Summary

Material humidity is actually water in the pores of porous substances or on the surface of solids in the form of minor droplets or uniform thin film.
Humidity is a parameter of key importance in biomass combustion, gasification and granulation (pelletising). In the case of wood burning boilers, in order to maintain combustion, the required humidity content is less than 55% to 65%. The optimum humidity content for wood combustion boilers is much lower, and amounts to about 10%-15%. In the case of biomass gasification, the maximum feed humidity content in the material strictly depends on the type of reactor for gasification. For small units with a permanent medium the humidity should not exceed 20%. Gasification can be carried out in a fluidised bed at the feed humidity between 50% and 65%.
In order to obtain durable and stable pellets from the biomass material, the raw material humidity should not exceed 15%.
Humidity content is so high in some forms of organic waste from biomass that its combustion or gasification is not commercially viable. In such cases other biomass treatment technologies are applied before its final use.

The need to dry biomass before final use is related to the following issues:

• the calorific value of the fuel strictly depends on the fuel's humidity,
• in order to ensure optimum course of biomass combustion, a fixed, strictly specified humidity level of the feed fuel is required. Combustion of fuel with different humidity requires using a complex control system, which entails a significant increase in investment,
• the longer storage of humid biomass causes biological decomposition of biomass ingredients, which makes its energy content lower, and deteriorates the hygienic conditions of the storage space,
• due to technological, economic and ecological reasons, humidity content in biomass based fuels should be 10-30% of the wet biomass,
• the humidity of biomass raw materials used for making briquettes and pellets should be about 10% of the biomass weight. Drying as one of the biomass preparation processes has a direct impact on the final cost of the fuel.

Since the already available high capacity biomass drying technologies have not been commercially attractive, the applied large scale biomass combustion technologies allow using biomass with a wide humidity range in power production.

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