I am still learning about Biomass myself but found this snippet very interesting.
The vital difference between biomass and fossil fuels is one of time scale.
"Biomass takes carbon out of the atmosphere while it is growing, and returns it as it is burned. If it is managed on a sustainable basis, biomass is harvested as part of a constantly replenished crop. This is either during woodland or arboriculture management or coppicing or as part of a continuous programme of replanting with the new growth taking up CO2 from the atmosphere at the same time as it is released by combustion of the previous harvest.
This maintains a closed carbon cycle with no net increase in atmospheric CO2 levels"
More info
http://www.biomassenergycentre.org.uk/portal/page?_pageid=76,15049&_dad=portal