Effect enhanced limestone

Effects of Aqueous Chemical Equilibria on Wet Scrubbing of Sulfur Dioxide With Magnesia-Enhanced Limestone... 

Data for magnesia-enhanced limestone scrubbing of SO2 by a Turbulent Contact Absorber were obtained from the 10-MW equivalent EPA Alkali Scrubbing Test Facility. These data and the chemical model have been used to predict the enhancement effect of magnesia-induced dissolved sulfite concentration on S02 removal. 

It was discovered that viscosifying the acid showed a remarkable improvement in acid fluid loss control. The enhancement was most pronounced in very-low-permeability limestone cores. The nature of the viscosifying agent also influenced the success. Polymeric materials were more effective than surfactant-type viscosifiers [682]. 

Pyritic sulfur can be removed by froth flotation, which takes advaniage of the differences of specific gravity of the two types of sulfur. To be effective, the coal must be pulverized into particles in the micron region. The process can be enhanced by adding limestone, catalyst, und soda ash to the coal dust. After treatment, the coal is formed into briquettes for ease of handling by convenlional conveyors. 

Figures 5, 6, and 7 demonstrate the effect of organic acids on the dissolution rate at 25°C. Additives that provide buffer capacity between the bulk solution pH (4.0 to 5.5) and the pH at the limestone surface (5.5 to 8.0) enhance dissolution rate by providing an additional means of diffusing acidity to the limestone surface. Figure 5 shows that at pH 5.0, 3 mM total acetic acid enhances the dissolution rate a factor of 7. This enhancement is somewhat greater at higher pH, where H+ diffusion is much more limited.

The addition of adipic acid to limestone-based FGD wet scrubbers results in improved limestone utilization and enhanced S02 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 

Since forced oxidation converts sulfite to sulfate, it has an adverse effect on SO2 removal in an unenhanced lime system in which sulfite is the major SO2 scrubbing species. This is also true in MgO-enhanced lime and limestone systems in which the promotion of SO2 removal relies on an increased sulfite-bisulfite buffer. When adipic acid is used with lime, calcium adipate becomes a major buffer species therefore, both good SO2 removal and sulfite oxidation can be achieved using within-scrubber-loop forced oxidation. 

The relative effects of 10% of fly ash, limestone, or sludge on the hydration of portland cement have been investigated by DTA/DTG/TG and XRD. Based on Ca(OH)2 estimation, it has been concluded that lime sludge and limestone enhance hydration of cement compared to that by slag or fly ash.l ]... 

Process Control Process response to changes in SO2 concentration is slower than with a Mg-enhanced lime system due to the time it takes CaCOs to dissolve. Process response to SO2 concentration changes is more flexible and forgiving than for a limestone system due to the buffering effect of the magnesium sulfite. 

Absorption Enhancement. The problem of pH reduction near the interface can be mitigated by the use of an additive, which buffers the solution so that its pH is not drastically reduced by the addition of an acid former (in this case SO2). Typical buffers that have been employed in wet limestone FGD plants are weak organic adds such as DBA (a mixture of dibasic adds containing adipic, glutaric, and succinic acids) and formic acid. DBA is more effective on a molar basis than formic acid because it is dibasic with buffering pHs of 4.3 and 3.3. Formic add buffers at a single pH of 3.73 (Stevens et al., 1991). 

---