Nah, opinions are worse. Some people have MORE than one opinion. Don't know of anybody with more than one of those holes in their rear.
I too am intrigued by this lava rock idea, but I would really have to see some hard numbers on it. Something I don't think the OP is in a position to accomplish at this time.
As far as the byproducts of combustion are concerned, Alex is correct. With PERFECT combustion, the byproducts are heat, light, and water.
With hydrogen and oxygen being the only two fuels present, you get complete combustion where the only byproduct is heat, light, and water. Problem is it is really, really, really hard to get just hydrogen and oxygen in the burn.
Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present such as carbon monoxide and pure carbon (soot or ash). Additionally, any combustion in atmosphericair, which is 78 percent nitrogen, will also create several forms of nitrogen oxides.
http://en.wikipedia.org/wiki/Combustion