Today, the European bioenergy sector utilises a wide variety of processes to upgrade or convert the energetic potential of biomass feedstocks. In fact, most modern bioenergy installations require more advanced fuel to work at their maximum capacity. On the other hand, biomass processing industries are developing more and more different fuels and/or bio-based materials out of the same initial feedstock. Thanks to these technologies, today bioenergy is one of the most reliable sources of clean energy adopted by companies, municipalities and households all across Europe. This section offers a brief overview of the specific processes and technologies.
Solid Biomass Conversion Processes
Most solid biomass is used in thermal applications. If you ever try to make fire using green wood, you already know that combustion performs better when moisture is limited. This is why most technologies developed over the past decades by the solid biomass processing sector have focused on the way to reduce the presence of water in the final fuel, either using traditional drying methods or more advanced ways. In addition to being essential for improving combustion, having a very dry fuel is key as it allows in the end to increase the energy potential for a same volume of fuel, improving its storage and its transportation.
- Logging, grinding, screening and/or drying operations are mechanical processes carried out in order to enhance solid biomass fuels. These operations help transform the biomass into a more homogeneous fuel that is easy to handle with an higher energy recovery (eg. wood chips, wood logs, agrofuel). In general, moisture content in wood chips used for energy generation in municipal or industrial plants ranges from 20-30%.
- Densification is another popular way to transform woody material into an advanced fuel with high calorific value. This process involves compressing biomass and lowering moisture levels in a range of 8-10%, allowing for a more homogenous fuel–either in the form of pellets or briquettes. The heat during compression fuses the lignin in the biomass, naturally binding the biomass together in its new, enhanced shape. Thanks to densification, the homogenous biomass fuel is easier to transport and can be used in automated biomass installations, such as pellet stoves and boilers.
- Thermo-chemical conversions are now used to produce fuel with even higher calorific value such as torrefaction or steam explosion technologies. During the torrefaction process, wood is subjected to 230 to 300ºC at atmospheric pressure in the absence of oxygen. Comparable to some extent to coffee torrefaction, this method allows the creation of a fuel with very interesting characteristics. Compared with conventional wood, torrefied wood has a very low (>5%) moisture content, is easily grindable and is relatively hydrophobic.
Thanks to advanced technologies, woody biomass can also produce liquid or gaseous fuels. Pyrolysis for instance is a thermal-chemical conversion that requires a high temperature (>400 °C) and little oxygen to convert the biomass into other forms including gas, liquid fuels (pyrolysis oil), and biochar. Gasification is another thermo-chemical conversion that takes place at high temperatures (>800 °C), with limited oxygen and/or steam, converting solid biomass into synthesis gas, or syngas, which contains carbon and hydrogen and can be used to produce liquid fuels such as biodiesel.
Wet Biomass Conversion Processes
Wet biomass are also materials present in high volume across Europe (manure, agricultural waste or by products). As they contain too high of moisture content to be turn efficiently into energy via a direct combustion process, other conversion pathways and energy outputs have been developed especially by the biogas (anaerobic digestion) and biofuel sector (fermentation). Anaerobic digestion and fermentation are the two main conversion pathways used to turn wet biomass feedstocks into advanced fuels.
- Anaerobic digestion is microbiological process of decomposition of organic matter, in the absence of oxygen, common to many natural environments and largely applied today to produce biogas in airproof reactor tanks, commonly named digesters. A wide range of micro-organisms are involved in the anaerobic process which has two main end products: biogas and digestate. Biogas is a combustible gas consisting of methane, carbon dioxide and small amounts of other gases and trace elements. Digestate is the decomposed substrate, rich in macro- and micro nutrients and therefore suitable to be used as plant fertiliser. Biogas can be used for direct combustion to produce heat but also power if converted in a cogeneration plant or directly in adapted gas motors. Biogas can also be “upgraded” through purification processes to obtain a biomethane than can be injected in the existing natural gas networks, used as a chemical products or as vehicule fuel.
- Fermentation is a biochemical conversion whereby microorganisms including yeast and bacteria convert bioenergy into products such as ethanol and liquid transport fuels (biodiesel). This process is done through several stages. First, sugary and starchy feedstocks such as corn must be crushed and combined with water, allowing the enzymes to convert these starches into sugar. This then ferments along with the addition of yeast, and is distilled into bioethanol. Bioethanol can also be produced from cellulosic biomass such as grass, wood, and stalks, via fermentation, however this process is more complex and involves a mechanical pre-treatment and the addition of enzymes or breakdown of the lignocellulose into sugar through hydrolysis, before following the same procedure.