petr-
sorry, but any time you burn a carbon-containing sobstance--including woodgas--the carbon is converted to carbon dioxide (or carbon monoxide, if you starve the combustion.) If you burn biofuel, the net result is a zero increase in CO2--the problem arises when we burn fossil fuels.

If you can store the combustion products, there would be no problem (except for those of storage space and energy consumed in producing absorbent compounds).Fuels such as methane produce less CO2 per BTU produced solely because aome of the energy produced results from oxidation of the hydrogen in the molecule--coal contains virtally none, oil has some and methane has more.