256
Water for Energy and Fuel Production
9.4.7 FuTure direCTionS For CelluloSiC eThAnol
While the future for cellulosic ethanol is very bright, the future efforts need to
address following issues:
1. While each step, pretreatment, hydrolysis, and fermentation are separate and the needs to further develop their interconnections separately
are also very important. For optimization of the cost, an integration of
these steps (such as the SSF process described earlier) needs to be further
evaluated.
2. More work on the development of enzymes and yeasts that are more tolerant to the product inhibition needs to be carried out. This can be helped by
the use of genomics, proteomics, and metabolic engineering techniques for
plant systems that are applied to other living systems.
3. From the cost point of view, full use of all parts of plants, namely, cellulose,
hemicellulose, and lignin for coproduct development needs to be further
considered. The development of the enzyme and yeasts that can simultaneously convert both glucose and xylose needs to be further evaluated. More
efficient lignin separation and refining should be further explored.
4. The energy consumption for various unit operations such as distillation and
extraction should be further optimized. The transport and storage of biomass feedstock is also an issue that needs to be addressed. Larger-scale
operations need to be considered.
5. The work on cellulosic ethanol should be extended to other alcohols, especially butanol, which is discussed in Section 9.5.
9.5 FermentatiOn OF sUGar tO isOBUtanOl
Recently, Quereshi et al. [1] presented a review of recent advances in fermentation
of isobutanol from various carbohydrates and starch materials. They examined the
effectiveness of a number of microbes for the fermentation of various feedstock
such as wheat and barley straws, corn stover, switchgrass and dried distillation
grains and solubles. Isobutanol is produced in two phases and always found in the
mixture of acetone–butanol–ethanol (ABE). Some of their conclusions are outlined as follows:
1. The experiments performed so far gave low productivity due to the toxicity
of butanol to the culture.
2. Clostridium beijerinckii was found to be the best overall culture followed
by Clostridium actobutylicum for butanol production.
3. Escherichia coli strains and S. cerevisiae microbes have also been examined, but they gave low butanol production.
4. Simultaneous removal of butanol while fermentation significantly improved
the production rate of butanol (from 1.2 g/l to 461 g/l in batch operation).
5. More butanol-tolerant strains using genetic engineering techniques need to
be pursued.
Water for Energy and Fuel Production
9.4.7 FuTure direCTionS For CelluloSiC eThAnol
While the future for cellulosic ethanol is very bright, the future efforts need to
address following issues:
1. While each step, pretreatment, hydrolysis, and fermentation are separate and the needs to further develop their interconnections separately
are also very important. For optimization of the cost, an integration of
these steps (such as the SSF process described earlier) needs to be further
evaluated.
2. More work on the development of enzymes and yeasts that are more tolerant to the product inhibition needs to be carried out. This can be helped by
the use of genomics, proteomics, and metabolic engineering techniques for
plant systems that are applied to other living systems.
3. From the cost point of view, full use of all parts of plants, namely, cellulose,
hemicellulose, and lignin for coproduct development needs to be further
considered. The development of the enzyme and yeasts that can simultaneously convert both glucose and xylose needs to be further evaluated. More
efficient lignin separation and refining should be further explored.
4. The energy consumption for various unit operations such as distillation and
extraction should be further optimized. The transport and storage of biomass feedstock is also an issue that needs to be addressed. Larger-scale
operations need to be considered.
5. The work on cellulosic ethanol should be extended to other alcohols, especially butanol, which is discussed in Section 9.5.
9.5 FermentatiOn OF sUGar tO isOBUtanOl
Recently, Quereshi et al. [1] presented a review of recent advances in fermentation
of isobutanol from various carbohydrates and starch materials. They examined the
effectiveness of a number of microbes for the fermentation of various feedstock
such as wheat and barley straws, corn stover, switchgrass and dried distillation
grains and solubles. Isobutanol is produced in two phases and always found in the
mixture of acetone–butanol–ethanol (ABE). Some of their conclusions are outlined as follows:
1. The experiments performed so far gave low productivity due to the toxicity
of butanol to the culture.
2. Clostridium beijerinckii was found to be the best overall culture followed
by Clostridium actobutylicum for butanol production.
3. Escherichia coli strains and S. cerevisiae microbes have also been examined, but they gave low butanol production.
4. Simultaneous removal of butanol while fermentation significantly improved
the production rate of butanol (from 1.2 g/l to 461 g/l in batch operation).
5. More butanol-tolerant strains using genetic engineering techniques need to
be pursued.