I've searched all over the internet, and haven't been able to find an
answer to this question, so I'm hoping that someone here may be able
to.  My question is this:

How would mixing lava rocks with coal affect such things as the
combustion rate, efficiency, etc. (as compared to just coal)?

I can make a few guesses, but I'm hoping someone here will be able to
answer with more confidence than my guesses.

Thanks!
Mark

I've searched all over the internet, and haven't been able to find an
answer to this question, so I'm hoping that someone here may be able
to. My question is this:

How would mixing lava rocks with coal affect such things as the
combustion rate, efficiency, etc. (as compared to just coal)?

I can make a few guesses, but I'm hoping someone here will be able to
answer with more confidence than my guesses.

Thanks, Jitney and Damon for your responses!  No, I'm not trying to
build a fluidized bed combustor.  And I would try this out myself, but
I'm not the person doing this stuff.  I have a friend in Cameroon who
is building and selling "coal cookers" that use an electric motor to
blow air over the coals.  These cookers are pretty small, but they
have two burners on top and a little oven beneath.  Anyway, I'm
helping to finance his business--don't worry, I'm not the victim of a
scam--and he was just telling me that he's started putting lava rocks
in with the coals.

I asked him about this, and he said "The volcanic rock plays a major
[part in this product,the rock is added on the coal,reducing the
amount of coal to be used,when the coal is ignited,the heat from the
coal is transmitted to the rock,the rock becomes very hot and does not
produce ashes or smoke. The size of the rock  remains same after
cooking and retains heat for over forty minutes after switched off.
This is the first time in Cameroon that a form of energy is gotten
from solid matter,we are familiar with gas and electrical energy in
our country.I am looking forth a many other things that this energy
will be useful."

His comments almost make me think that he thinks he's getting
additional energy from the lava rocks, but that's certainly not the
case.  I've asked him to elaborate on the benefit the lava rocks are
providing, but haven't heard back from him yet.  Anyway, I was just
trying to do a little independent research to either confirm what he
says or to help him better understand the benefit from a scientific
perspective (assuming there is a benefit).  I have also encouraged him
to try making a few simple measurements to help quantify the effects
of the lava rocks.

--Mark