off to comply with the agreed power intake from the electrical power system. The system supplies relevant messages to help the operator in the control center to do this. The operator then decides, for example, whether to shut down a cement mill completely. Such a system for measuring, managing and analyzing energy consumption data further increases the benefits of energy management programs. As these examples show, an investment in technologies for efficient energy use pays off within a short period of time.
This benefit becomes particularly clear when the efficiency of the use of energy and resources is increased. If they use less energy and resources, they can work more profitably.
Information for this article courtesy of Siemens, www.siemens.com/cemat.