The major cost factor in a cement factory is no longer the primary fuel but electric energy, due to the increasing use of secondary fuels, Although many energy-saving measures have already been considered and implemented, the search for further solutions continues to counter increasing power costs. Siemens suggests some areas to consider.
New and upcoming solutions, which were developed in other industries such as paper, chemical and steel, can be easily adapted to the cement sector. There are low-cost solutions which can be financed from the opex budget, while the larger investments will require a capex evaluation. Siemens is continually searching for new ways to save energy and advises cement producers to consider several areas in the plant which offer potential to further reduce electricity consumption. Figure 1 shows the typical investment costs and the expected ROI period for variable-speed drives (VSD), IE2- IE3 motors instead of IE1-motors (IE), optimising auxiliary equipment (AUX), continuous vibration monitoring (VIB), total harmonic distortion check (THD), management information system (MIS), secondary fuel management (FM), mill control optimisation system (MCO) and waste heat recovery and power generation (WHR).
Variable speed control If flow of processed air is presently being controlled using a damper, a variable speed drive can save energy because at lower speeds, less energy is drawn from the grid (see Figure 2). This energy-saving method is already typically used for larger drives. However, since small frequency inverters are more affordable today, every attempt should be made to save energy in this power range. Furthermore, many of the belt conveyors in a cement plant can run at lower speeds and still transport material. Low motor speeds will also reduce wear on the mechanical system. It is important to ensure that the drive motor is suitable for frequency inverter operation. Normally, an insulated bearing in the motor is sufficient potential by reducing operation speed.
International efficiency motors Extensive legislation on energy efficiency has been passed in the EU with the objective of reducing energy usage in the private and industrial sectors, and therefore reducing overall CO2 emissions. The new IE2_IE3 motors have higher efficiencies than IE1 motors (see Figure 3). Since electric power costs are much higher than capital investment costs for motors, a typical ROI period is within two years. The EU directive for the new IE motors has been in place since June 2011, and the high quantities of small motors in a cement plant represent an ideal potential for slashing energy consumption as motor efficiencies can be increased drastically in this area. As a result, it is more cost effective to replace a number of small motors than one big motor (see Figure 4). If an existing motor is to be replaced, the higher efficiency class motor will have a payback time of between 1.5 and three years.
Optimising auxiliary equipment A complex cement plant often means that extensive auxiliary equipment is required, eg pressurised air supply, cooling water and process water, which could be linked together to increase availability. The capacity of these sections is normally oversized since they are calculated for worst-case conditions. However, this equipment frequently operates in idling mode. Therefore, speed can be controlled to match actual consumption. This reduces power consumption, and therefore reduces energy consumption for these high numbers of auxiliary units with a low power rating. If necessary, the next step is to evaluate reducing the total number of auxiliary units. When it comes to lighting process areas, LED floodlights can slash power consumption by 80 per cent. In addition, all lighting systems should be intelligently controlled to reduce energy consumption even more. A detailed on-site review of an entire cement plant will bring to light potential opportunities for saving energy.
Continuous vibration monitoring Energy can also be saved by keeping the plant operational, because ultimately, the most important value to be considered is the kW/t of cement produced. Therefore, all measures should be taken to guarantee continuous operation. Unplanned downtimes negatively affect plant efficiency as start-up procedures are unproductive and costly. Continuously monitoring the vibration of all units especially the bearings of all machines is extremely important (see Figure 5). It is essential to ensure that all parts are replaced before equipment fails, and, therefore, that spare parts are available on-site in the required time. If part of the production facility is down due to damaged equipment, all efforts must be made to resume operation as soon as possible. This is why it makes sense to invest in advance, and avoid unscheduled downtimes.
Total Harmonic Distortion (THD) The increased use of variable speed drives in a cement plant will save energy but can also result in a higher harmonics in the electrical network. These increase losses in other connected loads and more power is consumed.
Furthermore, harmonics in the network also reduce the service life of many consumers. Power utility companies stipulate stringent limits on the harmonics generated by the electrical equipment in a cement plant. As a consequence, periodic checking of the THD level in a cement plant will show whether the equipment is still working within its limits. If the value is too high, a harmonic filter (ie a tuned power-factor-compensation unit) should be installed to reduce the THD value in the power supply. A periodic check will also show if measures should be taken to keep the THD within its limits. Siemens recommends a check every four years.
Combining MIS reports with EMS The Energy Management System (EMS) archives energy data from each department. It evaluates data to identify energy trends, time overlay trends and key performance indicators. A plant can reduce energy consumption based on analysis of historical data.
Furthermore, the data allows forecasts to be made about future energy consumption. At the same time, demand can be tracked and, when necessary, controlled.
For instance, typically, a mill section of a cement factory should not consume any energy when the mill is switched off. This can easily be checked using a Management Information System (MIS) report.
MIS continuously monitors on cement kiln line power consumption, and if consumption is higher than normal, this could mean that a component has failed. The report helps identify a pending failure, and allows time for the necessary corrective measures to be implemented. In short, continuous evaluation of MIS reports can help save energy and detect potential equipment failures.
Secondary fuel management Due to decreasing resources and increasing market prices for primary fuels such as oil, gas or coal, cement manufacturers must search for alternative energy sources. Currently, available secondary fuels include tyres, plastics, waste paper, waste oil, industrial waste, Tetrapak, old carpets, foam plastics, animal waste, wood shred, etc. All these secondary energy sources have different heat capacities. A kiln that has a high percentage of secondary fuels will operate at higher temperatures. Energy management to handle up to ten different fuels is becoming increasingly important, as secondary fuel costs have risen tremendously (these were previously free). Many secondary fuels result in changes in exhaust gas, clinker characteristics and kiln temperatures. A secondary fuel control management system to plan steady and continuous kiln operation can be implemented, based on Siemens standard CEMAT/PCS7 function blocks in the DCS or as a stand-alone system (see Figure 6).
Mill control system Mill Control Optimisation (MCO) provides additional possibilities to improve grinding and mill efficiency. The system shortens the start-up phase and allows the mill to operate stably at maximum production levels. To operate the plant with a low electrical energy tariff, mills must be started and stopped. This means it is necessary to ensure a smooth and efficient start-up, as it will reduce the energy consumption per tonne of ground material. The MCO can operate as a standalone system or fully integrated CEMAT/PCS7 application in PLCs (see Figure 7). The system especially helps night shift operators, when there is only a few personnel staffing the central control room.
Waste heat recovery Hot process gases produced from a kiln contain thermal energy, which is normally used for other process areas such as the raw or coal mill. Although this allows energy to be saved, the use of excess thermal energy can still be optimised. The excess energy can be converted into electrical energy (see Figure 8). The gases from the pre-heater tower and from the cooling area are used to generate high- pressure steam in boilers. A steam turbine drives a synchronous generator, which in turn feeds electrical energy back into the medium-voltage system of a cement plant. Optimum results can only be achieved based on a tailor-made system, which also takes into consideration any future cement plant modifications. Siemens provides cement manufacturers with continuous and ongoing assistance to identify a plant's energy-saving requirements. Only through continuous energy-saving solutions can cement plants be optimised on a sustainable basis.
If flow of processed air is presently being controlled usinga damper, a variable speed drive can save energy.
The increased use of variable speed drives in a cement plant will save energy but can also result in a higher harmonics in the electrical network.
Maarten Holland & Karen Chong, Siemens, Germany