The value of a particular type of biomass depends on the chemical and physical properties of the large molecules
from which it is made.
Man for millennia has exploited the energy stored in these chemical bonds, by burning biomass as a fuel and by eating plants for thenutritional content of their sugar and starch.
More recently,fossilised biomass has been exploited as coal and oil. However, since it takes millions of years to convert
biomass into fossil fuels, these are not renewable within a time-scale mankind can use. Burning fossil fuels uses
‘‘old’’ biomass and converts it into ‘‘new’’ CO2; which contributes to the ‘‘greenhouse’’ effect and depletes a non-renewable resource. Burning new biomass contributes
no new carbon dioxide to the atmosphere, because
replanting harvested biomass ensures that CO2 is absorbed
and returned for a cycle of new growth.