Posts Tagged ‘combustion’

Coal Power Plant Fundamentals – Combustion

Sunday, February 13th, 2011
     Ever have a small child threaten to hold his breath until he passes out and he actually managed to do it?  It’s not that unusual.  And if his body were prevented from acting in self preservation, that is, taking in breaths while he was unconscious, leading to his eventual awakening, he would die.  While the human body can survive about a month without eating and three days without water, under normal conditions it can survive only a matter of minutes without breathing.  Power plants, too, require oxygen to function, and this process is called combustion.

      Human lungs, along with the diaphragm which works to expand and release the lung cavities, enable our bodies to breathe in air, then expel the waste product, carbon dioxide.  Oxygen is needed to metabolize, that is burn, our food, enabling the food cells’ energy to be absorbed by our bodies and converted into energy to live.  Like us, coal power plants need to breathe in oxygen in order to convert coal’s latent energy into a usable form.

     Previously we learned how coal is fed to a coal mill where it is pulverized into a fine powder.  This powder is then sucked out of the mill by the exhauster and blown through a serpentine path of pipes leading to the burners on the furnace.  The burners will then act upon the coal, combining it with the oxygen in our atmosphere to create a chemical reaction capable of releasing coal’s energy in the form of heat.  All this activity looks to a bystander like a massive, sustained fire in the furnace.  See Figure l.   

Figure 1 – Coal Power Plant Combustion

     The boiler is contained within the furnace and is situated so it is exposed to fire from the combustion process.  Heat energy from the fire transfers into the water in the boiler, much like when you boil water for tea in a kettle on your stovetop.  If you’ve ever boiled water, you know that once it gets hot enough it will turn into steam, and the same for our furnace boiler.  The steam emitting from the boiler will cause a turbine-generator to spin, and the end result will be electricity for our use.  In the simple diagram of Figure 1,  waste products from the combustion process, like carbon dioxide, go up the smoke stack and are released into the atmosphere.  Incidentally, this is the same type of carbon dioxide that we exhale from our bodies when we breathe.  

     Please keep in mind that Figure 1 is a very simplified diagram.  In reality waste products leaving the furnace go through various pollution control devices where most pollutants are removed before they reach the smoke stack.  These details, and many more, are the type of information that would be covered during my training seminar, Coal Power Plant Fundamentals.    

     Next time we’ll learn how the heat energy in steam is converted into mechanical energy capable of spinning a turbine generator to make electricity.



Coal Fired Boiler Explosions

Sunday, July 25th, 2010

     Try this for a tongue-twister:  Coal fired electric utility power plant boiler…  If you’ve been reading along with us for the last couple of weeks, you now have a pretty good idea of what these are and what they do.

     These boilers are contained within furnaces in coal fired power plants.  The furnace’s job is to combine coal and air to create a combustion process.  It is like a big, insulated enclosure that keeps the heat energy from the combustion process from escaping before it can be absorbed by the water and steam in the boiler tubes.  The heat energy is then funneled to the steam turbine to spin an electrical generator, creating the energy which will eventually find its way into our homes and businesses.

     During the operation of the boiler, coal and air must be introduced into the furnace at carefully measured rates to maintain a proper fuel-to-air ratio which will enable the release of heat energy from the coal at a safe, controlled rate.  Fuel-air ratio is the amount of coal entering the furnace divided by the amount of air entering the furnace.  If this ratio isn’t precisely maintained, conditions may be right for an explosion to occur.  Specifically, the ratio has to fall within an “explosive range.”  Once within this range, all that is needed is an ignition source, such as hot ash, or even mere static electricity, and the result may be a furnace explosion.  

     There are certain times at which furnace explosions are more likely to occur than others, such as when the boiler is being started, operated at less than full capacity, or shut down.  When a furnace explodes, a pressure wave moves out from the center of the blast.  This pressure wave will bear up against the sides of the furnace with great force, and if the pressure is high enough the sides of the furnace, which are made of heavy steel components, will actually bend and split open.  Boiler tubes may even rupture, releasing high pressure steam and water into the power plant and furnace.  At the very least, the boiler will be down for expensive repairs and no electricity can be produced by its turbine generator.  This down time can last for many months and results in lost revenue to the energy producer.

     Aside from an explosive fuel-to-air ratio, there are other potential causes of furnace explosions.  For example, poor coal quality can lead to incomplete combustion, or the flame going out completely, encouraging unburned coal particles to settle and accumulate in the furnace.  The accumulation of coal can grow to the point where it forms an explosive mixture when combined with the right amount of air.

     So how can boiler explosions be prevented?  The National Fire Protection Association (NFPA) looked into the problem and developed an industry standard.  This standard is known as NFPA 85, Boiler and Combustion Systems Hazards Code.  Its purpose is to contribute to operating safety and prevent uncontrolled fires, explosions, and implosions of coal fired boilers.  NFPA 85 lays out guidelines to follow when designing, building, and operating boiler fuel handling systems, air handling systems, and combustion control systems.  Following its guidelines will certainly significantly decrease the probability of explosions occurring.

     Another means of explosion prevention includes implementing a boiler operator training program.  These enable attendees to better understand operating procedures and equip them with the knowledge to safely control the combustion process, particularly when a furnace explosion is most likely to occur.  This training can be done with a combination of classroom instruction along with time on a simulator and may be followed up with hands-on training in the plant itself.

     Lastly, boiler explosions can be prevented by implementing an effective inspection and maintenance program to locate and repair or replace boiler components, averting the possibility of a potential disaster occurring.  Things such as check lists can be used to ensure that nothing is missed.  This is a strategy that all pilots must use before starting their planes, and it is now being used in hospitals as well to cut back on the rate of patient infection due to carelessness on the part of hospital staff.

     Hey, we’re all human, and humans are not perfect.  But remember that an ounce of prevention is truly worth a pound of cure, and then some.  A properly placed check on the list could mean lives will be saved.