There are a number of different fuel types these include Biomass, Coal, Hydro, Nuclear, Wind etc. 

The BMU Fuel Type spreadsheet provides a mapping for which balancing mechanism units (BMUs) makes up specific fuel types; the fuel type categorisation by balancing mechanism unit (BMU) is undertaken by National Grid ESO using the primary fuel type of each power station and is not based on the balancing mechanism unit (BMU) registration data held by Elexon.