Word of the Day: Torrefaction

What a surprise. They want a subsidy to turn wood into “biocoal”. They want to use energy to roast wood and turn it into coal. Even though coal exists. And there is lots of it. Some of the processes use microwaves.EP-301209997

The torrefaction process is a thermal treatment of biomass, in a temperature range between 200-300° C, in absence of oxygen and with a variable residence time (around 1 hour). The biomass gets roasted during the process and releases various volatile compounds. The final solid product has a higher energy density, better storability and better grindability than wood pellets or woodchips. For these reasons it can be used as a valuable renewable feedstock for co-combustion in small percentages in coal power plants, or it can be used in higher percentages in coal fired combined heat and power plants, hence the name biocoal.

Torrefaction occurs by steam inertization and through an accurate process control. Wood chips are conveyed from a chip silo to the unit where all the phases of torrefaction take place: pre-drying, post-drying, torrefaction, and cooling of solids. After this step, the biomass is delivered to a pelletizing unit and packed in big bags of 1 m3.

Several different woody biomass feedstock were tested: forest chips made from coniferous trees (spruce, pine) and broadleaf (birch), as well as by-products such as bark. Results showed that the quality of torrefied pellets depends to a large extent on the quality of raw material, the particle size distribution should be as homogenous as possible and fine particles should be avoided. No binders were used in the pelletization phase, and this is an advantage as binders concur to increasing the production costs. The lower heating value of the biocoal pellets varied between 17.96 and 18.44 MJ/kg (4.99–5.12 kWh/kg) with an initial moisture content of 6.57%. Higher energy density and lower moisture are expected with new continuous process tests which will be conducted at a higher temperature and with a separate drying system.  Biocoal is typically hydrophobic, however, trials with outdoor covered storage during winter revealed a slight increase in the moisture content of the torrefied pellets.

Biocoal cost estimates

 … this study indicated a cost of 208 €/ton, or 37–39 eur/MWh, for a full scale 50 kton/y plant. Several different factors can affect the final production cost of torrefied pellets. In the conditions considered by the study, raw-material accounted for 57% of the final production cost, while the torrefaction process accounted for 33%, and transport for 10%. This price level is clearly higher than the current price of coal used in combined heat and power plants, around 26 eur/MWh in Finland, however 1.2 million tons of torrefied pellets could replace half of the coal used in CHP plants in Finland annually.

For this reason the authors suggest there is a need for a subsidy system, such as feed-in tariffs, to make energy from biocoal competitive in the current market conditions.

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