Limestone process

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... 

Manganese dioxide can be used to absorb the initially low concentration SO2 to produce the sulphate in a Mitsubishi Process (23). In this case the absorbing phase is itself the oxidising agent. Regeneration with ammonia and air simultaneously produces ammonium sulphate which can be directly marketed as a fertiliser, thus the calcium ion problem of the dry limestone process is replaced by a plant nutrient ion -... 

Widest application has been for the recovery of and drainage of very fine material overflowing coarser washing units in glass sand, concrete sand, coal, and limestone processing plants. 

For flue-gas desulfurization, lime and limestone processes are utilized in which the furnace gas is scrubbed with an aqueous liquid containing the necessary quantity of ealcium hydroxide or calcium carbonate for neutralizing the acidic furnace gas components. The end products are calcium sulfite/calcium sulfate sludges or on complete oxidation gypsum ... 

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. 

TABLE A-I. CONVENTIONAL LIMESTONE PROCESS SUMMARY OF CAPITAL INVESTMENT... 

TABLE A-IV. CONVENTIONAL LIMESTONE PROCESS ANNUAL REVENUE REQUIREMENTS... 

Fig. 8.18 Sulfur absorption in the flue system of a coal-fired power plant (EVS, Heilbronn, Germany) left) absorber right) limestone processing and gypsum recovery (briquetting) systems...

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