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Stirring up the biofuel mix

8 March 2012 Dr Gareth Evans

The world's thirst for biofuel is growing but there is a clear conflict of interests when it comes to land use. With rising food prices as much of a global issue as climate change and energy security, Dr Gareth Evans finds out why recent chemical advances to help improve biofuel production could not be timelier.

"We are poised to move beyond the obvious inefficiencies of corn-based ethanol," said Ronald T. Raines, Henry Lardy Professor of Biochemistry and Professor of Chemistry at the University of Wisconsin-Madison, and the man behind the first, one-step synthesis of a biofuel precursor from horticultural waste.

Non-edible biomass

"Chemical approaches to biofuel production could arguably hold greatest promise in facilitating the utilisation of non-food biomass, thereby reducing competition with agriculture over land capacity."

Back in 2009, his team successfully converted 'corn stover' - the inedible residue of maize production - directly into 5-hydroxymethyl-fufural (HMF), a compound which can be further processed into a range of other commodity chemicals, including the promising potential fuel dimethylfuran (DMF).

The process used N,N-dimethylacetamide (DMA) containing lithium chloride as a solvent, and a chromium catalyst, potentially providing a simple and efficient route for chemical biofuel production.

Three years on and the work to develop this continues. Raines still believes that furanics have enormous long-term potential, though he concedes that they face a range of political, regulatory and other hurdles, at least for the immediate future.

"We need to make fermentable sugar inexpensively from non-edible biomass, that should be the major focus of researchers in this field" Raines said. While he acknowledges that there are many attractive ways - both chemical and microbiological - to go from sugar to producing biofuels, he suggests that chemical processing is the most sensible way to make that sugar in the first place.

"I believe that organic chemistry is a far more feasible approach to sugar production than are biochemical - enzymatic - approaches. Lignin is not an impediment, and protons are much cheaper and less fragile catalysts than enzymes are."

Chemical approaches to biofuel production could arguably hold greatest promise in facilitating the utilisation of non-food biomass, thereby reducing competition with agriculture over land capacity.

With some estimates suggesting that around a sixth of the world's corn supply - enough to feed 350 million people for a whole year - is burned in the form of the 10% ethanol additive in US gasoline, the logic is compelling.

Biofuel from waste

Researchers at the University of Maine took a major step along this path recently, with the discovery of a revolutionary new chemical process which can transform a range of waste materials, including forestry residues, domestic rubbish and construction waste into a novel hydrocarbon fuel oil.

Described as a "spin on the chemistry used for acetone production in the 1800s" by team-leader Associate Professor Clayton Wheeler, the process involves thermal de-oxygenation (TDO) - a simple approach which requires neither catalysts nor hydrogen.

"Finland's Aalto University, for example, recently developed a novel process for producing bio-butanol from wood waste."

The cellulose content of the feedstock is first converted to organic acids, which then undergo an acid-base reaction with calcium hydroxide to form calcium salts. Heated to 450°C in a constantly stirred reaction vessel, these salts react to form a dark coloured oil, containing a mixture of more than 200 individual hydrocarbon compounds, with boiling points which encompass the same range as conventional petrochemical-based jet fuel, gasoline and diesel.

With characteristics like those, it seems it is shaping up to be the perfect 'drop-in' fuel, and since most of the oxygen is removed during processing - only around one percent remains in the resulting oil - it is calorifically dense, retaining most of the initial energy potential of the original material.

Perhaps the most important feature of the process, however, is that at a stroke, it appears to open the way for the commercial production of biofuel from 'dirty' feedstocks.

While many other methods require clean raw materials or purified intermediate derivatives, all of which add expense and complexity to the production cycle, the TDO process can utilise contaminated sources, such as grocery store waste, which adds to its obvious appeal. As Wheeler himself has said, "anytime you can use something without having to separate it, your costs go down."

New chemical insight

It is not only such novel processes that can benefit from new chemical research. The complicated series of chemical reactions, for instance, that occur during long-established, high-temperature biofuel production methods, such as pyrolysis and gasification, had always remained unknown since the molecules involved are simply too large and the chemistry too complex, for computers to model adequately.

It seems that, now, this is about to change, with the discovery of a 'mini-cellulose' molecule by University of Massachusetts chemical engineers - reported in January 2012's issue of 'Energy & Environmental Science' - allowing the chemistry of high-temperature conversion to be investigated for the first time.

The molecule, a-cyclodextrin, behaves in exactly the same way as cellulose when it is converted into biofuel, and according to Assistant Professor Paul Dauenhauer who leads the team, it allows the reaction cascade that would otherwise take 10,000 years to simulate to be modelled in around a month.

With these faster computer models has come the ability to track the process of converting wood to its chemical vapour products - and particularly the formation of furans which are important for biofuel production.

It is what Dauenhauer describes as "the basic tool necessary to optimize biofuel reactors", and the team hope that this knowledge will feed into bioreactor design, ultimately enabling the derived bio-oils to be tailored to requirements in a very controlled way.

Long-standing issues

"As Wheeler himself has said, "anytime you can use something without having to separate it, your costs go down.""

Elsewhere, other long-standing biofuel issues are also receiving renewed investigation. Finland's Aalto University, for example, recently developed a novel process for producing bio-butanol from wood waste, while the UK's University of Sheffield is pioneering methods to improve algal harvesting, possibly helping this feedstock realise its long-vaunted promise.

The ongoing quest for efficient catalysts likewise received a boost with the demonstration of Ceimici Novel's aptly named 'Smart Catalyst' at last year's, 2011's, Biofuels International Expo - although details of its composition were not made public.

Promising a five-fold lift in processing capacity, the product facilitates trans-esterification within minutes rather than hours - and takes place in a single stage, which allows for continuous operation.

While the biofuel industry remains at a relatively early stage in its evolution, it is developing fast and is seemingly destined to make an increasingly important contribution to the future global energy mix. The innovative chemical solutions currently being developed appear certain to help it towards that goal.

Rapeseed is the main bioethanol crop in Europe, with corn playing the same role in the US.
Distilling sugar-cane ethanol on a commercial scale.
Work at University of Maine could open the way to producing biofuel from 'dirty' sources.
An F404 engine running on biofuel at the US Navy's Aircraft Test and Evaluation Facility.