How water in coal particle evaporate?

Question

One of my projects is to remove moisture in the coal using heating air forcing into the Bradford Breaker which is used to crushes the coal into smaller pieces. My assumption is air going out will have 100% relative humidity. This means, whatever the air coming in will pick up more water and reaches 100% r.h. on the way out. And all the water added into the air out is from the coal. Agree or not is up to you decide but tell me why? The main question I have is how to know how much water can evaporate from the coal into the air. I have confused while reading latent heat and not sure at all. If there is no answer, I will have to assume that the coal can eject that much water in the air after all. I would be appreciated your contribution.

Answer 1

I hope this is a school project and not a real one.

If the latter, hire an engineer.

How much moisture the air picks up depends on a lot of factors, such as temperature of the coal and of the air, flow rates, surface area. And whether it reaches 100% depends on those factors.

Is all of the moisture in the coal on the surface?

Don't forget evaporation cools, both the coal and the air.

"all the water added into the air out is from the coal" where else would it come from?

Answer 2

Assuming 100% relative humidity of the exit air seems inappropriate to me. If the air exiting the system is saturated, then you almost certainly haven't dried the coal completely. If the coal could give up no more water, than the air wouldn't reach saturation or, conversely, if the air is at 100% humidity, it cannot pick up more water, regardless of how much more water may remain in the coal.

The best way to determine how much water can evaporate from the coal is to weigh a sample (or multiple samples), dry it in an oven until you are certain that it is dry and weigh it again. The difference is how much water, on average, the coal contains. Now you know how much you can hope to achieve from your heated air method. By weighing samples coming out of your air dryer and comparing with the expected totally dry weight (based on the oven-dried samples), you can determine what percentage of the available water your drying process removed.

Answer 3

the air will be 100% saturated before you heat the air up.  The hot air will lose heat to the coal based on the coals heat capacity, the hot air will lose heat to the atmosphere, and finally, the heat of vaporization of the water.  The sum of these heat uses will give you the amount of energy it will take to dry the coal.  Next, you'll need to know what temperature is required to get the water molecule to break out of the pores in the coal.

That temperature is higher than the 212F needed to vaporize it and in other pours media used to capture water, we use temperatures over 400F.

So you need enough air heated to 450F such that it exits at 400F from the coal loseing all the energies calculated earlier.