Model for flue‐gas desulfurization in a circulating dry scrubber

Abstract

A simple model was developed to describe the absorption of SO, in a circulating d y scrubbing (CDS) process, which is a semidy, lime-based, flue-gas desulfurization (FGD) process that utilizes a circulating fluidized bed arrangement for contucting a sorbent with SO, -laden flue gas under "coo1side"conditions. The reaction chemisty is thought to be similar to that of spray-dying absolption. The liquid-phase mass-transfer coeficient was successfully modeled as a function of the sorbent particle spacing on the wetted surfaces. Gas-phase mass-transfer resistances were assumed to be insignificant. Due to the high surface area available in a CDS reactor, the evaporation rate of water @om the slury was modeled as "constant-rate "dying according to classic spray-dryer theory. However, the "falling-rate 'jand "difision "evaporation stages were negligible in CDS since sorbent particle bunching at the surface of the sluny is nonexistent. IntroductionWet flue-gas desulfurization (FGD) processes were the first SO, control systems to be developed on a commercial scale in the United States. These systems were initially plagued by reliability problems due to a combination of the early status of development at the time, poor design, and questionable operating procedures. In addition, most electric utility plant operators had no previous experience with "chemical plant" operation. By the late 1980s, most of these problems were overcome and some technological advances were made in the scrubbing area that improved wet FGD reliability. However, wet FGD systems remain costly to operate due to large work force requirements, the generation of large quantities of solid waste by-products that must be disposed, and high parasitic energy requirements.Wet scrubber systems are characterized by high liquid-togas ratios (L/G) of liquid sorbent slurry to flue gas. The L/G ratios, ranging from 5,400 to 13,400 1 Nm-3 (40 to 100 gal/std. kft3), are high enough to completely saturate the treated flue gas with water vapor. Typically, some untreated flue gas is bypassed around the scrubber modules to reheat the scrubbed gas and prevent troublesome condensation in the duct and stack.Because of the many shortcomings with wet FGD systems, the power utility industry is continually seeking new processes that will lower the capital, operating, and disposal costs associated with these conventional SO, control systems. in the early 1980s, pilot studies of semidry FGD processes, such as spray-drying absorption, were conducted to determine their potential as SO, control technologies. Many semidry FGD technologies have reached commercial scale in the last decade. As a result, a large number of studies have been performed to research and further develop these processes.As a contrast to wet FGD systems, semidry systems operate with low L/G ratios (about 28 to 40 1 Nm-3 or 0.2 to 0.3 gal/std. kft3) as compared to 5,400 to 13,400 1 Nm-3 or 40 to 100 gal/kSCF or higher for wet FGD systems). Since massive slurry pipe circuits and the associated pumps are not require...

Experimental and Mechanism Studies on a Different‐Velocity Circulating Fluidized Bed for Flue Gas Desulfurization

“…This indicates that if a higher desulfurization efficiency is required, then water should be sprayed to form a film on the surface of Ca(OH)* particles, because SO2 reacts with Ca(OH)2 much more quickly in the liquid phase than in the solid phase. Neathery [5] showed the same results.…”

Modeling study on the impaction and humidification process in desulfurization activation reactor

“…Therefore, the reaction of the dissolved sulfur species with the sorbent can be considered instantaneous, and assume that a reaction front with the liquid phase will exist where the concentration of both the dissolved sorbent and sulfur species are zero (Neathery, 1996). Based on the two-film theory, as shown in Fig.…”

Transport—Reaction process in the reaction of flue gas desulfurization

“…1.553x l O-t° Otso2,ori2 CPA Nso 2 - The absorbent utilization ratio is defined as the ratio of the used up Ca(OH)2 in the reaction and initial Ca(OH)2before the reaction happens, or is expressed as: f wetlifetime I7 4 3 Ns°~dt…”