EGEE 101
Energy and the Environment

Electricity from Natural Gas

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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/

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
 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 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. 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.