PVC4: Intermediate bioenergy carriers for power and heat
Biomass such as wood pellets, wood chips and also straw can be directly combusted or gasified for the production of power and heat. However, in existing fossil-fired (heat and) power plants, it can be easier to use torrefied material. Torrefaction significantly improves the suitability of biomass for co-firing in coal fired power plants and has the potential to enable higher co-firing percentages at reduced cost.
Torrefaction is a partial carbonisation or slow pyrolysis process. It is a thermochemical process typically at 200 - 350 °C in the absence of oxygen, at atmopsheric pressure with low particle heating rates and a reactor time of one hour. The process causes biomass to partly decompose, creating torrefied biomass or char, also referred to as 'biocoal'. Biocoal is stable, brittle and water resistant, and is thus easier to grind than the original biomass material and also harder to be biodegraded. If combined with pelletisation, biomass materials can be converted into an intermediate bioenergy carrier that is easier to transport, handle and store and also has superior properties in many major end-use applications.
In the 20th century, big plants of Lambiotte (France) and Lurgi (Germany) were operating, but stopped charcoal production because it was too expensive. Currently, worldwide several small-scale units are operating, typically in clusters forming bigger production units. Examples are Plantar in Brazil (some 80 units together), GreenCoal in Estonia and a number of others in Poland.
An overview of torrefaction technology and market potential is provided in the report: Status overview of torrefaction technologies.
Fact Sheet: Torrefied pellets
Pyrolysis is the chemical decomposition of organic matter by heating in the absence of oxygen. The feedstock decomposes into organic vapours, steam, non-condensable gases and char. The technology can in principle use any low moisture content (preferable below 15%) organic material as a feedstock. The feedstock potential for producing advanced biofuels lies in forest and forest industry residues, as well as agricultural and agro-industrial residues. Plastic wastes can also be used as feedstock, but the resulting fuel will be termed recycled carbon fuels, not biofuels.
The pre-treatment of the feedstock typically includes drying to less than 15 % moisture and crushing/milling to particles of less than 5 mm. The highest yield of the desired liquid fraction, up to 65 wt% on a dry feed basis, is obtained by thermal fast pyrolysis. Fast pyrolysis takes place in order of seconds at around 500 °C. The heating medium is typically circulating sand, but also other forms of heating have been used. On cooling, the organic vapours and the steam condense to a dark brown viscous liquid called fast pyrolysis oil (FPBO) or Fast Pyrolysis Bio Oil (FPBO). The char and gas are used internally to provide the process heat required, and additionally also energy for export.
The word “oil” used in this context is misleading, the energy content is only half of that of fuel oil, it contains ash solids, the oxygen content is almost as high as for biomass (35 - 40 %), it is acidic (pH usually below 2) and non-miscible with either conventional oil or with water. Nevertheless, this liquid is transportable, storable and can without upgrading to some extent be used as a fuel oil substitute, in particular when using a catalyst during pyrolysis. By using a catalyst during pyrolysis or in the vapour phase, the oxygen content and acidity of the oil can be reduced, at the expense of a lower mass and energy yield. There is also a development of a pressurised pyrolysis in a hydrogen atmosphere, whereby the bioliquid generated has a yet lower oxygen content and acidity and being more similar to hydrocarbon fuels.
Pyrolysis oil can be directly combusted or co-combusted in boilers, furnaces or used in turbines to produce heat and power.
Fact Sheet: Pyrolysis oil
 Status overview of torrefaction technologies. Marcel Cremers, DNV GL, Netherlands, Jaap Koppejan, Procede Biomass, Netherlands, Jan Middelkamp, DNV GL, Netherlands, Joop Witkamp, DNV GL, Netherlands, Shahab Sokhansanj, UBC, Canada, Staffan Melin, UBC, Canada, Sebnem Madrali, CanmetENERGY, IEA Bioenergy Task 32, 2015.