Biofuels offer several advantages — economic, environmental and practical — compared to traditional fuel sources. Biofuels can be produced from local sources of readily available material, and localized fuel reduces transport costs and CO2 emissions from transit. Further, biofuels can achieve CO2-neutral combustion and are suitable for automatic combustion if properly processed.

To achieve these benefits, the biggest challenge associated with biofuels availability must be overcome: proper processing. Extensive drying typically is necessary to remove moisture from the organic material before use.

Advantages of Biofuels Drying

A variety of material can be used as a source of biofuels. Wooden pellets can be produced from wood chips, waste wood, green garden waste, bark and rejected material from paper recycling. Other common organic materials — fermented substrate of sugar beets, used coffee grounds, horse dung, straw, grass, bagasse and fruit pumice — also are used to create biofuels. Although the utilization of readily available and local organic materials is an advantage, each material provides different levels of energy when used as a fuel source.

Drying Organic Materials

Biofuels can be economically advantageous, environmentally friendly and achieve the high temperatures necessary to be a good source of fuel. In many cases, biofuels benefit from drying. Because the raw input material is often bulky and uneven, biofuels provide a challenge to processing and drying.

In addition to the size and weight variance, several other variables are involved with processing biofuels. Many biofuels sources have high levels of contamination, are sensitive to temperature and may emit volatile matter or explosive dust. These variables pose unique challenges to traditional methods of drying. Each drying method has a unique advantages and disadvantages.

Drum Dryers. Also known as rotary dryers, drum dryers are comparatively large machines that run gas parallel with the solids while drying. Although drum dryers have comparatively simple operation, they are capable of emitting fine material with a low “de-dusting” effect. Accessibility for maintenance may be limited.

Fluidized Bed Dryers. Fluidized bed drying designs provide perpendicular gas flow to the solid flow on a shallow bed. Fluidized bed dryers have demonstrated advantages when material is standard size; however, biofuels materials often have non-standard sizes. 

Fluidized bed dryers can form “carpets” when drying bulky and fibrous products, which is a disadvantage when drying organic materials for biofuels. Many organic materials such as tree cuttings can be bulky and fibrous, which can cause them to stick to internal parts. Also, fluidized bed dryers can potentially enable the drying air to become saturated with moisture easily. Finally, energy consumption can be a factor.

Belt Dryers. Belt dryers have perpendicular gas and solid flows similar to fluidized bed drying. While belt dryers solve the issue of potential “carpet” formation encountered by fluidized bed dryers, they can only be used for low temperature drying and do not “de-dust” products. Further, belt dryers are limited in the ability to mix the product during drying. The product remains stationary relative to the other material during the process. As a result, the quality of dried material may be inconsistent.

By comparing the advantages and disadvantages of the current drying technologies, it is possible to create a list of ideal dryer characteristics:

  • Ability to process a scope of different waste.
  • Deep bed of bulk material.
  • Long retention time.
  • Extensive mixing of the material.
  • Ability to run on waste heat (low heat/secondary heat).
  • Low air velocities.
  • Low pressure drop.
  • Low electrical consumption.
  • Efficient “de-dusting” of the product.
  • Removal of impurities (sand, etc.).
  • Simple design.
  • Acceptable overall dimensions.
  • Low weight for the moving/rotating parts.
  • Easy access for maintenance.

Addressing the Challenge of Organic Drying

For organic drying, the combination of challenges to traditional dryer technologies includes the ability to:

  • Provide sufficient residence time for the material.
  • Prevent partial overheating of the material.
  • Use secondary heat.
  • Prevent poor solid mixing and heavy construction expense.

One solution is to combine the features of fluidized bed drying with rotary drum drying to create a hybrid dryer. Called a rolling bed dryer, it combines the advantages of fluidized bed and rotary dryers. Figure 1 shows a cross-sectional view from the front and side.

The hybrid dryer provides gentle drying at low temperatures, which allows for energy efficiency while thoroughly drying organic materials. A compact bed helps ensure optimum heat exchange with a design that allows for long retention time. The result is homogeneous drying of the organic materials. In addition, the hybrid drying of system allows for an adjustable velocity of the drying air to allow processing of different waste types.

Tests on this dryer have demonstrated that a wet input material with 42 percent residual moisture (irregularly sized material and bulk density of 160 kg per cubic meter) was converted to dry material with a residual moisture content of 4 percent (bulk density of 99.5 kg per cubic meter).

This new approach has made drying organic materials for biofuels less challenging than it has been in the past.