Chemistry World, UK
06-28-07
To chew or to burn?
There's a positive buzz of research and bold investment surrounding second
generation biofuels. Turning woodchip, prairie grasses, corn stover and general
biomass waste into fuel to drive our cars ticks all the energy boxes. It reduces
dependence on imported oil, cuts carbon emissions from fossil fuels, and
crucially doesn't deplete valuable food resources. Yet while politicians get excited
and companies try to lower engineering costs for commercial development,
scientists are still asking which are the best - and cheapest - ways of unlocking
the energy stored in the cellulose, lignin and hemicellulose bound up in plant
waste.
Burning the lot is one option, producing heat and electricity. But cars don't
traditionally run on electricity. Lignocellulosic biomass has to be converted to
liquid fuel to make inroads on petrol and take advantage of that industry's
existing infrastructure.
In the familiar biochemical route, adapted from the mature US grain fermentation
industry, chemicals and enzymes break up tough lignocellulose into sugars which
are then fermented, usually to ethanol. There is another option: adapting
thermochemical processes, first developed for coal, to biomass. High
temperatures and pressures convert lignocellulose to carbon monoxide and
hydrogen (syngas), which is transformed via the Fischer-Tropsch reaction into
methanol, ethanol, or diesel, or kept as hydrogen. Variations on these themes
range from pyrolysing biomass into liquid oils, to producing biobutanol.
"I get tired of claims that one costs more than another, when no-one can point to
a meaningful comparative study"
- Robert Brown, Iowa State University, US
Which is best? A new meta-analysis has concluded that the current costs of
biochemical and thermochemical ways of turning waste biomass into liquid fuels
are surprisingly similar. Only combining the two can make cellulosic biofuels
economically competitive, say researchers. At first glance it's not obvious which
options have most to offer. 'I get tired of claims that one costs more than another
- when no-one can point to a meaningful comparative study,' Robert Brown,
who works on renewable fuels at Iowa State University, US, told Chemistry
World. Thermochemical and biochemical routes have different optimum plant
sizes, feedstock costs, byproducts (which may add value), output fuels, and
relationships to existing industry: so working out the resources each uses to
produce the equivalent of a gallon of petrol is not easy.
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.
"I have all but given up on cellulosic ethanol"
- David Bransby, Auburn University, US
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.'
Richard Van Noorden