Recovery and Reuse of SO 2 from Thermal Power Plant Emission

Air pollution has always been a trans-boundary environmental problem and a matter of global concern for past many years. High concentrations of air pollutants due to numerous anthropogenic activities influence the air quality. There are many books on this subject, but the one in front of you will probably help in filling the gaps existing in the area of air quality monitoring, modelling, exposure, health and control, and can be of great help to graduate students professionals and researchers. The book is divided in two volumes dealing with various monitoring techniques of air pollutants, their predictions and control. It also contains case studies describing the exposure and health implications of air pollutants on living biota in different countries across the globe.

intensity of solar radiations, temperature and degree of air pollution. Under favourable conditions the oxidation of sulphur dioxide can occur in the atmospheric aqueous phase at significantly faster rates than in the gas phase. It is believed that, on a global scale, more than 70% of the global oxidation of SO 2 to SO4 2− occurs within cloud droplets (Langner and Rodhe 1991).The oxidation of sulphur dioxide has been one of the most frequently studied reactions in aqueous atmospheric droplets. Three reaction pathways are considered to be dominantly responsible for oxidation of SO 2 in atmospheric water droplets. These are the oxidation of dissolved SO 2 by H 2 O 2 , O 3  . It is claimed that at pH 4, transition metal catalysed pathways could account for up to half of the oxidation of S(IV) to S(VI) (Graedel et al. 1985). According to present knowledge, iron(II/III) and manganese (II/III) are the most important catalysts in atmospheric droplets (Coichev and van Eldik 1994;Brandt and van Eldik 1995; Seinfeld and Pandis 1998). These metals are the only efficient catalysts at low pH. In addition, both iron and manganese are common constituents of tropospheric aerosols and water droplets even in remote areas due to their generation from erosion of the earth's crust. Other transition metals such as Cu(II), Co(III), Sc(III), Ti(III), V(III) and Cr(III), are also catalysts, but with a substantially lower effect on the reaction rate (Ibusuki et al. 1990; Grgić et al. 1991;Sedlak and Hoigné 1993). The catalytic oxidation of S(IV) is a free radical chain reaction. Its mechanism and kinetics are so complex and sensitive to the conditions under which the process occurs that even a minor change in experimental conditions can cause a change of the dominant path of the reaction course, and thus lead to diverse results. Despite numerous studies of the metal catalysed S(IV) oxidation there still exist serious discrepancies in rate expressions, rate constants, pH dependencies, activation energies, reaction mechanisms etc. Recent studies show that the sulphur(IV) oxidation in atmospheric water droplets can be affected by other reactions. In particular, organic chemistry may be especially important. Organic compounds may dissolve into water droplets and react with sulphoxy radicals and transition metal ions, and thus alter the rate of catalytic S(IV) oxidation (Martin et al. 1991;Pasiuk-Bronikowska et al. 1997;Grgić et al. 1998;Pasiuk-Bronikowska et al. 2003a,b;Ziajka andPasiuk-Bronikowska 2003, 2005).Recently, the inhibiting effect of such organic ligands as oxalate, acetate and formate in the iron-catalyzed autoxidation of sulphur(IV) oxides in atmospheric water droplets has been suggested. Grgić et al. (1998Grgić et al. ( , 1999 and Wolf et al. (2000) reported the strong inhibiting effect of oxalate on the Fe-catalysed S(IV) oxidation in aqueous acidic solution. Acetate and formate also inhibit the reaction, but to a much lesser extent than oxalate (Grgićet al. 1998). Very recently, the influence of some low weight mono-(formic, acetic, glycolic,lactic) and di-carboxylic acids (oxalic, malic, malonic) on the Mn(II)-catalysed S(IV) oxidation has also been investigated (Grgić et al. 2002;Podkrajšek et al. 2006). It has been established that mono-carboxylic acids inhibit the oxidation, with the strongest influence 2 J Atmos Chem (2008) 60:1-17 found for formic acid. The lowest inhibition was caused by acetic acid. From among dicarboxylic acids, oxalic acid slows down the S(IV) oxidation, although to a lesser extent than mono-carboxylic acids, while malic and malonic acids have practically no influence. The effect of organic compounds in atmospheric water on the transition metal-catalysed oxidation of sulphur(IV) is not fully known yet and more work in this area is needed to understand these processes better. The purpose of the present study was to study the kinetics of the Mn(II)-catalysed S(IV) oxidation and to determine the inhibiting effect of acetic acid on this process under different experimental conditions representative for heavily polluted areas. The experiments were carried out at Mn(II) and CH3COOH concentrations in the range 10−6-10−5 and 10−6-10−4 mol/dm3, respectively, and at initial pH of the solution in the range 3.5-5.0; initial concentrations of S(IV) were around 10−3 mol/dm3. S(IV) liquid-phase concentration of 1×10−3 mol/dm3 corresponds to 0.6 ppm SO 2 in the gas phase over a solution of pH=5, or 7 ppm over a solution of pH=4, or 20 ppm over a solution of pH=3.5. Such high SO 2 gas-phase concentrations are found in heavily polluted areas as well as in power plant and volcanic plumes. In highly polluted locations, for example in large urban areas where coal is used for domestic heating purposes, or for poorly controlled combustion in industrial installations, SO 2 concentrations are rather high and vary between 0.1 and 0.5 ppm, and sometimes they are even higher (Ferrari and Salisbury 1999). High sulphur dioxide concentrations are being recorded in some of the megacities in developing countries where burning of coal is the main source of energy. The greatest problems related to sulfur dioxide occur in Asia (mainly in Chinese cities and some Middle-East cities such as Teheran, Tbilisi and Istanbul) (Baldasano et al. 2003). In Asia there are cities [e.g. Guiyang (424 μg/m3), Chongquing (340 μg/m3)] with average annual values of more than six times the WHO guideline value (Baldasano et al. 2003). Also in Africa some of the urban areas, and especially industrial areas, experience high concentrations of sulphur dioxide (WHO 2006). Weekly average concentrations in Zambia's copper belt (Nkana, Mufulira and Luanshya) were found to range from 167 to 672 μg/m3, the highest weekly average being 1,400 μg/m3. Studies undertaken on the impact of the Selebi Phikwe copper smelter in Botswana show that there are large areas experiencing concentrations above 100 μg/m3. Short term measurement indicated 1-h average concentrations of more than 1,000 μg/m3 (WHO 2006). Also some of the heavily industrialized areas in Europe may still be experiencing high levels of sulphur dioxide. In some cities in the north western corner of the Russian Federation, close to large primary smelters, daily concentrations of sulphur dioxide exceed 1,000 μg/m3 (WHO 2006). From the point of view of atmospheric chemistry, especially fast chemical reactions, concentrations averaged for shorter periods e.g., for 1 h or even for several minutes, are more relevant. These concentrations are closer to actual concentrations at which fast reactions proceed in the atmosphere. Concentrations averaged for shorter periods are considerably higher than those averaged for longer periods. Peak concentrations over shorter averaging periods may still be very high, both in cities with a high use of coal for domestic heating and when plumes of effluent from power station chimneys fall to the ground (fumigation episodes). Transient peak concentrations of several thousand Concentrations of Mn(II) and acetic acid in solutions used in our experiments correspond to those found in rain-, cloud-and fogwater in heavily polluted urban and industrialized areas. Manganese is one of the most abundant transition metals in atmospheric liquid phases (wet aerosol, cloud, fog, rain). The only source of these metals in the atmospheric aqueous phase is the dissolution of aerosol particles incorporated in water droplets. The common particles containing trace metals are emitted from both anthropogenic (fossil fuel combustion, industrial processes) and natural (windblown dust, weathering, volcanoes) sources. Particles from anthropogenic sources contribute significantly to metal distribution in atmospheric droplets due to their high metal content and solubility. In consequence, trace metal concentrations in atmospheric waters are higher in urban and industrial areas (Colin et al. 1990). In atmospheric waters, manganese is mainly found as Mn(II), which is more soluble than manganese (

Basics and scopes of the work -Flue Gas Desulphurization (FGD)
Flue gas desulphurization (FGD) is the current state-of-the art technology used for removing sulphur dioxide from the exhaust flue gases in power plants. SO 2 is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO 2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using Ca (OH) 2 and NaOH slurry produces CaSO 3 and Na 2 SO 3 and can be expressed as: Some FGD systems go a step further and oxidize the CaSO 3 and Na 2 SO 3 to produce marketable CaSO 4 · 2H 2 O (gypsum) and Na 2 SO 4 (Sodium Sulphate): [5][6] CaSO 3 (solid) + ½O 2 (gas)

Mechanism
When sulfur dioxide (SO 2 ) in the flue gas contacts scrubber slurries, the pollutant transfers from the gas to the liquid phase, where the following equilibrium reactions are fundamentally representative of the transfer process.
when lime hydrated powder or caustic flakes introduced to water will raise the pH according to the following mechanism.
However, Ca(OH) 2 is only slightly soluble in water, so this reaction is minor in and of itself. In the presence of acid, calcium hydroxide reacts much more vigorously and it is the acid generated by absorption of SO 2 into the liquid that drives the lime dissolution process. NaOH Equations 1, 2 and 3 when combined illustrate the primary scrubbing mechanism. 2NaOH In the absence of any other factors, (for example, oxygen in flue gas) calcium and sulfite ions will precipitate as a hemihydrate, where water is actually included in the crystal lattice of the scrubber byproduct.
However, oxygen in the flue gas has a major effect on chemistry, and in particular on byproduct formation. Aqueous bisulfite and sulfite ions react with oxygen to produce sulfate ions (SO 4 -2).
Calcium sulfite-sulfate hemihydrate is a soft, difficult-to-dewater material that previously has had little practical value as a chemical commodity. Gypsum, on the other hand, is much easier to handle and has practical value. These factors are driving utilities to install forced oxidation systems for gypsum production.
There are three control technologies which have major application in the field of Sulphur di Oxide control. 7 Adsorption is a control technology for control of SO 2 from stack gases but suffers from several following drawbacks viz: Absorption is a control technology for control of SO 2 from stack gases is most widely practiced.
However this technology also suffers from following drawbacks: 1. Stack gas cooling and reheating is required. 2. Mist elimination is required.
However these problems can be easily encountered with proper engineering design used. Besides this less operator's intensiveness, less cost and ease of handling of liquid sorbent makes it an attractive option. It is one of the most widely used control technology employed for removal of SO 2 9-10

Material and methods
All experiments were conducted on Stack monitoring Kit (Model No. and Make -VSS1, 141 DTH -2005,Vayubodhan). First of all Stack monitoring kit of SO 2 monitoring were set up for experiment at chimney inlet of Boiler of thermal power plant. Flue gas containing SO 2 were supplied from chimney via probe connected with flexible pipe of stack monitoring kit. The flow of flue gas were controlled using an inlet line Rota meter and was maintained at a value of 3 liter per minute and other end of flexible pipe carrying air and SO 2 respectively were connected to a impinger of 10 cm diameter and 100 cm length. The impinger were filled with 100 ml of scrubbing media in this experiment i.e. Sludge solution, Calcium hydroxide solution, Sodium hydroxide solution.
The concentration of SO 2 in flue gases was first measured by Stack monitoring Kit. V a = Volume of sample aliquot-titrated, ml.

Result and discussion
Table -2 to 7 reports that relation between recovery of absorption of SO 2 using varying concentration of Sodium hydroxide, Calcium hydroxide, and Sludge with analysis results of precipitate. As can be seen from figure -1 that recovery of SO 2 using, Calcium hydroxide, and Sludge is far below that using Sodium hydroxide. Figure -2 shows the results of % SO 3 (Gravimetric) of precipitate which was prepared by three different reagents and SO 2 contained in flue gases. It is reported that % SO 3 is higher in case of NaOH as to others. Figure -3 shows the results of % SO 2 (Volumetric) of precipitate which was prepared by three different reagents and SO 2 contained in flue gases. It is reported that % SO 2 is higher in case of NaOH as to others. Figure -4 shows the results of % respective Sulphate of precipitate which was prepared by three different reagents and % SO 2 in flue gases. Figure -5 shows the results of % alkalinity of precipitate which was prepared by three different reagents. We know that alkalinity is the reverse of % SO 2 and it is confirmed by figure -5. Figure -9 and table -8 reports that effect of pH of NaOH solution and absorption of SO 2 and it is confirmed that when increase in the time period for absorption of SO 2 in NaOH solution, then there is a significant decrease in pH. Figure -10 reports that with the increase of time period for absorption of SO 2 in NaOH solution there is a significant decrease in conc. of NaOH Solution. Table -9 shows recovery of SO 2 using different parameters like time period for reaction , temperature of Solution and flow of flue gases in impingers with analysis results of precipitate. Figure -6 reports that recovery of SO 2 with different parameters. Figure -7 reports that % SO 3 in precipitate which was prepared by exhaust SO 2 using different parameters. Figure -8 reports that amount of % Sulphate which was prepared by SO 2 using different parameters. Figure -14 reports that amount of % SO 2 (Volumetric) which were prepared by exhaust SO 2 using different parameters. Figure -8 reports that amount of % Alkalinity which were prepared by SO 2 using different parameters.

Conclusion
From the comparative study of three different reagent regarding to removal of SO 2 , it is observed that Sodium hydroxide is superior as compare to calcium hydroxide and sludge. The initial rate of absorption is higher for Sodium hydroxide as compared to calcium hydroxide and Sludge. All the absorption methods coupled with a chemical reaction. It may be suggested that Sulphur dioxide is a weak acid and it is a well known fact that reaction of a weak acid with a strong base is fast, meaning stronger the base faster would be the reaction Therefore Sodium hydroxide is a strong base compared to calcium hydroxide and sludge so this evident that Sodium hydroxide is a better solvent for removal of SO 2 .
The lower Conc. of the reagent is found to be optimum. Increasing conc. of solution is not very fruitful for maximum absorption of SO 2 in exhaust flue gases. This is because of load of SO 2 in flue gases is very low (at ppm level), so the reagent remains as it is in solution after completely absorption of SO 2 .
The pH of the solution should be alkaline. Because of nature of SO 2 is acidic and reaction is restricted in acidic solutions The temperature of solution should be lower i.e. 20-25 o C. Because of at higher temperature reversible reaction may be take place and partially formed product may be change in to initial reactants.
The time period of the absorption of SO 2 should be maximum for completely absorption of SO 2 .
The direct flow of flue gases in to impingers containing solution will results maximum absorption of SO 2 instead of indirect flow of flue gases because of in indirect SO 2 react with water form sulfurous acid.
On the basis of our study we can recommended that if flue gas desulphurization system (FGD System) is set up before Chimney then maximum SO 2 is trapped, resulting lowers the SO 2 conc. in environment and lowers the air pollution.

Purpose
Air pollution is one of the very important issues world-wide. The wet limestone-gypsum process has been the most popular method adopted to eliminate SO 2 emitted from thermal Power Stations. However, due to the relatively high construction cost, its further implementation has inevitably limited and the development of more economical FGD technology has been sought.
Hence, Hitachi Compact FGD System was developed, for the purposes of simplification and cost reduction utilizing features of the latest FGD technology fully. The first System was delivered to Peoples Republic of China under the"Green Aid Plan", which has been organized and managed by the Ministry of International Trade and Industry (MITI), Japan, in order to implement their policy to transfer environmental preservation technology to neighbouring countries and it contributes to global environmental preservation and the technologies, such as, higher gas velocity in the absorber and adoption of horizontal flow spray absorber instead of conventional vertical flow spray absorber shortened duct length. Eventually it helps to accomplish a lower construction cost.

Performance
The absorption and forced oxidation mechanisms are the same as the conventional wet limestone gypsum FGD technology, so it is possible to achieve more than 80% of SO 2 removal efficiency. Also, because of higher gas velocity under the same conditions of gas versus liquid ratio, it is possible to maintain the same SO 2 removal efficiency.
In the method of horizontal spray tower, it is poss ibl e to achieve high dust removal efficiency as in the vertical spray tower.