BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 433

fermentative production routes of biofuels can be a daunting task, especially when
metabolic routes toward these fuels are hypothetical or even unknown. Still, research-
ers and people funding and judging research have to make proper choices early on.
This section is meant to partly simplify such choices. Using basic stoichiometry
and thermodynamics, one can determine the potential of fermentative processes
for hypothetical biofuels and compare these to ethanol. Kinetic limitations are of
a less fundamental nature and will not be taken into account in the following
discussion.
Assuming glucose or an equivalent carbohydrate as the feedstock, the overall
fermentation reaction considered is

C 6 H 12 O 6 ! x C a H b O c + y CO 2 + z H 2 O ð RX : 13 : 5 Þ

Thus, only anaerobic fermentative production of biofuels is considered. Aerobic
fermentation is excluded from this evaluation as the reaction with O 2 leads to partial
combustion of the available glucose and thus does not lead to the highest possible
biofuel yields. In addition, anaerobic fermentations may be carried out in simpler
equipment than aerobic fermentations, since no aeration is required. However, under
anaerobic conditions, the microorganisms may not generate sufficient ATP to grow.
Therefore, we will assume that (previously grown) cells are retained in the fermentor.
We will also assume that the small amount of ATP produced by the reaction of the
aforementioned equation (RX. 13.5) is consumed in cell maintenance reactions.
If thermodynamics show that no ATP can be produced by the reaction, the reaction
product will be excluded as potential biofuel. The cell is considered as a black box, so
this analysis also includes products for which the metabolic pathway is presently
unknown.
The product may accumulate extracellularly up to a concentration that is toxic to
the cell. At that point, maintenance processes require ATP beyond what is produced.
Because this constraint has a kinetic nature and can potentially be solved by using
product-tolerant strains, it will be neglected in this analysis. Instead, it is assumed that
the achievable product concentration is determined by the glucose feed concentration
and the reaction stoichiometry. The fermentation is assumed to occur at thermody-
namic standard conditions (25 C and 1 bar). For thermophilic fermentations, how-
ever, this may be a poor assumption.
Using the three elemental balances (for C, H, and O) derived from the equation
(RX. 13.5), the stoichiometric coefficients x , y , and z can be calculated using the
product values for a , b , and c . For certain products (mainly carboxylic acids),
negative values of the stoichiometric coefficient result for CO 2 , implying that CO 2
is a cosubstrate rather than a coproduct. For the production of H 2 , water is a
cosubstrate.
The maximum theoretical mass yield of the biofuel product on glucose, Y max

prod = glc ,
can be calculated using the molecular weights (MW) according to Equation (13.1):

prod = glc = x ×MW product
MW glucose

Y max kg −1

kg : ð Eq : 13 : 1 Þ