Plant power


By Harvey Black FUEL made from plants could soon be providing cheap energy for rural areas in developing countries. Researchers in the US have found a way to generate much more concentrated gas from farm waste. Heating biomass breaks it down into gases such as methane and carbon monoxide. Traditional gasifiers generate the necessary heat by burning the biomass in air. Unfortunately the nitrogen in air dilutes the gas produced. “The gasifiers typically produce a gas with a heating value that’s only 4500 kilojoules per cubic metre,” says Robert Brown of the Center for Coal and the Environment at Iowa State University in Ames. “That’s roughly an eighth of the heating value of natural gas.” By separating the burning and heating processes, Brown and his team have produced gas with a heating value of 14 800 kilojoules per cubic metre. In the prototype gasifier, which is a tube 0.5 metres in diameter and 2.5 metres tall, the biomass is first burnt in air for about eight minutes. The heat from this process is absorbed by metal ballast, which reaches temperatures of up to 760 °C. Next, steam generated externally is forced into the cylinder to transfer the heat from ballast to biomass. Cooling the mixture makes the steam condense, leaving a gas rich in methane, hydrogen and carbon monoxide, but largely free of nitrogen. Brown expects the final version to be just as good at producing gas as other methods such as anaerobic digestion and fermentation. Gary Staats, a fuel development specialist with the US Department of Energy, says the simplicity of the gasifier boosts its appeal. “It’d be a real positive [development] in reducing the cutting of trees to produce fuel,” he says. The gasifier could be particularly useful in India, which produces enormous amounts of bagasse,
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