We can not use biomass to replace all fossil fuels used today for the following reasons:

A) the conversion of biomass into oil has very low efficiency. In simple words, a large quantity of plants is needed to get a small amount of biofuels (in rough calculations 10 kg of corn give about 3.8 litres of ethanol, however, this has as much energy as 2.5 liters of oil).
B) to cultivate biomass e.g. corn need fertilizers, irrigation, transportation to the factory, processing, etc. which require energy which comes from fossil fuels.
C) areas that can be cultivated, are specific. Therefore if they are used to grow plants that will go for biomass, there will be no areas to be cultivated for food.