The background

“Biomass energy” is the general designation for energy produced from waste materials, or similar materials produced by agriculture and forestry. Because of their biological origin, these materials inherently have large moisture contents, typically in excess of 70%, and the most feasible way to extract the energetic content from such materials is to imitate the natural process from the generation of natural gas – namely, the process of anaerobic fermentation.

Anaerobic fermentation

The advantage of this process is that the very applicable gaseous fuel methane is produced in large quantities, but the main disadvantage is that the process is rather slow and requires fairly bulky, however simple, equipment. Anaerobic conversion has therefore been used primarily in relatively small systems used on individual farms for the energetic utilisation of animal manure and similar waste materials.

Throughout human history, various fuels have been burned to produce heat. In recent centuries, fossil fuels, i.e. natural gas, oil, peat and coal, created naturally from biological materials over very long periods, have been increasingly used. However, it has recently been recognised that the burning of fossil fuels formed hundreds of thousands of years ago may have to be restricted because the release of carbon dioxide from their combustion may be seriously affecting the climate.

Serious attention has therefore been paid over the last few decades to reducing the dependence on fossil fuels and improving the use of biomass for energy generation. The reason for this is that the carbon dioxide “recycling rate” is very short – a couple of years – as compared with that of fossil fuels.

State-of-the-art thermal processes

The process of burning – or combustion – may be a perfectly feasible process for conversion of biomass to energy if the moisture content of the fuel is below about 55%. This is because the energy released during combustion exceeds the heat needed to evaporate the moisture in the biomass. The net result will therefore be positive.