Today, the most widely used forms of bioenergy are firewood, bio-ethanol, biodiesel and biogas.
Plant biomass as a renewable source of biofuels is presently concentrated on a relatively narrow range of species : stovers of conventional crops such as maize, rice, sorghum and sugarcane, on the one hand, and dedicated plant species such as Miscanthus, switchgrass, and hybrid poplar [1].
As the crops used as feedstock were not specifically bred for providing energy, the yields of biomass used for energy production, in terms of carbohydrate and oil, are not optimal. The energy content of biomass and the efficiency of processes will have to be dramatically improved.
Preliminary results from research conducted by the European Environmental Agency indicate that the potential of environmentally compatible primary biomass for producing energy could double (increase from around 180 Mtoe in 2010 to about 300 Mtoe in 2030) (Mtoe, million tons equivalent ; EEA Briefing, 2005) [2].
Sustainability is a key criterion when extracting energy from biomass. Importantly, there should be :

• no conflict between energy and traditional biomass uses ;
• no impact on food prices ;

• a large positive energy balance. Energy output should be several (say 4–10) times greater than the energy put into the system ;
• a neutral or positive greenhouse gas (GHG) emissions balance compared to fossil fuels ;
• no threat to biodiversity and (rain)forest conversion to bioenergy crops ;
• no adverse effects on soil, water and air quality, along the whole production chain.

Obviously, such criteria are not yet met by in use biofuel crops [3].

By using a worldwide agricultural model to estimate emissions from land-use change (i.e. forest and grassland diverted to biofuels), it was reported that corn-based ethanol nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years. Biofuels from switchgrass, if grown on U.S. corn lands, increase emissions by 50%. This result raises concerns about large biofuel mandates and highlights the value of using waste products [4].

1. Demura T, Ye ZH : Regulation of plant biomass production. Current opinion in plant biology 2010, 13:298-303.
2. Research and Development on Renewable Energies, ISPRE Preliminary Global Report on Biomass [http://www.icsu.org/publications/reports-and-reviews/ispre-biomass/ispre_biomass.pdf]
3. Book Bytes - The Limits and Potential of Plant-Based Energy [http://www.earth-policy.org/book_bytes/2010/pb4ch05_ss5]
4. Searchinger T, Heimlich R, Houghton RA, Dong F, Elobeid A, Fabiosa J, Tokgoz S, Hayes D, Yu TH : Use of US croplands for biofuels increases greenhouse gases through emissions from land-use change. Science 2008, 319:1238.