Spray-drying Flue Gas Desulfurization Research Articles

Effects of Flue Gas Components on the Reaction of Ca(OH)2 with SO2

A differential fixed-bed reactor was employed to study the effects of the flue gas components, H2O, CO2, NOX, and O2, on the reaction between Ca(OH)2 and SO2 under conditions similar to those in the bag filters of a spray-drying flue gas desulfurization (FGD) system. The presence of CO2 with SO2 in the gas phase enhanced the sulfation of Ca(OH)2 only when NOX was also present. When either NOX (mainly NO) or O2 was present with SO2, the enhancement effect was slight, but became great when both NOX and O2 were present, and was even greater when CO2 was also present. The great enhancement effect exerted by the presence of NOX/O2 resulted from the rise in the NO2 concentration, which enhanced the oxidation of HSO3- and SO32- to SO42- in the water layer adsorbed on Ca(OH)2 surface and the formation of deliquescent salts of calcium nitrite and nitrate. The enhancement effect due to the presence of NOX/O2 was more pronounced when the relative humidity was above that at which the salts deliquesced; the extent of sulfation was more than twice that obtained when SO2 alone was present. The presence of H2O, CO2, NOX, and O2 in the flue gas is beneficial to the SO2 capture in the low-temperature dry and semidry FGD processes. The presence of NOX/O2 also enhanced CO2 removal when SO2 was absent.

Kinetic Analysis of Iron Blast Furnace Slag/Hydrated Lime Sorbents with SO2 at Low Temperatures

A differential fixed-bed reactor was employed to study the reaction between iron blast furnace slag (BFS)/hydrated lime (HL) sorbent and SO2 under conditions similar to those in the bag filters of a spray-drying flue gas desulfurization system. The BFS/HL sorbent was prepared at a weight ratio of 30/70 with a slurrying time of 16h and a slurrying temperature of 65℃. The sorbent was highly reactive towards SO2 as compared with hydrated lime alone. Increasing the relative humidity of the gas significantly increased the initial reaction rate and the ultimate Ca utilization of the sorbent. The temperature and SO2 concentration had slight effects on the initial reaction rate and negligible effects on the ultimate Ca utilization. The reaction kinetics of the sorbent with SO2 was well described by the surface coverage model, which assumes that the sulfation rate is controlled by the chemical reaction on the sorbent grain surface and takes into account the surface covered by the product. The results of this study are useful for the design and operation of dry or semidry processes that use BFS/HL sorbents to remove SO2 from flue gas.

Effect of [formula omitted] ash weight ratio on the kinetics of the reaction of [formula omitted] ash sorbents with [formula omitted] at low temperatures

Ca ( OH ) 2 / fly ash sorbents were prepared with different weight ratios. Their reactions with SO 2 have been studied under the conditions similar to those in the bag filters of the spray-drying flue gas desulfurization system. The sorbent reactivity increased in general with decreasing Ca ( OH ) 2 / fly ash weight ratio. The reaction kinetics was well described by the surface coverage model which assumes that the sorbent was made up of plate grains and the rate was controlled by the chemical reaction on the grain surface and takes into account the surface coverage by products. For sorbents with Ca ( OH ) 2 / fly ash weight ratios than 10/90, the effect of weight ratio on the reaction was entirely represented by the effects of the intial specific surface area and the Ca content of the sorbent.

Kinetic Model for the Reaction of Ca(OH)2/Fly Ash Sorbents with SO2 at Low Temperatures

Sorbents prepared with different Ca(OH)2/fly ash weight ratios were reacted with SO2 under conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction kinetics was well described by a generalized surface coverage model developed in this study. The model assumes that the sorbent was made up of plate grains and the rate was controlled by the chemical reaction on the grain surface and takes into account the variation in sorbent Ca molar content (M-1) and the surface coverage byproducts. The effect of Ca(OH)2/fly ash weight ratio on the reaction was represented by the effects of the initial specific surface area (Sg0) and M-1 of the sorbent on the kinetic parameters. Under the same reaction conditions, the sorbents with weight ratios ≥ 30/70 had about the same initial reaction rate per unit surface area of the sorbent; their ultimate conversions increased linearly with increasing Sg0, being independent of M-1. The effects of relative humidity, SO2 concentration, and temperature on the reaction were unaffected by the Ca(OH)2/fly ash ratio. The results of this study are useful to the design and operation of the dry and semidry processes using Ca(OH)2/fly ash sorbents to remove SO2 from the flue gas.

Kinetics of the reaction of iron blast furance slag/hydrated lime sorbents with SO 2 at low temperatures: effects of sorbent preparation conditions

Sorbents highly reactive towards SO 2 have been prepared from iron blast furnace slag and hydrated lime under different hydration conditions. The reaction of the dry sorbents with SO 2 has been studied under the conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction was well described by a modified surface coverage model which assumes the reaction rate being controlled by chemical reaction on sorbent grain surface and takes into account the effect of sorbent Ca molar content and the surface coverage by product. The effects of sorbent preparation conditions on sorbent reactivity were entirely represented by the effects of the initial specific surface area ( S g0 ) and the Ca molar content ( M −1) of sorbent. The initial conversion rate of sorbent increased linearly with increasing S g0 , and the ultimate conversion increased linearly with increasing S g0 M −1. The initial conversion rate and ultimate conversion of sorbent increased significantly with increasing relative humidity of the gas. Temperature and SO 2 concentration had mild effects on the initial conversion rate and negligible effects on the ultimate conversion.

A Surface coverage Model for the Reaction of Ca(OH)2 with SO2 at Low Temperatures

Hydrated lime (Ca(OH)2) is widely used as a sorbent in the dry and spray-drying flue gas desulfurization processes. The reaction of Ca(OH)2 with SO2 at low temperatures in these processes is well described by the equations obtained from a surface coverage model. The model assumes that the rate is controlled by the chemical reaction occurring on the water-adsorbed sorbent surface and considers the surface coverage by the product. The results of this study are useful for the design and operation of dry and spray-drying processes using hydrated lime.

Kinetics of the Sulfation of Ca(OH)2 at Low Temperatures

Hydrated lime [Ca(OH)2] is frequently used as a sorbent in the spray-drying flue gas desulfurization (FGD) and injection/humidification FGD processes to remove SO2 from the flue gas. In the present study, the kinetics of the reaction of Ca(OH)2 with SO2 has been studied by using a differential reactor and a gas mixture of N2, SO2, and H2O under the conditions of 0−80% relative humidity, 60−90 °C, and 1000−5000 ppm SO2. The reaction ceased before 1 h, and Ca(OH)2 was incompletely converted. The relative humidity of the gas significantly affected the initial reaction rate and the maximum conversion of Ca(OH)2. The temperature and SO2 concentration had mild effects on the reaction. The sulfation of Ca(OH)2 was well described by the asymptotic equation, which is obtained by assuming that the rate was controlled by the chemical reaction occurring on the water-adsorbed sorbent surface and by considering the surface coverage by the product. The results of this study are useful to the design and efficient operation of the processes using hydrated lime to remove SO2 from the flue gas.

Kinetics of the reaction of Ca(OH) 2/fly ash sorbent with SO 2 at low temperatures

A differential fixed-bed reactor was employed to study the reaction between Ca(OH) 2/fly ash sorbent and SO 2 under the conditions similar to those in the bag filters of the spray-drying flue gas desulfurization system. The Ca(OH) 2/fly ash sorbent was prepared at weight ratio of 70/30, slurrying time of 16 h , and slurrying temperature of 65°C. Foil-like calcium silicate hydrates were found in the sorbent. The Ca(OH) 2/fly ash sorbent was highly reactive towards SO 2 and its initial reaction rate and maximum conversion were much higher than pure Ca(OH) 2. Increasing the relative humidity of the gas significantly increased the initial reaction rate and the maximum Ca utilization of the sorbent. Temperature and SO 2 concentration had slight effects on the initial reaction rate and negligible effects on the maximum conversion. The reaction kinetics of Ca(OH) 2/fly ash sorbent with SO 2 was well described by the surface coverage model which assumes the sulfation rate being controlled by chemical reaction on sorbent grain surface and takes into account the surface coverage by products. The results of this study are useful to the design and operation of the dry or semidry processes using Ca(OH) 2/fly ash sorbent to remove SO 2 from flue gas.

Evaluation of a dry FGD material as a flowable fill

Many flue gas desulfurization (FGD) materials have low unit weight and good shear strength characteristics and thus hold promise for flowable fill applications. This paper focuses on the potential of using spray dryer FGD ash in flowable fill as a replacement for conventional fly ash. Several design mixes were considered. The design mixes consisted of varying amounts of FGD, cement, lime, admixture, and water. The mixes were tested in the laboratory for flowability, unit weight, moisture content, unconfined compressive strength (UCS), erodibility, set time, penetration, and long-term strength characteristics. Tests were conducted for up to 90 days of curing. The FGD flowable fill without any additives was observed to be comparable to regular (normal set) flowable fill in terms of placeability, UCS, and ‘diggability’. FGD flowable fill with additives and admixtures compared favorably with the characteristics of conventional quick-set flowable fills.

Transport—Reaction process in the reaction of flue gas desulfurization

A theoretical investigation was conducted to study the transport-reaction process in the spray-drying flue gas desulfurization. A transport-reaction model of single particle was proposed, which considered the water evaporation from the surface of droplet and the reaction at the same time. Based on this model, the reaction rate and the absorbent utilization can be calculated. The most appropriate particle radius and the initial absorbent concentration can be deduced through comparing the wet lifetime with the residence time, the result shows in the case that the partial pressure of vapor in the bulk flue gas is 2000Pa, the optimum initial radius and absorbent concentration are 210–310 µ m and 23% respectively. The model can supply the optimum parameters for semi-dry FGD system designed.

The Effect of Hygroscopic Additives on Ca(OH)2 Utilization in Spray Dryer Flue Gas Desulfurization

Soluble additives to enhance spray dryer performance for sulfur dioxide (SO2) removal from flue gas have been evaluated in a pilot plant system under controlled conditions. These additives have included NaOH, NaHCO3, NaCl and CaCl2. These additives were chosen because of their hygroscopicity as solids, and the understanding that SO2 reactivity is significantly enhanced in the liquid phase of the spray drying operation. All of the additives enhanced SO2 removal above baseline Ca(OH)2 slurry usages, with NaOH providing the greatest degree of enhancement at approach to adiabatic saturation temperatures below 16.7 °C. The percent of enhancement per unit mass of additive was found to decrease with additive concentrations above 100 mg l−1. The enhancement effect may be explained by analyzing the vapor pressure lowering effect of the slurry droplets containing the additives. This procedure may by appropriate for estimating the enhancement by other hygroscopic solids for spray dryer flue gas desulfurization impro...

95/00897 Mercury emission reduction using activated carbon with spray dryer flue gas desulfurization

95/00897 Mercury emission reduction using activated carbon with spray dryer flue gas desulfurization

Mathematical Model of Sulfur Dioxide Absorption into a Calcium Hydroxide Slurry in a Spray Dryer

Abstract This paper reports on the development of a comprehensive model to simulate the absorption of gaseous SO2 into a calcium hydroxide slurry in a spray dryer flue gas desulfurization system. In this study, the model was modified to incorporate the input of surface area of the calcium hydroxide particles. Previous studies assumed spherical particles. A sensitivity analysis was then conducted to determine the sensitivity of the model to surface area at particle size of 2, 3, 4, and 6 micrometers.

Effects of Recent Operating Experience on the Design of Spray Dryer FGD Systems

The information presented in this paper is directed to those who are involved in specification or purchase of flue gas desulfurization (FGD) systems. Spray dryer FGD systems have been in service on full-scale electric utility applications for about 4½ years. When introduced, this process appeared to provide reduced capital and annual costs and easier waste handling and disposal, compared to the wet scrubber FGD system. However, at the time that the first generation of this new FGD process was under design, a number of design questions were unresolved. These questions involved spray dryer sizing criteria, the number and type of atomizers, optimum reagent preparation and waste handling subsystems, and the effects of low temperature operation on the particulate removal equipment. The operating experience of these first generation spray dryer FGD systems has both confirmed their suitability for the electric utility environment and identified areas where improvement is needed. Problem areas include deposit bui...

Ocean Disposal of Consolidated Spray Dryer Flue Gas Desulfurization Wastes

Combustion of coal in electric power generating stations results in the production of large quantities of fly ash and flue gas desulfurization end products. These wastes must be disposed of by environmentally acceptable means. Ocean disposal of these wastes is an attractive option for power stations located on coastal waterways. Dumping of fly ash and untreated wet FGD sludge is environmentally deleterious. However, wet scrubber sludge stabilized with fly ash can be formed into solid dense blocks which have been shown to be acceptable for ocean disposal. The Marine Sciences Research Center has been investigating this disposal option for the past five years. A new FGD technology, Spray Drying Absorption, was introduced about five years ago. This process produces a dry end product composed of fly ash, desulfurization reaction products and excess reagents. When lime is the reagent the end products can be dampened and consolidated into self-hardening dense blocks. Niro Atomizer has been investigating dry FGD end product disposal and utilization for the past three years. An assessment of the feasibility of using solid blocks formed from dry FGD end products for ocean reef construction is being performed by the Marine Sciences Research Center. Consolidated dry FGD end products from Niro Atomizer's Riverside dry FGD demonstration facility and Copenhagen dry FGD pilot plants were tested. These wastes represent low sulfur western and European coal applications. The end products were dampened and consolidated into Proctor cylinders. After curing, the cylinders were exposed in sea water systems including the open seas for up to 6 months. Block structural integrity was maintained during this period and porosity remained essentually unchanged. The unconf ined compressive strength remained high during the test period.