Removal of SOX from Industrial Gases
A. R. Warade*
Dept. of Chemical Engineering, Pravara Rural Engineering College, Loni, Dist: Ahmednagar (MS)-413736
*Corresponding Author E-mail: arwarade@gmail.com
ABSTRACT:
This paper deals with the Sulphur dioxide pollution & its removal. In this study statistical data on worldwide, regional and local emission of SOX have been analyzed. In this study, comparison of typical and allowed emission of SOX has been achieved. In this paper main sources of SOX have been identified with environmental effect. A brief study to analyze a typical case of heterogeneous chemical equilibrium has been done. Flue Gas Desulphurization (FGD) process has been designed with elementary FGD cost elimination.
KEYWORDS: SOX pollution, Statistical data, Sources, Flue Gas Desulphurization (FGD), Cost estimation.
INTRODUCTION:
Collect and analyze statistical data on worldwide, regional, and/or local emission of sulfur dioxide.
This paper updates estimates of global anthropogenic sulfate emissions through 1993 and provides a time series of estimates for each year. We extend the methodology developed by Hameed and Dignon (1989) to include emissions from copper smelting and use estimates for US emissions between 1900 and 1940 which were not previously available. Emissions since 1986 show a slight rise due to an increase in the US followed by a slight decline and a continuing decline in emissions from the other OECD countries. Emissions from the rest of the world peak in 1989 and show a steep decline associated with recession and economic restructuring in Eastern Europe. The various emissions series are consistent with the historical record for the atmospheric concentration of non-sea sulfates that is reconstructed from an ice core recovered from Greenland.
Identify the SOX removal targets by comparing typical and allowed emission.
The OECD Secretariat has developed a multi-country, multi-sector, dynamic applied general equilibrium (AGE) model called GREEN to quantify the economy-wide and global costs of policies to curb emissions of carbon dioxide (CO2) 2. The possibility of extending GREEN via the introduction a module to calculate Sox emissions was envisaged in order to give an indication of potential secondary benefits of carbon abatement associated with emission reduction in those gases 3. Computation of Sox emissions raises many additional difficulties. Namely, emission factors have to be specific to fuels, sectors and regions 4. Moreover, country specific regulations and abatement technologies and their changes through time must also be taken into account. This implies that Sox emission coefficients cannot be assumed to remain constant. The sectoral breakdown of GREEN does not allow for a comprehensive treatment of Sox emissions. In addition, within the framework of GREEN, technological progress is exogenous. Thus, changes in SOX emissions can only be modeled as an indirect impact of policies to curb CO2 emissions through substitution among fossil fuels, introduction of back-stop energies or energy conservation effects 5.
Main Sources of SOX
Main sources of Sulphur dioxide are the combustion of fuels, especially coal. Therefore its concentration in the air depends upon the sulphur content of the fuel used for heating and power generation. The sulphur content of fuel varies from less than 1% for good quality anthracite to over 4% for bituminous coal.
· Most crude petroleum products contain less than 1% sulphur, a few contain up to 5%. Refining processes tends to concentrate sulphur compounds in the heavier fractions. Fuel gas also contains sulphur but in small quantity.
· About 80% of the sulphur in coal and nearly all that in liquid and gases in the form of sulphur dioxide.
· Another common sources of sulphur dioxide in the atmosphere, is metallurgical operations. Many ores, like zinc, copper, and lead, are primarily sulphides. During the smelting of these ores, sulphur dioxide is evolved in stack concentration of 5-10% (sulphur dioxide). But this can be recovered in the form of sulphuric acid.
· The natural sources such as biological decay and sea spray emit about 130 million tones of sulphur per year and the anthropogenic sources such as coal combustion, petroleum and smelting operations release an additional 132 million tones of SO2 annually into atmosphere.
· The largest single contribution to the total anthropogenic emissions, about 70% is made by coal combustion.
· Small amount of SO2 is probably present in gases emitted through volcanic activity.
ENVIRONMENTAL EFFECTS OF SOX ON HUMAN
SO2 is comparatively soluble gas and is absorbed completely soon after breathing in upper respiratory tract.
A low level concentration inhalation of SO2 results in temporary spam of the smooth muscle of bronchioles.
Higher concentration of SO2 causes increased mucus production on the wall of upper airways.
SO2 not only constrict bronchi but also damage protective mechanism of respiratory tract like cilia.
In general, the effect due to SO2 is more acute than chronic.
This method is applicable to conventional combustion chambers; finely ground limestone (CaCO3) is injected directly into the combustion chamber. The limestone is calcined to CaO by heat of combustion and it react with SO2 contained in flue gas to form sulphites and sulphates.
The solid reaction products, unreacted material and fly ash are removed either by dry collector or wet scrubbing. Reaction (3) is the most favorable at temperature above 1000oC; however, limestone does not completely react with SO2 to produce a stichometric yield of CaSO4 because of insufficient gas residence time in the combustion chamber. Furthermore at temperature above 1200oC, CaSO4 is unstable and SO2 removal either by sulphites and sulphate formation is inhibited. As a result the sulphur removes less than half of sulphur dioxides.
The process chemistry is basically the same as that in case of dry lime stone technique but the operating temperatures are of the order of 700-1000oC within the bed due to high heat transfer capabilities of the fluidized beds. These temperatures are much lower than those in the conventional combustion chamber (1500oC). At 900oC, CaCO3 dissociates into CO2 and CaO and the latter reacts with SO2 to form CaSO4 .The degree of desulphurization in this process may amount to 90%, which is more than twice as high as that in dry lime stone technique.
MATERIAL AND METHOD:
Briefly analyze flue gas desulphurization (FGD) process
WET FGD SYSTEMS
Wet FGD Systems are normally based upon the type of Scrubbing Fluid used. There are number of absorbents used for removal of SO2. Depending on the absorbent the Wet FGD Systems can be classified as:
A. Lime Based System
B. Caustic Based System
C. Double Alkali System
D. Ammonia Based System
E. Sea Water Based System
Above mentioned systems are followed normally on the commercial scale. 85 % of systems installed for Flue Gas Desulphurization worldwide are of Wet Type. Normally Lime is used as absorbent as it is one of the most economical absorbent available.
In the forthcoming Section a detailed System Description on Lime Based FGD is presented.
DESCRIPTION OF LIME BASED FLUE GAS DESULPHURIZATION SYSTEM:
Lime based FGD system can be broadly divided in three sections:
1. Quenching Section
2. Absorber Section
3. By-product Treatment Section
1. Quenching Section:
Normally the Flue Gases coming out of combustion process are at elevated temperatures. These gases are cooled down to the saturation temperature in the quenching section. A Direct contact type Quencher or a Venturi Scrubber is used for this purpose. Because of the corrosive nature and elevated temperatures special steel is used for the construction of the Quenching Section.
2. Absorber Section:
The quenched gases enter SO2 Scrubber at bottom and travel upward. Lime Slurry is sprayed in counter current fashion in this tower by means of nozzles arranged in stages. The circulating fluid absorbs SO2 from flue gas. The scrubbing fluid is collected at bottom tank. The scrubbing liquor is recirculated by means of pumps. The pH of scrubbing liquor is measured and lime slurry is added to bottom tank to maintain the pH value between 5-6. A part of scrubbing liquor is bled off to remove calcium compounds (CaSO4, CaSO3) formed due to reaction of SO2 with lime slurry. The cleaned gases pass upward in the scrubber over mist eliminators to separate entrained droplets. The clean gases then leave the scrubber at top. The scrubber is made in Stainless Steel or Rubber Lined construction.
3. By-Product Treatment Section:
The bleed from recycle tank is taken off at predetermined concentration and quantity by separate bleed pump. In case of throw away system the bleed is sent to clients effluent treatment plant. In case of gypsum recovery system oxidation air is injected Roots Blower in recycle tank and the bleed is sent to dewatering unit like centrifugal decanter or clarifier or Vacuum Belt Filter, etc. Gypsum is removed in the form of wet cake.
Analyze elementary FGD cost estimation
The objective of an economic evaluation is to identify the proposal with the lowest total cost considering both the initial capital cost and the annual cost for operation and maintenance. This simply stated objective can be a difficult and time consuming process because it assumes that all the capital and annual costs can be identified and quantified.
The first step in the economic evaluation is to identify the capital and annual costs of each proposal. After these costs are quantified, economic evaluation methods are applied to compare the total costs of owing and operating the FGD system alternatives.
1) Determination of Total Initial Capital Cost
It is the vital to economic evaluation process for the initial capital costs of all the alternatives to be evaluated on a technical basis. In order to accurately compare the total initial capital costs of several proposals, the following additional cost items must be quantified and added to the base proposal cost:
· Technical adjustment cost
· Scope-of-supply adjustment cost
· Differential support equipment adjustment cost
· Economic adjustment cost
2) Determination of Annual Operating and Maintenance Costs
Annual operating and maintenance costs encompass the recurring costs associated with operation of an FGD system and include the following:
· Electric power costs
· Reagent and additives costs
· Byproduct disposal costs
· Makeup water costs.
· Wastewater treatment costs
· Operating labor costs
· Maintenance Material and Labor costs
CONCLUSION:
In this paper we discuss the main sources and effects of SOX and analyze the statistical data on worldwide, regional and local emission of SOX. Along with we also analyze the typical case of heterogeneous chemical equilibrium. We also discuss the Flue Gas Desulphurization (FGD) process and its cost elimination.
REFERENCES:
Environmental Pollution control Engineering, By C.S.Rao (page no. 60 and 242)
Pollution control in Process industries, By S.P. Mahajan, (Page no.24)
Air pollution, By M.N.Rao and H.V.N. Rao (Page no.46, 260)
Report on Inter Agency Project Assesment and mgt. of health and environmental risk from industries in trans thane Creek Area (Indian case study) may 1998
www.lanl.gov/projects/cctc/resources/library/bibliography/misc/bibm-tgd.html
www.spargerlink.com/so2 /
www.mpcb.com
www.cpcb.com
www. quencel.gov
www. Carbon abentment .gov
Received on 05.11.2011 Accepted on 18.11.2011
©A&V Publications all right reserved
Research J. Engineering and Tech. 2(4):Oct.-Dec. 2011 page195-198