INDUSTRY SECTORS CEMENT

Transcription

1 INDUSTRY SECTORS CEMENT (NOTE: This chapter has to be expanded, updated and edited.. A more detailed version will follow in the final Toolkit which will cover the information relevant to the headings listed below) SECTOR DESCRIPTION...1 PROCESS FLOW...1 MAJOR PROCESS EQUIPMENTS...5 ENERGY EFFICIENCY OPPORTUNITIES REFERENCES Sector Description This section briefly describes the Industry Sector and gives a short introduction to the main features about the sector Cement and CO 2 The global cement industry contributes around 20% of all man-made CO2 emissions and is consequently responsible for around 10% of man-made global warming (Global cement technology magazine). The energy consumption by the cement industry is estimated at about 2% of the global primary energy consumption, or almost 5% of the total global industrial energy consumption. Cement production increases at about 3%/year at the moment. This rate is set to increase as developing nations rapidly become richer, and spends proportionately more on cement-intensive infrastructure. There are two sources of CO 2 emissions from the cement plant. One by virtue of the energy it uses and secondly the evolution of CO 2 as a by-product in the calcination process. The cement plant releases one tonne of CO2 for every tonne of cement produced, half of it from the fuel it uses and the other half from calcinations process. Process Flow This section includes the description of each step and the main inputs and outputs The basic process of Cement production as shown in fig involves 1. Acquisition of raw materials 2. Preparation of the raw materials for pyroprocessing 3. Pyroprocessing of the raw materials to form Portland cement clinker, and, Company Toolkit for Energy Efficiency 1

2 4. Grinding the clinker to Portland Cement Limestone quarry High grade limestone Raw Mill Exit gas to raw mill or abatement Stage 1 Stage 2 Crusher Limestone stockyard Iron Alumina Ore Raw meal blending and storage silos Gypsum and other constituents Cement Mill Upto 60% Fuel in Cement silos Stage 3 Stage 4 Precalciner Rotary Kiln Grate cooler Fuel in To clinker storage Clinker storage Description of production processes Fig Mining: Limestone, the key raw material is mined in the quarries with compressed air drilling and subsequently blasting with explosives. The mined limestone is transported through dumpers or ropeways to the plant. Surface mining is gradually gaining ground because of its eco friendliness. Bagging M/cs Bag Loading Crushing: The limestone as mined is fed to a primary and secondary crusher where the size is reduced to 25 mm. Of late even a tertiary crusher is used to further reduce the inlet size to the mill. The crushed limestone is stored in the stockpile through stacker conveyors. The crushed limestone, bauxite and ferrite are stored in feed hoppers from where they are fed to the raw mill via a weigh feeders the required proportion. in Raw Materials Preparation:. Roller mills for grinding raw materials and separators or classifiers for separating ground particles are the two key energyconsuming pieces of equipment at this process stage. For dry-process cement making, the raw materials need to be ground into a flowable powder before entering the kiln. Company Toolkit for Energy Efficiency 2

3 Generally ball mills and vertical roller mills are used. Coal Milling: In plants using coal, coal mills are part of the system to provide dried pulverized coal to kiln and precalciner. The raw coal from stock yard is crushed in a hammer crusher and fed to the coal mill. The coal mill can be an air swept ball mill or vertical roller mill where the coal particles are collected in the bag filter through a grit seperator. The required size is 80 % on 90µ and less than 2% on 212µ. Hot air generated in a coal fired furnace or hot air from clinker cooler/preheater exhaust is used in the drying of coal in the mill. Pyro processing: The function of the kiln in the cement industry is to first convert CaCO3 into CaO and then react Silica, Aluminum Oxide, Ferric Oxide, and Calcium Oxide with the free lime to form clinker compounds: C3S, C2S, C3A, and C4AF.. The raw material mix enters the kiln at the elevated end, and the combustion fuels generally are introduced into the lower end of the kiln in a countercurrent manner. The materials are continuously and slowly moved to the lower end by rotation of the kiln. Pulverized coal, gas, pet coke or Oil are the fuels generally used. This system transforms the raw mix into clinkers, which are gray, glass-hard, spherically shaped nodules that range from 0.32 to 5.1 centimeters (cm) in diameter. The chemical reactions and physical processes that constitute the transformation are quite complex, but they can be viewed conceptually as the following sequential events: 1. Evaporation of uncombined water from raw materials as material temperature increases to 100 O C 2. Dehydration as the material temperature increases from 100 O C to approximately 430 O C to form oxides of silicon, aluminum, and iron; 3. Calcination, during which carbon dioxide (CO2 ) is evolved, between 900 O C and 982 O C to form CaO; and 4. Reaction of the oxides in the burning zone of the rotary kiln to form cement clinker at temperatures of approximately 1510 O C Pre heater and Pre calciner: Preheaters are cyclones are arranged vertically, in series, and are supported by a structure known as the preheater tower. Hot exhaust gases from the rotary kiln pass counter currently through the downward-moving raw materials in the preheater vessels. Compared Company Toolkit for Energy Efficiency 3

4 with the simple rotary kiln, the heat transfer rate is significantly increased, the degree of heat utilization is more complete, and the process time is markedly reduced owing to the intimate contact of the solid particles with the hot gases. The improved heat transfer allows the length of the rotary kiln to be reduced or in other words for the existing kiln if retrofitted, increases the production.. Additional thermal efficiencies and productivity gains have been achieved by diverting some fuel to a calciner vessel at the base of the preheater tower. This system is called the preheater/precalciner process. While a substantial amount of fuel is used in the precalciner, at least 40 percent of the thermal energy is required in the rotary kiln. Upto 95 % of the raw meal gets calcined before entering the kiln. Calciner systems sometimes use lower-quality fuels (e.g., less-volatile matter) as a means of improving process economics. From pre-heater and pre-calciner, 60 % of flue gases travel towards raw mill and 40 % to conditioning tower where water injection is used to condition the gases. These gases are ultimately passed through electrostatic precipitator (ESP) for the maximum removal of particulate matters. Clinker Cooler: The hot clinker is cooled from 1100 O C to 90 O C in the grate cooler with a series of fans. The cooler has two tasks: to recover as much heat (upto 30% of kiln system heat) as possible from hot (1450 O C) clinker so as to return it to the process; and to reduce the clinker temperature to a level suitable for the equipment downstream. The hot air from recuperation zone is used for main burning air (secondary air) and precalciner fuel (tertiary air). The remaining air is sent to the stack through multiclones or ESP. Once clinker leaves the kiln it must be cooled rapidly to ensure the maximum yield for the compound that contributes to the hardening properties of cement. The main cooling technologies are the reciprocating grate cooler and the tube or planetary cooler. Finish Milling: In this final process step, the cooled clinker is mixed with additives to make cement and ground using the mill technologies described above. These materials are then sent through mills which perform the remaining grinding. The grinding process occurs in a closed system with an air separator that divides the cement particles according to size. Material that has not been completely ground is sent through the system again. Finish milling is the grinding of clinker to produce a fine grey powder. Gypsum Company Toolkit for Energy Efficiency 4

5 (CaSO 4 ) is blended with the ground clinker, along with other materials, to produce finished cement. Gypsum controls the rate of hydration of the cement in the cement-setting process The cement thus produced is collected in the bagfilter and taken to cement silos through a vertical pneumatic pump.. The energy used for cement grinding depends on the type of materials added to the clinker and on the desired fineness of the final product. Cement fineness is generally measured in a unit called Blaine, which has the dimensions of cm 2 /g and gives the total surface area of material per gram of cement. Higher Blaine indicates more finely ground cement, which requires more energy to produce. Portland cement commonly has a Blaine of cm 2 /g. Major Process equipments This section includes a general description of the equipments used in different processes (available and used) and comparing the energy efficiency of these Energy flows: The cement making process is highly energy intensive accounting for nearly % of the production costs. This provides ample opportunities for reducing energy consumption as many of the cement plants in developing countries consume much more than the the best achieved figures in developed countries. Electrical Energy: The energy flows in a typical cement plant is given in the figure below. The major electrical energy consumption areas are mill drives, fans and conveying systems. Company Toolkit for Energy Efficiency 5

6 Energy Flows Diesel for loaders, dozers and compressors Limestone Mining Diesel for dumpers and trucks/ Electrical energy for ropeway Transport Electrical Energy for crushers Electrical Energy for Mill drive and fans Heat Energy from kiln off gases Heat Energy from fuel input Electrical Energy for Kiln drive, fans and ESP Heat Energy from fuel input Electrical Energy for fans, drive and clinker breaker Electrical Energy for Mill drive and fans Crushing Raw Milling Pre calcination Pyro Processing Clinker Cooling Cement Grinding Bauxite, Ferrite Gypsum Electrical Energy for mill drive and fans Coal Milling Heat Energy from fuel input/waste heat from clinker cooler Packing & Dispatch About 30% of electric power is consumed for finish grinding, and a little under 30% each is consumed by the clinker burning process. Raw mill circuit is another major consumer accounting for 24 % of the energy. The raw mill circuit and finish grinding process mainly consumes electric power for the mill, and the clinker burning process mainly for the fan. Typical distribution of electrical energy is provided in the table below for a cement plant operating at 75 kwh per tonne of cement. Electrical energy distribution Section / Equipment Fig Electrical energy consumption (kwh / ton of cement) % Energy Consumption Mines, crusher & stacking Reclaimer, Raw meal grinding & transport Kiln feed, kiln & cooler Coal mill Cement grinding & transport Packing Lighting, pumps & services Total Thermal Energy: Company Toolkit for Energy Efficiency 6

7 Thermal energy accounts for almost half the energy costs incurred in cement manufacture. A variety of fuels such as coal, pet coke, gas and oil in addition to unconventional fuels such as used tires, incinerable hazardous wastes, agro residues etc are used in the cement plant. The major use of thermal energy is in the kiln and precalciner. In plants using coal, an external coal or oil fired furnace is used for generation of hot air required for coal mills. The number of stages in the pre-heater system has a major bearing on the thermal energy consumption of the kiln as shown in the table below. Kiln process Specific heat consumption in various kiln systems Heat consumption (kcal per kg clinker) Wet process with internals Long dry process with internals stage cyclone preheater stage cyclone preheater stage cyclone preheater stage cyclone preheater plus calciner stage preheater plus calciner plus high efficiency cooler stage preheater plus calciner plus high efficiency cooler less than 700 Material and Energy balance Material and energy balance in a cement plant The cement process involves gas, liquid and solid flows with heat and mass transfer, combustion of fuel, reactions of clinker compounds and undesired chemical reactions that include sulphur, chlorine, and Alkalies. It is important to understand these processes to optimize the operation of the cement kiln, diagnose operational problems, increase production, improve energy consumption, lower emissions, and increase refractory life.. A typical balance is shown in the figure Company Toolkit for Energy Efficiency 7

8 Fig Mass balance for production of 1 Kg cement Based on figure from Austrian BAT proposal 1996, Cembureau Instruments required: Conducting a material and energy balance would require apart from various parameters available in the control panel, measurement of flows, dust concentrations and electrical energy consumptions. The following instruments are suggested as minumum requirements; No Parameter Purpose Instrument 1 Velocity To calculate gas flows Pitot tube with manometer 2 Static pressure To measure pressure drops across various equipment such as cyclones, bag filters, ESPs, 3 Dust concentration Mills etc To calculate powder loading, collection efficiency, material losses etc. 4 Surface temperature To calculate radiation losses 5 Power To calculate specific electrical energy consumption Points to consider: High vacuum sampler Infra red thermometer Portable power analyser The plant has to be under stabilized condition so that the measurements taken are representative of normal operating conditions. The number of measurements to be taken depends upon the repeatability of the data. Since the temperature, pressure and flow rate are always variable during operation, a little skill and patience are required to keep the error up a Company Toolkit for Energy Efficiency 8

9 minimum. Since it becomes necessary to set up a measuring point at a place different from the usual working place, consultation shall be held with a relevant party in advance taking the safety of work into consideration, and the measurement shall be made in cooperation with that party. Necessary sampling points if not available have to be provided. What do we get out of it? Raw Mill Example This example illustrates how a material and energy balance is to be carried out for a raw mill. Under stable operating conditions the following were measured: Mill throughput by weigh feeder at the mill inlet Velocity measurements at various points for calculating flow Static pressures at various operating points Energy measurements by a portable power analyzer which directly measures kw Fan operating speed by tachometer The fig represents the balance carried out for the raw mill circuit Feed Kiln gases 290 O C m 3 /hr Dilution air Grit rejects Raw Mill 70 TPH Grit Separator Raw Mill circuit Cyclone CA fan Efficiency 52 % 85,000 m O C 550 mmwc 200 kw 1000 RPM CA fan Fig Bag filter DC fan 1,83,115 m O C 165 mmwc 140 kw 750 RPM DC fan Efficiency 44 % Roots blower 3600 m 3 /hr 5000 mmwc 75 kw To Raw meal Silo Air Lift pump The following were the outcomes of the balance leakage between mill outlet and CA fan inlet CA and DC fans are not operating in the best efficiency points resulting in poor efficiencies (Efficiencies upto 80 % is possible by impeller change or fan replacement) Air infiltration is observed in the bag house T h e r e i s h u g e Company Toolkit for Energy Efficiency 9

10 Necessary rectification and retrofitting can bring about a 20 % reduction in energy consumption per ton of raw meal. Samples of formats for undertaking material and energy balance are given in the next few pages. However it would be desirable for each plant to make its own formats depending on the depth of the balance and the nature of the plant. Energy Efficiency Opportunities This section includes practical tips, CPEE options checklists etc Option to reduce CO 2.Reduction in CO2 emissions from the cement plant involves a two pronged strategy. 1. By improving energy efficiency 2. By promotion of blended cements which can decrease the clinker percentage in cement, thus reducing the process CO 2 emissions CP-EE in cement plants, starts from the software including operation control and process control, then extends into the field of hardware including equipment improvement and process improvement. Generally, CP-EE measures can be classified into the following three steps: First step Raw material process Clinker burning process Finish process 1) Management of 1) Prevention of stoppages fineness 1) Selection of raw material due to failure 2) Management of 2) Management of fineness 2) Selection of fuel optimum grinding 3) Management of optimum 3) Prevention of leak media grinding media Second step Third step 1) Use of industrial waste material (fly ash) 2) Replacement of fan rotor 3) Improvement of temperature and pressure control system 4) Improvement of mixing & homogenizing system 1) From wet process to dry process 2) From ball and tube mills to roller mill 1) Use of industrial waste material (waste tires) 2) Recovery of preheater exhaust gas 3) Recovery of cooler exhaust gas (drying of raw material and generation of electricity) 4) Replacement of cooler dust collector from multiclone to E.P. 1) From wet process to dry process 2) Conversion of fuel (from existing to cheaper alternatives) 3)From SP to NSP 4)Use of industrial waste (slag and pozzolana) 1) Installation of closed circuit dynamic separator) 2) Installation of feed control system Company Toolkit for Energy Efficiency 10

11 5)From planetary and under coolers to grate cooler Capacity Utilisation High capacity utilisation is very essential for achieving energy efficiency. This brings down the fixed energy loss component of the specific energy consumption. Survey of excellent energy efficient companies show that 80% of the companies attribute capacity utilisation as one of the foremost reason for a major drop in specific energy consumption. Atleast 90% capacity utilisation is to be ensured for achieving low specific energy consumption. Also achieving high capacity utilization is under the control of plant personnel. Hence the first and foremost step for an aspiring energy efficient unit should be on increasing capacity utilisation and reduce the specific energy consumption. Fine Tuning of Equipment This is another opportunity for saving energy. On achieving high capacity utilisation, the fine tuning of equipment should be taken up by the energy efficient plants. Various energy audit studies reveal that Fine-tuning, if efficiently done can yield 3 to 10% of energy saving. The greatest incentive for resorting to fine tuning is that it requires only marginal investment. Technology Upgradation But quantum jumps in energy saving can be achieved only by application of new technologies/upgradation of existing technology. A list of energy efficient technologies are given in Chapter CP-EE in a cement plant 10%-20% of electrical energy reduction has been achieved in a cement plant by: Raw meal mix design change Elimination of run-on equipment Finish Mill Optimization Avoidance of air supply leakage Installation of more efficient fan motors Employees Awareness Power monitoring and targeting Process Replacement Measures Company Toolkit for Energy Efficiency 11

12 Energy Efficient technologies: Technique Description Emission reduction/ energy improvement Process Control and Management Systems Raw Meal Homogenising Systems Conversion from Wet to Dry Process Conversion from dry to multi-stage preheater kiln Conversion from dry to precalciner kiln Conversion from Cooler to Grate Cooler Optimisation of Heat Recovery in Clinker Cooler High efficiency Motors and Drives Adjustable Speed Drives Efficient Grinding Technologies Automated computer control may help to optimise the combustion process and conditions Use of gravity-type homogenising silos Complex operation, leaving only the structural parts intact Four or five stage preheating reduces heat losses, and sometimes reduces pressure drop Increase of capacity, and lowering specific fuel consumption Large capacity and efficient heat recovery. Heat recovery improved by reduction of excess air volume, control of clinker bed depth and new grates. Variable speed drives, improved control strategies and highefficiency motors Reducing throttling and coupling losses by replacing fixed speed AC motors High-pressure mills (like the Horomill) has improved grinding characteristics High-efficiency Classifiers Material stays longer in the separator, leading to sharper separation, thus reducing overgrinding Fluidised bed Kiln Advance Comminution Technologies Mineral Polymers Rotary kiln replaced by stationary kiln leading to lower capital costs, wider variety of fuel use and lower energy use Non-mechanical milling technologies as ultrasound. Not commercially available in coming decades Mineral polymers are made from alumino-silicates leaving calcium oxide as the binding agent. Typically 2.5-5% Reduction power use (1.4-4 kwh/t clinker) Estimated at 2.2 GJ/t (increase of power by about 10 kwh/t) Depending on original process. In one example reduction from 3.9 to 3.4 GJ/t Depending on original process. Estimated at 12% (0.44 GJ/t) Reduction of GJ/t (increase in power by 3 kwh/t) Estimated at 0.5 GJ/t in the US, and 0.2 GJ/t in India Estimated power savings ranging from 3 to 8%. Estimated at 10 kwh/t cement Estimated at kwh/t (40-50%) Estimated at kwh/t cement (8%) Fuel use of 2.9 to 3.35 GJ/t clinker (also lower NOx emissions) Expected (theoretical) savings are large Preliminary estimates suggests 5 to 10 times lower energy use and emissions Company Toolkit for Energy Efficiency 12

13 Case studies MADRAS CEMEMTS LIMITED, INDIA The plant achieved savings equivelent to 16.5% of the total energy cost by achieving the bench marking figures for the cement industry. Specific Electrical Energy Consumption : 69 Kwh/tonne of Cement Specific Thermal Energy Consumption : 705 Kcal/kg Clinker The major measures implemented during last three years include: 1 Installed new generation of MMD Crusher for limestone crushing. Power consumption is 60% less than the conventional crusher. 2. Utlisation of hot gas to Raw mill and Coal mill from Kiln exit gas thereby saving 160 Kcal/ Kg. Clinker. 3. Effective utilization of hot gases from cooler to Cement mill. This is equivalent to saving of 55 Kcal/ Kg clinker. 4. Up-gradation of Kiln and Cooler capacity improvement from 2350 to 3000 TPD by modifying the top of cyclone diameter and introducing CIS/CFG system to cooler for higher heat recuperation. This has saved 1.2 kwh/t of cement and 10 Kcal /Kg cl. 5. Up-gradation of Raw mill capacity from 180 TPH to 220 TPH by modifying the classifier. This has saved about 2 Kwh / Tonne of cement. 6. Optimization of Cement mill with changes in the mill internals. This has saved about 4 Kwh/ Tonne of cement. 7. Installed Fuzzy Logic Software System for better process stability and increased throughput 8. Installed Variable Frequency Drive (VFD) for cooler fans to save electrical energy. 9. Saved energy by optimizing process cooling water pump capacity. 10. Optimized DG Set operating voltage and frequency to 6.4 KV and 48 Hz 11. Optimized pressure setting of identified air compressor. 12. Replaced conventional chokes with energy efficient electronic chokes in fluorescent lamps and filament indication lamps in control panels with LED lamps. 14. Reduced voltage drop in pump house MCC feeder by shifting capacitor bank from SS to center. Company Toolkit for Energy Efficiency 13

14 SONADIH CEMENT PLANT, INDIA The plant incorporates the latest state-of-the-art technology comprising 5-stage preheater with in-line precalciner and vertical roller mills for raw material and coal grinding. The cement grinding system consists of a ball mill attached to roller press and hybrid classifier. The plant employs the latest concepts in instrumentation, and is fully automatic with centralized process control and operations equipment. 1. A wide range of measures as below have been adopted: 2. Proper raw mix composition for easy grindability and better burnability 3. Optimization of coal mix 4. Monitoring of process parameters and false air leakage, and optimization of process parameters 5. Replacement of table liner and roller tyres of raw mills and coal mill at optimum wear 6. Elimination of dampers from DC drive fans 7. Use of variable speed control fan and belt drives by v/f, slip power recovery system (SPRS) thyristor control devices for energy conservation. 8. Replacement of refractory at optimum wear to avoid radiation losses 9. Uninterrupted power supply to plant by running main grid and DG power grid in auto parallel control 10. Burning waste oil emulsions in the kiln Through these measure, the plant has been able to achieve specific power consumption of 63.5 kwh/ per tonne of clinker and a specific heat consumption of 730 kcal/kg of clinker Company Toolkit for Energy Efficiency 14

15 GHG linkages In the cement production process, carbon dioxide emissions can be grouped as energyrelated, referring to emissions that result from the combustion of fossil fuel, and processrelated, referring to the emissions from the decomposition of calcium carbonate. Studies have shown that one ton of carbon dioxide gas is released into the atmosphere for every ton of Portland cement which is made anywhere in the world. The only exceptions are so-called 'blended cements', using such ingredients as coal fly ash, where the CO2 emissions are slightly suppressed, by a maximum of 10%-15%. Cement, (Portland cement), results from the calcination of limestone (calcium carbonate) at very high temperatures of approximately C, and silico-aluminous material according to the reaction 5CaCO 3 + 2SiO 2 --> (3CaO,SiO 2 ) + (2CaO,SiO 2 ) + 5CO 2 this means that the manufacture of 1 metric tonne of cement generates 1 metric tonne of CO 2 greenhouse gas. Evaluating Carbon Dioxide Emissions due to energy savings Evaluating Carbon Dioxide Emissions due to cement blending Company Toolkit for Energy Efficiency 15

16 The efficiency scenario leads to energy savings at each step which can then be translated into annual carbon reductions a total of 22 kilotonnes of carbon or 10.4 kg C per tonne of cement. In the cement blending scenario there are no energy savings from efficiency improvements, but because the clinker-to-cement ratio is benchmarked at 0.95, total cement output of 3.1 Mt leads to an expected clinker production of 2.95 Mt. Since the plant operates with a 0.65 clinker-to-cement ratio, 0.95 Mt of clinker are avoided, saving 2,950 TJ of fossil fuels, or 62 kilotonnes C if fuel oil is used in the kiln 10. Also, since 165 kg C per tonne are generated through calcination, an additional 152 kilotonnes of carbon emissions are avoided. The blending project avoids 214 kilotonnes of carbon emissions, or nearly 70 kg C per tonne of cement. This is almost 10 times the total amount avoided by the efficiency project or 7 times when taken on a per tonne of cement basis. This example demonstrates that blending cement can lead to significant carbon emission reductions. These savings can be much larger than those that energy efficiency projects may attain. References 1.National Productivity Council- Energy Audit reports in Cement Industries. 2.Reports of Lawrence Berkley Laboratory 3. Web Sites: India Cements Ltd, Australian Cement Institute Company Toolkit for Energy Efficiency 16