How Gasification Works

There's another old process -- one you probably don't know much about -- that's gaining in popularity and may join wind and hydropower in the pantheon of clean, renewable energy. The process is known as gasification, a set of chemical reactions that uses limited oxygen to convert a carbon-containing feedstock into a synthetic gas, or syngas.

It sounds like combustion, but it's not. Combustion uses an abundance of oxygen to produce heat and light by burning. Gasification uses only a tiny amount of oxygen, which is combined with steam and cooked under intense pressure. This initiates a series of reactions that produces a gaseous mixture composed primarily of carbon monoxide and hydrogen. This syngas can be burned directly or used as a starting point to manufacture fertilizers, pure hydrogen, methane or liquid transportation fuels.The heart of a power plant that incorporates gasification isn't a boiler, but a gasifier, a cylindrical pressure vessel about 40 feet (12 meters) high by 13 feet (4 meters) across. Feedstocks enter the gasifier at the top, while steam and oxygen enter from below. Any kind of carbon-containing material can be a feedstock, but coal gasification, of course, requires coal. A typical gasification plant could use 16,000 tons (14,515 metric tons) of lignite, a brownish type of coal, daily.

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:

 with mechanical casting: 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.1989 specifically for small-scale gasification in the event of a petroleum emergency. The agency's report includes detailed, illustrated instructions for the fabrication, installation and operation of a downdraft biomass gasifier generator. (A link to the report is included in the source list under "Lafontaine" on the next page.) The unit requires a galvanized metal trash can, a small metal drum, common plumbing fittings and a stainless steel mixing bowl and can be mounted on a vehicle to provide syngas for internal combustion. With the gasifier in place, the vehicle can run reliably using wood chips or other biomass as the fuel.