The three primary routes for biomass to power are: Combustion, Gasification and Anaerobic Digestion. Combustion is easy to understand – instead of using coal or other fossil fuel, use biomass to produce steam that runs a turbine. Combustion of biomass for power could either be in the form of co-firing (when it is burned along with coal) or pure play biomass based combustion.
In the case of gasification, the biomass is first gasified and this gas turn produces power in a gas engine. Anaerobic digestion is usually applied to biomass that typically have a high amount of water in them (anaerobic digestion is most used for treating organic waste such as kitchen waste and sewage waste into energy). Under this route, microorganisms act upon the organic matter present in the biomass under anaerobic (absence of air) and convert it into biogas.
An emerging route for biomass based power is pyrolysis. In this, the biomass is rapidly heated to 450 – 600 °C in absence of air, and results in a bio-oil called the pyrolysis oil, which can in turn be used for firing the boilers. Typically, 50 – 75 % (by weight) of the feedstock is converted into pyrolysis oil.
Pyrolysis as a method for power production is not well established currently in India or elsewhere in the world. Typically pyrolysis plants work well beyond 2 MW scale, while gasification plants work well until 2 MW scale, at the current technological progress. Thus, it can be said that pyrolysis takes off where gasification ends.