Testing the new green biofuels

There is a rush to find new biofuels to replace declining stocks of oil products but how "green" are these new fuels? Will they provide clean alternatives to fuel oil or only create a new environmental problem?

Some of these questions are being answered by a specialised research group based in NUI Galway's Environmental Change Institute. Galway's Combustion Chemistry Centre carries out detailed tests on new and emerging fuel products and can measure just how "green" they are.

The centre is the only research unit of its kind in Ireland or Britain, explains its director, Dr Henry Curran. "We try to find the cleanest, most efficient fuels and identify the fuel and oxygen mix that will optimise performance," he says.

What happens to fuel and air as it burns is enormously complex as molecules are formed, broken and reformed in response to temperature and pressure. The centre's test equipment allows fuel combustion and the release of by-products to be studied in exquisite detail. "Some of the biofuels might be clean but can produce more harmful emissions," says Curran. "We need to understand the chemistry behind these reactions."

The centre, founded in 1992 by Galway's Prof John Simmie, remains the only such research group in the world. Its work will prove central to the safe introduction of any biofuels that the market here begins to consume, Curran believes.

"We look at how fuels burn in engines but we don't use an engine, we use a shock tube and a rapid compression machine," Curran explains. "We try to take out all of the physics behind this and only look at the chemistry."

Curran's group studies how fast fuel burns as a function of pressure, temperature and the fuel/air mix. The piston-driven compression machine simulates what happens inside an engine, working at temperatures in the 330 to 730 degree range. This range has expanded up to 1,730 degrees however following the purchase of a new device, a high-pressure "shock tube". The 12m-long device is ideal for studying the burning characteristics of second generation biofuels, Curran says.

"Current biofuels such as ethanol have some negative properties and the search is on for new biofuels which can be sourced from waste or from crops not used for human foodstuffs," he says. "So the presently used maize, soyabeans, sugar cane etc are not really suitable as feedstocks for producing renewable fuels."

The shock tube uses a high-pressure shock wave to pressurise, heat and ignite a fuel/air mix. About two-thirds of the tube length is filled with helium gas and pressurised up to 400 atmospheres. A diaphragm separates this side of the chamber from a low pressure fuel/ air/ argon mix held at about a 10th of an atmosphere. The diaphragm is broken and the helium rushes into the low pressure end, producing a shock wave that heats the mix before hitting and rebounding from the far end to further heat and pressurise the mix to ignition point. The researchers look for the delay before ignition, the additional pressures ignition causes in the tube and ignition temperature. They control fuel/air inputs so they know what is going into the tube and can analyse the combustion emissions afterwards. The rig allows them to test second generation fuel options such as dimethyl ether. It is a cleaner alternative to existing liquid or gas fuels and can be made from natural gas, coal or biomass.