Lime-limestone process

Removal of SO2 from flue gases is very important to prevent air pollution. There are many processes available for SO2 removal from power plant flue gases. Among them, the lime/limestone process as absorbent has been widely used. AU the conventional SO2 removal processes generate sludge that requires disposal. Therefore, the use of membranes which could separate both CO2 and SO2 in a single step process is very attractive. 

The ammonia-based double alkali process has the same advantage as the sodium-based system, compared to wet lime/limestone processes, of using a clear solution in the absorption step. However, both double alkali processes have the disadvantage of greater complexity... 

Dry Lime/Limestone Processes Furnace Sorbent Injection Processes... 

LIMB [Lime/limestone injection into a multi-stage burner] A flue-gas desulfurization process used in Germany and Finland. Dry, ground limestone is injected directly into the combustion chamber. This reacts with the sulfur dioxide, and the dry particulate product is collected downstream together with the ash. The process is suitable only for those systems which limit the maximum combustion temperature by staging, in order to minimize the production of oxides of nitrogen. 

The lime and limestone processes, as indicated in Figure 3, produce a sludge consisting mainly of calcium sulfite and calcium sulfate by the following reactions (limestone) ... 

The calcium sulfite or sulfate solids are allowed to settle from the solution. The regenerated solution is returned to the absorber. The solids are concentrated to around 70%. Because these solids are not a mixture of the sulfite and sulfate, their properties are far superior to lime or limestone process sludge (unless oxidation is used) and disposal should be easier. 

Where most utility installations are the lime or limestone processes, it can be seen from Table IV that a very small percentage of industrial installations are of this type. Most of these installations are the once-through sodium carbonate, sodium hydroxide, and double alkali processes. Where the utility installations have been plagued with corrosion, erosion, scaling and fouling problems, the industrial installations, have to date performed much better. A number of systems showed a process reliability of greater than 85%. 

Table I. Equilibria Present in Flue Gas Scrubbing Slurries for the Lime or Limestone Processes...

Equations 8 and 9 can be used for values of I up to 1. M. The second term in these equations accounts for the reversal of slope of activity coefficient versus ionic strength from negative to positive as ionic strength increases. Equations 8 and 9 have been widely used in the equilibrium calculations of the lime or limestone processes. With coals of moderate chloride content and for systems without extensive sludge dewatering, the ionic strength is well below 1.0 M, and equations 8 and 9 reasonable. 

The scaling tendency of the lime or limestone processes for flue gas desulfurization is highly dependent upon the supersaturation ratios of calcium sulfate and calcium sulfite, particularly calcium sulfate. The supersaturation ratios cannot be measured directly. They are determined by measuring experimentally the molalities of dissolved sulfur dioxide, sulfate, carbon dioxide, chloride, sodium and potassium, calcium, magnesium, and pH. Then by calculation, the appropriate activities are determined, and the supersaturation ratio is determined. Using the method outlined in Section IV, the concentrations of all ions and ion-pairs can be readily determined. The search variables are the molalities of bisulfite, bicarbonate, calcium, magnesium, and sulfate ions. The objective function is defined from the mass balance expressions for dissolved sulfur dioxide, sulfate, carbon dioxide, calcium, and magnesium. This equation is... 

Reactions between gases and liquids may involve solids also, either as reactants or as catalysts. Table 17.9 lists a number of examples. The lime/limestone slurry process is the predominant one for removal of S02 from power plant flue gases. In this case it is known that the rate of the reaction is controlled by the rate of mass transfer through the gas film. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

A preliminary-level economic evaluation (13) performed by EPRI (Electric Power Research Institute) and TVA (Tennessee Valley Authority) indicates that a combination of electrostatic precipitators (or bag house), ammonia-based SCR system, and wet lime/limestone FGD scrubber range between 20% to 185% cheaper than wet process for complete control of particulates, N0X and S02 The lower percentage is for the second type and higher percentage for the first type of process. Therefore, the second type of process appears to be more promising and will be the subject of further discussion in this paper. 

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