Published on EGEE 101: Energy and the Environment (https://www.e-education.psu.edu/egee101)

Home > Lessons > Lesson 10: Acid Deposition

Lesson 10: Acid Deposition

Overview

Dr. Mathews and the Nittany Lion shrine (this will be you after graduation!)
Click Here for a Transcript of the Lesson Introductory Video

[Dr. Mathews is standing next to the famous Penn State Nittany Lion Shrine.] Dr. Mathews: Most of you all know that I am standing by the Nittany Lion Shrine. A beautiful artistic rendering of a puma or a mountain lion. It has been standing here since 1942. And every home game it has its own honor guard to protect it from visiting fans from desecrating it with other colors or symbols or even paint. But how would you feel if it had been under attack every single day? Ever since 1942, it’s been made under attack from acid rain or acid deposition. We're in the center of the northeast and we have some of the highest PH rainwater in the United States. This is made out of limestone and it is a single 32-ton block that has been beautifully rendered into this rather wonderful lion. But when it rains and this acid-y content it's going to start eating away at this limestone block. There is also acidic snow, there is also dry deposition, and there is acid fog. All of this is attacking this statue. But since 1990 and the clean air act of 1992 we have tried to reduce our sulfur dioxide emissions and reduce or NOx emissions. How we did that and the impact that it has had on our air quality and on our PH of the water is what we are looking at today. [Video ends]

Credit: JPM

Lesson Objectives

This lesson will be successful if, when you're through with it, you're able to do the following:

  • Explain acid deposition formation and its effects
  • Articulate policy for emission reduction
  • Explain technologies for abatement
  • Discuss efficiency increases for pollution reduction

Rain, Rain Go Away - The Story of "Acid Rain"

Watch this 2 minute overview of Acid Rain.

Click here for a transcript of the What is Acid Rain video.

What is acid rain? Acid rain is any form of precipitation with high levels of nitric and sulfuric acids. It can occur in the form of snow, fog, and even dry materials that settle to earth. Most acid rain is caused by human activities.

When people burn fossil fuels, sulfur dioxide and nitrogen oxides are released into the atmosphere. These gases react with water, oxygen, and other substances to form sulfuric and nitric acid. Winds may spread these acidic solutions over hundreds of miles.

After it falls to earth, acid rain enters water systems as runoff and sinks into the ground. This can make water toxic to prey fish, clams, fish, and other aquatic animals. The rest of the food chain, including non-aquatic species such as birds, is often affected as well. Acid rain also harms forests by damaging trees' leaves, robbing the soil of essential nutrients, and making it hard for trees to take up water. By designing cleaner power plants and using fewer fossil fuels, we can reduce the number of pollutants that create acid rain

Credit: National Geographic

The media is keen on using simplistic terminology to explain environmental issues, acid rain is one of those terms. What is it that the media has got wrong here? Glad you asked! The simple answer is that "acid rain" is not just rain. It can come in all forms of precipitation that you might encounter — snow, hail, fog, etc. But once again, this doesn't cover all the bases. The meteorology chaps use the term precipitation to indicate water falling to earth in any of its many forms (there is a big difference between 2 inches precipitation as water and 2 inches precipitation as snow). But here is the crux - not all precipitation is wet - sulfate aerosols are precipitation and they are dry.

So, now you know that people who use the term acid precipitation make me much happier than those who use the term acid rain. And students like you, who understand that acid deposition is a combination of all the components below, make me happiest of all (Now all I need to do is get you all jobs in the media).

Components of Acid Deposition

  • Rain
  • Snow
  • Hail
  • Fog
  • Ice
  • Sulfate aerosols

Who is to blame?

Blame is something that we like to assign, especially as it can be passed away from our own consumption patterns, but remember the pollution is generated as a by-product when electricity or fossil fuels are used. We could blame the utilities (but not nuclear or renewable utilities) but they did generate electricity cheaply. Back in 2000, 19 of the 20 cheapest electricity generation utilities were coal-fired. We like cheap electricity. We complain when the prices go up. But we also require clean air and water — but we don't want to pay for it. It is similar to gasoline. Additional removal of the S costs money, it is a cost that is passed on to you, the consumer. The cost is around 7 cents a gallon to meet the latest Clean Air Act amendments-API estimates. Less pollution typically cost additional investment and change that come at a cost ($).

The presence of SO2 and NOX in the atmosphere is what makes the deposition (rain) acidic.

SO2 + 1/2 O2→SO3

SO3 + H2O → H2SO4

NOx + H2O → HNO3

These acids, when dissolved in water, exist in the ionic state so that the hydrogen ions are free to wander around: 2H+ and SO4 2–. The pH scale is essentially a measure of the ions in the solution.

Acid Rain Cycle
The Acid Rain Cycle
Credit: USGS

Source of SO2 and NOX emissions

As shown in the charts below, the culprits for the emission of these gasses were vehicles for NOx, and utilities (mostly coal-fired) for SO2.

2 Pie charts. anthropegenic NOx sources, 56% is from vehicles and 22% from utilities and SO2 comes mostly from electric utilities (66%)
Left: US NOx Emissions. Right: US SO2 Emissions in 2011
Credit: Nasa

As you can see in the chart below, emissions of SO2 have been decreasing as we use less coal (changes in our electricity sources), are using cleaner coal, and through the use of SO2 scrubbers on coal-fired utilities. Some of these changes are due to emission reductions in the Clean Air Act. The largest reduction is due to the adoption of natural gas (and renewables). The natural gas switch was due to changing economics rather than policy.

Plot showing the reduction in SO2 emissions being mostly from coal between 2000 and 2020
Emissions of SO2 from 2000 - 2019
Source: EPA

Natural Sources of SO2 and NOX?

There are natural sources as well as anthropologic. Here is some more volcano information concerning gasses [1].

 Scientist observing a volcano from the rim
Volcanoes emit SO2 and CO2 (important in the next lesson) into the atmosphere. They are an important natural emission source.
Credit: USGS Hawaiian Volcano Observatory [2] by Janet Babb, USGS

Listen

Click on the link to listen to this audio file about Natural sources of acid precipitation [3].

Click here for Acid Precipitation Audio Text Version

Dr. Mathews: One whole day I had as a young man, we went to the Canary Island, which is a Spanish speaking set of islands off the North African coast; beautiful volcanic islands. And I remember two things very vividly. One was going up Mount Haidi. It was an incredible drive. I was amazed how religious these people were; there were crosses everywhere. And upon getting to the top there was a weather observatory and an astronomical observatory because the air was nice and clean there. And also there was sulfur lying around. There were little holes everywhere where you could actually pick up chunks of sulfur if you wished to. It smelt of rotten eggs, the volcano was dormant but giving off lots of gasses. The other thing that I remember vividly is on the way down the crosses were not there. The crosses were not there just for religious reasons, they were there when someone had gone over the edge. And with a thousand foot drop I was terrified the whole way back done. Anyway, volcanoes give off lots of gasses.

 Lightning in the night sky
Lightning is hotter than the surface of the sun so it should be no surprise that the temperature is high enough to break the N=N bonds (nitrogen-nitrogen triple bond) to form NOx.
Credit: adobe.stock.com

Where's the Problem? and pH issues

Map showing the west and south of the United States has less acidic rain than the northeast.
The natural pH of rain is slightly acidic. But clearly, there is an issue with increased acidity in the northeast of the United States due to the high population and industry concentration. The good news is that the acid rain problem has been much reduced by environmental legislation.
Credit: EPA

Much of Europe has been impacted by acid deposition, the famous Black forest has been severely hit and many lakes in Sweden no longer hold any fish. Europe is also a special case as this is where the industrial revolution started (also lots of high S-coal) and so it should be no surprise that the impacts have been felt there. The international aspect of pollution is also a major issue. Pollutants from the UK (SO2 & NOx) drift on the winds and pollute Sweden. Thus, even if you clean up your own backyard you are not tackling the issue of your neighbor's trash blowing into yours.

Here in the US, it is the Northeast that is suffering the most from the impact of acid deposition. Why?

  • The population is high in the NE
  • Coal was the major utility fuel (and still has a significant contribution)
  • Industrialization is heavier in the NE
  • Weather patterns

pH

So how do we measure the acidity of the rain? We use a pH meter and the pH scale to determine how acidic (or its opposite - alkaline) the water is.

Do you think you could put these relatively common household chemicals and bodily fluids into the correct order?

  • Ammonia cleaning solution (NH3 based)
  • Vinegar (carboxylic acid HCO3)
  • Urine
  • Clorox (cleaning solution, chlorine based)
  • Sulphuric Acid (H2SO4)
  • Nitric Acid (HNO3)
The pH scale
The pH Scale
Credit: USGS

Looking at this pH scale, can you tell what pH actually is?

Pregnancy tests are based primarily on pH readings (peeing on a stick)- a woman's urine changes pH when she is pregnant. This is actually old technology that has been rediscovered (modern tests now detect hormones.)

So what is the pH of pure rain?

Rain is naturally slightly acidic as it can absorb CO2 from the atmosphere which alters the ionic chemistry of the water. Thus, to be acidic, the rain (or snow, or............) must be below (more acidic) 5.3 on the pH scale.

Impact on Fishes, Trees, & Statues

 Black and white drawing of a salmon and a rainbow trout.
Salmon and Rainbow Trout (yummy). Their presence indicates the water quality is high. Bring on the Chips! (French Fries).
Credit: Oregon Fish & Wildlife

Impact on Fish

As the rivers become more acidic, the problems with acidification of waterways are:

Biodiversity Reduction

As the acidity becomes significant, the life that the river can support is reduced to the more resilient fish.

Acid Shock

Recall that not only can rain be acidic, but hail and snow too. When spring arrives the snow melts. If the snow was acidic, then there can be an acid event that can kill many of the fish in the stream. This tends to happen in streams because of the dilution factor in some of the rivers. The technical term is an episodic shock (or acid shock).

Aluminum

There are all sorts of metals and minerals in the rocks and soil that are dissolved by the acidic water and washed into the streams. Aluminum (Al) is of major concern as it can interfere with the breeding cycle.

Mercury

There is also mercury (Hg) that is dissolved by the acidic water, washed into the streams, into the fish food, and into the fish. The issue is not that the mercury impacts fish, they don't care, but we should if we eat the fish. Mercury concentrates in the liver! Bears, Eagles, and other animals that eat the contaminated fish are also at risk. See the U.S. EPA Website on Mercury [4] for more information if you are interested.

Impact on Trees

The impact on trees is dependent on the acidification, type of tree, and interaction with the tree. The acid deposition can attack the leaves; sulfate aerosols lie on the leaves like a layer of dust (ever dust your houseplants?). Regional haze reduces the amount of light reaching the trees. Nutrients can be washed out of the soil. An acid fog can be in contact with the trees for long periods of time. Aluminum uptake is also a problem for the trees! Certain trees are more susceptible, however, and it is not uncommon to see what appears to be a healthy forest only to find an obviously distressed forest higher up (when the tree type changes).

 Map of US showing forest types. See link in caption for details.
Four major forest types assessed by NAPAP and areas identified as sensitive ecosystems.
Credit: NADP
Comparison of Red Spruce and Sugar Maple. See hyperlink in the caption for more details.
Acid Deposition Effects on Trees.
Trees are under attack from the rain and the fog directly but of greater concern is the loss of nutrients from the soil and the aluminum which the tree can uptake. Certain trees are impacted more severely than others.
Credit: USGS

Impact on Statues

Coming from the UK, I have seen a great deal of acid deposition damage. We used a great deal of limestone in our building and for decorative structural features, including statues (marble is by far a more expensive medium but is slightly more resilient). Limestone is common in PA and in large areas of the UK. Chemically it is CaCO3, calcium carbonate. It easily dissolves in slightly acidic water (stalactites and stalagmites are lime [stalactites are the ones that grow down, think tights go down!]). If you are interested, visit the Great Basin National Park website for some beautiful images of stalagmites [5].

When limestone is dissolved it forms calcium oxide (CaO) which is lime. (This is like the part in the movie that seems to be extraneous information but is important later on in the movie). One way to treat acid streams is with lime. 

Unfortunately, many statues, countless gravestones, and the information they contained have been lost due to acid rain exposure. The acid deposition has "eaten away" the limestone. We now use special paint on our automobiles to protect the paint from acid damage. It costs about $13 more per car, but think of how many cars there are! How much is the loss of a statue? How much is the loss of a healthy stream in PA?

 3 pictures. 1-statue of woman in 1908 you can see details of face. 2-same statue in 1969. You can not see details of face. 3-gravestone, you can not see any of the markings on it.
(left) Having survived a couple of centuries this statue is showing some signs of old age. (center) Some 61 years later, the impact of acid deposition has destroyed this artistic rendering of a young woman. (right) Can you read the inscription on this PA gravestone?
Credit: left and center: USGS, right: JP Mathews

Acid Mine Drainage

Acid does not all come from the sky! Pennsylvania has about 3,000 miles of polluted streams, creeks (I am not sure what the difference between creeks and streams are, we don't have "creeks" in the UK!). The culprit is not thousands of miles away, it is much closer to home: abandoned coal mines & culm piles. Recall when we discussed coal mining activities that the pyrite in the coal is uncovered and dissolves in the water, migrating through the abandoned coal mine. When the mines are abandoned, the pumps are taken out and the mines often flood with acidic water that can flow into the streams and pollute the water table. The Culm piles also expose the pyrite to the elements (wind, rain, hail, snow, etc.) and so they too contribute to the acidification of the local waterways. Pyrite is fools' gold and contains iron and sulfur.

(left) yellowish stream flowing out of an old mine entrance (right) discolored stream through the woods.
On the left, an old mine entrance, or perhaps a ventilation entrance, which long ago flooded, leaving visual evidence of acid mine drainage (AMD) here that adds to an already high pH level for rainwater of the North East. On the right, a more picturesque stream (or sulfur creek), in Northeastern PA, discolored because of the Iron Hydroxide (orange color) deposits. The iron comes from the pyrite, and the sulfur will be dissolved, forming sulphuric acid, which lowers the pH and makes the stream more acidic.
Left: Office of Surface Mining. Right: EPA
A short movie about acid mine drainage.
Click Here for a Transcript of the Acid Mine Drainage video

[Video opens panning down a discolored stream.] Dr. Mathews: This is a beautiful area of the anthracite region. This is one of the many sulfur creeks. It gets that name from its yellow or orange color in nature. This is due to acid mine drainage. What happens is the iron discolors the bed, the stream bed, with this yellow coloration, it is called yellow boy, and it is actually iron hydroxide. Unfortunately, it means that the stream is in very poor health and it is not a good spot to go fishing. It is very sad to see the beautiful areas of this anthracite region devastated in this manner. [Video ends panning the stream.]

Credit: JPM
Anthracite breaker in front of a culm pile.
An old anthracite breaker stands in front
of the culm pile.
Credit: JPM

To the left, an old anthracite breaker stands in front of a culm pile. Recall that this culm can be cleaned and used to fuel fluidized bed combustors. The breaker is where the coal was crushed and sized. The coal which was rejected (because the particle size was too small or the coal obviously contained significant quantities of other rocks) was thrown away on top of the pile. The exposed pyrite in the pile will produce acid mine runoff when it rains.

The effects of AMD on wildlife can be significant, as evidenced by the map below. Look at the distribution of fish on this map, then click on the arrow to see where the major coal deposits are in Pennsylvania. See any connection?

Region Haze-Mostly Sulfate Aerosols

Remember the difference between acid deposition and acid precipitation?

The sulfate aerosols (suspended fine solid particles of sulfate or tiny droplets of a solution of a sulfate or of sulfuric acid) that are also released into the atmosphere, are in many ways more damaging than the rain and hail. Their small size permits them to enter into our lungs where they cause breathing difficulties. The small particles also scatter light, which reduces visibility. After a good rainstorm, the visibility will often improve as the sulfate aerosols (and small particles) have been washed out of the air. In the US National Parks, 60% of the visibility reduction is attributed to sulfate aerosols. Dust from unpaved roads and small particles from forest fires (recall the PM2.5 challenges) also contribute. This has been so bad in the past that wildfires in Mexico caused unhealthy air in Houston (not a city renowned for healthy air in the first place). Thus, regional haze is a greater hazard when sulfate aerosols are present.

Recall that we looked at the regional haze challenges in the previous lesson.

Coverage Map: Acid Deposition Issues

NOTE This coverage map, unlike all the others you've encountered, simply illustrates the issues involved in Acid Deposition. There are not any active mouseovers on this one, but it remains a visual quick-reference summary of what I consider important from this first half of the lesson.

Acid Deposition Impaces fish, trees, material damage, acid mide drainage, international Issues, cost for Avoidance, breathing issues, and Regional Haze

The Clean Air Act 1990 (and ammendments)

The Clean Air Act was very successful legislation for improving air quality (with an initial focus on acid "rain"). The three images below show significant reductions that occurred between 1980 and 1999 concerning the SO2 and NOX concentrations. These reductions have continued into the 2020's and will continue to improve with renewable energy and vehicle electrification (see the chart below the maps).

Interactive map showing NOx emissions in 1990 and 1999.  See text version for details
Total Utility NOx Emissions in the US from 1990-1999.
Roll your mouse over the years below the image to see the change in utility NOX emissions in the U.S.
Text Version
Source: USGS

Interactive map showing Total Utility SO2 Emissions 1980.  See text version for details

Total Utility SO2 Emissions from 1980-1990-1999.
Roll your mouse over the years below the image to see the change in utility SO2 emissions in the U.S.
Text Version
Source: USGS
1989 1997
Interactive map showing the change in utility SO2 emissions that occurred over the course of about 8 years in Pennsylvania and the surrounding regions.
Roll your mouse over the years below the image to see the change in utility SO2 emissions that occurred over the course of about 8 years in Pennsylvania and the surrounding regions.
Source: EPA

These reductions have continued! The reduction is even more impressive when you factor in population growth, increase in the gross domestic product, and the increases in miles driven and energy used. Here the 6 common pollutants are CO, lead, NOx, VOC's, PM10 / PM2.5, and SO2.

Graph compares growth areas and emissions, 1970-2015.
Comparison of Growth Areas and Emissions, 1970 - 2019
Credit: EPA [6]

Acid deposition problems prompted the “Acid Rain Program” within the Clean Air Act of 1990. This, along with the Amendments of 1992 and some later amendments, tackled the point sources of SO2 and NOX. Mostly the larger coal-fired utilities were impacted (about 110) and started to reduce emissions in 1995, and further reduced emissions in 2000. Obviously, from the figures above, we have had great success in reducing the emissions of these gases. So how was this achieved?

Limits were placed on how much these large, mostly coal-fired utilities, could emit. Simple, right? Well, perhaps not as simple as you might expect. If all the utilities had to reduce emissions by the same percentage we would lower the emissions but at drastically different costs. So what? Well, this had been the approach used in other countries and the approach used in the past, but there is a better way: Emissions Trading!

This was the centerpiece of the acid rain program and now the preferred approach for some emission reductions. Limiting the total number of permits controls the quantities of emissions permitted. By requiring that a utility provide a permit for every ton of SO2 (the program started with SO2 so let's look at that scenario) they emit. If a permit is not available they have to clean up those excess emissions. Okay, so far this is not any different from mandated reductions. Here is the key; you can buy or sell permits at market value. The market controls the cost of permits. Overall this is a lower cost reduction strategy since a utility company can calculate if it will be cheaper to purchase permits, clean up the emission to the minimum (depending on how many permits they have), or reduce emissions to below the number of permits and sell off the excess permits to lower the cost of meeting the reduction. This lowers the cost of meeting the emission reduction, and so hopefully the cost associated with meeting the reductions will not impact the price of electricity as much (lower increases in the cost of electricity for you and me)! Given that most companies own at least several utilities much of the “trading” can occur in-house.

How do utilities get permits?

There are several methods:

  • An old utility will be given permits to the point where they can release 1995 numbers - 2.5 Lbs of SO2 per million BTUs of thermal input. After 2000 those numbers were reduced to 1.2 Lbs SO2 per million BTUs of thermal input. If you built a utility after 1995 tough luck for you! No permits for you, you have to buy what you need (essentially you will have to employ modern technology to prevent or capture emissions.)
  • There are several auctions during the year where the EPA will sell off permits.
  • There are also several brokerage firms that will help you buy or sell permits in the open market.
  • Permits do not run out so you can save them to use later.

The EPA provides a nice site explaining the Air Markets Program [7]. Give it a quick look over. - It is a good reiteration of the acid rain story (helpful for good exam scores).

This market-based approach worked well and was a major US success when it was included in the Kyoto Protocol (more on that in the next lesson). The NOX emissions were more complex as it depends on boiler type and size.  The next few pages look at how the emissions were reduced either by preventing their formation or by cleaning up the flue gas prior to it going out of the stack.

A summary of the Clean Air Act aimed at creating a healthy, productive environment, linked to sustainable economic growth and sound energy policy:

  • encourages the use of market-based principles and other innovative approaches, like performance-based standards and emission banking and trading;
  • provides a framework from which alternative clean fuels will be used by setting standards in the fleet and California pilot program that can be met by the most cost-effective combination of fuels and technology;
  • promotes the use of clean low sulfur coal and natural gas, as well as innovative technologies to clean high sulfur coal through the acid rain program;
  • reduces enough energy waste and creates enough of a market for clean fuels derived from grain and natural gas to cut dependency on oil imports by one million barrels/day;
  • promotes energy conservation through an acid rain program that gives utilities the flexibility to obtain needed emission reductions through programs that encourage customers to conserve energy.

A more modern examination of the Acid Rain Program results [8] can be found on the EPA website.

Buy a Cleaner Coal

Reducing S02 by Buying Cleaner Coal

Coal quality is very variable, which is one reason that we take the measurements of proximate, ultimate analysis along with calorific value. Shown in the graph below is the acceptable line for emissions of 1.2 pounds of sulfur dioxide per million BTU of chemical input. If you purchase a coal below the green line and you have an efficient utility (normal efficiency should be >35%) then you will not have to spend money on cleaning the coal or installing a scrubber. As in the case of our crude oil, the quality of our coal is decreasing because we have consumed much of the cleaner coals. Many of our coal regions are not under the green line and so a sulfur reduction strategy is necessary to ensure compliance with the Clean Air Act. One of the easiest methods of reducing the sulfur dioxide emissions is to buy the cleaner (and thus more expensive) coal from another coal seam.

Line graph.  Percent Sulfur by weight vs Calorific Value.  See caption hyperlink for details.
We have discussed coal quality measures before. Here is why S content and calorific value are very important for emissions. By increasing the calorific value (selecting another coal with a similar S content but higher calorific value), or selecting a lower S content coal (but with the similar calorific value) we can reduce the SO2 emissions. Of course, we could always use other energy sources (renewable, nuclear), or capture the SO2 in a scrubber.
Credit: USGS

Unlike most coal properties, the S content (from the ultimate analysis) has very little to do with rank. Anthracite does have a low S content because the high temperatures necessary for anthracite creation would have been sufficient to remove the S as H2S. However, increasing in rank has very little to do with S content, rather you need to go back millions of years and look to see where the sea shore was. Seawater infiltration into the existing coal seam was responsible for much of the enhanced S content of some coals. Fortunately for the Western states, their coal was not exposed and they tend to have the cleaner (lower S) coals. So as Wyoming coal has a lower S content there was a massive switch to this cleaner coal.

Graph showing the rise of low-S coal production and the decline of higher S coal fields.
The high S content of the Since this figure above was produced there was a more significant drop in coal production [9] as shown earlier in the course.Northern Appalachian region, uneconomic mining conditions (room and pillar underground mining), the Clean Air Act, and the closing of many underground mines because of coal depletion contributed to the reduction of the percentage that our region contributed to the total US coal production. Increases in productivity (Longwall mining) enabled the total tonnage to remain about the same for the 1990's to 2000. The major increase was the purchasing of much lower S coal (slightly lower calorific value) from the strip-mining operations in the West (Wyoming and Montana). 
Credit: USGS

Coal use is declining in the U.S. as we use more natural gas for electricity generation and industries also move towards natural gas. A smaller contribution comes from more wind power generation.

Enter image and alt text here. No sizes!
Recall that I have indicated that the economy has a role to play in our emissions of pollutants. During the recession, there was much less industrial activity and less coal was consumed, and pollution emissions dropped accordingly.
Credit: eia [9]

Catalytic Converter, Scrubbers, & Low NOx burners

Scrubbers - To Remove SO2 from the Flue Gas

The formula for this process looks like this: SO2 + CaCO3 + 1/2O2 + 2H2O → CaSO4 + 2H2O + CO2

 Babcok & Wilcox Absorber Module. Described the paragraphs below.
Babcock & Wilcox Absorber Module.
Credit: Babcock & Wilcox Company

The schematic above is of a device known as a scrubber. Their role is to take the flue gas with the products of combustion and remove the SO2 emissions prior to the flue gas being released out of the stack. This wet device also has the advantage of reducing some of the particulate matter. There is a dry version as well, but the wet scrubber is my favorite!

  1. SO2 and Flu Gas go into the Babcock & Wilcox Absorber Module.
  2. Multiple Interspatial Spray Levels. A fine spray of water is used to create a mist through which the flue gas must flow up (counter-current flow).
  3. Absorption Zone. The mist absorbs the SO2...
  4. which then falls to the bottom of the tank into a pool of slightly acidic water, after which lime is added.
  5. Oxidation Air Supply. Air is also added, which then oxidizes the product, allowing it to be converted into a solid product that can either be disposed of or sold as gypsum.

Most of you are, quite literally, surrounded by a version of this solid byproduct much of the time - GYPSUM is the primary material in the drywall, or wallboard, that makes up the interior walls of most new houses, apartments, and buildings today. If you're sitting in a place right now that has a smooth or textured painted surface or has wallpaper, then it's a pretty safe bet that gypsum lies underneath.

Catalytic Converters—to remove NOx from the flue gas

We have discussed approaches to reducing NOx from automobiles. One approach used was the use of a catalytic converter. The same technology can be used to remove NOx from power plants. The catalyst works in exactly the same way but on a much larger scale. The buildings can be 8 stories high and are the size of an apartment complex. A building that size will hold millions of dollars worth of catalyst that has to be replaced every few years. So, NOx reduction is an expensive process.

 Schematic of a Catalytic Converter. See caption for details.
In a similar approach to the catalytic converter on the gasoline car, reducing NOx via a catalyst can be used for NOx removal from flue gas. The catalyst is close to the boiler, as the gases need to be hot for the catalyst to work. There is a greater challenge to reducing NOx under these conditions because of particulates and SO2 in the flue gas.
Credit: LANL

Low NOx Burners - To Reduce the Formation of NOx in the Boilers

Don’t worry too much about the complexities of low NOx burners. It is enough that you know that they work by controlling the mixing of the air (oxygen) and fuel, and lowering the temperature of the flame (by controlling the mixing). You need to produce a radical-rich zone so that you can attack the formed NOx. This has to happen early in the flame or else the nitrogen in the char will most likely form NOx. Hopefully, the result will be more ubiquitous nitrogen and we will release less NOx into the atmosphere.

 NOx Reduction Boiler.  System View and a 45degree Rear-View Turbine view.  See caption for more details.
Here you see a couple of images portraying low-NOx burners. By careful mixing of the air and the fuel, you can lower the flame temperature and lower the NOx emissions. The right image illustrates the different air flows coming from the burner, and the swirling process that enhances the mixing of the fuel and the air (mostly nitrogen and oxygen).
Credit: left: IEA, right Fluent

Fluidized Bed Combustion

 Picture of a valley with grass and trees.
Here an old coal strip mine has been reclaimed
using the ash and lime particles (with a gypsum
layer) from the fluidized bed. This also helps
prevent acid mine drainage from this site and from
the culm pile.

There is a method of burning coal that uses existing technology to produce electricity with very low emissions of both NOx and SO2: Fluidized Bed Combustion. The fuel need not be coal - fuel flexibility, in fact, is one of the reasons that this is an attractive boiler configuration, but most PA fluidized beds tend to be in the anthracite region, where there is abundant and FREE fuel: Culm Piles (or so called "gob piles" in Western PA).

These "manmade mountains" of reject coal actually contain a significant quantity of coal (recall these culm piles help produce acid mine runoff - from the pyrite contained in the anthracite). By removing the culm pile, producing electricity, employment, and taxation and cleaning open-pit operations (more on that in a bit) these operations significantly improve the local area, environmentally and economically.

The reject coal is cleaned then combusted. The relatively large pieces of coal burn in the presence of limestone. Suspending the particles with air flowing from underneath enables the coal to be combusted slowly, the ash forms most of the bed (where the suspended material is is called the "bed") along with some coal and lime (from the limestone CaCO3 heat-----‡ CaO (Lime) + CO2). This gives the coal a long residence time in the bed, allowing lower temperature combustion (thus less NOx) and the S is not emitted into the atmosphere because it is captured in-situ (in place) by the limestone. CaO + SO2 + 1/2 O2-‡ CaSO4 (gypsum)

Like the output from S scrubbers explained above, the gypsum becomes a solid and can be used to fill in the old strip mining holes to restore the damage from old abandoned mines. But, the output from this process produces a less pure form of gypsum than from scrubbers, so this gypsum is not usable for wallboard materials (it is just a coating over the lime particle).

Efficiencies & Conservation

You've heard me say this before, and I'll say it again, because if you leave this course in a few weeks remembering only one thing (which I hope is a massive under-estimate), then it is this; When discussing possible solutions for pollution from fossil fuels, Efficiencies & Conservation are the 2 answers that are nearly always right. We have spent much time throughout this course talking about these two issues, so below are simple summaries of the main efficiency and conservation points as a reminder and refresher.

Automobile efficiency

This is not going to do much for SO2 emissions from gasoline engines (as the S content in gasoline is already so low, and getting lower (30 ppm)) but will lower emissions from diesel engines (low sulfur diesel achieves this too). However, increased efficiency in automobiles (mpg and passenger occupancy) will reduce NOx emissions. We talked about this in detail in the previous lesson on SMOG. Hybrid cars, electric cars, fuel cell cars and most alternative fuels will also reduce emissions either directly or through increased efficiency. There are also public transportation options.

Electricity Use

More efficient electrical devices in the home mean less electricity demand. Less electricity demand means less pollution.

Electricity Generation

There are plenty of non-fossil fuel utility options (renewable & nuclear). Biomass does not contain much S and so its utilization would help SO2 emissions. Natural gas combustion is a more efficient method of electricity production than existing coal utilities. While there are practically no S emissions there will be NOx emissions but at lower levels than old coal utilities (via a combination of increased efficiency and no fuel-NOx). If we use gasification we can clean the S and N from the gaseous fuel and obtain higher efficiencies in the generation of electricity (clean transportation fuels too).

Electricity Choice

You can, of course, pick from certain options how you want your electricity to be generated. Coal remains perhaps the cheapest, but if you are willing to pay the higher cost you can be green. In some cases, we are now seeing starting to see select renewable energy sources as the cheaper option. 

Vehicles, Sulphur, & NOx

If you recall, much of the material in the Transportation Lesson dealt with the consequences of our vehicle use. This page provides a quick visual refresher of that material most directly to pollution in the form of acid deposition. If you don't know why it is being shown go back and re-read the lesson.

 Car exhaust= Carbon Dioxide, NOx, Unburned Hydrocarbons and Carbon Monoxide. Speed vs MPG (15 mph - 25 mpg, rises to around 33 mpg from 25 mph - 60 mph and then goes slow decline to 25 mpg after 60 mph.
(left) Car exhaust (top right) Speed vs. MPG (top left) traffic jam on a highway
Credit: Left: JPM, top right: EPA, bottom right: Ibl
Illustration shows that only 1% of cars produce 35% of emissions.
Hydrocarbon Pollution from cars. [10]
Credit: EPA
Picture of a smart car.
Smart car (39 mpg highway).
 Catalytic converter
Catalytic Converter
Credit: SANDIA

Lesson 10 Coverage Map

You know the deal with these coverage maps by now. For an interactive version of this map, go to the Exam Three Study Guide module.

Accessible Version (word document) [11]

Deliverable

After looking at this map, please take the L10 quiz.


Source URL: https://www.e-education.psu.edu/egee101/node/656

Links
[1] http://volcanoes.usgs.gov/hazards/gas/index.php
[2] https://www.usgs.gov/media/images/usgs-hvo-geochemist-measuring-gases-released-k-lauea-volcano
[3] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson09/L09_Teidi.mp3
[4] https://www.epa.gov/mercury
[5] https://www.nps.gov/media/photo/gallery.htm?pg=3312826&id=609C1A91-F3AE-4FA7-886C-A4DD1A76FA54
[6] https://www.epa.gov/air-trends/air-quality-national-summary#air-quality-trends
[7] https://www.epa.gov/airmarkets
[8] https://www.epa.gov/acidrain/acid-rain-program-results
[9] https://www.eia.gov/todayinenergy/detail.php?id=44536
[10] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson09/fleet_emissions_EPA_LD.html
[11] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson10/Lesson%2010%20Coverage%20Map.docx