ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(CHAPTER-13:ENERGY PERFORMANCE ASSESSMENT OF CEMENT INDUSTRY)

 

ENERGY PERFORMANCE ASSESSMENT FOR EQUIPMENT AND UTILITY SYSTEMS

(CHAPTER-13:ENERGY PERFORMANCE ASSESSMENT OF CEMENT INDUSTRY)

Introduction

Cement acts as a bonding agent, holding particles of aggregate together to form concrete. Cement production is highly energy intensive and involves the chemical combination of calcium carbonate (limestone), silica, alumina, iron ore, and small amounts of other materials. Cement is produced by burning limestone to make clinker, and the clinker is blended with additives and then finely ground to produce different cement types. Desired physical and chemical properties of cement can be obtained by changing the percentages of basic chemical components (CaO, AI,O,, Fe,O,, MgO, SiO,, etc.). Mostly the cement produced is of Portland cement. Other cement types include white, masonry, slag, aluminous, and regulated-set cement. Cement production involves quarrying and preparing the raw materials, producing clinker through pyroprocessing the materials in huge rotary kilns at high temperatures, and grinding the resulting product into fine powder.

Cement Manufacturing Process

The basic process of Cement production as shown in Figure 13.1 involves 

¢ Acquisition of raw materials

¢ Preparation of the raw materials for pyroprocessing

¢ Pyroprocessing of the raw materials to form Portland cement clinker, and,

¢ Grinding the clinker to Portland Cement

Description of production processes

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.

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 weigh feeders in the required proportion.

Raw Materials Preparation: For dry-process cement making, the raw materials need to be ground into a flowable powder before entering the kiln. Generally ball mills or vertical roller mills are used. Modern cement plants mostly use vertical roller mills. Roller mills for grinding raw materials and separators or classifiers for separating ground particles are the two key energy consuming pieces of equipment at this process stage. Along with grinding simultaneous drying of raw materials using hot gases from the preheater exhaust also takes place.

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 separator. The required size is 80 % on 90 micron and less than 2% on 212 micron. 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 CaCO, 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  ountercurrent 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:

o Evaporation of uncombined water from raw materials as material temperature increases to 100 °C

o Dehydration as the material temperature increases from 100 °C to approximately 430 °C to form oxides of silicon, aluminum, and iron;

o Calcination, during which carbon dioxide (CO, ) is evolved, between 900 °C and 982 °C to form CaO

o Reaction of the oxides in the burning zone of the rotary kiln to form cement clinker at temperatures of approximately 1510 °C

Pre heater and Pre calciner: Preheaters are cyclones which 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 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 rawmeal 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 output from the kiln (clinker) is cooled from 1450 °C to 130 °C in the grate cooler with a series of fans. The cooler has two tasks: to recover as much heat as possible from hot 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 earlier. 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 (CaSO,) 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 1s generally measured in a unit called Blaine, which has the dimensions of cm? /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 3000-3500 cm2 /g.

Energy flow

The cement making process is highly energy intensive accounting for nearly 40 — 50 % 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 best achieved figures in developed countries.

Electrical Energy:

The energy flows in a typical cement plant is given in the Figure 13.2 below. The major electrical energy consumption areas are mill drives, fans and conveying systems. About 30% of electric power is consumed for finish grinding, and below 30% 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.

Thermal Energy:

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 average thermal energy consumption kcal/kg of Clinker is given in the table 13.1.

* Note : 1. The specific energy consumption of cement manufacture depends upon several factors such as raw meal quality (grindability index of limestone), fuel quality, pre-heating stages, percentage of blending of fly ash, slag and other mineral matter with clinker, fineness of the final product (blain number), plant vintage, capacity and technology employed etc. 2. The best reported values by Indian cement plants were 66.2 kWh/t cement and 687 kcal/kg clinker in 2008-09 (source: National Energy Conservation Award - 2009)

Material and Energy balance

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 heat balance should be constructed for the preheater, kiln, cooler and the heat output values should be compared with standard values.

Example 13.1. Heat Balance (Indirect Method) of a Six-Stage Preheater Inline Calciner

(ILC) Kiln

Note: (1) Average measurements of surface temperature to be taken on the surfaces of each cyclone of

preheater and calculations for radiation and convectional losses to be worked out as shown above

(2) Here, wind velocity of 1.5 m/sec considered

Radiation and convection losses from kiln = 4013284.69 kcal/h

                                                                    = 23.38 kcal/kg Clinker

Radiation and convection losses from tertiary air duct = 662684 kcal/h

.                                                                                     = 3.86 kcal/kg Clinker

Radiation and convection losses from preheater = 3071821.42 kcal/h

                                                                             = 17.89 kcal/kg Clinker

Radiation and convection losses from cooler assumed to be 3 kcal/kg Clinker.

Q, = Total radiation and convection losses = 23.38+3.86+17.89+3 kcal/kg Clinker

                                                                    = 48.13 kcal/kg Clinker

Inference

Significant abnormal heat loss is almost invariably associated with preheater exhaust and/or with cooler exhaust. As can be seen from the typical heat balance given here, of a total heat consumption of 771 kcal/kg of Clinker, 404 kcal/kg of Clinker is for chemical reaction. Of the remaining 367 kcal/kg of Clinker which goes as heat losses, 75 % of it is due to losses through kiln and cooler stacks. The challenge is to reduce these exhaust gas heat losses.

High exhaust heat loss may be mitigated by:

1. Reduce losses due to CO formation

o Good combustion in the main burner to avoid local reducing conditions in the presence

of excess oxygen

o Sufficient air/fuel mixing and retention time in the calciner for complete combustion

2. Reduce precalciner exhaust gas quantity

o Maximize heat recuperation from the cooler

o Minimize excess air without compromising combustion

Avoid over-burning with consistence kiln feed chemistry and with constant feed and

fuel rates (every 0.1 % free-lime below the optimum waste upto 14 kcal/kg Clinker)

o Minimize false air at kiln seals and preheater ports

3. Reduce preheater exhaust gas temperature

o Good meal distribution in gas duct by design and maintenance of splash plates and

splash boxes

o High cyclone efficiency so that hot meal is not carried up the preheater

4. Reduce cooler exhaust

o Maximum heat recuperation by control of air flow, Clinker distribution and Clinker

granulometry.

o Minimize false into firing hood and kiln discharge seal

Raw Mill

Raw milling, as one of the major cement process step, must produce sufficient kiln feed meeting targets for fineness, chemical composition and moisture to sustain required kiln production. The preheater gas at around 300 °C is used in the mill for drying and sweeping. Drying is also aided by heat dissipation from mill draw power which equates to approximately 1 ton moisture per 1000 kWh. Specific power consumption depends upon material hardness and mill efficiency. For ball mills, the range is approximately 10 kWh/ton (mill drive only), for soft, chalky limestone to 25 kWh/ton, for hard materials. For vertical roller mills the range may be 4.5 — 8.5 kWh/ton, and although ID fan power is increased system power is generally about 30% lower than for ball mills. A typical raw mill circuit is shown in the Figure 13.3. As can be seen the preheater exhaust gases (kiln gases) at 290 °C goes into the mill along with feed material. The material is dried and ground in the mill and fine powder is carried away by kiln gases to multiclones and to bagfilters where the product is collected. The transport of material through the gases is aided by circulating air (CA) fan and dust collector (DC) fan which also help to overcome pressure drop in the system.

Raw Mill Heat Balance

It will be useful to perform heat balance of the raw mill to optimize the air flow in the mill to improve

productivity besides saving energy. A typical heat balance of raw mill is given in the example below.

Example 13.2.

To determine hot gases required for drying of material in raw mill using heat balance The hot gases required for the drying of the feed moisture in the raw material while grinding in close circuit ball mill is calculated as below.

Calculations:

Gh Required hot gas quantity = (Nm3/hr)

Heat output:

Step:-1 Heat to raw material

Hop = Fq x Fs x (Egt-Rt-5) keal/hr

        = 100x1000x0.21x (105-20-5)

          = 1680000 = 1.68 x 10° kcal/hr

Note: The raw material temperature is normally less by 5 deg. C than the exit gas temperature.

Note: In the above calculation, heat input is less than the heat output. Hence it has to be balanced

by heat input from hot gases.

Solved example

During heat balance of a 5 stage preheater Kiln in a cement plant, the following data was measured at

Preheater (PH) Fan Inlet and clinker cooler vent air fan inlet:

Note: Take Pitot tube constant as 0.85, reference temperature 20 °C and atmospheric pressure 9908 mm WC.

Other Data:

Calculate the following:

i. Specific volume of PH gas as well as cooler vent air (Nm*/kg clinker)

il. Heat loss in pre-heater exit gas (kcal/kg clinker)

iii. Heat loss in cooler vent air (kcal/kg clinker)

iv. Ifthe measured specific volume of PH gas (Nm°/kg clinker) exceeds the design value, calculate the heat loss (kcal/kg clinker) and annual monetary loss due to excessive specific volume of PH gas.

Solution:

i) Density of Pre-heater gas at PH Fan Inlet at prevailing temp., pressure conditions

Velocity of PH gas

Similarly density of cooler vent air at cooler vent air fan Inlet at prevailing temperature, pressure

conditions:

ii) Heat loss in PH exit gas

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Chapter 12

Chapter 14