Cement is one of the oldest and commonest construction materials across the world. It is the second most used commodity worldwide after water. The material is mainly used for making concrete that is used for construction of different structures such as buildings, roads, dams, bridges, etc. Global demand and consumption of cement has continued to rise over the years leading to constant increase in cement production. In 2016, the global cement production is estimated to have reached 4.6 billion tons and is projected to reach 5.8 billion tons by 2022 (International Market Analysis Research and Consult Group, 2017). This demand is driven by rising construction activities in different parts of the world.
Despite the growing demand for cement and market size of cement industry, there are several concerns in this industry. One of the major concerns is the industry’s environmental impacts. Cement production is one of the most energy intensive manufacturing processes. The cement is produced from a blend of limestone/chalk, clay, sand, slate, shells, etc. Extraction and preparation of these materials require a lot of energy. These raw materials are then burned in a kiln to temperatures ranging from 1400 to 1500°C to produce cement clinker through a process known as calcination. After that, the clinker is ground to produce a fine powder that is then mixed with gypsum to produce cement (Global Cement, 2011). The total energy required to produce 1 ton of cement is above 4.7 billion BTUs (British thermal units), which is equivalent to approximately 400 pounds of coal. As a result of this, cement industry accounts for a significant percentage of carbon missions that is a major contributor of global climate change. Cement production alone is estimated to account for 5-6% of total carbon dioxide produced by human activities (Rodrigues & Joekes, 2011). Considering the increasing cement demand and production, carbon dioxide emissions emanating from cement industry will also continue increasing unless appropriate measures are put in place to cut them down.
All activities related to cement production and shipping produce significant amount of carbon emissions, besides other environmental impacts (Chen, et al., 2010). This has led to creation and development of various strategies aimed at reducing the amount of energy used in the industry and their associated environmental impacts. Some of these strategies include: increasing efficiency of the kiln and the overall cement production plants; use of production processes that consume less energy, such as dry process instead of wet process (Madlool, et al., 2011); use of renewable energy sources; increase energy efficiency of cement production plants (Worrell, et al., 2013); recycling and reuse of wastes generated from cement production; and monitoring and control of greenhouse gas emissions origination from cement production.
Analysis of cement industry impact on the environment is very important because it provides useful information that can be used to establish the most effective practices of reducing these impacts. This has seen some governments formulate policies that set limits of maximum acceptable quality of air released from cement production facilities. As a result of this, cement production companies have come up with mechanisms of monitoring and controlling the quality of air they release into the environment.
As stated before, cement industry produces 5-6% of the total carbon dioxide emissions produced by human activities. Other emissions include particulate matter, carbon monoxide, sulfur dioxide, nitrogen oxide, volatile organic compounds, hydrogen chloride, chlorine and ammonia. These emissions have huge impacts on the environment and therefore the need to monitor and control them before being released into the environment cannot be overemphasized (Selvakumar & Kim, 2016). Considering the projected global cement production over the coming years, there is need to find ways of reducing these impacts. Monitoring and controlling the quality of air that is produced from cement production activities is one of the most effective strategies of minimizing environmental impacts originating from the cement industry. There are several devices that are used to monitor and control air quality and pollution in cement industry. Some of these devices are discussed below.
ESPs are air quality monitoring and control devices that separates and eliminates fine particles, such as dust, smoke and liquid droplets, from air using electrostatic forces. These devices have high voltage, DC (direct current) discharge electrodes that attract oppositely charged dust and smoke particles. The negatively charged electrodes (discharging electrodes) are positioned between positively charged grounded electrodes (collecting electrodes). When air is passed through the ESP, discharging electrodes release a negative charge to the dust particles which then get attracted and stuck on the collecting electrodes. Once these particles are attracted on the electrodes, they get discharged or removed and dropped to the bottommost part of the ESP by vibrating or rapping the electrodes. These particles or wastes are very hazardous to the environment (Darweesh, 2017) and therefore should be disposed appropriately. There are different types of ESPs including: positively charged 2-stage ESP, negatively charged dry ESP and negatively charged wetted-wall ESP (Khattak, et al., 2013). These devices minimally obstruct flow of air through them and they have very high efficiencies of up to 99.9% in controlling particulate matter emissions in cement industry. Other advantages of ESPs are: can operate at very high temperatures ranging from 371°C-704°C; they experience minimal temperature and pressure changes; can collect and filter even difficult materials such as tars and acid; require less power for cleaning; they withstand very corrosive materials; and can achieve very high flow rates. Figure 1 below is a schematic diagram of an ESP
Figure 1: Schematic diagram of an ESP (Mitsubishi Hitachi Power Systems Environmental Solutions, 2015)
A baghouse is an air quality control device comprising of several fabric bags that collect particles from air on their surfaces (Roberts, et al., 2014). Baghouses are capable of removing 99.99% of particles from air. A baghouse has several compartments. The first compartment has a fibre mesh that traps larger particles. The smaller particles get collected on filter media/bags in subsequent compartments, creating a cake or layer. Only clean air is allowed to pass through the filter bags. When the air bags get clogged with dust particles to an extent of impeding flow of air, cleaning process gets started (Carr, 2012). Depending on the type of baghouse and cleaning method used, the cleaning process can be done as the baghouse compartment continues to function or it can also be isolated first. Baghouses are also known as fabric filters or fabric dust collectors. Advantages of baghouses include: very high efficiency for removing different sizes of particulate matter; flexible and versatile; cost effective if design and maintained properly; and are available in modular and robust designs making them easy and convenient to install. The main drawbacks of baghouses are that they have high maintenance needs and their efficiency is affected by corrosive materials and high temperatures exceeding 288°C. Different types of baghouses include: reverse air baghouses, reverse-jet or pulse jet baghouses and shaker baghouses (Engineering360, 2017).
These are devices that remove pollutants from air using a scrubbing liquid, which is usually water (Bhargava, 2016), but there are several other liquids that can be used. Polluted air is directed into the wet scrubber then it mixes with the scrubbing liquid therein. The method of contact can be by forcing the air to flow through the scrubbing liquid, spraying the liquid on the air or any other method. When the dirty air comes in contact with the scrubbing liquid, the liquid collects all gases or particulate matter and only allows clan air to pass through it. Types of wet scrubbers include: gas or chemical scrubbers, ammonia scrubbers, venturi or particulate scrubbers, dust scrubbers, chlorine scrubbers and sulfuric acid scrubbers. Main advantages of wet scrubbers are: require small space; have minimal explosion and fire hazards; can collect both particulate matter and gases (Han, et al., 2016); can handle high-humidity and high-temperature air; and can compensate corrosive air. Some of their drawbacks include: they require more energy; corrosion problems; and need mist removal so as to improve their efficiencies.
These are devices that are used to eliminate particulate matter from gases using the principle of inertia. A cyclone separator has a chamber that contains a vortex that is analogous to a whirlwind or tornado. Particulate matter and gas particles have different inertia. When dirty air enters the cyclone separator, the difference in inertia between particulate matter and gas particles automatically forces the gas particles to move to the upper part of the chamber whereas the particular matter drops to the bottom of the chamber. As a result of this, the gas at the top is free from any particulate matter while the dirt gets deposited at the bottom, as shown in Figure 2 below. It is therefore important to note that cyclone separators are only suitable for removing particulate matter from air.
Figure 2: Schematic diagram of cyclone separator (Rodriguez, 2016)
A gravity settler is a device that controls air pollution by separating particles from the air through settling. This device consists of a long compartment called settling chamber. Polluted air is directed into this chamber and held for some time so that larger particles can settle and be removed at the bottom while particle-free air flows at the top. In some cases, baffles may be used to disrupt air flow so as to improve the effectiveness of gravity settlers. These devices can be used in applications with very high pressure and temperature, and in corrosive environments. They also have low operating and maintenance costs. Gravity settlers can only be used to remove particulate matter from air.
References
Bhargava, A., 2016. Wet scrubbers – design of spray tower to control air pollutants. International Journal of Environmental Planning and Development , 2(1), pp. 68-73.
Carr, R., 2012. Fabric filter technology for coal-fired power plants. Journal of the Air Pollution Control Association, 32(4), pp. 362-366.
Chen, C., Habert, G., Bouzidi, Y. & Jullien, A., 2010. Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. Journal of Cleaner Production, Volume 18, pp. 478-485.
Darweesh, H., 2017. A Review Article on the Influence of the Electrostatic Precipitator Cement Kiln Dust Waste on the Environment and Public Health. Composite Materials, 2(1), pp. 8-14.
Engineering360, 2017. Baghouses and baghouse filters information. [Online] Available at: https://www.globalspec.com/learnmore/manufacturing_process_equipment/air_quality/baghouses[Accessed 21 September 2017].
Global Cement, 2011. Cment 101 – An introduction to the world’s most important building material. [Online] Available at: https://www.globalcement.com/magazine/articles/490-cement-101-an-introduction-to-the-worlds-most-important-building-material[Accessed 21 September 2017].
Han, Z. et al., 2016. An investigation on NO removal by wet scrubbing using NaClO2 seawater solution. SpringerPlus, 5(1), p. 751.
International Market Analysis Research and Consult Group, 2017. Cement market: global industry trends, share, size, growth, opportunity and forecast 2017-2022, Virginia, U.S.: IMARC Group.
Khattak, Z., Ahmad, J., Ali, H. & Shah, S., 2013. Contemporary dust control techniques in cement industry, electrostatic precipitator – A case study. World Applied Science Journal, 22(2), pp. 202-209.
Madlool, N., Saidur, R., Hossain, M. & Rahim, N., 2011. A critical review on energy use and savings in the cement industries. Renewable and Sustainable Energy Reviews, Volume 15, pp. 2042-2060.
Mitsubishi Hitachi Power Systems Environmental Solutions, 2015. Dry type Electrostatic Precipitator. [Online] Available at: https://www.es.mhps.com/en/products/atmosphere/dustcollection/electrostaticprecipitator/dryelectrostaticprecipitator/index.html
[Accessed 21 September 2017].
Roberts, P., Els, L. & Kornelius, G., 2014. Design considerations for a continuous emission measurement system for pressure type bag houses. The Clean Air Journal, 24(2), pp. 1-4.
Rodrigues, F. & Joekes, I., 2011. Cement industry: sustainability, challenges and perspectives. Environmental Chemistry Letters, 9(2), pp. 151-166.
Rodriguez, K., 2016. How to hire cyclone separator mechanic. [Online] Available at: https://musicinterprete.com/how-to-hire-cyclone-separator-mechanic/[Accessed 21 September 2017].
Selvakumar, K. & Kim, M., 2016. a numerical study on the fluid flow and thermal characteristics inside the scrubber with water injection. Journal of Mechanical Science and Technology, 30(2), pp. 915-923.
Statista, 2017. Global cement production from 1990 to 2030 (in million metric tons). [Online] Available at: https://www.statista.com/statistics/373845/global-cement-production-forecast/[Accessed 21 September 2017].
Worrell, E., Kermeli, K. & Galitsky, C., 2013. Energy efficiency improvement and cost saving opportunities for cement making, Washington, D.C.: Energy Star.
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