Simply put, it is all the organic matter we see around us – paper, wood, grass, husk etc.
It basically results from the photosynthesis process in plants which use carbon dioxide, water and solar energy to produce complex organic compounds (biomass).
Thus, biomass could be imagined as a huge reservoir of 'locked' carbon and solar energy.
Burning of wood to produce heat is the simplest example of using biomass energy. Use of biomass energy is a carbon neutral process only if trees are not cut explicitly for this usage. Wood waste, paper, dead organic material would have released CO2 into the atmosphere anyway on decay. Thus their usage doesn't really add to the carbon in the atmosphere. In fact, certain biomass on decay releases methane into the atmosphere which is 10 times more effective than CO2 in global warming. Using them for biomass energy instead releases CO2 thereby making it a 'net carbon negative' source.
Biomass can be obtained from a huge number of sources like trees, forest waste, urban waste, agricultural waste etc.
Some trees and grass varieties grow very tall and are quite 'woody' and can be harvested at regular intervals for more than a decade without ever replanting – a feature called 'coppicing'. Oil plants like soya bean and sunflower and food crops like corn and sorghum are used to produce bio-fuels world wide.
The use of corn to produce bio fuels is however mired in controversy over its impact on green house emissions and also food security in general. Other sources of biomass include forest waste (dead branches, twigs, leaves etc), agricultural waste (residue after processing of corn, wheat, sugarcane etc) and the organic waste produced in cities (food, paper, wood waste etc).
As can be seen, there are a huge number of sources of biomass and industry research on biomass energy is currently focused on how best to extract energy from these sources.
Simply burning biomass is a very inefficient and polluting method of extracting energy from it.
Today there are many thermal, chemical and biochemical techniques of processing biomass to generate fuels and combustible gases with much higher energy content. Some simple processes allow decay and fermentation of biomass to produce methane, ethyl alcohol (which can be used as fuels) and fertilizer.
More advanced processes like pyrolysis, gasification and hydrogenation are more efficient and help tap much more of the inherent energy content in the biomass. For instance, in pyrolysis, organic waste is burned with char and with inadequate air to yield combustible gases. In fact, engines based on this technology were widely used during World War II in the absence of petroleum.
Biomass has many interesting and varied applications and has great potential to address the energy demands of the future. It is touted as the only 'net carbon negative' source and could open up vast opportunities for entrepreneurs willing to experiment with new ideas and take bold initiatives.