There can be three methods for this and they are decomposition, fermentation and burning.
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There are three primary ways to convert biomass into energy :-
1. thermal:- the oldest and most common way is to burn biomass to create heat . this can be directly used for heating , cooking and industrial purposes or indirectly for production of electricity
2. thermo chemical :-by heating plant matter , it is possible to breakdown biomass to gases ,liquids and solids ,which can be further processed into gas and liquids like methane and alcohol . biomass reactors heat biomass in a low-oxygen enviroment to produce methane , which can then fuel steam generators,combustion turbines,combined cycle or fuel cells
3. biochemical:-when bacteria,yeasts,and enzymes are used breakdown biomass , methane is produced and can be used to produce fuel or heat and power .

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