Dr Clark reviews the consumption of the essential services of electricity, water and compressed air in a cement factory and concludes that many would benefit from a rigorous review of the use of these key inputs.
In cement manufacturing and the Technical Forum we are very much concerned with the raw material and fuel inputs to the cement manufacturing process and also the process and processing equipment itself. However, none of these inputs or equipment could succeed in the manufacture of cement without three essential services: electricity, water and compressed air. These services are an integral part of the operation of a cement factory.
Cement manufacture is an energy-intensive industry. On a modern dry-process cement plant 90 per cent of the energy is consumed as fuel for firing the cement kiln. The remaining 10 per cent of the energy is consumed as electricity to drive the crushers, mills, kilns and convey the materials between the process stages. A modern, efficient cement factory will consume around 100kWh of electricity per tonne of cement produced. An electricity supply of around 15MW will be required for each million tonnes of annual capacity.
Historically, where a reliable electricity supply is available from a public utility company, it has always been most economic for a cement factory to connect to the supply grid and pay for the electricity from the public utility. The investment costs for a transformer substation are only about 7-8 per cent of the investment cost of a captive power plant. However, there are many parts of the world where a reliable electricity supply from a public utility is not available and then installation of a captive power plant becomes necessary. A cement works cannot operate properly without a reliable electricity supply.
Water usage
A modern, dry-process cement factory consumes water in three ways: (i) for cooling the bearings of large machines, (ii) for spraying into the process for gas conditioning and evaporative cooling, and (iii) as potable, drinking water. Older, wet-process cement factories consume much more water for the preparation of the kiln feed slurry.
For the potable, drinking water the rule of thumb is to provide 0.5m3 per day per employee, so a cement factory employing 200 people would need 100m3 per day of potable water. This might be suppllied by connection with a public utility or the cement plant may have its own water purification systems. Consumption may peak at shift changeovers and therefore a storage tank with capacity sufficient for 25 per cent of the anticipated daily consumption is normally provided, ie 25m3 in the case of a cement factory employing 200 people.
Water consumption for bearing cooling and process water shows wide variation from one cement factory to another. Where a cement plant is located in a desert environment the process might be specifically designed to have low water consumption. This could include the provision of a sixth preheater stage to lower the preheater exhaust gas temperature and improve the thermal energy efficiency of the kiln. Baghouses would be used for major exhaust dedusting with ambient air being blown into the process by quench air fans ahead of such baghouses for temperature control.
The amount of water required for bearing cooling is dependent on the equipment installed and the number of water cooled bearings. Mill bearings, kiln roller support bearings, major process fans and compressors are likely to consume ~60l of water per minute per bearing. Smaller bearings and gas analysers might consume ~30l/min per bearing. In total a 3000tpd cement plant might require a bearing cooling flow of ~150m3/h.
This flow will also be dependent on the rise in water temperature that can be tolerated. If a rise in temperature of 30°C is acceptable then the water flow can be reduced. If only a 5°C rise in temperature will be tolerated then a higher flow of cooling water will be required. Electricity consumption for cooling water pumping will rise with the cooling water flow and might vary from 0.5 kWh/t of cement to >4 kWh/t of cement produced.
As the temperature of the cooling water rises it becomes more corrosive. There might also be problems with the deposition of carbonates from the water as temperature rises. These deposits will increase the resistance to cooling water flow and increase electricity consumption by the water pumps to force the required volume through the restricted pipes.
However, this bearing cooling water will usually be a closed recirculating system with cooling tower, reservoir and recirculating pump(s). Only ~10 per cent of the cooling water will be lost by evaporation and leaks, therefore a make-up of only ~15m3/h should be required for the 3000tpd cement plant example.
Water for injection into the process will also vary from one dry-process cement factory to another. The major consumers will be gas conditioning towers, water sprays into vertical raw mills for bed stabilisation and water sprays into cement mills for temperature control by evaporative cooling. The requirements for each of these will be dependent on the gas volumes and temperatures. A benchmark water consumption for a modern, dry-process cement plant would be ~0.2t/t of cement produced.
Total water consumption for bearing cooling and sprays for a 3000tpd cement plant can therefore be estimated to be ~40m3/h or 0.32t/t of cement produced.
Compressed air usage
Compressed air is used in a cement factory as an aid or the means for moving material. It has many advantages as a tool to assist in or be the means to move material in the cement-making process as compressed air tools can withstand the dust, moisture and high temperatures of the process. This makes it ideal for clearing build-ups and agglomerates. Obvious examples would be the build-ups in the kiln preheater or materials hanging up in the feed bins to the mills. Compressed air tools such as big blasters or air cannons might be fired at preset intervals to prevent the formation of build-ups clogging the process. Compressed air lances might also be used intermittently by skilled operatives to clear build-ups from areas of occasional build-up which are not provided with fixed air cannons.
There are examples where the use of compressed air is designed into the equipment for the moving of material. Obvious examples would be the pulsing of the accumulated dust from the filter bags of baghouses with compressed air, again at preset intervals. Other examples would be the pneumatic conveying of powder materials and the homogenisation of powdered material in silos. Kiln feed homogenisation is the clearest example of such homogenisation by compressed air.
Compressed air is also used for actuation of valves and the movement of levers and probes. In these applications the compressed air is being used to move equipment or components of equipment rather than to move material itself.
Some of these applications are provided with dedicated air compressors. The kiln feed blending silo will usually be provided with its own compressor room and compressors. Pneumatic conveying of raw meal, kiln feed, finely-ground coal or cement will normally, but not always, have its own dedicated compressor.
There will also usually be general service compressed air provided in the cement factory. This general service compressed air will be provided by banks of air compressors in a compressor room with the air being distributed around the cement factory by pipelines with outlet valves at strategic locations around the plant.
So compressed air is a very useful tool in the cement manufacturing process. It is used to provide the energy to move materials and items of equipment. However, it is a very expensive form of energy. The air is almost always compressed by electrically-driven screw compressors. The typical volume requirements of <1000m3/h and compression pressure <10bar mean that screw compressors are best. As a cement factory has to have an electricity supply, diesel-driven compressors do not make sense except as an emergency back-up in remote locations.
Unfortunately, only 10 per cent of the electrical energy input goes to producing useful mechanical work. Some 90 per cent is lost as heat with compression and idling losses being the greatest. This is why compressed air is such an expensive form of energy.
The expense of compressed air means that its use must be carefully controlled and not abused. It is all too easy to connect a compressed air line to assist with keeping the material moving through the cement process. Proliferation of the use of compressed air can soon lift the electricity consumption of a cement factory by a few kWh/t of cement.
One problem is that cement manufacture is such an energy-intensive industry, both thermally and electrically, that it is easy for a blasé attitude to energy consumption to develop. Because the grinding mills consume so much electricity air compressors probably only amount to <5 per cent of the electrical energy consumption of a cement factory or <5kWh/t of cement.
On the other hand, compressed air is an energy source which must be available when it is required. That is no problem where dedicated compressors have been provided for a specific application, such as blending silos or pneumatic conveying. More potentially problematic is the general service compressed air. There must be redundancy designed into this general service compressed air as the demand is not expected to be constant. Operational problems arise when the use of general service compressed air rises and the redundancy provided within the system does not keep pace with the rise in consumption.
The general service compressed air is provided by a bank of compressors. The base and peak requirements need to be known to design the system and the controls correctly. Normally a high capacity compressor(s) will run continuously to provide the base load of compressed air at the required pressure. A second compressor(s) will run in idling mode and switch to full load when the compressed air consumption rises to maintain the pressure in the system. As use of compressed air proliferates in a cement factory the standby compressor(s) are running on full load for a greater proportion of the time. The situation can be reached where the standby compressors are running all the time and there is no reserve left in the system to cater for further increases in compressed air consumption.
It must also be remembered that compressors will need to be taken down for maintenance from time to time. This will be when the shortage of compressed air becomes obvious. Eventually this can lead to loss of production due to shortage of compressed air to respond to a build-up or blockage.
The response to the redundancy in general service compressed air system being used up can take two forms: (i) an additional compressor is installed at significant capital cost, or (ii) the use of compressed air is reviewed with the aim of reducing consumption and restoring the redundancy in the compressed air system. Air lances permanently in place to move persistent build-ups, compressed air used for cooling, or baghouses pulsing too frequently are all examples of where compressed air might be saved.
Addition of an extra compressor might seem the easiest solution, but the capital cost is not the only consideration. Electricity consumption will obviously rise with no associated increase in the output from the cement factory. When a standby compressor is idling it will consume ~40 per cent of the electricity it would under full load. More redundancy in the compressed air system inevitably means more time with compressors idling. Cement factory operators can make a contribution to minimising such wastage of electricity. Where more than one standby compressors are idling then all but one compressor should be switched off to save electricity. When consumption rises additional standby compressors should be restarted as the one that is already running approaches 100 per cent running time.
There are then considerations of the design of the general service compressed air system. Different applications require different compressed air pressures and qualities. Control applications for actuators require ~7bar, while cleaning applications require ~10bar. Airslides only require low pressure and a blower is much more efficient. Compressed air should not be used in airslides.
The pressure in the whole system should not be raised to the level required by the highest application. There will be pressure losses and leaks in large compressed air systems. The application requiring the highest pressure should not be located at the far extremities of the system. Compressors should be located close to the applications requiring high pressure. Every 1bar increase in pressure increases electricity consumption by ~7 per cent. It makes no sense to try to make up for too low-pressure at the end of a distribution system by increasing the compression pressure. It is better to have a smaller system with adequate pipe diameters. The system should also be circular.
Compressed air quality is measured by its maximum dust particle size, its water pressure dew point and its residual oil content. Instrument air and baghouse pulsing needs cooled, dried, oil-free air. All the processing to achieve a particular quality involves filtering which increases the pressure drop in the system and therefore the electricity consumption to achieve a certain pressure at the consumers. The compressed air should not be over-processed for applications like jack hammers and air lances that don’t need it.
In general, pressure losses and leaks in large distribution systems are the biggest waste of compressed air. It is better to have smaller, decentralised general service compressed air networks serving each process section of the cement factory.
Summary
These essential services should present no problem in a well-designed and managed cement factory. However, it is too easy for them to be taken for granted and for their use and abuse to grow. Such abuses become ingrained in the way the cement factory is operated. Operating costs slowly rise and gradually the spare capacity in the systems is taken up. When the situation is reached where all the spare capacity is used up major losses through loss of production occur. Many cement factories would benefit from a rigorous review of the consumption of water, compressed air and electricity.
This article was first published in International Cement Review June 2011.
