Spray dryer absorbers

Wet/dry process. Lime slurry absorbs SO2 in vertical spray dryer forming CaSO —CaS, H2O evaporated before droplets reach... 

SO2 absorbed into Na2C02 solution in spray dryer, producing dry Na2S02 particles. 

Figure 30-2 illustrates a wet SO2 desulfurization system using a spray tower absorber. Figure 30-3 illustrates a rotary atomizer injecting an alk ine slurry into a spray dryer for SO2 control. 

Capital Equipment Costs The initial investment made by a utility to control pollutant emissions is in the equipment comprising the flue gas treatment system. The amount of the investment in this equipment purchase is directly related to the size and complexity of the equipment itself. The vessels required in this application must be large because of the volume of flue gas that must be treated. Some equipment, such as spray dryers or absorbers, can be 20 to 50 feet in diameter, and spray towers can be over 100 feet in height. Considerable preparation must go into sizing these vessels properly in order to maximize gas-sorbent contact time and minimize scaling and other operational problems. Much of this equipment, especially in wet systems, must be constructed of costly corrosion-resistant materials. Elaborate valving systems must be set up to control flows to the system. 

Spray drying has become increasingly important in recent years as an alternative to wet scrubbing for sulfur dioxide control. In the spray dryer the sulfur-containing flue gas is contacted with a fine mist of an aqueous solution or a slurry of an alkali (typically Ca(0H)2 or soda ash). The sulfur dioxide is then absorbed in the water droplets and neutralized by the alkali. Simultaneously, the thermal energy of the gas evaporates the water in the droplets to produce a dry powdered product. After leaving the spray dryer the dry products, including the fly ash, are removed with collection equipment such as fabric filters or electrostatic precipitators. 

SDA [Spray Dryer Absorber] A flue-gas desulfurization process in which an aqueous suspension of lime is injected into a spray dryer. Basically similar to DRYPAC. Developed by Niro Atomiser, Denmark. In 1986, it was in use in 16 plants in Austria, Denmark, Germany, Italy, Sweden, China, and the United States. 

It is typical to install the spray-dryer before the plant fly-ash collector, so that the existing dust control equipment can be used to collect the used absorbent. Slaked lime [Ca(OH)2] is the most common absorbent, although sodium carbonate (Na2C03) is used in some plants. Spray-dryers have also been used with regener-able magnesium oxide absorbent. " ... 

Spray-dryers are simpler and more compact than conventional wet scrubbers and have a lower capital and operating cost. Also, they do not produce large quantities of wastewater, and the spent absorbent is dry, thereby eliminating the need for thickening and filtration of the sludge. However, if the same dust precipitator is used for both the fly-ash and fhe spray-dryer product, the mixture of fly-ash and spent absorbent that they produce is unmarketable, and must be disposed of. Also, they require more expensive absorbents than conventional wet scrubbers. They are most suitable for retrofitting small plants that burn medium-sulfur coals, where capital costs and space restrictions are more of a consideration. ... 

An improved magnesium oxide (MgO) flue gas desulfurization process and its comparative economics are described. Innovations made include the use of a spray dryer, a cyclic hotwater reheater, and a coal-fired fluidized-bed reactor for regeneration of the MgO absorbent. Several technical concerns with the proposed design are addressed, including fly ash and chloride buildup. The economic evaluation shows the process to have a capital investment of about seven percent less than that of a conventional MgO scrubbing process and a 40 percent smaller annual revenue requirement. Finally, a sensitivity analysis is shown relating annual revenue requirements to the byproduct sulfuric acid price credit. 

Figures 1 and 2, respectively, show the old and new processes. The major innovations are use of (1) a spray dryer absorber in place of the wet venturi, absorber, centrifuge, rotary dryer combination (2) a cyclic hot-water reheat system interconnecting thermally the calciner product solids and the effluent gas from the spray dryer absorber and (3) a coal-fired, fluidized-bed reactor for conversion of magnesium sulfite (MgSO ) and sulfate (MgSO ) to MgO and SO gas. Otherwise, the two systems are very similar, utilizing a regenerable absorbent to recover the sulfur material as a usable commercial grade of concentrated sulfuric acid.

The initial and perhaps most critical design assumption is that both the MgO and the fly ash, which are carried out of the calciner and into the MgO product cyclone, have the same particle size distribution and density. This design (i.e., no possibility of physical separation of MgO and fly ash) is a conservative design assumption and adds complexity to the FGD process. It results in the need to recirculate large quantities of the MgO/fly ash mixture through the spray dryer and the calciner. In order to keep the MgO/ash recycle streams to a reasonable size, the mechanical collectors in the main flue gas ducts upstream from the SO2 absorber, which were used in the initial design because they are relatively inexpensive but yet remove only 80% of the fly ash, had to be replaced with the 95% efficient ESP mentioned earlier. 

The design for the MgO spray dryer-absorber is based on the data currently available from the lime-based spray dryer systems since data on an MgO-based spray dryer-absorber are currently not available. This assumption is considered to be reasonable since both lime and MgO produce an absorbent slurry and both are Group II element oxides, but primarily because no other data are available. 

The MgO system is based on a conservatively designed lime system (i.e., a MgO stoichiometry of 1.8 moles MgO/mole SO2 absorbed, a 20 F° approach to the flue gas saturation temperature in the spray dryer, etc.). All of the spray dryer design considerations will therefore have to be evaluated in a spray dryer MgO pilot plant. 

During phase 11, evaporation of water continues and SO is absorbed by the moist particle which is composed of a number of the elementary solid pedicles. The rale or absorption is determined by the diffusion of SO through the interstices between elementary particles and through the layer of spent reactant (CaSOj O.5 H 0) which builds up on each elementaty panicle, Although a quantitative model for spray dryer abeorption has not been developed, it has been observed that SO, absorption efficiency increases with increased lime/SOj ratio, reduced drop size, and reduced approach to water saturation of the ontlet gas. A plot of absorption efficiency as a function of lime/S02 mule ratio for a typical spray dryer SO absorption system is given in Fig. 6.4-12. ... 

Air-pollution control devices are widely but not universally used as follows Of 69 mass-bum rrrrits, 51 liave SDAs (spray dryer absorbers or scrabbers), 1 has a WS (wet scmbber), 0 have a C (cyclone), 9 have Lis (lime injection), 49 FFs (fabric Alters), 22 ESPs (elecAostatic precipitators), 25 SNCRs (selecAve noncatalytic reduction forNOx conAol), and28 CIs (activated carbon injections) of 13 modular systems, 2 have SDAs, 3 WSs, 1 has a C, 2 have Lis, 3 FFs, 7 ESPs, none has an SNCR, and 2 have CIs of 13 RDFs (refuse-derived-fuel) facilities... 

When separate vessels are used, one or more process chambers are inserted in the flue gas ductwork, and various sorbents are injected to remove the pollutants. The separate vessels provide a longer residence time for the absorbent to react with the gas, and pollutant capture is greater. This approach, at some increase in cost over the in-duct injection procedure, has the potential of capturing more than 90% of the pollutants. Technologies such as the spray dryer and SCR represent approaches that use separate vessels. 

The major limitation of rotary dryers is the relatively low volumetric load ratio of the drum, which typically ranges from 3% to 16%. Because sound energy is absorbed in the bulk of particulates, it is difficult to secure the efficient use of high-intensity sound energy in large rotary dryers. Much better energy utilization is in the case of spray dryers since sonic irradiation intensifies heat and mass transfer but also enhances Uquid atomization (see Figure 13.9). 

ESP, Electrostatic precipitator IWS, ionizing wet scrubber SDA, spray dryer absorber FF, fabric filter. 

Types of equipment classified as spray contactors include countercurrent spray columns, venturi scrubbers, ejectors, cyclone scrubbers, and spray dryers. The use of spray dryers as absorbers is of particular interest in the removal of sulfur dioxide from hot flue gas (see Chapter 7). 

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