The U.S. Department of Energy, through the EIA (Energy Information Administration) keeps up with the energy used by America. The measurement used is the BTU or British Thermal Unit. One BTU is the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit. Thus heating a pound of water from 32 degrees F. to 212 degrees F. requires 180 BTU’s. So, why is it important to know how many Btu’s are utilized? The reason is that 90% of the energy used in America is used by Heat-Engines to provide the motive force for vehicles, airplanes, drive electric generators and power industry. America in 2019 used about 100.2 Quadrillion Btu’s. Here is how these units of heat energy were utilized:
Most folks don’t think about Heat-Engines and their importance to provide our everyday high quality of lives. The facts are that Fossil Fuels, Nuclear and Thermal Power from Biomass together consume about 90% of our total energy. Did you ever consider that a Boeing 747 at cruise will consume about a gallon of jet fuel a second and burn about ten tons of jet fuel/hour of flight time. Everyday when you are driving you enjoy the convenience of your personal vehicle and see many large trucks delivering needed goods to local stores or shipping interstate. Electric vehicles are becomeing more common on the highway, but they too are using energy that originated (80+%) as either coal, natural gas or nuclear energy. An electric vehicle such as a Tesla uses stored electricity to power the vehicle. The electricity to charge the batteries (usually) comes from the American electric grid, most of which (about 80%) is provided by conventional energy of natural gas, nuclear and coal.
The intent of this chapter is to describe Heat-Engines. So lets first look at the example of a steam turbine commonly used in thermal power generation plants.
Each Btu is worth 778 foot pounds of mechanical Energy if converted from heat to work at 100% efficiency.
In essence, this is the process that takes place in all steam powered thermal power plants. Whether wood, coal, Biomass, natural gas or nuclear fueled. The heat is used to generate steam and then the steam is passed through a steam turbine to produce electricity. The figure below shows examples of heat engines that we depend on every day. Each of these converts heat energy from fossil fuels into motive force for either transportation or the generation of electricity.
Each of the examples above are heat engines. The auto and aircraft engines are internal combustion engines. The car uses a gasoline engine. The jet airliner uses a jet engine. The steam turbine (is really an external combustion engine) in the lower center has the top half of the shell removed to expose the turbine blading and rotor. The large reciprocating Diesel engine driven generator in the lower right is a drive engine for a ship and uses the Diesel cycle. Another version of a reciprocating internal combustion engine is the natural gas fueled, spark ignited gas engine. A natural gas reciprocating engine will look very similar to a Diesel or gasoline reciprocating engine. The steam turbine uses steam generated in a boiler, thus the combustion is external to the steam turbine and it would be considered an “External Combustion Engine”.
Modern Coal plants use the Rankine steam cycle to generate electricity.
The process of converting coal energy to electricity is shown below. In this example about 0.8 pound of coal is used to generate one kilowatt of electricity. One kilowatt hour of electricity is about the same as using ten 100 watt light bulbs or one toaster for an hour.
The illustration is greatly simplified. In order to achieve high efficiency a clean coal fueled power plant has lots of complicated equipment and the steam turbine is multiple stages of highly precise manufacture. A typical electric utility scale steam turbine is shown below. This is the prime mover to spin the rotor (magnet) of the generator.
A schematic of a modern coal plant which utilizes the Rankine steam cycle is shown below. This shows the water, steam, cooling water flows through the boiler, steam turbine-generator and condenser. The illustration does not include the enormous amount of equipment for fuel preparation and emissions control. Solid fuels require much more equipment to convert the heat to electricity than liquid or gaseous fuels.
Here below is a modern four unit, 2,400 MW coal power plant. In this example, the four boilers are the tallest structures and are about as tall as twenty story building. The stacks are about 300 ft tall. The plume of gases coming our of the stacks is water vapor. This is steam released as the exhaust flue gases are cleaned from sulfur using limestone slurry of water and powdered lime. Known in the industry as Flue Gas Desulfurization. So the white plumes are simply water vapor from the cleaned flue gases. Also within the plumes are the invisible gases of nitrogen, carbon dioxide and oxygen.
A 2,400 MW power plant when operating at full capacity (depending on the heating value of the coal fuel) will burn about 2,000,000 pounds per hour of coal. Supplied by 100 ton rail cars this is about 10 rail cars per hour of fuel. The raw coal supplied from the mines is pulverized to a fine powder slightly more coarse than face powder and is conveyed to the furnaces using fans to mix the powdered fuel with air. The coal is burned in suspension in the furnaces much like a huge gas flame. A schematic of a typical Utility scale Steam Generator (Boiler) is shown below.
This description is intended for High School students and to provide a brief overview of the various heat-engines that we depend on each day. Suffice it to say, a large coal power plant is a very complicated and very large complex of equipment. In essence a huge “Heat-Engine” that uses solid fuel, coal. The coal is burned in the furnace above reaching peak temperatures of about 3,000 degrees Fahrenheit. As the products of combustion pass through the boiler the water entering is heated to steam at up to 1,150 degrees F. and the flue gases (Products of combustion) are cooled to about 300 degrees F. The flue gases are then treated with emissions equipment to remove the oxides of nitrogen, the sulfur and the solid ash particulates. The solid ash particles are referred to as being Flyash and today much of the flyash is recycled to use in high strength concrete. Much of the FGD scrubber slurry waste (calcium sulfate) is recycled into sheetrock for home building.
A modern, clean coal plant such as the above example may require a staff of about 160 full time employees. One of the reasons coal plants have difficulty competing with natural gas plants is the large number of personnel and the costs of fuel preparation, maintenance and cost of chemical reagents to remove the sulfur and oxides of nitrogen from the exhaust gases. A Gas Turbine Combined cycle plant of four units and also producing 620 MW will require a staff of about 25 employees. A four unit 2,400 MW of power will require a much smaller staff than a similar sized coal plant, about 40 full time employees.
The choice of the fuel and prime mover depends on the geography and availability of coal, natural gas or nuclear fuel. Alaska for example is most suitable for coal because there is not a network of gas pipelines. Also, the demand for power is less than would be justified to construct a large nuclear plant. Coal plants are also common and competitive in Developing Nations such as African and Asian countries.
Background on How Natural Gas Plants Using Aeroderivative Gas Turbine Drives Have Replaced Much of America’s Coal Electric Power Generation
Up till about the year 2010 about 50% of America’s electricity was generated from coal and about 20% from nuclear. Then came Hydraulic Fracturing and oil and gas production in the U.S.A. from locations that were not anticipated before, such as North Dakota and Pennsylvania. The U.S.A. after ten years or so of successful and highly productive Hydraulic Fracturing and directional drilling, the U.S.A. has become the world’s #1 oil and gas producing nation. What does this mean? It means economic prosperity for the U.S.A. but also it has been a disruptive economic force in electric power generation. Because about 75% of the production cost of electricity from a coal plant is fuel, the new low cost natural gas has made natural gas a less expensive fuel option for power generation. In addition, the Gas Turbine Combined Cycle Power Plants have become the most efficient “Heat-Engines” ever developed. A GTCC Power Plant has far less components than a similar sized coal plant, thus a much smaller staff and overall has lower operations & maintenance production cost. (GTCC=Gas Turbine Combined Cycle) Because of the small staff the main production cost for electricity from a GTCC plant is fuel. The natural gas fuel is about 90% of the production cost of electricity from a GTCC plant.
Further, not only lower fuel costs than coal, the GTCC plants are now approaching 65% Thermal Efficiency. More on thatin a later chapter on economics of power generation.
How Did Jet Airplane Engines Fit Into Electric Power Generation?
Jet aircraft engines have been used for air travel and military uses since the 1950’s. The jet engine is another form of internal combustion engine. For jet airplanes, the engine creates hot air and gases that are forced out the rear of the engine thus causing thrust, a force to propel the airplane forward. For planes designed to fly less than about 450 mph, such as Regional Air Transportation, a Turbo-Prop is more efficient than jet thrust alone. A derivation of the jet engine is to use the hot air and combustion products to force over a gas turbine rotor and turn a shaft which in turn drives a propellor. This is called a “Turbojet”. Many Regional commuter airliners use propellors driven by a turbojet engine. A turbo-prop engine is shown below.
Turbo-Prop Engine fueled by jet fuel. The jet engine provides hot gases to spin the turbine which creates shaft power to drive the propellor.
The jet engines and turbo prop engines have advanced in power and efficiency drastically since first use during WWll. The advancements have been applied to Gas Turbines used for power generation in stationary power plants. This gas turbine engines which are derived from the best aviation jet engines to power generation have been referred to as “Aeroderivative”. That is because the R&D that was invested in developing the most powerful and highly efficient jet engines has been adapted to electric power generation using natural gas fuels. The G-E “F Class” Series of “Gas Turbine as installed in many modern natural gas fueled power generation plants. A G-E Gas Turbine is shown below. Siemens, Mitsubishi, Rolls-Royce, Pratt-Whitney and a few other world manufacturers also design and manufacture large Utility Scale Gas Turbines.
As described earlier, the gas turbine drives a shaft which is connected to a generator to produce electricity. Here is a schematic of the gas turbine combined cycle plant exhausting heat to a steam generator that then provides steam to a steam turbine. This is called a Gas Turbine, Combined Cycle Power Plant. The combined cycle is using two cycles for power generation, the internal combustion (Brayton) cycle and the external combustion (Rankine) steam cycle. By combining the two cycles overall efficiencies have reached the highest of any other thermal power generatioin cycle or process at up to 65% efficient.
A modern 620 MW Gas Turbine, Combined Cycle Power Plant is shown below. This is the Duke Energy, Buck Plant located near Salisbury, NC.
South Carolina has generated about 28.6% of the electricity consumed from natural gas fuel. Most of that is through gas turbine combined cycle plants similar to the Duke Buck Plant in the photo above. FYI, North Carolina generated about 33% of that states electricity from natural gas.
Up to this point we have discussed the reciprocating gasoline engine, the Diesel, coal plants, jet engines and gas turbines. All of these are variations of “Heat-Engines” they convert heat energy into thrust or shaft horsepower that is a motive force for either propulsion of a vehicle, train, ship or plane or for turning a shaft that drives an electric generator. These engines consume a high percentage of the total energy utilized in America. Next covered is nuclear which has generated about 18-20% of America’s electric power for decades. A nuclear plant produces steam using the heat of nuclear fission. The steam is then passed through a steam turbine, much the same as in a coal plant described above. Here below is a schematic of a nuclear power plant using a pressurized water reactor,
Electricity generated from nuclear power also utilizes the steam turbine prime mover as a massive heat engine. Instead of using combustion to provide the heat to produce steam, a nuclear power plant utilizes nuclear fission within the reactor vessel. The primary coolant for the nuclear reaction is water under high pressure so that it will not boil. This loop of hot, pressurized water is circulated through the steam generator where the heat is transferred to the secondary cycle which provides the steam flow to drive the steam turbine. Another version of a “Heat-Engine”
Other Heat-Engines that We Depend on
You have heard of renewable power generation which includes Hydro-electric, solar, wind and biomass. Solar and wind usually generate power directly, although there are a couple of hybrid designs that utilize sunlight to boil water and produce steam. These will not be discussed here.
Biomass fuel such as waste wood or processed wood chips is used much like coal in a boiler furnace to produce steam to drive a turbine generator, Then there is Bio-Fuels such as Ethanol made from corn which is added to gasoline and burned in Internal Combustion engines of our cars and trucks.
Another form of Biomass fuel is Landfill Gas which is methane that is generated from decaying garbage in landfills. This gas is collected and piped to power boilers to generate steam and electricity.
Lastly, a small amount of electricity is generated in the U.S.A. by geothermal power. This is usually in the western U.S.A. in California and Nevada where hot rocks below the ground are used to generate steam which is then passed through a steam turbine generator.
The above serves as a primer to become familiar with the importance of heat engines in our everyday lives.
Dick Storm, September 9, 2020