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Decision making on rehabilitation, modification and upgrading of existing cement plants dedusting systems

Mohsen Sadeghi JDEVS Air pollution control division Manager

No. 166, Heidarkhani St., Farjam Ave., Narmak, Tehran, I. R. Iran

Abstract: For establishing a new cement plants, selecting dedusting systems in green fields is more simple and a routine step in comparison with changing existing equipments to achieve higher levels of air pollution control demands with due consideration to all limitation such as required space, ducting routes, supplying electrical energy, new fans, new dust handling systems and of course essential economical considerations. In this study based on a literature survey and real experiences in upgrading of Iranian cement plants the subject has proceeded from managerial and technical decision making points of view. Introduction Case studies of dedusting systems upgrading in cement plant shows that selecting of optimum gas cleaning systems among different modern dedusting equipment is not simple. Traditional technical comparisons for investment, operation and maintenance cost, of one method against another method is necessary but are not enough. When managers of cement plant are in situation have to make decision for selecting a method to improving existing gas cleaning equipments ,they may be faced to a mixture of technical and economical limitations ,alternatives ,advantages and disadvantages. In this study it is focused on main and big filters of cement plants consisting of kiln/raw mill, clinker cooler and cement mills that are more important in investment and operation costs. In this regards the basic technical parameters for making right decisions is raw gas and dust composition: Achievable Emission Limits: Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment has a range of actual operating efficiencies of 90 to 99.9%. While several factors determine ESP collection efficiency, ESP size is most important. Size determines treatment time; the longer a particle spends in the ESP, the greater its chance of being collected. Maximizing electric field strength will maximize ESP collection efficiency. Collection efficiency is also affected by dust resistivity, gas temperature, chemical composition (of the dust and the gas), and particle size distribution. [13]

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Critical parameters of ESP operation: An ESP is a particulate control device that uses electrical forces to move particles entrained within an exhaust stream onto collector plates. The entrained particles are given an electrical charge when they pass through a corona, a region where gaseous ions flow. Electrodes in the center of the flow lane are maintained at high voltage and generate the electrical field that forces the particles to the collector walls. In dry ESPs, the collectors are knocked, or "rapped", by various mechanical means to dislodge the particulate, which slides downward into a hopper where they are collected. The hopper is evacuated periodically, as it becomes full. Dust is removed through a valve into a dust handling system, such as a pneumatic conveyor, screw conveyor or drag chain is then disposed of in an appropriate manner. In the wire-plate ESP, the exhaust gas flows horizontally and parallel to vertical plates of sheet metal. Plate spacing is typically between 20 to 40 cm . The high voltage electrodes are long wires that are weighted and hang between the plates. Modern designs use rigid electrodes (hollow pipes approximately 25 mm to 40 mm in diameter) in place of wire. Within each flow path, gas flow must pass each wire in sequence as it flows through the unit. The flow areas between the plates are called ducts. Duct heights are typically 6 to 14 m . The power supplies for the ESP convert the industrial AC voltage (220 to 480 volts) to DC voltage in the range of 20,000 to 150,000 volts as needed. The voltage applied to the electrodes causes the gas between the electrodes to break down electrically, an action known as a “corona.” The electrodes are usually given a negative polarity because a negative corona supports a higher voltage than does a positive corona before sparking occurs. The ions generated in the corona follow electric field lines from the wires to the collecting plates. Therefore, each wire establishes a charging zone through which the particles must pass. As larger particles (>10 μm diameter) absorb many times more ions than small particles (>1 μm diameter), the electrical forces are much stronger on the large particles. Certain types of losses affect control efficiency. The rapping that dislodges the accumulated layer also project some of the particles back into the gas stream. These reentrained particles are then processed again by later sections, but the particles reentrained in the last section of the ESP have no chance to be recaptured and so escape the unit. Due to necessary clearances needed for nonelectrified internal components at the top of the ESP, part of the gas may flow around the charging zones. This is called “sneakage” and places an upper limit on the collection efficiency. Another major factor in the performance is the resistivity of the collected material. Because the particles form a continuous layer on the ESP plates, all the ion current must pass through the layer to reach the ground plates. This current creates an electric field in the layer, and it can become large enough to cause local electrical breakdown. When this

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occurs, new ions of the wrong polarity are injected into the wire-plate gap where they reduce the charge on the particles and may cause sparking. This breakdown condition is called “back corona.” Back corona is prevalent when the resistivity of the layer is high, usually above 2 x 1011 ohm-cm. Above this level, the collection ability of the unit is reduced considerably because the sever back corona causes difficulties in charging the particles. Low resistivities will also cause problems. At resistivities below 108 ohm-cm, the particles are held on the plates so loosely that rapping and nonrapping reentrainment become much more severe. Hence, care must be taken in measuring or estimating resistivity because it is strongly affected by such variables as temperature, moisture, gas composition, particle composition, and surface characteristics . Precipitator size is related to many design parameters. One of the main parameters is the specific collection area (SCA), which is defined as the ratio of the surface area of the collection electrodes to the gas flow. Higher collection areas lead to better removal efficiencies. Collection areas normally are in the range of 40 to 160 m2 per sm3/second of gas flow with typical values of 80 (400) .[3] Advantages: Dry wire-plate ESPs and other ESPs in general, because they act only on the particulate to be removed, and only minimally hinder flue gas flow, have very low pressure drops (typically less than 13 mm water column). As a result, energy requirements and operating costs tend to be low. They are capable of very high efficiencies, even for very small particles. They can be designed for a wide range of gas temperatures, and can handle high temperatures, up to 700 ºC. [6]Dry collection and disposal allows for easier handling. Operating costs are relatively low. ESPs are capable of operating under high pressure (to 1,030 kPa (150 psi)) or vacuum conditions. Relatively large gas flow rates can be effectively handled. Disadvantages: ESPs generally have high capital costs. ESPs in general are not suited for use in processes which are highly variable because they are very sensitive to fluctuations in gas stream conditions (flow rates, temperatures, particulate and gas composition, and particulate loadings). ESPs are also difficult to install in sites which have limited space since ESPs must be relatively large to obtain the low gas velocities necessary for efficient PM collection . There can be an explosion hazard when treating combustible gases and/or collecting combustible particulates. Relatively sophisticated maintenance personnel are required, as well as special precautions to safeguard personnel from the high voltage. Today, ESPs are in use around the world in a variety of industries, including power generation, pulp and paper manufacturing, cement manufacturing, mining, waste

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incineration, petrochemical production, and steel manufacturing. ESPs usually operate very efficiently when new, but tend to lose efficiency as they age, despite proper maintenance efforts. Traditionally, ESP owners had to two options when dealing with an underperforming unit:(1) replace the old ESP with a new one, or(2)rebuild the inside of the ESP with new component parts. Both of these options have merit, but both continue to rely on principle of electrostatic precipitation as the basis for dust collection. Some cement manufacturer would like to change their method of dust collection to fabric filtration (using replace bag filters instead of steel collecting plates as the means to collect dust). However, the cost of replacing an existing ESP and replacing it with a fabric filter (bughouse) dust collector has historically been extremely high in terms of equipment cost, re-configuring production lines, employee re-training, etc. Because of this, most facilities have the idea of installing a new bughouse and instead have focused their investment on improving the performance of their ESP. Over the last several years, has designed and developed a conversion program that can change virtually any ESP into a pulse-jet bughouse without demolishing the old unit and without lengthy downtime. Through a series of successful installations, this ESP to Baghouse conversion has proven to work even better than originally estimated.[3] Characterization of exit gas and dust As a fact most existing dedusting equipments are ESPs to be modified, upgrades or completely changed with new EPs or Bughouses. Based on this fact this study starts with a review of the effect of gas and dust characteristic on efficiency of Electrostatic precipitator. The characteristics of dust emission from different unit operations depend upon the being processed; type of processing, i.e. heating, sintering, grinding; gas flow ratethrough the equipment; fineness of the product; type of handling operations, etc. Asthese characteristics vary widely, section-wise evaluation has been done in the following paragraphs.[1] kiln and raw mill The ESP has been successfully used to clean gas coming out from the preheater kiln. The gas is often taken through the raw mill for material drying before cleaning in ESP. The design and operation of a cement kiln has undergone substantial improvement over past 30 years. The fuel crisis made the old wet process change to semi-wet/dry to long dry and finally to multistage (4–6) preheater precalciner kiln. These improvements resulted in many changes in operating parameters of the ESP which are summarized in table 1. The following observations can be made from Table 1. 1. As a result of the modernization from wet to dry preheater kiln, the temperature of exit gas has increased and the dew point lowered with a corresponding increase in the dust resistivity (Figs. 1 and 2) .[1]

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2. The particle size distribution has shifted to a smaller size with 75–80% particles lying below 10 μm. In addition to the above, it is observed that the alkali content of preheater gas from the modern plant has been considerably reduced. Water-soluble alkalis (Na, K) are known for their property of reducing the dust resistivity. The alkali content of preheater exit gas reduces, because most alkalis get condensed at the cold raw meal feed or the top cyclones, unlike long dry kiln processes where they used to condense in ESP. Alkali by-pass The excess quantity of alkali compounds present in the raw material adversely affects the quality of the cement. Most of this alkali is evaporated inside the kiln and gets recycled due to condensation of alkali vapor at the cold raw meal feed or the top cyclones. In order to reduce this recycling a certain quantity of gas is bled off from the back end of the kiln. This gas contains around 80% alkali. As can be seen from the Table 4: (a) the temperature of by-pass gas can go up to 1050 ºC and the dew point is very low in the range of 35–45 ºC, (b) the resistivity of dust is not very high due to the presence of alkalis but a large fraction, around 90%, of particles lies below 10 μm. Clinker cooler The sintered material discharged from the rotary kiln is known as the clinker. A reciprocating grate cooler is largely used for cooling of clinker from a temperature of around 1200 ºC –70 ºC. This is done by blowing air, cross-current, through the grates. A part of the hot air coming out from the grate cooler is used as secondary combustion air in the kiln or for raw meal drying in the grinding mills, the rest is cleaned in ESP. On the basis of data tabulated in table 1 and other laboratory/plant observations, the following dust characteristics can be delineated. 1. The dust particles are coarse and abrasive with the majority lying above 10 μm size. 2. Under normal operation, the dust content of exit gas is in the range of 15–20 g/Nm3. However, under kiln upset conditions, excessive material may pass through the cooler. Under such conditions, exit gas temperature may reach 400–425 ºC and the dust content up to 50 g/Nm3.

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3. The dust resistivity is very high (1013–1014 Ωcm). The peak reaches around 150–200ºC (Fig. 1).

Figure 1. Dust resistivty vs. temperature Cement mill Modern cement grinding units consist of a ball mill in closed circuit with an air separator. The energy efficiency of the ball mill is very low, as only about 5–10% of the power supplied to the mill is used in grinding work, the rest is wasted in friction. The power lost in friction mostly gets converted into heat and increases the temperature of the product, cement. The cement temperature is not allowed to go beyond 100 ºC otherwise the gypsum, an additive to cement, will dehydrate. The cooling of cement inside the mill is achieved by air and water injection maintaining the dew point of exhaust gas at 50–60 ºC. Table 4 and laboratory/plant data provide the following information on the gas and dust emissions. 1. Water injection in the mill increases the dew point of exhaust gas and reduces the dust resistivity. Some typical values of dew point versus resistivity are given in Table 2 2. When air separator is added in close circuit with the mill, a part of the warm bleeds air from the separator circuit goes to the ESP along with the mill vent air. The addition results in an increase in dust loading, gas temperature and reduction of dew point and a corresponding increase in dust resistivity. However, when the

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separator arrangement incorporates cement cooling, a separate dust collector is normally used to clean this part of gas.

Figure 2. Resistivty of kiln dust vs. dew point The pulse-jet bag house: A typical pulse-jet bughouse contains 6 major components: (1) inlet; (2) fabric filters; (3) tube sheet; (4) pulse valve; (5) blowpipe; and (6) outlet. See Fig.3.Dust-laden gas enters the bughouse and directed toward the filters by a baffle. The goal of the inlet is to slow down the speed of the gas to let large particulate fall out (due to gravity) and to create a uniform gas distribution to the filters. This is done so that thetas stream does not cause excessive abrasion on the filters. As the gas is pulled through the filters, dust collects on the outside of the filter while clean air penetrates the filters and moves into the clean air plenum(the space above the tube sheet). From the clean air plenum, the clean gas moves to the fan and out to the stack. As the filters collect dust, they need tube cleaned periodically to maintain a desired level of resistance. To do this, compressed air is directed downward into

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the interior of the bags which momentarily removes the dust cake from the surface of the filters. The cleaning air is regulated by a pulse valve and directed to the filters via a blowpipe. In a pulse-jet bughouse, cleaning of the filters is done online. The regular re-arranging of the dust cake from air pulses causes the dust to move down the filter and eventually into the hopper. Sharp pulses of air are the only means required to keep the filters clean and operating correctly.[2] Exit gas Dust Quantity

(Nm3/kg product)

Temperature(°C)

Dew point (°C)

Content (g/Nm3)

Particle size (% < 10 µ)

Crusher 0.03-0.06 25-45 20-45 15-20 20-30 Raw mill 90-100 20-60 65-75 Ball mill (gravity discharge)

0.3-0.8 25-60

Air swept ball mill vertical roller mill

1.5-2.5 300-1000

Cement mill 0.3-0.8 65-75 20-60 20-80 15-50 Packing plant 0.06-1.2 35-5-45 20-25 20-40 15-50 Kiln (dry) 1.7-2.0 200-240 45-50 50-75 75-85 Coal mill (air swept ball vertical roller mill)

1.2-2.6 60-80 30-50 100-500 60-75

Clinker cooler (grate type)

2.-2.1 220-260 20-55 15-20 4-5

400-425 Up to 50 Kiln by-pass 1.2-1.4 1000-1050 35-45 75-150 80-95

Table 1: Exit gas and dust characteristics in cement industry Dew Point (°C) 26 30 42 50 Dust resistivity 1012 3 × 1011 6 × 1010 1010

Table 2: Variation in the resistivity cement mill dust with dew point Pulse-jet bughouses have several advantages over other types of bughouse designs. First, they can be compact and can come in a wide variety of footprints. They are typically easier to maintain since there are few moving parts and the parts that may

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need adjustment(valves) are located outside of teahouse compartment for easy accessibility. The pulse-jet bughouse is becoming the design of choice throughout the world. Many companies are converting their shaker style and plenum pulse style bughouses to become pulse-jet. Most bughouse suppliers now offer pulse-jet technology as an integral part of their product line. [2] One of the historical concerns about bughouses in general is their ability to handle extremely hot gas or to handle temperature spikes. This is one of the reasons why ESPs became so popular in industries such as power generation and cement. Another concern is that highly abrasive dust could cause premature filter failure (a problem not found in ESP technology). For a bughouse to work correctly in tradition ESP applications, it must be able to overcome these concerns.[7]

Figure 3. Typical Pulse-Jet Bughouse Converting an ESP to Baghouse

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With the need for increasingly higher efficiencies and the problems associated with aging EPs, this option has seen a renewed interest in recent years. The genius of the ESP to Bughouse conversions in its simplicity. (See Fig. 4).In most cases, the existing sidewalls and hoppers from the ESP can be used without modification. Existing ductwork and material handling systems can also be used. Usually, the biggest question is whether to create the clean air plenum within the existing footprint of the ESP inner chamber, or if the clean air plenum will need to be added to the top of the existing structure. This decision is based on a number of factors, primarily the temperature of the gas stream, the size of the existing EP, the desired gas flow, and the types of fabric filters to be used. The typical ESP to Bughouse conversion (on a moderate sized unit) requires five (5) basic steps, as follows:1. Removal of the internal components of the existing ESP. This includes plates, wires, rapping systems, T/R sets, upper and lower frames, per plates, etc. Basically, the working parts of the ESP are removed leaving an empty shell. 2. Installation of the tube sheet, baffles, and air directional systems. Some modifications may be required to inlets and outlets and to the ductwork for the system to work properly. If an external clean air plenum is required it is accomplished at this step. 3. Installation of access doors, walkways, and ladders. The placement of these items is dependent on the location of the clean air plenum and the ductwork. 4. Installation of filters. In many cases, the technology of choice is pleated filter elements. The decision concerning filter type is based on gas volume, gas temperature, size of collector, installation requirements and cost. Choosing the right filters is critical to the success of the overall project. Without the proper filter media, the bughouse may not perform any better than the ESP it replaced. Even worse, the wrong filters may wear out faster and require more maintenance than before. Properly design, sized, and installed filters will provide the efficiencies and reduced maintenance required.[2],[15]

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Figure 4. Converting ESP to bag house Hybrid Filters The hybrid filter integrates electrostatic precipitation and fabric filter to provide a compact filter, expected result is a system that offers the reliability of an ESP with the performance of a fabric filter .This technology is relatively new. An hybrid pilot unit has been in operation since July 1999, filtering 9000 acfm of flue gas from the Big Stone coal-fired power plant in South Dakota USA.[4] Hybrid is produced by some filter manufacturer such as Elex, Research Cottrell and FLSmith Airtech. If we neglect the very low experience of installation and operation of hybrid filters in cement plant in comparison between fabric filters and electrostatic filters also from the economical point of view it is not desired to select hybrid filter for new installation and only for special cases in upgrading of old ESPs can be studied .So in this study analysis is focused to ESP, Fabric Filter (Bughouse) and for some cases converting ESP to Bughouse.[5]

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Conclusion With the ever-increasing legislative pressure to reduce discharges from all sources , the owner/operator of a cement plant fitted with an electrostatic precipitator , or any form of particulate collection system ,is faced with the dilemma as to how to improve the performance of his particular plant to comply with these changes of emission, without necessarily replacing the collector and with the minimum down-time or interruption in production, while changes are made to the existing installation. In addition to improvement required for legislative measures, there are other instances needing precipitator efficiency enhancement such as: -Increase in production rate -Higher temperature -Gas flow rate initiates from uncontrolled air inleakage -Mechanical or electrical equipment worn out -Change in fuel or raw material In this situation manager for solving the problem has to select one of these three options: 1-Upgrading existing system 2-Adding dedusting capacity with series or parallel new units 3-Demolution of entire existing equipment and rebuilding a new system The first solution should be noted, is to look at upgrading possibilities, important upgrading advantages are: - Short plant down time - Low investment cost - Lower operating and maintenance costs - No additional space requirements - Reuse of existing equipment e.i. main structures, dust transport etc. - Life time extension The second solution can be seen as a complementary of first solution. Upgrading could consist of: - Replacement of some mmechanical or electrical parts [11] - Increase the height or length of the ESP - Change the number of fields in the ESP - Replace some parts in the cooling tower - Combine ESP and fabric filter in an existing casing [Change to Hybrid filter] - Change the ESP to a Bag house Plant upgrading experiences [14]shows the cost of different degree of upgrading as below: -Modification consist of alignment and repair of collecting and discharge system and gas distribution system 1-4% of complete cost -Installing new internal parts and new electrical energization system 5-30% of complete cost

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-New serial ESP chamber or new parallel chamber 35-50% of complete cost -New ESP or Bughouse 100% cost Many of the electrostatic precipitator problems faced by the cement industry are directly related to process economics. Kiln emission control, especially at older plants, was viewed as an expensive burden. Kiln emission control can therefore create solid waste and water pollution concerns which require additional expenditure .Consequently; there is no return on investment for cement kiln emission control. It is too difficult to understand why the cement industry has viewed electrostatic precipitator as a necessary cost of staying in business. A cost which should be minimised, with no room for extras. [12] It is important when we are making decision for upgrading existing gas cleaning system we should consider extra capacity for future operation problems like uncontrolled false air and aging of the facilities. Many existing precipitators provide little or no excess capacity, and maintenance program could be considerably improved. Unfortunately, both excess capacity and superior maintenance programs can be expensive. A further consideration is the useful life of the facility, especially with recent energy cost increases.

Figure 6. Series units

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Figure7. Parallel units References 1-J.D.Bapat, 2001"Application of ESP for gas cleaning in cement industry-with reference to India” Journal of Hazardous Materials B81 285–308 2-Larry Mc Connell, 2000 "The Electrostatic precipitator to Baghouse Conversion” BHA group Inc. 3-K. R. Parker, 1997"Applied Electrostatic Precipitation" pp 418-424 4-Richard Gebert,Ulich Leibacher, 2003"Commercialization of the advanced hybrid filter technology" . 5-Anders Debell, Yong Woon Chung, 1995"Rebuilding and upgrading of existing electrostatic precipitator" World Cement, June 1995 6-Jacob Katz, 1979"The art of electrostatic precipitator" pp 313-315 7-Howard E. Hesketh,Frank l. Cross,JR.1994" Sizing and selecting air pollution control systems" pp 36-40 8-Mark Sankey, Robert Mastropietro, 1997 "Electrostatic precipitator upgrade strategies get the most from what you have" 9-C.R. Cottingham, 1998,"Electrostatic precipitator upgrading meeting the challenge" 10-Senichi Masuda, 1977,"Hybrid-Type electrostatic precipitator"

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11-Ronald L. Hawks, 1986" Electrostatic precipitator operation and maintenance for maximum efficiency in cement industry applications" 28th IEEE cement industry conference 12-Jon F. Chadborne, April 1978,"Recent trends in cement kiln emission control”, Operation and maintenance of Electrostatic precipitator Conference proceeding, 13- EPA, 2003. U.S. EPA, Office of Air Quality Planning and Standards, “Air pollution fact sheet, Dry electrostatic precipitators" 14-Egon Skytte Jensen, 1999,"Upgrading of existing precipitator" Iran Seminar FLSmithAirtech 15-Mario G. Cora, 2002, Controlling industrial particle emissions:"A Practical Overview of Baghouse Technology" Environmental Quality Management/Summer 2002