[F]or the combustion phase, it is possible to burn fossil fuels more efficiently than biomass. Hence, the former ought to reduce CO2 emissions overall. But a more sophisticated analysis ought to consider life-cycle consequences (including CO2 released in extraction, preparation, transportation, etc., of the two forms of biomass). [...] Biomass may be renewable, politically correct, and fossil-fuel displacing. But it is unlikely to reduce atmospheric CO2 concentrations much, if at all.
http://www.masterresource.org/2011/09/biomass-vs-fossil-fuels-co2-battery/

So long as one uses carbon-based combustion, the chances of reducing CO2 emissions are nil, whether one uses new biomass or a fossil fuel. In fact, since newer carbon sources are also associated with higher moisture content in the fuel, burning them would increase CO2 per unit of usable energy.
http://wattsupwiththat.com/2011/09/02/batteries-from-the-carboniferous/

The construction of a biomass plant at the Anglesey Aluminium Metals (AAM) site in Wales has been approved by the UK government. The plant, which is powered by wood-burning, has the potential to fuel over 300,000 homes and will provide the area with 700 new jobs. Energy minister Charles Hendry commended the approval of the plant claiming that it will provide a “reliable, secure, flexible and renewable source of power”. Despite Hendry’s praise, critics claim that the plant will create as many environmental problems as it will cure.

http://stockholmnetworkblog.wordpress.com/2011/09/20/uk-government-approves-anglesey-biomass-plant/