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A Case Study Report on Air Pollution in Cement Industry

Air pollution is a major problem in Bangladesh. Cement industries are one of the most top contributors to GDP. They produce a lot of pollution in the environment. Local manufacturers do not maintain the requirement of the Department of Environment (DOE). This paper aims to study the pollution sources, emission inventory, emission monitoring, air pollution modeling, and pollution control equipment in the cement industry. Sample air pollution modeling is shown in AERMOD software. Finally, some recommendation was done in the paper.
DOI: 10.13140/RG.2.2.28260.60804

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A Case Study Report on Air Pollution in Cement Industry

  1. 1. 2 Bangladesh University of Engineering & Technology A case study on Air Pollution in Cement Industry Course Title: Industrial Pollution Control Course No: CHE 485 Submitted To, Submitted By, Nusrat Ara Irin Dr. Md. Easir Arafat Khan Jannatul Osman Arju Assistant Professor, Fahim Shahriar Sakib Department of Chemical Engineering, BUET Syed Alvi Sadat Ishmam Arkabur Rahman Arnob Date of Submission: 10.12.2020 Author’s email: fssakib98@gmail.com
  2. 2. 3 Table of contents Chapter Page number 1. Introduction 4 2. Cement Industry 5 3. Major Pollutants of Cement Industry & Their Sources 8 4. Emission Monitoring 13 5. Emission Inventory 17 6. Air Pollution Modeling 21 7. Pollution Control Equipment 25 8. Conclusion 29 9. References 29
  3. 3. 4 Introduction Bangladesh cement industry, one of the fastest growing cement markets in the world, grew at approximately 11.5% CAGR over the last seven years as demand doubled from 14.6 million MT per year to around 31.3 million MT per year. Per capita cement consumption in Bangladesh almost doubled from 95 KG in 2011 to 187 KG in 2018. Despite huge growth of the industry in last two decades, Bangladesh is still one of the lowest consumers of cement in the world. Approximately 81% of the total market share is held by top ten manufacturers. By the end of 2018, local manufacturers had grabbed 86% of the market, a reversal in scenario from 15 years earlier. Once a cement importer, Bangladesh is now a cement exporting nation. Bangladesh exported cement worth US$13 million during FY17-18, compared to US$9.5 million earned in the year before representing 16.68% growth in export earnings YoY with India being the main destination for cement exports. On the back of massive infrastructure investment by the government, rising remittance income, growing urban population and impressive GDP growth, cement demand is expected to grow at even higher than the historical rate. Although self- sufficient in cement production, Bangladesh needs to import almost all of the raw materials used in cement manufacturing. Moreover, increased raw materials price and intense price war have squeezed the profit margin for cement manufacturers. Recent changes in tax laws, higher fuel & transportation cost and cost of fund have added more sufferings for the industry players. Hence, although the outlook for cement demand growth is very robust; profitability still remains a concern for all.
  4. 4. 5 Cement Industry A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is only behind water as the planet's most-consumed resource. Types of Portland Cements In the construction industry, there are different types of cement. The differences between each type of cement are its properties, uses and composition materials used during the manufacturing process. Various types of cement are produced in Bangladesh but Portland cement and blended cement are by far the most common. Although they are primarily consisting of a kiln fired and fused material known as clinker and addition of some materials make them applicable for different purpose. Portland cement primarily consists of clinker that is ground and combined with small amounts of gypsum or a similar material and is produced in several grades designed to provide certain properties to concrete. The major ingredients of blended cement are clinker, gypsum, fly ash, slag and limestone. Besides the Portland cement and blended cement, smaller amount of specialty cements is produced in Bangladesh i.e., masonry, off-white Portland cements etc. Manufacturing Process of Cement The main stages in cement production can be discussed under the following sub-headings: 1. Raw Material Acquisition: Most of the raw materials used in cement production are extracted from the earth through mining and quarrying and can be divided into the following groups: lime, silica, alumina, and iron (Madsen et al., 2004). Quarry operations consist of drilling, blasting, excavating, handling, loading, hauling, crushing, screening, stockpiling, and storing. Naturally occurring calcareous deposits such as limestone, marl or chalk provide the source for lime whether silica, iron oxide and alumina are generally found in sand, iron ore, shale, and clay. Power station ash, blast furnace slag, and other process residues collected from different industry can also be used as partial replacements for the natural raw materials if they do not affect the cement properties.
  5. 5. 6 2. Raw Materials Preparation: Raw milling involves mixing the extracted raw materials to obtain the correct chemical configuration, and grinding them to achieve the proper particle-size to ensure optimal fuel efficiency in the cement kiln and strength in the final concrete product (Karstensen, 2006). Three types of processes- the dry process, the wet process, or the semidry process may be obtained during raw milling. In the wet process, water is added to the raw materials during grinding where in dry process, the raw materials are ground in dry condition. The water used in the wet process is about 45% of the raw material (Aziz, 1995). In the semidry process the materials are formed into pellets with the addition of water in a pelletizing device. 3. Clinker Burning: In pyro-processing, the mix after raw milling is heated into a rotary kiln to produce cement clinkers. A rotary kiln is a long steel cylinder used for pyro processing, inclined about half to one foot, and is about 8 to 12 ft in diameter and 200 to 400 ft in length. The raw mix is supplied to the system as a slurry (wet process), a powder (dry process), or as moist pellets (semidry process). The pyro-processing system involves three steps: preheating, calcining (a heating process in which calcium oxide is formed), and burning. The obtained cement clinkers after pyro-processing are hard, grey, spherical nodules with diameters ranging from 0.32 - 5.0 cm created from the chemical reactions between the raw materials. The fuel to be used for this purpose is coal, oil or natural gas (Aziz, 1995). However, the use of supplemental fuels such as waste solvents, scrap rubber, and petroleum coke has expanded in recent years to promote the environmental sustainability. 4. Clinker Grinding: In this stage; the clinker is ground with other materials to form a fine powder which is actually the “cement”. The hot clinker obtained from rotary kiln need to be cooled before grinding. Gypsum is added to regulate the setting time of the cement. Other chemicals or mineral additives or air entrainment or Cementous materials may also be added to the ground clinker to achieve the particular properties.
  6. 6. 7 5. Cement Packaging & Dispatch: The finished product is transferred using bucket elevators and conveyors to storage silos and then to packaging unit. After that cement can be delivered to customers in bags (normally having 50kg weight) or sometimes in loose amount by bulk conveyor. Basic steps involve in cement manufacturing process are shown as flow chart in Fig 1 Figure 01: Overall cement manufacturing process
  7. 7. 8 Major Pollutants of Cement Industry & Their Sources The entire process of cement manufacturing involves emission of enormous pollutants which can affect the human health as well as the whole environment. Emissions from CMF include particulate matter and combustion gases from cement kiln and other production process; wastewater from the cooling of process equipment; slurries and sediments from wastewater; plant maintenance waste; and research and laboratory wastes. However, the air pollution from CMF has significant environmental impact and thus this industry has been enlisted as the third largest industrial source of air pollution (USEPA, 2016). Emission to air from CMF can be categorized into two groups: (i) Gaseous Pollutants (ii) Particulate Matter (PM) Traditionally, compliance efforts have focused on pollutants including-  Dust and other particulate matter (PM)  Nitrogen oxides (NOx)  Sulphur dioxide (SO2)  Carbon monoxide (CO)  Ammonia (NH3)  THC (or TOC)  Mercury and other heavy metals  HCl  Dioxins/furans  Hydrogen fluoride (HF)
  8. 8. 9 Gaseous Pollutants Oxides of nitrogen (NOx), sulfur dioxide (SO2), and carbon oxides (CO2 and CO) are the most likely listed substances emitted during the production of cement as gaseous pollutants. Trace quantities of volatile organic compounds (VOC) including benzene and phenol, ammonia, chlorine, some listed metals, and hydrochloric acid may also be emitted (EETs Manual, NPI, Australia, 1998). Gaseous pollutants emit generally during clinker production in rotary kiln and also from preheater and clinker cooler. NOx Emission NOx releases from combustion of fuel at high temperature in the cement kiln. Three types of NOx form in the cement kiln- thermal, fuel, and feed NOX. In kiln exhaust gases, more than 90% of NOx is NO; with NO2 generally makes up the remainder (Nielsen and Jepsen, 1990). Thermal NOx is formed by oxidation of atmospheric nitrogen at high temperatures. The literature cites threshold temperatures for the formation of thermal NOx ranging from about 1200–1600°C (2200–2900°F) (Nielsen and Jepsen, 1990). Since the flame temperature in a kiln is significantly above these temperatures, considerable amounts of thermal NO are generated in the burning zone. NOx emissions also result from the oxidation of nitrogen compounds in the raw material feed to the kiln (feed NOx). It can also result from oxidation of nitrogen compounds in fuel (fuel NOx). SO2 Emission In process of cement manufacturing SO2 are generated both from the sulfur compounds in the raw materials and from sulfur in fuels used to fire a preheater/precalciner kiln system. The SO2 reacts with water vapor and other chemicals high in the atmosphere in the presence of sunlight to form sulfuric acids. The acids formed usually dissolve in the suspended water droplets, which can be washed from the air on to the soil by rain or snow. This is known as acid rain which is responsible for acidification of soils, lake and stream, corrosion of buildings and monuments and deforestation. Also, SO2 can form secondary particles like sulfates that cause haze and reduce visibility.
  9. 9. 10 Figure 02: SO2 Reaction Process in Cement Production Particulate Matter Emission Particulate matters emit almost throughout total process flow of cement manufacturing including quarrying, crushing, grinding (only in dry process), blending (only in dry process), and transportation of raw materials, kilns operation, clinker cooling, stock piles and packaging. Also, some of the gaseous pollutants and particulate matter may release due to the vehicular movement at CMF for the transportation of raw materials or to delivery final finished cement product. Although this vehicular emission may not be included in the emission from the core cement manufacturing process.
  10. 10. 11 Figure 03: Particulate matter emitting sources CO2 and CO Emission CO2 as a by-product is released during the production of clinker (occur in the upper, cooler end of the kiln, or a precalciner) in which CaCO3 is heated at temperatures of 600-900°C in a rotary kiln and results in the conversion of carbonates to oxides. The simplified reaction is: CaCO3+ heat CaO + CO2 (Gibbs et al., 2015) In cement production about 60% of CO2 is released in unavoidable chemical reactions of limestone (calcination process). The indirect emissions of CO2 and CO happen by burning the fossil fuel (natural gas, oil or coal) to heat the kiln and electricity consumption during cement production. Additional emission of CO2 and CO depends on the presence of organic matter in raw material of cement. CO2 is a green-house gas and it is estimated that 5-6% of all CO2 greenhouse gases generated by human activities originates from cement production (Rodrigues and Joekes, 2010). The estimated average carbon footprint is 0.83t CO2/t of traditional Portland cement clinker (ranging from 0.7 to 1.4t). CO is weak greenhouse gas but its presence influences the concentrations of other greenhouse gases like methane, tropospheric ozone and CO2 (carbon monoxide: its environmental impact, n.d., para 3).
  11. 11. 12 VOC, Organic Matters and Heavy Metals Emission Other cement related emissions in trace quantity include VOCs, dioxins, furans, methane, heavy metals etc. The main source of VOC emission from cement kiln is organic matter present in raw material. Occurrence of VOCs is also associated with incomplete combustion. VOCs are precursor to ozone formation, which can also contaminate soil and ground water. It has been identified that VOCs can cause retardation of plant growth, chlorosis and necrosis in broad leaves plants. In cement manufacturing dioxins are also formed in the combustion system when chlorine and organic compounds are present and it contaminates soil and ground water. Some contents of fuel and raw material, which is naturally present in low concentration is liable for heavy metal emission i.e. lead, cadmium, nickel, titanium, molybdenum. Heavy metals contaminate soil and water and can adversely affect plant functions and cell structure. Bioaccumulation of heavy metal can cause poising in aquatic and terrestrial life through biomagnifications.
  12. 12. 13 Emission Monitoring "Monitoring" means the collection and use of measurement data or other information to control the operation of a process or pollution control device or to verify a work practice standard relative to assuring compliance with applicable requirements. Emissions measurement, monitoring and reporting contributes to understanding, documenting and improving the industry’s environmental performance. Lack of emissions information can lead to local concerns about plant operations. With regards to EPA's air quality regulatory requirements, there are two basic types of monitoring with two different functions: • Ambient air quality monitoring collects and measures samples of ambient air pollutants to evaluate the status of the atmosphere as compared to clean air standards and historical information; and • Stationary source emissions monitoring collects and uses measurement data (or other information) at individual stationary sources of emissions (i.e., facilities, manufacturing plants, processes, emissions control device performance, or to verify work practices). Air quality monitoring is useful in better understanding the sources, levels of different air pollutants, effects of air pollution control policy, and exposure of various substances in the air. The Environmental Protection Agency has established ambient air monitoring methods for the criteria pollutants as well as for toxic organic compounds and inorganic compounds. The methods specify precise procedures that must be followed for any monitoring activity related to the compliance provision of the clean air act. These procedures regulate sampling, analysis, calibration of instrument and calculations of emissions level i.e. concentration. The number of monitoring stations should be minimum three. The location of station is dependent upon the area of coverage and the wind rose diagram which gives the predominant wind direction and speed. One station must be upstream of predominant wind direction and other two must in downstream of predominant wind direction. Gaseous pollutants should be monitored continuously while PM should be once every three days.
  13. 13. 14 Monitoring of particulate matter: Isokinetic manual gravimetric method is applicable for the determination of particulate matter emitted from stationary sources. A representative sample in the gas stream of a stack or duct is withdrawn iso-kinetically through a nozzle and collected on a pre-weighed filter. The mass of the particulate matter is then determined gravimetrically after removal of uncombined water and its concentration calculated by relating to sample gas volume. Except where approval is given by the EPA, Department of Environment and Natural Resources, the collection of the particulate matter must be carried out from within the stack. Iso-kinetic sampling is sampling at a rate such that the velocity and direction of the gas entering the sampling nozzle is the same as that of the gas in the sampled gas stream at the sampling point. In most stack, the velocity and concentration in the stack vary from point to point and from time to time .The differences in velocity and particle concentration from place to place are substantial . Higher nonuniformity is found near the bend or disturbance.It is required to take measurement at multiple position in a cross section to get the average velocity and concentration. Figure 04: Particulate matter sampling method
  14. 14. 15 Sampling must be representative. There should be no leak in the sampling lines. Touching stack walls with the probe when inserting the probe in the duct or when taking it out should be avoided otherwise more particles will be deposited on the filter. Temperature of the nozzle (opening), the probe and the filter must be controlled in order to avoid condensation. The sampling must be isokinetic. Gas sample needs to be properly dried before it is taken to the gas meter Gaseous pollutant monitoring can be quantified and analysed by Spectrophotometry, Chemiluminescence, Gas Chromatography-Flame Ionization Detector (GC-FID), Gas Chromatography-Mass spectrometry (GC-MS), and Fourier Transform Infrared spectroscopy (FTIR). Chemiluminescence Analyzer: NOx concentration is measured with this principle of chemiluminescence analyzer. Figure presented below shows the working principle of chemiluminescence analyzer. By introducing ozone (O3) to the sample gas, part of the nitrogen monoxide (NO) contained in the sample gas will react with ozone, and oxidized to nitrogen dioxide (NO2). Part of the generated NO2 will be in an excited state (NO2 * ), and when it returns to the normal state, it will radiate light. The phenomenon is called chemiluminescence. The reaction is extremely fast and only the present NO participates while being essentially unaffected by other existing species. Figure 05: Working principle of a NOx analyzer
  15. 15. 16 Non-Dispersive Infra Red (NDIR): An NDIR gas analyzer measures the concentration of CO, CO2 and NOx by determining the absorption of an emitted infrared light source through a certain gas sample. The analyzer has four major components: i) an infrared light source, ii) a sample chamber, iii) a wavelength filter and iv)an infrared detector. Figure below shows the working principle of a NDIR analyzer. In the figure it can be seen that there are two parallel sensors. One is sample cell, where sample gases are introduced and other is the reference cell. The infrared light rays are directed through the sample cell and reference cell. Infrared rays are absorbed by the sample containing of those gas samples. Thus signal of each gas concentrations is obtained by calculating the difference between the signal of reference cell and the sample cell. The quantity of the infrared radiation absorbed by each component is proportional to the concentration. Figure 06 : Working principle of NDIR gas analyzer Analysis of sulphur dioxide : Sulphur dioxide from air is absorbed in a solution of potassium tetrachloromercurate (TCM). A dichlorosulphitomercurate complex, which resists oxidation by the oxygen in the air, is formed. Once formed, this complex is stable to strong oxidants such as ozone and oxides of nitrogen and therefore, the absorber solution may be stored for some time prior to analysis. The complex is made to react with pararosaniline and formaldehyde to form the intensely colored pararosaniline methylsulphonic acid. The absorbance of the solution is measured by means of a suitable spectrophotometer.
  16. 16. 17 Emission Inventory An emissions inventory is a database that lists, by source, the amount of air pollutants discharged into the atmosphere during a year or other time period. The emissions inventory, is the primary task to identify the various sources of pollution from the particular plant. It provides the physical and geographical conditions of the source of pollutant. Emissions may come from different points in cement production processes, depending on the raw materials and fuels, preparation procedures and kiln systems, and emissions control systems used. The largest volume substances emitted during the production of cement are particulate matter (dust), oxides of nitrogen, sulfur dioxide, carbon dioxide and carbon monoxide. Trace quantities of volatile organic compounds, acid gases, some trace metals, and organic micro pollutants may also be emitted. The range of emissions of the main pollutants are shown below: Reported emissions from European cement kilns Pollutant Average concentration Concentration range from/to Average specific emission Dust 20.3 mg/Nm3 0.3/227 mg/Nm3 46.7 g/t ck NOx as NO2 785 mg/Nm3 145/2040 mg/Nm3 1.805 kg/t ck SO2 219 mg/Nm3 Up to 4837 mg/Nm3 0.504 kg/t ck CO Up to 2000 mg/Nm3 VOC/THC as C 22.8 mg/Nm3 1/60 mg/Nm3 52.4 g/t ck • Concentrations are reference concentrations, i.e. 273°k, 101.3 kPa, 10% O2 and dry gases • Specific emissions are based on kiln exhaust gas volumes of 2300 m3 /ton clinker Source: BREF on cement, lime and magnesium oxide manufacturing industries, May 2010 Quantum of dust generated in different process of cement industry is shown below: SOURCE NORMAL DUST GENERATION RANGE ( g / N m 3 ) Crusher 5 - 15 Raw mill : Gravity discharge Air swept 20 - 80 300 - 500
  17. 17. 18 Coal Mill : Gravity discharge Drying / grinding 20 - 80 100 - 120 Kiln : Dry Semi - dry Wet 50 - 75 10 - 20 30 - 50 Clinker Cooler 5 - 10 Cement Mill 60 - 150 Packing Plant 20 - 40 For the Emission inventory development source identification is essential. Systematic procedure of emission inventory involves the source identification which includes: • Release Type • Location • Emission of criteria air pollutants There are a number of methods for the estimation of the emission rate and emission concentration from each source .Such as: • Manufacturer’s design specifications, • Direct measurement • Emission factors An emission factor is a quantity derived to calculate the emission of a pollutant throughout the process. These factors narrate an average value of available data of acceptable quality, and is generally represented the long-term averages of the source type. The emission factors are used when other information is not available. The emissions from cement uses the general equation:
  18. 18. 19 Epollutant  ARproductionEFpollutant where Epollutant is the emission of a pollutant ARproduction is the annual production of cement EFpollutant is the emission factor of the relevant pollutant This equation is applied at the national level, using annual national total cement production data. This method assume an ‘averaged’ or typical technology and abatement implementation in the country and integrate all different sub-processes in the cement production between feeding the raw material into the process and the final shipment off the facilities. Figure 07: Emission factors of major pollutants in different country from cement industry
  19. 19. 20 The uncontrolled emission factor for cement manufacturing in different stages are shown in tabulated from below: Table 01: UNCONTROLLED EMISSION FACTORS FOR CEMENT HANUFACTURING--COAL COMBUSTION Process Particulate Sulfur dioxide Nitrogen oxide kg/Mg lb/ton kg/Mg lb/ton kg/Mg lb/ton Dry process kiln dryer 128 256 3.5 7.0 2.9 5.7 Wet process kiln dryer 120 240 3.0 6.0 4.1 8.2 Clunker cooler 4.6 9.2 - - - - Preheater kiln - - 0.4 0.8 2.8 5.5 Precalciner kiln - - 0.5 1.0 2.4 4.8
  20. 20. 21 Air Pollution Modeling Air pollution modeling is nothing but a numerical way to demonstrate the relationship between emissions, deposition, atmospheric concentration, meteorology and other factors. Mathematical modeling helps to give quantitative information about concentrations and deposition but only for a specific time and location (Daly 2003, p 15-28). As a nature of mathematical modeling, we assume some data for doing this like assuming the atmosphere in a predictable format. For numerical modeling, meteorology data and terrain data are needed. Emission rate is calculated from the volume and composition of a pollution source (https://www.aeroqual.com/air-pollution-modelling, accessed at 10 december 2020, 11:55 am). Pollution source is mainly two types and each has also two types. • Static Point source Area source • Dynamic Point source Area source Air pollution modeling has some advantages. It forecasts a hypothetical situation which helps to implement a new project. Modeling helps to determine the concentration and deposition of pollutants of new upcoming industry in an area. Regulatory board can get an idea of the pollution of that industry from modeling. It also helps them to compare with another alternative. The disadvantage is the model relies on the input parameters like meteorology data, terrain data, source particle data etc. So, they do not always reflect reality with accuracy. The concentrations of the pollutants in the atmosphere are determined by: • Diffusion • Transportation • Chemical transformation • Ground deposition
  21. 21. 22 There are several types of air quality models, all used for different purposes. Most common models are known as Atmospheric Dispersion Models (ADM). Here they are, 1. AERMOD: Aermod is a form of Gaussian plume model (GPM) and it is steady state. GPM is developed early due to overcome the challenge of understanding the diffusion properties of plumes which was very hard (Sutton 1932, Bosanquet 1936). It develops meteorological data from surface, onsite station, upper air and geophysical terrain data. It uses a single wind to transport pollutants. Here is a sample of aermod modeling simulation. Surface station: Bergman field, Alamosa, Colorado (USAF 724620 WBAN 23061 ICAO KALS) Upper air station: Grand junction airport, Alamosa, Colorado (WBAN 23066) Terrain location: Alamosa, Colorado (UTM 13S, NAD83 Datum) Pollutant: (SO2, emission rate 0.8 g/s, release height 40 m, stack inside diameter 10 m, gas exit velocity 5 m/s) Figure 08: Wind velocity and direction vector (blowing to)
  22. 22. 23 Figure 09: Wind class frequency distribution Figure 10: Aermod modeling pollutant concentration
  23. 23. 24 Figure 11: Aermod modeling of total deposition 2. CALPUFF: Calpuff is a form of Langrangian puff dispersion model and it is non steady state. It is used for long range simulation and rough weather. CALMET is a meteorological diagnostic model that uses data from surface, upper-air, over-water stations, precipitation stations and geophysical data to produce a fully 3-dimensional gridded wind field for the CALPUFF simulation. There is some photochemical modeling like CMAQ, CMAX, UAM, CALGRID. Some models are plume rise models, particle models, odor modeling statistical models which are not described here. For cement industry, Aermod and Calpuff is used worldwide.
  24. 24. 25 Pollution Control Equipment Cement industries are a major source of air pollution. Air pollution occurs mainly through dust. A few other pollutants include oxides of N (NOx) and SO2, which are produced during burning of raw materials. Since most of the cement industries in Bangladesh import clinker, emphasis has been given on dust pollution controlling equipment in this section. But some controlling measures for gaseous pollutants will be discussed at the end of this section. Pollution control in cement industries is done mostly using the following equipment 1) Bag filters 2) Electrostatic precipitators (ESP) 1)Bag Filter A bag filter (also known as Baghouse, Baghouse filter or fabric filter) is a popular choice of dust pollution control device in Power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies. It is also used in cement production during transport and packaging. Bag filters came into widespread use in the 1970s Figure 12: Bag Filter following the invention of fabrics with temperature tolerance of up to 350 ℉. The main appeal of bag filters is the fact that using simplified mechanisms, bag filters can achieve an efficiency of about 99 percent.
  25. 25. 26 A traditional bag filter usually employs cylindrical bags made of woven or felted fabrics as filtering medium. Pleated non – woven cartridges are used for cases with low dust loading and temperature below 250 ℉. Bag filters are usually classified by the methods used to clean them. There are mechanical shakers, reverse air and pulse jet baghouse. Air along with dust particles enters the filters, and the air passes through the fabric leaving all the dust particles. The dust accumulation is accompanied by a pressure drop. Cleaning process is initiated after a sufficient p-0ressure drop occurs. 2)Electrostatic precipitators Electrostatic precipitators are another form of air pollution control device that removes particulate matters from the air by using static electricity. Compared to wet scrubbing process, the energy is applied to the particulates being removed. Hence energy consumption is minimized. Figure 13: Plate type Electrostatic Precipitator Electrostatic precipitators have a fairly simple operational procedure. The Precipitator employs two electrodes for removing particulates. The shapes of the electrodes vary based on the type of precipitator used. One of the electrodes is charged with a high negative charge. This electrode imparts negative charges to the particulates of the passing gas. The second electrode,
  26. 26. 27 charged with a high positive charge is placed right after the first electrode. The negatively charged particulates are hence pulled towards this electrode. ESP are highly efficient, and their efficiency can reach up to 99 percent. But the efficiency of the precipitators varies based on two particulate properties – 1. Electrical resistivity 2. Particle size distribution A list of pollutant source along with dust accumulation and preventive equipment is given below- SOURCE DUST COLLECTOR Crusher Bag Filter Raw mill: Gravity discharge Air swept Bag Filter / ESP Coal Mill: Gravity discharge Drying / grinding Bag Filter / ESP Kiln: Dry Semi - dry Wet Bag Filter / ESP Clinker Cooler ESP / Bag Filter with H.E Cement Mill Bag Filter / ESP Packing Plant Bag Filter Pollution control measurement for gaseous pollutant Main gaseous pollutants are NOx and SO2 produced during burning of the raw materials in the kiln.
  27. 27. 28 NOx emission control NOx emission from the kiln can be reduced in three ways – 1. Using process modification: Reduction of excess air from 10% - 5%, NOx emissions can be reduced up to 15%. Addition of a small amount of steel slag can reduce NOx emissions up to 30%. Since NOx emission is directly proportional to the amount of energy consumed, use of low nitrogen alternative fuels are also recommended as it also reduces the amount of energy required. 2. Combustion control: Technical literature and industrial reports indicate that up to 2347% reduction in NOx emission can be achieved using low-NOx burners. Usage of alternative fuel can also contribute to reduced NOx emission 3. NOx emission: Ammonia is injected at the high temperature zone near the kiln exit. This ammonia reduces NOx to NO2 in the presence of O2. NO + 4 NH3 + O2 = 4 N2 + 6 H2O NO2 + 4NH3 + O2 = 3 N2 + 6 H2O SO2 emission control Using gas-based fuel instead of coal-based fuel reduces the SO2 content of the flue gas greatly. The produced SO2 is removed from the flue gas by injecting hydrated lime. This reacts with the sulfur dioxide to produce calcium sulfate salt. Ca (OH) 2 + SO2 + ½ O2 = CaSO4 + H2O This process can reduce SO2 emission to about 50-70%.
  28. 28. 29 Conclusion In this report we have discussed the overall pollution scenario and methods used to prevent them from a global perspective. But in Bangladesh such laws and regulations are not followed as much. Especially most local companies do not collect any excess data regarding the emission, and in some cases, emission was found out to be around thrice the acceptable value. The government policy should be reinforced, and sufficient measurements should be taken by both the appropriate authority. Government backing and encouragement should help speed up this process References 1. Daugherty, K. E. & Wist, A. O. Air pollution control in the cement industry. Amer.Inst.Chem.Engns Symp.Ser. 70, 50–55 (1974). 2. EMEP EEA. EMEP/EEA air pollutant emission inventory guidebook 2019: Cement production. 1–18 (2019). 3. Varma, S. A. K. Emissions Inventory and Emission factors for Cement Industry. 10, 402–408 (2017). 4. Canpolat, B. R., Atimtay, A. T., Munlafalioglu, I., Kalafatoglu, E. & Ekinci, E. Emission factors of cement industry in Turkey. Water. Air. Soil Pollut. 138, 235–252 (2002). 5. WBCSD, W. B. C. for S. D. Guidelines for Emissions Monitoring and Reporting in the Cement IndustryInitiative business solutions for a sustainable world Cement Sustainability Initiative. 40 (2012). 6. Brussel, V. U. Formation , measurement and analysis of emissions from stack gas. (2018). 7. CEMENT INDUSTRY POLLUTION CONTROL MEASURES. - Mechanical engineering concepts and principles (hkdivedi.com)
  29. 29. 30 8. https://chemicalengineeringworld.com 9. Shreve Chemical Process Industries, 5th edition, G. T. AUSTIN 10. Rao, D.N.1971. A study of the air pollution problem due to coal unloading in Varanasi, India. In: H.M England and W.T. Berry, eds. Proceedings of the second International clear air congress. Academic press, New York, 273-276 11. Shraddha, M, Siddiqui, N.A. "Environmental and health impacts of cement manufacturing emission", International Journal of Geology, Agriculture and Environmental Sciences. 2014: 26- 31. 12. Daly, A.; Zannetti, P. Air Pollution Modeling – An Overview. Ambient Air Pollut. 2007, I (2003), 15–28. 13. Air pollution modelling: what is it and what it can tell us? By Jason Thongplang (https://www.aeroqual.com/air-pollution-modelling, accessed at 10 december 2020, 11:55 am) 14. Sutton O.G. (1932) A theory of Eddy Diffusion in the Atmosphere. Proc. Roy. Soc. A, 135:143. 15. Bosanquet, C.H. (1936) The Spread of Smoke and Gas from Chimmneys. Trans. Faraday Soc. 32:1249.

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