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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.
A gasifier operates at higher temperatures and pressures than a coal boiler -- about 2,600 degrees Fahrenheit (1,427 degrees Celsius) and 1,000 pounds per square inch (6,895 kilopascals), respectively. This causes the coal to undergo different chemical reactions. First, partial oxidation of the coal's carbon releases heat that helps feed the gasification reactions. The first of these is pyrolysis, which occurs as coal's volatile matter degrades into several gases, leaving behind char, a charcoal-like substance. Then, reduction reactions transform the remaining carbon in the char to a gaseous mixture known as syngas.
Carbon monoxide and hydrogen are the two primary components of syngas. During a process known as gas cleanup, the raw syngas runs through a cooling chamber that can be used to separate the various components. Cleaning can remove harmful impurities, including sulfur, mercury and unconverted carbon. Even carbon dioxide can be pulled out of the gas and either stored underground or used in ammonia or methanol production.
That leaves pure hydrogen and carbon monoxide, which can be combusted cleanly in gas turbines to produce electricity. Or, some power plants convert the syngas to natural gas by passing the cleaned gas over a nickel catalyst, causing carbon monoxide and carbon dioxide to react with free hydrogen to form methane. This "substitute natural gas" behaves like regular natural gas and can be used to generate electricity or heat homes and businesses.
But if coal is unavailable, gasification is still possible. All you need is some wood.
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:
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