A new way of refining biodiesel so that it works in standard diesel car engines could help broaden the use of renewable fuels, according to scientists.
Biodiesel made from plant material could present a more eco-friendly alternative to the fossil-fuel-derived diesel, or petrodiesel, that is currently used. In the European Union (EU), commercial diesel already has to contain at least 7 percent biodiesel.
But the molecular makeup of these fuels means they boil at different temperatures from petrodiesel, which means only specially designed engines can run on pure biodiesel or blends that contain considerable amounts of the fuel. [Top 10 Emerging Environmental Technologies]
Now, scientists in Germany have found a way to transform chemicals derived from plants into a biodiesel that meets the boiling characteristics required by the EN 590 standard, set by the European Committee for Standardization, for commercial diesel sold in the EU.
Lukas Gooßen, a professor of organic chemistry at the Ruhr-Universität Bochum who led the research, said his motivation came during a trip to a biodiesel plant in Rwanda a few years ago. The problem, he said, was that only a single bus had been converted to run on the fuel because of a lack of funds.
“If you’re going to use biofuel, you need existing infrastructure you can tap into,” he told Live Science. “Anything else will be unaffordable to most countries and societies. This has to be the starting point.”
The reason conventional biodiesel doesn’t work in standard diesel engines is that about 95 percent of its constituent molecules are the same length and, therefore, boil at roughly the same temperature.
In contrast, petrodiesel is made up of a mixture of hydrocarbons of different lengths and structures that boil at different temperatures, giving petrodiesel a much broader boiling range. Importantly, these are the boiling characteristics for which modern cars have been designed.
“All engines were specifically made to fit this fuel,” Gooßen said. “Diesel fuel and diesel engines evolved together.”
There are existing processes that can convert plant oils into biofuelsuitable for use in standard diesel engines, but a significant proportion of the fuel is burned to carry out the refining. Gooßen and his colleagues decided to find a way to take advantage of catalysts — substances that accelerate chemical reactions — to do the same thing at low temperatures, using very little energy.
In a paper published online today (June 16) in the journal Science Advances, they report that using a combination of three catalysts, they were able to blend rapeseed oil methyl ester (RME) — a common raw material for making biofuels — and ethylene into a fuel with the same boiling profile as petrodiesel. Ethylene is an abundant hydrocarbon that can be generated from plant-derived ethanol or shale gas.
To demonstrate its potential as a motor fuel, the researchers built a model diesel car and used the biodiesel to propel it. The fuel is still a long way from commercialization, though; the first batch cost more than $1,125 per quart to produce. [Hyperloop, Jetpacks & More: 9 Futuristic Transit Ideas]
The main issue with the current process is that it uses expensive, short-lived catalysts aimed at making small amounts of high-value chemical products, Gooßen said. Finding cheaper, more robust alternatives suitable for commercial production will be a huge challenge, he added, as will designing effective high-throughput industrial processes.
But Gooßen said one of the biggest barriers to wider use of biodiesel is its incompatibility with standard engines. But his new study shows that this challenge can be overcome.
“This boiling point seemed to be a completely insurmountable hurdle,” he said. “Now, we are saying, ‘How can we make a catalyst that just does this cheaper?’ which is a completely different question to, ‘How can we even come from biodiesel to something we can put in a car?'”
There is a pressing need to find sustainable fuels that are “drop-in replacements” for current fossil fuels, said Duncan Wass, a professor of catalysis at the University of Bristol in the United Kingdom.
Using a cheap and abundant addition such as ethylene to transform biofuels makes good sense, he said, but he agreed with the study authors that the current catalysts are not fit for that purpose and that finding appropriate replacements will be hard.
“[I]t will be difficult to find a simple, cheap, long-lived, heterogeneous catalyst that can achieve the same results as these very sophisticated systems,” Wass told Live Science.
It is also too early to label the technology as “sustainable,” he added, until a full life-cycle analysis of the process and products is undertaken.