Because biomass is a widely available, renewable resource, its utilisation for the production of energy has great potential for reducing CO2 emissions and thereby preventing global warming. In this mini-review the 'state of the art' of several fermentation processes is discussed, starting with the most advanced process of ethanol production. This is followed by methane production, an established process for waste water purification which is gaining more attention because of the inherent energy production. Subsequently ABE fermentation is discussed and finally the biological production of hydrogen. The last section proposes a new way to assess and compare the different processes by relating their merit to 'work content' values and 'lost work' instead of the combustion values of their products. It is argued that, especially when dealing with energy from biomass, the application of this methodology will provide a uniform valuation for different processes and products. The described fermentation processes enable the supply of pure energy carriers, either gaseous or liquid, from biomass, yet the introduction of these processes is hampered by two major problems. The first is related to technological shortcomings in the mobilisation of fermentable components from the biomass. The second, having a much greater impact, is linked with socio-economics: until full externality costs are attributed to fossil fuels, accounting for their role in pollution and global warming, the competitiveness of the processes described here will hardly stand a chance.