By Brad Buecker, President of Buecker & Associates, LLC
It is common knowledge that many coal-fired power plants in the United States and other parts
of the world are being retired in response to concerns about climate change. However, in some
countries coal plants still provide a substantial portion of electrical needs. And, if carbon capture
and sequestration (CCS) continues to move forward, some coal plants may be with us for years
to come.
Regardless of one’s view on coal plant acceptability, a critical aspect continues to be
limiting sulfur dioxide (SO2) emissions. A technology to do so that has been around for decades
is wet-limestone scrubbing. But a question that may not be understood by many is: “How can
this natural mineral, which is a hugely important construction material and has very low
solubility in water, serve as a scrubbing reagent in a power plant?” This article examines the
unique chemistry behind this application.
Limestone is a common deposit in many global locations, including the U.S. The principal
component of limestone is calcium carbonate (CaCO3), and some stones may contain 95% or
greater CaCO3. Second in abundance is magnesium carbonate (MgCO3), which often constitutes
only a small percentage of the total carbonate, although some formations may include dolomite
that has an equal molecular mixture of calcium and magnesium carbonate (MgCO3·CaCO3).
Dolomite is rather unreactive in scrubbers. Lower quality limestones contain inert minerals such
as silicates in the form of quartz, shale, or clay. Some stones have minor concentrations of iron
and/or manganese carbonate (FeCO3 and MnCO3), which can influence some aspects of scrubber
operation.
An examination of limestone’s reactivity in natural waters provides a good foundation (pardon
the pun) for understanding why it can work well in scrubbers. Consider the lab experiment of
placing a limestone sample in pure water with a pH of 7.0. Limestone is only slightly soluble in
water.
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 only
6.8 * 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 overall
reaction 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) which
represents 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 alkalinity
concentrations 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 6
and 8, the acid concentration in neutral water calculates to 2.32 * 10-6 M, equivalent to a pH of
about 5.6. (A point worth noting here is that because H2CO3 is a weak acid, if few buffering ions
are present in the water, an external acidic influence can significantly lower pH. The classic case
is acid rain, which plagued the northeastern U.S. before power plants began installing sulfur
dioxide and nitrogen oxide (NOx) treatment equipment.)
This is where the chemistry becomes more interesting. Even though calcium carbonate is only
slightly soluble in neutral waters, and carbonic acid has a low dissociation constant, observe
again that the limestone dissolution produces hydroxyl ions (OH–) and the carbonic acid
produces hydrogen ions (correct is hydronium ions, H3O+ or multiples thereof, but we can ignore
that concept in this discussion), where the following reaction represents a simplified combination
of 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-fold
increase in CO2 dissolution and a four-fold increase in the calcium concentration. (1) This
explains why many natural waters have a significant bicarbonate alkalinity concentration and a
mildly basic pH range of 7 to 8.
OK, so now you may be asking, what does all this chemistry have to do with wet-limestone
scrubbers?
Briefly reconsider the concepts shown in Equations 5, 6 and 8. The following example uses data
from Reference 2 that has a table outlining the theoretical combustion product calculations for a
steam-generating unit burning 1.5% sulfur coal. The program calculates an SO2 concentration in
the 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 acid
solutions than carbonic acid.
Now we can examine how this chemistry plays out in a scrubber. Figure 1 outlines a general
flow 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 at
a 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 react
with 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 form
calcium sulfite-sulfate hemihydrate [(0.85CaSO3·0.15CaSO4)·½H2O]. Any sulfate above the 15
percent 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 value
as 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 designed
oxidation system will convert all of the sulfite to gypsum, which forms a cake-like material when
subjected 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 scrubber
performance 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 just
been commissioned a few months before. Grinding was performed in wet ball mills. The stone
was high purity with a typical CaCO3 concentration of 96-97%. The initial grinding specification
called for 70% ground particles passing through a 200-mesh screen as analyzed in the lab. But
even with this high-purity stone, it quickly became apparent that the initial grind size was too
coarse and did not allow sufficient reaction. The grind was adjusted over time to an eventual
specification of 90% through a 325-mesh screen. Following the grinding adjustments, the
limestone 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 a
two-year period to see if materials and transportation costs could be lowered from those of the
high-purity stone mentioned above, which was delivered form over 100 miles away. Some of
the test stones had a total carbonate alkalinity of greater than 90%, but where a significant
portion was dolomite. Others had CaCO3 concentrations in an 80-90% range, with the balance
made up of inert materials. In all cases, the lesser-quality stones performed very poorly and were
abandoned. Limestone utilization decreased dramatically, and some materials caused a
significant increase in scale formation. Furthermore, the much higher concentrations of inert
materials negatively impacted slurry-separating hydrocyclones.
The cyclone manufacturer was brought in to adjust the vortex finders of the units to improve
particle separation, but the results were marginal at best.
In another test, a small but significant concentration of FeCO3 in the stone converted to very fine
iron 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 are
now 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 prevent
channeling of the flue gas. A common problem in early scrubbers was nozzle plugging from
pieces of internal lining material that had broken loose in slurry circulating lines. This author
clearly recalls pulling pieces of fractured rubber liner from spray nozzles during periodic
inspections.
Forced-air oxidation efficiency
As has been noted, the handling characteristics of fully oxidized slurry are much better than for
slurry that is only partially oxidized. Accordingly, oxidation air system design and operation are
critical. Under-sizing of the oxidation air system during design is a noted problem, while at
other times the small bore-holes in oxidation air laterals can become encrusted with scale. The
analytical 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, this
technology still has value in some applications, and perhaps future CCS projects will require wet
scrubbing of SO2. Limestone is a plentiful and inexpensive material that can remove nearly all
SO2 from a flue gas stream. But wet scrubbing raises issues about byproduct and wastewater
disposal, particularly in regard to discharge of heavy metals and metalloids. The author and two
colleagues reported on an emerging selenium capture (along with other impurities) method in a
previous Power Engineering article. (4) Concerns about liquid discharge and disposal have had
a strong influence at some plants, where dry scrubbing (with more expensive lime reagent) was
chosen vs wet scrubbing. Even so, limestone still plays a critical role, as it is the base material
for the scrubbing reagent.
References
Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most
recently 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 beakertoo@aol.com.
Wisconsin headquartered SHINE Technologies has reported clearly visible Cherenkov radiation resulting from deuterium-tritium fusion.
The suppliers are expected to make up key elements of the supply chain for the next-generation, small nuclear plant.
Advanced optimization techniques, coupled with artificial intelligence and machine learning, can revolutionize agrivoltaics on a g…
The EPA cites carbon capture and sequestration (CCS) and hydrogen co-firing as bedrock strategies for achieving its proposed carbo…
Power Engineering®️ is the voice of the power generation industry including news, exclusive articles, and in-depth reviews. Subscribe today to receive critical information and industry updates to remain competitive in today’s market.
