Sulfur absorption efficiency

There are two more quantities that must be defined to complete the description of the fluidized bed combustor viz., the carbon combustion efficiency, ncCE> sulfur absorption efficiency, nsAE- These are ... 

The concentration profile of oxygen in the bed is fixed by establishing apriori a value for b as 4.5 and that of a as obtained from the assumed values of carbon conversion and sulfur absorption efficiencies. For a given oxygen profile the reaction rate constant, k3(To), and the size of the dolomite feed are varied. The changes in both of these parameters affect the value... 

Figure 8 represents the variation in limestone requirement as a function of sulfur absorption efficiency for various values of k3(To). The results emphasize that for a given value of k3(To), if the Imestone feed rate is increased which for a bed of fixed size implies a reduction in residence time, the sulfur absorption efficiency is correspondingly decreased. The implication of this result for an actual operating plant is important. It is Implicit in these plots that if the limestone feed rate is held constant, nsAE> increases with an Increase in k3(TQ). 

Figure 9 illustrates the effect of changing limestone average size, r , in the feed stream on the dependence of limestone feed rate, F, and on sulfur absorption efficiency, risAE plots refer to a constant value of 1c3(Tq). 

These results suggest that if the feed size of limestone is kept fixed, an increase in the limestone feed rate will result in the reduction of sulfur absorption efficiency- These results also emphasize that if the same sulfur retention is to be obtained when the size of the limestone particles is decreased the feed rate must be increased. However, for the same feed rate of limestone, a decrease in the size of limestone particles results in an increased sulfur retention. This may be explained on the basis of an increase in the overall surface area per unit volume of the bed when the average diameter of the particles decreases. It may be noted from Figure 9 that regardless of the limestone particle size, if sufficient residence time is allowed for limestone particles in the bed, it is possible to obtain sufficiently high sulfur retention. 

The influence of carbon conversion efficiency on the requirement of limestone for a fixed value of sulfur absorption efficiency is also computed. The generation of sulfur dioxide is found to be directly related to the amount of carbon combusted. 

The generation rate of sulfur dioxide reduces with the decrease in carbon conversion efficiency and hence the limestone requirement also decreases. A reduction in the carbon conversion efficiency from 99.5 to 70.0% causes a reduction in dolomite requirement from 27.5 to 18.9 g/s for a 99% sulfur absorption efficiency. 

For absorption controlled by diffusion through a gas film, it is necessary to provide a large enough interface area the interface area depends on the liquid to gas flow rate ratio, VL/VG, essentially for a defined dispersity of the absorbent. On the other hand, increasing VJV,c must lead to increased power consumption so it is important to optimize the flow rate ratio.. The experimental results on the influence of VL/VG on the sulfur-removal efficiency are shown in Fig. 7.13. To keep the conditions of atomization essentially the same, all the experiments in this set were carried out at a fixed volumetric liquid flow rate, VL while the gas flow rate, VG, for each run varies according to the required liquid to gas flow rate ratio and, simultaneously, the corresponding concentration of Ca(OH)2 was used to keep the ratio of Ca/S the same as 1.4. 

At the Ronnskar works of Boliden AB in Sweden, which include both a copper and a lead smelter, a cyclic process using water as the absorbent concentrates sulfur dioxide both to produce liquid sulfur dioxide and for feed to acid plants (20, 21). The process is reported to give an absorption efficiency of about 98% on gas containing 2% sulfur dioxide. Water is not normally a favorable solvent for such an application but can be used in this case because it is available at a low temperature, less than 5°C for most of the year. Recovery of sulfur dioxide from the complete smelter is to be increased from 90 to 95% by applying water-cooled collecting hoods and waste heat boilers to all the copper converters (20). 

The reagent stream must be controlled to permit calcium salt desupersaturation external to the scrubber and absorber while maintaining adequate concentration levels for good absorption efficiency. In order to do this a reagent stream containing 8-15% solids is circulated. The solid portion is composed of some fly ash components but mainly calcium carbonate, sulfite, and sulfate. Sulfur dioxide removal efficiency dictates the carbonate level. Sulfite crystals enhance and control desupersaturation of calcium sulfate while providing nucleation sites for crystal growth... 

The reason lies in the fact that the alkazid solution is highly sensitive to HCN (forming potassium ferrocyanide) and that the sulfur collection efficiency is unsatisfactory, which is essentially due to the poor CX)S absorptivity of the solvent... 

Wherever the absorption or adsorption processes used to clean the gases have a high absorptivity for H2S but can eliminate COS only at high cost or not at all, the processes are designed in such a way that the sulfur contained in the gas reaches the gas purification unit in the form of H2S. Since, however, most modem gas purification units are capable of removing not only H2S but also COS and other organic sulfur components efficiently enough to meet the requirements for methanol production, COS hydrolysis is today used only in special cases. 

Formic acid acts in a similar manner in stabilizing die pH however, it differs from other carboxylic acids used as buffering scents in several important respects (1) it is less expensive than other pure acids on a weight or molar equivalent basis, (2) it can be purchased, stored, and added as a neutral salt such as sodium formate, and (3) it has the unique ability to inhibit sulfite oxidation while improving sulfur dioxide absorption efficiency (Moser et al., 1990). 

Double-Absorption Plants. In the United States, newer sulfuric acid plants ate requited to limit SO2 stack emissions to 2 kg of SO2 per metric ton of 100% acid produced (4 Ib /short ton Ib = pounds mass). This is equivalent to a sulfur dioxide conversion efficiency of 99.7%. Acid plants used as pollution control devices, for example those associated with smelters, have different regulations. This high conversion efficiency is not economically achievable by single absorption plants using available catalysts, but it can be attained in double absorption plants when the catalyst is not seriously degraded. 

A variation of the n on regen erabi e absorption is the spray dry process. Time slurry is sprayed through an atomizing nozzle into a tower where it countercurtendy contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite-sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scmbbing operations. 

Regenerable absorption processes have also been developed. In these processes, the solvent releases the sulfur dioxide in a regenerator and then is reused in the absorber. The WelLman-Lord process is typical of a regenerable process. Figure 11 illustrates the process flow scheme. Sulfur dioxide removal efficiency is from 95—98%. The gas is prescmbbed with water, then contacts a sodium sulfite solution in an absorber. The sulfur dioxide is absorbed into solution by the following reaction ... 

The effects of various catalysts (47,48), contaminants (49,50), acid concentration (51), temperature (52), and pressure (53—57) on the rate of absorption have been studied. The patent Hterature indicates that absorption can be improved by making the contact between the gaseous ethylene and hquid sulfuric acid more efficient (58—61), by suitable design of the absorption tower (62), and by various combinations of absorption and hydrolysis (63-68). 

Maximize the recovery of sulfur by operating the furnaces to increase the SO, content of the flue gas and by providing efficient sulfur conversion. Use a double-contact, double-absorption process. 

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