Effect of moisture content

Reduction in the moisture content of biomass material may be required to achieve a number of purposes in energy applications. The moisture content also has an impact on transport and storage of biomass material.

Why is it important to reduce moisture content?

There is a direct, and strong relationship between how dry biomass fuel is and its energy content, or calorific value.  This Excel spreadsheet gives values of calorific value as a function of moisture content, as well as varous other paramenters such as density, for biomass fuel.

In combustion systems any water content in the fuel must be driven off before the first stage of combustion can occur, requiring energy, and thus reducing overall system efficiency and potentially reducing combustion temperature below the optimum.

Reduction in combustion temperature below the optimum may result in incomplete combustion of the fuel giving rise to the emission of tars and creosote which may condense in the flue, especially if it is long or includes changes of direction, and particulates.

The water may also recondense in the flue, and all these may lead to corrosion of the flue and the gradual accretion of material leading to the potential for eventual blockages or fire.

Impact on modern combustion systems

Most modern, high efficiency combustion systems are designed to operate within a range of parameters to ensure that performance meets emissions and efficiency specifications and a range of acceptable moisture content for the fuel is usually specified. If fuel outside this specification range is used the system may shut down automatically.

Not all modern biomass combustion systems require low moisture content fuel, however. Some are designed to handle fuel at much higher moisture content, e.g. as 'green' (freshly harvested) chips. These systems typically make use of some of the heat of combustion to dry the fuel as it approaches the combustion zone.

Many biomass gasifiers are designed to operate on very low moisture content feedstock, perhaps 10-20%. Other technologies, such as anaerobic digestion, fermentation, hydrothermal upgrading and supercritical gasification all make use of feedstock in an aqueous medium, and are particularly suitable for very high moisture content biomass, and for which drying is unnecessary.

Impact on storage and transport

High moisture content biomass has a much lower net energy density by mass, owing to the weight of the water, but also by volume owing to the energy required to evaporate the water. This means that transport is less efficient as a significant proportion of the load is water.

Storage is also less efficient, with less net energy available, but also storage of high moisture content biomass brings other problems with greater risk of composting, causing loss of biomass and potentially a fire risk from elevated temperatures and mould formation. Good ventilation and air flow help to minimise these problems.