Environmental Engineering Reference
In-Depth Information
was accomplished by early ethanol production by K. oxytoca, while ethanol produced in the
later stages was primarily by the more ethanol-tolerant strain [18].
2.4.4. Improved thermotolerance
. Ethanol fermentation at elevated temperatures (>55°C)
would facilitate product recovery, but thermophilic bacteria are poor ethanol producers. In
addition, thermophilic Clostridium and Thermoanaerobium species have been investigated for
potential as ethanol producers, but were consistently limited by end-product inhibition and
solvent-induced membrane damage [63].
In addition, efforts are underway to eliminate acid production during fermentation
through genetic engineering, enabling use of salt-intolerant thermophilic strains like
Thermoanaerobacterium thermosaccharolyticum, a microbe tolerant to high levels of ethanol
but intolerant of salt accumulation during pH-controlled fermentations. If such a
thermotolerant organism could be improved further to produce high-activity cellulases, a
highly productive, anaerobic, ethanol-producing strain could result. Cellulase production
could, however, pose an insurmountable energy burden to a fermentative organism; the
energetic considerations of this combination are being evaluated [5].
2.4.5. Fermentation of synthesis gas
. Rajagopalan and coworkers report the discovery of
a clostridial bacterium, P7, that converts mixtures of CO, CO
2
, and N2 into ethanol, butanol
and acetic acid, with high ethanol production and selectivity compared to previous isolates.
The authors report process parameters and consider options for improving ethanol yield [64].
2.5. Coproduct Development
Finally, the investigation of potential ethanol coproducts is underway. Biomass sugars
can support the production of many other products along with ethanol, including organic
acids and other organic alcohols, 1 ,2-propanediol, and aromatic chemical intermediates. If
these coproducts were sufficiently valuable, they could help greatly offset costs of ethanol
production. However, such coproducts must be chosen carefully to ensure that sufficient
markets are available [65].
Additional coproducts may be available from lignin: this material is present at 15-30
percent by weight in all lignocellulosic biomass, and any bioethanol production process will
have lignin as a residue. A team of researchers from the NREL, the University of Utah, and
Sandia National Laboratories is working to develop a process for making oxygenate fuel
additives from lignin; these processes are chemical in nature and are detailed in other
materials referenced [66- 68].
3. Research Priorities
The consensus among researchers and supporters of bioethanol research, in addition to
those engaged in commercial projects, is that the improvement of cellulase enzyme activity
and cellulase production, both to increase the efficiency of release of fermentable sugars from
biomass and to reduce cellulase cost, are two of the greatest advances needed in the effort to
commercialize fuel ethanol production [19, 5]. In addition is the development of enzymatic
pretreatment processes to release lignin from carbohydrate components [42, 9] and further
improvement of fermentative organisms [69, 64], with the particular goal of designing
microbes capable of consolidated bioprocessing [54].
