New Recipe For Biodiesel Calls For Lime

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CHEAPER THAN CROPS? Recycling used cooking oil with an environmentally friendly catalyst could offer an alternative to crops grown solely for biofuels.

A new method uses hydrated lime to transform soybean oil, recycled from other uses, into biodiesel, researchers report (Energy Fuels, DOI: 10.1021/ef200555r). The new method may provide an environmentally friendly route to biofuels, the team suggests.

Crops grown solely for fuel have been controversial because of the related loss of valuable food-producing land, forest, and other ecosystems, as well as the drain on water resources to grow some crops in water-strapped regions. But used vegetable oil is simply waste, and many researchers see it as a desirable source of raw materials for biodiesel, a replacement for fossil fuels made from plant or animal fats.

Likewise, Manuel Sánchez-Cantú of the Autonomous Meritorious University of Puebla, in Mexico, and his colleagues realized that hydrated lime, or calcium hydroxide, is also readily available because of its many uses. Lime plays roles in many commercial processes, such as food preparation, water treatment, and petroleum refining. The team recognized that other researchers had found that another type of lime, calcium oxide, known as quicklime or burnt lime, efficiently catalyzes the biodiesel transesterification process, which turns vegetable oil into the fuel by peeling away the oil’s glycerol backbone and reacting it with methanol.

The researchers performed some kitchen chemistry to see whether the process would work with hydrated lime. They used lime that they purchased in 25-kg sacks from a lime seller, in Perote, Mexico, and soybean oil that they bought from a local grocery store. After frying potatoes in the oil, the team added methanol and lime to the used soybean oil.

After two hours at reaction temperatures of around 60 °C, the process yielded conversion rates approaching 100%. Plus, they could reuse the lime several times. (By the fourth use, however, yields fell: Only about two-thirds of the waste soybean oil converted to biofuel.)

Pål Börjesson, a professor of Environmental and Energy Systems Studies at Lund University, in Sweden, says that using commercially available lime as a catalyst is novel. The nontoxic material is common, cheap, and environmentally friendly compared to other active catalysts, such as zinc oxide or alumina. However, he notes, products made from waste cooking oil could replace only a tiny percentage of commercial fuels because of the huge demand for fuel for transportation and other uses. The pool of waste oil is limited, he says, but “we should use it as much as we can.”

Since researchers have found success in creating biofuels with other kinds of solid catalysts, examining hydrated lime is worth trying, comments C.R. Krishna, an emeritus researcher of Brookhaven National Laboratory. “However,” he adds, “I am not sure these efforts translate to substantial cost savings,” because of costs to collect the waste oil, obtain methanol, and run the process.

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