Wood as a Fuel Source

Characteristics
By far the most popular and easiest to obtain fuel source is wood. Wood has some very important characteristics that make it a good fuel source for small scale gasification:
- Readily available in many parts of the world.
- Wood residue is a waste stream.
- Wood is generally low in nitrogen when compared to crops like grasses.
- Many woods have a low ash content.
- Wood is low in sulfur, making it possible to run engines without destroying them. (tar permitting)
- Wood is dense and occupies minimal space.
- Wood waste is cost effective.

The downside to wood:
- When it is gasified it creates tars that have to be thermally cracked and filtered.
- Wood has to be prepared in a uniform size and moisture content to be usable in most gasifier designs.
- Bridging can occur with small wood chips. Bridging is when the fuel heats up and sticks together, no longer allowing it to pass down through to the hearth zone.

Fuel Preparation
Wood can be chipped into blocks the size of your thumb or pelletized into a densified and uniform shape.
Moisture content of 20% or less is important to operation in small downdraft units. Industrial scale units with fluidized beds can handle moisture contents of up to 50% when the gas is used in an external combustion system.(ie steam, process heat)