Science - December 22, 2010

One step closer to producing ethanol from straw

To produce bioethanol from straw, you need organic acids, discovered Maarten Kootstra. Lab trials using wheat straw and maize stalks proved successful. All he needs now is funding for a test reactor.

It is quite easy to make bioethanol from, say, sugar beets. You just add yeast to beet pulp and convert the sugars to alcohol through fermentation. But producing bioethanol from straw is quite a lot trickier. The sugars in straw are locked into the lignocellulose fibres which give the plants their strength. You first have to break down the lignin component so that the cellulose becomes available. Then you can produce sugars for the yeast, using enzymes. You break down the lignin by adding an acid while you heat the straw in a reactor.

A commonly used acid in experimental biofuel reactors is sulphuric acid, which makes the cellulose more available for yeast, but also has several disadvantages. During the process, the sulphuric acid breaks down some of the sugars into compounds which obstruct the production of bioethanol by the yeast, lowering the yield. The use of sulphuric acid in large-scale production also creates a lot of waste. For this source of biofuel to be of any use, therefore, alternatives to sulphuric acid are needed.

Kootstra tested two organic acids: maleic acid and fumaric acid. Treating straw with maleic acid produced at least as many sugars as treatment with sulphuric acid, but without the above-mentioned side effects. 'The waste product is valuable and not rubbish', says Kootstra. Treatment with fumaric acid is somewhat less effective but has the advantage that you can make the fumaric acid yourself from some of the sugars released. That reduces the raw materials you need to keep the process going. Wageningen UR has applied for a patent on the discovery.

Livestock feed
Besides the production of ethanol, this process could also be used to produce livestock feed. Because a reactor using organic acids can pre-digest the fibre-rich feed, it makes it more digestible for the animals. That is why livestock feed producer Agrifirm is funding Kootstra's research.

The two organic acids are working well in the lab at Food and Biobased Research (FBR), but can the process easily be upscaled? Here, Kootstra refers us to Jokko Dekker at FBR. He is very keen to set up a testing facility to convert fifty kilos of straw per hour into livestock feed and bioethanol. However, there is a hitch, says Dekker: one vital piece of technology for such a reactor is missing. 'The question now is: who is going to develop that technology and fund it?'

Maarten Kootstra received his PhD on 13 December from Johan Sanders, professor of Valorization of Plant Production Chains.