To chew or to burn?

By Richard Van Noorden 
June 29, 2007

Brown presented his own comparative analysis at a June biofuels conference in Ghent, Belgium. His conclusions may surprise bug-lovers: using current technology, a gallon of petrol equivalent costs about the same ($1.78) to produce from an enzyme-run cellulosic plant as from a Fischer-Tropsch gasification plant of the same size. (Unfortunately, commercial scale cellulose plants are all extremely expensive to build compared to their grain ethanol predecessors.)

Bugs can chew as much as they like - and their access is aided by expensive pre-treatment of tough polymers - but currently they simply can't convert as much carbon to liquid fuel as hi-tech stoves that turn all plant waste, including indigestible lignin, into a gas. Of the energy stored in lignocellulose, only 38 per cent gets translated into liquid fuel by today's purely biochemical routes, Brown estimates. That figure may improve, but thermochemical routes already hit efficiencies of 45-50 per cent. 'I believe there's recent recognition that the thermochemical approach can be at least as good as the biochemical route in the transition from grain ethanol to cellulosic biofuels in the US,' says Brown.

Other researchers think he's understating the case. 'I have all but given up on cellulosic ethanol, and am looking much more optimistically at cellulosic diesel,' commented David Bransby, who works on energy crops at Auburn University, US. One company that agrees with him is Choren, which with Shell is opening a commercial scale (15 000 tonnes per year) cellulosic Fischer-Tropsch plant in Freiberg, Germany, at the end of 2007.

Combining bugs and burners together to attack biomass is the best option, says Lee Lynd, of Dartmouth University, US: for example, fermenting as much biomass as possible, then gasifying the lignin left over. Indeed, he points out, this joint approach is assumed by most biochemical proposals for making ethanol from cellulosic biomass. Brown suggests putting the bugs in last: fermenting the syngas which has been created from biomass waste, so bypassing the problems of expensive pre-treatment and wasted biomass. US firms BRI Energy and Coskata hope to make this vision a commercial reality.

With research breakthroughs, the picture could change rapidly. Lynd is working on microbes which can ferment ethanol directly from pre-treated cellulose. 'Projected mature biomass technology featuring fermentation has liquid fuels yields of 70 per cent,' he says. On the thermochemical side, Paul O'Connor, of BIOeCON, says his company has developed a catalyst system which converts biomass directly to oils, at lower temperatures and with more simple apparatus than required for gasification. Such catalytic cracking or depolymerisation, as it's known, should be less expensive than the Fischer-Tropsch process. Promising also are chemical ways of turning sugars into higher energy liquid fuels (see page 23) - if they can be adapted to work on cellulosic biomass at commercial scales.

For now, the main lesson from Brown's analysis may be that talking to the other side can result in benefits for all. 'People do thermochemical routes at one set of meetings, and biochemical at another,' he says. 'It's rare to find someone with interest and expertise in both areas.'

Source: Chemistry World