Energy management and conservation

Energy Management and Conservation

Part 2 Energy Survey

An Analysis of the Energy Consumption in the Manufacture of Cement

1. Introduction

The manufacturing of cement is an energy intensive process, and the cost of its energy has a high percentage out of the total production cost. In the cement industry, appreciable amounts of energy could be saved or conserved by preventing of leakage in the kilns, modifying the equipment to recover heat from the pre-heater and cooler in the process of cement-making and effective use of industrial waste materials (UNIDO & MITI, 1994). Optimising the energy consumption of the plant is required to achieve the objectives. To achieve these objectives, we must consider the constraints such as the product quality and maintaining the kiln's exhaust gas temperature at set level.

The quality of the coal and raw meal, combustions air quality, and air quenching system affects the product quality (Rasul et al., 2005).

The energy audit is the most effective measures for a successful energy management program. The major aim of the audit is to provide accurate information on the analysis of energy consumption of different components, which will be used for determining possible opportunities for energy conservation.

Although the potential methods of enhancing the overall kiln efficiency are waste heat recovery from hot gases and hot kiln surfaces, much has not been done on detailed thermal analysis of rotary kiln systems (Rasul et al., 2005; Engin & Ari, 2005).

The aim of this report is to evaluate the thermal energy consumption of the individual equipments and to determine the overall energy performance of the cement manufacturing process. Also, the potential energy saving opportunities in the unit will be analysed. Some recommendations will be made on the low cost energy efficient recovery and conservation measures of the manufacturing process.

2. Production Process of Cement

A cement production plant consists of the following three processes.

  1. Raw material process
  2. Clinker burning process
  3. Finish grinding process

The wet and dry process occurs in the raw material and clinker burning process. During the wet process as shown in figure 1, the raw materials are crushed to ~20mm diameter and the automatic weigher mixes it to an appropriate ratio. Water is added to the mixture which is further made finer using a combined tube mill into slurry with a water content of 35 to 40%. The slurry is sent to a storage tank, mixed to a homogeneous state and sent to a rotary kiln for clinker burning.

It is easier to mix the slurry in the wet process, but a large amount of energy is consumed as a result of water evaporation in the clinker burning (UNIDO & MITI, 1994). However, various degrees of wet processing exist: the fuels consumption in the kiln can be minimised by using the semi-wet (moisture content of 17-22%) (Martin et al., 1999). In the dry process, the raw materials are crushed at the mill into fine powder, blended and put in a storage tank, prior to pre-heating. Then, the crushed materials are transferred into the rotary kiln furnaces from the pre-heaters to the evaporation clinker for burning (Kabir et al., 2010; UNIDO & MITI, 1994). The moisture content in the (dried) feed of the dry kiln is typically around 0.5% (0 - 0.7%), thereby reducing the need for evaporation and reducing kiln length. The waste heat from the kiln could be used to further dry the raw materials before pyroprocessing (Martin et al., 1999).

In the cement production, the clinker production stage is the most energy intensive process. It accounts for more than 90% of total energy used in the industry. Clinker is produced by pyro-processing in large kilns. These kiln systems evaporate the free water in the meal, calcine the carbonate constituents (calcination), and form portland cement minerals (clinkerization) (Martin, Worrell, & Price, 1999).

3. Characteristics of Energy Consumption in Cement Production

Burning of the ground materials in a rotary kiln is the key process in cement production. The sequence of clinker production occurs in four steps, namely suspension preheating, precalcining, burning and cooling (Rasul et al., 2005). Rotary kilns are refractory lined tubes with a diameter up to 6 m. Their rotational speeds lie within 1-2rpm and they are usually inclined at an angle of 3-3.5o. The raw materials are preheated using the cyclone type pre-heaters before entering the kiln intake. In a typical dry rotary kiln system, pre-calcination gets started in the pre-heaters, and approximately one third of the raw material would be pre-calcined at the end of pre-heating (Engin & Ari, 2005).

The clinkerization process is where most of the energy, (over 85%) is being consumed, in the kiln. Finish grinding stage consumes about 40% of electric power, while the raw material and the clinker burning process consumes a little under 30% respectively. The finish grinding process mainly consumes electric power for the mill, and the clinker burning process mainly for the fan (UNIDO & MITI, 1994).

4. Energy auditing (Energy Audit Analysis) for dry type cement rotary kiln

Energy audit is the technique used to evaluate the thermal energy performance of the kiln systems. The kiln systems energy performance can be determined from energy balance over the control volume shown in Fig. 2. Heat energy to and fro the control volume are equated using simple steady state assumption. Therefore, relative distributions of outgoing materials and thermal energy can be computed accordingly, when internal heat transfer is neglected. Material and heat balances around the control volume are performed (Kabir et al., 2010).

4.1. Mass Balance

The average compositions for dried coal and pre-heater exhaust gas are shown in Fig. 3. Based on the coal composition, the net heat value has been found to be 30,600 kJ/kg-coal.

It is usually more convenient to define mass/energy data per kg clinker produced per unit time. The mass balance of the kiln system is summarized in Fig4. All gas streams are assumed to be ideal gases at the given temperatures.

4.2. Energy Balance

The assumptions used for carrying out the energy audit analysis include:

  • Steady state operating conditions
  • During the study period, the plant is under equilibrium conditions
  • verage surface temperatures of the kilns, preheaters and clinker coolers are constant
  • Throughout the study period, the average temperature is constant
  • The air leakage is negligible
  • Compositions of the raw material, fuel and clinker are constant (Engin & Ari, 2005; Kabir et al., 2010).

Combustion of fuel [e.g. low pour fuel oil (LPFO)] provides the heat required for the reactions in the rotary kilns. Low rate of fuel consumption should be applied during the operation, without affecting the quality of the clinker produced. To achieve this, a clear understanding on how the fuel is burnt and the efficiency of its utilization during burning should be known.

The thermal energy performance of the kiln systems is evaluated using the energy audit technique. The energy balance over the control volume can be used to determine the kiln systems energy performance as shown in fig. 4. The simple steady state assumption is used to equate heat energy to and fro the control volume. Hence, when internal heat transfer is neglected, the relative distributions of outgoing materials and thermal energy can be determined (Kabir et al., 2010)

5. Energy saving opportunities for pyroprocessing unit.

An improvement in cement pyroprocessing is one of the greatest means of reducing energy consumption and lowering emissions associated with cement/concrete (Choate, 2003).

Expanding the use of the secondary energy as much as possible is one of the objectives of energy conservation measures and this reduces the requirements for primary purchased energy. In any process industry, conservation opportunities can be divided into three categories namely: short term, medium term and long term measures.

Available technologies required in the short term measures generally do not require substantial investments. These are management practices, such as:

  • Inspection to encourage conservation activity.
  • Housekeeping, such as turning off motors and heaters when not in use.
  • Training programs for operating energy intensive equipment.

Switching to new and more efficient technologies are some of the medium term measures. At this stage, the objective is to reduce energy consumption and recovery without significant investment. Improving operation and maintenance techniques will ensure continuity of the process, improve inter-process delivery conditions and realise substantial energy savings. Moreover, improvements in production yields will equally add a significant input to energy savings. Table 3 shows the typical amount of the metric tonnes of clinker produced in a cement manufacturing pyroprocess system.

Large investment and new innovations are the long term measures. This involves capital investments of the interconnection of the processes. A combination of various measures below can be used to describe it:

  • Management and association of two energy sources [e.g. cogeneration of heat and power].
  • Optimisation of linking between equipment that helps in best utilization of intermediate products and energies.
  • Completely interconnected, centralised automation of the whole plant based on the earlier diagnostics of the actual operations of various equipments and an optimization algorithm developed.
  • Modification or redesign of the processes. Older technologies are replaced by new, advanced technologies, which are the major contributing factor for lower specific energy consumption.

Basically all output flows from kiln system could have potential for waste heat recovery. The main sources of energy conservation can be sensible heat of exhaust gas from kiln and cooler, latent heat of exhaust gases, and radiation heat of kiln and cooler. The recovered waste heat can be used for drying of raw material and coal, and preheating of combustion air as secondary air for kiln and tertiary air for Pyroclone. However, only thermal energy conservation measures are presented in this study (Rasul et al., 2005).

The typical energy balances for the major pyroprocessing systems are shown in Table 4. These balances show where energy losses occur (areas where no useful work is accomplished). Any area, other than the theoretical requirement, that is losing energy represents an opportunity for improving energy efficiency and lowering fuel-based emissions. The individual energy use areas (e.g., clinker discharge, kiln shell, etc) in Table 4 show the area and the magnitude of the opportunities available from managing energy losses by improving specific equipment or practices (Choate, 2003).

The following recovery and conservation measures have been considered: Waste heat recovery from kilns exit gases using steam generator; and Kiln shell heat losses reduction; and Raw material drying at the raw mills (Kabir et al., 2010; Carvalho & Nogueira, 1997; Engin & Ari, 2005).

5.1. Heat Recovery From the Kiln System

Thermal energy recovery and saving technologies demonstrates how the heat losses can be recovered. The effectiveness of the conservation measures are has been demonstrated toward improving thermal efficiency and environmental worthiness of kiln systems. (Kabir et al., 2010)

If the heat transfer and the flow characteristics prevailing inside the rotary cement kiln are known in advance, the reduction of pollutant emissions generated from cement plants and efficient use of energy can be realised. However, the process of producing cement cannot be interrupted but requires continuous monitoring and control. These tasks may be accomplished on a low-cost basis, using mathematical models for numerically simulating the flow and temperature fields inside the rotary cement kiln (Carvalho & Nogueira, 1997).

5.1.1. The Use of Waste Heat Recovery Steam Generator

The waste heat recovery steam generator (WHRSG) captures the heat that would have been wasted to the environment and utilises this heat to generate electricity. The clinker cooler discharge and the kiln exhaust gas are the most accessible and the most cost effective waste heat losses available. The average temperature of the exhaust gas from the kilns is 315oC, and the temperature of the air discharged from the cooler stack is 215oC. The air streams from both the kilns' exhaust and cooler stack would be directed through a WHRSG, and the available energy transferred to water via the WHRSG as shown in figure 5. The available waste generates steam which would then be used to power a steam turbine driven electrical generator. The electricity generated using the WHRSG would offset part of the purchase electricity, hence the electrical demand is reduced.

The available energy from the gas streams must be found in order to determine the size of the generator. The estimate obtained will be used to determine an approximation of the steaming rate for a specified pressure. A combination of the steaming rate and pressure will be used to determine the size of the generator (Engin & Ari, 2005).

Since planetary coolers recirculate the hot secondary air into the kilns to aid combustion, the use of coolers exit gas is restricted. The gases enter the conditioning tower at a temperature of 480oC, exchanges heat with water to produce high pressure steam. Sensible heat with the steam generated is mostly lost to the surroundings, but can be recovered using waste heat recovery technique.

5.1.2. Use of Waste Heat to Pre-Heat the Raw Material

Preparation of the raw materials is an electricity intensive process which requires about 25-35 25-35 kWh/tonne raw material (23-32 kWh/short ton). Pre-heating the raw materials before the clinkering process is one of the most effective methods of recovering waste heat in cement plants. Gas streams are directed into the raw material just before the grinding mill and this result to a more efficient grinding of the raw material in addition to increasing its temperature.

However, the temperature increase of the raw material is not generally effective because it will be stored in silos for a while before entering the clinkering process. Therefore, the fresh raw material taken from the mill is not directly sent to the kiln. On the other hand, some plants may have only kiln systems rather than grinding systems. In such cases, this may not be possible unless some additional modifications are made in the plant (Engin & Ari, 2005).

Kabir et al., (2010) has analysed a typical example of the mills heat conservation scheme is shown in Fig. 6. Here, the kiln exit gases are conditioned in the gas conditioning tower to leave at 250oC, to preheat and dry the feeds to the raw mills. The water content in the feed vapourises at 100oC, and estimated to leave the mills at 8.07 kg/s flow rate (Kabir et al., 2010).

5.1.3. Heat Recovery from Kiln Surface

A significant amount of heat is lost through the hot kiln surface. Here, the heat is lost through convection and radiation which accounts for waste energy of 15.11% of the input energy.

On the other hand, heat loss can be reduced significantly through the use of a secondary shell on the kiln surface. Insulating the kiln surface would not be considered because the kiln surface needs to be frequently observed by the operator so as to see any local burning on the surface due to loss of refractory inside the kiln. The basic principle of the secondary shell is shown in Fig. 7. (Engin & Ari, 2005).

5.1.4. Heat Recovery From the Cooler System

The main cooling technologies in the clinker production (pyroprocessing) unit are either the tube (planetary) cooler or the grate cooler. In the tube (planetary) cooler, the clinker is cooled in a counter-current air stream. In the grate cooler, the clinker is transported over a reciprocating grate passed through by a flow of air. The cooling air is used as combustion air for the kiln (Martin et al., 1999).

6. Energy Efficiency Technologies and Measures for the Cement Industry

6.1. Clinker Production - Dry Process Preheater Kilns

6.1.1. Low Pressure Drop Cyclones for Suspension Preheaters.

Power consumption of the kiln exhaust gas fan system will be reduced through the installation of newer low pressure drop cyclones for suspension preheaters, as cyclones are a basic component of the plant. About 0.6-0.7 kWh/ton clinker can be saved for each 50mm water column pressure loss reduced according to the efficiency. Although the installation of the cyclones can be expensive, it may often entail the rebuilding or modification of the preheater tower hence, the costs are very site-specific (Martin et al., 1999).

6.1.2. Dry Process Conversion to Multi-Stage Preheater Kiln.

Older dry kilns may have single- or two-stage preheater vessels. They may only preheat in the chain section of the long kiln, while some long dry kilns may not have any installed preheater vessels. This results to higher energy consumption and a low efficiency in heat transfer. Installing multi-stage (i.e. four- or five-stage) suspension preheating may increase the efficiency by lowering the heat loss. A reduced pressure drop is present in modern cyclone or suspension preheaters, leading to reduced power use in fans and increased recovery efficiency. In addition, radiation losses can be reduced by shortening the kiln length. If the old kiln needs replacement and a new kiln becomes too expensive, then the conversion of older kilns becomes the best option, assuming that limestone reserves are adequate (Worell & Galitsky, 2004; Martin et al., 1999).

6.2. Clinker Production - Wet Process Kilns

6.2.1. Wet Process Conversion to Semi-Dry Process (Slurry Drier).

Energy consumption can be reduced with increased productivity in the modernized wet kilns, by adding a slurry drier to dry the slurry before entering the kiln using waste heat from the kiln. This technique is different from what is practised in the semi-wet process as a gas drier is used in place of a slurry press drier (Martin et al., 1999). Gas suspension driers can be considered because they could increase drying efficiency and potentially reduce fuel consumption in the kiln by up to 1.4 MBtu/ton clinker (Grydgaard, 1998). Worrell et al., 2001 estimated the energy needs for water evaporation in a wet process kiln at over 2 MBtu/ton clinker. For comparison, a Lepol kiln as suggested by (Cembureau,1997) consumes about a quarter of that for evaporation, while increasing electricity use by approximately 5-7 kWh/ton clinker (Worell & Galitsky, 2004; Martin et al., 1999).

6.2.2. Wet Process Conversion to Pre-heater/Pre-calciner Kiln.

If a wet process kiln can be converted economically to a modern dry process productuin facility which includes either a multi-stage preheater/pre-calciner, fuel savings could be achieved. In the U.S. wet kilns, average specific fuel consumption can be estimated as 6.0 MBtu/ton clinker and this has produced an average fuel savings of 2.9MBtu/ton or less. However, converting a wet plant to a dry process plant may be costly, because it involves the full reconstruction of an existing facility. Costs may vary between $50/annual ton clinker capacity and $100/annual ton clinker capacity (Worell & Galitsky, 2004).

7. Energy Saving Opportunities in the Grinding Process

7.1. Advanced Grinding Concepts

Amount of energy needed for the grinding process depends on the fineness required for the final product and the additives used (in intergrinding). Electricity use for raw meal and finish grinding strongly depends on the hardness of the material (limestone, clinker, pozzolan extenders) and the desired fineness of the cement as well as the amount of additives. Slags from blast furnace use more grinding power, between 50 and 70 kWh/tonne (45 and 64 kWh/short ton) for a Blaine of 3,500 cm2/g, since they are harder to grind. Many plants use vertical roller mills in finish grinding, while ball or tube mills are being used traditionally. Modern ball mills may use between 32 and 37 kWh/tonne (29 and 34 kWh/short ton) for cements with a Blaine of 3,500 (Worell & Galitsky, 2004).

Modern state-of-the-art concepts such as the high pressure roller mill and the horizontal roller mill can use 20-50% less energy than a ball mill. Other new technology applications are the roller press, the Horomill, Cemax, the IHI mill, and the air-swept ring roller mill. The Horomill contains a horizontal roller, within a cylinder, is driven. The movement of the cylinder results to centrifugal forces which cause a uniformly distributed layer to be carried on the inside of the cylinder.

Finished cement is first reserved in silos, then tested and filled into bags, or shipped in bulk on bulk cement tucks or railcars. The use of conveyor belts during the packaging of cement requires additional power. However, the total consumption for these purposes is generally low and less than 5% of total power use. Total power use for auxiliaries is estimated at roughly 10 kWh/tonne clinker (9kWh/short ton clinker). The power use for conveyor belts is estimated at 1-2 kWh/tonne cement (0.8-1.8 kWh/short ton cement).The power consumption for packing depends on the share of cement packed in bags (Martin et al., 1999).

Modern finish mill systems may comprise several units of process equipment - twin-roll presses, high-pressure, ball mills, tube mills, and conventional or high-efficiency separators. Air flow, and material and heat balances should be measured and evaluated. Optimization of these systems may include:

  • Adjustment of ball charges
  • Studies of circulating loads
  • Analyses of Tromp curves
  • Modifying particle size distribution
  • Controlling gypsum dehydration (Choate, 2003)

7.2. High Efficiency Classifier

The use of high-efficiency classifiers or separators is useful in efficient grinding technologies. Finely ground particles are separated from the coarse particles using the classifiers. The coarse particles are recycled back to the mill. Over-grinding is reduced in high-efficiency classifiers since the material is more finely separated. After the installation of the classifiers in their finishing mills, Britain found a reduction in electricity use of 8% and a 25% production increase (Martin et al., 1999).

7.3. Improved Grinding Media

Improved wear resistant materials can be installed fr grinding media, especially in ball mills. The potential for reducing wear as well as energy consumption has been shown by the increases in the specific gravity and surface hardness of grinding media and wear resistant mill linings. High chromium steel material is one of the materials used for improved balls and liners. Grooved classifying liners are some of the other improvements included the used of improved liner designs. These have the potential to reduce grinding energy use by 5-10% in some mills (Worell & Galitsky, 2004; Martin et al., 1999).

8. Recommendations on Energy Savings or Alternative Processes

8.1. Operational Approach for Thermal Energy Saving Opportunities

The following operational approach could be used to improve the energy efficiency of the pyroprocessing unit of a modern cement plant:

  • Improving the combustion system
  • Adding multistage pre-heater with pre-calciner
  • Preventing air leakages through proper tightening of the pyroprocessing unit equipments
  • Adopting new pyroprocessing technologies
  • Using alternative energy such as energy from biomass and waste fuel for kiln firing
  • Switching from high carbon fuel to low carbon fuel (e.g replacing coal with natural gas)
  • Applying a low clinker to final cement mixture ratio (i.e. increasing the ratio of cement additives that do not require pyroprocessing) (Kabir et al., 2010).

9. General Measures

9.1. Preventive Maintenace

Personnel should be trained to be attentive to energy consumption and efficiency as part of the preventive maintenance process. Though many processes in cement production are primarily automated, preventive maintenance and process control measures are still required to increase energy savings in the cement production. Also, preventative maintenance (e.g. to the kiln refractory) can increase a plant's utilization ratio, since it has less down time over the long term. The reduction of false air input into the kiln at the kiln hood has the potential to save 11kcal/kgclinker (0.05 GJ/t). Martin et al., (1999) noted a reduction of up to 5 kWh for various preventative maintenance and process control measures.

9.2. Process Controls and Management Systems

Non-optimal process conditions or process management can cause heat to be lost from the kiln. The combustion process and conditions can be optimised using the automated computer control systems. The quality of the product (e.g. hardness and reactivity of the produced clinker) could also be improved through improved process control. This may lead to more efficient clinker grinding.

The use of on-line analysers is one of the additional process control systems. They enable the operators to ascertain the chemical composition of raw materials being processed in the plant instantaneously. Therefore, immediate changes in the raw materials blend are imminent. Steadier kiln operation is possible by using a uniform feed, thereby saving ultimately the fuel requirements. Energy savings from process control systems vary between 2.5% and 10%. ). Advanced process control systems are economically good and payback periods can be as short as 3 months (Martin et al., 1999).

9.3. High- Efficiency Motors and Drives

Throughout the cement plant, motors and drives are used to drive fans, rotate the kiln, transport materials and most importantly, for grinding. About 500-700 electric motors of different sizes may be used in a typical cement plant. The power consumption of cement kilns can be reduced by using variable speed drives, high efficiency motors and enhanced control strategies. At any time, motors may be replaced if it does not affect the process operation. Energy savings may vary considerably on a plant-by-plant basis, ranging from 3 to 8 (Martin et al., 1999).

9.4. Adjustable Speed Drives

Drives are high power consuming in the process of cement-making. By reducing the energy losses or by increasing the efficiency of the motor, the efficiency of the drive system can be improved. The installation of adjustable speed drives can reduce the energy losses in the system by reducing throttling and coupling losses. Adjustable speed drives are mainly applied for fans in the kiln, cooler, preheater, separator, and mills, and for various drives. The potential savings using ASD are estimated aas 9kWh/tonne cement (Worell & Galitsky, 2004; Martin et al., 1999).

10. Advanced Technologies

10.1. Fluidised Bed Kilns

The Fluidized Bed Kiln (FBK) started as early as the 1950s and it is a totally new concept to produce clinker. The rotary kiln in the FBK is replaced by a stationary vertical cylindrical vessel, in which the raw materials are calcined in a fluidized bed. The transfer of the clinker is regulated to the cooling zone through an overflow at the top of the reactor. The FBK has several advantages which include reduced capital costs because of smaller equipment, lower temperatures which causes lower NOx-emissions and a wider variety of the fuels that can be used, as well as lower energy use. Vleuten, 1994 stated that energy use is expected to be 10-15% lower in FBK compared to conventional rotary kilns. Although modern precalciner rotary kilns have shown fuel use of 2.6-2.7 MBtu/ton clinker, the fuel use of the FBK may be lower than that of conventional rotary kilns (Worell & Galitsky, 2004).

10.2. Advanced Communication Technologies

In modern cement-making, grinding is an important power consume. However, there are highly inefficient grinding technologies presently. Majority of the energy input (95%) is lost as waste heat in the grinding process, while a little amount of the energy input (1-5%) is used to create a new surface area. The raw materials could be dried using some of the heat, generated during grinding process. Compared with conventional ball mills, current high-pressure processes already improve the grinding efficiency. In the longer term, further efficiency improvements can be expected when non-mechanical "milling" technologies become available (OTA, 1993).

11. Conclusion

A detailed analysis of the Energy consumption in the manufacture of cement is studied in this report. Cement production process consumes a high amount of energy and it is highly cost intensive. Since energy cost of total cement production cost is large, energy conservation is an important matter in technical improvement activities. The energy audit is emphasised to be a medium for identifying areas of energy saving opportunities, in order to achieve effective and efficient energy management scheme.

It has been discovered that some amount of the input energy is being lost with the waste heat streams. Heat losses conservation systems such as: Waste heat recovery steam generator (WHRSG) and secondary kiln shell can be used to enhance the energy performance of the unit. They could be used to achieve substantial financial benefits as well as environmental benefit of reduction in Greenhouse gases (GHG) emissions.

In order to promote energy conservation, activities of "good housekeeping" and "equipment improvement" should be applied before improving a process.

References

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