Appendix l-D—The Impact of Genetics on Ethanol— A Case Study • 295 
Figure I D-2. Alternative Schemes for Acid Hydrolysis of Cellulosic Biomass for Ethanol Production 
Acid 
Acid 
Cellulosic 
biomass 
SOURCE; Massachusetts Institute of Technology. 
tion. Furthermore, if acids, alkali, or organic chem- 
icals are used, they must be recycled to minimize 
cost or disposed of in order to prevent pollution. 
In starch processing, prior to ethanol fermenta- 
tion, mechanical grinding, steam, and enzymes are 
employed. The energ\' requirements are small and 
contribute relatively little to the final ethanol cost. 
The objecth e in the development of cellulose-based 
processes should be to minimize both energy and 
chemical requirements. The development and scale- 
up of effective pretreatment technology are under 
acth e investigation^ and require continued financial 
support to better de\ elop se\ eral alternati\ e routes. 
The most promising routes are: steam treatment, sol- 
\ ent delignification, dilute acid, cellulose dissolution, 
and direct fermentation. 
Se\eral different acid hydrolysis schemes ha\e 
been proposed. Ho\ve\er, most appear as in flow 
scheme A or B in figure I-D-2. Dilute acid is used to 
hydrolyze the hemicellulose to pentose sugars pri- 
marily and then stronger acid at higher tempera- 
^Proceedings of 3rd Annual Biomass Energy System Conference, National 
Technical Information Service, SERI TP-33-285, 1979. 
tures is used to cause cellulose hydrolysis (scheme 
A). A major problem with this approach is the irre- 
\ ersible loss of sugars to undesirable side-product 
formation. After separation of residual solids (mostly 
lignin), which can be burned to provide energy for 
distillation, the sugar solution is fermented by yeast 
to ethanol. The pentose sugars also can be fer- 
mented, but by organisms other than the ethanol 
producing yeast, to other chemicals, some of which 
could be used as fuels (e.g., ethanol, acetic acid, 
acetone, butanol, 2,3-butanediol, etc.). 
An alternative (scheme B, figure I-D-2) to the above 
is to use a solvent, after pentose sugar removal, to 
dissolve the cellulose, allowing its separation from 
lignin. This cellulose solution is easily and efficiently 
hydrolyzed to sugars. The advantage of this ap- 
proach over the direct acid hydrolysis is that the 
yield of sugar is much higher. In the harsh acid hy- 
drolysis, considerable sugar is destroyed. However, 
the major disadvantage of both these schemes is that 
they require recycling or disposal of acids and 
solvents. A second problem is that almost nothing is 
known about how to scale-up some of the newly de- 
