EGEE 101
Energy and the Environment

Pulverised Coal Combustion

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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.

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
 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.

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.