Coal gasification is sometimes called "clean coal" because it can be used to generate electricity without belching toxins and carbon dioxide into the atmosphere. But it's still based on a nonrenewable fossil fuel. And it still requires mining operations that scar the Earth and leave behind toxic wastes of their own. Wood gasification -- or biomass gasification, to be more technically correct -- may provide a viable alternative. Biomass is considered a renewable energy source because it's made from organic materials, such as trees, crops and even garbage.
Biomass gasification works just like coal gasification: A feedstock enters a gasifier, which cooks the carbon-containing material in a low-oxygen environment to produce syngas. Feedstocks generally fall into one of four categories:
- Agricultural residues are left after farmers harvest a commodity crop. They include wheat, alfalfa, bean or barley straw and corn stover. Wheat straw and corn remnants make up the majority of this biomass.
- Energy crops are grown solely for use as feedstocks. They include hybrid poplar and willow trees, as well as switchgrass, a native, fast-growing prairie grass.
- Forestry residues include any biomass left behind after timber harvesting. Deadwood works well, too, as do scraps from debarking and limb-removal operations.
- Urban wood waste refers to construction waste and demolition debris that would otherwise end up in a landfill. Pallets -- flat transport structures -- also fall into this category.
The choice of feedstock determines the gasifier design. Three designs are common in biomass gasification: updraft, downdraft and crossdraft. In an updraft gasifier, wood enters the gasification chamber from above, falls onto a grate and forms a fuel pile. Air enters from below the grate and flows up through the fuel pile. The syngas, also known as producer gas in biomass circles, exits the top of the chamber. In downdraft or crossdraft gasifiers, the air and syngas may enter and exit at different locations.
The choice of fuel and gasifier design affects the relative proportions of compounds in the syngas. For example, wheat straw placed in a downdraft gasifier produces the following:
- 17 to 19 percent hydrogen gas
- 14 to 17 percent carbon monoxide
- 11 to 14 percent carbon dioxide
- Virtually no methane
But charcoal placed in a downdraft gasifier produces the following:
- 28 to 31 percent carbon monoxide
- 5 to 10 percent hydrogen gas
- 1 to 2 percent carbon dioxide
- 1 to 2 percent methane
Now you're ready to make your own wood gasifier. Keep clicking to see how.