I recently switched from Coal to Charcoal due to my coal supplier shutting his doors.  Still using the same basic forge layout with charcoal as I did with coal, but with two minor changes.

With coal, I used a hair dryer connected to a copper pipe, which fed into a stainless steel pipe with holes drilled in in for the air blast ( From the bottom ) , to get consistent heats, I had to all but seal the copper pipe into the Stainless pipe, With charcoal, even with the hair dryer on the lowest setting, in order to get a good heat and keep from burning my charcoal up very quickly, I had to pull the copper out of the stainless, now I leave about a 1-2 inch gap between the pipes for the airflow, and in some cases offset the copper so only about half of the opening on either side line up. 

Second thing, as mentioned above, is the fire size, with coal, I typically had a trench about 1.5 firebricks long, by about 1 firebrick deep, with charcoal I found it best to shorten up the length to about 8 inches ( .75 firebrick im thinking ) and nearly double the depth of the fire trench. 

I have yet to attempt to forgeweld since moving to charcoal, but I am certain I can get the temps needed as it takes about 3 minutes to get a piece of O1 half inch rod stock up to forging heat from cold, and once its been in the fire once, about 10-15 seconds to get it back to forging temps, I actually stick the piece back in the fire, and count out loud because the first piece of metal I stuck in the forge after tinkering with it came out sparking and very VERY yellow and burned, I think I left it in there for about 5 minutes as prior to the changes it would come out dull red after 5 minutes. 

good luck, play around with it. 

Bottom blast behaves very differently with charcoal. 

To get maximum from charcoa side blast is in my experince the best.  It that wonderfull spot fire that hat enables you heat the piece right where you need it to the welding temperature.

 

Ill actually be attempting my first forge weld with Charcoal this weekend, depending on how that goes I may rebuild to side blast as I wont be getting any coal any time soon. 

A side blast works a treat with coal as well so you're not making a commitment to one fuel.

Frosty The Lucky.

with a side blast, you are limited to about 6" of heat with charcoal, not so with coal. Further you will find that the hot spot is about an inch closer to the tyere with charcoal than coal. This isn't an issue, as you can pile an inch of coal on top of the hearth, and lay your stock on it. This is using a 3/4-1" ID tyere (3/4" schedule 40 pipe is about 7/8"). With charcoal your waisting fuel going larger, you need multiple tyere to do that.