FLUE GAS DESULFURIZATION

A review at the science,

research, economics, and

history in America

By Justin Tang

Case Western Reserve University ‘16

Civil Engineering Major

Includes source material

Abstract:

With coal being a fossil fuel, the side effects include the emission of “greenhouse gases” and

other wasteful toxins into the atmosphere. Fortunately, many means have been used to reduce the

amount of emissions from waste gas from power plants and industrial facilities using coal as an

energy source. One of these methods is Flue Gas Desulfurization.

The air pollution caused by the combustion of coal has been well known since the day of

industrialization. China has rapidly developed in recent decades, and is also the world’s leading

consumer of coal, with the United States of America not too far behind, now is facing the same

issue.

This is not unfamiliar to the United States, however. This report and the subsequent documents

included are meant to provide a look at the legislation, development, and cost of Flue Gas

Desulfurization within the U.S. and how it has battled the air pollution caused by the combustion

of coal and turning its waste products into something useful.

FGD and its related research have been around for roughly 150 years, but didn’t arise as a

practical mechanism until the 1940s. It is currently implemented in a number of industrial

facilities, such as coal-fired power plants, to remove the Sulfur Dioxide from the waste gas

produced. Sulfur dioxide (SO2) is a major waste product from the combustion of coal. It has a

detrimental effect on the environment, as it is one of the components of acid rain.

This paper talks about the history, costs, and benefits of Flue Gas Desulfurization and all its

different forms. I have split the paper into several sections, each one describing the Historical

context, Government Policies, Byproducts, Costs, and Safety.

Historical context and statistics:

Before 1970, America’s major cities were covered in smog from industrial facilities, and there

was widespread harm to many ecological systems. Facing the severe deteriorating situation, the

U.S. Legislature passed many laws and bills, including what are now called the Clean Air Act to

help clear up the environment and skies of pollutants.

The United States had become one of the most industrialized nations in the wake of the

Twentieth Century, but it was after World War II that it became the top industrial power.

Unfortunately, big side effects came as a result of where the U.S. was getting its electrical power

necessary to power itself: burning coal. This led to the release of Sulfur Dioxide (SO2) and

Nitrous oxides (labeled with the general equation NOx), the two gases most associated with acid

rain, when the two dissolve into rain droplets.

When these compounds dissolve into rainwater, it changes the properties of the water, and as a

result, ponds, lakes, and rivers become more acidic. Sulfur and nitrogen are essential nutrients

for many life forms, but when there is a surplus the soil, the resulting over-fertilization can have

drastic effects. When this happens, native life forms that fail to adjust to the newfound acidity die

off, and new, invasive species come in. Not to mention, the toxicity can be too much for aquatic

ecosystems to handle, and there have been so-called “Dead-zones” that have emerged in the

water where very little life is found.

In addition, other problems have been caused by the release of additional substances such as

Ozone (O3) spurred by NOx being released, as well as Mercury (Hg) that somehow is emitted.

Mercury is known to be a poisonous element for many life-forms, and it, like nitrogen, is prone

to seeping into rainwater as it can be released as a vapor. On the other hand, Ozone is very

beneficial to the planet when present in the right place. The substance is known as one of the

forms of oxygen mostly located in the earth’s Stratosphere that shield the planet from harmful

ultraviolet rays, but when it is present in the lower atmosphere, it has other effects. Too much

ozone in the lower atmosphere can lead to damage in plants, ecosystems, and even humans.

Breathing it in can cause asthma, lung damage, and premature death in those afflicted with

cardiovascular disease.

As being showed in above charts, the power sector sulfur dioxide emission has accounted for

most part of total sulfur dioxide emission; 70%, but only accounted for 20% total nitrogen oxides

emission. Other sources such as automobile emission, etc. has contributed most of nitrogen

oxides pollution. Therefore power sector de-sulfurization has become the most efficiency way

for reducing the sulfur dioxide pollution. The following charts

Other greenhouse gases emitted have led to many scientists becoming concerned with global

climate change as well, as the US National Oceanic and Atmospheric Administration (NOAA)

has documented climate change within the last two hundred years. CO2 levels had risen from 280

parts per million (ppm) in pre-industrial times all the way up to 382 ppm in 2006. Methane levels

had also risen by an astonishing 148 percent in the same time frame. This may or may not have

coincided with a 1.4 degree rise in average global temperature that also happened in this time

frame.

US Power Sector and Government Policies

To combat all these unfortunate trends, the U.S. government became concerned with the

environment and started up new research efforts and laws. As technology advanced, the U.S.

would only see more emissions as a result of increased power needs, which led to scientists

doing more research in reducing the wasteful components of emissions, as well as finding better

uses for them. This was especially true for coal.

Like China, the United States is another world power that heavily uses coal to obtain energy,

although much of the country’s 97.5 quadrillion (1015, or one thousand trillion) Btu comes from

Petroleum, which accounts for 36 percent of total energy. Natural Gas is second at 27 percent

followed by Coal at 19. Other U.S. energy sources come from Nuclear energy (8%) and

Renewable sources (10%), which include solar, geothermal, biomass, and hydroelectric.

When accounting for domestic energy production, however, coal takes up 24 percent of the

power source, and 49 percent in electricity generation, making the significance of coal that much

more important when talking about U.S. energy production.

A diagram below from a paper documenting the U.S’s experience in dealing with such problems

shows the U.S. power distribution as I described them, as of the year 2005:

On the following page is a more comprehensive set of data and statistics on U.S. electricity:

As mentioned earlier, coal comprises of a large chunk of American energy, not including the

energy utilized for transportation. This pie chart mentions just how much the U.S. depends on

coal:

Even though there are other gases that come out of flue gas, this paper will focus specifically on

Flue Gas Desulfurization in its efforts to extract sulfur dioxide out of the air.

Flue Gas Desulfurization came to prominence after the Clean Air Act was passed into Federal

law in 1970. It was revised several times in 1977 and in 1990. It covered multiple pollutants in

the air and multiple forms of energy, such as petroleum for transportation as well as industrial

and domestic use. The 1990 revision specifically touched down on the issue of acid rain, and

encouraged the use of flue-gas scrubbers in coal plants to reduce the amount of sulfur dioxide

emitted.

While Wet Flue-gas Desulfurization had actually been developed during World War II, it was

not widely used until the Clean Air Act was implemented in the 1970s. It was immediately put to

use because it was the only solution available. The “dry” FGD systems, also known as the

Circulating Dry Scrubber (CDS) were developed by the 1980s to be put to use in smaller

facilities where wet FGD was too costly to be implemented.

Implementation of such legislature has proven rather effective at decreasing SO2 levels in the

country, as seen on the maps above.

In 2012, the Environmental Protection Agency published a review of the Secondary National

Ambient Air Quality Standards for Oxides and Sulfur - Final Rule. The rule went into effect in

June 2012. More information about U.S. legislation and reasoning behind such legislation can be

found in that document.

The Desulfurization Process

The United States has implemented FGD for a while to not only clear up the skies from air

pollution, but to create synthetic gypsum.

So anyways, this is a little rundown on how FGD works. Exhaust gas from the coal-burning

facility is brought into a device called a “scrubber” before being released into the atmosphere.

The flue gas reacts with a compound (in this case limestone or calcium carbonate) in order to

extract the Sulfur Dioxide. Normally this is done in a scrubber separate from one used to weed

out ash, but there are currently efforts to combine the two processes into one scrubber for

convenience.

The equation to first desulfurize the flue gas from factories is as follows:

CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)

However, there are further steps to turn the byproducts into the desired byproduct, gypsum. The

one used to create gypsum from the reaction is as follows:

CaSO3 (solid) + H2O (liquid) + ½O2 (gas) → CaSO4 (solid) + H2O

This requires a lot more effort on behalf of the absorber than other methods, as the CaSO3 from

previous reactions is “re-watered” into a process called “forced oxidation”, but this is the

primary way of creating the compound gypsum (chemical formula CaSO4*H2O).

A simple diagram of the process can be displayed in the following diagram of a scrubber (from

Wikipedia):

This following diagram, found on FGDProducts.org, shows a more comprehensive look at

limestone- forced oxidation, showing the different chambers for “scrubbing” the waste gas. Note

the sophistication involved with the overall process, including the chambers “forcing” the

oxidation and facilitating water reclamation from the gypsum:

Below is a close up of the dewatering “hydroclone”, courtesy of French company Alstom:

There are actually two ways in which companies decide to dispose their byproducts. One way is

to dispose their byproduct gypsum into a pond in order to dewater it to about 5% moisture levels

by evaporation. It can then be reclaimed by a hydraulic dredge, re-watered, then set back into use.

Otherwise it can be sent to a mechanical dewatering system.

After dewatering, the final product may or may not be mixed with other additives such as ash.

The resulting gypsum looks a little like this: a powdered substance ready to be shipped out. (the

adjacent picture depicts dried gypsum within the pond)

(Image by the University of Kentucky)

While the U.S. still lags behind other countries in terms of Gypsum production, it is still the

fourth largest producer of the mineral, just behind countries like China, Russia, and Iran. Thanks

to FGD, the US has been catching up with its competitors. By 2015 the U.S. should be able to

crank out 20 million tons.

My research has shown that the U.S. has taken steps to put to use this synthetic gypsum, and

with great use. There are multiple ways of obtaining gypsum, including mining and FGD.

However, over the years, the U.S. has continued to see its amount of synthet ic gypsum increase,

and as of the year 2011, synthetic gypsum accounted for an astonishing 54 percent of domestic

supply in the country4.

The Desulfurization Profitable Byproducts

Now Gypsum is an important component in creating drywall, a crucial part to home construction,

but can also be utilized in the crafting of other construction materials such as gypsum blocks,

plaster, as well as mixing into concrete. Most of the synthetic gypsum is utilized to create

drywall and would be a welcome alternative to using gypsum from the mines. There have been

announcements for five new “wallboard plants” dedicated solely to drywall manufacturing, and

plans call for the plants to run mostly on FGD gypsum. Most of these plants figure to be located

on the East Coast:

Gypsum obtained from FGD has been shown to be a lot more pure than mined gypsum, as the

synthetic version has fewer impurities (the synthetic version is 96 percent pure as opposed to 80

percent in naturally occurring gypsum). Much of the gypsum obtained from wet FGD processes

is strong enough to be used in construction materials, whereas gypsum and other byproducts

obtained from dry FGD processes are not.

In that case, it is also widely used in agriculture, usually as a fertilizer. Gypsum provides plants

with two vital plant nutrients, calcium and sulfur, and it also plays a role in balancing salts in the

ground, as calcium ions from gypsum can scatter excess sodium in the soil and cause it to

dissolve. In some other cases, gypsum can be applied to manure in order to decrease the

solubility of harmful agents such as phosphorous, which, if dissolved into runoff water, can

cause problems such as excess algae growth in lakes and ponds.

According to a brochure from the US Department of Agriculture from 2006, the mineral

provides stability to plants, as it easily dissolves with rainwater and promotes root growth that is

necessary to hold soil in place, thus combating erosion. Sulfur, one of the two nutrients needed,

is essential to protein formation in plants. An experiment conducted by the USDA in 2003

determined that corn plants yielded nearly twice as much corn than corn plants in the control

group.

Actual statistics from another test, run over three years, can back up this claim. Though the

numbers are not as dramatic as described in the previous paragraph, they still are rather

impressive:

The following figure shows what gypsum can do not only to crop yield, but also to soil structure.

Note how the fields with gypsum applied to them have greater yield AND have less soil erosion:

(From the USDA factsheet on Gypsum)

This next chart shows the effects on infiltration rate that gypsum has on ground soil:

Here is a chart showing gypsum’s effects on alfalfa as it supplies vital sulfur to the plants, from a

test done in 1975:

It can also be utilized in many cosmetic and food products. In some cases it can be used in food

additives, or in breweries, beet farms, or pharmaceuticals.

In 2004, a study in North American [the US and Canada combined] production showed factories

churning out 12 million tons. Of these 12 million, 8.1 M was designated towards wallboard

construction. 0.74 M was designated to mixing into concrete, 0.13 M was designated to

agriculture, and the remaining 0.025 M was sent off to other uses. As the years pass, the latter

two figures will surely comprise of a larger segment of the total with further research.

FGD gypsum has been surprisingly profitable, as shown on the following chart:

Different Desulfurization Systems and Operation Costs:

There are three different types of scrubbers involved with FGD removal of Sulfur Dioxide: wet,

dry, and semi-dry.

Wet scrubbers utilize a funnel where flue gas reacts with a wet slurry based out of limestone or

lime, with limestone being significantly cheaper than lime despite lime being slightly more

effective at 95% compared to 90%. Essentially the flue gas is tossed into a solution of water and

lime/limestone, in which the water washes out the SO2 so that it more easily reacts with the

calcium ions and thus precipitates out. A lot of water and resources are required to facilitate

reaction, however, and thus it is the costliest of all three models. Nonetheless, its high turnout of

gypsum and scrubbing efficiency make it the preferred choice for heavy duty coal-fired facilities.

Semi-dry scrubbers are similar to wet scrubbers, but instead spray the solvent into the flue gas to

facilitate the scrubbing process. The tiny droplets of water evaporate easily, allowing for the

formation of a solid deposit in mid-air. Lime-based sorbents are more than often used in this case

due to their higher reactivity and cheaper cost than sodium-based sorbents. Typical users of

Semi-dry absorbers include electric facilities burning low to medium sulfur coal, industrial

boilers, and incinerators requiring 80 percent control efficiency.

For both of these scrubbers, the slurry may include magnesium sulfate, which serves to “buffer”

the pH of the slurry as it goes into contact with and absorbs the SO2 from the flue gas, leading to

better absorption. This is in part due to the constant pH (around 6.0) being optimal for better

solubility in the water. These magnesium salts also serve to prevent buildups on the surfaces in

the absorber.

Dry scrubbers essentially use a powdered form of the sorbent (usually limestone or lime). It can

either be the wet form of the powder to absorb SO2 in a dry environment, or the dry form of the

powder in a humid environment. There are two forms that come to mind when ta lking about “dry”

scrubbers: the lime dry sprayer (LDS) and the circular dry scrubber (CDS). They have an

efficiency of about 70 to 80 percent removal.

Alstom started manufacturing Dry scrubbers back in the year 1980, and have been working to

develop them to better efficiency. Babcock and Wilcox, another company, specializes more in

manufacturing wet FGD systems. They are

Either way, the byproducts of wet FGD, namely gypsum, is dewatered to 10% moisture, and they

are either sent to the landfill or sold, and can be mixed with fly ash or left alone. The water

obtained from the dewatering chambers contains the excess magnesium sulfate, and can be sent

to a waste treatment plant afterwards.

This picture above depicts a general lime/limestone FGD system, regardless of the system being

a “wet”, “semidry” or “dry” system. The next diagram should show a picture of a more specific

system: the wet FGD Limestone absorber:

Next is a diagram of a similar system that utilizes forced oxidation, also a “wet” or “semidry”

system:

Now take a look at a “dry” system. Notice that there is still water present in the process, and that

the sorbent is sent in as a dry powder, only to be mixed in with the water inside of the absorber

itself:

Power usage:

The following is a table showing each different process of scrubbing, as well as their byproducts

and percentage of total Megawatts, as of December 1989. It is found in the document “Lesson 9:

Flue Gas Desulfurization (Acid Gas Removal) Systems:

Process By-products Percent of total MW (as of 12/89)

Wet FGD technology clearly uses significantly more electricity than its dry counterpart,

especially for forced oxidation.

Costs:

The cost for the specific scrubbers, such as lime, limestone, and others, differ in magnitude,

especially over the long term. Firstly, although FGD works at a power station, there are some

minor energy needs to power the booster ID fans needed to overcome draft loss as well as the

recirculation pumps.

According to an analysis of wet FGD processes, the capital cost estimates include:

·Equipment and material (FGD system and balance-of-plant)

·Direct field labor

·Indirect field costs and engineering

·Contingency

·Owner's cost

·Allowance for funds during construction (AFUDC)

·Initial inventory and spare parts (1% of the process capital)

·Startup and commissioning

There can also be some transportation costs as well, as companies also need to pay for the

transportation of gypsum to other parts of the country far away from the base.

Below is a chart showing the changing prices for limestone and lime:

The biggest issue with wet FGD systems is the amount of water they take up as a result of the

process. In recent years, though, Alstom has been working on an absorber that takes in seawater

as opposed to fresh water, so that they would no longer worry about taking in public water.

Though at the minute they have lower efficiency than conventional wet FGD absorbers, the

scrubbing efficiency still goes up to 95%.

The bottom line is this: wet FGD systems that utilize a wider range of equipment needed to

create gypsum wind up being costlier to maintain, but the profits shown from the sales of

gypsum should be able to offset these costs.

In addition, one of the biggest issues for cost is that, in the quotes of a 2007 report, “it is

impossible to determine capital cost of an FGD system until the contract is signed with the

supplier”. The report goes on to say that there are multiple factors in determining “total plant

cost”, such as:

Disposing of FGD gypsum in the rim ditch ponds is actually rather cheap, at just $1 USD per ton.

However, once disposed, the cost cannot be recovered, so to avoid paying this cost, there must be

a way to divert the material away from disposal, but according to the Tennessee Valley Authority

(TVA), even doing that appears to be hard, as the relationship between tons and cost of disposal

is not linear:

The only way to divert the gypsum is by mechanically dewatering it. The TVA assumes the cost

of building such a facility to be anywhere from $3 M to $6 M, and operating such a facility cost

about $3 per ton. On the other hand, transporting the gypsum from the plant to the ponds

depends on the distance, as it estimated to be $1 per ton loaded on the truck, $1 for the first mile,

and $0.1 for each additional mile travelled. Thus a mechanical dewatering facility would only

make sense if the nearest pond is more than eleven miles away from the plant, and in that case, it

probably makes more sense for such facilities to market their gypsum to agricultural users.

As for operational costs, the following charts describe the overall capital cost for equipment, for

Wet scrubbers and dry scrubbers respectively, over the course of three years from 2008 to 2010:

The wet scrubber cost curve

Operational cost is described below:

Operational cost really depends on the cost of SO2 removal, which varies by how much sulfur

dioxide is present within the coal. The chart below takes into account the different costs between

coal from Pennsylvania and coal from Ohio, which means that different parts of the country have

different amounts of sulfur within each of their coal reserves.

Safety of Byproducts:

In the early years of the twenty-first century, there were some complaints from imported gypsum

wallboards, and that these wallboards utilized in American-built homes between 2001 and 2010

caused a wide range of problems in many homes in the American South. There were reports of

carbon disulfide, carbonyl sulfide, and hydrogen sulfide emissions from the drywall. Problems

such as health problems, corrosion of metal parts and appliances in homes, and ultimately, toxic

conditions were among the side effects, and lawsuits were filed.

The question is this: how can FGD-produced gypsum avoid the problems that occurred with the

gypsum used in the ill-fated drywall panels? How do we know that it is safe to use? For one, the

laws/guidelines for synthetic gypsum are generally much stricter when it comes to being put to

use in wallboard manufacturing. The federal limit as of 2014 for drywall gypsum is now at 1

percent fly ash.

However, other reports suggest that there is quite a large amount of ash placed into the gypsum

boards, which may lead to some concerns regarding what was inside the ash. Dust seems to be a

big problem, as it forms easily from the boards and in many cases, can be breathed in.

The EPA did a comprehensive study through the toxicity of fly ash and other emissions from

FGD gypsum drywall boards. Based on a lot of data, they were able to prove that FGD gypsum

was comparatively as toxic as mined gypsum. The organization published their report in 2014.

Here is some of the data regarding products made from FGD gypsum:

Here is an analysis of the ash content from a coal plant in the state of Alaska, detailing all the

different chemicals from the ash:

From the concrete that is created with this gypsum, some are concerned that it may leech its

toxins into the environment easily, affecting humans, animals, and plants alike. The 2014 EPA

report shows just a few ways that this is possible:

However this may sound, the real problem was finding whether the chemicals leaked out of the

drywall boards posed any actual hazard to human and environmental health. Some more stats on

fly ash are listed on the next table, showing the amounts in the ash and their detection.

Once this was calculated, some more equations were put into place, and the amounts of each

element were evaluated multiple times to ensure that they posed no threat to human health. This

next table shows that most of the elements screened were no threat.

The full report goes 91 pages long, but by the use of statistics, uncertainties, and other

mathematical techniques, the EPA determined that FGD gypsum wallboards posed no more of a

threat than their mined counterparts.

To further address the safety issue of desulfurization byproducts beneficial usages, EPA

developed and published the methodology for evaluating (included in section 7 of this report)

and has committed to continue examining available information. Independent external institute

has provided peer review on the methodology for evaluating. EPA used the methodology to

evaluate the potential environmental impacts associated from fly ash used as a direct substitute

for portland cement in concrete, and from FGD gypsum used as a replacement for mined gypsum

in wallboard. EPA’s evaluation concluded that the beneficial use of encapsulated CCRs in

concrete and wallboard is appropriate because they are either comparable to analogous products

or below the relevant benchmarks. This methodology supports beneficial use decisions by

allowing the user to determine whether releases from an encapsulated beneficial use of coal

combustion residuals are comparable to or lower than those from analogous products made

without CCRs, or are at or below relevant regulatory and health-based benchmarks, during use.

American Coal Ash Association also has conducted its own study on coal ash material safety and

the report is also included here in section 8.

Future uses/efficiency of uses:

In the year 2006, the American Coal Ash Association conducted a survey that recorded put the

amount of U.S.-synthesized gypsum at around 12 million tons, of which 9 million of it was used

and the rest dumped into landfills. But in the future, there have been talks about gypsum being

utilized in plastics and fiberglass, as well as reconstituting soil in mining areas and fields to make

it sturdier.

As FGD is more widely implemented in coal-burning electric facilities, FGD gypsum production

will continue to increase, perhaps double, in the next ten years. By 2015, the number is estimated

to rise up to 15 million tons per year, perhaps even 20 million. Currently more research is being

done on it to explore different uses of the mineral.

The deciding factor in where FGD gypsum will be used will continue to depend on the demand

for it. Currently, the construction sector in the United States is going through a rebirth, as not

only are more areas just starting to be redeveloped, but also homes in areas previously devastated

by natural disasters such as Hurricanes Sandy (2012) and Katrina (2005). But there are also new

buildings, such as schools, hospitals, and office buildings being constructed, as well as gypsum

being a component in concrete, and thus the need for gypsum should continue to go up.

Agriculture appears to be the most promising window of expansion for FGD gypsum usage, as

application rates are expected to average between 1 ton/acre to 5 tons/acre (or between

0.224170231 kg / m2 and 1.12085116 kg / m2). However, the heavy metals present in the gypsum

remain a concern, as more tests are likely to be done to study their affects and bolster acceptance

in this market.

Summary:

Air pollution has always been a problem as a result of coal combustion, but thanks to Flue Gas

Desulfurization, one pollutant has not only been taken care of, but it also has been proven to be

recycled into a useful product. The gypsum that is produced can be utilized in many ways, is

profitable, and helps clear the skies of sulfur dioxide. There remains a lot of research on the part

of scientists to ensure FGD gypsum’s future success, but the sky looks bright for this industry.

Acknowledgements:

I would like to thank Professor Tang Qing for giving me this opportunity to do research away

from an academic institution, and thus help a cause within China.