298 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
Fc = 0.9 - Fl - F„ (4) 
Substituting this into equation 3 gives: 
'^EIR ^E/H^H 
From equation 5, the effect can be calculated of 
hemicellulose content and conversion yield on the 
overall conversion of biomass to ethanol. Assuming a 
lignin content of 15 percent (F^ = 0.15) and using Y^/c 
= 0.57 g/g the following equation is obtained: 
Ye,b = 0.43 + Fh(Ye,h - 0.57) (6) 
The theoretical yield value on hemicellulose, 
Ye,h, is not well-defined because so little is known 
about the biochemistry of anaerobic pentose 
metabolism. If one mole of ethanol is produced per 
mole of xylose, the yield is 0.3 g ethanol/g xylose. It 
two moles of ethanol could be obained, Y^/h would 
be 0.61; however, neither the mechanism nor the 
thermodynamics of the conversion is sufficiently 
well-defined to allow one to expect this value. The 
maximum observed values are about 0.41 g 
ethanol/g xylose.® The sensitivity of the overall 
yield to this value is shown in figure I-D-4. The im- 
pact of pentose utilization depends on the amount 
^S. D. Wang and C. Cooney, Massacliusetts Institute of Technology, un- 
published results. 
Figure l•D•4.— Effect of Pentose Yield on 
Overall Yield of Ethanol from Cellulosic Biomass 
(Ye/b) with Varying Fractions of Hemiceilulose (Fh). 
SOURCE: Massachusetts Institute of Technology. 
of hemicellulose present. From the value in figure 
I-D-4 and the observation that 70 percent of the 
manufacturing cost is the raw material cost, it is 
possible to estimate tbe economic benefit of pen- 
tose utilization. Equation 7 relates the overall 
ethanol yield to the manufacturing cost: 
Cy = X M (7) 
Ye® C-7 
where is the manufacturing cost per gallon of 
ethanol, Cg is biomass cost (cents/lb), 6.6 is the con- 
version from pound to gallon of ethanol, and 0.7 is 
the 70-percent factor for relative biomass cost to 
ethanol cost. For a biomass costing 2 cents/lb and 
containing 20 percent hemicellulose, the manufac- 
turing cost is reduced from 59 to 43 cents/gal, when 
the yield on pentose goes from zero to 0.6. 
At the present time, there are few organisms that 
produce more than one mole of ethanol per mole of 
pentose and none of the usual alcohol producing 
yeasts will ferment pentoses to ethanol. Addition or 
improvement of the ability to use pentose will ha\ e a 
major impact on the economics of ethanol produc- 
tion. 
The second major cost in ethanol production re- 
lates to the cost of operation. Typically, 20 to 30 per- 
cent of the final manufacturing cost is accounted for 
by the sum of labor, plant o\ erhead, administration, 
chemical supplies, and fuel costs. The chemical suf)- 
plies represent less than 1 cent/gal ethanol and may 
be neglected. Tbe labor, overbead, and marketing 
costs vary with plant size, but represent 11 to 7 
cents/gal for a 20 to 100 million gal/yr plant, res[)ec- 
tively. Any improvement in tbe reduction of plant 
size or complexity will reduce this cost; howe\ (‘r. the 
economic impact is small. Fhe major component of 
the operating cost is the fuel charge for plant op(*ra- 
tion and for distillation. Plant operations, eg., mix- 
ing, pumping, sterilization, starch gelatinization, 
biomass grinding, etc., represent about 20 to 30 per- 
cent of the energy cost. The remainder is for ethanol 
distillation and residual solids drying. Considerable 
effort has been focused on methods to impnnc the 
energy efficiency of distillation to reduce it from the 
160,000 Btu/gal required for hexerage alcohol. While 
considerable differences in opinion exist as to the 
minimum, a reasonable e.x[)('ctation is about 40.000 
Btu/gal although current technology retiuires 6!). 000 
Btu/gal.® Forty thousand Btu is about half of the ener- 
gy content of ethanol per gallon. 
A discussion of process imiiroxcments relating to 
ethanol recovery has two coni|)on('nts I he first is 
"Report of the Casohol Sliulv Croup ol Ihe f.nerj^v He rar< h \d\i»or\ 
Board, Deparlnienl of Euiei'tw. U d!ihin«lon 1) ( *\l (,ilii. .out H 
D DeMo.ss, "Klhanol Formation m I’srinlomonHf lindnrr; \r, h n - ' •' 
Biophys. 34:47H-479, I9.X1 
