TYPES OF CEMENT,THEIR USAGE AND POLLUTION CONTROL MEASURES
India ranks world’s second largest Cement producing Country. There are about 130 large cement plants in the country with an installed capacity of 160.24 million tonne. During the year 2005-06 actual production was achieved 141.81million tonne. The number of mini cement plants accounts to 365 with an installed capacity of 11.10 million tonne. The recorded production by mini cement plants during the same period is 6.0 million tonne. There are about 1,35,000 manpower employed all over India with a turnover more than 9,700 million US $ (2005). The cement sector has been rapidly growing at a rate of 8% and is expected to grow further.
Cement industry is one of the major Air polluting industries among the 17 identified categories of industries*. In industrial sector, this is the second largest emitter of CO2.This accounts for 5% of global manmade CO2 emissions, of which 60% is from the chemical process and 40% from burning fuel. Cement manufacturing is an energy intensive process. Consuming energy from fossil fuels such as oil and coal creates carbon dioxide (CO2), the most important Greenhouse Gas (GHG) causing climate change. Emission of carbon dioxide, which causes Global warming, from Indian cement companies is significantly lower than European and American cement companies. The emission of CO2 also depends on the type of production processes, their efficiency, fuel used etc. Besides this, the cement industry in its various processes emits Suspended Particulate Matter (SPM) and oxides of nitrogen (NOx) which is produced during calcinations process. Particulate Matter is the main pollutant emitted from Cement industries.
Cement is a binder, a substance which sets and hardens independently and can bind other materials together. Cements used in construction are characterized as hydraulic or non-hydraulic. India produces different varieties of cement like Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), Portland Blast Furnace Slag Cement (PBFS), Oil Well Cement, Rapid Hardening Portland Cement, Sulphate Resisting Portland Cement, White Cement etc.
Hydraulic Cements are materials, which set and harden after combining with water, as a result of chemical reactions with the mixing water and, after cements today are hydraulic and most of these are based upon Portland cement, which is made primarily from limestone, certain clay minerals and gypsum in a high temperature process (1450oC) that drives off carbon dioxide and chemically combines the primary ingredients into few compounds.
The most common use for Portland cement is in the production of concrete. Concrete is a composite material composite material consisting of aggregate (gravel and sand), cement and water. Portland cement may be gray or white.
Portland Cement
This is the most common cement used around the world. It ia a fine powder with basic ingredients like concrete,mortea, stucco,non-specialty grout and aluminium and iron oxide as a flux to give strength.Portland cement clinker is made by heating in a kiln at 1450 °C.
Portland Pozzolana Cement(PPC)
This is widely used in industrial and residential buildings, roads, dams and machine foundations because of its quality to withstand aggressive gases, thermal cracks and wet cracking. It prevents formation of calcium hydroxide at the time of cement setting and hydration. Pozzolina is an important ingredient which is commonly used in the form of Fly ash, Volcanic ash, Silica fumes and Calcined clay, along with OPC clinker and gypsum. It is manufactured by blending them and grinded together or separately.
Portland Blast Furnace Slag Cement
The production of Portland Blast Furnace Cement has given major boost to Indian Cement Industry. This is a hydraulic type of cement. It is manufactured in a blast furnace where iron ore is reduced to iron. The molten slag than is quickly drenched with water, dried and then grounded to fine powder. The PBFSC consumes 75% energy that of the Portland cement.
Oil Well Cement
This is used for the grouting( cementing ) of the oil wells for both, the off-shore and on-shore wells where the pressure is around 20,000 PSI and the temperature is around 500 °C. It is manufactured from the clinker of Portland Cement and also from cement which are hydraulically blended. It has organic ‘retarders’ which prevent it from setting too fast. There are three grades of oil Well Cement namely Grade O-ordinary cement, HSR-high sulphate resistant and MSR-moderate sulphate resistant. With increased number of oil wells in India, this has also boosted the cement industry to a large extent.
Rapid Hardening Portland Cement
Rapid Hardening Portland Cement (RHPC) is used in concrete masonry works where quick repairs are required such as airfield and highway pavements, marine structures and bridge decks as well as in cold weather and pre-cast production of concrete. Cement clinker is produced by fusing together limestone and shale at extremely high temperature. Gypsum is added to this clinker and then finely grounded to produce RHPC.
To control pollution from the cement industry various steps have been taken which include Notification of emission standards for cement industry, adoption of Corporate Responsibility for Environment Protection (CREP) for 17 categories of industries including cement and regular monitoring for compliance of environmental standards. As far as Clean Development Mechanism (CDM) projects in cement sector are concerned, the National CDM Authority has accorded approval to about 40 cement projects. 20 of these have since been registered. These 20 registered projects would save 17.60 MT of CO2 till 2012.
The cement industry holds immense promise in terms of utilizing wastes from other industries, fly ash (from the power sector), blast furnace slang (from the iron and steel industry) and phosphor-gypsum (from fertilizer plant).These are used to manufacturer blended cement, without sacrificing the quality of cement. Today, about 12% of total fly ash generated in India is used by the cement industry.
Other than chemical pollutants, dust is the major pollutant emitted in the process of production of cement. The dust generation occurs from various sections of the cement plant during the process of cement manufacturing. The source of emission includes both point source and fugitive emission. The air quality in and around the plants is predominantly polluted by suspended particulate matter which can affect the surrounding. Considering the contribution of air pollution by the cement industry, the Central Pollution Control Board in close consultation with the State Boards and the Association of cement industry had evolved emission standards for cement plants of different capacities.
The main sources of fugitive emission in cement industry are:
Open air handling & storage of raw materials & clinker
Transfer Points
Leaking joints
Loading & unloading operation
Vehicular movement on unpaved roads
There are methods being adopted for controlling fugitive emission.
The methods employed for fugitive dust control in cement industry include exhaust ventilation system and water spray system. Proper house keeping aspects are also important to maintained the environment need and clean.
There are 6 types of Greenhouse Gases. They mainly include 6 gases namely, carbon dioxide, methane, nitrous oxides, hydroflurocarbons, perflurocarbons and sulphur hexafluoride.
In the process of calcinations, carbon dioxide emission in cement manufacturing originate from combustion of fossil fuels and from calcining the limestone in the raw mix. Calcium carbonate (CaCO3) in limestone when calcined gets converted into lime (CaO) and carbon dioxide (CO2).
CaCO3 à CaO + CO2
About 60% of emission caused by making cement are from this chemical process alone. The main sources of carbon dioxide emission are three. As a rough estimate, total carbon dioxide emissions range from 0.85 – 1.15 tonne of cement produce, assuming clinker to cement ratio as 0.95. the approximate contributions of each of the three main sources of carbon dioxide emissions are
- Calcination: 50-55%
- Fuel combustion: 40-50%
- Electricity: 0-10% (assuming that electricity is generated from fossil fuels)
Various types of Air Pollution Control Devices (APCDs) in the form of dust collectors are used in cement plants to control the emission of dust to the atmosphere. The types of pollution control equipment used in cement industry to control particulate emission are:
· Cyclone and Multi-cyclones
· Fabric filters/ bag filters
· Electronic precipitators (ESP)
· Gravel bed filters
The clean technologies have been adopted in cement industries besides other steps.
· Manufacture of clinker by Dry Process technology
· Manufacture of blended cements such as Portland Pozzolona Cement (PPC), Portland Blast Furnace Slag Cement (PBFS).
· Use of high calorific wastes as alternate fuels/source of energy.
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*Various polluting industries identified by CPCB are Aluminum, Cement, Chlor Alkali, Copper, Distillery, Dyes and dye intermediates, Fertilizers, Iron & steel, Oil Refineries, Petrochemicals, Pharmaceuticals, Pulp & Paper, Sugar, Tannery, Thermal Power Plants, Zinc and Pesticides.
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