Spray dryer scrubbing

The most popiilar dry scrubbing systems for incinerators have involved the spray drying of hme slurries, followed by dry coUection in electrostatic precipitators or fabric filters. Moller and Christiansen [Air Poll. Cout. Assoc. 84-9.5 (1984)] published data on early European technology. Moller et al. [U.S. Patent no. 4,889,698 (1989)] describe the newer extension of that technology to include both spray-dryer absorption and dry scrubbing with powdered, activated carbon injection. They claim greatly improved removal of mercury, dioxins, and NOx. 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

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. 

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. 

Since the MgO spray dryer FGD process collects particulate matter as an inherent part of the FGD system, it can be evaluated either as a combined particulate-S02 removal system or, with the inclusion of an ESP credit, as an FGD-only process. In this study the spray dryer MgO process evaluation is based on the combined particulate-502 removal system. The primary reasons are that the combined system is easier to explain and that similar evaluations for the limestone scrubbing/ESP and conventional MgO/ESP systems are available from previous studies (2, jj). The conventional MgO/ESP process cost information from the previous study was updated using area scale factors, relative product and gas rates, etc., to put the results on a consistant basis with the current evaluation of the spray dryer MgO process. 

The capital investment for the spray dryer MgO FGD process is 139.5M ( 279/kW) in mid-1982 dollars while that for a comparable limestone scrubbing/ESP process is 122.0M ( 244/kW) in mid-1982 dollars. The capital investment for the conventional MgO FGD process (including particulate control) is 149.7M ( 299/kW) in mid-1982 dollars. These costs are... 

The capital investment for the spray dryer MgO process is approximately 14K higher than that for the comparable limestone scrubbing process. This is not an unexpected result since the MgO process is a regenerable system while the limestone scrubbing process is a throwaway system. 

Likewise, the capital investment for the spray dryer MgO process is approximately 7 percent lower than that for the conventional MgO scrubbing process ( 279/kW vs. 299/kW). The conventional MgO scrubbing process has more equipment, and hence larger investment costs, particularly in the areas of chloride purge, slurry drying, and slurry processing equipment which are not needed in the spray dryer-based system. 

The first-year annual revenue requirements for the spray dryer MgO FGD process are 28.8M (10.47 mills/kWh) in mid-1984 dollars. The levelized annual revenue requirements for the spray dryer MgO process are 36.1M (13-13 mills/kWh). The first-year annual revenue requirements for the comparable limestone scrubbing process are 32.4M (11.78 mills/kWh) in mid-1984 dollars. The levelized annual revenue requirements for the limestone scrubbing process are 45.2M (16.43 mills/kWh). The first-year annual revenue requirements for the conventional MgO FGD process (including particulate control) are 40.4M (14.69 mills/kWh) in mid-1984 dollars. Levelized annual revenue requirements are 56.7M (20.61 mills/kWh). These costs are summarized in Table III. The complete details are presented in Tables A-IV, A-V, and A-VI in the Appendix. 

As is apparent from this figure the first-year annual revenue requirements can vary somewhat, depending on the price received from the byproduct sulfuric acid. The base-case credit of 65/ton of 10055 H2SO4 results in a first-year annual revenue requirement of 10.47 mills/kWh for the spray dryer MgO process and 14.69 mills/kWh for the conventional MgO process. At a byproduct credit of 35/ton which is approximately equivalent to the price of sulfuric acid in the past (or which could be the 1984 price netted after transportation costs are subtracted for a utility situated far from the ultimate consumer), the first-year annual revenue requirements for the spray dryer MgO process rise nearly 10 percent to 11.53 mills/kWh. This cost, although slightly lower than that for the limestone scrubbing process, is essentially equivalent given the accuracy associated with this study. For... 

The spray dryer MgO process appears to have a slight advantage (about 10 ) over a comparable limestone scrubbing process in terms of both first-year and levelized annual revenue requirements. This new MgO process, however, has a higher capital investment than the limestone scrubbing process. 

Four chapters address alternatives to throwaway slurry scrubbing. The development of the limestone dual alkali process is reviewed. Two chapters present results related to dry scrubbing with nahcolite or lime. A conceptual design and economics are given for MgO scrubbing using a spray dryer. 

The ACP system combines a sulfur dioxide scrubbing system based on spray dryer technology with a regeneration system based on a unique reduction step coupled to chemical recovery and Claus technologies. This combination results in an efficient and reliable process for application to sulfur dioxide pollution problems. The remainder of this paper discusses the details of the process and typical installation characteristics and also presents process economics which indicate that the ACP system is economically feasible a well as technically sound. 

Fig. 7.59 is the simplified flow sheet of a co-current spray drying system with recirculation of fines that is another possibility to accomplish agglomeration in a spray dryer. Often, the fines are returned to the liquid feed bin and redissolved or redispersed (see, for example Fig. 7.68b and c). However, if fine particles are returned to the spray zone, as shown in Fig. 7.59, they are captured by the droplets (similar to a wet scrubbing effect) and incorporated in the product. 

The main atmospheric pollution problem in soap manufacturing is odor. Odors emanating from the spray dryer may be controlled by scrubbing with an acid solution. The production of soap powder by spray drying is the single largest source of dust in the manufacture of synthetic detergents. 

Semidry Scrubbers. Semidry scrubbing systems start with a wet scrubbing medium (usually a lime slurry or a soda ash solution) and produce a dry waste product.The central device of a semidry scrubbing system is the spray dryer, similar to equipment that has been used for years in industry to manufacture everything from powdered coffee and milk to paint pigments and detergents. 

In a semidry scrnbbing system, the solntion or slurry is dispersed by nozzles or rotary atomization systems into a fine cloud of droplets.These droplets are brought into contact with a hot gas stream (and herein lies a disadvantage to semidry scrubbing systems) that proceeds to evaporate the water in the droplets. As the water evaporates, the acids in the gas stream react with the alkali material in the drying droplets and neutralize them, forming a fine powder. Most of this powder is removed from the bottom of the spray dryer, while the remainder is entrained in the gas stream and carried out to either a fabric filter or an electrostatic precipitator (ESP). 

DuPont recently commercialized a new process for the oxidation of butane to maleic anhydride using a CSR. The maleic anhydride is scrubbed from the reaction zone as maleic acid and then hydrogenated to tetrahydrofuran. The advantages are well documented in the references. A key to this process was the development of an attrition-resistant catalyst obtained by spray-drying a solution of micronized vanadium-phosphorus-oxygen (VPO) catalysts in polysilicic acid. In the spray dryer, a porous shell of very hard silica is formed to protect the soft VPO catalyst. 

Felsvang, K., Brown, B., and Horn, R., 1991, Dry Scrubbing Experience with Spray Dryer Absorbers in Medium to High Sulfur Service, Proceedings Acid Rain Retrofit Seminar The Effective Use of Lime, sponsored by the National Lime Assoc. (Arlington, VA), Philadelphia, PA, Jan. 9-10, pp. 139-186. 

Semi-dry scrubbers involve a chemical reaction with a wet slurry, and a dry scrubbing of the gases with dry collection of co-products. Typically the process is used to treat acid gases in a spray dryer, coupled with a pulse-jet fabric filter. In the scheme shown in Figure 7.16, with a vertical downflow spray dryer, an alkahne slurry is injected to neutralize the adds present in the flue gas. The gas temperature is controlled by the flow rate of the dilution water. 

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

To produce the 90-94% CaCli solid product, a portion of the concentrated liquid was sprayed into a dryer and directly heated by very hot flue gas. Solids were removed from the dryer, cooled and screened to the desired particle size, while the exhaust gas was scrubbed and then vented. The oversize was crushed and returned to the screen, while the fines were recycled to the 45% feed solution, or compacted into almond-shaped pellets. The 90-94% CaCl2 pellets were also shipped in either bulk or bags. 

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