In a post-fossil fuel age, the problem becomes obtaining carbon atoms in usable forms. Biomass (or, waste streams ultimately derived from biomass) would seem to be a good source for this, although not in quantities sufficient to replace the fossil fuels being burned for energy. The amount of carbon needed for chemicals and materials is much smaller.

This doesn't mean the biomass would be directly used; rather it could be converted to chemical feedstocks that could be purified just as petroleum and natural gas are fractionated, reformed, and purified to make such things. The outputs ultimately obtained might be little different from today's fossil fuel-based chemicals and materials.