ABSORPTION FLUE GAS DESULFURIZATION CREATED BY ZELİHA SALDIR ITIR SARI TUĞBA BEĞENDİ MUSTAFA ÖZGİRAY YAKUP TURGUT INTRODUCTION Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scrubbing a gas mixture with a suitable liquid . The fundamental physical principles underlying the process of gas absorption are the solubility of the absorbed gas and the rate of mass transfer. One or more of the constituents of the gas mixture dissolves or is absorbed in the liquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the liquid or the solvent, and in other cases , it reacts with the liquid chemically. The purpose of such scrubbing operations may be any of the following : gas purification (eg , removal of air pollutants from exhausts gases or contaminants from gases that will be further processed) , product recovery , or production of solutions of gases for various purposes. Gas absorption is usually carried out in vertical counter current columns as shown in figure 1.The solvent is fed at the top of the absorber , whereas the gas mixture enters from the bottom .The absorbed substence is washed out by the solvent and leaves the absorber at the bottom as a liquid solution . The solvent is often recovered in a subsequent stripping or desorption operation . This second step is essentially the reverse of absorption and involves counter current contacting of the liquid loaded with solute using and inert gas or water vapor . The absorber may be a packed column , plate column , spray column , venturi scrubbers , bubble column , falling films , wet scrubbers ,stirred tanks PACKED COLUMN The packed column is a shell either filled with randomly packed elements or having a regular solid structure designed to disperse the liquid and bring it Dumped-type packing elements come in a great variety of shapes and construction materials, which are intended to create a large internal surface but a small pressure drop. Structured ,or arranged packings may be made of corrugated metal or plastic sheets providing a large number of regularly arranged channels ,but a variety of other geometries exists. Packing materials may be classified as follows, rock 3-coke 4-stonaware shapes 4a-raching rings 4b-berl saddle 4c-sprial rings 1-wood slats 2-broken 4d-grid bloks 5-miscalloneous material Rashing rings are the most widely used form of tower packing. They are cylindrical rings, of the some length as the diameter of the cylinder and with the walls as thin as the material will permit. Rashing rings are almost always dumped into the tower at random and not stacked regularly. They offer the best combination of low weight per unit volume,free volume,free cross section and total surface of any type of packing. A packed bed column contains a support plate, a liquid distributor, and a mist eliminator. Mist eliminators are used to condense any vaporized scrubbing liquid. Support plates hold the packing in place. The advantages of packed columns include simple and,as long as the tower diameter is not too large,usually relatively cheaper construction. These columns are preferred for corrosive gases becuase packing, but not plates, can be made from ceramic or plastic materials. Packed columns are also used in vacuum applications because the pressure drop, especialli for regularly structured packings, is usually less then through plate columns. Usage examples Packed columns are used mostly in air pollution control.. The water soluble ethylene gas ishydrolyzed to ethylene gylcol. Packed columns are also used in the chemical ,petrochemical,food, pharmaceutical,paper, and aerospace industries. TRAY COLUMN Tray absorbers are used in applications where tall columns are required, because tall, random-type packed towers are subject to channeling and maldistribution of the liquid streams. Plate towers can be more easily cleaned. Plates are also preferred in applications having large heat effects since cooling coils are more easily installed in plate towers and liquid can be withdrawn more easily from plates than from packings for external cooling. Tray columns have got some disadvantage. These are slow reaction rate processes, higher pressure drops than packed beds and plugging and fouling may be occur. Tray absorbers are used in applications where tall columns are required,because tall,random-type packed towers are subject to channeling and maldistribution of the liquid streams. Plate towers can be more easily cleaned. Plates are also pereffered in applications having large heat effects since cooling coils are more easily installed in plate towers and liquid can be withdrawn more easily from plates than from packings for external cooling. Usage Examples Tray columns are used in a refinery dehexanizer to decrease the benzene content in the naptha feed to the process. This results in lower automobile exhaust emissions. STIRRED TANKS If the absorbtion process includes a slow liquid-phase chemical reaction, or close control of the process is needed, stirred tanks are used.the gas is introduced directly into the liquid and mixed by the stirred in a stirred tank. Usage examples Stirred tanks can be used in lime slurry carbonation,paper stock chlorication, regular oil hydrogenation,fermentation broth aeration,penicilin production, citric acid production,and aeration of activated sludge. BUBBLE COLUMN Structured catalytic bubble columns are new, very promising types of multiphase reactors. Their configuration lies basically between slurry reactors and trickle bed reactors. The solid phase, consisting of catalyst particles, is enclosed in fixed wire gauze wraps, which are mounted along the height of the column. The gas phase is dispersed into the liquid phase and it flows in the empty passages between adjacent envelopes. The liquid phase may be operated in a batch manner or it may also circulate in co-current or counter-current manner to the gas flow. The main advantages of this reactor type with respect with the conventional slurry bubble column are: 1.no problems for separating catalyst from the liquid; 2.improved conversion and selectivity due to staging of the liquid phase; 3.no scale up problems because the hydrodynamics is dictated by the size of the open channels of the catalytic structure. Usage Examples Bubble columns can be used to purify nitroglycerin with water, in the chemical industry for hydrogenation, oxidation, chlorination, and alkylation, and in the biotechnological field for effluent treatmet, single-cell protein productin, animal cell culture, and antibiotic fermentation. Bubble columns can be used for radioactive elements because there are no moving parts. Venturi Scrubbers Adjustable-throat venturi scrubber with movable plate Venturi scrubbers can be used for removing gaseous pollutants; however, they are not used when removal of gaseous pollutants is the only concern. The high inlet gas velocities in a venturi scrubber result in a very short contact time between the liquid and gas phases. This short contact time limits gas absorption. However, because venturis have a relatively open design compared to other scrubbers, they are very useful for simultaneous gaseous and particulate pollutant removal, especially when: •Scaling could be a problem •A high concentration of dust is in the inlet stream •The dust is sticky or has a tendency to plug openings •The gaseous contaminant is very soluble or chemically reactive with the liquid To maximize the absorption of gases, venturis are designed to operate at a different set of conditions from those used to collect particles. The gas velocities are lower and the liquid-to-gas ratios are higher for absorption. For a given venturi design, if the gas velocity is decreased, then the pressure drop (resistance to flow) will also decrease and vice versa. Therefore, by reducing pressure drop, the gas velocity is decreased and the corresponding residence time is increased. Liquid-to-gas ratios for these gas absorption applications are approximately 2.7 to 5.3 l/m3 (20 to 40 gal/1000 ft3). The reduction in gas velocity allows for a longer contact time between phases and better absorption. Increasing the liquid-to-gas ratio will increase the potential solubility of the pollutant in the liquid. Flooded elbow Venturi scrubbers can have the highest particle collection efficiencies (especially for very small particles) of any wet scrubbing system. They are the most widely used scrubbers because their open construction enables them to remove most particles without plugging or scaling. Venturis can also be used to absorb pollutant gases; however, they are not as efficient for this as are packed or plate towers. Venturi scrubbers have been designed to collect particles at very high collection efficiencies, sometimes exceeding 99%. The ability of venturis to handle large inlet volumes at high temperatures makes them very attractive to many industries; consequently, they are used to reduce particulate emissions in a number of industrial applications. This ability is particularly desirable for cement kiln emission reduction and for control of emissions from basic oxygen furnaces in the steel industry, where the inlet gas enters the scrubber at temperatures greater than 350 °C (660 °F). Venturis are also used to control fly ash and sulfur dioxide emissions from industrial and utility boilers. Falling film With high efficiency in absorbing HCl (hydrochloric) gas, H2S, HF, SO2, NH3 gas, graphite falling film absorbers comprise of absorption liquid distributor, cooling and absorption section and Gas-Liquid separator. Absorption liquid distributor is for film forming and flow into absorption tube in cooling and absorption section. On request, cooling and absorption section has two models basis of its heat transfer unit -Shell and Tube and Block. Gas-Liquid separator is to separate tail gas and product. Its convincing advantages and disadvantages is following: - Advanges Disadvanges High efficiency of Restricted by pressure absorption drop Film breakup Low outlet temperature Flooding No need after-cooling Low flow resistance Easy maintenance SPRAY COLUMN Spray columns are differetal contactors. The liquid stream enters the coloumn through one or more spray nozzles at different heights in the column. The droplets formed provide a large surface area for exposure to the gas stream,with smaller droplets resulting in a greater Exchange area. The liquid and gas streams can flow counter-currently or in paralel. An optimum droplet velocity is essential because low velocity will lead to low contact or turbulence and high velocity may cause flooding. A mist eliminator is used to separate any liquid that is entrained into the gaseous phase. Spray columns are used to absorb SO2 from coal-fired boiler exhaust gases. WET SCRUBBER Wetted packed towers are the simplest and most commonly used approaches to gas scrubbing. The principle of this type of scrubber is to remove contaminants from the gas stream by passing the stream through a packed structure which provides a large wetted surface area to induce intimate contact between the gas and the scrubbing liquor. the contaminant is absorbed into or reacted with the scrubbing liquor. The packing of the tower is normally a proprietary loose fill random packing designed to encourage dispersion of the liquid flow without tracking, to provide maximum contact area for the 'mass transfer' interaction and to offer minimal back pressure to the gas flow. The reactivity between the contaminant and the scrubbing liquor influences the system designer's determination of gas and liquor flow and the height and diameter of the packed bed. A demister is fitted at the top of the tower to prevent entrainment of droplets of the scrubbing liquor into the extraction system or stack. Wetted packed towers can be designed for very high efficiencies with relatively low capital and running costs. The low pressure drop associated with packed bed scrubbers permits the use of smaller more economical fans. Although efficiency may be affected, a packed tower will usually function when gas or liquor flows vary from its original design parameters. Usage examples Wet scrubbers are used by the food industry,such as in cheese proessing for dust and ambient moisture removal. FLUE GAS DESULFURIZATION SYSTEM Gas desulfurization can be accomplish by wet, dry, or alkali scrubing.These methods are covered in this section. THE WET FLUE-GAS DESULFURİZATİON SYSTEM The wet FDG system, also called a wet scrubber, is cammonly based on low-cost lime-limestone in the form of an aqeous slurry.this slurry, brought into intimate contact with the flue gas by various technique, absorbs the SO2 in it. The wet scrubbing process was orriginaly developed in the 1930s by Imperial Chemical Industries (ICI) in England.In the modern version of the process, the flue gas is scrubbed with a slurry that contains lime (CaO) and limestone (CaCO3) as well as the salts calcium sulfite (CaSO3 .2H2O)and calcium sulfate (in hydrate form, naturel gypsum, CaSO4 .2H2O).The SO2 in the flue gas reacts with the slurry to form additional sulfite and sulfate salts, which are recycled with the addition of fresh lime or limestone. The chemical reaction aren’t known with certainty but are thougth to be; CaO + H2O ----------- Ca(OH)2 Ca(OH)2 + CO2 --------- CaCO3 +H2O CaCO3 + CO2 + H2O------- Ca(HCO3)2 Ca(HCO3)2 + SO2 + H2O --------CaSO3 .2H2O + 2CO2 CaSO3 . 2H2O + 1/2O2 ------------ CaSO4 .2H2O One technique employs a spray tower downstream of the particulate-removalsystem (electrostatic precipitator or fabric fitler). The flue gas is drawn into the spray tower by the main steam-generator induced-draft fan where it flows in countercurrent fashion to the limestone-slurry spray. A mist eliminator at the upper exit of the tower removes any spray droplets entrained by the gas. The gas may have to be slightly reheated before it enters the stack to inprove atmospheric dispersion. The sprayed limestone slurry collects in the bottom of the tower and is recirculated back to the spray nozzles by a pump. A system of feed and bleed charges a fresh slurry, under pH control , and discharges an equivalent amount from the circulating slurry. The fresh slurry is prepared by mixing the lime-limestone with water in a ‘slaker-grinder’ and stirred in a slurry tank. The bled slurry is sent to a dewatering system, which is in the form of thickeners and filters or centrifuges, where water is removed from the calcium-sulfur salts. The reclaimed water is used to help make fresh slurry. The wet scrubber has the advantages of high SO2 removal efficiencies, good reliability, and low flue-gas energy requirements.In addition, it is capable of removing from the flue gases residual particulates that might have escaped the particulate-removal system. A main disadvantages is the build up of scale in the spray tower and possibilitiy of plugging. The prevention of such scale is essential to the reliable operation of the tower. Scaling occurs because both calcium sulfite and calcium sulfate have low water solubility, normally around 30 percent, and can therefore form supersaturated water solutions. A minimum liquid-to-gas ratio must therefore be used, its value depending upon the SO2 content of the flue gas and the expected extent of sulfite oxidation. Precipitation occurs at a finite rate, which necessitates holding the SO2-absorbing liquar in a delay tank after each pass. An insufficient delay time increases supersturation and promates scalling. Another tecnique for controlling scale is the use of seed crystals. These are calcium sulfite and sulfate precipitate crystals, in a supersaturated solution, that are maintained in the SO2-absorbing liquor. They provide sites around which preferential precipitation takes place and enhance the precipitation rate. Other disadvantages of the wet scrubers are the reheating of the flue gas, a larger gas pressure drop requiring higher fan power requirements than the dry FGD system (below), and typicallyhigher capital and operating costs. The waste material from wet scrubbers is a water- logged sludge that poses difficult and costy disposal problems. THE DRY FLUE-GAS DESULFURİZATİON SYSTEM Like the wet scrubber, above, the dry FGD system, also called a dry scrubber, utilizes an aqueous slıurry of lime, CaO, to capture flue gas SO2 by forming calcium sulfites and sulfates in spray absorbers. The slurry in the case, however, is atomized, usually by a centrifugal atomizer, into a fine spray that promotes the chemical absorption of SO2 and, because of the small spray paticle size, is quickly dried bye the hot flue gases themselves to a particulate suspension that is carried along with the desulfurized gas stream. The reaction particulates as well those carried by the flue gases (fly ash) are then removed, mainly by a fabric fitler, before the gas is drawn by the induced-draft fan to the stack. A major component of this system is the slurry- generating system. A ‘slaker’ meters lime and water into an agitated tank to prepare a slaked lime slurry which, in turn, is diluted by additional water and processed to remove inert impurities called grits, which are disposed of. The lime slurry is pumped to the spray absorber with the flow controlled by the amount of SO2 in the flue gas. Particulates both coming in with the flue gas and generated in the FGD are collected from the absorber and fabric-filter hoppers and sent to a recycling silo for disposal or for recycling of a portion of it with the slurry (depending upon the extent of original utilization of the reactant in the absorber). The recycled slurry is enriched by an alkaline material, such as CaO, MgO, K2O, or Na2O. The main advantages of the dry system are he dry, powdery nature of the waste material, which poses fewer and less costly disposal problems then the wet waste from the wet FGD system (thought these problems are still large), and the mechanical simplicity of the system. The main disadvantage is that the efficiency of SO2 removal is lower than that of the wet scrubber. 1979 NSPS (New Source Performances Standards) regulations, which specify only 70 percent SO2 removal in new plants, have encouraged the developmed of the system, however. Other disadvantages are the need for careful design optimization of the spray absorber and the slaker, and the storng dependence of collection efficiency on absorber outlet temperature, which neccessitates opereting as close as is safe to the saturation temperature that corresponds to the partial presure of the water vapor in the gas in order to avoid condensation (below the coresponding dew point). This poses problems with fitler-bag performance. SİNGLE ALKALİ SCRUBBİNG Clear water solutions of either sodium (usually in the form of sodium hydroxide, NaoH, or sodium sulfite, Na2So3) or ammonia (NH3) are excellent absorbers or SO2. The advantages of alkali scrubbing is tahat it avoids the scaling and plugging problems of slurry scrubbing by using alkaline earth. Ammonia scrubbing has the advantage that the scrubber product, ammonium sulfate, can be sold as a fertilizer, but the disadvantage that the process produces troublesome fumes. A well-developed sodium scrubber is the Welman- Lord SO2 recovery process, which has found use in powerplants, refineries, sulferic acid plants, and other industrial installations in the USA and Japan. The process utilizes a water solution of sodium sulfite (Na2SO3) for scrubbing and generates a concentrated SO2 (about 90%), in effect removing the SO2 gas from other flue gases. The flue gas from fossil powerplants (or nonferrous smelters) is first pretreated by cooling and removal of particulate matter, such as by electrostatic precipitators, prior to being sent to the absorber. In the absorber the water solition of sodium sulfite absorbs the SO2 in the pretreated flue gas to produce sodium bisulfite NaHSO3 according to SO2 + Na2SO3 + H2O ----------- 2NaHSO3 The desulfurized gas is reheated before going to the stack in order to improve atmospheric dispersion. The sodium bisulfite is sent to a forced-circulation evaporator-crystallizer via a surge tank. The evaporator- crystallizer is the herth of the system. The surge tank allows steady flow rates into it despite gas flow and concentration fluctuations. Through the application of low-pressure steam, such as from a turbine exhaust, the sulfite is regenerated in the form of a slurry according to 2NaHSO3 ----------- NaSO3 + SO2 + H2O The product SO2 may be utilized to produce liquid SO2 or sulfuric acid, on site or in a satellite plant, or to produce elemental sulfur. A well-known process for doing this is called the Claus process, which is based on the addition of H2S according to SO2 + 2 H2S ----------- 3S + 2 H2O NO REMOVAL A process for the removal of NO, also by the addition of H2S, is proposed. It is given by NO + H2S ----------- S +1/2N2 + H2O The combined removal of SO2 and NO is under study. In both reactions, the H2S must be completely consumed as it is a pollutant itself. In 1977 the system was estimated to add an additional $120/kW, or some 12 to 15 percent to the base capital cost of a powerplant. It was said operating costs would increase by about $60/MBtu. Most scrubbers in use by 1981 have been of the wet type. There is not sufficient experience with the dry type to establish which of the two may be selected by utilities in the future. Presently all scrubber systems are large and occupy a sizable area of a powerplant, have capital costs that run in the tens of millions of dollars for 500- to 1000-MW plants, and consume a sizable fraction of the gross electrical output of these plants. They also require a lot of maintenance , which results in the doubling of operation and maintenance personel and causes, consequently, larger operation and maintenance costs. In addition, they generate huge amounts of waste that has to be disposed of. There are two types of disposal of FGD wastes: wet disposal, called ponding, and dry disposal in landfills, which are getting scarce. In general utilities are not always eager to build these disposal systems. Nevertheless, some 19000 MW of FGD and sludge disposal systems were in operation, and 26000 MW were under construction or planned, in 1981. The Electric Power Research Institute (EPRI) has published the FGD Sludge Dİsposal Manual, which incorporates the latest waste-disposal technology and regulations and describes how to design an environmentally acceptable waste-disposal system and the options available for processing and disposal of the wastes.