Sulphur Dioxide and Our Planet
Sulphur Dioxide is a colourless pungent gas which is highly soluble in water. Fossil fuels such as coal, petroleum and natural gases contain sulphur. When burnt in motor vehicles, power stations and in other industrial applications, sulphur reacts with oxygen and its compounds in the combustion process to form Sulphur Dioxide. These are the artificial sources of Sulphur Dioxide. Apart from these, Sulphur Dioxide is produced naturally in huge quantities during volcanic eruptions and forest fires. It is the most chemically active compound produced by volcanoes. Apart from natural sources, Sulphur Dioxide is also produced in anthropological activities like burning of fossil fuels and other modern fabrication industries.
In order to power the industrial age, humans have burned gigantic amounts of coal to produce any form of power. Coal naturally contains high amounts of sulphur and burning of coal produces huge amounts of heat and the sulphur present in the coal combines with the atmospheric oxygen to produce Sulphur Dioxide. The smoke-stacks of a power plant throw out all the waste gases into the atmosphere which contains huge amounts of Sulphur Dioxide. It is said that thermal power plants are the largest contributor of Sulphur Dioxide into the atmosphere. It is estimated that more than half of the total amount of Sulphur Dioxide generated in the atmosphere comes directly from thermal power plants that run on coal.
Another anthropological source of Sulphur Dioxide gas as a pollutant is from paper production. The paper mills that produce pulp-wood through a chemical process, normally combust their spent pulping liquor in a combustion unit. The most widely used chemical processes are Kraft Pulping process, Sulphite Pulping and Neutral Sulphite Semi-Chemical Pulping (NSSC Pulping). Among these, Kraft Pulping process is the most widely used process as it can be used to make strong and durable paper products. This process consists of digestion of wood, or any other cellulite substance in a solution of sodium sulphite and sodium hydroxide, the waste products of which produce harmful pollutants including sulphur compounds. Sulphite Pulping and NSSC Pulping produce paper for the printing industry and for other general usage. Sulphite Pulping, as the name suggests, is the process of pulping wood in a basic solution of Sulphur Dioxide. And as is evident, the majority of waste products from this process is Sulphur Dioxide itself. In the case of NSSC Pulping, wooden raw materials are partially de-lignified in a buffered Sodium Sulphite solution and the rest of the pulping process is completed by mechanical processes.
Emissions from Kraft Pulping process mainly includes non-condensable gases which are usually collected in a tank which contains a mixture of terpenes, totally reduced sulphur compounds (TRS compounds) and methanol along with a variety of other organic compounds. These collected gases are inflammable and are used as fuel in other industrial uses which reduces the sulphur pollution significantly, but the Kraft process itself produces large quantities of Sulphur Dioxide which is a major pollutant. In the Acid Sulphite Pulping process, the Digestor and the Blow Pit systems contribute large amounts of Sulphur Dioxide pollution. Sulphur Dioxide is also present in the intermittent digestor relief gases. Sulphur Dioxide is also released from the recovery system, the various pulp washing, screening and cleaning processes. In a Magnesium Sulphite mill, Sulphur Dioxide emissions mainly occur in the recovery boiler. Neutral Sulphate Pulping is no different from all of the above. In this case, Sulphur Dioxide pollution is caused by the digestor systems, the blow tank systems and the recovery furnace. However, sulphur particulate emissions are a potential concern only when a fluidised bed reactor is used for chemical recovery.
The importance of transportation in modern day civilization is among the highest of all modern day requirements. The transport sector also adds to the global emission of Sulphur Dioxide, but this mainly comes from the fact that sulphur is evidently present in all fossil fuel including gasoline and diesel fuels. As a result, burning of gasoline and diesel also produces Sulphur Dioxide. It is impossible to achieve clean emission from motor vehicles without cleaning sulphur from the fuel.
Control of Sulphur Dioxide is a very important factor in modern day industries. Widely popular thermal power plants are a major contributor of Sulphur Dioxide as a pollutant as mentioned earlier. However, as technology has progressed over time, a wide variety of Sulphur Dioxide emission control methods have come up like Wet Flue Gas Desulphurization (FGD), Dry FGD utilizing a Spray Dryer Absorber (SDA), Circulating Dry Scrubber and Dry Sorbent Injection. Among these aforesaid methods, Wet FGD is perhaps the most popular method used worldwide because of it being very cost effective. The most common reagent for FGD system is Limestone, but other reagents like Lime, Magnesium enriched Lime, sea-water and Soda ash. However, Limestone-based designs can also use organic acid as an additive to enhance Sulphur Dioxide removal. Wet FGD systems are known to completely remove Sulphur Dioxide from the waste gases of all types of coal combustion like anthracite, bituminous, sub-bituminous, lignite and brown coals.
Dry FGD can be used as an alternative process to Wet FGD on utility boilers to control Sulphur Dioxide emissions. It is a misnomer because technically, the process is semi-dry. It utilizes an SDA system to spray an atomized Lime slurry into the hot exhaust flue gas to absorb the emitted Sulphur Dioxide and other acidic gases. The slurry loses its moisture in the heat of the flue gas and the resulting dry mixture along with fly-ash is collected in a particulate control device like an ESP or a fabric filter. A part of this collected mixture is differentially channelled back to the slurry mixture as a means to reduce Lime usage. The main advantage of such an SDA system when compared to a conventional Wet FGD system is that it leads the flue gas directly to the fabric filter or ESP which is a very efficient particulate collector and hence helps to trap the vast majority of particulates in the flue gas before it leaves the chimney stack of the plant into the atmosphere.
Circulating Dry Scrubber (CDS) system is another variation of the Dry FGD system which is mainly used on units which have a higher emission of Sulphur Dioxide owing to the burning of coal having a higher sulphur content. This system uses a bed which is fluidised for contacting the Sulphur Dioxide gas with the reagent, which is typically hydrated lime. In this process, the reaction of Sulphur Dioxide is highly promoted by the intensive gas-solid mixture that occurs in the reactor with the dry lime particles. The mixture of reacted materials, unreacted lime and other particulates thus formed is then taken away in a particulate collector of any kind from the flue gaseous mixture. Part of the mixture is removed as waste, but most of the mixture thus formed is recycled in the reactor by the addition of fresh calcium hydroxide.
Dry Sorbent Injection Systems are the cheapest to manufacture and run. A dry sorbent like Trona, sodium bicarbonate or lime is injected into the outlet manifold coming out of the boiler. Sulphur Dioxide is readily eliminated from flue gas in this process and the particulates are collected in a particulate collection system as usual. The absence of a separate absorber vessel and utilization of lower quantities of reagent makes this method the most economical of all the commercial methods of eliminating Sulphur Dioxide. However, this method has a very low efficiency in terms of removing the target pollutant and is used in scenarios requiring lower removal rates of Sulphur Dioxide.
Emission control at paper and pulp mills is extremely important. Emission control can be achieved through a wide variety of methods, some of which may include modifications to the production method, improved operating conditions or addition of emission control systems. In the case of Kraft Pulping, Sulphur Dioxide is emitted mainly by the oxidation of reduced sulphur compounds in the recovery furnace. The most widely acceptable and economical methods to reduce this emission includes changing of combustion parameters, increase in dry solid content of the strong black liquor, decreasing the Sulphur-Sodium ratio inside the furnace and an overall better control of the process. Apart from these, direct contact evaporator absorbs almost 75% of Sulphuric emissions and scrubbing further provides additional control.
In the case of Acid Sulphite Pulping process, placement of water showers in the vents of the blow tanks is a very efficient method of controlling Sulphur Dioxide emission. Apart from this a scrubber may also be used. The technological advancement of modern scrubbers allows them to remove almost all the Sulphur Dioxide from the emissions. Recovery systems with Magnesium, Sodium or Ammonium, which are highly efficient, can be used to capture Sulphur Dioxide from a variety of systems like recovery furnaces, acid fortification towers and multiple effect evaporators. Similarly, in the case of Neutral Sulphite Pulping process, Sulphur Dioxide from the emissions can be removed with very high efficiency with the help of scrubbers.
In the case of vehicular emissions, as is said earlier that making sulphur free fuel is the most important step of all. In today’s world, there are mainly three standards of low-sulphur automotive fuel. They are Reduced Sulphur Fuel (~150 ppm), Low Sulphur Fuel (~50ppm) and Near-zero Sulphur Fuel (~10 ppm). The technology necessary to produce low-sulphur fuels are predominant in most parts of the world as production costs are quite reasonable and constant development of higher quality active catalysts are bringing down the expenses of producing low-sulphur fuels by even further as time goes by. Thanks to strict environmental laws automobile manufacturers and fuel producers are having to employ a significant amount of research, manpower and capital into making cleaner cars and fuel.
Works Cited
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Ncasi. Environmental Footprint Comparison Tool - Effects of Decreased SOx and NOx Emissions (2013): n. pag. Environmental Footprint Comparison Tool. 2013. Web. 8 Mar. 2016.
International Energy Agency. "Emissions from Coal Fired Power Generation." New Delhi, India: International Energy Agency, 2011. N. pag. Print.
Samudra Vijay, Luisa T. Molina and Mario J. Molina. Estimating Air Pollution Emissions from Fossil Fuel Use in the Electricity Sector in Mexico (2004): n. pag. Commission For Environmental Co-operation. Massachusetts Institute of Technology, Apr. 2004. Web. 8 Mar. 2016.
