ENERGY CONSUMPTION AND ENERGY AUDITS IN THE CEMENT INDUSTRY
Because the chemical and physical reactions involved in the production of cement clinker from raw materials take place at high temperatures, cement manufacturing necessarily requires a lot of energy. With the cost of energy representing, something like a fifth of the gross price made of finished cement, every effort is made to conserve energy in manufacturing.
While strong increases in energy costs led to greater public awareness of the importance of energy conservation, maximum energy efficiency has been an industry goal of cement long before the so-called “energy crisis”.
By far, the greatest use of primary energy in the form of fuel is in the manufacture of clinker in the cement kiln, and in large part or the improvement in energy consumption by the cement industry has been the result of improvements in the design and operation of the kilns and their associated equipment.
The process used considers a direct relationship with the amount of energy required to produce a ton of cement clinker. Due to the need to evaporate the water from the raw suspension, the wet manufacturing process is considered the one that consumes the most fuel: even in the most modern wet process kilns, water evaporation represents 40 % or more of the total consumption of heat, and actually exceeds by a considerable margin the theoretical heat consumption of calcination reactions and clinker formation.
The considerably higher energy consumption of the wet process has led to a worldwide trend towards the use of dry or semi-dry processes, wherever the raw materials are suitable.
In addition to replacing the wet process with dry or semi-dry processes, which is not always technically or economically feasible, the adoption of suspension preheaters for dry process kilns (with savings of up to 14% in energy consumption compared to dry kilns without preheater), improvements in the design of the clinker cooler and the use of residual heat from the cooler for other purposes such as drying raw materials or fuel. Continuous improvements have been made in the design of kilns and associated equipment that have increased the energy efficiency of the kiln system. Among these are improvements in the design of the cyclones of the preheater tower.
As mentioned in the chapter on kiln operation and control, in recent years much attention has been paid to the use of low-quality fuels in the cement industry, and after preliminary tests, the consumption of alternative fuels is used regularly to complement the pulverized coal fuel in some cement plants.
While the firing of raw materials into clinker in kilns represents roughly 90% of the net energy used in cement manufacturing, the industry consumes substantial amounts of secondary energy in the form of electricity. Electricity represents approximately 11% of the total net energy consumption of the industry.
Large amounts of electricity are consumed in the grinding of raw materials and cement clinker: practically all the machinery used within the plants, and a considerable amount of the machinery used in the extraction and transport of raw materials.
With electricity, as with fuel, every effort is made to reduce consumption in relation to the amount of cement produced, both in absolute terms and in terms of cost to the industry. One means to reduce the cost of the electricity consumed is to operate the plant in such a way that the maximum advantage of the electricity tariff structure is obtained. As already mentioned, for example, grinding mills can be used wherever possible during periods when other demands in the electricity supply network are minimal to obtain the most favorable rates.
This figure can help cement plants compare their own energy use with other plants in the industry. The table classifies individual plants from the most efficient (1) to the least efficient (15). To position your plant among the others in the industry, complete the table on the next page with your most recent information to calculate the energy use per unit of your plant.
The energy use among the 15 plants represented in the upper figure, varies from a minimum of 3.68 to a maximum of 6.87 giga joules per ton of clinker. The average energy consumption for the 15 plants is 4.69 GJ/t. But the average for the four most energy efficient plants is only 4 GJ/t. In other words, there is a 15 percent difference between the most efficient plants and the industry average. This significant difference suggests that many plants occupy ample space to improve energy efficiency.
There are always extenuating circumstances and specific explanations for differences in energy use between individual companies and plants in terms of raw material resources, types of fuel, kiln capacity, technology and general operation and maintenance practices. However, improving energy use is important for industry and society, and deserves more attention from challenges and improvement achievements.
Typical patterns of fuel consumption in cement manufacturing processes (fossil fuels only)
|SPECIFIC FUEL CONSUMPTION IN KCAL/KG CLINKER
|Dry process (long kiln)
|Wet process (long kiln)
|Dry process (suspension preheater)
|Specific Fuel Consumption in (kcal/kg clinker) *
|Theoretical heat consumption
|Cooler loss (clinker cooler & gas ventilation)
* Based on the best operating plant.
A cement plant is an industry that consumes a large amount of energy, both in terms of thermal and electrical energy, and more than 40 % of the cost of production is due to the cost of energy.
With intense price competition in the market, energy conservation is without a doubt a very relevant factor that is offered as a low-cost option to reduce energy costs and create a competitive advantage in the market. Any effort to reduce the use of thermal and electrical energy will also directly affect the company’s profitability in a very positive way.
An energy conservation and auditing project are normally carried out in cement plants to evaluate the performance of their various equipment within the process, such as mills, kilns, fans, compressors, etc., for the manufacture of cement.
The energy audit also identifies possible ways to save energy and costs. We will focus on the different methods and formulas to carry out an energy authorship and to be able to evaluate the efficiency of a plant or equipment in relation to the market and other plants.
COLLECTION AND PROCESSING OF NECESSARY DATA
- Complete list of motors for plant (or area) with details of motor volts, amps, and KW ratings.
- Specific or real service of the equipment, that is, a fan rated at 300 m³/min and 200 mmwg.
- Using the equipment specifications, calculate the net power of the motor. The application of the equipment and motor efficiency factors calculate the theoretical power of the motor.
- Decide the running time of the equipment as% of the main equipment, i.e. (the mill, the furnace) the running time. This will normally be 100 %. Also decide if the operation of the equipment will be necessary during the period before or after the operation of the main equipment.
- Using a power meter, measure the actual KW, volts, amps, and power factor for motors that exceed the specified size (i.e. 10 KW).
- Energy consumption goals can be set for:
- Constant operation of the plant.
- Annual plant average allowing shutdowns.
- Acceptable energy consumption of the area when it comes to main elements.
These goals can be monitored using the online energy monitoring system.
- Inefficiently functioning equipment can be identified, and further diagnosis can be shown to be due to:
- Poor operating efficiency, eg fan.
- The actual service is in excess of the original, specified service.
- Mechanical defects.
- Motors that are large in size and therefore inefficient can be identified.
- Similarly, motors that are undersized and run at risk of failure can be identified.
- The effect of plant shutdowns on total energy consumption can be quantified.
- With comparisons made between different plants, firm cases of capital expenditures can be established to reduce energy consumption.
ELECTRICAL AUDIT FORMULASElectrical Audit Formulas
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