Limestone

By Brad Buecker, President of Buecker & Associates, LLC

It is common knowledge that many coal-fired power plants in the United States and other partsof the world are being retired in response to concerns about climate change. However, in somecountries coal plants still provide a substantial portion of electrical needs. And, if carbon captureand sequestration (CCS) continues to move forward, some coal plants may be with us for yearsto come.

Regardless of one's view on coal plant acceptability, a critical aspect continues to belimiting sulfur dioxide (SO2) emissions. A technology to do so that has been around for decadesis wet-limestone scrubbing. But a question that may not be understood by many is: "How canthis natural mineral, which is a hugely important construction material and has very lowsolubility in water, serve as a scrubbing reagent in a power plant?" This article examines theunique chemistry behind this application.

Limestone is a common deposit in many global locations, including the U.S. The principalcomponent of limestone is calcium carbonate (CaCO3), and some stones may contain 95% orgreater CaCO3. Second in abundance is magnesium carbonate (MgCO3), which often constitutesonly a small percentage of the total carbonate, although some formations may include dolomitethat has an equal molecular mixture of calcium and magnesium carbonate (MgCO3·CaCO3).

Dolomite is rather unreactive in scrubbers. Lower quality limestones contain inert minerals suchas silicates in the form of quartz, shale, or clay. Some stones have minor concentrations of ironand/or manganese carbonate (FeCO3 and MnCO3), which can influence some aspects of scrubberoperation.

An examination of limestone's reactivity in natural waters provides a good foundation (pardonthe pun) for understanding why it can work well in scrubbers. Consider the lab experiment ofplacing a limestone sample in pure water with a pH of 7.0. Limestone is only slightly soluble inwater.

CaCO3 ⇌ Ca2+(aq) + CO32-(aq) Eq. 1

Ksp (25o C) = [Ca2+] * [CO32-] = 4.6 * 10-9 (mol/L)2 Eq. 2

Straightforward calculations indicate that the initial CaCO3 solubility per Equation 2 is only6.8 * 10-5 moles per liter (M), equivalent to just under 7 mg/L.

However, carbonate is a relatively strong base, and it will react with water as follows:

CO32- + H2O ⇌ HCO3– + OH– Eq. 3

This influence drives the reaction shown in Equation 1 somewhat to the right, where the overallreaction can be written as:

CaCO3 (s) + H2O ⇌ Ca2+ + HCO3– + OH– Eq. 4

The CaCO3 solubility (25o C) rises to 9.9 * 10-5 M (~ 10 mg/L) per this effect, (1) whichrepresents a roughly 1/3 increase in solubility, but is still very slight.

However, this chemistry leaves two important unanswered questions.

• If CaCO3 solubility is so low, why do many natural water supplies have alkalinityconcentrations in the double to triple digits mg/L range?• How could such a material be effective in a flue gas scrubber?

The answers are directly related, as we shall now explore.

In surface waters, carbon dioxide from the atmosphere dissolves as follows:

CO2 + H2O ⇌ H2CO3 Eq. 5

The amount that enters solution can be calculated from Henry's Law:

KH = [H2CO3 (aq)]/P = 3.4 * 10-2 mol/L · atm (25oC), where Eq. 6 P = the partial pressure of CO2

The current atmospheric concentration of CO2 is near 420 ppm, which calculates to 0.00042 atm.So, for neutral water the H2CO3 concentration is around 1.43 * 10-5 M, which is not very large.

Research suggests that most solvated carbon dioxide remains as CO2 and does not dissociate.However, a small amount does, per the following reaction:

H2CO3 ⇌ HCO3– + H+ Eq. 7

The acidity this dissociation generates can be calculated from the following equation.

Ka = [HCO3–] * [H+]/[H2CO3] = 4.5 * 10-7 mol/L (25oC) Eq. 8

The very small value for Ka shows that H2CO3 (carbonic acid) is a weak acid. Per Equations 6and 8, the acid concentration in neutral water calculates to 2.32 * 10-6 M, equivalent to a pH ofabout 5.6. (A point worth noting here is that because H2CO3 is a weak acid, if few buffering ionsare present in the water, an external acidic influence can significantly lower pH. The classic caseis acid rain, which plagued the northeastern U.S. before power plants began installing sulfurdioxide and nitrogen oxide (NOx) treatment equipment.)

This is where the chemistry becomes more interesting. Even though calcium carbonate is onlyslightly soluble in neutral waters, and carbonic acid has a low dissociation constant, observeagain that the limestone dissolution produces hydroxyl ions (OH–) and the carbonic acidproduces hydrogen ions (correct is hydronium ions, H3O+ or multiples thereof, but we can ignorethat concept in this discussion), where the following reaction represents a simplified combinationof Equations 4 and 7.

H+ + OH– –> H2O Eq. 9

The acid-base neutralization drives both Reactions 4 and 7 to the right and causes a 35-foldincrease in CO2 dissolution and a four-fold increase in the calcium concentration. (1) Thisexplains why many natural waters have a significant bicarbonate alkalinity concentration and amildly basic pH range of 7 to 8.

OK, so now you may be asking, what does all this chemistry have to do with wet-limestonescrubbers?

Briefly reconsider the concepts shown in Equations 5, 6 and 8. The following example uses datafrom Reference 2 that has a table outlining the theoretical combustion product calculations for asteam-generating unit burning 1.5% sulfur coal. The program calculates an SO2 concentration inthe flue gas of around 0.11%, which is roughly three times greater than the atmospheric CO2 concentration as described above. The reaction of SO2 with water is analogous to Equations 5 and 7.

SO2 + H2O ⇌ H2SO3 Eq. 10

H2SO3 ⇌ HSO3– + H+ Eq. 11

But, the Ka for Equation 11 is 1.7 * 10-2, which is quite higher than carbonic acid.

So, the driving force for CaCO3 dissolution and reactivity is much greater in sulfurous acidsolutions than carbonic acid.

Now we can examine how this chemistry plays out in a scrubber. Figure 1 outlines a generalflow diagram of a spray-tower, wet-limestone scrubber.

The general equation for the initial scrubber reaction is:

CaCO3 + 2H+ + SO3-2 –> Ca+2 + SO3-2 + H2O + CO2↑ Eq. 12

In the absence of any other reactants, calcium and sulfite ions will precipitate as a hemihydrate,with water included in the crystal lattice of the byproduct.

Ca+2 + SO3-2 + ½H2O –> CaSO3·½H2O↓ Eq. 13

Proper operation of a scrubber is dependent upon the efficiency of the above-listed reactions,where pH control via accurate reagent feed is critical. Many wet-limestone scrubbers operate ata solution pH of around 5.6 to 5.8. A too-acidic solution inhibits SO2 transfer from gas to liquid,while an excessively basic slurry (pH > 6.0) indicates overfeed of limestone.

Oxygen in the flue gas greatly influences chemistry. Aqueous bisulfite and sulfite ions reactwith oxygen to produce sulfate ions (SO4-2).

2SO3-2 + O2 –> 2SO4-2 Eq. 14

Approximately the first 15 mole percent of sulfate ions co-precipitates with sulfite to formcalcium sulfite-sulfate hemihydrate [(0.85CaSO3·0.15CaSO4)·½H2O]. Any sulfate above the 15percent mole ratio precipitates with calcium as gypsum (CaSO4·2H2O).

Ca+2 + SO4-2 + 2H2O –> CaSO4·2H2O↓ Eq. 15

Calcium sulfite-sulfate hemihydrate is a soft material that tends to retain water. It has little valueas a chemical commodity. For this reason, many scrubbers are (or were) equipped with forced-air oxidation systems to introduce additional oxygen to the scrubber slurry. A properly designedoxidation system will convert all of the sulfite to gypsum, which forms a cake-like material whensubjected to vacuum filtration.

In many cases, 85 to 90% of the free moisture in gypsum can be extracted by this relatively simple mechanical process. High-purity, dried synthetic gypsum was once a favorite material of wallboard manufacturers.

Limestone utilization and scrubbing efficiency are critical issues. Factors that influence scrubberperformance include:

• Limestone grind size• Limestone purity, especially with regard to CaCO3 concentration• Performance of slurry separation devices• Spray nozzle efficiency• Adequate forced-air oxidation efficiency

Let's briefly review these concepts.

Limestone grind size

Grind size is quite important. This author first cut his teeth working with a scrubber that had justbeen commissioned a few months before. Grinding was performed in wet ball mills. The stonewas high purity with a typical CaCO3 concentration of 96-97%. The initial grinding specificationcalled for 70% ground particles passing through a 200-mesh screen as analyzed in the lab. Buteven with this high-purity stone, it quickly became apparent that the initial grind size was toocoarse and did not allow sufficient reaction. The grind was adjusted over time to an eventualspecification of 90% through a 325-mesh screen. Following the grinding adjustments, thelimestone utilization increased to 98% or thereabouts. (3)

Limestone purity and reactivity

The author was also part of a team that evaluated several limestones on a full-scale basis over atwo-year period to see if materials and transportation costs could be lowered from those of thehigh-purity stone mentioned above, which was delivered form over 100 miles away. Some ofthe test stones had a total carbonate alkalinity of greater than 90%, but where a significantportion was dolomite. Others had CaCO3 concentrations in an 80-90% range, with the balancemade up of inert materials. In all cases, the lesser-quality stones performed very poorly and wereabandoned. Limestone utilization decreased dramatically, and some materials caused asignificant increase in scale formation. Furthermore, the much higher concentrations of inertmaterials negatively impacted slurry-separating hydrocyclones.

The cyclone manufacturer was brought in to adjust the vortex finders of the units to improveparticle separation, but the results were marginal at best.

In another test, a small but significant concentration of FeCO3 in the stone converted to very fineiron oxide particles which plugged the cloth on the rotary vacuum drum filters.

Spray nozzle efficiency

Spraying technology has advanced immensely from early designs, and open spray towers arenow normal. Modern towers can potentially remove 98% or more of the entering SO2.However, the spray nozzle grid must be designed to provide uniform coverage and preventchanneling of the flue gas. A common problem in early scrubbers was nozzle plugging frompieces of internal lining material that had broken loose in slurry circulating lines. This authorclearly recalls pulling pieces of fractured rubber liner from spray nozzles during periodicinspections.

Forced-air oxidation efficiency

As has been noted, the handling characteristics of fully oxidized slurry are much better than forslurry that is only partially oxidized. Accordingly, oxidation air system design and operation arecritical. Under-sizing of the oxidation air system during design is a noted problem, while atother times the small bore-holes in oxidation air laterals can become encrusted with scale. Theanalytical technique outlined in Reference 3 can quickly detect loss of oxidation efficiency.

Gone is the heyday of massive scrubber installations at coal-fired power plants. However, thistechnology still has value in some applications, and perhaps future CCS projects will require wetscrubbing of SO2. Limestone is a plentiful and inexpensive material that can remove nearly allSO2 from a flue gas stream. But wet scrubbing raises issues about byproduct and wastewaterdisposal, particularly in regard to discharge of heavy metals and metalloids. The author and twocolleagues reported on an emerging selenium capture (along with other impurities) method in aprevious Power Engineering article. (4) Concerns about liquid discharge and disposal have hada strong influence at some plants, where dry scrubbing (with more expensive lime reagent) waschosen vs wet scrubbing. Even so, limestone still plays a critical role, as it is the base materialfor the scrubbing reagent.

References

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Mostrecently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He may be reached at [email protected].