Burning timber waste does produce CO2 emissions, however this CO2 would have been produced anyway if the timber was left to decompose. For energy crops the carbon is offset by the planting of replacement crops.
Biomass fuels remove carbon from the atmosphere when they are growing and release carbon when they are burnt. This is obviously better than fossil fuels that release carbon that has been locked away for millions of years and would otherwise remain so.
When considering Biomass the fuel source is critical and research needs to be under taken as to how this can be provided and the space required for storage. That said biomass fuel is becoming more readily available and a biomass system can be an ideal substitute for an oil powered system.