preheated secondary air

Does anyone have a idea how hot secondary air needs to be to ignite coal gas cooking out of coal? Im just looking for a rough number Im thinking it would be at least 300 or so.

I don't think it quite works that way.. For combustion you need a fuel (coal gas) and oxygen and a source of ignition, such as a flame. I don't think a hot secondary air of 300 degrees will ignite anything Somebody fix me if I'm wrong lol

This may give you a clue...

Code: Select all

Coal >  Ignition Temp/* >   Volatiles Initially Released Temp/*

Bit        400-500*C                   200-300*C
Ant        700-800*C                   380-400*C

The preheating of the secondary air is to prevent cooling down the volitiles so the won't burn. If you introduce cold, outside air, the volitiles will remain as soot, and wasted, unburnt energy.

The air above a fire is usually too low on oxygen to burn all the volitiles, and it takes the right combination of oxygen, heat and volitiles to create a fire. This 'right' combination often results in a 'puff-back' in handfed stoves.

With the hot blast stoves, the secondary air usually comes in through a duct that is located alongside the outside of the firepot, so the firepot is doing the preheating. The vent ring around the upper perimeter of the firepot is also very hot, so the air probably is heated to ??? 300-500* ?? just guessing, it would depend on the fire, firepot liner, airflow rate etc.

The design for the HotBlast stoves is well thought out, and I'd love to watch one light off the volitiles.. it would be an impressive sight I'm sure.

When I was burning Bit coal in my big hand fed boiler, I created a preheated air source, and when the conditions were right, the secondary fire was very impressive. and it reduced the soot buildup quite a bit.

Greg L.

OK that makes sense The heated secondary air helps combustion
Thanks for the detailed answer, learn something everyday!

I figured it wouldnt need to be super hot. Im seeing with my vigilant I sometimes have a good bit of unburnt coal gas going up the flue, and Im toying with the thought of replacing one or both of my glass windows with a steel plate with a vent and baffal to heat the air with a small window to see how its doing.

on lower heat output you'll always have unburned gas entering the flue; even with secondary combustion air, you'll lose most of that because at low burn, even with preheated secondary air you will not have enough temp to ignite the gasses. I wouldn't do anything about it, It doesn't cost you greatly in terms of efficiency and secondary air at lower burn rates won't improve things.