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

Home > Lessons > Lesson 2: Fossil Fuels & Nuclear

Lesson 2: Fossil Fuels & Nuclear

Overview

It is not uncommon in the U.S. to assume that electricity will simply always be there when we need it (unless you have an unpaid bill!). The lights go on, the hairdryer blows (and heats), and the cell phone charges – all of it thanks to the people and processes behind the plug. Oh, and lots of fossil fuel and increasingly more renewable energy. For most of these processes that use heat, the steam generated is what is doing the work (spinning the generator). Please watch the following (2:08) video on steam generation.

The technology upon which powered much of the 19th and 20th centuries.
Click here for a transcript.

[Dr. Mathews is standing at a table with a miniature steam engine.] Dr. Mathews: Well, I thought I would show you a very simple system for producing electricity. This is a little steam boiler. We are going to produce steam to power a steam piston that is going to reciprocate, spin the flywheel, and that is going to produce an electric current. That is what we are starting to talk about today. Electricity and the production of electricity from mostly steam. Then, we will start talking about other renewable energy types. But the bottom line is, what we are trying to do is spin a generator quite quickly to produce electricity.

Credit: JPM

This lesson is about all the things that happen before your electricity gets to you - how it is generated. These processes are connected to efficiency, conservation, and pollution (emissions).

Lesson Objectives

Your success in lessons 02  will be based on mastering the following objectives;

  • Describing how electricity is generated from: coal, natural gas, and nuclear approaches
  • Link energy generation with pollution
  • Identify the advantages and disadvantages of electricity generation for each approach
  • Know how the electricity demand profile changes and how it is met

Enjoy your trip "Behind the Plug" in Lessons 2 and 3 (remewables)!  

Wake Up Your Brain

Steam & the Industrial Revolution

The need for steam power was not initially to move people, goods, or services, but to pump water! Much of coal mining in the UK was deep underground mining where flooding is an issue. Thus, to extract more coal, the mines needed to continually pump water. This required a lot of work (work and energy are interchangeable, same units, so this means pumping required lots of energy [in a short amount of time—lots of POWER.) This was one of the first uses of the steam engine. The coal was also the fuel. We will see later that coal is a concentrated energy source that produces a lot of heat (high calorific value) so we can produce lots of steam and do more work with a small mass of coal. This was all happening in my "old" country, England, in the mid-1700s. The presence of coal and the engineers allowed the UK to pioneer the transformation from an agricultural society to that of an industrial society. The steam engine was used to turn looms (used in cloth manufacturing), pump water, turn belts, stir beer, and carry hops up to the upper floors of breweries. This revolution altered the distribution of the populace, enabled mass production, and created the intricate network of communications (road, rail, and canals) so that goods could be moved to different markets. 

We will see that most of our electricity is generated from steam. The generator was around in early 1831, but it was not until later in the century (1873) that a practical device was developed. Now, steam engines could turn a generator and produce electric power.  By 1936, electric wires had produced a grid that sent electricity throughout Great Britain. We don't even notice the miles of wires that follow the roads and branch off into our houses; it is part of the scenery.

The following (0:41) video gives you the basics of electric power production with steam. 

Click here for a transcript.

[Dr. Mathews is sitting at a table and we hear a steam engine in the background.] Dr. Mathews: The industrial revolution was powered by coal and steam. I have a little engine here. It's using a solid fuel to produce steam from water. We have it turning a little steam piston and we can use this to make work. I have certainly been to breweries, for example, where they use this type of technology to move the grain up into the beginning of the brewery and everything else would be gravity fed from there. If you were a Ludite (persons opposed to technology), this is what you hated. Because this is what took the jobs away from the people and started the industrial revolution in England in the 17th century - in the 18th century, sorry. Marvelous! [Video ends with Dr. Mathews smiling at the camera.]

Credit: JPM

Defining Energy

Lesson 2 is really all about the following relationships;

  • Energy is the ability to do work.
  • Power is the rate (speed) of performing work.

Imagine you have a flooded basement and you go to buy a pump. What you care about is the power of the pump. Either way, the same amount of energy is going to be used moving the water up and out of the basement, but the pump that will do it in a couple of hours has more power than the pump that will take the weekend to do the same amount of work. In essence, you need a pump with more horsepower. A lawnmower may have 5 hp, a garage door opener, 1/2 hp. If I swapped the engines, my garage door would fly open, but my lawn would require a long time to be cut with my now underpowered lawnmower. Much of the rest of the lecture is how we can generate electricity, lots of it and quickly, because, as you now know, we have a huge appetite for electricity (power) and energy in general.

One last thing to keep in mind:

When we generate energy, most of the approaches will also generate some pollution. Our leading contributors in the U.S. to electricity are natural gas, then coal, then nuclear. Even though crude oil is a fossil fuel it is mostly used in transportation. Note that crude oil is an insignificant contribution to electricity generation.

Electricity from Natural Gas

Natural gas combustion is currently the leading source of electricity in the U.S. at 38%. This is a remarkable change given it was the third leading fuel (behind coal and nuclear) in 1990.

There was a time when natural gas was worth practically nothing. In a single month, the Saudis would flare (burn-off) more than enough natural gas to run Europe for a whole year! Now, however, natural gas is prized and has significant use in home heating and electricity generation due to its current low cost due in part to improvements in our ability to extract natural gas from shale such as the Marcellus Shale deposit that is the largest gas play in the United States. Thus, in the U.S. we have abundant and cheap natural gas. In Europe, and Japan however natural gas is much more expensive.

Map of Marcellus Shale Base showing depth of Marcellus Shale Base and extent.
Depth of Marcellus Shale Base, 2009
Credit: Marcellus Center for Outreach and Research, http://www.marcellus.psu.edu/ [1]

The rise in popularity is somewhat influenced by the environmental issues of the other fossil fuels but is mostly influenced by price. While natural gas combustion is NOT pollution-free, it is much cleaner than coal or fuel oil when producing the same quantity of electricity.

Natural gas takes advantage of a technology that you might be familiar with: The turbofan engine on a plane, which is a type of gas turbine. The term gas refers to its state, not an abbreviation for gasoline. The secret to its success is in the compression of the air and the injection of already compressed natural gas. This mixture is then combusted producing the various (hot) products of combustion, which pass through the turbine vanes spinning the turbine like a windmill on a very, very windy day (it is actually much much faster). The turbine is connected to a generator that produces electricity. Watch this 2:39 minute video to see how a gas turbine works. We can also add a gas turbine to an existing steam turbine for a combined cycle system and achieve much higher overall efficiencies.

Click here for a transcript of the video.

Air; a lot of gaseous molecules floating all around us. It's great for breathing and it turns out it's great for getting lights turned on. That's because air along with abundant natural gas or other fuels are the ingredients that combine in a gas turbine to spin the generator that produces electric current. If you follow the electricity you use at home or work back through the power lines to your local power plant, you'll see that the process most likely starts with the work of the gas turbine-- the very heart of the power plant.

First, air is drawn in through one end of the turbine, in the compressor section of the turbine all those air molecules are squeezed together, similar to a bicycle pump squeezing air into a tire. As the air is squeezed it gets hotter and the pressure increases. Next, fuel is injected into the combustor where it mixes with a hot compressed air and is burned. This is chemical energy at work. Essentially, this is what happens in your family car's engine but at about twenty-nine hundred times more horsepower. Actually, it's exactly like the turbine engines on jet airplanes, the hot gas created from the ignited mixture moves through the turbine blades forcing them to spin at more than 3000 rpms. Chemical energy has now been converted into mechanical energy.

The turbine then captures energy from the expanding gas which causes the drive shaft -- which is connected to the generator --to rotate. That generator has a large magnet surrounded by coils of copper wire when that magnet gets rotating fast, it creates a powerful magnetic field that lines up electrons around the coils and causes them to move. The rotating mechanical energy has now been converted into electrical energy because, the movement of electrons through a wire is electricity. In what's called a combined cycle power plant the gas turbine can be used in combination with a steam turbine to generate 50 percent more power. The hot exhaust generated from the gas turbine is used to create steam at a boiler which then spins the steam turbine blades with their own drive shaft that turns the generator. What you end up with is the most efficient system for converting fuel into energy and that's your GE gas turbine 101.

Credit: GE Power [2]
 A natural gas turbine and one of the turbine's many vanes, or blade like fins.
In the main picture, a natural gas turbine arrives on a railroad flatbed truck. Look closely at the railing around the flatbed for a sense of the scale of this turbine. At its largest point in diameter, it measures roughly 16 feet across. Also shown is a close-up of one of the turbine's many vanes, or blade-like fins.
Credit: DOE

The General Electric Site [3] lists the key steps as:

The gas turbine can be used in combination with a steam turbine—in a combined-cycle power plant—to create power extremely efficiently.

  1. Air-fuel mixture ignites.

    • The gas turbine compresses air and mixes it with fuel that is then burned at extremely high temperatures, creating a hot gas.
  2. Hot gas spins turbine blades.

    • The hot air-and-fuel mixture moves through blades in the turbine, causing them to spin quickly.
  3. Spinning blades turn the drive shaft.

    • The fast-spinning turbine blades rotate the turbine drive shaft.
  4. Turbine rotation powers the generator.

    • The spinning turbine is connected to the rod in a generator that turns a large magnet surrounded by coils of copper wire.
  5. Generator magnet causes electrons to move and creates electricity.

    • The fast-revolving generator magnet creates a powerful magnetic field that lines up the electrons around the copper coils and causes them to move.
    • The movement of these electrons through a wire is electricity.

The key step in this process is the compression step, "The gas turbine compresses air and mixes it with fuel that is then burned at extremely high temperatures, creating a hot gas". This one step is the reason we can get more energy out of the combustion process. This makes natural gas the most efficient fossil fuel for producing electricity through the combustion process. Another reason for the higher efficiency is the higher temperature of the combustion process. Recall that we obtain energy through the combustion process of:

C + O Graphic of an arrow pointing to the right. CO
CO + 1/2 O2 Graphic of an arrow pointing to the right. CO2
and H2 +1/2O2 Graphic of an arrow pointing to the right. H2O

If I add all these steps together I get the overall equation:

CH4 + O2 Graphic of an arrow pointing to the right. CO2 + H2O

Natural gas has the greatest hydrogen to carbon (H/C) ratio of all the fossil fuels (4 hydrogen atoms per 1 carbon atom). As you obtain energy from the hydrogen steps and the carbon step, the temperature will be higher. Thus, the system will be more efficient.

Efficiency Problems

The problem with heat engines such as this (a heat engine is something that produces heat to do work, usually through steam), is that have limited efficiency because of the thermodynamics of the system. Don't worry too much about the rest of this section, the bottom line is the overall efficiency of these stems is capped due to thermodynamics and if electricity from combustion is the goal — we are limited to below 57% efficient.

  • Recall the First Law of Thermodynamics (conservation of energy).
    This indicates that we are simply transforming energy from one form to another (chemical to heat for example).
  • The Second Law (entropy) limits the efficiency of heat engines because we cannot use all the heat to produce work.

    Here is why. We need to use the heat engine at high temperatures to have an efficient system, let's say a few hundred degrees - 300 ° C - which is (300 +273) 573 Kelvin [4]. If the exhaust of the engine comes out at 100 ° C (373 K), then our efficiency is
    Efficiency = ((Thot - Tcold)/Thot) x100
    where T=Temperature, which must be represented in Kelvin!

The maximum (Carnot) efficiency is 57%, so no heat engine will have a higher efficiency. Pulverized coal power plants will have efficiencies of 37% or so. Natural gas combustion is more efficient because the Thot is higher. All heat exhausts tend to be around the same temperature. To achieve 100% efficiency, we would need to extract all the heat as work and have the exhaust exit at 0 Kelvin (not actually possible) - a very cold -273 °C. This is the main bottleneck of all our thermal (heat engine) systems, from electricity generation to transportation. Higher efficiencies are possible for electricity generation if you do not go through the thermal step (which hydroelectric or fuel cells do not).

Additional turbine information

There are a couple of other uses for the gas turbine. For example, instead of using the rapidly rotating turbine to turn a generator, you can use the thrust to power your plane, or the rotating action to lift a helicopter, or even power the 2-ton M1 tank.

Something to remember: the equations above forget one very important component, nitrogen! As we use air (which is free) for our combustion medium rather than oxygen (which is expensive) the correct equation should be:

CH4 + 2O2 + N2→ CO2 + 2H2O + N2

This is just like the bit in the movie that makes no sense and seems to be extraneous information, but, is important in the plot later on.

Advantages of Natural Gas

Compared to other fossil fuels, natural gas is cheaper, burns cleaner, making it more environmentally friendly, and is safer and easier to store.  And the U.S. has a plentiful supply of natural gas and new ways to extract it.

Disadvantages of Natural Gas

While plentiful, natural gas is not a renewable energy source and produces greenhouse gasses. It must also be handled with care due to it's combustible nature. The fracking process also causes concerns for water quality. We cover that and natural gas extraction in lesson 04 and 07.

Pulverised Coal Combustion

Electricity generation is not the only use for coal, but it is the dominant use both worldwide and in the US. The concept is simple; through the process of combustion, the trapped chemical energy from the coal produces heat, which we can change into electricity, which we then use to do work. Sounds simple right? Well, it can be, but it can also be a complex junction of engineering, chemistry, and environmental chemistry. Coal was the traditional fuel of choice for electricty generation in the U.S. supplying ~50% in the 1990's. It has since declined to ~25%. Ensure you know why (has already been revealed in this lesson).

Picture of coal being pushed by a bull dozer and a large power plant sitting by a body of water with piles of coal.
Left: Mountains of coal are used each year in the United States. Brave souls in underground mines or driving monster machines gather the coal that fuels (provides the energy–well, ~25% of it anyway) for the Internet, cooling, computers, television, and all our other electric appliances and gadgets.
Right: You can get an idea of the scale of a plant when you look at either the outside or the inside of the boiler. From the outside, these large coal-fired utilities can stand 10 stories high. Note that most utilities are close to water to help meet cooling needs.
Credit: driftwood [5] / adobe.stock.com [6]

Coal is stockpiled into great piles; this is the supply of the coal so the power plant (which I might also refer to as a utility site) can operate even if there are transport problems (rail strike), supply problems (miners strikes, etc.) or weather-related issues (snow blocking tracks, etc.) The coal is in chunks of varying sizes. We do not want the particle size to be too small yet, or the coal could blow away, wasting the fuel and causing dust problems. They will often spray water over these coal piles to keep the dust down.

 Graphic representation of a chunk of coal being split into hundreds of pulverized pieces.
Credit: JPM

Coal is moved via a conveyor belt to the pulverizers where the coal is crushed into a fine dust (about 35 microns in size — a human hair is about 100 microns.) Particle size reduction helps to speed up the process of burning the coal particles. If the particle size is smaller, then there is more surface area for the same weight. Thus, the coal particle will burn quicker and the combustor can be smaller in size (so that all the coal has time to burn). So with these small-sized particles, a 10-story high combustor is about the right size.

Combustion

Here is a very simple explanation of the process.

Click here for a transcript.

Coal-fired plants transform coal into electricity by pulverizing it and burning it in a furnace. The burning coal heats water in a boiler and turns it to steam. This steam, under tremendous pressure and at high temperature, provides the force to turn the turbine blades. The turbine spins an electromagnet inside copper coils in a generator and that produces the flow of electrons called electricity.

Credit: TVA [7]
 The inside of a water wall.
A water wall
Credit: DOE

Now that the coal is in small pieces, it is blown into the burner where the air is mixed with the coal in controlled quantities and a complex mixing profile to attempt to reduce pollutant formation. There might be 12 or 24 burners in a boiler wall with the burner outlets poking through the water wall of the boiler, in an opening that even I, even the "chubby" Dr. Mathews could pass through (251 lbs currently—but I am on a diet!). There are different configurations of burners, but simply put, the inside of the boiler is very hot (1,500 °C) causing the coal to devolatilize, give off gases, which combust, then the remaining char particle combusts. The process produces heat, lots of heat (the reaction is exothermic.) So much heat, that the tubing you can see in the inside of the wall (the waterwall) is necessary to stop the boiler from melting! On the inside, the "water-wall," or the multitude of pipes for producing steam, are visible. The holes are where the burners poke through. These are very expensive (high capital cost) to build.

Water flows through the pipes and is transformed into steam. But not just any steam. By using high pressures and special materials (so they can withstand the high temperature), the steam can reach very high temperatures above 500 ° C and is known as superheated steam). We use the heat generated in the boiler to produce high-temperature high-pressure steam. Because of this, we can do lots of work to generate lots of electricity. That is achieved by the combination of a turbine converting this heat energy into kinetic energy (a rotating turbine), which in turn spins a generator for electricity production. The turbine has multiple vanes that act in a similar manner to a wind turbine blade but uses steam pressure to spin the turbine. We get more electricity by using larger turbines and faster rotation speeds. This is why a coal-fired or nuclear power plant can generate much more electricity than a single wind turbine

steam turbine
This image of a turbine without the casing at a pulverized coal utility site where they generate electricity gives you an idea of the scale of the equipment.
Credit: alexrow  [8]/ adobe.stock.com [9]

The combustion gases that leave the boiler are known as flue gases. The flue gas is, in some cases, cleaned (depending on the coal quality and regulations governing the plant) and sent out of the stack (chimney).

Here the airflow and heat movement inside the coal-fired boiler is shown. We will see later that through careful mixing of the coal and air that we can reduce NOx emissions (covered later). The heat rises and heats the water in the waterfall pipes to generate the high-temperature and high-pressure stram.

 Graphic showing the effect of overfire air injection velocity.
This page and unit are important when we discuss methods for reducing air pollution issues, particularly acid deposition. Here the image shows how air-mixing controls in this boiler containing 20 burners can help lower NOx emissions.
Credit: DOE

Back to the high-pressure steam: the steam is superheated to allow it to carry even more energy the pipes containing the steam are special metal alloys (mixtures of metals) to tolerate these harsh environments (and are thus expensive),

The steam passes through a turbine, which spins at a very fast rate. If you recall that energy cannot be created or destroyed, then you will realize that making the turbine spin in excess of 1,000 revolutions a minute takes some of the energy from the steam (but not all of the energy!)

To help the flow process, the low-temperature steam is cooled to transform it back into water. Water takes much less volume than steam, and so the resulting pressure drop helps pull the steam through the turbine. This takes a lot of cold water to do, and so most power plants are also near large bodies of water (lakes, rivers, etc). The water, which was steam, is recycled as it is high-quality water; the water vapor produced by cooling the steam with river water evaporates from the cooling towers (just water vapor, no pollution!)

The turbine then spins a generator, which is a coil of wires inside a field of magnets. This produces an alternating current of electricity, which is what we use in our houses to power most of our devices, and if you're reading this on your computer, then you can thank (in part) the system just described.

One big problem (the challenge of efficiency)

We have multiple stages to produce electricity. Each stage will have its own efficiency. The stage with the lowest efficiency is the weakest link in a chain - the system cannot be more efficient (or stronger) than that one component. Here, it is the inability to extract all of the energy from the steam that is responsible for the overall poor efficiency in the entire system. Add in multiple losses due to inefficiencies at other stages, and you have pulverized coal combustion producing electricity with an efficiency of about 37%.

We'll cover much more on energy efficiencies in later lessons but if we can be more efficient we will burn less coal and produce fewer emissions. The U.S. is now moving towards emission controls for greenhouse gases so any new coal-fired utilities will likely be required to have the same emissions as a natural gas-fired utility. As it is easier to use natural gas, it is expected that few new coal-fired plants will be constructed in the U.S., many older plants have or are closing, and natural gas use for electricity will increase along with wind and solar.

Fluidized Bed Utilities

There are other approaches to the combustion of coal (not just pulverized coal combustion). In Pennsylvania, we have a coal mining legacy of abandoned culm/gob piles (a mixture of rock and rock containing coal that not of high enough quality for use — thus was rejected and piled at the mine site). One such man-made mountain is shown below. The term culm is used in the anthracite region of PA in the east, while the term gob pile is used in the bituminous regions that occupy the west of the state.

Photograph of a sizable hill of rejected rock and coal (culm/gob pile)
A man-made Culm/gob "mountain" from the waste from coal mining.
Source: Western PA Coalition for Abandoned Mine Reclamation

Watch the following 2:52 minute video "Circulating Fluidized Bed - CFB Boiler Process".

Click here for a transcript of the Circulating Fluidized Bed video.

Sumitomo SHI FW is advancing steam generation technology with its state of the art circulating fluidized bed technology. Due to its fuel flexible, low-temperature burning process, CFB technology can utilize all types of coals, lignites, petroleum cokes, and carbon-neutral fuels like biomass and recycled waste to produce clean and economical power generation. It offers a major advantage over the limitations of conventional combustion-- the ability to burn the widest range of fuels.

Conventional coal technology requires the fuel to be finely ground and dried before entering the furnace. These steps are not needed for the CFB. Instead, the fuel is coarsely crushed and dropped into fuel chutes, which lead to ports in the lower section of the CFB's furnace. Unlike conventional boilers that burn the fuel in a massive high-temperature flame, CFB technology utilizes circulating hot solids to cleanly and efficiently burn the fuel in a flameless combustion process.

Its low uniform combustion temperature minimizes the formation of nitrogen oxides and allows the injection of limestone to capture acid gases as the fuel burns giving this CFB the lowest furnace emissions. Since the fuel's ash doesn't melt, heat transfer surfaces stay clean, allowing the hot solids to conduct their heat efficiently throughout the entire boiler, while fouling and corrosion are minimized. The payoff-- low plant emissions, low maintenance, and high plant reliability.

To achieve very high combustion efficiency for all fuels, the solid particles in the furnace are collected by steam-cooled, solid separators which recycle most of them back to the furnace. Before re-entering the furnace, the particles pass through a high-performance INTREX heat exchanger where steam coils are submerged in a bubbling bed of hot solids to efficiently produce high temperature superheat steam.

To further reduce plant operating costs and emissions, our CFBs can utilize vertical tube supercritical steam technology to achieve the highest plant efficiency. And for full capture of greenhouse gas emissions, we offer Flexi-Burn CFB technology, which can produce a CO2 rich flue gas for beneficial use by the oil, agricultural, chemical, and construction sectors. circulating fluidized bed technology is an important part of the solution to meet the world's energy needs while conserving natural resources and preserving our environment. Visit us at shi-fw.com for more information on our exciting technology.

Credit: Sumitomo SHI FW

After the coal has been separated from the rock the coal can be combusted but we use a different approach for the combustion process. This video shows the process of coal combustion via a circulating fluidized bed, showing the heat exchange to produce high-temperature steam, the circulation process for the coal char, with SO2 capture, and electricity production. In some locations, the hot water from the turbine is also used by local municipalities (such as for prisons, etc.) When this occurs it is called co-generation.

Circulating Fluidized Bed combustion is another method of burning coal that uses existing technology to produce electricity with very low emissions of both Nitrogen Oxide (NOx) and Sulphur Oxide (SO2) both of which contribute to the acid deposition challenges. Note: the fuel need not be coal but we will use coal for this discussion since we are in a lesson about electricity generation from fossil fuels. Fuel flexibility is one of the reasons that this boiler configuration is attractive, 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). Culm piles are the waste material left behind from the mining process (mostly rock but some coal too). Fluidized Bed combustion is a way to reprocess this waste. They burn a much larger particle size than pulverized coal combustion (mm in size or larger), at a much lower temperature, over a longer period of time. They also have an in situ (in place) mechanism to capture the SO2 that would otherwise contribute to acid deposition. So here is how it works:

These "man-made mountains" of reject coal actually contain a significant quantity of coal. By removing the culm pile, producing electricity, there are employment, taxation, and cleaning open-pit operations (more on that when coal mining is discussed) opportunities.

The various stages are shown in this interactive figure (click on an asterisk for more information). Start with coal and work your way through the diagram in a counterclockwise direction.

Click here for a text description of the figure above.

Diagram of an Atmospheric Circulating Fluidized Bed Boiler.

Coal and Limestone are fed into the combustion chamber. The combustion chamber has air intake feeds and a center partition. Ash waste exit the combustion chamber at the bottom and are disposed of. Products enter the two attached cyclones at the top and then head to the heat exchanger. From the heat exchanger, the products enter fabric filters. Fly ash heads to disposal and flue gas exits via the stack.

Steam is a byproduct of both the combustion chamber and the heat exchanger. This is used to power a steam turbine which powers a generator and produces electricity. Excess steam is returned to the heat exchanger or to the boiler as feedwater.

Additional information:

  • Coal: This can include cleaned reject coal from culm piles or other fuels (biomass, etc.)
  • Limestone: A sorbent (absorber) for the Sulphur dioxide
  • Cyclones: Separates particles from the fluid using centrifugal forces
  • Fabric Filters: Prevents the larger particles from escaping with the flue gases. It is similar to a vacuum cleaning filter bag but at much higher temperatures.
  • Disposal/Truck: Transports the waste to old, abandoned coal mines to fill in the open pits.
  • Steam Turbine: Spins quickly so the generator spins quickly (1,000’s of revolutions per minute)
  • Generator: Spins quickly so it can generate lots of electricity

Credit: DOE

Once the coal is cleaned to reject the associated rock, it is crushed and the relatively large pieces of coal are combusted in the presence of limestone. By suspending the particles with air flowing from underneath, enables the coal to be combusted slowly, the ash that forms can be rejected. The lime is formed from the added limestone (CaCO3), the heat causes calcination which is the formation of lime (CaO) + carbon dioxide gas (CO2). This combustion approach 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 lime (the gas forms a sold coating on the lime particle).

CaO + SO2 + 1/2 O2 --> CaSO4 (gypsum)

The lime/gypsum is a solid that can be used to fill in the old strip mining holes to restore the damage from old abandoned mines. Unlike the output from the scrubbers - described later, this gypsum is not pure enough to be used as a wallboard material (it is just a coating over the lime particle). More on this when we discuss acid deposition solutions in unit 3.

Nuclear Power

Nuclear power provides ~20% of the US electricity supply. To understand how it works, we need to first understand the nucleus of an atom.

Atoms

We tend to think of atoms as solid balls linked together with chemical bonds forming molecules. The reality is that the "hard" shell that represents an atom is a representation of the electrons that are whizzing around the centrally placed nucleus. The "solid" ball is a poor representation; instead, think of a ping-pong ball. Most of the ball is empty; in the case of an atom, the shell(s) (mass of electrons) does not contribute much to the mass, as electrons are very low in mass. The bulk of the weight is in the nucleus where the proton(s) and neutron(s) reside. These each have a mass of 1 unified atomic mass units (also called Dalton). So, hydrogen, the simplest atom has 1 proton and 1 electron. It is the number of protons that determines the element, so 1 proton is hydrogen represented as 1H. If there is also a neutron, then the element is still hydrogen, but it is a different isomer. We call these deuterium atoms 2H or D, and if we produce water from it (D2O rather than H2O) we have "heavy water" (the two hydrogens being replaced with the heavier mass deuterium atoms). 

Atoms and Half-Life
Atom/Isotope Half-Life
Uranium 235 (235U) 0.7 x 109 YEARS
Plutonium 23,924,000 YEARS
Radon 222 3.8 days
Polonium 218 3 min

There are many elements that are radioactive. For some, only certain isotopes are radionuclide (they are radioactive). However, some will have very long half-lives; others decay very quickly. During the decay, the nucleus splits into two or more atoms producing heat (the kinetic energy of these new atoms) and neutrons to continue the reaction. Given the long half-life of 235U, we can't wait for natural decay for electricity production. Instead, we induce the splitting of the atoms with a neutron. Basically, when the 235U (uranium) is induced into decay via capturing a neutron, the atom breaks apart violently, producing 2 fragments that are the new atoms and several neutrons. Thus, when at the critical state, 1 neutron is allowed to induce another 235U to decay. To increase the probability of this occurring the moderator has the role of slowing the neutrons down. As all of the fragments are moving quickly; they have lots of kinetic energy, or in the case of atoms and molecules we can say they have a high temperature. Thus, we can create steam, as discussed in the video, to turn a turbine (and spin a generator). If you wanted less fission, the control rods can be lowered — absorbing the neutrons and controlling or stopping the heat and thus the electricity produced.

 Splitting atoms makes heat and releases neutrons
Splitting Atoms

How a Nuclear Reactor Works

Watch the following video (beginning at minute 1:30) to learn how nuclear energy creates electricity. What you watch here is fair game for the exam.

Click here for a transcript of the Nuclear Energy video.

MR. ANDERSON: Hi. It's Mr. Anderson, and this is AP Environmental Sciences video 25. It's on nuclear energy. You're probably familiar with the Richter scale. It's a log scale by which we measure the size of earthquakes. But you're not familiar with the INE scale, or the International Nuclear Event Scale. It's also a log scale, and we use it to measure the size of nuclear accidents. We've only hit 7 twice.

The first time was in 1986 in Chernobyl. We had a collapse and a meltdown of the reactor. 31 people died from exposure to radiation. In 2011 in Fukushima, we also hit a level 7. We had three other reactors meltdown after an earthquake and tsunami. In the US, the highest we've ever gone is a level 5 at Three Mile Island. It released a little bit of radioactive material into the surrounding area. But it scared people. These accidents scare people, and radiation scares people because we can't see it.

And so the amount of energy we're getting from nuclear reactors has remained static for decades, but it's starting to be revisited again. And the reason why is there's also something in the environment that is scary, and it's also invisible. And that's carbon dioxide. If we look at the amount of carbon dioxide being produced by nuclear power plants, it's on the level of the same as wind generation or hydropower. If we compare that to gas and oil and coal, there's way more carbon dioxide being created. So new technology and a decrease in carbon emissions could see a resurgence of nuclear energy.

Where does the energy come from? It comes from the fusion of radioactive material, generally uranium-235. So as it decays, it breaks down into two fragments-- barium and krypton. And as it does that, it gives off energy, and it gives off neutrons that can trigger more fission and more radioactive 235. So the way this is controlled, unlike in a weapon, it's controlled in a reactor. Most of the reactors in play right now are light water reactors or normal water reactors.

What you do is you put fuel rods inside it. And as they decay, it produces a little bit of energy. And that energy inside the water heats it up. And we can use it to generate steam and then generate electricity. Now when it melts down, this goes out of control, and we get a release of that radiation into the environment. And so by having it in water, we can contain some of that energy. And we can also use control rods. These are actually going to take in some of those neutrons. And by lowering them between the fuel rods, we can slow down the reactor.

Now the disadvantages are pretty apparent. Nuclear waste is going to be created, and it can be around for thousands and thousands of years. So we have to keep track of that. Each of the radioactive materials has a different half-life, but it's going to be on the order of thousands of years. And also we have these accidents where we can have explosions, malfunctions, and it releases that radiation into the environment and can cause things like thyroid cancer.

Why do we still have it? Well, the advantage is that it creates a huge amount of energy. And it can do that without increasing the amount of carbon emissions in the environment. So if we look at uranium-235, now we're looking just at the nucleus. And so we're looking at the protons and the neutrons. And so if we were to hit one of those uranium atoms with a neutron, what it'll do is it'll break in half. It breaks apart into these two fragments. And as it does that, it releases a certain amount of energy. You can see it's also liberating three of these neutrons. And each of those have the potential to hit another uranium-235, and we can break it down.

So it's not an out of control chain reaction like this that we might see in a nuclear bomb, but it goes slow over time. And so if we look at what those fuel rods are like, most of the uranium is actually going to be uranium-238. A few of it is uranium-235. And so as those neutrons are given off, by having it in water, we can absorb some of that energy, and we can control that radiation. And also, we can lower these control rods. They absorb the neutrons, and so we can slow it down.

So if we look at a typical light water reactor, we're going to have the fuel rods and the control rods in the core. We're then going to heat up a fluid. And that fluid is going to be in a closed system. So as it moves through these pipes, it returns back where it was. But it's bringing with it a huge amount of heat. Now that heat moves into a separate loop. And so in this loop, what we're doing is we're heating up the water. It's forming steam up at the top. And then that steam is moving through a generator, so we're generating electricity.

And then finally, we still have a lot of heat right here. So before we pump it back in, we have to get rid of some of that heat. And so we're going to do that by pumping the water and another loop into a cooling pond. And so as long as we have energy contained within those fuel rods, we can generate electricity. But what happens when we decay too much of that uranium-235? Now it becomes waste. It's still radioactive, but it's not generating enough electricity for the plant to go. And so now we've generated waste. So that's one form of nuclear waste.

But we're also generating a little bit of heat over here into the environment as well. And so how do we deal with that waste? Well, how do we deal with the fuel rods? We're going to put him in a pool. And as we put them in the pool, we're going to absorb some of that energy here. But eventually, we're going to have to put it in some kind of a container. And a lot of these are on these concrete slabs. And we've got that nuclear waste contained inside there. There's no real long-range plan of what we're going to do with his nuclear waste. And it's going to be a problem that we'll have to deal with generations down the line.

If we look at how long this could occur, you have to understand what a half-life is. A half-life is going to be the amount of time it takes for half of the material to decay or to break apart. And so if we look at time 0, let's say the half-life is one year, at time 0, we'd have 100% of the radioactive material. At time 1, we would have 50% of it. In other words, half of it would have decayed. In another year, it would be half of that and a half of that and a half of that and a half of that.

And so in an AP Environmental Science class, you should be able to calculate the half-life. And let me give you a problem. Let's say radium has a half-life of 1,500 years. How long will it take for 250 kilograms of the radium to decay down to less than 10 kilograms? And so we're saying the mass of radium at the beginning is 250 kilograms at time 0. And so in one half-life, in other words, in 1,500 years, we would have decayed half of it down to 125. In another 1,500 years, we'd be down to 62.5. And you can just keep doing this. And you can see at 7,500 years, we're less than 10 kilograms left. But you can see a lot of that is still going to be radioactive.

Now what happens in accidents, something happens where we're not able to contain this core. And so if we look at Chernobyl, they were testing the reactor, and it got out of control. It heated. We have a melting or an explosion that actually collapsed the roof and released a lot of radiation. If we're looking at Fukushima, it's like three levels of protection that failed. We have an earthquake, but we also have this giant tsunami. And if we're looking at Three Mile Island, it was a problem with a valve but also a problem with user error as well.

And so all of these, for the most part, are human error, either we had a mistake at the reactor or had a mistake in the design. And what it does is it releases some of this radioactive material into the environment. So, for example, radioactive iodine can cause thyroid cancer. So we eat it in our food. It causes cancer years down the line. And we're going to see this-- wherever there's a nuclear accident, we're going to have increases in thyroid cancer after that.

So if we look at these accidents, so this is at-- here's Three Mile Island, here's Chernobyl. So we had the heyday of nuclear reactor creation during this oil crisis. But then after these accidents, you can see the amount of reactors that we have has remained static. And you can say even though we could produce this amount of energy, we're producing less of that. And the reason has to do with this fear of radiation and the fear of accidents as well.

And so what's the future hold for nuclear power? Well, they're going to be increases in new technology. Thorium reactors are going to be working much better than uranium, light water reactors. And we can have these third-generation reactors where we can actually reuse some of that waste. And then finally, we have to reduce carbon emissions, and nuclear energy is going to be part of that discussion.

So could you pause the video and fill in the blanks? Let me do that for you. Nuclear energy is the fusion of something like uranium-235. We break it apart into fragments. We also get energy and some neutrons that can cause fission in other atoms. We've got the fuel rods. That's where the radioactive material is, but we also have these control rods. Disadvantages, nuclear waste. It takes a long time due to the half-life of these radioactive materials for the waste to go away.

We can have accidents that increase the amount of cancer. Thyroid cancer is an example of that. But the advantages, again, nuclear power can help us reduce the amount of carbon dioxide in the environment, reduce global warming. And that's why it's being revisited. And I hope that was helpful.

Credit: Bozeman Science [10]

Below is a static representation of a nuclear power plant. The caption provides a short explanation.

Diagram of a Nuclear power plant. See caption for details
The nuclear reactor consists of a reactor with fuel rods, control rods, and a moderator. The heat produces steam (in a boiling water reactor) which is heat exchanged (so the water is not radioactive) before going through the turbine to turn the generator which produces electricity. A cooling tower helps to cool the low-temperature steam back into water before it is returned to the reactor. A containment structure prevents radioactivity from escaping.
Credit: DOE

Advantages of Nuclear Energy

There are two primary advantages of nuclear energy: low cost (once the plant is constructed) and no greenhouse gas emissions - unlike many other major electricity generation sites!

Disadvantages of Nuclear Energy

Nuclear Power is quite controversial and comes with a number of concerns. 

students in hard hats standing near a mountain.
Penn State students near Yucca Mountain
Credit: PSU CAUSE
  • Cost. It is very expensive to build the plants, but remember, once built, it is relatively inexpensive to produce electricity.
  • Safety. Nuclear disasters like the ones in Chernobyl [11](Ukraine), Three-Mile Island [12] (Pennsylvania), and Fukushima [13](Japan) 
  • Waste storage. Currently, all nuclear waste is stored on the site where it is generated (initially in pools then in dry casks [14]). In the U.S. there was a plan for a permanent disposal site at Yucca Mountain [15] in Nevada but it was ultimately not used due to objections by Nevada. Thus, we do not currently have an appropriate storage plan for the waste —this will be a more important issue when we continue to close down nuclear power plants and have much more waste to store. 
  • Proliferation. The key risk here is that facilities originally constructed for nuclear energy production could be used to create nuclear weapons.

Future of Nuclear

We currently have 96 reactors in the US producing electricity in 56 nuclear power plants in 29 states, far more reactors than any other single country! The oldest one running is the same age as I am (~50'ish)! They are licensed for 40 years with a 20-year extension possible. As most were built in the 1970s and 1980s, soon we will be looking at dismantling a large number of nuclear reactors. Cheap natural gas is accelerating the time frame to decommissioning nuclear plants.

The figure below shows the trend of our nuclear contribution. Given the age of the existing reactors, we can expect a decline in the number of reactors in the next decade. A good question is; Will we build new ones? In an era of climate change expectations, nuclear is an excellent means to produce electricity without greenhouse gases. However, after Fukushima (earthquake and resulting Tsunami in Japan), this event stopped new nuclear construction in many nations with some moving away from nuclear power completely. The few "new" nuclear reactors have been in construction for a long time due to many delays. One of the large advantages of nuclear power is no greenhouse gas emissions. So closing down nuclear utility sites after ~2025 will cause an increase in greenhouse gas emissions if we replace nuclear power with power produced by fossil fuels (such as natural gas). We do have one nuclear reactor activated in 2016 in Tennesee, it was the first new plant in the new century with two more under construction in Georgia expected to be completed in the 2020s. However, 17 nuclear reactors have been closed and are being decommissioned. So, nuclear power is likely at its peak production of electricity in the U.S.

Graph showing the growth of nuclear power into the 1980's with some closures and new additions expected in 2016
U.S. Operable Nuclear Generation, 1960 - 2020
Credit: EIA

Nuclear Power Plants Around the World

World map showing the locations of nuclear power plants (concentrated mostly in the eastern United States and Western Europe).
Nuclear Plants are scattered around the world, but it's only the rich nations that can afford them! We are not happy that some "Axis of Evil" countries are joining the nuclear club.
Credit: International Nuclear Safety Center at ANL

Many nations are moving away from nuclear energy (despite the lack of greenhouse gas emissions) and are in the process of phasing out or reduce nuclear power. 

Lesson 2 Coverage Map

This map is a quick and dirty summary of the lesson you just finished. It's a quick way to get a refresher on the main points in the lesson. This is interactive so move your mouse over the topics. Pay particular attention to the sections on coal, natural gas, and nuclear. The rest of the items will be covered in Lesson 3. 

Accessible Version (word document) [16]

Deliverable

When finished here, take the lesson quiz.


Source URL: https://www.e-education.psu.edu/egee101/node/6

Links
[1] http://www.marcellus.psu.edu/
[2] https://www.youtube.com/channel/UCUNcvG7nwi7blC7AslmBRwA
[3] https://www.ge.com/power/resources/knowledge-base/what-is-a-gas-turbine#c3916284c4e44623b72230b270cbb357
[4] https://en.wikipedia.org/wiki/Kelvin
[5] https://stock.adobe.com/contributor/200488604/driftwood?load_type=author&prev_url=detail
[6] https://stock.adobe.com/images/power-plant/33352727?prev_url=detail
[7] https://www.tva.com/energy/our-power-system/coal/how-a-coal-plant-works#:~:text=Coal%2Dfired%20plants%20produce%20electricity,to%20start%20the%20process%20over.
[8] https://stock.adobe.com/contributor/204517492/alexrow?load_type=author&prev_url=detail
[9] https://stock.adobe.com/images/disassembled-steam-turbine-in-the-process-of-generator-repair-at-power-plant/212224245?prev_url=detail
[10] https://www.youtube.com/channel/UCEik-U3T6u6JA0XiHLbNbOw
[11] https://en.wikipedia.org/wiki/Chernobyl_disaster
[12] https://en.wikipedia.org/wiki/Three_Mile_Island_accident
[13] https://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster
[14] https://www.nei.org/news/2019/what-happens-nuclear-waste-us
[15] https://en.wikipedia.org/wiki/Yucca_Mountain_nuclear_waste_repository
[16] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson02/Lesson%202%20and%203%20Coverage%20Map.docx