I just wanted to know what will happen when we decrease the particle size of coal and then use it for combustion? Will the heating value of the powdered form be different.? Will the ignition temperature required be less? Has this size reduction something to do with "Low Temperature Combustion"?
The first thing you need to consider is the type of coal-fired reactor or boiler. There are many types such as tangential, wall type, fluidized bed type, . . . etc.
Another type called the Stokers which burn lump coal. There are two main types of stokers: 1) spreaders stokers and 2) chain-grate stokers (also known as “traveling grate” stokers). The spreader stoker distributes a continuous feed of coal evenly onto the bed. The chain-grate stoker feeds the coal and then distributes it with a moving grate. I may also add another type called the pulverized coal (PC) fired furnace which uses an optimized size of the coal particles that maximize the combustion in the burner zone of the boiler.
Let us take the fluidized bed type. Very fine coal particles with this type will lead to entrainment of the coal particles (without combustion) with the rising combustion gases which will reduce the thermal efficiency and will cause separation problems in the downstream unit operations.
Hope this helps answer your question.
Professor Yehia Khalil, Yale University, USA
Fellow of the University of Oxford, UK
The heating value is a chemical property (kJ/kg), so the physical structure of the coal is immaterial. What will change is that it will ignite easier and the duration of burn is likely to be less, although this depends on whether you can manage to effectively separate the powder and distribute it through the air. As the previous poster mentioned, there is an optimal size: if it is too big, the surface-area-to-volume ratio is too small, then the air doesn't interact with as much fuel; if it is too small, then the pyrolysis from the coal and the burned products surround the fuel too much and the air can't get to it.
Low-Temperature Combustion is more about the design of the combustor than the fuel. For a furnace, MILD combustion will give you LTC, and the way it does this is to dilute the air with the burned products. This preheats the air, making it easier to ignite the mixture, but being a diluent, there is more mass to heat for the same energy release, so the temperature cannot become as high.
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